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Detailed Description of TRUE Block Scale Structure #20 Based on Intercepts in Boreholes and Intercepts in the TASS Tunnel

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Detailed Description of TRUE Block Scale Structure #20 Based on Intercepts in Boreholes and Intercepts in the TASS Tunnel R-12-09 Detailed description of TRUE Block Scale Structure #20 based on intercepts in boreholes and intercepts in the TASS tunnel Anders Winberg, Conterra Aaron Fox, Anders Pettersson Golder Associates Henrik Drake, Isochron Geoconsulting September 2013 Svensk Kärnbränslehantering AB Swedish Nuclear Fuel and Waste Management Co Box 250, SE-101 24 Stockholm Phone +46 8 459 84 00 ISSN 1402-3091 Tänd ett lager: SKB R-12-09 P, R eller TR. ID 1389690 Detailed description of TRUE Block Scale Structure #20 based on intercepts in boreholes and intercepts in the TASS tunnel Anders Winberg, Conterra Aaron Fox, Anders Pettersson Golder Associates Henrik Drake, Isochron Geoconsulting September 2013 This report concerns a study which was conducted for SKB. The conclusions and viewpoints presented in the report are those of the authors. SKB may draw modified conclusions, based on additional literature sources and/or expert opinions. A pdf version of this document can be downloaded from www.skb.se. Summary The primary goal of this study, targeted on the TRUE Block Scale Structure #20, was to produce an integrated description of a deformation zone at the centimetre to decimetre length scale, by com- bining observations from cored boreholes, image logging, tunnel mapping, fracture mineralogical sampling, hydraulic testing and hydraulic pressure responses. The resulting composite description includes comments on the lithologies associated with the deformation zone, a description of the visible fracturing, descriptions of relationships in the ductile fabric at macro and micro scales, the hydraulic connectivity and hydraulic properties of the deformation zone, and detailed mineralogical descriptions of brittle and ductile fabrics. The lithology of Structure #20 is dominated by a mix of Äspö diorite and fine-grained granite. The structure consists of a mix of fault rock; mylonite, cataclasite, fault gouge, occasional breccia and of various open, partly open, and sealed fractures. The structure is also found to show at least weak association with fine-grained granite intrusions which suggests attention needs to be paid to igneous intrusions, as they may follow pre-existing ductile shear zones or may be coincident with ongoing ductile shearing. Three categories of fracturing associated with Structure #20 are assumed to be applicable to defor- mation zones in general within the Äspö/Laxemar/Simpevarp region; brittle deformation in the core of the deformation zone; increased fracture intensity in the damage zones surrounding the deformation zone core; and epidote-filled fractures that form so-called “marker beds” for the structure. Structure #20 constitutes an oblique-slip fault with predominantly sinistral displacements developed from a ductile shear zone pre-cursor. Mineralogical kinematic indicators suggest both dextral and pure dip- slip. These kinematic indicators are mostly of ductile character so this statement is incomprehensive. However, brittle indicators exist as well and numerous generations of fracture fillings indicate multi- ple brittle reactivation. Structure #20 exhibits a fairly typical deformation zone configuration includ- ing a relatively narrow core composed of both ductile and brittle structures, surrounded by a damage zone largely characterized by increased fracture intensity. Results indicate that, at least over the scale of the investigation area, Structure #20 is largely planar, with only local-scale undulation. Mineralogical analysis of intercepts of Structure #20 in boreholes and the TASS tunnel show simi- lar general features regarding the fracture re-activation history and deformation events. This is evi- denced by small deviations in fracture mineral assemblages and whole-rock and mineral chemistry. The overall characteristics were similar between borehole intercepts but showed some heterogeneity. Wall rock alteration showed very similar features (alteration of Fe-Mg-minerals, fragmentation of feldspars, re-crystallisation of quartz). The extent of the alteration varied from decimetre to metre scale, depending on the intercepts.The thickness and amount of mylonite and cataclasite varied between different intercepts. The general mineral assemblages were; mylonite (ductile deformation) and cataclasite and extensional fractures (brittle deformation). Apart from geometrical extrapolation the intercept of Structure #20 with the TASS tunnel was also interpreted from pressure responses observed in the TRUE Block Scale borehole array during drill- ing of the TASS tunnel pilot boreholes. The resulting intercept corresponds well with the geometri- cally extrapolated