Sterkfontein Cave Entrance Repairs

In December, 2022, the Johannesburg area received significant rainfall as intense storms and prolonged heavy rains. Ordinarily, rainfall seeps slowly through the dolomite that hosts the caves and take several months to permeate deep into the system. The rain in 2022 resulted in significant amounts of water to rapidly enter the caves through cracks and open entrances. Ordinarily, cave entrances are cleared of debris that may wash into the cave on a regular basis as part of the site maintenance plan. During the December 2022 rains, excessive water saturated the soils around several cave entrances and caused localised collapses of sediments, including rocks and organic debris. The tourist entrance to the caves was particularly badly effected and a large slump of sediments filling an adjoining crack in the dolomite broke handrails and washed deep into the cave. Upon immediate report of this by Sterkfontein and Maropeng staff on 13 December, 2022, the caves were closed for immediate safety reasons and remained closed while pending safety assessments and implementation of any required safety plan. The lateness in the year precluded rapid assessment of the damage, and early in Jan 2023, SRK, civil engineers with long-term experience of work at Sterkfontein, were engaged to assess the damage and propose solutions that would allow the safe opening of the caves. SRK, summarised the damages as follows:

• The affected area of the caves seems to be limited to the first 50 m of the visitors cave entrance;
• The instability occurred along a blocky rockmass at the visitors cave entrance where a fault contact exists. It is made up of a weak soil sediment and a more competent rock. The sediment had washed out between the rock contacts;
• Significant unravelling and erosion of the sediments occurred following heavy rains on the left side of the cave entrance. Damage also occurred to the handrailing which will need to be repaired or replaced;
• The eroded sediments has left a large cavity in the hanging wall between the rock/fault contacts. Tree roots were noted inside the cave cavity which indicates that the cavity has almost broken through to the surface (within 1-2 m from the surface). There is possibility for additional sediments to be eroded into the cave;
• There is a possibility that the continuous eroding of the sediments will result in further undermining of the rockmass at the cave entrance which could result in a collapse of the cave entrance; and
• Loose rocks and blocks with the potential to fail into the cave were also noted during the inspection.

Mitigation:
A multiphase approach for mitigation of these safety issues was proposed by SRK in collaboration with New Concept Mining (NCM), who have agreed to supply the materials for the support structures. Phase 1 focused conducting laser scans of the entrance area to enable structural geological assessment and planning of support structure placement and specifications. Phase 2 focused on installing both wooden pillars and mesh on walls to reduce potential movement of soils and collapsed rock. Phase 3 involves clearing debris from the vertical cave entrances that are close to the tourist route or research areas.

Progress:
Phase 1 and 3 have been completed. For Phase 2, support structure plans have been proposed by SRK. NCM have been engaged regarding the materials procurement and installation process. Some specifications of chemicals related to the structures to be installed are yet to be received. Upon receiving these, Dominic Stratford will apply for the necessary SAHRA permit (type s27 – Site Maintenance permit). Delays have been unavoidable and have stemmed from service provider agreements that are part of the Wits procurement system. Given the sensitivity of the area and its high visitor volumes, the standards of work and legal aspect of service provider appointment are relatively complex but are near completion.