Natural Slope Stability - Assignment Example

Introduction Slope stability is an important factor in the design and operation of open pit mining, quarrying and civil engineering excavation projects. In mining the design of stable slopes will have significant impact on the economics of an open pit or quarry project. Long term monitoring of slope stability is essential to ensure that the risk to personnel, equipment, buildings and other infrastructure located close to the toe or crest of a man-made slope can be properly managed. In this study we will be able to show the influence of rainfall in the stability of natural slope such as hillsides and mountain. Softening of slope cause many hills and mountain to have a landslide causing damage to infrastructure and injury to some. In UK an increasing number of landslides has been occurring because of the late study of the stability of the slope. A case study will be presented to show the evaluation of the slope geotechnically. The study considers the causes of landslides and what measures could be taken to manage the risks of these occurring and is reported elsewhere. This study also show things to consider in the effect of that climate change might have on the design and operation of roads, to identify whether any changes in current practices are required. The focus of this study is the influence of rainfall in the stability of the natural slope.This study includes in how to predict the possible continuous rainfall to avoid landslides in many areas in UK. This paper shows the study in geotechnical way. Possible solution on many landslides are given to prevent loss of life and possible damage to properties. Case Study The case study presented in this paper was done by the Scottish engineers, to predict possible slope instability that causes landslide because of continuous rainfall. The study location is in the road network of Scotland In August 2004 a series of landslides in the form of debris flows occurred in Scotland. Some of these affected the A83, A9 and A85, which form part of the trunk road network. These incidents were well reported in the media. While debris flows occur with some frequency in Scotland, they only rarely affect the trunk road network or for that matter the main local road network. However, when they do impact on the road network the degree of damage, in terms of the infrastructure and the loss of utility to road users, can have a major detrimental effect on both economic and social aspects of the use of the asset. Additionally, there is a high potential for such events to cause serious injury and even loss of life although, fortuitously, such consequences have been limited to date. The events of August 2004 followed a sustained period of heavy rainfall and, in addition, intense localised storms contributed to the triggering of at least some of the resulting debris flows. Rainfall of up to 300% of the monthly average fell in certain parts of Scotland during August 2004. Within the recent past, debris flow activity in Scotland has occurred largely in the periods July to August and November to January, but there is no certainty that such a pattern will be continued in the future. However, eastern parts of Scotland do receive their highest levels of rainfall in August. Additionally, climate change models indicate that rainfall levels will increase in the winter but decrease during the summer months but that intense storm events will increase in number. These factors, therefore, may change both the frequency and the annual pattern of debris flow events. The impacts of such events are particularly serious during the summer months due to the major contribution that tourism makes to Scotland's economy. Nevertheless, the impacts of debris flow events during the winter months should not be underestimated. Evaluation of the Study What is a Natural Slope Slope as defined in Encarta is the maximum angle at which soil can be banked without slipping. The slope can be natural (natural cliffs and hillside) or artificial or constructed (cuttings, embankments, quarries, spoil tips and dams). If the ground surface is not horizontal, a component of gravity will tend to move the soil downward. Sloping surfaces are subjected to forces associated with gravity and seepage which cause instability. Resistance to failure is derived mainly from a combination of slope geometry and the shear strength of the soil or rock itself. What is a Landslide A landslide is a type of "mass wasting." Mass wasting is down slope movement of soil and/or rock under the influence of gravity. A landslide is a movement of mass rock, debris, or earth down a slope. The failure of the slope happens when gravity exceeds the strength of the earth materials. Landslide occurs