Retaining walls offer homeowners several advantages, including the reclamation of unusable space, stabilization of soil and visual appeal. They are not difficult to construct for the average do-it-yourselfer, but retaining walls must be built in adherence to a few basic design principles. Otherwise, your hard work will be short lived, as failure of the retaining wall will be all-but-certain within a few years. Below are four important things to keep in mind when designing and building your retaining walls:
Use the proper size and shape drain aggregate
Hydrostatic pressure is the force that subterranean water exerts on a buried object, such as retaining walls. The weight of the water contained in the soil will eventually cause a retaining wall to collapse unless it is accounted for during the design and construction phase. One important way to prevent destructive hydrostatic pressure from building is to drain moisture from the space behind the wall. This will keep the weight of the water off the wall and maintain manageable loads against it.
To drain the area behind the retaining wall, fill in at least a foot of space immediately behind the wall with medium-sized angular gravel. The irregular shape of each piece will cause the gravel to "lock" together, and there is also plenty of space between each piece to permit water to flow freely downward. Never use pea gravel, river rock or other small, round aggregates. These pieces will not lock together and will instead churn in the space behind the wall, contributing to erosion. Further, such small, round rocks will also trap water which, defeats the purpose of building a drainage zone behind the wall.
Build to allow unrestricted water overflow
Another important principle when building your retaining wall is to build the top of the wall lower than the material behind it. This design feature will enable rainwater to flow over the top of the wall, rather than directing it downward into the space behind the wall. Good drainage as described above will also help prevent trouble, but you should avoid making the wall into a dam whenever possible.
Use solid, pinned blocks to build the wall
Retaining wall blocks come in a variety of shapes, sizes and internal configurations; some are solid throughout, but many others are hollow. Hollow blocks are easier to handle due to their lighter weight and may also be less expensive to purchase; however, hollow blocks should not be used on a retaining wall if you want it to have the longest possible lifespan. Water is easily trapped inside hollow blocks, and during the winter months, freezing can cause the blocks to crack and break apart. Ice can also cause block heaving, which is the process of blocks rising up and pushing over the layer beneath them.
That is why the use of solid, pinned blocks is your best choice for building retaining walls. These blocks are heavier, which enables them to better weather the forces upon them. In addition, solid walls can't accumulate water, and the pins that hold them together will add rigidity to the structure.
Build a single wall whenever possible
On a long slope, it is tempting to use multi-tiered retaining walls to divide the surface into a variety of terraces. While this may have some practical benefits, such as making each wall lower and simpler to build, multi-tiered walls can cause failure, especially in the lowest wall. The reason is due to the weight placed upon the soils beneath the walls; this weight is then transferred to the lower wall and adds a huge burden that it may not be designed to handle. If a multi-tiered wall seems unavoidable due to practical reasons, then be sure to build each wall as far apart as possible to lessen the pressure on the soil.
For more tips and information, talk with landscape or hardscape services in your area.