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    Charles, J. A. (2008). Gé otechnique 58, No. 7, 541–570 [ doi: 10.1680/geot.2008.58.7.541]

    541

    The engineering behaviour of ll materials: the use, misuse and disuseof case histories

    J . A . CHARLES*

    A number of developments in our modern world, includ-ing increasing urbanisation, major land reclamationschemes and the disposal of vast quantities of solid wastegenerated by mining and industrial activities, shouldensure that, for the foreseeable future, lls will be of increasing signicance in geotechnical engineering. Anengineered ll, which has been heavily compacted in thinlayers under closely controlled conditions, should be arelatively uniform material and have behaviour that iseasily predicted on the basis of average properties. Incontrast, poorly compacted ll dumped with little controlin deep lifts is likely to be in a loose state and exhibitgreat diversity in its geotechnical properties: the behav-iour of such heterogeneous ll will bear little relation toaverage properties, and will be controlled largely byzones of ll in a metastable state with unpredictablebehaviour. Case histories that include eld measure-ments—that is, quantitative data—are of particular valuein gaining an understanding of the performance of llmaterials. Case histories of ll behaviour are examinedin four areas of practical interest to the geotechnicalengineer: (a) the geotechnical behaviour of opencastmining backlls; (b) the performance of rockll dams;(c) the effectiveness of ground treatment; and (d) thecondition assessment of embankment dams. In each of these areas the lecture focuses on the results of long-termeld monitoring at a number of sites, from which some

    general conclusions are drawn. Discernment is requiredin the study of case histories, but despite shortcomings,they provide a much needed counterweight to excessivetheorisation.

    KEYWORDS: case history; compressibility; dams; deformation;embankments; eld instrumentation; ground improvement;monitoring; rockll; settlement; time dependence

    Un certain nombre de de ´veloppements survenant dansnotre monde moderne, y compris l’augmentation del’urbanisation, la pre ´sence d’importants plans d’ame ´n-agement du territoire ainsi que l’e ´limination de quantite ´sconside´rables de de ´chets solides, produits par les exploi-tations minie `res et les activite ´s industrielles, devraientassurer que, dans un avenir pre ´visible, les terrains rap-porte´s auront une importance toujours majeure en ge ´o-technique. Un terrain rapporte ´ spécialement ame ´nagé ,fortement compacte ´ en couches de faible e ´paisseur etdans des conditions contro ˆ lées de tre`s prè s, devrait e ˆtreun mate´riau relativement uniforme, pre ´sentant un com-portement facilement pre ´visible sur la base de proprie ´té smoyennes. Par contraste, un terrain rapporte ´ mal com-pacte´, rempli de façon peu contro ˆ lée avec des couchesprofondes est susceptible d’e ˆtre peu compacte ´ et de pre´-senter une grande diversite ´ de proprie ´té s géotechniques :le comportement de ces terrains rapporte ´s a trè s peu encommun avec ces proprie ´té s moyennes, et sera de ´termine ´en grande partie par des zones de remplissage a ` l’étatmé tastable et au comportement impre ´visible. On examinedes é tudes de cas de comportement de terrains rapporte ´sdans quatre d’inte ´rê t pratique pour le ge ´otechnicien :(a) le comportement ge ´otechnique de remblais de mises a `ciel ouvert ; (b) le comportement de barrages en enroche-ment ; (c) l’efcacite ´ du traitement du sol ; et (d) e ´valua-tion de l’e´tat de barrages en remblai. Dans chacune de

    ces caté gories, la communication se penche sur les re ´sul-tats de contro ˆ les à long terme sur le terrain dans uncertain nombre de sites, et a ` partir desquels elle tire desconclusions ge ´né rales. L’examen d’e ´tudes de cas ne ´cessiteun certain discernement, mais en de ´pit de ses insuf-sances, il fournit une compensation bien ne ´cessaire a` unetendance excessive aux grandes the ´ories.

    INTRODUCTION Importance of case historiesQuotations from great geotechnical men of the past haveoften been included in Rankine Lectures. In a deviation fromthis precedent, the following quotation is from a nineteenth-century prime minister. Benjamin Disraeli wrote:

    Read no history: nothing but biography, for that is lifewithout theory.

    In the nineteenth century, as in the twentieth and twenty-rstcenturies, not everyone was prepared to accept the advice of a prime minister, and undoubtedly this particular recommen-dation was based on an oversimplication of the true position. To appreciate life in a vanished age does requiresome general understanding of the times. However, an

    exaggeration can establish an important point, and biogra- phies—that is, accounts of the lives of real people—can givean insight into the human condition at a particular time and place that a general historical narrative, which is likely to beheavily biased by the preconceived ideas of the historian,may fail to do.

    Substituting ‘case history’ for ‘biography’, an analogousrecommendation to a geotechnical engineer would be: ‘Studyno theory: read nothing but case histories, for that is actualground behaviour undistorted by preconceived theoreticalconcepts.’ Such a recommendation would be unwise, for several reasons.

    (a ) The complexity of ground behaviour means that,without some preconceived ideas and a basic con-ceptual model of soil behaviour, it will not be possibleto make sense of eld observations, which will remain

    as unconnected facts. It is necessary to have atheoretical framework within which information fromcase histories can be assimilated.

    (b) As with biographies, case histories will not be full and

    Discussion on this paper closes on 2 March 2009, for further details see p. ii.* Building Research Establishment, UK.

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    impartial records of events, but inevitably will beinuenced by the perspective of the writer in theselection and presentation of material.

    (c) The subjects of case histories will tend to be theunusual, and the neglect of the ordinary case will meanthat they are unlikely to be representative.

    Nevertheless, despite these limitations, case histories have a

    vital role in geotechnical engineering, and are of far greater importance than in other branches of civil engineering.The overwhelming need for reliable experimental geo-

    technical data has long been appreciated. In 1881 BenjaminBaker published a paper on ‘The actual lateral pressure of earthwork’ . It might be questioned why the word ‘actual’was included in the title. Would not papers on lateral earth pressures always deal with ‘actual pressures’? A perusal of technical papers from Baker’s time up to our own day willsoon expose the error of such a naı ¨ve notion. Baker deplored the lack of experimental data, because it meant that ‘indivi-dual judgement has to be exercised in each instance’, and hereminded his readers that Professor Rankine (1864) con-cluded the section on earth pressure in his Manual of civil engineering with the following warning: ‘There is a mathe-matical theory of the combined action of friction and adhe-sion in earth; but for want of precise experimental data its practical utility is doubtful.’ The same comment could bemade about the use of any numerical soil model which lacksexperimental data to conrm its validity.

    Fifty-ve years after Baker’s paper was published, thegreat need for eld data was stressed by Karl Terzaghi(1936) in his presidential address to the First InternationalConference on Soil Mechanics and Foundation Engineeringwhen he afrmed that:

    . . . successful work in soil mechanics and foundationengineering requires not only a thorough grounding intheory combined with an open eye for the possible sources

    of error, but also an amount of observation and of measurement in the eld far in excess of anythingattempted by the preceding generations of engineers.

    The importance of making eld measurements was alsorecognised by Leonard Cooling (1945) of the Building Re-search Station, which later became the Building ResearchEstablishment (BRE), when speaking at the Institution of Civil Engineers in June of that year:

    As regards the future development of soil mechanics, Ithink the emphasis needs to be placed on the more practical engineering research side of taking measurementsand observations of full-sized structures and constructionalwork and of linking these with the properties of the soil

    strata at the site. Laboratory tests and theoreticalconsiderations, vital as they are, must be related to eld experience.

    In his Rankine Lecture, Cooling (1962) again stressed theimportance of eld measurements. As a diagnosis of whatwas needed and a recommendation of the direction thatgeotechnical engineering should take, the comments of Terzaghi and Cooling showed great insight, but when Terza-ghi added a word of prophecy to the effect that ‘the centreof gravity of research has shifted from the study and thelaboratory into the construction camp where it will remain’,he proved to be overly optimistic.

    The most valuable case histories are those where eld measurements have been used to monitor the geotechnical

    performance of structures over their working life, not justduring construction. Such case histories are relatively rare, because long-term monitoring projects are expensive and require continuity over a lengthy period. The long, contin-

    uous history of geotechnical research at BRE has made it possible to undertake long-term measurements of ground movements associated with many types of buildings and civil engineering works (Charles et al. , 1996).

    Signicance of ll

    Mankind has been creating ll throughout recorded his-tory. Some 4000 years ago, over a considerable period and for purposes that we cannot now determine, the 40 m highSilbury Hill in Wiltshire was carefully engineered in a seriesof six stepped horizontal layers. Its complex internal struc-ture was created by concentric rings of chalk block walls,which together with radial walls, formed compartments thatwere inlled with chalk rubble. Cross-sections of someengineered lls constructed during the last 4000 years areshown in Fig. 1, and details are provided in Table 1.

    It is a sad reection on human ‘progress’ that, 3000 yearsafter the building of Silbury Hill, William the Conqueror was throwing up mounds of earth, in what by then had become England, on which to build crude fortications for military purposes that are only too easy to recognise. In1069 a 15 m high mound was built on low-lying ground adjacent to the river in York to facilitate the subjugation of the north of England. The mound was built in horizontallayers of ll comprising stones, gravel and clay. Initially themound provided a base for a timber structure; the stonetower known as Clifford’s Tower was built on the mound inthe middle of the thirteenth century. Major cracking of thetower occurred in 1315–16 during severe oods, whichsoftened the ll.

