Report No: 1088/1/03

February 2003


South Africa has several inter basin water transfer schemes, as indicated in Figure 1-1, with the most recent one the Lesotho Highlands Water project (LHWP) transfer tunnels bringing water under gravity from Lesotho through about 80 km of concrete lined tunnels. South Africa also has the largest irrigation tunnel in the world, the 5.33 m diameter Orange-Fish Tunnel, 81 km in length.

Figure 1-1 Water Transfer Schemes in South Africa

Figure 1-1 Water transfer schemes in South Africa

A concrete lined tunnel can lose as much as 30 percent of its hydraulic capacity over a period of 30 years. Engineers have a sound knowledge of the hydraulic roughness of new tunnels, as has been confirmed during the recent (1998) commissioning tests of the LHWP Transfer and Delivery Tunnels. Furthermore, recently completed research funded by the Water Research Commission (Pegram and Pennington, 1998) helps to assess the hydraulic capacity of new tunnels. There is however a need for a better understanding of the tunnel ageing process in order to plan for the phasing in of future possible new tunnels as water demands increase.

South Africa will have to rely more and more in future on water transfer schemes. These tunnels are extremely expensive, with estimated capital costs of about R30 000 /m length.

The main aim of the project was to provide a reliable hydraulically based methodology with which concrete lined tunnel ageing and the associated decrease in hydraulic capacity can be predicted. Both the ageing mechanisms of sliming (with sediment deposition) and soft water corrosive conditions were investigated. This will ensure the timeous phasing in of new schemes when tunnel ageing decreases the hydraulic capacity.

Furthermore, the aim was to establish whether remedial measures can be implemented to reverse or contain the tunnel ageing process.

Tunnel design and operation guidelines were also developed to limit tunnel ageing.

This study combined laboratory and field test data to obtain a better understanding of the ageing process in large diameter conduits such as water transfer tunnels. Field tests were carried out since 1998 at the Orange-Fish Tunnel, Roode Elsberg Tunnel and Theewaterskloof Tunnel, while laboratory investigations at the Universities of Stellenbosch and of Pretoria were also carried out on pipes. The following are key findings of this research project:

No cement extenders were used in the concrete of the Theewaterskloof Tunnel and it was found that the leaching rate of Ca-ions increase exponentially with an increase in the water operating velocity in the pipe system. The main chemical process taking place in the concrete during soft water attack in the Theewaterskloof Tunnel is the leaching of Ca(OH)2. A maximum and minimum theoretical concrete deterioration depth rate was determined for the concrete. These rates also increase exponentially with an increase with water operating velocity in the pipe system. The permeability of this concrete is very high and therefore the leaching rates are also high.

The concrete mixture of the LHWP linings of the delivery tunnels contains fly-ash as a cement extender. Two different Ca-ion leaching rates for a specific water operating velocity were found for the LHWP concrete. It indicates that as a result of the fly-ash content in the mixture, decalcification of the C-S-H gel takes place at a low rate and Ca(OH)2 leaching at a higher rate. The theoretical concrete deterioration depth rates for the LHWP concrete are much lower than the rates for the Theewaterskloof Tunnel. There are not enough data available at this stage, but it seems that the concrete deterioration rate increases with an increase of water operating velocity.

Although the soft water attack cannot be stopped with a specific concrete mix design it is possible to decrease it significantly with an optimum design. The optimum concrete mixture as designed by Castro & Mcintosh (1994) for the LHWP tunnels will still be the best mixture for resistance to soft water attack because the Ca(OH)2 content is very low in this design mix.

It is recommended that the design guidelines (Chapter 10) are followed during the design of a tunnel. Even more important however, is that the tunnel should be operated according to the design.

Attention to the following chemical aspects regarding soft water corrosion is necessary: