ISRM Suggested Methods for Rock Stress Estimation—Part 5: Establishing a Model for the In Situ Stress at a Given Site

Abstract

1 IntroductionThis contribution relates to the updated suggested methodfor rock stress estimation and concerns the final rock stressmodel (FRSM) of a site or an area. The previous foursuggested methods are (1) Part 1: strategy for rock stressestimation (Hudson et al. 2003), (2) Part 2: overcoringmethods (Sjo¨berg et al. 2003), (3) Part 3: hydraulic frac-turing (HF) and/or hydraulic testing of pre-existing frac-tures (HTPF) (Haimson and Cornet 2003), and (4) Part 4:quality control of rock stress estimation (Christiansson andHudson 2003).The aim of a site or an area characterization forunderground works is to produce a three-dimensionalmodel containing information about topography, soils, rockmass lithology, structural geology, hydrogeology andmechanical data, including rock stress. Such a geologicalmodel is needed in analyzing the cause and effect onstresses from lithology boundaries, geological structures,faults and fracture zones intersecting the site or area.Although it is impossible to know all the details of thegeological evolution of a site or an area, it is worththe effort of trying to ascertain the in situ stress state fromthe bulk knowledge of the site morphology, topographyand geology, and if possible, to verify this informationwith additional data from boreholes and drill cores. Weadvocate that stress measurements to be conducted afterthe best estimate stress model (BESM) has been compiled.Sometimes numerical models can be of assistance inestimating the effect of geological parameter variations inthe establishment of a stress model for a site. In thiscontribution, a strategy and flow chart is presented toestablish the FRSM from a combination of available stressdata from the BESM, new stress data from stress mea-surement methods on site (SMM) and integrated stressdetermination (ISD) using previous data plus numericalmodeling (see Fig. 1). We are aware that sometimes theeconomic constraints prevent application of all the stepsin establishing the FRSM. However, the goal of a stressmeasurement campaign is to collate and harmonize thedata in the best way to describe the in situ stress conditionof a site or area.2 Overview of the Final Rock Stress Model (FRSM)In situ stress exists in a rock mass prior to any manmadedisturbances. Figure 1 presents the way forward forderiving the FRSM for a site or an area (Zang and Ste-phansson 2010). The BESM is established by collectingexisting information from databases and analyzing fielddata on morphology, topography, geology, borehole anddrill core information. Before any in situ stress measure-ments, the development of the BESM of the site or area isrecommended. The established stress model should be usedin selecting the appropriate stress measurement techniqueand assist in planning the measurements. After the BESMis established and stress measurement conducted, an inte-grated stress determination (ISD) should follow. In this last