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French Drains

Introduction
The term “french drain” is commonly used to refer to shallow subsurface drainage in landscapes, parks, golf courses and athletic fields. People use the term with a wide variety of intents. The exact meaning varies from one geographic region to another and the connotation has shifted with the passage of time. In 2004 alone, nearly 600,000 people googled the term “French drain!” Generally speaking, the term has followed this course of development.

Evolution of the French drain
No, the French drain is not some exotic European drainage method. Although the method, as well as the term, is widely used in Europe, it originated in 19th Century New England. It derives its name from Judge Henry French from Concord, Massachusetts, who published Farm Drainage in 1859.

Stage 1 French drain
Early French drains, and some still today, were merely egress trenches cut through low- lying wet areas and filled with gravel, crushed rock, or fragments of brick. Trenches ranged from 6 inches in width to two feet. They were dug to the depth that desaturation was desired. This method has certainly provided for drier basements, less soggy gardens, and more usable athletic facilities for many years. However, this type of French drain had two chief shortcomings:

1) Low flow rates: These gravel filled trenches are certainly more porous than the surrounding soil and clay, but water oozes through them at a trickle at best. While eventually they can desaturate soil quite effectively, they take a long time to do so.

2) Short lifespan: These unprotected trenches can wash full of fines in a surprisingly short time. Particles of clay and silt are carried along with the water and deposited in the empty spaces until these are packed full. Drainage in these trenches eventually slows and stops.

Stage 2 French drain
Placing a clay drain tile or a slotted or perforated plastic pipe at the bottom of the gravel filled trench dramatically increases the flow rate. The water saturated aggregates readily transfer water to the drainage pipes, which in turn provide a speedy escape path. Although the flow rates differ considerably depending on the size and type of drain tile, all provide flow rates far superior to a simple rock filled trench.

Unfortunately, this improvement addressed only the first shortcoming of the Stage 1 French drain. These trenches were as prone to blockage as were their predecessors. Sometimes, especially following dry periods, even the drainage tile became blocked. Placing a drain tile in the trench did very little to extend the life of the system.

Stage 3 French drain
To address the issue of life expectancy, installers began the practice of lining the trenches with a geotextile filter fabric. These drains were also generally more selective in their choice of backfill, usually requiring uniform crushed rock.

Unfortunately, these trench liners extend the life of the French drain only marginally. They succeeded in keeping the rock and pipe clean but the fines that the filter held away from the rock tended to cake up on the fabric eventually restricting and blocking the water entry.



Stage 4: Narrow Trench Installation Technology (NTIT)

A Multi-Flow drainage system addresses all of the inadequacies of previous French drain systems employing a methodology known as Narrow Trench Installation Technology Multi-Flow has pioneered this revolution in drainage technology.
Multi-Flow is comprised of multiple, interconnected, high density polyethylene flow channels wrapped in a geotextile fabric. Multi-Flow is typically installed in a 4 inch wide trench and embedded in clean coarse sand.
Narrow Trench Installation Technology™ has several significant advantages over traditional French drains:

1) High flow rates: Not only does Multi-Flow carry water away at a rapid rate, due to its shape, but it also collects the water very efficiently. The net result is that more water leaves the saturated area in less time.

2) Long lifespan: Multi-Flow’s high quality filter wrap keeps its flow ways clear. A second level of protection comes from the sand backfill which protects the geotextile from blockage. An inch to an inch and a half of filter sand provides substantial protection from filter blinding and consequently it dramatically extends the life of the drain system.

3) Simpler and less costly installation: Traditional French drains usually employ a trench that is a minimum of 12 inches in width. A back hoe is generally used to dig the trench. A large amount of spoil is produced and a similar amount of backfill is required. NTIT systems are installed in a 4 inch wide trench created by a chain trencher. The result is less surface disruption, less spoil to remove, less backfill to bring in, and of course, less effort and cost.

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