Recently, biopolymers have emerged in soil stabilisation. The efﬁciency of biopolymers in ground improvement is mainly dependent on biopolymer types, soil types, biopolymer contents, curing periods, thermal treatment and mixing methods. However, the effect of the initial moisture content during sam- ple preparation stages, on the mechanical behaviours of biopolymer-treated soils, has not been fullyunderstood. The ﬁrst part of this study probed the role of initial moisture content, in treating Shanghai clay with the xanthan gum by performing standard proctor compaction tests, Atterberg limit tests, unconﬁned compression strength (UCS) tests and microstructural analysis, while the second part con-tributed to capture the fatigue behaviours of the samples treated with an ideal moisture content by per- forming constant-amplitude and stepping-amplitude fatigue loading tests. Our results showed that the improvement appeared to occur from an average optimum moisture content for the treated soils (treatedoptimum), which was 3% wet of the untreated optimum. As the initial moisture content increased, theUCS values were elevated. However, there existed an ideal initial moisture content leading to the maxi- mum strengthening efﬁciency. For xanthan gum content (i.e., the mass of xanthan gum with respect to the mass of dry soil) ranging from 1.0% to 5.0%, this ideal value was between 1.1 and 1.2 times the treatedoptimum. Our results also indicated that xanthan gum, as a biopolymer soil strengthener, was efﬁcient in increasing either fatigue life or bearing capacity, under repeated loading for xanthan gum-soil matrices, when compared to untreated soils. While the untreated soils failed at the stress level of only half the UCS,the xanthan gum-treated soils with a 3.0% xanthan gum content sustained at the end of the tests. These data imply the potential use of xanthan gum in soil stabilisation, under repeated loads. /C2112020 Elsevier Ltd. All rights reserved. 1. Introduction During the past two decades, the application of biological pro- cesses to soil stabilisation has emerged in geotechnical engineering [1,2] . Bio-mineralisation, which leads to mineral precipitation using the bio-technological mediation route, has been extensively investigated as a bio-treatment for soils [3–8] . More recently, thedirect use of biogenic excrement (i.e., biopolymers) as a ground improvement method has gained increased academic interest, as biopolymers are abundant in nature and are considered eco- friendly materials for soil treatment [9], and on the other hand able to improve soil strength, even under a very low biopolymer-to-soil ratio (e.g., 0.5%) [10–12] . As a commonly applied contemporary biopolymer type in vari- ous practices, polysaccharides have been evaluated for their capac- ity to treat soils. Beta-glucan ( b-1, 3/1, 6-glucan) and guar gum have been found operational and effective in enhancing themechanical strength of soils [13,14] . Gellan gum and agar gum https://doi.org/10.1016/j.conbuildmat.2020.120039 0950-0618/ /C2112020 Elsevier Ltd. All rights reserved.⇑Corresponding author. E-mail address: xueyu.geng@warwick.ac.uk (X.-Y. Geng).Construction and Building Materials 263 (2020) 120039 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/lo cate/conbuildmat have similar properties, including thermo-gelation and signiﬁcant efﬁciency in strengthening soils [15–19] . The application of car- rageenan and chitosan as additives to earthen constructions con- tributes positively on water durability [20–22] . In addition to polysaccharides, the incorporation of protein-based biopolymers (e.g., casein and sodium caseinate salt) to improve the mechanical properties and durability of natural soils has also been reported [23,24] . Throughout the literature, it has been recognized that the efﬁ- ciency of biopolymers in ground improvement is mainly depen- dent on biopolymer types, soil types, biopolymer contents, curing periods, thermal treatment and mixing methods. The effect of the moisture content on the performance of biopolymer-treated soils, however, has not been fully understood. Some researchers have revealed that the biopolymer-treated soils [12,24] , like other geopolymer-treated soils [25], appear to have a higher mechanical strength with a lower value of moisture content. During the sample preparation stage, a reduction in the initial moisture content may lead to an increased strength [19], which is illustrated in Fig. 1 (Zone A). However, the strength is unlikely to continue increasing when the initial moisture content decreases ( Fig. 1 , Zone C), as too little water results in a poorly dissolved biopolymer