Safety Analysis of Structural Foundations Built on Abandoned Solid Waste Sites
Chapter One
Objectives of the Study
The specific objectives were
- To obtain the properties of ASWSS and adjourning Natural Soil (NS) for comparison with those reported in similar and recent works of other areas.
- To obtain design values of soil properties using Eurocode 7 and reliability methods for both ASWSS and adjourning natural ground.
- To evaluate the reliability indices and probabilities of failure of foundation designs for both ASWSS and adjourning natural soil in all cases using reliability based computer program (FORM5).
- To categorize, from the results of ii and iii, the safety indices of ASWSS foundation designs and the proportion of deviation from those of the natural adjourning soil and make recommendations regarding the use of ASWSS for developments.
CHAPTER TWO
LITERATURE REVIEW
In a study of this nature, a review of existing knowledge, theories and outcomes of other people’s works is unanswerably necessary. This chapter presents the published works and results of other scholars and researchers and the relation which their procedures and findings bear to this study in the following order of subsections:
- Properties of ASWSS;
- ASWSS as foundation soil;
- Safety analysis of ASWSS and
- Classification of reliability of ASWSS.
In this chapter the engineering phenomena taking place in the life of ASWSS, the characteristics of its geotechnical properties and the factors modifying them are well described. The purpose of this chapter, inter alia, is to establish a vocabulary for the entire study and present brief accounts of new approaches to safety analysis of ASWSS that address the problem posed by its inconsistent composition.
Properties of Solid Waste Soil
The term waste refers to those materials that have negative value to its current owner especially in its current form and location (Baccini and Brunner, 1991; Tchobanoglous et al., 1993). Municipal solid waste entails wastes generated within a town, city or districts and excludes those from commercial, industrial and building construction or demolition processes. They are mainly the tangible by-products of domestic and council activities.
In a health conscious and organized society, solid wastes are found in strategically located and designated dump sites called sanitary or landfills. These sanitary fills may be owned and privately operated to accept waste only from the owner (on site) or operated on behalf of municipal authorities and licensed to accept wastes from generators other than owner (landfill). However in a poorly managed system, they may be found in arbitrary and uncontrolled locations within or outside residential areas. Its unique geotechnical properties which result from wide range of its material composition are of primary importance in describing its engineering behavior. These properties may be examined in three basic perspectives namely, classification, compressibility and shear strength characteristics.
Classification of municipal solid waste
Solid Waste classification is a system that allows the separation of Solid Waste composition into its material types, identified in percentage by weight and could be related or compared with similar data from different sources. A number of classification systems obviously exist, but the basis for a choice of classification appears to be the purpose and nature of composition. For the purpose of management and pollution control, wastes may be classified according to the composition of its waste stream, disposal routes and generating source. A comprehensive sample of this mode of classification is shown in Australia Draft Solid Waste Classification of September 1993 in Table 2.1 For the purpose of engineering services Solid Waste may be classified based on compositional characteristics, aging level and degradation potential with reference to a particular location in space or time along the continuum. When information is required on variability characteristics of geotechnical properties of ASWSS due to degradation and compressibility, then Landva and Clerk (1990) classification of SWSS into organic and inorganic constituents becomes most relevant.
The material properties and grain size of a soil allow it to be assigned to one of the limited number of classification groups namely clay, silt, sand, gravel and stone. The particle size distribution curve actually reveals the composition of these groups in a particular soil mass. However, there are larger particles in ASWSS than are commonly found in soil and which are capable of influencing test results. Approximately then, two approaches to the classification of material composition of ASWSS are necessary in engineering measurement. The first approach is the general classification in which soil component is treated as a single material. The second approach is the classification of soil material portion into its standard groups, that is, stone, gravel, silt and clay.
stages. In the first stage (immediate settlement), stress dependent primary settlement occurs quickly within 300 days of stress application in reaction to the compression of SWSS under loading conditions.
A linear relation with gentle slope is observed between strain and logarithmic time in this settlement curve. This initial phase of settlement occurs in both new and old SWSS fills and continues for a period of about one year after which decomposition in recent deposits is activated. Acute degradation of organic composition sets in, with the resultant production of leachate and gas, causing the development of destabilizing pore pressures. The continuation of this phenomenon leads to a secondary phase of creep and non-stress dependent long-term settlement. This phase accounts for the greatest part of SWSS settlement and may be sustained for many years. Extraction of leachate and gas at this stage especially in an engineered repository system may be carried out if the fill is to be considered as the foundation ground for engineering structure.
