The Effects of Effective Stress and Overconsolidation Ratio on the Undrained Shear Strength of Soil

This study investigates the effect of effective stress and overconsolidation ratio (OCR) on normalized undrained shear strength, which represents the soil deformation and soil strength from applied load. Theoretical frameworks indicate that undrained shear strength (Su) is affected by soil parameters such as void ratio and is linked to the soil consolidation condition, stress path, and stress history. Furthermore, Su is functionally related to the OCR. The purpose of these test are to test two hypotheses: 1) that the undrained shear strength conforms with normalized soil engineering properties, this is true if the following equation describes the undrained shear strength Su = σ'v(Su/σ'v)nc·OCRm, 2) that undrained shear strength is a function of void ratio. If hypothesis 1 and 2 are true, the exponent (m), should be: m = (1 – Cr/Cc). The laboratory test examines Helmer clay, a type of Kaolinite clay that typifies the properties of low-plasticity, soft, compressible clay. Two laboratory experiments were utilized: the one-dimensional consolidation test, which examines effective stress, soil deformation, and overconsolidation ratio (OCR), and the fall cone test, which determined undrained shear strength based on the penetration of cone at different angles and weights. Consolidation tests were conducted using a 500-pound load cell, with load increments varying from 100 psf to a maximum of 12,800 psf. The results demonstrated significant correlations between the final void ratio (ef) and undrained shear strength (Su), as well as between effective vertical stress (σ'v) and the end of primary void ratio (e100), agree with results from previous research. These correlations indicate the exponent (m), which exhibits a slight variation due to the soil sample disturbance and the friction from the consolidation ring. The normalized undrained shear strength (Su/σ'v)nc or c/p ratio for soils with equivalent stress path and consolidation state has a higher value than prior studies related to the unique Su value using fall cone test and the soil specimen employed.