solution, which could adversely affect the workability of the biopolymer- soil matrix and its consequent mechanical strength [26].A sa result, there might exist an ideal initial moisture content ( wI) lead- ing to the highest strength as illustrated in Fig. 1 , i.e., inﬂection point from Zone Bto Zone A. Therefore, the initial moisture content appears to play an important role in the efﬁciency of soil treat- ment. Up to now, little research has been performed in this area. Usually, the initial moisture content has been set as a ﬁxed value, e.g., liquid limit [13,16,26] , optimum moisture content [21,23] , and natural moisture content [27]. In this study, the effect of the initial moisture content on the efﬁciency of soil treatment will be probed by an experimental study on the xanthan gum-treated Shanghai clay. Xanthan gum has been extensively researched in the soil treatment due to its unique functional properties. It has excellent cold water dissolving capacity, pH stability, storage stability, ionic salt compatibility, and pseudo-plastic ﬂow characteristics [28,29] . For geotechnical engi- neering, xanthan gum has been used as polymer support ﬂuids to stabilize deep excavations [30,31] . Recent studies have reported that xanthan gum, providing bonds between soil particles, has the potential to act as a soil strengthener [12,14,26,27] . In addition, the presence of xanthan gum in soils reduces the hydraulic con- ductivity due to the ‘‘clogging effect”, and enables hydrogen bonds to enhance the strength and corrosion resistance of sands [14,26,27] .On the other hand, xanthan gum are available at reasonable prices. In general, the market prices of biopolymers are more expensive than conventional soil stabilizers and inorganic binders (e.g., geopolymers). However, taken into account the global green- house gas reduction efforts (e.g., carbon emission trading), biopoly- mers will become remarkably competitive in soil treatment, e.g., for a unit amount (1 ton) of soil treatment, 0.5% xanthan gum is only 3.6% more expensive than 10% cement [9,32] . In addition, the biopolymers have higher production costs due to their wide- spread use in the ﬁelds of food production, agriculture, and medi- cal treatment. For geotechnical applications, it is expected that the price of biopolymers will be greatly reduced due to the unneces- sary edible standards [9,14] . To the authors’ knowledge, limited research has accessed the fatigue behaviours of the xanthan gum-treated soils. Soils sub-jected to repeated loads can fail under a stress level consisting only a fraction of a static strength [33–41] . Therefore, a lack of under- standing of the mechanical behaviours of the biopolymer-treated soils under fatigue loading will restrict the evaluation of their long-term resilience and practical engineering performances (e.g., highway pavements, railway tracks, and airport runways). Given this background, this study involved investigating the perfor- mances of xanthan gum-treated soils from two aspects, i.e., the ideal treatment condition (e.g., ideal initial moisture content) to achieve the maximum strengthening effect and fatigue behaviours under the ideal treatment condition. Standard proctor compaction tests and Atterberg limit tests were also performed along with the SEM images to better understand the underlying strengthening mechanisms. 2. Materials and methods 2.1. Shanghai clay Shanghai lies in the Yangtze River delta alluvial plain in China. For this study, natural soils were sampled from a construction site of Metro Line 15 in the northwest Shanghai, at a depth of 8.9 m. Table 1 shows some of the basic engineering properties for Shang- hai clay, e.g., optimum moisture content (OMC) and maximum dry density ( qdmax), and gravimetric-volumetric parameters. Particle size distribution analysis was performed on the soil using a Mastersizer 3000 particle size analyser (Malvern, UK), which caters for the particles in the range of 10 nm to 3.5 mm. As shown in Fig. 2 , the natural soil had 33.41% by weight sand con- tent, 54.86% silt content, and 11.73% clay content based on the soil particle size ranges as recommended in ASTM D422. The Atterberg limits, i.e., the liquid and plastic limits of the natural soil, were found to be wL= 37.9% and wP= 22%, respectively. The correspond- ing plastic index was IP= 15.9%. Combined with the particle size distribution curve and Atterberg limits, the natural soil was there- fore classiﬁed as sand lean clay, as per ASTM D2487.Strength Initial moisture conten t wIZone A Zone BZone C Fig. 1. Variation of strength with initial moisture content for biopolymer-treated soils.Table 1 Basic engineering