Many soils can prove problematic in geotechnical engineering since they can expand, collapse, undergo excessive settlement, have a distinct lack of strength or be corrosive. Thus different soils have different weaknesses and cause different problems this problems can range from a small crack in the wall to a sinkhole that destroys a town. During the viability analysis and planning stages of projects that involves infrastructure, it is important to identify problematic soils since this could save costs and/or redesign of the project later on. If it is noted before the project is started the project can be relocated or the soil adjusted to meet the projects demands.
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A portion of the Gautrain rail, approximately 16 km, from Pretoria to Centurion traverse on dolomitic grounds. Of this 16 km about 5.8 km of the rail were constructed on viaduct with the remaining portion directly on ground level. It is known that the construction on the problem soil dolomite is difficult. When a development is undertaken on dolomite it requires special investigations that are conducted by specialist in the investigation of dolomitic terrain. Developed areas such as Gauteng have high levels of urbanisation. The construction on dolomite in these areas poses a potential risk to the safety of many people and the structures in which they work and live.
In this report the geology of dolomite, were it can be found, why the soil is considered problematic as well as the solutions and improvements that can be done to be able to build on dolomite will be discussed.
2. Location and distribution
Detailed soil maps would be a first choice of information source in a civil construction project when information on the soil type is needed. But with the exception of certain metropolitan areas of the Western Cape and the Gauteng Provence, detailed soil maps are not often available (P Page-Green, 2008). A combination of aspects such as topography, climate and the soil pattern are the basic fundamentals of South African soil maps.
There are two major dolomite occurrences in South Africa namely, in the Transvaal Sequence the Chuniespoort Group and in Griqualand west Sequence the Campbell Group (Wagener F von M, 1985). Soils that develop on dolomite have unique problems. These soils are best identified from standard geological maps. When constructing on dolomite it is crucial that the extent of the problem is identified well in advance thus the use of soil maps during construction is normally redundant. It is not always easy to detect dolomitic soils since it is not normally directly exposed to the surface. Roughly speaking about 25% of the Gauteng province, and parts of Mpumalanga, Limpopo and the Northern Province are underlain by dolomite. These areas can be seen on the geological maps below were the blue parts are the dolomite.http://t3.gstatic.com/images?q=tbn:ANd9GcSGUgF0OReutcRPt8uC2klISB-nMc-Adm_G0YlBqowa1WYU14FPZQhttp://t3.gstatic.com/images?q=tbn:ANd9GcSGUgF0OReutcRPt8uC2klISB-nMc-Adm_G0YlBqowa1WYU14FPZQ
Figure : Distribution of dolomite in Gauteng
Figure : Distribution of dolomite in South Africa (Council for Geocience, 2008)
On the map bellow it can be seen that the area between centurion and Pretoria were the rail of the Gautrain was constructed is underlain by dolomite. The band of dolomite surrounds the granitic dome of Johannesburg
Figure : Geological map of the area surrounding the Gautrain site (Gautrain,2009)
3. Geology of Dolomite
Ancient carbonate rocks contains predominantly two minerals namely calcite (CaCO3) or dolomite (CaMg(CO3)2). A carbonate rock is known as limestone if it is dominated by calcite (more than 95% with less than 5% dolomite), when it is dominated by dolomite (the mineral) it is called dolomite (the rock) (Warren, 2000). When dolomite is in a rock formation it contains more than 90% dolomite with the remaining portion being calcite, detrital minerals and chert. Very few sedimentary dolomites are strictly stoichiometric, i.e. CaMg(CO3)2, and can be better represented as: Ca(1+x)Mg(1-x)(CO3)2, by encompassing the range from calcian to magnesian dolomites (Warren, 2000).
Dolomite is one of the 8 major problem soils (Expansive, Dispersive, Collapsible, Saline, Acid sulphate containing material, Compressive, dolomitic, and soils prone to liquefaction) found in South Africa (P Page-Green, 2008). Dolomite which is a rock containing calcium-magnesium carbonates have a distinctive elephant skin texture when weathered by even slightly acidic water.
Figure : Elephant skin weathering of dolomite (Council for Geocience, 2008)
This weathering occurs when water takes up carbon dioxide from either the atmosphere or the soil to for a weak carbonic acid. It takes up the most carbon dioxide from the soil since it contains 90% more than the atmosfhere. Dolomite has a higher solubility that other rocks with the significant solution observed in months or years since the dissolution processes is slowly in slightly acidic water. Elephant skin weathering of Dolomite
This process may be represented with the following chemistry equation:
CaMg(CO3)2 + 2 H2CO3 → Ca(HCO3)2 + Mg(HCO3)2
The dissolution process thus leads to the formation of underground caves and or cavities. After this weathering process has taken place the formation of ‘Wad’, a complex residual soil mantle occurs which then overlays the dolomite bedrock. The known characteristics for this weak Wad material are low density, highly erodible and highly compressible. These characteristics of the soil make it unsuitable for foundation building on top of it. Within this Wad layer very hard chert can be found ranging from 7mm to 1m in depth. This chert (silica) forms bands that are discontinuous and since it is found in the soft Wad it is unsuitable to support a foundation.
