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Civil and mining engineering projects usually include heavy demolition and excavation activities that lead to the formation of excavated rock slopes. It is essential to maintain the stability of these slopes till the end of the design life for the successful delivery of a project. Therefore, the site selection should be such that the orientation of joint/bedding planes is favorable for a stable excavation.
A well-inspected and thoroughly studied site can reduce the cost of stabilization. Moreover, the site for excavation should be selected in a way that the geological formation of bedding planes dips away from the excavation plane.
There are numerous scenarios where site selection cannot be made merely based upon geological formations due to certain technical requirements. In those cases, slope stabilization techniques should be used to increase the stability of slopes.
1. Slope Stabilization Methods and Classification
The most commonly used slope stabilization techniques are categorized as follows:
1. Geometric techniques: The application of geometric techniques brings about a change in the geometry of slope.
2. Hydrological techniques: The adoption of hydrological techniques lowers the water content of soil/rock material by reducing the groundwater table.
3. Chemical and mechanical techniques: Chemical and mechanical stabilization techniques increase the shear strength of the critical plane of soil/rock mass by external means. In addition, the shear strength of the slope can also be increased by minimizing the external forces triggering the slope failure.
1.1 Geometrical Techniques
Slope stabilization using geometrical techniques can be achieved by:
- Flattening the slope
- Eliminating part of the soil/rock
- Eliminating load from the top of the slope and therefore reducing the shear stresses on critical planes
- Constructing pressure berms at the toe of the slope and thereby providing extra safety against toppling failure
- Replacement of slipped material by free-draining materials and therefore reducing the build-up of pore water pressure
- By re-compaction of slip debris to provide more resistance against loading
1.2 Hydrological Techniques
Slope stabilization using hydrological techniques can be achieved by:
- Installing surface and subsurface drain pipes and therefore reducing pore water pressure
- Use of inverted filters
- Use of thermal techniques, such as ground freezing and heating methods
1.3 Chemical and Mechanical Techniques
Slope stabilization using chemical and mechanical techniques can be achieved by:
- Using grouting to increase the shear resistance of slope
- Constructing restraining structures, such as concrete gravity or cantilever walls
- Construction of gabion structures, baby crib walls, and embankment piles in order to provide resistance against toppling
- Constructing lime and cement columns
- Installing ground anchors, rock bolts, root piles, etc. to provide effective tension to rock blocks
- By planting shrubs and grasses to reduce soil erosion
2. Construction Techniques of Slope Stabilization
Slope stabilization techniques are categorized into three groups:
i) Reinforcement support: It includes rock bolts, dowels, tied-back walls, shotcrete, buttresses, etc.
ii) Unstable Rock removal: It involves methods like re-sloping, cutting, etc.
iii) Protection: This comprises the construction of ditches, mesh, catch fences, warning fences, rock sheds, tunnels, etc.
2.1 Rock Reinforcement Support
Rock reinforcement support involves the application of external elements to strengthen the rock to avoid failure.
2.1.1 Rock bolts and Anchors
The most beneficial supports are rock bolts and anchors as they protect blocks of rock from sliding away from the discontinuity planes.
The installation mechanism of rock bolts and anchors governs their effective compression capacity. The most effective way to install rock bolt is by fixing them perpendicular to the joints so that the joint discontinuities are easily trapped.
In the case of fractured rock slope, rock bolts and anchors are used in combination with concrete walls to cover the locations of fractured rock.
2.1.2 Steel Rods
Steel rods, also known as dowels bars, are installed and grouted into the rock mass to act as reinforcement.
The difference between rock bolts and steel rods lies in their installation methods as rock bolts are stressed during the installation, whereas steel rods are not.
Fine aggregates and mortar are the main constituents of shotcrete. Generally, shotcrete is applied pneumatically and placed in a layer of 50 to 100 mm.
The application of a layer of shotcrete to the rock face can protect the zones or beds of closely-fractured rock. Besides, shotcrete also prevents small blocks of rock from falling. Thus the process of progressive failure of producing large, unstable overhangs on the face reduces. Although its primary function is surface protection, shotcrete also provides some support against sliding of the overall slope.
Shotcrete improves the tensile and shear strength of slopes, thereby reducing the chances of slope failure.
Grouting is a technique of injecting a fluid grout into the rock mass to replace the air or water present in its fissures and cracks. The grout consists of a mixture of cement and water. However, sand, clay, rock flour, fly ash, and other similar materials can be used as a replacement to cement. As a result, the cost of stabilization work reduces, especially where fissures and cracks are large in volume.
If a cavity is present in the slope face, a concrete buttress can be built to avoid rock falls and support the overhang.
3. Stabilization Strategies to Reduce Slope Failure
The objective behind slope stabilization is to reduce the risk of slope failure to enhance public safety. Some standard stabilization techniques used in practice to improve public safety are mentioned below:
- Flattening of overburden slope
- Cutting of unstable rock blocks
- Scaling of loose materials/blocks
- Providing drain pipes and drain holes
- Use of dowel bars
- Installing rock anchor to avoid moving along discontinuity joints
- Use of rock bolts to enhance the jointed rock mass
- Constructing concrete or masonry walls with weep-holes
- Constructing rock trap ditches at the toe of the slopes
- Providing rock catch fences/walls along the slope to make the surrounding locations safe for public usage
- Providing hanging chains or webs to slow down toppling of blocks
- Providing free-hanging mesh net to direct loose rock pieces to fall only near the slope toe
- Constructing berms/benches as a rockfall collector
- Providing mesh secured by bolts and gunited to protect friable formation
- Constructing rockfall barriers (gabions and concrete block, reinforced soil barriers, etc.) at the toe of slopes
- Building and constructing rock sheds and tunnels
- Providing caution signals in rockfall locations
Stabilization methods such as rock bolts and anchors avoid the detachment of rock blocks from their initial position. For this reason, they are recognized as active procedures.
Walls, ditches, capture fences, rock sheds, and tunnels are passive methods as they do not interfere in the procedure of rock detachment.
The conditions for selection of the stabilization technique are:
1. Geotechnical requirement (geology, rock/soil properties, groundwater, and stability analysis)
2. Construction requirement (types of construction equipment, access to the construction site, construction expenses, etc.)
3. Ecological requirement (garbage disposal, aesthetics, etc.)
The choice depends upon the level of stabilization required, its design life, and the costs involved. The preliminary expenses will also influence the selection of the stabilization technique, which will ensure its efficiency for a longer period.