Chapter 8 REMEDIAL MEASURES 8.1 FACETS IN HAZARD CATEGORY

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1 Chapter 8 REMEDIAL MEASURES 8.1 FACETS IN HAZARD CATEGORY Stabilization of rock slope happens when the driving force acting on a potentially unstable rock slope is reduced, while the resisting force increases (Giani 1992). The remedial measures to increase such resisting forces will depend upon the type of landslides and slope stability analysis carried out by using different methods. The rock slope stability analysis was carried out by using Hoek and Bray method for the critical slope sections under consideration. The soil slope stability analysis was carried out using Circular Failure Chart (CFC) method and Limit Equilibrium (LE) Method. The factor of safety for critical rock and soil sections were evaluated before recommending any remedial measures. In Kolli hills Ghat road section, the facets 3 and 4 were categorized under high hazard zone and facets 1, 2, and 5 were grouped under moderate hazard zone. These sections required landslides mitigation measures to protect the stability of slopes. The facets 3 and 4 mostly covered by rock slope sections whereas, the facets 1, 2, and 5 are covered by soil slope sections. The proposed remedial measures should consider the terrain condition and major influencing parameters. Out of various causative factors, the facets 3 and 4 are mainly controlled by structure, weathering, and slope morphometry, which requires specific recommendations. Moreover, the facet 3 and 4 need top priority, since these sections fall in high hazard zone. The rock slope stability analyses also indicate that, the rock sections RS-3, RS-4, and RS-5 located in facet 3 and 4 are under partially stable to unstable condition. Similarly, the soil slope SS-4 located in facet 3 also unsafe condition. The facets 1, 2, and 5 are once again mainly controlled by structure followed by land cover, slope morphometry, weathering, and hydrogeological conditions. This sections are mainly covered by soil slope and required different types of remedial measures. The soil slope study indicate that, the sections SS-3 and SS-5 respectively fall in facet 2 and 5 are 182

2 unsafe condition. On the basis of hazard parameters, stability analyses and field observations, the following remedial measures were suggested. 8.2 REMEDIAL MEASURES International Union of Geological Sciences Working Group on Landslides, Commission on Landslide Remediation (IUGS WG/L) has prepared a short list of landslide remedial measures (Table 8.1), categorized into four practical groups, namely; modification of slope geometry, drainage control, retaining structures and internal slope reinforcement (Popescu, 2001). The remedial measures can be adopted individually or in combination based on the requirement in the site. Mostly, remedial practices involved the combination of two or more from the aforesaid categories. In several cases, all type of measures being exercised (Farooq 1988) Modification of Slope Geometry In order to improve the stability of the unstable or potentially unstable slopes, the profile of the slope is sometimes changed by excavation or by filling at the toe of the slope. The main slope excavation remedial measures involve; reducing the slope height, reducing the slope angle, removing unstable or potentially unstable materials, and incorporating benches in the slope (Figure 8.1). Apart from excavation of the slope, filling at the toe of the slope and creating a step also increases the stability of the slope. The excavation procedure considered as an effective method for correcting shallow forms of instability, in which movement is confined to soil layers close to ground surface. In the study area, the facets 2 and 5, mostly covered by soil slopes may require modification of slope geometry at few locations, where the slope angle is in critical. The modification slope should be carried out along with the road side cemented and contour drain Drainage Control The presence of water in joints or in soil slope has a fundamental influence on the slope stability. Hence, the knowledge of the water pressure distribution constitutes a basic input data for stability analysis. 183

3 Table 8.1 Brief list of landslide remedial measures Modification of slope geometry Removing material from area driving the landslide (with possible substitution by lightweight fill) Adding material to area maintaining stability (counterweight berm or fill) Reducing general slope angle Drainage Surface drains to divert water from flowing onto slide area (collecting ditches and pipes) Shallow or deep trench drains filled with free-draining geomaterials (coarse granular fills and geosynthetics) Buttress counterforts of coarse-grained materials (hydrological effect) Vertical (small-diameter) boreholes, pumped or self draining Vertical (large-diameter) wells with gravity draining Sub-horizontal or sub-vertical boreholes Drainage tunnels, galleries or adits Vacuum dewatering Drainage by siphoning Electro-osmotic dewatering Vegetation planting (hydrological effect) Retaining structures Gravity-retaining walls Crib-block walls Gabion walls Passive piles, piers and caissons Cast-in-situ reinforced concrete walls Reinforced earth-retaining structures with strip/sheet- polymer/metallicreinforcement elements Buttress counterforts of coarse-grained material (mechanical effect) Retention nets for rock slope faces Rock fall attenuation or stopping systems (rock trap ditches, benches, fences and walls) Protective rock/concrete blocks against erosion Internal slope reinforcement Rock bolts Micropiles Soil nailing Anchors (pre-stressed or not) Grouting Stone or lime/cement columns Heat treatment Freezing Electro-osmotic anchors Vegetation planting (root strength mechanical effect) (after Popescu 2001) 184

