Experiment Study on the Atterberg Limits of Clay Contaminated by Oil

Transcription

1 Experiment Study on the Atterberg Limits of Clay Contaminated by Oil Fa-xing Huang Institute of Porous Mechanics, Wuhan Polytechnic University, Wuhan, , China Hai-jun Lu* Institute of Porous Mechanics, Wuhan Polytechnic University, Wuhan, , China ABSTRACT For the evaluation of the restoration and reuse of the oil-contaminated soil, the experimental study of Atterberg limits of soil contaminated by oil were carried out and explored the law of Atterberg limits under the condition of oil polluted.the study showed that oil palyed a significant role in changing the value of Atterberg limits of oil-contaminated soil, the oil rate of 3% was the boundary, the influence of oil on the Atterberg limits of soil could be ignorance when the oil rate was less than 3%, but when the oil rate was higher than 3%, the liquid limit of the soil contaminated by crude oil increased while the plastic limit reduced, but the reduction of plastic limit was low, thus plastic index increased by 31.8% when the oil rate was 12%.The liquid limit and the plastic limit of soil contaminated by diesel oil were both reduced while plastic index increased, liquid limit and plastic limit were 73% and 46.6% respectively compared with the soil that haven t been polluted.due to the non-polar, low dielectric constant and adsorption of oil itself, in addition, the difference of viscosity, causing the difference of Atterberg limits under the condition of different shape of oil and oil rate. This research will provide a theoretical basis for the prevention, treatment and optimizing process of oil-contaminated soil, and also has significant practical and application value in the assessment or control of oil-contaminated sites. KEYWORDS: oil-contaminated soil; Atterberg limits; viscosity INTRODUCTION As an important industrial material and energy, petroleum and its products are important material basic in developing and improving economy and people's living standard, but it is inevitable that there will be a lot of crude oil and petroleum products spilled and leaked during the process of exploration, extraction, refinement, storage and different stage of use, these oil products go through the vadose zone and adhere to the surface of soil directly by the rain, making the vadose zone, groundwater and soil contaminated. In front of oil-contaminated soil remediation issues present, oil-contaminated soils is one kind of contaminated soil, it is important

2 Vol. 19 [2014], Bund. M 3038 to understand the mechanism of the soil first before reusing it, and Atterberg limits was widely used for the identification, description and classification of soil among the practice of geological engineering, and was regarded as the basis for the evaluation of the mechanism of soil, and also influenced the assessment of the renovation of soil.therefor, the study of Atterberg limits of soil contaminated by oil is of significance. For the engineering properties of soil contaminated by petroleum products, scholars conducted a series of research concerning the correlation between Atterberg limits and engineering properties of contaminated soil. Xiangran Li [1] investigated the variation tendency of physical and mechanical property of foundation soil polluted by acid and alkali from field investigation and laboratory simulation test and obtained that liquid limit, plastic limit, plasticity index and modulus of compression of the clay decreased while liquidity index and coefficient of compressibility increased; Jianbin Yuan [2] indicated that the factors affected the content of loose bound water were the factors that influenced its Atterberg limits from analyzing the relationship of the content of bound water and Atterberg limits of clay; Yongli Xu [3] found that the salts of chloride would cause the reduction of the water content, Atterberg limits and plasticity index of soil and the increase of its mechanical strength when analyzing and assessing the fields stacked by salt; Ashraf K.Nazir [4] studied the engineering properties of over-consolidated soil which was polluted by motor oil and found that unconfined compressive strength of the soil dropped by 38% and its Atterberg limits were reduced obviously; Mashalah Khamehchiyan [5] stated that permeability coefficient of soil contaminated by crude oil decreased and so done its liquid limit and plastic limit when investigating the field polluted by petroleum in the south coastal of the Persian Gulf; The mechanism of soil is closed related with its Atterberg limits to a certain extent, the ropiness of sample decide its macro mechanical parameters. We should grasp its physical and mechanical properties when reusing the soil contaminated by oil, and Atterberg limits is an important index among them, but the laws of Atterberg limits under the condition of petroleum pollution haven t been studied systematically, thus the study of Atterberg limits under the condition of different shape of oil and different oil rate are of significance. MATERIALS The clay used in the study were taken from the East Lake of Wuhan, Hubei, a depth of 5-7 meters, its natural dry density and natural moisture content were 1.55g/cm 3 and 19.6% respectively, other physical parameters were shown in Table 1.The soil was sieved through 2mm sieve after air drying, light compaction test was carried out, and the optimal moisture content and the maximum dry density were: 18.5% and 1.64g/cm 3, soil mineral composition was measured by X-ray diffraction analyzer and the results were shown in Table 2. ρ dmax / (g/cm 3 ) W opt / (%) Table 1: Characteristics of the clay used in the tests W L / (%) W p / (%) I p / (%) particle size distribution / (%) >0.05mm 0.05~0.005mm 0.005~0.002mm <0.002mm Table 2: The chemical composition of the clay used in the tests SiO 2 AL 2 O 3 Fe 2 O 3 CaO MgO K 2 O Na 2 O

3 Vol. 19 [2014], Bund. M 3039 The oil used was supplied by Qilu Petrochemical Company. The crude oil for the test belongs to the light oil, the diesel oil was also light, their physical parameters were shown in Table 3. It can be seen from Table 3 that the viscosity of the oil was smaller, but it still one million times larger than that of water, the viscosity of diesel fuel is small, only 2-3 times large than that of water, but its liquidity was better. Table 3: Physical characteristic of the oil used in the tests diesel oil crude oil standard density / (g/cm 3 ) freezing point / coefficient of viscosity /mpa s standard density / (g/cm 3 ) freezing point / coefficient of viscosity /mpa s 0.854* * METHODS Test apparatus Atterberg limits were determined by the FC-III digital display liquid plastic limit of soil tester from Shanghai Zhehao Test Instrument Equipment CO., LTD, the quality of drop hammer is 76g and the inner diameter of the soil cup is 40mm with its height is 30mm.Control the test temperature in the range of 25±1 strictly in order to eliminate the influence of temperature. Methods of making samples Dried the soil and sieved through 2mm sieve then dried them by oven at 105 for 24h, then prepared soil with different oil rate based on soil test method standards GB_T [7], among the oil rates were 0%, 1%, 3%, 6%, 12% in order to simulate the different degree of contamination, add crude oil and diesel oil into the soil, after the soil was equilibriumed, the mixture was stirred in a sealed box containing oil and soil mixture, then the samples were moved into the curing oven (25, 95% humidity) and kept for 10 days, after 10 days open the seal cover and drying the soil in the natural environment. Experimental process and data processing Mix the dried soil with distilled water and make them uniform, then transfer them into a plating for a night. Stir the soil until it is uniform, then fill the soil cup with the soil, insure that there is no pore in the sample, last make the surface of the sample smooth. The test from low water content, read the different drop height at different water content, then add distilled water into the soil to vary the water content. Control the drop height in the range of 3~4mm, 7~9mm and 15~17mm strictly and measure the corresponding water content. Fitting straight line in the double logarithmic coordinates with moisture content as the abscissa and droped depth as the ordinate, and find the water content with the droped depth is 17mm, named it liquid limit, and 10mm named plastic limit, and 2mm for shrinkage limit, all the value of water content expressed in a percent sign and accurate to 0.1%. Plasticity index is determined by the following equation:

4 Vol. 19 [2014], Bund. M 3040 where I p plasticity index, I p = w w L liquid limit, L w P w P plastic limit. Concerning that there is the volatilization of oil during the drying process in the oven, so the real value of liquid limit and plastic limit are moisture content, not water content. In order to calculate the real Atterberg limits, we need to measure the oil content before and after drying in the oven and the real water content is calculated by the following equation: where, ω % W t wet weight of contaminated soil; W d dry weight of contaminated soil; m oil residual after drying; n oil content before drying. Wt = ( 1+ mn) ( 1+ n) W d RESULTS Processing of Atterberg limits of clay contaminated by oil h/mm 10 0% 1% 3% 6% 12% h/mm 10 0% 1% 3% 6% 12% w (%) 1 10 w (%) Figure 1: the relationship of h and w of the clay contaminated by diesel oil Figure 2: the relationship of h and w of clay contaminated by crude oil

5 Vol. 19 [2014], Bund. M 3041 Figure 1 and 2 are the relationship of the drop depth and water content of soil contaminated by different content of crude oil and diesel oil separately in the double logarithmic coordinates, from the analyzing result, we can find that the linear relation of drop depth and water content of soil clearly in the double logarithmic coordinates, and their relation can describe by the fitting equation: log 10 h = K log10 w + B, the degree of fitting are both higher than 96%, and h represents drop depth while w represents water content in the equation, the parameters of the fitting equation are shown in Table 4. Table 4: Parameters of fitting equation from the tests Type of oil and fitting Parameters Crude Oil Diesel Oil Crude Oil Diesel Oil Concentration of contamination Parameter: Parameter: Parameter: Parameter: K K B B Case1: 0% Case2: 1% Case3: 3% Case4: 6% Case5: 12% From Figure 1 we can find that: (1) the linear relationship of the 5 cases results are obvious, the value of 3 points in each line are almost the same (controlled by the process of experiment) ; (2) the line shift from left to right as the concentration increase, the corresponding phenomenon in the test is that: the soil tend to be more hard as the concentration increase when the water content is almost the same, thus making the drop depth decrease. From figure 2, we can find that: (1) the slope of the line is different and the line tends to be leveler, we also can find that the value of K decrease from Table 4. (2) water content are more different when drop depth in the range of 3~4mm, and it show that the water content decrease as the concentration increase. The influence of oil on the Atterberg limits of clay Figure 3, 4 and 5 show the law of Atterberg limits under the condition of different shap of oil and oil rate, we adopt DLL (Liquid Limit of Diesel oil Contaminated clay), CLL (Liquid Limit of Crude oil Contaminated clay), DPL (Plastic Limit of Diesel oil Contaminated clay), CPL (Plasticity Limit of Crude oil Contaminated clay), DPI (Plasticity Index of Diesel oil Contaminated clay), CPI (Plastic Index of Crude oil Contaminated clay) in the description of the results.

6 Vol. 19 [2014], Bund. M 3042 Liquid limit/% DLL CLL Oil rate/% Figure 3: the change of liquid limit of clay polluted by oil Plastic limit/% DPL CPL Oil rate/% Figure 4: the change of plastic limit of clay polluted by oil Figure 3 indicates that liquid limit haven t changed largely under the condition of the pollution of oil when the oil rate are under 3%, but liquid limit show different tendency when the oil rate are higher than 3%, in detail, the value of liquid limit of crude oil-contaminated soil increase while the value in the diesel oil-contaminated soil decreased.from the figure, we can find that liquid limit of crude oil contaminated soil increases by 10.5% and the value decreases by 27% in the case of diesel oil contaminated soil when the oil rate reach 12%, it illustrate that different shape of oil make the liquid limit of soil distinct.

7 Vol. 19 [2014], Bund. M 3043 Figure 4 shows that plastic limit of crude oil contaminated soil decrease, but the reduction is scarce, the value of plastic limit is 92.3% of the soil that hasn t been polluted.when the soil is polluted by diesel oil, plastic limit changes obviously, the value decreases rapidly when oil rate is higher than 3% and the value is 46.6% of the clean soil when oil rate reachs 12%. The influence of oil on the plasticity index of clay DPI CPI 27 Plasticity Index Oil rate/% Figure 5: the change of plasticity index of clay polluted by oil Figure 5 shows the change of plasticity index under the condition of oil, the figure indicates that plasticity indexes of the soil under different rate of oil tend to be larger.the tendency is obvious in the oil that polluted by crude oil, in detail, the value of plasticity index doesn t change a lot when the oil rate is less than 3%, by contrast, it is clear that the value increases rapidly when the oil rate are large than 3%, and the value increases by 31.8% when the oil rate is 12%.The value of plasticity index shows weak growth and when the oil rate is 12% the corresponding value increases by 4.0%. DISCUSSION Atterberg limits based on plastic limit, liquid limit and plasticity index. Liquid limit is the minimum water content at which soil particles flow under their own weight and the plastic limit is the minimum water content at which a soil is molded without breaking. These boundary change under dry and wet conditions and control the consistency of the soil. Atterberg limits are widely used in geotechnical engineering for identification, description and classification of soil, and as a basic for the preliminary assessment of their mechanical properties. Although it is easy to determine these indicators and their relationship is relatively easy to describe, the exist of pollution factors make the interpretation the changes in these indicators become more and more complex. Water molecules are polar, thus a water molecule has a positive charge while the other side has a negative charge, and is considered as dipole, dipolar water is attracted both by the negatively charged surface of the clay particles and by the cations in the double layer. This attraction force between the water molecules decreases with distance between clay and water molecule. Because of the attraction force, so that the water molecules adsorbed on the surface of clay particles form a double layer, the innermost layer of the soil is strongly adsorbed to clay particles and forms strongly bound water, all the water around clay and clay show plasticity [8], dried clay and the nonpolar liquid do not have plastic property [9]. A non-polar oil fluid, as

8 Vol. 19 [2014], Bund. M 3044 previously described, the oil is sprayed onto the clay and mixed homogeneously, this will ensure the oil concentration in the clay balance, Therefore, as the oil is mixed with soil, it surrounds the soil particles, and then the water reaction with soil particle reduces. As a result, the thickness of double-layer water reduces and we see a decrease in Atterberg limits as oil is added to a soil. The change of Atterberg limits under the condition of crude oil As is known to all that physical properties of diesel oil and crude oil are different at 25, crude oil has high viscous, poor fluidity, and the viscosity of crude oil is times than that of water while the viscosity of diesel oil is only three times than that of water., The clay particles surface is wrapped with crude oil at high oil content, couples with the hydrophobic nature of the oil itself, these make the water reach the surface of the clay particles interact difficultly, the thickness of double layer is thin, thus, plastic limit decreases. When the clay transform from plastic state to flow (liquid) state, crude oil adheres to the surface of clay just like the role of small clay particles, thus the particles need another water to eliminate the friction of crude oil and particle. The role of crude just like a "pseudo-viscosity", enhancing the plasticity and making the soil reach flow state difficultly. Therefore, the reduction of the thickness of double layer of water lead to the reduction of liquid limit and plastic limit. The change of Atterberg limits under the condition of diesel oil As is described above, oil molecules adhere to the surface of particles under the action of the molecular forces, because of the the polar water molecules obtain, the water molecules adsorbed on particles would compete with the oil molecules, oil molecules that originally adsorbed on the surface of the clay particles were desorbed. Therefore, when the oil rate is low, oil has a little influence on the adsorption of oil on the clay particle because of the low adsorption sites of diesel oil, thus oil rates has a little influence on the liquid limit and plastic limit of clay when oil rate is less than 3%. When oil rate is large than 3%, diesel oil occupies more adsorption sites and some part of oil is desorbed from the particle as the water content increases. Then oil acts as lubricate liquid in the clay, thus the water content that needed to be the state of plasticity and liquidity decreased, leading the deduction of liquid limit and plastic limit. Plasticity index indicates the range of water content in the state of plasticity, the content of bound water in the surface of particle determines the value of plasticity index. Due to the big polar of water and non-polar of oil, though they both adsorb on the surface of particle, but the competitive power of water is bigger than that of oil, water may desorb a large proportion of oil and adheres to the surface of particle, making little change of plasticity index under the condition of the pollution of diesel oil.

9 Vol. 19 [2014], Bund. M 3045 CONCLUSION Based on the purpose of the assessment of the remediation effect of oil-contaminated, Atterberg limits test concerning different oil rate and shapes of oil were taking into account and carried out tests systematically, the study obtain the conclusions as follows: 1. Liquid limit show different tendency when the oil rate are higher than 3%, in detail, the value of liquid limit of crude oil-contaminated soil increases while the value in diesel oilcontaminated soil decreased. Liquid limit of crude oil contaminated soil increases by 10.5% and the value decreases by 27% in the case of diesel oil contaminated soil when oil rate reach 12%. 2. Plastic limit of crude oil contaminated soil decreases, but the reduction is scarce, the value of plastic limit is 92.3% of the soil that hasn t been polluted. When the soil is polluted by diesel oil, plastic limit changes obviously, the value decreases rapidly when oil rate is higher than 3% and the value is 46.6% of the clean soil when oil rate reaches 12%. 3. Plasticity index of the soil under different rate of oil tend to be larger. The tendency is obvious in the oil that polluted by crude oil, in detail, the value of plasticity index doesn t change a lot when the oil rate is less than 3%, by contrast, it is clear that the value increases rapidly when the oil rate are large than 3%, and the value increases by 31.8% when the oil rate is 12%. The value of plasticity index shows weak growth and when the oil rate is 12% the corresponding value increases by 4.0%. 4. Due to the non-polar, low dielectric constant and adsorption of oil itself, in addition, the difference of viscosity, causing the difference of Atterberg limits under the condition of different shape of oil and oil rate. REFERENCES 1. Li Xiang-ran, Yao Zhi-xiang, Cao Zhen-bin (2004) Study on physical and mechanical property variation of polluted erosive foundation soils in typical district of Jinan, Rock and Soil Mechanics, 25 (8) : Yuan Jian-bin (2012) The study for properties of bound water on clayey soils and their quantitative methods, Guangzhou: South China University of Technology. 3. Xu Yong-li, Wang Xue-li (1996). Analysis and assessment of polluted soil in a place of Baoji. Geology of Shaanxi, 14 (2) : Ashraf K. Nazir (2011) Effect of motor oil contamination on geotechnical properties of over consolidated clay, Alexandria Engineering Journal 1, 50: Mashalah Khamehchiyan, Amir Hossein Charkhabi, Majid Tajik (2007) Effects of crude oil contamination on geotechnical properties of clayey and sandy soils, Engineering Geology, 89: Zhu Chun-peng, Liu Han-long, Shen Yang (2011) Laboratory tests on shear strength properties of soil polluted by acid and alkali, Chinese Journal of Geotechnical Engineering, 33 (7) : People's Republic of China Ministry of Construction (1999) GB/T soil test method, Beijing: China Planning Press.

10 Vol. 19 [2014], Bund. M 3046 Das, B.M. (1994) Principle of Geotechnical Engineering, 3rd edition, PWS Publishing Company: 436. Gillott, J.E. (1987) Clay in Engineering Geology, Elsevier ejge