Effects of oil contamination and bioremediation on geotechnical properties of highly plastic clayey soil

Araz Salimnezhad,Hossein Soltani-Jigheh,Ali Abolhasani Soorki

a Department of Civil Engineering,Ozyegin University,Cekmekoy,Istanbul,34794,Turkey

b Department of Civil Engineering,Azarbaijan Shahid Madani University,Tabriz,Iran

c ACECR-Research Institute of Applied Sciences,Shahid Beheshti University,Tehran,Iran

Keywords:Oil contamination Bioremediation Geotechnical properties Clay mineralogy Soil microstructure Highly plastic soil Fine-grained clayey soil Marl

ABSTRACT Leakage of oil and its derivatives into the soil can change the engineering behavior of soil as well as cause environmental disasters.Also,recovering the contaminated sites into their natural condition and making contaminated materials as both environmentally and geotechnically suitable construction materials need the employment of remediation techniques.Bioremediation,as an efficient,low cost and environmentalfriendly approach,was used in the case of highly plastic clayey soils.To better understand the change in geotechnical properties of highly plastic fine-grained soil due to crude oil contamination and bioremediation,Atterberg limits,compaction,unconfined compression,direct shear,and consolidation tests were conducted on natural,contaminated,and bioremediated soil samples to investigate the effects of contamination and remediation on fine-grained soil properties.Oil contamination reduced maximum dry density (MDD),optimum moisture content (OMC),unconfined compressive strength (UCS),shear strength,swelling pressure,and coefficient of consolidation of soil.In addition,contamination increased the compression and swelling indices and compressibility of soil.Bioremediation reduced soil contamination by about 50%.Moreover,in comparison with contaminated soil,bioremediation reduced the MDD,UCS,swelling index,free swelling and swelling pressure of soil,and also increased OMC,shear strength,cohesion,internal friction angle,failure strain,porosity,compression index,and settlement.Microstructural analyses showed that oil contamination does not alter the soil structure in terms of chemical compounds,elements,and constituent minerals.While it decreased the specific surface area of the soil,and the bioremediation significantly increased the mentioned parameters.Bioremediation resulted in the formation of quasi-fibrous textures and porous and agglomerated structures.As a result,oil contamination affected the mechanical properties of soil negatively,but bioremediation improved these properties.

1.Introduction

Ruthless exploitation of natural resources by humans as well as industrialization devastated the environment with contaminating air,water,and the soil.Considering the importance of oil and its derivatives as well as the widespread activities of oil-related industries,these are being considered as one of the most common contaminants (Kermani and Ebadi,2012).Oil spills are defined as leakage and release of oil from petroleum extraction,storage,distribution,and refinement sites into the environment,which can threaten the marine,coastal,and land ecosystem.Oil spills have disastrous impacts on society,economy,and environment(Broekema,2016).Deepwater Horizon oil spoil in 2010 and Exxon Valdez oil spoil in 1989 are the major oil spills with the most severe environmental impacts.Predicting the severity and extent of oil spills in the marine or inland environments as well as providing rehabilitation strategies according to properties of the affected area is the concerns of engineering geology and geoenvironmental engineering.It is a multidisciplinary approach that can be done through geological hazard assessment and environmental impact analysis (Michel et al.,2013;Laustsen,2016).There can be other causes for oil contaminations like intentional spillages due to the destruction of Kuwait’s oil production facilities through the Gulf War in 1991 (Al-Awadhi et al.,1992;Al-Sanad et al.,1995).Soil contamination,as one of the most critical cases of contamination,not only changes the chemical,physical and biological properties of soil but also affects the geotechnical properties of soil.Moreover,propagation of the contamination in the foundation soil of structures can provoke irreparable effects (Puri,2000).

Evgin and Das (1992) studied the shear strength of motor oil contaminated quartz sand via triaxial tests,and they found a significant decrease in internal friction angle (φ) and a substantial increase in the volumetric strain of loose and dense sands.Al-Sanad et al.(1995) reported a slight decrease in the strength and permeability and an increase in the compressibility of Kuwaiti sand due to oil contamination.They noticed that the influence of crude oil on the strength parameters of soil is greater than the effects of light gas oil and benzene.

Oil contamination decreased the strength,permeability,maximum dry density (MDD),optimum moisture content (OMC),and Atterberg limits of CL(clay of low plasticity,lean clay),SM(silty sand),SP (poorly graded sand) soils (Khamehchiyan et al.,2007)and weathered basaltic rock of grades V and VI (Rahman et al.,2010).The similar results were obtained by Soltani-Jigheh et al.(2018) on silty sand collected from the vicinity of crude oil storage tanks in Tabriz oil refinery site in Iran.

Khosravi et al.(2013)indicated that by changing gas oil content from 2% to 20%,an increase in cohesion (c) and a decrease in the internal friction angle and kaolinite compressibility occur,and changing gas oil content has no considerable effect on the shear strength of the soil.

Oil contamination increased the internal friction angle,MDD,compression index,and Atterberg limits of fine-grained CL soil taken from fields of Tehran petroleum refinery and decreased OMC and the cohesion of soil (Kermani and Ebadi,2012).

Crude oil contamination of clay/silty coastal sediments increased the coefficient of compression significantly and did not change shear strength parameters meaningfully (Jia et al.,2011).Estabragh et al.(2014)investigated the consolidation behavior of CL and CH (clay of high plasticity,fat clay) soils contaminated with water-soluble glycerol and ethanol.For the samples with stress history,the preconsolidation pressure increased,and the compression index Ccdecreased due to the increase of organic fluid.

After demonstrating the considerable decrease of MDD,cohesion,internal friction angle,and unconfined compressive strength(UCS) of CL soil due to fuel oil contamination,Shah et al.(2003)improved the geotechnical properties of contaminated soil using stabilization agents such as lime,fly ash and cement.

Nasehi et al.(2016) reported an apparent reduction in MDD,OMC,and internal friction angle of SP,ML (low plasticity silt),and CL soils with increasing gas oil content,while an increase occurred in the cohesion of soils.In silty soil,UCS decreased due to contamination.They demonstrated the critical effects of the flocculated structure of soil and dielectric constant of pore fluid on the strength of soil.

Safehian et al.(2018) found that diesel contamination reduced MDD,cohesion,internal friction angle,and UCS of illite and increased OMC value.Contamination also affected the consolidation settlement of illite adversely.SEM images revealed that illite particles behave individually due to contamination,and the behavior is similar to silt.

