reducing compressibility of the expansive soil by

Research ArticleReducing Compressibility of the Expansive Soil byMicrobiological-Induced Calcium Carbonate Precipitation

Xiaobing Li1 Chunshun Zhang 2 Hongbin Xiao 1Weichang Jiang1 JunfengQian 13

and Zixiang Li1

1Institute of Geotechnical Engineering Central South University of Forestry and Technology Changsha 410004 China2Department of Civil Engineering Monash University Clayton VIC 3800 Australia3Technology RampD Center Hubei Road amp Bridge Group Co Wuhan 430056 China

Correspondence should be addressed to Chunshun Zhang ivanzhangmonashedu andHongbin Xiao t20090169csufteducn

Received 12 August 2020 Accepted 19 January 2021 Published 10 February 2021

Academic Editor Songtao Lv

Copyright copy 2021 Xiaobing Li et al is is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Most of the research studies on the improvement of expansive soils are focused on reducing their expansive properties howeverthere are few studies on the impact of the soil compressibility after the improvement In this paper through indoor high-pressureconsolidation tests the recent microbial-induced calcium carbonate precipitation (MICP) technology is studied to improve thecompression characteristics of the expansive soile significant effect of different microbial concentrations (achieved by differentnumber of treatments) on the compression deformation is revealed with the hyperbolic function that involves two parameterswith clear physical meanings In particular after 6 times of treatment with the microbial solution the compression characteristicsof the expansive soil reach the best improvement effect continuing to increase the number of microbial treatments is otherwisenot conducive to improving the soil compression performance Also a dramatical increase of the structural strength of themicrobial-treated expansive soil is presented and investigated Moreover we performed a scanning electron microscope (SEM)experiment and confirmed the existence of crystals due to mineralization is study shows that MICP is an effective andenvironmentally friendly means of reducing the compressibility of the expansive soil

1 Introduction

Expansive soil is a special kind of catastrophic clay and iswidely distributed in the world In order to use land re-sources rationally it is usually necessary to improve the poorengineering characteristics of the soil so that the expansivesoil can be used to fill the embankment or the subgrade of abuilding

One way is to add a certain amount of inorganic materialssuch as lime cement or fly ash for chemical and physicalimprovement of the expansive soil [1 2] For examplePhanikumar and Nagaraju [3] proposed a comparative studyof an expansive clay using fly ash and rice husk ash e testresults showed that liquid limit plasticity index and free swellindex decreased significantly with increasing fly ash and ricehusk ash contents However coefficient of permeability in-creased with additive content Bian et al [4] studied the

physical and mechanical characteristics of lime-modifiedexpansive soil through experimental research and found thatas the ash content increases the cohesion internal frictionangle and CBR value of the expansive soil increase after thatthey proposed that the CBR and direct shear strength of 7-daycured lime soil could be adopted to determine the best ashcontent Voottipruex and Jamsawang [5] analyzed theswelling and strength characteristics of the expansive soilusing cement and fly ash and then they concluded that theswelling percentage can predict the swelling and strengthcharacteristics of the soil ough these are useful findingsthere are some limitations such as the uniform mixture oflime and expansive soil is difficult to achieve long periodhigh construction cost and environmental pollution due tothe mixing process erefore it is necessary to find a newenvironmentally friendly and more applicable alternativemethod for improving the expansive soil

HindawiAdvances in Civil EngineeringVolume 2021 Article ID 8818771 12 pageshttpsdoiorg10115520218818771

Alternatively microbially induced carbonate precipita-tion (MICP) technology is an emerging technology that hasdeveloped rapidly in recent years and has been widely usedin many fields is new technology has also been used ingeotechnical engineering and gradually formed a new mi-crobial geotechnical technology with great significance insoil mechanics and engineering applications [6ndash14] emechanism of MICP technology is to catalyze the hydrolysisof urea through urease generated during microbial meta-bolism this process generates ammonia and carbon dioxidethat are dissolved in water in an alkaline environment togenerate ammonium and carbonate ions once these car-bonate ions meet sufficient calcium ions calcium carbonateprecipitates with gelling effect are generated is calciumcarbonate precipitation can not only fill the pores betweenthe soil particles but also form glue on the surface of the soilparticles to make the soil particles adhere to each otherWhen calcium carbonate precipitates and solidifies a rel-atively high-strength calcium carbonate crystals are formedwhich improve many engineering properties of the soil[15ndash17] e chemical reaction process of calcium carbonateprecipitation induced by microorganisms can be simplifiedas shown in the following

CO NH2( 11138572 + H2O⟶ CO2 + 2NH3

2NH3 + 2H2Oharr2NH+4+2OHminus

CO2 + OHminus ⟶ HCOminus3

Ca2++ OHminus

+ HCOminus3⟶ CaCO3 + H2O

(1)

Whiffin [18] used Bacillus pasteurii to induce calciumcarbonate to precipitate and cement sand particles therebyimproving the shear strength of the sand also the solidi-fication effect of Bacillus pasteurii and urease-producingstrains isolated from the soil was compared thereafter theeffects of calcium ion concentration and grouting methodson the strength of microbially reinforced sand were studiedHis experimental results showed that the uniaxial com-pressive strength of the sand sample treated by Bacilluspasteurii was as high as 58MPa Sharaky et al [19] foundthat Sporosarcina pasteurii plays an important role in thesand biocementation process and the research resultsshowed that the compressive strength of sandy soil increaseddue to the precipitation of calcium carbonate by the bacterialactivity rough experimental research Liu et al [20]systematically analyzed the strength characteristics ofMICP-reinforced sand and proposed a unified strengththeory for the reinforcement of calcareous sand Based onMICP technology Khaleghi and Rowshanzamir [21] had acomparison study on sandy soil using single and mixedcultures e research results verified that the mechanicaland physical properties of sandy soil with both single andmixed media improved through the MICP especially theoutcome of the mixed medium was much better than that ofthe single medium Wang et al [22] used different treatmentcycles of MICP technology to improve the wind erosionresistance of the sand it showed that the density and winderosion resistance of the sand increase with the number oftreatments so using MICP technology could reduce and

prevent the increase of desertification Liu [23] analyzed thedynamic characteristics of calcareous sand and its cemen-tation mechanism based on the MICP technology andconcluded that after the MICP treatment the dynamicstrength and resistance to deformation of calcareous sandwere improved to a certain extent Canakci et al [24] usedMICP technology to improve organic soil e test resultsindicated that the bacterial treatment increased shearstrength and reduced compressibility of the organic soil

e above application of MICP technology in domesticand international geotechnical engineering has made somepreliminary research advances [25ndash28] However the resultsof the above studies are intended to improve the strength ofsandy soils [29ndash32] and there is little research on the effectof MICP on improving the compressibility of clay especiallyexpansive soil Considering the increase in engineeringdemand in expansive soil areas this requires a new envi-ronmentally friendly and economical method to improveexpansive soil erefore we try to apply the emergingMICP technology to study its effect and mechanism on theimprovement of expansive soil focusing on soil com-pressibility rough laboratory consolidation tests thecompression characteristics and deformation laws of theexpansive soil before and after the MICP improvement arecompared and studied

