By: Phuti, Mawande and Tshilidzi
Engineering geology
Engineering geology is a field of science concerned with the application of geological principles in investigating and understanding the natural materials varying from soil, rock, surface water to ground water that impinge on the construction, operation and design of civil engineering structures and projects.
The Huguenot Tunnel, Western Cape
Geotechnical considerations in tunnelling design
When planning for a tunnel project, maximum information on aspects such as rock characteristics (rock type), structural system (discontinuities; joints, faults, folds, fractures, etc) and ground water conditions need to be obtained. This information is of paramount importance to the tunnel designer in that it should enable him to anticipate the behaviour of the rock surrounding the tunnel and the type of support methodology required to maintain the tunnel in a stable condition.
Huguenot Tunnel
The construction of the Huguenot Tunnel started in 1984 and the tunnel started operating in 1988. It is located along the N1 road, connecting Worcester, Paarl and the City of Cape Town in the Western Cape Province. The Huguenot Tunnel is considered the longest in South Africa, with the maximum length of 3.9 km. It traverses through the sandstone and quartzite rocks of the Du Toitskloof Mountains which are part of the Cape Fold Belt. The tunnel is shorter a route in comparison to the older Du Toitskloof Pass, and it shortens the travelling distance by 11km.
Geotechnical methods used to assess the geological setting of the Du Toitskloof (Huguenot) Tunnel
The methods included but not limited to the following assessment criterion:
(a) Drilling a pilot tunnel along the line of the second main tunnel;
(b) A detailed geological mapping and surface geology of the area through which tunnel was excavated.
(c) In situ stressed measured under various overburdens
(d) Strength and behaviour of rock types over the tunnel were also determined and
(e) The geotechnical aspects and support required over geological zones of the main tunnel were assessed.
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