The mechanical properties of clays are also affected by the characteristics of the fluid in the pore space.The liquid limit reacts differently to the relative permittivity or dielectric constant of the fluids.Considering this influence,Spagnoli et al.(2018)investigated the relationship between index properties and the undrained shear strength of various clay minerals with different dielectric constants of pore fluid.They changed the values of the dielectric constant by adding different amounts of ethanol to distilled water.Their test results showed that the undrained shear strengths of Na-and Casmectites correlate with the actual fluid ratio normalized to the liquid limit.As kaolinites and illites hold remarkably less exchangeable cations than smectites,this yields significantly smaller ranges for Atterberg limits and decreases the influence of dielectric constant to almost pure particle-particle interactions.Also,the much larger particle sizes of the kaolinite and illite may govern the undrained shear strengths (Spagnoli et al.,2018).

Recovering the contaminated sites into their natural condition and making contaminated materials as both environmentally and geotechnically suitable constructing materials need the employment of different remediation techniques.Based on the type and level of the contaminants,soil type,characteristics of the contaminated site,cost and duration of the process,and environmental impact and efficiency,there is a wide range of remediation methods such as biological,chemical,thermal,physicochemical and integrated remediation technologies which can be chosen.

Considering the physico-chemical properties,Fine et al.(1997)reviewed the retention,volatilization,and transport of hydrocarbons in soil.They found that by decreasing the soil moisture content,vapor phase transport and soil retention of hydrocarbons increase,and transport of nonaqueous phase liquid (NAPL) decreases.Bioremediation is being considered as a successful and widely-accepted method for the treatment of contaminated soils,because it can degrade the oil contamination enduringly,it does not cause any lasting negative side-effects,tolerates low costs,and is an environment-friendly approach (Lim et al.,2016).Sarkar et al.(2005) compared two types of biostimulation during the application of monitored natural attenuation for degrading dieselcontaminated Tarpley clay soil with low carbon content.They used inorganic fertilizer containing nitrogen (N) and phosphorus(P),and also sterilized bio-solids containing carbon(C)along with N and P,as rapid-and slow-release nutrients,respectively.After 8 d of incubation,both methods showed 96% of oil degradation in comparison with monitored natural attenuation that resulted in 93.8%degradation.It proved that using bio-solids as a bio-stimulator is more effective than the use of inorganic fertilizers,and the monitored natural attenuation is applicable for remediating certain contaminated soils with a large population of indigenous microorganisms.

Xu and Lu (2010) reported that indigenous bacteria fixed on peanut hull powder is a more effective way of removing crude oil compared to the use of free-living bacteria due to porous structure,large surface area,and strong adsorption capability of this biocarrier powder.

Makadia et al.(2011) investigated the feasibility of reusing remediated soils for the bioremediation of waste oil sludge.By comparing different methods of bioremediation,they noticed a considerable reduction in total petroleum hydrocarbons content(TPH)in naturally attenuated technique.Due to the existence of oildegrading bacterial and fungal communities in naturally attenuated and amended microcosms,reusing of remediated soils can be considered a low-cost and effective way of bioremediation without any need for other treatments.

Soltani-Jigheh et al.(2018) investigated the efficiency of bacterial biodegradation in sandy soil contaminated with crude oil.By augmenting oil-degrading bacteria and mineral nutrients,they achieved approximately 50%-80%oil degradation.Considering the mechanical aspects,they reported an increase in the values of MDD,cohesion,and UCS,and they suggested the usability of biotreated soil as the road basement or erosion controller.

Although there have been comprehensive and precise investigations on the effect of oil contamination on the geotechnical properties of different soils,there are no prominent researches that cover the impacts of crude oil contamination and bioremediation on the geotechnical properties of highly plastic clayey soils,and there are some contradictions between the results.In this research,Atterberg limits,compaction,unconfined compression,direct shear,and consolidation tests were conducted on natural,contaminated,and bioremediated soil samples to investigate the effects of contamination and bioremediation on the geotechnical properties of soil.To determine the physico-chemical effects of oil contamination and bioremediation on the soil microstructure,Xray diffraction (XRD),X-ray fluorescence (XRF),Brunauer-Emmett-Teller (BET-N2),and field emission scanning electron microscopy (FE-SEM) analyses were also performed on the samples.

2.Experimental procedure

2.1.Materials and methods

2.1.1.Soil

Tabriz yellow marl is the most common type of subgrade and bedding soil of Tabriz City and outcrops in extensive areas.The soil was taken from a construction trench in Marzdaran region from a depth of 0.5 m-1 m to avoid unwanted entry of the debris,trash,or organic soil.After separating and eliminating rock and stone particles from the main body of soil mass,it was pulverized and then dried in the oven at(110±5)°C for 24 h,then the soil was used in the preparation of all samples.This soil is generally known as plastic and sticky,difficult to handle,and a very poor quality subgrade and embankment material (Sadrekarimi et al.,2006).The basic properties of soil are listed in Table 1,and the grain size distribution curve is presented in Fig.1.

2.1.2.Contaminant (crude oil)

The crude oil provided by Tabriz Oil Refinery was selected as the contaminant,and its essential properties are listed in Table 2.

2.1.3.Oil-degrading bacteria and nutrients

For bioremediation purposes,the oil-degrading bacteria isolated from Kharg Island and oil-polluted wastes,muds,and sediments of southern Iran were used,which were provided by the Research Institute of Applied Sciences,Tehran,Iran.Bacterial strains,including five different species,were fixed on the diatomite powder(Table 3).Among identified strains,KE1 belongs to Rhodococcus ruber species,which is the producer of the biosurfactant (Philp et al.,2002),PM07 belongs to Alcanivorax dieselolei species,which play a significant role in the long-chain alkane metabolism (Wang and Shao,2014),PM01 belongs to Thalassospira xianheensis species that have high potential in polycyclic aromatic hydrocarbonsmetabolism (Zhao et al.,2010).K24 strain appears to be a new species of microbacterium.This strain with 97.1%resemblance to the 16S rRNA sequence was most similar to DSM 16089 strain,Microbacterium hydrocarbonoxydans.No study has been done yet in the case of the role of MK1 strain,a halotolerant species which is most similar to Gracilibacillus dipsosauri,in petroleum hydrocarbons decomposition.

Table 1Basic properties of soil.

Fig.1.Grain size distribution curve of the marl.

Table 2Properties of crude oil.

Table 3Bacterial strains selected from optimization experiments identified based on the 16S rRNA gene sequence.