2 Soil Samples and Testing Scheme

21 Preparation of Soil Samples e test soil material wastaken from the expansive soil excavated from the ring roadengineering in Nanning Guangxi According to the codeTest Methods of Soils for Highway Engineering (JTGE 40-2019) the measured basic physical properties and chemicalcomposition of the soil are shown in Tables 1 and 2respectively

According to the free expansion rate in Table 1 and theclassification as specified in the code JTGE 40-2019 it can bedetermined that the soil samples used in the tests are me-dium expansive soils e microorganism used in our ex-perimental study is Bacillus pasteurii with strain numberATCC11859 which was purchased through the ChinaNational General Microbial Species Collection ManagementCenter (CGMCC) It has high-yielding urease and is widelyused Bacillus which is harmless to humans and theenvironment

During the test the first step was to quickly propagateand cultivate Bacillus pasteurii the bacteria were taken fromthe refrigerator cells were activated and the culture solutionwas inoculated on a sterile operation platform e culturemedium used in the test was mainly composed of nutrientssuch as urea casein peptone soy peptone and sodiumchloride en the inoculated culture solution was culturedon a shaker for 48 hours and then the concentration of thebacterial solution was measured using a spectrophotometere concentration of the bacterial solution is usuallyexpressed by the absorbance OD600 value and it can be usedfor the test when it is greater than 10 [33ndash36]

e bacterial solution and cementation solution (mixedsolution of calcium chloride and urea) were added into the

2 Advances in Civil Engineering

soil sample at a volume ratio of 1 1e concentration of thecementation solution was determined to be 02M [37] econcentration of the bacterial solution and cementationsolution used in this experiment was found to be the op-timum concentration and beneficial to the generation ofmicroorganism mineralization We collectively refer tobacterial solution and cement solution as treatment solutionDuring this test we perform multiple processing on theexpansive soil using treatment solution which can ensurethe cumulative concentration of the treatment solution issufficient and the soil sample is in a plastic state And the soilsamples were subjected to 0 2 4 6 and 8 treatments re-spectively All soil samples after curing and drying are testedwith the optimummoisture content [38ndash41] All soil samplesare 618mm in diameter and 20mm in height to meet therequirement of the consolidation test

22 Testing Scheme e consolidation instrument used inthe test is a GDG-4S Triplex high-pressure consolidationtesting apparatus Consolidation tests were performedaccording to code JTGE 40-2019

First 1 kPa pressure was applied and prepressed toensure that all parts of the consolidation instrument were inclose contact en the preload was removed and the firstload was quickly applied After the first-level load wasstabilized for 24 hours the next-level load was applied andthe total load was divided into eight levels e load of eachlevel was 125 kPa 25 kPa 50 kPa 100 kPa 200 kPa 300 kPa400 kPa and 800 kPa During the loading process theconsolidated specimens were wrapped around with a dampcloth to prevent evaporation of water in the soil sampleduring the long-term loading

3 Test Results and Analysis

31 Variation of the Compression Curve According to thetest results the e-p curves of the expansive soil after mi-crobial improvement (the number of treatments is repre-sented by x which is 2 4 6 and 8 respectively) and theuntreated expansive soil as shown in Figure 1

It can be found from Figure 1 that the initial porosityratios of the expansive soil samples subjected to differentnumber of times of microorganism treatment are slightlydifferent from those of the untreated microorganism is isbecause the amounts of calcium carbonate precipitated inthe soil particles after different number of times of microbial

treatment are different so the bulk densities of the soilsamples changed slightly although the initial water contentof each soil sample is the same

It can also be found from Figure 1 that the porosity ratiosof the expansive soil treated with different number of timesof microorganisms decrease with the increase of the con-solidation pressure at is to say the expansive soil hasundergone compression deformation under different con-solidation pressures On the contrary with the increase ofthe consolidation pressure the e-p curves of the expansivesoil samples with different number of times of microbialtreatment decrease significantly at different rates Except forthe soil samples after 8 times of treatment the e-p curves ofother soil samples become smoother with the increasingnumber of treatments that is the compression coefficientbecomes smaller and the compression modulus becomeslargeris law shows that the compression characteristics ofthe expansive soil have been improved to varying degreesafter microbial treatment particularly when the number oftreatments is less than six the more the number of treat-ments is the more obvious the compression characteristicsimprove Note that when the number of soil modificationsreached 8 times the compression characteristics of the soilsamples became significantly worse is is because duringthe test after eachmicrobial treatment the soil samples were

Table 1 Physical properties of the expansive soil

Soil source Natural density (gcm2) Max dry density (gcm2) Relativegravity Gs

Liquidlimit ()

Plasticlimit ()

Plasticindex ()

Optimummoisture

content ()

Free swellingrate ()

Nanning 194 188 270 608 224 384 162 613

Table 2 Chemical composition of the expansive soil

Si4+ () Al3+ () Fe3+ () K+ () Mg2+ () Ca2+ () Ti4+ () Cu2+ () S2+ () Mn2+ ()235 576 89 42 23 16 09 07 02 01

0 200 400 600 800040

045

050

055

060

065

070

e

Untreated soil Treating 2 times Treating 4 times

Treating 6 times Treating 8 times

p (kPa)

Figure 1 e-p curves of the expansive soil

Advances in Civil Engineering 3

air-dried and crushed which affected the mesostructure ofsoil particles resulting in increasing fine particles some ofwhich might not be sufficiently cemented by the calciumcarbonate precipitates erefore in order to ensure theimprovement effect of the expansive soil it is necessary toavoid excessive crushing of the soil sample during the test Inaddition as the number of soil modifications increases theprecipitation efficiency of calcium carbonate between soilparticles may decrease [42]erefore we do need to controlthe number of soil modifications

