How to Control Mass Movements or Sliding of Rocks ?

Since most slides are very quick in their occurrence, taking not more than a few minutes, it will be futile effort to check the falling or failing mass when it has already left its original place. In most cases, however, where the weakness of the area has been established through its ‘past history or has become evident on the basis of recent studies, the strength of the mass may be improved quite effectively by using one or more of following techniques and methods.  Before actually deciding about the methods for controlling a potentially unstable area, it is always essential, as a first step, to compile a history of a slide area (actual or potential). Such a study should reveal the areal extent as well as the depth up to which the mass is potentially unstable and the extent and the frequency with which it has failed in the recent past. 

This should be followed by a detailed geotechnical examination of the slide area which should throw light on : 

1. Composition of the failing mass whether it is entirely soil or rock or a mix of the two; 
2. Structural disposition of the mass - especially dip and strike in stratified rocks and presence of planes of weakness
3. Position of the groundwater table within and around the critical mass
4.  relation of the mass prone to failure to any surface water body 
5. the slope of the ground

 Such an analysis will yield enough data for determining an economical factor of safety against  sliding for different possible modes of failures. Many methods for controlling the potential slides are available and choice of any method will depend on factors like nature of the potential slide, the underlying cause for it, the nature and amount of material (likely to be) involved in it and the economic considerations. Of many such methods, the most important are as follows :

• Drainage
• Restraining Structures
• Slope Reinforcement by Rock Bolting
• Slope Treatment

So , These 4 terms to Control Mass movements or sliding of Rocks is Well Explained in the Upcoming Sections of this Post .

Drainage

A very old saying about landslides quite close to hearts of most engineers even today is “Water is always the cause, and, drainage is the first cure” seems quite fundamental to this hazard. Drainage involves the removal of water from within the mass as well as preventing any further water to reach the material susceptible to failure. This may be achieved either by surface drainage or by subsurface drainage or by both methods. For diverting the surface flow, a series of drainage ditches at the top of the slope may be necessary. These ditches may be lined to prevent erosion of their sides by water. Similarly, in very water-sensitive slopes cut-off trenches back-filled with asphalt or concrete may be given to seal-off surface flow from a particular direction to reach the slope. Slopes may also be covered by granular material resting over filter fabric to remove excess precipitation without much risk of infiltration. Similarly careful scanning of cracks and fissures on the surface of the slope and their filling with cement, bitumen or clay mixture will help in reducing the infiltration component to a good extent.

Interception of groundwater, especially where there is enough evidence of its presence, must form an integral part of drainage system in an unstable slope. This may require digging deep interception drains at proper levels. Such drains will help in reducing the risk of development of high pore pressures. Use of counterfort drains has been advocated by many that serve the dual purpose of removing the groundwater and providing some support to the mass. In this method, deep trenches are cut into the slope. These are lined with filter fabric and then filled with granular material which acts as a supporting buttress. It is absolutely essential that such counterfort drains should be located below the potential failure surface otherwise the material used for buttressing will increase the weight and chances of failure. Oiling of slope surfaces, electro-osmosis, and heating of the slope material have been also used in different countries to stabilize slopes by reducing chances of infiltration during heavy rains. 

Restraining Structures 

Many a minor potential slides can be and are treated with the help of restraining structures such as retaining walls or buttresses. All such constructions at a proper location across the slope are aimed at stopping the moving mass by force and hence their success depends, to a great extent, upon a careful and correct analysis of the status of forces and weights acting in a given slope.Thus, in a simple case of a slope with an inclined potential plane of failure, the rock mass has two forces acting on it. 

W sin K -  in the direction of potential slip, the driving force. 

W cos K* tan (phi)  as resisting force. 

The slope will become unstable if driving forces exceed the resisting forces. As such, retaining structure ‘R’ will have to provide for this additional component to the resisting forces. Obviously, this additional force by way of the retaining structure will be useful only when it has actually added to the total resisting forces, otherwise the retaining structure may also get moved downslope on failure of the mass from above. 

