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