1. GENERAL

Most failures in natural or man made slopes occur due to particularly weak geological features, or groundwater pressures, or a combination thereof. There is a similar story in regard to construction difficulties and failures in tunnels. To reduce the risk of slope failures it is necessary to reduce groundwater pressures, but in tunnels the issue is to reduce groundwater flows. Pressure and flow are not synonymous, and engineers and geologists frequently confuse these issues. It is quite possible to reduce groundwater pressures to zero, without changing flow quantities and without lowering the water table, simply by changing the direction of flow. Sketch 1 is a classic undergraduate soil mechanics question which illustrates this matter.

Proper design of depressurisation and dewatering systems, and appropriate implementation of grouting systems demands good understanding of geological structures controlling flow, and good understanding of the fundamentals of groundwater flow. With such understanding the design of depressurisation, dewatering and grouting can be taken out of the “black art” box and put in the scientific box, where they belong.

2. DEPRESSURISATION

Depressurisation forms a key part of PSM’s design strategy for open pit slopes. Such work usually involves the installation of near horizontal drain holes installed progressively as the pit is deepened. The Kelian open pit, which is one of the Selected Projects presented in this website is a classic case where wall stability depended critically on proper operation of the depressurisation system. The only time during the life of the pit that a significant slope failure occurred was when the installation of drains was neglected.

Pit slope depressurisation is important in most open pit operations and some particular projects where this issue is carefully managed are:

• Mt Newman, iron ore, WA
• MacCraes, gold, NZ
• Cadia, gold, NSW
• Sunrise Dam, gold, WA

3. GROUTING

Traditionally grouting has seen its widest application for creating cutoffs beneath embankment and concrete dams. The Nakan dam, which is discussed in the Capability Statement on Hydraulic Structures and Water Management, includes lengths of grout curtain designed by PSM.

In recent years much of PSM’s attention in the are of grouting has related to tunnels. This work has involved Ordinary Portland Cement, ultrafine cements, acrylates and polyurethanes.

In 2001 and 2002, PSM was responsible for the design and implementation of the remedial grouting for the Burnley tunnel in Melbourne, a project which reduced tunnel inflows from about 60 lit/sec to ~ 8 lit/sec and involved:

• 12100 holes for ultrafine cement grouting
• 280 000 litres of cement grout
• 58 000 litres of acrylate grout
• 2400 litres of polyurethane grout

PSM’s approach in assessing the requirements for tunnel grouting is set out in the following flow chart.