Importance of the Geology for the Construction of the Highway and Associated Structures - Assignment Example

A) The geology of the area Geology of limestone: Limestone, comparatively a soft stone, is a sedimentary rock made from the mineral calcite. Usually this mineral Calcite (Calcium Carbonate, CaCO3 ) comes from the beds of dried seas and lakes and from sea animal shells. In naturally found Limestone some other substances like quartz, chert, clay, Iron rusts, organics substances, and Dolomite are also found in dissemination, nodules or layers. This rock is used in preparation of concrete and is a superlative building stone for soggy regions. Limestone is the most widely spread carbonate rock in the Earth's crust. It is almost 10% of the total volume of all sedimentary rocks. Along the streams, layer after layer of Calcium Carbonate are cemented with the seasonal evaporation of water. A hard white or light-coloured Limestone can also come into being by precipitation of calcite grains in hot springs and caves. Limestone is widely used in constructions since times immemorial. As a building material Limestone was very popular among the ancient Egyptians and after a long time only, it was replaced by the use of sandstone (Lucas and Harris, 1962). Blocks of Limestone were cut and a great many tombs and temples were constructed of it. Even chambers for tombs were made of limestone, resulting into depletion of hills of limestone. The ancient Egyptians carved it for everything other than tools and weapons. Being a soft stone, it didn’t suit to be a cutting tool. Yet the same ‘softness’ and construction of a fine grain textures, it suits well to carving and other engraving and lapidary techniques. Geology of alluvium Alluvium is what we understand as the common soft silica mud along the river banks. It is young sediment— of all the rock particles, fine particles of silt and clay and larger particles of sand and gravel, freshly eroded by the jumping rivers off the hillside and carried by streams. This image shown above describes the process of formation of Alluvium deposits. This is the Kern River valley in the southern Sierra Nevada, where boulders as large as 1 meter mix with sand and gravel, are seen washed down into a side canyon, forming an alluvial terrace. The Kern has undercut the terrace here and carried away parts of the alluvium deposit, starting a new cycle of transport and deposition somewhere further downstream. (About.com: Geology, Alluvium) Each time it moves downstream, wherever the river overflows its banks or wherever the velocity of a river is checked, the Alluvium is pounded and grounded into finer and finer grains. This is an eternal process and takes thousands of years. Sometimes the entire heap of alluvium is all of gravel size and even smaller. Most of such material finally ends up in the sea, to be slowly buried and turned into new rocks. But the journey from hills to sea bed takes a million years or so. Geology of shale Shale is also a product of moving water. It too is a thinly grained sedimentary rock which breaks up easily. Shale might contain fossils also, particularly of those small floating sea animals that lived in colonies between about 550 million and 325 million years ago (also called Graptolites). Even recent animal tracks/burrows and even raindrop impact craters are sometimes preserved on shale bedding surfaces. Shales may also contain hard solid mass formed by parts uniting into a whole. Typically deposited in very slow moving water, some excellent shale outcrops can be found along the coast around Donaghadee.(Northern Ireland Geology - Shale and Greywaches). The odd name ‘greywacke’ (also called Gritstone) means ‘grey grit’ for rock made of sand grains surrounded by clay particles. It consists of sand grains enclosed in a fine grained matrix. Greywache is a hard rock which is usually used for road stone, as it gives a high level of skid resistance. It’s heat resistant quality makes it favourite for chimneys. Greywaches are formed in deep water, where muddy currents or underwater 'avalanches' of sediments deposit huge quantities of sediments. They accumulate at the base of steep slopes. 'Black shales' are dark in colour and are rich in unoxidized carbon. B) The possible problems that the geology may pose for the construction of the highway and associated structures such as embankments, bridges or cuttings; and Limestone: For heavily loaded structures, the geology and nature of the limestone causes pile foundation problems. The limestone has a number of cavities and pointed peaks, cantilever slabs- like long branches of some tree having only one support base. Any large body of limestone is as hazardous as some mine field, as it includes floating slabs, pockets of soft clay and loose sand. Each of them is capable to cave in any structure, over them. Thus, the underlying limestone bedrock presents formidable challenges to foundation engineers. Irregular and sloping bedrock surfaces pose the problem of insufficient seating and damage to pile tips. Detecting the location and extent of cavities is also very