Translational Failure Analysis of Landfills

The scope of this paper is to develop a new approach to the two-part wedge method for translational failure analysis of landfills. The upper and lower bound solutions for the landfill stability, i.e., FSmax and FSmin, can be readily determined using this method. Using an average factor of safety, FSave, to replace the true factor of safety, FStrue, the differences between FSave and FStrue are within 5% for most considered cases. The solutions from the new method ensure that the waste strength is not exceeded anywhere within the waste mass. The calculated results agreed well with the results computed from the computer code PCSTABL6. The ability of the new method to calculate the translational failure of waste mass with a predetermined sliding failure face is demonstrated. Also, a waste filling sequence to keep the factor of safety above a stipulated value during the operation phase of the landfilling process can be generated.     

Modification to Translational Failure Analysis of Landfills Incorporating Seismicity

This paper extends a previously developed method of the writers by using a pseudostatic approach to conduct seismic stability analysis for potential translational failures of multilayer lined landfills. Assuming that the designed seismic coefficient, ks, is known, the factor of safety against translational failure for lined landfills can be calculated from the new developed equations. The critical interface with the seismic yield coefficient, ky, is generally located at different interfaces along the back slope and along the base. The failure surface is seen to transfer from one interface to another with changes of dimensions of the top width, depth, and front slope of the waste mass. Using this method, a waste filling sequence can be generated so as to maintain the ky-value above a stipulated value during landfill operations. It is important to ensure that the minimum ky-value at the critical interface is always equal to or greater than a targeted value during the waste filling sequence and after closure corresponding to the different geometric dimensions of the waste mass.     

Effect of Gas on Pore Pressures in Wet Landfills

The waste in a landfill may become saturated due to many reasons, including leachate recirculation or extreme precipitation. As high saturation levels in waste are achieved, the permeability of the waste to landfill gas decreases. This may result in pore pressures that are greater than what would be predicted by fluid statics. A theoretical model for estimating the excess pore pressure at the bottom of saturated waste is derived. A finite difference procedure is then presented as an approximate solution to the model. It was found that below the level of saturation, the steady-state excess pore pressure distribution increases linearly similar to a hydrostatic distribution. Combining its effect with the static water pressure, the excess pore pressure may be accounted for by using an equivalent unit weight of fluid that is artificially higher than water. A parametric study of the input parameters showed that the equivalent unit weight of the pore fluid was highly dependent on the hydraulic conductivity of the waste, particularly if the hydraulic conductivity of the waste is lower than about 2×10?6?m/s.     

Evaluation of Decomposition Effect on Long-Term Settlement Prediction for Fresh Municipal Solid Waste Landfills

A considerable amount of settlement occurs due to the decomposition of municipal solid waste (MSW) in landfills over a period of years. Therefore, the effect of biological decomposition governs the long-term settlement characteristics of municipal solid waste landfills. In this study, we investigated the long-term settlement characteristics by applying a number of prediction methods to fresh MSW sites and predicting the settlement curves. Most proposed methods, excluding the power creep law, successfully predicted long-term settlement only if accelerated logarithmic compression due to decomposition of biodegradable MSW was included in the settlement prediction.     

Fugitive Methane Emissions from Landfills: Field Comparison of Five Methods on a French Landfill

The study presented in this paper has been initiated by the Veolia Environment research center for waste management and supported by the French Environmental Agency. A comparison of five field-scale measurement methods for measuring fugitive methane emissions has been conducted on a French landfill site. The five methods evaluated consisted of a tracer gas technique, laser radial plume mapping, inverse modeling technique, differential absorption light detection and ranging (LiDAR), and helicopter-borne spectroscopy. These methods are evaluated on their abilities to measure emissions from a practical user-oriented aspect (metrological, technical, and economical criteria). High disparities in terms of quantitative results and applicability were observed from all methods. Techniques that used Gaussian dispersion modeling appeared less applicable to landfill sites due to topographic complexity and did not provide high confidence in the results. However, the method using optical remote sensing (radial plume mapping) methods showed that a spatially detailed analysis is achievable (cell level), and the LiDAR method showed very promising approach and technical performances.     

