Description
Drystack masonry walls consist of precast concrete special blocks and occasionally bricks designed to interlock with each other and to produce a solid wall face; interlock provides adequate shear resistance between each layer of blocks and assists accurate placing of successive layers of blocks.
Drystack masonry walls must be constructed on a mass or reinforced concrete foundation. In order to provide additional stability it is possible to build the walls thicker at the base. It is also prudent to specify the top layers of blocks to have some form of mortar pointing, adhesive or capping to avoid them being dislodged by vandals.
The individual blocks are usually designed to be placed by manual handling. The walls can be constructed vertically or with a batter to provide better stability for greater heights. In order to facilitate construction the backfill is placed and compacted keeping it to the same level reached by the wall.
Most of the drystack masonry walls are relatively free-draining; however, where appropriate or necessary, surface and/or deep drainage systems will be provided to keep the backfill materials free from groundwater pressures.
Drystack masonry walls may be used in association with top, bottom or wall –face planting; wall-face planting may be in spaces provided by open joints filled with a suitable growing medium. 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.
Design methods
Drystack masonry walls should be designed as gravity mass walls.
The wall specification should stipulate the materials to be considered for filling 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.
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 beneath the wall;
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Unacceptable transport of soil grains beneath the wall;
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Internal stability. The resultant force at any horizontal sections shall be within the middle third of the section. Checks should be made of horizontal sections above the base of the wall that there is adequate resistance to sliding.
Functional suitability criteria
Type of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Fall | 0 | Only suited to rotational or pseudo-rotational slides which are fully stabilized with no further movement |
| Topple | 0 | |
| Slide | 4 | |
| Spread | 0 | |
| Flow | 0 | |
Material type |
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| Descriptor | Rating | Notes |
|---|---|---|
| Earth | 8 | Mainly applicable to landslides involving earth and debris. Applicability in rock limited by typical slope geometry and failure mode |
| Debris | 5 | |
| Rock | 3 | |
Depth of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Surficial (< 0.5 m) | 9 | Typically applicable to shallow landslides, fully stabilized. |
| Shallow (0.5 to 3 m) | 7 | |
| Medium (3 to 8 m) | 2 | |
| Deep (8 to 15 m) | 0 | |
| Very deep (> 15 m) | 0 | |
Rate of movement |
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| Descriptor | Rating | Notes |
|---|---|---|
| Moderate to fast | 0 | Should be carried out preferably on very or extremely slow landslides which become fully stabilized. |
| Slow | 0 | |
| Very slow | 5 | |
| Extremely slow | 8 | |
Ground water conditions |
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| Descriptor | Rating | Notes |
|---|---|---|
| Artesian | 8 | Applicable in all groundwater conditions. Adequate drainage must be provided to wall and at the interface between low permeability backfills, if any, and natural soil |
| High | 8 | |
| Low | 7 | |
| Absent | 8 | |
Surface water |
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| Descriptor | Rating | Notes |
|---|---|---|
| Rain | 6 | Not applicable in contact with watercourses. |
| Snowmelt | 6 | |
| Localized | 5 | |
| Stream | 1 | |
| Torrent | 1 | |
| River | 1 | |
Reliability and feasibility criteria
| Criteria | Rating | Notes |
|---|---|---|
| Reliability | 4 | Reliability penalized by susceptibility to loss of integrity on further movement. |
| Feasibility and Manageability | 6 | Relatively simple technique, but applicability in landslide remediation must be proven. |
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) | 8 | Low to moderate, provided the work does not involve diversion of major water courses or interference with existing infrastructure. |
References
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Chapman T., Taylor H., Nicholson D. (2000). “Modular Gravity Retaining Walls – Design Guidance”. Publication C516, CIRIA, London.
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Coppin N.J., Richards I.G. (1990) ”Use of vegetation in civil engineering” CIRIA Book 10, CIRIA/Butterworths, London .
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Geotechnical Engineering Office (1993) ”Geoguide 1 – Guide to Retaining Wall Design” Civil Engineering Department, The Government of the Hong Kong, Special Administrative Region.