Groundwater Interceptor Drains

An interceptor drain is a gravel trench that is excavated into a relatively impermeable soil layer and installed to collect and remove groundwater as it flows across the impermeable layer as shown in Figure 12 (below). The trench is typically placed across a contour of a slight to moderate sloping area to intercept groundwater prior to influencing slope stability. Generally, trenches are constructed 2 to 3 feet wide and are lined with a quality geotextile that does not clog. There is a one to two foot overlap of the geotextile above the gravel and below the backfill in the trench. Water carried by the trench pipe should be conveyed to a tightline (solid pipe) which transfers water down the slope to an appropriate discharge point. Trenches can be excavated with curves and bends to prevent cutting tree roots and hitting underground utilities. It is wise to confirm utility locations before you dig. Trenches can be covered with topsoil and replanted to conform to your existing ground conditions. Do not forget about safety during construction. Trenches over four feet deep (on level ground) should be protected against collapse.
 

Figure 12. Interceptor Drain.


Limitations:

Trenches should not be greater than 4 feet deep without shoring to hold up the sidewalls and protect workers in the trench. Shoring is typically provided by a "trench box." Alternatively, the side of the trench could be sloped back to a flatter, more stable configuration. Trenches should be set back at least 20 to 30 feet from the slope crest. Technical assistance should be obtained where trenches are deeper than 5 feet below grade and within areas closer than a distance equivalent to the slope height measured horizontally from the slope crest. Additionally, if you can stand over the proposed trench location and look upslope and see the slope at your eye level within 20 feet (about 4H:1V), you should obtain technical assistance to evaluate the slope stability consequences of trenching at different depths and locations. You should not consider trenching until you are into a dry weather period and slope conditions reflect the dry conditions. Also, you must be aware of the location of waste-water treatment systems so you do not inadvertently collect incompletely treated wastewater effluent.

Getting heavy equipment onto a slope face is usually risky and impractical without significant disturbance to existing soils and vegetation. To reduce the potential for initiating a landslide, limit the length of trench open at any one time. If you have a 10 foot high slope behind the trench and you open a trench ten feet deep you essentially have 20 feet of overburden that may fall into the trench opening. This situation is dangerous and should not be taken lightly. Get technical assistance when trenching on slopes.

Advantages:

Good option to intercept groundwater which is perched above a relatively impermeable soil. Good coverage technique. Construction technique is widely practiced.

Disadvantages:

Pipes in the system are often undersized. Must be able to slope pipe in trench. If not properly backfilled and compacted, surface water can flow into trench and cause additional problems for the slope and drainage system. The tightline trench which connects with the interceptor trench should be adequately sealed and compacted to prevent water from moving down the trench to the slope face.

French drains are sometimes confused with interceptor drains. However, French drains generally use large gravel without a pipe and the trench itself conveys the water across and down the slope. Consequently, the trench must be sloped. These drains must still convert trench flow to pipe flow in order to get drainage down the slope. French drains are not generally the system of choice for slope applications.

Applicability

Compatibility