Description
This mitigation technique consists in excavating the material in the driving area and replacing it with a lightweight backfill material (Figure 1).
Lightweight fill is also used to minimize the extent and cost of other mitigation measures by minimizing the adverse effect of construction, for example where alignment constraints may dictate that fills for a new highway be placed in a potentially destabilizing position across an actual or potential landslide.
A wide variety of lightweight materials have been proposed and used in this context, depending on local availability and practice and reguloatory constraints, including naturally (geological) lightweight materials such as pumice or shells, manufactured materials, such as expanded clay, polystyrene slabs, cellular concrete, and waste materials or byproducts, such as soil mixed with shredded tyres (‘pneusol’), pulverized fly ash, slag, woodchips or logging slash. Clearly, manufactured materials are typically more expensive and synthetic material may have limited durability, but they afford greater reliability in terms of homogeneity of results; the use of waste materials or byproducts may also be subject to environmental constraints and concerns about possible long term pollution.
This technique operates on the principle of reducing the driving forces more than the resisting forces by altering the mass or load distribution on the slope, in the same manner and subject to similar considerations and limitations as described in fact-sheet 2.3 on “Removal of material from the driving area”. It is most suitable in cases where the instability mechanism occurs as a rotational or pseudo-rotational slide, e.g. where the displaced mass moves as a relatively coherent mass along a spoon-shaped (curved upward) failure surface with little internal deformation, while it is generally ineffective on translational slides on long, uniform planar slopes, or on flow-type landslides.
It should always be kept in mind that the resisting forces are also reduced, especially in the long term, as a result of the reduction in normal stress on the failure surface. It is therefore necessary to locate the excavation in such manner that the reduction in driving forces exceeds the reduction in resisting forces. The neutral line concept, described in fact sheet 2.0 on “mitigation by modifying the slope geometry / mass distribution; general aspects” can be used for a preliminary evaluation of the relative merits of the proposed excavation.
Generally it is most practical where it is necessary to remediate or prevent small slumps or small rotational failures, while at the same time maintaining a specific function. It is generally impractical and not necessary to carry out large scale substitution as would be necessary on large landslides.
Compared to the simple removal of the landslide mass in whole (2.1) or in part (2.3), substitution affords long term surface protection to the excavated surface. However, the permeability of the lightweight fill is typically much higher than that of the original soil and special care must be paid to drainage, both at the surface and at the interface with the natural soil.
The main limitations of the technique relate to the following issues:
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Excavation and replacement with lightweight fill may actually destabilize the ground farther up-slope by ubdercutting;
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Excavation and replacement with lightweight fill increases safety factor by only a limited amount, which tends to decrease with time in low permeability saturated soils;
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Excavation results in large volumes of material to be disposed of off-site in a controlled manner, with attendant difficulties;
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Excavation may interfere with existing structures and services; This is potentially significant when considering this type of mitigation for “potential” landslides, while on actual landslides the residual value of existing structures and facilities can be very low;
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Work on active landslides requires special care to ensure the safety of workers; in particular, it is necessary to assess the possibility of sudden accelerations and to have in place well drilled evacuation plans.
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Vibration necessary to compact certain lightweight fills may be detrimental to slope stability.
Design methods
For general considerations on the geotechnical design of mitigation by removal of material from the driving are, reference shall be made to the general fact sheet 2.0 on hazard mitigation by changes in slope geometry and/or mass distribution.
For the mechanical characteristics of manufactured materials, reference may be made to published guidelines (see for example Stark et al., 2004 on geofoam; Di Prisco, 2007 on expanded clay).
