The piezocone in lightly over consolidated clay
A laboratory experimental programme was carried out to investigate the behaviour of the piezocone in lightly over consolidated clay. The clay samples were prepared from reconstituted kaolin. The powdered form of this clay was mixed with water to form a slurry at a moisture content of approximately 120% under vacuum. The clay slurry was consolidated and then allowed to swell in chambers which were well instrumented. Penetration tests were carried out with two sizes of piezocones. The smaller piezocone had a cross sectional area of 1cm² with pore pressure measurement at the cone shoulder. The larger piezocone with a cross sectional area of 5cm² had pore pressure measurements at four locations. In addition to penetration testing, further information on the strength and consolidation characteristics of the test chamber samples was obtained form shear vane, restricted flow consolidation, flow pump permeability and consolidated undrained triaxial tests. The horizontal effective stress and the vertical stress ratio were found to influence the generated excess pore pressure at all four different locations on the piezocone shaft and the net tip resistance. This led to the establishment of direct correlations for the tip resistance factor Q with the coefficient for lateral earth pressure at rest Ko and the vertical effective stress ratio (OCR). Direct correlations for the excess pore pressure ratio at all four measuring positions with Ko and OCR were also established. Several methods of estimating over consolidation ratio were examined. One of the factors examined was Bq which was found to correlate poorly with OCR before tending asymptotically to a value of approximately 0.4. Other examples of factors which showed promising results as estimators of OCR were Bmi, the excess pore pressure ratio, and Q, the tip resistance factor. The excess pore pressures well behind the cone shoulder, which are partly generated by the shear stress in the area, showed promising correlations with the undrained shear strength. However, because correlations with pore pressure on the piezocone are dependent upon the position of the filter element, the particular results from this series may only be used for piezocones whose filter elements are at equivalent positions. The undrained strength ratio was also correlated with the tip resistance factor Q and the result was very promising for future interpretation of piezocone data.