Description
The stability of slopes may be increased by substitution of the original materials with materials of higher strength and, possibly, higher permeability. In the latter case, provided that drainage is permitted, additional increases in shear resistances may derive from changes of effective stresses due to the lowering of pore pressures. It is possible to distinguish between:
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“Shallow” substitution, where the unstable materials is partially or totally removed in bulk excavation and replaced with materials with adequate strength and permeability characteristics using standard earthworks equipment. Depending on the size and shape of the landslide, “shallow” substitution may involve from a few cubic metres to tens of thousand of cubic metres.
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“Deep” substitution, where conventional earthworks become impractical or uneconomic and substitution of unstable materials can be obtained by means of special techniques, typically vibro replacement/vibrodisplacement and jet-grouting.
“Shallow” substitution of a significant portion of the landslide by conventional earthworks is described here.
Partial “shallow” substitution to form drainage trenches or to form structural counterforts to transfer loads to stable layers below the landslide are discussed in Sections 4 and 6 of this Annex respectively.
Special techniques for deep substitution are described in the relevant fact sheets of Section 5.
Large scale excavation and replacement or recompaction of the landslide body have become feasible and progressively more widely practiced with the introduction of large self-propelled hydraulic-powered earthmoving and compaction equipment in following World War II (Rogers, 1992).
Where the volume of the sliding mass is relatively small and shallow, so that there is limited space for compaction, the excavated material is often replaced with crushed gravel or stone fill, which requires limted compaction and provides excellent drainage characteristics. The development of geosynthetics and in particular of filter fabrics has provided a fast and economic solution to the problem of migration of fines from the underlying fine fill or natural soil to the gravel or stone fill. However, considering that typically the shear strength at the soil-geotextile interface is lower than within the soil itself, it is necessary to step the base of the excavation to prevent forming an artificial discontinuity where further sliding can take place.
For larger slides the use of imported high quality fill is expensive and implies a significant environmental impact, not only associated with the quarrying and transportation of the imported fill but also with the disposal of the excavated material. Accordingly, in larger slides the excavated soil is normally used to backfill the slide, relying on a number of techniques to prevent further sliding, where required.
Drainage installed at the heel of the basal shear keys and drainage layers within and at the base of the backfill prevent future rises in pore water pressures in the backfill.
In clay, excavation and recompaction destroys the slip plane at the base of the slide, where only residual strength is available, and replaces it with homogeneous material; if necessary, the clay backfill can be improved by lime stabilization, both to ease handling and compaction and to improve its mechanical characteristics.
Additional reinforcement may be added to the backfill, effectively forming a reinforced soil structure, as described in greater detail in Section 7. This is especially useful where failure has occurred in very steep slopes, which cannot be reconstructed with standard fill.
Sisson (2010) describe a recent example of a major landslide repaired by substitution in Oceanside, California.
Design methods
Shallow substitution
For shallow substitution with unreinforced fill, the design process is the same as would be carried out for new fill, typically based on limit equilibrium analyses. Reference should be made to Section 2 of the Annex for further discussion on the applicability and limitations of these methods when used to evaluate “first time slides”. Special care should be paid in ensuring the stability of temporary excavations.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Best suited to rotational slides. Also suitable for translational slides, depending on geometry and extent. |
| Topple | 0 | |
| Slide | 8 | |
| Spread | 0 | |
| Flow | 0 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Rock may imply difficulties in excavation and the need to form steep slopes,, which require some form of reinforcment. |
| Debris | 6 | |
| Rock | 8 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 8 | Very small, superficial slides may be repaired by granular fill with limited compaction. Larger slides (medium deep) allow conventional construction equipment to operate efficiently. Intermediate slides cannot be addressed efficiently in either way, while very large, deep landslide involve large earthmoving with significant potential environmental and cost impacts. |
| Shallow (0.5 to 3 m) | 6 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 4 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | While excavation can be carried out without special difficulty when the rate of movement is slow (5cm/day) or less, backfilling presupposes that the slide is stable or moving at most very slowly. |
| Slow | 2 | |
| Very slow | 6 | |
| Extremely slow | 10 | |
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 backfilling, limiting the applicability of this techniques, when these conditions occur, unless the long term stability of the backfill is improved by combination with deep drainage. |
| High | 4 | |
| Low | 8 | |
| Absent | 10 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Surface flows must be diverted to prevent them from accumulating in the backfill. Drainage to be provided both on surface and at interface between fill and natural soil. |
| Snowmelt | 8 | |
| Localized | 8 | |
| Stream | 2 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the evaluation of the stability of the treated slope. |
| Feasibility and Manageability | 8 | Concept an techniques well developed. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Can be implemented with widely available equipment. Possible difficulties with excavation in rock and with the disposal of arisings. Construction control. |
| Environmental suitability | 6 | will be updated |
| Economic suitability (cost) | 8 | Low to moderate, depending on the material used (imported or from excavation). |
References
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Rogers J.D. (1992). “Recent developments in landslide mitigation techniques”. In “Landslides/Landslide mitigation”, Slosson E., Keene A.G., Johnson J.A., eds.,. Reviews of Engineering Geology, Volume IX, Geological Society of America, Boulder, Colorado.
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Sisson P. (2010). “Oceanside: Crews repair soil at site of devastating landslide”. North Country Times – The Californian, 17 July, http://www.nctimes.com/news/local/oceanside/article_fa30ca61-85a8-5750-8de5-9e4826fea809.html.