Soil Contamination And Shear Strength Lab
Contamination in the soil generally arises due to the elevated concentrations of various artificial chemicals in the natural soil. The extensive use of industrial chemicals- pesticides, hazardous chemical compounds including heavy metals, fertilizers- the over the past decades have led to enhanced concentration of inorganic pollutants in the soil. This, in turn, has affected the various organic, biological, and geological cycles of the soil. The altered structure of the soil due to contamination, mostly in certain industrial areas, poses numerous health problems and environmental challenges. High rate of the soil contamination is, thus, of utmost concern over environmentalists and scientists. Among the available remedial solutions for controlling the soil pollution, a smart selection- which takes into account all technological, economic and local issues- of remedial technology is very crucial for its effective implementation.
Present study is intended to gain knowledge of soil contamination and various site remediation technologies. The present study consists of following four objectives:
1. To understand the various processes involved in soil contamination, its causes and its effects on human health and environmental functioning.
2. To examine various remediation technologies available to produce a decontaminated site.
3. To test soil samples for their shear strength using Direct Shear Test, and assess the contaminated soil and possible source of contamination.
4. To suggest a solution for remediation.
1.2 Report Structure
After introduction in Section 1, Section 2 of the report discusses the processes involved in soil contamination. The causes of site contamination, hazardous chemicals, and various sources/ human activity responsible for contamination are addressed in this section, along with the effects of such contamination on human health and environment. Section 3 introduces the various available remedial solutions and evaluates the social, technological, environmental and economic issues related to their implementation. Section 4 begins with describing the Direct Shear Test experiment and finally suggesting the possible remedial solutions for the given specimen of soil. Finally, the last section, Section 5 gives the discussions and conclusions of the report.
2. SOIL CONTAMINATION, ITS CAUSES & EFFECTS
Following the Industrial Revolution, the overwhelming use of chemicals have resulted in contamination of soil, mainly in industrial and waste disposal sites. Petroleum compounds, pesticides, inorganic fertilizers, solvents and heavy metals are a few most common soil contaminants. The soil becomes contaminated if the concentration of contaminant(s) surpasses the specified threshold concentration level. Knowledge of nature such contaminants, their natural and threshold concentrations is, thus, essential for devising effective remediation.
2.1 Substances Responsible for Soil Contamination
Soil is contaminated by a wide range of hazardous chemicals. The medium through which these chemicals can be ingested by human beings are also different.
Table 2.1.1 Natural and Threshold concentrations for Metals, Semimetals and Cyanide
Substance(symbol) Natural Concentration(mg/kg) Threshold Concentration(mg/kg) Risks Antimony (Sb) 0.01-0.2 2 Health/ecological Arsenic (As) 0.1-25 5 Ecological Mercury (Hg) 0.005-0.05 0.5 Ecological Cadmium (Cd) 0.01-0.15 1 Ecological Cobalt (Co) 1-30 20 Ecological Chrome (Cr) 6-170 100 Ecological Copper (Cu) 5-110 100 Ecological Lead (Pb) 0.1-5 60 Health/ecological Nickel (Ni) 3-100 50 Ecological Zinc (Zn) 8-110 200 Ecological Cyanide (CN) - 1
Table 2.1.2 Natural and Threshold concentrations for Chlorinated Hydrocarbons and Polychlorinated biphenyls (PCB)
Substance(symbol) Natural Concentration(mg/kg) Threshold Concentration(mg/kg) Risks Dichloromethane 0.01 1 Health/ecological Vinyl Chloride 0.01 0.01 Health Dichloroethenes 0.01 0.05 Health Trichloroethene 0.01 1 Health/Ecological Tetrachloroethene 0.01 0.5 Health Trichlorobenzenes 0.1 5 Health/Ecological Tetrachlorobenzenes 0.1 1 Health/Ecological Pentachlorobenzene 0.1 1 Health/ecological Hexachlorobenzene 0.01 0.05 Health/Ecological PCB 0.1 Health/Ecological
2.2 Exposure to Contaminated Soil and Health Risks
Exposure to contaminated soil poses health risks for the people living in affected areas. The exposure can occur through a number of ways:
a. Direct ingestion of contaminated dust and soil.
b. Inhaling dust and vapors of contaminated site.
c. Skin contact with dust particles and soil (dermal absorption).
d. Eating contaminated vegetables and soil adhering to them. Hazardous substances on crops and livestock on contaminated land can get accumulated and ingested by human beings (Fookes, 1997).
e. Contaminants from soil go into surface and ground water which is used by humans of drinking and other purposes. They can also get accumulated with the contaminated soil in the man-made waterways.
