GEOL 1 CHAPTER 10

• DAM CONSTRUCTION - causes more interference with natural conditions than any other civil engineering operation.
• FAILURES OF DAM

  • to provide adequate spillway capacity
  • defective foundation-bed conditions
• EXAMINATION - is generally restricted to those components which can be seen-together with the inspection of galleries, tunnels, and shafts that have been left often after the completion o construction.
• AERIAL PHOTOGRAPHIC INTERPRETATION - is an economical way of initially assessing dams. Aerial photographs, however, cannot take the place of detailed examination of ground conditions on foot, especially in the vicinity of dams, where occasionally quite minor details of observation may prove to be unusual significance
• FOUR MAIN GEOLOGICAL PROBLEMS:

  1.Determination of the soundness of the underlying foundation beds and of their ability to carry the design loading.

  2. Determination of the degree of water tightness of the geologic foundation and of the measures, if any, required to make these materials watertight.

  3. A study of the effect of exposure to water.

  4. An investigation of the possibility of earth movements below the dam and determination of measures to be taken in safeguard
• PROPOSED DAM LOCATIONS - are generally restricted by topographic, economic, and social consideration
• EXPLORATORY TUNNELS - were widely used for the main dam of Le Sautet water power development in the south of France on Le Drac
• The essential SOUNDNESS OF BEDROCK below the dam is a prime requirement for investigation.
• EXPLORATORY PITS AND CORE DRILLING - may give an indication (although certain

  types of weak rock may yield surprisingly good cores), showing the bedrock's deleterious nature only when

  exposed to the atmosphere
• lBEDROCK DISPLACEMENT - of dam foundation is of vital importance. The rigid nature of many dams is such that any movement of the foundation beds may lead to serious structural damage in seismic regions, unless due allowance has been made in design.
• EMBANKMENT - is constructed over the natural alluvial soils in the valley floor which consist of pervious interbeds of silty clays, sands, gravelly sands, and clayey gravels.
• DAM DESIGN - here is based on a need to dissipate underflow head by seepage travel
• SHALE - exposed at final grade was excavated in relatively small areas and immediately covered with a bituminous coating
• LEAKAGE - from reservoir is an obvious source of trouble, though attention has not always been given to such a possibility
• RESERVOIR LEAKAGE is generally visually obvious and often substantiated by water balance computation.
• GROUTING OPERATIONS - may also be employed for remedial work if leakage is discovered after a dam has been completed
• NEW LAKE AUTSIN - on the Colorado River in Texas, which lost 95.6 percent of its capacity in 13 years
• LA GRANGE RESERVOIR - on the Tuolumme River in California, which lost 83 percent in 22 years.
• LAKE MCMILLAN RESERVOIR - of the Pecos Irrigation District in the United States, which lost 55.5 percent of its storage capacity between 1894 and 1920
• DIVERSIONS - of water between watershed are infrequent, but those that do exist are all of unusual interest