Site Investigation

HYDRAULIC DESIGN 2 Why is site investigation essential in dam design and construction?

What is a site investigation? A site investigation is an assessment of the site geological conditions, methods of construction and evaluation of cost for a proposed project location prior to undertaking.

WHY A SITE INVESTIGATION To know: • the depth of overburden that must be removed to reach an acceptable foundation for the dam wall; • the rock types which make up the foundation and to what extent are they affected by surface weathering; • what are the engineering properties of the foundation rock types (important properties are strength, deformability and durability); • the geological structure of the foundation (ie jointing, faulting and folding of the rock strata). A full description of the defect pattern in the rock mass should include orientation, spacing, extent or persistence and aperture or openess;

• how permeable is the rock foundation (ie to what extent are the rock defects such as joints, faults and bedding open); • where can adequate supplies of construction materials such as clay, sand, gravel and rock fill be obtained, preferably as close as possible to the dam site; • will the rock that must be excavated to provide a spillway for the dam be acceptable for use as rock fill in the construction of the dam embankment; • will the spillway require concrete lining and an energy dissipation structure at its downstream end or is the spillway rock sufficiently erosion resistant that these can be omitted.

• The effects and impacts of the proposed project on its environment. • The possible construction problems

PROCESSES INVOLVED IN SITE INVESTIGATION DESK SYUDY • Looking at geological maps • Topographical maps • Aerial photographs • Previous site investigation reports • Asking local people and authorities • Satellite photography • Google earth

Preliminary Investigation Aerial Reconnaissance - An initial commissioned flyover is essential, providing the Engineer with an idea of the topography and enabling him to form an opinion of the probable hydrological characteristics of catchment Ground Reconnaissance - Features that should be sought during early reconnaissance include old and potential land slides, geological faults and major joints parallel with the valley.

Geophysical Investigation • Geophysical methods provide an indirect evaluation of certain underground conditions. Several procedures have been developed, all of which measure some force pattern in the earth. Methods include: Seismic refraction surveys Electrical resistivity surveys

Exploratory Investigation Purpose - to secure accurate information about the soil and rock stratification, the composition of the materials and the location of ground water. • Boring and Sampling (test pits) • Laboratory testing • Correlation of results

  Detailed Investigation

• The evaluation of preliminary desk and field work should assess the potential for major hazards and qualitatively assess the likelihood of encountering any more hazards. This should allow a ranking of the potential sites in order of their probable suitability.