9.3.2 Subsurface Drainage
The purpose of subsurface drainage is to lower the water table and, therefore, the water pressure to a level below that of the potential failure surfaces. Methods of subsurface drainage include drain holes, pumped wells, and drainage galleries or adits (figure 13). Methods that can be used to accomplish subsurface drainage are
(1) Subsurface Drainage Blankets
(3) Horizontal Drains
(4) Relief Wells
(5) Drain Wells and Stone Columns
(6) Wellpoints and Deep Wells
(7) Drainage Galleries
Figure 13: Surface and subsurface drainage in the slope.
Subsurface Drainage Blankets: Of a thin layer of poor quality saturated soil at a shallow depth is present, it may be practical to remove the poor quality layer and replace it with a well-draining soil fill. The bottom of the excavation should be covered with a layer of filter fabric wrapping a 15 to 60 cm filter stone layer with a perforated pipe embedded in it to capture the flow. In order to avoid blockage of holes by vegetation, the first 1.5 m of the outlet end of the pipe should left unperforated. A drainage ditch should be installed to convey water flow from the outlet of the pipe to a suitable discharge point to minimize surface erosion.
Trenches: Deep trenches should be constructed when subsurface water or soil of unknown strength is found at such great depths that stripping of the soil is not practically feasible. Trenches are usually excavated at the steepest stable side slopes for the construction period. Trench so excavated should extend below the water- bearing layer and it should be backfilled with a layer of pervious material encased in filter fabric that has an underdrain pipe running through it. The number of adequate trenches, depends on the hydrogeology and geomorphological condition. If, the slope is in a natural depression of limited aerial extent, one trench normal to the centerline of the site may be sufficient.
Horizontal drains, sometime called Hydrauger drains are maintain to insert perforated pipes in a drilled holes of a slope to provide underground drainage. They usually slope upward into the slope to permit groundwater to drain by gravity. The drain pipes are commonly perforated or slotted PVC pipe. The drains are installed by drilling into the slope using a hollow-stem auger, inserting the drain pipe, and withdrawing the auger, leaving the drain in place. The hole is allowed to collapse around the drain pipe. There is no filter between the pipe and the soil. Flows usually decline with time after installation and then fluctuate seasonally through wet and dry periods. Horizontal drains are most effective when placed low in the slope provided that the slope does not contain distinct layers of high permeability above the drains (Figure 14 &15).
Horizontal drains can be used where the depth to subsurface groundwater is so great that the cost of stripping or placing trenches is very expensive. It should be designed specifically to lower the seepage pressures in slopes and prevent failure. The length of horizontal drains largely depends on the geometry of the zone. The length can be determined by drawing a cross section of the slope with its probable critical circle superimposed on a geologic cross section depicting aquifers. The length of the holes should extend beyond the critical failure surface. The length of the holes alos depends on the orientation to the critical discontinuities; the optimum design is to intersect the maximum number of significant discontinuities for each unit length (mater, foot, etc.) of hole drilled. If the holes tend to collapse, then perforated drain pipe should be inserted.
Installing horizontal drains is difficult in fine silty sands and soils that contain boulders, rock fragments, open cracks, and cavities. Silty sand tends to collapse and form cavities during drilling, as the initial hole is usually not cased for economic reasons. Such drains should be installed in such a position that it can be cleaned and flushed by pumping water into the drains.
Figure 14: Sub-horizontal drainage to lower groundwater levels for slope stabilization (http://www.sigra.com.au)
Figure 15: Slope Drainage methods (Duncan and Christopher, 2005)
Relief Wells: Relief wells are vertical holes with a diameter of about 40 to 60 cm. A perforated pipe of 10 to 20 cm diameter is placed inside the hole. The annular space between the borehole and the pipe should be filled with filter material. The main function of relief wells is to lower the water pressures in layers that are deep down in the subsoil. A water disposal system using a submersible pump or surface pumping and discharge channels is required to dispose the water from the wells. Disposal of the water may be very costly as effective dewatering system requires frequent maintenance. The spacing between relief wells is very important because it affects the performance and cost of the system. Spacing of 5 to 13 m are common. The depth of relief wells depends on the unstable zone in which stability needs to be improved.
Drain Wells and Stone Columns
Horizontal drains do not provide the most effective means of intercepting seepage where soil strata of varying permeability are oriented horizontally. Vertical drains, which cross the layers, are more effective in such condition. These wells can be drained using deep pumps, but the requirement for continual power and pump maintenance makes this a less desirable alternative.
Wellpoints and Deep Wells
Wellpoints are small-diameter vacuum wells driven or jetted into place. Vacuum is applied to the top of the wellpoints through a header to suck water up the wellpoints. These wells work better in clean sand and poor in fine-grained soils. Maximum effectiveness of these well is limited to 7 to 8 m. Deep wells use submerged pumps to push water to the top of the well and are not limited to a lift of 7 to 8 m. Each well has its own pump and operates independently. The wells are usually 30 to 60 cm in diameter and have filters surrounding a perforated casing. Like wellpoints, they must be operated continuously to remain effective. Dewatering wells are designed primarily to lower the ground-water level to a predetermined depth and to maintain that depth, until all below ground activities have been completed.
Where drainage is needed deep within a hillside, a drainage gallery (tunnel) can be used. Drains can be drilled outward from the tunnel, extending the drainage through the slope. Drainage adits or galleries driven under a pit or into a slope or highwall to intercept the groundwater-can provide an effective method of drainage. Where employed, drain holes should be drilled from the adit upward in a fan pattern to increase drainage effectiveness. For high rock cuts, installation of drain holes at different levels is suggested. Where rock is taken out in several lifts, drain holes should be drilled at the toe of every lift.