A collaborative team effort between Transnet, the client; WBHO, the principal contractor; JG Afrika, the consulting engineer; and Maccaferri Africa, a supplier of various geotechnical products; helped to quickly restore a vital transport link that was damaged by the heavy floods in KwaZulu-Natal in April 2022.
Work commenced on the project at the end of May 2022. The first railway line was opened in July 2022. In October 2022, the second railway line started operating again, reinstating the transportation of freight via rail between Johannesburg, Gauteng, and one of Africa’s busiest ports in Durban. This immediately relieved pressure on the province’s already-strained road infrastructure.
“Despite the complexity of this project, we succeeded in reopening this strategically important general freight line in the Transnet Freight Rail network in a very short period. I attribute this achievement to excellent dynamics between all members of the professional team. This is one of the strong points of the NEC contract under which this project was undertaken. It promotes honesty, fairness and good faith between members of the professional team. Risk mitigation meetings were held prior to claims being submitted and issues were amicably resolved through discussion. Moreover, Transnet responded quickly to approvals and queries which helped to mitigate delays on this fast-track project,” Jan Norris, a JG Afrika Director, who led the engineers working on this project, says.
WBHO and JG Afrika’s scope of work entailed rehabilitating 18 slope-embankment failures, including four in-cuts above the railway tracks. These failures were located at four different locations along the railway line in Klaarwater, Mount Vernon, Burlington and Seaview, just one of the many challenges associated with this construction project.
In order to meet the very tight deadline, engineering designs were undertaken simultaneously with construction. The initial expectation was that JG Afrika would turn designs around in 48-hours considering the urgent need to reopen the railway lines. Bearing in mind the many challenges that this project presented, on the whole, this expectation could not be met. However, the engineering team worked tirelessly to complete the designs within a reasonable timeframe to keep the project on track.
One of the biggest challenges was the lack of existing geotechnical information and insufficient time in which to undertake a new investigation at the various sites. As-built information was also unavailable. Geotechnical engineers and engineering geologists, therefore, assessed the slopes visually and used past literature on the geology of the various areas. Based on this information, the engineers relied on their own judgement as professionals when undertaking the designs.
The skills and experience of seasoned JG Afrika Engineering Geologist, Keval Singh, were harnessed for visual assessments of the aftermath of the floods.
As Mr. Singh points out, landslides are not uncommon in the greater Durban area due to the geological setting.
“There were previously three major railway embankment failures in the area. All three sites are located within the Natal Group sandstone, which comprises sandstone, siltstone and subordinate shales. A lot of the failures that we observed were primarily within the overburden and not within the rock mass. At the Mount Vernon site, the large circular failures mainly occurred within the fill material and as result of the washout of fines on the cut slope. At the Burlington site, an extensive mass of fill material failed. The material that we profiled there consisted of loose sand; a lot of boulders; and other irregular-sized fragments. At the Seaview site, the large translational failure occurred in the fill material. Notably, all the failed material that accumulated at the toe of the embankment was washed away during the flood leaving very little remnants behind,” Mr. Singh says.
Andrew Leibnitz, a Technical Director of JG Afrika, was part of the team that helped to devise innovative ways of rehabilitating the failed embankments.
At the Seaview site, the solution entailed extending an existing mass concrete wall that provided improved protection to a longer section of the embankment. It was extended by about 85m and tied into the cliff face at the end of embankment to avoid the same type of scouring effect in the future. It is the largest structure that was built as part of the project.
The wall height was determined by estimated future scour and flood levels using HEC-RAS software. Once the height of the wall had been determined, the engineers had to decide on the type of wall that would be constructed, while also considering several factors. They included stability, constructability and flexibility, bearing in mind the lack of available geological and as-built information for the designs.
