How to recognize and prevent slope failure
Whether a slope is made of natural soil or manmade and maintained, steep inclines are prone to failure. In the case of natural slopes, the soil composition, degree of vegetation cover, moisture content, and water level are all factors that can accelerate slippage and sloughing in slopes. Unfortunately, slope failure can be incredibly dangerous both in structural integrity sense as well as to human safety. Here are some important things to know about recognizing and preventing slope failure on your property before it’s too late:
Slope Steepness
Each soil type has its own internal friction angle. This angle effectively informs the engineer to what degree that soil can be stacked without sliding. For example, if a grain of sand has an internal friction angle of 35 degrees, then any pile of that sand you create would tumble down to a natural settlement whose sides match about 35 degrees as well. If you know the approximate internal friction angle of your soil type, then you can judge if it has been compacted in too steep of a slope for it to maintain its shape and not slip dangerously over time. Sometimes altering the soil’s composition is another technique for increasing the internal friction angle of the whole.
Moisture Content
Soil is compacted to a certain degree and always contains a void space. This void space is either air or, especially during saturation, water. Water can affect numerous factors, including a high moisture content that creates over-saturation, an elevated water table that changes compression and compaction characteristics, and even the freeze-thaw cycle as higher amounts of water in the soil will freeze, expand, then melt and shrink again. Increased water content also increases the density of soil as it displaces the much lighter air pockets. Monitoring the water content of your soils can prevent the disasters that accompany buckling and other geotechnical phenomenon.
Vegetation
Planting different species on a freshly compacted soil slope is a key strategy for adding strength to it. With their roots, plants are able to hold soils together against the impacts of heavy rains, winds, and other forms of erosion. As erosion and scouring can be a major source for undermining slope integrity, a simple solution such as adding vegetation is a great way for controlling the slope against failure.
Rock Integrity
The integrity of rock located below upper soil layers can greatly affect the overall integrity of the slope once damaged. Sometimes the bedding planes in rock can be layered with inadequate materials that react differently as water content levels change. In addition, rocks naturally form cracks such as fractures and joints. These changes are often due to cooling or erosion that impacts rocks in upper layers. As cracks increase in the rocks, the structural integrity decreases so that, over time, the slope is more prone to fail due to these slight changes. Of course, sudden shocks to the rocks – and the soil – also have the potential to cause sudden failure like sloughing in the slope.
Solutions for Fixing Failing Slopes
Structural engineering provides us with many techniques – even beyond vegetation – for fixing slopes we fear might fail. To ensure slope stability in constructed soil slopes, one common method is to take soil core samples, determine the stratigraphic layout of the soils, and then cut benches into the weaker soil. Next, a more stable and uniform soil type can be placed overtop to promote slope stabilization. Similarly, digging out soft soils and replacing them with gravel that is less compressive is one way to prevent failure above slopes which trickles down to the slope stabilization below. Other solutions structural engineering provides solutions like retaining walls, thorough soil compaction, and drainage methods to prevent saturation and seepage.
Slope stability is a serious safety issue that can be both remedied and prevented. By knowing these things to look for in a slope, one can address issues before they become threats to structural integrity. If constructing a new slope, it is vital to collect the information that will show what kinds of soils and soil behaviors you’ll be working with. Once you know your stratigraphy, moisture contents, and water levels, you can predict the kind of challenges you need to plan for and ensure your inspector oversees every detail of the compaction process. If you are monitoring an existing slope, you know the key factors to keep an eye out for such as slight changes in shape or saturation due to a natural storm event. Rainfall, however, is not the only culprit: Earthquakes also are capable of causing severe destruction to slopes. Just being a little proactive can prevent many structural disasters – and could possibly save people from being seriously injured.
