On Slope Monitoring Methods
- 1. Slope Monitoring A Seminar by N.Nagadatt Sharma MT18EXV011
- 2. Introduction Many rock slopes move to varying degrees during the course of their operational live. Such movement indicates that the slope is in a quasi-stable state, but this condition may continue for many years, or even centuries, without failure occurring. Slope movement is most common in open pit mines, and many mines continue to operate safely for years with moving slopes that are carefully monitored to warn of deteriorating stability conditions. Monitoring may also be suitable for large landslides that threaten facilities such as reservoirs, transportation systems and residential areas.
- 3. Why do we need Slope Monitoring? Sometimes the forces of nature — earthquakes, hurricanes, or even something as common as heavy rains — produce slope failures that cannot be predicted, much less prevented. But many slopes produce warning signs that the earth is about to move, with costly consequences. Conducting a slope-stability analysis in response to these warnings can forestall litigation, keep traffic flowing on busy highways, and perhaps even save lives. A slope stability analysis requires a comprehensive understanding of the interplay between moisture, soils, structures and gravity.
- 5. 5 SIGNS YOU MAY NEED A SLOPE STABILITY ANALYSIS All it takes is good eyesight to detect the warning signs of slope instability, but determining what to do about it is another matter altogether. It takes experienced, qualified geotechnical engineers to assess the forces at work in a hillside and recommend the optimum course of action to prevent slides, repair existing damage and mitigate future risks. Five common symptoms of slope instability are: 1. Visible cracks; 2. Leaning or curved trees, guardrails and other structures; 3. Toe erosion; 4. Hummocky slope surface; and 5. Moisture changes.
- 6. Indications of Impending slope failure Visible cracks. If a roadway or bridge surface has cracks large enough to be seen with the naked eye, there’s a strong possibility that the underlying structure has shifted. Leaning or curved trees, guardrails, and other structures. Trees normally grow vertically toward the sun, and light poles and guardrails normally appear level. If they are starting to lean, you could have a slope issue. Toe erosion. If you see signs of erosion at the natural bottom of a slope — fallen dirt, debris, rocks, and such — the slope could already be starting to give way. Hummocky slope surface. Hummocks are small mounds, so a hummocky surface of a slope means it has humps or bumps that aren’t natural features and may be telling you the underlying ground is moving. Moisture changes. Water is the chief culprit in most landslides, loosening soil structures and causing the combination of gravity and shear forces to trigger a slide. If water starts showing up in new places, or if places that used to be wet or hold standing water are suddenly dry, there’s a strong possibility that the local geology has shifted.
- 8. Indications of Impending slope failure Many more symptoms will show up in a comprehensive slope stability analysis conducted by experienced geotechnical engineers who have studied these risks in depth. Each slope is subject to the unique characteristics of its gradient, underlying soil and rock, and moisture content. Any roadway or building on the slope adds to the complexity of the stability equation. Furthermore, geographic regions have distinct soil characteristics that make them either prone to slides or resistant to them. Sound A rumbling sound that is getting louder or the sound of trees cracking and boulders crashing against each other likely indicates that a sudden mass ground movement is in progress.
- 9. Common Causes of Slope Failure In order to determine where to begin the slope failure mitigation in your property, you must first take the time to understand the factors that lead to slope failure in the first place. Here are some of the common causes of slope failure: Steepness of the Slope Water and Drainage Soil Composition Vegetation Bedding Planes Joints & Fractures Sudden Shocks
- 10. Common Causes of Slope Failure
- 11. Common Causes of Slope Failure Steepness of the Slope The natural tendency of steep slopes is to move some of its materials downwards until the natur.al angle of repose is found. Any form of slope modification, whether it be through natural means such as a stream undercutting the banks of a river or by workers removing a section of the slope’s base to build roads, will impact the stability of a slope Water and Drainage During heavy rains when the soil becomes saturated and water takes the place of air between the grains of soil, the earth in slopes becomes a lot heavier. Water also reduces grain-to-grain contact which, in turn, reduces cohesiveness and the soil’s angle of repose
- 12. Common Causes of Slope Failure Soil Composition Different types of soils will have very different characteristics when it comes to frictional resistance to erosion and cohesion among the grains. Soils that have a large amount of clay, on the other hand, tend to expand when exposed to water; this makes them heavier and more prone to movement. Lack of Vegetation Vegetation, specifically its roots, holds the soil in place and makes it more resistant to erosion. The bigger the size of vegetation, the more widespread its roots are and the more it is able to hold the soil in place.
- 13. Common Causes of Slope Failure Bedding Planes A bedding plane is basically a surface that separates a layer of stratified rock or bed from another. Because of their nature, exposed beds in a slope are also at a high risk of slope failure. Joints & Fractures Joints and fractures are natural cracks in the rocks forming a slope. Because of these cracks, the cohesion between the rocks that make up the slope is greatly reduced, increasing the likelihood of a landslide in the slope.
