SLOPE STABILITY AND STABILIZATION METHODS

Second Edition

LEE W. ABRAMSON Hatch Mott MacDonald Millburn, New Jersey

THOMAS S. LEE Parsons, Brinckerhoff, Quade & Douglas San Francisco, California

SUNIL SHARMA University of Idaho Moscow, Idaho

GLENN M. BOYCE Parsons, Brinckerhoff, Quade & Douglas San Francisco, California

A Wiley-lnterscience Publication JOHN WILEY & SONS, INC.

TJ FA 410 PAGE 001 CONTENTS

PREFACE xvii ACKNOWLEDGMENTS xix ABOUTTHE AUTHORS xxi at permitted by Section GENERAL SLOPE STABILITY CONCEPTS e permission of the r information should be Lee W. Abramson Inc. Introduction / 1 le information in regard Aims of / 2 hat the publisher is not Natural Slopes / 2 medical, psychological, etent professional Engineered Slopes / 3 Embankments and Fills / 3 Slopes / 15 / 18 Retaining Structures / 24 / 25 lization. 1.5.1 Features and Dimensions of Landslides / 25 1.5.2 Rates and Types of Movements / 29 926917 Factors Contributing to Slope Failures / 33 Basic Concepts Applied to Slope Stability / 34 Typical Input Data for Slope Stability Analyses / 36 1.8.1 Geologic Conditions / 36

TJ FA 410 PAGE 002 vi CONTENTS

1.8.2 Site Topography / 36 1.8.3 Possible Effects of Proposed Construction / 37 1.8.4 Material Properties / 39 1.8.5 / 42 1.8.6 Groundwater Conditions / 48 1.8.7 Seismicity / 49 1.9 Subsurface Model and Back-Analysis for Slope Stability Analyses / 51 1.10 Conclusions / 53 References / 53

2 ENGINEERING PRINCIPLES Thomas S. Lee 56 2.1 Introduction / 56 2.2 Types and Characteristics of Geologic Deposits / 56 2.2.1 Alluvial Deposits / 57 2.2.2 Glacial Deposits / 65 2.2.3 Eolian Deposits / 65 2.2.4 Residual Deposits / 67 2.2.5 Colluvial/Talus Deposits / 69 2.2.6 Marine Deposits ! 70 2.2.7 Melanges / 71 2.2.8 Other Types of Deposits / 72 2.3 Types and Characteristics of Rocks / 73 2.3.1 Shales / 74 2.3.2 Sandstones / 75 2.3.3 Limestones and Related Carbonate Rocks / 75 2.3.4 lgneous Rocks / 75 2.3.5 Pyrocrastic Volcanic Rocks / 76 2.3.6 Metamorphic Rocks / 76 2.4 Geologic Features Associated with Slopes / 76 2.4.1 Soil/Rock Fabric / 77 Geological Structures / 77 Discontinuities / 78 Groundwater / 78 Ground Stresses / 79 Weathering / 79 Preexisting Landslide Activities / 81

TJ FA 410 PAGE 003 CONTENTS vii

2.4.8 Mineralogy / 82 uction / 37 2.4.9 Seismic Effects / 84 2.5 Landslides / 84 Landslide-Prone Occurrences / 85 Fundamentals of Landslides / 93 Useful Clues to Landslide Investigations and 9pe Stability Identifications / 95 References / 99

GROUNDWATER CONDITIONS 102 Thomas S. Lee 56 Introduction / 102 Review of Groundwater Fundamentals / 103 :posits / 56 3.2.1 Movement of Groundwater / 104 3.2.2 Principles of Groundwater Mechanics / 106 Site Conditions / 108 Groundwater Levels / 108 Zones / 108 Aquifers / 111 Aquicludes / 112 Perched Water / 112 Artesian Water / 114 Springs / 114 Types of Groundwater Flow / 115 Runoff / 115 / 117 s/75 RegionalFlow / 118 Fluctuation of Groundwater Levels / 121 Rainfall / 121 / 123 Snowmelt / 124 Sudden Drawdown / 125 3.6 Influence of Geological Structures on Groundwater Flows / 125 Pore Pressures / 127 Positive Pore Pressures / 128 Negative Pore Pressures / 131 Measurement of Pore Pressures / 133

TJ FA 410 PAGE 004 viii CONTENTS

3.8 Water Levels forDesign / 138 3.8.1 General / 138 3.8.2 Wetting Band Approach / 139 3.9 Field Identification and Interpretation of Groundwater Conditions / 142 3.9.1 Field Identification of Groundwater Conditions / 142 3.9.2 Interpretation of Groundwater Conditions / 142 3.10 Groundwater in Slope Stability Analysis / 144 3.10.1 Developing a Groundwater Model from the Field Data / 144 3.10.2 Groundwater Effects on Slope Stability / 146 3.10.3 Groundwater in Rock / 150 3.11 M°nit°ring of Groundwater Pressures / 151 3.11.1 and Observation / 151 3.11.2 Installation ofPiezometers / 157 3.11.3 Fluctuating Groundwater Levels / 159 3.12 OtherInstruments--Rainfall Gages / 159 References / 159

GEOLOGIC SITE EXPLORATION Thomas S. Lee 162 4.1 Introduction / 162 4.2 Desk Study / 166 4.2.1 Available Existing Data / 166 4.2.2 Previous Geologic Explorations / 172 4.2.3 Identification of Landslide-Prone through Topographic Expressions / 173 4.2.4 Air Photos / 175 4.3 Field Study / 183 4.3.1 Site Reconnaissance / 183 4.4 Exploration Methods / 197 4.4.1 Introduction / 197 4.4.2 Auger Drilling / 198 4.4.3 Rotary Wash Drilling / 200 4.4.4 Limitations of Auger and Rotary Wash Drilling / 203 4.4.5 Sampling in the Ground / 203 4.4.6 Large / 204 4.4.7 Test Pits / 205

TJ FA 410 PAGE 005 CONTENTS ix

4.5 Testing Methods / 209 4.5.1 In Situ Testing / 209 :h / 139 4.5.2 Geophysical Testing / 223 ~tation of Groundwater 4.5.3 Downhole Geophysics Logging / 229 4.5.4 Mineralogy Tests / 232 Groundwater Conditions / 142 4.5.5 Radiocarbon Dating / 235 ~dwater Conditions / 142 4.6 Exploration Program Design / 235 Analysis / 144 4.6.1 Locations and Number of Boreholes / 235 ,ater Model from 4.6.2 Depth of Boreholes / 237 References / 238 Slope Stability / 146 150 ssures / 151 5 LABORATORY TESTING AND INTERPRETATION 242 vation Wells / 151 Sunil Sharma ters / 157 5.1 Introduction / 242 er Levels / 159 5.2 Concepts / 243 ges / 159 5.3 Mohr Circle / 244 5.4 Mohr-Coulomb Failure Criterion / 245 5.4.1 Mohr-Coulomb Failure Envelope--Unsaturated / 247 162 5.4.2 Mohr-Coulomb Envelope in p-q Space / 249 Effective/Total Stress Analysis / 250 5.5.1 Factors of Safety / 252 5.6 Stress Paths / 254 / 166 5.6.1 Typical Field Stress Paths / 257 orations / 172 5.7 Shear Strength of Soils / 259 de-Prone Terrains through Shear Strength of Granular Soils / 260 ~s / 173 Shear Strength of Fine-Grained Soils / 260 Stress-Strain Characteristics of Soils ! 261 Discrepancies between Field and Laboratory Strengths / 263 Strength Testing ! 269 Selection and Preparation of Test Samples / 271 Laboratory Test Conditions ! 272 200 The SHANSEP Method ! 274 t Rotary Wash Drilling / 203 Triaxial Tests ! 276 ! 203 ! 283 Direct Simple Shear (DSS) Test / 287 Unsaturated Tests ! 288

TJ FA 410 PAGE 006 CONTENTS

5.8 Pore Pressure Parameters / 291 Skempton’s Parameters / 291 Henkel’s Parameters / 291 Interpretations of Strength Tests / 293 Triaxial Tests / 293 Direct Shear Tests / 298 Unsaturated Tests / 302 Selection of Design Shear Strengths / 302 5.10 Other Properties / 305 Consolidation Tests / 306 Permeability Tests / 306 Compaction Tests / 307 Classification Tests / 308 Interpretations of Classification Tests / 310 Shrink/Swell Potential / 312 Slake Durability / 313 Collapsibility / 314 Dispersivity / 315 Chemical Tests / 316 X-Ray Diffraction Analysis / 318 Quality Control/Quality Assurance / 319 References / 321

SLOPE STABILITY CONCEPTS Sunil Sharma 329 Introduction / 329 Modes of Failure / 330 Factor of Safety Concepts / 332 Pore Water Pressures / 334 Phreatic Surface / 335 Piezometric Surface / 336 Example / 337 Negative Pore Pressures / 339 6.5 Block Analysis / 339 Example / 341 Infinite Slope Analysis / 343 Infinite Slopes in Dry / 343 Infinite Slope in c--¢ Soil with Seepage / 344

TJ FA 410 PAGE 007 CONTENTS xi

~,91 6.7 Planar Surface Analysis / 345 ters / 291 6.7.1 Planar Surface Example / 348 s / 291 6.8 Circular Surface Analysis / 349 ~ts / 293 6.8.1 Circular Arc (~b, = 0) Method / 349 6.8.2 q~, = 0 Example / 350 ’298 6.8.3 Circle Method / 350 302 6.8.4 Friction Circle Example / 352 Shear Strengths / 302 6.9 Method of Slices / 353 6.9.1 Ordinary Method of Slices (OMS) / 358 / 306 6.9.2 Simplified Janbu Method / 360 306 6.9.3 Simplified Bishop Method / 363 307 6.9.4 Generalized Limit Equilibrium (GLE) Method / 364 t 308 6.9.5 Janbu’s Generalized Procedure of Slices (GPS) / 367 tssification Tests / 310 6.9.6 Method of Slices--An Example / 370 al / 312 6.9.7 Control of Negative Effective Stresses / 375 13 6.9.8 Comparison of Limit Equilibrium Methods / 376 6.10 Selection and Use of Limit Equilibrium Methods / 378 6.10.1 Essential First Four Steps / 378 6.10.2 Selection of Analysis Method / 379 ~alysis / 318 6.10.3 Considerations for All Types of Analyses / 380 nce / 319 Design Charts / 380 6.11.1 HistoricalBackground / 381 6.11.2 Stability Charts / 381 329 Seismic Analysis / 393 Pseudostatic Method / 394 Newmark’s Displacement Method / 396 Accelerogram Selection for Newmark’s Method / 398 6.12.4 Computed Permanent Displacements / 399 6.12.5 Tolerable Permanent Displacements / 408 336 Other Factors Affecting Slope Stability Analysis / 409 Effect of Tension Cracks on Stability Analysis / 409 ~s / 339 Effects of Vegetation / 410 Loads on Slopes / 411 Three-Dimensional Analysis / 412 Rock Slope Stability / 413 ’,and / 343 The Finite Element Method (FEM) / 415 )il with Seepage / 344 6.16.1 Example of FEM Analysis of Slopes / 416