    It was only in the nineteenth century, in the great age of embankment dam building, that earthworks as high as Sil- bury Hill were again constructed in England. In the twen-

    Clifford’s Tower Mound

    SilburyHill

    Dale Dyke Dam

    BrianneDam

    Nurek Dam

    0 50 m

    Scale

    Fig. 1. Some engineered lls, 2000 BC to AD 2000

    542 CHARLES

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    tieth century lls were placed on an unprecedented scale toconstruct embankment dams and road embankments, to formsites for buildings by inlling excavations, and in other forms of land reclamation. In the late twentieth century the90 m high Brianne Dam, which is the highest dam in GreatBritain, was built in Wales. Elsewhere in the world muchhigher embankment dams have been constructed: the 300 mhigh Nurek Dam, which was built in the old Soviet Union,is the highest dam in the world.

    These engineered ll structures are quite small compared with the vast quantities of non-engineered lls being dumped as mining, industrial, chemical, building, dredging, commer-cial and domestic wastes. The total volume of mining wastecurrently produced annually in the world is probably of theorder of 10 billion cubic metres (10 3 109 m3). This notonly dwarfs the volumes of engineered ll used on thelargest dam projects, but is also greatly in excess of the 1943 106 m3 placed in the massive land reclamation works for Chek Lap Kok Airport in Hong Kong.

    Use and nature of eld measurementsIn studying case histories of ll behaviour that include

    eld measurements, three basic questions need to be ad-dressed: What can be obtained from eld measurements?What types of properties or behaviour should be measured?How can eld measurements be made?

    (a) Benets of eld measurements . The most important benet for those responsible for the design, constructionand subsequent performance of an instrumented struc-ture is that its behaviour can be better assessed.Assuming that the case history is made publiclyavailable, the measured behaviour can also provide a benchmark for use in calibrating the behaviour of similar structures. The monitoring results also should enhance the general understanding of geotechnical behaviour.

    (b) Geotechnical properties and behaviour to be monitored .In different situations, pore water pressure, total stress,vertical and horizontal displacement and strain may bemeasured. Settlement is often the simplest parameter tomeasure, and in many cases it is the most critical performance criterion.

    (c) Instrumentation and equipment . Reliable and accurateeld measurements on large civil engineering sites aredifcult to achieve, requiring not only considerableskill, experience and perseverance in difcult condi-tions, but also signicant expenditure. In the casehistories that are presented here, surface settlement hasgenerally been measured using precise levelling techni-ques, but in some situations a theodolite has also been

    used. Subsurface movements have been monitored usingmagnet extensometers installed in boreholes in opencastmining backlls (Marsland & Quarterman, 1974;Charles et al. , 1977) and horizontal plate gauges

    installed during construction in embankment dams(Penman & Charles, 1973a, 1982). Descriptions of theinstrumentation can be found in references cited in thetext.

    Scope of the lectureHaving emphasised the growing signicance of ll and

    the importance of case histories, it is appropriate to movefrom general comments to particular case histories of ll behaviour in four areas of practical interest to the geo-technical engineer:

    (a ) the geotechnical behaviour of opencast mining backlls(b) the performance of rockll dams(c) the effectiveness of ground treatment(d ) the condition assessment of embankment dams.

    Since these are all large subjects that cannot be dealt with ina comprehensive manner, each section focuses on the resultsof BRE eld monitoring at a number of sites, from whichsome general conclusions are drawn.

    GEOTECHNICAL BEHAVIOUR OF OPENCAST MININGBACKFILLS

    In 1949 the Building Research Station (now BRE) pub-lished Digest No. 9, Building on made-up ground or lling ,which stated that

    Suitable sites for new buildings and estates in industrialareas are becoming more difcult to nd and it is morefrequently necessary to build on made-up ground or lling.

    The passing of nearly 60 years has not invalidated thisstatement, but the qualifying phrase ‘in industrial areas’ isnow no longer needed. Opencast mining has been a major producer of deep lls whose geotechnical behaviour is of critical importance when restored opencast sites are consid-ered for building development. The principal practical inter-est concerns the potential for long-term settlement of the backll. Where use of a restored site for building purposesis foreseen prior to backlling, the ll should be placed inlayers and heavily compacted to an appropriate specicationunder controlled conditions: such an engineered ll should be reasonably uniform, with a potential for settlement that is both limited and predictable. Where such a future use is notforeseen, or was ignored and backlling was carried outwith little control and without systematic compaction, thesituation is very different.

    Creep settlement soil model Early work on the settlement of ll was carried out at

    BRE by Meyerhof (1951), who, from a literature review, presented the long-term creep settlement data shown in Fig.2. Although the settlement of the ll materials varied from30% for domestic refuse to less than 1% for compacted

    Table 1. Engineered lls 2000 BC to AD 2000

    Structure Location Purpose Date built Height: m Volume:106 m3

    Surface area:ha

    Silbury Hill Wiltshire, England Unknown pre-2000 BC 40 0.25 2Clifford’s Tower Mound York, England Military 1069 15 0 .04 0.4Dale Dyke Dam Shefeld, England Retain water 1864 29 0 .4 4Brianne dam Llandovery, Wales Retain water 1971 90 2 .0 7

    Nurek Dam Tajikistan Retain water 1980 300 58 60Chek Lap Kok Airport Hong Kong Platform for airport 1996 25 194 1248

    THE ENGINEERING BEHAVIOUR OF FILL MATERIALS 543

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    rockll, the creep rate of all the lls diminished with time,leading to the conclusion that, if ll is left long enough, therate of settlement of the ground surface will become negli-gible. This comforting conclusion suggests a simple solutionto developing a lled site: leave the site for long enoughand signicant settlement will cease. But is this so, and if itis so, for how long must the site be left?

    For many rockll dams in the United States, Sowers et al.(1965) found an approximately linear relationship betweencrest settlement and the logarithm of time that had elapsed since the middle of the construction period, as shown in Fig.3. The values of the creep compression rate parameter Æ

    ranged from 0 .2% to 1.1%, where Æ is the compressionoccurring during one logarithmic cycle of time. This behav-iour is similar to the secondary compression of clay soils.The value of Æ was not related to the type or strength of the parent rock, the form of dam construction (e.g. the positionof the watertight element), or the embankment height. Thesignicant factor was the method of placement of the rock-ll. The greatest Æ values were obtained where rockll had been dumped, whereas in the dam with the smallest Æ valuethe rockll had been compacted by rolling while beingsluiced. In all these dams the upstream location of thewatertight element meant that crest settlement would be littleaffected by changes in stress in the rockll embankment dueto uctuations in reservoir level or the consolidation of alow-permeability core.

    For most types of ll there is a linear relation betweencreep compression and the logarithm of time that haselapsed since the load was applied, and a simple settlementmodel can be derived for self-weight creep: that is to say,the settlement that occurs when stress and moisture condi-tions do not change, which can be expressed by the equation

    ˜ s ¼ Æ H log t 2

    t 1 (1)

    where ˜ s is the settlement of ll of height H between timest 1 and t 2 after ll placement, and Æ is the vertical compres-sion occurring during one logarithmic cycle of time (e.g. between 1 and 10 years since ll placement).

    Since this simple settlement model is based on self-weightcreep, it might seem appropriate for estimating the settle-ment of the shallow foundations of low-rise buildings wherethe extra loading imposed by the buildings is often of littlesignicance. As shown by equation (2), the creep settlementsoil model indicates that the rate of settlement of the ground surface will be proportional to the depth of the ll and inversely proportional to the length of time that has elapsed since the ll was placed, such that

    ˜ s˜ t

    ¼ 4:34Æ H

    t mm=year (2)

    where Æ is in %, ˜ s/˜ t is the rate of settlement inmillimetres per year, H is the height or depth of ll inmetres, and t is the time in years that has elapsed since ll placement.

    Not only does the creep rate diminish with time, it alsodoes so in an orderly and, provided the magnitude of the parameter Æ is known, predictable way. Kilkenny (1968)quoted Æ ¼ 0.74% for an opencast mining backll at

    1. Well-compacted,well-graded soil

    2. Medium-compactedrockfill

    3. Lightly compacted

    clay and chalk

    4. Uncompacted sand

    5. Uncompacted clay

    6. Well-compactedmixed refuse

    Time since fill placement: years

    Settlementas percentage ofheight of fill

    5

    10

    15

    20

    25

    30

    0 1 2 3 4 5 6 71 2 3

    4

    5

    6

    Fig. 2. Settlement of lls (after Meyerhof, 1951)

    Dam

    Dix River

    Nantahala

    Lewis Smith

    0

    0·5

    1·0

    1·5

    Time from middle of construction period: years

    Crestsettlement as percentage ofheight ofembankment

    Lewis Smith

    DixRiver

    Nantahala

    Date Rockfill

    1·1

    0·5 1 2 5 10 20 35

    α : %

    0·7

    0·2

    Dumped limestone

    Dumped graywacke

    Compacted sandstone

    UD

    SC

    SC

    TypeHeight: m

    84

    78

    97

    1925

    1942

    1961

    Fig. 3. Settlement of three USA rockll dams (derived fromSowers et al. , 1965): Æ, creep compression rate; UD, upstreamdeck; SC, upstream-sloping core

    544 CHARLES

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    Chibburn in Northumberland, and data presented by Lange(1986) suggested that, for backlls in the Rhenish browncoal area, typically Æ ¼ 0.5% to 1.0% for backlls less than100 m high, with greater values for deeper lls.

