FIRST .ApPLICATION OF ,A TOTALLY PROTECTED ANCHORAGE

Première application d'un ancrage à protection totale

by

c. MASTRANTUONO et A. TOMIOLOIng. G. RODIO et C.S.p.A.Casalmaiocco, Milano· Italia

SOMMAIRE

Cet article décrit la première application d'unnouveau système d'ancrage entièrement protégécontre les dommages causés par la fissurationet· la corrosion des barres d'acier.

Cet ancrage appartient à la famille des ancragesà bulbe comprimé.

La technologie d'exécution du bulbe de scelle·ment injecté est celle utilisée dan's le systèmebien connu IRP à injection répétée en pression.

Chaque barre est protégée sur toute sa longueurpar une gaine plastique jusqu'à la plaque du fond,sur laquelle se fixe chaque ancrage.

La plaque du fond est enrobée dans un mortierde résine époxy. Ainsi, même si quelques fissuresse développent dans le bulbe de scellement injectéprécontraint, aucun point ne peut être attaqué parla corrosion, l'acier étant entièrement protégé parla gaine en résine époxy.

Dans cet article, on présente les résultats deprototypes réalisés dans le rocher et dans dessols alluviaux mous.

Les essais effectués dans le sol mou ont étéexécutés à l'aide de dispositifs de mesure mon­trant la distribution de la transmission des chargesau terrain. Ces distributions ont été comparées àcelles de deux tirants IRP traditionnels exécutésdans le même sol.

SUMMARY

This paper describes the first experimental appJj·cation of a new anchoring system thoroughlyprotected against dangers connected with fissur·ing and corrosion of the steel rods.

This anchorage is similar to the type of anchorswith a compressed bulb.

The grouted bulb placing technology is the sameas used in the weil known IRP system, i.e., withrepeated high pressure, grouting.

Each rod is protected, along its entire length,by a plastic sheath down to a bottom plate onwhich each anchor is fixed. The steel bottomplate itself. is embedded in epoxy resin mortar.

ln this way even if some fissures develop in thestressed, grouted bulb, no point is left open toattack by corrosion, the steel being entirely pro·tected by the plastic sheath of epoxy resins.

This paper reports the results of prototypes inrock, and in soft alluvial soils. .

The tests in a soft soil were carried out withinstrumentation detecting distribution of the trans­ferred loads to the soil. These distributions havebeen compared to those of two traditional IRPanchors built in the same soil.

INTRODUCTION

During the last few years the application of ancho­rages has become more and more widespread.

The generalized use of these systems involves pro·blems of various types. Among them the protection ofanchorages against attack by corrosion and qualitycontrol on the job sites are of particular importance.

In the first part of this paper a new type of anchorcalled TPT, entirely protected' .against corrosion, isdescribed. The results of two series of tests on proto·

types of such anchor are reported. Prototypes in thefirst series have been' installed in rock whilst thoseof the second series in normally consolidated clayeysilts.

Anchors of the second series, in soft soils, wereinstrumented so as to n1easure distribution of stressesalong the length of the grouted bulb. These measu­rements were, also carried out on traditional anchorprototypes placed in the same soil thus allowing acomparispn between the two anchoring systems.

1. DESCRIPTION Of TPT ANCHOR

1 ( ....

As shown in figure 1 the characteristics of the TPTarrchor may be summarized as follows:

- The TPT anchor consists of a number of cablesplaced aIl around a pipe equipped to performrepeated high pressure grouting.

107

- Each strand is protected by a plastic sheath downto the bottom plate, thus preventing .any possiblecontact of the steel with waters present in theground, even if fissuring of the grouted bulboccurs.

LONGITUDINAL SECTION

DETAIL OF THE BOTTOM ANCHOR DEVieE

350 mm

EEo~

section A-A

LEGENDA

®

plastic tube equippedwith multi - stagegrouting devices

-- -- tendons

114-lJc.....:.--..'-.-l--- steel spiral

-metallic net

section' B-B

'",

G) mortar of epoxy resins

® cement grout

@ tendons protected with a plastic sheath

@ devlce for positioning the tendons

® tube equipped with grouting device

- The bottom plate equipped with strand fixingdevices is embedded in epoxy resin mortar. Inthis way all the steel parts are thoroughly protected.'

- The prestressing device may be any one of thoseemployed for other traditional anchors.

- The TPT anchor belongs to the family of anchorswith compressed grouted bulb in that the steelcables may run ,within the plastic sheaths andtransfer the load to the bottom plate.

® reinforcement steel spiral

(j) terminal steel plate

® devices for fixing the steel tendons

® tendons 'without plastic sheath protection

Fig. 1. - TPT anchor system.

- The TPT anchor grouted bulb is carried out usingthe same construction method employed for theIRP anchor. (4)

- Hence it is possible to combine the advantages ofrepeated high pressure grouting with those °relatingto protection against corrosion both of cables andbottom plate.