- 14. Common Causes of Slope Failure Sudden Shocks sudden shocks like earthquakes, hurricanes, volcanic eruptions, the passage of heavy trucks, blasting, and others may trigger the sudden mass movement of the soil in slopes. Based on the above discussion on the common causes of slope failure, you should realize that repairing a slope and preventing a landslide is not a simple task Band-aid solutions will only be a waste of money; they could also give you a false sense of security, which could ultimately cost your a risk.
- 15. Types of slope movement In setting up a movement monitoring program it is useful to have an understanding of the type of movement that is occurring. This information can be used to select appropriate instrumentation for the site, and assist in interpretation of the result. The type of movement is related to the failure mechanism and this information can be used to ensure that an appropriate type of stability analysis is used. That is, outward and downward movement at the crest and bulging at the toe would indicate a plane or circular failure, whereas horizontal movement at the crest only would be more indicative of a toppling failure.
- 16. Types of slope movement The following is a discussion on common types of slope movement, and their implications for slope stability. Initial response When a slope is first excavated or exposed, there is a period of initial response as a result of elastic rebound, relaxation and/or dilation of the rock mass due to changes in stress induced by the excavation. This initial response will occur most commonly in open pit mines where the excavation rate is relatively rapid. The rates of movement during initial response periods decreased with time and eventually showed no movement. Based on the monitoring carried out at Palabora, the following relationship has been established between the rate of movement V (mm/day) and the time t (days). V = A e−bt where A and b are constants that are a function of the rock mass properties, the slope height and angle, the mining rate, external influences and the ultimate failure mechanism
- 17. Regressive and progressive movement Following a period of initial response and then possible stability, slope “failure” would be indicated by the presence of tension cracks at, or near the crest of the slope. The development of such cracks is evidence that the movement of the slope has exceeded the elastic limit of the rock mass. However, it is possible that mining can safely continue under these conditions with the implementation of a monitoring system. Eventually, an “operational slope failure” may develop, which can be described as a condition where the rate of displacement exceeds the rate at which the slide material can be safety mined.
- 19. Long-term creep In contrast to the rapid excavation, and the consequent large scale, relatively fast movements that take place in open pit mines, mountain slopes may creep over periods of hundreds of years. Long-term creep may occur where there is no defined failure surface, such as a toppling failure Or where the change in slope geometry is very slow, for example, due to stress relief following glacial retreat or erosion at the toe by a river. Other causes of such long-term movement are historical earthquakes that each cause displacement, and climatic changes that result in periods of high precipitation and increased water pressures in the slope.
- 20. Surface monitoring methods In general, monitoring of the surface of a slide is likely to be less costly to set up and maintain than sub-surface measurements that will require drilling holes to install the instruments. However, surface measurements can only be used where the surface movement accurately represents the overall movement of the slope. For example, it would not be appropriate to make surface measurements where loose blocks of rock on the surface were toppling and rotating independently of the main slide movement. MONITORING EQUIPMENT Crack width monitors Surveying Laser imaging Tiltmeters Global positioning system Synthetic aperture radar
- 22. Sub-surface measurement of slope movement is often a useful component of a monitoring program in order to provide a more complete picture of the slope behavior. Incases where surface monitoring is not feasible, then sub-surface measurements will be the only measurements available. The main purpose of these measurements is to locate the slide surface or surfaces, and monitor the rate of movement; In some cases the holes are used for monitoring both movement and water pressures. Borehole probes Inclinometers Time–domain reflectometry Sub-Surface monitoring methods
- 23. Analysis of Highwall Slope Failure –Case Study
- 24. Geology of the mine A total of 153 boreholes were drilled in the block with a cumulative meterage of 14472.03 m. The density of bore holes is 32.95 per Sq. Km. The sub-surface data reveals that the Gondwana sediments rest un- conformably over the basement rocks of Proterozoic age, namely, the sullavai group of sequence. The general trend of the coal measures is NW-SE with northeasterly dips which are in conformity with regional trend.
- 26. Hydrological conditions of the Highwall slope The dip side area of Medapalli OCP from bank of the Godavari River is characterized by flat to gently undulating terrain. The surface RL is about 740 mRL. River Godavari forming NorthEastern boundary of the OCP. The highest flood level of Godavari river was recorded as to 746.60 mRL during the monsoon of 1983. The river sand bed level is at 735 mRL. The usual water level in rainy season is about 739 mRL. The opencast area is situated on the southern bank of Godavari River
- 27. Slope stability analysis Stability Analysis of Highwall using FLAC/SLOPE
- 28. conclusion Prevention, as they say, is better than a cure and in the case of slope failures, prevention measures are cheaper than repairing the damage afterwards If your property is on a slope, you need to know the common indications of an impending slope failure. he more you know about the symptoms of slope instability, the earlier you can deal with the problem and, consequently, the less your expenses will be This will also enable you to prevent (or at the very least substantially decrease) the likelihood of people getting hurt because of slope failure.