The Effect of Oil Contamination on Geotechnical Properties-Mahmood Ahmadi; Taghi Ebadi – Atran International Group

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Every day, petrochemical activities, oil spills, and pipeline or reservoir leakage contaminate the ground. In addition to environmental concerns, such as groundwater pollution, the alteration of geotechnical properties of the contaminated soil is also cause for worry. There are many ways which could enhance the leakage of gas oil including; corroded storage tanks, processing plants and petroleum transportation. Contamination has been proven to alter the geotechnical properties of soil, and researchers have extensively studied the properties of contaminated granular soils (sand) and fine-grained soils (clay and silt).

This paper summarizes the results relating to the effects of crude oil and gas oil contamination on the geotechnical properties of soils such as Atterberg limits, compressibility, hydraulic conductivity, shear strength, bearing capacity and the cohesion of the soils. In addition, this investigation was undertaken to evaluate the effect of temperature on the strength, permeability, and compressibility of oil-contaminated sand. The samples were artificially contaminated by mixing the soils with crude oil and gas oil in the amount of 2%, 4%, 8%, 12%, and 16% by dry weight.

The results indicated a decrease in strength, permeability, optimum water content and Atterberg limits and an increase in compression index and maximum dry density occur as the oil content increases. Knowledge of these effects of oil contamination is important in coastal engineering and environmental remediation activities of the studied coastal plain.

                   Keywords: Oil-contaminated soil, Geotechnical properties, Sand and clay, Granular soil, Fine-grained soils

Introduction

Leakage of crude oil and its derivatives into the surrounding environment can be proceeded by many means for example; damaged pipeline, petroleum transportation facilities, tanker accidents, oil drilling processes, corroded tanks and natural seepage. The leakage causes the contamination of soil and changes its physical, chemical and geotechnical properties. In connection with the cleanup works and for any possible applications, knowledge of the geotechnical properties and behavior of contaminated soils is required.

This information is also required when oil leakage from storage tanks and processing plants cause oil pollution in the surrounding soils. In this case, it is necessary to determine the effects of oil contamination on existing structures. The soil-bearing capacity, foundation settlement, shear resistance, compressibility, and plasticity are the factors that must be taken into consideration. The extent of the contamination depends on physical specification of soil (infiltration and retention properties) and the volatilization and viscosity of the contaminants (Fine et al. 1997).

The subject was the main interest of many researchers in recent years. For example, Meegoda and Ratnaweera (1994) concluded that the type, the amount and the viscosity of chemicals in pore fluids affect the compressibility of contaminated soils. Evgin and Das (1992) and Evgin et al. (1989) carried out triaxial tests on clean and oil-contaminated quartz sand. They found that full saturation with oil caused a significant reduction in the friction angle of both loose and dense sands and a drastic increase of volumetric strains.

They also showed by finite-element analysis that settlement of footing increased due to oil contamination. Al-sanad et al. (1995) reported a small reduction in permeability and an increase in compressibility due to oil-contaminated in Kuwaiti sand. Al-sanad and Ismael (1997) determined an increase in the strength and stiffness due to aging of contamination of sandy soil with oil. Temperature effects on engineering properties of contaminated sand studied by Aiban (1998). The result showed that the compressibility and permanent deformation of an oil-contaminated sand increase as the temperature increases above room temperature. Puri (2000) evaluated the geotechnical aspects of oil-contaminated soils through laboratory testing on sand samples.

The test results indicated that the compaction properties are influenced by oil contamination. The angle of internal friction of the sand based on the total stress condition was found to decrease with the presence of oil in the pores. Bearing capacity of unsaturated oil-contaminated sand examined by Shin and Das (2001). Results showed the ultimate bearing capacity of a surface strip foundation drastically reduced by oil contamination. Ratnaweera and Meegoda (2006) performed a series of unconfined compression tests on fine-grained soils contaminated with varying amounts of chemicals.

Glycerol, propanol and acetone were used as contaminants. The results showed a decrease in shear strength and stress–strain behavior of the soils. Khamehchiyan et al. (2007) carried out a laboratory testing program to determine the influence of crude oil contamination on the geotechnical properties of clayey and sandy soils. Results indicated a decrease in strength, permeability, maximum dry density, optimum water content and Atterberg limits with increase in the crude oil content.

