Landslides and slope failures have become chronic problem in many parts of the world. In the recent years a wealth of experience has been accumulated by the geo-scientists in understanding, recognition and treatment of landslide hazard. Landslides are responsible for considerable losses of both money and lives, and the severity of this problem worsen with increased urban development and change in land-use pattern. It is not surprising that landslides are rapidly becoming the focus of major scientific research throughout the world. At the international level workers concerned with the fields of geology, geomorphology, soil and rock mechanics have recently been contributing to improve our understanding of landslides, notably in the framework of the United Nations General Assembly Resolution 236 session 44 on 22 December 1989 (retrieved 2008-09-18).

A great majority of slope failures in Jammu and Kashmir occurs along National Highways and roads. The loss caused by landslides may by reduced substantially by implementing recommendations based on geological and geotechnical studies. Such studies can help identify potentially unstable sections of highways in advance so that proper remedial measures are undertaken in time. The present study is a modest beginning to achieve this objective.

The Himalaya constitute youngest mountain chain formed mainly during Miocene (7­– 25MY) and Pliocene (3- 7My) times. An uplift of about 4000 m of the rocks during the last 3 My (Pleistocene and Recent) has largely shaped the topography of Himalaya seen today. Himalaya is a geodynamically active mountain chain. Several NW-SE trending fault zones 'are still active in the region. The present uplift is estimated at around 5mm per year, which is almost five times the present uplift of the Alps. Erosion of alluvial fans (sand deposits) in the main valleys indicates a recent, relative lowering of the erosion base. Due to the high relief and active uplift, the rate of natural erosion is expected to be high. In addition, the generally folded and fractured nature of rocks and intensive rain fall make many parts of Himalaya susceptible to erosion and landslides. A general denudation rate for Himalaya ranging from 0.09 to 1.7 mm/yr has been estimated for the region (Valdiya and Bartarya, 1989). In parts of the Middle to Lower Himalaya, a characteristic gradient in landslide frequency can be observed from the lower slopes to the upper slopes of the valleys. Upper slopes representing an older geomorphological surface are chemically weathered.

The word landslide is used to denote the moment of Earth mass (Landgren, 1986). Various movements of earth are designated using different terms by different authors, e.g. , Rock fall, Earth flow, Slip, Mass movement, Subsidence, etc. Zaruba and Mencl (1969) broadly classified landslides as "Recent", "Ancient" and “Fossil” on the basis of their ages. Similarly slopes have been classified as active or inactive depending upon the evidence of movement within the last seasonal cycle.

Six basic types of slope movements i.e., falls, topples, slides (transitional and rotational), lateral spreads, flows and complex (combination of two or more types of movement) have been described by Varnes (1978). This classification is most commonly used and is based on Sharpe (1938) and supplemented by other sources (e. g., Zaruba and Mencl, 1969; Skempton and Hutchinson, 1969; Nemcock et aI., 1972; deFreitias and Watters, 1973).

The area of study selected for the present investigation is a part of Doda tehsil of District Doda, Jammu and Kashmir and falls in the Jammu region of Northwest Himalaya. This area is demarcated by latitude 33° 05/ and 33° 10/ N, and Longitude 75° 20 and 75° 30 E and is covered by Survey of India toposheet 43 0/8 on 1:50000 scale (Figure 1). The study area is approachable from Jammu by National Highway NH-1A (between Jammu and Srinagar) upto Batote (128 Km NNE of Jammu). From Batote another National Highway NH-1B bifurcates towards east linking Batote with Kishtwar. The area is traversed by Panjal Thrust within Wadia's (1928) Nappe Zone and Jamwal's (1992) older metamorphics. Two major tectonic planes bound the area i.e., the Murree thrust in the south and the Panjal thrust in the north.