1. Retaining structures may prove exceptionally successful where :
2. the ground is neither too fine nor too plastic, 
3. the sliding mass is likely to remain dry, and 
4. the movement is of a shallow nature and limited extent. 

Retaining walls may prove costly failures when they are designed to resist slides of great volume and thickness or long rising slopes. 

Reinforcement by Rock Bolting 

In recent times rock bolt and anchors have been used extensively for stabilizing rock slopes that were prone to failure. When the area of the potential failure is limited and the rock involved is jointed but not too soft, rock bolts are used to tie-up the different blocks together thereby improving its general stability Rock anchors are used when large areas require stabilizations, e.g. for stabilization of foundations for major engineering structures like dams and power houses, A rock bolt is a steel bar of suitable diameter (02-25 mm) and length (60 cm to 5 meters) one end of which 18 designed for expanding and the other end is threaded to take a nut and washer. Such a bolt is variably inserted into a hole drilled 1n the rock at a proper angle with the plane of weakness and then its end within the rock 1s made to expand whereby it fits tightly into the rock. The other :an is tied on a plate with the help of a nut and washer. The rod ls generally prestressed, and is always placed in tension When tied up in the above fashion, the rock block held up within the two ends of the bolt gets compressed and hence stabilized against falling off/or sliding easily. 

There are many varieties of rock bolts, of which the slotted bolts, the expansion-bolts and the groutable rock bolts are very commonly used in stabilization of slopes and also for roofs in tunnels. The slotted bolts have flame-cuts in the end to be inserted in the rock. A wedge of proper dimensions is first inserted in the hole and then the bolt is hammered in. The slotted end expands and thus the bolt gets fixed within the body of the rock. In the expansion-type bolt, the end which goes into the rock is provided with a shell that opens up on giving a torque to the bolt. 

Rock anchors are structural elements made up of cables, strands or bars that are, like bolts, placed in previously drilled holes and then whole or a part of them is bonded to the rock using a proper technique. They may be tensioned after their placing in the hole during, before or after grouting which is an integral part of the anchorage system. Anchor system may exceed 20-30 m in length and once installed they modify the original stress field of the rocks to a considerable extent. Many improvements have been made in the use of rock anchors for achieving slope stabilization such as : 

1. by using resin bonds instead of plain cement grouting
2. by introducing corrosion-resistant designs and materials for the cables that are supposed to remain within the ground permanently as an integral part of the rock system. 

Slope Treatment 

Under this heading we may include all those methods that are used to stabilize the slope, which is likely to fail by treating the top layers. If it is a rock or mixed rock-soil slope, guniting may often help to a great extent. Gunite, as we know, is pneumatically applied mortar or concrete. The mixture of cement and sand (1:3) with little water is applied on the face under pressure and is ]mom to develop sufficient strength on setting and hardening. When the slope is made up of soil, treatment may involve stability computations for the particular type of soil and the slope conditions.

If Such computations indicate that a given slope of soil will not be stable under the given conditions, then the solution may lie in: 

1. flattening the slope to ensure stable limits, 
2. decreasing the load.

Digging rock traps in the form of ditches at the foot of a slope and providing benches at proper intervals are also useful measures. Afforestation of potentially unstable slopes reduces the risk of their failure considerably. Vegetable cover, especially of deciduous trees and plants reduces the quantity of infiltration. It also contributes to the loss of moisture by evapotranspiration thereby reducing the volume of water for causing failure. While devising a slide-control program for an unstable area, it is always useful to weigh the relative merits of methods available. More often, it may be a combination of methods rather than a single method that may have to be used for stabilizing the slope. 

I hope that every term related to Mass Movement Prevention and Taking Care of Sliding of Rocks is Now Cleared , If not then Please go through all the sections of the post carefully.