When the consolidation pressure is 0 to 400 kPa afterthe microorganism treatment of 0 2 4 6 and 8 times thecorresponding reduction ratios of the porosity ratio are175 164 139 117 and 130 respectively whenthe consolidation pressure is further increased from 400 kPato 800 kPa the corresponding porosity ratios are furtherreduced to 38 37 33 26 and 30 for the 5 timesof treatment ese reductions in porosity ratios underincreasing the consolidation pressure are listed in Table 3

It can be seen from Table 3 that on the one hand re-gardless of whether the expansive soil is processed by theMICP technology the overall compression characteristics ofthe soil remain unchanged at the initial compression stagethe slope of the e-p curve is larger and the amount of soilcompression is larger reflecting the large decrease in soilporosity ratios at low compression levels when the verticalpressure exceeds 400 kPa the e-p curve tends to be gentleand the amount of compression of the soil body graduallydecreases reflecting the decrease in the reduction of theporosity ratio of the soil bodyis is because when the soil iscompressed the soil particles will rearrange and becomedensely packed with each other and also water and gas inthe soil are squeezed out of the pores of the soil All thesecontribute to the soil compactness so the movement of thesoil particles becomes more and more difficult resulting insmaller and smaller compression deformation

On the other hand under the same consolidationpressure the reductions of porosity ratios of the microbiallymodified expansive soil samples are less than those of theuntreated soil sample As the number of microbial treat-ments increases the change of the porosity ratio of the soildecreases first After the treatment for 6 times the change ofthe porosity ratio is the smallest is evidently confirms theeffect of using microorganisms to improve the compress-ibility of the expansive soil that is related to the number oftreatments In our study the compressibility of the ex-pansive soil obtained the best improvement after 6 microbialtreatments

32 Variation of the Index of Compressibility According tothe test results the relationships between the coefficient ofcompressibility a1-2 a1-2 is the compression coefficient ofconsolidation pressure between 100 kPa and 200 kPa thecompression modulus Es and the final compression amountSf of the microbially modified expansive soil are shown inFigures 2 and 3 respectively

It can be seen from Figure 2 that after 6 times of mi-crobial solution treatment the coefficient of compressibility

a1-2 of the expansive soil decreased from 044MPaminus1 to022MPaminus1 and its compression modulus increased from374MPa to 748MPa the final compression of the soilsample was reduced from 257mm to 170mm Both thecompression coefficient and the final compression amountof the expansive soil decrease first and then increase with theincrease in the number of microbial treatments the com-pression coefficient and the final compression amount of theunimproved plain expansive soil are the largest After 6times of microbial treatment both reached the minimume expansion modulus of the expansive soil increased firstand then decreased with the increase of the number ofmicrobial treatments e untreated expansive soil has thesmallest compression modulus After 6 times of microbial

Table 3 Reduction of the void ratio

p (kPa)x (times)

0 () 2 () 4 () 6 () 8 ()0ndash400 175 164 139 117 130400ndash800 38 37 33 26 30

a1-2

Es

020

025

030

035

040

045

a 1-2

(MPa

ndash1)

35404550556065707580

E S (M

Pa)

2 4 6 80x (times)

Figure 2 Relation between a1-2 or Es and x

2 4 6 80x (times)

16

18

20

22

24

26

S f (m

m)

Figure 3 Relation between Sf and x


treatment the expansive soil has the highest compressionmodulus

In practical engineering the compression coefficient a1-2is usually used to judge the compressibility of the soil

(1) For a1-2le 01MPaminus1 it is a lowly compressible soil(2) For 01MPaminus1le a1-2le 05MPaminus1 it is a moderately

compressible soil(3) For a1-2ge 05MPaminus1 it is a highly compressible soil

From the above range it can be known that the untreatedexpansive soil with a1-2 044MPaminus1 is a medium-highcompressive soil After the microbial treatment for 6 timesa1-2 022MPaminus1 which is a moderately compressible soilerefore the above results show that the compressioncharacteristics of the expansive soil can be significantlyimproved by using the MICP technology e reason isassociated with the increase of calcium carbonate precipi-tation induced by microorganisms e generation of mi-croorganisms has been confirmed rough the scanningelectron microscope (SEM) as shown in Section 4 a thinlayer of calcite covering soil particles was observed isfinding is consistent with that of Islam [43] Calcium car-bonate precipitates not only cement the surface of the soilparticles but also fill the pores between the soil particles sothat the fine soil particles may form aggregates serving as anadditional skeleton to resist external loads erefore thecompression characteristics of the expansive soil are im-proved and the amount of compressive deformation isreduced

On the contrary the improvement effect of the expansivesoil is related to the number of microbial treatments Whenthe microbial treatment reaches 6 times the treatment effectreaches the best When the number of treatments reaches 8times the coefficient of compressibility of the expansive soilsample becomes larger which indicates that when thenumber of times of treatment with treatment solution is notthe more the better As the number of treatments increasesthe number of times that the expansive soil is crushed(following the processing requirement) also increasesresulting in an increase in fine particles in the soil that mightnot be sufficiently wrapped by the calcium carbonate pre-cipitates and thereby a larger coefficient of compressibilityMoreover as the processing number of times increases theprecipitation efficiency of calcium carbonate between soilparticles may reduce erefore in order to achieve the bestimprovement effect not only excessive crushing should beavoided but also there is a need to reasonably control theprocessing number of times during the test

By processing the test data the change curves betweenthe coefficient of compressibility a the compression coef-ficients at consolidation pressures of 125ndash50 kPa50ndash100 kPa 100ndash200 kPa 200ndash400 kPa and 400ndash800 kPaand the consolidation pressure p of the expansive soil afterdifferent number of times of microbial treatment wereobtained as shown in Figure 4 For convenience the con-solidation pressures of the expansive soil in the figure arerepresented by the midpoints of 125ndash50 kPa 50ndash100 kPa100ndash200 kPa 200ndash400 kPa and 400ndash800 kPa respectively

From Figure 4 it can be found that under the sameconsolidation pressure the coefficients of compressibility ofthe expansive soil samples after microbial improvement areall smaller than those of the untreated soil sample Also theslopes of the curve of the microbially treated soil samples aremore or less similar to those of the untreated sample at lowconsolidation pressure say less than 75 kPa as the con-solidation pressure increases say over 150 kPa the slopes ofthe curves of the microbially treated soil samples are thensignificantly reduced compared to those of the untreated soilsample e above findings show that as the consolidationpressure increases the microbially treated soil samples be-come more and more difficult to be compressed due to theimprovement of the MICP effect and also the increasingconsolidation pressure As a comparison when it exceeds150 kPa to 600 kPa the coefficient of compressibility of theuntreated soil samples decreases dramatically which indi-cates that the soil samples still undergo considerable com-pression deformation within this high-pressure range

