Slope stability

Site investigation

In relation to slope stability, the main aims of site investigation are:

to obtain an understanding of the development and nature of natural slopes, and of the processes which have contributed to the formation of different natural features;
to assess the stability of various forms of slopes under given conditions;
to assess the risk of instability in natural or artificial slopes, and to quantify the influence of engineering works or other modifications to the stability of an existing slope;
to facilitate the redesign of failed slopes, and the planning and design of prevention and remedial measures;
to analyse slope failures which have occurred and to define the causes of failure;
to assess the risk of special external factors on the stability of slopes, e.g. earthquakes.

 

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Organisation of site investigation

Any site or ground investigation is performed under the constraints of time, money and the complexity and variability of the geological environment. It is therefore often necessary to reach a compromise between the precise details at a site and the time and money available. The investigation must be tackled in a logical and scientific manner. In addition, site investigation is a skilled operation, and must be entrusted only to suitably trained operators.

Site investigations can be considered under 3 main headings:
desk study,
field study,
laboratory work.
Here the discussion will be concentrated upon topics which are unique or have special importance in the context of slope stability.

 

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Desk study

The aim here is to obtain all available information with regard to the site and its geological environments. It will involve a search through records, maps, (topographical and geological), and any other information which is relevant to the geology, history and present condition of the site. A useful list of sources of information and the procedure to be followed in carrying out the desk study has been given by Dumbleton and West.

It is helpful at this stage to attempt a preliminary analysis of the geology by preparing sections etc. - this exercise may help to define where further information is required. A visit to the site must also be made to confirm observations and predictions already made.

Dumbleton, NLJ., and West, G. (1971). Preliminary Sources of Information for Site Investigations in Britain. RRL Report No. LR403, Transport and Road Research Laboratory, Crowthorne, Berks.

 

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Field study

The basic aims of the field study are to record accurately the topography of the site, to determine the precise nature of the geological deposits underlying the site and to determine their engineering properties, either by the collection of good quality samples which can be tested subsequently in the laboratory, or by performing tests in-situ.

Conventional surveying techniques may provide sufficient information to permit cross-sections etc. to be prepared. In some cases, useful input can also be obtained from air-photos.

In the context of mass movements on slopes, however, a recent development has been the introduction and application of geomorphological mapping. The employment of such techniques has been shown to usefully precede and supplement standard geotechnical and geological investigations.

In civil engineering it is quite usual in the case of light structures to limit the subsurface investigation to only a few relatively shallow trial pits. It should, however, be recognised that trial pits have an important function in investigations for other, more important structures and, particularly, on sites where landsliding and other forms of mass movement have already taken place or may be expected to occur in the future.

An essential part of any investigation concerns the groundwater conditions pertaining on the site, and the accurate measurement of water pressures in the ground. It must be stressed that great care is needed in order to obtain reliable data on this topic, which is of comparable importance in assessing the stability of a slope as the determination of the shear strength properties.

 

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Geomorphological mapping

The mapping is performed using the techniques described by Waters and Savigear. It is based on identifying breaks and changes of slope, and the resulting delimitation of slope units. The direction and value of maximum slope, when measured across each unit, is recorded. In the case of large units or in areas characterised by complex forms, the number of slope measurements per unit is increased. An example of the detail and impression of topographical form which can be obtained using this technique can be seen here. If necessary, a clearer visual impression of the topography can be obtained by using the slope information to prepare a slope category map, as described by Brunsden and Jones.

Further descriptions of the development of geomorphological mapping and its application in engineering projects have been given by Brunsden et al.


# Waters, R.S. (1958). Morphological mapping. Geography, 10-17.


# Savigear, R.A.G. (1965). A technique of morphological mapping. Mapping Assoc. Amer. Geogr., 55, 514-38


# Brunsden, D., and Jones, D.K.C. (1972). The morphology of degraded landslide slopes in South West Dorset. Quart. J. Eng. Geol., 5, 205-222.


# Brunsden, D., Doornkamp, J.C., Fookes, P.G., Jones, D.K.C., and Kelly, J.M.H. (1975). Large scale geomorphological mapping and highway engineering design. Quart. J. Eng., Geol. 8,227-254.


