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Concrete In Australia : September 2008
PROJECTS TRAFFICBARRIER SHARED PATH RAMP Bridge substructures SOIL NAILS Figure 6. Typical approach section showing soil nail walls. the site is located on the Para Fault line, which is actually a series of parallel north-south oriented fault lines. This area was expected to have a complicated and highly variable soil profile. This was indeed proven to be the case following extensive additional investigation. An additional drilling program, which included two 40m-deep holes, was conducted under the guidance of the geotechnical sub-consultants URS Australia. Unfortunately, due to the presence of the rail lines, deep bores could not be located at the actual bridge abutment and the entire design relied on inferred geotechnical information. The approaches General layout/description The underpass section extends for a total length of 470m with a maximum depth of 8.2m below natural ground level near the middle. The western entry is on level ground whereas the eastern entry is higher and on an upwards slope into the city. The eastern approach is located in the Adelaide City 1 in 100 year fl ood water detention basin thus requiring a longer approach and higher walls than might have been expected. The majority of the soils encountered on the approaches were stiff clays. Soil nailed retaining system Using experience gained on Victorian projects, CW-DC investigated and adopted soil nailed walls as the final design solution for the walls as shown in Figure 6. This consists of a grid of passively reinforced soil nails installed into the wall face and anchored into a shotcrete facing. Substantial cost savings were achieved by adopting this wall construction over conventional methods. The close proximity of property boundaries at some locations meant that the only practical alternative would be cantilevered bored piles, which are extremely expensive. The Bakewell Underpass project is the fi rst major application of soil nail walls in South Australia. The lack of experience in the local soils was a challenge for both the geotechnical engineers and CW- DC’s soil nail designer. The design was undertaken using the Slope W wall analysis software and allowances were made to account for the expansive nature of the clays in the wall structure. 52 Concrete in Australia Vol 34 No 3 1350 BORED PILE General layout/ description The design concept and detailing of the substructure of both the road and rail bridge is essentially the same. The bridges are supported by concrete headstocks on bored cast- in-place concrete piles. The piles also act as lateral soil retaining elements and have shotcrete arches between them above the fl oor of the underpass. In order to keep the piles to a reasonable size they span between the soil at the underpass fl oor, which provides support by passive resistance, and the bridge superstructure, which props the top of the pile as shown in Figure 7. Geotechnical overview As discussed earlier, the geotechnical conditions at the site are variable and diffi cult with lenses of soft clay in the foundation gravels. In order to establish a viable footing system, fi ve 750mm trial holes were bored and the stability of the excavations assessed. This program confirmed that the sand and gravel soils were reasonably stable above the watertable and that it was not practical to consider bored piles below this. The water was reasonably free-fl owing and the excavations collapsed readily below watertable level. Based on the trial drilling and the preliminary boreholes, it was decided that the maximum practical depth for a bored pile was about 1m above the watertable (a depth of 15m). Rail bridge substructure The initial construction methodology for the rail bridge proposed that the bored piles be constructed during night time rail closures, within windows of activity of about eight hours. To remove the need for trackworks the piles were laid out between tracks with a typical spacing of 4.5m and a maximum diameter of 1500mm. The risk of not completing a pile of this size within eight hours was very high and leaving an open hole adjacent to an operating rail track was considered unacceptable. Also, the preliminary geotechnical design of the piles indicated that a 1500mm-diameter bored APPROACHSLAB RAIL TRACKON BALLAST 1800 DEEP SUPER-TEE HEADSTOCK THRUST PAD Figure 7. Rail bridge long section showing abutment and deck detail.