SLOPE SAFETY: FACTORS AND COMMON MISCONCEPTIONS

Transcription

1 SLOPE SAFETY: FACTORS AND COMMON MISCONCEPTIONS By Ir. Dr. Gue See Sew* & Fong Chew Chung** *Managing Director **Geotechnical Engineer, Gue and Partners Sdn Bhd 1.0 INTRODUCTION The collapse of Block 1 of Highland Towers and the recent tragic landslide at Taman Hillview had prompted many experts to put forward their hypothesis or likely causes of the landslide in the area. Some of the hypothesis are quite factual while some are misleading or without proper basis. In addition, the general public who are not familiar with slope stability, are concerned about hill slope developments, especially if they live near hill slope areas. are analysed in terms of the total driving forces and total resisting forces. The factor of safety (FOS) is determined from the ratio of resisting forces to driving forces. The lowest FOS is the critical stability of the slope. Water Table Partially Saturated Soil Saturated Soil WEATHERED ROCK This article aims to explain to those who are not familiar with slope stability and intend to highlight the main factors affecting slope stability. It also presents some common misconceptions on landslides. How does landslide occur? What are the important factors affecting it? What are some of the common misconceptions about landslides? What should we do with abandoned projects near hill slopes? These are some of the questions this article will answer along with illustrations to simplify the complex phenomenon. Figure 1: Anatomy of a Typical Slope POTENTIAL SLIP SURFACES Figure 2: Potential Slip Surfaces ROCK ROCK 2.0 ANATOMY OF A SLOPE Figure 1 shows a typical slope consisting of (i) ground profile with some vegetation, (ii) ground water table, (iii) partially saturated soil above ground water table, (iv) saturated soil below ground water table and (v) weathered and/or competent rock. In the analysis of slope stability to determine whether a slope is safe, potential slip surfaces (Figure 2) are postulate on a slope cross-section. These slip surfaces With the above features of a typical slope, this article introduces several fundamental concepts found in slope stability. The first concept is friction. Friction is generated between two bodies when the bodies are moving against each other as shown in Figure 3. From the illustration, there is a normal force (N) causing the two bodies to come in contact, a driving force (T) and frictional resistance (F). Two important events arise: (1) If T increases, F also increases until a limit in Page 1 of 1

2 which the two bodies will slide against each other; (2) As N increases, F increases as well. F is a function of soil properties and the weight of the two bodies in contact. W T Soil F N Figure 3: Concept of Friction In slope stability, the main properties of soil for slope analysis are soil unit weight (γ), apparent cohesion (c ) and friction angle (φ). Relating the earlier concept of friction to slope stability, the forces N and T can be replaced by the force components in slope; N is analogous to the self weight of the soil, F is the shear resistance at the potential slip surface and T is the driving forces caused by soil self weight and/or surcharge (Figure 4). The governing equation for the resistance of the potential slip surface to shearing is based on the Mohr-Coulomb equation: ( σ u) tan( ') c' τ = φ n + T F Figure 4: Friction Concepts in Slope Where τ is shear stress, σ n is the normal vertical stress, u is the pore water pressure, φ and c are the friction angle and apparent cohesion of soil respectively. Therefore, in a slope stability analysis, a slope is unstable when the summation of shear forces or resistance along the potential slip surface is less than the driving forces. The second concept is the role of water pressure in slope stability analysis. In soil, water pressure exists if the soil is below the ground water table (saturated soil). The main effect of water pressure on a sliding plane is the reduction of normal pressure or forces on soil particle to soil particle at contact. Thus the shear stress is reduced and correspondingly the shear resistance is also reduced. The third concept is suction. Suction occurs in partially saturated soils where water is drawn out of the voids between soil particles mainly through evaporation. This creates a vacuum effect pulling the soil particle together, which increases normal pressure, or forces on the soil particles thereby increase the shear resistance. However, the suction effect in slopes is temporary and is easily diminished when water re-enters into the voids (for example, infiltration during prolonged rainfall). 3.0 IMPORTANT SLOPE STABILITY FACTORS There are many factors influencing the stability of slopes. Here, only the common important factors are covered and explained. Firstly, the properties of the soil such as friction angle, apparent cohesion and unit weight are important in slope stability. As an illustration, consider these two extremes: The first is a near vertical rockface with a building on top and is able to do so without much stability concerns (Figure 5). The second is gentle beach at a seaside where the gradient is very gentle and yet is not stable to build a structure directly on it (Figure 6). These two examples Page 2 of 7

