Landsliding: What To Do

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.

What are Cofferdams?

Cofferdams are temporary water barriers which are created to allow for dry construction to occur in a specified area. The water barrier most typically consists of sheet pile and internal bracing. Single wall cofferdams which have sheet pile sufficiently embedded into the soil to resist the outside water pressure are called a cantilever design.

FIGURE 1: Example of constructed cofferdam.


Where internal lateral support is provided to assist to resist the water load, the cofferdam is considered a braced cofferdam.  Where the retained water gets quite high, cellular cofferdams are used. Cellular cofferdams are connected cells, or sheet pile bins, which are filled with soil to provide dead weight and resistance against the external water pressure.

 Once the cofferdam enclosure is completed it is pumped out and the specified construction can begin.  Figure 1 shows photographs of some examples of the constructed cofferdams. Figure 2 is a photograph of a cellular cofferdam.

FIGURE 2: Photo of Cellular Cofferdam. Photo from C.J. Mahan Construction Company.


More information on cofferdam design can be found in: Update #47: Updated Improved Methods of Cofferdam Design

 

What to Look for in a Mine Subsidence Expert

The most appropriate mine subsidence expert for your case depends on the nature of the problem to be investigated. Mine subsidence investigations can require various expertise depending upon the focus of the problem. Some of the questions that may need answers are:

• Is the damage from mine subsidence?

• Will the underground mine result in subsidence in the future?

• If subsidence were to occur, what is the range of movement you would expect?

• If subsidence does occur, how severe could the resulting damage be?

• Was the mine designed properly so that mine subsidence would not result in the future?

• How can we mitigate the subsidence risk to a tolerable level?

• Can you stabilize the mine and how much would that cost?

As you can see from the above, the subject of mine subsidence can actually involve a number of expertise depending upon focus of the investigation. Also, there is the context of the expert mine subsidence investigation: Is it being done for existing or new construction, mine design, review of a mining permit application, or is it for tort litigation? Therefore, in addition to having the technical know-how, and expert with oratorical skills may also be necessary.

You may also be interested in:
Issue #14: Establishing Mine Subsidence Risk
Traits to Dig For in an Engineering Expert
Issue #24: Anatomy of Grouting Mine Voids
Issue #25: Transmission Pipeline Subsidence from Mining
Issue #27: Borehole Radar Used to Identify Deep Coal Pillars

What to Look for in a Karst Subsidence Expert

Karst subsidence are typically in the form of sinkhole to bowl-shaped depressions. They can occur unexpectedly and fairly abruptly and can cause significant damage. Because of their erratic geologic nature, karst it is often difficult to quantify the subsidence risk and associated damage potential. Therefore, the expert which is hired should be well versed in all the subsidence engineering aspects of interest.

More common questions the karst subsidence expert will be asked to answer are:

• What is the chance that there will be subsidence in the future?

• If there is a subsidence(s), how severe will it be?

• If there is subsidence, how much damage can we expect?

• What are my options to reduce the risk of subsidence in the future?

• A sinkhole has appeared, what do I do?

• Is there any way to virtually eliminate the risk of subsidence in the future?

Subsidence investigations in karst terrain are most commonly related to new construction or encountering unanticipated subsidence or other karst features which disrupt construction progress, insurance claims, or subsidence damage. Given the context of the investigation and the amount of risk which may be involved should determine the level of expertise that the subsidence engineer should have. If tort litigation is involved the karst subsidence expert should also be competent in this area. Also, with greater knowledge and experience in karst subsidence problems, the more cost effective the solution.

You may also be interested in:
Issue #34: Risk Investigation of Karst on Sinkhole-Subsidence Prone Lands
Traits to Dig For in an Engineering Expert

Direct Shear Tests

Introduction:
An accurate determination of soil shear strengths in the laboratory is essential to cost effective design and planning for geotechnical projects. One such laboratory method available at MEA is the Direct Shear Test. The test is commonly employed for geotechnical projects where soil shearing along a horizontal plane is anticipated. This applies to landslide remediation, slope stabilization, and pipeline analysis projects. The direct shear machine at MEA can perform tests over a large range of strain rates ranging from 0.00001 to 0.49999 in/min; these rates are suitable for both geotechnical and research projects. MEA’s direct shear machine offers a more customizable strain rate than standard direct shear apparatuses whose rates range from 0.0001 to 0.3 in/min. Figure 1 is a picture of our direct shear machine.

FIGURE 1: MEA’S DIRECT SHEAR MACHINE

Method:
The purpose of a direct shear test is to determine the shear strength of a soil; this is done by forcing the soil to shear along an induced horizontal plane of weakness at a constant rate. A minimum of three subtests are run where the sample is consolidated using three different weights and then sheared at the same constant rate. The peak shear stress and residual shear stress of each subtest are recorded and graphed. Using this data, the cohesion and friction angle of the soil can be determined. Figure 2 below shows an example test plot with five separate subtests used to complete the test. The soil in question had no cohesion and a peak friction angle of 40.5º and a residual friction angle of 37.9º.

FIGURE 2: AN EXAMPLE TEST PLOT FOR A DIRECT SHEAR TEST WITH FIVE SUBTESTS. “P” IS PEAK SHEAR STRESS AND “R” IS RESIDUAL SHEAR STRESS.

Landslides

Landsliding refers to the downward shifting of the ground on a slope. The slope can naturally exist and/or be manmade. The reason for the shifting is either there has been added force to the slope or the slope has been weakened. There are a number of ways downward force can be added to the slope. Some of the more common applications which can cause landslides are:
  • The slope is steepened and/or heightened with fill soil
  • Slope becomes water logged and thus made heavier
  • A structure is constructed on or close to the slope
  • The water level in the waterway at the bottom of the slope dropped significantly causing downward seepage forces.
More typical causes for landsliding from slope weakening are: waterway bank erosion; man-made undercutting or excavations along the slope; removal of root reinforcement from vegetation; and weathering of the soil mantle. Landsliding can occur slowly to abruptly with little warning. More typical tell-tale signs of slope instability are ground cracking along the slope which is most commonly towards the upper portion of the slope; trees, poles, fences, etc. which are leaning downslope and ground surface bulging of heaving near the bottom of the slope. 

Rotational Landslides

Diagram of an idealized landslide showing commonly used nomenclature for its parts. Courtesy of the Utah Geological Survey.