Study On Characteristics Of Collapse Deformation For Collapseible Loess, Foundation Treatment Method And Tests

Supported by the key scientific and technical tackling researches of the State Grid and by ways of cases studies in fields of site submerging test, pile test, theoretical research and a lot of engineering foundation treatments, profound and systematic researches were carried out on problems including the soaking deformation rules and computing methods, foundation treatment methods of large-thickness self-weight collapsible loess foundation, the elasticity and plasticity theories of composite foundation, analysis on the thermal consolidation and temperature field-seepage field-stress field coupling of lime pile composite foundation and the dynamic consolidation of down-hole dynamic compactions. Main works and innovative results are as follows:(1)Theoretical analysis on the deformation process of the large-thickness self-weight collapsible loess submerging tests and discussed the causes, soil layers and distribution of the loess as well as the factors influencing the loess collapsibility were carried out. The mechanism and computing methods of the collapsible loess soaking deformation and selected rational, easy and practical constitutive model were studied. The loess collapsibility was calculated via the finite element method (FEM) software ADINA through the secondary development. Finally a simple and practical collapsibility calculating formula was given by using the statistical regression, test and the valued numerical calculated results and the method discussed in the paper was proven by taking reference from the on-site test. The method is a new approach and provides theoretical basis for the loess soaking deformation.(2) Large-scale submerging tests were carried out on the site with36m thickness of collapsible soil layer in Heping Town of Lanzhou City and large-area submerging test was carried out on the condition that the diameter, depth and water-head height of the test pits have been confirmed. The moisture meter and tensiometer were buried at the vertical directions outside and inside the pits respectively. Layering deformation and settlement observation points were set inside the pit and the surface settlement points were set on the earth surface to further analyze the self-weight collapsibility features and study on the regional distribution rules of the collapsibility amount of the site when the site been immersed in saturated, unsaturated and non-Darcy flows. It proves that the soaking deformation of large-thickness self-weight collapsible loess has three features which are significantly different with the collapsibility of moderate-and-small-gauge (less than15m) self-weight collapsible loess, including①The soaking settlement amount goes through five stages along with the progress of the submerging duration, including the gentle stage at the beginning, the submerging drop-off stage, the gentle stage at the medium term, the drop-off stage after the water flow stops and the gentle stage at the later stage;②The collapsibility rate changes following the rule of "gentle-rapid-gentle-stable" during the submerging duration and then "rapid-gentle-stable" rule after the water flow stops;③The collapsibility amount, the crack width around the test pits and the height difference of cracks are greatly higher than all those in the previous similar research records;④Large-thickness loess soil layer at different depth will all go through several times of collapsibility and this quantity will decrease along with the increase of soil layer thickness;⑤Volatile water consents of soil layer of different depth changes in different rules; moisture infiltration within a depth limit of25m is easy and will slow down outside this limit due to the collapsibility of the upper soil mass and the increased gas pressure in the gap.(3) A computation model was founded for the expansion, elasticity and plasticity of the lime pile composite foundation according to the cavity expansion theory and that model was analyzed and solved and thus computation formulas of the compaction effective radius of the lime pile and the expansion and pressure increment of pile lining were obtained; the analyzing model for the thermal consolidation of lime pile composite foundation was established and an approximate analyzing and solving method was given for the thermal consolidation of the saturated soil mass around the cylindrical heat source of the lime compaction piles; a governing equation for the temperature field-seepage field-stress field coupling of the lime pile composite foundation was established and was numerically solved by way of finite elements; the creep simulation of the composite foundation was carried out based on the Prandtl-Reuss code; the simplified dynamic computation model of down-hole dynamic compaction was founded and analyzed and the functional relationship between the mass and distance of the dynamic compaction pounder as well as the relationship between the dynamic compaction depth and the soil body around the pile were obtained; finally, design and analyze one case by using the foundation treatment technology and finally it proved that the theoretical analysis and the actual measurements were rather similar. It shows that the treatment results have met related regulations. Comprehensive comparison shows that DDC is better than the lime pile treatment technology and thus it is proved that these two foundation treatment skills are applicable and reliable for the large-thickness collapsible loess.(4) Field tests were designed for the lime-soil compaction pile and DDC foundation treatment skills and field measuring and analysis were carried out and some conclusions which may provide major reference for the engineering design were obtained:①For areas treated with DDC piles, the moisture can hardly seep in from the periphery of pile caps. No great settlement appeared in the pile caps at the three treatment areas and the surface settlement resulted by expansion was even greater than that caused by the pile caps settlement and the soil mass collapsibility;②All the three different pile-length treated foundations can resist the load of20t/m2; using the DDC pile with length of15m can effectively reduce the engineering cost;③Pile distance between1.0m to1.4m can meet related regulation requirements both from the compaction coeffecient and the collapsibility coefficient. Choosing bigger pile distance may reduce the foundation treatment cost by approximately30%;④Construction investment on buildings on the large-thickness self-weight collapsible loess field may go rising since the foundation treatment for Grade Ⅱ and Ⅲ buildings covering large area is too strict; Lime-soil and pure soil may differ slightly in regard of load capacity when treating the large-thickness self-weight collapsible loess site and thus the pure soil pile shall be preferred directly in the future engineering construction in order to cut down the engineering price;⑤When inject water into the deep layer, all the pile cap, deep layer and the surface settlements will also develop in a three-stage development rules:stable at the initial stage. Slow down in the middle stage and drop off in the later stage;⑥When the pile is6m,10m and12m in length and inject water into the deep layer and the load is20t/m2, great settlements appear around the pile caps and the site surroundings. In the contrast, the settlement at the15m area is slightly gentle but the residual collapse amount here failed the requirements. When the foundation treatment depth reached20m-25m, the residual collapse amount may meet requirements. It agrees with the foresaid conclusion that the collapsibility can hardly appear at the20m-25m depth area.