properties of Shanghai clay. Properties Values Proctor compaction test Optimum moisture content (OMC), % 23Maximum dry density ( qdmax), g/cm31.59 Gravimetric-volumetric parametersUnit weight ( c), kN/m317.3 Speciﬁc gravity ( Gs) 2.73 Natural water content ( x), % 42.7 Natural void ratio 1.21Degree of saturation ( S r), % 962 J. Ni et al. / Construction and Building Materials 263 (2020) 120039 An X-ray ﬂuorescence (XRF) test on the soil was performed using a portable QUANTAX energy-dispersive X-ray microanalysis (EDX) spectrometer (Brucker, Germany), equipped with a 2.5 kg probe, 50 W pulse processor and Esprit analysis software. The XRF spectrum ( Fig. 3 ) indicated that the most abundant elements were Si, C, O, Al, K, Mg, Na, and Fe. Mineralogical analysis of the natural soil was conducted using the X-ray powder diffraction test (XRPD), using a D8 ADVANCE diffractometer (Brucker, Germany). Data was automatically col- lected for phase angles (2 h), ranging between 5 /C176and 90 /C176, at 0.02 /C176 intervals. Identiﬁcation of the crystalline phases was carried out using JADE software. XRPD patterns are shown in Fig. 4 , including main peak identiﬁcation, SiO 2. 2.2. Xanthan gum Xanthan gum is a polysaccharide formed by the fermentation of sugar (e.g., glucose or sucrose) by the bacterium Xanthomonas cam- pestris. It was chosen to treat Shanghai clay for its unique func- tional properties and reasonable prices as previously demonstrated. The xanthan gum powder for this study was manu- factured by Shandong Fengtai Biological Technology Co., Ltd. 2.3. Sample fabrication Sample preparation for both unconﬁned compression strength tests and fatigue loading tests followed the same procedures. The Shanghai clay was oven dried (105 /C176C) for 24 h, broken down care-fully with rubber hammers, and sieved using a 2 mm sieve to obtain a base soil. The base soil was thoroughly mixed with xan- than gum, before distilled water was added. To increase the solu- bility, the distilled water was heated to 80 /C176C[26]. A metal trowel was used for mixing manually for about ﬁve minutes until a homogeneous xanthan gum-soil mixture was obtained. The mix- ture was then placed inside a cylindrical mould with an inner diameter of 39.1 mm and a height of 80 mm in three layers. For each layer, 25 blows were applied through a rammer (305.5 g) dropping from a height of 247 mm. The compacted samples were extruded after compaction, and cured in a controlled environment at 60% relative humidity and 20 /C176C for 28 days. 2.4. Experimental programme The experimental program involved exploring the inﬂuence of using xanthan gum on the behaviours of Shanghai clay from ﬁve aspects: (1) compaction properties; (2) Atterberg limits; (3) micro- scopic structures; (4) mechanical properties under unconﬁned compression and (5) fatigue loading. The following subsections present the test procedures for each component of the experimen- tal programme. Each test was replicated at least three times to achieve a reliable average. 2.4.1. Standard proctor compaction tests The standard proctor compaction tests were conducted in accordance with ASTM D698 to determine the maximum dry den- sity and its corresponding optimum moisture content for xanthan gum-treated soil samples. Five xanthan gum contents (the mass of xanthan gum with respect to the mass of dry soil), e.g., mb/ ms= 1.0%, 2.0%, 3.0%, 4.0%, and 5.0% ( Table 2 ), were used to inves- tigate the variation of compaction properties with the increasing xanthan gum content. 012345 020406080100 0.001 0.01 0.1 1 10 Percentage passing, %Accumulative percentage passing, %Accumulative percentage passing Percentage passing Particle size, mmClay Silt Sand Fig. 2. Particle size distribution curve of Shanghai clay. 02468 1 0 1 2 1 4 1 6 1 8 2 0051015202530 CaNaMgIntensity (cps/ev) Energy (keV)C OSi Al K Fe Fig. 3. Chemical analysis of Shanghai clay (X-ray Fluorescence spectrum). Fig. 4. Mineralogical analysis of Shanghai clay (X-ray powder diffraction pattern). Table 2 Test matrix for standard proctor compaction, Atterberg limit and unconﬁned compression strength tests. Test Xanthan gum-to-soil ratio ( mb/ms), %Initial moisture content (mw/(mb+ms)), % Standard proctor compaction test1.0, 2.0, 3.0, 4.0, 5.0 – Atterberg limit test ( wP and wL)1.0, 2.0, 3.0, 4.0, 5.0 – Unconﬁned compressionstrength test1.0, 2.0, 3.0, 4.0, 5.0 23, 26, 28, 30, 32J. Ni et al. / Construction and Building Materials 263 (2020) 120039 3 2.4.2. Atterberg limit tests The Atterberg limits ( wPand wL) of the soils treated with ﬁve xanthan gum contents ( Table 2 ) were determined by Liquid- Plastic Limit Combined Test (GB/T 50123-2019, Part 9) by using a cone with a mass of 76 g and a tip angle of 30 /C176. The penetration depth of the cone into the sample (55 mm diameter and 40 mm height) was measured 5 s after the cone had been released. For each biopolymer content, the test was repeated on ﬁve samples with an increased moisture content. The water contents corre- sponding to 17 mm and 2 mm determined with the aid of a linear ﬁtting curve in the semi-log plane of cone penetration-moisture content were regarded as liquid limit and plastic limit, respectively. 