Unweathered and sound rocks are always considered as an ideal base for building foundations. But in the case of unfavorable rock conditions at any location, if the loads transferred by the columns fail to reach the underlying strata, heavy structural instabilities are liable to occur.
To prevent this from happening, it is important to understand the nature of rock to select the foundation suitable for the area. If the satisfactory compressive strength of rock is available at a smaller depth, then a shallow foundation would work; otherwise, engineers may have to look for sufficient depth to fulfill required safe bearing capacity value.
For a smaller depth where the rock is exposed, the engineers may be able to evaluate the quality of the rock, whereas, in the case of deep foundations, it is not feasible to evaluate the quality of rock. For instance, in case of construction of dam foundation, and bridges on the rocks.
The main problems an engineer has to face while designing a structure are:
1. Heavy column loads getting transferred to a foundation situated on weak rock.
2. Seepage pressure for the construction of dams and bridges on unsound rock.
3. Piers of a bridge transferring heavy loads to the foundation constructed on canyon walls.
To prevent the occurrence of the abovementioned problems, the characteristics of the rocks should be evaluated, and they should be checked for stability to overcome both deformation and rupture of rock surfaces. Certain treatments used to enhance the properties of rock mass are discussed below.
Remedial Measures to Improve Rock Quality
1. If soft, weathered, and highly fissured rock is present to a certain depth, then it is advisable to excavate that portion to improve the quality of rock.
2. If highly permeable and fissured rock is present beneath the dam, it is advisable to excavate that portion to reduce the effect of seepage pressure.
3. To improve the rock quality, it is advisable to use grouting operations to increase the strength of rock mass, and consequently, it will improve the supporting capacity for the foundation of dams, tunnels, etc. Grouting operation will also reduce the permeability of the rock mass, thereby reducing the chances of uplift pressure.
4. The rock bolts and anchors are very effective in reducing the uplift pressure, and therefore they should be used to improve the uplift resistance of the foundation of concrete dams.
5. For slope stability, the exposed loose rock can be treated by the use of shotcrete and gunite. The shotcrete or gunite will act as reinforcing material and improve the strength of rock mass.
6. Drains should be provided to release the pressure created by pore water. Such drains may include galleries, pumping systems, deep gravity wells, etc. Therefore, by reducing the pore water pressure, the chances of occurrence of seepage pressure head reduces.
How to Reduce Uplift Pressure and Seepage Control
Generally, the poor, weathered rocks can be present either at a shallow or deep level. Therefore, based upon the location of these rocks, some treatments are discussed below.
If the poor quality rock is present at a shallow level, then that portion of the poor quality rock can be removed, and a cut-off wall should be constructed at that position.
In deep foundations, if the poor quality rock is adjacent to the foundation, then the grout curtains can be provided. Such grout curtains should be connected to the grout blankets. The combination of grout curtains and grout blankets helps reduce the seepage head pressure.
Moreover, grout curtains should also be provided with drain valves to release the pore water pressure. Such a mechanism is very useful in dam construction, especially in the earthen dam, where the core of the dam is porous.
Reinforcement for rocks can be provided in the form of rock bolts, cable anchors, shotcrete, and compaction grouting to improve the rock quality and stability.
1. Rock Bolts and Cable Anchors
The strength of rock mass can be increased with the help of rock bolts and cable anchors. They are generally used to improve the slope stability, provide resistance against uplift in concrete dam foundation, and reduce the squeezing of rocks around the tunnel periphery. Rock bolts and cable anchors can also be used in supporting open excavation due to poor ground stability.
Rock bolts are used in place of grouting operations at the locations where grouting can induce displacement of fissured rock.
The mechanism through which rock bolts act as an effective compression member is governed by the installation method. The most effective way to install rock bolt is by installing it perpendicular to the joints so that the joint discontinuities are easily trapped.
2. Compaction Grouting
If the columns are transferring the heavy load on the foundation, then the rock quality can be improved using compaction grouting. The spacing for compaction is usually between 2 to 10m, and the depth can be 5 to 25m depending upon the quality of rocks. The compaction process should always be followed by grouting.
For the rock joints that are filled with clayey material, it is difficult to adopt this method due to secondary compression around the clayey layer, and therefore this method is mostly used for poor rock with open joints.
Shotcrete can be used as an alternative to rock bolts. In weak rock, the joints are highly fissured, and therefore the cracks propagate along the major discontinuity line. By applying shotcrete, all the joints in fissured rock can be filled to improve the overall quality of rock.
Shotcrete is made up of wet mix mortar and fine to medium-sized aggregates. Shotcrete is projected on the face of rock by the use of air jets. It uses the force of air jets and compacts the rock surfaces, acting as a reinforcing material.
Frequently Asked Questions
Limestone, Gypsum, calcite, dikes of metamorphic rock, and even the sandstone if it is cemented with gypsum. These rocks are highly soluble in water, and therefore the construction of foundation beneath such rocks is a concern to the engineers.
As the water table comes in contact with the soluble rocks, the weathering process commence, consequently the material from the outer surface will get washed and with time and again this process may form a cavity in the soluble rocks. To prevent this process, the grout curtains should be employed. The grout curtains will stop the flow of water towards the soluble rocks and therefore lowers the risk of weathering action.