In addition,monopotassium phosphate (KH2PO4) and ammonium chloride(NH4Cl)were added to biologically treated soil(BTS)samples as inorganic nutrients to stimulate the growth and metabolism of the micro-organisms.These substances contain N,P,and potassium (K) as macro-nutrients and chlorine (Cl) as a micronutrient.The addition of P and N,as growth-limiting nutrients,enhances the oil degradation rate (Nikolopoulou and Kalogerakis,2009).To achieve the optimum condition of bioremediation,the producer suggests pH value of 6.5-9.5,temperature of 20°C-50°C,salinity of 0-80,000 parts per million(ppm),the moisture content of 40%of soil,and regular ventilation and stirring.The ventilation of samples was achieved by the regular and manually deep daily stirring of the samples.

2.2.Sample preparation

Three types of samples,including natural soil (NS),oilcontaminated soil (OCS),and BTS,were prepared.Used soil was sterilized by putting it in the autoclave chamber,and then 4%,8%,and 12%of crude oil by the dry mass of soil were sprayed to samples using low volume,low pressure (LVLP) air gun spraying system.OCS samples were then put into the thick plastic bags to pass 30-d aging process,without any oil evaporation and moisture absorption by soil particles.In BTS samples,firstly the samples were prepared just like OCS samples,and after 30-d aging,the bacterial powder was dissolved in distilled water (containing 1 × 107colony-forming units of degrading bacteria per milliliter of the medium) and sprayed to OCS samples to 1% of net mass of hydrocarbon compounds.The amount of oil-degrading bacteria powder,which was applied over 1 kg of OCS,was 0.4 g,0.8 g,and 1.2 g,respectively.Accordingly,the mass of the required microbial powder can be obtained from Eq.(1):

Due to the instruction of the Research Institute of Applied Sciences,Tehran,Iran (the provider of oil-degrading bacteria and nutrients),the inorganic nutrients were added to soil to 1% of the net mass of hydrocarbon compounds by dissolving them in distilled water,spraying over the soil and stirring the mixture manually.

The abbreviation format for the naming of all samples is given in Table 4.

Table 4Naming format of the samples.

Because of coexistence of the water and crude oil as two different evaporative pore fluids and to measure the moisture content of OCS and BTS samples precisely,the following equation developed by Zheng et al.(2014) is used:where Wwis the moisture content by weight mass (%),mtis the mass of porous media before drying (g),mris the mass of porous media after oven-drying(g),mwis the mass of volatilized water(g),mdis the dry mass of porous media solids(g),n is the oil content of the porous media before drying,and γ is the oil drying loss coefficient.The difference between mtand mrvalues equals the mass loss of oil caused by drying(i.e.the mass of volatilized oil),thus the calculation formula of oil drying loss rate(wo) is

By fitting a linear line to the data of the plotted oil drying loss rate woversus oil content graph,the slope of the fitting line is oil drying loss coefficient γ,which was calculated to be 0.3839 experimentally due to the procedure that elaborated by Zheng et al.(2014) (Fig.2).

2.3.Tests

In order to evaluate the geotechnical properties of NS,OCS,and BTS samples,a series of laboratory tests including Atterberg limits(ASTM D4318-17e1,2017),standard Proctor compaction (ASTM D698-12e2,2012),one-dimensional (1D) consolidation (ASTM D2435/D2435M-11,2011),direct shear (ASTM D3080/D3080M-11,2011),and UCS (ASTM D2166/D2166M-16,2016)tests was carried out.

Direct shear tests were executed in a square shear box(6 cm × 6 cm) with a constant shear rate equal to 0.03 mm/min,under normal stresses()of 100 kPa,200 kPa,and 300 kPa.Also,all samples were prepared by compacting to 95%of their MDD and related OMC.

The unconfined compression test was carried out on all samples by a loading rate of 0.5 mm/min.Like the direct shear test,all samples were prepared by compacting to 95% of their MDD and corresponding moisture content.

The 1D consolidation test was carried out on NS,OCS,and BTS samples.All samples were prepared by compacting to 95%of their MDD and corresponding OMC with a height of 20 mm and a diameter of 50 mm.The incremental stresses were 5 kPa,25 kPa,50 kPa,100 kPa,200 kPa,400 kPa,800 kPa,and 1600 kPa in the loading process,and 800 kPa,400 kPa,and 5 kPa at the unloading stage.

Fig.2.Calculation of the oil drying loss coefficient γ.

For better understanding and interpreting the mechanical and geotechnical properties of soil samples,different analyses were carried out.Regarding this goal,XRF analysis was conducted on all samples to determine their elemental and chemical structure quantitatively.XRD analysis was used to specify the mineralogy of the natural clayey soil.BET-N2analysis was also performed on all samples to determine the changes in soil specific surface area(SSA).For visualizing the soil microstructure and distinguishing the effects of oil contamination and bioremediation,FE-SEM analysis(imaging) was conducted on all samples.

It is essential to measure TPH as a general indicator of petroleum contamination in the soil (Todd et al.,1999).Gas chromatography with flame ionization detector (GC-FID) was used to measure TPH values in OCS and BTS samples (SW-846,2003).

3.Results and discussion

3.1.Efficiency of bioremediation

To evaluate the efficiency of oil-degrading bacteria,it is essential to measure the amount of crude oil remained in soil samples after bioremediation.Fig.3 depicts TPH values for OCS and BTS samples after 30 d of aging and bioremediation.After 30 d,the bioremediation degrades approximately 50% of crude oil.

Comparing the results of this research with other studies(Soltani-Jigheh et al.,2018) demonstrates that the used oildegrading bacteria have better efficiency in sandy soils.Because highly plastic clays are finer than sandy soils,they have larger SSA and lower permeability comparatively.As a result,the contamination will be more severe,and the efficiency of the bioremediation will be minor.To differentiate the effect of oil evaporation in the laboratory environment from the function and efficiency of the bioremediation,the effect of oil evaporation was considered.Fig.4 shows the oil evaporation percentage and rate of OCS samples in the laboratory environment.OCS4,OCS8,and OCS12 samples were left in different containers and were weighed in consecutive days.The observations show that after 4 d or 5 d,the oil evaporation rate decreases dramatically and becomes almost equal to zero.Also,it is evident that the evaporated oil percentage is negligible,and it has a minor share of the initial contamination.

Initial observations of NS,OCS,and BTS samples showed that,in NS samples,the fine gradation and clean structure of yellow marl are visible (Fig.5a).Due to oil contamination,the yellow color of soil has turned to brown.However,the texture of soil is smooth yet(Fig.5b).In BTS samples,the brown color of samples has returned to yellow again,indicating the efficiency of the bioremediation and biodegradation process.Moreover,the bioremediation has changed the soil structure to an agglomerated and porous one,and lumps and clods of soil are visible.In addition,the white spots on the samples are bacterial biomass and by-products of biodegradation of crude oil (Fig.5c).