TJ FA 410 PAGE 008 xii CONTENTs

6.17 Computer Analysis / 419 6.17.! Available Computer Programs / 419 6.18 Probabilistic Analysis of Slopes / 420 6.18.1 Sources of Uncertainty / 420 6.18.2 Basic Probability Concepts / 421 6.18.3 Reliability Index / 424 6.18.4 Probabilistic Formulation for Slopes / 427 6.18.5 Probabilistic Analysis of Performance Function / 427 6.18.6 Quantifying Uncertainty / 436 6.18.7 Examples / 438 6.18.8 Summary / 452 References / 454

7 SLOPE STABILIZATION METHODS Lee W. Abramson 462 7.1 Introduction / 462 7.2 Unloading / 463 7.2.1 Excavation / 463 7.2.2 Lightweight Fill / 468 7.3 Buttressing / 470 7.3.1 Soil and Rock Fill / 474 7.3.2 Counterberrns / 474 7.3.3 Shear Keys / 475 7.3.4 Mechanically Stabilized Embankments / 477 7.3.5 Pneusol (Tiresoil) / 480 7.4 Drainage / 482 7.4.1 Surface Drainage / 482 7.4.2 Subsurface Drainage / 483 7.5 Reinforcement / 497 7.5.1 / 497 7.5.2 Stone Columns / 507 7.5.3 Reticulated Micropiles / 511 7.5.4 Geosynthetically Reinforced Slopes / 512 7.6 Retaining Walls / 520 7.6.1 Gravity and Cantilever Retaining Walls / 523 7.6.2 Driven Piles / 523 7.6.3 Drilled Shaft Walls / 524 7.6.4 Walls / 524

TJ FA 410 PAGE 009 CONTENTS xiii

Vegetation / 530 ms / 419 General Design Considerations / 531 ~20 Vegetation Species / 537 20 Mats and Blankets / 538 / 421 Biotechnical Stabilization / 540 Surface Slope Protection ! 541 ~r Slopes / 427 General Design Considerations / 542 rformance Function / 427 Shotcrete / 543 ~36 Chunam Plaster ! 544 Masonry / 546 Rip-Rap / 546 Soil Hardening / 547 Compacted Soil-Cement Fill ! 547 Electro-osmosis / 548 462 Thermal Treatment ! 549 Grouting / 549 Lime Injection / 550 Preconsolidation / 553 Rock Slope Stabilization Methods ! 559 Removal of Unstable Rock / 559 Catchment ! 562 Flattening of Slope ! 565 Buttresses ! 565 Surface Protection / 566 Lnkments / 477 Reinforcement ! 568 Drainage ! 568 Use of Explosives / 571 Rock Slope Stabilization Case Histories / 573 7.11 Alternatives to Slope Stabilization / 584 Complete Removal of Slide Zone / 585 Facility Relocation / 585 Bridging / 587 opes / 512 7.12 Selection of Stabilization Methods ! 587 7.12.1 Goals / 588 g Walls / 523 7.12.2 Technical Constraints / 588 7.12.3 Site Constraints / 589 7.12.4 Environmental Constraints / 590 7.12.5 Aesthetic Constraints / 590

TJ FA 410 PAGE 010 xiv CONTENTS

7.12.6 Schedule Constraints / 590 7.12.7 Other Constraints / 591 7.12.8 Cost / 591 7.13 Probable Cost Analysis of Stabilization Alternatives 591 7.13.1 Introduction / 591 7.13.2 Developing Alternative Schemes / 592 7.13.3 Estimating Outcomes and Probabilities / 592 7.13.4 Determining Probable Costs / 593 References / 598

8 DESIGN, CONSTRUCTION, AND MAINTENANCE Glenn M. Boyce 604 8.1 Introduction / 604 8.2 Contract Documents / 604 8.2.1 Contract Drawings / 605 8.2.2 Specifications / 606 8.2.3 Geotechnical Design Reports / 606 8.3 Inspection during Construction / 607 8.3.1 Inspection Guidelines / 607 8.3.2 Quality Control/Quality Assurance / 608 8.3.3 Instrumentation / 608 8.3.4 Instrumentation Monitoring / 627 Inspection following Construction / 630 Introduction / 630 Frequency of Inspections / 631 Technical Inspections / 632 Engineering Inspections / 632 Inspection Reports / 632 8.5 Maintenance / 633 Access ! 633 Slope Performance Observations 633 Instruments / 637 Drainage / 637 Adjacent Utilities / 638 Domain Reflectometry (TDR) 639 Introduction / 639 Mechanics of TDR / 639

TJ FA 410 PAGE 011 CONTENTS xv

590 Advantages and Disadvantages of TDR over / 640 References / 641 lization Alternatives 591

Schemes / 592 SHALLOW FAILURES 643 ad Probabilities / 592 Thomas S. Lee ;osts / 593 Introduction / 643 Seepage Flow Mechanism due to Infiltration / 644 Mechanism of Rainfall-Induced Landslides / 645 Field Loading Conditions / 647 ~IAINTENANCE 604 Correlations between Land Slides and Rainfall / 648 9.5.1 Antecedent Rainfall / 650 Rainfall Thresholds for Prediction of Shallow Failures / 651 Types of Soils, Hydrogeologic, and Geomorphologic Features / 652 Colluvium / 652 )orts / 606 / 653 607 Debris Flows / 655 607 Residual Soils / 656 \ssurance / 608 Rapid Snowmelt / 657 Effect of Permeability of Surficial Stability / 657 ag / 627 Standard Codes for Shallow Slope Stability / 658 / 630 Design Practice for Shallow Slope Stability / 661 9.10.1 Debris Flow Hazard Mitigation / 662 / 631 9.10.2 Design of Loess Slopes / 663 32 9.11 Attending Landslide Incidents / 664 632 9.12 Summary / 665 References / 665

STABILITY OF SLOPES 669 ’ations / 633 Lee W. Abramson Unique Nature of Landfills / 669 Typical Landfill Configurations / 669 Landfill Waste Engineering Properties / 675 ’ / 639 in Landfills and Engineering Properties / 679 10.4.1 / 680 10.4.2 / 680

TJ FA 410 PAGE 012 xvi CONTENTS

10.4.3 Geonets / 680 10.4.4 Geogrids / 682 10.4.5 Geosynthetic Clay Liners (GCLs) / 684 10.4.6 Engineering Properties of Geosynthetics / 685 10.4.7 Anchor / 686 10.5 Landfill Construction / 688 10.6 Slope Stability Considerations / 692 Excavation Slope Stability / 693 Waste Fill Stability / 696 Cover System Stability / 700 References / 702 INDEX 703

TJ FA 410 PAGE 013 1.4 ENGINEERED SLOPES 15

EMBANKMENT

FOUNDATION SOIL

STRAIN Figure 1.7 Example of stress-strain incompatibility.

Peak strengths of the and the foundation soils cannot be mobilized simultaneously because of stress-strain incompatibility (Figure 1.7). Hence, a stabil- ity analysis performed using peak strengths of soils would overestimate the factor of safety. Many engineers perform stability analyses using soil strengths that are smaller than the peak values to allow for possible progressive failure.

Shale Embankments Embankments constructed of shale materials often have slope stability and settlement problems. According to DiMillio and Strohm (1981), the underlying causes of shale fill slope failures and excessive settlement frequently appear to be: (1) Deterioration or softening of certain shales over time after construction (2) Inadequate compaction of the shale fill (3) Saturation of the shale fill TIME a clay foundation. (From These types of failures have been found to be typical in many areas from the Ap- palachian region to the Pacific coast. In general, severe problems with shales in embankments are found in states east of the Mississippi River rather than west of the river (DiMillio and Strohm, 1981). Embankments can use fill originating from shale tly stronger and stiffer formations successfully if the borrow source is not particularly prone to long-term ~.mbankment will crack decomposition and if adequate compaction and drainage are required. In addition, nd to the possibility of shale embankments should be keyed into any sloping surfaces by using benches and tween the embankment installing drainage measures to intercept subsurface water that may enter the foun- t Duncan (1977), using dation area. Guidelines for design and construction of shale embankments have been and progressive failure established by Strohm et al. (1978). harts may be used as a ynthetic reinforcement 1.4.2 Cut Slopes t failure in these cases. ion materials or locate Shallow and deep cuts are important features in any civil engineering project. The aim in a slope design is to determine a height and inclination that is economical

TJ FA 410 PAGE 014 GENERAL SLOPE STABILITY CONCEPTS

40 N( stren~ 35 tions agree: 30 f DATA COMPILED BY BRANDON ET AL., 1991

25 1.10 ’~ ~ ~ TREND UNE FROM VOIGHT. 1973 ~" 2o Probh UJ engin, diffen and (2 lO histor Wt edge ~ 5 mero[ can b~ 25 S0 75 100 125 150 stress- PLASTICITY INDEX (%) detern equilil Figure 1.32 Plasticity index residual friction angle relationship. (Voight, 1973, and Brandon the sk et al., 1991.) ataf~ decre[ Often to simplify the analysis, c’ is assumed equal to zero and only the residual are wi friction angle is used. Be~ (2) A stability analysis is performed using slope geometry, groundwater levels, slopes and external loading conditions at the time of failure. The analysis yields a establi factor of safety, FOS, that corresponds to the trial strengths from Step 1. (3) The trial strengths from Step 1 are adjusted using the safety factor computed REFE in Step 2, according to the following formulas: Alshun ~b~r (adjusted) = tan - t tan {~br (trial) } (Eq. 1-15) FOS No. Crr(adjusted) = Cr(trial) (Eq. 1-16) Bishop FOS Nos If extensive local experience with a particular material is available, that expe- Bishop St~ rience, as as Equations 1-15 and 1-16, could be used to adjust the strength Soil of some materials more than others. Note that, in general, q~’r is fixed at a cer- ~erru~ tain value and c’r is varied based upon local experience to expedite the time Cl~ involved in the back-analysis. Brando (4) The results of Step 3 can be verified by reanalyzing the slide using the newly I1 calculated strengths. The final back-calculated strengths that produce a safety Co~ factor equal to unity are appropriate for the existing sliding surface, where the Byme, i shear strength has been reduced to residual. tan,

