Analysis Of Effective Stress For Saturated Soils

In soil mechanics,the principle of effective stress for saturated soils is one of the most important theories.The concept of effective stress is also useful for analyzing various engineering problems,such as the water and earth pressures,shear strength of saturated soils,etc.On a micro scale,it is shown that the traditional formula of effective stress seems doubtful for cohesive soils.Thus some related problems such as earth pressures and uplift force in saturated soils are still controvertial issues.However,the final purpose of effective stress priciple is to describe the macro behaviour(i.e.deformation and strength)of saturated soils appropriately.Analysis on a micro scale cannot provide a direct evidence for the macro problems such as water and earth pressures.On the basis of former achievements,the principle of effective stress is studied from two aspects:a new mechanical model is derived for the effective stress,during which the physical meanings of the parameters in that model are explained;based on various experimental evidence,traditional expression of effective stress is tested to see if it describes the macro behaviour(i.e.deformation and strength behaviour)of saturated soils appropriately,and the errors are analyzed quantitatively.Some engineering problems related to the effective stress principle are also studied.Conclusions are drawn as follows:(1)According to the equilibrium analysis,the fraction of pore pressure in the effective stress expression(factor η)equals α—nα+ n,where a denotes the transfer coefficient from the pore pressure to the particle stress,and n denotes the porosity.As a E[0,1],the natural boundaries of η should be n and unity.For solid materials,both n and a equal zero,the effective stress expression reduces to σ’＝σ.(2)For cohesionless particles,a decreases linearly with increasing effective stress according to contact theory.The slope of the α—σ’ curve is influenced by the modulus and Poisson’s ratio of the solid material.Experimental evidence for quartz sand and gypsum sand is re-examined.It is shown that the real contact area is extremely small.Thus the decrease in factor η is extremely slow even for the particles as soft as gypsum sand.For cohesionless soils,factor η is very close to unity in the range of stresses in which most geotechnical structures are found.(3)A method is developed to calculate the factor η for cohesive soils based on the shear strength tests.A sketch of η and a is obtained.It is clear that η for cohesive soils is significantly smaller than that for cohesionless soils.Factor η is close to unity for clays at low effective stresses.But for clays with high PL,η decreases significantly with increasing effective stress.For two cohesive soils,the relationship between η and the effective stress is shown repeatable from the shear strength tests under different back pressures.(4)Based on the results of consolidation as well as the permeability tests on five cohesive soils,η-value which controls the deformation of saturated clays is obtained from the consolidation theory.It is shown that η decreases with increasing vertical stress.The obtained values of η are also shown to between the boundaries of n and unity.It agrees with the theoretical derivation,as well as Schiffman’s intuition.The values of ηobtained from the consolidation and shear strength tests show good agreement with each other.Thus the modification of Terzaghi’s effective stress is supported by the experimental evidence on both the deformation and strength behaviour of saturated soils.(5)For two cohesive soils,the obtained values of η were applied to the calculations of earth pressures.The relationship between η and the effective stress is fitted by exponential decay.Then the active and the passive earth pressures of saturated soil can be obtained from an iterative method.Results based on real η were compared with the results of traditional methods,i.e.,effective stress analysis(ESA)and total stress analysis(TSA).The results of ESA are close to that calculated from real η.Meanwhile,ESA tends to be conservative:the active earth pressure is overestimated,while the passive earth pressure is underestimated.Such conservatism is more significant for the high plasticity clay.Contrastingly,TSA results are risky for the two soils examined.Thus TSA is not appropriate for the estimation of long term earth pressures.(6)A model test of up-lifting is conducted in saturated soils,such that the reduction coefficient of buoyancy is obtained during the Ultimate Limit State of Up-lifting(ULSU,i.e.,the effective stress approaches zero).The results indicate that the reduction coefficient of buoyancy is close to unity for different saturated soils.The uplift force in saturated soil is approximately the same as that in pure water,no significant reduction is observed.For saturated soils,theoretical analysis shows that the reduction coefficient of buoyancy during the ULSU is the reciprocal of Skempton’s B value before consolidation.However,B-value significantly greater than unity is not observed for varied saturated clays.Thus significant reduction in buoyancy is unlikely to be true during the ULSU.