Description
In some situations, complete removal of the actually or potentially unstable mass can be an effective and economic form of mitigation, removing the potential hazard at source. Probably the most high profile example of the application of this technique is the construction and maintenance of the Panama Canal (Duncan, 2008). Generally, however, it is only practical on small slumps or small rotational slides. Large scale excavation of larger landslide areas is usually not recommended for several reasons (Highland and Bobrowsky, 2008):
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Excavation is not always effective. For large planar failures, excavation may not cause movements to stop and may allow the landslide to expand.
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Excavation may actually destibilize the ground further upslope by undercutting, which weakens the slope, even to the point of triggering a larger slide than is being mitigated.
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In certain soil profiles, where there are several actual or potential failure surfaces at different depths, excavating down to the top failure surface might trigger sliding on deeper failure surfaces.
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Excavation may interfere with surface runoff and water courses; unless this is properly addressed, it may cause backdrops and areas of temporary or permanent stagnant water with resulting changes to infiltration and the groundwater regime of the slope, or favouring erosion in areas previously protected by the slide mass.
Complete removal of the landslide body is only effective at mitigating the hazard of further movement if the slope can be reprofiled at a lower inclination compared to the original, failed, slope and/or additional hazard mitigation measures are implemented. If this is not carried out, removing the landslide debris is equivalent to accelerated erosion at the toe of the slope where this was a cause of the original landslide, recreating the conditions for further sliding to take place.
Excavation may alter drainage patterns, with potentially detrimental effects on the stability of the area; care should be paid to diverting surface water flows away from the excavated areas and to ensure that reprofiling does not create conditions for stagnant water to accumulate in low lying areas. Similarly, it is necessary to ensure that the materials exposed by the removal of the landslide body are not susceptible to or are adequately protected from rapid weathering which could cause renewed landsliding. To facilitate construction and maintenance of drainage and surgface protection works, excavated surfaces are typically shaped to form a number of benches, typically at 6 to 10 m vertical interval.
The equipment and methods of excavation will need to be selected to suit the nature of the material to be excavated and local conditions in general. Even when the parent, undisturbed material is rock, landsliding may have broken up the mass sufficiently for it to be excavated by conventional equipment. However, it is not rare for the landslide debris to retain sufficient remnants of the original structure and consistency of the parent material that excavation and removal of the landslide debris requires specialist equipment, such as hydraulic hammers or even explosives. In these situations, careful consideration will be given to the need to minimize vibrations, if there is a risk of these triggering further movement.
Design methods
In all cases a careful review of ground, groundwater and drainage conditions needs to be undertaken before any excavation is carried out. When considering complete removal of the landslide body, it is necessary to evaluate the stability of the slope in the proposed final configuration, with particular attention to the stability of the slope above the excavated area. The principles and methods of analysis are decribed in the general fact-sheet 2.0 on “Hazard reduction by modifying the slope geometry or mass distribution”.
The design should consider the method and sequence of excavation, to ensure stability at all times, especially when excavating active landslides; typically, excavation should proceed from the top of the slope downwards, rather than from the toe, to ensure that the work is carried out safely. The design should also consider the final disposal of the excavated material, which can be a serious problem in some cases. Uncontrolled tipping of the material downslope of the excavated area, as often practiced in emergency rehabilitation of rural roads in mountainous terrain, should be avoided since it can damage the existing vegetation and it can create a serious hazard of further sliding downhill.
Special care needs to be paid if the landslide mass is suspected to contain contaminated materials, for whatever reason, since this may require special provision with respect to ensuring the safety of both workers and the public and with respect to arrangements for the disposal of arisings.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 6 | Most applicable to slides, although might cause further sliding in certain conditions. Applicable in principle also to rock slopes subject to falls or toppling. The complete removal of source material for potential flows may be considered in special circumstances but it is unlikely to be applicable in practice. |
| Topple | 6 | |
| Slide | 6 | |
| Spread | 1 | |
| Flow | 2 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 7 | Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by difficulty of excavation. |
| Debris | 7 | |
| Rock | 7 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Typically applicable to relatively small and/or shallow landslides. The implications of large scale excavation and disposal typically make this technique impractical for deep and very deep slides. |
| Shallow (0.5 to 3 m) | 7 | |
| Medium (3 to 8 m) | 5 | |
| Deep (8 to 15 m) | 2 | |
| Very deep (> 15 m) | 1 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 3 | Can be carried out without special difficulty when the rate of movement is slow (5 cm/day) or less; in certain circumstances and with due care, it is possible to excavate slides moving moderately fast (up to a few metres per day), especially if it is possible to place the equipment on stable ground. |
| Slow | 7 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 2 | High or artesian groundwater conditions pose special problems both to the excavation and to the stability of the slope after removal of the landslide mass, limiting the applicability of this technique when these conditions occur. |
| High | 4 | |
| Low | 7 | |
| Absent | 7 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | Surface flows must be diverted to prevent them from reaching the excavated area. |
| Snowmelt | 4 | |
| Localized | 3 | |
| Stream | 1 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique as a mitigation measure depends on the reliability of the evaluation of the stability of the treated slope. |
| Feasibility and Manageability | 10 | Simple technique. Potential benefits and limits of applicability are well established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Easily implemented with widely available equipment. Possible difficulties with excavation in rock and with the disposal of arisings. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 6 | Moderate, provided the work does not involve contaminated material. |
References
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Duncan J.M. (2008) “Managing landslides in the Panama Canal”. Proc. 33d International Geological Congress, Oslo.
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Highland L.M., Bobrowsky P. (2008). “The landslide handbook – a guide to understanding landslides”. Circular 1325, U.S. Geological Survey
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Hutchinson J.N. (1977). “Assessment of the effectiveness of corrective measures in relation to geological conditions and types of slope movement”. General Report. Bulletin of Engineering Geology and the Environment, 16, n°1, 131-155.
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Turner A.K., Schuster R.L. eds. (1996). “Landslides: Investigation and Mitigation”. Special Report 247, Transportation
Research Board, Washington.