Comparison of Mechanical Behaviors of Loess Based on Two Different Modes of Oil Contamination

37 In loess oil-production areas, oil leakage not only contaminates the loess, but also changes its 38 mechanical properties. This study aimed to evaluate the effect of diesel oil contamination on 39 mechanical behaviors of loess though extensive laboratory tests conducted on loess with 40 different oil contents (0% to 16%) and dry densities (1.35, 1.45, and 1.55 g/cm3). Two different 41 modes of oil contamination were proposed and applied in compression tests, direct shear tests 42 and unconfined compressive strength tests to study the compressibility and strength 43 characteristics of diesel-contaminated loess. Results show that oil-contamination modes have 44 certain effects on the mechanical behaviors of loess. Under the first mode of oil contamination, 45 compared with clean loess, the compressibility of contaminated loess increases and its 46 unconfined compressive strength and shear strength all decrease. The compression modulus, 47 friction angle, unconfined compressive strength of diesel-contaminated loess using the second 48 mode of oil contamination are larger than those in the first mode of oil contamination at the 49 same oil content and dry density. Understanding these effects of oil pollution can significantly 50 guide soil and environment-remediation activities in oil-production areas.


55
In the process of oil exploitation, transportation, and use, incidences such as pipeline damage, 56 oil-tank accident, coastal-facility discharge, and oil-product leakage occur. Indeed, the 57 problems of soil pollution caused by oil spillage are increasingly becoming serious (Umoren 58 et al. 2019; Wu et al. 2017). Oil leakage causes serious contamination to soil and water, 59 significantly affecting impact the surrounding environment (Wang et al. 2009). Oil spillages 60 cause frequent pollution events to the soil and environment, which is a serious problem of oil-61 production areas. Oil leakage will not only contaminate the soil, but also change its physical 62 and mechanical behaviors (Abousnina et al. 2019). 63 The Gulf War in Kuwait has caused the largest oil leakage in history, seriously contaminating 64 water and land at a large scale (Randolph et al. 2018). Concern on the problem of soil 65 contaminated by oil spillage is increasing at home and abroad. AlSanad et al. (1995) conducted 66 a series of laboratory tests to study the influence of crude oil on the mechanical properties of 67 sand soil in Kuwait during the Gulf War. Subsequently, they considered the aging problem of 68 crude oil contaminated Kuwaiti sand and found that the strength and rigidity of oil-69 contaminated Kuwaiti sand can be improved by the aging of oil and the decrease of oil content 70 (AlSanad et al. 1997). In the study of Kuwaiti crude oil contamination on the mechanical 71 properties of sand, Shin et al. (1999) studied the shear strength and bearing capacity of sand 72 polluted by crude oil through the direct shear test and the bearing capacity test of strip shallow 73 foundation, and the results showed that crude oil had a significant influence on the shear 74 strength characteristics of sand, and the bearing capacity of foundation was significantly 75 reduced. The above studies on the mechanical properties of Kuwaiti sand contaminated by oil 76 spillage have started an upsurge of domestic and foreign scholastic research on the mechanical 77 properties of oil-contaminated soil. 78 To evaluate the mechanical properties of oil-contaminated sand, Aiban (1998)  However, in the above studies, the preparation method of oil-contaminated soil specimen is 115 relatively single, and few studies on the comparative analysis of mechanical properties of soil 116 with different modes of oil contamination have been conducted. In addition, little information 117 is available for dealing with the evaluation on the mechanical properties of oil-contaminated 118 loess. The oil leakage pollutes the soil in the loess oil-production areas. As a typical organic 119 contaminated soil, oil-spillage pollution is bound to have a certain impact on the mechanical 120 behaviors of loess. Therefore, the mechanical behaviors of oil-contaminated loess under 121 different modes of oil contamination are necessary to compare. 122 In the current work, the mechanical properties (i.e., compression properties, shear strength 123 characteristics, and compressive strength characteristics) of clean loess and contaminated loess 124 were evaluated through compression tests, direct shear tests, and unconfined compressive 125 strength tests, respectively. The results can serve as a reference for evaluating the geotechnical 126 properties and developing treatment and restoration methods for loess contaminated by oil 127 leakage in the loess oil-production areas. 128 (1) Soil 131

Experimental procedure
The loess used in this study was obtained from the construction site of the slope engineering 132 in the loess oil-production area of Northwest China. The depth of soil collection was 2.5-3.5 133 m. The main physical parameters of loess are shown in Table 1. 134 135 (2) Oil 138 In this study, diesel oil was selected as a typical representative of petroleum oil, which was 139 obtained from Sinopec. Compared with other light oil fluids such as gasoline, kerosene, engine 140 oil, and lubricating oil, diesel oil has poor flammability and low solubility in water, and it is 141 not volatile. Therefore, the laboratory tests are relatively safe. Viscosity of diesel oil is also 142 lower than that of other light oil fluids, and it is generally 3-5 times higher than that of water. 143 The physical properties of diesel oil at room temperature of 20 °C are shown in Table 2. 144 145 contamination are shown in Fig. 1. 162 Fig. 1(a) presents the specimen preparation method under the first diesel oil contamination 163 mode. Air-dried clean loess is initially passed through a sieve and then dried at the temperature 164 of 105 °C in an oven for 24 h. Diesel oil is added into a predetermined quantity of dried loess 165 to ensure that the oil contents of contaminated loess specimens range from 0% to 16%, and that 166 the mixture of diesel oil and loess come to equilibrium in a closed container at room 167 temperature (20 ℃) for 7 days. Then, water is sprayed onto a predetermined quantity of diesel-168 contaminated loess to ensure that the water content of diesel-contaminated loess is 12% 169 (natural water content). The sample is mixed until homogeneity in a closed container at room 170 temperature (20 ℃) for 24 h. Finally, the diesel-contaminated loess specimens are molded and 171 tested. 172 The specimen preparation method under the second mode of oil contamination is shown in Fig.  173 1(b). Air-dried clean loess is passed through a sieve. The natural water content (12%) is ensured 174 by adding water to quantitative clean loess and allowing it to stand for 24 h to achieve 175 homogeneity at room temperature (20 ℃). A predetermined quantity of diesel oil is then added 176 to loess to prepare contaminated specimens with different oil contents (0% to 16%  Under the first mode of oil contamination, it can be concluded from Fig. 2(a) Fig. 2(b) indicates that the compression 229 modulus of diesel-contaminated loess sharply decreases initially with the increase of oil content 230 and then increases gradually (n>4%). Moreover, the trend of increasing amplitude is relatively 231 slow. The compressive modulus of clean loess is larger than those of contaminated loess, which 232

