Twoextra-longtunnels,withtwotubeseach,wereexcavatedinacorridorofabout200 min Qinling Mountain in the central part of China, for railway and road lines from Xi’an to Ankang, respectively.Duringtunnelexcavations,severerockburstshaveoccurredinseveralgneiss sections.Therockburstfeaturesindicatethepronouncedeffectofthefabricoftheschistous structure of gneiss on the strength and deformation characteristics of intact rocks. The analysis on the geological conditionsinterms ofinitial geostatic stress, rock strength andthe occurrence of the gneiss foliation planes indicates that the schistous structure of gneiss is in favor to the severe rockburst inthe section of magmatic gneiss withlarge overburden. The influence of anisotropic featureofthegneissontherockburstbecomessignificantontheconditionthatalargeinitial principalstressisnearlyparalleltothefoliationplanesandthefoliationstructurecomposed mainly of biotite and quartz provides a weak plane to brittle failure.
Theoriginalpropertiesofarockorrockmassneara tunnel arechangedduetoanexcavationandstress redistribution occursaroundtheexcavatedspace.The characteristics ofanexcavationdamagedordisturbed zone vary withrelated geological conditions, excavation method,andopeninggeometry.It is evident that the overburden of tunnels and underground caverns has been increasing in recent years. As a result, rockburst becomes probably one of the major concerns for the stability deep underground structures in hard rocks.
Two extra-long tunnels, with two tubes each, were excavatedinacorridorofabout200minQinling Mountain in the central part of China, for railway and road linesfromXi’antoAnkang,respectively.During tunnel excavations,severerockburstshaveoccurredin several gneisssections.The features of the rockbursts indicate thatthe occurrenceoffoliationinthe gneiss is beneficialtotherockbursts, especiallythefailure of spalling. The influence of the shistous structure of the gneissontherockburstwillbediscussedin terms of the brittle damage mechanism of the gneiss.
2 FEATURES OF ROCKBURST
2.1 Geological conditions of wall rocks with rockburst
At both railway and road tunnels, moderate rockburst and severe rockburstoccurinthesectionsofmigmatitic gneiss withoverburdenmorethan600m.The rock masses inthesesectionsaregenerallyofhighstrength and integrality.The rock mass has been subjected to relatively lightalterationbyfaultsandjoints.The strength of the gneiss varies from 75 to 250 MPa, with an average value around 145 MPa.
2.2 Modes of rockbursts
Following Kaiser and Maloney (1997), the rockbursts that occurredintheQinlingtunnelsbelongt