Miniature piezocene tests and effects of smear due to vertical penetration in layered soils
Knowledge of the soil profile is necessary for ground engineering projects and piezocones are widely used in situ test devices that can supply some of this knowledge. This thesis describes an investigation of the performance of a specific piezocone when used in thinly layered soil. A miniature piezocone, with a cross sectional area of 1 cm2, was driven at a speed of 20mm/sec into artificial layered soil samples that were constructed in the laboratory and consolidated under a vertical pressure in a 254mm diameter test cell. The layered samples contained alternating layers of pre-consolidated Speswhite kaolin clay about 20mm thick and layers of more permeable, silty or sandy soil about 2mm thick. The pore pressure filter of the piezocone was located either at the cone tip or cone shoulder. During driving, the cone resistance and pore pressure responses were recorded at a rate of at least 200 readings/sec. Once the piezocone was stopped, in a clay layer, the dissipation of excess pore water pressure was monitored. In terms of the pore pressure response, though not the cone resistance, the piezocone was able to detect the more permeable layers located between the clay layers. Both dilation and localised drainage in the more permeable layers, deformed during penetration, could have significantly influenced the pore pressure responses. Despite the proximity of permeable layers, values of the coefficient of consolidation obtained from pore pressure dissipation at the piezocone tip agreed fairly well with values obtained independently during unloading or reloading of the clay in one-dimensional consolidation tests. At the cone shoulder, the permeable layers had some influence and larger values were obtained. The layered soil samples used for piezocone testing were also used for investigating the effects of soil disturbance, or "smear", caused by vertical penetration of objects with different sectional shapes in the context of permeability measurement and soil drainage. A mandrel carrying a vertical drain, either circular (23.5mm diameter) or rectangular (50x6.5mm) in section, was driven into the centre of the soil sample at a speed of 5mm/sec. The effects of smear were evaluated by performing radial flow permeability tests in which pressure distributions across sample were recorded. The effect of smear increased substantially as the permeability of the more permeable layers increased, but only when it exceeded the permeability of the clay by a factor of about 100. For a given layer combination, the rectangular drain always produced a significantly smaller smear effect than the circular drain.