#

 

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Trial pits

Mobile rubber-tyred excavators can be used to excavate trial pits to depths of about 4-5m, and they are economical since hiring can be made on a time basis. Deeper trial pits may require the use of tracked excavators, which are more expensive since they have to be transported to and from the site by low-loader. Great care must be taken when using trial pits to avoid the risks associated with collapse of the sides of the pits. As a general rule, the spoil from the pit must be placed well clear of the top of the pit, and adequate bracing used to provide stability for the sides. deep trial pits with full timbering have been used for specialist purposes (see, for example, Hutchinson et al.), but this is an expensive operation. It has been found to be economical to use a rotary power auger in order to sink deep inspection shafts in soils or soft rocks, but these shafts generally need to be not smaller than 1 m diameter if the strata are to be examined or tests conducted in situ.

The major benefit derived from the use of trial pits, compared with boreholes, is that they permit a physical examination of the soils en masse to be carried out in their natural habitat. It is then possible to establish the degree of variation which may be found in a particular soil and it is also possible to search for discontinuities which are frequently damaged or disturbed during a sampling operation in a conventional borehole. An example of the information which can be found in a trial pit is shown here. Judicious positioning of a series of trial pits, or the excavation of trenches, are of great use in the investigation of mass-movement processes, particularly since it is possible to locate the surfaces along which movements have occurred. Block samples can be taken to include these shell surfaces and appropriate tests, performed in the laboratory, will detertmne the shear strength along them. An accurate stability analysis can then be performed.


 

Hutchinson, J.N., Somerville, S., and Petley, D.J. (1973). A landslide in periglacially disturbed Etruria Marl at Bury Hill, Staffordshire. Quart. J. Eng. Geol., 6, 377-404.

 

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Laboratory work

The object of performing laboratory tests is to obtain information, additional to that obtained from in situ tests, on the composition and properties of the materials encountered on any site. Laboratory tests can be grouped under three main headings:

tests for classification and identification;
tests for engineering properties;
tests for special purposes in engineering construction.

The first group include tests to determine the particle-size distribution of the material, index property tests (Liquid and Plastic Limits), specific gravity tests, and tests to determine the bulk density and water content of the soils. Since these are very common tests they will not be discussed further here (see British Standard 1377).

The second group of tests includes those to determine the engineering properties of the soils, i.e. permeability, compressibility and shear strength.

In the third group are special tests devised for earthworks and roads and airfields. They have little relevance for mass-movement studies.

BS 1377 (1990). Methods of Testing Soils for Civil Engineering Purposes, British Standards Institution, London.

 

Measurement of shearing resistance

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. As stated previously, 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.

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.


Bishop, A.W., and Henkel, D.J. (1962). The Measurement of Soil Properties in the Triaxial Test, Edward Arnold, London.

 

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Shear box tests

The shear box was probably the first type of apparatus used for the measurement of the shearing resistance of soils. The apparatus, which is shown in the figure, 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:

  • it may be difficult to install an undisturbed sample in the apparatus;
  • the stress distribution across the sample is complex;
  • failure occurs along a plane dictated by the design of the apparatus;
  • the area under shear reduces during the test;
  • there is no direct control over drainage conditions in the sample.

    Typical results from tests on well-graded sand are illustrated here.



     

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    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.


     

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    Measurement of residual shear 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' + s' tan f'

    and the residual strengths can be expressed as:

    sr = cr' + s' tan fr'

    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
    f' = angle of shearing resistance
    f'r = residual angle of shearing resistance
    sp = peak shear strength
    sr = residual shear strength
    s' = applied effective stress

     

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    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:

    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.

    # 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.

    # 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.

     

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    Methods of measurement of residual 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.

    Shear box
    (a) Tests on natural shear surfaces
    (b) Reversal-type tests
    (c) Cut-plane tests
    Triaxial
    (a) Tests on natural shear surfaces
    (b) Cut-plane tests
    Ring shear


    # Skempton, A.W. (1964). Long-term stability of clay slopes. Geotechnique, 14, 75-102.


    # Skempton, A.W., and Petley, D.J. (1976). The strength along structural discontinuities in stiff clay. Proc. Geot. Conf. on Shear Strength of natural Soils and Rocks. Oslo, 2, 3-20.

     

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    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.


    #

     

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    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.

     

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    Further reading

    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.

    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.

