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Stabilization of Soil Slopes

Stabilization of Soil Slopes

LChapter 17

ROBERT D. HOLTZ AND ROBERT L. SCHUSTER STABILIZATION OF

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 are quite the design of stable slopes. First, because of the known. The properties of soils as they nature of soils and the geologic environments in relate to 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 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 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 . 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; 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 , 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 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 to identify and often are the most costly Details of 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 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 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 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 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 and soil-structure systems fore excavation; where mations; must penetrate analyses are required. right-of-way is limited well below sliding surface Install anchors Where right-of-way is Requires ability of Study must be made of in limited soils to resist shear forces situ soil ; by anchor tension economics of method. depends on anchor capac- ity, depth, and frequency Increase internal Drain subsurface At any landslide where Requires experienced strength water table is above personnel to install and shear surface ensure effective operation Use reinforced On embankments and Requires long-term Must consider stresses backfill steep fill slopes; land- durability of imposed on reinforcement slide reconstruction reinforcement during construction Install in situ As temporary structures Requires long-term Design methods not well reinforcement in stiff soils durability of nails, established; requires anchors, and micropiles thorough soils investiga- tion and properties testing

(continued on following page) 442 Landslides: Investigation and Mitigation

Table 17-1 (continued)

CATEGORY PROCEDURE BEsT APPLICATION LIMITATIONS REMARKS Increase internal Use biotechnical On soil slopes of modest Climate; may require Design is by trial and error strength stabilization heights in dry seasons; longevity of plus local experience (continued) selected plants Treat chemically Where sliding surface May be reversible; long-term Laboratory study of soil- is well defined and soil effectiveness has not been chemical treatment must reacts positively to evaluated; environmental precede field installations; treatment stability unknown must consider environ- mental effects Use electroosmosis To relieve excess pore Requires constant direct Used when nothing else pressures and increase current power supply and works; emergency shear strength at a desir- maintenance stabilization of landslides able construction rate Treat thermally To reduce sensitivity of Requires expensive and Methods are experimental soils to action of carefully designed system and costly water to artificially dry or freeze

unstable materials should be considered. Figure include limited excavation of near-surface unstable 174 shows an example of one such study. Stability materials as well as the use of other stabilization analyses indicated that removal of Volume B was techniques such as subsurface drainage. more effective than removal of Volume A. Eco- Bridging may also be a suitable alternative in nomics, as well as the potential increase in slope very steep mountainous country where construc- stability, will decide the final course of action. tion may cause unsightly scarring because of unsta- Removal of potentially unstable materials can ble excavated slopes or excavations that "daylight" range from simple stripping of near-surface materi- high on steep mountain slopes. Several spectacular als a few meters thick, as shown in Figure 17-2, to examples of this bridging approach can be found on more complicated and costly operations such as the autobahns in the Austrian Alps and on the those encountered in a sidehill along the autostrade in Italy. Willamette River in West Linn, , where a section of 1-205 required extensive excavation to 6. REDUCTION OF DRIVING FORCES depths as great as 70 m (Gedney and Weber 1978). Since the forces tending to cause movements 5.3 Bridging downslope are essentially gravitational, a simple approach to increasing stability is to reduce the In some instances, removal of especially steep and mass of soil involved in the slope. Techniques for long, narrow unstable slopes is simply too costly or this include flattened slopes, benched slopes, too dangerous. One alternative design is to span reduced excavation depths, surface and subsurface the unstable area with a structure (bridg) sUp- drainage, and lightweight fill (Table 17-1). All of ported on driven piles or drilled shafts placed well these possibilities reduce driving forces and all have below the unstable foundation materials (Baker been successfully used at one time or another. and Marshall 1958). Site investigations and stabil- As explained in Chapters 12 and 13, the stabil- ity analyses must ascertain that the bridge is indeed ity of embankment slopes cannot necessarily be founded at sufficient depth below the unstable ma- approached in the same manner as that of natural terials. If the bridge foundation must penetrate or excavated slopes. For example, the stability of through the moving soil, as shown in Figure 17-3, embankment slopes tends to increase with time the foundation piling must be designed to with- because of consolidation and the resulting strength stand the predicted lateral forces, but predicting increase of the fill and foundation. A notable these forces is not a simple task. Bridging may also exception to this would be embankments corn- Stabilization of Soil Slopes 443

FIGURE 17-1 FACTO RS OF SAFETY Stabilization of EXISTING SLOPE(ASSUMED)=1.00 Cameo slide above VOLUME A REMOVED'=101 VOLUME B REMOVED =1.30 railroad in Colorado VOLUME A= VOLUME B River Valley, Colorado, by partial WEST 830 1600. removal of materials (Peck and Ireland 0 1953; Baker and o.. Marshall 1958).

1550

B 2 / I- w // MESA VERDE SANDSTONE 1500 A LU o / /

O OLD / ,' LNEW TUNNEL 1450

FILL MANCOS SHALE DARKGRAY.HARD 026 40 60m 0 66 132 10198ft 1400 posed of degradable shalest and other soft rocks, Talus slopes often have marginal stability which may deteriorate with time and result in set- (Ritchie 1963) and deserve special consideration tlement or even failure (see Chapter 21). In natural (see Chapter 20). Runoff from normal rainfall or or excavated slopes, however, the long-term stabil- snowmek may cause sufficient increase in seepage ity may be significantly less than that available at pressures to initiate movement. Recognition of the end of construction. Design conditions that are talus slopes is important in preliminary location appropriate for these cases are discussed in Chap- designs because of the potential for dangerous ters 12 and 13. movements. Such slopes may also be disturbed by

PROPOSED ROADWAY EMBANKMENT -\ FILL TO PROVIDE DRAINAGE

OR I G INAL GROUND LINE

UNSTABLE FIGURE 17-2 STRIPPE Stripping of t .. FILTER MATERIAL unstable surface FIRM MATERIAL material to reduce .0 5 lOm landslide potential for sidehill 0 16.5 33ft embankment OUTLET AT LOW POINT PROVIDED (Root 1958). 444 Landslides: Investigation and Mitigation

trate talus to a depth of 10 to 30 cm and will .?kL.. improve both stability and resistance to raveling.

TTc;k1 6.1 Change of Line or Grade or Both Early in the design stage, cut-and-fill slopes should - - be evaluated for potential instability. If conditions warrant, adjustments to the line and grade can be made to minimize or possibly completely elimi- nate potential stability problems. This approach can also be applied to landslides discovered during / and after construction. Feasibility will be con- trolled by the economics of various alternative

.- - solutions. A general example of a grade revision to - prevent movement of an excavated slope is shown -. in Figure 17-4. --- A specific example occurred in design and con- struction of 1-70 across Vail Pass, Colorado, where FIGURE 17-3 construction activities, and if such activities cannot active landslides were encountered on Landslide avoidance be avoided, special construction procedures and opposing slopes adjacent to the right-of-way. by bridging near stabilization methods are necessary. R. Barrett Stabilization measures included filling the valley Santa Cruz, (personal communication, 1992, Colorado Depart.. and then transferring the thrust of one landslide California (Root ment of Transportation) noted that mechanical against the other and placing the stream and high- 1958). stability of cuts in talus slopes is not a major prob- way on the fill (Robinson 1979). Edit (1992) lem; instead, the faces of these cuts tend to ravel. described a case in which regrading was success- To prevent surface raveling of cuts along 1-70 in fully used to stabilize an unstable bluff along Lake Glenwood , Colorado, backfill was placed Superior in northern Wisconsin. FIGURE 17-4 between the cut surface and the structure for which Line or grade changes often result in reduction Grade change made the cut was made. In addition, the Colorado of driving forces. For example, a roadway may be during construction Department of Transportation is experimenting relocated away from the toe of the potential or to prevent possible with improving the resistance of talus to raveling existing landslide to avoid removal of toe support. failure of excavated by spraying on fiber-reinforced shotcrete or ure- Changes to cause redi.ction in driving forces during slope (Gedney and Weber thane liquid. Tests on Colorado Highway 82 near construction operations are both difficult and 1978). Aspen have shown that liquid urethane will pene- expensive. To flatten embankment slopes often

ScARF COLLUVIUM AND DISPLACED SAPROLITE

REVISED ORIGINAL _ ORIGINAL GROUND N

SURFACE -

FINAL GROUND SURFACE AFTER FAILURE - ,.....

PREDICTED FAILURE ZONEE

z"o."NALPROPOD GRADE

~q__HIGHEST WATER LEVEL 7'P.ING Stabilization of Soil Slopes 445 requires additional right-of-way and can involve and increase the strength of the materials in the alignment shifts that affect the design of facilities slope. Increasing the strength of soil materials on either side of the problem area. Thus, changes using drainage is discussed in Section 7.2.1. in line and grade after construction may be so costly as to prevent their use. These comments emphasize 6.2.1 Surface Drainage the fact that the cost-effectiveness of geotechnical investigations is greatest when they are carried out Surface drainage measures require minimal engi- during the preliminary design phases of the project. neering design and offer positive protection to Another technique for reducing the driving slopes. Thus they are among the first approaches forces, especially for known unstable areas, is the that should be considered in preventing potential partial removal or excavation of a sufficient quan- stability problems. tity of slope material at the top of the landslide to Adequate surface drainage is necessary in new ensure stability of the potential sliding mass excavations as well as in maintenance of con- (Figure 17-1). If an infinite slope condition exists structed slopes where movement has already or certain types of flow or debris slides are in- occurred. The design of cut slopes should always volved, this method may be ineffective. The take into consideration the natural drainage pat- quantity of material required to be removed is pre- terns of the area and the effect that the constructed dicted by trial and error using ordinary stability slope will have on these drainage patterns. Two analysis (Chapters 12 and 13). As usual, economic conditions that should be evaluated are considerations and potential use of the excavated materials may dictate whether unloading proce- Surface water flowing across the face of the slope, dures are feasible for a particular project. In some and instances—for example, when the project needs Surface water seeping into or infiltrating into the additional borrow materials—removal of the head of the cut. entire sliding mass may be feasible provided the material volumes are reasonable. The design of Both of these conditions cause of the face landslide removal should always consider the and increase the tendency for localized failures on stability of the slope behind or above the area to be the slope face. Diversion ditches and interceptor removed. Of course, this technique is most appro- drains are widely used as measures, priate during the design stage. These procedures especially in situations where large volumes of usually involve slope flattening; in some cases, runoff are anticipated (Figure 17-5). FIGURE 17-5 Good surface drainage is strongly recommended Surface drainage benching has been used to reduce the driving of slope using forces on a potential or existing landslide. Specific as part of the treatment of any landslide or poten- diversion ditch and geometric constraints of the site will determine if tial landslide (Cedergren 1989). Every effort should interceptor drain benching is appropriate. As discussed in the fol- be made to ensure that surface waters are carried (Gedney and Weber lowing section, benches also serve to help control away from a slope. Such considerations become 1978). surface runoff and provide work areas for place- ment of horizontal drains.