33 Variation of Vertical Strain and Consolidation PressureAfter the expansive soil samples were treated with micro-organisms for a different number of times the changes of thesoil vertical strains with the increase of consolidationpressures are shown in Figure 5

It can be known from Figure 5 that (1) in all soilsamples whether or not being improved their verticalstrains increase with the increase of the consolidationpressures however increase rates of the vertical strainsgradually slow down so the increase in the consolidationdeformation gradually decreases (2) under the sameconsolidation pressure the vertical strain of the soilsamples after microbial improvement is significantlylower than that of the unimproved soil samples (3) at thesame consolidation pressure the number of microbialtreatments increased to the sixth and the vertical strainsof the soil samples reduce to be minimum continuous



3125 15075 600300p (kPa)

00

02

04

06

08

10

a (M

Pandash1

)

Figure 4 Relations between a and p


improvement of soil samples say the 8th time results inthe strain to rebound considerably

e above three aspects can be explained as follows onthe one hand as the consolidation pressure increases thesoil particles are rearranged and gradually compacted andthe soil coefficient of compressibility decreases so theincremental amount of compression decreases on theother hand the generated calcium carbonate precipitatesin the microbially treated expansive soil samples arecemented on the surface of the soil particles or filled in thepores of the soil which provides additional skeletonstrength to the soil so the compressive strength of the soilis increased resulting in a decrease in the incrementalvertical strain of the soil e changes between the verticalstrains and the consolidation pressures shown in Figure 5adequately show that the compression characteristics ofthe expansive soil after microbial improvement have beensignificantly improved

By observing the curve of the vertical strain and con-solidation pressure in Figure 5 the hyperbolic function canbe used for fitting e relationship between the verticalstrain and the consolidation pressure before and after theimprovement of the expansive soil can be obtained

ε p

a + bp (2)

where ε indicates the vertical strain under consolidationpressure p and a and b are fitting parameters as listed inTable 4

From equation (2) it is straightforward to obtain theinverse of the pressure-dependent instantaneous elasticmodulus 1Et

1Et

dεdp

a

(a + bp)2 (3)

From equations (2) and (3) the physical meanings of theparameters a and b are clear

(1) 1a indicates the initial slope of the strain to theconsolidation pressure

1E0

1a

(4)

In another word a itself means the initial elasticmodulus E0

(2) 1b represents the asymptotic strain when p in-creases to infinity

εult 1b (5)

Also from equation (2) the consolidation pressure canbe expressed as a function of the vertical strain

p aε

1 minus bε (6)

Substitute equation (6) into (3) to obtain

1Et

(1 minus bε)2

a (7)

erefore the relationship between the vertical strain ofmicrobially modified expansive soil and the consolidationpressure can also be expressed in the incremental form asfollows

dε (1 minus bε)2

adp (8)

Also from Table 4 it is found that the coefficient agradually increases with the increase in the number ofmicrobial treatments while b increases till the 6th treatmentand then decreases at the 8th treatment

34 Preconsolidation Pressure of Microbially Modified Ex-pansive Soil As another critical soil index preconsolidationpressure normally reflects the stress history of the soil epressure is normally determined by Casagrandersquos empiricalmethod Nevertheless it is challenging to determine thepoint of minimum radius of curvature so as to followCasagrandersquos empirical method

Alternatively we adopt ln (1 + e)minus lgp the double log-arithm method ie the compression curve of the expansivesoil is represented by double straight lines and the inter-section of the two straight lines is considered to be thepreconsolidation pressure pc e double logarithm methodwas first proposed by Butterfield [44] Afterwards Onitsukaet al [45] and Hong and Onitsuka [46] validated the methodthrough a large number of experiments It can be seen fromFigure 6 that the bilogarithmic characteristic of eachcompression curve is significant in line with the bilinear

HyperbolaModfitting curves

000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)



Figure 5 Relations between ε and p


Untreated soilLinear fitting

pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa

Treating 2 timesLinear fitting

036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)

Figure 6 compression curve (ln (1+e) - lgp) of microbially-treated expansive soil samples


rule and the inflection point (pc) value of each curve can bestraightforwardly identified

As shown in Figure 7 it is obvious that the pc values ofthe expansive soil improved by microorganisms are higherthan those of the unimproved the pc value of the expansivesoil increases with the number of treatments particularly adramatical increase is found after 4 times of microbialtreatment is phenomenon shows that the microbialtreatment has a significant effect on the increase of pc in theexpansive soil However it should be noted that the changeof the inflection point pc shown in Figures 6 and 7 is ob-viously not the ldquorealrdquo preconsolidation pressure but a re-flection of the structural strength

e increase of the structural strength can be illustratedas follows the microorganisms which entered the pores ofthe soil were mineralized to generate calcium carbonateprecipitates that were found to be mainly calcite crystals[36] ese crystals were generated in irregular particles onthe surface and gaps of the soil particles wrapping up andtightly connecting the soil particles As a result the contactarea between the soil particles is increased and the con-nection capacity between the soil particles is enhancedwhich cause more integrated and denser internal structure ofthe soilerefore the structural strength of the soil is largelyimproved

Furthermore in Figures 6(a)ndash6(e) by fitting lines of thecompression curves after pc the slopes correspond to the(post-pc) normal compression lines ese slopes are ob-tained under the double logarithmic coordinates of Figure 6similar to the compression index in the e-p curve hencereflecting the soil compression characteristics e results of

each slope are shown in Figure 8 It can be seen that (1) whenthe consolidation pressure reaches the inflection point theslope of the compression curve of the expansive soilmodified by microorganisms is less than that of the un-modified soil (2) the slope of the normal compression curvedecreases first and then increases with the increase of thenumber of treatments in particular the slope is the smallestwhen the microorganisms are treated 6 times

e above two findings again show that the com-pression properties of the expansive soil after microbialtreatment have been improved Nevertheless excessivetreatment (such as the eight treatments in our study) is notconducive to improving the compression performance of theexpansive soil when the compressive stress is higher than pce reason is that too many treatments mean excessivegrinding of the soil samples which may severely disruptsome cementation of the soil and produce a large amount ofmicroparticles to small particles minimizing the amount ofmedium to large particles In addition excessive treatmentsof the soil may result in a decrease in the precipitation ef-ficiency of calcium carbonate between soil particles isleads to significant changes of the structure of the particlesand pores of the expansive soil Such changes in the internalstructure of the soil are very complicated and need furtherobservation of the development of calcite crystals

