Research On The Special Geostress Phenomenon Of Duwen Road After Strong Earthquakes And The Prevention Of Rockburst In The Tunnel

In the tunnel and underground engineering, surrounding rocks of the brittle rock massunder high geostress condition, due to the excavation of the free face appears, resulting in radialstress is reduced, stress differentiation as shear stress increased, make elastic which is stored inthe rock mass of elastic strain energy released suddenly, accompanied by burst loose, peelingand catapult even throw in the surrounding rock mass, the phenomenon known as the rockburst.Rockburst is an accompanying energy violent release of nonlinear dynamic phenomena whichis distinguished from the surrounding rock mass deformation and ductile failure. High geostressas a necessary condition of rockburst, is due to the complexity of the rock mass structure,lithological diversity and a variety of geological, environmental conditions and other factors.The field of paleo-tectonic residual stress field and recent tectonic stress field domestic andoverseas rock mechanics fields and engineering of high field stress outstanding comprehensivein-depth. The high geostress after Wenchuan earthquake on12th May,2008should be releasedmostly, but Duwen highway Futang tunnel is located in the hangingwall of the centralcoseismic fault of Longmen Mountain, in the construction of the occurrence of intenserockburst phenomenon, not only to delay the construction schedule, and caused a huge impacton the lives and safety of the workers and equipments and property. Find out the causes ofspecial high geostress phenomenon, and the mechanism of rockburst research, developengineering control measures, to ensure that the smooth construction of Duwen highway Futangtunnel significance play a key role to make teaching practice successful, continuous andeffective the same time, enrich and development of high geostress and rockburst theory willalso have important academic value.This paper described the engineering geological conditions of Duwen highway Futangtunnel in Wenchuan, distribution characteristics and the formation of the reasons for the initialground stress field in the tunnel site. Tracking based on the construction of geological research,indoor and outdoor testing, analyze the causes of high field stress, clarified causes of the Futangtunnel special high geostress phenomenon. On the basis of mounts of site tracing investigations,the analysis in engineering geology, applying the modern support theory and constructionconception of underground engineering, and combining with modern monitor technique andnumerical simulation, discussed the nursing methods rockburst prevention and treatment fromthe excavation and support to improve the mechanical properties of surrounding rock, changes in rock stress condition. Hereby, obtain these chief research results:（1）Through site tracing investigations, the Duwen hiahway Futang tunnel located in thehangingwall of the central coseismic fault of Longmen Mountain still has phenomenons offrequent rockburst and the destroy of discal drill core after Wenchuan earthquake on12thMay,2008. Different from the characteristics of the normal release of the earthquake situ stress,the aforementioned the high geostress phenomenon is tentatively scheduled for after theearthquake special high geostress phenomenon. Adopt stress-relif method on spot, the3Dgeostress was obtained. Using the site geological survey data and the numerical simulation ofthe initial stress field inversion analysis the maximum principal stress value is20~25MPa. Themaximum principal stress direction of the tunnel site in the assessment of earthquake area to theNE small angle of the tunnel site Minjiang River flows to intersect. According to Standard forengineering classification of rock masses (GB5021894), the surrounding rock of tunnel wasplaced in high stress state comparatively.（2）In order to research the initial geostress distribution characteristics, by means ofnumerical simulation, combined with the stress value which were measured before and afterWenchuan earthquake on12th May,2008. By calculating, the stress field of Futang tunnel sitebefore strong earthquakes was in a high geostress condition, the stress of the tunnel A5tendersto reduce by8~14MPa than before the earthquake, the stress of the Futang tunnel A6tendersto reduce by1~2MPa, local higher2~3MPa. After strong earthquakes, the stress of the A5tenders between12~18MPa, it was released of30to70percent, is no longer in the highgeostress condition. The stress of the tunnel A6tenders was between20~25MPa, it wasrelease by4percent to12percent, which was still in the High geostress condition, may be themain reason of the special High geostress phenomenon.（3）Through the method of optimization of model and considering the impact of the faultbelt of the tunnel site, this paper discovered initial geostress distribution characteristics of thetunnel site after strong earthquakes. Initial geostress distribution characteristics of the tunnelaxis can be divided into the stress field on superficial slope impacted by epigenetic andsuperficial reconstruction of rock mass and the relatively stable stress field in deep part. Thetwo stress fields also can be further divided into five zones, i.e., regions of stress lowering, localadjustment affected by faulting, rising, concentration and stable.（4）On basis of the geological survey in Futang Tunnel Site which have specialtopography of the deep V-shaped valley, found out the general laws using of the theory onepigenetic and superficial reconstruction of rock mass by numerical simulation calculation. Theresults show that: tunnel stress field of the principal stress parallet bank slope made of tectonicstress, self-weight stress and residual stress. Hereby, the bank slope stress field of the tunnel sitecould be divided into three-zone which was stress relaxation, stress concentration and stressstable region from the outside to the inside. Most of rockburst occured in stress concentrationand stress stable region. The causes of Futang special tunnel high geostress was made byLongmenshan thrusting functions, topography and geomorphology，epigenetic reformation ofrock mass and materials contrast, causing the special phenomenon of high geostress like the frequently rockburst after strong earthquakes, the destroy of discal drill core.（5）There were frequently rockburst phenomena for the Futang tunnel A6tenders afterthe stronge earthquakes, this paper classified and summarized the rule of rockburst, analysis ofthe secondary stress field characters of surrounding rock which occured after the excavation oftunnel in the separately zoning of original structure of rock mass. This paper classifyed therockburst on principle of energy source which affords energy during the rockburst. The multiplestresses IV-1was the main types according the result of the rockburst energy sourceclassification.（6）By improving physical and mechanical properties, stress conditions, reinforcementmeasures of the surrounding rock, this paper discussed the prevention measures of rockburst inthe special high geostress conditions, for the purpose of providing suggestions of constructionand design.