TJ FA 410 PAGE 015 5.9 INTERPRETATIONS OF STRENGTH TESTS 301

TABLE 5.6 Comparisons of Existing Empirical Correlations for Drained Residual Friction Angle Solid Index Properties Reference Liquid limit and clay fraction Stark and Eid (1994) Clay fraction Collotta et al. (1989) Lupini et al. (1981) Skempton (1964, 1985) Plasticity index Kanji (1974) Lambe (1985) Mitchell (1993) Voight (1973) ~nt Liquid limit Bishop et al. (1971) Mesri and Cepeda-Diaz (1986) Mitchell (1993 )

¯ The proportion of platy particles to spherical particles in the soil and the coef- ficient of interparticle friction of the platy particles are factors that control the residual shear strength of the cohesive soils. ¯ Correlations between residual strength and soil index properties and/or gradation values derived from the cannot be general and should be used judiciously. comparison to the almost Stark and Eid (1994) suggested that the drained residual strength failure envelope is nonlinear for cohesive soils with a clay fraction greater than 50 percent, and a " than for a deep slide liquid limit between 60 and 220 percent. They also proposed that the drained residual friction angle is a function of liquid limit, clay fraction, and effective normal stress, ,ive soils reveal that the as shown in Figure 5.47. To model the effective stress-dependent behavior of the and the quantity of clay- fineralogy, and the clay-

2 millimeter. Therefore, [ Effective- Nor~a ase the drained residual [ Stress (kPa) 24 ~, ,,, -- -- 4oo radation, clay fraction, -- 700 e widely reported in the t and existing empirical zal correlations are very the basic soil properties ,. Based on many years l; Collotta et al.. 1989: nd Cepeda-Diaz, 1986; ions can be drawn: 40 8=0 120 160 200 240 280 320 LIQUID LIMIT (%) :lay content of cohesive Figure g.47 EmpMcal relationship between liquid limit ~d residual friction angle. (From Stark and Eid, 1994, reproduced by pe~ission of ASCE.)

TJ FA 410 PAGE 016 302 LABORATORY TESTING AND INTERPRETATION residual strength, Stark and Eid recommended that the nonlinear failure envelope or TABLE 5.7 a secant residual friction angle corresponding to the average effective normal stress Factor on the slip surface be used in a stability analysis. Sample distu of foundal 5.9.3 Unsaturated Tests (for relatix undisturb{ Constant water and controlled suction tests are adequate for testing unsaturated soils. It is preferable to test unsatnrated soils at their in situ moisture contents and in situ Effect of fiss stress conditions. Dead load tests may be valuable for obtaining threshold suction in clays, e (i.e., minimum suctions required for stability) and for studying the mechanism of highly ow failures that are initiated by infiltration. clays and Fredlund’s stress state variables may be used to assign unsaturated strengths, which effects no may be used directly for slope analysis using the total approach. To use this in small s~ approach it is important to obtain the "correct" saturated ~b’ value at low confining Rough caps pressures and the correct ~bb value from appropriate unsaturated tests. It must be noted in laborat, that it is difficult to obtain the "corrected" saturated ~b’ value. Laboratory evidence Triaxial corr (Howat and Shen, 1981) has shown that the conventional practice of back-saturation tests inste can destroy the fabric of an originally unsaturated soil specimen to such an extent compress that both dense and loose specimens can become medium dense. Typical values of ~bb shear, anc for seven different soils were given earlier in Table 5.1, which also includes values tests of conventional ct and ~b’ Mohr-Coulomb parameters. Triaxial inst If it can be shown that suction is maintained in a slope after heavy rainfall, then it strain test may be included in conventional stability analyses as an increase in apparent strength. Back-pressv If the variation of suction as a function of moisture content is high, as in the case of the volcanic and granitic residual soils in Hong Kong, then a small change in moisture content will result in a large change in suction. For such cases, it may be prudent to disregard the effects of suction for the slope stability analyses.

Convention~ 5.9.4 Selection of Design Shear Strengths of CU te~, as total sl The in situ strength of the soil that will be used to evaluate the factor of safety (FOS) envelope: for a slope must selected carefully with full consideration given to the many complex facets that may have affected the laboratory determination of shear strength. Table 5.7 Isotropic, ir anisotrol: attempts to present a quantitative appraisal of the many features that may lead to an consolid~ overestimation (unconservative) or an underestimation (conservative) of the in situ triaxial c, shear strength near a potential failure zone in a slope. tests Johnson (1974) states: (a) A f (b) Aj Evaluations of this type have little meaning unless they are done for a specific site Anisotropk and conditions, but even then required data are usually not available to permit reliable behavior conclusions. vertical i suitably Thus it is desirable that the engineer consider each of the factors listed in Table 5.7 test samt and assign a quantitative factor of confidence or uncertainty to each factor, as it may have influenced the reported laboratory test data.

TJ FA 410 PAGE 017 :303 e envelope or TABLE 5.7 Factors Influencing Design Shear Strengths Remarks normal stress Factor Influence (percent) -(5 to 20) Remolding may increase Sample disturbance strength of slickensided of foundation materials specimens. Disturbance (for relatively good, undisturbed samples) is greatest for deep borings ~turated soils. and soft soils tts and in situ Generally a factor for Effect of fissures +(25 to 1,000) shold suction highly overconsolidated ~n clays, especially soils only aechanism of highly overconsolidated clays and clay shales engths, which effects not reflected ’h. To use this in small samples ow confining Rough caps and bases +5 ~nust be noted in laboratory tests Especially important tory evidence Triaxial compression +(20 to 30) tck-saturation tests instead of for foundation soils ach an extent compression, simple 1 values of~bb shear, and extension cludes values tests Triaxial instead of plane -(5 to 8) finfall, then it strain tests trent strength. Back-pressure saturation Depends on field May cause grossly in the case of conditions excessive strengths :in CU tests at low ;e in moisture confining stresses; be prudent to conservative at high confining stresses Conventional plotting -(15 to 20) Effect may be eliminated of CU test data by plotting data according to Taylor’s safety (FOS) as total stress envelopes method aany complex Values shown assume gth. Table 5.7 Isotropic, instead of anisotropic, test envelopes for ray lead to an isotropic consolidation of the in situ consolidation in CU triaxial compression interpreted as rf tests versus ofO’f ’ "c ," 1.e., as 1 used by designers (a) Af > ~ -(0 to 30) (b) Af < ~1 +(0 to 20) in stability analysis ;pecific site +(10 to 40) mit reliable Anisotropic material behavior--use of vertical instead of suitably inclined I in Table 5.7 test samples ¯ .tor, as it may (continued)

TJ FA 410 PAGE 018 304 LABORATORY TESTING AND INTERPRETATION

TABLE 5.7 (Continued) Factor Influence (percent) Remarks Conventional rates +(5 to 200) Effect depends on rate of shear in the of testing, soil type, laboratory rate of consolidation in field, etc. Progressive failure +(0 to 20) Depends on soil; mainly a factor for foundation soils. May-be more serious than shown for some soils Source." Johnson (1974), reproduced by permission of ASCE.

After evaluating the quality of the laboratory data, it is strongly recommended that the engineer compare the appraised shear strength values with available correlations Figure 5. and local experience. For granular soils, there are several correlations between field (From Fil measurements such as SPT and CPT and drained strength. These have been discussed in the previous chapter on field exploration. For fine-grained soils such as clays, the engineer may use the correlation between The [ the plasticity index (PI) and the peak drained angle of internal friction shown in (i.e.. co~ Figure 5.48. Alternatively, if presheared soils are being evaluated, Figure 5.47 and OCR >_- 5.49 may be used to correlate the PI with the residual angle of internal friction. strength Also, it should be noted that the cohesion component, Cr, will be negligible at large expecte~ displacements where residual strength controls. a cohesi soils, th( drained As o : 50 i ¯ Kenney (1959) mally c( ¯ Bjerrum & Simon~ (1960) i ¯ Ladd, etal. (1977) 0.23 m ( ~ 4o S and m If the

x 30 beused i I~ ~-~ . :- ---~- .... i : : this tabl

~ 20 databas~ be expe

.~. lO Averag~ (Bjerrum & Sirnon~ 1960) 5.10 10 20 30 40 50 60 70 8~0 90 100 PLASTICITY INDEX, PI In this their in. Figure 5.48 Correlation between plasticity index (PI) and angle of internal friction. ~b’. geotect

TJ FA 410 PAGE 019 5.10 OTHER PROPERTIES 305

4O Remarks 35 "ect depends on rate 3f testing, soil type, 30 ,ate of consolidation tn held, etc. pends on soil; mainD a factor for foundati ;oils. May be more ;erious than shown br some soils 10

¢ recommended 1ailable correlati Figure 5.49 Correlation between plasticity index (PI) and the residual angle of friction, itions (From Filz et al., 1992.) have been discus,

The undrained strength of a fine-grained soil will depend on the type of loading friction shown (i.e., compression or extension) and the overconsolidated ratio (OCR). For an ~d, Figure 5.47 OCR > 4.0, the undrained strength in compression will be greater than the drained of internal friction strength in the short-term, but this higher strength should not be used if the slope is negligible at expected to maintain long-term stability. Also, overconsolidated soils, will exhibit a cohesion value, c, below their preconsolidation stress. For normally consolidated soils, the undrained strength in compression will generally be about 50 percent of the drained strength, or qSc,=~4~.~ ~ As our database of undrained strengths grows, it has been found that for nor- 9) mally consolidated clays, the c,/Cr~m (or c,/~r£o) is nearly a constant value equal to mona (1960) ’1977) 0.23 -t- 0.04 (Jamiolkowski et al., t985; Mesri 1975). Typical values of the constants S and m, used by the SHANSEP approach, are given in Table 5.8. If the engineer is reviewing laboratory test data for unsaturated soils, Table 5.1 may be used to assess the quality of the reported magnitude of the Cb parameter. Admittedly, this table only includes data from seven different soils from eleven studies, but this database is likely to grow as more and more tests are performed for slopes that can ~__’_._~ ~ ~; be expected to remain unsaturated during their life.