Within the soil strata flouters or otherwise known as boulders of solid rock are present. These flouters are formed due to pinnacles that have either fallen or have been undermined. The floaters are surrounded by soil making construction on it hazardous since the size of the floater and the strength of the soil undelaying it is not known.
The bedrock of dolomitic strata consists of a series of rock pinnacles. These pinnacles are normally between 10 – 20 m in length. In boreholes drilled just 10 m apart the bedrock depth can differ by 30m or more. The depth of the solid bedrock from ground level can vary from a few meters to depths that are greater than 100 meters. In the case of the Gautrain the solid bedrock was found 30 meters below the ground surface at some of the sites.
It is not easy to determine where the bedrock is. Thus specialised drillings (inspection holes) need to be made to determine the location of the solid bedrock. These holes are drilled to ensure that a foundation is not build on a floater or on the hard chert layer.
4. Why the soil is problematic
Two of the mayor problems associated with dolomite are the formation of sinkholes and dolines.
4.1 Sinkholes
As the dolomite dissolute cavities form which leads to the formation of cracks in the form of an arch. These cracks get wider and longer as the soil is eroded and the cavities get bigger. When the underlying soil is triggered in the middle by a disturbing agent or the cavities get big enough a sinkhole is formed. With small sinkholes the cross-section resembles a bottleneck as soil falls through a cavity. Sinkholes can occurs suddenly or over time and forms a hole ranging in sizes. The sinkhole can be classified in terms of its size as proposed by Buttrick and Van Schalkwyk, as shown in the table below.
Maximum diameter of surface manifestation (m)
Terminology
< 2
Small sinkhole
2 – 5
Medium-size sinkhole
5 – 15
Large sinkhole
> 15
Very large sinkhole
Table 1. Suggested classification of sinkholes in terms of size
(Buttrick & Van Schalkwyk, 1995)
Figure : the formation of a sinkhole
The formation of sinkholes can directly be linked to the changes in the water table. Almost all sinkhole formations are due to human activities. These activities include the dewatering due to mines, leaking utility services and abstraction of ground water. Sinkholes can be disastrous and can lead to loss of property or live as noted in the past.
4.2 Dolines
Dolines can be described as an enclosed depression. Dolines form as a result of the compression of the dolomite residuum at certain depths. There are two main types of dolines namely dewatering type and saturation type. There is another type of doline that is referred to as a partially developed sinkhole which is caused by the erosion of the subsurface materials (Council for Geocience, 2008).
Dewatering-type Doline
A dewatering-type doline occurs gradually till it forms a large enclosed depression at the end of the process. The mechanism behind the formation of this type of doline can be summarised as follow:
Within the dolomite rock profile there is a zone that is deeply weathered which is filled with potentially highly compressible material. A part of this material is usually submerged below the existing groundwater level.
When the groundwater level falls rapidly the previously submerged and unconsolidated soil is exposed which results in a decrease of the pore water pressure.
The thick layer of wad that is exposed by the lowered water table may cause excessive compression and rapid surface settlement.
A dip otherwise known as a depression of the surface is caused by the settlement.
Due to deferential movement surface tension cracks occur in the surrounding area.
Surface Saturation-type Doline
Surface saturation type dolines are usually less than 5m in diameter thus relatively small. The mechanism behind the formation of this type of doline can be summarised as follow:
Occurs in situations where compressible dolomitic material underlay an area at relatively shallow depths with the ground water table either within or below the compressible material. Varying depths of the ground water table does not influence the ground surface movement.
The materials at the surface are not saturated by the ground water table but due to for instance poor drainage or a leaking pipe services.
The water penetrates the surface and continues till it reaches the low density material.
The deeper low density materials settle into a denser state since it is saturated. This causes a surface depression due to the increasing load on the near surface materials.
When the cause of the drenching is stopped the movement will rapidly decrease in general.
The size of the depression is determined by the saturated profile underlying the area. The factors include the thickness, the depth the low density material is present, the configuration, and the extent of the saturation and also the location of the bedrock dolomite.
Partly developed sinkholes
When the subsurface erosion due to the ingress water is terminated it may also result in settlement of the surface which can appear to be similar to a doline.
5. Solutions and soil improvements when soil is present
There are many ways to construct foundations to make it feasible to construct on dolomite. Some of these methods include:
5.1 Piles
Piles are constructed out of circular concrete forms that are reinforced and socketed into the hard dolomitic bedrock. The construction of piled foundations into rock is not usually favoured in dolomitic or karst conditions. This is because of the serious installation constraints concerning the presence of the chert bands, rock floaters and also due to the nature of the bedrock that forms pinnacles. Where space is a constraining factor, for instance when there is a need to build close to roads or major services, it is considered to use pile to rock construction. (Gautrain,2009)
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5.2 Raft Foundations
Unlike piles that sits directly on the bedrock, raft foundations are basically large pad footings that ‘floats’ in the soil mass. As discussed below the soil mas on which the raft is constructed are usually pre-treated to improve its density and strength by means of ground improvements. Another way is to pile the rafts itself by extending down to a more competent established horizon.
