Katı atıkların depolanmasında karşılaşılan geoteknik problemler

Abstract

Hızlı sanayileşme ve nüfus artışı sonucunda büyük boyutlarda kirlenme sorunuyla karşılaşılmıştır. Çevre sorunları olarak özetlenen bu konunun önemli bir kısmını genelde çöp olarak adlandırılan, kapsamındaki genişlik sebebiyle -atık- da denilmekte olan tüketim ve üretim yan çıktıları oluşturmaktadır. Bunlardan katı ve sıvı fazda olanlarının kontrol altında, tasarlanmış sıhhi atık depolama tesisleri, hendekleri ve biriktirme ünitelerine tasfiyesi gerekmektedir. Bu çalışmada, katı atıkların depolanmasında karşılaşılan sorunlar incelenmiştir. Atıkların genel sınıflandırılma biçimleri anlatıldıktan sonra, kirlenme olayının fizyolojisi hidrolik ve jeolojik açıdan açıklanmaya çalışılmıştır. Kirleticinin hareketinin tariflenmesi ve ortamın çerçevesinin tespit edilmesinin kirlenmenin yayılımını önleyebilmek ve kontrol edebilmek için gerekli olduğundan bahsedilmiştir. Kirletici yeraltısuyu akımının olduğu ortamda, mevcut hareketin hızından etkilenerek ilerlemekte ve bu esnada zemin ortamında kimyasal reaksiyonlara girerek kirliği taşımaktadır. Bu hareket sırasında içerdiği kirlilik ve yayılma hızı düşebilmektedir. Katı atıklar uygun biçimde katı atık depolama tesislerine depolanmalıdır. Bu tesislerin tasarımının en önemli kısmını oluşacak çöp suyu sızıntısını kontrol etmek oluşturmaktadır. Bu amaçla geçirimsizlik için oluşturulan şilte sistemleri anlatılmıştır, önceleri doğal kil tabakalar şilte olarak kullanılmıştır. Sıkıştırılmış kil şilteler yetersiz durumları gidermek için denenmiştir. Daha sonraları sentetik malzemeler kullanılarak, daha ince ve daha geçirimsiz, emniyetli tabakalar oluşturulmuştur. Katı atık depolama tesisleri içinde oluşacak çöpsuyu sızıntısı için özel toplama ve uzaklaştırma sistemleri tasarlanması gerekmektedir. Bu sistemlerin tasarım özellikleri ayrıntılı olarak anlatılmıştır. Geoteknik mühendisliğinde yaygın olarak kullanılan -sonlu elemanlar- yöntemiyle kirliliğin yayılımı analiz edilmeye çalışılmıştır. Bu amaçla CTRAN/W yazılımı ve destek yazılım olarak SEEP AV programı kullanılmıştır. Sözkonusu programlar yardımıyla çeşitli atık depolma tesislerinde karşılaşılabilecek sorunlarla ilgili parametrik çalışmalar yapılmıştır.

Rapid increments on industrialization and urbanization around the globe are caused vast amounts of environmental problems. Geotechnical engineers are increasingly challenged to solve these problems related to waste disposal facilities and cleanup of contaminated sites. A new discipline called environmental geotechnology is established. Environmental geotecnologists must not only be acknowledged with geology and civil engineering, but also accustomed with principles of hydrogeology, chemistry and biological processes, as well. In first three sections term 'environmental geotechnology' and contamination processes are explained and investigated. Until the late 1960s, dumping and incineration were the primary methods of waste disposal in the world. The environmental awareness of public and the subsequent legislation and regulation governing waste disposal required upgrading of existing waste dumps to sanitary landfills. Increases in population and rate of generation of waste put economic pressure on utilization of existing waste disposal sites with due regard for environmental concerns. All types of pollution have direct or indirect effects on soil properties. For example, rain falling on a garbage dump will pollute both surface and ground water systems. The polluted water will attack foundation structures such as footings, caissons, piles and sheet piles. If polluted water is used for mixing concrete, it will affect the durability and workability of the concrete. In embankment construction, the moisture-unit weight relationship of soil will also be affected. Numerous investigations have reported on air and water pollution, but little effort has been made to determine how the ground soil responds to these hazardous or toxic substances. At present, most geotechnical project designs and construction are based on the test results following ASTM and AASHTO standards. These standards are based on control conditions at room temperature with distilled water as the pore fluid. Since field conditions and the standard control condition are significantly different, many premature or progressive failures frequently occur. To understand soil behavior under in-situ conditions, it is necessary to examine soil behavior as close to the actual condition as possible. To accomplish this goal, we must understand the environmental conditions as they exist on the ground and their interactions over a long-term period. The field of practice called 'environmental geotechnics' evolved over a peroid of about two decades starting in the 1970s. It is traced nuclear power industry. One of the part of the process of construction a nuclear power plant was development of an 'environmental impact statement'. No nuclear power facility could be granted a construction permit until an environmental impact statement was completed. Concerns about the safety of nuclear power plants have always been voiced, but in mid to late 1970s, on of the most frequently expressed concerns was over the ultimate disposal of high-level radioactive waste. Geotechnical engineers played an important role in these early studies through