5.10 OTHER PROPERTIES

) 90 100 In this section, several supplementary laboratory tests are reviewed on the basis of their indirect influence within the overall framework of slope stability and practical aternal friction, .

TJ FA 410 PAGE 020 ydraulic Conductivity," ~st, pp. 717-725. CHAPTER 10 s Triggering a Shallow ~gists, Vol. XXV, No. 3,

Hillslopes of Coastal ~0, pp. 79-95. l Environment & Soil ed and ," n, Ed., May, pp. 67-82. ?pes in London Clay;’ )undation Engineering,

~ning," The Governor’s vey, Denver, Colorado, STABILITY OF LANDFILL SLOPES ornia Geology, Vol. 30, es of 1978 and 1980," rizona 1978 and 1980, 10.1 UNIQUE NATURE OF LANDFILLS ational Academy Press, rn California: Rainfall Landfills pose unique slope stability issues chiefly because construction involves tgineering Geoscience, different combinations of cuts and fills in a variety of materials including soil. . water, landfill waste, and geosynthetics. There are three critical periods in the life of )amage, January 1982, the landfill when slope stability is considered: ~. 139-152. ¯ Siting and preparation to receive waste ering Failures," Journal ¯ Waste placement ¯ .ils," Highway Research Capping and final closure 10--27. Initially, when the landfill is being sited and prepared to receive waste, the stability of 2olluvium and Talus," the host site materials must be analyzed for behavior depending on.whether the landfill is being placed in a pit, in trenches, against a hillside, in a canyon, or above 3ebris Flows in Central ground. As the waste is placed in the landfill, temporary fill slopes are constantly being 9cess, Recognition and created as different sections of the landfill are filled and then buttressed by additional g Geology, J. E. Costa filling. Finally, when the landfill is full, a cap is placed above the waste and the landfill is closed. Each of these phases presents unique combinations of materials in contact -Time Warning System ’erican Society of Civil with each other, unique combinations of material and strength properties, and thus, unique slope stability issues. the Initiation of Debris oscience, Vol. 1, No. 1, 10.2 TYPICAL LANDFILL CONFIGURATIONS

Five typical landfill configurations are shown in Figure 10.1. The most common landfill configuration in areas of deeper water table is probably the covered pit design 669

TJ FA 410 PAGE 021 STABILITY OF LANDFILL SLOPES

on maxin the ridge~ Figure construct a liner c~ (Figure 1, (a) or valleys the bounc and cany, side slopt Figure 113 surface. "l near the g soil requi (b) soil impo Alan( be accept The type are possit

¯ Top( ¯ Soil ¯ Lain ¯ Tl’an ¯ Wat~ ¯ Floo ¯ Geo] ¯ Aeri (d) Other sift ability of and fundi rivers’ flo or water ~ (e) ment (FiE Figure 10.1 Typical landfill configurations. (Sharma and Lewis, 1994.) of a landf items sho shown in Figure 10.la. The advantage of this configuration is that soils excavated The la from the pit may be used for daily cover. Depending on their nature, these excavated Figure 1C soils may also be suitable for clay liners or granular drainage material. The trenched and layer,’ landfill shown in Figure 10.1 b has advantages similar to those of the covered pit. It is, if needed~ however, typical of older landfilling practices, prior to the emphasis in landfill design material t reduce od

TJ FA 410 PAGE 022 10.2 TYPICAL LANDFILL CONFIGURATIONS 671

on maximizing airspace. As is evident in Figure 10. lb, significant airspace is lost to the ridges of native soil remaining between cells. Figure 10.1c depicts an upslope landfill. These types of landfills are generally constructed in regions of rolling or hilly terrains. The waste is generally placed on a liner constructed on an excavated natural slope. The canyon or valley landfills (Figure 10. ld) are so named because they are typically constructed in natural canyons or valleys. They can be similar to covered pits if the canyon is bowl-shaped; however the boundary side slopes are generally much higher and often steeper. For both upslope and canyon landfills, soil requirements are often satisfied by excavating the natural side slopes, selecting an on-site borrow area, or importing soil. Finally, as shown on Figure 10. I e, above-ground landfills are constructed entirely above the natural ground surface. These landfills typically exist in soft regions, where groundwater is near the ground surface, or where it would be impractical to excavate a pit. Operational soil requirements for these landfills are satisfied by on-site borrow areas or off-site soil import. A landfill site must meet several locational and geotechnical design criteria and be acceptable to the public. It must be in reasonable proximity to waste generators. The type of site, whether hilly, flat, or whatever, will dictate the configurations that are possible at each site. Data used in the selection of a site often include:

¯ Topographic maps ¯ Soil survey maps ¯ Land use plans ¯ Transportation maps ¯ Water use plans ¯ plain maps ¯ Geologic maps ¯ Aerial photographs

Other siting considerations include waste type, waste volume, landfill volume, avail- ability of landfill equipment, recycling and incineration options, existing landfill sites, and funding (Bagchi, 1994). No landfills should be constructed near lakes, ponds, rivers, flood plains, highways, public parks, critical habitat areas, wetlands, airports, or water wells. In fact, a landfill will be an integral part of the landscape and environ- ment (Figure 10.2) and must be evaluated in that context. In analyzing the feasibility of a landfill, the entire life cycle costs of the landfill must be evaluated, including the ts. (Sharma and Lewis, 1994.) items shown in Table 10.1. The landfill waste materials can be placed in a variety of ways, as shown in :onfiguration is that soils excavated Figure 10.3. All wastes received by the landfill are spread and compacted in cells ling on their nature, these excavated and layers within a confined area. At the end of each working day, or more frequently alar drainage material. The trenched if needed, the area is covered completely with a thin, continuous layer of daily cover nilar to those of the covered pit. It is, material that is then compacted. Daily cover is used to control moisture, control litter, ’ior to the emphasis in landfill design reduce odors, limit rodent and bird contact, provide vehicle access, help prevent fires,

TJ FA 410 PAGE 023 Z

z

n(/~N

z

and im materia a lift. T The tw~ are the Afte infiltrat purpos~ discour shapin[ cover. 672

TJ FA 410 PAGE 024 10.2 TYPICAL LANDFILL CONFIGURATIONS 673

TABLE 10.1 Components of Landfill Cost Predevelopment Costs Site characterization Environmental assessment Engineering design Hydrogeologic investigation Professional service fees--design/approvals Legal consultation Construction Costs Land clearing Excavation Liner and collection system installation Leachate management--pumping station and/or treatment systems Surface water control and final cover construction ’~ Gas management system Groundwater monitoring systems Site structures Operations Equipment and personnel Leachate and landfill gas management Environmental monitoring costs Community relations Impact management--dust, odors, and birds Closure costs Cap/final cover Seeding Runoff control Long-Term Care Costs End use plan costs, including trees and shrubs Site inspections Land service care Leachate and gas management Environmental monitoring Insurance

Source: McBean et al., 1995. and improve the appearance of the landfill. The compacted wastes and daily cover material constitute a cell. A series of adjoining cells, all of the same height, make up a lift. The completed landfill consists of one or more lifts, as depicted in Figure 10.4. The two basic methods of operation for landfilling, or sequencing, of the daily cells are the method and the area method (Figures 10.5 and 10.6). After the landfill is completely filled, a cover must be constructed to minimize water infiltration and isolate the waste from the environment, as shown in Figure 10.7. The purposes of the final soil cover over a landfill are to encourage surface water runoff. discourage erosion, retain moisture for vegetation, manage gas migration, provide shaping and contouring, and provide a base for the establishment of a suitable ground cover. Six typical layers of a final landfill cover are shown in Figure 10.8.

TJ FA 410 PAGE 025 674 STABILITY OF LANDFILL SLOPES

(FRE~ OPER (a) CELL

(b)

10.3

The e Land[ tic, le~

(c)

(d)

Figure 10.3 Landfill waste placement phases. (Sharma and Lewis, 1994.)

TJ FA 410 PAGE 026 10.3 LANDFILL WASTE ENGINEERING PROPERTIES 675

INTERMEDIATE COVER

CELL CELL HEIGHT

DALLY COVER Figure 10.4 Configuration of landfill waste lifts and cells. (McBean et al., 1995.)

10.3 LANDFILL WASTE ENGINEERING PROPERTIES

The engineering properties of landfill waste are quite variable and unpredictable. Landfill waste may contain a multitude of materials including food, rubber, plas- tic, leather, textiles, wood, furniture, paper, metal, glass, machinery, transportation

REFUSE PLACED COMPAC~ON BY BULL-DOZER

.ewis, 1994.) COMPACTED SOLID WASTE Figure 10.5 Trench method oflandfilling. (McBean et al., 1995.)

TJ FA 410 PAGE 027 676 STABILITY OF LANDFILL SLOPES

PLACID

BY BULL-DOZER

ORICJNAL GROUND

COMPACTED SOtlO WASTE

Figure 10.6 Area method of landfilling. (McBean et al., 1995.)

Top deck

equipm ing pro study a~ slope st some as values. Ther.

.. :.:.e includin ity, Atte Additiol mine th( (6) to slope Figure 10.7 Landfill final cover configurations. (Sharma and Lewis, 1994.) weight, shown iJ plants m

TJ FA 410 PAGE 028 10.3 LANDFILL WASTE ENGINEERING PROPERTIES

COMPAC~ON BY ,Cover ~uLL-DOZER Soil

,er (soil or

t al., 1995.)

