Types
There are many ways to categorize slope failures, and your book gives a table and series of figures in Figure 10.5 (p 332) which categories slope failures based on “Materials,” essentially, consolidated materials (rock) and unconsolidated materials (soil). Here we are going to add another category to the table above “Materials” to help us to relate slope failure to the resisting and driving forces we just discussed on the pervious pages. As you can guess, that category is going to be the “Role of Water.”
The Role of Water
Water plays an important part in determining slope failure as it is both a means to resist or prevent slope failure and a means to drive or promote slope failure. How is this? Several reasons that we can think of in terms of short-term and long-term effects:
Short-term
Resisting Force – In small amounts, water aids in creating cohesion between particles. Think about what happens when you get the pages of your book wet: the pages stick together. This happens at the microscopic level between particles, especially, small grained particles. We learned this earlier in the Soils module when we discussed soil strength. This is especially noticeable in clay particles that have a platy shape. (Click here to see this page from the Soils module.)
Driving Force – In large amounts, sudden episodic downpours of water can actually overrun cohesive strength and result in sediments moving like mud.
Long-term
Resisting Force (weathering) – Exposure over long periods of time to water can assist in chemical weathering processes, making particles smaller and more able to stick together due to cohesion.
Driving Force (weathering) – Exposure over long periods of time to water can assist in chemical weathering and physical weathering (e.g., frost wedging) processes, fracturing and breaking up large consolidated rocks into smaller chunks. This is important in the formation of underground voids, sinkholes, and caves. (Think about infiltration and how groundwater resources form.)
Resisting Force (vegetation) – A climate which includes regular precipitation can result in rich and diverse vegetation growing along a slope. Plant roots can add to slope stability.
Driving Force (vegetation) – A climate which promotes the growth of plants can result in biological weathering of consolidated rock, fracturing and breaking up large consolidate slopes into smaller chunks. (Remember too, that vegetation itself has its pros and cons to slope stability.)
Stream bank erosion: This shows how streams can differentially erode a hillside (slope). The stream has undercut the hillside, resulting in an overhanging surface that eventually slides down toward the stream, widening the stream channel. (Image credit: Maryland DNR)
Resisting Force (erosion) – Remember that streams function is dominated by erosion at higher slopes. Over the long-term slow erosion of lose sediments can leave behind only the strong consolidated rock, creating a slope that doesn’t move suddenly due to slope failure, but one that is slowly eroded away. This, in essence, adds to the resisting force by leaving behind a slope that is mostly made of strong consolidated rock.
Driving Force (erosion) – The act of erosion itself can lead to slope instability, especially, if specific areas of the slope are differentially weathered and eroded. This happens in valleys when streams widen their stream channel. The stream “undercuts” the hillside creating an “overhanging” slope which eventually slides down as there is nothing to hold it up. See the picture below.