What is a retaining wall and how are they built?
What is a retaining wall? A retaining wall is any structure that holds back another element. This element could be earth, stone, water, sand, etc. The retention of a material is required for various reasons, however, there are typical common occurrences in construction, where the need to retain a material is required.
1.) Severe elevation differences due to site work conditions. This refers to a sloped site that must have various levels of terraces supported by retaining walls. As the good parcels of land are being developed, the remaining parcels are in many cases on a hillside or down in a valley. The varying elevations and differences in site gradients will require the construction of more retaining walls to make the site work. Retaining walls allow the use of a hillside to locate a driveway, the use of a low land area for a building pad, etc. As the site become more complicated, the need to retain material with the use of retaining walls becomes increasingly necessary.
2.) Driveway access. In many situations a driveway that accesses a parking structure, or garage lower than the grade around the building, will require retaining walls on one or both sides of the drive. In situations where the simple grading of a slope will not allow the creation of a safe and sustainable driveway, the construction of retaining walls becomes necessary. In some cases, the simple need to create flat areas to construct level drives is necessary and the only way to manage the elevation changes is to construct retaining walls.
3.) Property line demarcation, where elevations are severe enough to warrant the need for a structure to designate property line and location. In many instances, again, especially as the available properties become increasingly challenging, the need for retainage structures increases. There are particular cases where a structure might be so close to the property line that the only way to manage the elevation requirements is to construct a retaining wall to correct the elevation discrepancies.
4.) Water course retention. In areas of water courses, such as rivers, streams, etc, there may be a need to retain the banks of the water course. If the water course is a means of draining the property, then the use of retaining walls will become necessary to establish level and consistent grades for property development.
5.) Most foundations surrounding a basement are retaining walls. If the grade is higher than the interior floor elevation of the basement, the perimeter walls act as retaining structures. In most residential situations the actual building structure over the top of the foundation walls acts as a lateral support to hold the foundation from imploding. Therefore these types of retaining walls are usually not heavily reinforced and not heavily loaded laterally.
Failure of retaining walls is a common structural occurrence which causes severe damage and destruction. Incorrect methods of retaining wall construction has resulted in serious damage to both property and lives.
This is the reason that engineering and construction of retaining walls is so important. The proper alignment, weight distribution and assembly of the retaining wall is important to its structural integrity.
What are some basic retaining walls constructed of?
1.) Retaining structures known as gravity walls consist of heavy independent blocks of material. This material could be concrete, stone or any other type of heavy, dense material that can be stacked one upon the other. The structural integrity of this type of wall is based upon the weight, or the gravity of each element, as they are placed one on top of the other. The built up weight of the system is what causes the material on the higher side of the wall to be retained. The simple weight of the system is too heavy for the lateral forces to move.
2.) Random stone retaining walls are basically gravity walls also. They function as a system of interlocking pieces, that will fit, one into the other for support and strength. A good stone wall that is used for retainage purposes, is an excellent method of constructing a retaining wall, due to its ability to freely drain. The ability of a retaining wall to shed the water from the high side, is critical to the proper functioning of any retaining wall. Water must not be allowed to gather on the backside of the retaining wall. If water is allowed to accumulate the lateral pressure on the wall can become substantial and cause the retaining wall to fail.
3.) A concrete reinforced retaining wall, consists of two heavily reinforced concrete elements. One element is horizontal on the ground and is basically a reinforced slab of concrete. The second element is a vertical element of reinforced concrete that is positioned 90 degrees to the horizontal slab. This second element, is normally another heavily reinforced concrete slab, that is constructed with reinforcing steel that is designed to attach to the first concrete slab. This type of concrete retaining wall typically requires a structural engineer, to calculate the lateral forces on the retaining wall, as well as the design of the concrete elements of the wall. The concrete wall has what is referenced as the toe and the heel of the wall. This is basically determined by the placement of the vertical concrete element on the horizontal element. The piece of horizontal footing that is in the front of the wall is normally referenced as the toe and the back of the footing is called the heel. The heel can be remembered by the portion of the footing that is under the loaded side of the wall. Reinforcing is positioned to allow the tensile stresses that form in any retaining wall, to be absorbed by the steel, and the compressive forces to be absorbed by the concrete portion.
4.) Another type of retaining wall is a battered wall or a buttressed wall. Both types of walls incorporate the stepping back of the wall into the loaded side of the wall. This is a battered wall. The elements of the wall are each set back from each other by a small amount to further strengthen the vertical portion of the wall. The buttressed wall is similar to the large tower looking structures on the outside or inside of high walls that surround a home or another type of protected area. The buttress provides additional strength to the wall, by placing an additional bracing effect on the vertical portion of the retaining wall.
