Ground Improvement:
Ground Improvement:
Grouting has proved effective in reducing percolation of water through dam foundations, and its introduction into dam construction has allowed considerable cost savings by avoiding the use of deep cut-off and wing trenches. Consequently, many sites that previously were considered unsuitable due to adverse geological conditions can now be used.
Initial estimates of the profitability of ground frequently been have based on the results of pumping-in tests, in which water is pumped into the ground via a drill hole. Lugeon (1933) suggested that grouting beneath concrete gravity dams was required when the permeability
exceeded 1 lugeon unit (i.e. a flow of 1 l m-1 min-1 at a pressure of 1 MPa).
The improvement of the ground, however, has been loosened in modern practice, especially for seepage acceptable foundations (Haulsby, 1990), in relation to earth dams and lost storage and erosion of foundations or core materials.
The effect of the grout screen is to form a low permeability wall into the ground beneath the dam. The holes are drilled and grouted from the bottom of the cut-off or heel trench. Where joints are perpendicular, it is best to drill grout holes in a 10-15 ° rack, as these can be cut across the joints at various points, but the vertical holes may lose them.
The rate at which grout is injected into the ground usually increases with the increase of grouting pressure, but it is limited as excessive pressure causes the ground to break and rise (Kennedy, 2001). Safe maximum pressure depends on the weight of the load, ground strength, in situ stresses, and pore water pressures.
However, there is no simple relationship between these factors and safe maximum grouting pressures. Hydraulic fracture tests can be used to determine the most appropriate stresses, especially in crack rocks or stresses that may be related to overweight.
Once the accessibility criteria are determined, for the whole or part of the grout screen, it is achieved by split spacing or closure methods, in which the primary, secondary, tertiary, etc., grouting sequences are run-up to the water test. Grout holes approach the required criteria (Haulsby, 1990).
In multi-row screens, the outer rows must be completed first, causing the inner rows to close on the outer rows. The distance of 1.5 m between the lines is usually satisfactory.
The upstream line should be a tight line, and the stiffness is down. Single-line screens are usually constructed by drilling alternate pores and then completing the treatment with intermediate pores. Ideally, the grout screen is carried to a depth where the required stiffness is naturally available.
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This can be determined by the probe holes sunk before the grout screen design or by the primary holes sunk during grouting (Evert, 2005). The search usually does not exceed the depth equal to the height of the storage head on the ground surface.
Reinforcement grouting is usually shallow, holes rarely extend more than 10 m.
It is intended to improve Joint Rock by increasing its strength and reducing its permeability. Reinforcement in the foundation area increases grouting bearing capacity and reduces settlement. The range of upward and downward integration grouting of the grout screen depends on the conditions in the zone above the foundation (Kutzner, 1996).
Reinforcement grouting improves the connection between concrete and rock and slightly loosens the rock surface due to explosion operations. In addition, it gives uniformity to a desirable foundation in a different setting
And unbalanced stresses should be avoided.
In other words, the grout increases the rock stiffness and tends to bring the Young Modulus to the required higher uniform values. The holes are usually drilled to the surface of the foundation but in some cases they are directed to dissect specific features. Depending on the nature of the rock they are installed in grid pattern at 3 to 14 m centers.
Integration grouting must be completed before dam construction begins. Casagrande (1961) expressed doubts about the need for grout nets, asserting that a single-row grout screen often built before filling the reservoir was inadequate. Moreover, grouting is useless until water pressure is reduced, and the only effective means of controlling piezometric levels is through drainage systems and therefore increasing the strength at the dam foundation, he said.
He continued that drainage with good cracks is the only effective treatment for low hydraulic conductivity rock. The drainage can control the hydraulic capacity at the bottom of the dam so that the grout screen can achieve what is required, except that it does not reduce the rate of drainage leakage.
However, leakage does not affect rock masses where hydraulic conductivity is low. In other words, drainage is usually necessary for cracked rocks of lesser permeability (ie less than 5 lujan units), but grouting is a wasteful effort. In contrast, when permeability is high (more than 50 lugen units), grouting is necessary to control groundwater leaks beneath the dam.
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