Fig.3.Total petroleum hydrocarbons content of crude oil-contaminated and bioremediated (for 30 d) samples.

Fig.4.Oil evaporation percentage and rate of samples in the laboratory environment.

3.2.Mechanisms and physico-chemical factors affecting the oil contamination and bioremediation of soil

In general,lighter fractions of the crude oil degrade more quickly than the heavier ones by oil-degrading bacteria.Largersized organic molecules tend to be adsorbed more strongly than inorganic cations by clays due to the greater accessibility of London-van der Waals forces(Yong,2000),thus the bioremediation process becomes harder and more complicated over time.

Sorption of crude oil changes the properties of clay particle surfaces from hydrophilic to hydrophobic.Nonpolar compounds have low dipole moments (less than 1 Debye) and low dielectric constants (less than 3),hence their adsorption onto particle surfaces is a weak bonding and is generally limited to outer surfaces of soil(Yong and Rao,1991).The distribution of organic contaminants between the soil fractions and pore water,which is represented as partition coefficients and solubility of organic pollutants in water,has a substantial effect on contaminant-soil association and interaction.Most hydrocarbon molecules are insoluble in water because of their hydrophobic structure.These factors result in lower mobility and higher retention of contaminants in the environment.Light hydrocarbons can be leached and evaporate more easily,while heavier components will tend to be retained in the soil.The higher water solubility allows a more considerable number of chemical pollutants to be retained in the aqueous phase.It results in lower sorption by the soil solids.If the hydrocarbons have lower accommodation concentration in water,they will have a higher attitude to be connected to the surface of soil particles.Hence the aromatic fractions of petroleum products,which have higher accommodation concentrations and are the most toxic,would have the least tendency to be connected to the surface of soil particles (Yong,2000).

Bacterial biomass can adsorb hydrophobic organic molecules,and an adequate amount of biomass could stop or slow contaminant movement,which is called biocurtain (National Research Council,1993).Consequently,due to the mentioned mechanisms and characteristics,in clays,contaminants connect strongly to the surface of the particles,and the availability of contaminants to micro-organisms will reduce.Moreover,it is hard for the water and oxygen to reach and penetrate the whole soil mass and make the aerobic biodegradation and respiration possible and efficient.

Fig.5.Comparative initial observations from samples:(a) NS,(b) OCS,and (c) BTS.

3.2.1.Mineralogy and elemental-chemical composition of the samples (XRF and XRD analyses results)

Because of high plasticity and SSA,and low permeability,the soil may exhibit complicated behavior when exposed to contamination and remediation.Since clay minerals are very fine-grained and they have layered structure,the normal XRD analysis could not detect them.Therefore,to demonstrate clay minerals,it is required to remove silicates and overlapping minerals from the soil structure.In this process,carbonates,oxides,and organic matter were removed during four steps.After analyzing the normal XRD,the sample was saturated with potassium chloride.Then,to detect chlorite,the sample was heated at 550°C for 2 h to remove kaolinite.In the next step,the sample was saturated with magnesium chloride.In the final step,ethylene glycol was added to the soil for the detection of chlorite from montmorillonite.The results of the treatment process and the graphs of the quadruple stages are shown in Fig.6,with acronyms k,kt,mg,and mge from bottom to top,respectively.The approximate and semi-quantitative values of minerals are also given in Table 5.Tables 6 and 7 summarize the results of XRF analysis,which is performed to determine the chemical and elemental composition of soil samples.In XRF analysis,a portion of soil was first weighed,and then put into the furnace for heating at 1100°C for 2 h,and finally it was reweighed.In this analysis,the percentage loss due to ignition is reported as parameter loss on ignition(LOI).In OCS samples,by increasing the amount of crude oil contamination,LOI values increase.BTS samples have much higher LOI values than OCS samples do,because new materials have been produced due to the bioremediation by bacterial activity in BTS samples,which are mainly the bacterial biomass (and secretions) and by-products of oil biodegradation that are evaporated under high ignition temperatures.The results demonstrated that contamination and bioremediation do not have any significant effect on the chemical composition of soil.

3.3.Atterberg limits

Atterberg limits describe the changes in the consistency of finegrained soils in different moisture contents,and they are used to classify and characterize these soils.Fig.7 indicates as crude oil content increases,LL values of soil have small changes,while PL increases,and PI decreases.An almost similar pattern was observed for BTS samples.With the rise of oil content to 12%,PI values dropped from 31%to 21%(32.26%)for both OCS and BTS samples.A small difference between the consistency limits of OCS and BTS samples is related to the presence of more fine-grained particles in BTS samples due to the production of biomass.

Fig.6.XRD pattern of soil after quadruple stages.

Table 5Mineral composition of tested soil (%).

Table 6Chemical composition of soil samples (%).

The dipolar water molecules are absorbed by the negative surface charge of clay particles as well as by the cations like Ca2+,Mg2+,Na+,and K+.Besides,the oxygen atoms in the particle surface absorb hydrogen atoms of water molecules and some hydrated cations of pore water,which results in the hydrogen bonding.The total water,which is held by attraction forces in contact with clay particles,is named double layer water,and the innermost layer of double-layer water,which is more viscous than the free water,is called absorbed surface water.Placement of water around clay particles is responsible for the plastic behavior of the clayey soils(Das and Sobhan,2013).

Table 7Constituent chemical elements of soil samples (ppm).

Crude oil prevents the bonding of water molecules to clay particles,and water cannot reach the double layer of clay particles.As a result,more water will be needed for the clay particles to show their plastic properties,which leads to an increase in the plastic limit.

Crude oil molecules are not dipole,thus they are not able to establish a polar bond with clay particles like water molecules(Olson and Mesri,1970;Sridharan et al.,1973).Also,water and crude oil are immiscible,and their mixture consists of two distinct liquid phases,which are known as an emulsion (Manning and Thompson,1995).The presence of crude oil around clay particles lowers SSA of soil;as a result,it reduces the water reaction with clay particles.Other factors contribute to changes in soil plasticity properties such as the dielectric constant and the viscosity of pore fluid (Meegoda and Ratnaweera,1994).

3.4.Compaction test

Fig.8 shows the results of compaction tests in the form of dry density versus moisture content for OCS and BTS samples.In general,for both OCS and BTS samples,by the increase of oil content,MDD and OMC values decrease.The oil presence reduces water absorption and dissipates applied energy,which leads to low compaction.The changing trend of MDD and OMC versus oil content is compared for OCS and BTS samples in Fig.9.With the rise of oil content to 8% in OCS samples,MDD values decrease from 16.25 kN/m3for plain soil to 15.8 kN/m3for OCS8 sample,which is about a 2.77% decrease.The associated decrease for BTS sample is approximately 9.79%.In mentioned contaminant concentration,the major part of the oil is absorbed by the finer portion of soil,and oil cannot show any lubricating effect by overcoming water as an interlayer pore fluid when compared with sandy soils.Moreover,oil has higher viscosity in comparison with water,which causes dissipation of energy of the compaction hammer (Puri,2000;Soltani-Jigheh et al.,2018).

