Grouting of Rock Foundations
Grouting is needed to fill open cracks in a rock foundation so water cannot leak out of the dam and is carried out by pumping grout (a mixture of cement and water) under pressure into holes drilled into the rock foundation.
Important factors in grouting are;
- the angle to the vertical at which the holes are drilled (to intersect the maximum number of open cracks). One of the duties of the engineering geologist is to advise the grouting programme designers as to the most commonly occurring cracks (or defects) in the rock mass and the orientation of these cracks;
- spacing between adjacent grout holes. In dam construction the aim of foundation grouting is usually to produce a grout "curtain" when the grouting is completed. This means that the area of rock grouted from one hole will overlap the area grouted from the adjacent hole so there is a continuous curtain of grouted rock where the original open cracks have been filled by grout;
- the pressure at which the grout is injected (too great a pressure may cause new cracks in the foundation rock). A grout hole is usually grouted in stages and allowable pressures increase with the depth below the surface of the stage being grouted. A well known rule of thumb of 1 p.s.i. per foot depth is usually satisfactory for average to weak rock. Pressures can be doubled for hard, strong, sound rock masses;
- the thickness of the grout which is controlled by the ratio of water to cement in the grout. This ratio is quoted by volume in terms of loose volume of cement eg w:c = 3:1 indicates a grout made of three volumes of water to one of cement. If the grout is too thick ie not enough water, it may not penetrate into the open cracks in the rock. However too much water is also undesirable as it can produce non-durable grouts which may be susceptible to removal by aggressive groundwater.
It is highly desirable for cost estimating and construction timing planning reasons that the dam designers have a reasonably accurate idea, before construction starts, of how much foundation grouting will be required. Estimates of the required grouting (which may or may not turn out to be reasonably accurate) are usually based on water testing of exploratory drill holes during the investigation stage of the dam project. These tests attempt to measure the permeability of the dam foundation rock by pumping water into a hole drilled into the foundation and measuing the volume of water that is taken up by the rock mass (ie the water that does not return to the surface). If the foundation were completely impermeable then all the water pumped into the hole would return to the surface.
The Lugeon unit is a measure of permeability obtained from pump-in water tests and is defined as as a water take of 1 litre per metre length of hole per minute at 10 bars pressure. 1 Lugeon indicates a nearly tight foundation which requires almost no grouting at all; 10 Lugeons warrants grouting for most types of dams; 100 Lugeons is encountered in heavily jointed and cracked dam sites with relatively open joints or could possibly indicate a sparsely cracked foundation where the joints are very widely open.
The above information is taken from several publications by A.C. (Clive) Houlsby, Grouting Consultant of Sydney, Australia including;
- Grouting Manual. Water Resources Commission of NSW. 3rd edition (1980).
- Rockgrout, version 2.0 1994: a suite of grouting software programs for the PC
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