in highland such as hillside and cliffs. In the study done by the Scottish engineers they will be able to evaluate what causes these landslide and what is the possible way of preventing it. Landslide can be classify as follows: rate of movement , type of materials, and natural movement. (Landslides in British Columbia). The Scottish identify this case as the natural movement which includes the debris flow. Debris Flow Debris flows are fast moving flows of mud and rock and they are the most numerous and dangerous of all the landslides. Debris flows generally occur during periods of intense rainfall or snow melt. They usually begin on the top of steep hills with saturated soil as They are so dangerous because they move quickly, destroy without warning, and obliterate everything in their path. They can destroy homes, knock down trees, and obstruct streets and roadways. Their average speed is 10 miles an hour, but some have been known to exceed 35 miles per hour! Their viscosity ranges from thick, rocky mud to water mud. The following are several types of debris flows. (Talking About Disasters). Debris flow are further classify as follows: Earth Flow: The wet ground breaks up and falls down the hillside in a rounded shape. It usually occurs on clay or sand and it is the slowest and driest type of flow. Mud flow : Sometimes referred to as a mudslide, a mudflow is when the soil becomes so saturated with water that it speeds down the hill in a muddy river carrying debris. It is the fastest and wettest type of flow. Research Methodology To evaluate the landslide or instability of slope in the above case study it best to be able to identify certain things involving in this natural disaster. It must be appreciated that the causes of instability are often complex and any attempt at classification will be approximate and incomplete. The Working party on World Landslide Inventory have proposed a list of causal factors grouped under four main headings: Ground conditions Plastic weak material Sensitive material Collapsible material Weathered material Sheared material Jointed or fissured material Adversely oriented mass discontinuities (including bedding, schistosity, cleavage, faults, unconformities, flexural shears, sedimentary contacts) Contrast in permeability and its effects on ground water Contrast in stiffness (stiff, dense material over plastic materials) Geomorphological processes Tectonic uplift Volcanic uplift Glacial rebound Fluvial erosion of the slope toe Wave erosion of the slope toe Glacial erosion of the slope toe Erosion of the lateral margins Subterranean erosion (solution, piping) Deposition loading the slope crest Vegetation removal (by erosion, forest fire, drought) Physical factors Intense, short period, rainfall Rapid melt of deep snow Prolonged high precipitation Rapid drawdown following floods, high tides or breaching of natural dams Earthquake Volcanic eruption Breaching of crater lakes Thawing of permafrost Freeze and thaw weathering Shrink and swell weathering of expansive soils Man-made processes Excavation of the slope or at its toe Loading of the slope or at its crest Drawdown (of reservoirs) Irrigation Defective maintenance of drainage system Water leakage from services (water supplies, sewers, stormwater drains) Vegetation removal ( deforestation) Mining and quarrying (open pits or underground galleries) Creation of dumps of very loose waste Artificial vibration (including traffic, pile driving, heavy machinery) Effect of Continuous Rainfall in the Stability of the Slope Rain is one of the most important factors affecting the design and operation of the road network. It affects the design of drainage systems that collect and discharge surface water. It also affects the sizing of river bridges/culverts. Rain also creates a hazard to road users when it is not shed sufficiently quickly from the carriage way and is a frequent contributing factor in road accidents. Rain can cause landslide events, both through large volumes of surface water eroding the land surface, and through changes in groundwater levels reducing the stability of cuttings. In addition, rain, together with temperature, can significantly alter the soil moisture condition within a catchment, creating a situation where the volume of water that the catchment sheds may be much higher than the 15% to 50% currently used in the design of drainage systems. The rainfall events currently used in road design are based on historical records and therefore there is a concern that if rain is expected to increase, these records may no longer correctly describe the design storm events. Analysis of records shows that recent changes in Scottish rainfall may be identified. Trends in storm event rainfall have been the subject