SRW (Segmental Retaining Wall) Drainage

Water entering an SRW retaining wall system is indeed problematic. Failure to account for introduction of water into the reinforced zone of SRW walls is one of the primary causes cited during forensics as a reason for their failure. As such, water entering the reinforced zone must be accounted for during both the design and installation phase of the retaining wall to insure the wall maintains its structural integrity for the typical 75 plus year design life.

Always Include a Drainage Layer

All SRW retaining wall design and construction should include a drainage layer in the front of the wall. The most commonly used method is a 6" deep layer of gravel with a perforated pipe placed at the bottom that daylights out the front or sides of the structure. It is highly recommended that the entire system be wrapped in a lightweight drainage geotextile such as US 100NW to prevent fines from entering the system and clogging it. At the least, the pipe itself should be wrapped with a nonwoven filter fabric or sock.

US Fabrics offers AWD SITEDRAIN Strip Drains that can replace the pipe and geotextile when placed at the bottom of the gravel drainage layer. Additionally, a product such as SITEDRAIN Sheet Drains can replace the gravel entirely and connect to the Strip Drains to form a complete composite drainage system.

How Water Can Enter the Reinforced Soil Zone

Water can enter the reinforced zone of an SRW from four different areas: 1) It can enter from beneath as a result of a rising water table; 2) It can come in from behind the reinforced zone from the retained soil zone; 3) It can enter from above through surface water drainage; 4) It can emanate from the backfill soil.

Solutions

If a water table is within the reinforced soil zone, gravel backfill must be utilized for the entire reinforced zone up to the the maximum level the water is expected to rise. An geotechnical engineer with experience in SRW design and familiar with the specific site conditions should be consulted to determine the the gravel size, type and quantity required.

Water coming in from from the back of the retained soil zone will also require an experienced engineers' help. Ultimately, a chimney drain placed between the reinforced soil zone and the retained soil zone will be required to intercept the water before it enters the reinforced zone. Composite drains or a geonet with a filter fabric attached are nice fits for this application. Obviously, the chimney drain must connect to a drain system that runs under the reinforced soil zone and daylights out the front or sides of the wall.

Water flow from the top should be avoided by designing the area around the SRW so the runoff is redirected around the reinforced zone of the SRW wall. When this is not possible, an impermeable geomembrane above the reinforced zone should be utilized.

Prior to constructing the wall, review drainage plans and details with the general contractor or site civil engineer to identify all potential sources of water:

• Below grade storm sewer pipes
• Water lines, mains or fire hydrants
• Grading of site
• Parking lots
• Catch basin to storm sewer system
• Roof down spouts
• Slopes above walls

Regarding water in the backfill soil: Water will naturally dissipate from within the reinforced zone. If the above issues are accounted for this should not be a concern. If there is a unique concern, please contact an experienced engineer familiar with the site specific issues.

A properly designed SRW that accounts for water entering the reinforced soil zone will result in an economical, beautiful and long-lasting solution for you and your customers!