2.4.3. Unconﬁned compression strength tests The effectiveness of incorporating xanthan gum in improving the soil strength at different initial moisture contents was investi- gated by the unconﬁned compression strength tests, which were conducted under the strain-controlled condition at a loading rate of 1.5%/min in accordance with ASTM D2166. The xanthan gum contents used were the same as those adopted for the standard proctor compaction and Atterberg limit tests. For each biopolymer content, ﬁve initial moisture contents were employed ( Table 2 ). As suggested by Fatehi et al. [23] and Hataf et al. [21], the optimum moisture content of the untreated soil ( w= 23%) (hereafter referred as untreated optimum) was adopted as the minimum initial mois- ture content. The xanthan gum-treated soil samples with w= 23% had a lot of voids on the sample surfaces, indicating an unthorough mixing due to the poor workability of the xanthan gum-soil mix- ture. Then 3% wet of the untreated optimum was selected based on the results of the standard proctor compaction tests (section 2.4.1), which indicated that the mean value of the optimum mois- ture contents of the treated soils (hereafter referred as treated opti- mum) with various xanthan gum contents was w= 26%. Afterwards, the initial moisture content was increased at an inter- val of 2%. With the increment in the amount of water for sample mixing, the workability of the xanthan gum-soil mixture was improved. However, as the initial moisture content reached 34%, the compacted samples were visually deformed after they were extruded from the cylindrical mould. Therefore, w= 32% was adopted as the maximum initial moisture content. The untreated soil sample, prepared with the optimum moisture content w= 23%, served as a control sample. 2.4.4. Fatigue loading tests Apart from the unconﬁned compression strength tests, fatigue loading tests were performed on the xanthan gum-treated soil samples as well. These samples used w= 30%, and mb/ms= 1.0%, 2.0%, and 3.0%. They were cured under the controlled conditions for 28 days before being exposed to the fatigue loading tests. The ZWICK-100HFP5100 test apparatus (ZwickRoell, Germany) was employed to apply fatigue loads. For detailed descriptions see Chen et al. [42–44] . The test procedure started with the mounting of cylindrical samples on the loading frame and bringing the load cell into contact with the sample surface. Through the user-friendly visualisation window, the mean stress, stress amplitude, and max- imum loading cycles were fed into the load application device. The loading system was switched on at the same time as the automatic data acquisition system, and fatigue loading was initiated. The pro- cess continued until failure of the sample, or the maximum loading cycle was reached. In this study, the fatigue loading tests involved two strategies: constant-amplitude and stepping-amplitude. In the constant- amplitude tests, the mean stress ( rm) and stress amplitude ( ra) for each sample remained constant during the loading process. In the stepping-amplitude tests, the mean stress remained constant,but the stress amplitude increased stepwise. Details of the two components of fatigue loading test are described as follows. The loading schemes for the constant-amplitude and stepping- amplitude fatigue loading tests are shown in Fig. 5 . The test input parameters included the mean stress( rm) and stress amplitude (ra). The corresponding maximum stress ( rmax=rm+ra) and the minimum stress ( rmin=rm-ra) were calculated. The effects ofrm,raand their combination on fatigue life were investigated. The test matrix of constant-amplitude fatigue loading tests is given inTable 3 . The maximum stress were from 1/2 to 3/4 fractions of UCS values referring to Lekha et al. [45], corresponding to medium to high stress levels. The UCS for each treatment condition was used as a basis for the calculation of fatigue stresses (e.g., rm,ra, rmax, and rmin). The maximum number of loading cycles for each test was set as 2,000,000. The loading frequency was set as a reso- nant frequency between the sample and the apparatus, which was about 40 Hz to 45 Hz, rather than a ﬁxed value. For the stepping-amplitude fatigue loading tests, the mean stress ( rm) for each sample was constant, while the stress ampli- tude ( ra) increased step by step, at a constant interval. For each loading step, 500,000 cycles of fatigue loading were applied. The fatigue loading tests started from the ﬁrst step and stopped until either the designed number of cycles was reached, or the sample failed. The test matrix of the stepping-amplitude fatigue loading tests is given in Table 4 . 