Study On The Mutation Of Geostress Environment In Wenchuan Earthquake Area And Its Engineering Effects

Ms 8.0 earthquake occurred in Wenchuan where is near the Longmenshan faults zone on May 12th,2008.During Wenchuan earthquake,with strong vibration and faulting,the high geostress which were bearing inside the rocks have been released.By organizing the results of the investigations on a dozen of tunnels,it was found that most of the fractures were related to geostress releasing process.Specifically,according to the re-gained data,the ground stress was 26 MPa before Wenchuan earthquake in Longxi tunnel,and it became 14.2 MPa after the earthquake,which showed the geostress has been released about 46%.This case has proved that the geostress was releasing during the earthquake,and the whole geostress in earthquake influence areas has experienced a sudden change.However,during post-quake reconstructions,rock burst phenomena and high geostress concentrations were still be found near the epicenter areas where Futang tunnel and Zijin tunnel were located.These special findings gave rise to several questions:What are the distribution characteristics of the ground stress in different areas inside the earthquake influenced areas after the quake?What is the main regularity of the strong earthquake induced sudden changes in geostress?What are the geostress-changing induced engineering effects?These questions have been less studied systematically in Wenchuan earthquake influenced areas,either been studied worldwide.Based on this fact,this dissertation collected the in-situ measured geostress data before the Wenchuan earthquake and after the earthquake in Longmenshan areas,and made a brief analysis on the quantitative change characteristics of ground stress;By3D numerical simulation on the distribution of geostress inside the Wenchuan earthquake influenced areas(Longmenshan areas),the distribution regularity of high geostress concentration was obtained.Through comparing on the collected data,the distribution regularity of geostress as well as the reason for this sudden change was found.In the end,the engineering effect of abrupt changes in crustal stress environment.Through the studies,the following results and ideas can be obtained:(1)The measured data and materials about the crustal stress in the Longmen Mountain region before and after the Wenchuan earthquake were systematically collected to carry out complementary measurement to the crustal stress in the typical parts(e.g.the fault hanging and heading walls,hard and soft rock areas,the points and valley slope zones subjected to crustal stress measurement before the earthquake)in this region after the earthquake.As a result,a total of 79 measuring points and nearly 300 groups of geostress data were obtained;a database of geostress in the Longmen Mountain region was established to manage and use these data;and the quantitative(magnitude and direction)change characteristics and initial law of crustal stress before and after the earthquake were gained.(2)A large true 3D numerical model was built for the crustal stress field in the Longmen Mountain region.Through the geographic information platform,the DEM satellite pictures required for the area modeling were obtained;the Global Mapper software was employed for digital processing of these pictures to produce digital hypsographic map;the region 500 km long,100km wide in the southwest direction and 5.0×10~4km~2 in area along the Longmenshan Fault Zone was selected for modeling,the five major fault zones and 8 groups of stratigraphic units were taken into consideration to build the large true 3D numerical analysis model by comprehensively using the software such as CAD,Sufer,Ansys and Flac3D5.0.(3)According to the inverse analysis and numerical calculation of the crustal stress field,the crustal stress characteristics,change law and high crustal stress distribution in the Longmen Mountain region before and after the earthquake were compared and studied.Specifically,before the earthquake the entire Longmen Mountain region was under high geostress condition,especially in the middle and southwest end of the central fault zone(Z=1000m within the elevation plane the maximum principal stress is 50?70MPa);the geostress was released in general after the earthquake,but the magnitude of release was different in each area,in particular,the crustal stress released on the hanging wall of seismogenic fault decreased significantly;after the earthquake,the geostress on the heading wall of central fault was still high locally,in particular the river valley zone had high crustal stress and the Y-shaped tectonic region on NW side in the southwest end of the Longmenshan Fault Zone also had high crustal stress.(4)After the Wenchuan earthquake,the orientation of principal stress in the Longmen Mountain region remained unchanged basically,mainly NW and NE directions,but the azimuth angle of principal stress deflected to different degrees.The NW of the southwestern tip of the footwall of the seismogenic fault is deflected 5?10°to the north,azimuth of the NE is deflected 0?5°to the east,and the azimuth of the central NW is deflected 5?15°to the north.NE orientation The eastward deflection is 0?20°;the northeastern NW is azimuthally deflected 10?25°westward,and the NE azimuth is deflected 10?15°eastward.(5)According to the characteristics and mutation law of geostress in the Longmen Mountain region after Wenchuan earthquake,were proposed the five effects from the abrupt changes in the regional crustal stress caused by strong earthquake,namely the heading and hanging wall effect,distance effect,lithology effect,elevation effect and valley slope effect arising from abrupt changes in geostress.(6)Based on the law and effects from abrupt changes in geostress after the earthquake,were summarized the four controlling factors having influence on the abrupt changes in geostress,namely regional structure,seismogenic fault,topography and lithology.Among them,regional structure and seismogenic fault are internal causes,also the major controlling factors for geostress accumulation before the earthquake and crustal stress release after it;and topography(different elevations and valley slopes)and lithology are external causes,also the key factors contributing to difference and non-uniformity of abrupt changes in crustal stress after the disaster.(7)In regard to the law of mutation in geostress caused by strong shock and the crustal stress characteristics in the Longmen Mountain region after the earthquake,were elaborated the three effects of mutation in geostress on the construction of major projects in the Wenchuan earthquake disaster area,namely its effect on route selection of major project,its large deformation effect on tunnel surrounding rock and its rockburst effect on tunnel surrounding rock.