Technical Note Indirect determination of the modulus of deformation of rock masses based on the GSI system

1. IntroductionToday, as a result of the increase in the number oflargetunnelconstructionsandundergroundexcavationsrequired for civil engineering and waste disposal, andtheoperationofdeepopenpitmines,determinationofrock mass properties has become one of the mostimportant stages in design and construction for rockengineering.Whenrocksandrockmassesareclassifiedforgeotechnicalpurposes,thisneedstobeonthebasisofstrengthorstaticmodulusofdeformationtogiveanindication of their stability or deformational response.In addition, the strength and modulus of deformationarethenecessaryinputparametersoftherockmassestobeutilizedbynumericalmethods.The modulus of elasticity of the rock material isundoubtly the geomechanical parameter that bestrepresents the mechanical behaviour of rock material.Asimilarconclusioncanalsobemadeforrockmasses.However, the modulus of deformation of rock massesshould be determined in situ by carrying out specialtests, which normally implies considerable costs andoperationaldifficulties.Becauseofsuchdifficulties,thedetermination of the modulus of deformation of therock masses continues to attract the attention of rockengineersandengineeringgeologists.Given the importance of having the value of themodulusofdeformationavailableand,atthesametime,the operational and economical difficulties mentionedabove,manyresearchers,mentionedbelow,wereledtoconsider several cost-effective and not too time-con-suming indirect procedures that can provide informa-tion on the modulus of deformation. These attemptswerebasedonempiricalmodels,mainlyinconjunctionwith the rock mass classification systems. The firstempirical equation for prediction of the deformationmodulusofrockmasseswasproposedbyBieniawski[1]and was followed by other empirical models, such asthosesuggestedbyBartonetal.[2],SerafimandPereira[3],NicholsonandBieniawski[4],Mitrietal.[5],HoekandBrown[6],PalmstromandSingh[7],Barton[8]andKayabasietal.[9].The predictive models proposed by Bieniawski [1],Serafim and Pereira [3] and Mitri et al. [5] utilize theRMR of Bieniawski [10], while Barton’sestimate [8] isbased on Q-values. Although the prediction model byNicholson and Bieniawski [4] employs RMR values, italsoconsidersareductionfactor.TheequationofHoekand Brown [6] is a modified form of Serafim andPereira’sequation[3]andbasedontheGSISystem.TheequationbyPalmstromandSingh[7]employsRMi[11]valuesforpredictionofdeformationmodulus.Kayabasiet al.’s [9] equation considers the elasticity modulus ofintact rock, RQD and degree of weathering andcomparesthepredictedvalueswithinsitudeformationtestresults.Recently,twostudiesforthesamepurposewereconductedbyGokceogluetal.[12]andCaietal.[13]. The study by Gokceoglu et al. [12] aimed atchecking the prediction performance of the existingempirical equations and providing some contributionsthe work of Kayabasi et al. [9] for the estimation ofdeformation modulus of weak rock masses. Theseinvestigators suggested a prediction equation consider-ingthemodulusratioofrockmaterialðE