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Even hard and coherent rocks might become unsuitable as sites for engineering projects when these are profusely traversed by geological discontinuities such as joints, faults, shear zones and clay seams etc.
Methods commonly used to improve the strength and stability conditions in these rocks are:
1. Grouting;
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2. Backfilling, and,
3. Rock bolting.
1. Grouting:
Object:
Grouting involves injecting under pressure a good quantity of cement slurry with admixtures into the rocks of the site. The cement and admixtures are carried deep and all through the minute cracks where these fill, set, harden and virtually seal all the types of openings in the rocks. The grouted rock becomes impermeable, strong and free from defects associated with earlier planes of weakness or open spaces.
Method:
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In grouting, three main operations are involved:
(i) Drilling of bore holes at properly selected positions with predetermined diameter, depth and angle. Usually a number of bore holes are to be dug in a grouting programme, the number going as high as many hundreds or even thousands. Their aggregate depth may exceed thousands of meters.
(ii) Preparation of Grout Slurry – This is done separately under expert guidance. It involves thorough mixing of pre-calculated quantities of cement, and if needed, sand and other desired admixtures such as asphalt, saw dust, and chemicals like sodium silicate and calcium chloride. Ashes and even gravel have been used as ingredients in grouting.
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(iii) Injection – The slurry is injected into the ground through perforated injection pipes generally under definite pressure. In fact, on the basis of pressure applied, grouting is differentiated into low-pressure grouting (pressure below 10 kg/cm) and high-pressure grouting (pressure above 40 kg/cm). In some cases, grouting might be done in two stages, piercing the solidified grouted zone made earlier once again to achieve maximum imperviousness and strength.
Results:
Grouting has been adopted and remains as one of the best methods available for making rocks impervious and/or strong in critical places such as in the foundations and abutments of dams, sides and roofs of the tunnels and in stability of slopes.
Hundreds of thousands of tonnes of cement has been used as grout in each such major project and all of it becomes a part of the ground, invisible but strong part. The grouted zone may form a thin sheet (grout screen) to stop leakage, or, it may be the entire zone separating a whole mass from the rest of ground (grout curtain), or, it may be the entire mass below or along the sides which has been treated and transformed into a grouted zone.
2. Backfilling and Site Reinforcement:
In many cases, drilling investigations reveal the existence of faults, shear zones, clay seams and such other shattered or weak zones that the entire zone of rock is made virtually useless with respect to the requirements of the foundation. The weak rock zone might not be prone to improvement by grouting etc. at economical costs.
In such cases, the treatment of this zone would require:
(i) Excavation of the entire volume of the weak, objectionable material.
(ii) Cleaning (by washing) of the excavated zone and
(iii) Backfilling the excavated place with concrete of proper mix design.
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The depth to which this weak zone should be excavated and backfilled depends on a number of factors such as:
(a) Location of the weak zone viz-a-viz the structure e.g. dam foundation or abutment;
(b) Likely loads that are to act upon at that location.
From theoretical studies at many constructions made in the case of dams, it has been found that excavation of weak zones should be done to a depth given by:
d = 0.002 bH + 5 for H < 150 feet
d = 0.3 bH + 5 for H > 150 feet
where, H = height of dam in feet, above general level of foundation;
b = width of weak zone, in feet;
d = depth of weak zone, in feet, below surface.
It has also been suggested by various workers in the field that in clay gouge seems, ‘d’ should not be less than 0.1 H. However, the above account is only an empirical attempt. In actual practice, the factors have to be kept in mind.
Another method of ‘treatment’ of sites gaining favour with the engineers is ‘reinforcement’ involving construction of concrete blocks at predetermined places with prestressed reinforcement. Such concrete counterforts are then anchored to the sound rock by prestressed cables.
3. Rock Bolting:
Rock bolting is perhaps the most popular and prevalent method for keeping the fractured rocks together in and around tunnel openings. Hundreds of thousands of tonnes of rock bolts are inserted every year in important tunnel projects in many parts of the world.
A rock bolt is a specially designed steel bolt with main function of holding broken pieces of rocks together by providing reinforcement and support. It is inserted without disturbing the natural environment of the rock. Once within the rock body, a bolt provides, besides the usual grip, additional strength (due to its material) and thereby contributes to improve stability of the rock.
Rock bolts are manufactured in many types. In fact it is a well-established industry having patented types of rock bolts each with different qualities. The main types of rock bolts are the slotted type, the expansion-type and the groutable-rock bolts. The first type is the simplest and most commonly used in fissured rock.
The slotted rock bolt consists of a steel bolt, length varying between 80 cm to 100 cm, and diameter 2-2.5 cm. On one end, it is slotted to accommodate a wedge. It is also threaded and provided with a bearing plate, washer and nut.
For providing anchorage with rock bolts, process starts with drilling holes, slightly bigger in size than that of the bolts according to a predetermined scheme. Then wedges are placed in the holes and bolts are driven over them. After proper placement, each bolt is clamped, tightened and tensioned at a proper stress (Fig. 25.3).