3 illustrates that stronger soil or rock can support a building/load compared to weaker soil or rock. plane decreases due to increased water pressure between soil particles as explained earlier. In addition, the ground water table on the upslope acts as additional driving forces. All these factors decrease the FOS of a slope. Low Groundwater Table High Groundwater Table Figure 5: Building on Steep Rockface Figure 6: Gentle Beach Secondly, slope geometry is important as illustrated in Figure 7. Low and gentle slope is safer than high and steep slope for similar soil. It is because the latter has more mass on the upslope acting as driving forces (F) compared to that of a gentle slope. Figure 8: Effect of Ground Water Table Fourthly, slope maintenance is also an important factor. Poorly maintained slopes can lead to slope failure. These may include, amongst others, damaged/cracked drains, inadequate surface erosion control and clogged drains. Eventually, erosion of the slopes allow the formation of gullies (Figure 9) or cause localised landslips (Figure 10) which will propagate with time into bigger landslides if erosion control is ignored. Steep Slope Gentle Slope Figure 7: Effect of Slope Geometry Thirdly, ground water table profile is an influencing factor in slope stability. The ground water table for hillslopes is generally low and fluctuates with time and rainfall events. Figure 8 shows two general types of ground water table profile which may be found in a slope. High ground water table increases the risk of failure as the shear resistance in the potential failure Figure 9: Gullies on Slopes Finally, excavation or unengineered activities at the toe of the slope can cause slope instability. These activities disturb the stabilising soil mass at the toe of hill and hence reducing the FOS of the slope. In addition, activities such as stockpiling earth which imposes surcharge loads at the top/crest of the slope Page 3 of 7

4 also decreases the FOS of a slope as this surcharge increases the driving forces. Figure 10: Localised Erosion on Slopes 4.0 COMMON MISCONCEPTIONS Here we attempt to debunk some of the common misconceptions often appear in our media about slope safety and explain why they are misconceptions. (1) The first misconception is Soil tests showed that the slope is safe. Soil tests are factual reports of the soil properties at the location in which the test is carried out. Soil tests alone do not tell us whether a slope is safe. Rather, an engineer needs to study the overall slope and carry out engineering analyses of the slope using the soil tests results and slope geometry to determine the FOS of a slope. As iterated earlier, slopes are complex and they are not man made materials, hence its geology and composition can vary significantly over a short distance. Geological features, soil types and properties have significant influence on slope stability. Hence detailed investigations and analyses should be carried out to ensure safety. Soil tests only provide the parameters for analyses and designs of slopes. (2) Heavy rain causes slope failure. This is not correct, although it triggers landslips. Increased rainfall raises the ground water table and decreases the FOS of the slope. The minimum FOS generally ranges from 1.2 to 1.4 depending on the risk to life and economical ramifications. The threshold value at failure is unity. A simple analogy of FOS can be illustrated using the example of weight lifting. Suppose the maximum weight a person could lift is 50 kg, and when the person is given 50 kg, then the FOS at failure or threshold is 1.0 (50 divided by 50). If the person is given 40 kg, then the FOS is 1.25 (50 divided by 40). However, properly engineered slopes should not fail as the slopes should have been designed for the most probable water table during heavy rainfall. The exception is when the actual rainfall is greater than the designed return period of rainfall. (3) Erosion will not cause slope failure. This statement is also not entirely correct. Erosion can propagate a slip further and cause a bigger landslide. There are two types of slope failures due to erosion. One type is an erosion that starts at the toe of the slope, propagates upslope and eventually trigger the slope to fail. The other type is a propagation of erosion from slope crest towards downslope. In both cases, the small and localised erosion is further eroded by rainfall and surfacial water flow, causing more soil mass to fail. This is repeated until the whole slope is not stable and slides. Uncontrolled erosion can lead to slope failure. (4) Retaining walls always prevent slope failure. The public may think that structural solutions like retaining wall is very strong and hence can retain soil mass of the slope without problems. However, this may not be the case. Un-engineered walls can cause slope failure as shown in Figures 11 and 12. A properly designed retaining wall by a professional engineer should not fail as the retaining wall has been properly designed to our codes of practice to retain the soil mass and ground water table. Page 4 of 7