6.2 Drainage

Because of its high stabilization efficiency in rela- tion to design and construction costs, drainage of surface water and groundwater is the most widely used and generally the most successful slope stabi- lization method (Committee on Ground Failure Hazards 1985). Of all possible schemes to be con- sidered for the correction of existing or potential landslides, proper drainage is probably the single most important. Drainage will both reduce the weight of the mass tending to cause the landslide 446 Landslides: Investigation and Mitigation

especially important when a failure has already major importance. lithe preliminary site investiga- occurred. Unless they are sealed, cracks and fissures tion reveals the presence of groundwater, if design behind the scarp face of a landslide can carry sur- studies predict potential downslope movements, face waters into the failure zone and reactivate the and if positive subsurface drainage can reduce fail- landslide. Consequently, reshaping the surface of ure potential, it is worth preparing a suitable design the landslide mass can be very beneficial because for cost comparison with other alternatives. unnoticed cracks and fissures may be sealed and Subsurface drainage as a method of lowering water-collecting surface depressions eliminated. the groundwater table within an unstable slope There are a number of possibilities to treat the has traditionally consisted of one or more of the surface of the slope itself in order to promote rapid following procedures: runoff and improve slope stability. Some of these measures are Drainage blankets and ; Drainage ; Seeding, sodding, or mulching, and Drainage galleries, adits, or ; Using shotcrete, , thin masonry, Subhorizontal (commonly called "horizontal") paving, asphalt paving, and rock fills. drains drilled either from the slope surface or from drainage wells or galleries; and All of these techniques have been used successfully Subvertical drains drilled upward from drainage to protect slopes made of degradable shales or clay- galleries. stones and to prevent the infiltration of surface runoff. Techniques for controlling surface runoff Most often these systems drain by means of grav- are especially effective when used in conjunction ity flow; however, pumps are occasionally used to with various subsurface drainage techniques. remove water from low-level collector galleries or The Geotechnical Control Office of Hong wells. Figure 17-6 is a schematic drawing showing Kong (Geotechnical Control Office' 1984) has some of these techniques. presented useful guidelines for the maintenance of The effectiveness and frequency of use of the var- surface drainage systems. These guidelix\es partic- ious types of drainage treatments vary according to ularly recommend the use of surface channels as the hydrogeologic and climatic conditions. It is gen- opposed to pipes placed on the surface. erally agreed, however, that groundwater constitutes the single most important cause of the majority of 6.2.2 Subsurface Drainage landslides. Thus, in many areas of the world the most generally used and successful methods for prevention Because seepage forces act to increase the driving and correction of landslides consist entirely or force on a landslide, control of subsurface water is of partially of groundwater control (Cedergren 1989).

U PS LOPE DRAINAGE DITCHES PUMPED WELLS

VERTICAL GRAVITY

FIGURE 17-6 Schematic diagram of subhorizontal and vertical drains used to lower groundwater in natural slopes (Gedney and Weber 1978). Stabilization of Soil Slopes 447

Subsurface drainage is equally important in cuts if seepage is evident or if there is a possibility that and embankments, and most types of subsurface it may develop during wet periods, a blanket drain drainage treatments are applicable to the preven- of pervious material (clean free-draining or tion and correction of landslides in both situa- ) should be placed before the embankment tions. When embankments are constructed on is constructed. If springs or concentrated flows are potentially unstable slopes, landslides may occur encountered, drainpipes may also be required. because the increase in shear stresses imposed by When conditions are such that drainage blan- the embankment exceeds the shearing strength of kets are uneconomical, for example, when the the foundation soils. Instability may also result if depth of unsuitable material to be stripped is large, the embankment interferes with the natural drains may be appropriate. Trenches filled movement of groundwater. Therefore, in an in- with free-draining material have been used effec- vestigation of possible instability, two factors must tively for shallow subsurface drainage for several be considered: decades. The trenches are excavated by backhoe- type excavators to depths of 5 to 6 m and by Weak zones in the foundation that may be over- clamshells to greater depths; some type of tempo- stressed by the proposed embankment load, and rary support system may be required if the excava- Increases in pore-water pressure sufficient to tion is more than 5 to 6 In deep. Cancelli et at. cause a significant reduction in the shear (1987) provided a brief review of the theory for strength of the soil. the design of trench drains. Sometimes single drainage trenches perpendicular to the center line Because there often is no surface indication of un- of the facility are sufficient; in other situations, stable slope conditions, careful subsurface explor- larger, more extensive networks of interconnected ation is required if slope instability is to be drains may be necessary. In addition to facilitating FIGURE 17-7 predicted before construction. drainage, trench drains provide increased resis- Trench and augered Drainage is sometimes installed to intercept sub- tance to possible sliding because the compacted drain slope stabilization system surface water outside the limits of excavations, but backfill of the trench section acts as a key into it frequently is impossible to determine whether for gas pipeline firmer material beneath the trench. crossing of the such interceptor trenches will effectively cut off all In some cases drainage wells (discussed below) groundwater that might contribute to slope failure. Assiniboine River, are used under or at the bottom of drainage tren- Canada (modified Other methods that may be appropriate in sidehill ches (Figure 17-7) (Lew and Graham 1988). from Lew and embankment areas often are excessively expensive Graham 1988). in excavation areas. Each case must be examined REPRINTED FROM CH. 6.2.2.2 Drainage Wells BONNARD (ED.), on an individual basis. The cost of drainage systems Deep wells are increasingly being used to drain un- LANDSLIDESIGLISSEMENTS is generally lower when these measures are incor- DE , PROCEEDINGS stable slopes, particularly where the required OF THE FIFTH porated into the preliminary design process. When INTERNATIONAL drainage depths are too deep for economical con- SYMPOSIUM, LAUSANNE, they are included as remedial measures during or 10-15 JULY 1988, VOLUME 2, struction of drainage trenches. Sometimes the 1988-1989, 1604 PP., 3 following construction or landslide movement, the VOLUMES, A.A. BALKEMA, wells are drilled fairly close together, essentially to OLD POST , drainage systems may be expensive. BROOKFIELD, VERMONT In this section recent advances in subsurface form a drainage gallery, as in the case of 1-80 in 05036 drainage systems are discussed. In addition, a few less common means of drainage such as electro- osmosis, vacuum drains, siphons, and the use of 0.4 -m -dia and geocomposites will be briefly Original Surface 0.75 -m -dia mentioned. PiPelie TrenCh _AU9ered Drain

6.2.2.1 Drainage Blankets and Trenches River When an embankment is to be constructed over a surface layer of relitively shallow, weak soil un- - Clay'llfl II derlain by stable rock or soil, usually the most eco- Till 0 10 20m nomical treatment is to strip the unsuitable I I material, as shown in Figure 17-2. After stripping, 448 Landslides: Investigation and Mitigation

California described by Smith et at. (1970) and Drainage galleries are constructed using con- Gedney and Weber (1978). A similar system was ventional tunneling and techniques, and recently utilized in Kentucky, as described by in addition to acting as drainage facilities, they Greer and Mathis (1992), to correct a landslide. serve as exploratory adits for geologists to study Sometimes large-diameter vertical wells up to 2 in landslides at depth. They are also useful for in- in diameter are used, as in the cases described by stalling monitoring equipment and instrumenta- Collotta et at. (1988) (Figure 17-8) and Bruce tion. As with the case of deep drainage wells, (1992b); these wells can be as much as 50 m deep. gravity may be insufficient for positive drainage, Each well is connected to its neighbor either by and pumping may be required. Gillon et al. intersecting "belied" bases, which may overlap, or (1992) described a case in which drainage gal- by horizontal drill holes and drains. Occasionally leries were used for landslide stabilization adjacent some shafts are kept open for inspection, moni- to a in New Zealand (Figure 17-10). toring, and maintenance purposes. Schuster (1995) provided a number of additional examples 6.2.2.4 Subhorizontal Drains of landslide stabilization using drainage wells. In recent years small-diameter subhorizontal Vertical drainage wells have also been installed drains have become very common for landslide under embankments to accelerate the consolida- stabilization. As was the case with drainage tun- tion of weak and unstable foundation soils. This use nels, they were first installed as corrective mea- is similar to classical sand drains or modem prefab- sures, and although they are still used for this ricated vertical (wick) drains (Holtz et al. 1991). purpose, subhorizontal drains are now primarily installed as preventive measures against slope in- FIGURE 17-8 6.2.2.3 Drainage Tunnels, Adits, or stability. Although these drains are frequently Vertical drainage called horizontal drains, they really are subhori- shafts connected to Galleries horizontal PVC When the depth to subsurface water is so great zontal in that they are typically installed on slopes outlet drains, that drainage trenches or wells are prohibitively approximately 2 to 5 degrees above the horizon- Florence-Bologna expensive, drainage tunnels are sometimes used. tal. The drill hole is then fitted with a perforated Motorway, Italy Although originally used as a corrective treatment, pipe; in the past, steel pipe was commonly used, (modified from drainage tunnels are sometimes constructed as a but in the last few years polyvinylchloride (PVC) Collotta et al. 1988). preventive measure. They are expensive, and thus pipe has been preferred. REPRINTED FROM CH. BONNARD (ED.), are less commonly used in highway construction Subhorizontal drains are probably the most LANDSLIDESIOLISSEMENTS DE TERRAIN, PROCEEDINGS than in large slope-stabilization measures, which are commonly used subsurface drainage technique for OF THE FIFTH INTERNATIONAL sometimes required for hydroelectric projects. One landslide stabilization. In large excavations that SYMPOSIUM, LAUSANNE, 10-15 JULY 1988, VOLUME 2, example is the case described by Millet et al. (1992) are potentially unstable, the drains are installed as 1988-1989, 1604 PP., 3 in which drainage tunnels, among other techniques, the cut is excavated, often from one or more VOLUMES, A.A. BALKEMA, OLD POST ROAD, were used to stabilize a large landslide threatening benches in the slope. Drains may be installed from BROOKFIELD, VERMONT 05036 Tablachaca in Peru (Figure 17-9). the ground surface or from drainage galleries,

Full-Section Drainage Well1 370 Inspection Drainage Well 1rn1!_m1 . .

