Reinforced concrete stem walls

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Description

Reinforced concrete stem walls, called also cantilever walls or gravity cantilever walls, are L-shaped or inverted T-shaped walls which rest on the ground and act, from a geotechnical stability point of view, in conjunction with the mass of the retained fill above the foundation element. Tall or heavy duty walls may incorporate additional buttresses or slabs.

These walls may be cast in-situ or prefabricated; cast in-situ walls requires a range of site craft skills, but can be more effective in difficult situations, where the foundation soil is poor or where interaction with other structures is required; prefabricated walls, which are manufactured in a wide range of heights, can usually be installed without a specialist labour force and the construction process is relatively fast, but they require good access and the use of a crane for offloading and installation.

Different finishes can be applied, especially to precast walls, using stone or matrix formwork.

Reinforced concrete elements should have vertical joints. BS 8002 Clause 4.3.1.4.6 recommends that where necessary (generally where water penetration from the retained fill through the wall joints would be unsightly or could damage a facing structure), “joints should be lined with a resilient jointing materials about 10 to 20 mm thick and sealed with a proprietary sealing compound. Dependent upon the groundwater present, waterbars may be also required”.

A drainage layer is normally installed on the back of the wall to limit pressures on the stem. Additionally, where appropriate/necessary to keep the backfill materials free from groundwater pressures, surface and/or deep drainage systems will be foreseen.

Standard precautions for ensuring the durability of reinforced concrete should be followed (see for example BS 8110, EC2, BS 5400- Part 4).

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Design methods

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.

From a geotechnical point of view, these walls should be designed as a gravity mass walls considering the backfill on the foundation slab as an integral part of the wall (see for example BS 8002; Geotechnical Engineering Office, 1993 and Chapman et al., 2000); the vertical plane on which the earth pressure are evaluated is that through the back of the heel, not the stem. Even where “active” earth pressure conditions are applicable on the upslope face of the wall for geotechnical design, structural design should be based on “at rest” conditions, since normally the stem and the connection between the stem and the base will be too stiff to allow sufficient relative movement between wall and backfill to generate “active” conditions. Higher earth pressures may occur as a result of compaction, especially on the upper portion of the stem (Ingold, 1979; Duncan et al., 1991).

Wall design shall pay special attention to aspects related to water pressure and drainage. Rationale for drainage systems and related details 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:

• Bearing resistance failure at the base of the wall;

• Sliding failure at the base of the wall;

• Failure by toppling of the wall;

• Loss of overall stability around the wall;

• Overall stability of the slope, including the wall;

• Unacceptable leakage beneath the wall;

• Unacceptable transport of soil grains beneath the wall;

• Internal stability. The forces to be used for the design of the stem and heel will be evaluated according to guidance provided by for example BS 8002, EC2, BS 8110 (see also Geotechnical Engineering Office, 1993 and Chapman et al., 2000).

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Functional suitability criteria

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Reliability and feasibility criteria

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Urgency and consequence suitability

References

• BS 5400 – Part 4 (1990) ”Steel, concrete and composite bridges, code of practice for design of concrete bridges”.

• BS 8002 (1994) ”Code of Practice for Earth Retaining Structures”.

• BS 8110 – Part 1 (1997) ”Structural use of concrete, code of practice for design and construction”.

• BS 8110 – Part 2 (1965) ”Structural use of concrete, code of practice for special circumstances”.

• Chapman T., Taylor H., Nicholson D. (2000). “Modular Gravity Retaining Walls – Design Guidance”. Publication C516, CIRIA, London.

• Duncan J.M., Williams G.W., Sehn A. L., Seed R.B. (1991). ”Estimation Earth Pressures Due to Compaction”. Journal of Geotechnical Engineering, Vol. 117, No. 12, 1833-1847; discussion and closure, Vol. 119, No. 7, July 1993, 1162-1177

• EC2 ”Design of concrete structures”.

• Geotechnical Engineering Office (1993) ”Geoguide 1 – Guide to Retaining Wall Design” Civil Engineering Department, The Government of the Hong Kong, Special Administrative Region. 

• Ingold, T.S., (1979) The effects of compaction on retaining walls, Gèotechnique, 29, p265-283

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