Robert M. Koerner

Leachate in landfills: the stability issues

Abstract Of the many technical papers on the quantity of leachate within landfills, almost all of them focus on its generation, collection, removal, or (more recently) on its reinjection for the purposes of accelerated degradation of the waste mass. Few papers have addressed the influence of leachate on the stability of the waste mass, the exception being a few forensic papers dealing with waste failures. Thus, this paper attempts to present a unified approach explaining the influence of leachate on landfill stability in a sequential manner. Moving from relatively low quantities to high quantities, the various leachate distribution scenarios are as follows: (i) discontinuous leachate, (ii) perched or localized leachate, (iii) leachate head on the liner, (iv) leachate head with gas entrapment on the liner, and (v) leachate under excess pore pressure. All these scenarios have the effect of decreasing a given site-specific factor-of-safety (FS) value. Furthermore, the decrease in FS-value is in approximate accordance with the above listing. The various scenarios are illustrated using actual case histories of landfill failures whenever possible. It is shown that the quantity of leachate in a landfill and/or the site-specific liquids management program can be critically important both during waste placement operations and, depending on the geometry of the particular site, quite possibly for the landfills entire service lifetime with respect to the overall stability of the waste mass.

GEOSYNTHETIC REINFORCED SEGMENTAL RETAINING WALLS

Abstract Segmental retaining walls (SRWs) (primarily those with precast concrete block facing) reinforced by geogrids or geotextiles are in a period of enormous growth. Estimates are that 35,000 of these walls exist and that they cover the entire range of practical wall heights. This paper gives a perspective of the evolution of retaining walls in general, and follows with results of a recent cost survey. It is seen that geosynthetic reinforced walls are the least expensive of all wall categories and at all wall heights. Three design methods are then compared to one another with respect to their details and idiosyncrasies. This is followed by a numeric example showing that the modified Rankine method is the most conservative, the FHWA method is intermediate, and the NCMA method is the least conservative. A survey of the literature is included where it is seen that there have been approximately 26 walls which suffered either excessive deformation or actual collapse. The overwhelming causes for these cases of poor performance were (i) backfilling with improperly draining fine grained soil and (ii) contractors deficiencies which could have been avoided with proper quality control and inspection. The paper, which reflects North American practice, closes with a discussion of possible concerns most of which are under active investigation. Clearly, continued strong growth for geosynthetic reinforced SRWs is justified.

STABILITY ASSESSMENT OF TEN LARGE LANDFILL FAILURES

This paper presents and analyzes ten large solid waste landfills. Five are unlined or soil lined, and five are lined with one or more geosynthetic materials. The case histories are analyzed by a 3-D computer code adjusting variables (mainly interface shear strength) to arrive at a factor-of-safety (FS) equal to 1.0. A triggering mechanism, unique to each site, is then applied resulting in an FS less than 1.0. The same variables and triggering mechanisms are then used in a 2-D computer code with FS-values further decreasing by different amounts. The ratio of 3-D to 2-D factors-of-safety is called a wedge factor. Conclusions reached are as follows: (i) Interface shear strengths are the overriding considerations in varying FS-values. Accurate determination cannot be overemphasized. (ii) The triggering mechanisms were all liquid related, i.e., leachate buildup within the waste mass, wet clay beneath the geomembrane, or excessively wet foundation soil. (iii) The average wedge factor of all case histories without, then with, the triggering mechanisms is 1.24.

