What remedial measures could be adopted to overcome an imminent mass movement?

Answer 1

That depends on the exact nature of the mass movement!

Rock masses:Areas that may be prone to rock falls or rock slides are commonly assessed by an engineering geologist. They will attempt to identify regions where the rock mass forming the slope or cliff face is at risk of failure.

These regions are likely to be those where there is weak material (for example a band of shale in other higher strength sedimentary rocks) or areas where the structure of the rock mass makes a failure more likely to occur. An example might be where the bedding planes or joints within the rock mass dip steeply towards the slope face.

When they identify areas where failure is a potential risk, they will then suggest a solution depending on the exact nature of the potential problem. Solutions may include rock bolts which are used where discrete blocks in a rock mass might fall from the face of a cliff and the bolt or anchor essentially acts to secure the block to the stable rock mass behind. Rock anchors perform a similar role to rock bolts but are used where greater restraining forces are required.

Other solutions include the scaling of the face. This involves removing loose material and small loose blocks that have the potential to fall. The engineer may also recommend that retaining structures are built in front of certain portions of the weak rock mass to act to support weakened material. Rock catch netting may also be attached to the face of a potentially unstable slope to catch small blocks which may fall from the face. Larger collapse may be halted by rock catch fences which are high strength steel posts secures firmly into the ground and which are connected by rows of high tensile strength steel cable. These are placed at the base of slopes or cliffs to halt any larger blocks which may fall from the cliff face.

Another solution which is commonly used before the application of rock bolts is "shotcreteing". This is when a reinforcing steel mesh is bolted to the rock face and then high viscosity concrete is sprayed onto the face. This sets and acts to support the rock face. An extreme solution may be to actually trim the rock face back to an angle that is stable however this is commonly not possible / economic.

Soils / Soil slopes:If the mass movement is acting in soil slopes there are a number of potential solutions which commonly involve either increasing the strength of the soil mass or reducing the stress acting upon it.

Soil nailing is a process whereby steel bars are grouted into holes drilled into the unstable soil slope face. These are places in a regular array and act to reinforce the soil by increasing it's tensile and shear strength.

Mini piling is similar in concept to soil nailing however the piles are commonly of higher strength and larger dimensions than a soil nail and so would be used in situations where a greater restraining force is required to support the soil mass but also require more extensive and expensive works to place.

Other solutions include lime stabilisation which involves boring columnar holes into the soil and mixing a lime or other grouting material into the soil which acts to increase the soil strength, inhibiting the formation of shear planes and halting soil movement.

Another solution may be de-watering which is a process where the groundwater table is lowered. This acts to reduce the pore water pressure in the soil mass in turn increasing the in-situ stress and hence the shear strength of the soil which can halt movement.

Another potential solution is the planting of vegetation on the soil slope whereby the roots will act to bind the soil mass increasing it's strength and acting to inhibit the formation of shear surfaces which are commonly necessary for slope failures / movements to occur in soils. Vegetation will also act to reduce the infiltration rate of water into the soil which can reduce the fluctuations in pore water pressure which will act to increase the stability of the soil.

The above is a brief summary into methods that may be used to halt mass movements.