    Settlement of opencast mining backllsLong-term monitoring of settlement has been carried out

    by BRE at several restored opencast mining sites to investi-gate the behaviour of the backlls and their suitability for building developments. Information about ll properties isgiven in Table 2, but, since the lls are heterogeneous and at some locations contain large boulders, the values for thegeotechnical properties that are quoted can at best only beregarded as reasonably typical. A crucial issue in the inves-tigation was to examine the validity of the creep settlementmodel, which describes the settlement that occurs whenstress and moisture conditions do not change, bearing inmind that most poorly compacted, partially saturated llsundergo a reduction in volume when inundated or sub-merged for the rst time, which is commonly termed collapse settlement or collapse compression.

    Instrumentation could be installed and monitoring com-menced only when ll placement was complete. Preciselevelling of surface settlement stations was combined withsettlement observations at depth within the ll using settle-ment gauges consisting of magnet extensometers installed invertical boreholes. Groundwater levels also were monitored.

    Settlement of opencast mining backll at HorsleyBacklling took place between 1961 and 1970 at Horsley

    opencast coal mining site, near Newcastle. The ll iscomposed principally of mudstone and sandstone fragments,with mudstone predominating. Boulders occupy less than10% of the backll. In the upper part of the workings

    excavation of the overburden was carried out by faceshovels, and backlling was by end tipping from dumptrucks; in the lower part excavation was by dragline. Therewas no systematic compaction of the ll, which has amaximum depth of 70 m. During opencast operations therehad been a lagoon at one location, and another part of thesite had been preloaded with a 30 m high spoil heap. A eld

    test in a borehole at the site of the lagoon indicated a ll permeability greater than 10 4 m/s.

    Following restoration of the site in 1973, the settlement of the backll was monitored throughout a 19-year period (Charles et al. , 1977, 1984, 1993). The location of vemagnet extensometers (gauges A9, B2, C11, D1 and D15)and ve traverses of surface settlement stations (A, B, C, Dand E) are shown on the plan of the site in Fig. 4. Details

    of the ground conditions are given in Table 3.The surface settlement measured by precise levelling atvarious locations between 1973 and 1992 is shown in Fig. 5.The largest settlement, 0 .8 m, was measured at surfacesettlement station E12 (Fig. 5(b)), and particularly smallsettlements, 0 .1 m, were measured at gauge C11 (Fig. 5(a)),which was at the site of the lagoon, and at gauge D1 (Fig.5(b)),where the ll had been temporarily preloaded by aspoil heap. The special circumstances at the locations of C11 and D1, together with variations in the age and depthof the ll across the site, provide an explanation for some of the large differences in settlement, but by no means all of them. Furthermore, when settlement is plotted on a logarith-mic timescale, as shown in Fig. 6, a pattern of behaviour isrevealed that is remarkably different from the linear relation-ship between settlement and logarithm of time since ll

    Table 2. Opencast mining backlls

    (a) Coarse backlls

    Location Fill type Silt and clay: %

    rd :

    Mg/m 3w: % r

    s:

    Mg/m 3n: % V

    a: %

    Horsley Mudstone and sandstone 10 1 .70 7 2.54 33 21Blindwells Mudstone and sandstone 20 1 .56 7 2.45 38 23Tamworth Clay with shale fragments 45 1 .78 9 2.62 32 16

    (b) Clay backlls

    Location w: % wP : % wL : % cu: kPa

    Mean Range

    Ilkeston 19 12–25 23 41 150Corby 18 7–28 17 28 100

    The table presents typical values of geotechnical properties to give an indication of ll type, but itshould be noted that the most signicant property of these non-engineered lls is their heterogeneity.r d , dry density; r s , particle density; n, porosity; V a , percentage air voids; w, water content; wP, plasticlimit; wL, liquid limit; cu , undrained shear strength.

    Traverse of surfacesettlement stationsBorehole settlement gauge

    N

    CC11Lagoon

    Overburden heapD1

    D

    D15Most recentfill

    E

    Pump

    B2B

    Oldestfill A9

    A

    500 mScale

    Fig. 4. Plan of Horsley opencast coal mining site

    THE ENGINEERING BEHAVIOUR OF FILL MATERIALS 545

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    placement that the simple creep model would suggest and which Sowers et al. observed (Fig. 3).

    The crucial factor is that it was necessary to dewater thesite during opencast mining, and pumping continued for some time after the completion of backlling, keeping thewater table below the level of the ll over much of the site.During this period settlements were very small. When pump-ing stopped in 1974 the water table rose 34 m, reaching anew equilibrium level in 1977. During this three-year period large movements occurred. Table 3 lists the surface settle-ment measured at the gauges, rst during the three years(1974–1977) that the groundwater level rose, and second during the whole of the 19-year monitoring period (1973– 1992). The large arrows in Fig. 6 indicate the periods whenthe water table was rising through the ll, and show thecritical role of the submergence of the ll in causingcollapse compression, although an increased rate of settle-ment was noticeable for several years after the water table

    had reached an equilibrium level. The differences in the ageof the ll when subjected to a rise in groundwater level aredue principally to the different dates at which the variouslocations were backlled.

    The settlement at depth within the ll at gauge B2 for theentire monitoring period is shown in Fig. 7. The numbers in brackets in Fig. 7(a) are the depths below ground surface of the various magnet markers. Fig. 7(b) shows the rate atwhich the groundwater rose up through the backll. It wasduring the three-year period in which the groundwater levelrose that large movements occurred.

    In Fig. 8 the same data for gauge B2 are presented interms of vertical compression. In Fig. 8(a) the vertical strainat different depths within the backll (e.g. between magnetmarkers 7 and 8) is plotted against time, and Fig. 8(b) showsthe dates by which the groundwater had risen to the levelsof the different magnet markers (e.g. May 1975 for magnetmarker 7 and February 1976 for magnet marker 8). Thedepths of the magnet markers within the ll are shown inFig. 8(c). It can be seen that, as the water table rose,settlement took place at the depth where this rise wasoccurring. Collapse compressions locally were almost 2%,although the average compression measured over the fulldepth of inundated backll was smaller than 1%.

    Despite a clear link between surface settlement and col-lapse compression on inundation, Fig. 9 reveals not only a

    distinctly non-uniform distribution of vertical compressionwith depth at gauge B2, but also a large compressionoccurring above the water table. The distribution of settle-ment with depth at gauges C11 and D1 in Fig. 10 conrms

    Table 3. Ground conditions in opencast mining backll at Horsley

    Gauge Ground level:mAOD

    Rockhead:mAOD

    Fill depth:m

    Fill date Inundated depth: m

    Settlement: m Fill condition

    1974–1977 1973–1992†

    A9 98.6 38.0 60.6 1961 46 0 .31 0.40 OldestB2 101 .8 38.7 63.1 1964 45 0 .33 0.50 DeepestC11 94 .9 49.2 45.7 1965 35 0 .06 0.11 Lagoon

    D1 108 .1 52.6 55.5 1966 31 0 .10 0.10 Preloaded D15 119 .2 72.7 46.5 1970 11 0 .15 0.31 Most recentE12‡ 115 .8 68 48 1966 17 0 .35 0.79 Intermediate age

    Period during which the groundwater level rose.† Total monitoring period.‡ No magnet extensometer at E12, but listed because maximum settlement recorded at this location.

    0

    100

    200

    300

    400

    1974 1976 1978 1980 19841982 1986 1988 1990 1992

    C11

    D15

    A9

    (a)

    0

    100

    200

    300

    400

    500

    600

    700

    800

    Settlement: mm

    D1

    B2

    E12

    (b)

    Fig. 5. Long-term surface settlement of opencast backll atHorsley, 1973–1992 (after Charles et al. , 1993)

    5 10 20 30Time since fill placement: years

    C11

    B2

    E12

    0

    0·5

    1·0

    1·5Surface settlement as percentage ofdepth offill

    Fig. 6. Long-term surface settlement of opencast backll atHorsley with time plotted on a logarithmic scale

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    that settlement has been particularly small over the 19-year monitoring period at both these gauges compared with thesettlement monitored at B2 (Fig. 9). Settlement attributable

    to the rise in groundwater level was very small in the wetll at C11 and in the preloaded ll at D1 (Table 3). The llthat had been preloaded with a 30 m high surcharge of ll atgauge D1 could be expected to settle least, as the ll is

    overconsolidated and stiff, and at some depths there wassome heave. It is more surprising that the relatively ne-grained, soft, wet lagoon ll at C11 should not settle much, but before the rise in groundwater level, the settlement rateat C11 was greater than in the other parts of the site.Monitoring only commenced eight years after ll placementwas completed at C11, and it is likely that there was largesettlement in the early years when excess pore pressures

    were dissipating.Although the rise in groundwater level had a major inu-ence on settlement behaviour, there were large variations incollapse compression as ll was submerged at gauges A9,B2 and D15, as seen in Fig. 11, where it could have beenexpected that similar behaviour would be observed. Theheterogeneity of the ll has effectively masked any correla-tion between collapse compression and vertical stress in thestress range 250 to 800 kPa.

    Settlement of opencast mining backll at BlindwellsExcavation at Blindwells opencast coal mine near Edin-

    burgh began in 1978, and backlling was carried out using

    draglines, face shovels and end tipping. The ll has a maxi-mum depth of 60 m, and comprises mudstone, siltstone and sandstone. A 1 .4 km section of the Tranent bypass trunk road was to be built across the site and, to reduce settlement,the top 16 m of the backll was systematically compacted on the line of the road. Fill on either side of the road corridor did not receive any systematic compaction. Thetypical density of the uncompacted ll is given in Table2(a). The whole of the Blindwells opencast site was dewa-tered prior to the start of excavation, and the water tablewas held down below the maximum excavation depth.Magnet extensometers were installed in August 1984.