2.3Causes of Soil contamination
2.3.1Soil Contamination by Natural conditions
This contamination occurs due to natural conditions processes. In the vicinity of the mineral rocks containing compounds of metals like Nickel (Ni) and Chromium, the concentration of such metals is far above the threshold concentration.
2.3.2 Soil Contamination by Human activities
Human activities and inadequate use of land resources are responsible for far-flung soil contamination. Improper disposal of industrial and urban waste, inadequate quantities of herbicides and pesticides used and landfill contaminate the soil. Mining, if carried out without proper measures to cover the exposed minerals harms soil, vegetation, and aquatic ecosystems of a waste area. Leakage in the tanks containing hazardous chemicals including radioactive waste is another such cause.
2.4 Effects of Soil Contamination
Soil contamination can cause severe health problems for various animals including human beings and damage the environment in a variety of ways.
2.4.1 Ecological Impacts
Presence of hazardous chemicals has adverse effects on the structure and chemical composition of the soil. This, in turn, affects the biological cycles going on in the soil. The introduction of alien chemicals in the food chains may lead to eradication of some species from the chain which leads to their extinction. Contaminants also affect the biochemical processes in plants and crops leading to poor growth of crops and reduced crop yield.
As discussed above, soil contamination poses serious threats to human health and wellbeing, and the ecological systems and there is an urgent need to check and control it. Proper measures such as treatment of industrial waste, purification of waste water and various other solutions should be taken at various levels.
Remediation technologies employ mainly two methods- ex-situ and in-situ methods. In ex-situ methods the contaminated soil is removed and sent for treatment while in-situ methods the soil is treated at the contaminated site. A wide range of commercially available technologies are listed in Table 3.0, some of which are discussed in further detail.
Table 3.1. Remediation technologies available in the UK; not an exhaustive list (Studds, 2004)
Type Location Technology Biological In-situ Monitored natural attenuation, Bioventing, Enhanced Bioremediation Ex-situ Landfarming, Window turning, Bio=piles Chemical In-stiu Soil flushing, Chemical oxidation, Oxygen release compounds (ORC), Permeable reactive barriers Ex-situ Solvent Extraction Physical In-situ Air sparging, Soil vapour extraction, Dual phase extraction, Hydrofracturing, Electroremediation, Pump and treat Ex-situ Soil washing, solidification and stabilisation Thermal Ex-situ Thermal desorption, Incineration, Vitrification Containment In-situ Stabilisation, Impermeable barriers Ex-situ Stabilisation, Capping
Dredging/Excavation processes are carried out by excavating contaminated soil and subsequent treatment using chemical oxidation methods (Mechx, 2002). Dredging can be carried out for treatment of river beds and bay bottoms having contaminated mud or silt.
3.1.2Solidification and Stabilization
This method involves the reaction between a binder and soil to stop or reduce the mobility of contaminants. Adding the reagents to the contaminated soil to produce chemically more stable soil is known as stabilization. On the other hand, in solidification the reagents are added to contaminated soil for imparting dimensional stability for containing the contaminants in solid product and thus, limiting its access by external agents.
3.1.3Pump and Treat
This involves the process of pumping of contaminated groundwater by using a submersible pump and then treating it with certain chemical reagents. This is a quick method to reduce the concentration of contaminants.
3.2Criteria for Selection of Remediation
To check the inadequate use of remediation technologies, the various governments have come up with some criteria for selecting a remedial solution. In the UK, the three criteria- Cost, Flexibility and Speed of Reclamation- are required to be considered for choosing a remediation technology (Beckett and Cairne, 1993).
