Construction of embankment dams
As was discussed for concrete dams in Lesson 4.6, arrangements have to be made to divert the river while constructing an embankment dam. This temporary exclusion of river flow is necessary to provide dry or semi-dry area for the work to continue. However, for concrete dams, after the initial levels in touch with the foundation have been built, the river water during floods may be allowed to overtop the partially completed structure during monsoons. In case of embankment dam, it is rarely allowed as overtopping would generally lead to a washout of the downstream face first and the rest follows the collapse. Hence, for embankment dams, the diversion measures require the detouring of the whole flood water through bye-pass. In only special cases, has an embankment been constructed that has been allowed to overtop only when partly constructed. The downstream face has to be sufficiently protected in that case with concrete blocks or gabions. Details of river diversion for dams may be found in the Bureau of Indian Standards Code IS:10084-1984 (Part 2) “Design of diversion worksCriteria” from where the following has been called.
Figure 55 shows a typical example of a diversion channel for earth/rockfill dam project in a narrow river. The layout and principal dimensions, specifically the cross-section of the diversion channel is governed by several considerations such as topography, volume of flood to be handled, water levels during passage of monsoon and non-monsoon floods in consonance with raising of the dam and requirement of excavated material from the diversion channel for use in constructing the earth dam, etc. The coffer dams in such a case form an integral part of the earth and rockfill dam in the finally completed stage, and are also not allowed to be overtopped. Because of the considerable expenditure and time involved in the construction of diversion channel for earth dams, these channels are designed to be useful for other purpose also such as spillway tail channel or power house tail channel. Although, initially such channels may be without protective lining on the sides, they are protected at a subsequent stage when utilized for spillway or power house tail race channel.
In a wide river channel, provided the height of the earth dam is small enough, diversion could be managed by a temporary channel revolving a gap through the earthfill dam while the remainder of the embankment is being constructed (Figure 56). Before the stream is diverted, the foundation required for the dam should be completed in the area where the temporary opening will be left through the embankment. This preparation would include excavation and refilling of a cut-off trench, if one is to be constructed. The stream is then channelised through this area after which the foundation work in the remainder of the streambed is completed.
Figure 56, Diversion through a gap in a partially constructed earth dam in a wide river
In some rivers, the floods may be so large that provision of diversion channels even for average floods may be highly expensive. The only alternative then is to have the discharge passed through a conduit excavated through one or either abutment. Coffer dams, nevertheless, have to be constructed on the upstream and downstream of the working area to divert the stream flow into the diversion tunnel and to prevent the water on the downstream side of the river from flooding the work space.
The principles of foundation treatment for concrete gravity dams have been outlined in Lesson 4.6, Similar methods have to be adopted for embankment dams, too. However, the Bureau of Indian Standards code IS: 11973-1986 “Code of practice for treatment of rock foundations, core and abutment contacts with rock, for embankment dams” deals specifically with the requirement of an embankment dam. Some important points from this standard are explained below.
Basically, the surface under the entire core and under a portion of the upstream filter and downstream transition zone shall be completely excavated to such rock as will offer adequate resistance to erosion of fines in the core. All loose or semi-detached blocks of rock should be removed. The quality of rock shall be judged characteristic of core material. Rock of ‘Lugeon’ values in percolation test within 10 (Ten) will generally be free of cracks larger than 0.025 mm. Erosion of fines from core materials commonly used would not occur through such cracks. Grouting may be necessary to bring down ‘Lugeon’ values to above allowable limits in the contact zone.
The amount of care required in treating the rock surface is controlled by the character of the core material. If the core material is resistant to piping, especially if it contains considerable coarse material with adequate proportion of sand, surface treatment is less demanding than if the core material is susceptible to piping; for example, a fine silty sand and very lean clays. In the latter case, extreme care should be taken and the core material should be placed only after very careful inspection of the treated surface. For dispersive clays, special precautions, such as protection by filter fabric or plastic concrete may be required.
Small ribs and similar irregularities should be filled with plastic concrete to produce slopes not steeper than about 1:1 where the difference in elevation is a few centimeters to a meter or so. Surface treatment in this fashion should extend upstream to approximately the mid-point of the upstream filter and downstream at least 0.6 to 0.9 m beyond the downstream edge of the fine filter. In particularly adverse situations, such as where there are joints wider than the coarser particles of the filter, surface treatment as described may be necessary under the entire transition zone.