of less research. However, Osborn et al. (2000) found evidence that the intensity distribution of daily precipitation across Scotland has changed over the relatively short period 1961-1995. For 26 stations across Scotland they showed that the majority have recorded a general shift from light and medium events to heavier events in the winter and, to a lesser extent, also in the spring and autumn. The reverse was found to be true in the summer. They suggested that changes in winter weather types may have contributed to the increase in the proportion of precipitation provided by heavy events. The historical change in river flooding, which has a direct link with rainfall, has been the subject of research for some time. CEH Wallingford conducted research on long-term trends in the UK flood record (Robson et al 1997, Robson et al 1998). This found that no significant long-term trends in national flood behaviour could be detected. In a study of Scottish flood behaviour Werritty (1998) found that there had been little change in flood magnitudes during the period 1970-1996 for 44 river flow stations across Scotland. However, a greater increase in flood frequency was found in the same study. Werritty suggests that the significance of the increase in flood frequency should not be overstated as, although there have been a number of major catastrophic floods since 1989, there appears to be no consistent increase in the size of moderately high flood events across Scotland. Following the extreme flooding in England and Wales during Autumn 2000, the Met Office and CEH Wallingford were commissioned to assess whether the floods and rainfall could be linked to climate change (Met Office & CEH Wallingford, 2001). They concluded that although the events were consistent with model predictions of how human-induced climate change affects rainfall, it was not yet possible to say how far rainfall and flooding events can be attributed to climate change, as opposed to natural variability. Historic evidence on changes in Scottish groundwater levels is not available. In addition, very limited information exists on historical Scottish soil moisture conditions and so no evidence of sustained long-term trends can be identified. Geotechnical analysis on the effect of rainfall on the stability of the soil Geotechnical factors Soil properties including cohesion, grain size, shear strength, moisture content, void ratio, relative density and permeability are relevant to the occurrence of debris flows. These are likely to be known only as a result of a detailed ground investigation and should be picked up during a second stage detailed site appraisal. Analysis of the Scottish soil can be evaluated using the geotechnical factors which are described on the next pages. Measurement of Shear Strength or Shearing Resistance Laboratory Test 1. Drained and Undrained Test The accurate measurement of the shearing resistance or shear strength of a material is essential in attempting to predict future instability or to assess the present or past stability condition. Shear strength tests must be performed on samples of the highest quality if reliable information is to be obtained. Even when this condition is satisfied, however, there may still be cases where the shear strength measured in the laboratory differs from that mobilised in situ. Laboratory tests can broadly be divided into two types, depending primarily on the pore pressures set up within the sample during the test and whether dissipation of these pore pressures is prevented or permitted. Tests can therefore be categorised as either 'undrained' or 'drained'. In undrained tests, the pore pressures set up during the test are not permitted to dissipate, and the test may be performed relatively quickly. The existence of these pore pressures - which may or may not be monitored - influences the behaviour of the soil to a marked extent. It is generally considered that the results obtained from undrained tests are applicable to short-term stability conditions. In drained tests adequate time is allowed for the dissipation of pore pressures, so tests are much longer than most undrained tests. The results of these tests can be used to assess the long-term stability in slopes and cuttings. The British engineers may use the undrained test to be able to evaluate the stability of the slopes and cuttings in a shorter period of time and thus can be able to test a big numbers of areas with critical slopes. Shear strength properties of soils are defined by two parameters, apparent cohesion c and the angle of shearing resistance f . In undrained tests the parameters are expressed in terms of total stresses, whereas in drained tests the parameters are denoted by c' and f '. A summary of problems which can be analysed in terms of total or effective stresses has been given by Bishop and Henkel. 