Study On The Characteristics And Mechanisms Of Ground Fissure

Ground fissure is a kind of superficial rupture caused by internal and external geological dynamic process,which is the result of multi-field and multi-scale coupling.Its forming factors include tectonic and non-tectonic factors,or natural and human factors.Ground fissures are widely distributed in China,mainly in the Fenwei Basin,North China Plain and Suzhou–Wuxi–Changzhou area.The strong tectonic and human activities in these areas have induced the occurrence of large-scale ground fissures,which have brought serious threats to people's lives and property and brought huge economic losses to the country and society.Therefore,it is of great practical significance to systematically summarize and analyze the classification,distribution,characteristics,structure and mechanism of ground fissures.Based on the plan project of CGS "Comprehensive Study on the Causes and Disasters of Ground Fissures in Fenwei Area" and the special scientific research project of MLR "Study on the Causes and Monitoring and Early Warning of Typical Ground Fissures in North China Plain",this paper selected the Fenwei Basin,North China Plain and Suzhou–Wuxi–Changzhou area as the study areas,took the ground fissures in these areas as the study objects,and used surveying,mapping,trenching,drilling,geophysical exploration and related monitoring work to summarize the classification,distribution law,movement characteristics and fracture mode of ground fissures,to study the multi-field coupling mechanism of internal and external dynamics of tectonic ground fissures and the soil-water coupling mechanism of non-tectonic ground fissures.On this basis,the formation and evolution process of ground fissures and the catastrophe model are discussed,which provides theoretical basis and guiding suggestions for disaster prevention and mitigation of ground fissures.The main research results of this paper are as follows:(1)The classification and analysis system of ground fissures is presented and described.The classification of ground fissures is varied and can be divided into different types according to different elements,such as ground fissure properties(tectonic and non-tectonic ground fissures),depth and stress factors(surface external force-controlled ground fissure,shallow human activity-controlled ground fissure and deep internal force-controlled ground fissure),other factors(major and minor relationship,mechanical properties,length and width).The general analysis system of generalized ground fissures and the detailed analysis system of typical ground fissures are established.The causality and factor correlation between the formation essence and the appearance characteristics are analyzed.(2)The distribution of ground fissures and the structure of fracture are summarized.The distribution of ground fissures is related to many factors,such as regional tectonic,fault activity,groundwater exploitation,topography,stratigraphic lithology and strong rainfall.Stereo fracture mode: plane multistage bifurcation,strike difference segmentation,section deformation zoning,vertical structure stratification,combination rupture,non-tectonic single morphology.The motion features include vertical dislocation,horizontal tension,horizontal shear,combined motion.The characteristics of activity include gradualness,intermittency,jumping,difference,gradual extinction,hysteresis and periodicity,among which the difference includes segmental difference,seasonal difference and spatial difference.The gradual extinction and hysteresis are obvious for non-tectonic ground fissures.(3)The disaster pattern and its effect of ground fissures are summarized.The disaster modes of ground fissures for field roads include vertical steep ridges,beaded pits,horizontal grooves and banded fractures;for housing buildings,the disaster modes include vertical tension,oblique depression,horizontal shear,mirror cracking,plane fold,three-dimensional torsion and structural fracture.(4)The mechanism of formation and expansion of creep-type ground fissures is proposed.The internal motive force of fault creep-type ground fissures is derived from a series of deep geological processes.The mechanism of deep crevice includes removing,stretching,thinning,melting and spreading,gravity equilibrium traction and fracture creep cracking.The mechanism promotes the rupture and expansion of the loose layer in the upper part of the sedimentary cover,thus laying a structural prototype and foundation for the formation and exposure of the ground fissures.The multi-directional fracture mechanism of shallow stress field includes the upper disc strain model,the lower disc strain model and the stress-strain transfer model.(5)The mechanism of formation and expansion of pumping-type ground fissures is proposed.Uneven compression and differential deformation of strata are the prerequisite and foundation for the formation of the ground fissures caused by pumping.Quantitative analysis of stress and strain of aquifer from three-dimensional space is the breakthrough to study the fracture mechanism of this type of ground fissure,which includes the deformation analysis of diving layer and confined water layer.Typical ground fissures are often distributed along the edge of the funnel.these ground fissures often have limited depth and show more obvious tensile characteristics,which are caused by the larger tensile stress at the edge of the funnel,such as the edge of the regional settlement funnel,the shoulder of the ancient river channel and the bedrock ridge.(6)The mechanism of formation and expansion of raining-type ground fissures is proposed.The hidden fissures in the superficial rock and soil can be used as the dominant channel for the infiltration of water body.Many surface water seeps down along the fracture fissure of rock and soil,which is a process from unsaturated to saturated.The change of pore pressure has an important effect on crack expansion.The continuous action of the infiltration process leads to the strong water erosion effect in the filling soil,especially at the bottom of the crack.This includes hydraulic fracturing,pore pressure surges and burst expansion,which contribute to the rapid release of ground fissures.(7)The mechanism of formation and expansion of multivariate-coupling-type ground fissures is proposed.Ground fissures formed by the coupling of fault activity and overexploitation of groundwater often occur in large and medium-sized urban areas,causing serious damage.The coupling effects include: increasing shear,reducing resistance,water pressure surge and shear offset.Ground fissures formed by the coupling of regional tectonic activities and loess collapsibility occur in loess distribution areas,which seriously affects agricultural irrigation and economic development.The coupling process includes GreenAmpt horizontal infiltration process,loess wetting deformation process and gravity bending process.