Should it be economically feasible, all the materials under the base of a proposed dam should be removed that can cause excessive settlement, leakage and/or seepage Alternatively, the dam design should be modified to take care of such materials. However, the first action is more rewarding and optimum. This includes dental work, stripping and scaling.
Dental Work: Removal of loose, unconsolidated and undesirable materials locally under the dam foundation is known as dental work.
Stripping: The general removal of unconsolidated or unsound material throughout the foundation is called stripping, and
Scaling: is the removal of loose masses of rocks on the abutment zones.
However, only these process are not final and do not give clearance for a dam to be put on the scaled and stripped foundation. The foundation rocks beneath such unconsolidated material may exhibit structural weaknesses such as faults, joints etc. Therefore, additional treatment of foundation may be required in many cases. These discontinuities may include, seams of altered rocks which may be too soft for the expected dam loads, solution channels and caverns in limestone and other soluble rocks, flow tunnels and large cavities in basalts and soft layers or inclusions as are often found in shales and siltstones. Removing the unsuitable materials by tunneling and/or shafting may treat these weak zones. All openings thus created are filled by concrete. Where the leakage channels are rather large, subterranean cutoff concrete walls may ward off these zones. However, grouting is the most effective and common method for remedial treatment.
GROUTING: Grouting is a method of injecting suitable material of cement and water or other admixtures in the foundation for purpose of either creating a barrier or to consolidate. Therefore, two types of grout work are generally performed according to the situation and site conditions. Grouting is therefore of two types:
(i) Consolidation Grouting: In order to make any foundation strong enough and to act really like a monolith, this type of grouting is done. In this method grouting is done three-dimensionally by using pre-drilled grout holes and then injecting suitable grout mixture. Such a treatment was given to a riverbed fault occurring in the near middle of Sardar Sarovar Dam. A great danger to the dam was due to this. The treatment comprised of the excavation of fault zone in order to remove all the shattered and unconsolidated material. Figure shows the fault zone treatment given at Sardar Sarovar dam site. Hammock re-enforcement consisting of four layers of 36mm diameter of steel bars at 50 cms. spacing parallel to the dam axis in one direction and parallel to the fault zone in other direction was provided. Grouting of the form of a grid of 3m x 3m grouted with cement and water mixture. In order to provide positive contact between the concrete plug and rock surface of the trench, grout bottoms were duly connected with a system of grout piping provided with the contact in order to carry out grouting.
(ii) Screen/Curtain Grouting: This type of grouting is a two dimensional treatment, basically to provide a cut-off wall or screen to prevent seepages and leakages. This grouting is very extensively used in tunnel ground treatment where disturbances from ground water or surface water percolations are very common. Figure shows how a curtain grouting can prevent seepage/leakage into tunnels and can divert the seepage path away from it.
GROUTING MATERIALS: Though the choice of grouting material depends on its cost, availability and efficiency under given condition, the most common type of grout mixture is Portland cement and water. In many cases a mixture of sand with cement and water is also used. The viscosity or thickness of the grout mixture depends upon the type and size of the cracks. In wider cracks a more viscous mixture is used whereas in the narrow cracks less viscous mixture is used. In wider cracks, in stead of using cement grout, a mixture of cement-rock flour, cement-hydrated lime, cement-calcium chloride, cement-diatomaceous earth, quick cement, cement-clay or cement bentonite can be used. In cases where large water flows are encountered, hay or straw may be forced down to a large hole and saturated with a ashphaltic, emulsion.
GROUTING EQUIPMENT: The grouting unit generally consists of a mixer and one or more pumps. The mixer tank is filled with grout mixture and is continuously squeezed out into the ground by compressed air. Pressure Gauges are installed at the grout holes to maintain proper line pressure. Careful notes are also taken of the amount of grout placed in each hole in order to determine the "grout take". Excessive quantities of grout required in one or two particular holes and difficulty in maintaining adequate pressure are signs that grouting procedure is not up to the mark. "Grout holes" are generally 2.5 to 5" in diameter. The drilling technique depends upon the depth of the grout holes and the type and mechanical properties of the foundation rocks. Generally grouting is done in two stages which is as follows:
(i) Low Pressure Grouting: This grouting precedes the high pressure grouting. It is generally done before any concrete is placed within the body of the dam. The main purpose of this is to consolidate the cock and to seal all the major crevices and openings. The other purpose is to create a strong monolithic slab at the heel area of the dam to resist the high pressures that will develop in this area at second stage of grouting. Since this pressure may leave or destroy the rock at the heel area, low pressure grouting should be extended throughout the whole foundation area.
(ii) High Pressure Grouting: The main objective of high pressure grouting is to create a deep curtain at the heel of the dam which will prevent leakage from the reservoir and will prevent uplift of the dam by water under pressure caused by high levels of the reservoir. This grouting precedes filling of the reservoir and is done through the galleries made within the body of the dam or from pipes projecting through the heel of the dam. The orientation of the grout holes will depend upon the foundation conditions, design of the dam and size of the reservoir and the dam itself. In inched grout holes, the inclination is determined by the orientation of prominent joints and seams in the foundation rocks. Oblique holes generally make an angle between 100 to 150 with the vertical and the depth generally is from 20% of the hydrostatic head in hard rocks to as much as 70% in poor rocks.
GROUTING METHODS: There are two types of grouting methods, namely stage method and packer method.
(i) Stage Method: In this method the holes are drilling the seam to closest to the surface and it is grouted. The holes proper are cleaned of grout mostly by washing and flushing, and drilled down to the next seam. The process is repeated, using increasing grout pressure till the planned grouting depth is reached.
(ii) Packer Method: In this method all the grout holes are drilled to their fullest length. A zone of certain thickness is then grouted, and a seal or packer is inserted into the hole to a level corresponding to the top of the grouted zone. The overlying zone for a certain thickness is then grouted using decreased pressure. The process is repeated till the uppermost layer is reached and grouted.
GROUT LOSS: If the cracks in the foundation rocks are not well studied or estimated, even non-excessive pressure can result into grout losses because of leakages. It is usually controlled by caking the surface seams with wood wedges and "oakum" or by grout caps and gunnite blankets. It the grout pressure is excessive, the grout may more upward in the rock adjacent to the hole until it erupts on to the surface at some distance from the hole. It is usual for grout leakage to appear over 50 feet from the hole and in some cases, grout has been observed at the surface several hundred feet away from the grouted hole.