Review of Strength Improvements of Biocemented Soils

Biomineralization and bioweathering, which correspond to the bonding and debonding of soil grains, are two aspects of the interactions between organisms and minerals that form the unique characteristics of the biosphere (Hazen and Ferry 2010; Trower et al. 2017; Galvez et al. 2020; Murdock 2020). Mimicking biomineralization could be used to fabricate functional materials and solve engineering problems (Li and Qi 2008; Liang et al. 2015; Whittaker and Joester 2017). Such natural processes are especially attractive, because synthetic materials are dominant in engineering practices, for example, portland cement (Namikawa 2016; Rahimi et al. 2016; Xiao et al. 2017; Ranaivomanana et al. 2018), asphalt (Solanki et al. 2015; Wang et al. 2019a), gypsum (Haeri et al. 2005; Riccardo et al. 2010; Latifi et al. 2018), fly ash (Padade and Mandal 2014; Kang et al. 2016; Rios et al. 2017), and epoxy resin (Wang et al. 2019d). These traditional materials are increasingly regarded as incompatible with living environments (DeJong et al. 2011; Achal and Mukherjee 2015; Achal et al. 2015; Jia et al. 2019; Terzis and Laloui 2019b; Sharma et al. 2021b). A large number of microbes that inhabit the subsurface could induce and control the formation of minerals, such as hydrated silicon dioxide, carbonate, and phosphate salts (Kim et al. 2019; Tornos et al. 2019; Moore et al. 2020). Among these minerals, calcium carbonate (CaCO₃) minerals are particularly abundant and common in aqueous environments (Boquet et al. 1973; Boudreau et al. 2018; Hu et al. 2021). Biomineralized CaCO₃ could bind and trap fine grains and lead to the formation of sedimentary structures (Woods et al. 1999), such as stromatolites (Dupraz and Visscher 2005), and ooids (Diaz et al. 2017; Li et al. 2021). Recently, the potential of biomineralized CaCO₃ in soil improvement was noted and accessed by geotechnical researchers (Mitchell and Santamarina 2005; DeJong et al. 2006; Mitchell and Ferris 2006; Whiffin et al. 2007; Ivanov and Chu 2008). In addition, Whiffin (2004) successfully manufactured the first biocemented sand column using bacteria, which opened a new era for manipulating the engineering characteristics of geomaterials using biological methods. Various biotreated methods have been developed to tackle geotechnical problems (Decho 2010; Jiang and Soga 2017), for example, biocementation to improve soil strength and reduce erosion and seepage (Montoya and DeJong 2013; Cheng and Cord-Ruwisch 2014; Gomez et al. 2018a; Nassar et al. 2018; Li et al. 2019; Montoya et al. 2019; Wu et al. 2019b; Xiao et al. 2022c), biodesaturation to enhance liquefaction resistance (Rebata-Landa and Santamarina 2012; He et al. 2013, 2014; O’Donnell et al. 2017a; Nakano 2018; O’Donnell et al. 2019; Mousavi and Ghayoomi 2021; Wang et al. 2021a), bioclogging or biosealing (Davis et al. 2009; Mitchell et al. 2010, 2013; Phillips et al. 2013a, b; DeJong et al. 2016; Phillips et al. 2018; Kirkland et al. 2020), and the bioremediation of heavy metal and oil pollution in soil and water (Davis et al. 2003; Wang and Chen 2006; Keimowitz et al. 2007; Beolchini et al. 2009; Wang and Zhao 2009; Luo and Gu 2011; Zhao et al. 2015; Beolchini et al. 2017; Cheng and Shahin 2017; Cornu et al. 2017; Li et al. 2017b; Qi et al. 2018; Shapiro et al. 2018; Naguib et al. 2019; Asta et al. 2020; Rolando et al. 2020). Biotreatment in this study is within the scope of microbially induced calcium carbonate precipitation (MICP). This study systematically reviews and summarizes the strength of biotreated soils with different base materials shown in scanning electron micrograph (SEM) images and different particle size distributions (PSDs) as shown in Figs. 1(a–i) based on the microscopic to macroscopic testing techniques. An overview of the strength that was determined using different test methods, which included static and dynamic tests, is provided and discussed. The improving mechanisms in biocementation and failure modes in biotreated soils are interpreted based on the systematic review.

