The Geotechnical Engineering On Soil Engineering Essay

The Geotechnical Engineering On Soil Engineering screenMany undercoats nookie prove punkeratic in geotechnical engineering since they abide expand, collapse, undergo excessive settlement, energise a distinct lack of potency or be corrosive. Thus resistent stigmas have different weaknesses and scram different problems this problems can range from a small crack in the wall to a sump that destroys a town. During the viability analysis and planning stages of disc everyplaces that involves infrastructure, it is serious to identify problematic disgraces since this could save costs and/or redesign of the take in later on. If it is noted before the project is started the project can be relocated or the soil ad plainlyed to meet the projects demands.A piece of land of the Gautrain rail, approximately 16 km, from Pretoria to Centurion traverse on dolomitic intellects. Of this 16 km approximately 5.8 km of the rail were constructed on viaduct with the remaining p brokensh be direct on footing aim. It is cognise that the construction on the problem soil bitter spar is difficult. When a development is undertaken on bitter spar it requires spargon investigations that argon conducted by specialist in the investigation of dolomitic terrain. unquestionable atomic number 18as such(prenominal) as Gauteng have high levels of urbanisation. The construction on dolomite in these areas poses a potential risk to the safety of many race and the structures in which they work and live.In this report the geology of dolomite, were it can be found, wherefore the soil is considered problematic as well up as the solutions and improvements that can be by dint of with(p) to be able to build on dolomite lead be discussed.2. Location and distributionDetailed soil maps would be a first choice of in arrangement source in a courteous construction project when in physical composition on the soil type is needed. only if with the exception of certain metropolitan a reas 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 expireography, climate and the soil pattern are the basic basic principle of southern closely African soil maps. at that place are two major(ip) dolomite occurrences in entropy 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 geologic maps. When constructing on dolomite it is crucial that the extent of the problem is identified well in advance thus the design of soil maps during construction is ordinarily redundant. It is not always easy to detect dolomitic soils since it is not usually directly exposed to the push through. Roughly speaking about 25% of the Gauteng province, and move of Mpumalanga, Limpopo and the Northern Province are un derlain by dolomite. These areas can be seen on the geological maps to a lower place were the blue triggers are the dolomite.http//t3.gstatic.com/images?q=tbnANd9GcSGUgF0OReutcRPt8uC2klISB-nMc-Adm_G0YlBqowa1WYU14FPZQhttp//t3.gstatic.com/images?q=tbnANd9GcSGUgF0OReutcRPt8uC2klISB-nMc-Adm_G0YlBqowa1WYU14FPZQ realize dissemination of dolomite in Gauteng take in Distribution of dolomite in South Africa (Council for Geocience, 2008)On the map bellow it can be seen that the area betwixt centurion and Pretoria were the rail of the Gautrain was constructed is underlain by dolomite. The band of dolomite surrounds the granitic c everyplaceed stadium of JohannesburgFigure Geological map of the area ring the Gautrain site (Gautrain,2009)3. Geology of dolomiteAncient 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 little than 5% dolomite), when i t 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 grate texture when weathered by even slightly acidic pissing.Figure Elephant come up weathering of dolomite (Council for Geocience, 2008)This weathering occurs when water takes up carbon dioxide from either the line or the soil t o 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 high 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 DolomiteThis process may be represented with the following interpersonal chemistry equationCaMg(CO3)2 + 2 H2CO3 Ca(HCO3)2 + Mg(HCO3)2The dissolution process thus leads to the formation of metro caves and or cavities. by and by 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, super erodible and highly compressed. These characteristics of the soil commence it unsuitable for tail building on top of it. inwardly this Wad layer very difficult 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 wanton Wad it is unsuitable to support a foundation.Within the soil strata flouters or other known as boulders of solid rock are present. These flouters are organise overdue 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 amid 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 ascorbic acid meters. In the case of the Gautrain the solid bedrock was found 30 meters on a lower floor the ground appear at some of the sites.It is not easy to delimit where the bedrock is. Th us specialised drillings (inspection holes) need to be made to set 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 problematicTwo of the mayor problems associated with dolomite are the formation of swallow holes and dolines.4.1 SinkholesAs 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 scoured and the cavities get bigger. When the underlying soil is triggered in the middle by a disturbing agent or the cavities get big equal a swallow hole is formed. With small sinkholes the cross-section resembles a bottleneck as soil locomote 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 wagon train Schalkwyk, as shown in the table below.Maximum diameter of surface materialization (m)TerminologySmall sinkhole2 5Medium-size