A suggested method for reporting landslide remedial measures International Union of Geological Sciences Working Group on Landslides, Commission on Landslide Remediation (Chairman: M. Popescu)
a landslide (WP/WLI 1993a). These are summarized in the Multilingual Landslide Glossary (WP/WLI 1993b). Our working definition of a landslide is “the movement of mass of rock, earth or debris down a slope” (Cruden 1991). The Working Group has also set up Committees to extend the scope of the Landslide Report to the causes of landslides (WP/WLI 1994), their rates of movement (WP/WLI 1995), their geology, and the damage landslides may cause. The suggestions from the Commission on Landslide Remediation define the main landslide remedial measures and will be used to supplement the Landslide Report. The Working Group welcomes carefully documented proposals for additions on amendments to this (and other) suggested methods. They should be addressed to the chairman of the Introduction Working Group (currently Dr. D.M. Cruden, Department of Civil Engineering, University of Alberta, Edmonton, The International Union of Geological Sciences Working Alberta T6G 2G7, Canada). Group on Landslides (IUGS WG/L) is a continuation of the International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory (WP/WLI). It was Levels of effectiveness and formed from the International Association of Engineering Geology and Environment’s (IAEG) Commission on Landacceptability that may be applied slides and Other Mass Movements, the International in the use of remedial measures Society of Soil Mechanics and Geotechnical Engineering’s (ISSMGE) Technical Committee on landslides and nominees of national groups of the International Society for Terzaghi (1950) has written that “if a slope has started to Rock Mechanics. As a contribution to the International move, the means for stopping movement must be adapted Decade for Natural Disaster Reduction (1990–2000), the to the processes which started the slide”. For example, if Working Group is assisting the establishment of a World erosion is a causal process of the slide, consideration Landslide Inventory by suggesting standard terminology regarding remediation would include armoring the slope against erosion, or removing the source of erosion. An for describing landslides. The Working Group has suggested a method for reporting erosive spring can be made non-erosive by either blana landslide (WP/WLI 1990), for preparing a landslide keting with filter materials or drying up the spring with summary (WP/WLI 1991) and for describing the activity of horizontal drains, etc. The greatest benefit in understanding landslide-producing processes and mechanisms lies in the use of this understanding to anticipate and devise measures to minimize and prevent major landslides. The term major should be Received: 13 July 2000 7 Accepted: 29 August 2000 underscored here because it is neither possible nor M. Popescu (Y) feasible, nor even desirable, to prevent all landslides. There Department of Civil and Architectural Engineering, are many examples of landslides that can be handled more Illinois Institute of Technology, 3201 South Dearborn Street, effectively and at less cost after they occur. Landslide avoiChicago, Illinois 60616, USA dance through selective siting of developments is obviously e-mail: mepopescu6usa.net desired – even required – in many cases, but the dwindling Fax c1-312-5673519 Abstract A brief list of landslide remedial measures is presented and a format for reporting landslide remediation is suggested. They make useful additions to the Landslide Report proposed by the International Union of Geological Sciences Working Group on Landslides (formerly the International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory).
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number of safe and desirable construction sites may necessitate an increasing use of landslide-susceptible terrain. Just as there are a number of available remedial measures, so also are there a number of levels of effectiveness and levels of acceptability that may be applied in the use of these measures. We may have a landslide, for example, that we simply choose to live with; one that poses no significant hazard to the public, but which also requires periodic maintenance, for example, through removal, due to occasional encroachment onto the shoulder of a roadway. The permanent closure of the Manchester-Sheffield road at Mam Tor in 1979 (Skempton et al. 1989) and the decision not to reopen the railway link to Killin following the Glen Ogle rockslide in the UK (Smith 1984) are well-known examples of abandonment due to the effects of landslides where repair was considered uneconomic. Most landslides, however, must usually be dealt with sooner or later. How they are handled depends on the processes that led to and precipitated the movement, the landslide type, the kinds of materials involved, the size and location of the landslide, the place or things affected by the landslide or the situation created as a result of it, available resources, etc. The technical solution must be in harmony with the natural system, however, or the remedial work will be either short lived or excessively expensive. In fact, landslides are so varied in type and size and always so dependent upon special local circumstances, that for a given landslide problem there is more than one method of prevention or correction that can be successfully applied. The success of each measure depends to a large extent on the degree to which the specific soil and groundwater conditions are correctly recognized in investigation and applied in design. As many of the geological features, such as sheared discontinuities, are not well known in advance, it is better to put remedial measures in hand on a â€œdesign as you go basisâ€?; i.e. the design has to be flexible enough for changes to be made during or subsequent to construction of the remedial works.