However, US National Contingency Plan (NCP) gives following nine criteria to be considered before selecting a remedial solution (Lafornara 1991):
1. The overall protection of human health and the environment
2. Reduction of mobility, toxicity and volume through the treatment
3. Compliance with applicable and appropriate requirements
4. Long-term effects
5. Short term effects
6. Implement ability
7. Cost effectiveness
8. Acceptance of state
9. Acceptance of community
4. SHEAR STRENGTH AND ASSESSMENT USING DIRECT SHEAR TEST
The greatest shear stress that a material can bear without rupture is called its Shear Strength. It is a measure of resistance of material to deformation by continuous displacement of its individual particles. When stress is such that the particles slide or roll past each other, failure occurs.
4.1Shear Strength in Soils
The shear strength of soil depends on a number of factors including, grain size distribution, shape of particles, stress history, soil structure and the like. There are two types of forces responsible for shear strength:
i.) Cohesive forces between soil particles, which is due to the electrostatic attraction between particles. It is independent of shear stress applied to the soil.
ii.) Frictional Resistance between particles, which depends on the applied stress.
Two types of tests are primarily used, to determine the shear strength of a soil sample in lab, which are: Direct Shear Test and Triaxial Shear Test.
4.2Direct Shear Test
The Test equipment consists of a metal box which is horizontally split into two halves. In this box the soil specimen is placed. A vertical (normal) stress is applied to the specimen while the shear force is applied horizontally to the top half of the soil by moving one half of the shear box relative to the other. As the shear stress in continually increased, it eventually causes failure in the soil sample. The procedure is repeated for varying normal load. The angle of friction (f) can be determined by plotting shear stress with normal stress.
Two separate samples of soil were tested. First sample was 16 kg of clean soil and the other was 8 kg of soil contaminated with fuel ash.
The following steps are followed while performing the test:
1. Measure the diameter of shear box and calculate its area.
2. Make sure that both halves of shear box are in contact and fixed together.
3. Weigh out 150 g of sand.
4. Place the soil in three layers in the box using the funnel. Compact the soil with 20 blows per layer.
5. Place cover on top of sand.
6. Place shear box in machine.
7. Apply normal force.
8. Start the motor with selected speed (0.25mm/min) to keep constant the rate of shearing.
9. Observe and record the horizontal displacement gauge, vertical displacement gage and shear load gage readings until the horizontal shear load peaks and then falls.
1. Load at failure for sample one is 87.5 N
2. Load at failure for sample two is 29.1 N
4.3Recommended Remediation for Contaminated Sample
The selection of remedial solution with fuel ash as a contaminant depends on a number of factors concerning the physical properties and chemistry of the soil, the distribution and migration behavior of the contaminants and geological conditions at the site of contamination. The further information regarding the state of aggregation, stability and water solubility of the contaminant fuel ash is to be collected. Hydro-chemical characterization of the soil water also needs to be done for a fair assessment of the resulting health and environmental risks. If there is intense high risk, soil should be excavated to be treated at a different place. In this case, in-situ procedures should be avoided as they are slow and time consuming. The selection is to be made among the available ex-situ technologies. Considerations of cost and legal issues should be taken into account before finalizing the selection of remediation.
Results of the direct shear test in terms of the load at failure for two different soil samples give information suggest that the soil structure two specimens are very different. The load at the failure for the contaminated soil specimen is quite less than that of clean soil specimen indicating the weak structure of the former making it more vulnerable to the degradation by environmental and other factors.
Contamination of soil has become a serious environmental problem. It badly affects the complex functioning of ecosystem and the results have started being seen in many forms from degradation of human health to the extinction of certain species from the life cycle. There is, thus, and urgent need to take sincere steps for remediation of contaminated soil to control the ever increasing land pollution.
Soil contamination is of numerous types and intensities. Various environmental and man-made factors influence the rate and intensities of contamination. These have to be understood for devising effective site remediation. Till now various remediation technologies have been developed. Some of them are being used extensively across the globe while the use of many others is limited by various technological and economic factors. Appropriate technology is to be chosen after studying the nature and extent of contamination, evaluating technological challenges and considering social and economic issues.
In the last few decades, increased public and political concern of soil contamination has led to development of innovative remediation technologies which are based on chemical, physical and biological processes. In present study we understood and appreciated the various steps involved in assessment of contamination and selection of remedial solution. The Direct Shear Test was conducted for a contaminated soil sample which is a simple, fast and reliable test for measuring the shear strength and assessing the contamination. An appropriate remedy is selected after examining the results of test and studying environmental conditions.
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