New developments of the Integrated Stress Determination Method and application to the Äspö Hrad Rock Laboratory, Sweden

University dissertation from Stockholm : Mark och vatten

Author: Daniel Ask; Kth.; [2004]

Keywords: Integration; Rock Stress; ISDM; Äspö HRL;

Abstract: This thesis presents new developments of the IntegratedStress Determination Method (ISDM) with application to the AspoHard Rock Laboratory (HRL), Oskarshamn, Sweden. The newdevelopments involve a 12-parameter representation of theregional stress field in the rock mass. The method isapplicable to data from hydraulic fracturing, hydraulic testson pre-existing fractures (HTPF), and overcoring data fromCSIR- and CSIRO-type of devices. When hydraulic fracturing/HTPFdata are combined with overcoring data, the former may be usedto constrain the elastic parameters, i.e. the problem involves14 model parameters.The Swedish Nuclear Fuel and Waste Management Co. (SKB),have conducted a vast amount of rock stress measurements at theAspo Hard Rock Laboratory (HRL) since the late 1980s. However,despite the large number of stress measurement data collectedin this limited rock volume, variability in the stress fieldexists. Not only does the result vary depending on measuringtechnique, e.g. overcoring data indicated larger stressmagnitudes compared to hydraulic fracturing data; the resultsare also affected by existing discontinuities, indicated bynon-linear stress magnitudes and orientations versus depth.The objectives for this study are therefore threefold: (1)find explanations to the observed differences between existinghydraulic and overcoring stress data at the Aspo HRL; (2)explain the non-linear stress distribution indicated byexisting stress data; and (3) apply the ISDM, including the newdevelopments, based on the results obtained in step 1 and2.To evaluate the observed differences between existinghydraulic and overcoring stress data, a detailedre-interpretation was conducted. Several measurement-relateduncertainties were identified and corrected for when possible,which effectively reduced the discrepancies between thehydraulic and overcoring measuring results.Modeling studies managed by SKB have shown that theredistribution of the stresses at Aspo HRL to a large extentcan be correlated to the NE-2 Fracture Zone, which divides therock stress data into two stress domains. The effect of thiszone was confirmed in this study, and the re-analyzed datasuggest an orientation of ?1equal to 124oN±13ofor the NW domain and 139oN±18ofor the SE domain. Later, the application of theISDM further verified the influence of the NE-2 Fracture Zoneon the regional stress field. The results in the vicinity ofthe NE-2 Fracture Zone indicate that ?1is perpendicular to the zone, whereas the ?2and ?3coincide with the plane defined by the zone(strike 21oN, dip 77otowards SE). However, the principal stressmagnitudes seem less influenced by the zone.The amount of data located outside the zone of influencefrom the NE-2 Fracture Zone is limited, and the regional stresstensor is hence difficult to define. Most likely, theorientation of the regional ?1is trending about 140oN.