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NATURAL RESOURCES CONSERVATION SERVICE CONSERVATION PRACTICE STANDARD
LAND RECLAMATION
LANDSLIDE TREATMENT (No. and acre) CODE 453
DEFINITION available expertise in soil engineering is needed and expert consultants should be hired, if necessary. Treating in-place materials, mine spoil (excavated overburden), mine waste or overburden to reduce Slope stability analysis shall account for all critical soil down-slope movement. and loading conditions. The strength parameters of natural soil and rock or of waste materials shall be Scope based on the appropriate conditions for each slide. Long-term strength parameters (c=0 and internal This practice applies to landslides or potential friction based on residual shear) are of often required. landslides. The methods of slope stability analysis ate to be appropriate safety factors shall be provided based on PURPOSES the degree of uncertainty in the soil strength values used, the soil and water conditions assumed, and the To prevent or stabilize landslides to: protect life and detail of the analysis used. property; prevent excessive erosion and sedimentation; improve water quality and landscape resource quality; When there is a potential for loss of life or damage to and to create a condition conductive to establishing farmsteads, residential areas, frequently traveled roads, surface protection and beneficial land use. and occupied facilities, or important public utilities, the measure shall include removal of the material subject CONDITIONS WHERE PRACTICE APPLIES to sliding or any other control of ensure safety.
To areas where in-place material, mine spoil, waste, or Earthquake or seismic forces are to be considered on overburden is unstable, moving, or judged to have major high hazard sites. The criteria as contained in potential of moving downslope in a manner that will Technical Release No. 60 for earth dams shall apply cause damage to life, property, or the environment and for geologic investigations, seismic assessments, and produce excessive sediment and debris. Land minimum seismic coefficients associated with reconstruction is normally associated with this practice. earthquakes.
CRITERIA Water control
In most cases the unstable or potentially unstable Water creates problems in two ways. The addition of conditions cannot be attributed to one cause. water to the material above the slope zone increases the Therefore, the solution is usually a combination of load. It also acts as a lubricant, or increases pore treatment measures, each either increasing the internal pressure within the slide material and in the slope area, strength or decreasing the external load to the point thereby reducing internal strength. In both cases water where required stability is obtained. increases the potential for sliding.
Slope Stability There are three major sources of water within the slide area, surface runoff that finds its way onto the slide Measures developed to prevent or stabilize slides shall area, precipitation directly on the surface, and be based on engineering analysis and judgment made subsurface water from known or unknown source. A by an engineer trained and experienced in soil combination of these sources usually contributes to the mechanics. Slides are the most complex of excessive water problem. geotechnical problems requiring analysis. The best
Conservation practice standards are reviewed periodically, and updated if needed. To obtain the NRCS, CA current version of this standard, contact the Natural Resources Conservation Service. July, 2000
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Surface runoff water toe (downhill slide) of the slide usually will increase the instability and cause further slide movement. Runoff water from outside areas is to be controlled by using diversions, associated structures, and conveyance Slope reduction systems. Slopes can sometimes be reduced be grading and Water from, direct precipitation shaping to eliminate critical slopes within the slide area. It can also be reduced as a result of loading Infiltration can be limited and controlled by providing control measures. positive surface drainage, sealing the surface cracks and breaks on the slide and adjacent areas, and Increasing internal strength establishing vegetation. Grading and shaping may be required to provide positive surface drainage. Terraces Reducing the internal water of the slide material, structures, and waterways are to be installed as needed removing or replacing the slide material, incorporating to provide safe water disposal without erosion and with any admixture needed into it, and compacting it can positive grade to reduce seepage. Cut and fill to a depth increase the internal strength to resist a tendency to of 0.9 to 1.2 m (3 to 4 ft) may be required to reduce slide. surface infiltration and seal cracks and breaks. Compaction of the material will further reduce External restraints infiltration, but care must be taken to prevent excessive compaction that would restrict vegetative In some cases. buttresses, bulkheads retaining walls, establishment. Establishing a vigorous vegetative piling, tieback anchors, and gabions can be used to cover will increase evapotranspiration and control restrain further slide movement. These structures may erosion. provide the only practicable solution where high- valued improvements are involved and movement must Ground water be contained in a short distance. The structures are normally very expensive and are usually not Ground water that contribute to instability is to be practicable otherwise. They also require complex controlled. Many slides remain active during design analyses, using the expertise for geologists, soil reconstruction and further movement can be expected, mechanics engineers, and structural engineers. Therefore, drainage systems are to be designed to remain operative after limited movement. Pipes must Component practices be used with caution because of the potential of breaking and/or misalignment with further movement. All individual practices installed as a component of Flat or nearly flat gradients should not be used for the landslide treatment are to be designed and installed in same reasons. A properly designed filter shall be used accordance with applicable NRCS standards and to prevent clogging of the drains. specifications. If NRCS standards are not available, the practice if to be designed and installed using Earth material control current engineering technology.
