Strip or Sheet Drain Vs. French Drain

Q: What are benefits to using “strip or sheet drain” over conventional French drain construction with soils?

A: Sheet drains (prefabricated vertical drain board or drainage geocomposites) are typically used on below-grade foundations or retaining walls to drain water and eliminate hydrostatic pressure. Sheet drains create a void against the wall, allowing subsurface water to seep through the filter fabric (geotextile) dropping down to a drain pipe system. Sheet drains may be placed in vertical or horizontal strips and held in place temporarily with screws, nails or panel adhesive until backfill is placed. Adjacent sheet drains need to be overlapped and extra fabric on the sheet edges need to be adhered with glue, tape, sewing, or hot-air or wedge seaming. Sheet drains are also used as protection over waterproofing (barriers) and can relieve earth pressure on structures. When using slightly compressible sheet drains, they yield a bit and change the soil stress state from passive to active pressure. 

The following are advantages of sheet drains over conventional French drains: 

• The manufactured quality control of geosynthetics in a controlled factory environment is a great advantage over soil and rock construction. Most geocomposite factories are ISO 9000 certified and have their own in-house quality programs as well. 
• The low thickness of geosynthetics, as compared to their natural soil counterparts, is an advantage insofar as light weight on the subgrade, less airspace used, and avoidance of quarried sand, gravel and clay soil materials. 
• The ease of geosynthetic installation is significant in comparison to thick soil layers (sands, gravels or clays) requiring large earthmoving equipment. If approved, the smaller trench results in labor and material savings, as the usual gravel and pipe are eliminated. Also, time is money and strip drains will be installed much faster. 
• Finally, when comparing geosynthetic options to alternative natural soil alternatives, there are usually cost advantages (note: transportation costs have tripled in the last year) and invariably sustainability (lower CO2 footprint) advantages to be realized on most construction sites.