Olchawa and Kumor (2007) investigated the effect of diesel oil on the compressibility of organic soils. The results showed that compressibility of the soils increased with increase in diesel oil content. Olgun and Yildiz (2010) examined the effect of organic fluids on the geotechnical behavior of a highly plastic clayey soil. This research revealed that liquid limit and consolidation parameters generally decreased while shear strength increased with increase of organic fluid/water ratio and decrease of dielectric constant of the pore fluid. Jia et al. (2011) carried out a laboratory and in- situ testing-program to determine the effect of crude oil contamination on the geotechnical properties of soils.

The results showed the clay fraction (< 0.005 mm) is higher in the heavily polluted samples while the Atterberg limits increased with increasing of oil contamination. In-situ penetration tests showed a decrease in strength with depth for the heavily polluted samples. Contamination effect on the geotechnical properties of an over-consolidated clay contaminated with motor oil examined by Nazir (2011).

The result showed a significant decrease in both liquid and plastic limit with the increase of the duration of oil contamination up to approximately 3 months. In addition, a significant reduction in the unconfined compressive strength and an increase in the compression and swell index with increasing duration of contamination up to 6 months observed. Khosravi et al. (2013)

investigated the effect of gas oil contamination on the geotechnical properties of kaolinite. Results indicated an increase in the cohesion and a decrease in both the internal friction angle and compressibility of kaolinitic soils with increase of gas oil percent. This paper carried out a consideration on effects of oil-contamination on geotechnical parameters.

Scope of the problem

Several oil-derivatives production sites, storage facilities, pipeline basements, launcher and receiver instruments, and sewage facilities will be constructed over contaminated soils in the vicinity of the existing structures. Moreover, engineers are willing to beneficially use the contaminated soil in civil engineering projects. Therefore, the results of this research can be used in the first phase of studies for the development program.

Sample preparation

Soil samples were taken from 30 to 50 centimeters below the ground surface to prevent upper organic soil layers from entering the sample soils. Particle size distribution of the studied soils is according to ASTM D422 (1999). Specific gravity determined based on ASTM D854 (1999). The standard Proctor compaction tests carried out on the soils samples based on ASTM D698 (1999).

For direct shear and unconfined compression tests, standard Proctor compaction test carried out to find density and optimum moisture content of the samples at different percentages of contamination.

The liquid limit (LL) and plastic limit (PL) experiments were conducted with the ASTM D4318 and ASTM D4318 standards, respectively, and plastic index was calculated as PI = LL − PL.

The measurement of the Specific Surface Area (SSA) of soils may be useful for ranking soils for their ability to sorb organic compounds such as pesticides and pollutants. The SSA of a soil sample is the total surface area contained in a unit mass of soil. Soils with high SSA have high water holding capacities, more absorption of contaminants, and greater swell potentials.

After particle size classification, each sample was divided into five parts and then they were dried by oven at 105 °C. Then the samples were mixed with crude oil in the amount of 0, 4, 8, 12 and 16% by weight of the dry soil samples. The mixed samples were put into closed containers for 1 month for aging and equilibrium allowing possible reactions between soil and crude oil.

Results and Discussion

 

Atterberg Limits

The results of the Atterberg limits tests are shown in Figure 1. Atterberg limits are extensively used for identification, description and classification of cohesive soils and as a basis for preliminary assessment of their mechanical properties. The limits consist of liquid limit (LL), plastic limit (PL) and shrinkage limit (SL). Liquid and plastic limits control the consistency of the soils as wetting conditions change. The results indicated the liquid limit (LL) and the plastic limit (PL) increased in the clayey (CL) and silty (ML) soil samples with increasing the gas oil percent.

The increase can be explained by the theory of the diffuse double-layer. Water molecules are polar. As a result, a water molecule has a positive charge at one side and negative charge at the other side. It is known as a dipole. Dipole water is attracted both by the negatively charged surface of the clay particles and by the cations in the double layer. The other mechanism by which water attracted to clay particles is hydrogen bonding, where hydrogen atoms in the water molecules are share with oxygen atoms on the surface of the clay.

