Major Fracture Zones inFiskarfjärden, Stockholm

Abstract:

This Master thesis is written in connection with the Stockholm Bypass project which is Trafikverkets and Sweden’s largest infrastructure project to date. It will consist of 21 km of motorway running west of Stockholm city linking the southern and the northern parts. Out of the total 21 km of motorway, 17 km will run through rock tunnels and cross beneath the Lake Mälaren at 3 places. One of these places is the water passage Fiskarfjärden, situated between Sätra and the island of Kunghatt which has been carefully studied with geological (drill core mapping, BIPS, Water-loss measurements and outcrop mapping) and geophysical (reflection and refraction seismic) methods. This has been used to produce an engineering geological prognosis over the area which indicates that the rock mass in the water passage is heavily fractured and of poor quality. Due to this, additional investigations of the structural framework and the large-scaled structures in the area have been undertaken. The objectives of this thesis are therefore to analyze and classify available geological data, identify and model the major fracture zones situated in the water passage, investigate the kinematics of the fractures focusing on the relative movement between the blocks, if possible, determine the stress orientation and, predict how these major fracture zones will affect the Stockholm Bypass rock tunnel. The available data provided a good opportunity to examine the area. The drill core mappings showed that the dominating fracture orientation in the water passage is WSW-ENE with deformation structures supporting faulting along these fractures. The additional outcrop mapping and drill core mapping which focused on finding kinematic indicators revealed that fractures in the area have been reactivated both as strike-slip and dip-slip faulting and that dextral strike-slip faulting is dominating in a WSW-ENE orientation and sinistral strike-slip faulting in a NW-SE orientation.The orientation of these two fault surfaces matches the main faults on the geological map and indicates that the area is composed of a conjugate fracture set with conjugate shearing whereas the combination of the dominating fracture orientations in the drill cores with the kinematic observations and fractures found in the field correlates with Riedel shear fractures in a dextral shearing configuration. The majority of the zones of crushed rock that cut the tunnel show a dominating WSW-ENE orientation. At this angle to the NW-SE oriented horizontal stress field, these fractures are more likely to experience closure since they are oriented approximately normal to the stress field. The zones of crushed rock that are either oriented E-W or NNE-SSW are more likely to reactivate as strike-slip faults. The very few fracture zones that are oriented NW-SE and correlate to fractures with sinistral shearing are likely to reactivate as dip-slip faults as they are oriented parallel to the present day extensional forces or as strike-slip faults if they are located at depth.