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Geotechnical study of an engineering project: A geomatic approach

Dr. Jayant Sinha1 & Sri Sahendra Singh2
Department of Geology, St. Xavier's College,Ranchi
Tel: 91-22-8291320/0319/0321  Fax:91-22-8290585
1jayant_s_99@yahoo.com, 2sahendrasingh@eudoramail.com

The Stability, durability and cost effectiveness of engineering projects such as Dams, Tunnels, Bridges, Roads, hydal power sites, Scientific and Strategic establishments and buildings are invariably related to the terrain geomorphology as well as subsurface geological parameters. It is therefore essential that the person concerned with the construction work should have beforehand knowledge of the strata through which construction works are to be carried out. Proper investigation, interpretation and analyses of geological parameters of the construction site holds the key for the success of an engineering structure in terms of economy and its durability.

India is one of the few countries in the world that appreciated the value of applying geological principles to engineering problems as early as in the second half of the 19th century. In the case of applications of geology to the selection of dam sites, this happened about fifty years earlier than in western nations (Reddy, 1995).

Thomas Oldham (1852) was the first to analyse the geological factors influencing the choice of the proposed railway alignment between Calcutta and Patna, a distance of 500 Kms. Thomas Holland, in 1884, analysed the causes of the land slides in the Birehi Ganga valley, Himachal Pradesh.

Major engineering projects for water storage and utilizations were constructed in India in the 19th century and geological principles were taken into account by the construction engineers( Balasundaram and Rao, 1972).

The geotechnical knowledge played a key role in the successful completion of major engineering projects in India. Bhakra Nangal Dam is a very good Indian example in which, geological considerations at the time of construction were taken care off, which played a vital role in its stability and durability. Bhakra Nangal Dam is a straight high gravity Dam founded on the soft rocks and medium hard rocks belonging to Siwalik supergroup, with fault zones cutting across the foundations and abutment in different attitudes ( Krishnaswami, 1982). The preventive measures carried out comprised excavation of the heel clay stones and excavations of the spillway apron, which was tied down to the sandstone member overlying the downstream claystone band to prevent erosion. Beside these special treatments, the cross shear zones on the abutments were treated by providing concrete tunnel plugs and the entire foundation area of the Dam was grouted. Grouting and drainage curtains were further provided from foundation galleries in the Dam and through drainage and grouting, tunnels were provided in the abutments. No major problems have been faced during the post construction period of the Dam, except for cracks in the upstream (Reddy, 1995).

There are cases in which the post failure studies have revealed that ignorance of geological conditions that existed in and around these structures, were basically responsible for such failures. Dams constructed without taking geological factors of the region into account have resulted into their failure, leading to large-scale damage. Austin Dam ( Texas, U S A) is a classical example of Dam failure due to ignorance of geological factors at the time of its construction in 1893. The foundation of the Dam was cretaceous limestone, shale and clay, which were highly jointed and faulted. Due to heavy rainfall in the catchment region, flood overtopped the Dam and the central portion collapsed completely on 7th April 1990. This resulted in the adjacent blocks, about 75 meter long being carried along the water flood about 18 meter downstream. The Austin Dam failure was mainly due to construction of the Dam on porous sedimentary rock ( Reddy, 1995).

So far geological investigation of the site using traditional methodology have not yielded better results. Many projects could not stand the test of time either because proper geological considerations were not taken care off or due to faulty geological investigation. Now days, geological considerations are given due recognition in the construction of engineering projects. The traditional methodology alone is no longer followed, as the chances of error is just too much in them. The analysis of geological parameters has not only become fast but also highly accurate in modern time involving use of recent techniques.

Modern investigation techniques, such as GPS, GIS and Remote Sensing techniques can provide fast and desired accuracy to surface and subsurface parameters. Global Positioning Systems are capable of recording positions to a high level of accuracy. GPS positioning system is probably the most effective general-purpose tool created to date for the determination of movement, spatial specifications, track history and time. GPS system as a whole can be divided into three segments:
  • Space segment: constellation of satellites
  • User segment: GPS receiver, hand held or vehicle installed
  • Controll segments: unmanned ground station to monitor the satellites
The concept behind GPS is the distance measurements between the receivers and satellites by the method of space resections. Differential GPS is required to obtain the accuracy required by many GIS data capture applications in terms of subcentimeter scale. DGPS requires the use of a base station at a known location to remove systematic error from the GPS signals. Recent GPS / GIS data collection devises have the capability to collect a wide range of GIS attribute data in addition to position of interest such as points, lines and area features.

Taking into consideration, the inhospitable terrain, where most of the engineering constructions are carried out, the use of GPS supplemented by GIS and remote sensing techniques by a geologist, is highly recommended. GPS and remote sensing techniques are very useful tools for data capture. GPS in fact is highly useful to cross check the data collected by a satellite imagery analyst and the surveyor.

GIS technique is basically a data analysis and display tool. GIS is a computer based system, which is an exceedingly useful tool to analyze the geomorphological features present on the surface of the earth and various spatial changes taking place on it with respect to time. In major engineering projects, such as dams, bridges, tunnels, high relief roads, data collection is a very difficult job, due to the inhospitable environment in most of the construction sites. Hence, taking into account such hazards or any unforeseen circumstance, GPS and remote sensing technologies will provide not only fast and accurate but also reliable geomorphological and geological data, which can be very successfully analysed by GIS, to give complete picture about the stability and durability of the structure.

The advantages of GPS/remote sensing techniques/GIS can be summarized as below:
  • Multifunctional: apart from the location data collected by GPS, the other attributes, which can be recorded, are:
  • traverse distance
  • bearing of a station
  • track history
  • high precision clock
  • in built memory for data record
  • net displacement
  • datum option
  • Operational simplicity
  • Portability
  • Compatibility with modern electronic gadgets
GPS used in association with GIS may help on-site data capture, its analysis and its output. GPS with hand-held range finder, especially in case of inaccessible. Verification of location data collected from satellite imagery by GPS

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