Underground Construction - April 2021 - 21

Katse River dam

Ash River outfall

Mohale Reservoir

Phase II of LHWP entails construction of the 540-foot high
concrete faced rock ﬁll Polihali Dam, with a capacity of 2.2 billion
cubic meters of water.
The dam is to be connected with the 27.3-mile water transfer
tunnel to Katse Dam that was completed under Phase 1A in 1998. It
has a height of 607 feet, storage capacity of 1,950 million cubic meters
of water and spillway discharge of 6,252 cubic meters per second.
However, before the construction of the Polihali Dam, two
water tunnels have to be excavated to divert the waters of Senqu
River, where the dam is located, away from the natural riverbed.
" This will create a dry foundation to facilitate the construction
of the Polihali Dam, " said a project brief by LHDA. The two tunnels
will increase the capacity to carry ﬂoods and provide ﬂexibility to
work in one tunnel while the river ﬂows in the other one.

Tunnel specs
The ﬁrst tunnel is about 0.6 miles long with a diameter of almost
23 feet, while the second also runs for 3,281 feet and is 29.5 feet

wide. Both tunnels run parallel to each other from the intake point
upstream to the outlet downstream of the Polihali Dam.
Although concrete casting and reinforcement of the diversion
tunnels, initially slated to take 18 months from contract signing
date, was scheduled to start in the ﬁrst quarter of 2020, LHDA
reported in June 2020 the works were disrupted by the COVID-19
outbreak.
" Full resumption of the Phase II advance infrastructure
construction works and the Project's social and environmental
programmes will be incremental as consultants and contractors
meet their Covid-19 mitigation obligations and travel restrictions
are lifted, " said LHDA CEO Tente Tente in a June 2020 statement.
No further updates have been given on new deadlines for
the diversion tunnels. However, LHDA had earlier said such
construction " usually goes along with the building of coﬀerdam,
one upstream and one downstream of the proposed dam, which
together allow the river ﬂow to bypass the dam foundation area. "
Construction of both river diversion tunnels will use the drilling
and blasting method " supported by rock bolts and shotcrete as
required, " according to LHDA.
In April 2019, the SCLC Polihali Diversion Tunnel Joint Venture
had been awarded the $34-million contract for the construction
of the tunnels. JV members include Salini Impregilo SpA (South
Africa), Cooperative Muratori Cementristri CMC di Ravenna
(South Africa), LSP Construction (Pty) Ltd (Lesotho) and CML
Infrastructure Ltd (South Africa).
The two tunnels have been designed and will be supervised
during construction by Metsi Senqu Khubelu Consultants JV
comprising South African ﬁrms Aurecon, Knight Piesold, Hatch
Goba, SMEC and FM Associates of Lesoth. They are part of a long
network of water transfer and diversion tunnels meant to deliver
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Underground Construction - April 2021

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