difficult. (D. T. Bergado & A. N. Selvanayagam) However, empirical design methods and local construction techniques have been successfully used to overcome these problems. They are: (i) bridging limestone cavities and slabs by concrete fillings, (ii) putting up small diameter high yield stress piles in large numbers to distribute the loads evenly and thus to withstand high driving stresses, (iii) using both (1) & (2) i.e.filling cavities with concrete, and (iv) using micropiles to redistribute the loads. Alluvium: Limestone poses recurring difficulties for design, construction and maintenance. Primarily because of unseen sinkholes sometimes prompting apprehension about the possible existence of larger, deeper cave systems and cavities which may lack surface expression and that might cause a potential collapse. More so within the areas of highway construction. In such a case, elaborate geological investigations need to be carried out. The character of the foundation conditions will have to be ascertained, w3ith the help of boreholes, geophysics, probe drillholes, trial pits and borehole photography. Subsurface studies can reveal instead dissolution-enlarged joints and fractures forming an integrated network and generally infilled with loose clay. But this may fail to disclose cavernous openings in the limestone. Normally, the glacial sediment cover consists of stiff clays and medium dense sand and gravel, but the disturbance effects may have developed much weaker strata in the vicinity of sinkholes. The risk potential in the highway design and construction is sufficiently high, requiring a number of mitigating measures. These include geological surveillance of earthworks, excavation and infilling of identified sinkholes and geotextile reinforcement of the proposed stretch of highway. Regular monitoring will have to be carried out to check the structural integrity of the carriageway and ensure public safety. (Douglas Nichol) Shale: The shale is the third major problem to be dealt with in the construction of the carriageway, part from limestone and alluvium. It is associated with a big cost as well. When developers are forced to drill deep for a secure foundation, they're also forced to dig deeper into their pockets. If a commercial construction project has to have piers drilled rather than a simple shallow foundation, the developer can expect a 35-50 percent increase in cost of the foundation. A foundation typically constitutes 5-10 percent of the overall cost of the construction. "The culprit in all of this is the Fayetteville shale," says Walter Manger, professor of geology at the University of Arkansas. "Shale expands and contracts, caused from the clay's weathering." In all shale rests at root of construction problems. C) Possible geological sources of materials within the map which could be used in the construction of the highway and its associated structures The possible geological sources in the map that could be used for construction are the same that poses major or minor problems to it. For example, limestone can be quarried for roadbeds, landscapes around the carriageway, and also to cement manufacturing factories. Many architectural landmarks around the world are made of Limestone. The reason behind its popularity is the relative ease with which Limestone can cut into blocks or for more elaborate carving. It is long-lasting durable and stands up well to exposure. However, being a very heavy material, it can not be used in high rising buildings. Though the limestone is used for construction is good for humid climates, it’s chemical composition makes it vulnerable to acids. Calcium Carbonate quickly disintegrates in the contact of acids and even acidic gases, making acid rain a huge problem when it occurs in places where limestone is used extensively. Even regular rain, in which rain drops might have absorbed some Carbon Dioxide from the atmosphere, can cause a slow erosion of the limestone. Similarly shales are very worthwhile. The heat and pressure alters them into a hard, fissile, metamorphic rock known as slate, which can be used in carriageway construction wherever relevant. Reference: 1. Lucas, A. & Harris, J.R. (1962) Ancient Egyptian materials and industries. E. Arnold, London, 523 p. 2. About.com: Geology, Alluvium, Available at http://geology.about.com/library/bl/images/blalluvium.htm, Accessed on February 10 2008 3. Northern Ireland Geology - Shale and Greywaches, Available at http://www.geographyinaction.co.uk/Geology%20files/Shale.html, Accessed on February 10 2008 4. D. T. Bergado & A. N. Selvanayagam, Pile foundation problems in Kuala Lumpur Limestone, Malaysia, Quarterly Journal of Engineering Geology and Hydrogeology; 1987; v. 20; issue.2; p. 159-175. 5. Douglas Nichol, Sinkholes at Glan Llyn on the A55 North Wales Coast Road, UK, Available at http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V63-3VWP1YR-8&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=55716d45911e6703e57dbb35632f19fc, Accessed on February 10 2008 6. Shale, Available at http://en.wikipedia.org/wiki/Shale, Accessed on February 10 2008 Read More