Assessing changes in geotechnical properties of soil caused by oil contaminants

Soil pollution seriously threatens the integrity of geotechnical soil properties, considerably altering physical and mechanical characteristics. Therefore, the geotechnical characteristics of contaminated soils should be studied. Samples used in this study were obtained from Peshawar, Khyber Pakhtunkhwa Province, Pakistan. Tests such as Atterberg limits, direct shear strength, sieving analysis, consolidation, and unconfined compression were conducted. The internal friction angle, cohesiveness, and shear strength of the soil decreased when oil pollution was introduced, probably owing to the lubricating effect of oil, which also causes a decrease in the dielectric constant. Surprisingly, in certain cases, bioremediation could reduce soil swelling, relieve pressure from swelling, and prevent soil settlement. Oil contamination resulted in decreased permeability and mechanical strength in the analyzed soil samples. Importantly, the effect of oil contamination on the shear strength parameters is not uniform and depends on the soil type. A thorough study is required because of the long-term consequences of oil contamination, particularly the long-term aging effects on geotechnical soil properties and behavior. Therefore, these results must be compared with those of the most recent tests. Furthermore, future research should consider the complex interplay between the functional groups inherent in soil solids and the injected contaminated oil.


Introduction
The exploitation of natural resources by human beings and the industrial revolution have devastated the environment, causing soil contamination by several materials such as to petroleum products, which severely affects the essential standards of soil, freshwater, and climate.Almost all oil spills result in an increase in hydrocarbons on land, which slowly causes soil contamination [1].Oil spillage mostly occurs due to oil exploration, movement, production, and processing; diesel product leaks from oil tankers, spills from car accidents, leaks from underground pipelines, and engine oil pollution of soil are highly common in locations near auto repair shops [2].Oil-based products, such as gasoline, alter the physiochemical and biological characteristics of the soil and lower the productivity of tillable and cultivable crops.It is recognized as one of the most dangerous contamination cases because it alters the geotechnical features of the soil as well as its chemical, physical, and biological aspects.[3].Oil spillage contaminates soil and also has catastrophic effects on the economy, society, and environment [4].The chemical composition of the contaminant and the characteristics of the soil have a considerable impact on the extent of contamination.Certain chemicals, such as sulfur compounds found in contaminated soils, attack concrete, having a negative impact on the hydration of fresh concrete.Additionally, oil contamination degrades properties such as the maximum dry density (MDD), ideal moisture content, and strength [5].
Researchers have proposed different methods for treating oil-contaminated soils, of which bioremediation and chemical oxidation are the most widely used.Other methods include containment in a large number of burial sites, incineration, biochemical methods, absorption methods, soil washing methods, vacuum extraction, and separation by centrifuge and screen systems.The modern method is impressive for treating oil sludge remaining in areas recently covered by standing oil.A considerable decrease in petroleum hydrocarbon content has been observed in naturally attenuated procedures when comparing various bioremediation methods [6].Some experimental studies have demonstrated that oil pollution has a considerable impact on soil parameters such as moisture content, accessible phosphorus, and total organic carbon [7].Oil has a similar impact to water in that it increases the likelihood of interparticle slippage, which lowers the shear strength of polluted soils [8].Because the specific surface area is increased and a biosurfactant is created, bio-remediated soil has higher shear strength, cohesiveness, and internal friction angle than polluted soil [5].Additionally, contamination has a negative impact on the geotechnical characteristics of soils, making them unsuitable for engineering applications without remediation or stabilization procedures in their original pure states [9].The degree of saturation caused by the contaminating oil and the starting viscosity both affect the water permeability [10].It is preferable to avoid using contaminated soil for shallow foundation projects because it negatively affects the angle of friction and cohesion of the soil [11].In addition, the ideal moisture content, liquid limit, and permeability generally decrease because of oil contamination [12][13][14][15].According to previous studies, the ratios of the bending moment, shear forces, and normal forces are affected by variations in the severity of oil contamination in soil of 40, 34, and 20%, respectively [16].When gasoline contamination occurs, the unconfined compressive strength increases earlier than normal.However, as the gasoline content increases above a certain threshold, the unconfined compressive strength decreases by 4% [17].Ammonium hydroxide has a reduced impact on soil characteristics compared to kerosene, copper sulfate, or lead nitrate [18].Adding more Pb to oilcontaminated soil results in a decrease in the diffuse double layer, which improves compaction when using the same amount of compaction energy [19].

Impact of oil contamination on Geotechnical Engineering and construction integrity
Oil contamination is a major concern in geotechnical engineering, particularly for building projects.Oil can considerably change the mechanical and physical characteristics of soil, change permeability, reduce shear strength, and influence compaction.These alterations may impair the ability of the soil to support loads, which could result in structural instability and project failure.Furthermore, the need for specialized methods to clean oil-contaminated sites can increase project costs and schedules.Resolving oil contamination has become essential to guarantee the longevity, safety, and environmental compliance of construction projects in light of growing environmental standards and the drive for sustainable construction methods.Therefore, oil contaminants require coordinated efforts for efficient management and mitigation because they pose a geotechnical challenge and a larger construction and environmental issue.
Depending on the type and concentration of pollutants, the presence of soil contaminants, particularly oil-based compounds, drastically changes the geotechnical properties.Reduced soil density, moisture content, strength, and swelling pressure are the results of such pollution, and compressibility increases.The goals of this study were to assess land contamination and examine the changes in geotechnical properties caused by oil contamination.However, we recognize the importance of quantifying oil contamination levels by providing a more comprehensive understanding of the relationship between contaminant levels and the extent of their impact on soil properties.Such studies will be strengthened by offering a clearer linkage between contaminant levels and observed changes in geotechnical characteristics.The conclusions can direct efficient remediation techniques that solve geotechnical and environmental issues while guiding safe construction practices for contaminated soil.The complex interaction between oil contamination and the geotechnical and environmental properties of soil, which are crucial to many different industries and construction projects worldwide, was also examined.Overall, this study aimed to ensure environmentally friendly building methods and provide insights and solutions for the problems caused by soil pollution.