In the absence of fresh deposits (in the case of abandonment) and after a sufficiently long period of time (20 years and above), there is a drop in settlement scale (third stage) and a linear strain/logarithmic time scale relation develops again with a minimal slope due to gradually ending process of decomposition. Depending on the magnitude of loading , the creep of non-degradable but compressible materials and residual decomposition of organic substances may induce further but small scale settlement and progressive redistribution of stresses at this stage. From the analysis of these three stages of settlement, Bjarngard and Edgers (1990) modeled the settlement of SWSS as follows: Where settlement, initial thickness of landfill, initial effective stress, stress increment, compression index (slope of strain versus logarithmic effective stress), coefficient of mechanical secondary compression and coefficient of secondary compression due to residual decomposition.
Shear strength characteristics of ASWSS
The shear strength of ASWS is that property that binds its element together and enables it to remain in equilibrium when its surface is not level as a result of a combination of inter particle attraction and resistance to inter particle slip and mass deformation (Smith, 1998; Bowles 1996). Shear strength which is often correlated to its parameters of cohesion and angle of internal resistance, is of primary importance in describing the strength characteristics of ASWSS.
It is often expressed using coulomb failure criterion as (Smith and Smith, 1998)
Where, However the heterogeneous composition, time-dependent and inconsistent properties of SWS have made its shear strength evaluation from field and laboratory measurements difficult and costly.
Generally, the common approaches include triaxial compression test, shear box test, in-situ tests (like CPT, SPT, etc) and back computation from plate loading test. The conventional application of these test methods and their resulting data in ASWSS investigation and foundation design has proved to be inadequate, unreliable and most of the time bears no meaningful correlation with the field realities (Mitchell, 1993 and Kavazanjian, 2001). This explains why foundation failures are recorded in ASWSS despite claims of investigation. The question then is ‘what must be done to obtain design input data that mimics approximately the geotechnical characters of ASWS’. The answer to this question which is the focus of this study as well as of many other researches currently going on in the area is provided in the next section.
CHAPTER THREE
PLAN AND METHODOLOGY
Applying conventional and simple methods in describing the properties of homogenous formation makes much sense. However, in a complex and heterogeneous field of soil and non-soil material composition like ASWSS, such methods obscure the geotechnical identity of the tested sample while erratic departure of the interpolated properties from their actual field values of unobserved locations results. Employing geotechnical reliability in characterizing and designing on such fields is, no doubt, rigorously based. However, the current simplification of reliability – based design (RBD) is gradually giving it an emerging structure that is quite accessible to practical application. Additional tools and effort are required to meet the challenges of the compositional nature of ASWSS. This chapter highlights the proposed scheme of execution and distribution of such efforts in the following subsections:
- Location of study area
- Design of study
- Population and sampling techniques
- Instruments for data collection and administration
- Procedures and methods of data analysis
CHAPTER FOUR
RESULTS AND DISCUSSION
The failure of structural foundations develops in different forms. It often starts from an isolated yielding event that triggers off sequence of subsequent failure mechanisms. In all cases and regardless of components interdependencies, the contribution of the bearing soil as an individual component is investigated by examining its various geotechnical properties which generally define the strength status. ASWSS reliability studies therefore begin with the determination of these geotechnical properties, followed by the estimation of the probabilities of their state leading to failure.
The distribution of properties of engineering materials exhibit variations at different scales depending on the material types and phenomena governing the variability. Where the scale of scatter is large, decision making becomes difficult and the consequences of unreliable judgment may be severe.
The pattern of variations may be investigated by establishing the relationship between pairs of observed data especially where there is an inclusion of data that has not been significantly altered. . In this way statistical description of a random field may be given more comprehensively and accurate engineering information may be usefully derived from such statistical data via reliability-based procedures. These procedures are often structured to capture the remnant of field realities and data that have escaped the conventional and empirical field and laboratory examinations.
The ability of the procedures to generate true and sufficiently large representatives of field population makes it adequate in describing the uncertain field. The capabilities of such engineering project sites with respect to loadings can be accurately evaluated and presented in forms of reliability indices and probabilities of failure by relevant models. Derived mainly from laboratory tests this chapter presents the geotechnical properties of ASWSS under study and their range of applications in models of quantitative safety analysis.
CHAPTER FIVE
CONCLUSION AND RECOMMENDATIONS
A common thing in engineering judgment is that confidence is placed on the likelihood of the measured material properties while past events which may possibly explain the likely order of present circumstances are relegated. For instance, if ASWSS investigation reveals satisfactory sample data for engineering purpose, the tendency is to assume that the result is similar to that of the parent population, without regard to high base-rate of failure, if any. In all cases, a balanced posterior probability is influenced by prior probability and likelihood of data. This chapter summarizes the conclusive remarks and recommendations based on the combined application of these sources of evidence in analyzing the reliability of ASWSS for structural developments.
