DETERMINATION OF OCR IN CLAYS BY PIEZOCONE TESTS USING CAVITY EXPANSI0N AND CRITICAL STATE C0NCEPTS

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For piezocone penetration in clays, cavity expansion theory describes both the point resistance and induced excess pore water pressure in terms of the undrained shear strength (C_u) and rigidity index (I_r=G/C_u). Modified Cam Clay provides a simple effective stress representation of undrained behavior in terms of stress history. Therefore, the two theories may be combined to provide an approximate determination of the in-situ overconsolidation ratio (OCR = σ_p'/σ_<υo>') in terms of the effective stress friction angle (φ') and normalized piezocone parameter (q_T - u_m)/σ_<υo>', in which q_T=corrected cone resistance and u_m=measured pore water pressure. The approach distinguishes between piezocones which measure pore water pressures on the cone tip/face (u_t) and those which measure just behind the tip (u_<bt>). Specific examples are presented to show the general applicability of this simple effective stress model for estimating the in-situ stress history of clay deposits with 1<0CRs<60 and 20°<φ'<40.
1991-06-15