Sample Collection
The predominant subgrade and bedding soil within the confines of Peshawar City are abundant in substantial outcropping.Soil samples were meticulously extracted from trenches located near Shoba Market, specifically along the Kohat Road and Barra Road Motor Mechanics Workshops.The distance between the two sample collection points was approximately 10 km.The main reason for selecting these two sites was that newly developed projects such as commercial plazas and recreational areas were launched by the Khyber Pakhtunkhwa government.The sampling depths ranged from 0.5 m to 1 m, chosen deliberately to preclude unauthorized deposits of waste, organic matter, or debris.The area of the car repair workshop selected for sample collection was contaminated with fuel oil, such as gasoline and diesel fuel, brake fuel, transmission fuel, and gear oil.

Sample Preparation
Initially, the soil underwent a meticulous process of grinding and desiccation within an oven, where it was subjected to controlled temperatures of 105 °C and 110 °C for 24 h.Subsequently, all soil aggregates were meticulously fragmented into smaller particles before being passed through a series of sieves; the entire procedure was conducted in laboratory.The presence of oil in the pores of the soil and between soil particles reduces cohesion.Moreover, oil interferes in the bonding and adhesive characteristics of pores or interparticle fluids.Oil is a nonpolar fluid and its molecules cannot bond to water or the charged surface of clay minerals.This produces an immiscible phase in the soil-water system, which acts as a lubricant and decreases the cohesion and UCS of the contaminated soil.Furthermore, a continuous decrease in the failure strain with an increase in oil content was observed.Oil contamination caused a quick increase in the UCS, and a substantial reduction in the UCS was observed with an increase in.

Direct Shear Test
Direct shear tests were performed to determine the shear strengths of the samples.In the oilcontaminated soil samples, the stress-displacement diagram moved downwards with increasing oil content, and the soil strength decreased.These tests showed that the role of oil is similar to that of water, which increases the probability of interparticle slippage, thus reducing the shear strength of contaminated soils.

Discussion
A comprehensive laboratory study was conducted to investigate the impact of oil contamination on the geotechnical properties of soil at two distinct locations: the Kohat Adda and Barra Road Motor Mechanic Workshops.As shown in Figures 3, 4, 5, and 6, an increase in oil contamination resulted in the non-passage of soil particles.The soil exhibited poorly graded characteristics, with the coefficient of curvature/gradation falling beyond 1 to 3.This parameter was determined by analyzing the gradation curve using sieving analysis.To categorize and define these soils, Atterberg limits were used to describe how the consistency of fine-grained soils changed with various moisture contents.
Table 1 shows that as the soil oil content increased, soil LL values barely changed, but soil PL values increased, and soil PI values decreased.The results show that the Kohat Road sample was more contaminated than the Barra Road sample because of the high amount of oil contamination in the soil, and the Atterberg limits decreased.Because its role is similar to that of water, oil increases the possibility of interparticle slippage and lowers the shear strength of contaminated soils.Because oil dissipates the energy of the compaction hammer and inhibits water absorption, it lowers MDD and OMC.Oil lowers cohesiveness by clogging soil pores and spaces between soil particles.Moreover, as a pore and interparticle fluid, oil does not exhibit bonding and sticky properties.The nonpolar nature of oil prevents its molecules from forming bonds with either water or the charged surfaces of clay minerals.As shown in Figures 7,8,9,10,11,12,13, and 14, it reduces the cohesiveness and UCS of the contaminated soil and creates an immiscible phase in the soil-water system that functions as a lubricant.Additionally, when the oil concentration increased, the failure strain decreased continuously.The stress-displacement diagram for the oil-contaminated soil samples shows a downward tendency as the oil content increases and the soil strength decreases, as shown in Figures 15 and 16.The findings indicate that oil has an effect similar to that of water in that it increases the likelihood of interparticle slippage, which lowers the shear strength of contaminated soils.

Conclusion
The lubricating effect of oil and reduction in the dielectric constant are caused by oil contamination, which lowers the shear strength, cohesiveness, and internal friction angle of the soil.Under rare circumstances, oil contamination and bioremediation can cause a decrease in soil swelling and an increase in swelling pressure and soil settlement.Typically, oil pollution reduces the permeability and strength of soil samples.Although the impact of oil contamination on shear strength metrics varies depending on the soil type, it always results in a reduction in the peak shear strength.It is important to evaluate and compare the results of the current studies with the effects of oil pollution over time (aging) on the geotechnical characteristics of the soil and its behavior.Therefore, the

Figure 1 .
Figure 1.Satellite image of the two sites, Bara Road and Kohat Road.
The resulting soil was used for the preparation of multiple sample types, each carefully sealed within robust plastic containers to prevent moisture absorption by soil particles or the evaporation of oil during the subsequent experimental procedures.A series of laboratory tests, including sieving analysis (ASTM D6913M-17), moisture content (ASTM D2216-19), Atterberg limits (ASTM D4318-17e1, 2017), specific gravity (ASTM D854-14), direct shear (ASTM D3080/D3080M-11, 2011), and unconfined compressive strength (UCS, ASTM D2166/D2166M16, 2016) tests, were conducted to assess the geotechnical properties of NS and OCS samples.