2.4.5. Microscopic structures Scanning electrons microscope (SEM) and ﬁeld emission scan- ning electron microscope (FESEM) have been used to explore the microstructural characteristics of biopolymer-treated soils, includ- ing shapes, sizes, and aggregation of soil particles [19,23,27] . In this study, SEM images were taken for both untreated soils and xan-mσaσσ N(a) (b)σ Nmσaσ Fig. 5. Loading schemes for (a) constant-amplitude and; (b) stepping-amplitude fatigue loading tests. Table 3 Test matrix of constant-amplitude fatigue loading tests. mb/ms,% rm/UCS ra/UCS rmax/UCS rmin/UCS Number of cycles at failure ( Nf) 0.0 0.3 0.25 0.55 0.1 233,750 0.0 0.65 0.1 0.75 0.55 38,584 1.0 0.4 0.15 0.55 0.25 1,256,2471.0 0.4 0.3 0.7 0.1 377,524 2.0 0.4 0.15 0.55 0.25 1,945,328 2.0 0.6 0.15 0.75 0.45 525,2713.0 0.4 0.3 0.7 0.1 16,375,3643.0 0.55 0.15 0.7 0.4 1,386,0724 J. Ni et al. / Construction and Building Materials 263 (2020) 120039 than gum-treated soils ( mb/ms= 1.5% and w= 28%). Microstructural analysis was conducted to better understand the connection between xanthan gum and Shanghai clay, and furthermore the underlying mechanisms of soil stabilization. 3. Results and discussion 3.1. Effect of biopolymer contents on compaction characteristics The introduction of xanthan gum altered the compaction char- acteristics of Shanghai clay. The optimum moisture content was observed to increase from 23% to 26.7% and the maximum dry den- sity had a descending trend falling from 1.59 g/cm3to 1.51 g/cm3, as the xanthan gum content increased from mb/ms= 0.0% to 5.0% (Fig. 6 ). Ayeldeen et al. [14] reported that this alteration may depend on both the biopolymer characteristics (e.g., chemical properties and solution viscosities) and soil ﬁne-grained content. In the current study, xanthan gum molecules contain a large num- ber of hydrophilic groups (e.g., –OH and –COOH) and therefore have a large capacity to attract water molecules or dissolve in water, and retain water. As a result, elevating the xanthan gum content increased the amount of absorbed water, and therefore increased the optimum moisture content and decreased the maxi- mum dry density.3.2. Effect of biopolymer contents on Atterberg limits The liquid limit increased obviously from 37.9% to 44.9% as the mb/msratio increased from 0.0% to 1.0%, and then slightly increased with the increasing xanthan gum content, to a maximum value of 51.1% at mb/ms= 5.0%, as shown in Fig. 7 . The increasing trend of liquid limit of xanthan gum-soil matrix was also observed by Chang et al. [46]. The mechanism responsible for this phe- nomenon is similar to that works in compacting xanthan gum- treated soil (e.g., hydrophilic properties and water retention capac- ity). Treated samples with a higher xanthan gum content had a higher content of hydrophilic xanthan gum hydrogel, leading to a higher liquid limit. Simultaneously, the plastic limit increased gradually from 22% to 25.7% as the xanthan gum content increased from 0.0% to 5.0%. 3.3. Results of unconﬁned compression strength tests 3.3.1. Effect of initial moisture content on stress–strain behaviour The typical stress–strain curves for ﬁve xanthan gum contents at an initial moisture content w= 30% are shown in Fig. 8 . The trends of axial stress versus axial strain of xanthan gum-treated soil samples differed gradually from that of the untreated soil sam- ple ( mb/ms= 0.0%) with the increasing mb/msratio, indicating that adding xanthan gum to soils provided strengthening effects. On the other hand, the xanthan gum-treated soil samples were moreTable 4 Test matrix of stepping-amplitude fatigue loading tests. mb/ms,% rm/UCS ra/UCS rmax/UCS rmin/UCS Number of cycles at failure ( N) 0.0 0.45 0.05 0.5 0.4 500,000 0.15 0.6 0.3 6,2350.25 0.7 0.2 –0.35 0.8 0.1 – 1.0 0.45 0.05 0.5 0.4 500,000 0.15 0.6 0.3 500,0000.25 0.7 0.2 142,587 0.35 0.8 0.1 – 2.0 0.45 0.05 0.5 0.4 500,000 0.15 0.6 0.3 500,0000.25 0.7 0.2 500,0000.35 0.8 0.1 95,547 3.0 0.45 0.05 0.5 0.4 500,000 0.15 0.6 0.3 500,000 0.25 0.7 0.2 500,000 0.35 0.8 0.1 215,869 22232425262728 1.501.521.541.561.581.601.62 0 . 01 . 02 . 03 . 04 . 