MICP has been recognized as a novel and effective biotreatment technique for soils and rocks in geotechnical engineering (Wang et al. 2017; Kumar et al. 2019; Saffari et al. 2019; Bhutange and Latkar 2020; Choi et al. 2020; Jiang et al. 2020; Omoregie et al. 2020). Microorganisms produce specific enzymes, induce biomineralization, and bond granular grains when they are alive. Two solutions are used in the MICP: bacterial and cementation solutions. The bacteria could be either cultivated in a laboratory or stimulated in situ (Gomez et al. 2017; San Pablo et al. 2020), and the calcium source could be calcium salts, which include calcium chloride (CaCl₂) (Cheng and Cord-Ruwisch 2012; Cunningham et al. 2013; DeJong et al. 2013; El Mountassir et al. 2018; Tobler et al. 2018; Gomez et al. 2019; Wu et al. 2019c), calcium acetate (Zhang et al. 2014; Pham et al. 2018; Wang et al. 2021b), and extracts from natural materials, such as eggshells (Choi et al. 2016b), limestone (Choi et al. 2017), and tofu wastewater (Choi et al. 2016b, 2017; Fang et al. 2019). When both solutions are mixed, CaCO₃ nucleation occurs in a supersaturation state, and then CaCO₃ crystals grow at the particle surfaces and interparticle contacts by consuming the calcium and carbonate ions. Irregularly shaped amorphous CaCO₃ crystals transform into spherical vaterite and then into stable rhombohedral calcite during this process (Yin et al. 2009; Rodriguez-Blanco et al. 2011; Baek et al. 2019; Wang et al. 2019c; Yoon et al. 2019; Kim et al. 2020; Zambare et al. 2020; Xiao et al. 2022a). The complex biogeochemical reactions that occur in MICP (Nemati and Voordouw 2003; Ivanov and Chu 2008; Chu et al. 2013) are NH₂–CO–NH₂ + 2H₂O → 2 ${\mathrm{NH}}_4^{+}$ + ${\mathrm{CO}}_3^{2-}$, and Ca²⁺ + ${\mathrm{CO}}_3^{2-}$ → CaCO₃↓. Previous researchers demonstrated that the mechanical properties of soils could be significantly improved with biotreatment, which includes increases in the unconfined compressive (UCS) and splitting tensile strengths (STS) (DeJong et al. 2006; Whiffin et al. 2007; Chou et al. 2011; Venda Oliveira et al. 2015; Choi et al. 2019a), an improvement in stiffness (van Paassen et al. 2010b; Montoya and DeJong 2015), a reduction in compressibility (DeJong et al. 2006; van Paassen et al. 2010a; Lee et al. 2013) and grain crushing (Xiao et al. 2020a), an increase in dilatancy (Chou et al. 2011; Tagliaferri et al. 2011; O’Donnell and Kavazanjian 2015), an enhancement in resistance to liquefaction (Burbank et al. 2013; Sasaki and Kuwano 2016; Feng and Montoya 2017; Xiao et al. 2018), a decrease in hydraulic conductivity (Chou et al. 2011; Al Qabany and Soga 2013; Chu et al. 2013; Martinez et al. 2013; Jiang et al. 2017), a reduction in soil erosion (Ham et al. 2018; Wang et al. 2018; Jiang et al. 2019; Fattahi et al. 2020; Do et al. 2021; Dubey et al. 2021; Kou et al. 2021; Liu et al. 2021; Meng et al. 2021; Xiao et al. 2022b), an enhancement in the bearing capacity of piles (Lin et al. 2016b; Xiao et al. 2020b, 2021b), and an increase in thermal conductivity (Venuleo et al. 2016; Martinez et al. 2019; Wang et al. 2020b; Cheng et al. 2021; Xiao et al. 2021c; Wang et al. 2022).

Engineering responses to biotreated soil are reported to be affected by many factors : (1) environmental, which includes rainwater flushing (Mortensen et al. 2011; Tang et al. 2020), contamination (Cheng and Shahin 2017; Yu and Jiang 2020), temperature (Ferris et al. 2004; Nemati et al. 2005; Sun et al. 2019b), acid rain (Gowthaman et al. 2021), oxygen content (Li et al. 2017a; Su and Yang 2021), salinity (Mortensen et al. 2011; Tang et al. 2020), wetting–drying cycles (Sharma and Satyam 2021), and freeze–thaw cycles (Sharma et al. 2021a; Sun et al. 2021b); (2) treatment and testing factors, which include chemical concentrations (Al Qabany et al. 2012; Zhao et al. 2014), flow rate and direction of chemical media (Martinez et al. 2013), calcium resource (Choi et al. 2016b; Wang et al. 2021b), treatment strategy (Al Qabany et al. 2012; Martinez et al. 2013; Cheng and Shahin 2016), urease source (e.g., indigenous or exogenous microbes) (Dhami et al. 2013b; Park et al. 2014; Cheng et al. 2017a; Bibi et al. 2018; Gomez et al. 2018b; Nayanthara et al. 2019; Marin et al. 2021), bacterial concentration (Bosak et al. 2004; Soon et al. 2014), pH (Wu et al. 2019a; Zehner et al. 2020), and curing time for the soaking method (Zhao et al. 2014; Sharma and Satyam 2021); (3) characteristics of base materials, which include particle mineralogy (Lioliou et al. 2007; Rodriguez-Navarro et al. 2012; Dyer and Viganotti 2017; Dikshit et al. 2021; Zhang et al. 2021b), roughness (Safavizadeh et al. 2019; Song et al. 2021a), shape (Xiao et al. 2019d; Song et al. 2021b), and size (Morales et al. 2015; Venda Oliveira et al. 2017; Nafisi et al. 2020), and fines content (Mahawish et al. 2018a; Venda Oliveira and Neves 2019), gradation (Sasaki and Kuwano 2016; Cardoso et al. 2020; Pan et al. 2020), and relative density (I_D) (Rowshanbakht et al. 2016); (4) the addition of other materials, which include nanosilica (Liu et al. 2022), clay (Cardoso et al. 2018; Ma et al. 2021; Won et al. 2021), polymer (Wang and Tao 2018), tire (Cui et al. 2021b), fiber (Li et al. 2016; Hao et al. 2018; Wen et al. 2018; Choi et al. 2019b; Xiao et al. 2019b; Lv et al. 2021; Yao et al. 2021), geosynthetic (Gao et al. 2021), steel slag (Kaur et al. 2021), and fly ash (Dhami et al. 2013a; Li et al. 2018b); and (5) stress and saturation, which include stress path (Montoya and DeJong 2015), confining pressure (p_c) (Feng and Montoya 2016), and degree of saturation (Zhang et al. 2017; Chen et al. 2021). The implementation of biotreatment can be classified into four methods: (1) injection or percolation (Yang and Cheng 2013; Cheng and Cord-Ruwisch 2014; Zamani et al. 2019); (2) soaking (Stabnikov et al. 2011; Li et al. 2016; Cardoso et al. 2018; Liu et al. 2019b; Wen et al. 2019; Cheng et al. 2020); (3) compaction (Morales et al. 2015; Sadeghi et al. 2015; Venda Oliveira et al. 2015; Pham et al. 2018; Morales et al. 2019); and (4) spraying or brushing (Xu et al. 2014; Chen et al. 2016; Maleki et al. 2016; Naeimi and Chu 2017; Jiang et al. 2019; Liu et al. 2020a; Liu and Gao 2020; Liu et al. 2020c; Xiao et al. 2022d). The widely used injection method has been adapted in different ways into: (1) a low pH one-phase method where the bacterial and cementation solutions were mixed under low pH conditions and the mixed solution was injected into the specimens (Cheng et al. 2019; Wu et al. 2019a); (2) a low temperature one-phase method where the bacterial and cementation solutions were mixed under a low temperature state then injected into the specimens (Xiao et al. 2019a, e); (3) a two-phase injection method where the bacterial solution was injected into the specimens and retained for a period at the first phase, and the cementation solution was then injected into the specimens during the second phase (Weil et al. 2012; Al Qabany and Soga 2013; Dejong et al. 2014; Khan et al. 2015; Kakelar and Ebrahimi 2016; Terzis et al. 2016; Jiang et al. 2017; Jiang and Soga 2019; Wang et al. 2020a, b; Kashizadeh et al. 2021; Saracho et al. 2021); (4) a three-phase injection method where the bacterial, fixation, and cementation solutions were successively injected into the specimens during one treatment cycle (Harkes et al. 2010; van Paassen et al. 2010b; Yang and Cheng 2013; Kakelar et al. 2016; Rowshanbakht et al. 2016); (5) a four-phase injection method where the bacterial and cementation solutions were successively injected into the specimens twice during one treatment cycle (Mahawish et al. 2016, 2018c, 2019a, b); (6) a reversed injection method (DeJong et al. 2006; He and Chu 2014; Keykha et al. 2015; Montoya and DeJong 2015; Cui et al. 2017; O’Donnell et al. 2017b; Lam et al. 2018; He et al. 2020); (7) a bioslurry-assisted injection method (Cheng and Shahin 2016; Wu et al. 2019a; Wu and Chu 2020); and (8) an electroosmosis-assisted injection method (Keykha et al. 2014, 2015). Of note, these methods can be used singly or integrated to develop flexible implementation strategies (Chu et al. 2013; Gao et al. 2019b; Riveros and Sadrekarimi 2020a).

The parameters that characterize the strength properties of biotreated soils include UCS, STS, shear strength, yielding from compression, and liquefaction resistance. These characteristics can be obtained from UCS, STS, one-dimensional compression, direct shear, monotonic triaxial shear, and cyclic triaxial shear tests as shown in Fig. 2(a). The precipitation of CaCO₃ generates a substantial improvement in strength (DeJong et al. 2010). Potential mechanisms that are responsible for the enhancement in the strength characteristics of biotreated specimens include surface coating, which increases particle roughness, interparticle bonding that increases cohesion (c) strength, and void filling which increases specimen density, as shown in Fig. 2(b). The confirmation of the exact mechanism for the specimens from different soils with various cementation levels relied on the particle scale examination techniques, such as microfluidics (Wang et al. 2019b, c; Marzin et al. 2020; Wang et al. 2021c; Xiao et al. 2021a, 2022a), nuclear magnetic resonance (Kirkland et al. 2017; Thrane et al. 2020), scanning electron microscopes (DeJong et al. 2006; He and Chu 2014; Keykha et al. 2015; Montoya and DeJong 2015; Cui et al. 2017; O’Donnell et al. 2017b; Lam et al. 2018; He et al. 2020), and X-ray computed microtomography (Armstrong and Ajo-Franklin 2011; Kirkland et al. 2019). In addition, potential failure mechanisms in biotreated specimens that were subjected to external loading (e.g., compression, tension, and shearing) include CaCO₃–sand interfacial fracture (DeJong et al. 2010; Feng et al. 2017), CaCO₃–CaCO₃ fracture (Azadi et al. 2017; Xiao et al. 2021f), and sand–particle breakage (Tagliaferri et al. 2011; Qian et al. 2018; Xiao et al. 2020a), as shown in Fig. 2(c). For example, the CaCO₃ fines in the void space and the remaining CaCO₃ that precipitated on the surface of sand particles proved that CaCO₃–CaCO₃ breakage occurred (Azadi et al. 2017). The breakage of sand particles in biotreated specimens might be expected when the loading level is high, or the host sand is as weak as the CaCO₃ minerals (i.e., calcareous sands) (Liu et al. 2019a; Xiao et al. 2020a).