“We designed a traditional cantilever-type wall. It was constructed in panels that varied in height from 6,1m to 6,6m with movement joints between them. These heights were selected partly to simplify the layout of the reinforcing and hence to facilitate constructability. Because these are standard bar lengths, this approach also ensured the timely delivery of the material to site to avoid delays. A modular approach also simplified construction by enabling site staff to adapt the design of the wall quickly and efficiently to site topography within acceptable parameters,” Andrew Leibnitz, a Technical Director of JG Afrika, says.
The wall was mainly founded on exposed bedrock. Extensive use was made of mass concrete to create a level platform for the wall foundations. Only the first three panels with a combined length of 18m that adjoined the existing wall had to be supported on a large mass concrete raft foundation. This is due to unsuitable founding material along this section of the proposed wall.
The wall was then dowelled into the underlying bedrock by means of 32mm diameter grouted dowels. This ensured that the structure was stable and would not wash away in a future flooding event, while also aiding with sliding and over-turning stability. The depths of the dowel bars were also alternated so that they did not all terminate at the same level and, in doing so, further ensuring greater stability of the structure.
At the Burlington site, the engineers were primarily concerned with stabilising the railway track to ensure that it did not erode in future without placing additional pressure on the two unstable embankments. They could not be stabilised by means of conventional retaining walls due to their very steep gradients and poor founding conditions, which was a typical challenge on this project, considering that some of the embankments are about 100 years old.
The solution comprised an L-shaped wall, with the heel portion of the wall extended in length to sit underneath the outside railway track. In this way, the weight of the train and the ballast provide stability to the wall. Moreover, the front portion of the wall has been designed in such a way that it can be undercut by at least a metre and remain stable. This has successfully addressed concerns about future undercutting of the slopes during a future flood.
The wall was constructed in 6m panels with buttresses for electrification masts, where required.
Wherever possible, the slopes were flattened to achieve Transnet Freight Rail’s standard 1:1,5 slope. However, there were places where this was not possible due to insufficient space. The benching was undertaken into the existing profile at 900mm lifts. A G5 back fill layer was then placed in 300mm layers and compacted to 95% MOD AASHTO density. Hereafter, geogrids were introduced at 600mm lifts and extended into the benches. Where this was not possible, Soilcrete was used to fill the gaps. A Geocell or similar approved material was then placed with a 150mm topsoil layer that was hydroseeded.
“A design such as this one would usually involve months of geotechnical investigations, accompanied by complex laboratory testing. Not having the luxury of time to do this, we used our experience and professional judgment on the geotechnical parameters to complete the project within the tight timeframe, dictated by the urgency to get the rai line up and running,” Ms. Norris says.
A number of retaining walls were also designed and constructed as part of the project. At tender stage, one of the requirements was to reinstate the slopes to 1:2 with backfill. However, this was not always possible due to space constraints.
Rockfall protection also had to be installed in areas where there were exposed rocks to protect the tracks and trains below. One of the designs entailed constructing a small catch wall at the toe of the embankment. The embankment was then covered with a steel-mesh drapery that was anchored into place to retain the loose rocks. A polymer type drapery was also used at various sections. Maccaferri Africa provided exceptional technical support to the engineering and contracting teams throughout this aspect of the project, especially with regards to the correct installation of the steel-wire mesh with its unique applications.
Among the other challenges with which the professional team had to contend was restricted access to some of the sites. For example, two temporary river crossings had to be constructed to access the toes of the embankments on the banks of the Umhlatuzana River. Both were washed away in September 2022 and had to be reconstructed.
Because this was an emergency project, there were some waivers on the environmental permits. However, there were still restrictions on the way in which operations could be undertaken in a water course and compliance was monitored by Transnet Freight Rail.
Work adjacent to an operational railway line also had to be carefully sequenced.
“It was an absolute pleasure to work with a large and established contractor that continuously demonstrated professionalism in the way in which it undertook its work. Certainly, Transnet’s hands-on and supportive approach also contributed to a very successful outcome,” Ms. Norris concludes.
