Theoretical Study On Two-dimensional Consolidation Of Layered Foundation Under Surcharge

For the soils with alternating clay and sand layers,under the action of preloading,the sand plays a role of natural horizontal drainage body to discharge the pore water out of the field,and then the soils will be consolidated.At present,most of the theoretical studies on the consolidation of this kind of soils are based on the element calculation model — a clay layer in the middle and two sand layers above and below.Obviously,this model cannot take into account the interaction of the clay layers in the actual soils,therefore,when the sand cannot fully meet the drainage requirements of the clay,the results of this calculation model may cause large errors.In view of this situation,this thesis firstly establishes an element consolidation calculation model that can consider the interaction of the clay layers under the condition of alternate distribution of clay and sand layers,and the excess pore pressure of each layer in the model is deduced and solved combing with the numerical transformation method;then,the main factors affecting the position of undrained surface of the clay under this kind of soils are discussed;in addition,the consolidation model for overall analysis is established,and the governing equations are derived and solved by introducing numerical transformation method and boundary transformation method;finally,the obtained theoretical formula is applied to a practical engineering case.The main results are as follows.(1)On the premise of the determined position of the undrained surface in the clay layer,the element consolidation calculation model can calculate the excess pore pressure and the degree of cross-section consolidation of the soils effectively,and the necessity of considering the interaction between clay layers in the calculation process is verified by examples,at the same time,it is obtained that the maximum difference in the average consolidation degree of each vertical section in the horizontal direction is more than 20%.(2)When the ratio of volumetric compression coefficient between sand layer and adjacent clay layer is less than or equal to 0.1,the sand can be considered as incompressible;under the condition of large-area preloading,the influence of the distribution of additional stress in the soils on the section consolidation degree does not exceed 2.0%;regardless of the total load,the total time required for the section consolidation degree at the center of the site to reach 85% is approximately linear with the total time in the loading stage;applying an overload of 0.1 times the building load during the stacking phase can save more than 15% of the total time when the post-construction settlement is used as the control index of unloading.(3)The permeability coefficient and thickness of the sand have the greatest influence on position of the undrained surface of the intermediate clay,these two parameters can make the undrained surface move from one side of the clay to the other side;the thickness and compressive modulus of the clay layers spaced apart from the target clay also have a significant effect on the position of the undrained surface,and the influence range can reach0.6 times the thickness of the clay layer;The effect of site parameters can be ignored.(4)The theoretical solution of the overall consolidation calculation model of the alternately distributed clay-sand layer can well describe the dissipation law of the excess pore pressure of the multi-layer soils;when only one sand layer is arranged in the foundation,the fastest consolidation occurs at 2/3 times the calculated depth from the surface;the consolidation rate of two sand layers with equal spacing is slower than that of single sand layer at 2/3 times the calculated depth from the surface under the condition of the total thickness of sand layer is the same;when the ratio of the horizontal permeability coefficient of the sand to the adjacent clay is greater than or equal to 10,it is approximately consistent with the assumption that the excess pore pressure in the sand layer is uniformly distributed along the vertical direction in the previous literature;the variation law of the position of the undrained surface of the clay layer obtained by the theoretical formula is basically consistent with that obtained by the numerical method.(5)Under the influence of the actual soils distribution conditions and soil structure,the surface settlement and the average consolidation degree of the section obtained by the theoretical formula in Chapter 4 have a certain difference with the actual engineering monitoring values;the conventional soil constitutive model cannot describe the soft soils consolidation problem well in numerical calculation;the variation law of the calculated value at the DB7 observation point is roughly the same as the measured value;the final settlement obtained by theoretical calculation can be multiplied by a coefficient between 1.0-1.3 as the reference value of the final settlement of the site.