A brief list and short comments on landslide remedial measures Correction of an existing landslide or the prevention of a pending landslide is a function of a reduction in the driving forces or an increase in the available resisting forces. Any remedial measure used must provide one or both of the above results. Many general reviews of the methods of landslide remediation have been made. The interested reader is particularly directed to Hutchinson (1977), Zaruba and Mencl (1982), Bromhead (1992), Schuster (1992), Fell (1994) and Popescu (1996). In order to help include relevant information on landslide remediation in a standard format in the Landslide Report (WP/WLI 1990), the IUGS WG/L Commission on Landslide 70
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Table 1 A brief list of landslide remedial measures 1. Modification of slope geometry 1.1 Removing material from area driving the landslide (with possible substitution by lightweight fill) 1.2 Adding material to area maintaining stability (counterweight berm or fill) 1.3 Reducing general slope angle 2. Drainage 2.1 Surface drains to divert water from flowing onto slide area (collecting ditches and pipes) 2.2 Shallow or deep trench drains filled with free-draining geomaterials (coarse granular fills and geosynthetics) 2.3 Buttress counterforts of coarse-grained materials (hydrological effect) 2.4 Vertical (small-diameter) boreholes, pumped or self draining 2.5 Vertical (large-diameter) wells with gravity draining 2.6 Sub-horizontal or sub-vertical boreholes 2.7 Drainage tunnels, galleries or adits 2.8 Vacuum dewatering 2.9 Drainage by siphoning 2.10 Electro-osmotic dewatering 2.11 Vegetation planting (hydrological effect) 3. Retaining structures 3.1 Gravity-retaining walls 3.2 Crib-block walls 3.3 Gabion walls 3.4 Passive piles, piers and caissons 3.5 Cast-in-situ reinforced concrete walls 3.6 Reinforced earth-retaining structures with strip/sheet- polymer/ metallic-reinforcement elements 3.7 Buttress counterforts of coarse-grained material (mechanical effect) 3.8 Retention nets for rock slope faces 3.9 Rock fall attenuation or stopping systems (rock trap ditches, benches, fences and walls) 3.10 Protective rock/concrete blocks against erosion 4. Internal slope reinforcement 4.1 Rock bolts 4.2 Micropiles 4.3 Soil nailing 4.4 Anchors (pre-stressed or not) 4.5 Grouting 4.6 Stone or lime/cement columns 4.7 Heat treatment 4.8 Freezing 4.9 Electro-osmotic anchors 4.10 Vegetation planting (root strength mechanical effect)
Remediation has prepared a short checklist of landslide remedial measures (see Table 1). The measures are arranged in four practical groups, namely: modification of slope geometry, drainage, retaining structures and internal slope reinforcement. Hutchinson (1977) has indicated that drainage is the principal measure used in the remediation of landslides, with modification of slope geometry the second most commonly used method. These are also generally the least costly of the four major categories, which is clearly a factor in their wide use.
