Retaining walls are used for stabilizing and controlling erosion of steeply sloped areas of the lot. In some cases, retaining walls are used in conjunction with terracing to provide a level area for recreational purposes. In either case, they should be designed to withstand the lateral pressures being exerted on them, by the soil and the hydro-static pressures from behind the wall.
Retaining walls may be built with concrete, construction timbers, railroad ties, stone, concrete or concrete blocks. Some concrete and concrete block walls have stone or brick veneer facings. On occasion, you may find a retaining wall of steel baskets filled with stones. This is referred to as a “gabion” retaining wall. As the gabions age, the steel baskets tend to corrode and deteriorate, especially on the side facing the embankment. Over the years, the stone inside may begin to move.
The design of the retaining wall should incorporate provisions for drainage of water that normally accumulates behind the wall. Otherwise, the hydrostatic pressures built up will cause structural failure of the wall. Drainage may be provided by installing continuous perforated drain lines at the lower portion of the wall, and backfilling the areas with stone and gravel.
In concrete retaining walls, the perforated pipe is replaced by weep holes in the bottom of the walls that allow the water to exit through the front of the wall. The grade or soil at the base of the drainage system behind the wall should direct any water that accumulates to the weep holes. Unfortunately, retaining walls are often built with inadequate drainage provisions. The gravel backfill or the drain line may be omitted, or the weep holes may be too few or too small to be effective. The weep holes should be kept clear so that the water behind the wall can adequately drain.
A retaining wall built with construction timbers or railroad ties should be anchored to the hillside to provide resistance to the lateral forces. If the wall is not tied back into the earth, it can bow, buckle or heave and eventually collapse. Anchoring the wall is achieved by using tiebacks or dead-men. A tie back is a construction timber that is placed perpendicular to the wall. The front end is flush with the wall and is fastened to the wall itself with large spikes. The rear end is fastened to a dead-man or a small section of lumber perpendicular to the tieback and parallel to the wall itself. Because the construction timbers and railroad ties have open spaces between the members, weep holes may not be necessary.
Many railroad tie and timber retaining walls are not constructed with anchors. You can tell whether anchors were used by looking at the wall. If tiebacks were used, end sections will be visible in the face of the wall. However, from a visual inspection you cannot determine the length of the tie back or whether dead-men were, in fact, installed. All retaining walls should be vertical or inclined slightly towards the embankment. They should not be leaning forward or away from the embankment. When you encounter a retaining wall that is leaning, it is an indication that the wall has not withstood the lateral forces that have been exerted. Once the wall begins to lean, crack and heave, the pressure that caused its condition should be relieved.
Retaining Wall Design
Retaining walls are used to give support to vertical sloping land. By installing retaining walls, such sloping land can be leveled and hence be of greater use. Plants, trees and gardens can be incorporated, to once useless land.
Retaining walls also have a less functional use, which is a visual or decorative purpose. Retaining walls can be used in the garden to add to the landscaping design, such as surrounding flower beds or to accentuate feature gardens.
With the use of colored bricks and pavers, decorative stones and various timbers, retaining walls can add that aesthetic appeal to the garden grounds.
Retaining walls are also used for the construction of highway walls, parking lots, erosion walls and bridges. Factors to Consider in Building a Retaining Wall:
Building a retaining can be easy enough for do-it-yourself installation, however the higher the slope the more difficult it becomes to construct. When the slope of the land becomes too high and steep, a structural engineer is required. The reason for this is that many structural factors must be taken into consideration and resolved to prevent destabilization of the wall.
So the gravity of the situation where a retaining wall is being considered is very important. The retaining wall has to be able to do the same job that the soil presently does. Therefore the type of soil behind the wall must be considered, or better yet a drainage system can be put in place. This will allow water to be released from behind the wall, and hence put less weight pressure on the wall.
Bulkheads
Bulkheads are very similar in design and construction as standard retaining walls. The primary difference in definition between a bulkhead and retaining wall is that a bulkhead is retaining earth on one side, and is partially surrounded by water on the other. Materials used in the construction of bulkheads vary, but generally are the same as those used for the construction of the piers. Timber construction generally uses a pressure-treated marine grade material. A creosote material was used for some years, but it has been discouraged lately. Creosote grade material does not last as long, deteriorates and needs constant maintenance and replacement.
Masonry is often used as a bulkhead material. Masonry can take the form of brick, block, or poured concrete. This type of construction, unlike the piers, should rest upon a footer that carries the weight of the wall. Provisions should also be incorporated in the masonry wall to provide tiebacks up into the earth. Masonry bulkheads, much like retaining walls, generally have a granular backfill and a drainage system that would allow water that accumulates and drains from behind the wall. Brick and block masonry walls are used in residential applications, they require considerable maintenance and generally have a very short life expectancy. Displacement of the brick or block and the lateral pressures being exerted from the earth behind the wall cause breaking of the mortar joints. Tiebacks can be installed through the faces of the brick and block areas utilizing steel channels or wooden planks secured back through the earth with tie rods. This method, at best, is temporary.
Riprap or rubble is often used as a bulkhead system where the height of the bulkhead does not have to be above 2 or 3 feet. Riprap, by definition, is simply the “haphazard arrangement of loose, irregularly sized or broken stones used as protection from erosion along the shoreline of a lake, stream, or waterfront.” The material can be installed in low-lying applications to retain the earthen surface and prevent erosion. However, the applications are limited to low-lying applications, due to the inability to stack this material to a sufficient height and depth to prevent movement from lateral forces.
The construction of wooden bulkheads is much the same as that of any other retaining wall system. On the waterways, the supporting structures could be driven piles, which are sometimes referred to as “tie piles.” Attached to the earthen side of the piles is a horizontal-supporting member referred to as a “whale.” These horizontal members are also called “rangers” or “whalers.” The whalers are attached to the piers with galvanized fasteners, generally in 3 to 4 locations, depending upon the height of the retaining system. The size of the whalers depend upon the spacing between the piers and the amount of earth retained behind the bulkhead. Attached to the horizontal whalers is a sheathing material, generally of dimensional pressure-treated lumber. This dimensional lumber can range from 2 x 6 to 2 x 8 and are attached to the whalers. Gravel back-fill should be installed behind the bulkhead system with a geotextile fabric that permits dirt from accumulating into the granular back-fill. This provides a weeping system so that the water and moisture that accumulates at the rear of the bulkhead can drain away.
The largest problem with the bulkhead systems is erosion of the back-fill area. As the back-fill begins to erode, the granular surface is affected, as is the drainage. The increase in hydro-static pressure, coupled with the lateral forces of the earth itself, tend to push the units over.
Wave and water action on the front side of the bulkhead area also creates a number of problems. It is important that the structural material be constructed of a marine grade pressure-treated lumber, or creosote material; as pieces begin to deteriorate, break, etc., they should be replaced promptly. Damage and rot to the sheathing boards and whalers is time-consuming and fairly expensive, since the entire earthen surface and granular back-fill behind it will need to be removed. In general, the cost of repair or replacement of an existing bulkhead system utilizing pressure-treated lumber is approximately $80-$120 per linear foot of bulkhead, depending upon the height of the bulkhead system. As with retaining walls, the bulkhead system should not be displaced, leaning, cracked or broken. The earthen surface on the retaining side of the bulkhead system should be to the top of the bulkhead so that surface water runs down over the top of the bulkhead and does not accumulate behind the wall.
