WASTE DUMPS AND HAUL ROADS

1. GENERAL

Effective design, operation and maintenance of waste dumps, tailings disposal areas and haul roads is critical to efficient mining operations. Unfortunately these are facets sometimes given less attention than they deserve, which can lead to expensive failures.

2. WASTE DUMPS

Waste dumps often constitute two quite different issues, namely:

• stability and safe tipping operations, and
• generation of contaminated runoff, particularly acid drainage.

While assessment of waste dump stability involves well established geotechnical design methods, the real difficulty arises in assessing appropriate design shear strength parameters for the waste materials and dump foundations. Two recent examples illustrate these issues.

Ernst Henry Mine, Queensland

The mine is in an area of flat terrain comprising a surface layer of overconsolidated, reactive, fissured clays. The “pancake” dumps are constructed in 15m lifts and the question is: to what height can they be built? The key issue is the shear strength of the fissured clays and to this end, use was made of monitored dump failures on fissured clay foundations published by Blight in 1969. Finite element modelling confirmed simple design equations for use in future dump management.

Griffin Coal Mining Company, WA

The issue at Griffin was that waste dump heights were notionally restricted to 30m by statutory guidelines, and the mine wishes to create a 250m high dump off the haul road and into the pit. The key issue is the shear strength of the waste material, which includes rock fragments of a tonne or more, and is therefore not amenable to normal laboratory testing. There is also the issue of the appropriate density to use for determining design shear strengths.

The procedure adopted has involved the use of laboratory shear strengths of the fines component of the waste couple with back analysis of existing dump performance. To date dump heights have been increased to 90m.

The control of acid mine drainage is a substantial issue in many modern mining operations.

At the Kelian Gold Mine in East Kalimantan, PSM was responsible for designing the Nakan dam which stores 40 million tonnes of potentially acid producing waste rock under water, in perpetuity. The main embankment is 126m high and the saddle dam is 76m high. The facility had to be designed with a variable level decant structure so that the reservoir could be gradually filled with water to cover the rising level of waste.

At the Cadia Mine in NSW, potentially acid generating waste is encapsulated in low permeability clay layers. PSM were responsible for developing method specifications for the clay encapsulation layers and for developing a field measurement procedure for assessing the permeabilities of these layers. This procedure made use of a Guelph Permeameter, which is now operated by the mine on a routine basis.

3. HAUL ROADS

The design and construction of mine haul roads is often left to mine personnel to develop on an ad hoc basis, sometimes successfully and sometimes not.

In late 2001, the Rio Tinto group realised the important ramifications of poor haul road performance. Rio Tinto Technical Services (RTTS) in Melbourne was commissioned to produce design, construction and maintenance guidelines for the worldwide operations. PSM was retained by RTTS to develop materials evaluation and pavement design procedures, and design guidelines for hydraulic structures such as drains and culverts. The project included climatic areas from frozen ground in Canada to swamp areas in Borneo.

The final version of the new Rio Tinto Haul Road Manual was published in early 2003.