Lec8 Instrumentation 2009

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  • Instrumentation

    According to Peck (1988) "every instrument installed on ag ( ) e ery nstrument nstalled on aproject should be selected and placed to assist inanswering a specific question," but also in sufficient

    b " d f l f hnumber "to provide a meaningful picture of the scatterin results " and "to allow for inevitable losses resultingfrom malfunction and damage "from malfunction and damage

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    Reasons for Instrumentation

    Site investigation (eg. Pore pressure; permeability) Design verification Construction control Construction control Quality control of workmanship Safety (eg. Early warning of impending failure requires quick retrieval and processing)q p g)

    Legal protection I i f ( P ) In-service performance (eg. Pore pressure)

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    To ensure that the actual ground response closelyg p ycorresponds to the one assumed, the following have to beclosely monitored:

    Rate and magnitude of horizontal wall displacements

    Rate and magnitude of settlements

    Strut forces

    Wall bending moments

    Dissipation of pore pressure

    Damage to Adjacent structures

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    Inclinometer (in-soil and in-wall)

    Measures the average inclination along given depthintervals inside a pipe/tube installed in the borehole. Fromthe inclinations, the horizontal movement of the soil/wallcan be determined. The toe has to be anchored in a hardstratum as a lateral displacement will lead to anstratum as a lateral displacement will lead to anunderestimation of the absolute lateral movement.

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  • Settlement markers/displacement markers/extensometers

    Settlement markers are installed on the ground in linesperpendicular to the wall to establish the settlement of theground surface profile using survey levelling methods.

    Displacement markers are used to measure lateralpmovements of the ground surface using precisiontheodolite and electronic distance measurement (EDM).S ttl t k d di l t k lSettlement markers and displacement markers are alsoplaced at the columns and floors of surrounding buildingsor services to measure movementsor services to measure movements.

    Deep extensometers are sometimes installed to monitorheave of the subsoil due to excavation and as an earlyheave of the subsoil due to excavation and as an earlyindicator of basal heave.

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    settlement marker

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    Extensometer

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    Crack Meter Tiltmeter

    Tiltmeters are used to monitor changes in the inclination of a structure.

    Tiltmeter consists of an electrolytic tilt sensor housed in a compact waterproofsensor housed in a compact, waterproof enclosure. The tilt sensor is a precision bubble-level that is sensed electrically as a resistance bridge The bridge circuitresistance bridge. The bridge circuit outputs a voltage proportional to the tilt of the sensor.

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  • Vibrating wire strain gauge, Load cell

    Used to measure forces in the struts/anchorsstruts/anchors.

    www.geokon.com

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    Earth pressure cellEarth pressure cell

    www.geokon.com

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    Survey instruments to measure tunnel movements

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    Piezometer

    The piezometer allows the changes of the groundwaterlevel in the subsoil to be monitored It should be located inlevel in the subsoil to be monitored. It should be located inlines perpendicular to the wall to establish the profile.Changes in groundwater level are very important becauseC a ges g ou d a e e e a e e y po a becauselowering of the groundwater level could cause settlementof the retained soil. On the other hand, rising ofgroundwater unexpectedly to a higher level than thedesign value can cause additional lateral pressure leadingto failure of the wallto failure of the wall.

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  • Piezometer

    Instrument for measuring pressure in the pore water.

    vibrating wire piezometerpneumatic (uses gas)

    electrical (contains a vibrating-wire transducer on which water pressure acts through a ceramic tip)

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    which water pressure acts through a ceramic tip)

    Open standpipe piezometerC i t f t b i hi h th l l f t iConsists of an open tube in which the level of water is measured by sounding or lowering a tape into the tube.

    Sand is placed around the filter tip. Bentoniteseal is placed above the sand to isolate the

    t t th ti Apore water pressure at the tip. As pwpincreases or decreases, the water level insidethe standpipe rises or falls. The height of the

    Installed in sand formations

    water above the filter tip is equal to the pwp.

    Installed in sand formations

    Also used to verify drainage condition of bottom layer

    Casagrande

    condition of bottom layer

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    CasagrandeSlopeindicator.com

    Observation wells use the same components as the standpipepiezometer but are installed differently. The tip is not sealed off andthe borehole is backfilled with sand or gravel.

    Observation wells create anundesirable vertical connectionb t t t d h ld lbetween strata and should rarelybe used. The water level with theobservation well is likely toobservation well is likely tocorrespond to the head in themost permeable zone and willusually be misleading.

    Observation well

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    Location of InclinometersBecause of arching effects, the wall deflection at oraround corners are smaller.

    Deformation of the shorter side of the excavation is lessthan on the longer side.g

    wall ? ?wall ? ?

    ??

    Plan view of ti

    Wall deflection

    CV4352 19excavation

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    I65I65

    I104

    CV4352 24Source: Aviva ER

  • I104I104

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    I65 I104I65 I104level 7 17 Apr04

    20 Apr04 ~ 1 pmp p

    Source: Aviva ER

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    Circle Line C824 Soil Inclinometer (IS-104)

    Source: JSE

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