Figure 1. Location Map of the Study Area

The study area falls within the Chenab basin. The southern slope includes Devi Ka Padar, Akar, Poni, Malas, Dedni, Hud and Kailinag forests whereas the northern side is scantly forested with Kundi, Dharvi and Malvas forests. The other parts of the northern side are either barren or covered by little grassy patches only. The vegetal cover has an impact on the hill slope stability. Trees intercept rain, lessening the impact that individual raindrops have on soil, and shade from trees keeps the forest floor moist and cool.

The important geomorphological features observed in the study area are formed by the course of river Chenab which flows transverse to the regional topography. The valley is cut by river Chenab and its tributaries are deep and wide. Development of multistage terraces at the slopes is a direct effect of an antecedent drainage system and an indication of neotectonic activity. The lithology clearly controls the drainage pattern and topography in the study area. The harder rocks such as quartzites have resulted in formation of small cliffs and narrow water ways whereas the soft rocks like phyllite and schist have given rise to broader tops and wider valleys. The flow direction of some tributaries along the strike direction is a common feature in areas where antecedent drainage of river Chenab is common.

The Himalayan Mountains have been formed by the collision of Indian and Eurasian plates that took place during Late Cretaceous to Early Eocene times and followed by post collision convergence (Dewey and Bird, 1970; Molnar and Tapponier, 1975; LeFort, 1975; Powel, 1979; Thakur, 1987; [1]LeFort, 1989; Searle, 1991 and Thakur, 1992). The collision phase followed by the intercontinental thrusting and thickening of the continental crust in the Himalayan belt, and lateral expulsion of material by strike slip faults. In addition many important structural and tectonic features like folds, faults, and thrusts developed during this stage of Himalayan uplift.

Geologically, Kashmir Himalaya is fascinating on account of its stratigraphy, paleontology, structure and petrology with rocks of all ages from Precambrian to recent exposed with the limited geographic extent that is bounded by three Himalayan ranges: Karakoram, Zanskar and Pirpanjal. The study area falls within the Outer Himalaya tectonic zone of Northwestern Himalaya. In the study area Murree Group and older metamorphics are exposed. In the metamorphic belt fall the Assar, Korapani (Figure 2) and Malhori landslides. The Murree Group mainly consists of semi-consolidated to consolidated red sandstones, shales with intervening clay beds (Figure 3) and the older metamophics are composed of phyllites, schists, quartzites, crystalline limestones, slates, basic volcanics, etc.

The area falls within the two major tectonic planes i.e., Murree and the Panjal thrusts. Lithostratigraphy of the area is given in Table 1.

Figure 2. View of landslide near Korapani

Figure 3. National Highway NH1B through Murree strata near Batote

Table 1. Geological sequence of the study area (After Jangpangi, 1986)

Kinematic analysis is the reconstruction of movements that take place during the formation and deformation of rocks. This analysis describes translation and rotation of bodies of rocks that have moved in rigid fashion during deformation and, describes changes in size and shape of bodies of rock that have behaved non-rigidly. Kinematic analysis is applied to both the secondary structures formed in solid rock as a result of crustal deformation and to the interpretation of movements that accompany the primary formation of rocks. The former is of main interest in this study. Kinematic movements are evaluated at all scales of observation.

As pointed out by Mead (1925) “A fractured rock body expands and attracts any available fluid into its fracture-formed voids”. The most universally occurring anisotropic characteristic of all the rock masses is the presence of the distinct breaks. The breaks include bedding surfaces, joints, faults and well developed metamorphic foliations. A freshly exposed joint appears as a hairline crack. Joints may be microscopic (micro-joints) as well as large scale ranging for a few meters as master joints. With the increasing intensity of weathering the openings may increase. Some joints are filled with secondary minerals. If the mineral fills are very thin (a few mm) it is still regarded as a joint when observed on the outcrop scale. When the same mineral fill is observed under the microscope it is described as a vein.