Earth material and internal water are the load factors Environmental that contribute to the unstable conditions that cause slides. Treatment consists of removing earth material All distributed areas are to be provided with adequate to reduce the load and slope, increasing the internal water disposal systems and established to vegetative strength of the earth material, and providing external cover, or otherwise protected, to control erosion and restraints to movement. sediment as soon as practicable. Temporary protective measure will be necessary if long delay is anticipated Loading control in establishing permanent cover. Foot and vehicular traffic is to be controlled to protect the area. In most cases loading control consists of removing excess material to a safe location. However, in some Visual resources are to be given the same consideration instance the solution may be adding a material to the as other design features during planning, design, and toe of the slide area to increase the load, resisting installation. All disturbed areas shall be reshaped and further movement. Removal of slide debris from the regraded to blend in with the surrounding land features.
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CONSIDERATIONS Investigations
1. Geology of the area and associated subsurface Investigations are to be made to determine: conditions. 1. Surface profiles, cross sections, and topographic 2. Type and amount of spoil or waste. features.
3. Topography of the slide and adjacent areas 2. Geologic profiles and cross sections showing including known estimated pre-mine, attitude and conditions of strata and details preconstruction, or pre-slide conditions. of the slip zone.
4. Surface drainage and runoff patterns. 3. Soil properties, including gradation, density, strength, and physical and chemical characteristics. 5. Groundwater profiles, seepage patterns, and sources of subsurface water. 4. Ground-water conditions
6. Land use, dwellings, roads, structures, and water 5. Depth and volume of material involved. disposal systems. 6. Extent of problem or potential problem area. 7. Procedures used during mining operations or construction. 7. Estimated preslide profile and subsurface conditions. 8. Slide potential during investigation and construction. 8. Conditions where slopes are stable in similar materials. 9. Rainfall and runoff. Water Quantity Landslides result from a combination of several factors, the most important being static load, slope of 1. Effect on and discharge capacity of water courses the surface and slip zone, the soil characteristics in the affected by the landslide. slope zone, and the presence of water. The key to control is to bring about a favorable balance between 2. Water budget effect on volume and rate of runoff, the load that created the tendency to move and the evaporation, deep percolation, and ground water resisting forces that restrain movement. This can be recharge. done by reducing the load, reducing the slope, increasing internal strength, and providing the slope, 3. Potential for a change in plant growth and increasing internal strength, and providing restraining transpiration because in the amount of soil forces. A good reference on landslides is the moisture in the vicinity of the structure. publication "Landslides: Analysis and Control," 1978. Transportation Research Board (TRB), National Water Quality Academy of Sciences, Special Report 176, 234 p. The 1996 publication "Landslides: Investigation and 1. Potential to reduce erosion and related movement Mitigation", TRB Special Report 247, 673p., represents of sediment or sediment-attached substances. an update of Special Report 176. 2. Short-term and construction-related effects on Extreme caution must be exercised and careful downstream water courses. planning is required before permitting any drilling equipment, construction machinery, or personnel in the 3. Potential to alter the discharge of toxic materials to slide area. A slide is often active only during wet ground or surface waters. periods and may be comparatively stable during dry periods. With this in mind, heavy drilling and 4. Effects on the visual quality of water resources. machinery work should be scheduled during dry periods.
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Endangered Species Considerations
Determine if installation of this practice with any others proposed will have any effect on any federal or state listed Rare, Threatened or Endangered species or their habitat. NRCS's objective is to benefit these species and others of concern or at least not have any adverse effect on a listed species. If the Environmental Evaluation indicates the action may adversely affect a listed species or result in adverse modification of habitat of listed species which has been determined to be critical habitat, NRCS will advise the land user of the requirements of the Endangered Species Act and recommend alternative conservation treatments that avoid the adverse effects. Further assistance will be provided only if the landowner selects one of the alternative conservation treatments for installation; or at the request of the landowners, NRCS may initiate consultation with the Fish and Wildlife Service, National Marine Fisheries Service and/or California Department of Fish and Game. If the Environmental Evaluation indicates the action will not affect a listed species or result in adverse modification of critical habitat, consultation generally will not apply and usually would not be initiated. Document any special considerations for endangered species in the Practice Requirements Worksheet.
PLANS AND SPECIFICATIONS
Plans and specifications for slide treatment shall be in keeping with this standard and shall describe the requirements for applying the practice to achieve its intended purpose.
OPERATION AND MAINTENANCE
The maintenance plan is to include periodic inspections because of the potential for additional movement. Failure of water disposal systems, failure of vegetation, and other problems. The water disposal system, subsurface drainage system, access roads, and vegetative cover are to be maintained to accomplish their intended purposes. Necessary maintenance and repair activities are to be initiated promptly.
NRCS, CA July, 2000