OVERVIEW OF THE DOLOMITIC GEOLOGY UNDERLYING CENTURION

Posted on Mar 5, 2010 in General

Background

Between Centurion and Pretoria, in the northern section of the Gautrain route, the rail alignment traverses about 16km of dolomitic ground, of which 5.8km will be on viaducts (elevated structures) with the remaining portion constructed at ground level.

This band of dolomites is part of a broad swathe of dolomite bedrock which surrounds the Johannesburg Granite Dome. The dolomites are notorious for the occurrence of sinkholes which can be triggered by changes in the ground water regime and which, in the past, have resulted in sudden and tragic loss of life and property.

Figure 1 : Geological map of the Witwatersrand (Norman & Whitfield)

Weathering processes

Dolomite is a calcium-magnesium carbonate rock with a distinctive “elephant skin” texture. As a result of this chemical composition, the dolomites are susceptible to dissolution resulting from the percolation of rainwater and the flow of sub-surface water. This dissolution results in the formation of underground caves and cavities with well known examples being the Sudwala Caves and Wonder Cave in the Cradle of Humankind.

The long process of solution weathering also results in the formation of a complex residual soil mantle known as “Wad”, overlying the dolomite bedrock. Wad is a very low density, very weak material that is highly erodible and highly compressible and therefore completely unsuitable for foundations of any type.

Interspersed in the wad are layers of very hard chert (silica) in discrete bands ranging from several millimetres to over a metre thick. These bands are discontinuous and, as they are contained within the soft wad, also unsuitable for supporting foundations.

Figure 2 : Typical dolomite cross-section showing voids and wad.

Bedrock topography

The topography of the dolomite bedrock consists of a succession of rock pinnacles. These pinnacles are generally between 10m and 20m tall, but variations in bedrock of up to 30m or more can be found in boreholes drilled 10m apart. The depth of the solid rock can range from a few metres below the ground surface to depths of greater than 100m.

“Floaters”, or very large boulders of solid rock, are also present. These are isolated blocks of detached dolomite (pinnacles that have fallen or have been undermined) that are now surrounded by soil. Constructing a foundation on a floater may be hazardous since, without conducting exploration drilling, there is no way of telling just how big the floater is or the strength of material underlying it.

Figure 3 : Near-surface dolomite pinnacles exposed.

Sinkholes

The presence of cavities or very loose material in the dolomites and overlying sediments can lead to the formation of sinkholes. Sinkholes vary in size and may form gradually or very suddenly. Such events can be disastrous and have, in the past, lead to loss of life and severe damage to property.

Almost all sinkholes are associated with a change in ground water regime. The vast majority of sinkholes are induced by human activity, such as ground water abstraction or leaking utility services. Induced sinkholes account for at least 95% of all recorded events.

The challenge to construction

The requirement to minimize disturbance of the ground water regime was a large driver of the decision not to try to tunnel beneath the city of Centurion but rather to construct the Gautrain through the city on an elevated viaduct structure which would still allow transverse access to pedestrians and vehicles underneath.

Further northwards in the open countryside, where regular across-track access is not required, the Gautrain tracks are built on the ground, either on specially prepared track-beds or on a unique “U-beam” ground bridge designed to span across even relatively severe ground subsidences.

The foundation solutions developed to support the Gautrain over these dolomite areas are discussed in greater detail in associated Factsheets.

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