Civil Engineering November 2022 | Vol 30 No 10

1940s and outlined by Peck in his 1969 paper, to investigate the causes of failure and aspects that determined the stability of Wall No.2. SMEC began by investigating the ro- bustness of the original design. This process was based on information from the as-built drawings and the geomechanical properties derived from the geotechnical investigation. Following applicable standards, the latest standard used in South Africa for designing such structures, the external stability of the as-built wall was satisfactory, but rupture and adherence issues were noted. To investigate the behaviour of the retaining structure and its interaction with the slope on which it was founded, a numerical simulation was conducted using Phase2. Multi-stage models were used to represent the critical section based on the construction sequence. The objective was to mimic the progressive stress below and within the wall as construction progressed. According to the results, the construction of the wall had a negligible effect on slope stability, and the maximum lateral dis- placement of the wall facing and settlement below the wall remained within acceptable limits for MSE walls. The sensitivity of the slope to changing boundary conditions such as groundwater, internal erosion and external loading was investigated using numerical analysis cali- brated based on the findings of earlier field and laboratory investigation stages. The observations made by SMEC during the in-loco inspection of the wall corresponded to those given by the nu- merical analysis investigation, whereby the wall had rotated downslope. The results from the numerical analysis investiga- tion further confirmed that the failure mode would be exacerbated through the introduction of moisture conditions within the fill or a change in frictional properties of the fill due to, for example, chemical weathering, dispersion, or internal erosion. It also confirmed that the overall slope was sensitive to changes in groundwater. HOW DO WE FIX IT? Maintaining the status quo was not an option, as it was likely to collapse, posing severe safety issues for road users and the general public. Two options were proposed as future slope stabilisation measures. The first option involved the demolition of the existing wall and replacing it with a new 46 November 2022 Civil Engineering The supporting wall began to exhibit longitudinal cracks along its length Cracking caused significant damage to the road pavement and drainage system