On the other hand,the crude oil has a lower density compared to water,thus as the oil occupies a part of the pore space instead of water,it will lower the density of pore fluid totally and consequently MDD values of OCS samples (Mustafa et al.,2018).An increase in MDD of OCS12 sample may be attributed to the fact that when oil content increases to a critical degree,it also efficiently surrounds the coarser soil particles and acts as a lubricant between the soil particles.The results are consistent with those of Khamehchiyan et al.(2007),Nasehi et al.(2016),and Safehian et al.(2018),and contradict the findings of Kermani and Ebadi (2012).The compaction curves of BTS samples have a flatter shape in comparison with those of OCS samples,which indicates the presence of fine-grained biomass in these samples.

Fig.7.Effect of crude oil content on plasticity characteristics of (a) OCS and (b) BTS samples.

Fig.8.Compaction curves of (a) OCS and (b) BTS samples.

Fig.9.Effect of oil content on soil:(a) maximum dry density,and (b) moisture content.

Fig.9a displays that BTS samples have lower MDD values in comparison with similar OCS samples.It may be interpreted by the generation of bacterial biomass and by-products of the biodegradation process.Besides,the activity of oil-degrading bacteria can lead to agglomeration of soil particles.In this situation,water cannot access all of soil particles,and it will only wet the outer surface of the aggregated soil lumps.Therefore,the water cannot play an efficient lubricating role in compaction.Moreover,the agglomeration due to bioremediation creates very porous structures with large pore spaces that are filled with very loose and low-density biomass.As a result,the overall density of compacted soil will decrease.Osinubia et al.(2012)reported a decrease in MDD and an increase in OMC due to an increase in fines content.Because finer particles have lower specific gravity,the addition of fine biomass particles will decrease the ultimate MDD;moreover,finer particles have a large SSA,which increases the soil water absorption to reach its MDD.

3.5.Direct shear test

The shear strength of soils has significant influences on stability,deformation,and stress distribution of foundations,tunnels,and slopes (Gaziev and Erlikhman,1971).To determine the shear strength parameters of samples,direct shear tests were carried out.The behavior of the contaminated and bioremediated soils are almost similar(Fig.10).In the OCS samples,the stress-displacement diagram moves downwards with the increase of oil content,i.e.the contamination has reduced the shear strength.In BTS samples,the stress-displacement curve of the sample containing 4%oil is higher than the others.However,as the oil content increased up to 8%and 12%,the soil strength decreased.

Fig.11 depicts the changes in the shear strength of soil with oil content for OCS and BTS samples.The oil contamination has decreased the shear strength of OCS samples.For example,at normal stress of 300 kPa,by the increase of the oil contamination up to 12%,the shear strength of OCS samples reduced from 82.14 kPa to 49.38 kPa,which is almost a 40% decrease.The shear strength of BTS4 sample is higher than that of other samples.Similar to OCS samples,by increasing contamination concentration,the strength of BTS samples decreases.For example,at σ′n=300 kPa,the shear strength of soil has increased from 82.14 kPa to 95.7 kPa for BTS4 sample.With an increase of the oil content from 4% to 12%,the shear strength has reduced from 95.71 kPa to 69.01 kPa.All BTS samples have higher shear strength compared to OCS samples at all normal stresses.The difference in the shear strength of OCS and associated BTS samples is related to changes in the soil structure.

Contamination and remediation also changed the internal friction angle(φ)and cohesion(c)of tested soils(Fig.12a).By increase of the crude oil content,the internal friction angle reduced from 12.36°for natural soil to 7.85°for OCS12 sample,which shows a 36.5%reduction in φ value.A similar reduction in φ values has been reported for sandy and clayey soils(Ghaly,2001;Shin et al.,2002;Khosravi et al.,2013).The decrease in the internal friction angle due to the presence of crude oil might be associated with the lubrication effect of oil on the surface of the particles,which reduces interparticle friction (Khosravi et al.,2013).

The internal friction angles of BTS8 and BTS12 samples have not changed in comparison with natural soil,and the values are about 12°,while the corresponding value has risen to 14.27°for BTS4 sample.Due to the presence of residual oil and biomass,a reduction is expected to occur in φ values of BTS samples.At the same time,there is a slight increase for BTS4 sample,which is related to the formation of the flocculated and agglomerated structure.Besides,the internal friction angle of BTS samples is higher than the associated value of OCS samples due to the degradation of a considerable amount of oil contamination and the agglomeration of soil structure.

Except BTS4 sample,oil contamination,as well as bioremediation,resulted in reduced soil cohesion.As is known,double layer water is responsible for the plastic behavior of clayey soils,and it is absorbed to the surface of clay particles electrically (Das and Sobhan,2013).Changes in the dielectric constant of pore fluid can affect the thickness of the double layer (Mitchell and Soga,2005),which occurs in contaminated soils.Since the dielectric constant of the crude oil(i.e.2-2.2)is lower than that of water(i.e.80),adding the oil to the soil reduces the thickness of the double layer(McBride,1994),which leads to a decrease in the cohesion of soil (Safehian et al.,2018).

Spagnoli et al.(2011) investigated the effect of pore fluid’s dielectric constant on the shear strength of clay minerals(kaolinite,illite,and Na-smectite),and their studies revealed that the interaction between mineralogy of clay particles and pore fluid’s dielectric constant has a significant influence on shear strength of swelling clays like Na-smectite.To change the dielectric constant of pore fluid,pure ethanol was mixed to deionized water in different ratios.Hence,in Na-smectite,decrease of the pore fluid’s dielectric constant will increase the shear strength considerably.Besides,the shear behavior of kaolinite and illite seems to be controlled by particle contacts,thus the chemistry of pore fluid will not change their mechanical properties.Nevertheless,it should be carefully considered that in the study of Spagnoli et al.(2011),the organic agent to change the dielectric constant(ethanol)is a totally watersoluble liquid,and it cannot form a different phase in pore fluids such as crude oil and other insoluble organic fluids.

Fig.10.Shear stress-displacement curves of OCS and BTS samples at (a,b) σ′n=100 kPa (c,d) σ′n=200 kPa,and (e,f) σ′n=300 kPa.

