Construction of buildings in itself is a big, big project. It’s something that sets human beings apart as a species. And we have broken so many barriers when it comes to choosing a site to construct anything. When you look at construction projects being taken up under the sea floor, or hundreds of feet below ground, the remarkability of constructions on a slope diminishes. Nevertheless, this is one of the basic challenges a civil engineer faces and plays an important role in preventing damages through disasters like earthquakes and landslides.
An important factor taken into consideration in such situations is the slope stability analysis.
Slope stability analysis
Slope stability analysis is conducted to assess the stability of man-made or natural slopes and it’s equilibrium conditions. The objective of this analysis is to find areas that are vulnerable to failure/collapse and take appropriate remedial action.
Some of the places where this analysis is used is in construction of buildings on mountain sides, road cuts, excavations, open-pit mining etc.
Before the use of technology, this analyisis was conducted by hand-held calculators or graphs. Currently, many softwares have been written that can conduct this analysis with upto a millimetre in precision. The trend of involving risk assessment method in this analysis is also a recent one that is gaining wide popularity.
Some of the methods used are-
- Limit equilibrium analysis- under which we have many different methods like Spencer’s method, Sarma’s method etc.
- Limit analysis
- Kinematic analysis
- Rock fall simulators
- Numerical methods of analysis- under which we have modelling techniques like continuum modelling, discontinuum modelling etc.
Whatever the method, there are some things we need to keep in mind while performing this analysis, and that is the focus of this paper.
Preventive measures while conducting the analysis
The preventive measures for slope stability analysis depend on the method used to conduct the analysis, and the purpose of analysis. In light of this, here are the preventive measures for some of the popular methods of analysis.
- Method of slices– This is the most popular limit equilibrium technique. It divides the soil mass over the slope into vertical slices. The ambiguity that arises from this method is that different limit equilibrium methods use different assumptions in the use of this technique that leads to different values of the factor of safety. It is therefore possible that one limit equilibrium method may classify a slope as stable (meaning factor of safety is above 1) while another may classify the same slope as unstable (meaning factor of safety is below 1).
It is therefore, important to recognize the purpose of the analysis and accordingly select the appropriate limit equilibrium method to conduct it.
In this view, there are 2 types of methods of slices; non rigorous and rigorous methods. Rigorous methods satisfy all 3 equilibrium conditions; force equilibrium in horizontal and vertical direction and moment of equilibrium conditions. Therefore, it provides a more accurate factor of safety than non rigorous methods.
- In terms of methods used in general, a method that is more rigorous than limit equilibrium analysis is the limit analysis. The limit equilibrium analysis makes certain assumptions in its calculations, which is not done in limit analysis. This enables us to identify the upper and lower bounds of the factor of safety.
Nevetheless, the purpose of the analysis needs to be taken into consideration here. If the purpose is something as simple as determining the collapse of an embankment, then we need not waste time and energy doing the limit analysis. Our purpose will be solved with just the limit equilibrium analysis.
Limit analysis should be done when the project is large, and high level accuracy is required.
- If we opt for numerical method of analysis, it is important to note that some of the inputs that are required for this modelling are not measurable and the input data is therefore poor. Care should be taken accordingly, and only an experienced modeller will be able to provide accurate results.
- The analysis that we take is generally for the area under consideration for a structure. But that does not mean that the areas surrounding our area of interest are also as stable as our area of interest. This needs to be taken into consideration, especially when it comes to building protective measures.
For example, consider a house being built on a hill slope which has a factor of safety as 1.1. The slope is therefore stable and it is safe for us to build our structure. Nevertheless, it is seen that there is an imminent danger of rockfall due to instability upslope of this area. Embankments will have to be build to shield our structure from a possible rockfall. If this is not taken into consideration, then we are risking the destruction of our structure.
- Risk assessment- This is the qualitative or quantitative assessment of two factors of a particular object; the magnitude of potential loss and the probability that that loss will occur.
It is wise to conduct a risk assessment of the slope along with slope stability analysis as this provides another dimension to our analysis which was not known previously. It takes into consideration things that a slope stability analysis does not. For example, the possibility of landslides, earthquakes, massive rainfall etc.
Structures to protect/prevent slope failures
Once the analysis is complete and the structure has been built, we can take some measures to protect or prevent the slope from failing. These are generally structural enhancements done on our part to improve the stability of the slope.
Below is a picture describing some of the human structures that can be made to increase slope stability-
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
Prevention of landslides is another effective way to ensure that the slope stability remains high. This can be done by the elimination method, where you re removing the danger of the landslide.
- You can do this by rerouting the sliding material via drains.
- You can create a trench for the sliding material to collect in.
- You can remove all unconsolidated material on the slope to ensure nothing will slide at all.
- You can create a bridge between the structure and the sliding material.
The second method for preventive landslides is the controlling method, which involves-
- Control of surface water, by redirecting it via drainages and not allowing it to carry earth material that could lead to slope failure.
- Buttresses can be constructed that are a good foundation to any structure and provide it with extra stability. Not just buttresses; any kind of retaining wall like gabion walls or piling can be used.