2. STRENGTH Of GROUTING 'MORTARS

A particularly important problem involved in groutedanchors, especially those' with compressed bulb, isthat related to strength of the injected miXe

This mix undergoes a state of triaxial stresseswherein the principal stresses are due to:

108

a) stresses transmitted by bottom plate;b) confining effect due to the soil surrounding the

grouted bulb.1t is well known (1) that by varying the mechanical

properties of the grouted bulb and surrounding soil,

./~

/V

VV --

// Rendulic

planef-

~oo 100 200 300 400 500 Vi"- 62 = V2- 6

3

( Kg/cm2)

200

300

100

400

500

900

800

61

1000

Fig. 2. - Variation of strength of grout under triaxial. state of stress.

distribution and magnitude of stress states in thegrouted mix will also vary.

The author has carried out tests with a view to deter­mining the strength range of a grouting mix of the typecommonly employed, subject ,to triaxial stress states.

These tests were performed at ENEL (GovernmentElectrical· Agency) Laboratory at Niguarda, (Milan).

Tests were carried out on eight sets of specimens of agrouting mix with the following characteristics:

- size of specimens . la x .10 X la cm;- pozzolana cement 325;- water/ cement ratio 0.55 ;Results of these tests are given in figure 2 where two

curves are plotted, representing, in Rendulic's plane,the strehgth range limits of the grouting mix subjectto triaxial stress states. The maximum stresses usuallyacting on the mix injected near the anchor bottom platevary from 500 to 600 kg/sq.cm. By comparing thesestresses with the stresses shown in figure 2. it will beseen that these states of stress do not cause breakingof the mix when the surrounding soil is able to exertsuch a confining pressure that the ratio between theprincipal stresses is equal to 0.3.

3. TEST ON PROTOTYPES

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Tests on TPT anchor prototypes were carried out ontwo test sites representative of two limit conditions.

. In the first case the soil consisted of rocks showingthe best mechanical characteristics, while in the secondcase the soil was formed by clayey silts of very poorgeotechnical properties. In both cases the aim of thetests was to provide information on the followingpoints:

- verify that the TPT anchor actually worked as acompressed bulb anchor;

- verify that the strength of th~ injected mixessubmitted to triaxial stress states \vas consistentwith strength measured by the laboratory testsdescribed in the preceding paragraph;

- compare the behaviour of compressed bulb TPTanchors with that of the traditional IRP anchors.

The tests performed led to some conclusions whichmay be sumlned up as follow.s:

- in practice, the measured free lengths coincidewith the theoretical lengths thus showing that theanchors actually behave in the same way as anchorswith compressed bulb;

- stresses transferred by the bottom plate to thegroute~ mix were 800 kg/sq. cm. approx. andno failure phenomenon occurred.

This confirms the validity of values found throughthe experimental tests referred to in paragraph 2.

Following the successful outcome of the tests, TPTanchors have been employed in general in an jobsitesinvolving anchors in rock which required a definitiveprotection against corrosion.

3.1. Tests of TPT anchors in rockon the Sorrento jobsite (Naples)

Six TPT anchors were built in soil consisting ofcalcareous rock.

The tests. were carried out from July to September1976. The characteristics of these anchors were thefollowing:

length: 25 mdrilling diameter: 120 mmtesting load: 11 7 tThe test load value is determined by the yield point

of the steel. The tests were performed accordingto the procédure described by Portier (2), Le. the cyclemethod. This systen1 offers the possibility of inter­polating the free length of the anchor taking intoaccount the loading and unloading curves measuredduring the tests.

M·easurements taken were highly satisfactory for theyshowed differences of 7 % from the theoretical freelengths ..

As an example, we show in figure 5 the stress/straincurve measured during the test on a TPT anchor. Fig. 3. - Rupture caused by uniaxial state of stress.

109

120 ,--------,

actual freelength

70 f------f--------A'--f---f'----+---t---I-----

80 t----------t-- -----H'II~__t'+__-H----___j

100 f------+-------t---U----t----t-t'------l

~ 60.2

40 f-----H--H--f--I----IF----f----------j

20 f-----fJ-H-t'------I-I-----------l!-------

10,w-r----f--.t-f-------t--------j

30 f---+-I--:II---+----c3-----f-----

50 f----------I-f-Ji'---JL------H--f----------j

Fig. 4. - Rupture caused biaxial state of stress.

At present (March 1977) a jobsite where about150 anchors of this type are being installed, is beingset up. In this jobsite measurements, of the freelengths are systematically carried out.

3.2. Tests on TPT anchor in soilof poor geotechnical ~haracteristics

In February 1977 a series of tests were begun onTPT anchors, installed in soils of very poor geotechni­cal characteristics, that is alluvial clayey silt soils in thePo river delta.

The anchors were set out in the same site wheretraditional IRP anchors were under construction. Thisallowed comparison on the behaviour of the two typesof anchors in the same soil.