2. Shear Box Test The shear box was probably the first type of apparatus used for the measurement of the shearing resistance of soils. The apparatus consists essentially of a square brass box split horizontally at the level of the centre of the soil specimen which is held between metal grills and porous stones. The horizontal force acting on the upper part of the box is gradually increased until the specimen fails in shear. The shear force at failure sf is divided by the cross-sectional area A to give the shearing stress t f at failure. The vertical stress sn is provided by a vertical load on the sample, normally by dead-weights and a lever system. The horizontal load is applied by pushing the lower part of the box by means of an electric motor and gearbox. Volume changes are monitored by a dial gauge mounted to show the vertical movement of the top loading platen. The size of the shear box normally used for tests on fine-grained soils is 60 mm square, and the sample is approximately 20 mm thick. For soils containing gravel, a shear box 300 mm square is frequently used; in dealing with some soils even larger specimens may be required since, as a rough rule, the maximum particle tested should not exceed one-eighth of the length of the shear box. Tests in the shear box are relatively simple to perform, but the test is open to a number of criticisms. The most important of these are: a. it may be difficult to install an undisturbed sample in the apparatus; b. the stress distribution across the sample is complex; c. failure occurs along a plane dictated by the design of the apparatus; d. the area under shear reduces during the test; e. there is no direct control over drainage conditions in the sample. Typical results from tests on well-graded sand are illustrated here. 3. Triaxial tests The triaxial compression test is the most widely used technique to determine the shear strength of soils. The apparatus is shown diagramatically in the figure. The sample, which is cylindrical, is tested inside a perspex cylinder filled with water under pressure. The sample under test is enclosed in a thin rubber membrane to seal it from the surrounding water. The pressure in the cell is raised to the desired value, and the sample is then brought to failure by applying an additional vertical stress. One of the major advantages of the triaxial apparatus is the control provided over drainage from the sample. When no drainage is required (i.e. in undrained tests), solid end caps are used. When drainage is required, the end caps are provided with porous plates and drainage channels. It is also possible to monitor pore-water pressures during a test. Full details of the basic apparatus and refinements, and procedures for a wide range of tests in the triaxial apparatus, are given by Bishop and Henkel. For cohesive soils, the size of sample normally used in the triaxial apparatus is 38 mm diameter and 76 mm long. When gravel is present, for example in boulder clay, larger samples may be used, the most common being 100 mm diameter and 200 mm long. For coarse gravelly soils, rockfill and artificially prepared granular material such as railway ballast, even larger samples are required if realistic values of the shearing strength are to be obtained. This is also true for fissured cohesive soils, where the sample tested must be of sufficient size to contain a truly representative collection of all the structural features which may affect the shear strength. To obtain the shear strength parameters of the soil, a number of specimens (normally at least three) are tested at different values of cell pressure. For each test, the vertical stress s 3 at failure are determined and are used to plot a Mohr circle. The envelope to these circles then defines the shear strength parameters. It is important that the values of the shear strength parameters c' and f' are obtained from the Mohr's circles obtained by tests on similar material. In markedly heterogeneous materials, it may be difficult to obtain sufficient samples for testing, and the technique of 'multi-stage' testing may be employed. This form of test is normally perforated on 100 mm diameter samples. The sample is initially tested at a particular cell pressure and the vertical stress is increased until failure is approached. At this point the cell pressure is increased and shearing resumes until failure is again approached under the new cell pressure. The process is repeated a number of times. There has been some criticism of this type of test, but it does appear to give reasonably acceptable results if the test is performed with care. Residual Strength When a soil is subjected