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Rocks and civil engineering
Published in A.C. McLean, C. D. Gribble, Geology for Civil Engineers, 2017
Construction of the new Strome Road involved making cuttings in a variety of metamorphic rocks and ensuring that the rock slopes of the cuttings were stable. The rock consisted of Lewisian feldspathic and hornblende-gneisses (that is, members of the engineering granite group) with intercalated chlorite-schists and hornblende-rich rocks (schist group), and younger Moine quartzite and quartz-rich gneisses (quartzite group). The structure is complex. The Moine rocks have been thrust westwards over the Lewisian rocks along a major plane of faulting, the Moine Thrust, below which there is extensive fracturing (Fig. 7.10). In the southern part of the site, Lewisian rocks are present above, as well as below, the thrust. This part of the new road was examined in detail by one of the authors (C.D.G.) and the discontinuities were logged systematically. They are all joints and comprise five distinct joint sets. The results are shown in the orientation diagram (Fig. 7.11), where the diameters represent the strike (in degrees magnetic) of the sets, and angles of dip are also given. Figure 7.12 shows the relationships of the five main joint sets in a block diagram. Details of each joint set are listed in Table 7.8. At least three sets were present in each of the nine road cuttings examined, and in one, all five were seen together. The rock mass here possesses a very small block volume, even before blasting. The joints are opened by blasting and a highly unstable slope is produced. At the planning stage, slopes of 70° to the horizontal were considered to be practicable, but in the event, slopes of about 45° were the best that could be achieved in most cuttings. Pre-splitting techniques (Langefors & Kihlstrom 1973, pp. 304-7) were tried as a possible means of producing a safe permanent slope. In this procedure, close-set small-diameter drill holes are drilled along the position of the intended face of a rock cutting. These are then detonated using a weak charge to produce a fracture. Then the rock mass is drilled and blasted in the usual way, but the rock is removed only up the pre-split surface (see Fig. 7.13). A safe angle of slope (see Table 8.1) is probably about 35-40°, so the present slopes may decrease in angle still further before stability is finally reached. To allow for this, the road was constructed as far from the high side of the deeper cuts as possible, and roadside trenches were dug to trap any falling rock. Fortunately, most cuts were only about 10 m deep and the remedial measures appear to have worked. Many of the geological problems encountered could have been predicted at the planning stage if the engineers had fully understood the effect of the Moine Thrust on the rocks that were to be excavated, but this was not brought out in the original site investigation report.
The ‘8Gs’—a blueprint for Geoheritage, Geoconservation, Geo-education and Geotourism
Published in Australian Journal of Earth Sciences, 2019
In Figure 13, in descending order, from small scale (geosites) to extensive and more complex areas (geoparks), a number of site-specific examples are listed in relation to their geology and geological features; references to these sites, areas and geological features are noted as follows: Talbragar Fossil Fish Bed (Dulhunty & Eadie, 1969); Siccar Point (Barclay et al., 2005; Brocx & Semeniuk, 2007); Kalinjala Mylonite Zone, Port Neill (Drexel et al., 1993); Moine Thrust (Lapworth, 1885); Pilbara dravite (Brocx & Semeniuk, 2010); orthocone site in Morocco (Errami et al., 2015); Muderup Rocks (Semeniuk & Johnson, 1985); fossils at Mannum (Drexel & Preiss, 1995); Fossil Hill at Cliefden Caves (Brocx, Semeniuk, & Percival, 2019; Percival, Webby, & Burkittm, 2019); Goat Point, Shark Bay (Logan, Read, & Davies, 1970); Broome Sandstone (Geological Survey of Western Australia, 1990); Tims Thickett Limestone at Yalgorup (Semeniuk, 1995, 1997); Jack Hills, WA (Wilde et al.,2001); Gubbio, Italy (Alvarez, 2009; Alvarez et al., 1980;); Hallett Cove (Cooper et al.,1972; Drexel & Preiss, 1995; Giesecke, 1999); The Chalk, The Seven Sisters (Gallois, 1965; Larkin, 2006; Melville & Freshney, 1982; Mortimore et al., 2001); Iguerda Inlier (Errami et al., 2015); and the Pilbara Craton (Geological Survey of Western Australia, 1990).