Functional suitability criteria
Type of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Only suited to rotational or pseudo-rotational slides. |
| Topple | 1 | |
| Slide | 8 | |
| Spread | 2 | |
| Flow | 1 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by difficulty of excavation. |
| Debris | 6 | |
| Rock | 3 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 5 | Typically applicable to relatively small and/or shallow landslides. It is generally impractical and not necessary to carry out large scale substitution as would be necessary on large landslides. |
| Shallow (0.5 to 3 m) | 6 | |
| Medium (3 to 8 m) | 8 | |
| Deep (8 to 15 m) | 6 | |
| Very deep (> 15 m) | 4 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 1 | While excavation can be carried out without special difficulty when the rate of movement is slow (5 cm/day) or less, backfilling with lightweight fill presupposes that the slide is stable or moving at most very slowly. |
| Slow | 7 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 6 | High or artesian groundwater conditions pose special problems both to the excavation and to the stability of the slope after backfilling with lightweight fill, limiting the applicability of this technique when these conditions occur. |
| High | 7 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | Surface flows must be diverted to prevent them from accumulating in the lightweight fill and/or infiltrating the portion of the landslide mass left in place. Drainage to be provided both on surface and at interface between fill and natural soil. |
| Snowmelt | 6 | |
| Localized | 4 | |
| Stream | 2 | |
| Torrent | 1 | |
| River | 1 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 6 | The reliability of the technique depends on the evaluation of the stability of the treated slope and on the homogeneity and durability of the fill used. |
| Feasibility and Manageability | 6 | Concept is well developed but knowledge of mechanical properties and applicability of different lightweight fills still not fully established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Can be implemented with widely available equipment. Possible difficulties with excavation in rock and with the disposal of arisings. Construction control. |
| Environmental suitability | 4 | will be updated |
| Economic suitability (cost) | 6 | Moderate to high, depending on the material used. |
References
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Di Prisco C. (2007). “Applicazioni geotecniche e caratterizzazione meccanica dell’argilla espansa Leca”. Leca Soluzioni leggere e isolanti, Laterlite S.p.A. (in Italian).
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Dubreucq T., Pezas N. (2009). “Technique du pneu-sol et glissement de terrain dans les Pyrénées Atlantiques”. Centre d’Études Techniques (CETE) du Sud-Ouest, http://www.cete-sud-ouest.equipement.gouv.fr/article.php3?id_article=433
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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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Holtz R.D. (1989). “NCHRP synthesis of highway practice 147: Tratment of Problem Foundations for Highway Embankments”. TRB, National Research Council, Washington, D.C., 14-16.
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Nakano J., Miki H., Kohashi H., Fujii A. (1999). “Reduction of land cutting effects by the application of lightweight embankments”. In: Slope stability engineering. N. Yagi, T. Yamagami, J. Jiang (eds.), Balkema, Rotterdam, Netherlands.
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Negussey D. (2007). “Design parameters for EPS Geofoam”. Soil and Foundations, Vol. 47 (1), 161-170.
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Nelson D.S., Allen W.L. (1974). “Sawdust and lightweight fill material”. Report FHWA-RD-74-502, FHWA, U.S. Department of Transportation, 24 pp. (reprinted in Highway Focus, Vol. 6 (3), 53-66).
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Nguyen-Thanh-Long. (1996). “Lightweight Pneusol’, the stability of slopes and recovering the residual value of industrial byproducts”. In: Landslides – Glissements de terrain, Kaare Senneset (eds.), Balkema, Rotterdam, 1733-1737.
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Payette R., Busby H.R., Wolfe W.E. (1996). “Landslide remediation through the use of coal combustion by-products”. In: Landslides – Glissements de terrain, Kaare Senneset (eds.), Balkema, Rotterdam
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Read J., Dodson T., Thomas J. (1991). “Experimental project use of shredded tires for lightweight fill: Post construction report”. FHWA Experimental Project n° 1, Oregon Department of Transportation, Salem, 31 pp.
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Reuter G., Rutz J. (2000). “A lightweight soultion for landslide stabilization”. GFR Magazine, Vol. 18, n.7.
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Sharma S., Buu T. (1992) “The Bud Peck Slide, I-15, Near Malad, Idaho”. In Transportation Research Record 1343, TRB, National Research Council, Washington, D.C., 123-129.
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Stark T.D., Arellano D., Horvath J.S., Leshchinski D. (2004). “Guideline and Recommended Standard for Geofoam Applications in Highway Embankments” NCHRP Report 529, Transportation Research Board, Washington, D.C.
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Wolfe W.E., Busby H.R., Kim S.H. (1996). “Highway embankments repair and stabilization using power plant waste products”. In: Landslides – Glissements de terrain, Kaare Senneset (eds.), Balkema, Rotterdam.
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Yeh S., Gilmore J. (1992). “Application of EPS for slide correction”. In: Proc. of Specialty Conference on Stability and Performance of Slopes and Embankments, R.B. Seed, R.W. Boulanger (eds.), Berkeley, California, Geotechnical Special Publication 31, ASCE, 1444-1456.