Addition of material to the area maintaining stability

Category: MODIFYING THE SLOPE GEOMETRY - mass distribution

Description

The addition of material to the toe or resisting area (or more in general, buttressing, counterweight fills and toe berms) operates by increasing the resisting forces, thereby improving overall slope stability, by providing sufficient dead weight or restraint near the toe of the unstable slope (Figure 1).

Figure 1: Coarse grained or rock buttress for slope stabilization (source: Gedney and Weber, 1978)
Figure 1: Coarse grained or rock buttress for slope stabilization (source: Gedney and Weber, 1978)

 

This method 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. It is generally ineffective on translational slides on long, uniform planar slopes, or on flow-type landslides.

Generally it is most practical on small slumps or small rotational failures, but several examples exist where this technique has been applied successfully on large landslides where conditions allowed large scale earthmoving to be carried out. In these cases, this mitigation measure is typically supplemented by drainage and/or other mitigation measures.

It should always be kept in mind that when fill is placed on the landslide body itself, the driving forces are also increased. It is therefore necessary to locate the fill in such manner that the increase in resisting forces exceeds the increase in driving forces. This is typically achieved by placing the fill at or very near the toe of the landslide. 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 fill.

lt is worth noting that the increase in resisting forces associated with the fill will tend to increase in time as a result of the increase in normal effective stress on the failure surface as consolidation takes place. Thus, the most critical conditions typically occur during construction, when the Contractor is still on site and it is easier to respond to unexpected performance.

Butress fills are normally constructed of blasted quarry rock, boulders and cobbles and coarse gravel fill, which are relatively free draining. If fine grained material is used, it is essential to include a drainage layer at the interface between the buttress and the underlying natural soil (Figure 2)

Figure 2: Drainage layer at interface between natural soil and low permeability fill (source: Millet et al., 1992)
Figure 2: Drainage layer at interface between natural soil and low permeability fill (source: Millet et al., 1992)

This technique can be incorporated economically in highway or railway projects if it is possible to design the alignment to match the stabilization requirements, as was done for example with the Taren Landslide (Kelly and Martin, 1985)

The main limitations of the technique relate to the following issues:

  • Filling may actually destabilize the ground farther down-slope;

  • Satisfactory solutions may involve significant modification of the landscape and possible interference with water courses at the toe of the landslide;

  • Filling may require large volumes of material, to be procured off-site; availability of suitable fill may limit application of this technique;

  • Filling 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;

  • Filling on or at the toe of 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.

Examples of large landslides stabilized by this technique (alone or in combination with other mitigation measures are provided by Gedney and Weber (1978);  Edil (1992); Kropp and Thomas (1992).  Figures 3 shows a similar example described by Millet et al. (1992).

Figure 3: Stabilization of Tablachaca Dam Landslide, Peru, crossection (source: Millet et al., 1992)
Figure 3: Stabilization of Tablachaca Dam Landslide, Peru, crossection (source: Millet et al., 1992)

 



Design methods

For general considerations on the geotechnical design of mitigation by addition of material to the resisting area, reference shall be made to the general fact sheet 2.0 on hazard mitigation by changes in slope geometry and/or mass distribution.

The basic design of buttress fills is similar to the design for external stability of conventional gravity retaining structures, including check of the following limiting situations, evaluated taking into account the loading induced by the landslide body.

  • Overturning

  • Sliding at or below the base

  • Bearing capacity of the foundations, including evaluation of the stability of the slope downhill of the buttress

It is also necessary to evaluate the possibility that the landslide body overrides the buttress, especially on slides with a significant translational component.

Possible internal failure modes should also be checked to ensure that the buttress does not fail by shear.



Functional suitability criteria

Type of movement

Descriptor Rating Notes
Fall 0 Most suited to rotational or pseudo-rotational slides; may be useful to reduce toppling hazard in certain conditions.
Topple 2
Slide 8
Spread 2
Flow 1

Material type

Descriptor Rating Notes
Earth 9 Mainly applicable to landsliding involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode.
Debris 7
Rock 4

Depth of movement

Descriptor Rating Notes
Surficial (< 0.5 m) 4 Typically applicable to relatively small and/or shallow landslides. The implications of large scale filling and procurement typically make this technique impractical for deep and very deep slides. On the other hand, it may be the only suitable technique in very large landslides, besides drainage.
Shallow (0.5 to 3 m) 7
Medium (3 to 8 m) 8
Deep (8 to 15 m) 7
Very deep (> 15 m) 6

Rate of movement

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.
Slow 8
Very slow 9
Extremely slow 9

Ground water conditions

Descriptor Rating Notes
Artesian 7 Applicable in all groundwater conditions. Adequate drainage must be provided at the interface between low permeability fills and natural soil.
High 8
Low 8
Absent 8

Surface water

Descriptor Rating Notes
Rain 5 Possible limitations in applying this technique where the landslide is caused by or impinges on a water course, although examples exist where rivers have been diverted to implement this type of solution. Adequate protection must be provided in this case against toe erosion by wave or current.
Snowmelt 5
Localized 4
Stream 2
Torrent 0
River 0

Reliability and feasibility criteria

Criteria Rating Notes
Reliability 10 The reliability of the technique depends on the reliability of the evaluation of the stability of the treated slope. More reliable than excavation.
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 the procurement and/or control of compaction of fill.
Environmental suitability 4 will be updated
Economic suitability (cost) 8 Moderate, provided the work does not involve diversion of major water courses or interference with existing infrastructure.

References

  • Edil T.B. (1992). “Landslide cases in the Great Lakes: Issues and Approaches”. In: Transportation Research Record 1343, TRB, National Research Council, Washington, D.C., 87-94.

  • Gedney D.S., Weber W.G. ( 1978). “Design and construction of soil slopes”. In: Landslides: analysis and control, Special Report 176, Chapter 8, R.L. Shuster and R.J. Krizek (eds.), TRB, National research Council, Washington D.C., 172-191.

  • Millet R.A., Lawton G.M., Repetto P.C., Varga V.K. (1992). “Stabilization of Tablachaca Dam”. In: Proc. of a Specialty Conference on Stability and Performance of Slopes and Embankments, Berkeley, California, R.B. Seed and Boulanger R.W. (eds.), Geotechnical Special Publication 31, ASCE, 1365-1381.

  • Kelly J.M.H., Martin P.L. (1985). “Construction works on or near landslides”. In: Landslides in the South Wales Coal Field, Polythecnic of Wales, C.S. Morgan (eds.), 85-106.

  • Kropp A., Thomas M. (1992). “Partial landslide repair by buttress fill”. In:Transportation Research Record 1343, TRB, National Research Council, Washington, D.C., 108-113.

  • Holtz R.D., Shuster R.L., (1996). “Stabilization of soil slopes”. In: Landslides: Investigation and Mitigation, Special Report 247, Chapter 17, A.K. Turner, R.L. Shuster (eds.), Transportation Research Board, Washington.

  • Turner A.K., Schuster R.L. (eds.) (1996). “Landslides: Investigation and Mitigation”. Special Report 247, Transportation Research Board, Washington.

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