Due to pore fluid viscosity and change of the particles-pore fluid interaction,the frictional properties will decrease.On the other hand,the change of the pore fluid’s dielectric constant alters the physico-chemical interaction.An increase in viscosity,as well as the existence of the immiscible phase in soil structure,can reduce the shear strength (Ratnaweera and Meegoda,2005).Moore and Mitchel (1974) concluded that if the dielectric constant of the pore fluid becomes different from that of the clay particles (i.e.4),the net interaction force becomes increasingly positive.As the inter-particle attraction force increases,the parameters reflecting the soil resistance to deformation should also increase.It is worth mentioning that all pore fluids used in their study were infinitely miscible with water.In contrast,Sridharan and Venkatappa Rao(1979) showed that the shear strength decreases with increasing dielectric constant.It may be due to the fact that the increase of the dielectric constant increases the electric repulsive pressure,and lowers the electrical attractive pressure.Finally,a decrease in effective contact stress decreases the shear strength of soil.They demonstrated that the cohesion and internal friction angle decrease due to an increase in the dielectric constant of pore fluid.

Fig.11.Influences of oil contamination and bioremediation on the shear strength of (a) OCS and (b) BTS samples.

Fig.12.Effect of oil content on (a) internal friction angle and (b) cohesion of soil.

Moreover,since oil coats the surface of soil particles,SSA of soil will decrease(Ur-Rehman et al.,2007),which is in agreement with the results of BET analysis.The decrease in soil SSA will decrease the cation exchange capacity(CEC)of soil.As a result,the contact of soil particles with each other and water molecules reduces,and soil particles would adsorb a lesser amount of water.Hence,shear strength and cohesion of soil decrease (Kermani and Ebadi,2012;Safehian et al.,2018).

All of the biotreated samples have a higher cohesion in comparison with OCS samples.Fine particles of bacterial biomass increase the inter-particle bond due to the larger surface area in contact and finally increase the cohesion.Because oil and hydrocarbons are insoluble,oil-degrading bacteria produce biosurfactants to be able to access pollutants and make contact between them and microbial cells.Biosurfactants lower the surface or interfacial tension between two liquids,between a gas and a liquid,or between a liquid and a solid.Therefore,the produced biosurfactant will cause the water-crude oil composition to act as an integrated system in the soil structure.

3.6.Unconfined compression test

For both OCS and BTS samples,by increasing the oil content,the UCS decreases (Fig.13).These results are in agreement with previous researches (Khamehchiyan et al.,2007;Nasehi et al.,2016;Safehian et al.,2018;Soltani-Jigheh et al.,2018).For more detail,the influence of oil content on UCS and failure strain of soil is displayed in Fig.14.

As mentioned previously,the presence of oil in pores of soil and also between soil particles reduces the cohesion.Moreover,oil cannot show the bonding and adhesive characteristics as a pore and inter-particle fluid.Because crude oil is a non-polar fluid,and its molecules cannot bond to water or charged surface of clay minerals.It will produce an immiscible phase in the soil-water system,which acts as a lubricant,and it decreases the cohesion and UCS of contaminated soil.By increasing the oil content to 12%,UCS value of soil decreases from 321.07 kPa to 203.83 kPa.

The increase of UCS at oil content of 8% could be explained by agglomeration of soil particles bonded together by oil film to form a larger but weakly bonded soil matrix.This bonded soil matrix initially resisted the impact of the loading but thereafter failed with the increase in oil content which has weakened the interstitial force of cohesion between the particles of soil (Oluremi et al.,2015).Another reason for this behavior can be the formation of flocculated(congested) soil fabric.At the same void ratio,flocculated soil has higher strength and permeability and lower compressibility than the same soil in a dispersed condition(Lambe and Whitman,1991).

This strength for BTS samples decreased from 321.07 kPa to 96.35 kPa,i.e.UCS values of OCS and BTS samples have decreased by about 36.52%and 70%,respectively.Moreover,all BTS samples have lower UCS values than OCS ones(Fig.14a).The biological treatment and bacterial activity make the soil structure porous and agglomerated.Also,bacterial secretions,which are very loose and weak materials,fill these pores,thus the 1D strength of the BTS samples decreases considerably.

Furthermore,there is a continuous decrease in failure strain with the increase of the oil content.By increasing the oil content to 12%,failure strain values decrease from 3.8% to 2.4% for OCS samples.The failure strain of BTS samples increases to 5.1% when oil content increases to 4%,and then it gradually decreases to 3.1%corresponding to 12% of oil content.It can be concluded that,among all samples,the failure strains of BTS4 and BTS8 samples are higher than that of other samples.Due to oil contamination,the behavior of OCS samples becomes brittle,while bacterial secretions cause the soil to show a softer and more plastic behavior than NS and OCS samples.Also,all BTS samples display higher failure strain values compared to OCS samples.The described behavior is also evident in the failure pattern of the samples (Fig.15).

Fig.13.Stress-strain curves of (a) OCS and (b) BTS samples.

Fig.14.Influence of oil content on (a) unconfined strength and (b) failure strain of soil.

Fig.15.Failure pattern of soil samples at the end of the unconfined compression test.

3.7.Consolidation

3.7.1.Swelling characteristics

The main reason for swelling of some soils by absorbing water is the existence of certain types of clay minerals such as smectite,montmorillonite,nontronite,vermiculite,illite,and chlorite (Jia et al.,2011).The presence of non-swelling minerals such as quartz and carbonates can also dilute the swelling effects of these soils(Kemp et al.,2005).XRD results(Fig.6)show that the used soil consists of montmorillonite,illite,and chlorite as swelling minerals and quartz and calcite as non-swelling minerals.In the 1D consolidation test,the free swelling is defined as the percentage of changes in the sample height due to water absorption to its initial height,under a surcharge pressure of 5 kPa.Fig.16a shows the free swelling values of NS,OCS,and BTS samples.A reduction was observed in free swelling of OCS4 sample in comparison with NS sample;thereafter,as oil content increases,the free swelling begins to rise,and even its value for OCS12 is more than the associated value of NS sample.In BTS samples,a decreasing trend is evident with the increase of the oil content.Due to oil-degrading bacteria activity,fine grains are stuck together,and agglomeration and aggregation occur in the soil mass.Also,the bacterial products with a large SSA absorb a significant amount of water,thus montmorillonite,illite,and chlorite minerals within the soil cannot absorb enough water to expand the interlayer spaces.In addition,entering the bacterial biomass into the soil structure,especially among swelling minerals,reduces the swelling potential of BTS samples.

Fig.16.Influence of oil content on (a) free swelling and (b) swelling pressure of the samples.

