Study On Instability Mechanism And Protective Measures Of Expansive Soil Cutting Slope In Mombasa-Nairobi Railway

The Mombasa-Nairobi Railway passes through a large area of expansive land in the form of deep roads in the Mombasa area.Due to the lack of research results on the engineering characteristics of expansive soils in this area,and the lack of research results on the instability mechanism of such cutting slopes,the design of protective structures such as pile-slab walls is relatively conservative.Therefore,it is necessary to carry out special research on the engineering characteristics of expansive soil in the area,the instability mechanism of the curb slope and the optimal design of the pile-slab wall structure.Firstly,through the field investigation,the instability characteristics of the expansive soil slope in this area were studied,and the design characteristics of the protective structure were analyzed.Secondly,through the indoor geotechnical test and on-site artificial rainfall test,the swelling characteristics,shear strength characteristics of the expansive soil and the influence characteristics of rainfall infiltration on the soil water state inside the slope are explored.At the same time,the effects of various characteristics on the slope stability are also analyzed.Combined with the above research,the inducing factors of slope instability are comprehensively analyzed,and the instability mechanism of the expansive soil cutting slope is revealed.Finally,the three-dimensional numerical simulation model of pile-slab wall is established by using Midas-GTS finite element software.The force and deformation characteristics of pile-slab wall structure at different stages are simulated and analyzed.In addition,based on the orthogonal experimental design method,the design parameters of the pile wall protection structure are optimized.Through the above research,the main conclusions are as follows:(1)The instability of expansive soil slopes in this area is characterized by seasonal,shallow,fissive and progressive.The appearance of the above characteristics is closely related to the climatic characteristics of the rainy season and dry season in the region,as well as the swelling deformation characteristics and shear strength characteristics of the soil.(2)The greater the expansion potential of the expansive soil in this region,the larger the expansion quantity of the soil will be when its expansion is stable.Moreover,the expansion ratio of the soil when it is expanded stably has a linear correlation with its own dry density,water content and overburden load.(3)The shear strength of expansive soil in this area decreases with the increase of its own water content,the decrease of dry density and the increase of the number of dry and wet cycles.The reasons for the above phenomena are related to the weakening of the intermolecular forces,the decrease of the soil suction and the loss of the cementation force between the soil particles caused by the development of the soil cracks.(4)The influence of rainfall on the slope soil is about 2m.The soil within this range will undergo volume expansion deformation and shear strength reduction due to water absorption.The above effects will reduce the slope stability.(5)The occurrence and development of fissures,the attenuation of shear strength of weathered soil and the infiltration of rainfall are the three inducing factors of instability and failure of expansive soil cutting slope.The occurrence and development of fissures and the attenuation of shear strength of weathered layers are the internal factors of slope instability,which are closely related to the low permeability and strong expansion and contraction of the soil.Rainfall infiltration is an external cause of slope instability and its direct predisposing factor.(6)The instability of the expansive soil cutting slope is the result of long-term effects of various factors.The slope will go through five stages from excavation to instability.They are the stage of the occurrence and development of shallow fissures,the stage of low shear strength zone formation,the stage of rainfall Infiltration,the stage of slope foot instability and the stage of expansion of instability.(7)The excavation unloading action and expansion force of the soil between the piles not only affect the stress and deformation characteristics of the pile-slab wall structure,but also reduce the stability of the slope.Based on the orthogonal test results,the optimized design parameters of the pile-slab wall structure are obtained.And the simulation results show that the optimized pile-slab wall structure is improved in terms of safety and economy.The above conclusions can provide some reference for the treatment and protection of expansive soil cutting slope in similar areas.