    Bishop, A.W., Green, G.R., Garga, V.K., Andresen, A., and Brown, J.D. (1971). A new ring-shear apparatus and its application to the measurement of residual strength. Geotechnique, 21, 273-328.

    Bishop, A.W., and Henkel, D.J. (1962). The Measurement of Soil Properties in the Triaxial Test, Edward Arnold, London.

    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.

    BS 1377 (1990). Methods of Testing Soils for Civil Engineering Purposes, British Standards Institution, London.

    Bromhead, E.N. (1 979), A simple ring shear apparatus. Ground Eng., 12, 40-44.

    Broms, B.B. (1980). Soil sampling in Europe; state of the art. Journ. Geot. Eng. Div.,

    Brunsden, D., Doornkamp, J.C., Fookes, P.G., Jones, D.K.C., and Kelly, J.M.H. (1975). Large scale geomorphological mapping and highway engineering design. Quart. J. Eng., Geol. 8, 227-254.

    Brunsden, D., and Jones, D.K.C. (1972). The morphology of degraded landslide slopes in South West Dorset. Quart. J. Eng. Geol., 5, 205-222.

    Dumbleton, NLJ., and West, G. (1971). Preliminary Sources of Informati on for Site Investigations in Britain. RRL Report No. LR403, Transport and Road Research Laboratory, Crowthorne, Berks.

    Hutchinson, J.N., Somerville, S., and Petley, D.J. (1973). A landslide in periglacially disturbed Etruria Marl at Bury Hill, Staffordshire. Quart. J. Eng. Geol., 6, 377-404.

    Hvorslev, M.J. (1937). Uber die Festigkeitseigenschaften gestorter bindiger Boden. Ingenior Skriftor A., Copenhagen, 45.

    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.

    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.

    Savigear, R.A.G. (1965). A technique of morphological mapping. Mapping Assoc. Amer. Geogr., 55, 514-38

    Skempton, A.W. (1964). Long-term stability of clay slopes. Geotechnique, 14, 75-102.

    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.

    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.

    Tidemann, B. (1937). Uber die Schubfestigkeit bindiger Boden. Bautechnik 15,

    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.

    Waters, R.S. (1958). Morphological mapping. Geography, 10-17.

     

    back to Slope stability

    Remedial measures

    The factor of safety of a slope in soil possessing cohesion and friction can be written as

    If, for a particular slope, the computed or actual factor of safety Fs is inadequate, clearly Fs can be increased by:
    increasing the numerator (i.e. S[c'l+(Wcosa-ul)tanf] ), or
    decreasing the denominator (i.e. SWsina ), or
    a combination of the above.

    The main methods for achieving this increase in Fs, are: replacement; modification of slope geometry; drainage; use of restraining structures.
    Detailed reviews of the wide range of remedial methods used in improving the stability of slopes are given by Hutchinson and Zaruba and Mencl.


    Hutchinson, J.N. 1977. Assessment of the effectiveness of corrective measures in relation to geological conditions and types of slope movement. Bulletin of the Int. Assoc. Eng. Geol., 16, 131-155.


    Zaruba, Q. and Mencl, V., 1982. Landslides and their control. Elsevier, Amsterdam; Academia, Prague.

     

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    Drainage

    Drainage is one of the most widely used methods for improving stability. Clearly surface water must be removed and build-up of water pressures in tension cracks prevented. Subsurface drainage must be designed to reduce the water pressures acting on actual or potential slip surfaces; in this way, the value of the pore pressure (u) is reduced, thereby producing an increase in the factor of safety.

    Several methods exist for subsurface drainage, including:
    trench drains
    horizontal drains
    vertical drains (or wells)
    galleries

    Drainage may also be achieved by the use of electro-osmosis and by planting suitable vegetation.

    Of these various methods, trench drains are frequently the cheapest and most widely used method. They are applicable to slips of moderate depth, but for deeper failures other methods may be more appropriate.

     

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    Trench drains

    Trench drains are normally constructed by machine; the drains are typically 0.5 to 1.0m wide and up to 7 or 8m deep. They are back-filled with suitable free-draining material with a porous pipe at the base to collect and remove the water. Provision to prevent clogging must be incorporated in the design. Ideally, the drain should penetrate through the slip surface (such drains are referred to as "counterfort" drains) and then in addition to the improvement in stability as a result of reduced pore water pressured on the slip surface, some additional restraint is achieved by the replacement of the weak slipped material by the stronger material in the drain.