4 Microstructural Analysis

rough using the scanning electron microscope (SEM) themicrostructure characteristics of the soil sample under theappropriate magnification were observed as shown in

2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)

Figure 7 Relationship between pc and times of treatment x

Table 4 Fitting parameters

Soil samples a b R2

Untreated soil 1543202 5735 0997Treating 2 times 1761237 5902 0996Treating 4 times 2035680 7000 0996Treating 6 times 2069820 9086 0993Treating 8 times 2087056 7628 0992


Figure 9 As can be seen from the figure a number of ir-regularly shaped angular crystals are distributed around thesoil particles ese crystals are calcium carbonate precip-itates produced by microbial mineralization ey wrap onthe surface of soil particles cement the soil particles andimprove the stability of the soil particles is helps tounderstand the mechanism of the improved compressibilityand structural strength of the expansive soil By furtherobserving Figures 9(a)ndash9(d) it is found that the degree ofcrystal development andmorphology and cementation effectvary with the number of treatments is shows that dif-ferent treatment times have an impact on the mineralizationof microorganisms However only based on the current

preliminary SEM results the relationship between treatmenttimes and mineralization is not significant enough ere-fore future work needs to be continued from a microscopicperspective to further observe the shape of the crystal and theeffect of the interaction between the crystal and the soilparticles

5 Mechanism Analysis of MicrobialImprovement of CompressionCharacteristics of the Expansive Soil

51 Filling Effect In the process of calcium carbonate pre-cipitation induced by the microorganisms some crystalline and

001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)

Figure 8 Relationship between the slope of the compression curve and x

Figure 9 SEM micrographs of (a) 2 times (b) 4 times (c) 6 times and (d) 8 times Some crystals are marked with circles


noncrystalline inorganic compound precipitates are producedand their main component is calcium carbonate colloid emicrobial process that produces these colloids followed bycolloidal crystallization and solidification is also calledmicrobialmineralization From our study this mineralization process inthe expansive soil can be efficiently completed using Bacilluspasteurii After the mineral compounds are mineralized theyprecipitate in the pores between the soil particles so that a largeamount of pores are filled which results in a decrease in soilporosity and an increase in soil compactness

52 Cementing Effect In the process of MICP calciumcarbonate colloids are precipitated on the surface of the soilparticles as shown in Figure 9 e soil particles arecemented with each other and they are firmly connected toeach other In this process on the one hand because themicroorganisms enter the soil pores and undergo miner-alization the fine particles (such as colloidal particles andclay particles) in the soil are consolidated with each other toform aggregates and become coarse particles (such as silt andsand) As a result the content of colloidal and clay particlesin the soil decreases the content of silt and sand particlesincreases and the particle grading of the expansive soilchanges On the other hand due to the cementation of thesoil particles the connection between the soil particles isenhanced erefore the structural strength of the soil isimproved and the ability of the soil to resist external de-formation is significantly improved

6 Conclusions

Most of the current research studies focus on reducing theexpansive properties of the expansive soil however after theimprovement of the soil the research on how the com-pressibility of the expansive soil will affect is rare In thispaper through the experimental study of the compressioncharacteristics of the improved expansive soil by MICPtechnology the effects of different microbial contents (viathe number of treatments) on the compression character-istics of the expansive soil were investigated and the mi-crostructure of mineralization was analyzed e mainconclusions obtained in this paper are as follows

(1) It is feasible to improve the compression charac-teristics of the expansive soil based on the MICPtechnology by adopting a suitable treatment methodAfter the microbial improvement the coefficient ofcompressibility of the expansive soil was significantlyreduced and the soil compressibility changed fromhigh to low level Among them in our study thecompressibility improvement effect was the best at 6times of microbial treatment

(2) After microbial improvement the relationship be-tween the soil vertical strain and consolidationpressure can be simply and accurately represented bya hyperbolic function that is controlled by twophysically meaningful parameters a and b

(3) As the numbers of treatment increase the pre-consolidation pressure (here reflecting the structuralstrength of the expansive soil) increases significantlyis increase in soil strength is closely related tochanges in the internal structure of the soil body

(4) We observe the existence of crystals between soilparticles by SEM test and validate the microbialmineralization based on MICP

In practice the expansive soil treated by MICP mightalso be used with other granular soils [47] and mixtures[48 49] to improve the performance of embankments whichis worthy of further study

Data Availability

e data that support the plots within this paper and otherfindings of this study are available from the correspondingauthor upon reasonable request

Conflicts of Interest

e authors declare that they have no conflicts of interest

Acknowledgments

e work described in this paper was supported by a grantfrom the National Natural Science Foundation of China(Project no 50978097)

References

[1] J James and P K Pandian ldquoPlasticity swell-shrink andmicrostructure of phosphogypsum admixed lime stabilizedexpansive soilrdquo Advances in Civil Engineering vol 2016Article ID 9798456 10 pages 2016

[2] Y Liu Y Su A Namdar G Zhou Y She and Q YangldquoUtilization of cementitious material from residual rice huskash and lime in stabilization of expansive soilrdquo Advances inCivil Engineering vol 2019 Article ID 5205276 17 pages 2019

[3] B R Phanikumar and T V Nagaraju ldquoEffect of fly ash andrice husk ash on index and engineering properties of ex-pansive claysrdquo Geotechnical and Geological Engineeringvol 36 no 6 pp 3425ndash3436 2018

[4] J M Bian L Jiang and B T Wang ldquoStrength test of ex-pansive soil improved by limerdquo Journal of Changrsquoan Uni-versity vol 33 pp 38ndash43 2013

[5] P Voottipruex and P Jamsawang ldquoCharacteristics of ex-pansive soils improved with cement and fly ash in northernailandrdquo Geomechanics and Engineering vol 6 no 5pp 437ndash453 2014

[6] W Deng and Y Wang ldquoInvestigating the factors affecting theproperties of coral sand treated with microbially inducedcalcite precipitationrdquo Advances in Civil Engineering vol 2018Article ID 9590653 6 pages 2018

