HARDSCAPE WALLS - RETAINING WALLS
Purpose of Your Wall
Most importantly, you should decide what is going to be the purpose of your wall. If you are retaining soil more than 3 feet high, a natural stone, dry stacked gravity wall should not be your first option. Depending on soil conditions and what is being retained an engineered wall system may need to be explored. This is where a segmental retaing wall system would be best suited. If however, you are simply holding back topsoil in a small garden or retaining mulch or decorative stone along a path, a gravity wall could be a perfect solution for your project.
The geography and topography in this part of the country can range from slightly rolling to mountainous so we are used to seeing retaining walls. These walls allow homeowners and contractors alike to create steps or level areas on a sloped surface. However, if constructed poorly, the forces of nature will topple a retaining wall. The three basic types of retaining walls are:
SRW (Segmental Retaining Wall)
Segmental Retaining Walls
Segmental retaining walls are modular block retaining walls used for vertical grade change applications. The walls are designed and constructed as either gravity retaining walls (conventional) or reinforced soil retaining walls. The system consists of dry-cast concrete units that are placed without mortar (dry stacked) and rely on their unit to unit interface and mass to resist overturning and sliding. Unit to unit interfaces include friction, shear elements, and interlock. The systems may also employ soil reinforcement that extends into the backfill and allows for the construction of walls with significant height (e.g. in excess of 50 ft (15.24 m)) that could not be accomplished with the units alone.
Segmental retaining walls are considered flexible structures, so the footing does not need to be placed below the frost line, provided there is sufficient foundation bearing capacity. SRW units are manufactured in conformance with industry standards and specific cations to assure that units delivered to a project are uniform in weight, dimensional tolerances, strength, and durability—features not necessarily provided in site cast materials.
A gravity retaining wall is usually less than 3 to 4 feet tall and depends on its own weight or mass to retain (hold back) the earth behind it. This type of retaining wall is constructed with a volume of material so that when stacked together, the weight and friction of the retaining wall exceeds the forces of the earth behind it. The wall is thicker at the base than at the top and slants backwards. This is referred to as ‘battering’. The thickness of a gravity wall at its base should be one half to three fourths its height.
There are several basic types of gravity-stacked natural wall stones:
Wall Stone - Flat, Angular rock used for stacking
Field Stone - Rounder, usually thicker rocks that assist in wall stability
Boulders - Larger stone that can be angular or round in shape
Masonry walls are constructed out of brick, block or CMU's, or natural stone. They are used in the construction of building walls and retaining walls. When Concrete Masonary Units (or CMU) are used for a retaining wall, typically a veneer stone or stucco coat is applied.
A cantilever retaining wall is one that is a uniform thickness, which is tied to a footing. A cantilever wall typically holds back a significant amount of earth, so these types of retaining walls should be engineered. An example of a cantilever retaining wall would be a typical basement wall in a house. The width of the footing for a cantilever wall is very important because it resists tipping or sliding forces of the earth behind the retaining wall. These types of retaining walls require significant steel reinforcing in both the footer and the wall structures so that the two pieces actually become one integral unit. This is why this type of retaining wall needs to be designed by structural engineers. Don’t try this at home!!
Counterfort Retaining Walls
A counterfort retaining wall is very similar to a cantilever wall, except that this type of retaining wall has a triangular shaped wall that connects the top of the wall to the back of the footer. This is necessary added support. The wall is hidden within the earthen or gravel backfill of the wall. The footer, retaining wall and support wall must be tied together with reinforcing steel. The support walls add a great deal of strength to the retaining wall and make it virtually impossible for the wall to become detached from the footer. Just like cantilever walls, a counterfort wall should be designed by a competent structural engineer and not attempted by a weekend engineer.
Buttressed Retaining Walls
A buttressed retaining wall is just like a counterfort wall except that the support wall is on the outside of the retaining wall. They are visible but add incredible strength to the wall system. They are so strong that in order for the retaining wall to fail or tip over, the buttresses would have to be crushed. This buttress concept was used in the construction of many cathedral walls in Europe. The buttresses helped to stabilize them. If you intend to build one of these walls, you should hire an engineer. A mistake in this type of retaining wall could not only be costly, but dangerous.