to shear, an increasing resistance is built up. For any given applied effective pressure, there is a limit to the resistance that the soil can offer, which is known as the peak shear strength sp. Frequently the test is stopped immediately after the peak strength has been clearly defined. The value sp has been referred to, in the past, as simply the shear strength of the clay, under the given effective pressure and under drained conditions. If the shearing is continued beyond the point where the maximum value of the shear strength has been mobilised it is found that the resistance of the clay decreases, until ultimately a steady value is reached, and this constant minimum value is known as the residual strength sr of the soil. The soil maintains this steady value even when subjected to very large displacements. Typical results for a drained test on clay, taken to displacements large enough to mobilise the residual strength, are shown below. Further tests could be made on the same clay but under differing effective pressures. The results previously described would again be obtained, and from a number of tests it would be noticed that the peak and residual shear strengths would define envelopes in accordance with the Coulomb-Terzaghi relationship, a. Thus the peak strengths can be expressed as: sp = c' + ' tan ' and the residual strengths can be expressed as: sr = cr' + ' tan r' The decrease in shear strength from the peak to the residual condition is associated with orientation of the clay particles along shear planes. c' = apparent cohesion c'r = residual apparent cohesion ' = angle of shearing resistance 'r = residual angle of shearing resistance sp = peak shear strength sr = residual shear strength ' = applied effective stress Measurement of Residual Shear Strength The residual shear strength condition is of considerable practical importance since, if the soil in situ already contains slip planes or shear surfaces, then the strength operable on these surfaces will be less than the peak strength, and if sufficient displacement has taken place, the strength may be as low as the residual strength. There are a number of circumstances, as a result of which shearing of the soil may have taken place, and the principal processes, summarised by Morgenstern et al , are: landsliding, tectonic folding, valley rebound, glacial shove, periglacial phenomena, and non-uniform swelling. The identification of the existence of shear surfaces is a problem of great importance during any site investigation, particularly where mass movements are involved. It is generally accepted ( Skempton and Hutchinson) that the residual shear strength of a soil is independent of stress history effects, not influenced by specimen size, and rate-dependent to only a small extent. The major difficulty in determining the residual shear strength lies in the fact that large displacements may be necessary to achieve the required degree of orientation of the particles. Methods of Measurement of Residual Shear Strength The methods of measuring residual shear strength in the laboratory are given in the table. The most satisfactory methods, in many ways, are to obtain undisturbed samples which contain a natural slip surface and then test them either in the shear box or triaxial apparatus so that failure occurs by sliding along the existing slip plane. Alternatively, an artificial slip plane can be produced by cutting the specimen with a thin wire-saw. Much of the early work on determining the residual shear strength of soils in the laboratory was performed using multi-reversal type tests in the shear box on previously unsheared material ( Skempton). The results of tests to measure residual shear strength in the shear box and triaxial apparatus have been reported by Skempton and Petley. There are practical difficulties with each of these tests, and they also have the major disadvantage that none of them permits the complete shear-stress-displacement relationship to be obtained. 1. Shear box (a) Tests on natural shear surfaces (b) Reversal-type tests (c) Cut-plane tests 2. Triaxial (a) Tests on natural shear surfaces (b) Cut-plane tests 3. Ring shear The large displacements required to define the complete shear-stress-displacement relationship can be obtained by using the ring-shear (or torsional shear) apparatus. The apparatus consists of two pairs of metal rings which hold an annular sample. The sample is subjected to a normal stress and then one pair of rings (normally the lower pair) is subjected to rotation. It is therefore a form of direct shear test, and failure occurs along a predetermined plane, as with the shear box. this type of apparatus was probably first used by Hvorslev and Tiedemann. More recent designs of the ring-shear