Centrifuge Shaking Table Tests Of Stone Column-Improved Saturated Sand Dam Foundation

China has the largest number of earth-rock dams in the world,which are used for electricity generation,water supply,flood control and irrigation,and are an important part of our nation’s water conservation and hydropower infrastructure.China is also a country with frequent earthquakes,and numerous post-earthquake investigations have shown that earthquake-induced liquefaction of the foundation is one of the major causes of earth-rock dam failures.Stone columns have the advantages of simplicity of construction,economic convenience and so on,and are widely used in the foundation improvement of liquefiable sites.The seismic response and the effectiveness in mitigating liquefaction of stone column-improved foundation are the hot and challenging topics for both domestic and international researchers in recent years.However,there are few researches on stone column-improved dam foundation at present.Due to the interaction between dam foundation and dam body,the seismic response of dam foundation is significantly different from that of free field,and the effectiveness of stone columns at different positions of dam foundation needs to be assessed.Therefore,based on the actual background of a water conservancy and hydropower project in western China,two centrifuge shaking table tests were designed and conducted to analyze the dynamic response of saturated sand dam foundation,assess the influence of stone columns on the seismic performance of foundation-dam system.Experimental results were complemented and extended by elastic-plastic fluid-solid fully coupled numerical simulations.The main work and results of this thesis are as follows:(1)Two hyper-gravity shaking table tests were designed and conducted for saturated sand dam foundation and stone column-improved foundation,focusing on the densification and drainage effects of stone columns.Various molds and processes were designed and adopted for the dam foundation,dam body and stone columns to ensure the reliability of the model.(2)The differences in seismic response of the two models were systematically compared and analyzed: The acceleration amplification factor of the untreated foundation decreases with increasing elevation under the center of dam,and decreases firstly and then increases with increasing elevation under the toe of dam and at free field;stone column-improved dam foundation has greater stiffness and lower damping,which may result in a larger seismic energy transmission to the dam body;compared with the soil under the dam body,the soil under the toe of dam and at free field are more likely to liquefy;stone columns slow the buildup of excess pore pressure,accelerate the post-earthquake pore pressure dissipation,reduce the peak of excess pore pressure and duration of high pore pressure in the treatment area,but the soil outside the treatment area still liquefies under a strong earthquake;stone columns effectively reduce the settlement of the dam crest and uplift of the soil surface,improving the seismic performance of the foundation-dam system.(3)The dynamic shear stress-strain response of the dam foundation under earthquakes with different seismic intensities was compared and analyzed: The soil is basically in the linear elastic stage under small earthquakes,and its nonlinearity and hysteresis become increasingly significant as the seismic intensity increases.Under strong earthquake,shear stiffness of the soil under the toe of dam and at free field is completely lost and the cyclic mobility is exhibited,while there are no substantial losses in shear stiffness of soil below dam body.Stone columns can mitigate the degradation of shear stiffness of the soil in the treated area by inhibiting the development of excess pore pressure,but have little effect on the soil outside the treatment area.Normalized shear modulus of soil at different depths of the dam foundation decreases gradually with increasing shear strain,while the results of the damping ratio are scattered and have no obvious regularity.(4)Based on Open Sees platform and PDMY constitutive model,two hyper-gravity tests were simulated by using elastic-plastic fluid-solid fully coupled analysis method.The numerical results are consistent with the experimental results.Further parameter analysis shows: In the context used in this thesis,the densification effect of stone columns plays a major role in mitigating the softening of soil and reducing excess pore pressure of site.Surface surcharge can alleviate softening and dilation of foundation soil,inhibit the development of excess pore pressure and shear strain.In a 0.2g earthquake,the soil at different positions of stone columnimproved dam foundation will not liquefy,while under the strong earthquake,although the stone columns can reduce the rate of increase of excess pore pressure and the settlement of the dam crest,the peak value of excess pore pressure ratio is not significantly reduced,and the effect of increasing the area replacement ratio is not obvious.Increasing the permeability coefficient of the stone columns can effectively prevent the liquefaction of the soil in the treatment zone,while the peak value of excess pore pressure ratio of the soil outside the treatment area is still high.The research results of this thesis reveal the seismic response law of saturated sand dam foundation,verify the effectiveness of stone columns in improving seismic performance of the foundation-dam system,which can provide some scientific basis for the seismic design of saturated sand dam foundation.