Noncohesive granular particles can attain sufficient bonding strength after biotreatment to form a columnar specimen that could be tested under unconfined conditions. UCS, which is one of the important parameters in geotechnical engineering, can indicate the strength improvement in biotreated soils. The UCS is widely adopted in studies on biotreated soils, because the determination of the UCS from biotreated specimens is simple and efficient. Various soils with different minerals, particle sizes, gradings, and particle shapes from the standard testing materials to nature deposits were used to examine the feasibility of biotreatment and MICP. Based on the PSDs shown in Figs. 1(d–i), these soils that were improved with MICP for the UCS tests were divided into three categories: (1) fine-size grained soils with plastic or nonplastic fines with a mean diameter (D) <0.425 mm; (2) medium-size grained soils with a mean D from 0.425 to 2 mm; and (3) coarse-size grained soils with a mean D from 2 to 4.75 mm. The fine-size grained soils with plastic or nonplastic fines that were improved with MICPs for the UCS tests cover Hubei expansive soil (Li et al. 2018b), Tangshan hydraulic fill fine sand (Lian et al. 2019), Srinagar dredged soil (Saquib Wani and Mir 2019, 2021), Fujian Tulou silt (Liu et al. 2020d), tropical residual silt (Soon et al. 2013, 2014), sand with white kaolin clay (Cardoso et al. 2018), Yangtze River sand or a sand–kaolin clay mixture (Sun et al. 2018, 2019a), Idaho natural or Montana natural soils and a sand–clay mixture (Islam et al. 2020), Cape Flats dune sand (Lambert and Randall 2019), Kerman Desert dune sand (Khaleghi and Rowshanzamir 2019), Cook Industrial fine sand in Western Australia (Cheng et al. 2014; Cheng et al. 2017b, 2020), British standard grade D silica sand (Al Qabany and Soga 2013), Ottawa fine sand (Chu et al. 2013; Choi et al. 2016b; Hoang et al. 2020), Itterbeck fine sand (van Paassen et al. 2010b; Terzis and Laloui 2019a), Aeolian fine sand (Li et al. 2018a; Tian et al. 2018), Anzali Beach fine sand (Moosazadeh et al. 2019), Nansha calcareous fine sand (Deng and Wang 2018), Xisha calcareous fine sand (Liu et al. 2019a), and Caspian Sea coastal fine sand (Kalantary and Kahani 2019). The medium-sized grained soils that were biotreated for the UCS tests involved Cook Industrial medium sand from Western Australia (Cheng et al. 2013, 2014, 2019), tropical beach sand from Singapore (Stabnikov et al. 2013), Fujian medium sand (Cui et al. 2017; Li et al. 2017a; Ma et al. 2021), Itterbeck medium sand (Terzis and Laloui 2019a), Ottawa 20/30 sand (Hoang et al. 2019), commercial medium sand from the Australia Company (Mahawish et al. 2018b, 2019b), and Nasha calcareous medium sand (Fang et al. 2020). The coarse-sized grained soils that were treated with MICPs for UCS tests included Pakenham Blue Metal (old basalt) coarse aggregate (Mahawish et al. 2018a), and a Leman Lake natural sand–gravel mixture (Terzis and Laloui 2019a). In addition, the biotreatment to improve the UCS was applied to synthetic sand mixtures, for example, fiber–sand mixtures (Choi et al. 2016a; Li et al. 2016; Xiao et al. 2019b; Fang et al. 2020; Zhao et al. 2020a, b), and organic material–sand mixtures (Wang and Tao 2018; Nawarathna et al. 2019). Figs. 3(a–d) show the relationship between the UCS and CaCO₃ content (m_c) for selected biotreated soils. Most of the specimens had an m_c < 32% regardless of the soil types, except for the specimens from Fang et al. (2020). In general, the higher the m_c the higher the UCS for a given treated soil (Cheng et al. 2013; Mahawish et al. 2018a, 2019b; Venda Oliveira and Neves 2019; Islam et al. 2020). The relationship between the UCS and m_c depends on many factors, as noted in the previous section, and particle size is one of the main factors. It is important to determine what type of soils can be effectively treated with MICP, because natural soils can be categorized into different classifications based on particle size. DeJong et al. (2006) proposed an inherent criterion to examine the workability of MICPs based on the relationships between the size of the particles and microorganisms and highlighted that the mobilization of bacteria requires ≥0.4 μm pore throats, which indicated that most soils (e.g., from silt to gravel) could be strengthened using MICP (Mitchell and Santamarina 2005). The distribution of CaCO₃ within grain matrixes could affect the UCS of biotreated specimens. Al Qabany and Soga (2013) observed that low urea and CaCl₂ concentrations resulted in stronger specimens compared with high concentration cases, which was due to the better distribution of CaCO₃ at the interparticle contacts. Cheng et al. (2014) reported that the UCS of specimens that were treated with a low concentration of calcium ions in seawater were higher than those treated with a high concentration cementation solution. Cheng et al. (2017a) studied the environmental conditions that affected the UCS of biotreated fine sands and found that a high UCS could be obtained with low urease activity and an ambient temperature where large-sized crystals and uniform precipitation distribution were observed. In addition, Mujah et al. (2019) suggested that a combination of a low concentration cementation solution and high urease activity produced stronger specimens, because large-sized rhombohedral-shaped crystals were precipitated at the soil pore throat. All of these demonstrated the importance of precipitation patterns on the strength of biotreated soils. In addition, microscale meniscus-shaped water adsorbed at the interparticle contacts could effectively retain bacteria and calcium ions to produce CaCO₃ precipitations (Cheng et al. 2013), and these precipitations could effectively bond soil particles (Xiao et al. 2021c). Therefore, Cheng et al. (2013) suggested that a higher soil strength could be obtained with biotreatment at a lower degree