[7] R Gui Y-X Pan D-X Ding Y Liu and Z-J ZhangldquoExperimental study on the fine-grained uranium tailingsreinforced byMICPrdquoAdvances in Civil Engineering vol 2018Article ID 2928985 10 pages 2018

[8] J K Mitchell and J C Santamarina ldquoBiological consider-ations in geotechnical engineeringrdquo Journal of Geotechnical


and Geoenvironmental Engineering vol 131 no 10pp 1222ndash1233 2005

[9] W De Muynck N De Belie and W Verstraete ldquoMicrobialcarbonate precipitation in construction materials a reviewrdquoEcological Engineering vol 36 no 2 pp 118ndash136 2010

[10] M Umar K A Kassim and K T Ping Chiet ldquoBiologicalprocess of soil improvement in civil engineering a reviewrdquoJournal of Rock Mechanics and Geotechnical Engineeringvol 8 no 5 pp 767ndash774 2016

[11] J T DeJong K Soga E Kavazanjian et al ldquoBiogeochemicalprocesses and geotechnical applications progress opportu-nities and challengesrdquo Geotechnique vol 63 no 4pp 287ndash301 2013

[12] X Sun L Miao T Tong and C Wang ldquoImprovement ofmicrobial-induced calcium carbonate precipitation technol-ogy for sand solidificationrdquo Journal of Materials in CivilEngineering vol 30 Article ID 4018301 2018

[13] C Dupraz R P Reid O Braissant A W DechoR S Norman and P T Visscher ldquoProcesses of carbonateprecipitation in modern microbial matsrdquo Earth-Science Re-views vol 96 no 3 pp 141ndash162 2009

[14] J T DeJong M B Fritzges and K Nusslein ldquoMicrobiallyinduced cementation to control sand response to undrainedshearrdquo Journal of Geotechnical and Geoenvironmental Engi-neering vol 132 no 11 pp 1381ndash1392 2006

[15] M G Gomez C M R Graddy J T DeJong and D C NelsonldquoBiogeochemical changes during bio-cementation mediated bystimulated and augmented ureolytic microorganismsrdquo Scien-tific Reports vol 9 Article ID 11517 2019

[16] D Gat M Tsesarsky D Shamir and Z Ronen ldquoAcceleratedmicrobial-induced CaCOrdquo Biogeosciences vol 11 pp 2561ndash2569 2014

[17] V Achal and X L Pan ldquoCharacterization of urease andcarbonic anhydrase producing bacteria and their role incalcite precipitationrdquo Current Microbiology vol 62pp 894ndash902 2011

[18] V S Whiffin Microbial CaCO3 Precipitation for the Pro-duction of Biocement Murdoch University Perth Australia2004

[19] A M Sharaky N S Mohamed M E Elmashad andN M Shredah ldquoApplication of microbial biocementation toimprove the physico-mechanical properties of sandy soilrdquoConstruction and Building Materials vol 190 pp 861ndash8692018

[20] L Liu H Liu A W Stuedlein T M Evans and Y XiaoldquoStrength stiffness and microstructure characteristics ofbiocemented calcareous sandrdquo Canadian Geotechnical Jour-nal vol 56 pp 1502ndash1513 2019

[21] M Khaleghi and M Rowshanzamir ldquoBiologic improvementof a sandy soil using single and mixed cultures a comparisonstudyrdquo Soil amp Tillage Research vol 186 pp 112ndash119 2019

[22] Z Wang N Zhang J Ding C Lu and Y Jin ldquoExperimentalstudy on wind erosion resistance and strength of sands treatedwith microbial-induced calcium carbonate precipitationrdquoAdvances in Materials Science and Engineering vol 2018Article ID 3463298 10 pages 2018

[23] H L Liu ldquoExperimental study on dynamic characteristics ofMICP cemented sandrdquo Chinese Journal of Geotechnical En-gineering vol 40 pp 38ndash45 2018

[24] H Canakci W Sidik and I H Halil ldquoEffect of bacterialcalcium carbonate precipitation on compressibility and shearstrength of organic soilrdquo Soils Found vol 55 pp 1211ndash12212015

[25] V Ivanov and J Chu ldquoApplications of microorganisms togeotechnical engineering for bioclogging and biocementationof soil in siturdquo Reviews in Environmental Science and Bio-technology vol 7 pp 139ndash153 2008

[26] J Do ldquoDebonding of microbially induced carbonate pre-cipitation-stabilized sand by shearing and erosionrdquo Inter-national Journal of Geo-Engineering vol 17 pp 429ndash4382019

[27] J F Tang ldquoDevelopment of microbially induced calciumcarbonate precipitation technology in soil improvementrdquoInternational Journal of Advanced Research and Technologyvol 2 pp 26ndash29 2016

[28] Y-M Kwon I Chang M Lee and G-C Cho ldquoGeotechnicalengineering behavior of biopolymer-treated soft marine soilrdquoInternational Journal of Geo-Engineering vol 17 pp 453ndash4642019

[29] A Almajed H K Tirkolaei E Kavazanjian and N HamdanldquoEnzyme induced biocementated sand with high strength atlow carbonate contentrdquo Scientific Reports vol 9 p 1135 2019

[30] M Burbank ldquoGeotechnical tests of sands following bio-induced calcite precipitation catalyzed by indigenous bacte-riardquo Journal of Geotechnical amp Geoenvironmental Engineeringvol 139 pp 928ndash936 2013

[31] A Gurbuz Y D Sari Z N Yuksekdag and B Cinar ldquoCe-mentation in a matrix of loose sandy soil using biologicaltreatment methodrdquo African Journal of Biotechnology vol 10pp 7432ndash7440 2011

[32] A Mahawish A Bouazza and W P Gates ldquoImprovement ofcoarse sand engineering properties by microbially inducedcalcite precipitationrdquo Geomicrobiology vol 35 pp 887ndash8972018

[33] B C Martinez ldquoExperimental optimization of microbial-lnduced carbonate precipitation for soil improvementrdquoJournal of Geotechnical amp Geoenvironmental Engineeringvol 139 pp 587ndash598 2013

[34] H A Keykha A Asadi B B K Huat and S KawasakildquoLaboratory conditions for maximal calcium carbonate pre-cipitation induced by Sporosarcina pasteurii and Sporosarcinaaquimarina bacteriardquo Environmental Geotechnics vol 6pp 1ndash20 2018