Results and Discussion
it can be concluded that diesel oil increases the ease of compression under this condition. 233 Under the second mode of oil contamination, Fig. 3(a) shows that the compression coefficient 234 of loess firstly decreases, and then increases before gradually decreasing again with the 235 increase of oil content. The fluctuation in compression modulus of loess at ρd=1.35 g/cm 3 is 236 larger than that of loess with ρd=1.45 and 1.55 g/cm 3 . Compared with clean loess, the 237 compression coefficient of diesel-contaminated loess fluctuates with increasing oil content. In 238 Fig. 3(b), it can be also observed that the compression modulus of diesel-contaminated loess 239 initially increases and then decreases with the increase of oil content before increasing again 240 (n>8%). The variation in compression modulus trend of diesel-contaminated loess is contrary 241 to the compression coefficient trend at the same condition. 242 Comparative analysis of Figs Under the first mode of oil contamination, the influence of diesel oil on the cohesion of loess 264 is presented in Fig. 4(a). With the increasing oil content, the cohesion of contaminated loess 265 initially decreases and then increases before decreasing at the same dry density. At three 266 different dry densities (ρd=1.35, 1.45, and 1.55 g/cm 3 ), diesel oil (n=16%) was added to the 267 clean loess specimens to reduce the cohesion value of loess by almost 60.7%, 57.6%, and 58.6% layer's thickness of loess decreases, as shown in Fig. 6, thereby reducing the cohesion of 295 contaminated loess. Fig. 5(b) shows the effect of diesel oil on the friction angle of loess. The 296 friction angle of contaminated loess became larger than that of clean loess at the same dry 297 density with the change in oil content. Under the first mode of oil contamination, Fig. 7 illustrates that diesel-contaminated loess has 314 a larger unconfined compressive strength with the higher dry density at the same condition. 315 With the increase of oil content, the unconfined compressive strength of diesel-contaminated 316 loess all initially decreases and then slowly increases at three different dry densities (ρd=1.35, 317 1.45, and 1.55 g/cm 3 ), reaching the minimum value with oil content approaching 5%. At n=4% 318 oil content, the unconfined compressive strength of contaminated loess decreases the most at 319 ρd=1.55 g/cm 3 , a 43% reduction, which is nearly half than that of clean loess. By comparing 320 the contaminated loess specimen (n=4%) with ρd=1.55 g/cm 3  Under the second mode of oil contamination, Fig. 8 shows that with increasing oil content, the 336 unconfined compressive strength of loess initially increases (n<2%) and then reduces before 337 slowly increasing again (n>8%). The unconfined compressive strength value reaches the 338 maximum value with oil content approaching 2% at the same dry density. Compared with clean 339 loess, the unconfined compressive strength of oil-contaminated loess is greater, except for 340 several oil contents. In these tests of specimen preparation method, clean loess is initially 341 combined with water and then added with diesel oil to prepare contaminated loess. When the 342 oil content of loess is in a low level, diesel oil fills the pores of soil particles, and its viscosity 343 is greater than that of pore water among specimen particles, thereby increasing the viscosity of 344 soil particles (Khamehchiyan et al. 2007). Accordingly, the cementation ability of 345 contaminated loess is strengthened, and the unconfined compressive strength increases with 346 low oil content. With increasing content of oil, it gradually fills the pores around soil particles. 347 At this time, the viscosity of diesel oil is far less than its own lubrication effect, and the relative 348 sliding of soil particles becomes relatively easy, which reduces the unconfined compressive 349 strength of diesel-contaminated loess.

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A series of extensive laboratory tests were carried out on clean and diesel-contaminated loess 372 to compare and analyze the effect of oil contents and dry densities on mechanical behaviors of 373 diesel-contaminated loess by using two different modes of oil contamination. The conclusions 374 drawn are as follows.

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(1) The compression properties, shear strength characteristics, and compressive strength 376 characteristics of loess are greatly affected by different modes of oil contamination, which 377 leads to various mechanical behaviors of loess. 378 (2) Under the first mode of oil contamination, with increasing oil content, the changing law of 379 compression modulus, cohesion, and unconfined compressive strength of diesel-contaminated 380 loess are nearly identical at the same density. Due to diesel oil spillage, the compressibility of 381 loess increases and its unconfined compressive strength and shear strength decrease. Most 382 changes occur in loess with low oil content. 383 (3) By comparing the test results of the two different modes of oil contamination, it can be 384 concluded that the compression modulus, friction angle, and unconfined compressive strength 385 of diesel-contaminated loess subjected to the second mode of oil contamination are larger than 386 those subjected to the first mode of oil contamination at the same oil content and dry density. All the data used to support the findings of this study are included within the article. 390

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The authors declare that there is no conflict of interest. 392