Consolidation Analysis For Layered-Ground Improved By Sand Wells And Its Engineering Application

Drainage preloading has been found the most economical method for ground improvement for these soft deposits and it is widely used in soil improvement. Sand drains including prefabricated vertical drains and other vertical drain channels are usually used to accelerate the consoliation process of soft clay, silt and other compressible materials. According to technology development and economic growth some buildings are required to have smaller value of the post-construction settlemnt. The design criterion for the ground with sand drains has been changed from controlling the stability to controlling the post-construction deformation. To adapted to the change, consolidation theory of ground with sand drains is studied systematically, and an applied design method is put forward.The main research work in the paper is listed as followed:(1) The consolidation theory on ground with partially sand drains is discussed and the continuity conditions at the plane of penetration were improved so that it can reflect pore pressure gradient in radial direction and rationalize pore pressure dissipation in vertical direction. An engineering case is analyzed using the analytical methods and the FEM numerical method. It is shown that results obtained by the present analytical method is closer to those obtained by FEM numerical method than by the existing analytical method. It can reflect consolidation of ground with partially pentrated vertical drain wells well and truly. Based on general Viogt rheological model, consolidation theory of homogeneous ground with partially penetrated sand drains has been developed.(2) The existing consolidation theory of double-layered ground with sand drains and of ordinary ground with partially penetrated sand drains has been extended for consolidtion analysis of general layered ground with full or partially penetrated sand drains. Analytical solutions and derivation process for it have been offered. The theory can be applied widely because it can take into account the condition that the section with or without vertical drains containe multi- strata. An application program based on the solution principle has been coded.(3) Based on the principle of superposition, solution for consolidation of layered ground with vertical drains under time-dependent loading has been developed. The surcharge can be loaded arbitrarily, such as stiff loading and ramp loading, which will induce some breakpoint when differentiated with respect to time. Solution for vacuum combined with surcharge is also been obtained through some mathematical processing. It is shown that, the faster the stratum consolidated, the more it is affected by the loading condition. It is also shown that the differences of average degree consolidtion affected by loading condition is larger during loading period than at any other time. There is hardly difference when time is large enough. Multi-ramp loading can be replaced by simple ramp loading to calculate average degree ofconsolidation in late stage in engineering design.(4) The problem of consolidation of ground with sand drains for impeded boundaries has been studied. Analytical solutions have been developed for double-layered ground with vertical drain, for simple ground with partially penetrated and for general layered ground with sand drains. Analysis for impeded boundaries shows that impeded boundaries parameters has great effect on the system consolidation and it affected the conjoint stratum more than that away from. Impeded boundaries should be take into account when drainage path is hindered in sand cushion or there is partially drainage path in the bottom strata of the system.(5) In practical engineering, initial excess pore pressure is dependent on depth; preconsolidated pressures for all strata are not a unit value; stress-strain relation is nonlinear. These factors mentioned above are taken into consideration for nonlinear consolidation analysis of ground with sand drains. In the nonlinear consolidation analysis, stratum and time are divided into many parts so that every...