of saturation where the microscale meniscus-shaped water could form. Sufficient and wide distribution of interparticle contacts per unit volume in fine and medium-size grained soils provided an advantageous matrix for biotreatment. Based on this, these sands were frequently used to investigate the factors that affected the strength of biotreated soils (Qian et al. 2010; Cheng et al. 2013; Bernardi et al. 2014; Chu et al. 2014; Azadi and Pouri 2016; Cheng et al. 2017a; Porter et al. 2017, 2018; Al Imran et al. 2019; Hoang et al. 2019; Lian et al. 2019). The treatment of gravel with MICP originated from the reinforcement of stone columns in soft ground. The strength improvement in biotreated coarse-sized grained soils has been investigated by many researchers (Mahawish et al. 2016, 2018a, b, c, 2019a; Terzis and Laloui 2019a; Wu et al. 2019a; Pan et al. 2020; Wu and Chu 2020). A conventional two-phase injection method might not be effective, because bacterial and cementation solutions could be easily flushed out due to the large-sized pore throat in gravel soils. Modified injection strategies were proposed to improve the efficiency, such as biogrouting with a soil–lift treatment strategy (Mahawish et al. 2016), and the biogrouting containing bioslurry strategy (Wu and Chu 2020). However, it can be seen from Figs. 3(a–c) that the UCS values of biotreated coarse-sized grained soils were in a broader range than those of biotreated fine and medium-sized grained soils at the same m_c, because the distribution of CaCO₃ was more heterogeneous in the coarse-sized grained soil specimens than in fine or medium-size grained soil specimens (Mahawish et al. 2016). The properties of biotreated soils could be further improved by admixtures. Fibers that are used in biotreated soils include polyvinyl alcohol (PVA), basalt, polypropylene, and carbon (Choi et al. 2016a; Li et al. 2016; Liu et al. 2019b; Xiao et al. 2019b; Zhao et al. 2020a, b). As shown in Fig. 3(d), the UCS of biotreated soils with fibers was comparable with or slightly higher than that without fibers; however, the ductility was greatly improved with the help of fibers (Choi et al. 2016a; Li et al. 2016; Xiao et al. 2019b). This was attributed to the combination of CaCO₃ bonding and fiber bridging, which would benefit the application of the tensile strength of the fiber and, therefore, enhance the ductility of biotreated sands (Xiao et al. 2019b). Apart from fibers, organic materials can benefit from biocementation (Li et al. 2015; Park and Kim 2016; Nawarathna et al. 2018, 2019). Wang and Tao (2018) proposed a polymer-modified MICP approach in which PVA powder was dissolved in a cementation solution. Specimens that were treated with the polymer-modified MICP approach presented a larger UCS and higher m_c than those without PVA. Based on the strength model that was established by Consoli et al. (2007), Xiao et al. (2019b) proposed a semiempirical equation for the UCS for the porosity and volumetric cement content of biotreated specimens, where the volume of CaCO₃ in the void of the specimens was considered. This model can be expressed as $q_u$ = $A_{qu}[\eta /{(C_{iv})}^{b_{qu}}{]}^{{-\alpha }_{qu}}$, where the UCS is q_u, the porosity is η, the volumetric cement content is C_iv, and the fitting parameters are A_qu, b_qu, and α_qu. The parameters b_qu and α_qu for the UCS in biotreated soils were taken as 0.3 and 3.8, respectively, as shown in Figs. 3(e–g). The UCS of biotreated sands shown in Figs. 3(e and f) could be predicted by the semiempirical equation. In addition, the comparison of surfaces for the UCS predictions shown in Fig. 3(f) shows that the biotreated basalt fiber–improved sands produced a higher strength than the biotreated PVA fiber–improved sands.

Tensile strength is an important property of soil with c strength, which can be determined by direct or indirect tensile strength tests (Tang et al. 2016). The STS, which is an indirect tensile strength test, has been widely used in geotechnical engineering (ASTM 2011), and could be conducted to evaluate the performance of MICP-cemented sands (Venda Oliveira et al. 2015; Wen et al. 2018; Choi et al. 2019a; Liu et al. 2019a; Xiao et al. 2019b; Liu et al. 2020b; Zhao et al. 2020a). The STS can be calculated as q_t = 2P/(πLD) (ASTM 2011), where the STS is q_t, the applied compressive strength is P, the length of the specimen is L, and the diameter of the specimen is D. Fig. 1(e) shows the typical PSDs of tested soils that were used in STSs, which included silica sand, sand–clay mixture, and fly ash. Similar to the UCS of biotreated soils, the STS increased with an increase in the biotreatment level for a given soil that was improved by the same strategy (Choi et al. 2016a, 2017; Liu et al. 2019a; Xiao et al. 2019b), and the STS depended on the physical properties of soils, for example, degree of saturation, and additives. Cardoso et al. (2018) reported that the STS of a biotreated sand was further improved by adding clay to the specimens. In addition, the STS of biotreated soils could be affected significantly by adding fibers. Several types of fibers were introduced into sands to enhance their mechanical properties (Choi et al. 2016a, 2019a; Xiao et al. 2019b; Zhao et al. 2020a, b). Fibers can promote the immobilization of bacteria, which increased m_c when the specimens were treated with the same procedure. In addition, the combination of CaCO₃ bonding and fiber bridging could enhance the soil’s capability to the external force. Therefore, the STS increased with an increase in fiber content under the same treatment cycles (Choi et al. 2016a, 2019a; Zhao et al. 2020a, b). Xiao et al. (2019b) found that the STS of the biotreated basalt fiber–improved sands increased from 110 to 350 kPa with an increase in the fiber content from 0% to 0.8% at m_c = 11%–13%. However, the STS remained constant once the fiber content exceeded a threshold value (Zhao et al. 2020a). Furthermore, the reinforcing effect depended on the type of fibers. For example, Zhao et al. (2020b) tested the effects of carbon, basalt, polypropylene, and polyester fibers on biocementation. The carbon fiber that was used for biotreatment showed the optimal performance, followed by the basalt and polypropylene fibers, and the polyester fiber showed the worst performance. Figs. 3(h–j) show that an increase in volumetric cement content and a decrease in porosity resulted in an increase in the STS for the biotreated basalt–fiber improved sands and biotreated PVA fiber–improved sands. Similar to the predictions of UCS shown in Figs. 3(e and f), the semiempirical phase volume model could capture the variations in the STS of the biotreated sands with the addition of fibers (Choi et al. 2016a, 2019a; Xiao et al. 2019b), which is given as q_t = $A_{qt}[\eta /{(C_{iv})}^{b_{qt}}{]}^{{-\alpha }_{qt}}$, where the fitting parameters are A_qt, b_qt, and α_qt. The values of b_qt and α_qt for STS of biotreated soils were taken as the same as that of b_qu and α_qu for the UCS of biotreated soils, respectively. The comparison of surfaces for STS predictions in Fig. 3(j) shows that the biotreated basalt fiber–improved sands produced a higher STS than the biotreated PVA fiber–improved sands. The value of the fitting parameter (A) was 3.2 times larger in the UCS prediction than in the STS prediction for the biotreated basalt fiber–improved sands and 3.7 times larger in the UCS prediction than in the STS prediction for the biotreated PVA fiber–improved sands.

MICP improvement could transfer the interparticle contact force and restrain grain crushing, as highlighted by Xiao et al. (2020a), which is a novel finding that could be used to enhance the capability of piles laid in crushable sands, for instance, calcareous sand (Yasufuku and Hyde 1995; Zhang et al. 2013; Mao et al. 2018; Xiao et al. 2020b). Based on Xiao et al. (2020a), the fragments of grains were of a comparably similar extent for the untreated and heavily biotreated sands; however, a vertical pressure of 10 MPa was used for the untreated sand and a vertical pressure of 30 MPa was used for the biotreated sand with m_c = 9.4%. When the vertical pressure of 10 MPa was applied to the biotreated sand with m_c = 9.4%, the fragments were few and some grains were intact. As shown in Figs. 4(a–d) for the grading evolution, the uplift in PSD at a given pressure was effectively restrained with biotreatment, especially at <20 MPa. When the pressure increased to 30 MPa, the grading for all specimens in the log–log plane changed to linear, which indicated that the sands under high pressure became fractal, especially for the untreated sand. The change in grading due to the increase in stress from 10 to 30 MPa was slightly larger for the biotreated (m_c = 9.4%) than for the untreated sand, which further illustrated that the residual potential for a change in grading was smaller for untreated than for biotreated sands after the application of a vertical stress of 10 MPa. As shown in Figs. 4(e–h) for grading density evolution, the grain size density curve of the untreated sand shifted, apart from the original density curve, to left at 10 MPa; however, the grain size density curve of the biotreated sands changed slightly under the same pressure. As the pressure increased to 20 MPa, the grain size density curve of the biotreated sands gradually changed from the original density curve to that of the untreated sands. When the pressure was 30 MPa, the grain size density curve of the biotreated sands that follows that of the untreated sands shifts away from the original density curve, which indicated that the restraint of particle breakage due to biotreatment was limited under such high stress. Similarly, the change in grading density due to the increase in stress from 10 to 30 MPa was slightly larger for biotreated (m_c = 9.4%) than untreated sands. The deformation of the biotreated soils, which included silica sand, fly ash, and residual soil under a given vertical stress, was often less than that of the untreated soils with the same composition, gradation, and density (Lee et al. 2013; Xiao et al. 2020a), which was due to the protection of the biotreatment. However, the deformation in biotreated sand–clay mixtures was larger than that of untreated ones when the vertical load increased from 10 to 400 kPa and converged when the vertical load was >400 kPa (Cardoso et al. 2018). Xiao et al. (2021f) carried out a series of compression tests on biotreated sand with different gradations. For a given gradation, the compression curve of biotreated sands was above that of untreated sands, which was due to CaCO₃ filling the void spaces of the specimens and CaCO₃ bonding between particles. Furthermore, the stress–strain curves showed none collapsed phenomena, which implied the gradual breakage of CaCO₃ around the particle contacts under compression. Xiao et al. (2021f) divided the compression process into three stages for biotreated silica sands according to the potential failure mechanisms of biotreated sands mentioned previously. In Stage I, the compression behavior of the specimen was dominated by interparticle friction and the possible abrasion or attrition of the CaCO₃–sand interface. In this stage, little breakage of CaCO₃ occurs, and the volume change in the specimen is limited. In Stage II, the compression of the specimen was attributed to the rearrangement of the sand particles, CaCO₃–CaCO₃ breakage, and the abrasion or attrition of sand grains. A transition stress point could be distinguished in the compression curve to separate Stages I and II, which could be regarded as the initiation of bond breakage. In Stage III, the volume change was caused by the breakage of the sand particles. Another stress point indicated the gradual transition from CaCO₃–CaCO₃ breakage to sand–particle breakage could be deduced.