5.) There are also geo- technical walls that rely on the placement of geo- tech fabric that is installed within the jointing of the blocks of the retaining wall. This fabric is then installed horizontally, from the face of the loaded side of the retaining wall to a pre engineered length, in back of the wall. This system uses the weight of the block wall, as well as the lateral strength of the horizontal geo fabric embedded in the block joints, to hold the wall in place against the lateral pressures of the retained material. These types of walls are usually seen along highways or around the perimeter of structures where the pavement areas are either elevated or dropped down from the elevation of the structure. This type of wall requires a structural engineer to properly design the wall and will require stamped engineered drawings from an engineer, to allow the building department to approve these types of retaining wall.
6.) Driven piles with lagging is another common method of constructing a retaining wall. In this instance piles are driven at a pre-engineered on center spacing. Lagging or boards, or aluminum panels, or any other type of designed retainage material, is then installed between the driven piles to secure the retained material. In some instances, a temporary means of support for the material is performed, and a concrete wall is placed, with the driven piles an integral part of the retainage system, embedded within the concrete wall.
There are several basic engineering elements that if understood, will allow the lay person to better understand the functioning of the wall, as well as enhance an understanding of how to properly build a retaining wall.
1.) It is imperative that the side of the wall retaining the material be freely drained. The water from the back side of the wall cannot be allowed to collect in the rear of the vertical portion of the wall. This is normally accomplished by backfilling the wall with a very porous material such as stone. In addition a drain should be incorporated into the rear of the wall to allow water to drain from the backside. Weep holes are also recommended which are actual holes within the retaining wall to allow the drainage of water from behind the wall. Proper drainage is required to prevent the build- up of hydrostatic pressure in the back of the wall and to allow the water to drain freely. Water is extremely heavy and fluid, which places all the lateral pressure of the full column of water, onto the back of the retaining wall. It is imperative that the water is allowed to drain.
2.) In most situations the majority of the heavy reinforcing within the vertical portion of the wall is alongside the surface facing the loaded side. In other words, the side being retained has the rebar on the vertical section. The horizontal or footing element of the wall will usually have the majority of the reinforcing bar close to the bottom of the footing. Reinforcing bar is located within the tension portion of the stress diagram for this wall and the concrete mass is located on the compression side. Basically rebar is close to the loaded side of the retaining wall and along the bottom of the footing.
3.) The material beneath the retaining wall is an important function of the integrity of the wall. The material below the footing, or the base of the gravity wall must be free draining, and of an adequate compressive strength, to not allow the wall to compress the sub-base. If the sub base of the wall is compressed, this could allow the wall to tip over, or shift the expected orientation of the lateral pressure on the wall itself. In many instances, the base of the wall will be over excavated and a stone layer, properly compacted, will be installed. This will ensure that the bearing of the wall footing on the sub base is adequate to support the pressures and load of the retaining wall. The free draining element of the sub base is necessary to eliminate any standing water that may accumulate under the wall itself. The drainage of water is a primary component of a properly engineered retaining wall.
4.) In some instances a fabric is installed at the rear of the retaining wall, to eliminate the build- up of silt and sand against the back of the retaining wall. This concern for build-up of silt and sand could cause a blockage of water behind the sediment, causing a hydraulic lateral pressure to develop. It is extremely important that this does not occur due to the extreme forces that retained water can produce.
Retaining walls for residential purposes are sold in all the large box home centers. This type of wall is normally a total gravity wall that relies on the weight of the block or structural element to create a collective barrier to retain material. Various wall types, different blocks, different interlocking systems are available and can be constructed by the homeowner. However, the larger retaining walls that are commercially required, are precisely engineered walls, that are installed by professional contractors using large equipment.
In many cases, the retaining walls on a complicated site will cost considerable money. If the site has various elevations and several means of travel, there could be hundreds of feet of retaining walls. An excellent means of value engineering, which is the changing of a specification to save money on the project, is sometimes found in the design of the retaining walls. If the walls are designed, for example, with a concrete footing with block walls and a brick veneer, then the change to a stone wall system, built of site material can save the owner substantial material. The proper application of retaining walls, as well as the proper design of the walls is important to the overall cost and success of a project. Retaining walls can allow a relatively difficult property to be more easily developed. The retainage of materials allows the creation of buildable level areas for structures.
Unfortunately the costs of large commercial retaining walls can be substantial, however the properties that their construction can modify, into proper developable parcels, may be worth the cost of the retainage structure. The use of professional architects, site planners and engineers will allow the developer to determine if the results are worth the costs.