Masonry stone gravity walls can operate in a variety of soil conditions, from weak clays to diabase bedrock, as well as in water environments. Stone walls are able to withstand water for long periods of time with no long-term damage to the wall. This means that water applications such as ponds, streams, etc. are possible with this system.
This system is highly flexible, and can be manipulated to fit in tight spaces, such as along property lines or near buildings. Additionally, maximum wall height is not an issue with masonry stone gravity walls; walls have been constructed taller than 30 feet.
The advantage of using natural materials is that these walls are generally more aesthetically pleasing than concrete-based or segmental block alternates. If recycled concrete is available on a project, it can be used in the mass of the wall as well. This can help with LEED requirements. Another advantage of the materials used is the design life of the structure. This system can perform as intended for hundreds of years.
This system can be broken down into three main components: footing (below grade), stem (above grade), and drainage. Together, these components create a system that can handle virtually any condition, from varying surcharges, soil conditions, water, and onsite factors. For example, all of Metroplex’s retaining walls are professionally engineered for site-specific conditions and take into consideration sliding, overturning, bearing capacity, and global stability.
The footing is typically a mix of grout and stone that is placed to support the entire system. Sitting roughly 6 inches below grade, and extending down to frost depth, the footing locks the wall in place and supports the stem.
One of the ways that this is accomplished is with a “toe” on the front of the footing. This mass of stone keeps the wall upright, prevents overturning, and reduces bearing pressures. When problem soil areas are encountered — whether soft silt or fatty clays — the footing can be extended through these soils until competent bearing is encountered. This is a cost-effective alternative to other systems where it is common to remove and replace weak soils with structural fill, or install a mechanical support system such as caissons or H-piles.
Also, in areas of bedrock, most systems require that rock be blasted and excavated prior to wall construction. With the masonry stone system, the bedrock can be excavated to a fraction of the depth and the footing can be locked into it. In cases where the rock is not fractured, the stem of the wall can be drilled and doweled directly into the bedrock, removing the need to excavate any material for the footing.
The stem is the visible portion of the wall, and is typically thinner than the footing as there is no toe. The driving factor in reducing this thickness and the key to Metorplex’s gravity wall system is the batter of the stem. Similar stone systems are constructed in a “pyramid” shape. However, the Metroplex system has the batter on the face of the wall parallel with the batter on the back, forming a parallelogram. Generally, this batter is 1:6 (although it can be designed more vertical or with more batter). This means that for every 6 inches of vertical face, there is 1 inch of setback toward the rear of the wall.
The advantage of the lean back is that the pressure from the soil onto the wall is reduced. In the pyramid system, the soil mass retained is larger and the pressures are greater. Geometric advantages of the parallelogram system are that it creates thinner walls with no reduction in factors of safety, which saves materials and labor.
Finally, the most important component is the drainage. Water build-up behind a retaining wall, or hydrostatic pressure, is a great concern, and this system has two methods to address it. The first is a drainage zone behind the retaining wall. Based on the surrounding soils, this can be either a drain board material attached directly to the back of the wall or a chimney of gravel/#57 stone. This zone extends from the top of the wall to the top of the footing.
Collecting the water from saturation of the backfill from rain, or other subsurface water sources, this water is funneled down toward the footing to the second system — the weep holes. The weep holes are typically 3-inch PVC pipes that collect the water at the back of the wall and extend to the face of the retaining wall. Set roughly 6 inches above final grade and spaced approximately every 8 feet, these weep holes give the water a place to go after it has been collected.
By maintaining a consistent 12-inch-wide cap, fence and handrail can easily be core drilled into the cap and maintain the clean look.
Control joints and cap
Other important aspects of the masonry stone retaining wall system are expansion joints and cap. Expansion joints are 6-inch-deep grooves set in the face of the wall every 16 feet that contain a 4-inch piece of expansion material similar to what is used in sidewalks. The purpose of this expansion, or control, joint is to provide a place for cracking the wall if it settles or moves. Masonry walls can accept vertical movement of a few inches with only aesthetic damage to the wall. The cracks in the wall do not reduce the weight of the wall and therefore typically have little impact on the structural aspects of the wall. Where these kinds of cracks form, they can typically be repointed very easily.
The next item is the wall cap. Constructed using the same stone as the face of the wall, the cap of the parallelogram masonry wall is different than similar systems. A pyramid-shaped stone wall may have a cap that, by nature of its batter, is 2 to 3 feet wide, while a parallelogram wall will always have a 1-foot-wide cap. This saves on materials, creates an area where sidewalk and curb can be integrated, and gives a clean look.
Sidewalk and curbing is usually found close to these walls, given that every inch on a job site is valuable. The top of the wall can be adjusted to accommodate these structures, and is generally not an issue. By maintaining a consistent 12-inch-wide cap, fence and handrail can easily be core drilled into the cap and maintain the clean look. If needed, this system can also be brought above grade to form a parapet. Parapets can be aesthetically pleasing in place of handrail, and can be designed as vehicle impact resistant to eliminate the need for guardrail.
Rubble stone gravity walls are flexible, aesthetically pleasing, and do not require perfect site conditions to install. They are also highly customizable to fit site-specific needs, clients’ tastes, and local weather impacts. These walls have been built all over the Washington, D.C., area and have stood the test of time.
Information provided by Metroplex Retaining Walls of Virginia, Inc. (www.metroplexwalls.com).