Most joints are easily recognized in the field. They often occur parallel to one another. An array of parallel joints constitutes a joint set. Planar parallel joints may also be described as joint sets. In order to describe, classify and analyze joints, several important characteristics are taken into account. These characteristics include attitude of joint planes, spacing or density of joints, their closeness, etc. In the present study data on these characteristics have been recorded and analyzed for knowing the mode and direction of the probable landslide, wedge and slope failures.

Joints play an important role in destabilizing the slopes. Landsliding, slumping and other mass wasting processes are enhanced by the saturation of jointed rocks by the rain or underground water particularly in terrains of high topographic relief. The fluid pressure exerted by rain water or ground water in cracks in rocks weakens the level of normal stress on the fracture surface and, therefore, enhances the potential of slip. When fluid filled joints in rocks beneath a hill slope dip outward towards the free face of the hill, the steady force of gravity in concert with fluid pressure exerted by the water in cracks cause sliding of earth and houses outward and down slope. Kinematic analysis makes it possible to predict most probable failing areas along with their mode and direction of failure.

It is impossible to examine all joints and joint related structures in a given structural domain. Therefore, standard practice is to evaluate jointing through detailed structural analysis at a few selected stations. The aim is to study the nature of the overall joint system through systematic examination of randomly selected sub-areas within the domain. A sampling station is to be chosen where joints and joint related structures are classified and measured. In most studies the stations are simply outcrop areas of varying size and shape. In a geometric sense stations can be demarcated by circular or square inventory areas of specified dimension or as relatively short sample lines of specified traverse length and direction. The present study area involves highly fractured and jointed rocks. Sixteen different outcrop-scale joint sites (demarcated by square inventory area of 2.25 m² in each cell) in the area of study along the highway were chosen for collection of data on joint characteristics, which have been used for structural stability analysis of different rock types.

The data sets recorded on average orientation were plotted as stereographic projections (on the stereonet). Besides, the sliding friction angles for different rock types were superimposed over these projections as concentric circles, which demarcate the wedge failures free to fall from the wedge failures free to slide along with the demarcation of their sub-divisions into slide equilibrium, stable equilibrium and limiting equilibrium. Also spacing of the joints affects the overall strength of the rock body and is therefore, considered as an important joint characteristic. The spacing of the joints in the present study was calculated using the formula:

ρ_j = L/S²

Where,

ρ_j, is joint density,

L, is cumulative length of all joints in a square area, and

S, is side of the square.

The joint density is expressed in the unit of length/area (e. g., ft/ft², cm/cm², m/m² or km/km²). In practice the values of the density are converted to reciprocal form e.g., ft^-1, cm^-1, m^-1, km^-1.

Data on joint and slope orientation at the sixteen sites (Batote, 9th km, 3 locations at Baggar, 2 locations at Assar, 2 locations at Kora Pani, 3 locations at Raggi, 2 locations at Malhori and 2 locations at Khellani) along the NH-1B was collected and is given in Table 2. These data were plotted on stereographic nets to decipher the nature of potential failures at the sample sites.

Table 2. Joint set data from the 16 locations of the study area

Stress–strain diagrams have been used to work out rankings of rock strength for specific conditions of confining pressure and rate of loading (strain rate). However, these rankings are approximate because rocks are strongly influenced by the composition, texture, and general condition of the test samples (rocks) for each lithology [1] (Davis, 1984). Moreover, nature and orientation of mechanical heterogeneity (anisotropy) resulting from fractures, layering, foliations, and the like, profoundly influence the rock strength. In spite of these limitations the stress-strain based rankings of rock strength can be helpful in evaluating the impacts of various kinematic processes in comparable rock types in an area of investigation.

The present study records four major joint sets in the rocks studied. These joint sets are generally tight but sometimes open up to a few millimeters. Stereographic projections of these joint sets have been plotted to depict the formation of unstable wedges. This structural stability analysis reveals the following combinations, which serve as useful parameters for analyzing various possible modes of structural instability in the strata of the study area.