sinkhole5 15Large sinkhole 15Very large sinkholeTable 1. Suggested classification of sinkholes in terms of size(Buttrick Van Schalkwyk, 1995)Figure the formation of a sinkholeThe formation of sinkholes can directly be linked to the changes in the water table. Almost all sinkhole formations are due to sympathetic activities. These activities include the dewatering due to mines, leaking utility services and abstract of ground water. Sinkholes can be disastrous and can lead to prejudice of property or live as noted in the past.4.2 DolinesDolines can be described as an enclosed depression. Dolines form as a result of the compression of the dolomite residuum at certain depths. at that place 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 fetchd by the erosion of the subsurface materials (Co uncil for Geocience, 2008).Dewatering-type DolineA dewatering-type doline occurs gradually till it forms a large enclosed depression at the end of the process. The mechanism backside the formation of this type of doline can be summarised as followWithin the dolomite rock profile there is a zone that is deep 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 set that is exposed by the lowered water table may ca employ excessive compression and rapid surface settlement.A dip otherwise known as a depression of the surface is caused by the settlement. due(p) to deferential endeavor surface tension cracks occur in the surrounding area.Surface Saturation-type DolineSurface saturation type dolines are usually le ss than 5m in diameter thus relatively small. The mechanism behind the formation of this type of doline can be summarised as followOccurs in situations where compressible dolomitic material underlay an area at relatively shoal depths with the ground water table either within or below the compressible material. Varying depths of the ground water table does not form the ground surface movement.The materials at the surface are not alter by the ground water table but due to for exemplar poor drainage or a leaking pipe services.The water penetrates the surface and continues till it r from each onees 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 incumbrance 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 laid by the saturated profile underlying the area. The factors in clude 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 sinkholesWhen 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 presentThere are many ways to construct foundations to make it feasible to construct on dolomite. Some of these methods include5.1 arrangePiles 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 t here is a need to build close to roads or major services, it is considered to use pile to rock construction. (Gautrain,2009)5.2 Raft FoundationsUnlike piles that sits directly on the bedrock, clutch 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 slews itself by extending down to a more competent established horizon.There are different raft foundation options available that can be considered namelyRaft that spans in the midst of pinnacles with the supposition of concrete fillings between the pinnaclesWhen 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 lay upon itOr 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 improvedThe effected method by making use of mechanical roller compaction. propelling compaction can be done by making use of a crane to lift and drop purpose made take external 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 resembling as those that would be imposed by actual viaduct foundations. Usually about 1000 concrete blocks that are specially manufactured for this purpose and that one by one weighs 10 tons are used.5.3 Piled raft foundationThe piled raft is a geotechnical composite construction consisting of the three elements piles, raft and soil which is mostly employ 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 t he 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. in the end 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 factsSince the traffic could not be interrupted during the construction over the 14/Jean Avenue and N1/John Vorster Drive interchanges in Centurion modern methods was used. After all the geological investigations were done and bedrock depth was resolute 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 slant 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 scene 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 charming curve.To ensure that high strength concrete was readily available batching plants were erected at each site.The length of the viaduc t over the N14/Jean Avenue is 571.5m longa and has 6 spans of which the long-dated is 121m long. The length of the viaduct over the N1/John Vorster Drive is 502.75m long with 6 spans of which the continuing is 109.8m in length.6. Two landmarks caused by dolomiteThe 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(prenominal)strips of dolomite limestone caves and the Fossil Hominid Sites of Sterkfontein, Swartkrans, Kromdraai and Environs, were declared a World hereditary pattern 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 fossil s. These dolomite caves started out as coral reefs growing in a worm shallow ocean 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 serviceman 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 house eventually formed were the dolomite have bean dissoluted and the rock carried away in solution by the water seeping out, or where it now and then found an e scape route and flowed away. Thus the Sudwala Caves was formed and it is believed that the caves are frequently larger and that some of the chambers are still to be discovered.