investigation and characterization of suitable host rocks for waste repositories, analysis of long-term performance of the earth materials under realistic temperatures and pressures, evaluation of probable ground water impacts and assessment of potential risk. Some catastrophic events caused that national attention was focused on adverse impacts of improper disposal and management of chemical wastes. A significant discussion concerning appropriate design standards for waste containment facilities is begun. Geotechnical engineers at this time began to be brought into the process of designing waste containment facilities. In last two decades many of the industrialized countries have been regulating generation, disposal and management of waste. In the United States the primary emphasis has been on waste disposal rather than waste reduction. In Western European countries Governments have played a much larger role in pollution management. The primary emphasis has been on waste reduction through reuse, recycling, clean up technology development, etc. Waste may be generated in the form of solids, sludges, liquids, gases, and combination thereof. With increasing industrialization the quantity of waste has increased immensely. Depending on the source of generation, some of these wastes may degrade into harmless products whereas others may be non-degradable and hazardous. Approximately 10-15% of the wastes generated in the United States is considered hazardous. Hazardous wastes pose potential risk to human health and living organisms. Such wastes are not degradable And may have a cumulative detrimental effect. The Resource Conservation and Recovery Act (RCRA) non-hazardous wastes are municipal, household hazardous, sewage sludge, water treatment sludge, MSW combustion ash, industrial non-hazardous, small quantity generator hazardous, construction and demolition, mining, agricultural, oil and gas and etc. The primary disposal techniques are landfills, surface impoundments, land application, deep well injection, etc. It is stated that 90% of the off-site disposal of hazardous waste in the United States is on land. VI Hazardous wastes cover a broad spectrum of materials, particularly when one considers newly generated, treated wastes and old, untreated wastes buried in the ground some years in the past. Investigations indicated that most of the industrial wastes are consisted of chemical and allied products, primary metals and petroleum and coal products. Many wastes are mixtures of materials. Any attempt to characterize such materials would be pointless; virtually all waste forms can be found. The wastes range from strongly acidic to neutral, and neutral to strongly alkaline. Some wastes are rich in metals, some are rich in organic, and some are mixtures that contain both metals and organics. One special type of waste deserves mention at this point to introduce terminology. Organic liquids may divide into two groups: water soluble liquids (e.g., ethyl alcohol) and non-soluble liquids (e.g., trichloroethylene). Actually, virtually all organic liquids are soluble to some extent water, but in with many organic liquids the solubilities are measured in parts per million or parts per million or parts per billion organic constituent that can be dissolved in water. Non water soluble organic liquids are called non aqueous phase liquids (NAPLs). There are two types of non aqueous phase liquids; dense non aqueous phase liquids (DNAPLs) are heavier than water and light non aqueous phase liquids (LNAPLs) are lighter than water. One would expect to find a LNAPL (e.g., gasoline) floating on the water table and a DNAPL (such as the chlorinated solvent perchloroethylene) perhaps below the water table at the interface between an aquifer and a lower hydraulic conductivity formation. Cleanup of LNAPLs and especially DNAPLs poses unique challenges. Municipal solid wastes (MSW) are somewhat more consistent than industrial waste. The distribution of MSW produced varies to the conventional habits of the countries. Investigations that is carried out at developed countries indicated that most of the MSWs are paper, metals, food and yard wastes. It is pointed out that recycling attempts reduced the amount of glass and paper wastes. AS recycling becomes more prevalent, the distribution would change. An important feature of MSW is that the waste decomposes and produces gas, including methane, that can be very dangerous if not properly controlled. The liquid is derived from waste is called leachate. Leachate can be produced directly from buried liquid wastes or consolidation of fluid-bearing wastes, by decomposition or chemical reactions, or by the leaching action of water moving through the waste. The control of leachate and gasses the single most important design requirement for new waste disposal facilities. vn It is important to understand the attenuation process. As it is told that environmental geotecnology is an interdisciplinary subject, geotechnical engineers would communicate with the geochemist, to ask the right question and to understand the significance of geochemical data. Most geological material in contact with ground water contains some percentage of clay minerals. Such geological material usually consists of complex mixtures of clay minerals, iron and manganese hydrous oxides and the organic matter. Under