Effect of Toe Excavation on a Deep Bedrock Landslide

Observations, data, and analyses that were used to investigate the cause of distress to a single-family residence located adjacent to a major highway cutslope are presented herein. The investigation revealed that the distress in the single-family residence was caused by a deep, large excavation-induced landslide. The excavation, which was made to widen an existing highway, helped trigger the landslide by exposing geologic structures on the cutslope and by unloading the toe of the slope. This case history illustrates some of the ramifications of large highway excavations in natural slopes surrounded by urban areas, e.g., exposing significant geologic features such as shear zones, faults, and folds; the importance of investigating and explaining signs of movement at both the top and toe of a slope; the impact of rainfall on the movement of a large slide mass; and that large slide masses can undergo slow, episodic movement instead of sudden, large movement.     

Effect of Progressive Failure on Measured Shear Strength of Geomembrane/GCL Interface

This paper presents experimental data and numerical modeling results that illustrate the effects of progressive failure on the measured shear strength of a textured geomembrane/geosynthetic clay liner (GMX/GCL) interface. Large direct shear tests were conducted using different specimen gripping/clamping systems to isolate the effects of progressive failure. These tests indicate that progressive failure causes a reduction in measured peak shear strength, an increase in the displacement at peak, an increase in large displacement shear strength, and significant distortion of the shear stress–displacement relationship. A numerical model was developed to simulate progressive failure of a GMX/GCL interface. Measured and simulated shear stress–displacement relationships are in good-to-excellent agreement at four normal stress levels. The model was then used to investigate mechanisms of progressive interface failure and factors that control its significance. The results indicate that accurate measurements of shear stress–displacement behavior and strength are obtained when gripping surfaces prevent slippage of the test specimen and the intended failure surface has the lowest shear resistance of all possible sliding surfaces. The use of proper gripping surfaces is expected to reduce difficulties in test data interpretation and to increase the accuracy and reproducibility of test results.     

Shear Strength Parameters of Municipal Solid Waste with Leachate Recirculation

The objective of this study was to characterize relative changes in waste shear strength parameters during waste decomposition. Twelve direct shear tests (100?mm diameter by 50?mm thickness) were performed on waste specimens ranging from fresh to well-decomposed residential refuse. In addition, nine direct shear tests were performed on selected waste components including fresh paper, partially decomposed refuse, and plastics. Results indicate that the friction angle of refuse decreased with decomposition. As refuse decomposed, the plastic content increased, which contributed to a decrease in friction angle as the friction angle of plastics is 18–19° as compared to 33° for fresh shredded waste. The extent of refuse decomposition was characterized by the cellulose plus hemicellulose to lignin ratio [(C+H)/L]. The measured friction angle decreased from 32 to 24° as (C+H)/L decreased from 1.29 to 0.25. The shearing pattern for decomposed refuse showed a peak, followed by residual, which was then followed by a steady increase in shear stresses with displacement; the final rate of increase was similar to that observed in fresh paper specimens. Results from this work were comparable to data from previous reports, though it is important to characterize the extent of solids decomposition for a valid comparison with published studies.     

Lessons Learned: Failure of a Hydroelectric Power Project Dam

The case of a hydroelectric power project is presented to show how lack of understanding and poor filter design, along with insufficient control of construction quality, caused the failure of a dam. This paper summarizes the forensic investigations and attempts to explain the conditions that led to the formation of sinkholes on the embankment slope and the subsequent initiation of progressive failure of the embankment. The steps necessary to restart the power project, including the final remedial design, are then detailed. This paper also discusses how the functions of various engineering design components are integrated in the context of failure of the embankment. This case serves as a lesson for practicing engineers concerning the difficult technical, professional, and procedural issues that may arise during the design and construction of critical infrastructures.     