There are different raft foundation options available that can be considered namely:
Raft that spans between pinnacles with the possibility of concrete fillings between the pinnacles;
When the bedrock is less than 15m below the ground and the voids and cavities are grouted to reduce occurrence of sinkholes, soil improvements can be done and the raft placed upon it;
Or the raft can be placed on unimproved soil but still with the voids and the cavities grouted to reduce the occurrence of a sinkhole.
Methods on how the soil can be improved:
The conventional method by making use of mechanical roller compaction.
Dynamic compaction can be done by making use of a crane to lift and drop purpose made steal pounders on the soil.
Another method is preloading the soil with an additional load by making use of concrete blocks. This additional load almost the same as those that would be imposed by actual viaduct foundations. Usually about 1000 concrete blocks that are specially manufactured for this purpose and that individually weighs 10 tons are used.
5.3 Piled raft foundation
The piled raft is a geotechnical composite construction consisting of the three elements piles, raft and soil which is mostly applied for the foundation of tall buildings in an increasing number. The foundation concept of piled rafts differs from traditional foundation design, where the loads are assumed to be carried either by the raft or by the piles, considering the safety factors in each case. The method used in this project was conducted by firstly pre-loading a 20m x 20m area, were the structure will be placed on, by using concrete blocks. Thereafter the substrata within the 20m x 20m column, that was constructed, are improved by grouting. This is done to reduce the existing voids and cavities present that can lead to sinkhole formations. After completing the grouting works, the piles are then installed within these grouted columns. Finally concrete raft also referred to as a pile cap were then casted over the newly constructed pile. (Gautrain,2009)
5.4 Solution used in the project and interesting facts
Since the traffic could not be interrupted during the construction over the 14/Jean Avenue and N1/John Vorster Drive interchanges in Centurion innovative methods was used. After all the geological investigations were done and bedrock depth was determined the concrete shafts or piles where constructed.
The shaft foundations were approximately 7 m in diameter and on some cases reached depths of 30 m deep. After the pier is finally secured the placement of the viaducts can start. There were made use of a cast-in-place method by constructing the viaducts form both directions. The sections are constructed so that the span of the viaducts will be post tensioned. To conduct this method a sophisticated hydraulically controlled concrete shutter was used.
The viaducts were placed at an angle so that the rail segment crosses the intersection diagonally. This asked for innovative engineering as the pears needed to be shaped elliptically to be slender enough to fit in the confined space available but strong enough to support the superstructure of the viaducts. Since the design is that of an arch the mid-spans are shallower that the segments that rests on the piers this gives the viaducts a graceful curve.
To ensure that high strength concrete was readily available batching plants were erected at each site.
The length of the viaduct over the N14/Jean Avenue is 571.5m longa and has 6 spans of which the longest is 121m long. The length of the viaduct over the N1/John Vorster Drive is 502.75m long with 6 spans of which the longest is 109.8m in length.
6. Two landmarks caused by dolomite
The destruction of dolomite is not all bad. It also provides tourist attractions such as the Cradle of human kind and the Sudwala Caves.
On December 2 1999 The Cradle of Humankind which consists of several strips of dolomite limestone caves and the Fossil Hominid Sites of Sterkfontein, Swartkrans, Kromdraai and Environs, were declared a World Heritage Site. It contains the fossilised remains of prehistoric forms of animals, plants and most importantly, hominids. This declared area is 47 000 hectares and extends roughly between Oaktree, Hekpoort, Broederstroom and Lanseria in Gauteng. Most of the site is on dolomite which leads to two major consequences- the formation of caves and the formation of fossils. These dolomite caves started out as coral reefs growing in a worm shallow sea about 2.3 billion years ago. Currently there are over 200 caves in total on the site with a possibility for more to be discovered.
The Sudwala Caves contain the largest dolomite chamber in the world namely the Owen Hall. The caves have a chamber which is a naturally formed amphitheatre of approximately 37m in height and 70m in diameter. The caves have a floor surface of 14,000 m2 over a distance of about 600m that are open to the public. The tallest stalagmite in the caves is about 11m in length. The water table fluctuated as a result to the changes in the topography and climate. This caused acidic water to seep through the cracks into the dolomite thus slowly but surely dissolved the dolomitic rock. As a result a series of underground chambers eventually formed were the dolomite have bean dissoluted and the rock carried away in solution by the water seeping out, or where it occasionally found an escape route and flowed away. Thus the Sudwala Caves was formed and it is believed that the caves are much larger and that some of the chambers are still to be discovered.
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