Study On The Propagation Mechanism Of Ground Fissures

Ground fissures have been the main environment geological disaster in Xi'an city. Since 1950s, 14 ground fissures have been found in Xi'an area, which has caused 4 billion Yuan's direct losses. The worse is that new fissures are coming forth and developed. As a result, the utilization of building land and urban planning and development of Xi'an city are badly restricted, and great hidden trouble is buried for city's engineering construction. So the great geo-hazard difficult problem to be solved urgently is to perform the research on genesis and disaster-causing mechanism and work out economical and effective countermeasures.Presently, the distribution and activity characteristic of Xi'an ground fissures have been found out basically, but its genetic mechanism is immature being many different points of view and disputes on it for that there are some key basic problems being unresolved. Furthermore, the research on the countermeasures of minimizing and defending disaster is far from sufficiency because of the restriction of genesis and disaster-causing mechanism researching degree. The problems and deficiencies in the research of Xi'an ground fissures were analyzed. The key problems were explored and studied by the author through large-scale and small physical experiment and numerical simulation analysis, new viewpoint on the genetic mechamsm of Xi'an ground fissures were put forward. At the same time, beneficial opinion were present on the guarding against disaster of underground structure across ground fissures and the methods to evaluate the influence bandwidth of ground fissures. Finally, new appraisement on seismic effect of ground fissure site subjected to strong earthquake in Xi'an area was performed basing on further studying and re-recognizing the ground fissures.Some main innovative ideas are got as follows in this paper:1. Xi'an ground fissures are actually structure planes approaching to ground surface which have been exist in the past, and they emerged on the surface caused by differential land settlement induced by pump water and great rainfall infiltration. NNW regional tensile stress field, continuous tilt motion of Xi'an normal faults group and uninterrupted deposit of covered unconsolidated formation are the three simultaneous and concurrent factors in the essential formation procedure of Xi'an ground fissures. 2. Xi'an ground fissures are connected with underlying faults at end. They are synsedimentary structural fracture plane that underlying faults continue to the ground surface. There are two main causes for the dip angle becoming lower in deeper stratum, one is book-inclined motion of normal faults group, and the other is stratum compression. The main cause of supernormal activity of Xi'an ground fissures is overpumping.3. The cracking forms of walls over ground fissures are analyzed with non-linear finite element method. The originated position, dip direction, dip angle and later mode of the cracks on walls are accomplished when the walls crossing over ground fissures with three-dimensional activity, and the changing law varying with different ratio of sedimentation and tensor are got.4. The analysis result of ground fissure site seismic effect shows that the scale effect is the most obvious for far-field earthquake with higher low frequency, and the second obvious for middle-field earthquake, the least for near field earthquake. In Xi'an area, the ordinary intensity earthquake can cause the buried ground fissure rupture propagation, but the modes of them are diverse caused by different seismic waves.