Motorway Embankment

::: Grav els ion CD Ui Boflom Collector for Drained- Water DrillIng

3301 - Highly Stratified .1 Fractured Flysch Formation

Stabilization of Soil Slopes 449

large-diameter wells, and drilled shafts. The typi- cal drain hole is 120 to 150 mm in diameter and is lined with a slotted PVC plastic casing 60 to 100 mm in diameter. Drilling is usually by conven- tional rotary techniques, although precision drilling. may be necessary for very long drains Area of (Sembenelli 1988). Royster (1980) described tra- Lanoslide Rows of Drains _.- Buttress ditional drilling and installation techniques. Andors t Because a filter is ordinarily needed, the annular \\ ...... - space between the casing and the soil or rock of Drainage the drill hole is a problem. Filling this annulus Tunnels with sand is difficult if not impossible. Sometimes S - pipes with very fine slots are used; alternatively, Intal(& SarTrap coarse slotting can be protected on the outside by I "stockings." Power Several interesting case histories of successful Extsting Tunnel -. Dam applications of subhorizontal drainage to stabilize landslides have been published, for example, in Transportation Research Record 783 (Transpor- tation Research Board 1980), which includes ex- amples from California, Colorado, Mississippi, and Tennessee. Reagan and Jutkowfsky (1985) de- scribed horizontal-drain practice in New York 29504- . I State. Craig and Gray (1985) reported on hori- GrounoSuaca zontal drainage in Hong Kong, an area noted for 2900 . many unstable natural slopes. Isenhower (1987) described the stabilization of landslides in San 2950 .\ Acfive Sling Antonio, Texas, using horizontal drains, and / . Surfas(Typ) Anoent N. Sharma and Buu (1992) used horizontal drains 2800-i ng Surfaces -::: '-'• and lightweight fill to stabilize a landslide in Estimod Idaho. Roth et al. (1992) described the use of hor- 27504...... Baseot /...... ...... uj Lancslde izontal drains ("hydraugers") to stabilize the fa- Tunnel S.250 Buttreu Lu I mous Pacific Palisades landslide in California. 2700-4 . .-/

Smith (1980a, 1980b) presented a comprehensive Anchors . study of the long-term effectiveness of horizontal- 2650 Tunnel 5-200 E .... drain installations in California. His primary con-

clusions were that metal pipe casings will only last 4 S approximately 30 to 40 years; slotted PVC pipes 150 200 250 300 350 400 450 500 550 600 650 DISTANCE (m) will provide longer service life, are less susceptible to corrosion, and appear to allow considerably less sediment to enter the drains. On the basis of 6.2.3.1 Electroosmotic Dewatering FIGURE 17-9 California experience, periodic maintenance is re- Stabilization of quired; most drains need to be cleaned once every 5 In the mid-1930s the concept of using electroosmo- sis to promote drainage and consolidation of unsta- Tablachaca Dam to 8 years unless exceptionally fine-grained sedi- landslide, Peru: ble soils was introduced (Casagrande 1948). Despite ments or heavy root growth are present. overview and some success for slope stabilization (see, for exam- cross section ple, Section 7.2.4.2), this process has not received 6.2.3 Other Less Common Techniques (Millet et al. 1992). widespread usage, especially for drainage, probably REPRINTED WITH PERMISSION OF AMERICAN Schuster (1995) summarized other less common because of installation and operation costs and some SOCIETY OF CIVIL ENGINEERS techniques such as electroosmosis, vacuum dewa- remaining technical uncertainties about the process. tering, and blasting of rock slopes for improving However, Lo et al. (1991a, 1991b) used specially de- drainage. signed copper electrodes to prevent gas accumula-

450 Landslides: Investigation and Mitigation

FIGURE 17-10 Brewery Creek Key landslide- Slide Base stabilization 250 ------Groundwater Levels scheme, Clyde Power Project, Drainage Drill Holes New Zealand Existing Landslide (modified from Gillon et al. 1992). Zoned Earthfill State Hwy. 8 Blanket REPRINTED FROM 200- . Reservoir Level EIev 195 m D.H. BELL (ED.). Fan Drainage , LANDSLIDES/OLISSEMENTS E DE TERRAIN, PROCEEDINGS - Drilling OF THE SIXTH Groundwater level INTERNATIONAL .2 ' post Buttress SYMPOSIUM, engineering works ii "///// /Clutha River CHRISTCHURCH 10-14 FEBRUARY, 1992, VOLUME I, 1992-1994, I800PP.,3 VOLUMES, A.A. BALKEMA, OLD POST ROAD, 150 BROOKFIELD, VERMONT 05036 Base Drainage Gallery In-Situ Pumped Low-Level Schist Curtain Collector Drain 0 25 50 m 100 (normal to gallery) I I

tion around the anode and to allow free water to 6.3 Geotextiles and Geocomposites flow from the cathode without pumping. Geosynthetic products can be used for drainage 6.2.3.2 Vacuum Dewatering and slope stabilization in many of the situations Arutjunyan (1988) reported on the use of vacuum described above. The use of a geotextile filter is in drill holes to dewater fine-grained soil slopes in often cheaper and just as effective in situations the former Soviet Union. The technique has been where graded granular filters are required. Just as applied to landslides as a temporary expedient with graded granular filters, the geotextile filter until long-term stabilization could be carried out. should be designed for soil retention, system per- The vacuum treatment increases soil suction and meability, and long-term filtration characteristics. accelerates the process of . It was Christopher and Holtz (1985, 1989) and Koemer successful to depths of 30 to 35 m when the vac- (1990), among others, have presented methods uum was applied for a period of 2 to 4 weeks. for geotextile filter design. Geocomposites are products consisting of a geo- 6.2.3.3 Drainage by Siphoning filter to protect the drain and keep it free- Siphon drains for slope stabilization were installed flowing throughout its service life and a plastic net at 40 sites in France during the 5 years before or core that provides in-plane drainage. Geocom- 1992 (Gress 1992). These drains have the advan- posites can be installed in trenches on slopes, espe- tage of being able to raise water to the surface cially in areas where access is difficult, behind without pumping. Siphoning of water from unsta- retaining structures, and in other places where ble strata is accomplished by sealed PVC pipe sys- interception of seepage is desired. Geocomposites tems. One example of successful use of siphoning are manufactured in sheets or strips, and thus are to lower the groundwater table under a highway more easily and cheaply installed than conven- embankment on an unstable slope occurred at tional granular filters and drains. When properly de- Venarey-Les-Laumes near Dijon, France, where signed, they work as well as, or often better than, five vertical siphon drains spaced at intervals of 10 conventional aggregate filters and drains. m lowered the piezometric level from an original Figure 17-11 shows drainage applications in depth of 2 m to a depth of 8 m beneath the high- which geotextile filters or geocomposite drainage way (Gress 1992). products may be used. Stabilization of Soil Slopes 451

panded polystyrene, shredded and chipped tires, FIGURE 17-11 (a) and oyster shells and clamshells. The advantages Drainage and disadvantages of the use of many of these ma- applications using terials were described by Hokz (1989). geotextiles or geocomposites: Several examples of the use of lightweight fill toc trench drain, . lOOoO materials to stabilize landslides have been given in interceptor drain, the literature. Nelson and Allen (1974) reported and (c) drainage 42QQQ.. on a successful landslide correction using chipped behind structure bark and sawdust to produce a sidehill embank- (modified from ment weighing about 5 kN/m3. They used a 0.3-m Christopher and base under the pavement section; in addi- Holtz 1989). tion, an asphalt seal was placed on the exposed slope to retard deterioration and pollution. Since (b) then there have been many other similar projects in the Pacific Northwest. Yeh and Gilmore (1992) reported the use of expanded polystyrene (EPS) to correct a large landslide in southern Colorado. The landslide was successfully stabilized by using a counterweight berm at the toe and replacing the landslide material in the embankment with EPS with a unit weight of about 0.2 kN/m3. Sharma and Buu (1992) used pumice weighing about 12 kN/m3 in the correction of a landslide on 1-15 in Idaho. Shredded waste tires weighing about 6.4 kN/m3 were used as lightweight fill in the correc- tion of a landslide under a highway embankment on Highway 42 in Oregon (Read et al. 1991). Humphrey and Manion (1992) conducted tests on tire chips to determine their basic properties.

7. INCREASE IN RESISTING FORCES

The third general method for stabilizing earth slopes is to increase the resisting forces on a po- tential or existing landslide. Although techniques vary widely, they generally function by either

Applying a resisting force at the toe of a land- slide, or Increasing the internal strength of the soils in the failure zone so that the slope remains stable 6.4 Reduction of Weight without external assistance (Table 17-1).

