Geotechnical Characterization, Field Measurement, and Laboratory Testing of Municipal Solid Waste

The science of understanding fluid flow in waste materials is both complicated and in a state of evolution. In general, however, it is possible to apply soil mechanics and hydrogeological principles to waste, although care needs to be taken with standard phase relationships for a number of reasons. Firstly, there is a great potential for confusion between terms, as landfill science has borrowed definitions from different disciplines which very often have slightly different meanings and derivations. Secondly, the impact of gas is a complicating factor, especially as it is being generated in situ. And finally ongoing degradation can cause a volume reduction in solids, a factor not dealt with in any other area involving fluid flow through porous media. There has been a reasonable amount of research and understanding of saturated waste hydraulics. Darcy's law is routinely applied and appears satisfactory for most eventualities. Changes in effective stress on first loading appears to be the dominant influence on hydrogeological properties, especially hydraulic conductivity, with secondary influences coming from gas, differences in waste composition and possibly degradation. Waste anisotropy and heterogeneity are also other important considerations. Less is known about unsaturated waste hydraulics, and indeed the applicability of standard unsaturated flow theory to waste has not been proven. For example there is considerable uncertainty about the nature and form of water retention curves for waste. Standard theory usually relates parameters to saturated conditions, but the link between saturated and unsaturated waste hydraulics is difficult to define, partly because ongoing gas generation in landfills probably mean that truly saturated conditions never exist. At present there is not enough data on the impact of gas generation, distribution and movement on unsaturated flow parameters.

While the engineering profession's understanding of the response of municipal solid waste (MSW) to monotonic and cyclic loadings has improved over the past two decades, several important issues remain unresolved. Characterization of the shear strength of MSW is a critical step in performing reliable static and seismic stability analyses of landfills. Landfill stability analyses can be no more reliable than the reliability of the engineer's estimate of the shear strength of the waste. Furthermore, the stress-strain response of MSW needs to be considered to provide compatibility between the mobilized shear strength and the level of deformation along potential failure surfaces. Relevant studies of MSW shear strength are summarized in this state-of-knowledge chapter. Large-scale laboratory test data, which includes direct shear (DS), triaxial (TX), and simple shear (SS) test results, and back-analyses of failed and stable landfill slopes in the field are considered. Findings from a recent comprehensive study by Zekkos (2005) are emphasized. There is large variability in the shear strength of MSW reported in the literature. Obstacles to characterizing the shear strength of MSW include its age, heterogeneity and the difficulty in recovering and testing representative waste samples due to the large size of some waste constituents. Differences in testing procedures employed and in the assumptions made when interpreting the test results also contribute to the variability of the shear strength of MSW. A consistent conceptual framework to perform and to evaluate the results of the laboratory and in-situ tests performed on MSW is required. It is hoped that the recommendations provided as part of the first "International Waste Mechanics Symposium" will address this issue and work toward providing a common framework for advancing the profession's understanding of waste properties and mechanics through developing consensus on the performance and reporting of laboratory and field testing procedures. Published data reported in the literature on the shear strength of MSW are discussed in sections on large-scale tests and back-analyzed assessments. This literature review does not provide complete descriptions of the works completed by researchers, as this information is available in detail in the referenced papers. Instead, key findings are summarized. Following the literature review, a summary of the state-of-knowledge of MSW stress-strain response and strength is presented and recommendations are made for developing a consistent framework for performing and reporting shear strength data on MSW. Currently unresolved issues are also identified.

Total settlement in municipal solid waste (MSW) landfills can be considerable, from 25% to 50% of original fill thickness (Bjarngard and Edgers, 1990). Some occurs very quickly, i.e. during the filling phase (or short term), but there remains a significant amount of long-term settlement. Deformations of this magnitude and duration impact on the design, operation, closure and future use of landfills. Long-term settlement defines the pre-closure fill level and hence chargeable void space. If the eventual settlement is excessive or unforeseen, the integrity of the capping layer and any infrastructure for the collection of gas or recirculation of leachate may be compromised. Even if the capping layer remains intact it is necessary to avoid depressions on the upper surface where rainwater could pond. Finally, since settlement can continue for many years, it will pose a constraint on future development, including vertical expansion of the site. So, for engineers charged with the design of a landfill facility or site restoration, an understanding of, and ability to analyse, landfill settlement is required. It is well known that landfill settlement is the product of a combination of processes, including load and biodegradation-related phenomena. Whilst existing geotechnical settlement models can handle load effects, they are limited in their ability to deal with biodegradation. There already exist a number of good reviews of the form and capability of these models, see for example, El-Fadel and Khoury (2000); Elagroudy et al. (2008). In this chapter conventional methods of geotechnical settlement analysis are reviewed as the background to recent work to develop a more fundamental approach to landfill settlement analysis. In addition to the review papers, there are a number of research papers that have contributed to the preparation of this chapter but which have not been expressly cited; these papers are collected in the bibliography.

