Holding back the earth
The third of an occasional series1 on geotechnics looks at the design, specification, procurement and construction of gravity retaining walls
Of the common types of retaining wall (see box), it is the gravity retaining walls that rely on the mass of the wall itself to resist the forces applied by the retained ground. Examples include reinforced-concrete stem walls, masonry walls, crib walls, gabion walls and reinforced soil, all of which are are often less than 5m high.
Further guidance on the selection, design and construction of gravity walls is available in two ciria reports2,3.
Requirements for a wall depend on the height of soil which needs to be retained, how close to the wall any imposed load on the retained soil (surcharge) would be, and what measures might be required to fit the wall into the overall development, such as aesthetics or the need to preserve any existing trees. Specifiers need to identify any constraints, such as being unable to excavate far behind the proposed wall line due to the site boundary or an existing structure, or the presence of any particularly corrosive soil.
It helps wall selection if some forms of desk study and ground investigation have been carried out by this stage. As a retaining wall will usually be a small part of a larger development, much of the information gathered for that development will be useful for the wall's scheme design.
Using available information, such as the ciria reports2,3 and manufacturer's brochures, a preferred wall type can be identified early in design, though as design progresses different wall types might be found to be more economical for overcoming problems or for particular aesthetic requirements. The more flexible the specification is in permitting alternatives, the more economical will be the eventual solution. Considerations when choosing a wall type include:
the stability of any cut slope behind the wall while it is being built
the stability of any structures behind the wall affected by its construction
filling - the type of material, whether it is imported or reused site- won fill (to minimise the volume of fill for disposal off-site)
damage to the wall during compaction of the fill.
The specifier should make sure that:
responsibility is clearly allocated for each part of design
the desk study and ground investigation provide suitable information to the wall designer
the wall design is satisfactory in terms of external stability (the complete retaining-wall system can resist the sliding and overturning forces from the retained ground and has adequate bearing capacity) and internal strength (the wall and its component modules are strong enough not to break)
the design details are fully communicated to the contractor. There should be a requirement in the construction contract that significant variations in either construction or the ground conditions encountered during construction are fed back to the designers to check that the design takes account of them.
In choosing one of the procurement strategies given below, specifiers should consider how best to allocate responsibilities for design of the wall. Depending on which design approach is to be followed, tendering may take place before or after detailed design. When design responsibilities are split, one party should still take responsibility for the overall design and check that all elements of the design have been satisfactorily completed and are mutually consistent. There are four main procurement options, given in order of decreasing specifier input.
Specifier designs everything. The specifier is the designer of the retaining- wall system and the wall modules. A supplier produces these special modules to order. This approach is unlikely to be economical except for the largest contracts where the one-off production costs are offset by the quantity required.
Specifier selects a standard product but checks its external stability and internal strength. The specifier, although carrying out calculations of the external stability (under imposed loads) and internal stability (structural capacity of the wall), nevertheless relies on whatever structural capacity has been assigned to the system by the supplier.
Specifier provides supplier with an interpretative geotechnical report and requires the supplier to confirm strength and stability of the product in the particular conditions. In confirming the product is adequate for its situation, the supplier should inform the specifier of any deficiencies in the geotechnical interpretative report or of any reservations about the extent or quality of investigatory work relevant to the design.
Specifier requires supplier to confirm stability of the product in the particular conditions. The specifier shows the site to the supplier and describes the design requirements. The supplier provides a product appropriate to those conditions, having first carried out an appropriate desk study, ground investigations and analyses. Generally this option gives the specifier least control over the final outcome.
Water pressure and drainage
A rise in water-table level from the toe of a retaining wall to its top can more than double the horizontal pressure on the retaining wall. This can happen relatively suddenly and is why water is one of the main causes of retaining-wall failures. It is therefore important that the water-table level, direction of water flow and seasonal variations of water in the ground are well understood before construction starts. Also, rain storms or burst water mains can create particularly severe conditions.
Wall drainage is recommended if the wall backfill is relatively impermeable or if infiltrating water would have nowhere to go. Where no drainage is provided, water pressures should be calculated assuming the highest water- table level that is physically possible, eg before overtopping of a wall element occurs, or before another direction of water flow prevents the water-table level from overtopping the wall. Free-draining wall types such as gabions may be appropriate where water flows are anticipated to be high and maintenance infrequent.
Construction, inspection and maintenance
The table sets out the features of different gravity-wall constructions. A continuing programme of maintenance is recommended for all structures. With gravity walls there is usually some warning before a failure occurs, so preventative maintenance can avoid the need for later repairs. Typical danger areas that may be picked up during wall inspections are indicated in the diagram above.
Tim Chapman is an associate with Arup Geotechnics. Andrew Pitchford is with ciria