Unit.1 Earth and Rockfill Dams 2 General Fetures: The seepage of reservoir water through the body of earth and rock fill dam or at interfaces of dam seat with ground/steep ground surface or abutment or both, creates the following two problems, apart from causing excessive water loss and thereby reducing usable storage of reservoir: 1. Seepage force in the form of pore water pressures, and 2. Piping. ➢ The drainage system should be so devised that it tackles the problems mentioned. ➢ The design is mostly governed by type and permeability of base materials as well as filter materials, water depth in reservoir, topographical features of dam site, etc. The conventional type of seepage control and drainage features generally adopted for the embankment dam are: ➢ Impervious core, Inclined/vertical filter with horizontal filter, Network of inner longitudinal drain and cross drains, Horizontal filter, Transition zones/transition filters, Intermediate filters, Rock toe, and Toe drain. Selection Of Site:With a decision made about the necessity of having the dam across a river near about a broad area, attention is focused on narrowing down into one, or preferably two or three alternate sites that may be apparently suitable from visual inspection. Detailed investigations are carried next to examine the option that satisfies as being most economical, technically more suitable, convenient for construction etc. The various investigations that are carried out for finalizing a particular dam, specifying its type, height, method of construction, etc. are mentioned below.
Topographic Requirement:- 3 From a preliminary observation of the elevation contour maps of a region, one has to decide on an option that seeks a gorge which is most narrow, which would require minimum quantities of dam construction material. At the same time, an ideal location may also be decided from the volume of the water that may be able to store in the reservoir behind the dam. This may be observed from Figure 6 which shows two possible alternate sites that may be considered suitable from the available narrow gorge sites, A-A and B-B. Though the heights of the dams are nearly the same as may be observed from the corresponding elevation contours, the length of the dam for the latter is clearly more with an expected corresponding higher volume of construction material that would be required. Nevertheless, a further inspection of the elevation contours on the upstream indicates that a dam AA would have a much smaller reservoir as compared to that at B-B. Submergence Possibilities:Though a preliminary investigation indicates one location to be more preferred than others, it may have to be seen what losses have to be sustained if each of these alternatives are selected. There could be a valuable forest area which might get submerged. Of course, it is possible to replenish the loss by planting more saplings at other places not subjected to submergence. However, there could be
some industry located upstream, or some mines, whose relocation may not be economically possible. Geotechnical suitability:- 4 Since a dam is a massive structure, the foundation should be geo technically sound to sustain the high stresses that is expected to be developed due to the self weight of the structure, water pressure of the reservoir and earth quake vibration induced forces at the dam body and the water in the reservoir. The geological and geotechnical investigation of a dam site for detailed evaluation is to be directed towards determining the geological structure, stratigraphy, faulting, foliation and jointing, and to establish ground water conditions adjacent to the dam site, including the abutments. The objective of these investigations is to: ➢ Determine the engineering parameters which can be reliably used to evaluate the stability the dam foundation. ➢ To determine the seepage patterns and the parameters for enabling the assessment of the probable seepage pressures. ➢ To make sure that there are no bad patches within the would–be reservoir area, like lime stone caves, through which reservoir water may leak out. ➢ It needs also to be established by geotechnical investigations about available construction materials in the economical vicinity of the dam site. The tasks that need to be performed for geotechnical testing include the following: ➢ Logging of all natural and excavated rock exposures and borehole records. ➢ Correlation between the exposures and borehole data and inferring out a spatial pattern of subsurface rock jointing patterns, layers or seams of weak materials, etc. ➢ Excavation of additional trial pits, boreholes, shafts and exploratory adits wherever considered necessary. All the above investigation techniques are used in one way or other to evaluate the following engineering parameters that are necessary inputs to dam designs: 1. Depth of overburden. 2. Permeability and porosity parameters with reference to seepage control. 3. Compressibility characteristics of sandy strata and their relative density. 4. Shear strength and consolidation properties of cohesive strata.
Hydrologic adequacy:- 5 At any site, the construction of a dam for creating a reservoir for water storage would require the following points to be adequately satisfied: 1. The average quantity of water available in the river through out year. 2. The minimum flow of the river, both as the absolute minimum and the minimum average over a period of a month or a year. 3. The maximum flow that has been recorded and estimates of what might occur in future. When records of sufficient time are not available or are of doubtful accuracy, correlation is attempted with rainfalls and flows that have been recorded in the surrounding catchments. Hydrometeorology studies can give estimates of future predications. Some times visual inspections may also lead to signs of high flood marks. Apart from the flow parameters mentioned above, which are required for designing the size of the reservoir (and consequently the dam height) and that of the spill way for passing the largest flood flow, local rain fall records are also essential to prepare reasonable construction schedule of dam. Continuous heavy rains during most of the year could deter the possibility of constructing an earth dam. Sedimentation possibilities:The average quantity of sediment carried by the river has to be known, as precisely as possible, which would give an idea of the rate at which a proposed reservoir way get filled up. If the rate of sedimentation is too excessive on a certain river, then the project may have to be dropped or suitable engineering design, like provision of large sluices using the a concrete dam at lower levels of the dam, have to be incorporated by the design engineer. Availability of a suitable spillway site:All dam should have an adequate spillway for passing flood flows. If a river gorge is narrow, then there may not be sufficient spillway width available and a suitable location on the periphery of the reservoir has to be found to locate a spillway. If that is not possible, then the proposed dam site has to be abandoned and other alternatives are selected. A good judgment is also required to decide the suitable type of dam based on spillway consideration.