05 . 0 Optimal moisture content, %Maximum dry density, g/cm3Maximum dry density Optimal moisture content Xanthan gum-to-soil ratio in mass ( mb/ms), % Fig.6. Variation of maximum dry density and optimal moisture content versus biopolymer contents. 1820222426 303540455055 0.0 1.0 2.0 3.0 4.0 5.0Liquid limit Plastic limit Xanthan gum-to-soil ratio in mass ( mb/ms), %Liquid limit, % Plastic limit, % Fig. 7. Variation of liquid limit and plastic limit versus biopolymer contents. 0.01.02.03.04.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0Axial stress, MPa Axial strain, %mb/ms=1.0% mb/ms=2.0% mb/ms=3.0% mb/ms=4.0% mb/ms=5.0%mb/ms=0.0% Fig. 8. Stress–strain curves for different xanthan gum contents ( w= 30%).J. Ni et al. / Construction and Building Materials 263 (2020) 120039 5 deformable, i.e., the peak axial strain corresponding to the peak axial stress increased from 1.67% to 5.12% as the xanthan gum con- tent increased from 0.0% to 5.0%. When the initial moisture content was reduced to the treated optimum w= 26%, the xanthan gum-treated soils also had improved stress–strain behaviours ( Fig. 9 ), however, not as promi- nent as those for 30% initial moisture content. This implies thatusing the treated optimum as an initial moisture content did not necessarily lead to the best strengthening effect. This may be due to the undesirable workability caused by insufﬁcient water for xanthan gum-soil matrix mixing. As described in the previous sec- tion 3.2, the plastic limit of the xanthan gum-soil matrix increased as a result of increasing the xanthan gum content. Therefore, the initial moisture content of 26% was slightly larger than the plastic limit of the soil samples after they had been treated with xanthan gum at m b/msratio ranging from 1.0% to 5.0% ( wP= 23.5% to 25.7%). The moisture-dependent stress–strain behaviour is further illustrated in Fig. 10 . For a given value of mb/ms= 1.0%, the efﬁ- ciency of xanthan gum in improving the soil mechanical behaviour was intimately related to the initial moisture content. Four of the ﬁve stress–strain curves for xanthan gum-treated soils lie above the untreated soil, indicating the strengthening effect can only be obtained for a moisture content larger than 26% (treated opti- mum). The treated samples prepared with the untreated optimum (23%) seemed not to differ from the untreated sample. The similar phenomenon was also observed for mb/ms= 2.0% prepared with theuntreated optimum. This could be explainable that w= 23% was even smaller than the plastic limit of the xanthan gum-treated soils, which resulted in an unthorough mixing and prevented the xanthan gum to be an efﬁcient binder. In addition, the strength was observed to reach a maximum value of 3.47 MPa at the initial water content w= 30%, where either a smaller value w= 28% or a larger value w= 32% resulted in a reduced peak axial stress. This phenomenon indicates that there exists an ideal initial moisture content that can lead to a maximum strengthening effect. 3.3.2. Effect of initial moisture content on UCS The variations of the unconﬁned compression strength (UCS) deﬁned as the peak axial stress, with an increasing initial moisture content for untreated soils are shown in Fig. 11 . The UCS under- went an increase and then a decrease as the initial moisture con- tent increased from 17% to 30% with a peak value of 1.98 MPa at the optimum moisture content w= 23%. Fig. 12 shows the trends of UCS with an increasing moisture content for ﬁve biopolymer contents. For a given value of mb/ ms= 1.0%, the UCS initially increased from 2.67 MPa to a peak value of 3.42 MPa as the initial moisture content increased from 26% to 30%. Afterwards, additional amount of water led to a reducedUCS value of 2.98 MPa. This trend retained when m b/ms= 1.0% 0.01.02.03.04.0 0.0 1.0 2.0 3.0 4.0 5.0 6.0Axial stress, MPa Axial strain, %mb/ms=1.0% mb/ms=2.0% mb/ms=3.0% mb/ms=4.0% mb/ms=5.0%mb/ms=0.0% Fig. 9. Stress–strain curves for different xanthan gum contents ( w= 26%). 0.01.02.03.04.0 0.0 1.0 2.0 3.0 4.0Axial stress, MPa Axial strain, %w=23% w=26% w=28% w=30% w=32%w=30%w=23% Untreated soil with OMC Fig. 10. Stress–strain curves for different initial moisture contents ( mb/ms= 1.0%). 1.01.21.41.61.82.02.2 16 20 24 28 32Initial moisture content, %UCS, MPa Fig. 11. Variation of UCS versus initial moisture content ( mw/(ms+m b)) for untreated soils. 