When a site is experiencing landsliding, it is a good idea to have some basic understanding of what might have caused this ground movement in the first place. Landsliding in soil occurs when the slope is weakened or loaded. Weakening typically occurs when the soils weaken over time (i.e. weathering), and the slope’s vegetative root structure, which was anchoring the soil, is removed. Undercutting the slope either naturally (e.g. stream erosion) or by man weakened or reduced the slope’s resistance to sliding. Loading the slope can occur when temporary or permanent loads are added to or placed on the slope (such as storage containers or stock piles of materials at the top of the slope), or when the soil slope gets soaked by excessive precipitation or when previously submerged slope is now exposed. Based on the above, it stands to reason that stripping and steepening the slope during land development causes the greatest damage to the slope and should be carefully evaluated.
Given the various phenomena which can exist as discussed above, the rate of sliding can vary significantly from a slow creep to a rapid failure. When dealing with an abrupt/quick sliding event some actions that can be taken are the following:
- Block off area to reduce hazard
- Can easily progress upward (rarely expands sideways without some causation component) – consider this as part of the hazard area.
- Cover/seal ground cracks from precipitation runoff
With slowly developing slope events, there will be signs of instability. These could include:
- Settlement at the top of the slope resulting in downslope tilting and separations in adjacent structures and flatwork.
- Cracking in the slope especially if roughly along the slope (e.g. not random network of cracking)
- Fence posts, poles, etc. titled downslope, trees leaning down slope, or if very slow trees curving upwards to compensate for very slow slope movement.
The above is illustrated in Figure 1.
It is important to note that any point in time a slow-moving event can turn abrupt.
Of course, there are other phenomena and technical issues involved when there is a sliding event than is given above. To properly assess and understand the sliding conditions and any hazards and to know how to properly remediate the event, a geotechnical investigation should be performed. For information on how to select the appropriate geotechnical engineering companies see: What to look for When Selecting a Geotechnical Engineering Company. It is important to note that is not advertised that contractor be hired to provide the fix without adequate engineering.