Especially in embankment construction, the use of Both approaches should be considered during pre- lightweight backfill materials can be very effective liminary design investigations to ensure the best in reducing the gravitational driving forces tend- technical and economical solution. The tech- ing to cause instability. Lightweight materials that niques described in this section to correct land- have been successfully used in highway embank- slides are appropriate for those that occur both ments include chipped bark, sawdust, dried , during and after construction. , slag, cinders, cellular concrete, expanded A number of procedures have been developed clay or shale, lightweight geologic materials, ex- that increase the resisting forces at the toe of a p0- 452 Landslides: Investigation and Mitigation- -

tentially sliding soil mass. These include various Buttresses, counterweight fills, and toe berms; methods of adding mass to the toe areas, various Structural retention systems such as cantilever structural retention systems to deflect or redirect and gravity retaining walls, externally, braced the driving forces, and a variety of earth matérial walls or walls supported by anchors or tiebacks, reinforcement systems. Although reinforced soil , root piles, conventional piles, and structures may be used as buttresses or structural drilled shafts; and - retention systems, an important- aspect- - of iheir A -variety of'reinforced-soil systems. stability is internal, and they will be discussed for convenience under the internal-strength category 7.1.1 Buttresses, Counterweight Fills, and below. Toe Berms Methods used to increase the internal strength of the potentially sliding soil mass include subsur- The principle behind,the use of buttresses, coun- face drainage, a wide variety of reinforcing sys- terweight fills, or toe berms is to provide sufficient tems, vegetative and biotechnical stabilization, dead weight'or restraint near the 'toe of the unsta- and miscellaneous methods such as chemical ble mass to prevent slope movement. In-other treatment; electroosmosis, and thermal stabiliza- words, the buttress must be heavy enough to prO- tion. Sometimes a combination of external and vide the additional component of resistance near internal treatments is employed. In the following the toe of the slope required for stability. Figure description of these techniques, emphasis is on de- 17-12 shows a rock buttress used to stabilize an un- velopmetits since 1978. - stable slope. . - The basic design of a buttress is similar to the 7.1 Application of External Force design for external stability, of conventional gravity-retaining structures. The buttress must be Application, or in some cases redirection, of ex- stable against ternal forces so as to increase the resistance of the FIGURE 17-12 slope to - pbtential movements will increase the Rock 'buttress used Overturning, to stabilize unstable stability of a slope. Such resisting forces are most Sliding at or below its base, and slope (Gedney and often applied to the toe of the potentially moving Bearing failure of the foundation. Weber 1978). mass-by a variety of methods, including - Conventional analyses for these three possible failure modes should be carried out with the 'usual factors of safety applied. Although not strictly a factor in stability, a settlement analy- sis should 'also be performed if the'foundation is compressible to. ensure that the 'final grade of the buttress is consistent with the geometric design re- quirements of the facility. Depending-on the geom- etry and the internal strength of the buttress, possible internal failure modes should also be checked to ensure that the buttress does not fail by shear within-itself. - Buttresses are commonly constructed of blasted quarry rock, boulders and cobbles, and coarse gravel fill. These materials are also commonly used to repair small slope failures, or "pop-outs," in highway cuts. If these occur duriiig construction, the highway contractor usually makes repairs as part of the normal construction contract. If the landslide occurs after construction and is not too large, similar reconstruction is normally carried out by highway -maintenance crews. Stabilization of Soil Slopes 453

For larger landslides, a more involved geotech- on adjacent property, conventional retaining struc- nical analysis and design are required. Gedney tures, piles, and reinforced-soil slopes and walls may and Weber (1978) described the reconstruction of provide workable alternative solutions. Because the a slide in a shale embankment in Indiana on 1-74 design principles for retaining walls and deep foun- by the use of a large earth-and-rock counterweight dations used for this purpose are fairly well under- buttress. stood, only a few pertinent references will be gh'en. Millet et al. (1992) described an interesting use One major drawback to the use of relatively stiff of a large buttress (nearly 500 000 m3) constructed structural elements for slope stabilization is that of granular materials to stabilize a large, poten- they are not able to tolerate much movement. tially unstable landslide threatening Tablachaca Consequently, elemental systems such as crib walls, Dam, a major hydroelectric power dam in Peru walls, and the various soil reinforcement sys- (Figure 17-9). The project required treatment of tems are becoming increasingly more common. the sediments in the reservoir bottom under the Figure 17-13 summarizes the current methods of buttress with the use of stone columns. On a more earth retention; the methods are divided into two modest scale, Edil (1992) reported on the use of a groups depending on whether they provide exter- terraced berm constructed of concrete nal or internal stabilization. Examples of both debris to stabilize an unstable slope along the types of systems are shown in Figure 17-14. Exter- western shores of Lake Michigan. Kropp and nally stabilized systems rely on some type of exter- Thomas (1992) used buttress fills to successfully nal structural walls or fill against which the stabilize an existing landslide area in California. stabilizing forces are mobilized. Before the late 1960s, the predominant types of permanent re- 7.1.2 Structural Retention Systems taining structures were concrete and masonry gravity walls and reinforced-concrete cantilevered In situations where a buttress fill is not feasible be- walls. More recently, Reinforced Earth and a num- cause of geometry or cost or because it encroaches ber of similar reinforcing systems using, for exam-

EXTERNALLY STABILIZED INTERNALLY STABILIZED SYSTEMS SYSTEMS

Gravity Reinforced In-Situ In-Situ Walls Walls Soil Reinforcement timber masonry metallic, polymeric • soil nailing precast concrete concrete and organic • reticulated sheet piles cantilever reinforcing strips micro piles soldier piles countertort and grids • soil doweling cast in-situ gabion anchored earth - slurry walls crib - secant pile bin - tangent pile ceIlul bored-in-place coffe (piles not contiguous) FIGURE 17-13 soil-cement Classification scheme for earth retention systems Braced Tied-Back HYBRID SYSTEMS - SPECIAL MATERIALS (modified from cross-lot • augered tailed • polymer-impregnated soil O'Rourke and Jones rakers • belied tailed masonry • low-density fills 1990). pressure- - low-density concrete REPRINTED WITH PERMISSION OF injected - expanded polystyrene AMERICAN SOCIETY OF CIVIL ENGINEERS

454 Landslides: Investigation and Mitigation

Externally Stabilized Systems

Potential Failure Wedge In-Situ Wall tn-Situ Wall / Cantilever _____ ____ / Gravity Wall 4lElements nchors / (Interlocking / Tiebacks / J

a) Cantilever b) Gravity Element c) Braced d) Tied-Back

Internally Stabilized Systems

-Strips or Grids Facing y a els / / Facing ll,c otential Failure Surface Grouted

I Bars PanE

e) Reinforced Soil f) Soil Nailing

FIGURE 17-14 pIe, have become common for earth- tion, the pile wall must be designed to resist the Examples of retaining structures. Earth-reinforcing systems are full earth pressure imposed by the soil in the slope externally and discussed in Section 7.2.2. as the excavation progresses. internally stabilized earth retention There were two sessions on the use of walls to One of the more successful applications of systems (modified control landslides at the National Convention of drilled shaft walls to stabilize a slope was described from O'Rourke and the American Society of Civil Engineers in Las by Palladino and Peck (1972) and Wilson (1970). Jones 1990). Vegas in 1982. The papers by Morgenstem (1982) During the construction of 1-5 in Seattle, heavily REPRINTEDWrni and Schuster and Fleming (1982) are particularly overconsolidated lacustrine clays were supported AMERICAN SOCIETY OF recommended in this connection. by very large diameter (up to 4 m) drilled concrete The use of closely spaced vertical driven piles shafts reinforced with heavy steel H-pile sections or large-diameter drilled shafts is quite common. [see also the discussion by Gedney and Weber They may be placed as a preexcavation retaining (1978)]. Nethero (1982) described the use of can- system, or they may be used following slope move- tilever drilled-shaft walls in the Ohio River Valley ment to stabilize landslides. If the piles are placed for the correction and prevention of slope stability adjacent to each other, the wall is known as a tan- problems. Several interesting case histories of gent pile wall; if there is some spacing that may landslide stabilization using drilled-shaft walls later be filled in with timber lagging, reinforced- were described by Rollins and Rollins (1992). concrete panels, or shotcrete, the wall is known as Although the shafts were not very deep, they were a secant pile wall. These walls may be designed as designed to function as cantilevers and they per- cantilevers or, more commonly, they are supported formed excellently in all cases. by tied-back anchor systems, which are discussed The use of the finite-element method to analyze in the following section. In the case of an excava- drilled piers used for slope stabilization was re- Stabilization of Soil Slopes 455 ported by Oakland and Chameau (1984), and in landslide stabilization. A spectacular use of Reese et al. (1992) presented a method for the ground anchors was described by Millet et al. analysis of drilled shafts used for increasing the sta- (1992) in which high-capacity rock anchors were bility of slopes. A design procedure for walls em- used together with a granular buttress to stabilize a bedded in failed slopes vas proposed by Isenhower large landslide at Tablachaca Dam in Peru (Figure et al. (1989), who reported that such walls 17-9). Interesting case histories describing the use supported by 0.5- and 0.6-m-diameter shafts are of tiebacks together with drilled shafts and driven much more economical than conventional earth- H-pile walls were presented by Hovland and retaining structures used for landslide correction. Willoughby (1982) and Tysinger (1982). According to Gedney and Weber (1978), at- Other related systems such as soil nailing tempts to stabilize landslides using driven steel or [Figure 17-14(f)] and micropiles increase the in- timber piles have seldom been successful. ternal strength of the sliding mass rather than ap- Depending on the geologic conditions at the site, plying external force. Thus, these systems are piles may be difficult to drive to the desired depth, discussed in the following section. and in the case of steel H-piles, for example, it is difficult to drive them vertically or in tangent. In 7.2 Increase in Internal Strength Scandinavia there has been some success in the use of both timber and steel piles to stabilize mov- Techniques that are used to increase the internal ing slopes and embankments (Bjerrum 1972); at strength of the potentially unstable soil mass for both these sites the subsurface conditions were rela- backfills and in situ stabilization are discussed next. tively soft glacial and postglacial marine clays. These include subsurface drainage, soil- reinforcing systems, vegetative and biotechnical stabilization 7.1.3 Anchor Systems techniques, and miscellaneous techniques such as chemical, electrical, and thermal stabilization. One of the more effective measures for stabilizing landslides is the use of anchor systems fo increase 7.2.1 Subsurface Drainage the resisting forces by applying external restraint to a moving soil mass [Figure 17-13 and Figure As noted by Gedney and Weber (1978), one of 17-14(d)]. These developments followed from the the most effective treatments for landslides and use of tiebacks to support walls for temporary ex- other unstable slopes is to increase the shear cavations for the construction of buildings and strength of the soil by means of subsurface other structures. drainage. Drainage reduces the excess pore pres- According to Gould (1990), anchors sures in the soil mass, which increases the effective were developed as early as the 1930s from Euro- on the potential failure surface. Subsurface pean experience with permanent rock anchors in drainage techniques are discussed in detail in dam stabilization. In the early 1960s their use in Section 6.2.2. Milwaukee and New York for building excavations clearly indicated the advantages of an uncluttered 7.2.2 Soil Reinforcement working space achieved by eliminating external supports [compare Figure 17-14(c) and 17-14(d)], Soil reinforcement is the inclusion of tensile resis- and this successful application led to rapid devel- tant elements in a soil mass to improve its overall opments in the use of high-capacity prestressed shearing resistance. One of the most exciting de- anchors to support deep excavations. Tiebacks velopments in practice were also used to stabilize landslides in in in the past 25 years, soil reinforcement is a tech- the mid-1960s, and, soon after, the technology nically attractive and very cost-effective tech- was being employed in North America for similar nique for increasing the stability of natural and purposes. An excellent description of the use of embankment soil slopes and for reducing the earth tiebacks for landslide stabilization was provided by pressures against retaining walls and abutments. Weatherby and Nicholson (1982). They described As noted in Figure 17-13, internally stabilized the design, installation, and corrosion protection soil systems can be conveniently divided into re- that are necessary for permanent ground anchors inforced soil [Figure 17-14(e)] and in situ rein- Landslides: Investigation and Mitigation

