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Gravity Retaining Wall Basics
It is sometimes necessary to maintain level differences in adjacent areas of a project so that both areas are able to provide the function that is required. This may be to maintain required grades for roads or for creating additional area for structures. This can be done by creating slopes, but these solutions do require space because of the angle of repose of the earth. Retaining walls provide an easy solution for such problems. They are structures that can retain earth or other material where such level differences need to be maintained. These walls can be designed as a structure to resist the lateral pressure provided by the earth and can be composite structural elements or gravity walls which resist the pressure by their sheer weight. Pressures on retaining walls vary from zero at the top of the wall to a maximum pressure at the lowest portion of the wall. Gravity walls are designed so that the effective thrust calculated from the lateral pressure of the earth and the weight of the wall falls within the base of the wall so that it does not overturn. As a safety factor, designers try to limit the effective thrust so that it falls within a distance of one third of the base width from the center.
Image source: Wikimedia: Retaining Wall
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Reinforcing Gravity Walls
Properly designed gravity walls do not require any sort of reinforcement. However as a matter of caution, reinforcement gravity walls are suggested especially to reduce any maintenance requirement. Brick or stone gravity walls can be reinforced by the introduction of steel bar reinforcement vertically and horizontally during the construction of the wall. Sometimes walls may deteriorate over time and such walls can be reinforced by using mesh reinforcement with shotcrete. Facing the gravity wall with precast elements anchored into the wall is also a means of providing reinforcement to an existing wall.
Gravity walls can also be reinforced by using the properties of the soil being retained. Providing geo-textiles at the back of a wall before backfilling it with material helps to reinforce the wall, as these geo-textiles can themselves serve as some sort of retaining element for the soil. Gabions with rock are positioned just behind the wall before any backfilling and can serve to strengthen the gravity wall. Quite often for low height retaining walls, the gabions themselves are used as a structural element and may function directly as a wall. Walls can be reinforced by the introduction of anchor bolts at regular intervals. The anchor bolts will be introduced into holes drilled in the wall and to a sufficient length in the soil behind it. These holes are then grouted with cement slurry. Chemical treatment of the soil behind the retaining wall can help to increase its angle of repose. This in turn reduces the lateral pressure on the wall and acts like reinforcement of its strength. High water content in the soil can conversely serve to decrease the angle of repose and thus increase lateral pressure on the gravity wall. This can be offset by dewatering. Most retaining walls will have some sort of weep holes at regular intervals in the height and length of the wall, which are meant to drain out water from the fill and prevent a reduction in the angle of repose.
Image source: Wikimedia: Gabions