Estimation of undrained shear strength of fine grained soils from cone penetration resistance

摘要

Abstract Background Many previous authors have proposed empirical methods for the evaluation of undrained shear strength S ~( u ) of soft to stiff saturated cohesive soils?from CPT cone resistance q ~( c )?data. Published works are rare on this topic for the cases of unsaturated and overconsolidated clays and therefore further studies are needed to study the S ~( u ) versus q ~( c ) relationship for such soils. This paper investigates the q ~( c )– S ~( u ) correlation for Sudanese fine grained soils taking into account the soil type, moisture condition?and stress history effects. Methods Database pertaining to many soil samples representing low to high plastic clays and silts collected from thirty different Sudanese states was used in this study for analysis. The S ~( u ) values were determined from laboratory by the UU triaxial test method on undisturbed soil samples taken from adjacent borings and the CPT was performed by a mechanical adhesion jacket cone type. Results The S ~( u ) values varied from 30.3 to 460?kPa in the clay soils and from 16 to 252?kPa in the silt soils with average values of 129 and 91?kPa respectively. The?consistency of most clay soils could?be described as firm to very stiff with few soft and hard samples.?The values of empirical cone factor N ~( k )? were determined for the soil types considered and found to vary over a wide range of 35.1 to 55.6 with average values of 37.5 for clay soils and 44.1 for silt soils. Discussion A rigorous analysis of soil database was performed to model the S ~( u )– q ~( c ) relationship taking into account the effects of plasticity index and overconsolidation ratio soil parameters. The cone factor ?N~(k) values determined were analyzed and compared to those experienced in previous studies from different countries. The study results revealed that for a given soil the N ~( k ) is not a simple constant but depends on several factors such as moisture condition and degree of stiffness prevailing in the field during testing. The N ~( k ) values obtained are in close agreement with and confirm the 35 figure reported in previous studies for highly plastic clay and silt soils. The average N ~( k ) values obtained are much higher than those reported in several countries for normally consolidated soft to firm saturated cohesive soils but are lower than values found in few studies for hard and overconsolidated soils.?Interpretation of the CPT?and S ~( u ) database is different for saturated and unsaturated soils which makes?comparison of the N ~( k ) values difficult. Conclusions A reliable correlation cannot be directly developed between S ~( u ) and q ~( c ) for soils?with different types and characteristics as?both variables are influenced by several factors. The effects of soil type, moisture condition and stress history should be considered in studying the? q ~( c )– S ~( u )?relationship.?Reliable empirical relationships have been developed to estimate? S ~( u ) from CPT? q ~( c ) data for Sudanese clay and silt soils of different OCR values.?