Many dirts can turn out debatable in geotechnical technology since they can spread out, prostration, undergo inordinate colony, have a distinguishable deficiency of strength or be caustic. Therefore different dirts have different failings and do different jobs this jobs can run from a little cleft in the wall to a swallow hole that destroys a town. During the viability analysis and planning phases of undertakings that involves substructure, it is of import to place debatable dirts since this could salvage costs and/or redesign of the undertaking subsequently on. If it is noted before the undertaking is started the undertaking can be relocated or the dirt adjusted to run into the undertakings demands.
A part of the Gautrain rail, about 16 kilometers, from Pretoria to Centurion crossbeam on dolomitic evidences. Of this 16 kilometer about 5.8 kilometers of the rail were constructed on viaduct with the staying part straight on land degree. It is known that the building on the job dirt dolomite is hard. When a development is undertaken on dolomite it requires particular probes that are conducted by specializer in the probe of dolomitic terrain. Developed countries such as Gauteng have high degrees of urbanization. The building on dolomite in these countries poses a possible hazard to the safety of many people and the constructions in which they work and live.
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In this study the geology of dolomite, were it can be found, why the dirt is considered debatable every bit good as the solutions and betterments that can be done to be able to construct on dolomite will be discussed.
2. Location and distribution
Detailed dirt maps would be a first pick of information beginning in a civil building undertaking when information on the dirt type is needed. But with the exclusion of certain metropolitan countries of the Western Cape and the Gauteng Provence, elaborate dirt maps are non frequently available ( P Page-Green, 2008 ) . A combination of facets such as topography, clime and the dirt form are the basic basicss of South African dirt maps.
There are two major dolomite happenings in South Africa viz. , 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 alone jobs. These dirts are best identified from standard geological maps. When building on dolomite it is important that the extent of the job is identified good in progress therefore the usage of dirt maps during building is usually excess. It is non ever easy to observe dolomitic dirts since it is non usually straight exposed to the surface. Approximately talking about 25 % of the Gauteng state, and parts of Mpumalanga, Limpopo and the Northern Province are underlain by dolomite. These countries can be seen on the geological maps below were the bluish 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 country between centurion and Pretoria were the rail of the Gautrain was constructed is underlain by dolomite. The set of dolomite environments the flinty dome of Johannesburg
Figure: Geological map of the country environing the Gautrain site ( Gautrain,2009 )
3. Geology of Dolomite
Ancient carbonate stones contains preponderantly two minerals viz. calcite ( CaCO3 ) or dolomite ( CaMg ( CO3 ) 2 ) .A A carbonate stone 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 stone ) ( Warren, 2000 ) . When dolomite is in a stone formation it contains more than 90 % dolomite with the staying part being calcite, detrital minerals and chert. Very few sedimentary dolomites are purely stoichiometric, i.e. CaMg ( CO3 ) 2, and can be better represented as: Ca ( 1+x ) Mg ( 1-x ) ( CO3 ) 2, by embracing the scope from calcian to magnesian dolomites ( Warren, 2000 ) .
Dolomite is one of the 8 major job dirts ( Expansive, Dispersive, Collapsible, Saline, Acid sulphate incorporating stuff, Compressive, dolomitic, and soils prone to liquefaction ) found in South Africa ( P Page-Green, 2008 ) . Dolomite which is a stone incorporating calcium-magnesium carbonates have a typical elephant tegument texture when weathered by even somewhat acidic H2O.
Figure: Elephant tegument weathering of dolomite ( Council for Geocience, 2008 )
This weathering occurs when H2O takes up C dioxide from either the ambiance or the dirt to for a weak carbonaceous acid. It takes up the most carbon dioxide from the dirt since it contains 90 % more than the atmosfhere. Dolomite has a higher solubility that other stones with the important solution observed in months or old ages since the disintegration processes is easy in somewhat acidic H2O. Elephant skin weathering of Dolomite
This procedure may be represented with the undermentioned chemical science equation:
CaMg ( CO3 ) 2 + 2 H2CO3A A a†’A Ca ( HCO3 ) 2 + Mg ( HCO3 ) 2
The disintegration procedure therefore leads to the formation of belowground caves and or pits. After this weathering procedure has taken topographic point the formation of ‘Wad ‘ , a complex residuary dirt mantle occurs which so overlays the dolomite bedrock. The known features for this weak Wad stuff are low denseness, extremely erodible and extremely compressible. These features of the dirt make it unsuitable for foundation edifice on top of it. Within this Wad bed really hard chert can be found runing from 7mm to 1m in deepness. This chert ( silicon oxide ) forms sets that are discontinuous and since it is found in the soft Wad it is unsuitable to back up a foundation.
