RETAINING WALL TYPES
Simple gravity retaining walls are commonly encountered on hillside properties. These walls are often employed as landscape planter walls and are constructed with broken pieces of concrete and stone. Sometimes the individual pieces are mortared together, and in some walls the stones or pieces of concrete are simply stacked against each other. Department of Building and Safety building codes do not allow the construction of simple gravity walls. These walls are especially susceptible to damage during seismically-induced ground shaking. Periodic maintenance and repairs are normally required when these walls are present on the site, as illustrated in the second image. The main problem associated with gravity walls is the unpredictability of long term performance. Some gravity walls can last for many years and show no signs of distress, whereas other gravity walls can fail without notice. When supporting only planter areas, the risk involved in maintaining these retaining devices on a site is relatively low. A higher risk is associated with simple gravity walls which support structures (pools, decks, additions, etc.) or offsite property and slopes, as shown in the third image. This unreinforced stone and mortar wall is on the order of 7 feet high and supports a steep backslope.
BRICK RETAINING WALLS
Brick retaining walls are also commonly employed on hillside properties. Brick retaining walls lack steel reinforcement and usually are not provided with a proper foundation. Brick walls are commonly grouted together, providing some added strength over simple gravity walls. Surface and subsurface drainage control devices are also not usually provided for brick walls. Water pressure behind brick walls is a common source of distress and/or failure of brick walls. Brick retaining walls are also not permitted under the current building codes. The second image shows inventive ways homeowners will increase the height of their retaining walls. In this case, the brick was simply added to the top of the wall, increasing the weight above the wall. Some deflection of the lower masonry wall has resulted. The third image shows a 10 foot high retaining wall constructed of brick and poured concrete. Rotation and failure of the wall threatens the side yard area and offsite building.
CRIBLOCK RETAINING WALLS
Criblock retaining walls consist of interlocking concrete "cribs". The individual cribs lock together and the interior cells are commonly filled with compacted fill. Criblock walls have the attraction of being able to be planted, as the image demonstrates. Criblock retaining walls are a form of gravity wall, lacking steel reinforcement and a conventional type foundation. Criblock walls have experienced some dramatic failures in the city of Los Angeles, damaging slopes and structures. Criblock retaining walls are considered best suited to support planter areas. Criblock walls are not considered to be suitable for support of slopes or structures which could be damaged in the event the wall fails. Failure of criblock walls typically occurs due to faulty installation or buildup of hydrostatic pressure behind the wall due to improper subsurface drainage provisions. Various governmental agencies have differing views on the appropriateness of criblock walls. The city of Los Angeles has historically been reluctant to issue permits for criblock walls. When the city has issued permits, they often require that the cells be filled with concrete, eliminating the main aesthetic appeal of the criblock system. Caltrans and Public Works, who are not under the jurisdiction of the Department of Building and Safety, often employ criblock retaining systems along roadways and in public developments.
UNREINFORCED MASONRY WALLS
Concrete retaining walls which have not been provided with steel reinforcement were commonly constructed in hillside areas during the 1920's through the 1940's. Unreinforced walls are more susceptible to damage due to soil pressure, water pressure and seismic shaking.
RAILROAD TIE RETAINING WALLS
Another commonly employed method of retaining soil or slopes is the use of railroad ties. The Los Angeles Department of Building and Safety specifically prohibits construction of retaining devices out of wood (Section 1819.6). Wood walls are susceptible to water and termite damage and can fail catastrophically. When used as planter walls and not supporting structures, slopes or neighboring properties, the risk involved with maintaining wood railroad tie planter walls is relatively low. Railroad tie planter walls tend to last longer and perform better than other wood retaining devices due to the thickness of the ties.
OTHER WOOD RETAINING DEVICES
Other types of wood can sometimes be employed on hillside properties to retain soil and slopes. As with railroad tie walls, all types of wood walls are prohibited under the Los Angeles building codes. Thin wood retaining devices are highly susceptible to failure when exposed to water, termites and soil pressure. The first image shows a poorly constructed wood retaining device holding up about 4 feet of loose soil under a house. The wall is in a state of failure and is a safety concern. Wood batterboards are often used on hillsides to help control erosion, as shown in the second image. These devices can be helpful, but require maintenance and repairs. When improperly constructed, as shown in the image, the walls become a potential liability. These type of walls typically fail catastrophically, potentially causing damage to downslope property and structures. It is usually better to address the root cause of the slope erosion than to apply a temporary solution. Slope erosion is usually caused by the lack of proper slope vegetation and/or inadequate surface drainage control.
The Los Angeles Department of Public Works employs the use of wood retaining walls to support public walkways and streets. A battle often ensues between Public Works and the Department of Building and Safety regarding reliability of wood walls where hillside properties adjoin Public Works wood retaining walls.
Here is a unique application of telephone poles. The wall supports about 4 to 5 feet of fill, behind which is a 1950's residence. Subsurface exploration at the site revealed that the residence was not provided with a standard foundation, with the stem wall resting in fill supported by the telephone pole retaining wall system. Recommendations were provided for support of the residence foundation system into the underlying bedrock and ultimately replacing the wood wall system.
CHAIN LINK FENCES
Either deliberately or not, chain link fences are sometimes employed as retaining devices. Chain link fences are not constructed to retain soil and debris. Accumulated material behind chain link fencing will eventually cause the fence to rotate and fail. Failure can be catastrophic, causing damage to downslope structures and property.