apparatus have been described by Bishop et al. and Bromhead. Difficulties The aim of laboratory testing is to define a shear strength which is applicable to the field situation. Unfortunately, there are many reasons why laboratory tests may give values for shear strength different from those which apply in the field, and the main reasons are considered below. The most obvious source of error is bad or indifferent sampling. it has already been emphasised that samples must not only be truly representative, but they must also be of the highest quality and this entails the use of well-designed apparatus which is in good condition and operated by skilled personnel. As a general rule sampling disturbance will tend to reduce the strength of the soil, and, as a further generalisation, it is likely that the shear strength parameters in terms of total stresses, (i.e. simple undrained tests) will be more greatly affected than the parameters in terms of effective stresses. The effect of sampling disturbance on the stress-strain relationship for brittle and ductile soils is given in the figure. Most triaxial tests are performed on samples with a vertical axis, and the majority of shear box tests are performed so that failure occurs along a horizontal plane. In the field, however, a failure plane may be appreciably curved. In clays, anisotropy is likely to occur as a consequence of their mode of formation, and the presence of discontinuities such as joints and fissures which may exhibit some degree of preferred orientation. Some results indicating the anisotropy of undrained strength in London Clay are presented in the table. In terms of effective stress, it has been found that results of tests where shearing occurs in a horizontal direction are lower than of tests where failure occurs at other orientations, but there is a paucity of information on this topic. To obtain realistic results from laboratory tests, it is essential that the tests are performed on samples which are sufficiently large to be representative of the in situ state. For intact clays, it is likely that typical laboratory specimens (i.e. 38 mm diameter for triaxial tests) are adequate for practical purposes, but in boulder clays or fissured clays, this may not be true. For London Clay, for example, it appears that the in situ undrained strength is around 65-70 per cent of the strength measured on a conventional 38 mm diameter sample, and accurate laboratory estimates of strength would only be obtained from tests on larger samples (possibly up to 300 mm diameter). Again there is a lack of information on the effect of sample size on the effective stress parameters c' and f ' of stiff fissured clay. Marsland and Butler report the following results for Barton clay: 38 mm diameter samples: c'= 11kPa, f =24deg 76 mm and 125 mm diameter samples: c'= 7kPa, f =23.5deg A discussion of other factors which may lead to discrepancies between field and laboratory shear strengths has been given by Skempton and Hutchinson. # # Ratio of undrained strength of London Clay parallel to bedding cB and in compression specimens with their axis normal to bedding cN (after Skempton and Hutchinson) site clay size of specimens cB cN Reference Maldon brown London Clay, shallow 38 x 76 mm 0.88 Bishop and Little (1967) brown London Clay, shallow 100 x 200 mm 0.86 Bishop and Little (1967) Walton blue London Clay, shallow 38 x 76 mm 0.78 Bishop (1948) Wraysbury blue London Clay, shallow 38 x 76 mm 0.75 Agarwal (1967) blue London Clay, shallow 300 x 600 mm 0.76 Agarwal (1967) Ashford blue London Clay, deep 38 x 76 mm 0.83 Ward et al. (1965) Marsland A., and Butler, F.G. (1967). Strength measurements in stiff fissured Barton Clay from Fawley, Hampshire, Proc. Geot. Conf. on Shear Strength of Natural Soils a Rocks, Oslo., 1, 139-146. Other Computer Programs in Analyzing the Stability of the Slope 1. STABL is a computer program for the general solution of slope stability problems by two-dimensional limiting equilibrium methods. It allows also the analysis of reinforced soil slopes with geosynthetics, nailing, and tiebacks using the Bishop, Spencer and Janbu methods. STABL features unique random techniques for generation of potential failure surfaces for subsequent determination of the more critical surfaces and their corresponding factors of safety. One technique generates circular; another, surfaces of sliding block character; and a third, more general irregular surfaces of random shape. Specific trial failure surface can also be specified by the user.(Salgado, 2. Tagasoft - Slope stability, finite element analysis, pile and pile groups, settlement and consolidation, seismic analysis and foundation design software. 