1. The intersection points of the joint sets depict the most potential gravity wedges, free to fall, and

2. Most vulnerable slide intersections of these joint sets fall within the sliding friction angle area on the stereographic projection.

The site-wise analysis and results are given in the following sub-headings:

The Batote site is located at 128 km from Jammu on the NH-1B at Batote. The rocks at the site are composed of shales and sandstones. The general slope angle varies from 34⁰ to 42⁰. The three discontinuity planes have varying attitudes which are given in Table 2. The spacing between the joints is 13 – 30 cm. At all the sites joint walls are rough and irregular because of weathering. The joints are generally open with a minimum gap of 1mm and maximum gap of 11mm. The analysis of these discontinuity planes reveals that there is a possibility of sliding in the 48⁰ / 124⁰ direction (Fig. 4 a). The prominent wedge failure, free to fall, is formed by the intersection of S2 and S3 and the most vulnerable sliding for the wedge would be in the 48⁰ / 124⁰ direction.

The 9^th Kilometer site is located at 137 km from Jammu on the NH-1B at the 9th Kilometer from Batote. The rocks at the site are composed of phyllites. The slope orientation is 52⁰ / 180⁰. The four planes of weakness have varying attitudes which are given in Table 2. The spacing between the joints is 17 to 50 cm. The joints are generally open with a minimum gap of 2.5 mm and maximum gap of 37 mm. The analysis of these discontinuity planes reveals that there is a possibility of sliding in 19⁰ / 134⁰ and 05⁰ / 242⁰ directions, and stable equilibrium in the 35⁰ / 118⁰ direction (Figure 4 b). The prominent wedge failure, free to fall, is formed by the intersection of S1, S2, S3 and S4 and most vulnerable sliding for the wedge would be in the 05⁰ / 242⁰ and 19⁰ / 134⁰ directions.

The Baggar site-I is located at 141 km from Jammu on the NH-1B at Baggar. The rocks at the site are composed of gneisses. The slope orientation is 52⁰ / 166⁰. The four planes of weakness have varying attitudes which are given in Table 2. The spacing between the joints is 30 to 75 cm. The joints are generally open with a minimum gap of 5 mm and maximum gap of 25 mm. The kinematic analysis reveals that there is a possibility of sliding in the 25⁰ / 131⁰, 27⁰ / 232⁰ and 06⁰ / 175⁰ directions (Figure 4 c). The prominent wedge failure, free to fall, is formed by the intersection of S1, S2, S3 and S4 and most vulnerable wedge failure would be in the 06⁰ / 175⁰ and 25⁰ / 131⁰ directions.

The Baggar site-II is located at 143 km from Jammu on the NH-1B at Baggar. The rocks at the site are composed of gneisses. The slope orientation is 50⁰ / 134⁰. The attitudes of the four major joints sets recoded at the site are given in Table 2. The spacing between the joints is 28 to 82cm. The joints are generally open with a minimum gap of 3 mm and maximum gap of 36 mm. The kinematic analysis reveals that there is a possibility of sliding in the 10⁰ / 344⁰, 09⁰ / 242⁰, 15⁰ / 211⁰ and 32⁰ / 155⁰ directions (Figure 4 d). The prominent wedge failure, free to fall, is formed by the intersection of S1, and the slope angle and most vulnerable wedge failure is likely in the 15⁰ / 211⁰ direction.

The Baggar site-III is located at 144 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses. The slope orientation is 82⁰ / 195⁰. The attitude data on the four joint sets is given in Table 2. The spacing between the joints is 34 to 75 cm. The joints are generally open with a minimum gap of 6 mm and maximum gap of 25 mm. The likely failures at this site are sliding in the 26⁰ / 198⁰ and 17⁰ / 224⁰ and 03⁰ / 347⁰ directions (Figure 4 e). The prominent wedge failure, free to fall, is formed by the intersection of S1, S2, and slope angle and the most vulnerable wedge failure is likely in the 03⁰ / 347⁰ direction.

