| Africa


providing them with breathable air and protection from hazards until rescue teams arrive. These cutting-edge engineering techniques not only improve productivity but also ensure the integrity of the tunnel and the safety of the workforce. The two TBMs which will tunnel from Katse and Polihali were designed by Robbins and manufactured by CCCC TH in China. All drill and blast tunnels are excavated and supported following the NATM (New Austrian Tunnelling Method) principles, with a constant adaptation of the rock support to the rock mass behaviour. Rock support, which must be installed immediately after every blast, consists in general of dowels, wire mesh and rock bolts, aided by other support elements in case of poor ground conditions. In-situ reinforced concrete lining is used for permanent connecting tunnels excavated by drill and blast method. Safety consideration is built into the design of every aspect of the tunnelling methods and regular safety risk audits are carried out to ensure compliance. This project is a testament to modern tunnelling innovation and is critical in securing a reliable water supply for the region.


Hydropower integration: the Oxbow


Scheme The Oxbow Scheme, with an installed capacity of 80MW, is part of LHWP Phase II. The head reservoir dam is located on the Malibamatso River in Lesotho’s northern highlands. Due to the limited reservoir size, the Oxbow Scheme will be operated strategically as a peaking facility for Lesotho’s power demand, with the primary aim of curbing the costly energy imports and enhancing the country’s energy independence prospects. The Oxbow tunnel intake will be connected to a 6.6km headrace tunnel (3.6m diameter) emerging at a portal and valve-house on the west side of the mountain. A 3.6m diameter surface penstock will convey the water from the portal some 3km in the westerly direction to the ‘Malefiloane surface powerhouse which will be housing four Pelton turbines each rated at 20MW. The Oxbow Scheme will utilise a design net head of 720m. The generation discharge from the powerhouse will flow into a weir about 1km downstream of the powerhouse from where there will be an intake structure onto a 4.2km tailrace tunnel which will in turn convey the water to a secondary 10MW Nqoe powerhouse located just upstream of the ‘Muela Power Station tail-pond. The fact that the Oxbow Scheme will dam up water on the Malibamatso River which would otherwise be flowing into the Katse reservoir, implies an obligation from the project to redirect the generation discharge back into the LHWP water transfer infrastructure, hence the ultimate discharge into the ‘Muela reservoir. As per the existing LHWP water conveyance infrastructure, from the ‘Muela Dam the water will join flow from Katse (which will include flows from the Mohale and Polihali dams) into the delivery tunnel towards Clarens in South Africa to the north. One benefit that will be realised from damming the Malibamatso River for Oxbow power generation is that possible impact of wasteful spillage of excess water which would otherwise occur at the Katse Dam, can now be minimised and regulated (by holding and managing the flow upstream), as discharge from


the Oxbow Dam will be managed and regulated as environmental releases.


Logistics and deployment in remote terrain


Deploying Double Shield Hard Rock TBMs in the highlands of Lesotho presents significant logistical challenges, requiring meticulous planning and coordination.


Following the factory acceptance tests in China in May 2024, the first of the two TBMs was dismantled and its components transported by sea to Durban, South Africa. From there, the components were transported by road to Lesotho arriving in early September. The transportation of oversized TBM components, some weighing up to 55 tonnes in a single piece, along the steep and winding roads including the Northern Access Road, demands specialised hauling equipment and careful route planning, with contingency measures to traverse the mountainous terrain in adverse weather conditions. Due to the site’s remote location, the re-assembly and commissioning of the TBM requires on-site heavy lifting equipment, skilled personnel, and efficient logistical support to ensure smooth operations. The challenges extend beyond the transport to site


and reassembly of the TBM. Establishing power supply, water management


systems, and ventilation for tunnelling operations in a remote area is complex and requires significant investment in supporting infrastructure. Housing, catering, and medical facilities were also required to be set up for a large workforce working in challenging remote locations


Continuous logistic support is also required


throughout the tunnelling operation. Importing TBM parts, spare components, and consumables involves navigating customs regulations and ensuring timely delivery through difficult terrain. Stockpiling critical spare parts is necessary due to the long lead times for replacements, which could impact construction progress if delays occur. Careful planning, innovative logistics solutions,


and strong collaboration between contractors and stakeholders help to overcome these challenges and ensure the successful deployment and operation of the TBM in this critical infrastructure project. The second TBM is expected to arrive in mid-2025 also by sea to Durban and then by road to the Polihali site.


www.waterpowermagazine.com | May 2025 | 15


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