4. Landslide Mitigation Works

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 line (Fig.39),(Fig.40).
• Drainage Wells
  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

• Drainage Tunnels
  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).

e)River Structures

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

a)Pile 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 concrete (Fig.48).

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).