Strength Properties And Steady-State Behavior Of Sandy Soil

It was known that clean sandy isn't in extension all the while, and a significant amount of fines exist in alluvial and hydraulic deposit silty sand. And the role of nonplastic fines content on the strength properties and steady-state behavior of sandy soils has been a topic of debate for some time. In this paper the research was done with CU tests on the Nanjing sand with sheet particle to analyze and evaluate the effects under low confining pressures. The main research of this dissertation is as follows:1. The undrained compression tests were performed on Nanjing sand under lower confining pressures.The residual strength is increasing with the increasing of relative density at the same confining pressure and the samples at the same relative density are from instability to stability and to instability again under the higher confining pressure. The complete static liquefaction has occurred for the looser Nanjing sand at 1.0750e =. The metastable particle structure of Nanjing sand makes for the static liquefaction. By normalizing the stress path, the relationship of peak strength and residual strength is:2. The basic physical properties indices of 10 kinds of silty sand are measured to know their particle distribution. The change of specific gravity is little with fine content; neither are the changes of the changes of dry density, uniformity coefficient, coefficient of curvature and void ratio. All show the particle size distribution of silty sand is from worse to better when the fine contents are increased.3. The CU tests on 10 kinds of silty sand are performed under lower confining pressures. The fine contents have small effect on the deviatoric stress which arrives at the peak stress, but after the peak stress, the effect on the stress becomes obvious very much. And it shows there is different not only the position of the steady state lines but also the slope of theirs when the fines are changed. The steady state line of clean sand is on top of that of silty sands and the steady state lines move down as the fines increasing up to 25%, then move upwards up to 60%. So the limit fine content of silty sands is 25%.4. Contribution from the coarser and the finer grains in silty sand to stress-strain ralationship is affected by the intergranular matrix structure. New intergranular state parameters, such as intergranular relative density ( D rs), intergranular and interfine void ratio( e sand e f), are induced to characterize the behavior of silty sands. As e s < emax,HSthe soil behavior of silty sand at an intergranular void ratio e s is similar to that of the host sand at a void ratio e equal to e sand the fine content and confining pressure have little effects on the stress-strain-strength relations. When the fine content exceeds 25%, the peak strength and residual strength are affected by the confining pressure. And the residual strength is more sensitive to the confining pressure than the peak strength. Even more, at e s in the vicinity of emax, HS, the fines and confining pressure have obvious effect on the stress-strain behavior of silty sand. The stability of specimen is better with the increasing fines and confining pressure.5.The effect of fines on the stability was discussed by the empirical correlation between SPT N-value and relative density, average grain diameter for sandy soils. The critical SPT N-values in steady state line of silty sands with different fines were calculated. It is validated that the flow potential of Nanjing sand is overestimated in Code for Seismic Design of Building.6. The results are analyzed for triaxial compression tests by superior sand model. Compared with the test results, the superior sand model can reflect the stress-strain behavior of Nanjing sand with medium confining pressures. But the result of the looser Nanjing sand under lower confining pressure can't be simulated when the test was terminate at a shortfall axial strain of 20%.