forcement. In situ reinforcement systems include 2. Passive soil bearing resistance on reinforcement soil nailing [Figure 17-14(J)], micropiles, pin piles, surfaces that are oriented normal to the direc- and root piles. Reinforced soil is applicable to sit- tion of relative movement between the rein- uations in which the reinforcement and backfill forcement and the soil. are placed as the slope or wall is constructed. Common reinforcing elements include steel strips Because of their inherent flexibility, reinforced- (Reinforced Earth), welded wire sheets, bar mats soil structures are considered to be relatively resis- and meshes, geotextiles, , and fibers. tant to seismic loadings. However, in some Excellent general discussions of internally sta- seismically active regions, the use of reinforced bilized soil reinforcement systems include those by slopes is still somewhat restricted because of the Lee et al. (1973), Jones (1985), Mitchell and lack of definitive research in this area. For rein- Villet (1987), Christopher et al. (1990), Mitchell forced-backfill retaining structures, the connec- and Christopher (1990), and O'Rourke and Jones tions between the reinforcing and the facing are (1990). This discussion will deal with recent critical during seismic events (Allen and Holtz advances in the use of internally stabilized soil re- 1991). inforcement systems. 7.2.2. 1. 1 Strip Reinforcement In systems using 7.2.2.1 Backfill Systems strip reinforcement, a coherent material is formed The concept of reinforcing the backfill behind by placing the strips horizontally between succes- retaining walls was developed by H. Vidal in sive layers of backfill (Figure 17-15). When the France in the mid-1960s. Since then, thousands of French architect and engineer Vidal introduced walls reinforced with all types of inclusions have the concept in the 1960s, he named his develop- been successfully built throughout the world. ment Terre Armée (Reinforced Earth). Reinforced Reinforced-soil structures have the advantage Earth has since become almost a generic term, over more traditional retaining walls (Mitchell often being used to describe all forms of soil rein- and Villet 1987) in that they forcement. However, in some countries, including the and Canada, Reinforced Earth Are coherent and flexible and thus can tolerate is a trademark (Jones 1985). The first Reinforced relatively large settlements, Earth wall was built near Nice, France, in 1965. Can use a wide range of backfill materials, As of 1991, 16,000 Reinforced Earth walls with a Are easy to construct, Are relatively resistant to seismic loadings, Can form aesthetically attractive retaining walls and slopes because of a variety of available facing types, and Are very often less costly than conventional re- taining structures, especially for high steep slopes and high walls.

Steep slopes of reinforced soil or vertical retaining walls take up less of the right-of-way, and thus they are especially attractive for landslide repairs adjacent to transportation facilities. Common types of backfill reinforcement inclu- sions include steel or polymeric strips, steel or FIGURE 17-15 polymeric grids, geotextile and sheets, and Component parts of steel cables or bars attached to different anchor Reinforced systems. The two main mechanisms of stress trans- Earth wall (modified fer between the reinforcement and the soil are from Lee et al.1973). REPRINTED WITH PERMISSION OF 1. between the surface of the reinforce- AMERICAN SOCIETY OF CIVIL ENGINEERS ment and the soil, and Stabilization of Soil Slopes 457 total face area of 9 600 000 m2 had been con- structed worldwide (D. McKittrick, personal com- 40, a I I munication, 1991, Reinforced Earth Company, I McLean, Virginia). Schlosser (1990) observed I that about one-third of the Reinforced Earth wall area in the world has been built in Europe and about one-third in North America. After the introduction of Reinforced Earth, knowledge of soil reinforcement increased rapidly, a) Vertical geotextile facing e) Sloping geotextile facing primarily because of the research sponsored by government agencies, notably the Laboratoire I Central des Ponts et Chaussées (LCPC) in France, the Federal Highway Administration in the I United States, and the Department of in the United Kingdom. Early experiments with fiberglass-reinforced polymers (Schlosser 1990), stainless steel, and alu- minum strips for Reinforced Earth walls were not successful, so all Reinforced Earth walls currently Vertical precast concrete f) Sloping gunite or structural are constructed using galvanized steel strips. element facing facing However, corrosion rates of metals in soils are very difficult to predict, and even galvanized steel is subject to corrosion; thus free-draining sand-and- gravel backfills are specified to reduce corrosion potential. Epoxy-coated steel strips have been de- veloped and may offer higher resistance to corro- sion. Elias (1990) discussed the corrosion of epoxy strips as well as traditional steel-strip reinforcing. 1.1 In 1973 nonmetallic reinforcing strips were in- Vertical cast in-place g) Sloping soil and vegetation troduced in construction of a highway retaining concrete/masonry facing facing wall in Yorkshire, England (Holtz 1978; Jones 1978). The reinforcing strips were made of con- tinuous glass fibers embedded lengthwise in a pro- tective coating of epoxy resin. The Paraweb strip, in which the fibers are made of high-tenacity poly- ester or polyamarid, is another example of a geosynthetic strip reinforcement.

7.2.2. 1.2 Sheet Rein forcément In sheet rein- forcement, geotextiles are placed horizontally be- d) Vertical masonry facing h) Geotextile gabion tween layers of embankment soils to form a composite reinforced-soil system. Figure 17-16 shows a number of possible uses of geotextiles in reinforced walls and slopes. The mechanism of and geotextiles (Christopher and Holtz FIGURE 17-16 Possible reinforced stress transfer in sheet reinforcement is predomi- 1989; Koerner 1990). Granular soils ranging from walls and slopes silty sands to gravels commonly are used as back- nantly friction (Mitchell and Villet 1987; Chris- using geosynthetic topher and Holtz 1989; Christopher et al. 1990). fill, and the most common facings are formed by reinforcement A large variety of geotextiles with a wide range of wrapping the geotextile around the exposed soil (modified from mechanical properties is available; they include [Figure 17-16(a) and 17-16(e)]. Because geotex- Mitchell and Villet nonwoven needle-punched or heat-bonded poly- tiles are subject to vandalism and deterioration 1987). ester and polypropylene and woven polypropylene from ultraviolet light, for long-term protection the 458 Landslides: Investigation and Mitigation

exposed material must be covered with shotcrete thought to be more resistant to corrosion than or asphalt emulsion or with soil and vegetation. steel, but geosynthetic walls have been found to be Typical applications of geotext ile -reinforced walls 30 to 50 percent less expensive than most other re- include landslide stabilization on remote moun- inforcement systems. Still, there is some doubt as tain , highway retaining walls on steep to the durability and longevity of geosynthetic re- slopes, embankment walls for temporary or per- inforcing materials because of chemical and bio- manent road widening or diversion, and highway logical attack. One of the most comprehensive embankment walls on soft foundation soils studies of the effects of outdoor exposure on geo- (Mitchell and Villet 1987). was conducted in Hong Kong (Brand and The use of geotextiles in reinforced-soil walls Pang 1991). The study confirmed that geotextiles followed shortly after the introduction of Rein- should be protected and that their use in critical forced Earth. The first geotextile-reinforced wall reinforcement situations should be limited to was built in France in 1971. In the United States, short-term projects. Elias (1990) provided some in- the first such wall was one 3.3 rn high built by the formation on the durability of geosynthetics, as did U.S. Forest Service to reconstruct a failed road fill Allen (1991). A U.S. Federal Highway Admin- above the Illinois River in Siskiyou National istration study is currently under way on the dura- Forest, Oregon (Bell and Steward 1977). The ge- bility of geosynthetics for soil reinforcement. otextile was a 440-g/rn2 needle-punched nonwo- yen polypropylene that was permeable and 7.2.2. 1.3 Grid, Bar, and Mesh Reinforcement resistant to rotting. However, it was subject to de- In grid reinforcement, polymeric or metallic ele- terioration when exposed to ultraviolet light; ments are arranged in a rectangular grid shape. The two-dimensional grid-soil interaction in- therefore, the wall was finished with a shotcrete volves both friction along the longitudinal grid el- facing to protect it from sunlight. An example of a ements and passive bearing resistance against the very successful short-term use of geosynthetic re- transverse elements. Because of this passive resis- inforcement was reported by Allen et al. (1992). tance, grids are more resistant to pullout than flat A 12.6-rn-high wall supporting a 6-rn-high sur- steel strips; however, full passive resistance devel- FIGURE 17-17 charge was used to preload a compressible founda- ops only after relatively large displacements (5 to Geosynthetic- tion for a bridge abutment (Figure 17-17). 10 cm) (Schlosser 1990). reinforced wall on During the 1980s, the use of geotextile soil re- Probably the greatest development in the use of -90 in Seattle, inforcement for retaining walls increased signifi- grids for soil reinforcement has been polymeric ge- Washington. cantly; more than 80 projects were completed in COURTESY OFT. M. ALLEN, ogrids. Geogrids are relatively stiff, netlike materi- WASHINGTON STATE North America during this period (Yako and DEPARTMENT OF als, with open spaces called apertures that usually TRANSPORTATION Christopher 1988). Not only are geosynthetics measure 1 to 10 cm between the elements. They are made of polypropylene, , or PVC- coated polyester. Applications of grids to reinforced walls and slopes are shown in Figure 17-16. The first use of polymeric geogrids was by Japanese engineers in the 1960s to reinforce subsoils for railway embank- ments (Jones 1985). Because these original grids were made of nonoriented polymers, they were rel- auvely fragile and lacked the necessary tensile strength to sc'rvc i,, reinforceixieui. I luwever, they were quite effective as aids (Iwasaki and Watanabe 1978). In the 1970s, advances in the lormulatioti of polymers led to significant improve- ments in rheir strength and stiffness, and geogrid were developed with polymers oriented in orthogo- nal directions, which provided increased direc- tional strength. These high-strength geogrids were lust used in 1979 to construct a reinforced-soil wall Stabilization of Soil Slopes 459

