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Assessment of Ground Water using GIS and its importance in Trenchless Technology

By
Korukonda Vinay

Abstract
This paper describes the results of a ground water potentiality assessment and its importance in the field of trenchless technology. Water plays a vital role in the development as well as for depletion of any activity in the area . Thus, the availability of surface and ground water governs the process of planning & development of any activity. The surface water resources are inadequate to fulfill the water demand. Productivity through groundwater is quite high as compared to surface water, but groundwater resources have not yet been properly developed through exploration. Keeping this in view, the present study attempts to identify and understand groundwater potential zones of the study area using GIS. The methodology includes the construction of groundwater potentiality map using Geographical Information System (GIS) to protect the groundwater resources in the study area and to formulate recommendations to reduce the water scarcity. The ground water potentiality of the area has been assessed through integration of the relevant layers which include geomorphology, geology, slope and land use/ land cover, in ArcGIS environment. Criteria for GIS analysis have been defined on the basis of ground water conditions and appropriate weightage has been assigned to each information layer according to relative contribution towards the desired output. The ground water potential zones map generated through this model was verified with the yield data to ascertain the validity of the model developed and to find its corresponding influence in the subsurface constructions.
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What are the rationales of Observational Method in geotechnical works?

The idea of Observational Method was first discussed by Peck in the Rankine Lecture in 1969. The Observational Method is commonly adopted in geotechnical works in construction phase, though it is also feasible in design stage.

In essence, in the conforming design by engineers during planning stage, the design is usually based on over-conservative approach or most unfavourable conditions owing to a lack of precise and actual site information. During subsequent construction, with precise site information and condition available the Observational Method is adopted in which the original design is revised based on most probable conditions with instrumentation monitoring. If the monitoring results show that performance of the revised design approaches the limit of acceptable level of risk, then it shall be reverted to planned modification which is based on most unfavourable conditions and hence the level of risk is lowered back to the original design. Otherwise, the revised design shall continue and this results in cost reduction without comprising safety of works.

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However, care should be taken in implementing Observational Method when rapid deterioration of the site may occur so that there is insufficient time for carrying out the planned modification. For instance, rapid deterioration can result from development of high pore water pressure in heavy rainfall or burst watermain.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

What are the measures to reduce the effects of soil liquefaction?

To reduce the effect of soil liquefaction, it is intended to reduce the pore water pressure induced during earthquake shaking. This can be achieved by providing better drainage in soils (e.g. wick drains, sand drains etc. ) and densification of soils (e.g. vibroflotation, dynamic compaction etc.).

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Liquefaction hazards can be reduced by improving the drainage ability of the soil. If the pore water within the soil can drain freely, the build-up of excess pore water pressure would be reduced accordingly.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Does liquefaction occur to sand only?

In liquefaction, the pore water pressure builds up steadily and eventually approaches a value equal to the confining pressure. In an earthquake, however, there is not enough time for the water in the pores of the soil to be squeezed out. Instead, the water is trapped and this avoids the soil particles from moving closer together. Consequently, this results in an increase in water pressure which reduces the contact forces between the individual soil particles, thereby softening and weakening the soil. Eventually, soils particles lose contact with each other and behave like a liquid.

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Hence, the type of soils which is susceptible to liquefaction is the one like sand whose resistance to deformation is mobilized by frictional forces between particles under confining pressure. In case the soil is fine grained, cohesive forces tends to develop between these fine particles and it is difficult to separate them. Therefore, sand with increasing content of fines tends to increase its resistance to liquefaction.

The consequence of liquefaction is that the subsequent settlements after liquefaction may damage the overlying structures. Moreover, for sloping ground lateral flow may result which is undesirable. Liquefaction only occurs to saturated soils.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

Can sheets or geo-grids replace reinforcing elements in soil nails?

Where soil nails are intended for improving the slope stability of existing ground, sheets or geo-grids can hardly replace reinforcing elements in soil nails. Practically speaking, the reinforcing of existing slopes limits the types of reinforcing elements to be adopted. For instance, sheets or geo-grids do not have sufficient bending stiffness to be inserted into exiting slope and they are usually placed in soils as soil layers are built up. The reinforcing element of exiting ground requires steel bars with good tensile strength.

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This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.