    During the rst 13 years of monitoring 0 .5 m settlementwas measured in ll that had not been systematically

    compacted and 0 .2 m where the upper zone had beencompacted (Watts & Charles, 2003). Typical Æ values in theuncompacted ll were about 1%. In 1997 the groundwater level began to rise, and there was an increase in surfacesettlement of 0 .3 m during the period in which the ground-water level rose by 15 m. Most of this settlement can berelated to a vertical compression of 1 .4% in the submerged ll, as seen in Fig. 12. Clearly, the long delay between ll placement and the rise in groundwater level did not signi-cantly reduce collapse potential.

    Settlement of opencast mining backll at TamworthBacklling at an opencast coal mining site at Tamworth

    in the English Midlands was completed in 1972. Theexcavation was backlled mainly by scrapers, with some end tipping, and the maximum depth of ll was 32 m. The llwas composed of clay and shale fragments, and the site wasrestored with a sloping ground surface. Monitoring of ground movements and water levels within the ll com-menced in 1977 and continued until surcharge operations began in 1995, prior to development of the site for housing(Charles & Burford, 1987; Watts & Charles, 2003).

    In Fig. 13 the settlement measured within the deepest partof the backll is shown in relation to the change in ground-water level within the ll. Surface settlement between Juneand December 1977 was small, equivalent to a creep rate of about 10 mm per year. In early 1978 the rate of creep

    increased signicantly, and a further 40 mm of settlementoccurred during the following two years. During 1980 asubstantial rise in the rate of settlement was recorded, and by the beginning of 1983 about 250 mm of settlement had

    (b)

    1974 1976 1978 1980 1982 1984 1986 1988 1990 1992

    4 (49·1 m)

    6 (37·0 m)

    8 (25·8 m)

    9 (19·7 m)

    10 (13·8 m)

    13 (0 m)

    0

    100

    200

    300

    400

    500

    Settlement: mm

    (a)

    0

    10

    20

    30

    40

    50

    60

    Depth: mm

    Groundwater level

    Fill

    Bedrock1

    4

    6

    8

    9

    10

    13

    Fig. 7. Relationship between settlement at different depths inthe ll and rise in groundwater level at gauge B2, Horsley,1973–1992 (after Charles et al. , 1993)

    1973 1974 1975 1976 1977 1978 1979

    (a)

    (b)

    (c)Bedrock1

    23456789

    10111213 GL

    Fill63 m

    3 4 5 6 7 8 9

    0·5

    0

    0·5

    1·0

    1·5

    2·0

    Verticalcompression: % 6–7

    7–85–6

    9–108–9

    Fig. 8. Vertical strain and groundwater level at gauge B2,Horsley, 1973–1979 (after Charles et al. , 1993)

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    occurred. Surface settlement then continued at a rate of almost 40 mm per year for the next 10 years.

    Such movements are greatly in excess of any anticipated creep settlement. It seems likely that groundwater was penetrating into the opencast backll from old unsealed workings and then seeping through the ll. A rise in ground-water level in 1994–1995 between magnet markers D and Ecaused a signicant increase in settlement at that depth,conrming that the backll had some collapse potential.Ground treatment by preloading with a 7 m high surchargeof ll was carried out in 1995 and induced some 0 .2 m of surface settlement.

    Settlement of opencast mining backll at IlkestonThe opencast coal mining backlls at Horsley, Blindwells

    and Tamworth were essentially granular, as illustrated by thetypical properties given in Table 2(a). The susceptibility to

    collapse compression of opencast backlls that are predomi-nantly clay has been investigated at sites at Ilkeston and Corby, and some typical properties are given in Table 2(b).The stiff clay backll at Ilkeston was placed by scraperswithout any additional systematic compaction in 1959. Therewas no water table within the backll.

    In 1973 a newly constructed block of eight two-storeyhouses suffered some settlement when excavation for drains

    began close to the north gable end and, following heavyrain, further movement took place in the centre of the rowof houses. Soon all the houses in the block were affected,and movements continued, although underpinning and pres-

    0 0·1 0·2 0·3 0·4 0·5Settlement: m

    0

    20

    40

    60

    Depth belowground level: m

    Rise ingroundwater level

    0 1 2 3Vertical compression: %

    Fig. 9. Settlement and vertical compression against depth at gauge B2, Horsley

    100 200Settlement: mm

    0 100 200

    D110 11

    9

    7

    6

    5

    4

    3

    2

    1

    0

    10

    20

    30

    40

    50

    Depth belowground level: m

    C11

    109

    8

    7

    6

    5

    4

    3

    2

    1

    Fig. 10. Comparison of settlement with depth at gauges C11and D1, Horsley

    1000

    800

    600

    400

    200

    0

    Verticalstress: kPa

    0 1·0 2·0Collapse compression: %

    Fig. 11. Effect of submergence on Horsley backll at settlementgauges A2, B9 and D15

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    sure grouting were carried out. A year after the houses werecompleted, oor levels showed a maximum differential set-tlement of 0 .14 m across the 9 m wide block, and the eastwall was 0 .065 m out of plumb. The houses were never occupied, and the block was demolished in 1982, by whichtime there was an estimated total settlement of 0 .3 m.

    It was suspected that water penetrating into the llthrough drain trenches had initiated collapse compressionwithin the backll, and a eld inundation test was carried

    out (Charles & Burford, 1987). When 3 m deep trencheswere lled with water to a depth of 1 .8 m, the rates at whichwater levels fell in the different trenches varied from as littleas 0.04 m/h (0 .08 m 3/h) to as much as 1 m/h (2 m 3/h). With-

    in 24 h of lling the trenches with water, additional settle-ments of up to 50 mm had been recorded, conrming thatwater penetrating into the opencast backll via surfacetrenches could cause signicant collapse compression. Fig.14 shows the settlement measured at various depths withinthe ll at a magnet extensometer installed 1 .5 m from two of the trenches. Settlement occurred immediately water was putinto the trenches, and the compression was located between

    magnet marker ‘g’, 0.5 m below ground level, and magnetmarker ‘e’, 7 m below ground level. Six days after the start

    of the test 40 mm settlement had occurred, and the trencheswere backlled. However, settlement continued at a signi-cant rate. A temporary increase in the rate of settlement inlate 1976 followed a period of heavy rainfall. The averagevertical compression between magnet markers ‘g’ and ‘e’over the whole monitoring period was 2 .6%.

    Settlement of opencast mining backll at CorbyAt the Snatchill experimental site, the 24 m deep opencast

    ironstone mining backll had been placed by a large walkingdragline, so that the upper part of the backll was composed of large lumps or clods of stiff clay, with an undrained shear strength of 100 kPa. There was no water table within the backll. Restoration had been completed in 1970, and in1975 three 50 m square areas were treated with differentforms of ground treatment prior to housing development.Houses were also built on untreated ground (Charles et al. ,1978; Burford & Charles, 1991).

    One area was inundated via 1 m deep trenches at 10 mcentres. The trenches were lled with water in February1975 and backlled in June 1975. The average surfacesettlement induced by this inundation was 0 .1 m. During therst 10 days of the experiment about 90 m 3 of water wasabsorbed by the backll, but comparatively little was ab-sorbed subsequently. About half this volume was lost fromone trench. The largest settlement was measured at a magnetextensometer that was only 2 m from this trench, and, asshown in Fig. 15, settlement continued after the trencheshad been backlled. Prior to the inundation test the ground surface settled at about 1 mm per month, whereas during theinundation test the ll surface settled 165 mm in 6 months, principally as a result of vertical compression of 5 .6% between magnet marker ‘g’, 2 .3 m below ground level, and magnet marker ‘f’, 4 .5 m below ground level. In the 6 yearssubsequent to the end of the inundation test the ll surfacesettled 118 mm, mainly as a result of compression of 1 .8% between magnet marker ‘f’, 4 .5 m below ground level, and magnet marker ‘e’, 12 .1 m below ground level. During thefollowing 9 years the ll surface settled 25 mm, principally because of some small further compression between 4 .5 mand 12 .1 m below ground level.

    1985

    1986

    1987

    1988

    1989

    1990

    1991

    1992

    1993

    1994

    1995

    1996

    1997

    1998

    1999

    2000

    2001

    2002

    2003

    0

    200

    400

    600

    800

    Settlement: mm

    05

    1015202530354045505560W

    ater level: m belowground level

    CE

    G

    IK

    K

    I

    G

    E

    C

    A

    Newinundation

    BackfillNatural ground

    Fig. 12. Settlement of opencast backll at Blindwells, 1985–2003(after Watts & Charles, 2003)

    1977

    1978

    1979

    1980

    1981

    1982

    1983

    1984

    1985

    1986

    1987

    1988

    1989

    1990

    1991

    1992

    1993

    1994

    1995

    0

    200

    400

    600

    800

    Settlement: mm

    0

    10

    20

    30Water level: m belowground level

    C

    E

    E

    C

    A

    Newinundation

    Backfill

    Natural ground

    D

    FH

    B

    D

    F

    H

    B

    Fig. 13. Settlement of opencast backll at Tamworth, 1977– 1995(after Watts & Charles, 2003)

    0

    100

    200

    Settlement: mm

    1974 1975 1976 1977 1978 1979 1980 1981de

    f

    gh

    Inundationtest

    12 m

    hg

    f

    e

    d

    cba

    Backfill

    Naturalground

    Fig. 14. Inundation test on opencast mining backll at Ilkeston,1974–1981 (after Charles & Burford, 1987)

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    Settlement of colliery spoil at CoalvilleThe vulnerability of poorly compacted opencast mining

    backlls to collapse compression on inundation is just oneaspect of their metastable condition—that is, an apparentlystable condition that requires only a minimal disturbance toinitiate a change to a truly stable state. The change from ametastable state to a stable state can involve a substantialreduction in volume of the ll, and hence substantial settle-ment of the ground surface. The behaviour of colliery spoilat Coalville demonstrated that inundation is not the only phenomenon that can trigger a sudden reduction in thevolume of a non-engineered ll in a metastable state(Skinner et al. , 1997).