Experimental Observation on the Effectiveness of Fiber Sheet Strip Stirrups in Concrete Beams

Experimental observations were made for the effectiveness of fiber sheet strips (FSSs) as internal stirrups in comparison with fiber-reinforced polymer (FRP) rod stirrups and steel stirrups. A total number of 10 concrete beams were tested under three-point loading. Each beam measured 1,400 mm long, 150 mm wide, and 250 mm deep. Their shear span-depth ratios were 2.5. The beams were composed of different shear reinforcements: one without stirrups, two with steel stirrups, one with carbon FRP rod stirrups, and the rest with different types of FSS stirrups. The main variables include stirrup types, strengthening of bent portions of FSS stirrups, impregnation, and shear reinforcement ratio for FSS stirrups. Test results indicated that concrete beams reinforced with FSS stirrups had enhanced shear strength over the beam without shear reinforcements. Moreover, the FSS stirrup-reinforced beams could maintain comparable shear behavior to that of the concrete beam reinforced with steel stirrups in overall load-deflection relationships, shear strengths, crack patterns, and crack widths at maximum load.     

Relationship of Compressibility Parameters to Municipal Solid Waste Decomposition

Recently, there has been substantial interest in the enhancement of refuse decomposition in landfills, which results in increased settlement. In this paper, changes in waste compressibility as a function of the state of decomposition are reported. Samples representative of residential refuse were decomposed under conditions designed to simulate decomposition in both control and bioreactor landfills. Twenty four one-dimensional oedometer tests (63.5 mm cell) were performed on refuse prepared in laboratory-scale reactors for measurement of primary (Cc) and secondary (Cαi, representing creep, and Cβi, representing biological) compression indices. The state of decomposition was quantified by the methane yield and the cellulose (C) plus hemicellulose (H) to lignin (L) ratio. The magnitude of compressibility was shown to increase as refuse decomposed and compressibility parameters were correlated with the state of decomposition. Initial settlement increased with decreasing (C+H)/L ratio while the creep index was fairly independent of the state of decomposition. The coefficients of primary compression (Cc) for bioreactor samples showed an increasing trend with decreasing (C+H)/L ratios. Cc increased from 0.16 to 0.36 as (C+H)/L decreased from 1.29 to 0.25, and similar values of Cc were obtained with control samples at similar (C+H)/L ratios. The creep index range was estimated at 0.02–0.03 for control and bioreactor samples in various states of decomposition. The magnitude of the biological degradation index (Cβi) depended on the degradation phase with the highest value of 0.19 obtained during the phase of accelerated methane production. Proposing a single Cc for landfill settlement calculations may lead to inaccurate predictions. Properties of each waste sublayer will change as a function of the decomposition stage, and dominating processes with appropriate compressibility parameters should be applied to individual sublayers.     

Damage-Coupled Progressive Failure and Yield Line Analyses of Metal Plates

Continuum damage mechanics based progressive failure analysis of an aluminum alloy AL2024-T3 plate has been carried out. Isotropic continuum damage mechanics model proposed by Chandrakanth and Pandey in 1995 has been implemented in a nonlinear finite element computational scheme based on damage-coupled and damage-uncoupled elastoplastic constitutive relationship. In order to model the progressive growth of damage and plasticity from extreme fibers toward the neutral axis, discrete layered approach has been adopted in the formulation using Ahmed’s degenerate isoparametric shell element, which accounts for shear deformation. A critical damage criteria is used for determining the onset and propagation of failure in the plate. Damage-coupled and damage-uncoupled analyses have been carried out on rectangular and triangular plates of aluminum alloy Al2024-T3. Yield line patterns have been generated using extensive nonlinear progressive failure analysis and comparison with conventional yield line analysis has been made. It is envisioned that employing the methodology presented herein, yield line pattern generation for structural components with complex shapes can be obtained, which would significantly assist engineers in analysis and design of structures.     