STEEL-REINFORCED MASONRY RETAINING WALLS
Properly designed and engineered retaining walls are constructed of concrete block or poured concrete and provided with steel reinforcement. Steel-reinforced retaining walls have the greatest potential for long term favorable performance. In hillside areas, retaining walls are normally required to be engineered. The geologist and soils engineer must recommended the design pressure that the wall must support as well as the foundation system to be employed. The structural engineer then uses this information to engineer the wall and provide a set of construction plans to be used by the masonry contractor.
SOLDIER PILE RETAINING WALLS
When temporary shoring is required to construct a retaining wall, it may be advantageous to incorporate the shoring system into the permanent design of the wall. Soldier piles are commonly used in hillside development as a means of making high temporary vertical excavations. Soldier piles are typically 24 to 36 inches in diameter and consist of a cylinder of steel-reinforced concrete. Solider piles are typically spaced 8 feet on center, providing the main support for the excavation. Shotcrete is used between the piles to cover the space between. A block retaining wall can also be structurally connected to the soldier pile system.
RETAINING WALL COMPONENTS
Retaining walls can be provided with a variety of different foundation support systems depending upon the soil conditions and local topography. The geologist and soils engineer must evaluate site conditions and provide the structural engineer with a recommendation of the foundation system to use and the bearing material which is to provide support for the foundation system.
Conventional Foundations - The most common form of support of retaining walls is conventional footings. The geologist and soils engineer must determine the foundation bearing material and the minimum depth of embedment. The structural engineer will determine the actual depth of embedment and the width of the footing needed to support the wall. A keyway is also sometimes employed in a conventional retaining wall foundation.
Deepened Foundations - Deepened foundations, consisting of either friction piles or caissons are commonly used to support hillside retaining walls. Deepened foundations are necessary when retaining walls are located over or near descending slopes or when necessary to penetrate deep unsuitable earth materials. Deepened foundations are also sometimes used to avoid surcharging lower walls or structures.
Tie Back Anchors - Tie back anchors are sometimes employed to resist lateral forces imposed on retaining walls. Tie back anchors are drilled at an angle into the slope and spaced at pre-determined intervals. Tie back anchors are commonly used in construction of temporary shoring for high excavations. The Los Angeles Department of Building and Safety does not allow the use of tie back anchors in the permanent design and construction of retaining walls.
Several methods can be utilized to prevent the building up of hydrostatic (water) pressure behind the retaining wall. Water pressure will exert 62.4 pounds per cubic foot of pressure on a retaining wall. Typical retaining wall designs employ a 30 to 43 pound per cubic foot equivalent fluid pressure. If water pressure is allowed to develop behind the wall, most walls will experience rotation or failure. To prevent this condition from developing, several methods can be employed which include weepholes, open headers in concrete block and subdrains.
Subdrains are a commonly used method of providing subsurface
drainage control behind retaining walls. A subdrain typically consists of
a 4-inch diameter perforated PVC pipe. The subdrain pipe must conform with
ASTM standards. Flexible subdrain pipe is normally not used due to its
susceptibility to crushing when the wall backfill is placed. The approved
subdrain pipe is laid on a bed of 3/4 inch gravel at the base of the retaining
wall. The subdrain pipe is placed with the perforations pointing down,
allowing the small trough at the bottom of the pipe to conduct drainage.
The perforated pipe is connected to solid drain pipe which discharges to an
approved location (street).
A commonly employed method to allow subsurface water to drain through the retaining wall is the use of weepholes. This image shows 3-inch diameter weepholes installed into the base of the retaining wall. Weeholes can be installed at the time of wall construction or can be provided later.
When retaining walls are constructed from concrete blocks, the bottom course of block can have the joints between the blocks left ungrouted. This allows drainage to seep through the open grout spaces, often referred as "open headers". Open headers are considered to be less effective than conventional weepholes or subdrains.
In some applications, use of conventional subdrain systems is
not possible. MiraDrain Company offers a variety of filter fabrics which
can be used to provide some degree of drainage behind retaining walls which have
not been provided with enough free space behind the wall to employ a standard
drainage system. Filter fabrics are not as effective as conventional
subdrains and weepholes, but in special circumstances, filter fabrics represent
an alternative which can provide some degree of protection.
Retaining walls must be properly backfilled. The backfill material must be selected and approved by the geologist or soils engineer. Typically, the subdrain pipe or weepholes must be covered with a minimum of 12 cubic inches of 3/4 gravel to allow free drainage to the subdrain pipe and to help prevent clogging of the pipe with finer drained silt and clay particles. The remaining height of wall is typically backfilled with compacted fill. Compaction testing must be performed on the wall backfill to ensure the require degree of compaction. In some circumstances, gravel is placed the entire height behind the wall to within 2 feet of the top of the wall to provide better drainage conditions. The upper 2 feet must consist of a compacted fill blanket to reduce surface drainage infiltration into the wall backfill.
This exterior concrete block retaining wall has been constructed at the base of a compacted fill slope and has not been provided with waterproofing. As a result, a significant amount of moisture penetration has occurred through the face of the wall.
Surface Drainage System
This picture illustrates several surface drainage control features of a typical retaining wall constructed at the base of an ascending slope. The wall has been provided with a concrete drainage swale or "V" drain, to intercept upslope drainage. The drainage swale is designed to collect and discharge slope drainage to an approved location, usually the street or storm drain system. The retaining wall has also been provided with freeboard, which is the portion of the wall which extends above the drainage swale. The purpose of freeboard is to prevent upslope drainage and debris from overtopping the wall. The height of freeboard is determined by the geologist, but typically is 2 feet.