the proper conditions the clay minerals, the hydrous oxides and organic matter impart to natural earth material the ability to scavenge and to concentrate cations and anions from seepage solutions or migrating ground water. A mechanism with the potential to affect significant attenuation is cation and anion exchange between clay minerals and ions in solution. Ion-exchange or ion replacement reactions can occur to some extent in all clay minerals. Analogous reactions resulting in ion exchange from solution can occur not only with clay minerals but also at the surface of iron and manganese hydrous oxides and organic matter that can be found associated with natural geological materials. Precipitation and coprecipitation can remove chemical constituents from solutions as in soluble precipitates. Often, precipitation reactions are initiated by changes in chemical parameters such as pH. As a general rule, neutralization of pH optimizes conditions for geochemical removal of many constituents from solutions. Geochemical attenuation mechanisms are most active in a pH range between 5 and 8. The extent to which geological materials will function as a geochemical trap and attenuates the movement of potential ground water contaminants will depend upon: 1. the chemical composition of the seepage solution or ground water; 2. the geochemical and mineralogical properties of the geological material; and 3. the pH conditions that are established during contact of water with the geological material. Geologic characteristics such as permeability, porosity, anisotropy, and homogeneity, are functions not only of the type of rock or soil material, but also of the depositional environment, diagenesis, and tectonic processes such as folding and faulting that may have occurred after deposition. This information is needed to build a geological framework in which hydrogeologic principles may be applied before ground water flow and contaminant transport can be evaluated. VIII An understanding of the interaction between hazardous and toxic wastes and the engineering behavior of ground soil involves several requirements: for instance the measurement of the relevant soil properties; the test equipment, that must be durable and resistant to hazardous waste pore fluid, especially organic acids, as well as usable for long-term performance study; and a knowledge of clay mineral elements, with understanding of the chemical, phsico-chemical and microbiological behavior of soil. It is said to be evident that significant work has been done to provide an understanding of the interaction between soil-pollutant and soil-water systems. To adequately apply and understand these phenomena, a characterization of hazardous and toxic waste is necessary. Geotechnical engineer must also study site characteristics and understand the general properties of the given waste and how these properties influence the soil behavior from the environmental point of view. Present soil mechanics concepts are based on short-term performance and consider only loading conditions. Revisions or modification of these concepts must include test equipment, testing procedures and constitutive laws of soils, to provide proper design and construction of foundation structures, especially in polluted water systems. In section four landfills and waste disposal facilities are investigated. Their design conditions are overviewed. Landfills are the final repositories for unwanted or unusable wastes. Until the middle of this century, nearly all wastes were discarded in open, unengineered dumps. Waste was often burned to conserve space. The sanitary landfill began to become commonplace shortly after World War II. A sanitary landfill consists of a refuse disposal area in which the waste is disposed of in cells that range in up to about 5 m. Within each cell, waste is covered with a 150-300 mm. thick layer of soil ( called daily cover ) at the end of each working day. The sanitary landfill represented a dramatic improvement over the open dump. Controlled placement of waste in sanitary landfills (Particularly daily covering) greatly reduced the number of rodents and insects, dramatically reduced public health risks, and generally contributed to major aesthetic improvements in waste disposal. Engineered liners for waste disposal facilities did not become until the 1970s. Sophisticated waste containment units with multiple liners and fluid collection systems did not become common in the United States until they were mandated by USEP A for hazardous waste landfills in the early 1980s. The objective of a waste disposal facility is to contain the waste in a manner that is protective of human health and the environment. Because no endeavor of mankind can be undertaken without some risk, there is always a risk that a landfill will fail to perform up to expectations. Monitoring systems are installed around landfills to determine if the facility is performing in an unexpected manner. Regulations dictated IX the minimum technology that is required to keep risks associated with waste containment facilities small. There are several issues that have impact for site selection for a solid waste disposal facility on land. In broad terms three major issues are environmental, economic, and political. The geotechnical and hydrogeological parameters fall within the environmental