A 'number of anchors were instrumented with remo­vable strain gauges.

Measurelnent were taken by instruments and techni­cians of «Laboratoire Central des Ponts et Chaussées»of Saint-Brieuc (3). The tests made it possible to takemeasurements on stress distribution within the bulb onboth TPT anchofs (loaded by a bottom plate)' andtraditional IRP anchors where the load transfer tothe bulb occurs by adhesion between, steel and cementmix).

At present (March 1977) the testing programme isstill under way, hence it is impossible to give generalresults on the behaviour of TPT anchors in soil of poorgeotechnical characteristics.

Nevertheless we think it is of sorne interest to referto the results of the straingauge measurements detect­ed along the bulb length, referred to in figure 6, 7and 8.

In figure 6 a typical section of the work is given,showing both anchor position and points where straingauge measurements were carried out. Measurementswere taken only at the grouted bulb and strain gaugesplaced in the same position for aIl anchors.

ln figures 7 and 8 strain measurements carried outboth on a IRP anchor and a TPT anchor are reported.

A numerical evaluation of these results will form thesubject of a future. paper that will be published after

O"----------"------ "------ ~

o ~ 100 ~O

extension (mm)

Fig. 5. - Stress/strain curve.

completion of the present investigation programme.Here we are just making a few remarks of a qualitativecharacter.

ln figure 7 bulb deformations as a function ofapplied load for a TPT anchor are given. We have fiveload/deformation diagrams each referring to one ofthe points indicated in figure 6.

1t should be noted how at the plate, the deforma­tions are very high while they decrease towards thetop of the bulb. This confirms the success of theanchor, from the technological point of view, in thatworking performance approaches theoretical perfor­mance, even in the range of the lowest loads.

ln figure 8, curves relating to IRP anchor, similarto those in figure 7 are shown.

As already stated, in this type of anchor, load trans'­fer occurs by adhesion of the strands to the groutedmix. Deformations decrease towards the end of thebulb.

1twill be seen that the strain gauge. placed nearthe înflatable packer on top of the bulb detects nodeformations.

1t is interesting to note that unit maximum per centdeformations, in anchors with bulb undergoing tensilestress, are much higher than those of anchors withcompressed bulb. This proves the risk of bulb micro­fissuring, as already pointed out by various authors(Ostermayer, 1974, for instance) (5).

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CROSS SECTION WITH ANCHORSand distribution of deformationsas a function of loads

-diaphragm wall

~ points of measuremenl

o 10 20 30 40 50 60 70 ( t)1 H--=-.__.--...,...-o--...,... ~ ......, 0~

~i-JL--._-----l....__....L-_---.:........L...:.--_--L-__.L..--_-----l...._----I + .?o

1

Fig. 6. - Cross section with an·chors. Position of points ofmeasurements.

U)~

~I-J

~_··......:..i_LIJ:::bb1-:00

o

Fig. 7. - Deformations in thefPT anchor. The loads are trans~

ferred to the bulb by a bottomsteel plate. The measurementpoints are referred to those offig. 6.

r---~--"""'--------.----r-----' - 400

1----+---+-----j----6"",.r.=-----1 - 200

- 600

- 400

- 200

0

- 600

- 400

- 200

00 10 . 20 30 50 (t)

Fig. 8. - Deformations in the 1RP anchor.The loads are transferred to the bulb by

JI adherence between tendons and groute

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~ Î\. +400

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AKNOWLEDGEMENTS

The authors wish to express their gratitude to Mr.Bellotti of the ENEL Laboratory of Niguarda for hisresearch on triaxial stress states in grouting mixes, andto Mr. Jézéquel of Laboratoire des Ponts et Chausséesof Saint-Brieuc, for measurements taken by means ofremovable strain gauges.

REFERENCES

(

[1] COATES (D.F.) and YU (Y.S.). - «Three-dimen­sional stress distribution around a cylindrical holeand anchor», Proc. 2nd Congress IRSM, Beograd,vol. 3, Theme 6-7 (1970).

[2] FENOUX (G.Y.) and PORTIER (J.-L.). - «Lamise en précontrainte des tirants», Travaux,n. 449/450, 33-43 (1972).

[3] JEZEQUEL (J.-F.), HERVE (G.) and LIBERGE(P.). - «Extensomètres amovibles pour la mesuredes relations contraintes-déformations des pieuxchargés. verticalement», Rapport interne Labora-

112

toire Régional des Ponts et Chaussées de Saint­Brieuc (1973).

[4] JORGE (G.R.). - «Le tirant IRP réinjectablespécial pour terrains meubles, karstiques ou àfaible caractéristiques géotechniques», Proc. 7thICSMFE, Mexico, Spec. Session 15, 159-163(1969).

[5] OSTERMAYER (H.). - «Construction, carryingbehaviour and creep characteristics of groundanchors», Proc. Conference on Diaphragm Wallsand Anchorages, London, Sept. Pap~r 18, 141-151(1974).