Landslides in the Sydney Basin - Geoscience Australia

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Seismic Hazard in SydneyProceedings of the one day workshopIn the late 1990’s, Tower Colliery conducted longwall mining beneath sections of the deep (+50 m)gorges associated with the Cataract and Nepean Rivers, see Figures 1 and 3. As part of the miningof longwall panels 16 and 17 (see Figures 1 and 3), an extensive surface monitoring campaign wasimplemented in conjunction with the NSW Roads and Traffic Authority (RTA) and a BridgeManagement Committee set up to assess the impact of this mining on the gorges and the DouglasPark bridges carrying the Hume Highway over the Nepean River. These bridges are over 600 mfrom the closest caving in longwalls 16 and 17 which were mined at a depth of about 450 m. Resultsof the monitoring are given in Hebblewhite et al. (2000).Quite separately to the monitoring of ground surface movements, interest in the possibility thatseismicity might also be occurring at this time was aroused following the chance recording of aseismic event during a conventional seismic reflection survey by BHP Billiton in July1998. This wasfollowed by a magnitude 4.4 event on 17 March 1999 located just to the south of BHP Billiton’smines. Local residents were also reporting noticeable ground vibrations. BHP Billiton thereforeundertook a preliminary program of seismic monitoring. Results are reported in Hatherly et al.(2000) and Vasundhara et al. (2000).DouglasParkHarris CkLW 17LW 16Figure 3. Detail of the Nepean and Cataract Rivers, longwalls 16 and 17 of Tower Colliery,the freeway, bridges and other infrastructure. Harris Creek flows south through a narrowravine to the Nepean River. It is thought that difficult ground conditions in in-seam drill holesand mine headings in the northwest corner of LW16 are due to a northerly continuation of anorth-south trending fault encountered further south in Tower Colliery. Black stars indicatelocations of some of the seismic events detected during the period September 1999 to January2000.111
Seismic Hazard in SydneyProceedings of the one day workshopIn this paper, results of these two monitoring programs are summarized and we present somethoughts on the mechanisms that may be involved.MONITORING OF THE DOUGLAS PARK BRIDGESThe monitoring program instituted by the Bridge Management Committee consisted of the following(Hebblewhite et al., 2000). GPS monitoring of targets across the terrain for horizontal movement. Conventional precise leveling and theodolite surveys in selected, accessible traverse lines acrossthe overall terrain, gorges and adjacent to, and on the bridge, pier and foundation structures.EDM surveys on gorge closure at nominated cross gorge locations.Remote electronic monitoring of critical locations on the bridge decks, expansion gaps, bearingand other key structural locations.The results of this study confirmed that although the bridge structure underwent some significantlateral movement towards the mining location, its serviceability was never compromised. In thevicinity of the bridges, the majority of the movement was of a regional ‘rigid body’ nature, with thebridge and the surrounds moving en-masse towards the mine. There was movement of 65 mm due tothe mining of LW16 and a further 70 mm due to LW17. Directly above the longwall panels,subsidence was also observed as well as gorge closure and some degree of relative ‘upsidence’. It isthe large amounts of regional, horizontal ground movement, extending over quite considerabledistances away from the mining that are of most interest in this paper.SEISMIC EVENTSThe seismic monitoring was undertaken between August 1999 and June 2000 using 15 short periodseismographs made available by the Australian National Seismic Imaging Resource (ANSIR). Withno knowledge of where the seismicity, if any, would be located, the units were deployed across BHPBilliton’s mining leases in the Appin area. The vertical component of the ground motion was mainlyrecorded.From this program of monitoring, it was found that there were occasional seismic events that couldbe correlated over distances of about 6 km. The locations of these (see Figure 3) were found to be inthe vicinity of Tower Colliery and the Nepean and Cataract River gorges. Some were also felt byresidents.The depths of the events are thought to have been less than 1 km. Their timing did not necessarilycoincide with periods of active mining as is the case with the much smaller microseismic eventscaused by rock fracturing during longwall caving (see Hatherly et al., 2003, for example). However,by June 2000 when the mining of longwall 17 was completed, the seismic activity ceased.DISCUSSIONHebblewhite et al. (2000), discuss potential mechanisms for the observed subsidence and the lateralmovement of the bridges towards the mine. To explain the lateral movement, three mechanismswere proposed.relaxation of strata in the direction of the principal horizontal stress towards the sides of thelongwall panels,activation of geological structures, andthe cantilevering of massive strata.112
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