The final rock surface should have smooth contours against which soil can be compacted by heavy equipment. Hand compaction is generally unsatisfactory and it is advisable to place plastic concrete in core contact areas of conduit trenches and other irregularities transverse to the dam axis for a width at least 0.5 H or preferably 1.0 H. Surface treatment as described may be difficult to accomplish on steeply sloping abutments. In this case, gunite may be used for filling depressions after the cracks and joints have been cleaned and sealed. If there is extensive jointing, especially if the joints slope upward away from the face, adequate sealing of the joints may require constructing a concrete slab, which is dowelled to the rock, and then grouting through the slab.
The depth of excavation necessary in weathered rock is difficult to establish during initial design. The depth of weathering is usually very irregular, being controlled by minor variations in joint spacing and rock type. Abrupt changes in elevation of the surface of ‘groutable rock’ probably will be found. Overhangs, some of large size, should be anticipated.
Usual practice is to select material, preferably a plastic soil, for the first lift over the rock surface. If plastic soils are limited, the most plastic soil available should be used. Gravel or stone exceeding about 50 mm in size should be removed or excluded from the material placed in this first layer over the rock to improve compaction at the contact. The surface on which the core material is placed should be moist but free of standing water, and the material when placed should be wet of optimum. In dry climates or during dry weather, difficulty may be experienced with this first lift becoming excessively dry where it feathers out on a gentle to moderate slope. In such a case the edge of the fill should be sloped slightly downward toward the contact with the rock. Against steep rock faces or adjacent to concrete structures, sloping the fill slightly upward near the contact is desirable to provide better clearance and better compaction at the contact.
Treatment of rock defects and discontinuities
In evaluating and planning for excavation and seepage control measures, special attention shall be given to discontinuities such as faults and relief (sheet) joints, which may extend for long distance as nearly plane surfaces. Relief joints may exist naturally or may open during excavation. They are most likely to occur in deep, steep-walled valleys, especially in brittle rocks, or where high modulus rock is underlain by low modulus rock. Since they are roughly parallel to the valley wall, they may cause slides during construction. Openings of several centimeters have been observed. Control of seepage through such joints becomes a major problem. Installation of concrete cutoffs across particularly bad joints may be warranted or extensive grouting may be necessary. Drainage from such joints shall be provided.
When seams are filled with silt, clay, etc, or in faults with gauge, it is essential to excavate and backfill the seam and gauge zones in the entire core contact zone. It is advisable to excavate and backfill a further length on the upstream for a distance equal to the reservoir head and backfill it with concrete. On the downstream side the seams should be excavated and backfilled with a well designed and adequate filter again for a distance equal to the reservoir head.
There are three main objectives in the grouting programme (see also IS : 6066-1984). These are as follows:
To reduce the seepage flow through the dam foundation; To prevent possible piping or washing of fines from the core into cracks and fissures in the foundation; and
To reduce the hydrostatic pressure in the downstream foundation of the dam. The latter is generally a problem only for dams on fairly weak foundations and critical abutment configurations. This is usually accomplished in conjunction with an abutment drainage system.
To prevent possible piping of the fine core material through the foundation, blanket grouting is accomplished as determined by the rock conditions. If the core foundation of the dam consists of closely fractured and jointed rock, a blanket grout pattern is used with holes spaced at 3 m to 5 m with depths of 6 m to 10 m. If the foundation rock is massive, no blanket grouting is done. Localised area consisting of faults, fissures, or cracks are generally grouted upstream of the cutoff and sometimes downstream.
The performance of an earth or rockfill dam depends upon the control exercised during construction, supervision and inspection. An entirely safe design may be ruined by careless and shoddy execution. Proper quality control during construction is as important as the design. The skill, experience and judgment required of the engineer in charge of construction, is in no way lesser than that of the design engineer. Hence, the Bureau of Indian Standards has published the following publication which provides guidance for construction of embankment dams with regards to quality control IS: 14690-1999 “Quality control during construction of earth and rockfill damsrecommendations”.