Description
Crib walls comprise a grillage of header and stretcher elements placed on a firm foundation, usually of mass or reinforced concrete; the spaces between the grillage of header and stretcher elements are filled with free draining coarse grained materials (sand and gravel), which must be durable, e.g. resistant to erosion and frost; crib walls can be built with either the front face or the rear face stepped; it is desirable to incline the wall with an angle corresponding to 1 H: 4 V (Figure 1).
The header and stretcher elements can be made of reinforced concrete or timber and are designed to be interlocking; durability is provided by appropriate covering of the reinforcement in the concrete elements, or by treatment to timber elements. These elements are usually designed for manual handling; some more complex cellular systems exist where the header and stretcher elements are integrated such that they require a crane for lifting.
Wall can be made with one or multiple row of cribs at the base, depending on the height of the retaining structure to be formed. In order to facilitate construction, the backfill is placed and compacted keeping it to the same level reached by the wall. In placing the backfill within and behind the wall attention should be paid to avoid causing damage to the header and stretcher elements.
Crib walls are permeable and will allow retained fill to drain freely; where appropriate or necessary, surface and/or deep drainage systems will be provided to keep the backfill materials free from groundwater pressures.
Planting is possible, typically with livepole cuttings long enough to reach the backfill; the front face provides suitable anchorage for climbing or cascading vegetation. Advice on planting vegetation can be found in Coppin and Richards (1990). Care should be taken both in the choice of plants suitable for locations within, above or below the wall and for the suitability of the growing medium (usually loose topsoil or growbags) which may require special water retention measures.
Crib walls are susceptible to vandalism and accidental damage due to the small section size of the header and stretcher elements; timber and concrete elements are susceptible to fire, although it is unlikely that fire will be intense enough to cause more than superficial damage. Once the headers and stretchers have been erected, it is possible to fill a crib wall with lean mix concrete, making it more akin to a masonry wall; in this case, the free-draining nature of the wall is lost and a drainage system may have to be incorporated to prevent the build-up of groundwater pressures behind the wall.
Design methods
The wall specification should stipulate the materials to be considered for filling within and behind the wall.
The properties of the backfill will depend on whether or not locally-won backfill is to be used, and if the material is required to be free-draining. Optimum backfill is: easy to compact, giving high strength and stiffness; and free-draining, to minimize the build-up of groundwater pressure. Backfill should not include: natural or contaminated soil which will be chemically aggressive; frozen materials; degradable materials such as topsoil, peat, wood, vegetation, etc.; materials which could be toxic, dangerous or prone to spontaneous combustion; soluble material or collapsible soils. The use of clays prone to swelling should be carefully considered as they can exert very high pressures on the back of retaining walls; the same applies for materials derived from argillaceous rocks such as shales and mudstones. Care should be taken to ensure that the infill material cannot escape from the crib wall. Sometimes it needs to be retained using geotextile.
Walls design shall put special consideration on aspects related to water pressure and drainage. Rationale and details for drainage systems can be found for example in Geotechnical Engineering Office (1993) and Chapman et al. (2000).
The following ultimate limit states (ULS) need to be verified:
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Bearing resistance failure at the base of the wall;
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Sliding failure at the base of the wall;
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Failure by toppling of the wall;
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Loss of overall stability around the wall;
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Overall stability of the slope, including the wall;
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Unacceptable leakage through or beneath the wall;
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Unacceptable transport of soil grains through or beneath the wall;
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Internal stability. The main aspect of internal stability that will concern designers is checking that sliding and overturning failures cannot occur at various levels within the wall. BS 8002 Clause 4.2.7.2.3 warns against the use of crib walls to retain unstable slopes; this is because the crib walls will not offer much resistance to failure planes passing through it. The detailed design of reinforced concrete elements will normally be undertaken by specialist suppliers. Information on the forces for which the crib modules should be designed is given in BS 8002 Cl. 4.2.7.4.2 and in greater detail in BD 68/97.
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 | 3 | |
| Slide | 8 | |
| Spread | 0 | |
| Flow | 1 | |
Material type |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 9 | Mainly applicable to landslides involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode |
| Debris | 7 | |
| Rock | 5 | |
Depth of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 0 | Typically applicable to shallow to intermedite depth landslides. Minimum size of elements makes this approach impractical for superficial landslides |
| Shallow (0.5 to 3 m) | 8 | |
| Medium (3 to 8 m) | 7 | |
| Deep (8 to 15 m) | 3 | |
| Very deep (> 15 m) | 1 | |
Rate of movement |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 1 | Should be carried out preferably on very or extremely slow landslides; with due care it can be carried out in slow landslides |
| Slow | 4 | |
| Very slow | 8 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | Applicable in all groundwater conditions. Stone filled crib walls are intrinsically free draining Adequate drainage must be provided at the interface between low permeability backfills, if any, and natural soil |
| High | 9 | |
| Low | 8 | |
| Absent | 7 | |
Surface water |
||
| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 7 | Not applicable in contact with watercourses. |
| Snowmelt | 7 | |
| Localized | 7 | |
| Stream | 0 | |
| Torrent | 0 | |
| River | 0 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 8 | The reliability of the technique depends on the reliability of the evaluation of the stability of the treated slope and of the foundations. |
| Feasibility and Manageability | 8 | Relatively simple technique. Potential benefits and limits of applicability are well established. |
Urgency and consequence suitability
| Criteria | Rating | Notes |
|---|---|---|
| Timeliness of implementation | 8 | Downgrade to 6 where elements need to be lifted using cranes in confined workplaces or on steep slopes |
| Environmental suitability | 6 | will be updated |
| Economic suitability (cost) | 6 | Moderate, provided local stone is used and the work does not involve diversion of major water courses or interference with existing infrastructure. |
References
BD 68/97 ”Crib retaining walls”
BS 8002 (1994) ”Code of Practice for Earth Retaining Structures”.
Chapman T., Taylor H., Nicholson D. (2000). “Modular Gravity Retaining Walls – Design Guidance”. Publication C516, CIRIA, London.
Coppin N.J., Richards I.G. (1990) ”Use of vegetation in civil engineering” CIRIA Book 10, CIRIA/Butterworths, London .
Geotechnical Engineering Office (1993) ”Geoguide 1 – Guide to Retaining Wall Design” Civil Engineering Department, The Government of the Hong Kong, Special Administrative Region.
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