category. The political factor is heavily impacted by public attitude. For a siting study to achieve public acceptance, citizen groups should participate in identifying the siting criteria and their relative importance. The ultimate goal is to select a site where the greatest protection to the environment is provided in the event that the technology, possibly affording protection, fails. In this regard some states in the USA are identifying areas where waste facilities could be located safely. Liner systems are basic components of an engineered waste disposal facility. Section five has a broad perspective on this issue. Clay liners are called to be the hearth of the waste disposal facility. Low-hydraulic conductivity soil liners are given various names, including soil liner and clay liner. The term clay liner is used even though other minerals in the liner material (i.e., sand) may be present in larger quantities than clay. The term clay is emphasized because clay is largely responsible for the low-hydraulic conductivity of earthen liners. There are three types of clay liners: 1. naturally occurring clay liners; 2. compacted clay liners (CCL); 3. geosenthetic clay liners (GCL). A clay liner serves as a hydraulic barrier to flow of fluids. Clay liners are used to minimize infiltration of water into buried waste (cover systems) or to control of release of leachate from the waste (liner systems). To meet these objectives, clay liners must have low-hydraulic conductivity over long periods of time. Further, one must be able to verify that the hydraulic conductivity will be suitably low, that is often the most difficult problem to be resolved. In addition, clay liners are expected to attenuate the movement of leachate, to prolong release of chemicals in leachate, and to serve other site-specific functions. In order to prevent leachate via geomembranes, rather than minimization via soil liners of leachate migration similarly produces better environmental results in the waste disposal facilities. Hydraulic conductivity data of different clay soils using various organic solvents which resulted in extremely high values over permeation of the same clays with water. The collection of liquids in waste containment systems and their proper removal represents an important element in the successful functioning of these important facilities. In section six, two major parts of the facility, primary and secondary (if necessary, leak detection) leachate collection systems beneath the waste and surface water removal in the cover system above the waste are investigated. The gas collection system in the closure system will also an important part of the collection and removal system of the waste disposal facility. The term 'liquid management1 referred to the timely and the efficient removal of liquids out of, and away form, the waste containment facility where they can be properly treated and then disposed. In section seven, various contaminant transport analysis are done by the help of a finite element software product CTRAN/W. Program modelled the movement of contaminants through porous materials such as rock and soil. The comprehensive formulation of CTRAN/W makes it possible to analyze problems varying from simple particle tracking in response to the movement of water, up to complex processes involving diffusion, dispersion, adsorption, and radioactive decay. Program is integrated with another finite element software product SEEP/W that computes the flow velocity for the CTRAN/W. Program utilizes the SEEP/W flow velocities to compute the movement of dissolved constituents in pore- water. A contaminant transport process can be illustrated by the flow of water in along pipe filled with sand. The water flows along the pipe as a plug with a constant velocity that is called advection. As the mass moves along with the moving water, it spreads out or disperses. Another way of viewing the contaminant transport process is to assume that the contaminant is injected continuously at a constant concentration. At some point in the pipe beyond the injection location, the contaminant initially appears at a low concentration and then gradually increases until the full concentration is reached. The spreading out of the contaminant is called dispersion. The movement of contaminant is consisted of advection and dispersion processes. After SEEP/W analysis computed the flow velocities; CTRAN/W compute the migration of the dissolved solutes. SEEP/W can analyse simultaneously saturated and unsatured flow, under steady-state and transient water flow conditions, with transient boundary conditions and also under anisotropic and heterogeneous ground conditions properly. In addition, CTRAN/W can analyse the contamination transport under transient concentration, with independent coefficients of dispersivity in two orthogonal directions. Program also considers the coefficient of diffusion as a function of water content and adsorption as a function of concentration. Various kinds of waste disposal facilities are studied in this thesis. Different concentration and flow conditions are used in parametrical analysis. Liner systems and their hydraulic conductivity performance are especially considered and investigated. Generally, the performances of facilities are directly affected by the hydrogeological, geochemical conditions, and anisotropy.