Civil Engineering May 2022 | Vol 30 No 4

foundation down to bedrock. A cantilever reinforced in-situ concrete wall would be constructed above the mass concrete foundation (see Figure 9). In-situ concrete construction was selected due to the required shape of the wall at the ends, limited time of working in low tides, lack of space on the site, the need for monolithic structure, and to minimise the number of joints. Founding the wall on bedrock meant that there would be no settlement or differential settlement at wall joints that would have to withstand water flow of high velocity from waves. To minimise resultant hydrostatic pressures acting behind the wall, weepholes were used in the base of the wall to allow drainage. The weepholes were filled with no fines (i.e. no sand content) concrete to reduce water flow of high velocity penetrating the weepholes and causing erosion of the backfill. The narrow space available for the new wall and the high level of ground and steep slope of ground behind the wall of Erf 234 required the base of the new cantilever wall to be constructed with a counterweight located above the concrete encased sewer. In order to construct the mass in-situ concrete below water and in an active tidal zone, it was decided to use a sheetpile cofferdam that would be installed down to the bedrock (see Figure 12). Sand material would be exca- vated from within the cofferdam down to bedrock and mass concrete continu- ously cast directly onto bedrock. The sheetpiling would also provide temporary lateral support and scour protection to the adjacent concrete encased sewer during the construction. There was sufficient sand thickness on site for vertical sheetpiles to be supported in the sand and sufficiently protrude above the sand level to form a vertical barrier that would protect the reinforced concrete works for the cantilever wall (behind the sheetpiles) from oncoming waves. The sheetpiles also restricted the ingress of water into the cofferdam, making excavation in the dry possible. The toe (i.e. bottom) of the sheetpiles was restrained horizontally by drilling and installing vertical bars into the bedrock on the inside of the sheetpiles with grouting between the bar and sheetpiles prior to the excavation of the sand inside the cof- ferdam. This ensured that when excavation was done, the sheetpiles would not move inward from the outside earth and water pressures. The contractor also required ad- ditional lateral ground support insurance to be in place during the project. The sheetpile cofferdam was inte- grated into the permanent works design for the following reasons: Q Q The high risk and significant damages that could be caused to the properties and CCT sewer in a relatively short time when extremely vulnerable to the sea during construction Q Q Safety of construction personnel and divers in the cofferdam after sand excavation. CONSTRUCTION SOLUTIONS Measurement of ground motions from the vibration of the sheetpiling was done at the house foundations to ensure motions were within the allowable limits for residential structures in accordance with building codes of practice. This was needed as two- and three-storey houses were located relatively close to the construction activi- ties and had shallow footings founded on sand with high moisture contents. Effective use of the excavator located directly behind the failed wall allowed ex- cavation of the backfill material to be done in very limited space. This required the excavator to climb up the wall by pulling itself up a very steep steel ramp. A stable footing had to be prepared as the backfill of sand and rubble behind the wall was uneven and unstable due to the erosion. 26 May 2022 Civil Engineering Figure 10 Cofferdam sheetpiling taking place at Erf 234 Figure 11 Excavation of backfill complete and sheetpiles installed first against concrete encased foul sewer at Erf 234 Concrete encased CCT 200 mm diameter sewer