30 at a railroad station in Yorkshire (O'Rourke and 1 to 5 Jones 1990). 1 to 4 - 25 In 1981 geogrids were used to repair slope fail- > Clay fill with e added ures in cuts on the Ml and M4 motorways in , 20 Geosynthetic layers England (Murray and Irwin 1981; Murray 1982; Jones 1985). Considerable cost savings resulted because the landslide debris was reused together with the geogrid reinforcement (Figure 17-18). No new backfill material was required. In 1983 the first geogrid wall in the United States was built to I Limit of Drainage layer stabilize a landslide near Newport on the Oregon excavation 01 1 L L _____L ______coast (Szymoniak et al. 1984). This 9-rn-high ge- 80 70 60 50 40 30 20 ogrid wall with a face slope of 80 degrees was se- Distance from barrier (m) lected over other alternatives because FIGURE 17-18 Location of boreholes and inclinometers after remedial measures It had the lowest estimated cost, and (Murray 1982). The open face of the grid wall allowed estab- lishment of vegetation that provided a natural appearance compatible with the surroundings of the adjacent state park. .,-..-.-. At about the same time, Forsyth and Bieber 4• _i.. (1984) reported on the construction of a geogrid- reinforced slope in California to repair a slope fail- ure. It was 9.5 in high and had a slope of 48 degrees. Figure 17- 19(a) shows the slope under : construction, and Figure 17-19(b) shows the com- 3 pleted structure. Since the construction of these early walls, more than 300 polymeric geogrid walls and slopes have been constructed in the United States (Mitchell and Christopher 1990). Figure 17-20 shows another, nearly completed landslide repair on a state highway in California. Several bar-and-mesh reinforcement systems have been developed that rely on hoth frictional and passive resistance to pullout. In 1974 the first "bar mat" system of soil reinforcement was devel- oped by the California Department of Trans- portation (Caltrans) to construct a 6-rn-high wall along I-S near Dunsmuir, California; these crude grids were formed by cross-linking steel reinforcing bats to fouii a LOaie bat titat (Fo1yil1 1978). Laboratory tests by Chang et al. (1977) showed that this bar-and-mesh reinforcement could produce more titan live times the pullout resistance of longi- tudinal bars. In an agreement with the Reinforced Earth Company, the Caltrans bar-and-mesh rein- forcement technique was designated Mechanically FIGURE 17-19 of the difficul- Stabilized Embankment (MSE). One La Honda, California, landslide repair using geogrids: (a) under construction ties with MSE in the field has been corrosion of the and (b) completed slope face. bar-and-mesh reinforcement. Evolving from the COURTESY OF TENSAR EARTH TECHNOLOGIES. INC. 460 Landslides: Investigation and Mitigation

FIGURE 17-20 introduced commercially in 1983 on New Mexico Nearly completed State Highway 475 northeast of Santa Fe, where landslide repair four reinforced-soil structures were built with a total using geogrids, of 1600 m2 of wall facing. By 1990 more than 50 ad- Highway 9 near ditional RSE walls had been constructed (Mitchell Santa Cruz, California. and Christopher 1990). ;sTry orrL;,\ The VSI_ Retained F.arrh system utilizes 0.5- to 0.75-rn-wide strips of steel grid (bar-inal) rein- forcement that is bolted to hexagonal precast con- crete panels. The first VSL Retained Earth wall in the United States was constructed in 1983 in Hayward, California. By 1990 more than 600 VSL Retained Earth walls with some 465 000 m2 of wall facing had been built in the United States (Mitchell and Christopher 1990). The system is licensed in the United States under a Reinforced Earth patent, but it uses its own patented system to connect the bar-mat reinforcement and the concrete facing panels. The Georgia Stabilized Embankment was de- veloped by the Georgia Department of Transpor- tation and uses a steel-grid reinforcement and precast-concrete face elements. It is licensed in Caltrans project and other independent develop- the United States under a proprietary agreement inents were the Hilfiker Welded Wire Wall with the Reinforced Earth Company. (WWW), Hilfiker Reinforced Soil Embankment As with other reinforcement systems, geogrids (RSE), VSL Retained Earth, and the Georgia Sta- and steel bar mats are susceptible to environmen- bilized Embankment systems. tal deterioration and corrosion, and the prudent The Hilfiker WWW uses the same type of designer makes certain of the chemical compati- welded-wire reinforcing mesh that is commonly bility of the backfill and reinforcement materials. placed in concrete slabs; the facing is formed of the Full-scale experiments with the sidevalls of used same mesh that serves as the horizontal reinforce- tires hooked together with a bent No. 3 rehar clip to ment. The material is fabricated in 2.4-rn-wide reinforce embankment fills have been carried out by mats with grid spacings of 15 by 61 cm. To the Caltrans (Forsyth and Egan 1976; Forsyth 1978). casual observer, WWW may appear to be a type of Sometimes the entire tire casing is used. The tires gabion wall [Figure 17-16(h)]. Gahion walls, how- are placed in layers similar to geotextiles and other ever, are gravity walls made by encasing coarse- geogrids. Results to date have been very promising, grained fill in wire or geogrid baskets; they are based although there is some difficulty in properly com- on the principle of confinement and gravity retain- pacting backfill in and around the tires. The French ment rather than internal tensile reinforcement Ministry of Transport also has used similar systems (Hausmann 1990). The first commercial WWW of used tire casings for reinforcing slopes, embank- was built for the Southern California Edison Power ments, and walls. Research began at the LCPC on Company in 1977 for road repair along a power line pneusol (tiresoil) in 1976, and so far more than 80 in southern California. During the 1980s, the use of structures have been built with pneusol in France WWW for retaining structures expanded rapidly. and Algeria (Long 1990). By 1990 about 1,600 such walls had been com- pleted in the United States (Mitchell and 7.2.2. 1.4 Earth-Anchor Reinforcement Embank- Christopher 1990). The tallest WWW constructed inent soil slopes can he reinforced during construc- so far is nearly 17 m high (Anderson et al. 1987). non by slender steel-rod reinforcements bent at one The Hilfiker RSE, which resembles the Caltrans end to form anchors. This type of is MSE, is a continuous welded-wire reinforcement still in an experimental stage. Stress transfer is system with precast concrete facing panels. It was mainly by passive resistance on the anchor, which Stabilization of Soil Slopes 461 implies that the system provides stability in the FIGURE 17-21 same manner as tied-back retaining structures and Schematic diagram thus may not be a true reinforced-soil system of Anchored Earth retaining wall (Mitchell and Villet 1987). However, the system is (modified from discussed here because it is analogous in placement Murray and Irwin technique to other methods of soil reinforcement in 1981). embankrnents and fills. The concept of earth-anchor reinforcement was developed and patented by the Transport Research Laboratory (TRL) of the United Kingdom as Anchored Earth (Figure 17-21). The reinforcement consists of 16- to 20-mm-diameter mild-steel bars. The outer end of each bar is threaded to fit into concrete facing panels; the other end is formed into an anchor in the form of a Z or triangle. Anchored Earth is designed to rely only on passive resistance developed against the deformed ends (anchors) of the reinforcing bars. The first Texsol wall, with a face angle of 60 de- Because Anchored Earth is still in the research grees to the horizontal, was built in France in 1983 and developmental stages, none of its applications (Leflaive 1988). By the end of 1988, 85 Texsol can be considered to be routine. However, it does projects had been built in France, using 100 000 appear to be a promising approach to soil rein- m3 of Texsol-reinforced soil (Schlosser 1990). forcement, especially on projects where clean Most projects involved landslide repairs and granular backfill is not available. failed-slope reconstruction [Figure 17-22(a)]. A similar concept was used successfully by These walls and slopes have a high bearing Fukuoka and Irnamura (1982) in Japan in the late capacity and relatively rapid growth of vegetation 1970s to construct a 5-m-high fabric-faced retain- on the slope face [Figure 17-22(b)] and are self- ing wall with multiple anchors. Each of the 20-mm- healing when subjected to erosion. diameter steel tie bars was attached to a 40- by 40-cm concrete plate embedded in the backfill soil. Another material that has been suggested for use as fiber reinforcement is bamboo, which is one of the fastest-growing and most replenishable biologi- 7.2.2.1.5 Fiber Reinforcement The engineering use of fiber reinforcement of backfill soil, which is cal materials in existence. Bamboo can also be used analogous to fiber reinforcement of concrete, is still as continuous elements in other types of reinforced in the developmental stage. Materials being investi- soil. Fang (1991) has published data on the strength gated for possible use include synthetic fibers, con- and durability of bamboo as soil reinforcement. tinuous synthetic filaments, metallic fibers (metal threads) and powders, and natural fibers (reeds and 7.2.2.2 In Situ Systems other plants) (Mitchell and Villett 1987). A recent In situ soil reinforcement methods allow for the innovation is a three-dimensional reinforcement reinforcement of existing soil masses. These meth- technique that was developed in France at the ods include techniques described as soil nailing, LCPC in 1980 (Schiosser 1990) and known as the application of soil anchors, or the use of root Texsol. Texsol is made by mixing the backfill soil, piles, micropiles, or pin piles. usually a clean sand, with a continuous polyester fil- ament with a diameter of 0.1 mm (Leflaive 1982). 7.2.2.2. 1 Soil Nailing Soil "nails" are steel bars, Approximately 0.1 to 0.2 percent by weight of the rods, cables, or tubes that are driven into natural composite material consists of the filament and a soil or soft rock slopes or are grouted into. pre- total length of reinforcement of about 100 km/rn3 of drilled boreholes. Soil nailing can be used to re- reinforced soil. In the field the sand is deposited strain either using a shotcrete system (without cement) and the filaments are spun from bobbins and carried along Potentially unstable slopes where little or no with the high-velocity sand. movement is occurring but where safety factors 462 Landslides: Investigation and Mitigation