Stability and tension considerations regarding cover soils on geomembrane lined slopes

Abstract The occurrence of cover soil instability in the form of sliding on geomembranes is far too frequent. Additionally, there have been cases of wide width tension failures of the underlying geomembranes when the friction created by the cover soil becomes excessive. While there are procedures available in the literature regarding rational design of those topics, it is felt that a unified step-by-step perspective might be worth while. It is in this light that this paper is written. Included are four separate, but closely interrelated, design models. They are the following: 1. • cover soil stability on side slopes when placed above a geomembrane, • cover soil reinforcement provided by either geogrids or geotextiles, • wide width tension mobilized in the geomembrane caused by the interface friction of the soils placed above and below the geomembrane,and • circumferential tension mobilized in the geomembrane by subsidence of the subgrade material beneath the geomembrane. Each of these designs are developed in detail and a numeric problem is framed to illustrate the design procedure. Emphasized throughout the paper is the need for realistic laboratory test values of interface friction, in-plane tension and out-of-plane tension of the geomembranes. By having realistic experimental values of allowable strength they can be compared to the required, or design, strength for calculation of the resulting factor-of-safety against instability or failure.

ARRHENIUS MODELING TO PREDICT GEOSYNTHETIC DEGRADATION

Abstract A methodology for the determination of the degradation of polymeric materials used in long-term applications such as landfill liners and covers, vertical and lateral expansions of solid waste facilities, retaining wall and slope reinforcement, etc., is the focus of this paper. Attention is placed on geomembrane barriers, but geotextile filters, geonet and geocomposite drains, geogrid and geotextile reinforcement and plastic pipe might also be involved. The predictive technique most widely used for polymer degradation (and explained throughout this paper) is based on a time-temperature superposition principle, and is called ‘Arrhenius modeling’ in this paper. It uses high-temperature incubation of the polymeric material in question followed by some type of laboratory testing, in order to extrapolate the experimental behavior to the site-specific, and lower, temperature. The tacit assumption involved is that the materials behavior within the high-temperature incubation range (as indicated by the activation energy, ‘Eact’) is constant within this range and can be extrapolated to the lower temperature behavior of practical interest. Assuming this to be the case (it is obviously a critical assumption and promises to be difficult with oriented geosynthetic products), the nature of the experiments used to identify the amount of degradation becomes very significant. We have outlined 13 candidate tests (there are undoubtedly more) for the evaluation of polymers commonly used to manufacture geosynthetics. They are grouped in the mechanical, physical and chemical test method categories. Each test method is described in some detail. Furthermore, an attempt has been made to predict how the various responses might behave in their elevated temperature test environments. Using these trends, an Arrhenius plot can be generated, and a prediction of degradation to a site-specific temperature should be possible to obtain. It is hoped that this paper will stimulate discussion (and even controversy) and, if warranted, an intensive research effort in this area. With a strong enough effort, a single geosynthetic material can be investigated by a number of different testing techniques to see if a consensus regarding degradation times is reached. At the minimum, such a broad-based approach should identify which testing method is preferred for use in Arrhenius modeling. At this point, that particular method(s) can be used on all of the different polymer materials used in a particular application to see which one possesses the best resistance to a specific type of degradation.

Waste containment facilities : guidance for construction quality assurance and construction quality control of liner and cover systems

This book presents a comprehensive description of recommended procedures for construction quality assurance (CQA) and control (CQC) for waste containment facilities. Chapter 1 describes the CQC/CQA process, identifies key responsibilities of individuals and organizations, and discusses the important components for proper CQC/CQA. Chapter 2 summarizes the construction process for waste containment units and discusses the importance of proper CQA. Information on the cost of CQC/CQA is also provided. Chapter 3-8 give detailed information on the CQC/CQA procedures (observations, tests, testing frequencies, corrective actions, and documentation) for compacted clay liners, geomembranes, geosynthetic clay liners, soil drainage materials, geosynthetic drainage materials, vertical cutoff walls, and other components such as pipes and appurtenances. The book is written as a technical guide for field technicians, field engineers, owners, regulators, and others interested in proper construction, CQC, and CQA of waste containment facilities.

