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Geotechnical study of an engineering project: A geomatic approach
Lithological mapping through thermal infra red (TIR) and microwave data
Sub-surface investigations for geological data has got a boost up in the light of use of electromagnetic radiation in the micro wavelength region ( 1 mm - 0.8 m) and thermal infra red (1 micro m - 1 mm.) region. Microwave remote sensing has a unique advantage of penetrations into the sub-surface and it is able to provide information on soil moisture, snow cover, surface condition below vegetative cover. Here we have two major types of sensor :
- Active microwave remote sensor
- Passive microwave remote sensor
Active microwave remote sensor illuminates the ground with microwave radiation and then receives the back-scattered energy from the object. Some of the active microwave remote sensors are :
- Side looking airborne radar (SLAR)
- Synthetic aperture radar (SAR)
- Wind scatterometer
- Altimeter
- Polarimeter
- Rain mapping radar
Passive sensors are called microwave radiometers, which receive and detect the radiation emitted from various objects on the earth.
In the TIR region the thermal and reflected properties vary and their intensity changes with the change in lattice structure and mineral compositions. Moreover, lithological mapping of an area can be carried out by the measurement of apparent thermal inertia (ATI) of a rock, which is different for different litho types. The ATI signature of a rock is especially affective in the nighttime as the effect of solar radiation is minimum during nighttime. For example, limestone and dolomite delineation is difficult in visible range of electromagnetic radiation, but dolomite having higher thermal inertia compared to limestone can be distinguished by nighttime thermal image. Different types of geological structures such as anticline and syncline also show warmer and cooler signatures. Similarly, structural discontinuity such as fault and fracture also show cool linear anomalies in both day and night time, hence making their delineation possible.
Hydrogeological parameter of the site
Investigation of this parameter is very important for the successful completion of any project. They decide the stability of the foundation of any structure. Generally preparing hydrogeomorphological map carries out such types of investigations. Hydrogeomorphological investigation comprises of following studies:
- Ground water table and their fluctuations
- Aquifer characteristics of the site
- Geochemical analyses of Water sample
- Informations regarding precipitation and surface discharge
- Rock characteristics of the recharge area
Remote sensing data can provide quick and useful baseline information on the factors controlling the occurrence and movements of ground water.
Geophysical investigation of the site
Geophysical surveys form an integral part of the site investigation for any project. This mainly includes Seismic and Electrical resistivity surveys. Seismic background of the area must be kept in mind at the planning stage itself, as it not only determines the stability, but also the design of the structure. Seismic maps can well be crosschecked with GPS, thereby minimizing any error in location of most suitable site for the construction.
Environmental considerations
Geotechnologists have always shown concern towards the environment, due to various mining and ongoing engineering projects. Environmental impact assessment needs to be given due recognition in the planning stage of any project. Latest Geomatic tools can be of great help in deciphering the impact on environment in post-construction phase.
Conclusion
The success of any engineering project, no doubt, depends on its location, design, construction, operation and maintenance, but consideration to the geological aspects of the project site should be at the forefront of the planning stage. There are many cases of structural failures, where geological parameters were overlooked at the time of construction. Latest geomatic tools to get fast, accurate and reliable information should supplement the study of geological factors. These type of investigations have been carried out using IRS- 1B data on Nayar river in Pauri district of UP Himalaya and Mahadevi river in Belgaun district of Karnataka.
Reference
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- Hukku B M (1975), Bhakra Nangal project, Punjab, G S I, Engineering Geology case studies, Misc Pub. Nos. 29, pp. 11 - 13.
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