A suggested method for reporting landslide remedial measures
Experience shows that while one measure may be dominant, most landslide remediation involves the use of a combination of two or more from the major categories. For example, while restraint may be the principal measure used to correct a particular landslide, of necessity drainage and modification of slope geometry will also be utilized to some degree. Modification of slope geometry is a most efficient method particularly in deep-seated landslides. However, the success of corrective slope regrading (fill or cut) is determined not merely by the size or shape of the alteration, but also by its position on the slope. Hutchinson (1977) provides details of the “neutral line” method to assist in finding the best location to place a stabilizing fill or cut. There are some situations where this approach is not simple to adopt. These include long translational landslides where there is no obvious toe or crest; where the geometry is determined by engineering constraints; and where the unstable area is complex and thus a change in topography which improves the stability of one area may reduce the stability of another (Leventhal and Mostyn 1987). Drainage is often a crucial remedial measure due to the important role played by pore-water pressure in reducing shear strength. Because of its high stabilization efficiency in relation to cost, drainage of surface water and groundwater is the most widely used and generally the most successful stabilization method. As a long-term solution it suffers greatly because the drains must be maintained if they are to continue to function (Bromhead 1992). Surface water is diverted from unstable slopes by ditches and pipes. Drainage of the shallow groundwater is usually achieved by networks of trench drains. Drainage of the failure surfaces, on the other hand, is achieved by counterfort or deep drains which are trenches sunk into the ground to intersect the shear surface and extending below it. In the case of deep landslides, often the most effective way of lowering groundwater is to drive drainage tunnels into the intact material beneath the landslide. From this position, a series of upward-directed drainage holes can be drilled through the roof of the tunnel to drain the toe of the landslide. Alternatively, the tunnels can connect up a series of vertical wells sunk down from the ground surface. In instances where the groundwater is too deep to be reached by ordinary trench drains and where the landslide is too small to justify an expensive drainage tunnel or gallery, bored sub-horizontal drains can be used. Another approach is to use a combination of vertical drainage wells linked to a system of sub-horizontal borehole drains. Schuster (1992) discusses recent advances in the commonly used drainage systems and briefly mentions less commonly used, innovative means of drainage, such as electro-osmotic dewatering, vacuum and siphon drains. Buttress counterforts of coarse-grained materials placed at the toe of unstable slopes are often successful as a remedial measure. They are listed in Table 1, under both “Drainage” when used mainly for their hydrological effect and “Retaining Structures” when used mainly for their mechanical effect.
During the early part of the post-war period, landslides were generally seen to be “engineering problems” requiring “engineering solutions”, involving correction by the use of structural techniques. This structural approach initially focused on retaining walls but has subsequently been diversified to include a wide range of more sophisticated techniques including passive piles and piers, cast-insitu reinforced concrete walls and reinforced earthretaining structures. When properly designed and constructed these structural solutions can be extremely valuable, especially in areas with high loss potential or in restricted sites. However, fixation with structural solutions has in some cases resulted in the adoption of over-expensive measures that proved to be less appropriate than alternative approaches involving slope geometry modification or drainage (Jones and Lee 1994). Over the last several decades there has been a notable shift towards “soft engineering” non-structural solutions including classical methods such as drainage and modification of slope geometry but also some novel methods such as lime/cement stabilization, grouting or soil nailing (Powel 1992). The cost of non-structural remedial measures is considerably lower when compared with the cost of structural solutions. On the other hand, structural solutions such as retaining walls involve opening the slope during construction and often require steep temporary cuts. Both these operations increase the risk of failure during construction, due to oversteeping or increased infiltration from rainfall. In contrast, the use of soil nailing as a non-structural solution to strengthen the slope avoids the need to open or alter the slope from its current condition. Environmental considerations have increasingly become an important factor in the choice of suitable remedial measures, particularly issues such as visual intrusion in scenic areas or the impact on nature or geological conservation interests. An example of a “soft engineering” solution more compatible with the environment is the stabilization of slopes by the combined use of vegetation and man-made structural elements working together in an integrated manner, known as biotechnical slope stabilization (Schuster 1992). The basic concepts of vegetative stabilization are not new – vegetation has a beneficial effect on slope stability by the processes of interception of rainfall and transpiration of groundwater, thus maintaining drier soils and enabling some reduction in potential peak groundwater pressures. In addition to these hydrological effects, vegetation roots reinforce the soil, increasing soil shear strength, while tree roots may anchor into firm strata, providing support to the upslope soil mantle through buttressing and arching. A small increase in soil cohesion induced by the roots has a major effect on shallow landslides. Although the mechanical effect of vegetation planting is not significant for deeper-seated landslides, the hydrological effect is beneficial for both shallow and deep landslides. However, vegetation may not always assist slope stability. Destabilizing forces may be generated by the weight of the vegetation acting as a surcharge and by wind forces on the exposed
