on the seabed, extending upwards to RL+13, to safely protect the pit from the high tide level rising to above RL+10. Geology The geology comprises the main ore body known formally as Seawall Hematite, dipping at 55 degrees towards the south into the seabed along the southern side of Cockatoo Island. The high-grade Hematite unit (68% iron content) with a thickness of about 30 m (98.4 ft) was uplifted by regional folding to its current dip angle, which is clearly visible from the air upon approach to the island. Earlier stages of open cut mining commenced in 1951 at an elevation of approximately 130 m (426.5 ft) above sea level, and, by 1985, approximately 2 km (1.25 mi) of accessible ore were removed down to sea level. Embankment Construction The various stages of seawall are founded on the existing seabed, effectively bearing on the very soft Coralline Sediments. Construction required strictly controlled stages of progressive placement of fill material and loading, allowing strength gain to occur from pore pressure dissipation in and consolidation of the weak Coralline Sediments. Seawall fill material comprised locally quarried quartzite and sandstone. The material was notoriously variable, heterogeneous and anisotropic. Permeability characteristics of the fill material were therefore difficult to predict both laterally and vertically within rockfill that was placed to form the seawall embankment. Placement criteria during low tide windows also meant that fines would be washed out more predominantly in the lower sections of seawall due to the massive movement of seawater across the construction zone during tidal cycles. Cutoff Wall System Selection Construction of the first seawalls (Stage 1) commenced in 2002, and Stages 2 and 3 followed and kept mining operations going until 2012. These earlier stages of seawall construction involved the placement of the rockfill obtained from the local quarry and during mining activity, combined with a low permeability clay core and filter zone material, and some sheet piling. Between 2003 and 2009, there were at least three major preventable failures associated with the first three stages of seawall construction, all causing the suspension of mining activity and major recovery work. The Stage 4 expansion needed to take advantage of the previous lessons learned to avoid any repeat of the earlier failures. Stage 4 required a new 261 m (856 ft) long seawall to be constructed. A rockfill seawall would again provide the structure for and stability of the barrier wall, but not the water cutoff properties required. The seawall cutoff would need to extend from Seawall Concept elevation RL+12 down into the Coralline Sediment aquiclude, where a suitable cutoff toe level was determined to be about elevation RL-5. Wagstaff Piling proposed the idea of injection grouting, which had the advantage of requiring light rigs and a batch plant that are relatively easy to mobilise, with ongoing raw material supply feasible by road and by barge to the island. The grouting method was developed based on a previous Wagstaff project that involved grouting of a rockfill seawall in Gladstone, Queensland. In that case, a rockfill seawall with limited fines was grouted successfully to create a seepage barrier using rapid gelling chemical grout to fill the voids, thereby preventing dredged fines from washing out through the seawall during tidal cycles. A significant advantage of grouting was that the seawall could be raised simply without the need of fines or clay core material, and then a grout curtain barrier injected centrally through the rockfill would provide sufficient seepage cutoff. The grouting was proposed to be done along the wall alignment using a combination of techniques, as summarized below. Stage 1 - Permeation Grouting Permeation grouting would be performed in the upper seawall fill materials from a depth of 0 to 10 m (0 to 33 ft). Using percussion drilling techniques, a top hammer drilling rig would bore through the rockfill with fluid flushing techniques, and then a rapid gelling grout formulation of bentonite, cement and chemicals (sodium silicate) would be injected into the borehole. Stage 2 - Jet Grouting From a depth of about 10 to 17 m (33 to 66 ft), utilizing the passage of the holes already drilled for the permeation grouting, jet grouting would be performed below the seawall in the lower seawall hematite sands and coralline sediments, beneath the permeation grouted upper seawall fill. DEEP FOUNDATIONS • MAR/APR 2017 • 15
