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Engineering Geology

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E n g i n e e r i n g G e o l o g y<br />

Figure 7.14<br />

Typical record of cone penetrometer test.<br />

structural load. This is especially the case where a weaker horizon occurs at depth but is still<br />

within the influence of the bulb of pressure that will be generated by the structure (Fig. 7.16b).<br />

The plate-loading test provides information by which the bearing capacity and settlement<br />

characteristics of a foundation can be assessed (Matthews and Clayton, 2004). Such a test<br />

may be carried out in a trial pit, usually at excavation base level. Plates vary in size from 0.15<br />

to 1.0 m in diameter. Tomlinson (2001) recommended that a 300 mm plate was the minimum<br />

size that should be used in stiff fissured clays in order to obtain their undrained shear<br />

strength. If the deformation modulus is required for such soils, then Tomlinson recommended<br />

a plate size of 750 mm. The plate should be bedded properly and the test carried out on<br />

undisturbed material so that reliable results may be obtained. The load is applied by a<br />

hydraulic jack bearing against beams supporting kentledge, or reaction may be provided by<br />

ground anchors or tension piles installed on either side of the load position. The load may be<br />

applied in increments, either of one-fifth of the proposed bearing pressure or in steps of 25<br />

to 50 kPa (these are smaller in soft soils, i.e. where the settlement under the first increment<br />

of 25 kPa is greater than 0.002D, D being the diameter of the plate). Successive increments<br />

338

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