    In a major land reclamation scheme some clay pits were backlled with freshly mined colliery spoil brought in lorriesfrom a local colliery and tipped in lifts 1 .5 to 2 m high.There was no compaction other than that provided by thelorries running over the surface of each layer. Typically silt-sized particles constituted about 40% of the spoil. The holeswere dewatered before and during the lling operation, butwater levels were not controlled afterwards. A sudden in-crease in settlement in the middle hole in the spring of 1982was caused by deep mining. The total settlement monitored by precise levelling at this location was 0 .6 m, but most of the mining subsidence occurred in the strata below the baseof the backll. A magnet extensometer measured the settle-ment relative to the base of the backlled hole, and thisshowed that the large, deep-seated movement in the under-lying strata had triggered 60 mm of compression in the backll, as shown in Fig. 16.

    The colliery spoil was also susceptible to collapse com- pression on inundation. At one location in the south hole,where there was 6 .5 m of ll, 0 .47 m of settlement occurred between November 1978 and January 1979, correspondingto an average vertical compression of 7%, which wasassociated with very heavy rainfall in December.

    ConclusionsThe eld data from the case histories of the long-term

    behaviour of opencast mining backlls have provided aconclusive answer to the question ‘Can building develop-ment safely take place on a deep opencast mining backll,which has been placed without controlled systematic com- paction, when a specied period of time has elapsed sincell placement?’ Poorly compacted opencast backll placed with little or no control is likely to be in a metastable

    condition and, irrespective of the age of the ll, there is arisk that some small disturbance, such as an increase inwater content, will cause a signicant reduction in volume.

    Collapse potential does not automatically reduce with time

    and, unless it can be established that the ll does not havesignicant collapse potential (e.g. a previous inundation or wet placement of the ll should have greatly reduced, if noteliminated the potential for further collapse), there will be arisk that collapse settlement could occur during or subse-quent to building on the site and, consequently, either deepfoundations or ground treatment are likely to be required.

    Collapse compression can result from a rising ground-water level, but may also occur above the water table fromdownward inltration of surface water or groundwater inl-tration into the backll through the high wall of an opencastmine. There is likely to be some time dependence in theresponse to wetting, particularly where inundation is due todownward percolation of water or in a clay ll. The eld experiments have demonstrated that water inltration fromthe ground surface via trenches can cause collapse compres-sion in clay ll that continues for many years, and can be aslarge as 6%. The non-uniform response to inundation atdifferent locations within clay ll indicated that it was not a practical form of ground treatment.

    The engineering behaviour of poorly compacted heteroge-neous lls is difcult to predict, and may bear little relation-ship to average values of geotechnical properties. Settlementthat damages buildings will be largely a function of the mostadverse properties encountered within the ll. Well-documen-ted case histories, with long-term eld measurements, providea helpful basis for assessing likely eld performance.

    Signicant creep movements can occur in poorly com- pacted ll, and for mudstone/sandstone ll Æ values of theorder of 1% are typically observed. Creep movements willgenerally be relatively small, unless the ll is very deep or has been placed quite recently. Where settlement predictionsare based, either explicitly or implicitly, on the simple creepsettlement model, building developments may appear to besuccessful simply because inundation of the ll has not yetoccurred. However, at some restored opencast sites and other types of lled site large collapse settlements have occurred subsequent to building development, with unpleasant conse-quences for the buildings (Charles & Watts, 2001).

    PERFORMANCE OF ROCKFILL DAMSThere is an instructive contrast between the performance

    of poorly compacted opencast backlls and the behaviour of

    19741975

    19761977

    19781979

    19801981

    19821983

    19841985

    19860

    100

    200

    300

    Settlement: mm

    Houseconstruction

    Trenchesbackfilled

    Inundation testcommenced

    gh

    f

    24 m

    hgf edcba

    Backfill

    Naturalground

    Fig. 15. Inundation test on opencast mining backll at Corby,1974–1986 (after Charles & Burford, 1987)

    0

    20

    40

    60

    80

    100

    120

    1401980 1981 1982 1983

    Settlementdue to

    deepmining

    Settlement: mm

    Fig. 16. Settlement of colliery spoil at Coalville due to deepmining, 1979–1983 (after Skinner et al. , 1997)

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    engineered lls placed in a heavily compacted state to formembankment dams. While the engineered lls have proper-ties that are much superior to those of the opencast backlls,the water retaining structures that they form must meet morecomplex requirements. It is important that unsatisfactory behaviour in an embankment dam is detected at an earlystage, and this is greatly assisted by a sound appreciation of what constitutes normal satisfactory behaviour.

    British rockll damsDeformations have been monitored by BRE at several

    rockll dams. and basic information about the heavilycompacted rocklls used in these dams is summarised inTables 4 and 5. A determining factor in the behaviour of these embankments is the nature and position of the im- permeable or watertight element. The materials forming thisvital element of the dam are described as ‘impermeable’ and ‘watertight’, in contrast to the highly permeable rockll; in practice they have a nite, although very small, permeabil-ity.

    Scammonden Dam in West Yorkshire has an unusuallywide crest, which carries the M62 motorway (Fig. 17(b)).The dam is founded on Carboniferous shales, and there is adeep grout curtain under the upstream-sloping clay core.The clay core was placed at a water content well aboveProctor optimum, and is protected on both upstream and downstream sides by lter material (Penman & Mitchell,1970). It was anticipated that its upstream location would ensure that settlement of the clay core would not affect themotorway. The rockll is formed from Carboniferous sand-stone, with three mudstone zones in the middle of theembankment. Prior to embankment construction, extensivetrials demonstrated that multi-row blasting produced a well-graded ll that could be placed and compacted to a highdensity, while ripping and single-row blasting were unsatis-factory (Williams & Stothard, 1967). The downstream slopei s 1 in 1.8. The upstream slope is 1 in 3 .1 close to thefoundation and progressively steepens to 1 in 1 .8 close tothe crest; there is a weight block at the upstream toe. Theembankment was completed in September 1969. Reservoir

    impounding commenced in July 1969 and the reservoir wasfull in June 1972. Four horizontal plate gauges were in-stalled on the major section of the dam during construction,and the measurements of the deformations of the embank-ment have been described by Penman et al. (1971).

    Brianne Dam is situated in central Wales (Carlyle, 1973).It is founded on a Palaeozoic slatey mudstone, and the area beneath the central clay core was grouted to a shallow depth.The upstream and downstream rockll shoulders wereformed by heavily compacting the plate-like fragments of the slatey mudstone rockll, with water added during place-ment. The upstream slope is 1 vertical in 2 horizontal, and the average downstream slope is 1 in 1 .75, as shown in Fig.17(a). The clay core was placed wet of optimum water content, and is protected on both upstream and downstreamsides by transition and lter material. Embankment construc-tion was completed in November 1971, and reservoir im- pounding commenced immediately. The reservoir was full

    Table 4. Field placement of rockll

    Dam Date built Height: m Rockll Vibrating roller

    Layer depth: m Number of passes Weight: t

    Scammonden 1969 73 Sandstone/mudstone 0 .9 5 11.5Brianne 1971 90 Slatey mudstone 0 .9 4 13.5

    Winscar 1974 53 Sandstone 1 .7 4 13.5Marchlyn 1979 47 Slate 1 .0 4 13.5Megget 1981 56 Gravel 0 .4 4 5.5Roadford 1989 41 Sandstone/mudstone 0 .45 8 9.1

    Table 5. Field density of rockll

    Location r d : Mg/m 3 w: % r s: Mg/m 3 n: % V a: %

    Scammonden 2 .02 7 2.69 25 11Brianne 2 .35 3 2.75 15 7Winscar 2 .03 6 2.60 22 10Marchlyn 2 .25 4 2.81 20 11

    Roadford 2 .07 4 2.74 24 15

    r d , dry density of ll; r s , particle density; n, porosity; V a , percentage air voids; w, water content.

    (b)

    (a)

    (c) (d)

    Fig. 17. Cross-sections of rockll dams showing location of horizontal plate gauges: (a) Brianne; (b) Scammonden; (c)Winscar; (d) Megget

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    by the beginning of January 1973. Three horizontal plategauges were installed on the major section of the damduring construction, and the deformation measurements have been described by Penman & Charles (1973b, 1973c). In1996 the spillway crest was raised by 1 .0 m, and a concretewall was inserted into the top of the embankment above theclay core to extend the watertight element to the undersideof the crest road (Hughes, 1998).

    Winscar Dam (Fig. 17(c)) in west Yorkshire is founded onMillstone Grit sandstone that contains a little shale (Collins& Humphreys, 1974). The heavily compacted sandstonerockll is from the same Carboniferous series as found atScammonden, and water was added to the rockll during placement. The upstream asphaltic concrete membrane or facing was placed on the 1 in 1 .7 upstream slope after embankment construction was completed. The downstreamslope is 1 in 1 .4. The deformation measurements obtained from three horizontal plate gauges installed on the major section of the dam during construction have been described by Penman & Charles (1985a). Major leakage problems wereencountered during rst lling of the reservoir, and supple-mentary foundation grouting was carried out as well asrepairs to the asphaltic membrane. Although the crack in themembrane was only about the size of a matchbox, it caused serious leakage. The identication and repair of the asphalticconcrete facing of the dam have been described by Routh(1988). During the early impounding period the reservoir level reached more than 80% of its maximum height, butthe reservoir was completely emptied in 1980 to permitrepairs to the asphaltic membrane. The rst successfulcomplete lling of the reservoir was in 1981–1982, but problems recurred in the 1990s. Further leakage investiga-tions and repairs have been described by Wilson & Robert-shaw (1998) and Carter et al. (2002); a geocomposite liner was installed on the upstream slope in 2001.

