GEOTECHNICS & APPLIED GEOLOGY

Prepared by: Group D

The Geological Investigation for a dam site consist of the following stages: 1. Preliminary investigation (Reconnaissance) 2. Detailed investigation

In an area which apparently seems to be suitable for the location of a dam, preliminary investigation are made to ascertain its important merits and demerits. Preliminary investigation is necessary, so that detailed investigations which are much more expensive and laborious should be made only, if the site is approved. the important information collected at this stage is as follows:

Lithology provides the details of rock types occuring that area. The different varieties of rocks present, their nature and extent of weathering, the occurrence of soil, rock debris, etc. in the area are known. This gives a broad idea of the presence or absence of component rocks, the weathering it has undergone and other related information.

This study gives information like,  Strike and dip of beds  Faults, folds  Joints, unconformities  Foliation Details of these features are very important because they have a great influence on the stability of a site for dam construction.

This study gives information about important surface features like,  Valleys  Hills  Trend of river cources  Slopes and terraces present in the area These details indicate the stability of the slope and the scope for the occurrence of landslides.

Ground water conditions relate to the study of occurrence of springs, swamps, wells, seepages, etc. present in the area. Such a study indicates the water table position and the scope for leakage of water from the reservoir.

If a site is found to be good in the preliminary investigation, then it is taken up for detailed investigation. This process comprises of : 1. Surface investigation 2. Sub-surface investigation

These investigation include further close examination of lithology, structure, topography and ground water conditions which have already been studied roughly during preliminary investigations. Lithology: In this study a geological map of the area is prepared by undertaking extensive field work and noting carefully the outcrops, de, their attitudes, etc.

Structural features of rock beds like strike and dip, faults, folds, joints, foliation, unconformitie are studied in detail and marked accurately on the map. These details indicate the scope of leakage of water, the stability and durability of the dam. Topography : The topographical features like valleys, hills trend of river course, slopes, terraces are studied inn detail.

The very purpose of investigation, either preliminary or detailed, is to infer the lithology, structure and ground water conditions actually prevailing at the dam site. Planned geological and geophysical investigations help in interpreting the sub surface conditions faily accurately. But since they are surfacial studies, there is scope foe misinterpretation to some degree. A dam being a very expensive and giant structure.

The soft ground include those soft and unconsolidated materials which possess little or no cohesion and have a very low crushing strength. These materials consist of gravels, sands, silts, clays and soft shales. They may be dry or water bearing. Excavation through such a ground does not require blasting. Arch supports are always necessary.

Geological investigations : In soft ground tunneling, geological investigations should comprise of: 1. Preparation of geological profile along the centreline of the tunnel. 2. Through study of lithological characters and mode of origin of the sediments.

1. Ravelling ground The material of the ravelling ground flakes and breaks into pieces sometimes after it is exposed. E.g. residual soil. 2. Running ground Such a ground consists of clean loose gravels or sands which run into the excavation.

The squeezing ground consists of material that contains a large amount of clay. Such a material flows into the tunnel plastically. Thus, most important difficulties in soft- ground would be ravelling, running and squeezing of the ground.