Civil Engineering May 2022 | Vol 30 No 4

24 May 2022 Civil Engineering AIMS AND OBJECTIVE WSP was appointed in September 2017 by the Hartwig Trust to design a solution to repair the seawall at Erf 234 and provide environmental consulting services. From WSP’s initial inspection it was concluded that repair of the existing wall was not feasible and reconstruction was necessary. Temporary repairs were also ruled out as it was believed that the existing failed wall could still provide sufficient protection to the property until the planned reconstruction would start in March 2018. This would be the end of the summer season when the beach would be less busy and by which time the necessary approvals would be in place. As work progressed on Erf 234, WSP was appointed in April 2018 by the Jarvis Trust to design a solution for the repair of the seawall at their neighbouring Erf 636 and provide environmental consulting services. PROJECT DESCRIPTION The scope of work included: Q Q At Erf 234: demolition of the existing wall and design and construction of a new 43 m long reinforced concrete cantilever seawall with scour protec- tion, and the wall height increased for sea level rise of 25 years. Q Q At Erf 636: demolition of the ex- isting concrete wall and design and construction of a new 17 m long rein- forced concrete cantilever seawall with scour protection, and the wall height increased to reduce overtopping. PROBLEMS ENCOUNTERED The design and construction of the new wall had to address the following challenges: Q Q The walls were located in the swash zone, limiting much of the construc- tion work to low tide conditions. Low beach levels at the time of construc- tion further reduced the amount of time that the site was in the dry. Q Q No access was possible from the property side for construction plant due to lack of open space, making access from the beach necessary. The lack of working space at the front of the property re- stricted all construction activities. Q Q Wave action during high tides required robust temporary works to protect the construction works from waves and provide scour protection to the properties when they would be vulnerable to erosion by the sea. Q Q An existing CoCT foul sewer (draining Ons Huisie area), located directly behind the existing damaged seawall, was at risk of undermining. Q Q The close proximity of the existing house foundations behind the seawall, whose founding levels were similar to the top of the seawall. Q Q Public use of the beach during con- struction resulted in the public being inquisitive and getting close to the construction activities, as it was not practical to barricade the site on the sea side of the works as it would be removed by the sea. Q Q Excavating below the founding levels and close to existing seawalls of the neighbouring properties meant that these walls had to be protected from undermining. Q Q The environmental requirements involved with working on the beach and keeping the construction scope of works within the parameters of the original environmental authorisation. The most challenging design aspect was coming up with a scour protection solution for the seawall foundations that would be subjected to varying beach levels, water levels and waves during high tides. Being located on the sea side of the highwater mark and below mean sea level (MSL) imposed practical restrictions on what could be constructed. The footprint of all permanent works had to be within the erf boundary. The width of the new wall foundation was also restricted by the location of the concrete encased CoCT sewer along the front of the erven. The level of the sewer was the same as the existing wall founda- tion, which meant that the wall had to Figure 7 Forward rotation and settlement of the wall from scouring at Erf 234 in October 2017. Deformed sour protection mattresses from settlement of concrete wall Figure 8 Low beach levels, undermining and settlement of Erf 636 seawall scour protection mattress and missing wall cope units in March 2018