Within the dirt strata scoffers or otherwise known as bowlders of solid stone are present. These scoffers are formed due to pinnacles that have either fallen or have been undermined. The musca volitanss are surrounded by dirt doing building on it risky since the size of the musca volitans and the strength of the dirt undelaying it is non known.
The bedrock of dolomitic strata consists of a series of stone pinnacles. These pinnacles are usually between 10 – 20 m in length. In boreholes drilled merely 10 thousand apart the bedrock deepness can differ by 30m or more. The deepness of the solid bedrock from land degree can change from a few metres to depths that are greater than 100 metres. In the instance of the Gautrain the solid bedrock was found 30 metres below the land surface at some of the sites.
It is non easy to find where the bedrock is. Therefore specialized borings ( review holes ) need to be made to find the location of the solid bedrock. These holes are drilled to guarantee that a foundation is non construct on a musca volitans or on the difficult chert bed.
4. Why the dirt is debatable
Two of the city manager jobs associated with dolomite are the formation of swallow holes and dolines.
As the dolomite dissolute pits form which leads to the formation of clefts in the signifier of an arch. These clefts get wider and longer as the dirt is eroded and the pits get bigger. When the implicit in dirt is triggered in the center by a distressing agent or the pits get large plenty a swallow hole is formed. With little swallow holes the cross-section resembles a constriction as dirt falls through a pit. Sinkholes can happen all of a sudden or over clip and signifiers a hole runing in sizes. The swallow hole can be classified in footings of its size as proposed by Buttrick and Van Schalkwyk, as shown in the tabular array below.
Maximal diameter of surface manifestation ( m )
& lt ; 2
Small swallow hole
2 – 5
Medium-sized swallow hole
5 – 15
Large swallow hole
& gt ; 15
Very big swallow hole
Table 1. Suggested categorization of swallow holes in footings of size
( Buttrick & A ; Van Schalkwyk, 1995 )
Figure: the formation of a swallow hole
The formation of swallow holes can straight be linked to the alterations in the H2O tabular array. Almost all sinkhole formations are due to human activities. These activities include the dewatering due to mines, leaking public-service corporation services and abstraction of land H2O. Sinkholes can be black and can take to loss of belongings or unrecorded as noted in the yesteryear.
Dolines can be described as an enclosed depression. Dolines form as a consequence of the compaction of the dolomite remainder at certain deepnesss. There are two chief types of dolines viz. dewatering type and impregnation type. There is another type of doline that is referred to as a partly developed swallow hole which is caused by the eroding of the subsurface stuffs ( Council for Geocience, 2008 ) .
A dewatering-type doline occurs bit by bit till it forms a big enclosed depression at the terminal of the procedure. The mechanism behind the formation of this type of doline can be summarised as follow:
Within the dolomite stone profile there is a zone that is profoundly weather-beaten which is filled with potentially extremely compressible stuff. A portion of this stuff is normally submerged below the bing groundwater degree.
When the groundwater degree falls quickly the antecedently submerged and unconsolidated dirt is exposed which consequences in a lessening of the pore H2O force per unit area.
The thick bed of wad that is exposed by the lowered H2O tabular array may do inordinate compaction and rapid surface colony.
A dip otherwise known as a depression of the surface is caused by the colony.
Due to regardful motion surface tenseness clefts occur in the encompassing country.
Surface Saturation-type Doline
Surface impregnation type dolines are normally less than 5m in diameter therefore comparatively little. The mechanism behind the formation of this type of doline can be summarised as follow:
Occurs in state of affairss where compressible dolomitic stuff carpet pad an country at comparatively shallow deepnesss with the land H2O tabular array either within or below the compressible stuff. Changing deepnesss of the land H2O tabular array does non act upon the land surface motion.
The stuffs at the surface are non saturated by the land H2O tabular array but due to for case hapless drainage or a leaking pipe services.
The H2O penetrates the surface and continues till it reaches the low denseness stuff.
The deeper low denseness stuffs settle into a denser province since it is saturated. This causes a surface depression due to the increasing burden on the close surface stuffs.
When the cause of the soaking is stopped the motion will quickly diminish in general.
The size of the depression is determined by the saturated profile underlying the country. The factors include the thickness, the deepness the low denseness stuff is present, the constellation, and the extent of the impregnation and besides the location of the bedrock dolomite.
Partially developed swallow holes
When the subsurface eroding due to the ingress H2O is terminated it may besides ensue in colony of the surface which can look to be similar to a doline.