    Megget Dam (Fig. 17(d)) in southern Scotland has acentral asphaltic core, which is supported by heavily com- pacted, well-graded gravel shoulders (Gallacher, 1988). As- phaltic concrete cores are sometimes referred to as bituminous cores or diaphragms. The upstream slope is 1 in1.5, and the downstream slope steepens from 1 in 2 .1 at the base of the dam to 1 in 1 .5 at the crest. A 60 m deep groutcurtain was formed in the rock along the centreline of thedam. The embankment was completed in October 1981, and impounding commenced in May 1982. The reservoir reached top water level for the rst time in January 1986. The rip-rap on the steep upstream slope was damaged by severestorms at the beginning of 1984. Bituminous grouting wascarried out on the upper part of the slope in 1997 (Gallacher et al. , 1998). Three horizontal plate gauges were installed onthe major section of the dam during construction (Penman &Charles, 1985b).

    Marchlyn Dam in north Wales forms the upper reservoir of the Dinorwig pumped storage scheme (Baines et al. ,1983). The slate rockll embankment was built on a glacialmoraine, and the upstream asphaltic membrane was placed over both the rockll and the moraine, forming an impound-ing structure with a total height of 72 m. Beneath theinspection gallery at the upstream toe a grout curtainextends to a maximum depth of 120 m. First lling of thereservoir was completed in December 1982. BRE developed an inclinometer to measure deection of the 1 in 2 upstreamslope during reservoir impounding (Penman & Hussain,1984).

    Roadford Dam in south-west England has an upstream

    asphaltic concrete membrane that forms the watertight ele-ment (Duncanson & Johnston, 1988). The embankment was built of low-grade sandstone and mudstone rockll, whichwas excavated by face shovel loaders assisted by a ripper

    (Wilson & Evans, 1990). The rockll was placed at arelatively low water content (Table 5). Both upstream and downstream slopes are 1 in 2 .25. Sandll was placed next tothe inspection gallery at the upstream toe of the dam toreduce differential settlement at this critical location. Im- pounding commenced in October 1989, and rst lling of the reservoir was completed in April 1991. The deection of the upstream membrane was measured during reservoir

    impounding using electro-levels installed by BRE (Tedd et al. , 1995).

    Construction deformationsThe compressibility of the rockll materials was measured

    in tests on samples heavily compacted in layers in 0 .6 m and 1.0 m diameter oedometers. Rockll with a maximum parti-cle size of 125 mm was tested in the 1 .0 m diameter oedometer. Fig. 18 shows that the compressibilities of therocklls from Brianne, Scammonden and Winscar damswere quite similar, but the gravel ll from Megget was muchless compressible.

    Settlement occurs during rockll placement owing to the

    self-weight of the ll. The vertical strains measured in thedownstream rockll shoulders at Brianne, Scammonden and Winscar were quite similar, since the compressibility of therocklls measured in oedometer tests were not very differ-ent. The deformations at Megget were much smaller, be-cause the gravel ll was much less compressible. Thisdifference is illustrated in Fig. 19, which shows the settle-ment during embankment construction on the centreline of the Scammonden, Winscar and Megget dams. Brianne is notincluded in the gure because the maximum constructionsettlement of 2 .3 m was principally a function of the behav-iour of the soft central clay core, not of the rockllshoulders (Carlyle, 1973); the constructional horizontalmovements at Brianne induced by the lateral pressure fromthe soft clay core at the interface between the clay core and the granular lter are shown in Fig. 20.

    Although the vertical strains at Brianne, Scammonden and Winscar were quite similar, with a maximum of 3–4%compression (Fig. 21), the pattern of horizontal strain at thedams was very different. The horizontal strains shown inFig. 22 are for the upstream-downstream direction; horizon-tal strain along the axis of the dams was not measured. Theclay cores at Brianne and Scammonden were placed wet of optimum water content and had high pore water pressuresand low effective stresses during construction. At the end of

    0 500 1000 1500Vertical stress: kPa

    0

    2

    4

    6

    Verticalstrain: %

    M

    B

    S

    W

    Fig. 18. Compressibility of rocklls measured in large oed-ometer: B, Brianne slatey mudstone; M, Megget gravel; S,Scammonden sandstone; W, Winscar sandstone

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    construction both dams had a pore pressure ratio ( r u ¼ u/ª h)as high as 0 .7 at some locations in their cores. The claycores were much less stiff than the rockll shoulders, and the differences in embankment behaviour can be attributed largely to the positions of the clay cores (Penman & Charles,1973c; Charles, 1975). The horizontal strains show the major inuence of the lateral thrust of the soft central clay core onthe whole of the downstream shoulder at Brianne, with amaximum compression of a little more than 0 .6%, and themuch more limited effect of the upstream sloping core onthe downstream rockll at Scammonden owing to the widthof the crest and the upstream location of the clay core. At

    Winscar virtually the whole embankment cross-section hasnegative horizontal strain (i.e. extension), with a maximumof just over 0 .4% in the centre of the embankment (Penmanet al. , 1982).

    The stress–strain properties of rocklls measured in one-dimensional compression tests are generally non-linear, butthe internal distribution of settlement during embankment

    construction can be predicted with little error using aconstant equivalent compressibility, and the maximum settle-ment occurring during construction can be related to theconstrained modulus ( D). Assuming that a large-diameter

    1·0

    0·5

    0

    h H/

    Me WS

    0 0·5 1·0Settlement: m

    Fig. 19. Construction settlement against height on centreline of

    Megget, Scammonden and Winscar dams (after Charles &Penman, 1988): h / H , ratio of height above foundation level tomaximum height of embankment

    Top of embankment

    80

    70

    60

    50

    40

    30

    20

    10

    0Base of embankment 0 0·5 1·0 1·5

    Movement: m(a)

    Downstream movementSettlement

    0 0·5 1·0Movement: m

    (b)

    Fig. 20. Construction movements at (a) core/ne lter interfaceand (b) downstream slope at Brianne dam (after Penman &Charles, 1973b)

    1234

    (a)

    12

    33

    (b)

    1

    2 3

    4

    (c)

    Fig. 21. Contours of constructional vertical strain (%):(a) Brianne; (b) Scammonden; (c) Winscar

    0·3

    0·40·5

    0·6 0·3 0·2 0·1

    (a)

    0·1 0

    0·2

    0·1

    0

    0·2

    0

    (b)

    0·20·4

    00

    (c)

    Fig. 22. Contours of constructional horizontal strain (%):(a) Brianne; (b) Scammonden; (c) Winscar

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    oedometer test accurately represents the one-dimensionalcompressibility of the ll in the eld, for ll placed over awide area, it can readily be shown that (Charles, 1976,1990)

    For D ¼ a constant ¼ Dª H :

    smax ¼ 0:25 ª H 2

    Dª H

    ! (3)

    For D ¼ k ( v)0:5:

    smax ¼ 0:195 ª H 2

    Dª H ! (4)where smax is the maximum settlement during constructionof an embankment of height H , ª is the bulk unit weight of the ll, and Dª H is the secant-constrained modulus for avertical stress v ¼ ª H . Fig. 23 shows that the measured values of smax have mostly closely corresponded to equation(4), which therefore can be used to give an initial predictionof construction settlement. This approach works reasonablywell because stress changes during construction generallycorrespond to an increase in mean effective stress while the principal effective stress ratio remains roughly constant, butduring reservoir impounding stress changes are much morecomplex.

    Post-construction deformationsSubsequent to the construction of a rockll dam, move-

    ments will occur because of

    (a ) stress changes associated with reservoir impounding(b) consolidation of a clay core(c) creep compression in the rockll.

    Movements monitored during reservoir impounding illus-trate the determining inuence of the position and nature of the watertight element within the embankment. At Briannethe continuing settlement of the crest was largely a functionof the behaviour of the central clay core—that is, primaryconsolidation due to dissipation of excess pore pressuresfollowed by secondary compression. The upstream ll wassubmerged during impounding, thus reducing the effectivestresses within the ll. While in theory this should cause therockll to undergo a slight expansion or heave, in practiceany small creep movements or collapse compression willnegate such a tendency. Fig. 24(a) shows that the pointslabelled U on the upstream slope settled a very similar amount to monuments E and F on the downstream slopeduring reservoir impounding, indicating that the rockll had

    little if any collapse potential. The location of these surfacesettlement stations is shown in Fig. 24(b).

    Because of its wide crest and upstream-sloping clay core,much of the rockll at Scammonden was not signicantlyaffected by either reservoir impounding or consolidation of the clay core. The approximately linear relationship betweensettlement and time elapsed since construction plotted on alogarithmic scale is shown in Fig. 25 for different heights onthe centreline of the dam. The line marked ‘c’ represents thesettlement of the crest of the dam, which corresponds to avalue of Æ ¼ 0.17% for the full height of the embankment.

