Geoscience Reference
In-Depth Information
destruction with the help of accumulated surface and sub surface water. The
through flows are highly active in these regions and a high rate of 5
12 m/hr of the
subsurface flow even during spring has been observed in eastern Himalayan slope.
Water entering into the soil few meters upslope is released on the scar faces making
the slope instable. Generally a series of near parallel drains are to be constructed
above the gully head to collect the water to restrict it to take part in destruction.
These should have a non-eroding graded gradient with a cemented mortar and
should be drained into a non-erosive channel. In a sinking zone and on highly
permeable rocks, closure spacing of the drains are necessary. At Pagla Jhora region
such Catch Water Drains are to be constructed for arresting the surface water from
the large upper catchment and may be diverted to either 2nd or 4th sub-watershed.
-
8.3.6 Continuous Monitoring of Sub-surface Water
For ef
cient and effective planning for drainage arrangement in this distress area,
knowledge on the pore water pressure is needed. Thus, installation of Piezometer on
slopes at different levels in Paglajhora sinking zone is necessary. To assess the slope
instability condition, a network of monitoring instruments could be installed in
order to provide, together with others climatic data, the piezometric response deep
inside the landslide and to measure the rate of deformation at several points in the
interests of public safety (Angeli et al.
1994a
,
b
). The installation of high-strength
steel anchors and sub-horizontal drains are to be drilled into the slope to assist in
removal of ground water in accordance with an engineering design just in the
sinking area of Lower Paglajhora, Shiviter tea garden and Nurbong Tea Estate.
Basically, the sub-surface drainage network can be managed by the following
good engineering practice.
Introduction of sub-surface drain in the sinking zone and provision of
lter
around subsurface drain.
Provision of drain behind retaining walls.
Use of flexible pipelines with access for maintenance.
Prevention of inflow of surface water.
8.3.7 Landslide Mitigation by Improving Soil Strength
Usually, a simple method for increasing the shear strength of soil is to reduce the
pore water pressure and that can be accomplished by reducing the sub-surface
horizontal and downward flow of water through applying the following methods.
Search WWH ::
Custom Search