DEVELOPMENT OF A NEW CONE PENETROMETER AND ITS APPLICATION TO GREAT DEPTHS OF PLEISTOCENE CLAYS

Abstract

A new piezocone was developed and applied to Osaka Pleistocene clay layers as deep as 250 m. Its geometry is the same as that of the reference procedure defined by the ISSMFE and standard of JGS: i.e., the projected cross sectional area is 10 cm²; the angle of the cone is 60°; the pore water pressure is measured at the shoulder. This piezocone does not measure the skin friction. Due to great depth at the investigated site, the capacity of the point resistance (q_t) and the pore water pressure is as large as 30 MPa and 20 MPa, respectively. The test was carried out, using a borehole drilled prior to the penetration, because of stiff sand or gravel layers and large skin friction between the rod and the ground. The q_t measured by the cone penetration test (CPT) was correlated to the yield consolidation pressure (p_y) measured by the Constant Rate of Strain (CRS) oedometer test for the soil sample recovered near the point of the CPT investigation. The cone factor for the p_y value (N_pc) was defined by (q_t -p_vo)/p_y, where p_vo is the in situ total overburden pressure. The range of observed N_pc value is relatively narrow and between 2.5 and 2.8, which is in the middle of the range of N_pc factors measured in Holocene clays in the various areas in the world as well as Japan. The overconsolidation ratio (OCR) was also derived by CPT. Variation of the OCR estimated by the CPT is nearly equivalent to that measured by the CRS oedometer. It may be concluded from this investigation that the consolidation properties, especially the p_y value, derived from samples recovered from great depths, are quite reliable as design parameters.