Figure 10.8 Typical layers of landfill final covers. (Koerner and Daniel, 1997.)

equipment, electrical materials, petroleum, coal, chemicals, and other manufactur- ing products. While conventional geotechnical exploration tools can be used to study and evaluate them, no standard test methods are available. Yet, to carry out slope stability analyses involving these peculiar materials, the engineer must make some assumptions and develop the suite of required material properties and strength values. There are a multitude of properties related to landfill waste that could be discussed including grain size distribution, , moisture content, hydraulic conductiv- ity, , unit weight, strength, dynamic properties, and compressibility. Additionally, chemical and leach properties of landfill waste must be known to deter- mine the appropriate disposal locations and methods. The properties most germane to slope stability analyses are unit weight and strength. Some average values for unit tnd Lewis, 1994.) weight are given in Table 10.2. Strength properties are often expressed in a graph, as shown in Figure 10.9. The strength of wastes from industrial facilities such as power plants and other manufacturing facilities has received study and some examples are

TJ FA 410 PAGE 029 678 STABILITY OF LANDFILL SLOPES

TABLE 10.2 Average Unit Weights for Landfill Waste TABL] Unit Weight Source Refuse Placement Conditions kg/m3 lb/ft3

U.S. Department of the Sanitary landfill DescriI Navy (1983) Not shredded Coal re Poor compaction 320 20 Undl Good compaction 641 40 Effe~ Best compaction 961 60 Fly ash Shredded 881 55 Unit Sowers (1968) Sanitary refuse: depending on Unit compaction effort 481-961 30-60 Unit NSWMA (1985) Municipal refuse: Fly ash In a landfill 705-769 44-49 34% After degradation and settlement 1,009-1,121 63-70 a Slun Landva and Clark (1986) Refuse landfill 913-1,346 57-84 Com (refuse to soil cover ratio eft varied from about 2-1 to 10-1) b Com EMCON Associates (1989) For 6-1 refuse to daily cover soil 737 46 West V Source: Sharma et al. (1990). Shell aThese values were obtained from test-pit measurements of refuse at 11 municipal landfills in Canada. (c~ Values measured for the Halifax landfill and the August 1983 measurements at the Edmonton and Calgary Shell landfills have not been included, as suggested by the authors. (c~ bBased on tonnage records and areal survey maps recorded during the period from April 1988 through West V April 1989. Flue-ga sl~ Cons 120 Corn Red mu Unle~ 100 Leac] Mud: ~ 8o Source: © Z 6o © O presen! parameters ofc = "r" 40 Lewis, O and ot[ 2O © © 10.4 0 PROP 0 12 16 20 24 28 32 36 40 FRICTION ANGLE (Degrees) Fifty tc Figure 10.9 Typical shear strength values for landfill waste. (Singh and Murphy, 1990.) directl5

TJ FA 410 PAGE 030 10.4 GEOSYNTHETICS IN LANDFILLS AND ENGINEERING PROPERTIES 679

TABLE 10.3 Strength Properties of Mineral Landfill Waste Undrained Unit Weight Friction Compressive Water kg/m3 lb/ft3 Cohesion, c Angle, ~b Strength, qm Content, w Description (kPa) (degrees) (kPa) (%) Coal refuse 320 20 Undrained condition 10-40 10-28 641 40 Effective stress 0-40 25-43 961 6O Fly ash, Arizona 7-day 223 881 55 Unit wt. 12.6 kN/m3 331 Unit wt. 13.4 kN/m3 587 481-961 30-60 Unit wt. 13.8 kN/m3 Fly ash (silica 46%, aluminum 705-769 44--49 34%, calcium 7%) 1,009-1,12t 63-70 Slurry samples 0 37 913-1,346 57-84 Compacted, undrained 0 41 effective stress Compacted, drained 0 37 737 46 West Virginia fly ash Shelby tube samples 0 34 (consolidated, undrained) nicipal landfills in Canada. 0 37.5 the Edmonton and Calgary Shelby tube samples (consolidated, drained) 38-43 d from April 1988 through West Virginia bottom ash Flue-gas desulfurization (FGD) sludge Consolidated, drained test 0 41.5 Compacted 0--40 10-40 Red mud (bauxite residue) Unleached 63 52 49 - 20.585 psf Leached 0 41 Mud: sand (5:1) 38 Source: Oweis and Khera (1990).

presented in Table 10.3. For slope stability analyses, strength values for landfill waste of c = 400 pounds per square foot and q5 = 20° is a good starting point (Sharma and Lewis, 1994). Also, of great importance is interface friction angles between waste and other construction materials such as clay caps, geosynthetics, and so on. © © 10.4 GEOSYNTHETICS IN LANDFILLS AND ENGINEERING PROPERTIES 32 36 40 Fifty to one hundred years ago, landfills did not require linings. Waste was dumped ~gh and Murphy, 1990.) directly onto the native ground and then covered and vegetated as well as possible.

TJ FA 410 PAGE 031 680 STABILITY OF LANDFILL SLOPES

Environmental regulation now requires very specific multiple-layer redundant lining and cover systems (Figure 10.10). Concurrent with stiffer environmental regulation of landfills, the use of geosynthetic materials has forged its way into the geotechni- cal engineering community including usage at landfills. Geosynthetic materials are materials, mostly plastic, which are commonly used in place of, or to enhance the function of, natural soil materials. Landfill lining and cover systems generally require a variety of geosynthetic materials including geomembranes, geotextiles, geonets, geogrids, and geosynthetic clay liners (GCLs).

10.4.1 Geomembranes Geomembranes are flexible, polymeric sheets that have extremely low permeability and are typically used as liquid or vapor barriers (Figure 10.11). Table 10.4 lists the major types of geomembranes in current use. Advantages and disadvantages of some commonly used geomembranes are provided in Table 10.5. In landfills, base liners are placed below waste to minimize liquids expelled from and/or filtered through the waste (known as leachate) from contaminating the underlying ground and most important, the groundwater. Cover liners are placed above the final waste configuration to keep water, usually from rain or snow, from entering the waste and producing leachate.

10.4.2 Geotextiles Geotextiles are synthetic fabrics used in geotechnical engineering for various applica- tions. The majority of geotextiles are composed of polypropylene or polyester fibers; a small percentage are composed of polyamide or polyethylene (Sharma and Lewis, 1994). Geotextiles are manufactured from monofilament, staple, or slit film fibers that are twisted or spun together into yarn. The fibers or yarns are formed into geotex- tiles using either woven or nonwoven methods. Woven geotextiles are formed using traditional weaving methods and a variety of weave types (Figure 10.12). To create nonwoven geotextiles, the manufactured fibers are placed and oriented on a moving conveyor belt. Needle punching, melt bonding, or resin bonding bonds the fibers. At landfills, nonwoven geotextiles are most commonly used for filtration, separation, cushioning, and drainage. Woven geotextiles are usually used for reinforcement. Both types can be used as an alternative for daily cover.

10.4.3 Geonets Geonets are used for drainage and consist of two sets of parallel solid or foamed extruded ribs that intersect at a constant angle to form an open net configuration (Figure 10.13). Channels are formed between the ribs to convey either liquid or gases. To prevent intrusion by soils or other adjacent materials in the field, a geocomposite drainage net may be used that consists of a bonded to the geonet.

TJ FA 410 PAGE 032 ayer redundant lining tronmental regulation ~y into the geotechni- !nthetic materials are of, or to enhance the ems generally require geotextiles, geonets,

aely low permeability [). Table 10.4 lists the tisadvantages of some a landfills, base liners nd/or filtered through ~,ing ground and most tal waste configuration waste and producing

ng for various applica- ~ne or polyester fibers; ~e (Sharma and Lewis, ~, or slit film fibers that e formed into geotex- .tiles are formed using gure 10.12). To create l oriented on a moving ding bonds the fibers. ~r filtration, separation, or reinforcement. Both

~rallel solid or foamed 3pen net configuration y either liquid or gases. e field, a geocomposite to the geonet. 681

TJ FA 410 PAGE 033 STABILITY OF LANDFILL SLOPES

TABLE Synthetk Butyl rut

Chlorinm

Figure 10.11 Geomembrances.

10.4.4 Geogrids Chlorosu Geogrids are high-strength, soil reinforcement products composed of polypropylene, polyethylene, polyester, or PVC-coated polyester. All geogrids have an open mesh configuration with apertures ranging from ½ to 3 inches (Figures 10.14 and 10.15). They are formed by several different methods. The polyester and polyester PVC- coated geogrids are typically woven or knitted. The polypropylene geogrids are either Ethylene-

TABLE 10.4 Major Types of Geomembranes in Current Use Thermoset Thermoplastic Polymers Polymers Combinations Polyvinyl chloride (PVC) Low-dens Butyl or isoprene-isobutylene PVC-nitrile rubber polyetb (nR) Polyethylene (VLDPE, LLDPE, Epichlorohydin rubber PE-EPDM MDPE, HDPE, referring to Polyvinyl very low, linear low, medium, and high density) Chlorinated polyethylene (CPE) Ethylene propylene diene PVC-ethyl vinyl monomer (EPDM) acetate Elasticized polyolefin (3110) Polychloroprene (neoprene) Cross-linked CPE Ethylene interpolymer alloy Ethylene propylene terpolymer Chlorosulfonated (EIA or XR-5) (EPT) polyethylene Source: Ba (CSPE or Hypalon) Polyamide Ethylene vinyl acetate (EVA) Source: Koemer (1990).