Summary
Risk and reliability analysis in engineering entails mathematical approaches that are closely connected to the principles of probability and statistics. The general advocacy is that geotechnical reliability should not advance beyond FORM so as to avoid undesirable complexity in statistics and calculations because geotechnical data does not have sufficient physical robustness to withstand sophisticated mathematics. Besides, it is feared that further complexity in treatment would render the field and laboratory efforts towards predicting and inferring the geotechnical state of geological materials to mere statistical exercise. This study, therefore from field data acquisition/transcription to evaluation of safety indices, employed simple and available mathematical and statistical methods, especially in analysis and evaluation of safety indices of designs. Combined concepts of the probability of events based on long series of similar occurrence and the measurement of probability on the strength of empirical knowledge of events have been applied to assess the properties and uncertainties in the use of ASWSS for developments. Comprehensive acquisition of soil physical and chemical properties was done using laboratory analysis. Monte Carlo simulation and Hasofer-Lind Approach (FORM) were used side by side to evaluate the safety indices of foundation designs both on NS and ASWSS. As expected the safety indices of designs on NS are far better than those on ASWSS.
Conclusion
The conventional methods of site investigation and treatment of soil data have proved inadequate in accessing the exact geotechnical capability of ASWSS. In most cases the unusual field conditions like the presence of weak zones and ‘reinforced earth scenarios’ neither play appropriate roles nor have account in the evaluated representative value of the strength characteristics. This is in addition to statistical imprecision resulting from insufficient statistical data obtained from inadequate characterization of ASWSS. Occurrence of ‘reinforced earth scenarios’ and unnoticed weak spots on ASWSS hinder the efforts towards predicting the design values that are truly representative of the global average. Foundation failures, therefore, were recorded in the past because of over reliance on the raw data obtained via conventional methods of investigation and lack of adequate provision for experience – based guidance in the evaluation of average strength of ASWSS.
Up to seventy percent (70%) of triaxial test results ( and ) of ASWSS were significantly lower than those of the corresponding NS, with a scale of about one in two (1 in 2) in some cases. These are the main strength parameters of soil and large differences between their ASWSS and NS values indicate large differences between the strength characteristics of ASWSS and NS respectively. However, with just twenty five percent (25%) Atterberg Limits test results showing significant difference and one hundred percent (100%) compaction test results showing no difference at all between ASWSS and NS, the state of geotechnical properties alone could not have been responsible for the scale of failures recorded in the use ASWSS for developments.
The long – term effects of over seventy percent (70%) of sulphate, fifty percent (50%) of chloride and calcium/magnesium contents in excess of EPA/WHO prescribed maximum value certainly impact negatively on the stability and continuous performance of ASWSS foundation members. The subtle but salient effects of this source of foundation failure can persist for a long period of time before it finally results into obvious signs of failure.
The levels of degradation of ASWSS that have spent over twenty (20) years of abandonment no more threaten the ultimate or serviceability performance of foundation members. The fact that majority of the design cases fall in hazardous class, which is the worst state of failure tendency, is an indication of the risk involved in the use of ASWSS for building construction. Low values of safety indices of foundation designs on ASWSS showed clearly that it is unsuitable for heavy structures. However, light and minor structures can be sustained with relevant design techniques.
Recommendations
Based on the findings of this study, the following recommendations are made:
- Geotechnical characterization of ASWSS should be obtained via a sampling plan that is generally more detailed than that of natural ground. It should be an attempt to capture the majority of site irregularities that are associated with ASWSS.
- It is strongly submitted that the characterization plan of ASWSS and evaluation of design values should include informed percentages of ‘upper trim’ and ‘lower extended’ mean respectively.
- Where ASWSS is adequately investigated, a minimum foundation depth of 2.0 m with its relevant foundation width is capable of achieving average reliability; otherwise foundation depth should not be less than 2.5 m.
- It is reasonable also to increase the foundation width, within the acceptable limits of settlement provision however, so as to enhance the safety index of the design especially where the target reliability index has not been attained.
- Foundation members should be strictly protected against the damaging effects of chemical compounds by using sulphate resistant cement in all cases of ASWSS foundation constructions.
- The expected performance of structural foundations proposed to be built on ASWSS should be assessed via a reliability based risk assessment procedure or geotechnical solution that truly and tractably shed light on the prospect of foundation functionality.
- A minimum of ‘Above Average’ classification of structural foundation design of ASWSS should never be compromised except for minor and unimportant structures.
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