LChapter 17 ROBERT D. HOLTZ AND ROBERT L. SCHUSTER STABILIZATION OF SOIL SLOPES 1. INTRODUCTION 2.DESIGN CONSIDERATIONS he basic principles for design and construc- Several factors are basic and must be considered in Ttion of stable slopes in soils are quite well the design of stable slopes. First, because of the known. The engineering properties of soils as they nature of soils and the geologic environments in relate to slope stability are generally understood. which they are found, virtually every slope design Analysis capabilities for slope stability have im- problem is unique (Peck and Ireland 1953;Hutch- proved markedly in recent years because of the inson 1977). Second, the procedures used to esti- digital computer. In this report Parts 2, Investi- mate the stability of an excavated slope are the gation, and 3, Strength and Stability Analysis, same as those used to estimate the stability of an provide important background information for embankment slope. These first two factors are true this chapter. Specifically, Chapter 12 (Soil for the analysis of newly constructed slopes as well Strength Properties and Their Measurement) as for existing slopes and for the design of reme- gives the procedures for determination of the ap- dial measures. Third, designing a stable slope propriate soil parameters utilized in the stability includes field investigations, laboratory tests, sta- analyses that are discussed in detail in Chapter 13. bility analyses, and proper construction control. In this chapter the basic principles established in Because most of the details involved in this work Parts 2 and 3 are applied to the design of stable cannot be standardized, good engineering judg- slopes for new construction of both excavated and ment, experience, and intuition must be coupled embankment slopes. The procedures are also ap- with the best possible data gathering and analyti- propriate for the analysis of preconstructed slopes, cal techniques to achieve a safe and economical as well as for design of remedial works and correc- solution to slope stabilization. tion of existing landslides. This chapter is an update of Chapter 8 in Special 3. FACTOR OF SAFETY Report 176 (Gedney and Weber 1978), which in turn built upon earlier reports (Baker and Marshall In conventional practice the stability of a slope is 1958; Root 1958). Because much of the basic tech- expressed in terms of its factor of safety, although nical information given by Baker and Marshall, in recent years there has been increasing interest Root, and Gedney and Weber is still valid, empha- in developing a probabilistic assessment of slope sis in this chapter will be on recent case histories reliability (see Chapter 6). In the conventional and innovations in slope stabilization techniques approach, factors of safety less than 1 obviously since 1978. indicate failure, or at least the potential for failure, 439 440 Landslides: Investigation and Mitigation whereas stability is represented by safety factors Avoid the problem, greater than 1. The choice of the appropriate safety Reduce the forces tending to cause movement, factor for a given slope depends on a number of and considerations, such as the quality of the data used Increase the forces resisting movement. in the analysis, which in turn depends on the qual- ity of the subsurface investigations; laboratory and A summary of these three approaches is given in field testing; interpretation of field and laboratory Table 17-1. data; quality of construction control; and, in some cases degree of completeness of information about S. AVOIDANCE OF THE PROBLEM the design problem. The engineer must also con- sider the probable consequences of failure. In most A geological reconnaissance is an important part of transportation situations, slope designs generally preliminary project development for many trans- require safety factors in the range of 1.25 to 1.50. portation design studies. This reconnaissance Higher factors may be required if slope movements should note any evidence of potential stability have the potential for causing loss of human life or problems due to poor surface drainage, seepage an great economic loss or if there is considerable existing natural slopes, hillslope creep, and ancient uncertainty regarding the pertinent design param- landslides. As noted in Table 17-1, avoiding the eters, construction quality control, potential for landslide problem is an excellent approach if it is seismic activity, and so forth. Likewise, lower safety considered during the planning phase. However, a factors may be used if the engineer is confident of large cost may be involved if a landslide problem the accuracy of the input data and if good con- is detected after the location has been selected and struction control may be relied upon. the design completed. 5.1 Ancient Landslides 4. DESIGN PROCEDURES AND Ancient landslides can be one of the most difficult APPROACH ES landforms to identify and often are the most costly Details of slope stability analysis procedures are to deal with in terms of construction. Natural geo- given in Chapter 13. Analytical techniques allow a morphic and weathering processes, vegetation, or comparison of various design alternatives, includ- human activities may all but obscure these land- ing the effects of those alternatives on the stability, forms, and careful field investigation is necessary to of the slope and on the economy of the possible detect them. solutions. In addition, all potential failure modes As with the case of talus slopes, which are dis- and surfaces should be considered. As discussed in cussed below and in Chapter 20, old landslides are Chapter 13, preliminary analyses may utilize sta- often barely stable, and they may not have signif- bility charts with simplified assumptions; such icant resistance to new loadings or other changed simple stability determinations may be adequate in conditions that tend to reduce their stability. many cases to decide whether a standard slope Such slopes may continue to move, for example, angle can be used. More involved analysis and sta- during periods of heavy rainfall, and yet be bility calculations may be necessary for more com- relatively stable during other parts of the year. plex problems. In all cases, consideration must be Changing natural drainage patterns on the sur- appropriately taken of the environmental condi- faces of old landslides may significantly influence tions to which the slope is likely to be subjected their stability and cause unwanted movements. during its entire design life, including changes in Thus, the decision to construct transportation soil strength and groundwater conditions, possible facilities over ancient landslides must be carefully seismic activity, or other environmental factors. As investigated and appropriate consideration given a minimum, the analysis should include conditions to remedial measures and long-term stability. expected immediately after construction and at 5.2 Removal of Materials some later time after construction. Approaches to the design of stable slopes can If relocation or realignment of a proposed facility be categorized as follows: is not practical, complete or partial removal of the Table 17-1 Summary of Approaches to Potential Slope Stability Problems (modified from Gedney and Weber 1978) CATEGORY PROCEDURE BEST APPLICATION LIMITATIONS REMARKS Avoid problem Relocate facility As an alternative Has none if studied during Detailed studies of proposed anywhere planning phase; has large relocation should ensure cost if location is selected improved conditions and design is complete; also has large cost if reconstruction is required Completely or Where small volumes May be costly to control Analytical studies must be partially remove of excavation are excavation; may not be best performed; depth of unstable materials involved and where alternative for large excavation must be suffi- poor soils are encoun- landslides; may not be cient to ensure firm tered at shallow depths feasible because of right- support of-way requirements Install bridge At sidehill locations May be costly and not provide Analysis must be performed with shallow soil adequate support capacity for anticipated loadings as movements for lateral forces to restrain well as structural capability landslide mass Reduce driving Change line or grade During preliminary Will affect sections of roadway forces design phase of project adjacent to landslide area Drain surface In any design scheme; Will only correct surface Slope vegetation should be must also be part of infiltration or seepage due considered in all cases any remedial design to surface infiltration Drain subsurface On any slope where Cannot be used effectively Stability analysis should lowering of groundwater when sliding mass is include consideration of table will increase slope impervious seepage forces stability Reduce weight At any existing or Requires lightweight materials Stability analysis must be potential slide that may be costly or performed to ensure proper unavailable; excavation placement of lightweight waste may create problems; materials requires right-of-way Increase resisting forces Apply external Use buttress and At an existing landslide; May not be effective on deep- Consider reinforced steep force counterweight in combination with seated landslides; must be slopes for limited fills; toe berms other methods founded on a firm founda- right-of-way tion; requires right-of-way Use structural To prevent movement be- Will not stand large defor- Stability