13th Australia New Zealand Conference in Geomechanics, 1-3 April 2019 - Challenges encountered with a Cased CFA Secant Pile Wall over unused brick pit in the Botany Basin
Stefano Pirrello, Keller Pty Ltd, Sydney, Australia, +61 0421 000 127,
stefano.pirrello@keller.com.au (now deepfoundationsolutions@gmail.com)
Cased CFA Secant Pile (CSP) walls are usually built to reduce risks associated with achieving tight vertical tolerances. The latter becomes crucial where high water table and deep bedrock profiles characterise the site. This was the case at the site of a proposed two-level basement, part of a residential development located within the Botany Basin in Sydney, where a cut off-wall was required.
The proposed development’s north-east corner overlaid an unused brick pit. Available historical photos showed the brick pit in use up to circa 1930. Further aerial photos from 1949 showed the same area had been developed with industrial buildings. Clay mining activities within the brick pit exposed natural bedrock, typically sandstone. The closest geotechnical borehole available was located about 25m from the north-east corner of the proposed cut-off wall. This borehole showed sandstone at about RL 3.0m AHD (i.e. ≈15m below ground level). In view of this, a more comprehensive site investigation was required to assess the bedrock profile so as to reduce the risk associated with both the minimum required shaft interlocking and with the cut-off depth at the base.
The site investigation carried out by Keller included borehole drilling combined with Seismic Cross Hole Tomography (SCHT) and Surface To Borehole Tomography (STBT). This was carried out along the centre line of the North-East corner of the wall alignment.
The results of the additional geotechnical and geophysical site investigation will be discussed in relation to design and construction challenges. Design validation through interpretation of real-time data from the piling rigs’ on-board computer data, and the monitored wall movement by means of inclinometers installed within the piles will be discussed and compared against the predicted wall movements using WALLAP and PLAXIS.
Keywords: CFA, CSP, SCHT, STBT, Wall Displacement, WALLAP, PLAXIS.
