In cases where the manmade slopes can become too steep due to space restrictions, these slopes have to be restrained against failure by other means such as retaining walls or retaining structures.
There are numerous different types of retaining structures, from metal sheets to thick concrete. Depending on the slope design, the risks involved and the restraint required, the walls can extend to the full height and length of the slope, or only a part of it. However, even with retaining walls, water still plays a very important role, and much attention must still be paid to the control of water.
Regardless of the type, all retaining walls have similar drainage features:
Free-draining layer behind the wall
Cutoff drain at the top
Weepholes
Cascade drains
Drain at the toe (bottom of the slope)
Free-draining layer, like sand or similar composite, is provided between the back of the wall and the soil behind. This allows water from the soil to drain into the free- draining layer and exit via weepholes or similar. Where drainage is not properly constructed, free-draining conditions behind the wall do not exist, and the soil behind the wall gets saturated with water. The wall or the restraining structure now has to provide restraint against the forces created by both the soil and the water. The walls are not designed for this situation. These combined forces are too much and failure or large deformation in the wall will take place.
The types of retaining walls most commonly used include:
Brick walls
Rubble walls
Reinforced concrete walls
Gabian walls
Crib walls
Reinforced earth walls
HOW RETAINING STRUCTURES WORK
Simply put, all retaining structures work on the basis that the forces holding the wall in place (such as the weight of the wall or the holding power of anchors or tie backs) are greater than forces trying to topple the wall (the force of the earth pushing from behind the wall). There are many types of retaining structures, from reinforced earth, crib wall, fancy block wall to simple reinforced concrete walls and rubble or brick walls.
All walls and restraining structures are designed for the forces generated by the soil behind them. Designs of retaining wall assume damp but unsaturated condition of soil behind the wall. The designs ensure this condition is met during service by providing free-draining soil layer behind the wall and suitable weepholes for water to exit the wall.
This is to make sure that the soil behind the wall never gets saturated.
DRAINAGE IN THE RETAINING WALLS
The cutoff and toe drains in the walls have the same function and work similarly as those in slopes. The weepholes also drain water but work somewhat differently. The water entering the soil behind the wall is allowed to drain into the free-draining layer (consisting of sand and gravel) provided between the wall and original soil behind. Once this water enters the free-draining layer, it is easily channeled out via weephole (pipes) and out of the wall into the toe drains below.
This ensures that soil retained by the wall never gets saturated with water, and the soil behind does not create excessive pressure on the wall that could cause the wall to collapse. Preventing the soil retained by the wall from getting saturated by water is an important requirement of all types of retaining walls. Every type of retaining wall has to provide some method of removing water from soils behind the wall which may not necessarily be via weepholes only.
WEEPHOLES
Weepholes are designed to drain water from the soil behind the wall without allowing the fine soil particles to be carried out with this water. How the weepholes are constructed is very important as they are the only means by which water will drain out of the soil behind the walls, especially in retaining walls that are not self-draining. Various free-draining methods are aimed at allowing only the water to drain out without washing out the fine particles from the soil behind the wall.
When water cannot drain out because of bad construction of weepholes or problems with the soil filter, numerous problems can occur as the water builds up behind the wall. Walls can bulge or cracks can appear before collapsing.
Most blocked weepholes may be flushed with water using a garden hose or similar. Bad weepholes must be replaced. This may require excavation behind the wall and such action will require professional assistance.
WORKING WEEPHOLES
TYPES OF RETAINING STRUCTURES
BRICK WALLS
Brick walls are usually one brick thick and are about a meter or less in height. These walls are meant for covering soil for erosion protection or for decoration only. They are not meant to retain the soil behind the wall. If used as a retaining wall at the toe of the slope or as a small retaining wall, these walls must be made very massive and thick. In cases where the weight of the wall is important, rubble walls instead of brick walls are used.
RUBBLE WALLS
Rubble walls are made up of large blocks of stones and mortar. These walls provide restraint due to the weight of stones held in place by cement mortar. Proper functioning of weepholes and drainage control is very important as these walls cannot withstand too much soil pressure from behind. Rubble walls depend on their weight for stability.
REINFORCED CONCRETE WALLS (RC WALLS)
Reinforced concrete walls (RC walls) are made up of concrete and steel reinforcement. Walls are designed as retaining structures and depend on the strength of the concrete and steel to provide restraint. These walls are costly but do not easily get damaged. Maintenance and control of water, particularly via weepholes, are very important to these walls. Buildup of water behind the walls can cause them to fail.
CRIB WALLS
Crib walls are made up of interlocking concrete sections forming rectangular cages filled with granular soils (sand and gravel). They are basically gravity walls as they depend on their own weight for stability. These walls can be built up to about six meters by increasing the cross-section width of the wall so as to provide more stabilizing weight. Extremely high walls of this type have been built, but these are rare cases and maybe due to possible exceptional needs and problems.
Because of their open structure, these walls are self-draining and do not need weepholes. These walls are also flexible, that is, such walls can tolerate increases in pressure of soil by flexing or giving in a bit. This action can reduce the soil pressure behind and stop the wall from failing. When the soil behind the wall continues to move, such walls can undergo considerable deformation in the form of bulging which can be seen long before collapse occurs. When properly constructed, such walls can be very attractive, particularly when vegetation is carefully allowed to grow on it.
GABIAN WALLS
Gabian walls are made up of large stones held in place by wire cages. Because gabian walls consist of only rocks, they can allow water to easily flow through them and hence do not require weepholes. These walls are also flexible and behave similar to crib walls.
Gabian walls depend on the sheer weight of the stones inside the cages for their retaining strength. The walls depend on the wires for their durability. Once the wires get rusted or break up, stones fall out and the gabian walls become useless, unless there is enough vegetation growing around to keep stones in place. Gabian walls must be regarded as a temporary solution only.
Deteriorating gabian walls where wires are rusted or broken and stone fill is coming apart must be monitored and replaced.
REINFORCED EARTH WALL
Reinforced earth walls consist of concrete panels held in place by strips of steel (reinforcement strips) buried into compacted soil behind the wall. The strips are held in place by friction between the strips and earth. These walls are extensively used in highway construction. Like any walls, they have to be professionally designed and require stringent care during construction. These walls have an open structure, that is, water can easily seep out of the panel joints to drain water out of soil. These walls are not rigid like reinforced concrete walls and under load will behave similar to crib walls.