[35] G D O Okwadha and J Li ldquoOptimum conditions for mi-crobial carbonate precipitationrdquo Chemosphere vol 81pp 1143ndash1148 2010

[36] A A Qabany K Soga and C Santamarina ldquoFactors affectingefficiency of microbially induced calcite precipitationrdquoJournal of Geotechnical amp Geoenvironmental Engineeringvol 138 pp 992ndash1001 2012

[37] V Achal and X Pan ldquoInfluence of calcium sources onmicrobially induced calcium carbonate precipitation by Ba-cillus sp CR2rdquo Biotechnology and Applied Biochemistryvol 173 pp 307ndash317 2014

[38] M P Harkes L A Van Paassen J L Booster V S Whiffinand M C Van Loos-Drecht ldquoFixation and distribution ofbacterial activity in sand to induce carbonate precipitation forground reinforcementrdquo Ecological Engineering vol 36pp 112ndash117 2010

[39] J F Qian Y S Yao J Li H B Xiao and S P Luo ldquoResilientproperties of soil-rock mixture materials preliminary in-vestigation of the effect of composition and structurerdquo Ma-terials vol 13 no 7 p 1658 2020

[40] C Xia S Lv M B Cabera X Wang C Zhang and L YouldquoUnified characterizing fatigue performance of rubberizedasphalt mixtures subjected to different loading modesrdquoJournal of Cleaner Production vol 249 2020


[41] C Lin S Lv D J and F Qu ldquoLaboratory investigation for theroad performance of asphalt mixtures modified by rock as-phaltstyrene butadiene rubberrdquo Journal of Materials in CivilEngineering vol 33 2021

[42] P Anbu C-H Kang Y-J Shin and J-S So Formations ofCalcium Carbonate Minerals by Bacteria and its MultipleApplications Springer International Publishing New YorkCity NY USA 2016

[43] T Islam Studying the Applicability of Biostimulated CalcitePrecipitation in Stabilizing Expansive Soils Boise State Uni-versity Boise ID USA 2018

[44] R Butterfield ldquoA natural compression law for soils (anadv-ance on e-logp)rdquo Geotechnique vol 29 pp 469ndash480 1979

[45] K Onitsuka Z Hong Y Hara and S Yoshitake ldquoInter-pretation of oedometer test data for natural claysrdquo SoilsFoundvol 35 pp 61ndash70 1995

[46] Z Hong and K Onitsuka ldquoAmethod of correcting yield stressand compression index of ariake clays for sample distur-bancerdquo Soils and Foundations vol 38 pp 211ndash222 1998

[47] Y Yao J Ni and J Li ldquoStress-dependent water retention ofgranite residual soil and its implications for ground settle-mentrdquo Computers and Geotechnics vol 129 Article ID103835 2021

[48] C Liu S Lv D Jin and F Qu ldquoLaboratory investigation forthe road performance of asphalt mixtures modified by rockasphaltstyrene butadiene rubberrdquo Journal of Materials inCivil Engineering vol 33 2020

[49] S Zhang Y Ronald S Pak and J Zhang ldquoVertical time-harmonic coupling vibration of an impermeable rigid cir-cular plate resting on a finite poroelastic soil layerrdquo ActaGeotechnica vol 2 pp 1ndash25 2020


Alternatively microbially induced carbonate precipita-tion (MICP) technology is an emerging technology that hasdeveloped rapidly in recent years and has been widely usedin many fields is new technology has also been used ingeotechnical engineering and gradually formed a new mi-crobial geotechnical technology with great significance insoil mechanics and engineering applications [6ndash14] emechanism of MICP technology is to catalyze the hydrolysisof urea through urease generated during microbial meta-bolism this process generates ammonia and carbon dioxidethat are dissolved in water in an alkaline environment togenerate ammonium and carbonate ions once these car-bonate ions meet sufficient calcium ions calcium carbonateprecipitates with gelling effect are generated is calciumcarbonate precipitation can not only fill the pores betweenthe soil particles but also form glue on the surface of the soilparticles to make the soil particles adhere to each otherWhen calcium carbonate precipitates and solidifies a rel-atively high-strength calcium carbonate crystals are formedwhich improve many engineering properties of the soil[15ndash17] e chemical reaction process of calcium carbonateprecipitation induced by microorganisms can be simplifiedas shown in the following

CO NH2( 11138572 + H2O⟶ CO2 + 2NH3

2NH3 + 2H2Oharr2NH+4+2OHminus

CO2 + OHminus ⟶ HCOminus3

Ca2++ OHminus

+ HCOminus3⟶ CaCO3 + H2O

(1)

Whiffin [18] used Bacillus pasteurii to induce calciumcarbonate to precipitate and cement sand particles therebyimproving the shear strength of the sand also the solidi-fication effect of Bacillus pasteurii and urease-producingstrains isolated from the soil was compared thereafter theeffects of calcium ion concentration and grouting methodson the strength of microbially reinforced sand were studiedHis experimental results showed that the uniaxial com-pressive strength of the sand sample treated by Bacilluspasteurii was as high as 58MPa Sharaky et al [19] foundthat Sporosarcina pasteurii plays an important role in thesand biocementation process and the research resultsshowed that the compressive strength of sandy soil increaseddue to the precipitation of calcium carbonate by the bacterialactivity rough experimental research Liu et al [20]systematically analyzed the strength characteristics ofMICP-reinforced sand and proposed a unified strengththeory for the reinforcement of calcareous sand Based onMICP technology Khaleghi and Rowshanzamir [21] had acomparison study on sandy soil using single and mixedcultures e research results verified that the mechanicaland physical properties of sandy soil with both single andmixed media improved through the MICP especially theoutcome of the mixed medium was much better than that ofthe single medium Wang et al [22] used different treatmentcycles of MICP technology to improve the wind erosionresistance of the sand it showed that the density and winderosion resistance of the sand increase with the number oftreatments so using MICP technology could reduce and

prevent the increase of desertification Liu [23] analyzed thedynamic characteristics of calcareous sand and its cemen-tation mechanism based on the MICP technology andconcluded that after the MICP treatment the dynamicstrength and resistance to deformation of calcareous sandwere improved to a certain extent Canakci et al [24] usedMICP technology to improve organic soil e test resultsindicated that the bacterial treatment increased shearstrength and reduced compressibility of the organic soil

e above application of MICP technology in domesticand international geotechnical engineering has made somepreliminary research advances [25ndash28] However the resultsof the above studies are intended to improve the strength ofsandy soils [29ndash32] and there is little research on the effectof MICP on improving the compressibility of clay especiallyexpansive soil Considering the increase in engineeringdemand in expansive soil areas this requires a new envi-ronmentally friendly and economical method to improveexpansive soil erefore we try to apply the emergingMICP technology to study its effect and mechanism on theimprovement of expansive soil focusing on soil com-pressibility rough laboratory consolidation tests thecompression characteristics and deformation laws of theexpansive soil before and after the MICP improvement arecompared and studied