5 without signs of failure but the factor of safety could be low and near the threshold. Hence it is not safe to assume that natural slopes are usually safe. It has to be investigated and analysed. Figure 11: Collapsed Rubble Wall Figure 13: Failure of Natural Slope Figure 12: Failed RC Wall (5) Slopes are maintenance free. Slopes are not always maintenance free. The maintenance such as clearing of clogged drains and patching up localised erosion spots are required. Poorly maintained slopes will lead to slope failures. Clogging increases water pressure build-up through seepage and localised erosion can propagate landslides. Slopes should be regularly maintained following a maintenance manual. (6) The slope has been standing for more than 10 years! So it is safe!. This is not necessarily true as Figure 13 shows that natural slopes can fail suddenly without warning even though it s been standing for years. Natural slopes may be currently standing up (7) EIA report ensures slope stability. An EIA report is a study of the environmental impact for a proposed development will have in the area and surroundings. It is used as a planning tool for development. However, it does not examine the engineering of the slopes in detail to determine whether a slope is safe and the required stabilisation measures, if any. Detailed investigation, analysis and design would only be carried out after the approval of EIA report but before the approval of earthwork plans. (8) Geological report shows that the slope is safe. Geological report covers the history of the soil and the underlying bedrock to explain the geological formation of the site and highlight its geological features, types of rock present, soil stratification, weathering grade and minerals present. It does not cover the engineering and design of slopes. In the face of the public perceptions of these reports, only an engineer s report or a geotechnical report with interpretation of field and laboratory tests and detailed analyses for slopes, will show whether a slope is safe. If the natural slopes with its proposed platforms do not have adequate factor of safety, then strengthening Page 5 of 7

6 measures such as regrading of slope, retaining walls and soil nails should be recommended. Construction drawings and specifications would then be prepared for implementation. Site supervision by the team from the design consultant is a prerequisite component to ensure slope safety. 5.0 ABANDONED HILLSLOPE PROJECTS Hillslope projects may be abandoned due to financial difficulties or for any other reasons. However, partly developed hillslope is usually left as it is. This poses many risks to public safety and some of the risks are presented here. Incomplete Earthworks Hillslope developments mostly involve substantial earthworks to prepare the necessary platforms for building construction. These earthworks involve regrading the existing slopes and transporting its fill to form the required slopes. However, in an abandoned hill slope project such as in Figure 14, the earthworks are not complete and the cut and fill slopes are not fully graded to the design and safe gradient. In addition, soil erosion takes place and gullies formed could de-stabilise the slopes. Hence slopes in abandoned projects are often not stable in the long term and are susceptible to continued erosion and ingression from rainfall. Figure 14: Abandoned Hillslopes Incomplete Slope Strengthening Works In addition to earthworks, there are some earth retaining structures or soil reinforcement which were originally designed to stabilise and retain the slopes. However, if these slope strengthening works are not completed, they may not fully retain or strengthen the soil slopes originally designed for. Hence, the stability of the slopes is in doubt. Incomplete Drainage Works Similarly, incomplete drainage works reduces the stability of the slopes as it affects the ground water table. These incomplete drainage works may cause build up of water pressure by the incomplete channelling of water flow to the main drainage outlets. Subsequently, the build up water will seep into the hill slopes, raising the ground water table profile and therefore increasing the risk of slope instability. No Slope Maintenance Most abandoned projects would be left as it is without further maintenance. As a result, drainage paths gets blocked or silted by the accumulation of decayed vegetation and soil. In addition, ponding on several locations of the slope can occur, which may trigger progressive failures such as mudflow. 6.0 CONCLUSION Stability of slopes is affected by various factors but the important factors are soil properties, slope geometry, ground water table, slope maintenance and unengineered activities at toe and loading on top of a slope. Slopes must be properly planned, investigated, analysed and designed to ensure safety. Strengthening measures such as retaining walls and soil nails are usually needed with regrading to achieve the safe construction platform. Proper and adequate site supervision by the design consultant team is critical to ensure the slope safety. Page 6 of 7

7 With adequate measures taken, environmental and safety conscious hillslopes developments such as in Figures 15 and 16 can be safely constructed for living close to the nature. Figure 15: Properly Designed Slope Figure 16: Proper Hillslope Development Page 7 of 7