In areas of moderate to high seismicity, seismic loading typically controls the design of waste fills. In these areas, a reliable estimate of (strain-dependent) material properties of Municipal Solid Waste (MSW) is required for safe and economical design of waste fills. Our understanding of the dynamic properties of MSW has significantly improved since the promulgation of Federal (US) Subtitle D regulations in 1993. These landmark regulations mandated for the first time seismic design and analysis of landfills in areas of modest and high seismicity (PHGA in excess of 0.10 g; 2,475 years return period). This chapter aims to review the state of the art of our understanding of the dynamic properties of MSW. These dynamic properties of MSW include: (1) The shear wave velocity V_s and small-strain shear modulus G_max profile; (2) The shear strain-dependent shear modulus reduction G/G_max and material damping (λ) curves; and (3) The dynamic Poisson's ratio v.

To ensure stability of a construction, the physical properties of its components have to be known so that relevant analysis and design can be carried out. In a landfill, waste presents the largest structural element and influences stability and integrity of the lining system (Jones and Dixon, 2003) and operation of leachate and gas extraction systems. However, in spite of this critical role there is still a dearth of knowledge on behaviour of municipal solid waste (MSW) as an engineering material. To aid development of methodologies for measuring and reporting mechanical properties of waste materials, and to formalise analysis and design procedures, a universally accepted waste characterization framework is required. Waste characterization must provide sufficient information to enable designers and operators of waste containment facilities to appreciate the type and engineering properties of the waste mass and hence to anticipate likely behaviour and potential problems. A waste characterization framework should have two elements, a classification system for waste components, and description of the structure/fabric of the waste mass. Classification of components can be related to material characteristics and parameters. Description of waste body structure can be related to mass characteristics and state parameters. Knowledge of both is required to understand mechanical behaviour of waste bodies (e.g. compressibility, shear strength and hydraulic properties), to group wastes with similar mechanical properties and to facilitate the exchange and interpretation of measured properties by both researchers and practitioners. Although there are several waste classification systems in current use world-wide, none of these relate specifically to mechanical behaviour. Therefore, there is a need to both develop a waste characterization framework and to disseminate it to encourage use world-wide. This section summarises the state-of-the-art of waste characterization as reported at the Waste Mechanics Workshop, New Orleans, March 2008, and outlines a framework that was presented and discussed. Application of a component classification is demonstrated using data from the literature. The workshop defined the uses of a waste characterization procedure, it identified the key factors that should be included in a characterization system, and considered the methods that should be used to obtain measurements and the best approach to present characterization information. Consensus was achieved on the form of the characterization framework and it was agreed that it should be disseminated to those actively involved in waste mechanics research and practice, with the aim of promoting it's use and to stimulate further development.

Recovery and testing of representative, undisturbed or intact samples of Municipal Solid Waste (MSW) is difficult, if possible. Therefore, in-situ testing is the most reliable method for evaluation of the material properties of MSW. However, in-situ testing techniques are also subject to limitations that may significantly affect their applicability and consequently reliability of evaluated MSW parameters. Furthermore, because these measurements can only be made after the waste is in place, they are only directly applicable to analysis of existing landfills while extrapolations are required for design of new landfills. Material properties of interest to engineering community include unit weight and moisture content, shear strength, stiffness and compressibility, stress-strain behavior, hydraulic conductivity (saturated and unsaturated), and field capacity. These properties can roughly be categorized into mechanical and physical properties. Similarly, in-situ measurements may be divided into direct measurements of the behavior of MSW and indirect measurements that rely on correlations and/or back analyses to calculate properties of interest. It is generally difficult to categorize a certain in-situ test method as direct or indirect because each test method can directly measure certain properties and indirectly measures other properties. For example, wave propagation surveys are a direct measurement of wave velocity but an indirect measurement of stiffness; similarly, soundings with the Standard Penetration Test (SPT), Cone Penetration Test (CPT) and dilatometer (DMT) can be used to directly identify waste stratigraphy and indirectly to assess shear strength of MSW. Field measurements of MSW properties may also be divided into non-intrusive measurements that do not penetrate the waste mass and intrusive measurements that penetrate the waste mass. Classification of in-situ testing techniques into intrusive and non-intrusive is more distinct and will be used for categorizing in-situ test methods in the remainder of this Chapter. Over the past years, significant improvements in field measurements became possible. These include rapid and inexpensive measurement of ambient vibrations (e.g., REMI), accurate measurement of small deformations with laser beams (e.g., LiDAR), and include abundance of relatively accurate digital terrain measurements generated by regulatory-mandated annual flyovers over landfills. This is in addition to more in-situ test methods being adopted from geotechnical engineering for testing and sounding at landfills, such as SPT and CPT sounding, and testing with devices such as dilatometer and (self boring) pressuremeter. This Chapter aims to provide a critical review of the state-of-the practice of these methods as applied to waste mechanics.