1.52.02.53.03.54.04.5 26 28 30 32UCS, MPa Initial moisture content, %mb/ms=1.0% mb/ms=2.0% mb/ms=3.0% mb/ms=4.0% mb/ms=5.0% Fig. 12. Variation of UCS versus initial moisture contents ( mw/(ms+m b)) for biopolymer-treated soils.6 J. Ni et al. / Construction and Building Materials 263 (2020) 120039 was replaced with mb/ms= 4.0% and 5.0%. For mb/ms= 2.0% and 3.0%, the UCS values also underwent an increment and a decre-ment with the increasing moisture content, but the peak value of UCS was observed at w= 28%. The overall trend of UCS indicates that xanthan gum-treated soil samples with all ﬁve biopolymer contents grained strength as the moisture content increased from 26% to 28%. Further elevat- ing the moisture content from 28% to 30% led to an increased UCS formb/ms= 1.0%, 4.0% and 5.0% and a decreased UCS for mb/ ms= 2.0% and 3.0%. Afterwards, as the moisture content exceeded 30%, the UCS values decreased regardless of the xanthan gum con- tent. Therefore, the ideal initial moisture content corresponding to the maximum UCS for a given biopolymer content can be assumed to lie between 28% and 30%, which is 1.1 to 1.2 times the treated optimum. The maximum UCS values for various biopolymer contents denoted by solid circles are extracted from Fig. 12 and shown in Fig. 13 . Increasing the biopolymer content was effective in enhanc- ing the soil strength at the ideal initial moisture contents. The dosage of mb/ms= 1.0% increased the maximum UCS from 1.98 MPa to 3.42 MPa, by 72%. As the biopolymer content further increased, the maximum UCS increased gradually and reached 4.36 MPa at mb/ms= 5.0%, by 120%. The SEM images of the untreated soils and xanthan gum-treated soils ( mb/ms= 1.5% and w= 28%) are presented in Fig. 14 . It is clear 0.01.02.03.04.0 012345 Xanthan gum-to-soil ratio in mass ( mb/ms), %Maximum UCS, MPa Fig. 13. Maximum UCS for ﬁve biopolymer contents at the ideal initial moisture contents. (a) (b) (c) (d) (f ) (e) Fine particles conglomeration Fig. 14. SEM images of untreated soils (a), (c), and (e); and xanthan gum-treated soils (b), (d) and (f).J. Ni et al. / Construction and Building Materials 263 (2020) 120039 7 that the xanthan gum-treated soils were less porous than the untreated soils on all three magniﬁcations. This phenomenon is in line with the strength improvement. However, the SEM images for xanthan gum-treated Shanghai clay does not show inter- particle behaviour, e.g., clear biopolymer coating around the single soil particle and bridges between the detached soil particles, which is very common in the SEM images for the biopolymer-treated sands [13,14,18,26] . The cementation mechanism for xanthan gum in Shanghai clay can be explained by the biopolymer- particle interaction. As xanthan gum is negatively charged, the existence of natural cations (e.g., K+,C a2+,M g2+, and Na+inFig. 3 ) in Shanghai clay promoted the ionic bonding between xanthan gum and Shanghai clay. In addition, the carboxylic acid (–COOH) and hydroxyl groups (–OH) of xanthan gum can easily induce hydrogen bonding [26]. The similar SEM observations for the biopolymer-treated ﬁne soils can be found in [13,47,48] . 3.4. Results of fatigue loading tests The fatigue life, which is the number of cycles at failure in the constant-amplitude fatigue loading tests for the samples with dif- ferent xanthan gum contents, are given in Table 3 . For the samples with mb/ms= 1.0% and rm/UCS = 0.4, increasing the stress ampli- tude from ra/UCS = 0.15 to 0.3 led to a decreased fatigue life from 1,256,247 to 377,524 cycles. For the samples with mb/ms= 2.0% and ra/UCS = 0.15, a reduction in fatigue life from 1,945,328 to 525,271 cycles accompanied with an increment in rm/UCS from 0.4 to 0.6. Results from samples with mb/ms= 1.0% and mb/ms= 2.0% indi- cated that increasing either mean stress or stress amplitude, incurred more serious sample damage. To identify if mean stress or stress amplitude played a more important role in the fatigue life of the xanthan gum-treated soil samples, the samples with mb/ ms= 3.0% were designed to be exposed to identical rmax/ UCS = 0.7, but different components. It appeared that the soil was likely to have a short fatigue life with a higher ratio of rm/ UCS for a given ratio of rmax/UCS. Furthermore, in Fig. 15 , it is clear that the use of xanthan gum increased the fatigue life. For rmax/UCS = 0.55, the untreated soil sample failed at 233,750 cycles, while the xanthan gum-treated soil sample with mb/ms= 1.0% failed at 1,256,247. When the mb/ msratio increased to 2.0%, the sample almost sustained till the end of the test. For rmax/UCS = 0.70 and rmax/UCS = 0.75, similar trends were observed. For stepping-amplitude fatigue loading tests, all samples were subjected to a mean stress rm/UCS = 0.45 and repeated stressesincreasing from ra/UCS = 0.05 to 0.35, at an interval of 0.1. Results of the fatigue life are given in Table 4 . With the increment in mb/ms, the samples