Study On The Formation Mechanism And Prevention And Control Measures Of Bedrock Crack Ground Fissures In The Kenya Rift Valley

Ground fissures are a kind of shallow rupture phenomenon of the earth,which is closely related to the internal and external dynamics of the earth and human engineering activities.The Kenya Rift Valley belongs to the separation boundary between the Nubian plate and the Somali plate of the East African continent,and the current tectonic movement and volcanic activity are still very strong.Due to the unique tectonic geological background,stratigraphic lithology,meteorological and hydrological conditions and human engineering activities in the rift valley,the ground fissures in the Kenya rift valley are extremely developed.Ground fissures pose a great threat to the safety of local people’s lives and properties,have caused great damage to local engineering facilities and have a great constraint on future engineering planning,and have caused a large number of land to be idle and abandoned.It is of great scientific value and practical significance to find out the development law and genetic mechanism of ground fissures in the rift valley region for the subsequent engineering construction in the rift valley region and the prevention and mitigation of existing ground fissure disasters.Based on the research on the formation mechanism and disaster reduction measures of the Nairobi-Malaba Railway ground fissure(2019-ZJTDKJ-05),the research platform of the East African Rift Valley ground fissure disaster mechanism and prevention and control(41920104010)project and the cooperation of the Nairobi-Malaba Railway construction unit,this paper conducted a field investigation and research on the development characteristics of the ground fissure in the East African Kenya Rift Valley,and established a classification system of the ground fissure in the Rift Valley.Focusing on the bedrock crack ground fracture unique to the Kenya Rift Valley,this paper clarifies the structural genesis of bedrock crack ground fracture and the relationship between the bedrock crack propagation process and plate movement,basement structure,fault activity,volcanic activity,stratum lithology and structure,and hydraulic erosion through theoretical analysis,physical model experiment and numerical simulation.The engineering structure measures suitable for the geological environment and economic development level of the rift valley region and the comprehensive measures for disaster reduction and prevention of ground fissure disasters are proposed,which provide the theoretical basis for the subsequent linear engineering construction such as railway,and also fill the theoretical gap in the study of the formation mechanism of ground fissures in the active continental rift region in the ground fissure research system.The main research results of this paper are as follows:(1)The development characteristics of ground fissures in the Kenya Rift Valley are identified and the classification system of ground fissures in the Rift Valley is established.The ground fissures in Kenya Rift Valley are characterized by tectonic and hydraulic erosion.Tectonically,the bottom of the ground fissure is connected with the underlying bedrock fracture.The hydraulic erosion is manifested by the fact that the exposed surface of ground fissures is often accompanied by strong rainfall,and the hydraulic erosion of soil in the fissure zone is obvious.Tectonic ground fissures have obvious directional distribution characteristics in plane distribution,that is,the strike is concentrated in the range of about 45 ° from NNW to NNE direction,consistent with the direction of regional extensional tensile stress.Non-structural ground fissures are mainly in north-south and north-east directions,which are consistent with the regional surface runoff direction and road network direction;According to the development characteristics and disaster effects of the Rift Valley in Kenya,it can be divided into four types: bedrock fracture ground fissure,fracture control ground fissure,geological environment damage ground fissure and human engineering induced ground fissure;(2)The manifestation and structural origin of the underlying bedrock fracture connected with the lower part of the bedrock fracture type ground fracture are analyzed and studied.The genetic mechanisms of hidden bedrock fractures can be summarized as: deep thermal activity induced fractures,plate fault activity controlled fractures,early tectonic fracture induced fractures,volcanic cycle induced fractures,and coupled tensile stress induced fractures.The upwelling and spreading of asthenosphere material is the dynamic source of tectonic activity.The fracture structural plane in the early basement rock is the tectonic stress guide zone.The fault extension activity is the main controlling factor for the formation of bedrock fractures.The volcanic periodic activity is the main inducing factor.The coupled tectonic tensile stress field eventually forms bedrock fractures.(3)It reveals the process of fracture propagation and erosion mechanism of underlying bedrock fracture in the overlying Quaternary loose volcanic accumulation layer under the condition of rainfall infiltration.The erosion and failure mechanism of hidden bedrock rupture can be summarized as follows: loose soil is prone to erosion,surface runoff erosion,subsurface erosion of large and small particle sizes,preferential infiltration of hidden cracks,and erosion expansion and collapse.The erodibility of the shallow soil itself is the internal cause of hydraulic erosion and expansion of hidden bedrock fractures;The hydraulic erosion of surface runoff has a dominant development direction,namely the regional dominant runoff direction,along which ground fissures are extremely developed;The large differences in particle size and the alternating thickness of pumice and silt in the geological structure make it easy for the soil to form contact erosion and contact loss infiltration damage inside the soil;Hidden bedrock fractures are the dominant infiltration channels for water;Internal erosion spreads until the surface collapses.According to the law of water infiltration,the process of fracture propagation can be divided into four stages: uniform infiltration stage,dominant infiltration stage,erosion cavity expansion stage,and erosion collapse formation stage.(4)The engineering structural measures for crossing the ground fissure zone and the comprehensive measures for disaster reduction and prevention of ground fissure disasters suitable for the economic development level and geological environment of the rift valley are proposed.Through physical model test,the suitability of the pile-plate method to cross the bedrock fracture ground fissure is verified.The results show that the pile-plate method is applicable to the treatment of the ground fissure disaster in the Kenya Rift Valley,but a series of comprehensive treatment measures to reduce the erosion damage of the ground fissure should be taken at the same time,such as building intercepting ditches,grouting to strengthen the soil in the ground fissure zone,and building silt dams.Engineering management should follow the principles of adapting to deformation and failure laws,improving geological environment conditions,and meeting the level of economic development in the management of ground fissure disasters in rift valley areas.