Do you have standing water or muddy places in your lawn or yard? A slope on your project that is directing rain water toward a home or building, or maybe a parking lot or driveway with standing water every time it rains? If any of these scenarios sound familiar, then you have a water management problem. Adding a retaining wall and proper drainage options to your project is your solution. Water is an important resource, but in the wrong places can be a very destructive force. In fact, 90% of all wall failures are caused by improper water management. This is why it is important to consider where the water on your project goes both during and after your retaining wall is completed.
Fighting the War on Water
Before we can begin to wage a war against water on a project, we must first determine the type of water present. Only then can we develop the solutions available to fight the problem. There are two basic types of water found on any given landscape project.
- Surface Water
Surface water can be defined as a water source above the ground. These water sources will need to be identified and taken into account during the design of the wall. Slopes above the wall, driveways, parking lots, roof downspouts, irrigation systems and outdoor faucets are all examples of surface water.
Groundwater can be defined as water that occurs within the soil. Sources include surface water infiltration, water table fluctuation and water moving above the layers of permeable soils.
Groundwater movement must be managed before encountering the wall structure, including the soils behind the wall. Typically this type of water is not an issue on most projects, however, if during the excavation process you find water in your trench, be sure to seek help from a local engineer to ensure that the groundwater has been accounted for in your design. Now that we know what types of water there are we can determine how best to manage them. There are three main ways to address the water on a site:
- Site Grading
- Gravel Fill Zone
- Chimney & Blanket Drains
The main objective in any wall project where water is an issue is to divert the water away from the reinforced soil mass. Surface run-off is typically handled by diverting the water using berms or swales above the wall or by grading the site so that the normal flow of water is away from the back of the wall. The gravel fill zone is comprised of a clean well-graded gravel material that provides many positives to the design and function of your wall. Its primary function is to provide a quality material for compaction in and around the facing. Also, it aids in the connection with the geogrid and shear between block courses.
One of its secondary functions would be to provide a route for any incidental water that may get into the wall. The gravel fill provides an area behind the block and in the cores where incidental water can easily drop though to the toe drain and keep hydrostatic pressure from building up behind the wall. Toe drains are an important part of managing this incidental water. All drains need to be either vented to daylight or attached to an underground drainage system.
Chimney & Blanket Drains
A chimney or blanket drain is another line of defense in our war with water. This drain consists of a blanket of clean stone beneath the wall and a column of clean stone behind the geogrid reinforced structure. This allows the water to migrate downward toward the base of the wall where it can enter the drain pipe and make its way away from the wall.
Now that we know what types of water there are and the different ways we can combat them, we can better prepare for the water that will inevitably be present on the project site. As with any project, make sure you consult with your local engineer to ensure the water is accounted for within the design. Water will be defeated and the project will be the victor!
Resource: www.ncma.org – Concrete Masonry designs – June 2009