Management

Development and management of unstable slopes

Slip, Pohangina

Landslides generally involve relatively small losses over a wide area which means that there has been no concerted effort made to recognise landslide hazards. If the landslide prone areas are identified then these regions can be avoided. However, this has not been the case, and many unstable slopes have been developed. The highest risk areas are in cities where the demand for land means urban development has encroached onto steeper, unsuitable slopes. In general the cost of preventing landslides is less than the cost of correcting them (and certainly less than the costs associated with catastrophic slope failure).

This next section outlines some of the methods used to reduce the financial burden associated with slope instability, including planning studies, land-use controls, landslide mitigation and landslide prediction

Planning studies

Slope stability analysis is generally conducted at three different levels:

(1) regional, (2) local and (3) site
(Costa and Baker, 1981, p. 275-280).

Regional analysis takes the form of landslide hazard zonation, described in the previous section. At the local scale more specific information about the landslide causes and solutions is required, which establishes guidelines for zoning and construction practices.

At the site level detailed site evaluations may involve boreholes, sampling and soil mechanical analysis all of which will be used to develop a safe design for property development. Activities such as mining and quarrying produce large quantities of loose material that are often piled up next to the operation and these are a potential hazard.

One of the most quoted failures of these spoil heaps or tips occurred at the village of Aberfan coal pit in Wales, when a large 60 m high pile, 156 m above the village collapsed and killed 144 people including 116 children in a school (Costa and Baker, 1981, p. 274). Dams constructed from unconsolidated material are also potentially hazardous - in 1972 a coal waste dam in a tributary of Buffalo Creek, West Virginia, failed flooding the valley and killing 118 people. The cost of this disaster was set at US$50 million. These hazards are now well-recognised and planning should be made to prevent these events happening again.

Landuse Controls

In general the impacts of landsliding are felt locally and the costs of repairs and maintenance are the responsibility of local government. This means that it is in the interests of local government to put in place landuse controls which will help to maintain slope stability in low risk areas, avoid development in high risk areas and avoid development which might act to destabilise previously stable slopes.

Implementing legislation to achieve these goals is of vital importance in the eventuality of slope failure and subsequent insurance claims. Slope failure can be disastrous, and the examples discussed in this study guide show that many lives have been lost as a result. Landsliding also results in damage to property and possessions. Furthermore, the property damaged may not be developed on an unstable slope, but in the path of debris slides or flows created upslope.

Mitigating existing landslides

Figure 9.

There are a number of methods which can be used to treat existing landslides (see Table 9 and Figure 9). Water is the most common cause of mass movements and control of water movement in unstable sites and improving the drainage of the site is one of the most effective and least expensive way to reduce the landslide hazard. Surface water should be diverted from the sites through culverts and groundwater level lowered through pumping and draining. Both culverts and drains must be kept clear of silt and excess water. Covering the site with plastic sheeting allows surface water to runoff without being absorbed into the unstable slope material. Planting trees is also a useful corrective method. Forested slopes often become unstable after fires burn the trees growing on them. When the trees are alive they remove water from the soil to the atmosphere through evapotranspiration. Retaining structures are the second most commonly used corrective measure used after drainage control. Walls and buttresses built at the toe of slopes give them extra support. The instable material can be secured by driving piles into unconsolidated material, or by pinning loose rocks to the rock wall with bolts or pins. Steel mesh can be spread over rock faces to prevent loose rock from falling. Reducing the load at the head of unstable slopes, e.g. by removing buildings, and /or reconstructing the slope, will improve slope stability. Slope angle can be lowered or in extreme cases the entire unstable mass can be excavated.

Other corrective methods include hardening the soil, electro-osmosis (accomplishes the same result as drainage), ionic exchange (influences the strength of clay), freezing the soil water and grouting with cement.

Table 9. The type and effectiveness of remedial measures on different slope instabilities/mass movements (from Costa and Baker 1981).

