A retaining wall is a structure that holds back earth, or in some cases other materials such as wet or dry industrial waste, agricultural products, or recyclables. There will therefore be a difference in elevation between the two sides of the wall, which must be built to resist the lateral pressure exerted by the material on the higher side. The method by which this pressure is resisted defines the type of retaining wall.
Types of Retaining Wall
This is the simplest type of retaining wall, and depends on the weight of the wall itself to resist lateral pressure. Gravity retaining walls tend to be built of heavy materials such as masonry, and are generally wider at the base and tapering towards the top. Often the wall is built to lean back slightly against the weight of the earth it retains. Modern versions of this wall type include the gabion wall, constructed of steel mesh baskets filled with stones; these are environmentally friendly, because they are free-draining and in time the baskets will become overgrown. Gravity retaining walls are best used when the difference in level is not too great, and when the soil held back has an inherent degree of stability.
Cantilevered retaining walls are usually constructed from reinforced concrete. The upright part of the wall is called the ‘stem’; this attaches to the horizontal plate at the bottom, the ‘base’, producing a section profile like an ‘L’ or an inverted ‘T’. Together they convert the lateral pressure on the wall into downwards pressure on the ground. The part of the base below the backfill is referred to as the ‘heel’, and the forward part as the ‘toe’. Cantilevered walls need much less mass than a gravity wall to retain the same height of soil, and are therefore usually cheaper to construct. A concrete cantilevered retaining wall can be poured on site but is more often made of precast sections, and may be buttressed on the lower side to increase its strength. A decorative facing of stone or brick is sometimes added.
A cross between a cantilevered and a straightforward gravity wall, a semi-gravity wall is constructed of concrete which includes some tension reinforcing steel, enabling the width of the wall to be reduced. Like a gravity wall, it is unsuitable for use above a certain height.
A counterfort wall is like a concrete cantilevered wall, but with the addition of narrow concrete webs at intervals along the rear surface. These triangular webs are the counterforts, and they serve to tie the wall and base together, increasing its strength and resistance to shear and bending forces. The thickness of the wall can therefore be reduced, and for this reason a counterfort wall may be more economical than a simple cantilever once a certain height of wall is needed.
Sheet pile walls are often the preferred method of soil retention when the ground is reasonably soft and the wall does not need to be high. The piles need to extend below ground to a depth of about twice the wall height. Prefabricated sections are driven into the ground, and linked laterally to each other. The piles can be of various materials, including steel, wood, and even vinyl, but precast concrete is favoured in situations where corrosion could be a problem, such as near salt water. For higher walls, extra soil stabilisation such as ground anchors may be used in conjunction with sheet piling.
Retaining wales using bored, or cast-in-place, concrete piles have advantages over precast piles in settings where noise and vibration during construction could present structural or environmental problems. The holes are first excavated using specialist equipment, and the hole sides stabilised. The method used for stabilisation will vary according to the ground conditions. Concrete is then poured into the void. The piles may be linked together to decrease permeability, but in general this method does not work as well as precast piling in conditions where there is excessive ground water.
Methods of Soil Stabilisation
A few techniques can be used to increase the stability of the soil where a difference in elevation is required. These techniques can be used to increase the effectiveness of retaining walls, or in some cases as an alternative to a wall.
Any type of retaining wall may be given additional strength and stability by the addition of ground anchors, which need to be secured sufficiently far back from the wall to be behind the failure plane. Ground anchors, which are generally inserted by boring and expansion, can be steel but are often of pressurised concrete forced into a fabric sock which then expands. The anchor is generally attached to the wall by means of steel cables. Sometimes ground anchors are used to support a temporary geosynthetic facing during the construction of a permanent retaining wall.
Soil nailing is generally used on near-vertical slopes when the soil stability does not call for a full retaining wall, but a degree of reinforcement is required nevertheless. Usually, a grouted reinforcement bar is driven into the slope, generally at a slight downward angle. Other methods involve the use of sacrificial drill bits to form the bar. The surface of the slope is often faced with a sheet of material such as sprayed concrete; flexible materials such as geo-mesh which is an alternative, with the additional environmental advantage of allowing the surface to be grassed. Soil nailing is a cost-effective way of stabilising the ground.
The effectiveness of a retaining wall may be increased by applying various stabilisation techniques to the soil behind it. Often a chemically-active substance is mixed in with the soils to increase its cohesion. The addition of cement to siliceous soil, in proportions varying between five and twenty per cent according to the composition of the soil, will substantially reduce the lateral pressure on the retaining wall. Lime and bitumen are also possible soil-strengthening additives.
Mechanically stabilised earth (also known as reinforced earth) uses horizontal layers of reinforcement to increase soil stability. The reinforcement layers can be of steel mesh, but also of a variety of geosynthetic materials such as mesh or geogrids. This technique can be used on its own or with a retaining wall or facing. The flexible nature of the reinforcement layer enables it to cope with differential soil settlement, and even with seismic activity in areas of the world where this needs to be taken into consideration.
Contact Us to Discuss Your Requirements
At Shay Murtagh, we understand the need to work with our customers to understand their requirements and devise an individual solution. Our qualified and experienced engineers are on hand to discuss the needs of your project. Contact us by telephone or email: [email protected]