The swelling pressure is the pressure applied by swelling soils when they absorb water,and their volume change is restricted,or swelling pressure is the pressure that is needed to hold the soil at constant volume (Sridharan et al.,1986).Changes in the swelling pressures of samples (Fig.16b) illustrate that NS sample has the highest swelling pressure.As the oil content increases,there is a regular decrease in swelling pressure of both OCS and BTS samples.The swelling pressure of OCS samples is 1.55-3 times higher than that of BTS samples.Swelling pressure increases with the decrease of moisture content (Elsharief et al.,2015).Moreover,the samples which are compacted to low density will have lower swelling pressure.Contaminated samples have lower OMC and higher MDD than the bioremediated samples and show higher swelling pressure.The placement of bacterial biomass produced by the biodegradation of crude oil among the plates of clay particles,which is led to the formation of the porous and agglomerated structure,can be another reason for the lower swelling pressure of the BTS samples compared to the NS and OCS samples.

Another reason for the lower free swelling and swelling pressure values of BTS samples may be the lower MDD and higher OMC of these samples (Sridharan and Gurtug,2004;Villar and Lloret,2008;Nagaraj et al.,2010;Wang et al.,2012).By increasing the MDD,the number of the swelling clay particles within a given volume increases,which increases the swelling.

3.7.2.e-log10p curves

Fig.17 compares e-log10p curves of NS,OCS,and BTS samples.In OCS samples,the compressibility increases as the oil content increases.The inter-particle friction decreases due to the lubricating effect of the crude oil,and consequently,the particles roll and slip over each other,which ease the soil compressibility.Moreover,the initial void ratio of OCS samples is larger than that of plain clay;therefore,the compressibility of these samples is considerable.Due to bioremediation,the void ratio,porosity,and settlement of BTS samples increase.This is because the bacterial activity agglomerates the soil and changes the texture of soil to a more porous mass.

3.7.3.Compression and swelling indices

The results of the changes in compression and swelling indices of samples under the influence of oil contamination and bioremediation are depicted in Fig.18.Because the oil has a lubricating effect,it decreases the inter-particle friction and increases the compression index Cc,but the oil content ranging from 4% to 12%has a minor impact on Cc.Furthermore,the non-polar nature of crude oil molecules is another reason for the compressibility increment due to oil inclusion.Because charged clay particles cannot absorb non-polar crude oil molecules as well as dipolar molecules of water so that the crude oil can be drained out easily(Safehian et al.,2018).There is an increase in amount of Ccin BTS samples due to their porous and agglomerated nature,but it does not show a regular trend compared to OCS samples.On the other hand,the swelling index Csis constant up to 4% of crude oil contamination,and then it increases.However,in BTS samples,Cshas a lower value for samples with 8%and 12%of contamination in comparison with NS sample.

3.7.4.Settlement

The changes in the volume of clayey soils continue for a long time after the immediate settlement,and the consolidation settlement in clayey soils may be several times larger than the immediate settlement.Soil settlement is the most critical issue in the design of soil structures,and it is vital to pay attention to it.Fig.19 displays the final settlement at the end of each stage of loading for all samples.In OCS samples,for a given pressure by an increase of oil content,the settlement increases.This could be attributed to the lubricating effect of the crude oil,which decreases the friction between soil particles and makes them place in a tight and compressed packing near each other.Moreover,crude oil lowers the SSA of soil,and consequently,the adsorption of water by soil particles decreases and the water can be drained out more easily.As a result,the consolidation settlement increases (Safehian et al.,2018).

The settlement of BTS samples increases due to the increase of oil content to 8%,and then by a further increase of oil content,it decreases.As previously stated,bioremediation increases the soil porosity and void ratio by degrading the oil and producing the biomass and other by-products in the soil.These materials are very fine,and they have a very loose and weak structure against applied forces.

3.7.5.Coefficient of consolidation (CV)

Fig.20 shows the changes in CVvalues versus applied stresses for all samples.The influence of oil content on the coefficient of consolidation is shown in Fig.21.At the earlier stages of the loading process,NS samples have the highest coefficient of consolidation,and OCS12 has the lowest one among OCS samples.Still,at a higher applied stress level,the changes are minor and negligible.At lower stress levels,the highest CVis related to NS sample,but at higher stresses,BTS12 has the highest CV.In OCS samples at the lower stress level,CVdecreases sharply,but at higher stress levels,the changes are minor,and CVdecreases gradually.In BTS samples,at the lower stress level,by increasing the oil content,CV decreases,but at higher stresses after 8%oil content,CVincreases slightly.The results are consistent with the observations by Nazir(2011).

3.8.Field emission scanning electron microscopy (FE-SEM)

Fig.17.e-log10p curves for (a) OCS and (b) BTS samples.

Fig.18.Influence of oil content on (a) compression index and (b) swelling index of soil.

Fig.19.Influence of oil content on the final settlement of (a) OCS and (b) BTS samples under different stresses applied.

Fig.20.Variation of coefficient of consolidation with effective stress for (a) OCS and (b) BTS samples.

Fig.21.Influence of oil content on coefficient of consolidation for (a) OCS and (b) BTS samples.

Fig.22.FE-SEM images of (a) NS,(b) OCS4,(c) OCS8,and (d) OCS12 samples.

Soil microstructure and morphology are crucial in the interpretation of soil behavior in the macroscale.FE-SEM was used to determine the morphology and fabric of NS,OCS,and BTS samples instead of SEM because FE-SEM provides images with higher resolution,magnification,and well accuracy (Gnanamoorthy et al.,2014).The geometric arrangement of soil particles is defined as soil fabric(Holtz et al.,1981)and affects the engineering behavior of soils (Budhu,2010).There are two types of forces between clay particles,i.e.attraction forces due to London-van der Waals and repelling forces due to the diffuse double layer.The net interparticle forces influence the structural form of clay mineral particles.If there is net repulsion,the particles will acquire a faceto-face orientation,which is referred to as a dispersed structure.If there is a net attraction,the particles will take edge-to-face or edgeto-edge position,referred to as a flocculated structure (Knappett and Craig,2019).Olgun and Yıldız (2010) showed coagulation and aggregation between the soil particles due to the presence of organic fluids,and the degree of aggregation increased with decreasing dielectric constant and with increasing fluid/water ratio.Ur-Rehman et al.(2007)found that when the soil is subjected to the oil contamination,oil coats the individual clay particles,and the clay groups.This process forms flocs,which are similar to silt or even sand-sized particles.If then water is added to the oilcontaminated clay,due to the high dielectric constant (highest among all liquids),it can dissociate the oil-clay bond.FE-SEM images of NS and OCS samples show that the surface of soil particles in natural clay is completely clean and entirely separated(Fig.22).In OCS samples,as oil content gradually increases,the crude oil covers the surface of soil particles,leading to a decrease in the SSA of soil(Table 8).By increasing the oil content,the borders and sharp edges of particles are less apparent.