TJ FA 410 PAGE 034 10.4 GEOSYNTHETICS IN LANDFILLS AND ENGINEERING PROPERTIES683

TABLE 10.5 Advantages and Disadvantages of Commonly Used Synthetic Membranes Synthetic Membrane Advantages/Disadvantages Butyl rubber Good resistance to ultraviolet (UV) ray, ozone, and weathering elements Good performance at high and low temperatures Low swelling in water Low strength characteristics Low resistance to hydrocarbons Difficult to seam Chlorinated polyethylene (CPE) Good resistance to UV, ozone, and weather elements Good performance at low temperatures Good strength characteristics Easy to seam Poor resistance to chemicals, acids, and oils Poor seam quality Chlorosulfonated polyethylene Good resistance to UV, ozone, and weather elements tposed of polypropylene, Good performance at low temperatures :rids have an open mesh Good resistance to chemicals, acids, and oils Good resistance to bacteria igures 10.14 and 10.15). Low strength characteristics ;ter and polyester PVC- Problem during seaming ylene geogrids are either Ethylene-propylene rubber (EPDM) Good resistance to UV, ozone, and weather elements High strength characteristics Good performance at low temperatures Low water absorbance Poor resistance to oils, hydrocarbons, and solvents Combinations Poor seam quality Low-density and high-density Good resistance to most chemicals PVC-nitrile rubber polyethylene (LDPE and HDPE) Good strength and seam characteristics Good performance at low temperatures PE-EPDM Poor puncture resistance Polyvinyl chloride (PVC) Good workability High strength characteristics Easy to seam PVC-ethyl vinyl Poor resistance to UV, ozone, sulfide, and weather acetate elements Cross-linked CPE Poor performance at high and low temperatures Chlorosulfonated polyethylene Source: B agchi (1994). (CSPE or Hypalon)

TJ FA 410 PAGE 035 684 STABILITY OF LANDFILL SLOPES

Figure 10.12 Geotextiles.

extruded or punched sheet drawn, and polyethylene geogrids are exclusively punched sheet drawn. At landfills, geogrids may be used to support a lining system over a weak subgrade or to support final landfill cover soils on steep refuse slopes. They are also used sometimes in "piggyback" landfills between the old landfill and the new one. Typical ranges of tensile strengths for geogrids made of different materials are listed in Table 10.6. 10.4.6 There are thetics in 10.4.5 Geosynthetic Clay Liners (GCLs) their inter Geosynthetic clay liners (GCLs) are very low permeability barriers consisting of of the mat a layer of unhydrated, loose granular or powdered that is chemically or stability sl mechanically adhered to a geotextile or . They are generally used as an terials ant alternative to compacted clay liners. Differences between GCLs and compacted clay interface t liners are presented in Table 10.7. stability m ¯ Soil/~ ¯ Soil/~

Figure 10.13 Geonets.

TJ FA 410 PAGE 036 10.4 GEOSYNTHETICS IN LANDFILLS AND ENGINEERING PROPERTIES685

¯ e exclusively punched ng system over a weak ~ slopes. They are also drill and the new one. Figure 10.14 Geogrid. ent materials are listed 10.4.6 Engineering Properties of Geosynthetics There are a multitude of engineering properties that are of interest when using geosyn- thetics in landfills. Many of these deal with the ability of geosynthetics to perform their intended function as a barrier or pathway for liquid and gas migration. Many of the material properties of concern are listed in Table 10.8. However, from a slope barriers consisting of stability standpoint, the frictional behavior of these materials and between these ma- that is chemically or terials and other materials is of paramount importance. This property is known as re generally used as an interface friction. The possible interfaces that may be of interest in a landfill slope Ls and compacted clay stability analysis are:

¯ Soil/geotextile ¯ Soil/geomembrane

Figure 10.15 Geogrid. (Martin, 1998.)

TJ FA 410 PAGE 037 STABILITY OF LANDFILL SLOPES

TABLE 10.6 Typical Range of Wide Width Geogrid Strengths in Maximum TABLE Strength Direction

Tensile Strength, Ultimate Tensile Geosynthe Geogrid 5% Strain (lb/in.) Strength (lb/in.) Geomemb~ Polyester 750-2,300 2,600-8.500 Polyester, PVC coated 600-7,600 1,500-25,400 Polypropylene 50-110 70-190 Polyethylene 140-460 300-810 Source: Sharma and Lewis (1994).

¯ Geosynthetic/geosynthetic Geotextiles ¯ Geosynthetic/clay liner

Interface friction angles can be measured in the laboratory using ASTM D 5321. Using this method, each of the combinations of materials can be tested. There is also an abundant amount of information available in the literature, as shown in Tables 10.9. 10.10, 10.11, and 10.12. Geonets 10.4.7 Anchor Trenches

In addition to selecting the appropriate types and combinations of geosynthetics, Geogrids anchoring details must be specified to hold the geosynthetics in place and prevent

TABLE 10.7 Differences between GCLs and Compacted Clay Liners Geosyntheti~ (GCLs) Characteristic Compacted Clay Liner Materials Bentonite clay, adhesives, Native soils or blend of soil and geotextiles, and bentonite geomembranes tearing, As Construction Manufactured and then Constructed in the field installed in the field rectangular, Thickness Approximately 10 mm Approximately 0.5 to 1.0 m any particu] -1° -8 10 to 10 ends (typical) l0-s to 10-7 cm/s (typical) dimensional of clay of anchor tr Speed and case of Rapid, simple installation Slow, complicated construction the geosyntl construction at time Essentially dry; cannot Nearly saturated; can desiccate TABLE of construction desiccate during and can produce consol- construction and produces idation water no consolidation water Cost $5 to $11 per square meter Highly variable (estimated Geotextii range: $8 to $32 per square Woven meter) Nonwov~ Experience level Limited due to newness Has been used for many decades Nonwow Source: USEPA (1993). Source.’St

TJ FA 410 PAGE 038 10.4 GEOSYNTHETICS IN LANDFILLS AND ENGINEERING PROPERTIES687 in Maximum TABLE 10.8 Importance of Geosynthetic Material Properties Main Material Properties Ultimate Tensile Geosynthetic of Concern Common Test Methods Strength (lb/in.) Geomembranes Thickness ASTM D 374, D 751, D 1593 2,600-8,500 Tensile behavior ASTM D 412, D 638, D 882, 1,500-25,400 D 4885 70-190 Tear resistance ASTM D 1004 300-810 Punch resistance FTMS 101C Chemical resistance USEPA 9090 Seam strength ASTM D 4437 Geotextiles Thickness ASTM D 177, D 5199 Tensile behavior ASTM D 4632, D 4595 Bursting behavior ASTM D 3786 Tear behavior ASTM D 4533 using ASTM D 5321. Apparent opening size ASTM D 4751 be tested. There is also Permittivity ASTM D 4491 Transmissivity ASTM D 4716 as shown in Tables 10.9, Seam strength ASTM D 1683, D 4884 Geonets Thickness ASTM D 1777, D 5199 Crush strength ASTM D 1621 Transmissivity ASTM D 4716 Geogrids Tensile strength ASTM D 4595 ations of geosynthetics, (Also see geomembranes ics in place and prevent and geotextiles) Geosynthetic clay liners (Address each composite Liners (GCLs) material separately :ompacted Clay Liner including standard tests on bentonite materials) soils or blend of soil and ~tonite tearing. As shown in Figure 10.16, anchor trenches can generally be classified as flat, tructed in the field rectangular, or V-shaped. Selection of the appropriate anchor trench configuration for any particular site depends on the required holding capacity, access considerations, oximately 0.5 to 1.0 m dimensional constraints, and available construction equipment. The holding capacity -7 to 10 cm]s (typical) of anchor trenches is developed by the applied normal load of the soil placed above , complicated construction the geosynthetics that creates frictional resistance between the geosynthetics and the ly saturated; can desiccate TABLE 10.9 Typical Range of Reported Soil Geotextile Friction Angles d can produce consol ...... Sand Friction Angle Clay FricfionAngle ~tion water (degree) (degree) Geotextile (Efficiency) (Efficiency) ly variable (estimated age: $8 to $32 per square Woven 23-42 (0.68-1.0) 16-26 (0.61-0.93 ) eter) Nonwoven, needle-punched 25-44 (0.67-1.0) 15-28 (0.62-0.99) been used for many decades Nonwoven, resin- or heat-bonded 22-40 (0.56-0.91) 17-33 (0.60-0.85) Source: Sharma and Lewis (1994).

TJ FA 410 PAGE 039 688 STABILITY OF LANDFILL SLOPES TABLE Hydratii TABLE 10.10 Typical Range of Reported and Recommended Soil Geomembrane Fluid Friction Angles Distilled Reported Sand Recommended Reported Clay Recommended Friction Angles Sand Friction Friction Angles Clay Friction (degree) Angles, 3 (degree) Angles. 3 Geomembrane (Efficiency) (degree) (Efficiency) (degree) PVC 21-33 20-30 6-39 6-15 (0.62-0.93) (0.53-1.0) HDPE 17-28 17-25 5-29 5-10 (0.45-0.81) (0.47-0.88) Tap watt Textured 30-45 30-40 7-35 9-15 HDPE (0.86-1.0) (0.70-1.0) VLDPEa 21-28 (0.62-0.67) Source: Sharma and Lewis (1994). a Since VLDPE is a relatively new product, limited results were reported in the literature. It is anticipated that the range of efficiencies for VLDPE to sand interfaces is broader than shown. Blank ( ) means insufficient data at this time. Mild lea~ underlying soil. There is minimal friction resistance developed between the upper soil and the geosynthetic since the soil above the geosynthetic is likely to move with the geosynthetic. The soil depth, type of soil or other material underlying the geosynthetics, and geosynthetic anchorage length are therefore the key factors in developing the required anchor trench holding capacity. Sharma and Lewis (1994) provide additional information and design formulas for anchor trench design. Harsh le~

10.5 LANDFILL CONSTRUCTION

The construction of a landfill is a carefully planned, methodical, highly regulated, and dynamic process. Substantial planning, study, and design must take place before a landfill can be sited, opened, and/or modified. Additionally, constant monitoring, Diesel fu~ testing, and reporting are required throughout the life of the landfill. More relevant

TABLE 10.11 Typical Range of Reported Geosynthetic to Geosynthetic Friction Angles in Degrees

HDPE HDPE PVC Smooth Textured Geonet Woven geotextile 10-28 7-11 9-17 Source: U5 9-18 a Claymax Nonwoven, needle-punched 16-26 8-12 15-33 10-27 geotextile manufacttu Nonwoven, resin/heat-bonded t’Dry refers, 18-21 9-11 15-16 17-21 c Constrain, geotextile sheared at 1 Geonet 11-24 5-19 7-25 -- aFree swel Source." Sharma and Lewis (1994). and then sh

