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Dissipation tests

SITE INVESTIGATION

A piezocone was used to measure the pore water pressures generated during the piezocone cone penetration test (PCPT) in an area where a road underpass is proposed at Ascot.

At specified depths the progress of the cone was halted and a dissipation test carried out. This standard test measures the dissipation of the generated pore water pressure with time at a specific point. From published correlations the interpretation of these results can be used to estimate the horizontal coefficient of consolidation (Ch) and the insitu permeability (k).

A dense gravel layer near the surface had the effect of desaturating the cone and affected the results. To overcome this a prepush was undertaken using a standard cone through the top 2m gravel layer. The standard cone was then removed and a saturated piezocone used to complete the test.

Below the dense gravel layer a mixture of medium dense sands, clays and silts formed the main constituents of the soil. The standard U2 filter position did not generate a satisfactory pore water pressure response because of the ground conditions. For better pore water responses Lankelma suggested the filter was mounted in the U1 filter position on the face of the cone tip.

Within the ground conditions at Ascot, the U1 filter generated a higher pore water pressure response, unlike the U2 position, which is mounted on the shoulder of the cone. This produced enhanced dissipation results and enabled a better resolution for the interpretation of the PCPT results.

CPT testing at the location of the underpasses allowed better definition of ground conditions. Use of piezocones and the ability to undertake dissipation tests provided the means to retrieve direct information about groundwater conditions on site.

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