Soil nailing has been used for in situ stabilization of natural and excavated slopes for nearly 20 years. In North America the system was first used in Vancouver, British Columbia, in the early 1970s for temporary excavation support (Shcn ct al. 1978), In Europe the earliest reported soil-nailing projects were for retaining wall construction in Spain (1972), France (1973), and Germany (1976) in con- nection with highway or railway cut-slope construc- tion or temporary smipporr for huilding excavations (Elias and Juran 1991). Today the technique of soil imailing is widespread in Canada, Germany, France, Great Britain, Japan, and the United States. The stability of soil-nailed reinforcement relies upon

Development of friction or adhesion mobilized at Ilie soil-nail intel lace and Passive resistance developed along the surface perpendicular to the direction of the soil-nail relative movement.

Soil nailing is most effective in dense granular and low-plasticity stiff silty clay soils because a top- down sequential cuiIsu ucimoit piocedute is coin- monly used. The soil must have sufficient strength

(b)

FIGURE 17-22 (above) are low enough to indicate a strong possibility Texsol: (a) wall under for future movement, or construction in Creeping slopes, in which movement is actually French Alps; (b) 1 0-rn-high occurring. Texsol slope with vegetation 1 year Together with the in situ soil, the nails form co- after construction. herent reinforced-soil structures capable of stop- COURTESYOFJ.L ping the movement of unstable slopes [Figure WALKINSI LW, FEDERAL HIGHWAY 17-14(f)] or of supporting temporary excavations AD\IINNFRATION (Figure 17-23). Nailing differs from tieback sup- port systems because the nails are passive elements that are not posttensioned; also, the nails are usu- FIGURE 17-23 ally spaced more closely than tiebacks. Commonly, Soil nails extruding one nail is used for each 1 to 6 m2 of soil surface from shotcrete area. Stability of the surfce between the nails is surface of temporary provided by a thin layer (10 to 15 cm) of shotcrete excavation for tunnel portal, 1-70, reinforced with wire mesh (Figure 17-23) and by Glenwood Canyon, intermittent large steel washers or panels (which Colorado. later may be covered with shotcrete). Stabilization of Soil Slopes 463 to be able to stand in a vertical cut of about 2 in without failure or excessive deformation. Thus, soil Original nailing is generally not cost-effective or practical in Unstable loose granular soils and very soft clays (Mitchell Slope and Christopher 1990). A high groundwater table may also present construction difficulties. Anchored ..... Spider Netting •....• Suggestions for design of soil nailing include eci (in tension)..' those by Elias and Juran (1991) and Byrne (1992). Soil-nailing systems are relatively flexible and thus CP should be resistant to seismic loading. However, knowledge of the dynamic behavior of soil-nailed Nails or structures is limited (Felio et al. 1990), and re- Anchors search is needed to develop procedures for earth- lure Surface quake-resistant design. There are no proprietary restrictions on the use of soil nailing. However, some specific installation systems, nails, and fac- ings are patented. Instead of shotcrete facing, a geotextile or geonet FIGURE 17-24 (above) The technique of soil nailing has been used is attached to the nails, and as the nails are in- Schematic cross section of anchored mostly for stabilization of temporary excavations. stalled, they pull the surface netting or geotextile geosynthetic "spider There is some concern about the corrosion rate of into the soil, putting it into tension and at the netting" used with the steel nails used in the process. However, new same time constraining the near-surface soils in soil nails or anchors types of nails and coatings with high resistance to the slope (Koemer and Robins 1986). Design pro- to stabilize slope corrosion are being developed. To further increase cedures for anchored geosynthetic systems were (modified from confidence in the potential use of this method for described by Hryciw and Haji-Ahmad (1992). Koerner and Robins 1986). permanent slope stabilization, additional research is being done on the field performance of soil-nailed 7.2.2.2.2 Soil Anchors Stabilization of soil FIGURE 17-25 structures. For example, in 1986 a 4-year,$4 million slopes with long prestressed anchors is increasing. Cross section national research program named CLOUTERRE Hutchinson (1984) described the stabilization of of landslide in was initiated by the French Ministry of Transport to a landslide in glacial and glaciofluvial deposits in Quaternary deposits improve the status of knowledge and to develop de- southern Wales (Figure 17-25). Before treatment at Nantgarw, South Wales, United sign and construction guidelines for soil-nailed re- in 1980, movements of as much as 15 mm/year Kingdom (modified were occurring at the head of the landslide and 2 tention systems (Schlosser and Unterreiner 1991; from Hutchinson Schlosser et al. 1992). Plumelle and Schlosser to 5 mm/year was occurring at the toe. Because of 1977); slide has (1991) presented the results of three full-scale ex- severe spatial constraints, anchoring into the un- been stabilized by periments from this project. derlying bedrock proved to be the most effective deep anchors in A number of interesting case histories con- stabilization measure. underlying bedrock. cerning the use of soil nailing for support of temp- orary excavations are available in both the French and U.S. literature [for example, Lambe and Taft Vale Hanson (1990)]. Denby et al. (1992) described Trunk Road 0 10 20 m Tension the design, construction, and performance of a Crack 23-rn-deep excavation in Seattle, Washington, that was successfully supported by a temporary iff soil-nailed wall. Collin et al. (1992) used a timber crib wall as the facing and helical anchors as the Old Cardiff Quaternary soil nails for a permanent earth retention system for a slope. There appear to be no published cases in which soil nailing has been used to stabilize Fail .i osiBedrock Anchors active landslides or moving slopes. A related system is anchored geosynthetics (spi- der netting) for slope stabilization (Figure 17-24). 464 Landslides: Investigation and Mitigation

7.2.2.2.3 Root Piles, Micropiles, and Pin Piles jects that illustrate recent developments in U.S. Another approach to reinforcement of in situ soils pin-pile technology and discussed applications and soft rocks is the use of root piles, also known as more specific to slope stabilization (Bruce 1992b). micropiles or pin piles. Root piles are cast-in-place An interesting case history of the use of pin piles reinforced concrete piles with diameters ranging to control slope movements was described by from 7.5 to 30 cm. In the smaller-diameter range, Pearlman et al. (1992). the insertions are provided with a central rein- forcing rod or steel pipe, whereas those with larger 7.2.3 Vegetative and Biotechnical diameters may be provided with a reinforcing-bar Stabilization cage bound with spiral reinforcement. A root-pile system forms a monolithic block of reinforced soil Slope stabilization provided directly by vegetation that extends below the critical failure surface and by biotechnical slope protection (the use (Figure 17-26). In contrast to soil nailing, the of vegetation combined with structural slope- reinforcement provided by root piles is strongly stabilization elements) is reviewed briefly. The influenced by their three-dimensional, rootlike basic concepts of vegetative stabilization are not geometric arrangement. new, but recent research and development now Reticulated Root Piles were originally devel- enable more effective use of this technique than oped in the 1950s by Lizzi (1977) and were in the past. Additional information about bio- patented by the Italian firm Fondedile of Naples, stabilization was provided by Gray (1970), Gray which introduced and installed the system world- and Leiser (1982), Greenway (1987), and Wu wide (mainly for underpinning). The original (1994a, 1994b). patents have now expired. Root piles have been According to Wu (1994b), vegetation contri- used for slope stabilization only for the last 20 butes to stability of slopes through (a) root rein- years, and most root-pile slope stabilization works forcement and (b) rainfall interception and have been constructed within the past 10 years. evapotranspiration, which reduce pore pressures. Aurilio (1987) described the use of root piles for Case studies have shown that slope failures can be stabilization of a landslide in California. attributed to the loss of reinforcement provided by According to Bruce (1992a), during the past 20 tree (Wu et al. 1979; Riestenberg and years, U.S. practice using root piles has developed Sovonick-Dunford 1983; Riestenberg 1987). In quite differently from the original European de- spite of the fact that Greenway (1987), in his ex- velopment. He provided details from some 20 pro- tensive summary of the effects of vegetation on slope stability, included reports that vegetation tends to reduce slope stability, most researchers believe that vegetation is by far a positive aspect in the protection of slopes. Wu (1 994b) quantified this protection in terms of root reinforcement and reduction of soil moisture and pore pressures. Research into the engineering role of vegeta- tion for slope stabilization by the Geotechnical Control Office of Hong Kong may be the most comprehensive such program in the world (Barker 1991). Especially notable are the root reinforce- ment studies conducted on vegetated slopes by Greenway et al. (1984), Greenway (1987), and Yin et al. (1988). In addition, the Geotechnical FIGURE 17-26 Manual for Slopes (Geotechnical Control Office Schematic cross 1984) includes information on the mechanical section illustrating use of root piles for Failure Surfac and hydrological effects of vegetation. stabilization of slope In recent years trees have been planted on (modified from Lizzi many slopes worldwide to increase slope stability. 1977). One example is an element of a program to cor- Stabilization of Soil Slopes 465 rect an embankment failure on 1-77 in Caidwell, EXAMPLES Ohio. The slope was planted with black locust CATEGORY seedlings at a spacing of 1.2 m. The long-term ob- LlPlul @©TI1D©1fl©J jective was to help reduce the soil moisture and to Grass seeding develop root stabilization. As of 1994 the project Conventional Sodding Plantings * Transplants appeared to be successful. Another recent well-documented case of the - * Live staking planting of tree seedlings occurred as part of the Woody plants used Contour wattling stabilization program for the Cucaracha landslide as reInforcements * Brush layering and barriers to in the Gaillard Cut at the Panama . The his- * Soft gabions soil movement * toric Cucaracha landslide was reactivated in 1986, Brush mattress almost blocking the canal (Berman 1991). As part Breast walls with of a comprehensive stabilization program, begin- slope face plantings Piant/ structure * with ning in 1987 portions of the Cucaracha landslide associations slope face plantings and other landslide areas in the Canal Zone were 99 * planted with 60,000 fast-growing acacia and Tiered structures with bench plantings grnelina seedlings (Rivera 1991). Stabilization of slopes by the combined use of Woody plants grown * Live cribwalls vegetation and manufactured structural elements In the frontal openings Vegetated rock gabions © or Interstices of . Vegetated geogrid walls working together in an integrated manner is c retaining structures * Vegetated breast walls known as biotechnical slope stabilization. This relatively new concept is generally cost-effective * Joint plantings as compared with the use of structures alone; it has !L!J Woody plants grown * Staked gablon matresser * increased the environmental compatibility of such in the frontal openings Vegetated concrete or Interstices of treatments and allows the use of indigenous nat- block revetments porous revetments * Vegetated cellular grids ural materials. Although vegetative treatments Reintorced" grass alone are usually much less expensive than earth- retaining structures or other constructed slope protection systems, their effectiveness in arresting IIrir ©©T@TO©I1I * Concrete gravity walls * Cylinder pile walls slope movement or preventing soil loss under ex- Conventional * treme conditions may be much lower than that Structures Tie back walls of the structures (Gray and Leiser 1982). Figure 17-27 presents different biotechnical slope stabi- lization and protection measures and examples. Grasses and woody plants are used most often FIGURE 17-27 in biotechnical stabilization. They have a true re- species, such as willow and dogwood, that rooted Classification inforcing function and should not be considered readily from cuttings (Figure 17-28). The branches acted as reinforcement and as horizontal drains, and of different merely cosmetic adjuncts to the structure. They biotechnical slope may be planted on a slope above a low retaining rooting of the embedded stems provided secondary protection and wall, or the interstices of the structure may be stabilization. erosion control planted with vegetation whose roots bind together Thomas and Kropp (1992) described a very in- measures (Gray and the soil within and behind the structure. The sta- teresting scheme using vegetation to stabilize a Sotir 1992). large debris-flow scar in California. At the time REPRINTED WITH bility of all types of retaining structures with open PERMISSION OF AMERICAN the paper was presented in 1992, the vegetation SOCIETY OF CIVIL gridwork or tiered facings benefits from such veg- ENGINEERS etation (Figure 17-27). was quite effective and the project was considered Gray and Sotir (1992) described the use of by the authors to be a success. a crushed-rock-blanket toe berm combined with a brush-layer fill to stabilize a road cut along a 7.2.4 Miscellaneous Stabilization Methods scenic highway in . The crushed rock was placed at the toe of the cut; the brush- The following miscellaneous stabilization meth- layer fill included stems and branches of plant ods are discussed: 466 Landslides: Investigation and Mitigation