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vegetation, though both these are very minor effects. Roots of vegetation may also act adversely by penetrating and dilating the joints in rocks. For detailed information on the research into the engineering role of vegetation for slope stabilization refer to Greenway (1987) and Wu (1991). In addition, the “Geotechnical Manual for Slopes” (Geotechnical Control Office of Hong Kong 1981) includes an excellent table noting the hydrological and mechanical effects of vegetation. The concept of biotechnical slope stabilization is generally cost effective compared to the use of structural elements alone; it increases environmental compatibility and allows the use of local natural materials. Interstices of the retaining structure are planted with vegetation whose roots bind together the soil within and behind the structure. The stability of all types of retaining structures with open grid work or tiered facings benefits from such vegetation. An example of a composite vegetated geotextile/geogrid-reinforced structure named “Biobund” was presented by Barker (1991). Stabilization of large landslides (over 1 million m 3 in volume), which are to be reported for the World Landslide Inventory, is very difficult and extremely expensive. For these large landslides an increase of about 5% of the factor of safety might be aimed for, but even this modest percentage change would involve very large absolute changes in the forces acting on the sliding mass. Thus, if only structural measures were used they would be required on a massive scale, consistent with the scale of the landslide, and would be extremely costly (Gongxian 1985). Modification of slope geometry is applicable on a large scale, but is more easily applied to single rotational slides with a high thickness to length ratio. The consequences of redistributing the ground masses where a series of overlapping slip mechanisms exist are harder to predict and may easily be adverse; if material is moved to stabilize one mechanism another mechanism may be destabilized (Hutchinson 1977). Drainage is likely to provide the most attractive option for large landslide stabilization (Nakamura 1984).
Table 2 Landslide report section on landslide remedial measures – La Frasse Landslide (Novarez and Bonnard 1988) Modification of slope geometry Drainage Retaining structures Internal slope reinforcement
– 2.4 3.4, 3.10 4.4
La Frasse Landslide La Frasse Landslide (Novarez and Bonnard 1988), a very important instability phenomenon located in the Swiss Alps, probably occurred during the last glacier retreat, following the destruction of the thin reversed limb of a Jurassic limestone syncline under the action of glacial scouring. This abrasion set in motion the clayey schist rocks with a flysch facies which formed the heart of the syncline. The present depth of the sliding material varies between 40 and 50 m at the bottom of the valley and from 60 to 100 m in the central part of the landslide. A layer of stabilized material separates the lower rupture surface from the bedrock. The volume of the active landslide is some 60 million m 3 and the long-term velocity in the more active lower part of the sliding mass varies between 150 and 600 mm/year. Bearing in mind its large size and fairly significant activity, La Frasse Landslide has not caused a great deal of damage. The structures most affected were two main roads crossing the lower part of the landslide. A short section of the lower road has been partially stabilized by fixing the more superficial sliding mass by 9-m-long piles and 10 m long anchors. Thus the road could be maintained in good condition for a period of about 10 years. The rockfill levee built in 1988 along the Grande Eau River in order to reduce the erosion at the toe of the landslide was washed away less than 1 year later. In 1994, following a crisis period, a series of pumped drainage boreholes were built in the middle of the most active landslide zone. Although the velocity of the landslide has been reduced, a reduction is always observed after a crisis period, hence it is not yet clear whether it is related to the drainage measures introduced (Bonnard, personal commuDiscussion nication 1995). Information on the remedial measures applied to La Frasse The suggested method for reporting landslide remedial Landslide so far is summarized in the standard format in measures in the Landslide Report (WP/WLI 1990) divides Table 2. The numbers in Table 2 correspond to those given these measures into four practical groups: modification of in Table 1. slope geometry, drainage, retaining structures and internal slope reinforcement, as illustrated in Table 1. The LandJizukiama Landslide slide Report section concerning remedial measures should The second example concerns the Jizukiama Landslide in have these four headings and under each heading a list of Nagano City, Japan (Landslide Society of Japan 1988). The the remedial measures applied to the reported landslide. sliding area is largely composed of rhyolite tuff affected by Table 1 would make a useful checklist attached to the hydrothermal alteration and abundant montmorillonite. Landslide Report. The rock is fractured due to numerous faults. The landTwo case examples are briefly discussed below to illustrate slide, with a total volume of about 5 million m 3, had the the selection of remedial measures and filling out of the character of a sudden collapse in the weathered rock zone, thrusting the old colluvial material downward. corresponding section in the Landslide Report.