The Assar site-I is located at 151 km from Jammu on the NH-1B at Assar. The rocks at the site are composed of gneisses. The slope orientation is 41⁰ / 108⁰. The attitude of the joint sets is given in Table 2. The spacing between the joints is 14 to 65 cm. The joints are generally open with a minimum gap of 2 mm and maximum gap of 15 mm. The kinematic analysis reveals that there is a possibility of sliding in the 11⁰ / 125⁰, 03⁰ / 346⁰ and 16⁰ / 230⁰ directions (Figure 4 f). The prominent wedge failure, free to fall, is formed by the intersection of S1, S4 and slope angle and the most vulnerable wedge failure is likely to take place in the 11⁰ / 125⁰ and 03⁰ / 346⁰ directions.

Figure 4 (a-f). Stereo net plots of the wedge failures of the study area

The Assar site-II is located at 152 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses. The slope orientation is 77⁰ / 190⁰. The four joint sets exhibit varying attitudes which are given in Table 2. The spacing between the joints is 30 to 92 cm. The joints are generally open with a minimum gap of 5 mm and maximum gap of 20 mm. The kinematic analysis of these discontinuity planes reveals that there is a possibility of sliding in the 090 / 225⁰ and 14⁰ / 272⁰ directions and stable equilibrium along 38⁰ / 270⁰ direction (Figure 4 g). The prominent wedge failure, free to fall, is formed by the intersection of S1, S3 and slope angle and the most vulnerable wedge failure is like in the 09⁰ / 225⁰ direction.

The Kora Pani site-I is located at 160 km from Jammu on the NH-1B at Kora Pani. The rocks at the site are composed of gneisses and phyllites. The slope orientation is 45⁰ / 125⁰. The four joint sets show varying attitudes which are given in Table 2. The spacing between the joints is 40 to 90 cm. The joints are generally open with a minimum gap of 10 mm and maximum gap of 50 mm. The possibility of sliding at this site is in the 30⁰ / 105⁰ and 19⁰ / 195⁰ and 06⁰ / 357⁰ directions (Figure 4 h). The prominent wedge failure, free to fall, is formed by the intersection of S1, S3, S4 and slope angle and the most vulnerable wedge failure would be in the 06⁰ / 357⁰ and 19⁰ / 195⁰ directions.

The Kora Pani site-II is located at 162 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses and phyllites. The slope orientation is 15⁰ / 089⁰. The attitude of four joint sets at the site is given in Table 2. The spacing between the joints is 16 to 45 cm. The joints are generally open with a minimum gap of 7 mm and maximum gap of 25 mm. The kinematic analysis reveals that there is a possibility of sliding in the 08⁰ / 338⁰ and 03⁰ / 168⁰ and 09⁰ / 25⁰ directions (Figure 4 i). The prominent wedge failure, free to fall, is formed by the intersection of S1 and S3 and the most vulnerable wedge sliding is in the 08⁰ / 338⁰ direction.

The Raggi site-I is located at 166 km from Jammu on the NH-1B at Raggi. The rocks at the site are composed of gneisses. The slope orientation is 59⁰ / 132⁰. The data on the four joint sets is given in Table 2. The spacing between the joints is 30 to 103 cm. The joints are generally open with a minimum gap of 7 mm and maximum gap of 55 mm. The likely possibility of sliding at this site is in the 15⁰ / 168⁰, 20⁰ / 291⁰ and 09⁰ / 16⁰ directions, and stable equilibrium is in the 450 / 770 direction (Figure 4 j). The prominent wedge failure, free to fall, is formed by the intersection of S1, S3, S4 and slope angle, and the most vulnerable wedge falure is likely in the 15⁰ / 168⁰ direction.