4 Figure 8.1 Cross section view of modification of slope geometry The flow pattern significantly changes the slope safety factor (Sharp et al. 1972), which can be obtained with a limit equilibrium method. Drainage as a method of slope stabilization can be very effective, but as long term solution, it suffers greatly because the drains must be maintained if they are to continue to function. Deep drains modify the seepage pattern within the soil or rock mass and are much more costly than shallow drain (Figure 8.2). The following are the some of the effective drainage control measures, which maintain the slope stability. Reshaping the surface of the slope area to control flow and surface run off. Creating Impermeable layers at slope crest to prevent excessive water infiltration. Providing a flow line to divert undesirable surface flows into nonproblem area. Minimizing the removal of vegetation and establishing new vegetation growth. 185

5 Construction of toe drains to intercept the discharge and materials swept down by the flow. The surface drains designed in the light of above mentioned aspects will take care of the run-off before it reaches the area immediately behind the crest of the slope. It is advisable to keep these drains clean of silt and debris to maintain their efficiency. The diversion of the drainage channels in the slope area should be made with great care to avoid the chances of feeding water into the slope if incorrectly sited and improperly sealed. Figure 8.2 Schematic diagram of a drain trench The soil slope sections SS-3, SS-4, and SS-5 respectively located in facets 2, 3, and 5 in Kolli hills Ghat road are in unsafe category under partial and completely saturated condition. These sections required drainage control to prevent landslide occurrences. Construction of concrete drainages at toe in the edge of the road can intercept and drain the excess water further downward along the road and prevent infiltration. Surface protection is probably the best slope protection, particularly against erosion of soil slopes. A grass mat covering the slope will not only bind the surface material together, but it will also tend to inhibit the entry of water to the slope. Providing a vegetative cover to an eroding slope is the final protective measures through biotechnical stabilization of slope. In order to 186

6 provide immediate cover, grass and scrub species which are fast growing and hardy in nature can be selected. To provide permanent cover in damp or wet areas, trees and shrubs with deep tap root system, which absorb water from the sub soil during evapotranspiration. This method of slope protection through vegetation cover is not suitable on the locations, where scarcity of rainfall. In the study area is concern, in most of the facets the land cover is considered as one of the important causative factors in landslide occurrences. With reference to LHEF rating scheme, the facets 2, 3, and 4 required more vegetation cover through intensifying forest plantations Internal Slope Reinforcement Systems The aim of a rock slope stabilization with structural elements is to help the rock mass to support itself by applying external structures which are not part of the rock mass but support it externally. Rock bolts and cable bolts are reinforcing elements that are inserted into the rock mass in order to increase rock stiffness and strength (Panet 1987; Pelizza 1991; Coates and Sag 1973). Rock bolts or cable bolts or anchors in rock or soil can be of various types such as mechanically anchored bolts, grouted rock bolts, grouted cable bolts, and friction anchored bolts (Lang 1972). The rock bolts are made up of solid or tube formed steel while cable bolts are normally made up of steel wires which are laid in a rope configuration. Rock bolts and cable bolts can be installed untensioned or tensioned with cement grouting in the rock mass (Spang and Egger 1992). In the study area, the rock slope sections RS-3, RS-4 and RS-5 are respectively located in facets 3 and 4 are mainly controlled by structure. The results of slope stability analyses have shown that, these sections are under unstable condition. At present, few sections are protected by retaining wall. In addition, reinforcement supports such as rock bolt, grouting, and or anchoring are recommended to stabilize the slope condition. Additional provision can be made for effective drainage to reduce the pore water 187

7 pressure. The openings at the surface may be covered with impervious materials to prevent percolation of surface water into the slope Retaining Walls Construction of retaining wall along the problematic slopes of roads is a very common practice (Figure 8.3). Arbitrary construction of such retaining structures as a protective measure without estimating lateral earth pressure and the passive force will not serve the purposes and increase the cost of construction. The normal retaining walls have been considered as effective structure to prevent small scale movements involving minor lateral earth pressure. Figure 8.3 Various types of retaining walls: (a) rock: filled buttress: (b) gabion wall; (c) crib wall; (d) reinforced earth wall; (e) concrete gravity wall; (f) concrete-reinforced semi gravity wall; (g) cantilever wall; (h) counterfort wall; (i) anchored curtain (after Seehra 2012) 188