Rock Mechanics for Resources, Energy and Environment – Kwa´ sniewski & Lyd˙ zba (eds) © 2013Taylor & Francis Group, London, ISBN 978-1-138-00080-3 Influence of in-situ stress variation on results of back analyses – an open pit case study M. Groši´ c & D. Vidovi´ c Geotech Ltd., Rijeka, Croatia Ž. Arbanas University of Rijeka, Faculty of Civil Engineering, Rijeka, Croatia ABSTRACT: One of the crucial points in performing stress-strain back analyses is knowledge of the magnitudes and directions of insitu stresses. Although vertical insitu stress mostly depends on weights of overlaying strata, horizontal stress is more complex to determine and depends on vertical stresses, locked stresses of tectonic origin, topography and other complex geological conditions. To obtain the deformation modulus of rock mass, except from insitu measurements, back analyses based on monitoring data are often carried out. Direct approach from the back analysis is based on iterative procedure, correcting the trial values of unknown parameters by minimizing the error function. This paper presents a case study of an open pit excavation reinforced by rockbolts and shotcrete that was monitored during the construction phase. Back analyses were performed with variation of in situ horizontal stresses as an unknown in thenumerical model. Analyses have shown significant scattering of obtained deformation moduliof rock mass, which vary a few dozen times. Results of the back analyses and comparison with measured data are presented. 1 INTRODUCTION Knowledge of the virgin stress field is very impor- tant in many problems dealing with rock mechanics and related projects. The need to understand the insitu stresses in rock mass has been recognized by engineers for a long time and many methods to measure these stresses have been proposed since the early 1930s. These in situ stress measuring tests are relatively expensive and are used only at significant projects in civil and mining engineering. For smaller projects tests are not performed and certain indirect estimations of rock mass deformation properties are used. Horizontal stress direction and magnitudes vary sig- nificantly on local and global scales and incorrect predictions can lead to incomprehension of geotechni- cal construction behavior and to possible collapse and failure. 2 PREVIOUS RESEARCH 2.1 The deformation modulus of rock mass Deformability is characterized by a modulus describ- ing the relationship between the applied loadsand the resulting strains. The fact that jointed rock masses do not behave elastically has prompted the usage of the term modulus of deformation rather than modulus of elasticity orYoung’s modulus (Palmström & Singh 2001). According to the Commission onTerminology, Symbols and Graphic Representation of the Interna- tional Society for Rock Mechanics (ISRM),there are several important definitions of deformation proper- ties of rock and rock mass (ISRM 1975). Modulus of elasticity orYoung’s modulus (E) is defined as the ratio of stress to corresponding strain below the limit of pro- portionality of a material. Modulus of deformation of a rock mass (E m ) is the ratio of stress to corresponding strain during loading of a rock mass, including elas- tic and inelastic behavior. Modulus of elasticity of a rock mass (E em ) is the ratio of stress to corresponding strain during loading of a rock mass, including only the elastic behavior. 2.2 Indirect estimation of the deformation modulus The deformation modulus can be measured in situ using one of the following methods: flat jack tests, cable jacking tests, radial jack tests, dilatometer tests and pressure chamber. Because of the high costs of the tests, the deformation modulus of rock mass is rarely measured in situ and is often obtained by indirect estimations. Deformation parameters of rock mass are often esti- mated through approximate equations that are based on one of the rock mass classifications (RMR, GSI, Q classification, RMi). Several authors have proposed empirical relationships for estimating the value of rock 795