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A suggested method for reporting landslide remedial measures
Table 3 Landslide report section on landslide remedial measures – Jizukiama Landslide (Landslide Society of Japan 1988) Modification of slope geometry Drainage Retaining structures Internal slope reinforcement
1.3 2.1, 2.2, 2.5, 2.6, 2.7 3.4, 3.8 4.4
Table A1 A brief list of landslide causal factors 1. Ground conditions (1) Plastic weak material (2) Sensitive material (3) Collapsible material (4) Weathered material (5) Sheared material (6) Jointed or fissured material (7) Adversely oriented mass discontinuities (including bedding, schistosity, cleavage) (8) Adversely oriented structural discontinuities (including faults, unconformities, flexural shears, sedimentary contacts) (9) Contrast in permeability and its effects on groundwater (10) Contrast in stiffness (stiff, dense material over plastic material)
Cracks appeared in the upper part of the slope during the snow-melting season in 1981 and creeping of the slope continued intermittently. After unusually heavy precipitation during the rainy season of 1985, the slope suddenly slid down at 5 p.m. on 26 July, killing 29 people and totally destroying 50 houses. As an immediate measure to restore 2. Geomorphological processes (1) Tectonic uplift the equilibrium in the sliding area, 110 H-steel piles were (2) Volcanic uplift installed at the toe of the landslide and surface drainage (3) Glacial rebound works to divert water from flowing into the sliding area (4) Fluvial erosion of slope toe were undertaken. In the period 1985–1989 the following (5) Wave erosion of slope toe remedial works were put in place: 29 cast-in-situ rein(6) Glacial erosion of slope toe (7) Erosion of lateral margins forced concrete piers 5.1 m in diameter, 23 drainage wells (8) Subterranean erosion (solution, piping) 3.5 m in diameter, 157 steel piles 0.32 m in diameter, 818 (9) Deposition loading of slope or its crest anchors 18–52 m in length, 8,400 m sub-horizontal bore(10) Vegetation removal (by erosion, forest fire, drought) hole drainage works and 3 drainage tunnels with a total length of 1,630 m. In addition, land regrading works and 3. Physical processes slope protection with reinforced concrete network were (1) Intense, short-period rainfall carried out. (2) Rapid melt of deep snow (3) Prolonged high precipitation The Jizukiama Landslide Report section prepared (4) Rapid drawdown following floods, high tides or breaching of according to the checklist given in Table 1 is presented in natural dams Table 3. (5) (6) (7) (8) (9) (10)
Earthquake Volcanic eruption Breaching of crater lakes Thawing of permafrost Freeze and thaw weathering Shrink and swell weathering of expansive soils
Acknowledgements The draft of this suggested method was prepared by the IUGS WG/L Commission on Landslide Remediation: M. Popescu (Chairman), P. Anagnosti, Ch. Bonnard, R. Fell, E. Krauter, W.A. Lacerda, H. Nakamura, A. Onalp and L. Valenzuela. Interesting suggestions and comments by E. Bromhead, A. Federico, O. Hungr and J. Hutchinson are gratefully acknowl- 4. Man-made processes edged. The Commission is also grateful to UNESCO’s Earth (1) Excavation of slope or its toe Science Division for funding travel of some members to meetings (2) Loading of slope or its crest to discuss this suggested method. (3) Drawdown (of reservoirs) (4) Irrigation (5) Defective maintenance of drainage systems (6) Water leakage from services (water supplies, sewers, storm-water drains) (7) Vegetation removal (deforestation) (8) Mining and quarrying (open pits or underground galleries) There is an obvious relationship between landslide reme(9) Creation of dumps of very loose waste dial measures and landslide causal factors (Popescu 1996). (10) Artificial vibration (including traffic, pile driving, heavy machinery)
Appendix: Landslide causal factors