Method of treatment	General use		Frequency of successful use
	Pre- vention	Corr- ection	Fall	Slide	Flow	Position of treatment on landslide	Best applications and limitations
I. Avoidance methods. Effect on stability of landslide: Not affected
A. Relocation	x	x	2	2	2	Outside slide limits	Most positive method of alternative location economical
B. Bridging	x	x	3	3	3	Outside slide limits	Primary highway applications for steep, hillside locations affecting short sections (parallel to c/L)
II. Excavation Effect on stability of landslide: Reduces shearing stresses
A. Removal of head.	x	x	N	1	N	Top of head	Deep masses of cohesive material.
B. Flattening of slopes.	x	x	1	1	1	Above road or structure	Bedrock; also extensive masses of cohesive material where little material is removed at toe.
C. Benching of slopes	x	x	1	1	1	Above road or structure	Relatively small shallow masses of moving material.
D. Removal of all unstable material	x	x	2	2	2	Entire slide
III. Drainage Effect on stability of landslide: Reduces shearing stresses and increases shear resistance.
A. Surface: 1. Surface ditches	x	x	1	1	1	Above crown	Essential for all types.
2. Slope treatments	x	x	3	3	3	Surface of moving mass	Rock facing or permeable blanket to control seepage.
3. Regrading surface	x	x	1	1	1	Surface of moving mass	Beneficial for all types.
4. Sealing cracks	x	x	2	2	2	Entire, crown to toe	Beneficial for all types.
5. Sealing joint planes and fissures	x	x	3	3	N	Entire, crown to toe	Applicable to rock formations.
B. Sub-drainage: 1. Horizontal drains	x	x	N	2	2	Located to intercept and remove subsurface water	Deep extensive soil mass where ground water exists
2. Drainage trenches	x	x	N	1	3		Relatively shallow soil mass with ground water present
3. Tunnels	x	x	N	3	N		Deep extensive soil mass with some permeability
4. Vertical drain wells	x	x	N	3	3		Deep slide mass, ground water in various strata or lenses
5. Continuous siphon	x	x	N	2	3		Used principally as outlet for trenches or drain wells
IV. Restraining structures Effect on stability of landslide: Increases shearing resistance.
A. Buttresses at foot. 1. Rock fill	x	x	N	1	1	Toe and foot	Bedrock or firm soil at reasonable depth
2. Earth fill	x	x	N	1	1	Toe and foot	Counterweight at toe provides additional resistance
B. Cribs or retaining walls	x	x	3	3	3	Foot	Relatively small moving mass or where removal of support is negligible
C. Piling 1. Fixed at slip surface 2. Not fixed at slip surface
D. Dowels in rock		x x	N N	3 3	N N	Foot Foot	Shearing resistance at slip surface increased by force required to shear or bend piles
E. Tie-rodding slopes.	x	x	3	3	N	Above road or structure	Rock layers fixed together with dowels
	x	x	3	3	N	Above road or structure	Weak slope retained by barrier which in turn is anchored to solid formation
V. Miscellaneous methods: Effect on stability of landslide: Primarily increases shearing resistance.
A. Hardening of slide mass 1.Cementation or chemical treatment (a) At foot		x	3	3	3	Toe and foot.	Non-cohesive soils.
(b) Entire slide mass	x	x	N	3	N	Entire slide mass.	Non-cohesive soils.
2. Freezing			N	3	3	Entire	To prevent movement temporarily in relatively large moving mass
3. Electro-osmosis	x		N	3	3	Entire	Effects hardening of soil by reducing moisture content
B. Blasting		x	N	3	N	Lower half of landslide	Relatively shallow cohesive mass underlain by bedrock. Slip surface disrupted; blasting may also permit water to drain out of slide mass
C. Partial removal of slide at toe	-	-	N	N	N	Foot and toe	Temporary expedient only; usually decreases stability of slide

Landslide prediction and warning

Although it is unlikely that the occurrence of a landslide will be predicted to the minute or second certain factors can be used to predict or warn of likely future sliding. Engineers can calculate stability factors based on the engineering properties of the slope. The problem with this approach is that the results of an instability survey are only applicable to the particular slope, or part of a slope, that is analysed. A geologist or geomorphologists can attempt to identify and date old relic landslides in the same area. However, in this case the conditions that caused the old landslide may no longer exist, in which case comparing the old event to the modern slope is not useful.

The movement of slopes can be measured either using strain metres (which may be fence posts) which identify pressure being placed on them from upslope and electronic circuits which break during rock slides.

Heavy rainfall is a common trigger of landslides and through careful analysis of past records a comparison between rainfall duration and intensity and the occurrence of landslides can be made. In general there is a relationship between the amount of rainfall over a medium length of time, which will be absorbed into the soil, and increased rainfall in the short term, which brings the soil to field capacity. In the San Francisco Bay region the largest number of landslides occur during storms in which more than 150 mm of rain falls on steep slopes (> 15⁰) in areas where 250 - 380 mm of rain has already fallen. Another factor relates to the ability of the soil, or regolith, to absorb the water percolating through from the surface. If the rainfall intensity is so hight hat the water cannot escape downwards through the regolith then a perched water table develops, increasing the pore water pressure in the material. If this forms above the failure surface, shear strength reduces and failure may occur.

Thus, by carefully monitoring weather patterns and rainfall intensity using rain gauges in combination with slope maps (or slope stability maps) warnings can be issued to hazardous areas during heavy rain.

Summary

Movement occurs in natural slopes when the total disturbing forces applied to the soil or rockmass at risk exceeds the total strength of the ground (Taylor et al. 1977). This can be expressed as the safety factor (N) where :

The disturbing forces are the sum of:

1. The weight of the soil/rock mass
2. The weight of water in the soil/rockmass
3. Seepage drag forces (usually act downhill)
4. Earthquake induced forces
5. Superimposed loads, e.g. filling and buildings

The resisting forces are the product of the shear strength of the soil/rockmass, i.e. the factors that bind the particles together and prevent the mass from sliding, whether they be frictional, chemical or electrical, and the surface area of the failure surface or potential failure surface.

This resistance can be reduced by:

1. Excess pore pressure which reduces frictional contact between particles
2. Excavation or erosion of the toe of the slope which removes support
3. Weathering which weakens the soil/rock mass
4. Long-term small-scale strain movements which also weakens the mass
5. Removal of cementing material in the ground by seepage
6. Drying, shrinkage and cracking
7. Removal of vegetation and/or killing of roots that bind the soil/rock mass
8. Ground shaking, especially seismic vibration
9. Subsurface erosion by ground water flow

Landslides are one of the most manageable and predictable natural hazards. The list below identifies a general scheme for reducing the consequences of this natural hazard:

1. Conduct scientific and engineering studies of the physical processes to identify the location, size, recurrence interval, severity, triggering mechanism, path and ground response.

2. Translate the results of these studies into regional and local reports and maps, e.g. instability analyses and landslide-susceptibility/ hazard zonation, which can be understood by laypeople.

3. Transfer this translated information to those who will be or are required to use it, e.g. councillors, planners and building contractors, and help them to use the information through educational, advisory and review services.

4. Select and use appropriate landslide hazard-reduction techniques, e.g. drainage, retaining structures, piles and grouting, through legislation, regulations, design criteria, financial incentives and public or corporate policies.