icals in slope stabilization has been obtaining ade- CROSS SECTION VIEW: quate injection of the chemical into the soil in the LIVE curnNGs zone of rupture. Gedney and Weber (1978) presented four case OMPACTED FILL histories on the use of chemicals to stabilize land- . --. slides and other unstable slopes. Since that time, however, in spite of significant advances in grout- ing technology (Baker 1982; Borden et at. 1992), a only a few case histories could be located in which the use of grouting techniques for landslide stabi- : lization was described. PLAN VIEW: In recent years grouting techniques have been ...... :.. : used to inject lime slurry into the soil (Figure 17-29) (Boynton and Blacklock 1985). The slurry, which follows natural fracture zones, bedding sur- faces, and other surfaces of weaknesses, is injected through 40-mm-diameter pipes fitted with perfo- rated nozzles (Rogers 1991). The pipes are hydrau- 0 —Edge of fill lically pushed into the ground, and the slurry is injected to refusal at depth intervals of 300 to 450 mm. Typical injection pressures range from 350 to FIGURE 17-28 (above) Chemical treatment of the soils on the poten- 1300 kPa. In this way depths of more than 40 m Schematic of tial sliding surface, including grouting, ion ex- can be treated. brush-layer fill change, and injection of lime, portland cement, Blacklock and Wright (1986) discussed restora- construction (Gray and fly ash; tion of failed soil embankments on Interstate and Sotir 1992). highways in Alabama, Arkansas, and Missouri REPRINTED WITH Electrical treatment, primarily electroosmosis; PERMISSION OF using the lime- and lime-fly ash-slurry injection AMERICAN SOCIETY OF and CIVIL ENGINEERS Thermal treatment, including freezing. method of in situ . Baez et al. FIGURE 17-29 (1992) evaluated the use of lime/fly ash (L/FA) Schematic cross 7.2.4.1 Chemical Treatment slurry injection in the slope rehabilitation of a section illustrating The use of chemicals to treat unstable soil slopes is on the lower Chariton River in Missouri. process of lime-slurry an attractive possibility. Because of their success in Their studies showed that double injection of the pressure injection stabilizing compacted soils in highway and airfield L/FA increased the strength of the levee soil by 15 for stabilization of to 30 bercent. Furthermore, in the 4 years since embankment slope subgrades, portland cement, time, fly ash, and cal- the levee was treated with L/FA injections, no (modified from cium chloride have been suggested as suitable Boynton and chemicals for treatment of unstable slopes. How- slope failures have occurred in the rehabilitated Blacklock 1985). ever, a major problem in using any of these chem- stretch of the levee, whereas there have been fail- ures in adjacent unstabilized areas. Lime stabilization was very effective in improv- ing the strength of slope debris that was recom- Embankment pacted in the reconstruction of a landslide in Injection Rig England (Murray 1982). (This is an interesting case Slurry Path history from another viewpoint in that it is the first use of geogrids to increase stability of a failed slope.) Considerable economy was achieved by use of land- Original Ground slide debris as backfill rather than the traditional — --- excavation and removal of the debris and its re- 'SettIement Line placement with higher-quality granular materials at considerable additional cost. A geotextile-wrapped oft Clay T ension Cracks granular drainage blanket was provided behind the grid- and lime-stabilized slope. Probably all three Stabilization of Soil Slopes 467 treatment techniques (drainage, time stabilization, which leads to permanent increases in shear and geosynthetic reinforcement) contributed to the strength and consequent increases in slope stabil- success of the project. ity. However, the high cost of thermal treatment precludes its use on all but the most experimental 7.2.4.2 Electroosmosis slope remediation problems. Another special technique that increases the shear- has developed in recent years ing strength of soils in situ is electroosmosis. The to be a very effective technique for temporary sta- classic papers on electroosmosis were published by bilization of large excavations and tunnels. One of Casagrande (1948, 1952, 1953). There are a num- the most complete treatises on ground freezing is ber of case histories in the literature in which elec- by Jessberger (1979). troosmosis was used to stabilize embankment foundations; these were summarized by Holtz (1989). However, the classic application of elec- troosmosis has been to stabilize landslides and Allen, T. A., B. R. Christopher, and R. D. Holtz. 1992. slopes. An early case history was reported by Performance of a 12.6 m High Geotextile Wall in Casagrande et al. (1961) in which electroosmosis Seattle, Washington. In Geosynthetic Reinforced Soil was used to stabilize a 30-m-high slope in organic Retaining Walls, Proceedings of the International that had developed serious instability during con- Symposium, Denver (J.T.H. Wu, ed.), A.A. Bal- struction of a pier for the Little Pic River bridge on kema, Rotterdam, , pp. 8 1-100. the Trans-Canada Highway in Ontario. Casagrande Allen, T A., and R. D. Holtz. 1991. Design of et al. (1981) also described the use of electroosmosis Retaining Walls Reinforced with Geosynthetics. to stabilize a slope for an 80-m-deep excavation for In Proc., Geotechnical Engineering Congress 1991, the core trench of a dam in British Columbia. Boulder, Colorado (F. G. McLean, D. A. As summarized by Veder (1981), Griffin de- Campbell, and D. W. Harris, eds.), Geotechnical Special Publication 27, American Society of scribed the use of electroosmosis to stabilize a Civil Engineers, New York, 970-987. 30-rn-high slope of loose, sensitive silt above pp. Allen, T.M. 1991. Determination of Long-Term the Kootenay power plant in British Tensile Strength of Geosynthetics: A State-of-the- Columbia, Canada. As a result of electroosmosis, Art Review. In Proc., Geosynthetics '91 Conference, the slope, which originally was barely stable at Atlanta, Industrial Fabrics Association Interna- 1V:5H, was able to be safely steepened to 1V:2H. tional, St. Paul, Minn., Vol. 1, pp. 351-379. In previous discussion concerning electroos- Anderson, R. L., K. D. Sharp, and 0. T Harding. motic dewatering (Section 6.2.3.1), it was noted 1987. Performance of a 50-Foot High Welded Wire that Lo et al. (1991a, 1991b) developed special Wall. In Soil Improvement—A Ten Year Update (J. P. electrodes that allow free water flow without Welsh, ed.), Geotechnical Special Publication 12, pumping. In field tests conducted on the soft, sen- American Society of Civil Engineers, New York, sitive Leda clay of eastern Canada, undrained pp. 280-308. shear strength increased uniformly by about 50 Arutjunyan, R. N. 1988. Prevention of Landslide percent in a period of 32 days throughout the Slope Process by Vacuuming Treatment of Dis- Proc., Fifth International depth of the electrodes. Because no pumping was consolidated Soils. In Symposium on Landslides, Lausanne, July 10-15, required, both installation and electricity costs A.A. Balkema, Rotterdam, Netherlands, Vol. 2, were significantly lower than those in previous pp. 835-837. electroosmosis drainage installations. Aurilio, G. 1987. In-Situ Ground Reinforcement with Root Piles. In Soil Improvement—A Ten Year 7.2.4.3 Thermal Treatment Update (J. P. 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