Study Of Unsaturated Collapse Deformation Of Loess

It is common to use the collapse coefficient obtained in laboratory to calculate the collapse deformation or in-site infiltration test to monitor it. Thought results of the last method is accurate, it not only takes long time to perform but also is time consuming. The collapse coefficient recommended by regulations is more popular in most engineering. But there exists big difference between the calculation deformation and actual value. There are two reasons for the discrepancy. For one thing, the collapse coefficient is obtained in saturate conditions. While it is known that loess collapses as soon as the moisture content reaches a value which may be less than the saturate value. For another thing, in-site infiltration tests demonstrate that the total collapse deformation changes with the infiltration time, and during the no-infiltration time the deformation continues and the total value increases. Ignoring the time factor may also cause the difference between the calculation deformation and the actual one. Loess is a kind of typical unsaturated soil. According to the unsaturated seepage and deformation theories, the research presented herein put forward to a new method to calculate the collapse deformation which can obtain the collapse deformation versus time. And the new method is used to calculate the yearly settlement and total settlement since irrigation in 1967. And the result is comprised with relative literatures. Besides microscope is used to obtain the microstructure of undisturbed samples and compressed samples with different moisture content which is used to study the relationship between the macroscopic physical and mechanical properties.In Heifangtai, a well was digged to take samples to measure the basic physical and mechanical parameters versus depth, based on which the thick loess were divided to 4 layers. Take samples in each layer to perform oedometer tests and measure the soil water characteristic curve. A seepage model was modelled to get the water pressure versus depth during irrigation. Then settlement versus time was calculated with the results of the laboratory tests and the seepage model. Results show that since the irrigation in 1967, the total settlement has been 1.91 m which is close to the measured values 2.01 m in the position of taking samples. Before the groundwater increases, the yearly settlement is relative big which is the biggest in 1967 and is 0.4m. It fluctuates at the range of 20-40 mm after the ground water table increases in 1973.The microstructures of undisturbed soils and compressed soil with different moisture content under 900 kPa were obtained by the microscope. Analysis show that the size and number of the pores in undisturbed samples is directly determined the void ratio and dry density and the pore size distribution affects the SWCC of the samples. When the volumetric water content is the same, the more the big and middle pore size is, the smaller the matric suction is. Under the 900 kPa, the bigger the moisture content the smaller the dominating pore size and the more the micro pore. The characteristics of pore size distribution of different layers are similar. Parameters reflecting the deformation characteristics are mainly affected by the pore size distribution of undisturbed samples. The more the big and middle pore is, the bigger the collapse coefficient is and the smaller the elastic modulus is.