    Where an embankment dam has a central exible dia- phragm of asphaltic concrete, reservoir impounding sub-merges the upstream ll, decreasing the vertical effective

    stress, but the total horizontal pressure acting on the mem- brane increases. At Megget it was found that the increase inhorizontal thrust due to reservoir impounding was 0 .29ª whw ,where ª w is the density of water and hw is the reservoir

    1·0

    0·5

    0

    smax

    : m

    1 2 3 4 5γ H D2/ : mγ H

    M

    R

    Eqn (3)

    W

    Eqn (4)

    S

    Fig. 23. Maximum measured construction settlement, smax , as afunction of constrained modulus of rockll derived from largeoedometer test, D ª H : M, Megget; R, Roadford; S, Scammon-den; W, Winscar

    280

    260

    240

    220

    200

    Elevation: m AOD

    0 100 200

    Settlement during reservoir impounding: mm(a)

    F

    U

    E

    D

    C

    B

    A

    UF

    ED

    C

    B

    A

    (b)

    Fig. 24. Settlement during rst lling of reservoir at Briannedam (after Charles, 1987)

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    head at the location where the horizontal stress was meas-ured. However, as shown in Fig. 26, the downstream deec-tion of the asphaltic diaphragm was very small owing to thestiffness of the gravel ll.

    At dams with upstream membranes the reservoir water applies a loading normal to the membrane. First lling of the reservoir causes major stress increases and embankmentdeformations. Subsequent uctuations in reservoir level af-fect the stresses in the rockll in a similar way, but have asmaller effect on embankment deformations, as the rockllis much stiffer under these reloading and unloading stresses.When the water load is transmitted to the rockll immedi-ately beneath an upstream membrane the mean effective

    stress increases, but the principal effective stress ratio tendsto reduce. The modulus during rst lling of the reservoir istypically almost twice as large as that operating duringconstruction owing to the change in principal stress directionduring these two phases of loading.

    Deformations due to rst lling of the reservoir have beenmonitored at three dams with upstream asphaltic mem- branes: Winscar, Roadford and Marchlyn. At Winscar the

    movements, which were measured using horizontal plategauges, are shown in Fig. 27(a). The maximum deectionduring rst lling was 0 .2 m at just below half the height of the dam. The maximum compressive strain normal to themembrane was 1% near the toe of the dam (Fig. 27(b)).Elastic nite element analyses were used to predict deforma-tions during reservoir impounding with rockll parameters, based on the assumption that the bulk modulus had the samemagnitude during reservoir lling as it had during embank-ment construction, and Poisson’s ratio was zero. AssumingPoisson’s ratio to be 0 .33 during construction, elastic theoryled to the conclusion that the constrained modulus duringimpounding was twice as large as that during embankmentconstruction (Charles & Penman, 1988). This approach gavereasonable predictions of movements normal to the upstreammembrane except close to the crest of the embankment.

    Deformations at the toe of an embankment can damagean upstream asphaltic membrane, and the deection of theupstream membrane was measured during reservoir im- pounding near the toe of Roadford Dam using an electro-level system (Evans & Wilson, 1994; Tedd et al. , 1995).Prior to reservoir impounding, the deection of the mem- brane was estimated from the compressibility of the llmaterials measured in large oedometer tests. The analysiswas based on assumptions similar to those used for theanalysis of Winscar, except that, using a Poisson’s ratio of the rockll during construction of 0 .25, the ratio of con-strained modulus during impounding to that during construc-tion was 1 .67. The measured and predicted deectionsnormal to the upstream membrane are shown in Fig. 28.

    There is some correlation between the maximum deec-tion of an upstream membrane during rst lling of thereservoir ( nmax ) and the maximum settlement during em- bankment construction ( smax ), as demonstrated in Fig. 29 bythe monitored behaviour of some international concrete facerockll dams (Charles & Penman, 1988) and three asphalticconcrete upstream membrane dams where BRE made obser-vations. At Roadford the measurements of membrane deec-

    0

    100

    200

    300

    Settlem

    ent: mm

    0·1 1 10

    h H /

    0·25

    0·64

    1·00

    a

    b

    c

    Time since end of construction: years

    Fig. 25. Long-term settlement on the centreline of Scammondendam (after Charles, 1990): h / H , ratio of height above foundationto maximum height of embankment

    D

    C4

    B5

    A6

    Movementscale

    0 50 mm

    Fig. 26. Vector movements at Megget dam during reservoirimpounding (after Penman & Charles, 1985b)

    1 8 0

    1 6 0

    1 4 0 1 2 0 1 0

    0

    8 0 6 0

    4 0

    2 0

    (a)

    0·2

    0·2

    0·40·6

    0·81·0

    (b)

    Fig. 27. Movements during reservoir impounding at Winscardam: (a) movement normal to upstream asphaltic concretemembrane (mm); (b) strain normal to upstream asphalticconcrete membrane (%)

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    tion went only part way up the upstream slope: the hollowcircle in Fig. 29 represents the maximum measured deec-tion, and the solid circle is the estimated maximum deec-tion on the assumption that the deected shape of themembrane at Roadford was similar to that measured atWinscar. The dashed line on the graph corresponds to a ratio

    of nmax / smax ¼ 0.25, which would be expected if the con-strained modulus controlling the deformation due to reser-voir impounding was about twice as large as the moduluscontrolling construction settlement.

    The settlement of the crests of Brianne, Megget, Scam-monden and Winscar dams has been monitored for long periods following the end of construction. In Fig. 30 thedata are plotted as vertical strain. The vertical strain atBrianne is twice as large as that at Scammonden and Winscar, reecting the fact that at Brianne the consolidationof the clay core is the dominant effect. At Winscar the extraloading during reservoir impounding was a signicant factor:

    the increased rate of settlement that occurred between sixand a half years and nine years after the end of constructionwas associated with the rst full lling of the reservoir in1981–1982 after it had been emptied in 1980 for membranerepairs. At Scammonden the consolidation of the clay corehad only a minor effect on crest settlement.

    Quite large crest settlement continued at a reasonablyconstant rate for some time after reservoir impounding atRoadford, with no sign of the steady reduction in settlementrate observed at the other four dams. This additional settle-ment has been attributed to an increase in water content inthe embankment ll (Evans & Wilson, 1992). The crestsettlement rate eventually reduced with time (Hopkins et al. ,2002); nevertheless, by May 2001 the crest had settled 535 mm since the end of construction, of which 425 mmhad occurred since impounding began, corresponding tovertical strains of 1 .3% and 1 .0% respectively, which issurprisingly large. Vaughan (1994a) estimated that a littlemore than 1% collapse compression had occurred at a slowrate.

    Figure 31 shows the compressibility of two samples of Roadford rockll measured in tests in a 1 m diameter oedometer. The properties of the two samples are given inTable 6. Sample ‘a’ was heavily compacted at a relativelyhigh water content, and sample ‘b’ was less heavily com- pacted at a lower water content. Until sample ‘b’ wasinundated the compressibility of the two samples was notdissimilar, but when sample ‘b’ was inundated a collapsecompression of 2 .6% occurred. Sample ‘a’ was not inun-dated, but as the initial air voids were only about 4% therewould not have been any collapse potential. The two dashed lines for sample ‘b’ on the gure give estimates of the behaviour of sample ‘b’ if it was not inundated at all and if it was inundated before any load was applied. The initialdensity and water content of sample ‘b’ were close to theaverage eld values (Table 6).

    In Fig. 32 the crest settlement data for the ve dams are plotted using a logarithmic timescale. There is generally a

    D e f

    l e c t

    i o n :

    m m

    14 mRockfill

    18 box sections housing E-Ls8 at 0·5 m long10 at 1·0 m long

    Drainage layer

    Sand waste

    D i s t a n

    c e f r o m

    t o e : m 1 4

    1 2 1 0

    8 6

    2 0

    4

    0

    2 0

    4 0

    6 0

    8 0

    1 0 0

    1 2 0

    Predicted deflection

    1

    2

    3Date Res head: m

    1 November 19892 April 19913 July 1994

    14·633 (full)32

    Fig. 28. Deection of upstream asphaltic concrete membraneduring reservoir impounding at Roadford dam (after Tedd et al. , 1995)

    0·2

    0·1

    00 0·5 1

    uam

    cf Ma

    R

    W

    s max : m

    nmax

    : m

    Fig. 29. Relationship between maximum normal deection of upstream membrane during reservoir rst lling, n max , andmaximum settlement during embankment construction, smax(after Charles, 1990): uam, upstream asphaltic membrane; cf,concrete face; Ma, Marchlyn; R, Roadford; W, Winscar

    0 5 10 15 20Time since end of construction: years

    S

    M

    W

    B

    R

    0·2

    0·4

    0·6

    0·8

    s H/: %

    Fig. 30. Post-construction long-term settlement of crests of dams: B, Brianne; M, Megget; R, Roadford; S, Scammonden;W, Winscar; s, crest settlement; H , height of embankment

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    linear relationship between vertical strain and log time for Brianne, Megget, Scammonden and Winscar dams. This typeof relationship has been obscured at Roadford by ongoingcollapse compression.

    The creep behaviour of rockll can be evaluated in detailfrom the internal deformation measurements made using thehorizontal plate gauge system. At Scammonden much lessthan one third of the long-term compression of the embank-

    ment is located in the top third of the embankment (Fig.25). Stress dependence of creep settlement is explored inFig. 33, where the measured distribution of long-term settle-ment with height within the embankments at Scammondenand Winscar is compared with what would be expected if (a) Æ was a constant and not dependent on vertical stress, or (b) Æ was proportional to vertical stress. The results aregenerally closer to the latter assumption than to the former.

