Posted on Mar 5, 2009 in General
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. The viaducts are constructed in one of two ways, by installing pre-cast segments using underslung girders, or by means of the cast-in-place balanced cantilever method.1 In the Centurion area, the viaduct spans range from 40m to over 120m. This implies that foundation loads are extremely heavy.
The challenge to construction
The dolomite areas traversed by the Gautrain are characterized by extremely difficult founding conditions including sub-surface bedrock pinnacles, the presence of highly compressible wad material as well as hard rock “floaters” and extremely strong chert layers.
The dolomites are also 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.2
Designing and installing heavy foundations in these conditions has been a unique challenge for the project team and Bombela mobilized a team of international and South African experts in the fields of geology, ground investigation, foundation design and construction, in order to accomplish this.
As a result of the extensive ground investigations3, laboratory testing and full scale trials carried out, a suite of foundation solutions has been designed and validated. This Factsheet describes the foundation solutions developed for the viaduct structures in the Centurion area.
Various foundation types are utilised over the dolomite section depending on the local geological conditions encountered:
- Where bedrock occurs at shallow depth, conventional pad footings on dolomite bedrock/pinnacles with the use of a specially constructed mass concrete “mattress” are considered. Foundation proofing (by means of small diameter probe drilling) is required to ensure that founding takes place on rooted bedrock in all instances.
- In areas of deeper bedrock (either above or below the water table) the following solutions are considered:
- Viaduct pier founded on a raft bearing onto the in situ soil, with grouting of voids and cavities to reduce the risk of sinkhole occurrence to an acceptable level.
- Viaduct pier founded on a raft located on a dynamically compacted soil mattress overlying lower dolomite residuum improved by means of grouting of voids and cavities to reduce the risk of sinkhole occurrence to an acceptable level.
- Viaduct pier founded on a piled raft bearing onto lower lying dolomite residuum with grouting of voids and cavities to reduce the risk of sinkhole occurrence to an acceptable level.
- End-bearing piles socketed into hard dolomite bedrock.
- Large diameter shaft socketed into hard dolomite bedrock.
So-called raft foundations are essentially large pad footings “floating” in a soil mass (as opposed to sitting on bedrock). Often the soil mass is subjected to some form of pre-treatment to improve its density and strength (see below) or the raft may in fact itself be piled with the piles extending down to a more competent founding horizon. Different raft foundations options are considered as follows:
- Raft spanning between pinnacles (possibly with concrete infilling between pinnacles);
- Raft on improved soil (bedrock less than 15 m below ground) and grouting of voids and cavities to reduce the risk of sinkhole formation;
- Raft (with or without soil improvement) and grouting of voids and cavities to reduce the risk of sinkhole formation;
Methods of soil improvement applied include:
- Conventional mechanical roller compaction,
- Dynamic compaction. Compaction of the ground by repeated lifting and dropping of purpose made steel pounders by means of a crane.
- Preloading. Preloading is achieved by applying a surcharge load on the soil using concrete blocks. The surcharge loads are similar to those that would be imposed by an actual viaduct foundation. The surcharge typically comprises 1000 concrete blocks each weighing about 10 tonnes and specially manufactured for the purpose.
Piled raft foundations
This method initially consists of pre-loading a 20m x 20m plan area using the concrete blocks described above. The substrata within the 20m x 20m column are then improved by grouting the existing voids and cavities to reduce the risk of sinkhole formation. On completion of the grouting, piles are installed within the grouted column. The concrete raft (i.e. pile cap) is then cast over the piles.
This type of foundation consists of circular reinforced concrete piles socketed into the hard dolomite bedrock. Piled foundations to rock are generally not favoured in dolomitic conditions due to serious installation constraints relating to the presence of chert bands and hard rock “floaters” within the dolomite residuum and also due to the depth and pinnacled nature of the bedrock. Nonetheless, in areas where space is a significant constraint (e.g. road intersections, close to roads or other major services) the use of piles to rock is considered.
In several instances on the balanced cantilever viaducts (where the foundation loads are higher due to their greater spans) 7m diameter shafts have been excavated down to bedrock. These are concrete lined and socketed into the hard dolomite bedrock up to 30m below surface.
- See associated Factsheets
- The geology of the dolomites traversed by the Gautrain is described in greater detail in an associated Factsheet.
- The geotechnical investigation of the dolomites traversed by the Gautrain is described in greater detail in an associated Factsheet.