Civil Engineering November 2022 | Vol 30 No 10

M any rural roads remain unpaved in South Africa, primarily in rural communities and poorer areas. Consequently, government invest- ment in upgrading these roads has become essential, not only for creating new jobs but also for improving the lives of the people living in rural areas by facilitating easy ac- cess to schools, jobs, and essential services. Most road improvements involve en- larging the roadway to increase its capacity, and what better method of expanding a roadway than using a mechanically stabi- lised earth (MSE) wall? When used appro- priately, they are often the most economical way of retaining soil. They are also labour intensive, easy to construct, and offer the opportunity to use locally sourced materials. These walls can provide effective slope stability and fill retention if adequately de- signed and constructed. On the other hand, if they are not, they may fail with significant failure consequences. Failures can result from a variety of causes which are often re- lated. Therefore, in the event of a failure, the cause must be investigated to determine the mode of failure and the factors influencing wall stability before the appropriate options for remedial works are considered. The KwaZulu-Natal (KZN) Department of Transport commissioned SMEC South Africa to investigate the cause of the failure of an MSE wall located on Main Road P73 near KwaBombo, a rural community west of Hibberdene in the KZN province of South Africa. Main Road P73 connects Sipofu, some 20 km from the National Road N2 (N2) to the east, with the Highflats to the west, and is thus a vital link between rural and farming communities. The road was upgraded from a Type 6 District Gravel Road to a Type 3 Secondary Provincial Road with a modi- fied geometrical alignment. Additionally, the road’s construction required high fills that were accommodated by retaining walls consisting of MSE systems. The road upgrade was completed in 2017. In late 2020, approximately two to three years after construction, Wall No.2 supporting Main Road P73 at km 30+304 began to exhibit longitudinal cracks along the length of the wall, approximately 4 m to 5 m offset from the crest of the wall. Cracking caused significant damage to the road pavement and drainage system. INSPECTION The in-loco inspection report revealed that the wall was constructed on a very steep slope over a natural seep. The wall exhibited instability, as evidenced by longitudinal cracks extending along its length. The result was substantial damage to the road pavement and drainage system, posing a substantial risk to road users, aggravated by a limited view dis- tance and steep drop-off from Wall No.2. It was evident from the tension cracks that the wall had deformed laterally and vertically to such an extent that it was in a state of imminent collapse, and any further change in conditions, such as further water infiltration or additional loading, could lead to further deformation and failure. Further, significant soil staining was noted on the front face of the wall, which was indicative of water flowing over and through it over time. No bulging or cracking of the foundation soils were ob- served; however, vegetation was very thick in places; therefore, the possibility of global stability issues could not be excluded. This led to questions requiring further investigation, such as, “Why is the wall failing?”, “How can we fix it?” and “How do we prevent the failure from reoccurring?” WHAT LED TO THE FAILURE? SMEC employed the observational method, developed by Terzaghi in the SANRAL SA I CE 2022 Nat i onal Awards Category: I nvest i gat i ve /plann i ng report COMMENDATION Investigative/planning report category KEY PLAYERS Client: KwaZulu-Natal Department of Transport Consultant: SMEC South Africa Contractor: Geopractica Contracting Investigation and Design of Remedial Works for Wall No. 2 on Main Road P73 Civil Engineering November 2022 45 SMEC conducted an in-loco inspection and geotechnical investigation to determine the cause of failure