    ConclusionsField measurements have conrmed that carefully con-

    trolled placement and compaction can produce rocklls withrelatively uniform geotechnical properties and predictable

    behaviour. The case histories go some way towards establish-ing benchmarks for normal behaviour of different types of rockll dam. The monitored movements give a good indica-tion of the deformations likely to occur during successivestages in the life of a rockll dam, and substantial depar-tures from such behaviour in a dam could indicate the onsetof unsatisfactory behaviour.

    Although rockll behaviour is not elastic and not linear,movements during embankment construction can be pre-dicted using simple linear elastic models. The limiting factor in making such predictions is not the sophistication of thesoil model used in the calculations, but rather the difcultyof establishing representative soil parameters for a ll mate-rial containing large rock fragments. Movements during

    0

    5

    10

    Verticalstrain: %

    0 0·5 1·0 1·5Vertical stress: MPa

    a

    bInundation

    Fig. 31. Compressibility of Roadford rockll measured in 1 mdiameter oedometer

    Table 6. Properties of Roadford rockll in 1 m diameteroedometer tests and in the eld

    r d : Mg/m3

    w: % n: % V a: %

    Test a 2 .16 7.9 21 4Test b 2 .05 4.7 25 15Field 2 .07 4.4 24 15

    r d , dry density of ll; w, water content; n, porosity; V a , percentageair voids.

    0·5 1 2 5 10 20Time since end of construction: years

    0

    0·2

    0·4

    0·6

    0·8

    1·0

    1·2

    1·4

    R B

    W

    S

    M

    s H/: %

    Fig. 32. Post-construction long-term settlement of crests of damswith time plotted on a logarithmic scale; B, Brianne; M,Megget; R, Roadford; S, Scammonden; W, Winscar; s, crestsettlement; H , height of embankment

    1·0

    0·5

    0

    Height above foundation

    Heigth ofcrest above foundation

    0 0·5 1·0Long-term settlement

    Long-term crest settlement

    Creep rateproportional tovertical stress

    Uniform creep rate

    Scammonden

    Winscar

    Fig. 33. Creep rate of sandstone rockll at Scammonden andWinscar dams as a function of vertical stress

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    reservoir impounding require more sophisticated analysis,since the loading applied to the dam by the reservoir water causes rotation of the direction of principal stress. Casehistories that include long-term monitoring provide the onlysatisfactory basis for validating numerical models.

    Heavy compaction greatly reduces the compressibility,deformability and creep movements of rocklls, but theentire elimination of collapse potential may also require

    appropriate watering of the ll. Heavily compacted rockllsof different geological origin may have very similar behav-iour, but heavily compacted gravel ll was much lesscompressible than the sedimentary rocklls.

    Post-construction deformations of rockll dams occur from a variety of causes, including reservoir loading, claycore consolidation and creep of the ll materials. Deforma-tions examined at different depths in regions of the embank-ments not seriously affected by reservoir uctuations, and where the ll had no collapse potential, show that creepcompression increased linearly with the logarithm of timesince the end of construction. The creep compression rate parameter Æ is stress dependent for these heavily compacted lls.

    EFFECTIVENESS OF GROUND TREATMENTWhere opencast backll has been compacted in a manner

    similar to that used in the construction of rockll dams, theground so formed usually should be suitable for buildingdevelopment. However, building development is often pro- posed on restored opencast mining sites where the ll hasnot been systematically compacted. The backll could beexcavated, any unsuitable material discarded and the remain-der put back as an engineered ll to a suitable specicationunder close supervision (Trenter & Charles, 1998). However,where the ll is deep this approach is unlikely to be either economic or practicable, and piled foundations are also

    unlikely to be an economic solution for low-rise buildings(Charles & Burland, 1982; Charles, 2005).The question therefore arises as to the extent to which in

    situ ground treatment can convert a non-engineered opencastmining backll into an engineered ll with suitable proper-ties for buildings on shallow foundations. A closely con-nected question relates to the depth to which in situdensication methods applied at the ground surface areeffective.

    Evidence-based ground treatment In the preface to the First Gé otechnique Symposium in

    Print, Ground treatment by deep compaction , the editors,

    Burland, McKenna and Tomlinson (1975), referred to the

    mystique surrounding ground treatment methods, com-menting:

    Nearly all the papers have been produced by contractorsspecializing in these techniques and, not unnaturally, theyhave concentrated on the successes obtained by their methods. The failures, or at least the lack of apparentsuccessful applications, have remained unrecorded. Theresult of this has been the growth of a certain mystiquesurrounding the techniques, and claims have been made ontheir ability to ‘strengthen’ ground which cannot always besubstantiated when subjected to a critical review.

    During the subsequent 30 years well-documented case his-tories have had a substantial role in dispelling much of themystique, but poorly documented case histories can bemisleading. Case histories where a lled site has beensubjected to a particular form of ground treatment and subsequently used for building development can be easilyaccumulated. The apparent absence of problems gives theimpression that an appropriate and adequate form of treat-ment has been applied. What is not known is how theground would have behaved if there had been no treatment.Building development might have still been successful. Insome cases little may have been required from the ground treatment and little may have been achieved. The problemarises where much is needed from the treatment. With ll ina metastable state, it will only be known whether the ground treatment has adequately dealt with this when, say, water from a leaking drain starts to saturate some of the ll.

    A parallel can be drawn with medical practice. It mayseem surprising and indeed alarming that for centuries physicians made life-and-death decisions concerning thetreatment of their patients on the basis of little, if any,scientic evidence. It is only in the last 60 years that‘evidence-based medicine’ has come to the fore using rando-mised controlled trials. Civil engineers do not usually havethe resources to carry out randomised controlled trials, butwell-documented case histories with appropriate long-termmonitoring provide an alternative route to establishing ‘evi-dence-based ground treatment’. A controlled trial of ground treatment methods was carried out on the opencast ironstonemining backll at the Snatchill experimental site at Corby,and some of the results are summarised in Table 7.

    With low-rise building developments on opencast mining backlls bearing capacity is unlikely to be a major problem,and foundation design based solely on an allowable bearing pressure will not address the principal hazard. Fig. 34 showssettlements monitored at a housing development on clay llthat had been treated by dynamic compaction at the Snatc-hill experimental site. The houses have experienced 50 mm

    settlement over a period of 25 years. Ground adjacent to the

    Table 7. Settlement of clay ll at Snatchill experimental site, Corby

    Treatment Settlement of ground surface

    Settlement of houses (mm)

    induced bytreatment: m

    During houseconstruction

    Total during and after houseconstruction to 1999

    Mean Maximum Minimum

    Preloading 0.41 1.4 11 25 5Dynamic compaction y 0.24 7.0 52 74 23Inundation { 0.10 6.1 54 149 30 No treatment – 2 .7 33 53 14

    9 m high surcharge in position for one month.† 15 t weight, base area of 4 m 2 , dropped from 20 m, energy input 2800 kN/m 2 .‡ Via 1 m deep trenches at 10 m centres.

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    houses, which is not loaded, has settled a comparableamount, indicating that the settlement of the houses isunrelated to the weight of the buildings, and can be attrib-uted to creep compression under the self-weight of the ll.

    Many of the geotechnical hazards for buildings on non-engineered lls are associated with volumetric compressionof the ground: therefore densication of the ll prior to building is an appropriate form of ground treatment.Although uncompacted lls have low shear strength and highcompressibility compared with the same ll in a heavilycompacted state, this will not be a problem in manysituations. An angle of shearing resistance of 40 8, typical of a loose granular ll, is quite adequate for most purposes.Compression of the ll under self-weight occurs as the ll is placed, and low-rise housing applies only small extra loadsto the ll, so the greater compressibility of loose ll is of little practical importance; the real need is for ground treatment to convert a metastable ll into a stable ll.

    Preloading One of the most fundamental and simplest methods of

    ground treatment is to consolidate the ll by temporary preloading with a surcharge of ll prior to construction. Fillsare generally inelastic and strains are mostly non-recover-able: therefore, once the ll has been consolidated, it willremain in that denser state, and its subsequent vulnerabilityto volumetric compression and hence settlement will begreatly reduced. Consolidation makes the ground muchstiffer under subsequent applied loads: in effect a normallyconsolidated ll has been converted into an overconsolidated ll, with all the improvement in soil properties that areconsequent on this change. The settlement of preloaded lldue to the subsequent weight of a building will be verysmall provided that the stresses applied to the ll by the building foundations are, at every point within the ll,smaller than the stresses previously applied by the surcharge.

    The effect of preloading is illustrated by comparing the behaviour of ll that had been temporarily preloaded by aspoil heap during the opencast operation at Horsley at gaugeD1 (Fig. 10) with ll at gauge B2 that had not been preloaded (Fig. 9). The settlement at gauge D1 was less thanone fth of that at gauge B2. The large rise in groundwater level produced very little settlement in the preloaded ground.However, the spoil heap was 30 m high, which is far inexcess of what would be practicable or economic as aground treatment method in normal circumstances.

    The surcharge trial on the clay backll at the Snatchill

    site gives a more realistic example of the effectiveness of preloading as a ground treatment method. Fig. 35 shows therapid response of the clay ll to loading. This is because thesettlement of the ll was caused by the compression of air-

    lled macrovoids between lumps of clay, and not by the veryslow process of squeezing water out of the low-permeabilitylumps of clay. There was little recovery on unloading.

    Preloading from the ground surface has a limited depth of effectiveness, and an indication of this depth for the tempor-ary surcharge of ll at Corby is given in Fig. 36, whichindicates that the 9 m high surcharge had little or no effectat depths greater than 9 to 10 m below ground level. Theheterogeneity of the backll is demonstr