Dynamic Effect Of Pore-water Pressures In Different Patterns On Dynamic Properties Of Saturated Sand

The famous effective stress principle of saturated soil is an elemental principle to describe the effect of pore-water pressure that is a main stress characteristic of porous medium of multiple phases. Five different patterns of pore-water pressures: pore-water pressure wvdue to static water level, pore-water pressure u due to water flow, pore-water pressure ua due to transient stress change, pore-water pressureucdue to soil structure damage and pore-water pressure ur due to water diffusion and dissipation, may exist simultaneously in the process of dynamic loading according to the transient dynamics research of saturated sand, and they have different effects on dynamic characteristics of saturated soil because of their different producing reasons in mechanism. Based on these ideas about dynamic pore-water pressure, firstly, this paper analyzes thoroughly the main differences and properties of these pore-water pressures that may appear in the engineering practice, puts forward their modeling methods in geo-technique labs, improves the conventional dynamic tri-axial instrument according to the modeling requirements, and designs relevant experimental scheme firstly, and then makes a series of experiments by using this improved instrument and draws some useful conclusions which reveal the dynamic effect of pore-water pressures in different patterns. Lastly, current study of soil dynamics and corresponding practical problems in engineering are surveyed and discussed on the basis of the gained elementary conclusions.The research results show that Ux under the condition of uncontrollediiidrainage isn't concerned in the dynamic loading duration, and can't affect the dynamic properties of saturated sand. But under the same draining condition, the u produced due to the steady pore-water seepage, which isin equilibrium system of forces and takes no effect on the deformation of soil, can affect the dynamic deformation property of saturated sand when the dynamic residual pore-water pressure occurs owing to the destruction of its microstructure. This may result in the local or whole unsteady water permeation from the un-balanceable distribution of pore-water pressure, and consequently induce the obvious degradation of dynamic properties in comparison with that in condition without u . This phenomenon can be calledas the reviving effect of seepage forces, and its extent of affecting varies with the magnitude and combination of u .and also the percolation gradient. In addition, under the un-drained condition, the initial ua has no effect on the dynamic properties as it isn't concerned with the change of soil microstructure, and the dynamic properties of saturated sand with an initial static uc (the residual pore-water pressure in consolidation e. g. ) remains the same as that under the condition without initial uc but with the same effective stress state. That is, the excess pore-water pressure due to partial consolidation will not affect the dynamic property of saturated sand. But, the initial dynamic uc induced by pre-vibration can wholly improve the stability of soil structure and increase its dynamic strength as the same time as the soil structure is damaged and produces a certain residual pore-water pressure. As for ur due to pore-water pressure diffusion, it can result in the reduction of dynamic strength under the action of a larger dynamic load because of the reduction and relaxation of effective stress in soil. The over-consolidation of soil can always decrease the damage effect of dynamic loading. All in all, the following conclusion about dynamic effect of pore-water pressures in different patterns on dynamic properties of saturated sand can be arrived at that the dynamic properties of saturated soil will be affected as long as the existence and change ofpore-water pressures can result in the microstructure change by way of direct participation or indirect inducement. The research takes the problem about the dynamic reviving effect of seepage forces into consideration in the study of soil...