Table 8SSA and Vp of the samples due to the results of BET analysis.

FE-SEM images of BTS samples indicate the efficiency of the bioremediation.As can be seen,due to bioremediation,the oil content decreases in the soil mass,and the surface of soil particles begins to be clean;also,the borders and the shape of soil particles are visible (Fig.23).The biomass and by-products of bacterial activity in the soil appear in the form of intertwined strings,regularshaped porous structures,and quasi-fibrous textures (Fig.24).The biomass and the remainings of the oil degradation are soft and weak materials with high SSA(Table 8),which increases the water absorption capability of BTS samples.

Fig.23.FE-SEM images of (a) NS,(b) BTS4,(c) BTS8,and (d) BTS samples.

3.9.Specific surface area (SSA)

The SSA of soil is being recognized as one of the fundamental properties required in understanding the engineering behavior of soils.The engineering behavior of coarse-grained soils is dependent on the self-weight and the skeletal forces;however,fine-grained soils are influenced by electrical and capillary forces.Variation in the electrical and the capillary forces with particle size and SSA affects the engineering properties of fine-grained soils (Yukselen-Aksoy and Kaya,2013).SSA is the total surface area per unit of mass for materials,and finer particles have higher SSA.The SSA of soil gives essential information about the ability of soils to absorb or retain liquids and gases.Considering the confrontation of the different soils to various contaminants,SSA is a crucial factor because it can affect the intensity of contamination,the fate of the contaminants,and the choice of the remediation methods.Soils with higher SSA have higher water-holding capacities,more ability to absorb contaminants,and higher swell potentials(Khamehchiyan et al.,2007).BET-N2method was employed to measure the SSA of soil samples (Brunauer et al.,1938).In this method,a device measures the adsorption of nitrogen gas at a temperature of 77 K(-196°C)by soil particles surface.Because N2is non-polar,it cannot penetrate the interlayer of swelling clays,thus SSA values obtained from the BET method are subjected to the external particle surface.The results of BET analysis showed that the values of SSA and Vphad dropped considerably as crude oil content increases (Table 8).SSA and Vpvalues of OCS12 sample havedecreased by86.08%and 77.83%,respectively,in comparison with those of natural soil.

Fig.24.FE-SEM images showing the formation of the various porous structures and quasi-fibrous textures in (a) BTS4,(b) BTS8,and (c,d) BTS12 (different types of porous structures) samples.

Applicability of bioremediation and reuse of bioremediated soil in the practical works are essential.Bioremediation is an efficient,low-cost and environmental-friendly approach which is applicable as an in situ technique.There is a crucial need for strategies that can improve and optimize the efficiency of the oil-degrading bacteria and microbial powder in highly plastic clayey soils.These strategies can be classified as prolongation of the remediation period,increasing the amount of bacterial powder and nutrients,improving the ventilation and moisturization process,and the use of other remediation techniques in parallel.The bioremediated lands can be planted with vegetation and trees.On the other hand,the remediated soils can be reused for bioremediating other oilcontaminated lands.

Shale,mudstone,and marl are composed of clay minerals and,therefore,are potentially suitable for use in the formation of a lowpermeability liner (Sarsby,2000).Besides,clays can be utilized in different industrial applications as the source of alumina and silica.Calcareous clay,known as marl,is the raw material basis for the manufacture of the Portland cement (Gillott,1987).Thus,bioremediated soil has the potential to be used in the construction of liners and the production of Portland cement.Considering the significant reduction in contamination,the increase of the shear strength and the decreases of the free swelling and swelling pressure due to bioremediation,the bioremediated soils could be used in road construction as subgrade and subbase materials.

4.Conclusions

The main goal of the current research was to investigate the effects of crude oil contamination and bioremediation on geotechnical,physical,chemical,and microstructural properties of highly plastic clayey soil.Therefore,a broad range of laboratory tests,including Atterberg limits,compaction,unconfined uniaxial compression,direct shear,and 1D consolidation,were conducted on NS,OCS,and BTS samples.The main results are summarized as below:

(1) Bioremediation using bacterial powder degraded crude oil contamination about 50% after 30 d,and this process produced biomass and bio-product materials within the soil.

(2) Crude oil contamination reduced the SSA of soil and the water reaction with clay particles,which led to the rising of soil plastic limit and consequently decreased the plasticity index by about 32.26%.Similar behavior was observed for bioremediated soil.

(3) Adding crude oil to the soil reduced the MDD and OMC due to the dissipation of the compaction hammer energy by oil,lower density of oil,and the decrease of water absorption.Moreover,bioremediation decreased the MDD and increased the OMC of soil,which resulted from the generation of finegrained biomass and formed agglomerated structure.

(4) Oil contamination lowered the shear strength,cohesion,and internal friction angle of soil,which may be related to the lubricating effect of oil and the decrease of dielectric constant.Bioremediated soil has high shear strength,cohesion,and internal friction angle as compared with contaminated one due to the increase of the SSA and the production of biosurfactant.

(5) Irrespective of some exceptions,contamination and bioremediation resulted in reducing soil swelling and swelling pressure,and rising settlement of soil.

(6) XRF,BET,and FE-SEM analyses showed that oil contamination and bioremediation did not change the chemical and elemental structure of soil.FE-SEM images showed that biodegradation-induced biomass and by-products altered the soil microstructure to quasi-fibrous textures and porous one,which increased the void ratio and porosity of these samples and made them agglomerated.In addition,BET analysis demonstrated that oil contamination decreased the SSA of soil,and bioremediation increased the SSA of soil considerably.

(7) By applying some modifications,the bioremediation,as an efficient,low-cost and environmental-friendly approach,can be used in the case of highly plastic clayey soils.The bioremediated soil may be utilized in the construction of liners,road construction,manufacturing Portland cement,and bioremediating further contaminated sites.

Data availability statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

List of symbols

c Soil cohesion (kPa)

CcCompression index

CsSwelling index

mrCoefficient of consolidation (cm2/min)

e Void ratio of soil

mdMass of porous media solids (dry mass, g)

mrMass of porous media after oven-drying (g)

mtMass of porous media before drying(g)

mwMass of volatilized water (g)

n Oil content of the porous media before drying

VpTotal pore volume

WwMoisture content by mass (%)

φ Internal friction angle of soil

ω Moisture content (%)

γ Oil drying loss coefficient

ρdSoil dry density (g/cm3)

ρdmaxMaximum dry density of soil (g/cm3)