5. Solutions and dirt betterments when dirt is present
There are many ways to build foundations to do it executable to build on dolomite. Some of these methods include:
Hemorrhoids are constructed out of round concrete signifiers that are reinforced and socketed into the difficult dolomitic bedrock. The building of piled foundations into stone is non normally favoured in dolomitic or karst conditions. This is because of the serious installing restraints refering the presence of the chert bands, stone musca volitanss and besides due to the nature of the bedrock that forms pinnacles. Where infinite is a restraining factor, for case when there is a demand to construct close to roads or major services, it is considered to utilize heap to sway building. ( Gautrain,2009 )
5.2 Raft Foundations
Unlike hemorrhoids that sits straight on the bedrock, raft foundations are fundamentally big tablet termss that ‘floats ‘ in the dirt mass. As discussed below the dirt mom on which the raft is constructed are normally pre-treated to better its denseness and strength by agencies of land betterments. Another manner is to stack the tonss itself by widening down to a more competent established skyline.
There are different raft foundation options available that can be considered viz. :
Raft that spans between pinnacles with the possibility of concrete fillings between the pinnacles ;
When the bedrock is less than 15m below the land and the nothingnesss and pits are grouted to cut down happening of swallow holes, dirt betterments can be done and the raft placed upon it ;
Or the raft can be placed on unimproved dirt but still with the nothingnesss and the pits grouted to cut down the happening of a swallow hole.
Methods on how the dirt can be improved:
The conventional method by doing usage of mechanical roller compression.
Dynamic compression can be done by doing usage of a Crane to raise and drop intent made steal pounders on the dirt.
Another method is preloading the dirt with an extra burden by doing usage of concrete blocks. This extra burden about the same as those that would be imposed by existent viaduct foundations. Normally about 1000 concrete blocks that are specially manufactured for this intent and that separately weighs 10 dozenss are used.
5.3 Piled raft foundation
The piled raft is a geotechnical composite building consisting of the three elements hemorrhoids, raft and dirt which is largely applied for the foundation of tall edifices in an increasing figure. The foundation construct of piled tonss differs from traditional foundation design, where the tonss are assumed to be carried either by the raft or by the hemorrhoids, sing the safety factors in each instance. The method used in this undertaking was conducted by foremost pre-loading a 20m ten 20m country, were the construction will be placed on, by utilizing concrete blocks. Thereafter the substrata within the 20m ten 20m column, that was constructed, are improved by grouting. This is done to cut down the bing nothingnesss and pits present that can take to sinkhole formations. After finishing the grouting works, the hemorrhoids are so installed within these grouted columns. Finally concrete raft besides referred to as a pile cap were so casted over the freshly constructed heap. ( Gautrain,2009 )
5.4 Solution used in the undertaking and interesting facts
Since the traffic could non be interrupted during the building over the 14/Jean Avenue and N1/John Vorster Drive interchanges in Centurion advanced methods was used. After all the geological probes were done and bedrock deepness was determined the concrete shafts or hemorrhoids where constructed.
The shaft foundations were about 7 m in diameter and on some instances reached deepnesss of 30 thousand deep. After the wharf is eventually secured the arrangement of the viaducts can get down. There were made usage of a cast-in-place method by building the viaducts form both waies. The subdivisions are constructed so that the span of the viaducts will be post tensioned. To carry on this method a sophisticated hydraulicly controlled concrete shutter was used.
The viaducts were placed at an angle so that the rail section crosses the intersection diagonally. This asked for advanced technology as the pears needed to be shaped elliptically to be slender adequate to suit in the confined infinite available but strong plenty to back up the superstructure of the viaducts. Since the design is that of an arch the mid-spans are shallower that the sections that rests on the wharfs this gives the viaducts a graceful curve.
To guarantee that high strength concrete was readily available batching workss 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 devastation of dolomite is non all bad. It besides provides tourist attractive forces such as the Cradle of human sort and the Sudwala Caves.
On December 2 1999 The Cradle of Humankind which consists of severalA 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 fossilized remains of prehistoric signifiers of animate beings, workss and most significantly, hominids. This declared country is 47 000 hectares and extends approximately 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 dodos. These dolomite caves started out as coral reefs turning in a worm shoal sea about 2.3 billion old ages ago. Presently there are over 200 caves in entire on the site with a possibility for more to be discovered.
The Sudwala Caves contain the largest dolomite chamber in the universe viz. the Owen Hall. The caves have a chamber which is a of course formed amphitheater of about 37m in tallness and 70m in diameter. The caves have a floor surface of 14,000 M2 over a distance of approximately 600m that are unfastened to the populace. The tallest stalagmite in the caves is about 11m in length. The H2O tabular array fluctuated as a consequence to the alterations in the topography and clime. This caused acidic H2O to ooze through the clefts into the dolomite therefore easy but certainly dissolved the dolomitic stone. As a consequence a series of belowground Chamberss finally formed were the dolomite have bean dissoluted and the stone carried off in solution by the H2O oozing out, or where it on occasion found an flight path and flowed off. Thus the Sudwala Caves was formed and it is believed that the caves are much larger and that some of the Chamberss are still to be discovered.