In this article we will discuss about:- 1. Introduction to Mass Movements 2. Classification of Mass Movements 3. Monitoring and Control.
Introduction to Mass Movements:
In many regions of the world a temporary instability of superficial mass of soil and rock has always been an acute problem. These superficial masses may leave their original positions abruptly or extremely slowly and start either a downgrade movement or vertically downward sinking thus giving rise to baffling situations. These movements of the ground may entail loss to property and life, especially when they happen to occur in or near the populated areas, along highways, railway lines, dams and reservoirs, tunnels or under heavy structures.
Such movements of the superficial masses have been termed in common man language as Landslides or landslips; in technical language, however, these have been grouped as Mass Movements and form a major subject of study for a geologist and a civil engineer concerned with stability of the civil engineering projects.
Classification of Mass Movements:
Many classifications have been suggested for mass movements. This is attributed to a number of factors such as wide variety of geological situations in which these movements take place, to a great heterogeneity in the type and quantity of material involved in failure, the nature of the surface along which failure takes place and the speed with which failure occurs.
No single classification can encompass all these characters of Mass Movements and yet remain a simple classification.
Under the present classification, all mass movements are divided into three groups on the basis of type of failure:
B. Sliding and
Each group is further subdivided into classes on the basis of rate of movement, nature of the mass involved in failure and degree of saturation where applicable. The emphasis has been on the simplicity of classification rather on detailed technical characteristics. The classification is summarised in Table 20.1.
By flowage is understood a downgrade movement of mass along no definite surface of failure. Mass involved in this type of failure is primarily unconsolidated or loosely packed or rendered so by natural processes of decay and disintegration. In flowage of unconsolidated or poorly consolidated mass each grain or unit of grains behaves as if it has its own surface of shear failure. The result is that the movement is distributed throughout the mass and in a highly irregular manner.
Flowage is further distinguished into slow and rapid flowage. In the first group, failure is not easily perceptible. The ground may be moving downslope at as such low rates as few centimeters a year or even less. In rapid flowage, however, the movement of failing mass may be easily visible and the mass may travel a few metres or more a day. The conditions causing flowage in the two classes may be closely related or entirely different.
A true landslide is a type of mass failure in which a superficial mass fails by moving as a whole along a definite surface of failure. The surface of failure may be planar or semicircular in outline. It is often characteristic of a landslide that the mass above the failure surface is unstable whereas the material lying below this surface is generally stable.
Moreover, sliding may involve material of any composition, shape and of varying degree of consolidation – loose soil, rock fragments and whole blocks or slabs of rock. In unconsolidated deposits, loose, inherently weak rock masses and weathered top surface, sliding commonly takes place along curved shear surfaces.
But when the mass involved is hard, brittle and coherent, such as massive igneous, sedimentary and metamorphic rocks, shear surfaces are broadly planar in character. In such cases, a set of joint planes or bedding planes or fault planes may be the most convenient natural planes of failure under specific circumstances favouring sliding.
Landslides are further distinguished into Translational Slides, Rotational Slides and Rock Falls on the basis of the type of movement involved in the failure:
(i) Translational Slides:
The surface of failure is generally planar in character, speed of failure is quite rapid and the nature of mass involved in failing may be rock blocks, rock slabs, debris and soil cover or even a mixture of all of them. These slides are quite frequent on slopes made up of rocks and cohesive soils.
(ii) Rotational Slides:
In such slides, the failing surface is generally curved in character and the speed of failure is also quite rapid. Because of the nature of the failing surface, the movement of the mass takes the form of a sort of rotation, rather than translation. The material involved in failure tilts downwards at the rear end and heaves up at the front or toe.
There may be a single surface of failure or a number of them adjoining to each other. Generally, where a layer of clay underlies a slope made of mixed character, the former may develop conditions favourable for single or multiple types of slips in a variety of manners.
(iii) Rock Toppling and Falls:
These are grouped along with slides although there may be little or no sliding involved in their failure. This is for the simple reason that they are commonly associated with or accompany the landslides and because they are essentially a slope-failure phenomenon. In the falls, there is almost a free, sudden and fast decent from a steep slope.
Previous to fall, conditions leading to deterioration of the coherent rock or soil into disjointed mass with removal of link from the main strata dominate on the slope. Such conditions are favoured by weathering of rocks on the slopes due to climatic changes e.g. frost action, expansion and contraction, leaching of natural binders etc. Obviously, these are the surface layers of rocks and soils on the steep slopes that are likely to fail in the form of falls under the above conditions.
It is defined as sinking or settling of the ground in almost vertically downward direction which may occur because of removal of natural support from the underground or due to compaction of the weaker rocks under the load from overlying mass. The net result in each case is that the surface material – natural ground or artificial fill – suffers a sinking downward movement.
Monitoring and Control of Mass Movements:
Much work has been done in different countries to develop adequate methods by which slides could be predicted and prevented but not much success has yet been achieved in these directions. Even then, there has been some development in the direction of monitoring the stability of masses in critical areas in devising techniques that could improve the strength of rock slopes.
Prevention of slope failures or landslides, however, will remain an unsolved problem in its true perspective. Our attempt should be to determine the causes leading to instability of an area of some concern to man, monitor the conditions and check the failure before it actually takes place.
Mass movements taking place in mountainous areas or in periglacial regions and as seasonal phenomenon may be understood to enrich our knowledge about them as natural phenomena but their large scale monitoring and control are better reserved for future research.
In the following paragraphs we shall discuss only monitoring and control of critical slopes as those along highways, or reservoirs or tunnels:
By monitoring of slope movements is understood detection well in time (before the failure actually takes place) of conditions or symptoms that are indicative of imminent failure of the slope. It may or may not be possible to check the failure on the basis of such an advance information. At the same time the data collected through monitoring systems may be of great help in devising control methods for subsequent failures.
Monitoring of slopes may be achieved by using conventional surveying techniques where minor displacements could be recorded and interpreted as prelude to some major failure. In the more sophisticated studies the change in distances may be measured by using electronic equipment, laser equipment, settlement gauges and extensometers. Since pore water pressure is an important factor in causing mass displacements, this must also be monitored in a sensitive area, with the help of piezometers.
In devising a monitoring system, the cost-benefit ratio has to be kept in mind in all the cases because such systems are invariably expensive.
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 slidfe area which should throw light on:
(i) Composition of the failing mass – whether it is entirely soil or rock or a mix of the two;
(ii) Structural disposition of the mass – especially dip and strike in stratified rocks and presence of planes of weakness;
(iii) Position of the groundwater table within and around the critical mass;
(iv) Relation of the mass prone to failure to any surface water body; and
(v) 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:
(a) Providing drainage;
(b) Constructing retaining walls;
(c) Providing reinforcement by way of rock bolts or rock anchors; and
(d) Slope treatment.
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.
(b) 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 φ 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.
Retaining structures may prove exceptionally successful where:
(i) The ground is neither too fine nor too plastic,
(ii) The sliding mass is likely to remain dry, and
(iii) 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.
(c) Slope 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, for example, 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 metres) one end of which is 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 in the rock at a proper angle with the plane of weakness and then its end within the rock is made to expand whereby it fits tightly into the rock.
The other end is tied on a plate with the help of a nut and washer. The rod is 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:
(i) By using resin bonds instead of plain cement grouting, and
(ii) 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.
(d) 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, 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 known 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:
(i) Flattening the slope to ensure stable limits,
(ii) 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 evapo-transpiration thereby reducing the volume of water for causing failure.
While devising a slide-control programme 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.