All of the water held to clay particles by force of the attraction is known as double-layer water. The innermost layer of the double-layer water, which is held very strongly by the clay, is known as adsorbed water. This orientation of water around the clay particles gives fine-grained soils their plastic properties. The water in the pore space that is not absorbed by the clay particles and moves easily in the soils called free water. The free water determines the liquid behavior of the soil (Das 1994). Unlike the water molecule, the gas oil molecule is not dipole.

Therefore, as the gas oil is mixed with soil, it covers the soil particles and does not allow water molecules to develop the diffuse double-layer, more water needed for the soil to obtain plastic properties. This might be the reason for the increase in plastic limit. However, if the oil orients the soil particles, most of the water added to the soil during the test will join the free water, so liquid limit show a slightly increase with the increasing gas oil content (Kermani and Ebadi 2012).

On the other hand, because of increase in the cohesion of the CL and ML samples after contamination with gas oil and development of flocculation fabric in the soils, more water needed to flow the soil due to its own weight (i.e., shear strength equals zero). It means an increase in liquid limit. With increasing of liquid and plastic limit, plasticity index (PI) of the soil specimens decrease.

Figure 1. Influence of oil content on the Atterberg limits and plasticity index.

Compaction Tests

The results of compaction tests are plotted in Figure 2 in the form of dry density versus water content curves. Standard Proctor compaction tests were carried out on the three soil types. The compaction curves for contaminated soils generally moved to the left side of the uncontaminated soils curve as oil content increased. The variation of the density in the soils shows a significant drop when 4 % oil was added to the soils and for the heavily contaminated samples, the density remains almost constant.

The reduction of dry density in sandy soil is low, since the void spaces are larger and gas oil can move through the soil grains with the same rate as water and it has similar lubricating effect. The results are in agreement with those reported by Shah et al. (2003) and Khamechiyan et al. (2007), but inconsistent with the findings of Al-sanad et al. (1995) and Meegoda et al. (1998). The relationship between the moisture content and gas oil percent revealed a drawdown trend of optimum moisture content with increasing of gas oil content in all of soil types.

It implies that the water content needed to achieve maximum density has decreased when gas oil content increased. It is probably attributed to lubricating effect of gas oil that alters the soils to a state of looser material than uncontaminated soils (Rahman et al. 2010). It should be noted that optimum moisture content of uncontaminated sandy soil is relatively high which can be caused by the presence of montmorillonite in the soil and its water absorption

 

 

 

Conclusions

The following conclusions can be asserted from the study discussed above:

  1. The presence of an oil-contaminated sand layer under the footing resulted in a significant decrease in the bearing capacity and an increase in the settlement of footings.
  2. The bearing capacity factor (Nγ ) decreases significantly with the increase of the percentage of oil contaminating the sand.
  3. Atterberg limits decrease with increasing oil contamination in CL soil. The observed behavior is due to the nature of water in the clay minerals’ structure and performance of existing non-polar and viscous fluids in soil.
  4. Increasing of crude oil content in soils causes a reduction of optimum water content and increase in maximum dry density. The reduction in optimum water content is more in artificially oil-contaminated soil samples, indicating excess oil in the soil.
  5. In general, oil contamination induces a reduction in permeability and strength of all the soil samples. However, effect of oil contamination on shear strength parameters is not uniform and it depends on the soil type but it leads to decreased peak shear strength in all studied samples.
  6. The long-term effect (aging) of oil contamination on the selected soil properties and their behavior should be determined and compared with the results of the present tests. Also, the future study should consider the problem of interaction between the functional groups that exist in the soil solids and the contaminated oil.
  7. The compressibility and permanent deformation of the oil-contaminated sand increase as the temperature increases above room temperature.
  8. The shear strength parameters based on as-molded triaxial tests are not sensitive to the testing temperature when samples are compacted to their maximum dry densities.
  9. Increasing oil content and kinematic viscosity have a tendency to decrease the hydraulic conductivity of a soil.
  10. The total stress friction angle decreases with increasing oil content. The magnitude of decrease in the friction angle increases with the increase of kinematic viscosity of oil.

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