TJ FA 410 PAGE 040 TABLE 10.12 Direct Shear Test Results under Variable Hydrating Conditions Hydrating Measured Constrained Free C oil Geomembrane Fluid GCL Typea Property Dryb Swell Swelld Distilled water Claymax 0 (deg) 37 16 0 Clay Recommended c (kPa) 6.9 3 4 ngles Clay Friction Gundseal ~ (deg) 26 19 0 e) Angles, 3 c (kPa) 50 5 3 ~cy) (degree) Bentomat 0 (deg) 42 37 23 c (kPa) 14 6 5 ~ 6-15 Bentofix ~ (deg) 36 31 10 0) 68 7 9.0 9 5-10 c (kPa) Tap water Claymax ~ (deg) 37 18 0 88) 9-15 c (kPa) 6.9 3 3 5 Gundseal 0 (deg) 26 18 0 0) c (kPa) 50 5 3 Bentomat ~ (deg) 42 43 26 c (kPa) 14 6 10 Bentofix 0 (deg) 36 34 15 he literature. It is anticipated c (kPa) 68 6.9 7 n shown. Blank (--) means Mild leachate Claymax ~ (deg) 37 24 4 c (kPa) 6.9 6 3 Gundseal ~ (deg) 26 18 13 ped between the upper c (kPa) 50 5 4 letic is likely to move Bentomat 0 (deg) 42 39 25 c (kPa) 14 8.3 14 naterial underlying the Bentofix ~ (deg) 36 43 20 fore the key factors in c (kPa) 68 5 12 irma and Lewis (1994) Harsh leachate Claymax 0 (deg) 37 19 0 ~r trench design. c (kPa) 6.9 6 3 Gundseal ~ (deg) 26 13 0 c (kPa) 50 7.6 3 Bentomat 0 (deg) 42 45 32 c (kPa) 14 5 12 dical, highly regulated, Bentofix ~ (deg) 36 39 30 must take place before c (kPa) 68 4 8.3 y, constant monitoring, Diesel fuel Claymax ~ (deg) 37 44 38 landfill. More relevant c (kPa) 6.9 4 6 Gundseal ~ (deg) 26 24 29 c (kPa) 50 4 6 ~synthetic Friction Bentomat ~ (deg) 42 42 40 c (kPa) 14 6 5 Bentofix ~ (deg) 36 51 46 HDPE c (kPa) 68 4 5 Textured Geonet 9-18 Source: USEPA (1993). 9-17 aClaymax and Gundseal are unreinforced GCLs; Bentomat and Bentofix are reinforced. Claymax also 15-33 10-27 manufactures a reinforced GCL, but it was not used in this testing program. bDry refers to product as-received, placed under desired normal stress, than sheared at midplane. 15-16 17-21 CConstrained swell refers to product hydrated under desired normal stress, (i.e., constraihed swell) then sheared at midplane. 7-25 -- dFree swell refers to product hydrated under zero normal stress, then placed under desired normal stress, and then sheared at midplane.

TJ FA 410 PAGE 041 STABILITY OF LANDFILL SLOPES

U Anchor Trench Trench Backfill 1.5’deepxl’wide I ~ 3’ Min. ~ Fluid Level

V Anchor Trench Trench Backfill 2’ deep × 4’ wide I I ~ ~ I Fluid Level

Figure 10.16 Typical anchor trench configurations. (Martin, 1998.) to slope stability issues is the fact that numerous activities are going on at one time in multiple locations with a variety of materials and equipment. Also, precipitation, wind, and other environmental factors further contribute to the variability of site conditions. The basic sequence of construction at a landfill includes:

¯ Subbase preparation ¯ Liner construction o Waste placement ¯ Landfill closure

The subbase for a landfill refers to the ground surface on which the liner is constructed. Compaction and of the subbase are necessary so that the actual liner can be constructed easily. The subbase may be developed on existing ground surface or may be constructed on fill or cut. Great care must be taken so that the subbase provides a sound foundation for the landfill. Conventional earthwork equipment and methods are used to prepare the subbase for a landfill. Liner construction often consists of multiple layers of soil and geosynthetics (Figure 10.17). Placement of the soil follows conventional earthwork practices. Con- struction of geosynthetic liners requires specialized methods particularly related to connection of adjacent sheets/layers and prevention of damage (i.e., puncturing, tearing, blocking of drainage, etc.) during construction. As each new layer is placed,

TJ FA 410 PAGE 042 Fluid Level /

z ~- ~ Subgrade

Fluid Level

d Subgrade

~rtin, 1998.) going on at one time t. Also, precipitation, the variability of site

he liner is constructed. the actual liner can be c ,~round surface or may the subbase provides luipment and methods oil and geosynthetics hwork practices. Con- particularly related to ~age (i.e., puncturing, :h new layer is placed, 691

TJ FA 410 PAGE 043 692 STABILITY OF LANDFILL SLOPES

the shear strength between layers could change significantly. On a slope, the layer that possesses the lowest shear strength governs stability. It is conceivable therefore that as the liner layers are built up and then piled with waste, a slope could go from being stable to being unstable due to the introduction of a weaker layer. Waste placement is a unique process during the life of the landfill because a variety of waste materials are being placed in a variety of configurations. Temporary slopes are commonly made and modified on a regular basis. Some of these slopes may be in contact with the liner system; Some may not. Also, slopes made during this phase will be traversed several times by a variety of equipment and vehicles going a multitude of different directions. Since a slope stability analysis cannot be carried out every time a new load of waste arrives at the landfill, there must be governing restrictions on the steepness and location of temporary and permanent slopes. These restrictions must take into account precipitation and groundwater conditions. After the landfill has received its maximum amount of waste, it must be closed such that it performs well and does not impact the environment in the long term. It receives a final cover or cap that is similar to the underlying liner system. In other words, there are a series of layers that get placed over the waste to control infiltration of water into the landfill, thus minimizing leachate, controlling the release of gases from the landfill, and providing for a physical separation between the waste and environment for protection of public health. A typical cross section of a landfill cap is shown in Figure 10.18. Of the documented case histories of failures, most occur during liner construction and waste placement. Often, when failures have occurred, they were due to underesti- mation of interface friction angles between landfill, soil, and geosynthetic materials or the dynamic effects of equipment sequencing or loading. For instance, Martin (1998) reports on a failure in Washington that was due to bulldozer braking acceleration and deceleration loads. From a slope stability standpoint, all materials that are placed at GT (as nec, some angle from horizontal must be analyzed for stability relative to the adjacent ma- terials as well as all construction effects including sequencing, layering, equipment, weather, and so on. GT (as nec~

10.6 SLOPE STABILITY CONSIDERATIONS

Slope stability considerations for a landfill must include all phases of development in- cluding subbase preparation, liner construction, waste placement, and landfill closure. Figure 10.18 Slope stability issues during subbase preparation are the same for any conventional cuts or fills as discussed previously in Chapter 6. Landfill slope stability is unique because after subbase preparation, multiple layers of soil, geosynthefics, and waste 10.6.1 Ex get piled on top and underneath each other, creating a unique combination of materials Figure 10.2( that behave in a composite fashion with uniquely different shear strength properties excavation s (Figure 10.19). it is often sin All of the methods of slope stability analysis discussed in this book apply to requirement,~ landfills. However, it is important to note the potential modes of failure in a landfill property bou and the unique materials and material properties found at a landfill. a significant

TJ FA 410 PAGE 044 10.6 SLOPE STABILITY CONSIDERATIONS y. On a slope, the layer 3_ is conceivable therefore i’. ? ":~i!,Vegetated _150 (typ.) ~, a slope could go from eaker layer. 450(~n.) 9!30 (min.) andfill because a variety GT tions. Temporary slopes 300 sol! or ff these slopes may be in equiv. GC de during this phase will GM zles going a multitude of 450 or equiv. GCL ~e carried out every time ...... ¯ or GCL ’,rning restrictions on the 300 soil These restrictions must or" -equiv. GS te, it must be closed such the long term. It receives :m. In other words, there trol infiltration of water elease of gases from the (a) ~ waste and environment landfill cap is shown in -300 (typ.) during liner construction ey were due to underesti- geosynthetic materials or 9~0(~n.) r instance, Martin (1998) 600 (rain.) braking acceleration and GT (as aterials that are placed at GM lative to the adjacent ma- tng, layering, equipment, GT (as necessary)

~hases of development in- ment, and landfill closure. Figure 10.18 Examples of municipal solid waste final covers. (Koerner and Daniel, 1997.) ~me for any conventional slope stability is unique 10.6.1 Excavation Slope Stability geosynthetics, and waste Figure 10.20 illustrates some of the potential slip surfaces that may occur in landfill ~ combination of materials excavation slopes (Sharma and Lewis, 1994). For covered pit and trenched landfills, shear strength properties it is often simplest to adjust the excavation slope to meet stability and constructability requirements. Since the crest of the landfill excavation slope is typically limited by ed in this book apply to property boundaries, flattening an excavation slope from 2H-1V to 3H-1¥ can have des of failure in a landfill a significant effect on landfill airspace and revenue. Most landfill owners therefore t landfill.

TJ FA 410 PAGE 045 694 STABILITY OF LANDFILL SLOPES

1’-2’[GRANULAR DRAINAGE MATERIAL 772-F2-7777777~-2-7777~. G e o t e x tile

(a)

Geocomposite drainage net 1’-2~~ (b)

GRANULAR DRAINAGE MATERIAL ~,Geotextile ," Geomembrane

~,Geocomposite ...... drainage net Geornembrane 1’-2’ ~ CLAY LINER~ ~

GRANULAR DRAINAGE MATERIAL ~ . //////// Geomembrane 1 2 ///~, CLAY LINER~’~/ ~/

, , ~ /////// \Geomembrane 1-2 ~ CLAY LINER~’~/

(e) Figure 10.19 Types of landfill containment lining systems. (Sharma and Lewis, 1994.) attempt to excavate slopes to the maximum achievable. Typical excavation slopes for covered pit and trenched landfills are approximately 3H-1V. This is because most soil slopes are stable at 3H-1V inclination and clay liners may be constructed on the side slopes at this grade. Figure 1 Upslope and canyon landfills are often constructed at grades steeper than 3H-1V, due to the steep inclination of the natural slopes. Although these slopes may be trimmed back to meet a 3H-1V grade, a significant volume of excavation may be required due to the typically large slope heights. A groundwater table or weak soil

TJ FA 410 PAGE 046 10.6 SLOPE STABILITY CONSIDERATIONS 695

Interval extile excavation .~/~--,7"---~_~"~//.’>" slope ~ ,~," .~ ~-" [ _ Approx. , .." .- L 10’-50’ Final excavation slope (a)

~composite =nage net Approx. 10’-50’

(b)

,otextile =omembrane Approx. 50’-300’

ocomposite ~inage net (c) omembrane

,omembrane ;o¢omposite ainage net ~omembrane

arma and Lewis, 1994.)