3. G O C Rocsience - Geomechanics software and research 4. Mitre Software Corporation - Slope stability and inclinometer software. 5. Geotechnical database management and slope stability software - MZ associates limited has ceased trading. Mark Zytynski, the author of the packages SID and STABLE will continue to support these systems on an informal basis. 6. Wedge Failure Analysis - Wedge failure analysis is a wedge failure analysis package. It includes a stereonet view of the planes the user inputs and a printable summary of the analysis. Recommendations The possible way to prevent landslides during heavy rainfall in UK is first taking considerations the geotechnical factor of the slope specially in a high land areas. The factor that most of the study taking into consideration is the historical and the geotechnology way. This study and analysis shows that considering the shear strength and the residual strenght of the soil would help to prevent the said natural calamities in UK. Possible test of clays or soil in area which is considered to be prone in landslides should be studied first. Whatever method is useful in determining the stability of the slope. A combination of computer aided analysis and laboratory test can be an effective way of determining the stability. Conclusions Proper consideration of the geotechnical factors such as the shear strenght and residual strength could be of great help to those who are studying the effect or influence of the rainfall in the stability of the slope. This could be of great help to researchers and others who are conducting the study about this case. Further analysis should be done. Scottish Engineers would have a lot of work to do in order to prevent further destruction on the stability of the soil. References: 1. http://www.geology.wisc.edu/courses/g115/projects03/emgoltz/types.htm 2. Reynard NS, Crooks S, Wilby R and Kay A, 2004. Climate change and flood frequency in the UK. In proceedings of 39 th Defra Flood and Coastal Management Conference 2004, York, 11.1.1 - 11.1.12. 3. # Morgernstern, N.R., Blight, G.R., Janbu, N., and Resendiz, D. (1977). Slopes and excavations, 9th Int. Conf. Soil Mech. and Found. Eng., 12, 547-604. 4. # Skempton, A.W., and Hutchinson, J.N. (1969). Stability of natural slopes and embankment foundations. State-of-the-Art Report. 7th Int. Conf. Soil Mech. Found. Eng., Mexico, 291335. 5. UK Water Industries Research, 2004. Climate Change and the Hydraulic Design of Sewerage Systems. UKWIR Report 03/CL/10. 6. Marsland A., and Butler, F.G. (1967). Strength measurements in stiff fissured Barton Clay from Fawley, Hampshire, Proc. Geot. Conf. on Shear Strength of Natural Soils a Rocks, Oslo., 1, 139-146. 7. Prof. Rodrigo Salgado ,School of Civil Engineering,Purdue University West Lafayette, IN 47907-1284 8. Bishop, A.W., and Henkel, D.J. (1962). The Measurement of Soil Properties in the Triaxial Test, Edward Arnold, London. 9. Marsland A., and Butler, F.G. (1967). Strength measurements in stiff fissured Barton Clay from Fawley, Hampshire, Proc. Geot. Conf. on Shear Strength of Natural Soils a Rocks, Oslo., 1, 139-146. 10. Bishop, A.W., and Henkel, D.J. (1962). The Measurement of Soil Properties in the Triaxial Test, Edward Arnold, London. 11. Bishop, A.W., and Little, A.L. (1967). The influence of size and orientation of the sample on the apparent strength of the London Clay at Maldon, Essex, Proc. Geot. Conf., Oslo, 1, 8996. 12. BS 1377 (1990). Methods of Testing Soils for Civil Engineering Purposes, British Standards Institution, London. 13. Landslides in British Columbia 14. Talking About Disasters < www.fema.gov/rrr/talkdiz/landslide.shtm> 15. Malik Smade , Geotechnical Directory http://www.geotechnicaldirectory.com/page/Software/Slope_stability_(rock).html 16. Agarwal, K.B. (1967). The influence of size and orientation of samples on the strength of London Clay. Ph.D thesis, University of London, unpublished. 17. Bishop, A.W. (1948). Some factors involved in the design of a large earth dam in the Barnes Valley. Proc. 2nd Int. Conf. Soil Mech. and Found. Eng., Rotterdam, 2, 13-18. 18. Skempton, A.W., and Hutchinson, J.N. (1969). Stability of natural slopes and embankment foundations. State-of-the-Art Report. 7th Int. Conf. Soil Mech. Found. Eng., Mexico, 291335. 19. Skempton, A.W., and Petley, D.J. (1967). The strength along structural discontinuities in stiff clay. Proc. Geot. Conf. on Shear Strength of natural Soils and Rocks. Oslo, 2, 3-20. 20. Tidemann, B. (1937). Uber die Schubfestigkeit bindiger Boden. Bautechnik 15, 21. Ward, W.H., Marsland, A., and Samuels, S.G. (1965). Properties of the London Clay at the Ashford Common shaft: in situ and undrained strength tests. Geotechnique 15, 321-344. 22. Waters, R.S. (1958). Morphological mapping. Geography, 10-17. Read More