Study Of The Fissures, Volume Change And Permeability Of Expansive Soil Under Wetting And Drying Cycles

The research background of the present thesis is the expansive soil problem of S-N water transfer project, the objects investigated are swelling rock from Xinxiang and expansive clay from Nanyang. By conducting artificial rainfall test and laboratory test, the failure mechanic of expansive soil slope, the evolution law of fissures under wetting and drying cycles, the volume change law and the permeability change law of expansive clay under wetting and drying cycles are studied. The main research contents and results are as follows:In order to study the soil-water interaction mechanics and the cause of landslide of swelling rock slope under precipitation infiltration, artificial rainfall test was conducted on two bare swelling rock slopes with different gradients in the Luwangfen test section of the Middle-Route of the S-N Water Transfer Project. Study of the monitoring data proves that: The gradient of slope and the rain intensity make no difference to the infiltration intensity when the water supply intensity exceed the infiltration ability of slope. In the slope, the area affected most severely by rainfall is within the depth of fissures, the water content here fluctuates wildly in the course of wetting and drying cycles. The matric suction of the superficial layer will lose completely owing to the existence of backwater zone. The invasion of water is just the inducement of the failure of gentle swelling rock slope, whilst the development of fissures and the weakening of the shearing strength caused by wetting and drying cycles is the essential cause.Laboratory test is conducted to study the evolution law of the fissures of Nanyang expansive clay under wetting and drying cycles. By employing the vector diagram technology to vectorize the fissures photos, the geometric characters of fissures are extracted, before calculating the fissures ratios. Study of the regularity of fissures ratios changes proves that: the essential factor that influences the stretching degree of fissures is not water content, but the gradient of water content. Under wetting and drying cycles, the fissures of expansive clay will develop gradually, but the role of wetting and drying cycles is limited, the development of fissures will cease when it comes to a certain degree due to the undersize of the clods. Expansive soil mass will encounter plastic deformation under wetting and drying cycles, the development of fissures and the damage of soil mass integrity are the direct causes of this plastic deformation. Laboratory test is conducted to study the evolution law of the volume change of Nanyang expansive clay under wetting and drying cycles. Study of test result proves that, the plastic deformation of the expansive clay develops and accumulates gradually under the effect of wetting and drying cycles, both in the shrinkage limit state and saturated state. The plastic deformation of expansive clay can be reflected on the height, area and volume changes of soil body, but more significantly on height change due to the confining stress. The initial expanding rate of expansive clay rises gradually as the number of wetting and drying cycles increases during the wetting processes. The development of fissures is the key factor to the improvement of water absorbing capacity of expansive clay, which directly promotes the initial expanding rate of expansive clay. The increase of void ratio and water-holding capacity, as well as the accumulation of plastic deformation are all caused by the development of fissures in expansive clay under wetting and drying cycles, they are the three different outward manifestations of the same physical process.Laboratory test is conducted to study the evolution law of the permeability of Nanyang expansive clay under wetting and drying cycles. Conventional falling head permeability test and modified unsaturated falling head permeability test was conducted. By integrating the permeability test result and the fissures data, it proves that: the permeability of saturated expansive clay and the initial infiltration capacity of unsaturated expansive clay rises as the number of wetting and drying cycles increases. The total length of the fissures which run through the seepage path is the key factor that determines the permeability of expansive clay.