)le. Typical excavation 3H-1V. This is because ~ ers may be constructed (e) Figure 1!1.20 Potential slip surfaces in landfill excavation slopes. (Sharma and Lewis, 1994.) t grades steeper than ~ough these slopes may e of excavation may be ,ater table or weak soi!

TJ FA 410 PAGE 047 696 STABILITY OF LANDFILL SLOPES

layer also often influences the stability of upslope and canyon landfills. These items must be considered in the stability analyses. Sin For many excavation slopes, the worst-case conditions for analysis are either during ofl construction or immediately following construction. Often, waste placed against the unl excavation slope acts as a buttress and prevents movement. erie typ~ lini: 10.6.2 Waste Fill Stability or ~ Potential slip surfaces through landfill waste generally occur in one of three ways, as The plar illustrated in Figure 10.21: ¯ Through the waste alone and ¯ Along the liner system typi ¯ A composite surface through waste and along the liner grot it is eith~ the t / REFUSE

(a)

REFUSE

Lining sys ~

(b)

REFUSE

Lining system

Figure Figure 10.21 Potential slip surfaces in landfill waste. (Sharma and Lewis, 1994.) (Sharn~

TJ FA 410 PAGE 048 10.6 SLOPE STABILITY CONSIDERATIONS 697 landfills. These items Since waste is generally a heterogeneous material that may contain isolated masses of high-strength materials, such as discarded refrigerators or demolition debris, it is alysis are either during unlikely that slip surfaces through waste materials are circular. However, since the ~ste placed against the current state of technology affords no better solutions, waste fill stability analyses are typically performed using circular surfaces through waste materials. If a geosynthetic lining system is placed in a landfill, a translational block surface along the liner system or a composite slip surface through waste and along the liner system may be critical. There are several interface surfaces within a geosynthetic lining system where a weak one of three ways, as plane may exist. Figure 10.22 depicts typical slip surfaces that may occur through covered pit and trench-type landfills. For these types of landfills, the critical stability condition typically occurs in interim waste fills, prior to the pit or trench being filled to the ground surface (Figure 10.22a). Once the pit or trench is filled to the ground surface it is inherently stable (Figure 10.22b). If waste is placed above the ground surface, either a circular surface through the waste itself, or a block-type surface extending to the base lining system may occur (Figure 10.22c).

(a)

REFUSE

(b)

REFUSE

(c) Figure 10.22 Potential slip surfaces in waste for pit and trench landfill configurations. ta and Lewis, 1994.) (Sharma and Lewis, 1994.)

TJ FA 410 PAGE 049 698 STABILITY OF LANDFILL SLOPES

Figure 10.23 illustrates potential waste fill stability concerns in upslope or canyon- type landfills. From a stability standpoint, upslope landfills are perhaps the least desired waste fill configuration. This is because both interim slopes (Figure 10.23a) and final slopes (Figure 10.23b) have a significant potential for instability, especially

Lining system--~

(b)

Figure and Lewis’ -~ REFUSE ~ ...... ’~’ Lining system-~" ,~, /

when ov stable in fill condi Finall with sore (Figure l /REFUSE surface n Lining system---~ \ I \ are also c capacity of landfil In all many of Figure 10.23 Potential slip surfaces in waste for upslope and canyon landfill configurations. potential (Sh~a and Lewis, 1994.) how this

TJ FA 410 PAGE 050 10.6 SLOPE STABILITY CONSIDERATIONS 699 in upslope or canyon- are perhaps the least lopes (Figure 10.23a) instability, especially (a)

REFUSE

Lining system (b)

REFUSE \

WEAK SOILS "" "- ~__ ~ -""

(c) Figure 10.24 Potential slip surfaces in waste for aboveground landfill configurations. (Sharma and Lewis, 1994.)

when overlying a geosynthetic lining system. Canyon-type landfills are generally stable in their final configuration (Figure 10.23c), but may be unstable during interim fill conditions (Figure 10.23d). Finally, aboveground landfills are generally stable when founded over firm ground, with some potential for a circular slip surface occurring if the side slopes are too steep (Figure 10.24a). If a geosynthetic lining system is employed, a composite-type slip surface may extend through the waste mass (Figure 10.24b). Aboveground landfills are also often constructed over soft subgrades or in zones of high groundwater. subgrade failures (Figure 10.24c) should therefore be checked in these types of landfills. In all of the landfill configurations illustrated, proper fill sequencing can alleviate many of the stability concerns. The key is to maximize the "resisting" portion of the on landfill configurations. potential slip surface prior to increasing the "active" portion. Figure 10.25 illustrates how this is achieved for covered pit and upslope landfills.

TJ FA 410 PAGE 051 700 STABILITY OF LANDFILL SLOPES

Active block ~sting block LRB .~ LRB > HAB

(a)

~ Active LRB > HAB Resisting block Filling Lining system-~ ~ "<-’---- direction LR8 ,I (b) Figure 10.25 Active and passive stability blocks in covered pit and upslope landfills. (Sharma and Lewis, 1994.)

10.6.3 Cover System Stability During preparation of landfill site development plans, consideration should be given to the stability of the landfill final cover system so that appropriate final landfill grades may be selected. Most landfills are constructed with final grades of approximately 3H-1V, which are generally stable for soil cover systems. There is some concern, however, regarding the ability of soil cover systems to deform with the underlying Figure and Dani~ -----COVER SYSTEM

[ waste set they are i Poten material REFUSE along th~ interface: the wors! to the lar loads wo Figure 10.26 Shallow failure surface in final landfill cover due to construction equipment. when the (Sharma and Lewis, 1994.) illustrate~

TJ FA 410 PAGE 052 10.6 SLOPE STABILITY CONSIDERATIONS 701

~ eomembrane

LR8 > HAB (a)

upslope landfills. (Sharma

teration should be given riate final landfill grades grades of approximately There is some concern, ~rm with the underlying Figure 10.27 Effect of construction equipment loads on final landfill cover stability. (Koerner and Daniel, 1997.)

waste settlements. Geosynthetics are therefore often used in final cover systems since they are inherently more flexible. They may, however, create stability concerns. Potential slip mechanisms in final covers are generally planar, occurring between material interfaces or through the material itself. Potential slip surfaces could occur along the interfaces or materials shown on Figure 10.26; however, there are some interfaces that are more likely than others to be critical weak planes. In some situations, the worst-case condition for a final cover system is during construction. This is due to the large equipment loads applied during construction. Failure due to equipment loads would probably result in a localized shallow circular slip surface especially when the final cover is being placed from the top of the landfill down to the base as to construction equipment. illustrated in Figure 10.27.

TJ FA 410 PAGE 053 71)2 STABILITY OF LANDFILL SLOPES

REFERENCES

Bagchi, A., 1994. Design, Construction, and Monitoring of Landfills. New York: Wiley. EMCON Associates, personal communications between Larry Burch and Hari Sharma, July 1989. Koerner, R. M. 1990. Designing with Geosynthetics. Englewood Cliffs, New Jersey: Prentice Hall PTR. Koerner, R. M., and D. E. Daniel, 1997. Final Covers for Solid Waste Landfills and Abandoned Dumps. Reston, Virginia: American Society of Civil Engineers. Koerner, G. R., and Koerner, R. M., "Biological Activity and Potential Remediation Involv- ing Geotextile Landfill Leachate Filters," in Geosynthetics Testing for Waste Containment Applications, ASTMP STP 1081, Robert M. Koerner, Ed. ASTM, Philadelphia, 1990a. Landva, A. O., and Clark, J, I., "Geotechnical Testing of Wastefill," in 39th Canadian Geotech- nical Conference, Ottawa, Canada, 1986, pp. 371-385. Martin, S., 1998. "Stability and Constructability of Geosynthetics," University of Idaho, Moscow. IN[ McBean, E. A., E A. Rovers, and G. J. Farquhar, 1995. Solid Waste Landfill Engineering and Design. Englewood Cliffs, New Jersey: Prentice Hall PTR. National Solid Waste Management Association (NSWMA), Basic Data: Solid Waste Amounts, Composition and Management Systems, Technical Bulletin 85-6, National Solid Waste Management Association, October 1985, p. 8. A line, Oweis, 2. S., and R. P. Khera, 1990. Geotechnology of Waste Management. Sevenoaks, Kent, Access England: Butterworth and Company (Publishers) Ltd., 273 pp. Activit Sharma, H. D., and S. E Lewis, 1994. Waste Containment Systems, Waste Stabilization, and Agglor Landfills Design and Evaluation. New York: Wiley. Air ph~ Sharma, H. D., M. T. Dukes, and D. M. Olsen, 1990. "Field Measurements of Dynamic Moduli Alloph; and Poisson’s Ratio of Refuse and Underlying Soils at a Landfill Site," Geotechnics of Alluvia Waste Fills: Theory and Practice, ASTM STP 1070, A. Landva and G. D. Knowles, Eds. Alteratl Philadelphia: ASTM, pp. 57-70. Americ Singh, S., and B. J. Murphy, 1990. "Evaluation of the Stability of Sanitary Landfills," Geotech- M nics of Waste Fills: Theory and Practice, ASTM STP 1070, A. Landva and G. D. Knowles, Analog Eds. Philadelphia: ASTM, pp. 240-258. Analys arc (~ Sowers, G. E, "Foundation Problems in Sanitary Landfill," Journal of the Sanitary Engineering bloct Division, ASCE, Vol. 94, No. SA1, February 1968, pp. 103-116. circu U. S. Environmental Protection Agency (USEPA), 1993. Report of Workshop on Geosynthetic fricti. Clay Liners, USEPA, Washington, DC, August. infini U.S. Department of the Navy, Soil Dynamics, Deep Stabilization, and Special Geotechni- meth cal Construction, Design Manual 7.3, NAVFAC DM 7.3M, Naval Facilities Engineering plana Command, Alexandria, Va., April 1983, p. 7.3-79. proN three Anchor Andisol Angle o Anhydr Animal Anisotrt

TJ FA 410 PAGE 054