The Raggi site-II is located at 168 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses. The slope orientation is 45⁰ / 59⁰. The attitude of the four joint sets is given in Table 2. The spacing between the joints is 30 cm to 75 cm. The joints are generally open with a minimum gap of 10 mm and maximum gap of 25 mm. The kinematic analysis reveals that there is a possibility of sliding in the 22⁰ / 120⁰, 27⁰ / 246⁰ and 25⁰ / 41⁰ directions, and stable equilibrium is in the 55⁰ / 15⁰ direction (Figure 4 k). The prominent wedge failure, free to fall, is formed by the intersection of S2, S3 and slope angle and the most vulnerable wedge failure is likely in the 25⁰ / 41⁰ direction.

The Raggi site-III is located at 170 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses. The slope orientation is 42⁰ / 135⁰. The attitude of the joint sets is given in Table 2. The spacing between the joints is 15 to 70 cm. The joints are generally open with a gap ranging from 4 – 25 mm. The kinematic analysis reveals that there is a possibility of sliding in the 22⁰ / 72⁰ and 37⁰ / 169⁰ directions, and stable equilibrium in the 55⁰ / 153⁰ direction (Figure 4 l). The prominent wedge failure, free to fall, is formed by the intersection of S2, S3 and S4 and the most vulnerable wedge failure is likely in the 37⁰ / 169⁰ direction.

Figure 4 (g-l). Stereo net plots of the wedge failures of the study area

The Malhori site-I is located at 172 km from Jammu on the NH-1B at Malhori. The rocks at the site are composed of gneisses. The slope orientation is 25⁰ / 77⁰. The attitude of the joint sets at this site is given in Table 2. The spacing between the joints is 20 to 60 cm. The joints are generally open with a minimum gap of 2 mm and maximum gap of 90 mm. The kinematic analysis reveals that there is a possibility of sliding in the 13⁰ / 139⁰ and 05⁰ / 270⁰ directions, and stable equilibrium in the 46⁰ / 160⁰ direction (Figure 4 m). The prominent wedge failure, free to fall, is formed by the intersection of S1, S2, S3 and S4 and the most vulnerable wedge failure would be in the 05⁰ / 270⁰ and 13⁰ / 139⁰ directions.

The Malhori site-II is located at 173 km from Jammu on the NH-1B at Malhori. The rocks at the site are composed of gneiss and phyllites. The slope orientation is 25⁰ / 105⁰. The four joint sets show varying attitudes which are given in Table 2. The spacing between the joints is 20 to 75 cm. The joints are generally open with a minimum gap of 2.5 mm and maximum gap of 72 mm. The possibility of sliding at this site is in the 22⁰ / 71⁰ and 07⁰ / 181⁰ directions, and stable equilibrium is in the 17⁰ / 174⁰ direction (Figure 4 n). The prominent wedge failure, free to fall, is formed by the intersection of S2, S3 and S4 and the most vulnerable wedge failure is likely to occur in the 17⁰ / 174⁰ direction.

The Khellani site-I is located at 179 km from Jammu on the NH-1B at Khellani. The rocks at the site are composed of gneisses. The slope orientation is 14⁰ / 70⁰. The three joint sets show varying attitudes which are given in Table 2. The spacing between the joints is 16 to 90 cm. The joints are generally open with a minimum gap of 4 mm and maximum gap of 32 mm. The kinematic analysis reveals that there is a possibility of sliding in the 14⁰ / 84⁰ and 08⁰ / 176⁰ directions, and stable equilibrium in the 53⁰ / 193⁰ direction (Figure 4 o). The prominent wedge failure, free to fall, is formed by the intersection of S1, S2, and S3 and the most vulnerable wedge failure is likely in the 08⁰ / 176⁰ direction.

The Khellani site-II is located at 182 km from Jammu on the NH-1B. The rocks at the site are composed of gneisses and phyllites. The slope orientation is 32⁰ / 82⁰. The four joint sets show varying attitudes which are given in Table 2. The spacing between the joints is 22 to 45 cm. The joints are generally open with a minimum gap of 5 mm and maximum gap of 29 mm. The possibility of sliding is in the 20⁰ / 29⁰, 16⁰ / 247⁰ and 29⁰ / 112⁰ directions (Figure 4 p). The prominent wedge failure, free to fall, is formed by the intersection of S1, S3, S4 and the slope angle and the most vulnerable wedge failure is likely to occur in the 16⁰ / 247⁰ and 29⁰ / 112⁰ directions.