Shear strength parameters between geomembranes and cohesive soils

Abstract This paper focuses on the shear strength of various geomembranes and a number of different cohesive soils. The data base is felt to be necessary since geomembranes used in solid waste disposal are often placed directly on low permeability clay soils. Adapted direct shear tests were performed allowing for a determination of adhesion and friction angle values. It was seen that the adhesion of the soil to the geomembrane is significantly reduced from the cohesion value of the soil itself, unless the geomembrane is very soft or textured. Conversely, the friction angle at the interface between the geomembrane and the soil is relatively high, at least for the normal pressures used in these tests. The information presented is felt to be the type necessary for a number of practical design considerations.

Finite element modelling of soil-geogrid interaction with application to the behavior of geogrids in a pullout loading condition

Abstract The use of polymeric geogrids in reinforcement applications is progressing rapidly with the current commercial availability of the products of at least ten different manufacturers. While all of these products consist of an interconnected set of longitudinal and transverse ribs, their similarity ends at this point. Major variables consist of rib size and strength, aperture size, type of polymer, and method of joining the intersecting ribs. This paper attempts to model the use of a ‘generic’ geogrid in the anchorage, or pullout, mode of behavior. It does so by deriving an incremental finite-element formulation to simulate the non-linear response of the geogrid to pullout. Polynomial and hyperbolic functions are used to represent the load-extension behavior of the geogrid and the soil-geogrid-interaction frictional and bearing properties. Three models are incorporated in the analysis to simulate the deformation of the transverse ribs. Highly flexible ribs are assumed to take a parabolic shape and to act as strings, whereas short stiff ribs are considered not to deflect during pulling. Intermediate cases are analyzed by assuming the ribs to behave as beams deflecting under load. Upon development of the program, it is illustrated on an example geogrid. The behavioral trends of the various components of friction and bearing are generated with respect to the percentage mobilized of the pullout load. Of major interest is an understanding of the relative contribution of the various components, particularly the amount of force that must be transmitted through the rib junctions.

High pressure shear strength investigation of an anisotropic mica schist rock

Abstract Mica schist of the Wissahickon Formation is tested in tension and compression in order to investigate the nature, causes and effects of friction and intrinsic shear strength (cohesion) and their relation to the compressive strength of this metamorphic rock. The fracture surfaces are examined with the aid of a scanning electron microscope. Special consideration is given to the contribution of foliation planes, which are considered sources of weakness. Shear strength parameters are shown to be a function of confining pressure and petrographic characteristics of the sample. Orientation of weaker foliation planes also has a demonstrable effect on the parameters, which have minimal values when foliation planes are oriented at approximately 55° from the horizontal.

ACOUSTIC EMISSION BEHAVIOR AND MONITORING OF SOILS

As far as nondestructive testing methods are concerned, acoustic emission techniques are relatively recent additions to the rock monitoring area (dating from the late 1930s) and to the metal testing area (dating from the 1950s). Its application to soils is an even more recent event with little activity prior to the 1970s. However, over the past five to ten years, interest has been generated in the soils area to the point where at least five equipment manufacturers are currently marketing acoustic emission systems specifically for geotechnical engineering applications. This activity is seemingly well founded, for acoustic emissions are indeed generated by deforming soil masses and technical feasibility is now firmly established. This state-of-the-art paper on acoustic emission activity in soils presents these findings on the basis of fundamentals, small-scale laboratory tests, and large-scale laboratory tests. Furthermore, the technique has been applied to field situations in a number of cases. These include slope stability monitoring of dams and embankments, soil movements arising from horizontal and vertical deformations, seepage monitoring, and grout/hydrofracture monitoring. Specific case histories in each group are presented. Collectively taken, the information available seems encouraging enough for many investigators to use the technique for a wide variety of applications. With a multifaceted attack, the current qualitative status of the technique (that is, no acoustic emission indicates stability; low acoustic emission indicates small movement; moderate acoustic emission indicates larger movement; high acoustic emission indicates instability) should move into a better defined quantitative status. In this latter case, acoustic emission signatures of different soils could lead to instant assessment of actual stress levels in any given situation. (Authors)