Shear strength is an important characteristic to determine the shear resistance of soils that are subjected to external loads. The shear strengths of biotreated soils were examined extensively through drained (Feng and Montoya 2016; Lin et al. 2016a; Pham et al. 2016, 2018; Gao et al. 2019a; Liu et al. 2019a; Nafisi et al. 2019; Terzis and Laloui 2019a; Xiao et al. 2019e; Nafisi et al. 2020; Cui et al. 2021a; Wu et al. 2021; Xiao et al. 2021d), and undrained triaxial shear tests (DeJong et al. 2006; He and Chu 2014; Keykha et al. 2015; Montoya and DeJong 2015; Cui et al. 2017; O’Donnell et al. 2017b; Lam et al. 2018; He et al. 2020; Kashizadeh et al. 2021). Montoya and DeJong (2015) demonstrated that the shear behavior of biotreated Ottawa sand varied with the biotreatment level in the undrained triaxial shearing tests. The stress ratio versus axial strain relationship changed from strain hardening to strain softening, and the failure mode transited from bulging failure to localized shear band failure with an increase in cementation level. Meanwhile, the negative pore pressure increased with an increase in the biotreatment level, which indicated that increasing the biotreatment level increased dilatancy potential. Furthermore, a series of undrained triaxial tests on biotreated Fujian quartz sand that were performed by Cui et al. (2017) showed that the stress–strain behavior of specimens at p_c = 100 kPa transitioned from hardening to softening modes for deviatoric stress versus axial strain, as the biotreatment extent increased, which led to an increase in the maximum dilation in pore water pressure (PWP) versus axial strain. Choi et al. (2019b) showed that the addition of PVA fiber could enhance the transition of the stress–strain relationships. The test results based on a series of drained triaxial tests proved that the stress–strain behavior of biotreated sands could be influenced by the biotreatment level and p_c (Feng and Montoya 2016; Lin et al. 2016a; Gao et al. 2019a; Liu et al. 2019a; Nafisi et al. 2019; Cui et al. 2021a; Wu et al. 2021). In addition, extensive direct shear tests have been performed to evaluate the shear resistance of biotreated soils along a given plane (Azadi et al. 2017; Gui et al. 2018; Hataf and Jamali 2018; Pakbaz et al. 2018; Zamani and Montoya 2018; Amini Kiasari et al. 2019; Khaleghi and Rowshanzamir 2019; Saquib Wani and Mir 2019; Cheshomi and Mansouri 2020; Pakbaz et al. 2020; Riveros and Sadrekarimi 2020a). Based on the representative stress–horizontal displacement and vertical displacement–horizontal displacement curves for biotreated sands in direct shear tests that were conducted by Pakbaz et al. (2018), the biotreatment could result in a significant strain softening, and the shear stress of biotreated sands at a given vertical stress was higher than that of their untreated counterparts. In addition, the deformation changed from contraction to dilation due to the biotreatment; however, dilatancy was suppressed with an increase in the vertical stress. Figs. 4(i–m) show the representative stress–strain relationships in biotreated Ottawa 50/70 sand in drained triaxial tests from Feng and Montoya (2016) and Lin et al. (2016a). The stress ratio of biotreated Ottawa 50/70 sands at a higher level of treatment increased rapidly to a peak state within a lower axial strain then decreased sharply at p_c = 0.1 MPa. The influence of particle size on the stress ratio versus axial strain was marginal in untreated sands and considerable for biotreated sands at p_c = 0.1 MPa. In addition, p_c affected the stress ratio at a given axial strain for untreated and biotreated sands. The peak state stress ratio increased as p_c decreased, and an increase in p_c resulted in a decrease in the strain softening pattern in biotreated sands where m_c = 1.2%–1.6%. In addition, grain mineral has a great influence on the stress ratio and stress–strain relationship in untreated sands. As shown in Fig. 4(n), the untreated calcareous sand (Cui et al. 2021a) shows a higher peak state stress ratio than the untreated silica sand (Feng and Montoya 2016). However, the effect of grain mineral on the peak state strength could be mitigated with biotreatment in Fig. 4(o). The maximum dilatancy could be defined as the maximum ratio of volumetric strain to deviatoric strain increments (Cui et al. 2021a), because the plastic strain could not be directly measured during triaxial tests. A typical stress–strain relationship for calcareous sands with extremely heavy biotreatment (Cui et al. 2021a) and silica sands with heavy biotreatment (Feng and Montoya 2016) is shown in Fig. 4(p). The axial strain that corresponded to the peak state stress ratio in the biotreated calcareous sand was smaller than that which corresponded to the maximum dilatancy. In addition, this difference in the axial strains that corresponded to the peak state stress ratio and the maximum dilatancy in biotreated calcareous sands was larger than that in biotreated silica sands. For the biotreated silica sand, the CaCO₃ bonds between sand particles started to fracture before peak state strength. When the deviatoric strain was not high enough, the locked void spaces between sand particles that were formed by the cementation of CaCO₃ were not completely released and the CaCO₃ fine particles that formed during shearing could not enter the locked void spaces. The capacity of CaCO₃ clusters to fill voids was limited, which resulted in higher dilatancy. After the peak state strength, as deviatoric strain accumulated, more CaCO₃ bonds were broken and the release of the locked void spaces was magnified, which resulted in an inhibition of continuous dilation. In the biotreated calcareous sand, after the peak state deviatoric stress, the cementation of CaCO₃ between the sand particles was seriously damaged, which meant that a large amount of broken CaCO₃ could fill the voids within the sand–grain skeleton, and therefore, the density of the specimen