Landslide mitigation works are conducted in order to stop or reduce
the landslide movement so that the resulting damages can be minimized.
With a clear understanding of the causes and mechanics of the landslide,
the landslide control works can be implemented according to the following
flow chart (Fig.33).
The landslide mitigation works are broadly classified into two categories:
1) control works; and 2) restraint works. The control works involve modifications
of the natural conditions of landslides such as topography, geology, ground
water, and other conditions that indirectly control portions of the entire
landslide movement. The restraint works rely directly on the construction
of structural elements.
Specific measures included in the control works and restraint works are
listed in Fig.34.
Landslide Control Works
a) Surface Drainage Control Works
The surface drainage control works are implemented to control the movement
of landslides accompanied by infiltration of rain water and spring flows.
The surface drainage control works include two major works: drainage collection
works and drainage channel works. The drainage collection works are designed
to collect surface flow by installing corrugated half pipes or lined U-ditches
along the slopes, and then connected to the drainage channel. The drainage
channel works are designed to remove the collected water out of the landslide
zone as quickly as possible, and are constructed from the same materials
as the drainage collection works. The surface drainage control works are
often combined with the subsurface control works (Fig.35)(Fig.36)(Fig.37).
Fig.37 Surface drainage ditches
b) Subsurface drainage Control Works
The purpose of the subsurface drainage control works is to remove the
ground water within the landslide mass and to prevent the inflow of ground
water into the landslide mass from outside sources. The subsurface drainage
control works include shallow and deep subsurface drainage control works.
Intercept Under Drains and Interceptor Trench Drains
These systems are most useful to remove shallow ground water from up to
3m from the ground surface. The interceptor under drains contain impervious
sheets at the bottom of the trench, and the gravels are wrapped with filter
fabric and the drains are connected at groundsils and catch basin.
Structurally, the interceptor trench drain is a combination of the interceptor
under drain and surface drainage control, and are commonly used (Fig.38).
Horizontal Gravity Drains
In order to remove the shallow groundwater within about 3m from ground
surface, 30 to 50 m-long horizontal gravity drains are drilled. The pipes
could be either perforated P.V.C. (polyvinyl chloride) or steel construction,
and should be drilled at an angle of 10 to 15 degrees from the horizontal
Drainage wells of up to 25m deep and at least 3.5m in diameter are excaveted
within areas of concentrated ground water. A series of radially-positioned
horizontal gravity drains with multi-levels are drilled to collect the
ground water into the drainage wells where the water can be removed through
drainage tunnels. They are constructed of either steel or reinfored concrete
segment, and concrete is used at the well bottoms and the upper portion
of the well (Fig.41),(Fig.42),(Fig.43).
Fig.42 A drainage well usnig reinforced concrete segments
The primary purpose of the drainage tunnels (which are constructed below
the slide plane) is to remove collected water out of the landslide mass
by interconnecting the drainage wells. Instead of excavating the drainage
wells from the ground surface, they can be constructed upward from the
drainage tunnels. The series of gravity drains drilled from the tunnel
tends to increase the effectiveness of the drain system. This is the most
effective and reliable drainage work where numerous ground water veins
exist within the landslide mass (Fig.44),(Fig.45).Furthermore,
this work is effective to maintain existing facilities.
Generally, the diameter of the tunnel is between 1.8 and 2.5m, and the
drainage channel is constructed along the invert.
c) Soil Removal Works
This is one of the methods where the most reliable results can be expected,
and generally applies to small to medium sized landslides. Except for special
cases, the soil removal is focused on the head portion the slide (Fig.46).
d) Buttress Fill Works
The buttress fill is placed at the lower portions of the landslide
in order to counterweight the landslide mass. It is most effective if the
soils generated by the soil removal works are used (Fig.47).
Degradation and channel bank erosion reduce earth stability and often
tends to induce slide activity. In such cases, check dams, groundsils and
bank protection can be constructed to prevent further erosion.
Landslide Restraint Works
The pile works consist of driving piles as keys to tie together the
moving landslide and the stable ground to restrain the movement. Generally,
a thick walled steel pipe is used as the pile, and is then filled with
b) Large Diameter Cast-in-Place Pile Works
The large diameter cast-in-place works function similar to those of
the pile works and are designed to tie the moving landslide and the stable
ground together. However it involves much larger diameters. The construction
is similar to the drainage well, and generally consists of pile of 1.5
to 6.5m in diameter and filed with reinforced concrete. compared to the
piles, the large diameter cast-in-place type are much more resistant to
bending stresses (Fig.49).
c) Anchor Works
The anchor works utilize the tensile force of anchor bodies embedded
through the slide mass and into stable earth, and are connected to thrust
blocks located on the ground surface. The thrust blocks are anchored with
a tendon that counteracts the driving forces of the landslide to restrain
the slide movement. The advantage is that large restraint forces can be
obtained from a relatively small cross sectional tendon (Fig.50).
d) Retaining Walls
Retaining walls are constructed to prevent smaller sized and secondary
landslides that often occur along the toe portion of the larger landslides.
Because of the large-scale earth-movement and numerous springs that are
expected in landslide terrain, crib walls are common instead of conventional
reinforced concrete retaining walls (Fig.51).