intercept, suggesting that Structure #20 can be regarded as an essentially planar structure across its known extent. Transmissivity data evaluated from hydraulic tests performed in the TRUE Block Scale borehole array, the TASS tunnel pilot boreholes and short TASS grouting holes transecting Feature A were analysed. Analysis of statistics based on various means of pooling the data showed that the geometric mean and variance of the transmissivity of Structure #20 based on the TASS pilot and probe boreholes were close to those established based on on all available data on the structure, suggesting that the statistical sample for Structure #20 from the TASS pilot bore- holes (representing a length scale of metres) may be regarded as representative of the “global statisti- cal properties” of the structure (over length scale of several decametres). It was however noted that at best, this relationship is indicative for this type of NW conductive structures at Äspö (and possibly in the neighbouring Laxemar-Simpevarp area). SKB R-12-09 3 A simple flowchart was deviseded to distinguish and describe similar structures as a preliminary method of screening borehole data to determine if a particular fracture intercept is worthy of addi- tional investigation as a potential “large fracture”, the latter affecting the positioning of deposition holes/waste canisters through the (Full tunnel Perimeter Intersection) FPI-related design criteria. The results of the detailed geological and structural mapping were presented in conceptual synthesis images that add additional detail and structural-geological context. The synthesis is compatible with, and lend additional support to, the original micro-structural conceptualization reported by the TRUE Block Scale and TRUE Block Scale Continuation projects. Characterization efforts on individual major structures (MDZs/fractures) of the kind and type described in this work may turn out to be prohibitive in terms of time and costs. However, repeated targeted campaigns on representative type structures (and fractures) (e.g. representing typical orien- tations) may provide an important basis for constructing generalised conceptual models and may also help define a data base based on which “large fractures”, related to the FIP-related design criteria, may be identified. It was also noted that techniques employed in this work, HDR photography, UV photography and mineralogical kinematic indicators, may prove as valuable contribution to planned detailed site investigations. 4 SKB R-12-09 Contents 1 Introduction 7 1.1 Background 7 1.2 Objectives and scope 7 1.3 TRUE Block Scale hydrostructural model 8 1.4 Extrapolation of TRUE Block scale structures 9 1.5 Characterisation of TRUE Block Scale structures in conjunction TASS tunnel development 10 1.6 Selection of structures for further analysis 10 2 Methods 11 2.1 Analysis of hydrogeological information 11 2.2 Structural-geological mapping of drill cores 11 2.3 Photographical documentation in the TASS tunnel and associated analysis 13 2.3.1 Camera equipment and setup 14 2.4 Mineralogical sampling an analyses 14 2.4.1 Samples for fracture mineralogical investigations 15 2.5 Structural-geological tunnel mapping 16 3 Identification and characterisation of structures from hydrogeology 19 3.1 Objectives and scope 19 3.2 Overview of existing work 19 3.2.1 Tunnel sealing project 19 3.2.2 Planning for SWIW tests 20 3.3 Drilling progress versus time 20 3.4 Pressure responses observed during drilling 21 3.5 Hydraulic properties of selected structures 23 4 Geological information from cored boreholes 27 4.1 Review of mapping of Structure #20 27 4.2 Rock types 28 4.3 Fracturing 29 4.3.1 Fracture orientations 29 4.3.2 Fracture intensity 34 4.3.3 Fracture morphology 41 4.4 Structural geology 44 4.4.1 Drill core analysis 44 4.4.2 BIPS image interpretation of Structure #20 intercepts 48 4.4.3 Summary of Structure #20 characteristics in borehole data 67 4.5 Mineralogy/fracture mineralogy in cored boreholes 70 4.5.1 Description of fracture mineralogy in samples from the TASS pilot boreholes 70 4.5.2 Description of fracture mineralogy in samples from the TRUE Block Scale boreholes 83 4.5.3 Chemical analysis 88 4.5.4 Stable isotopes 92 4.6 Integrated description of Structure #20 from borehole information 93 5 Geological information from tunnel mapping 97 5.1 Objectives and scope 97 5.2 Tunnel image acquisition and initial processing 97 5.2.1 Photography at ordinary light (including HDR) 99 5.2.2 Photography in UV light 101 5.2.3 Lessons learned from tunnel imagery 104 5.3 Geological mapping of Structure #20 in the TASS tunnel 104 5.3.1 Analysis of fracture data from TMS 104 5.3.2 Mapping of Structure #20 in the TASS tunnel 105 SKB R-12-09 5 5.4 Tunnel maps from high-resolution photography 115 5.4.1 Methodology for using high-resolution images to map fracture traces 115 5.4.2 2D fracture trace maps 118 5.4.3 Limitations
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