The IUGS WG/L Commission on Causes of Landslides has prepared a short checklist of landslide causal factors arranged in four practical groups according to the tools and procedures necessary for documentation (see Table A1). The format of the table lends itself to the creation of simple databases suited to much of the database management software now available for personal computers. The information collected can be compared with summaries of other landslides and used to guide further investigations and mitigation measures. According to their function, landslide causal factors can be classified into two groups: (1) preparatory causal factors,
which make the slope susceptible to movement without actually initiating it and thereby tending to place the slope in a marginally stable state; and (2) triggering causal factors, which initiate movement. The causal factors shift the slope from a marginally stable to an actively unstable state. A particular causal factor may perform either or both functions, depending on its degree of activity and the margin of stability. Although it may be possible to identify a single
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triggering process, an explanation of ultimate causes of a landslide invariably involves a number of preparatory conditions and processes. Based on their temporal variability, the destabilizing processes may be grouped into slow-changing (e.g. weathering, erosion) and fast-changing processes (e.g. earthquake, drawdown). In the search for landslide causes, attention is often focused on those processes within the slope system that provoke the greatest rate of change. Although slow changes act over a long period of time to reduce the resistance/shear stress ratio, often a fast change can be identified as having triggered movement.
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Novarez F, Bonnard Ch (1988) Technical note on the visit of La Frasse Landslide. In: Proc 5th Int Symp on Landslides, Lausanne, Rep 3, pp 1549–1554 Popescu ME (1996) From landslide causes to landslide remediation. Special lecture. In: Proc 7th Int Symp on Landslides, Trondheim, Rep 1, pp 97–114 Powel GE (1992) Recent changes in the approach to landslip preventive works in Hong Kong. In: Proc 6th Int Symp on Landslides, Christchurch, Rep 3, pp 1789–1795 Schuster RL (1992) Recent advances in slope stabilization. Keynote paper. In: Proc 6th Int Symp on Landslides, Christchurch, Rep 3, pp 1715–1746 Skempton AW, Leadbeater AD, Chandler RJ (1989) The Mam Tor landslide, North Derbyshire. Philos Trans R Soc Lond A329 : 503–547 Smith DI (1984) The landslips of the Scottish Highlands in relation to major engineering projects. British Geological Survey Project 09/LS. Department of the Environment, HMSO, London Terzaghi K (1950) Mechanisms of landslides. Geological Society of America, Berkley Vol, pp 83–123 WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory (Chairman: DM Cruden) (1990) A suggested method for reporting a landslide. Bull IAEG 41 : 5–12 WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory (Chairman: DM Cruden) (1991) A suggested method for a landslide summary. Bull IAEG 43 : 101–110 WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory. Working Group on Landslide Activity (Chairman: DM Cruden) (1993a) A suggested method for describing the activity of a landslide. Bull IAEG 47 : 53–57 WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory (Chairman: DM Cruden) (1993b) Multilingual landslide glossary. Bitech, Richmond, British Columbia WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory, Working Group on Landslide Causes (Chairman: ME Popescu) (1994) A suggested method for reporting landslide causes. Bull IAEG 50 : 71–74 WP/WLI: International Geotechnical Societies’ UNESCO Working Party on World Landslide Inventory, Working Group on Rate of Movement (Chairman: Ch Bonnard) (1995) A suggested method for describing the rate of movement of a landslide. Bull IAEG 52 : 75–78 Wu TH (1991) Soil stabilization using vegetation In: Proc Worksh on Biotechnical Stabilization, University of Michigan, Ann Arbor, pp A-1–A-32 Zaruba Q, Mencl V (1982) Landslides and their control. Elsevier, Amsterdam, 324 pp
Abstract A brief list of landslide remedial measures is presented and a format for reporting landslide remediation is suggested. They make u...