Stability Analyses of Rainfall Induced Landslides

The slope stability issues concerning rainfall induced slope failures are investigated and presented. Specifically, the effect of both negative and positive pore water pressures on the stability of initially unsaturated slopes are carefully explained and coupled with infinite slope analysis methods in order to present a predictive formulation of slope failures that occur as a result of rainfall events. The formulation serves as a baseline analysis method for evaluating potentially unstable soil slopes that are subject to surface infiltration and explains the various triggering mechanisms that may occur based on individual combinations of the slope geometry, soil strength, and infiltration parameters. A procedural method is outlined for utilizing the analytical formulation to predict the change in the factor of safety for a slope subject to infiltration and a detailed analysis of a case study is presented to verify the method. Quantitative statements are made concerning the time and depth of failure in relationship to the soil, slope, and rainfall parameters.     

Characterization of Failure in Cross-Anisotropic Soils

Drained true triaxial tests on dense Santa Monica Beach sand deposited with a cross-anisotropic fabric have been performed to study the failure condition in the principal stress space. The failure surface was assumed to be symmetric around the vertical axis (on the octahedral plane of the principal stress space), but varying as a function of the Lode angle. Data from previously performed consolidated-undrained true triaxial tests on San Francisco Bay Mud and data from triaxial compression, triaxial extension, and plane strain tests on Toyoura sand showed similar behavior in terms of effective stresses. A three-dimensional failure criterion is proposed for characterization of failure in cross-anisotropic soils, under commonly occurring conditions when loading and depositional directions coincide and no significant rotation of principal stresses occur. This cross-anisotropic criterion is developed using a coordinate rotation of the principal stress space and utilization of an existing isotropic failure formulation. Derivation of the three required parameters is explained and illustrated. The proposed criterion is compared with various experimental results; and it is demonstrated that the failure criterion for cross-anisotropic soils captures the experimental behavior with good accuracy.     

Shear Behavior and Mode of Failure for ASTM C1433 Precast Box Culverts

This study evaluates the shear behavior and capacity of the precast concrete box culverts subjected to HS 20 truck wheel load. The most critical culvert behavior was considered by studying culverts subjected to zero depth of the fill and placed on a rigid bedding material. Full-scale experimental tests, with wheel load placed at the distance d from the tip of the haunch to the edge of the load plate, were conducted on 24 typical precast concrete box culverts designated as per ASTM C1433-05. The test results further indicated that flexure governed the behavior up to and beyond AASHTO 2005 factored load. Three-dimensional nonlinear finite-element models (FEMs) of the test specimens were developed and verified with the experimental results. The three-dimensional volumetric shear force distributions on the top slab of the 42 ASTM C1433-05 boxes were obtained by using the FEM from which the distribution width for each box was calculated. This was used to obtain the critical factored shear force for all the boxes which were then compared with the American Concrete Institute shear capacity equations. It was shown that the shear capacity exceeded the factored critical shear force for all the ASTM C1433-05 boxes. This study shows that the AASHTO 2005 provision with regard to the shear transfer device across the joint is unsupported.     

Investigation of Bridge Expansion Joint Failure Using Field Strain Measurement

This study was initiated after the sliding plate expansion joints of Millard E. Tydings Memorial Bridge, a long-span truss bridge, failed prematurely just a few months after their initial installation. The study included field investigation, material testing, analysis, and field testing. Test data records show numerous high spikes, which represent high tensile stresses, within their respective measured periods. By extrapolation, the failure was predicted by applying Miner’s linear cumulative damage rule. The calculation shows that the accumulative value would reach 1.0 when the first and subsequent failures were reported. The unexpected high stresses and poor workmanship caused early crack formation attributable to distress.     