    #

     

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    Design of trench drains

    An approximate method for designing trench and counterfort drains has been developed by Hutchinson using finite element analyses and assuming two-dimensional steady-state flow. Hutchinson used the results of the analysis to define the efficiency (h) of the drains and to relate the efficiency to the ratio s/ho where s is the spacing of the drains and ho is the depth of the drains beneath the groundwater level (see diagram). He suggested that Lines G and H can be taken as reasonable upper and lower bounds for the design of drains. For the purposes of preliminary design, curve G can be regarded as a conservative lower bound. Hutchinson also presented data from the long-term performance of drains at 6 sites, with encouraging results.

     

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    Example drainage design

    Consider a slope inclined at 9º to the horizontal in which a translational failure has occurred. The slip surface exists at a depth of 4m below ground level, and the phreatic surface has been located at 0.5m below ground level. If the shear strength parameters on the shear surface are given by c'r = 0 and f 'r = 16º, what is the factor of safety of the slope in its present condition and what will be the factor of safety if trench drains 4m deep are installed at 10m spacings?

    Take g = 20kN/m³ gw = 10 kN/m³

    The introduction of these drains in the slope increases the factor of safety from 1.02 to 1.34.

     

    Example drainage design

    Solution without drains

    For a plane translational slip,

    If c' = 0, then

    In this example, g = 20kN/m³ , gw = 10 kN/m³, z = 4m, h = 3.5m, f'r = 16º and b = 9º

    Therefore, without drains,

     

    Example drainage design

    Solution with drains

    The depth of the drains below the water table, ho = 3.5m and their spacing, s = 10m.

    Therefore s / ho = 10 / 3.5 = 2.9

    On the figure opposite, Hutchinson suggested that Line G can be taken as a reasonable upper bound for the design of drains. So, using Line G in this case,

    for s / ho = 2.9, we obtain = 0.48

    is the average efficiency of the drains, and is given by:

    Thus for = 0.48 and ho = 3.5m,

    = 1.82m

    The new value of Fs can be computed by substituting h = in the appropriate equation:

     

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    Restraining structures

    Retaining structures such as piles, walls and anchors may be used to improve stability. It must be appreciated that the forces and moments to which these structures are subjected may be very large and hence careful design is essential. The detailed design of these structures is outside the scope of this section. Useful discussions are given by Zaruba and Mencl, Bromhead and Leventhal and Mostyn.


    # Bromhead, E.N. 1992. The stability of slopes. Blackie, London.


    # Leventhal, A.R. and Mostyn, G.R. 1987, Slope stabilisation techniques and their application. In Slope Instability and Stabilisation, ed. by B. Walker and R. Fell, Balkema, Rotterdam.

     

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    Modification of slope geometry

    Changing the geometry of a slope to improve stability can involve the following:
    excavation to unload the slope,
    filling to load the slope,
    reducing the overall height of the slope.

    Where excavation and/or filling are used as remedial measures, it is essential that they are correctly positioned, and use should be made of the Neutral Point Concept.


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    Replacement

    Where the slip surface is not unduly deep, removal of all (or part) of the slipped material and replacement provides a relatively simple and straightforward remedial measure. The removed soil may be replaced by free-draining material (in which case some additional benefit may be achieved by drainage) or by the recompacted slip debris. If shear surfaces exist at shallow depth, they can be destroyed by digging out, remoulding and recompacting. A recent development has seen the incorporation of geotextile reinforcement within the replaced material.

     

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    Further reading

    Bromhead, E.N. 1992. The stability of slopes. Blackie, London.

    Chandler, R.J. 1991. Slope stability engineering. Thomas Telford, London.

    Hutchinson, J.N. 1977. Assessment of the effectiveness of corrective measures in relation to geological conditions and types of slope movement. Bulletin of the Int. Assoc. Eng. Geol., 16, 131-155.

    Leventhal, A.R. and Mostyn, G.R. 1987, Slope stabilisation techniques and their application. In Slope Instability and Stabilisation, ed. by B. Walker and R. Fell, Balkema, Rotterdam.

    Zaruba, Q. and Mencl, V., 1982. Landslides and their control. Elsevier, Amsterdam; Academia, Prague.