Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 245

justifi es them and when the surface in question allows them. The goal is
then to limit the infl ow of water in fractures and slope reversals (plugging
of fractures, local impermeabilization with polyane sheets, leveling of the
unstructured surface) and to encourage the outfl ow of water accumulated
on-site (opening of springs, creation of drainage ditches).
Superfi cial drainage can contribute to a return to equilibrium, when
runoff or sub-superfi cial fl ow originating upslope contributes in large part
to the supply of water. This is especially the case when the dominating
bedrock allows an important, concentrated runoff towards the sensitive
zone, either naturally (large impermeable catchment area), or due to human
activity (intensive irrigation, voluntary dumping of storm water runoff,
breaks and leakages from irrigation canals and buried pipelines).
The uphill drainage of these infl uxes and their lateral collection towards
stable outlets often allow a distinct amelioration of the situation.
2.3.2 Drainage ditches
This mechanism is often used when the aquifer horizon does not extend
below a depth of a few meters. It consists of excavating ditches, then fi lling
them with clean draining material, after having emplaced a perforated
pipe in the bottom, and with measures to prevent future clogging (choice
of particle size respecting fi ltration rules, or interposition of an anti-
contaminating geotextile). These ditches must have a high longitudinal
slope and be connected to a gravitational collection system.
Such projects are sometimes built to follow topographic lines in order
to intercept the groundwater fl ow from further upslope and to collect them
laterally, out of the sensitive zone. This geometry must nevertheless not be
recommended, due to the real risk of reinjection of the collected discharge
into the body of the landslide, through leaks due to local movements or to
poor maintenance.
In most cases, drainage ditches are opened along the line of the steepest
slope, or structured in oblique herringbone patterns connected to collection
ditches following the direction of the slope. Their purpose is to lower the
piezometric level by cutting across the maximum number of water-bearing
horizons or lenses.
These ditches can have very large dimensions when the stakes justify
it and when the depth of the slide allows it. Such is the case for the ditches
bulldozed down to the shear surface and fi lled with draining material.
They can reach tens of meters deep, and they are then essentially buttresses
encased over the entire height of the slide and adding a sizeable mechanical
reinforcement to the effects of the drainage (example of the Ardisson
landslide, see chap. D2-5). In a similar line of thought, other mixed structures