Shear Strength of Municipal Solid Waste

A comprehensive large-scale laboratory testing program using direct shear (DS), triaxial (TX), and simple shear tests was performed on municipal solid waste (MSW) retrieved from a landfill in the San Francisco Bay area to develop insights about and a framework for interpretation of the shear strength of MSW. Stability analyses of MSW landfills require characterization of the shear strength of MSW. Although MSW is variable and a difficult material to test, its shear strength can be evaluated rationally to develop reasonable estimates. The effects of waste composition, fibrous particle orientation, confining stress, rate of loading, stress path, stress-strain compatibility, and unit weight on the shear strength of MSW were evaluated in the testing program described herein. The results of this testing program indicate that the DS test is appropriate to evaluate the shear strength of MSW along its weakest orientation (i.e., on a plane parallel to the preferred orientation of the larger fibrous particles within MSW). These laboratory results and the results of more than 100 large-scale laboratory tests from other studies indicate that the DS static shear strength of MSW is best characterized by a cohesion of 15?kPa and a friction angle of 36° at normal stress of 1?atm with the friction angle decreasing by 5° for every log cycle increase in normal stress. Other shearing modes that engage the fibrous materials within MSW (e.g., TX) produce higher friction angles. The dynamic shear strength of MSW can be estimated conservatively to be 20% greater than its static strength. These recommendations are based on tests of MSW with a moisture content below its field capacity; therefore, cyclic degradation due to pore pressure generation has not been considered in its development.     

Effect of Sampling Disturbance on Properties of Singapore Clay

This paper examines results from triaxial unconfined compression tests and undrained compression tests on reconsolidated samples of a Singapore marine clay retrieved using two sampling methods that offer differing quality of samples. Both local internal strain measurements using a Hall-effect transducer and external strain measurements using LVDTs were employed in the triaxial tests. Bender elements were embedded in some of the samples to establish the maximum shear modulus. If the samples are not reconsolidated, the shear strength and stiffness determined from triaxial tests are found to be sensitive to the quality of the samples, and generally lower than that determined by in situ tests. However, if the samples are subjected to isotropic or K0 consolidation to the estimated in situ condition, there is little difference between the shear strengths of samples retrieved using different samplers, and also consistent with results from vane shear tests. However, for the maximum shear modulus, even with reconsolidation, there is still a 10% difference between the results from samples retrieved using different samplers. Further, the laboratory determined maximum shear moduli are about 10% lower than the value determined in an in situ seismic cone test.     

Effect of Pile Driving on Static and Dynamic Properties of Soft Clay

A full-scale closed-ended pile was driven into a deep deposit of soft clay that was instrumented with inclinometers and pore pressure transducers at three radial locations and three depths. This paper presents the results and interpretation of both field measurements of shear-wave velocity and the laboratory testing program performed on pre-pile and post-pile “undisturbed” specimens. A companion paper provides full details of the site investigation, field measurements of excess pore pressure, and the deformation field around the pile. Shear-wave velocity profiles at four radial distances were obtained as a function of time following pile driving using the suspension logging method. Compressibility characteristics for this soil were determined through one-dimensional constant rate of consolidation tests carried to very high stresses. Shear strength testing included anisotropically consolidated undrained triaxial tests performed on specimens at two confinement levels to study the effect of fabric and evolving anisotropy. Direct simple shear testing was performed on specimens in their normal vertical orientation, and rotated 90° to observe changes in structure/fabric orientation after pile installation.     

Energy-Based Modeling Approach for Debonding of FRP Plate from Concrete Substrate

For concrete beams and slabs strengthened with bonded fiber reinforced plastic (FRP) plates, plate debonding from the concrete substrate is a common failure mode. In this paper, the debonding process is modeled as the propagation of a crack along the concrete/adhesive interface, with frictional shear stress acting behind the crack tip. Crack propagation is taken to occur when the net energy release of the system equals the interfacial fracture energy. The analysis is first performed for the special case with constant shear stress along the debonded interface, and then for the general case with slip softening in the debonded zone. From the results, a direct correspondence between energy-based and strength-based analyses can be established for arbitrary softening behavior along the interface. Specifically, through the proper definition of an effective interfacial shear strength, the conventional strength-based approach can be employed to give the same results as the much more complicated energy-based analysis. Also, based on the relation between the effective shear strength and other material parameters, it is possible to explain the very high interfacial shear stresses observed in experimental measurements. As an application example, distribution of plate stress and interfacial shear stress for the linear softening case is derived. The model results are found to be in good agreement with experimental measurements, showing that the simple linear softening model can describe the debonding process in real material systems.