Research On Failure Mechanism Of Shallow Instability Of Expansive Soil Slope And Geogrid-confined Effect On Swelling Deformation

Expansive soil distributes widely throughout the world as a special soil, and it triggers many serious damages of geotechnical structures. The progressive failure of expansive soil slope at shallow depth occurs frequently in railway, highway, hydraulic engineering, etc. But the current methods of stability analysis and the failure mechanism of expansive soil slope have some serious defects and can not to reflect properly the particularity of expansive soil slope failure.Based on the stability problems of expansive soil slope of the South-to-North Water Transfer Project, this paper focused on the very essence property of expansive soil, swelling behavior, and carried out a series of large static physical model tests of the compacted expansive soil slope. The test results revealed the whole sliding process of expansive soil slope without fissure, overconsolidation, and wetting-drying circle, etc. Combining the multi-field coupling numerical analysis with unsaturation-saturation moisture field, swelling deformation field and stress field, the expansion failure mechanism for the shallow instability of expansive soil slope is proposed.Furthermore, this paper studied the geogrid-confined effects on swelling deformation of expansive soil by a series of expansion model tests and mastered the swelling deformation rules with different geogrids type, strength, numbers of laying layer and fixed ways of geogrid front end. The physical model test of geogrid-reinforcement slope of expansive soil showed that geogrids can reduce the swelling deformation and improve the whole stability of expansive soil slope. The feasibility of geogrid dealing with the expansive slope stability is proved. The research results provide a theoretical basis and experimental data for disposition methords of expansive soil engineering slope.The main conclusions are as follows:(1) For the expansive soil slope, a series of large static physical model tests(6mx2mx2.8m) of the compacted expansive soil slope and the multi-field coupling numerical analysis indicate that, the fundamental factor influencing the slope shallow instability is the swelling behavior, not the fissure or strength reduction by overconsolidation. The failure mechanism of the shallow progressive instability of expansive soil slope can be elaborated:the moisture filed of expansive soil slope becomes nonuniform in space-time after absorbing water from rainfall, and it generates the nonuniform deformation within the certain shallow layer of the slope, the stress-strain filed redistribution is obvious. The local shearing failure occures firstly at the shallow wetting-drying surface because of the shear stress concentration, then expands deeper into the slope, finally, the multiple shearing surfaces lead to the whole progressive sliding of the expansive soil slope.(2) For the stability analysis of the expansive soil slope, a multiple-filed numerical analysis method coupling unsaturation-saturation, expansion deformation, and stress filed is proposed. Furthermore, the analogue simulation of the physical model tests proves that this method can reappear and reflect truly the progressive sliding failure.(3) By referencing the tempreture expansion therory, the moisture swelling coefficient of expansive soil is back analyzed according to the indoor expansion tests with or without vertical loading.(4) The swelling deformation rules of expansive soil with geogrid-confinement are related to different geogrids type, strength, numbers of laying layer and fixed ways of geogrid front end, etc. The expansion model tests prove that geogrids can reduce the lateral swelling deformation significantly.(5) The large physical model tests show that the stability with geogrid-reinforced expansive soil slope is improved and the swelling deformation decreases to a certain extent.