Study On The Formation Mechanism Of Ground Fissures In Weibei Terrace Based On Earth's Critical Zone Theory

Ground fissure,as a complex geological process in Earth Critical Zone,is a surface process produced by the synergies of various catastrophic forces,and it is a common but inevitable rupture phenomenon of the surface rock mass in the long-term evolution of the Earth.It extensive development in North China area,especially Fenwei Basin is well known for the large-scale development of ground fissures and serious disasters.Based on the China Geological Survey research project "research on the causes of ground fissures and land subsidence in Fenwei Basin" and the "investigation of ground fissures and geo-hazard in the Weinan area,Guanzhong Basin",this article taken the Weibei Terrace in the southern Fenwei Basin as the research object.Firstly,through detailed field investigations,geological exploration,in-situ tests,and indoor experiments,this paper summarized the basic characteristics of the ground fissures in the Weibei Terrace and proposed the catastrophic areas and the catastrophic forces of ground fissures.Then,different levels of geological model were established based on the theory of the earth's critical zone,the ground fissure mechanism of earthquake ground motion and groundwater fluctuation was analyzed based on typical cases.Finally,the origin model of ground fissures in Weibei Terrace formation was obtained,and the theory of ground fissures formation was proposed.The main conclusions of this article are:1.Through field surveys and data collection,it was found that there were accumulated 252 ground fissures in the northern part of Weihe basin,mainly in the Jingyang County on the northern Weihe basin and Sanyuan County on the northeastern Weihe basin,accounting for 79.4% of the total.The spatial distribution pattern of ground fissures is “three bands and one zone”.They are Weihe fault band,Kouzhen-Guanshan fault band,Liquan-Pucheng-Heiyang fault band and Dali zone.The distribution of ground fissures is characterized by the concentrated development along the fault zone,along with the extension of the geomorphological variation zone,associated with land subsidence,and scattered distribution in the loess collapse area.The overall shape of the ground fissures is linear,polygonal,arc-shaped,and en echelon type;the local forms are jagged,stepped,and collapsed pits.The main structural sections of ground fissures are hopper-type,dumbbell-type,side-feather-type,step-type,"Y"-type,and "X-type".The movement pattern of ground fissures has obvious "three-way" movement characteristics.The activity characteristics have obvious seasonal differences,spatial differences,periodicity,and diversity of activity patterns.The hazard characteristics of ground fissures are banded,heterogeneous,diverse,progressive,and irresistible.2.The catastrophic areas in the Weibei Terrace are mainly along the structural fault zone,both sides of the geomorphic boundary line and the loess depression zone.The catastrophic forces of the ground fissures mainly include deep forces,middle forces and shallow forces.Among them,the deep force refers to the force of tectonic action,which mainly includes the fracture creep and seismic motion,and belongs to the external force of the earth's critical zone.The middle force refers to the collapsible deformation force of the loess caused by the change of the groundwater level and the settlement force of the soil.The shallow force refers to the percolation potential generated during surface water infiltration.Both middle force and shallow force belong to the internal force of the earth's critical zone.3.Tectonic action is a key factor in the birth and formation of ground fissures.The multi-level fracture system of earth's critical zone formed by tectonic activities which give birth to ground fissures.Fracture creep and seismic activity led to the rupture system extending to earth surface and forms ground fissures.Fracture creeping mainly caused the existing sub-hidden faults on the upper plate to extend to earth surface and forms ground fissures.Seismic activity changes the stress state on both sides of the hidden fault in the basin and the then formation shear cracks in the hanging wall and vertical tensile cracks in the footwall.4.Pumping,surface water infiltration,and loess collapse are important factors for the fromation and expansion of ground fissures in the Earth's critical zone.The pumping action will lead to the change of stress state on both sides of the earth in the critical zone and the formation of tensile cracks,shear cracks and tensile-shear cracks on both sides of the hidden fault.The angle between the crack and the horizontal plane is(?).The existing structural joints of underground water runoff lapping form underground tunnels,and then the tensional tension zones are formed at the top of the tunnel which is help to form tensile cracks.After ground fissures formation,surface water infiltration further aggravates the activities of ground fissures.The maximum horizontal influencing distance of the canal in the loess area is about 30 m.The inclined infiltration line resulted in the inhomogeneous collapsibility of the loess and the tensile stress formed on the surface is the catastrophic action force of the ground fissure,and the tensile cracks produced in the loess depression.The maximum development depth of ground fissures is generally 10 m.5.The model of the typical ground fissures in Weibei Terrace can be divided into four types,namely the structural fault control,the fault activity open,and the penetrating subsidence expand;the structural fault control,the earthquake open,and the groundwater exploitation recurrence;the structural joint control,seepage erosion open and expand;loess collapsible deformation model.Nowadays,the process of ground fissures can be divided into three stages,they are formed by the synergistic coupling of deep structures,tectonic activities and human activities.