Description
Caissons used to provide a mechanical stabilization of landslides typically range in diameter between 6 and 15 m (Brandl, 1988; Leoni and Manassero, 2003). They can be placed in earth and debris slopes, typically along specific alignements across the direction of movement at strategic positions within the landslide, at a maximum centre to centre spacing of twice the diameter.
Construction of caissons involves three main stages:
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Construction of the annular structure which is necessary to ensure that subsequent activities can be carried out safely;
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Excavation to the design depth, as necessary to ensure that each caisson is adequately keyed into the underlaying competent and stable strata;
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Backfilling with reinforced concrete (mass concrete may be used in relatively short, large caissons where shear behaviour predominates).
Depending on anticipated ground and groundwater conditions, the most common techniques used to form the annular structure constructed in the first stage are (De Paoli, 1989; Tambara, 1999):
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Progressive construction during excavation by alternate excavation and casting of consecutive concrete rings, although this may be problematic in unstable slopes.
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Advance formation of an annular structure by means of micropiles, jet grouted columns, piles or diaphragm walls, which is later supplemented by annular steel or concrete ribs as excavation proceeds.
Where ground conditions vary significantly along the depth to be excavated, different techniques can be used for different portions of the structure: for example by performing the annular structure to rockhead only and extending the excavation into rock with local support only.
Special care needs to be paid when excavating below the groundwater level, especially if more permeable ground is overlain by less permeable ground and/or where running conditions may occur. Temporary dewatering is necessary in these conditions and in extreme cases they may make this technique inapplicable.
The main advantages of this technique may be summarized as follows:
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Very stiff and robust structure;
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Applicable in deep landslides (up to 20÷25 m deep) where other techniques may prove inadequate;
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Main structural components are constructed under controlled, clean conditions, allowing inspection of reinforcement and controlled placement and compaction of concrete;
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May be adapted to suit a variety of ground conditions below the sliding mass, including rock;
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Allow installation of anchors and/or suborizontal drains from within the caissons, several metres below ground level;
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Allow direct inspection of sliding mass and underlying competent strata during construction.
On the contrary, it must be borne in mind that construction may take several months and it requires access roads and a level working platform for safe operation, which on relatively steep ground may require significant preliminary works.
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Figure 3: Excavation with temporary retaining structure consisting of bored piles and concrete annular beams
(source: SGI-MI project files)
Design methods
The design load on the caissons may be determined in 2D limit equilibrium analyses by calculating the reaction on the vertical section corresponding to the caisson alignement which is necessary to guarantee, with the appropriate factor of safety, the stability of the portion of the slide located upslope of the wall in the absence of the downslope portion; in any case, the load on the wall cannot exceed passive soil pressure.
The contribution of the downslope portion can be considered only if this portion remains stable with an appropriate factor of safety once the driving force from the upper portion is removed; even in this case, it may be prudent to consider this mass only as confinement for the stable soil below, since even very small deformation such as shrinkage in a dry season may be sufficient to reduce or completely remove downslope support to the caissons.
The design loads and the stability of the downslope portion in seismic conditions are normally determined from pseudostatic limit equilibrium analyses, taking into account the excess pore pressures that may develop in the slope, where applicable.
Once the net actions imposed by the landslide on the caissons are known, a suitable soil-structure interaction analysis is carried out by an appropriate method to determine both the reactions in the stable soil into which the caissons are anchored and the effects of actions on the caissons.
The spacing between the caissons must be determined balancing:
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economy and the need to avoid interference between adjacent caissons and/or with natural drainage;
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the need to ensure that soil arching develops between adjacent caissons and that the soil does not “flow” between them.
The check that soil arching develops between adjacent caissons and that the soil does not “flow” between them can be done by means of analytical (simplified) tools (see for example Ito and Matsui, 1975) or 3D numerical analysis.
Provided soil arching is guaranteed, plain strain 2D soil-structure interaction analysis is representative of actual conditions, with the effects of actions on each caisson being those derived from the 2D analyses, multiplied by their centre to centre spacing. The same analysis may be used to determine the optimal length of the caissons and the benefit of additional anchors, if used.
The calculation of the caisson capacity in relation to the soil/structure interaction may be carried out according to several approaches and simplified methods based on the simplified assumption that the caisson is infinitely rigid and is subject only to rotation (Pasqualini, 1975; Rocchi et al., 1992). A commonly used approach is that based on coupling the equation of global equilibrium with the deformations of the structure as determined using non linear spring; alternatively, soil- structure interaction analysis of horizontally loaded caisson may be carried out by 3D finite element analysis.