The evaluation of static and dynamic properties of MSW is a prerequisite for the performance of any type of meaningful landfill stability calculations, but also in many aspects of landfill design, operation and post-closure development. Field testing techniques can provide valuable data on the in-situ behavior of MSW properties under the specific conditions of the testing configuration. However, the constitutive behavior of MSW can only be studied in depth by systematic laboratory testing under carefully controlled confining stresses, and composition of waste samples. Although the collection of undisturbed samples of waste remains essentially impossible, sample disturbance may not be a major issue in laboratory testing of MSW due to the lack of "geologic history". On the other hand, the preparation of MSW samples in the laboratory, with conditions duplicating the field deposition and compaction, involves many critical issues that need to be carefully addressed. Presently, no standards or guidelines exist on the performance and documentation of laboratory testing of MSW and as a consequence different approaches are used. In some cases, complete documentation on various aspects of the testing is not presented limiting the value of the presented test results. The intention of this chapter is to examine these critical issues and provide some systematic guidance on laboratory testing of Municipal Solid Waste.

This statement of research needs assumes an ongoing need to deposit and contain waste in landfill. Further, that landfills are designed, operated and subsequently managed in as technically efficient and environmentally sustainable manner as possible. There follows a need to understand better the internal processes that control the behaviour of the contained waste because it is these processes — a complex mix of hydraulic, biochemical and mechanical phenomena — that define the operational conditions and long-term performance of the engineered containment system. This statement is the perspective of a researcher who happens to be working in UK; it is not the authorised statement of any UK authority, agency, or contractor, nor is it the agreed agenda of the UK landfill research community. Note that the perspective has international relevance. In fact, advances in landfill research may now be more beneficially exploited in countries outside the EU, since EU waste management policy has shifted away from landfill.

Waste Mechanics is completely different from Soil Mechanics, but perhaps out of convenience it is increasingly addressed based on Soil Mechanics concepts. Ideally Continuum Mechanics should be used, but even in Soil Mechanics past empirical approaches are adopted as numerical tools are less available and knowledge of Continuum Mechanics more limited. In Brazil, solid waste landfills were managed from a sanitary engineering viewpoint, and engineers operating the landfills were generally mechanical or electrical engineers, more concerned with waste collection and transport logistics than in fact with waste disposal and its possible impacts on the environment. It was only after the Bandeirantes landfill accident (see Figure 1) that geotechnical engineers were in fact called to play a more important role in landfill projects and management. It should also be mentioned that in most cases, landfill accidents do not involve people and when they do, they affect lives in areas that do not have the ability to engage the media so that the incident is not news or very quickly forgotten.

Significant advances have been made during the past two decades in the understanding of the mechanical behaviour of municipal solid waste, however challenges remain in improving the knowledge base and most importantly, in disseminating such knowledge to engineering practitioners. Large post-closure deformations have been observed in landfills, often interfering with buried infrastructure such as that used to collect landfill gas. Slope failures in landfills have been observed in all continents, in some cases with significant loss of life. Accumulating experience suggests that there are a limited number of key factors that govern the mechanical behaviour of landfilled waste masses. Since the effect of each individual factor is relatively predictable, it is proposed by the author that a quantitative index value be developed for comparison of the behaviour of different waste masses.

Municipal solid waste became a target of engineering studies merely a few decades ago when the size of cities became large and the amount of waste increased due to goods-consuming life styles. Most studies so far have addressed environmental issues so that ground water may be protected from pollution and odor as well as insects may be reduced. In this paper, the author would like to state that there are different problems concerning municipal solid waste that should be solved from a mechanical perspective. The first problem is a collapse of waste landfill that is triggered by heavy rainfalls. The collapse may kill people who are living nearby and hence there is a need to mitigate this waste hazard. This problem is particularly important in mega cities of developing countries where the population is increasing rapidly and efforts for mitigation are not adequate. The second problem is the reuse of closed landfill areas. This idea comes from the situation in Tokyo, particularly, that lacks space resource for future development.

Our understanding of the response of municipal solid waste to monotonic and cyclic loadings has improved significantly over the last decade. However, several important issues remain unresolved. Standardized field characterization procedures, waste sampling protocols, specimen preparation methods, and laboratory testing techniques are required if significant advances are to be made over the next decade. The profession can only advance so far if confusion regarding what was tested, how was it tested, and what the results were continues as it has in many cases. Currently, many of the inconsistencies in our understanding of the static and dynamic properties of solid waste are caused by inconsistencies and uncertainty in waste properties characterization procedures and testing or reporting protocols. The first "International Waste Mechanics Symposium" will hopefully address these issues as well as other important issues and provide a common framework for advancing the profession's understanding of waste properties and mechanics through developing consensus on the performance and reporting of laboratory and field testing procedures. Based on limitations in our current state of knowledge of waste properties and mechanics, a number of critical needs in waste mechanics research are identified.