Field Instruments

This appendix provides brief descriptions of some common instruments used in thefield in geotechnical engineering projects: the piezometer, earth pressure cell, loadcell, cone pressuremeter, dilatometer, and inclinometer.

Piezometer

The piezometer nleasures water levels and pore water pressure. The simplestpiezometer is the Casagrande piezometer (shown schematically in Figure 2.16) whichis installed in a borehole of 75-150 mm diameter. Figure A.1 shows a photograph of

Figure A. 1 Components ofCasagrande-type piezometer (Courtesyof N. Sivakugan, James Cook Univer­sity, Australia)

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28 Appendix A

the components of the Casagrande piezometer. The device consists of a plasticpipe joined to a filter tip that is placed in sand. A bentonite seal is placed above thesand to isolate the pore water pressure at the filter tip. The annular space betweenriser pipe and the borehole is backfilled with a bentonite-eement grout to preventvertical migration of water. Although they are simple, reliable, and inexpensive~ thestandpipe piezometers are slow to reflect the change in pore water pressures and fe·"

quire someone at the site to monitor their operation. The other types are pneumaticpiezometers, vibrating wire piezometers, and hydraulic piezometers. Piezometers areused to monitor pore water pressures to determine safe rates of fill or excavation; tomonitor pore pressure dissipation in ground improvement techniques such as PV s,sand drains, or dynamic compaction; to monitor pore pressures to check the perf{)r·~

mance of embankments, landfills, and tailings dams; and to monitor water drawdo\vnduring pumping tests. Its nonmetallic construction makes the piezometer suitable inany adverse environment for 10ng-terrrL operations in most soils.

Earth Pressure Cell

An earth pressure cell (Figure A.2) measures the total earth pressure, which includesthe effective stress and the pore water pressure. It is made by welding two circularstainless-steel diaphragm plates around their periphery. The space between the platesis filled with a de-aired fluid. The cell is installed with its sensitive surface incontact with the soil. Any change in earth pressure is transmitted to the fluidthe cell, which is measured by a pressure transducer that is similar to a diaphraf~m­

type piezometer. Earth pressure cells can be used to measure stresses within em­bankments and subgrades, foundation bearing pressures, and contact p~essures onretaining walls, tunnel linings, railroad bases, piers, and bridge abutments.

Figure A.2 Earth pressure cell (Courtesy of N. Sivakugan, James CookUniversity, Australia)

Field Instruments 729

Figure A.3 Load cell (Courtesy of N. Sivakugan, James Cook University, Australia)

Load Cells

Load cells (Figure A.3) measure the loads acting on them. The load cell can be of themechanical, hydraulic, vibrating wire, or electrical-resistant strain gauge type. Loadcells measure strains or displacements under the applied loads that are translated intoloads through calibration. Load cells are used in proof testing and performance mon­itoring of tiebacks, rock bolts, soil nails and other anchor systems, and pile load tests.They are also used to nleasure strut loads in braced excavations. In the laboratory,they are employed in triaxial devices and in load tests on model piles and footings.

Cone Pressuremeter

The cone gives a reliable classification of soils and good estimates of strength, but ispoor in estimating soil stiffness. The pressuremeter, on the other hand, is well suitedto measuring stiffness and strength parameters (Houlsby and Withers 1988). Thecone pressuremeter (Figure A.4) exploits the features of a piezocone al1d a pres­suremeter. It combines the continuous profiling capability of the cone with the abil­ity of the pressuremeter to measure the load-deformation characteristics ofsoils-particularly the shear modulus. Approximately 2 m in length, the probe con­sists of a piezocone at the lower end of the penetrometer shaft and a pressuremeter

730 Appendix A

Figure A.5 Inclinometer system (Courtesy ofN. Sivakugan,James Cook University,Australia)

Figure A.4 Cone pressuremeter(Courtesy of N. SJivakugan, JamesCook University, Australia)

of the same diameter above, as shown in the figure. The top of the probe can be con­nected by the cone penetration test rods to the ground, and the probe is generallypushed into the ground by a cone truck or by jacking against a kentledge. The his­torical developments of the cone pressuremeter are given in Dalton (1997).

Inclinometer

Inclinometers are quite popular for measuring lateral displacement in slopes, land­slides, retaining walls, piles, and bridge abutments. An inclinometer system (Figure A.5)is composed of a torpedo-shaped probe fitted with guide wheels and a tilt sensor

References 731

Figure A.6 Dilatometer and other equipment (Courtesy of N. Sivakugan, James CookUniversity, Australia)

and connected by a graduated cable to a readout unit. The probe is sent through theinclinometer casing iJlstalled vertically in a borehole, and the tilt sensor records thedeviation between the probe axis and the vertical plane. By installing the inclinome­ter hc)rizontally, the profiles of settlement or heave beneath embankments, storagetanks, and landfills can be obtained.

Dilatlometer

The d.ilatometer test was discussed in detail in Section 2.6. Figure A.6 shows two flatdilatolmeters with other instruments for conducting the test.

Daltol1, C. (1997). "Pressure for Change," Ground Engineering, Vol. 30, No.9, pp. 22-23.Houslby, G.T., and Withers, N.J. (1988). "Analysis of the Cone Penetrometer Tests in Clay,"

Geotechnique, VoL 38, No.4, pp. 575-587.