2 Soil Samples and Testing Scheme

21 Preparation of Soil Samples e test soil material wastaken from the expansive soil excavated from the ring roadengineering in Nanning Guangxi According to the codeTest Methods of Soils for Highway Engineering (JTGE 40-2019) the measured basic physical properties and chemicalcomposition of the soil are shown in Tables 1 and 2respectively

According to the free expansion rate in Table 1 and theclassification as specified in the code JTGE 40-2019 it can bedetermined that the soil samples used in the tests are me-dium expansive soils e microorganism used in our ex-perimental study is Bacillus pasteurii with strain numberATCC11859 which was purchased through the ChinaNational General Microbial Species Collection ManagementCenter (CGMCC) It has high-yielding urease and is widelyused Bacillus which is harmless to humans and theenvironment

During the test the first step was to quickly propagateand cultivate Bacillus pasteurii the bacteria were taken fromthe refrigerator cells were activated and the culture solutionwas inoculated on a sterile operation platform e culturemedium used in the test was mainly composed of nutrientssuch as urea casein peptone soy peptone and sodiumchloride en the inoculated culture solution was culturedon a shaker for 48 hours and then the concentration of thebacterial solution was measured using a spectrophotometere concentration of the bacterial solution is usuallyexpressed by the absorbance OD600 value and it can be usedfor the test when it is greater than 10 [33ndash36]

e bacterial solution and cementation solution (mixedsolution of calcium chloride and urea) were added into the












Liquidlimit ()

Plasticlimit ()

Plasticindex ()

Optimummoisture

content ()


Nanning 194 188 270 608 224 384 162 613



0 200 400 600 800040

045

050

055

060

065

070

e



p (kPa)











p (kPa)x (times)

0 () 2 () 4 () 6 () 8 ()0ndash400 175 164 139 117 130400ndash800 38 37 33 26 30

a1-2

Es

020

025

030

035

040

045

a 1-2

(MPa

ndash1)

35404550556065707580

E S (M

Pa)

2 4 6 80x (times)


2 4 6 80x (times)

16

18

20

22

24

26

S f (m

m)















3125 15075 600300p (kPa)

00

02

04

06

08

10

a (M

Pandash1

)






ε p

a + bp (2)



1Et

dεdp

a

(a + bp)2 (3)



1E0

1a

(4)



εult 1b (5)


p aε

1 minus bε (6)


1Et

(1 minus bε)2

a (7)


dε (1 minus bε)2

adp (8)





000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)






pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References
































































Liquidlimit ()

Plasticlimit ()

Plasticindex ()

Optimummoisture

content ()


Nanning 194 188 270 608 224 384 162 613



0 200 400 600 800040

045

050

055

060

065

070

e



p (kPa)











p (kPa)x (times)

0 () 2 () 4 () 6 () 8 ()0ndash400 175 164 139 117 130400ndash800 38 37 33 26 30

a1-2

Es

020

025

030

035

040

045

a 1-2

(MPa

ndash1)

35404550556065707580

E S (M

Pa)

2 4 6 80x (times)


2 4 6 80x (times)

16

18

20

22

24

26

S f (m

m)















3125 15075 600300p (kPa)

00

02

04

06

08

10

a (M

Pandash1

)






ε p

a + bp (2)



1Et

dεdp

a

(a + bp)2 (3)



1E0

1a

(4)



εult 1b (5)


p aε

1 minus bε (6)


1Et

(1 minus bε)2

a (7)


dε (1 minus bε)2

adp (8)





000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)






pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References






























































p (kPa)x (times)

0 () 2 () 4 () 6 () 8 ()0ndash400 175 164 139 117 130400ndash800 38 37 33 26 30

a1-2

Es

020

025

030

035

040

045

a 1-2

(MPa

ndash1)

35404550556065707580

E S (M

Pa)

2 4 6 80x (times)


2 4 6 80x (times)

16

18

20

22

24

26

S f (m

m)















3125 15075 600300p (kPa)

00

02

04

06

08

10

a (M

Pandash1

)






ε p

a + bp (2)



1Et

dεdp

a

(a + bp)2 (3)



1E0

1a

(4)



εult 1b (5)


p aε

1 minus bε (6)


1Et

(1 minus bε)2

a (7)


dε (1 minus bε)2

adp (8)





000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)






pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References


































































3125 15075 600300p (kPa)

00

02

04

06

08

10

a (M

Pandash1

)






ε p

a + bp (2)



1Et

dεdp

a

(a + bp)2 (3)



1E0

1a

(4)



εult 1b (5)


p aε

1 minus bε (6)


1Et

(1 minus bε)2

a (7)


dε (1 minus bε)2

adp (8)





000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)






pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References

























































ε p

a + bp (2)



1Et

dεdp

a

(a + bp)2 (3)



1E0

1a

(4)



εult 1b (5)


p aε

1 minus bε (6)


1Et

(1 minus bε)2

a (7)


dε (1 minus bε)2

adp (8)





000

002

004

ε006

008

010

012

014

200 400 600 8000p (kPa)






pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References























































pc = 439kPa

036

038

040

042

044

046

048

050ln

(1 +

e)

100 100010p (kPa)

(a)

pc = 461kPa


036

038

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(b)

pc = 477kPa


040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(c)

pc = 579kPa


040

042

044

046

048

050

052

ln (1

+ e)

100 100010p (kPa)

(d)


pc = 725kPa

040

042

044

046

048

050

ln (1

+ e)

100 100010p (kPa)

(e)











2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References






























































2 4 6 80x (times)

40

45

50

55

60

65

70

75p c

(kPa

)



Soil samples a b R2







001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References


























































001

002

003

004

005

006

007

008

009

010

Slop

e k

2 4 6 80x (times)






6 Conclusions







Data Availability




Acknowledgments


References
























































6 Conclusions







Data Availability




Acknowledgments


References






















































reducing compressibility of the expansive soil by

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