Finite element methods may be used instead to provide a simultaneous and consistent estimate of the soil-structure interaction both with the sliding mass and with the underlying stable soil. Finite element analyses in the time domain can also be used to refine the evaluation of the performance of the structure under seismic conditions.
The mechanical charateristics of the caissons must be adequate to sustain the actions and the effects of actions on them. The structural checks must satisfy all applicable codes and standards on the subject.
It is important that the designer considers the adequacy of the annular structureand of the stability of the temporary excavations, including consideration of base stability (reverse bearing capacity, piping, blow out). The methods of analysis must reflect the details of construction. It is prudent not to rely solely on the annular resistance of structures formed by adjacent vertical elements and the reduced annular stiffness of this type of construction compared to the axial stiffness of monolithic elements. Nonetheless, the structure needs to be designed to resist at-rest soil pressures.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Best suited to slides and the slide-like portion of complex landslides. May be applicable in some cases to prevent the triggering of slides with the potential to turn to spreads or flows, but are substantially ineffective once fuidification has occurred. |
| Topple | 0 | |
| Slide | 8 | |
| Spread | 4 | |
| Flow | 4 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Difficoult, very expensive and typically inappropriate in rock, but can be extended into rock if required. Method of construction to be selected taking into account ground and groundwater conditions. |
| Debris | 8 | |
| Rock | 0 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | Typically: · best suited where the movement is deep (> 8 m, up to 20 – 25 m), · inappropriate in shallower movements because excessive. |
| Shallow (0.5 to 3 m) | 0 | |
| Medium (3 to 8 m) | 4 | |
| Deep (8 to 15 m) | 6 | |
| Very deep (> 15 m) | 8 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Workers’ safety and end result require construction to take place when movement is extremely slow or very slow (maximum 1.5 m/year, corresponding to approximately 5 mm/day). |
| Slow | 2 | |
| Very slow | 6 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 2 | High groundwater levels associated with coarse grained materials and/or artesian groundwater conditions require special dewatering during construction, possibly making this technique not feasible in extreme cases. |
| High | 6 | |
| Low | 8 | |
| Absent | 8 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 8 | Water courses need to be temporarily diverted or reliably dry during construction. Potential pollution of watercourses by construction operations, especially for the first stage annular structure (for example by drilling fluid and/or by grout) may impose restriction on construction procedure. No problems once the works are completed, except possibly when caissons interfere with the banks of watercourses, modifying the erosion regime. |
| Snowmelt | 8 | |
| Localized | 6 | |
| Stream | 2 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | Reliable performance in well characterized landslides; in first time slides it depends on estimate of piezometric regime and apprporiate operational strength parameters of soil, which can be problematic. |
| Feasibility and Manageability | 8 | Technique and design process are well established and widely used in suitable conditions. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 6 | Requires specialist equipment and techniques; implementation may need temporary roads and working platform for safe operation. |
| Environmental suitability | 2 | will be updated |
| Economic suitability (cost) | 2 | Very expensive. |
References
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Brandl H. (1888). ”Stabilization of deep cuts in unstable slopes”. Proc. of 5th International Symposium on Landslides, Lausanne (Swiss), Balkema, 867-872.
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De Paoli B. (1989). ”La costruzione di fondazioni a pozzo alla luce dello sviluppo delle tecniche operative”. Proc. 17 Convegno Nazionale di Geotecnica, Taormina (Italia), AGI, 51-58, (in Italian).
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Leoni F., Manassero V. (2003). ”Consolidamento e rinforzo dei pendii in terra”. Proceedings of the 19th Conference of Geotechnics of Turin (CGT), Turin (Italy), (in Italian).
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Pasqualini E. (1975). ”Criteri di dimensionamento delle fondazioni a pozzo”. Proceedings of the 6th Conference of Geotechnics of Turin (CGT), Turin (Italy), (in Italian).
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Rocchi G., Collotta T., Masia A., Trentin M., Fittavolini C. (1992). ”Design aspects on piers foundations in landslip-prone seismic areas”. Proc. of 6th International Symposium on Landslides, Christchurc (New Zealand), Balkema, 1957-1969.
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Tambara F. (1999). ”Stabilizzazione dei pendii”. Hevelius editore, (in Italian).