The underlying components
The basic components of SRWs typically comprise the foundation soil, base or footing materials, segmental wall units, retained soil, drainage fill and, in some cases, the geogrid reinforcements. Contractors should, however, consult the manufacturer’s instructions regardless of the system being used. In most cases, these instructions will identify the components and provide basic descriptions, installation instructions, and other recommendations that will ensure proper installation and information regarding recommended maintenance or, in some cases, commissioning.
The following list is representative, however, the components may vary depending on the retaining wall system being employed, the site conditions, and other factors:
- Foundation soil: Soil supporting the gravel base, SRW units of various shapes and sizes.
- Base/footing: The footing typically distributes the weight of the system over the foundation soil and provides a working surface for construction. In most cases, the base consists of granular material, such as processed gravel, although non-reinforced concrete or flowable fill can be used.
- Segmental wall units: The concrete masonry units used to create the mass necessary for structural stability and to provide stability, durability, and visual presentation of the wall.
- Drainage stone: Crushed stone is typically placed behind the wall to facilitate the removal of groundwater and minimize buildup of hydrostatic pressure on the wall. It is also used to fill the cores of open-cell units to increase the weight and shear capacity. Geotextile fabric may be installed between the drainage stone and the infill soil to prevent the unit from becoming clogged. Backfill material is normally placed and compacted in the area immediately behind the drainage zone.
- Reinforced soil: The compacted backfill material containing the horizontal layers of geogrid placed behind the drainage zone.
- Retained soil: The undisturbed soil for ‘cut’ walls or the backfill soil compacted behind the infill or reinforced soils zone.
The designer of SRWs must consider the site-specific details of the area in which the wall is placed. Parameters include wall geometry, batter, site, soil and water conditions, surcharge loading, and design lifetime. Other selection criteria can include the availability of suitable backfill materials, project economics, and desired esthetics of the completed project. In most cases, the designer or contractor will also rely on relevant reports of soil conditions and recommendations prepared by an independent third-party non-technical consultant.
There are three main stability modes for SRWs. They include: internal, external, and overall stability.
- Internal stability considers the mechanics working within the structure to resist all forces such as pullout, tensile strength, and localized stress between grid layers.
- External stability considers local modes of failure such as overturning, base sliding, bearing capacity, and global stability, or more simply, the environment directly affecting the structure. This evaluation would also consider drainage factors such as surface run-off, embankment flow, and groundwater flow.
- Overall stability considers the effects of the environment surrounding the wall, such as slope stability and settlement.
Factors affecting the external stability include the wall’s height, the pressure and forces on the retained soil, and the weight of the wall. Walls can be designed by any conventional method and are normally calculated using limit state design (LSD), also known as load and resistance factor design (LRFD). Limit state is a condition of a structure beyond which it no longer fulfils the relevant design criteria.
Root barriers
Retaining wall installations are critically dependent on the integrity and stability of base materials below the surface. Any disruption or movement of the base will cause deficiencies to quickly appear. These defects generally manifest as heaving or displacement of retaining wall units, which represent a hazard and can be expensive to repair. (Root barriers are normally required for municipal or commercial applications. If the contractor is concerned about proximity of trees, he/she should consult the manufacturer’s instructions.)
In urban areas the growth and development of tree root systems near retaining wall installations can disrupt the base materials and other components in the manner described. In fact, a recent study cited defects directly attributed to tree root growth as the sixth most common cause of premature pavement failure of retaining wall installations. For this reason, root barriers, which prevent the growth of opportunistic tree roots under infrastructure components, are increasingly recognized as essential components of hardscaping installations. Root barriers have proven, over time, to be effective in eliminating deficiencies in new construction and have also been successfully used to retrofit and repair existing installations. Proper detailing and installation of root barriers is required at locations where tree roots intersect with hardscaping components to ensure successful long-term performance.
Earth retention
SRWs have features which make them practical for earth retention projects. The destabilizing pressure of the retained soil mass is nullified by the combined weight of the individual solid concrete blocks when a gravity SRW system is in use. (The SRW approach is generally referred to in the industry as limit equilibrium analysis.) Designers and engineers usually allow for any overlooked or unexpected pressure that may be exerted upon the wall; therefore, the wall must be able to perform above and beyond the requirements of the stability analysis.
Life cycle cost analysis
Life cycle cost analysis involves a comparative economic assessment of design, material, and construction alternatives to determine the best value for funds invested. This type of analysis reviews the initial installation cost as well as considers other factors such as maintenance, rehabilitation, inflation, interest, and user costs.
Brian Burton is involved with Award Bid Management Services, an innovative multidisciplinary firm which specializes in technical business writing. The firm assists companies interested in selling goods and services to governments and institutions. He can be reached via e-mail at burton@award-bid-management.com.