Figure 4 (m – p). Stereo net plots of the wedge failures of the study area

The present study reveals that almost all the slides, slope and wedge failures are concentrated around the thrust/fault planes. A few of the potential failure sites occur within the shear zones and Continuous strain domains. However, these sites are stable at the moment (Figure 5).

The overall rock mass strength is dependent on spacing of the joints in the rocks. Even the strongest intact rock is reduced to one of the little strength when closely spaced joints are encountered. If the joint spacing is large the rock mass strength is controlled by the intact rock properties. Therefore, orientation and frequency of joints in rocks greatly influence their strength. In case of intercepting joint sets and day lighting joint surfaces cutting a rock face (Figure 6), and closely spaced joint surfaces (Figure 7) tend to cause numerous rock falls. However, in similar conditions with widely spaced joints massive catastrophic block failures are expected. Strain produced in rocks under conditions of low differential stress through time results in mechanical response in the form of creep and under accelerating rate of strain, rocks fail by rupture. Through these ruptures percolation of rainwater reduces the frictional resistance against sliding. The interstitial fluids affect the strength of rocks considerably. Rehbinder and Lichtman (1957) noted that pore water reduces the strength of a single crystal to a tenth of the value it possessed when completely dry. Similarly Price (1960), Colback and Wiid (1965) in rock strength tests concluded that the uniaxial strength of completely saturated rocks was only about 45% of its own dry strength. The “Rehbinder effect” states that absorbed water reduces the surface energy of the walls of pores, flaw or micro-fracture surface under suitable stress conditions.

Figure 5. Landslide within the shear zone of the study area

Figure 6. Rock falls due to cutting of Rock faces in the area of study

Figure 7. Closely related joint surfaces in rocks of the study area

Generally landslides develop along stratification surfaces on the slopes with varying gradients and, along the fault and joint planes. Some slope failures are triggered directly by human activity by making scarps along road cuts and inducing fractures in the rocks. However, at the base of the chain of landslide triggering processes stays gravitation, which may act only when a sufficient gradient of slope develops.

In the present case, master joints and foliation are among the main causes of slides. Master joints cutting across the foliation planes have resulted in the formation of blocky structures at most of the places. The instability in the entire area has been activated by percolation of water during monsoons and toe cutting by the Chenab River. In addition steep slopes and shear zones are potential sites for landslides and rock falls in the area.

The present study records the average 3 to 4 sets of joints in the rocks and all the slides, slope and wedge failures are concentrated around the thrust/fault planes. A few of the potential failure sites occur within the shear zones and continuous strain domains. The overall rock mass strength is dependent on spacing of the joints in the rocks. Even the strongest intact rock is reduced in strength when closely spaced joints are encountered. If the joint spacing is large the rock mass strength is controlled by the intact rock properties. Therefore, orientation and frequency of joints in rocks greatly influence their strength. In case of intercepting joint sets and day lighting joint surfaces cutting a rock face and closely spaced joint surfaces tend to cause rock falls. The instability in the entire area activates by percolation of water during monsoons and toe cutting by the Chenab River. In addition steep slopes around fault and shear zones are potential sites for landslides and rock falls in the area.

The data generated on nature of the geology, structure and kinematic analysis, are likely to go a long way in mitigating the landslides, slope- and wedge failures in the study area. However, further site specific investigations on these lines in this and the other sectors of the NH-1B are suggested to be undertaken for furtherance of the understanding of the landslide problems.

Thanks are due to Prof. G.M. Bhat for providing laboratory facilities. Discussions with the BRO people provided stimulus to pursue this study.