underwent more loading steps. The untreated soil sample failed under rmax/UCS = 0.60 at the second step, while the sample with mb/ms= 1.0% failed under rmax/UCS = 0.70, at the third step. The sample with mb/ms= 3.0%, although failed at the same loading step as the sample with mb/ms= 2.0%, had a longer fatigue life 215,869 cycles under rmax/UCS = 0.80 compared to 95,547 cycles for mb/ms= 2.0%. Results from the two types of the fatigue loading tests revealed a common phenomenon. The untreated soil failed under a stress level approximately half of the UCS (e.g., rmax/UCS = 0.55 to 0.6), while the xanthan gum-treated soil with mb/ms= 3.0% sustained. With the increasing xanthan gum contents, the fatigue life increased regardless of the loading schemes. 3.5. Possible implementations of xanthan gum in geotechnical engineering practices Although this is a laboratory study, how soft clays can be trea- ted with xanthan gum in practice is brieﬂy discussed as follows. The incorporation of xanthan gum into soils on site can be con- ducted through deep mixing, spraying, grouting, high-pressure injection, and etc. [9,31,49] . Given the high viscosity of xanthan gum solution that is prone to block the voids in soils for further solution penetration, deep mixing might be recommended. In the existing laboratory research, biopolymers have been introduced into the soil in power or solution form (i.e., dry mixing or wet mix- ing). According to Arab et al. [50], wet mixing method is more effective in treating different cohesive soils with sodium alginate. Ayeldeen et al. [14] observed that the efﬁciency of xanthan gum or guar gum through wet mixing in reducing collapsibility was about 2–3 times more than dry mixing, while Chang et al. [26] reported that dry mixing is more effective than wet mixing by pro- viding a well-distributed xanthan matrix in soil given that the amount of mixing water is sufﬁcient for xanthan gum dissolution. As for practical applications, dry mixing is considered as an appro- priate method [51]. Therefore, the choice of mixing method in practice may need to be determined according to the solubility, the addition amount and types of biopolymers, and soil types. Fur- thermore, various factors such as workability, construction cost, relevant machinery, and environmental issue need to be assessed for practical and economic application on site. 4. Conclusions The incorporation of xanthan gum in treating Shanghai clay was experimentally studied by performing standard proctor com- paction tests, Atterberg limit tests, unconﬁned compression strength tests, fatigue loading tests (constant-amplitude and stepping-amplitude) and microscopic analysis. Results indicated that the initial moisture content played an important role in the effectiveness of soil treatment with xanthan gum. As the initial moisture content increased from the treated optimum 26% (3% wet of the untreated optimum), the unconﬁned compression strength increased until an inﬂection point was reached, indicating an ideal initial moisture content of 1.1 to 1.2 times the treated optimum to achieve a maximum strengthening efﬁciency, for all ﬁve xanthan gum contents from 1.0% to 5.0%. Furthermore, either constant-amplitude or stepping-amplitude fatigue loading tests provided evidence that xanthan gum, as an eco-friendly soil strengthener, was efﬁcient in increasing either soil bearing capac- ity or fatigue life, under repeated loads. 0510152025Fatigue lives ( Nf), x105 Xanthan gum-to-soil ratio in mass ( mb/ms), %0.0 1.0 2.0 1.0 3.0 3.0 0.0 2.0σmax/UCS=0.55 σmax/UCS=0.70 σmax/UCS=0.75 Fig. 15. Variation of fatigue life with mb/msandrmax/UCS.8 J. Ni et al. / Construction and Building Materials 263 (2020) 120039 CRediT authorship contribution statement Jing Ni: Conceptualization, Methodology, Formal analysis, Writ- ing - original draft, Supervision, Project administration, Funding acquisition. Shan-Shan Li: Validation, Investigation, Writing - orig- inal draft. Lei Ma: Validation, Investigation, Writing - original draft. Xue-Yu Geng: Writing - review & editing. Declaration of Competing Interest The authors declare that they have no known competing ﬁnan- cial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper. Acknowledgments The work described in this paper was supported by the National Natural Science Foundation of China (51608323, 51678319, 51978533). In addition, this project has received funding from the European Union’s Horizon 2020 Framework programme Marie Skłodowska-Curie Individual Fellowships under grant agreement No. 897701 and Marie Skłodowska-Curie Actions Research and Innovation Staff Exchange (RISE) under grant agreement No. 778360.