Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 246

are sometimes used at the toe of a slide in order to lower the water table
and create a favorable mechanical effect: masking earthworks made up
of draining material, implanted along topographic lines, or herringbone
drains, arranged following the line of greatest slope.
2.3.3 Subhorizontally drilled drains
These are widely used, most often in combination with other supporting
methods. They consist of small-diameter perforated tubes (generally made of
metal or of PVC), which are installed in boreholes sloping slightly upwards,
over lengths reaching or exceeding one hundred meters. Their principal
advantages are their fastness of construction, their great fl exibility in terms
of location and orientation, and the gravitational outfl ow of the collected
discharge. Their durability, however, is unfortunately not guaranteed, they
can become less effective over time:
• either as an effect of the reactivation of the landslide, causing the
rupture of drainage pipes, especially if they are made of PVC, and
therefore the reinjection of water into the moving mass;
• or as an effect of gradual clogging of the drains, particularly when the
water has a high mineral content and when it carries fi ne particles or
when the fl ow is too irregular.
These subhorizontal drains are generally drilled in diverging arrays
from a small number of specially equipped work chambers, which allows
a sweep of the entire landslide, sometimes relying on a succession of arrays
at different elevations.
When the zone to be drained is too deep, or the surface topography is
too gentle, the drains can be drilled from a specifi c work chamber encased
in the terrain, accessible via a short passage or a well.
2.3.4 Vertically drilled drains
Their use is less frequent, but they can complement other stabilization
mechanisms.
They consist of perforated tubing installed in vertical boreholes and
designed to inject water from the superfi cial aquifer into a deeper reservoir.
This operation requires a preliminary understanding of the deeper aquifer's
outlets, and verifi cation of its absorption capacity and peak piezometric
variations, in order to avoid an effect opposite the one desired due to sizeable
increases in hydraulic head during high-water periods.