Study On The Characteristics Of Collapse And Permeability Of Large Thickness Loess Ground Based On Water Immersion Test

With the implementation of “Western Development” and “One belt, one road” strategies and urbanization of China, there are more and more important large buildings in the loess area. The available site for construction extends from low terrace to high terrace, from river valley and plain to loess Yuan, Liang and Mao, even patches of reclaimed land from hilly areas are now used as construction sites. As a result, the risk that is associated with thick collapsible loess deposit is an urgent challenge for numerous cases of construction projects. The reasons behind are as follows: the characteristics of collapsibility of thick loess is not well understood, the method for evaluation of collapse settlement is yet to be modified for better precision, the efficient ground treatment for thick layer of collapsible loess and negative skin friction of pile foundation caused by loess collapse, etc. In addition, wetting is the direct cause of loess collapse, hence to understand the seepage rules in loess ground is a precondition for seepage prevention using engineering measures. In the past, due to constraint of in-situ water measurements technology, in-situ study on loess seepage is too rare to reach common understanding. The quantitative evaluation of loess collapse and civil engineering design for collapse damage prevention are mainly based on rule of thumb, which may lead to disastrous situation. Therefore, study on seepage in loess ground has both practical and theoretical implication.In this thesis, based on literature study on domestic and international study on collapse and seepage, through pit flooding test, laboratory test, theoretical analysis and numerical simulation, the collapse, depth limit assessment of collapse, pore water seepage and dissipation of sites respectively in central Shaanxi, west Henan and Qinghai are studied. The new findings obtained with this study are as follows:(1) The time-domain and 3D collapse deformation development under pit flooding was systematically studied. It is found that development of collapse induced deformation has four stages: slow increase, quick increase, slow increase again and finally reaches relatively stable status. Depending on testing sites, the duration of each of the four stages may differ. The severer loess collapse is, the quicker the development of deformation will be. Boundary condition clearly constrains the development of deformation. After flooding ended, due to pore water pressure dissipation, different level of consolidation deformation developed depending on site conditions. Larger deformation develops in the middle of the depth range, while loess at relatively shallow depth shows mild collapse due to wetting by rainfall or irrigation and coefficient of collapse of loess at much deeper depth decreases with increase of depth.(2) The comparison of collapse depth limit of self-weight collapse of loess respectively by field tests and laboratory tests found that generally and site-independently the results obtained from the two kinds of tests are objectively different, and a quantitative, multi-perspective analysis was carried out to understand the mechanism behind. For the first time, a modification method was proposed to modify the collapse depth limit got from the laboratory loess test for survey and design of constructions.(3) A comprehensive evaluation method of collapsible loess with large thickness is put forward. In view of the characteristics of large thickness loess, this paper puts forward a set of comprehensive evaluation method for collapsible loess, which based on the survey method, the test method of the collapsible index, the relationship between the basic physical index of the loess and collapsibility, calculation method of self-weight collapsibility and collapsibility.(4) Based on pit flooding test and numerical simulation, the patter and range of seepage were revealed quantitatively, both horizontal and vertical permeability coefficient were measured to provide reference for loess seepage damage control and drainage design.(5) An empirical formula was established to assess the residual water cut of large depth loess ground, which fills the blank in this study field. For the first time, based on large pit flooding test and in-situ water measurement technology, the time-space characteristic of pore water dissipation was studied quantitatively and the relationship between residual water content and dissipation duration has been established.