increased considerably. Therefore, a greater dilatancy was anticipated under shearing, and the maximum dilatancy point in the stress–strain curve could fall behind the peak state stress point. However, more test data on biotreated calcareous and silica sands are needed to further validate the previous phenomenon and mechanism. The improvement in shear strength due to biotreatment is reflected by increases in c and the friction angle (ϕ). Feng and Montoya (2016) found that the peak state ϕ increased with an increase in m_c in biotreated Ottawa 50/70 sand, or the slopes of the failure envelope increased in the stress plane (e.g., deviatoric versus mean effective stress). However, the effective c increased significantly only in the heavily treated specimens (m_c = 4.3%–5.3%). This might be explained by the evolution of the CaCO₃ precipitating mode. The CaCO₃ minerals precipitated on the surface of the sand particles at a low cementation level (m_c = 0.9%–1.4%), which might increase the roughness of the particles. As the content of CaCO₃ increased, the formation of interparticle bonds became apparent, which led to a significant increase in the effective c. Cui et al. (2017) found that the effective c and ϕ increased with an increase in the m_c of the biotreated silica sand. The peak state deviatoric stress and the effective c had exponential relationships with the m_c, and the relationship between the effective ϕ and m_c could be fitted with a linear equation. Pakbaz et al. (2018) found that the peak state ϕ and effective c in biotreated Karoon Shore sands increased with an increase in the biotreatment according to the direct shear tests. Choi et al. (2019b) showed that the addition of PVA fiber could further increase the effective c and peak state ϕ in biocemented specimens. Lin et al. (2016a) reported that the peak state ϕ were 35° and 32° for untreated Ottawa 50/70 and 20/30 sands, respectively, and the peak state ϕ were 32° and 31° for lightly biotreated Ottawa 50/70 and 20/30 sands, respectively. The effective c were 41 and 58 kPa for lightly biotreated Ottawa 50/70 (m_c = 1.5%–1.6%) and Ottawa 20/30 sands (m_c = 0.9%–1.1%), respectively. This indicated that the slope of the failure envelope slightly decreased under light biotreatment and the intercept increased in the stress plane compared with the untreated specimens. These results were different from other studies (Montoya and DeJong 2015; Feng and Montoya 2016; Cui et al. 2017; Gao et al. 2019a; Nafisi et al. 2019; Nafisi et al. 2020). The possible reason could be the lower p_c (e.g., 25, 50, and 100 kPa) in Lin et al. (2016a) under which the rolling and rotation of the sand particles could not be effectively inhibited during shearing. In addition, the test data were not abundant, which led to difficulties when estimating the changes in ϕ due to biocementation. Of note, the critical state or residual strength of biotreated sands was higher than that of untreated sands (Feng and Montoya 2016; Lin et al. 2016a; Cui et al. 2017; Nafisi et al. 2019; Xiao et al. 2019e; Xiao et al. 2021d). As shown in Fig. 4(i) for the CaCO₃ content effect, an increase in the biotreatment level resulted in an increase in the residual strength of the biotreated sand specimens. However, the residual strength could be further reduced under reloading. According to O’Donnell and Kavazanjian (2015), the stiffness and dilatancy of the first reconstructed biotreated silica sands (without interparticle bonds) improved significantly at small strains compared with the untreated ones. The stiffness and dilatancy of the second reconstructed biotreated sands improved little compared with the untreated sands. This suggested that the improvement in the stiffness and dilatancy of the remolded biotreated sands was caused by particle roughness from the CaCO₃ coating. During shearing, the role of the CaCO₃ crystals transformed from interparticle bonding to particle roughening. After repetitive reconstructions, the CaCO₃ crystal coating on the sand particles was abraded, which resulted in a marginal improvement in strength compared with the untreated sands. For calcareous sand, diverse findings on the changes in peak state strength with biotreatment level still exist. As proposed in Cui et al. (2021a), failure envelopes in the stress plane for biotreated calcareous sand showed that an increase in m_c resulted in a decrease in the slopes of the failure envelope and a substantial increase in the intercept, which indicated a dramatic increase in the effective c but a decrease in the peak state ϕ with an increase in m_c. In contrast, Liu et al. (2019a) indicated that biotreatment could significantly improve the effective c of calcareous sand and did not affect the peak state ϕ. The calcareous sand particles with abundant internal pores were subangular to subrounded in shape, and the surface roughness of the untreated calcareous sand was high (Xiao et al. 2020b; Cui et al. 2021a). The CaCO₃ crystals might fill the internal pores, which reduced the surface roughness of sand particles; therefore, the peak state ϕ might not change or even decrease with an increase in the biotreatment level. In addition, Gao et al. (2021) carried out pullout tests to investigate the biotreatment effects on shearing behaviors at the interface between geosynthetic and biotreated soils. The results showed that the pullout resistance of the biotreatment–geosynthetic system was significantly improved with a small amount of CaCO₃. Meanwhile, the interface adhesion and the average ϕ between the geosynthetic and biotreated soils increased with an increase in m_c. Therefore, CaCO₃ precipitation between soil particles and along the soil–geosynthetic interface enhanced the shear strength of the soil–geosynthetic system.

The liquefaction of soils that is induced by dynamic loadings could result in the collapse of foundations and superstructures due to the increase in excess PWP (u) and, therefore, a reduction in the effective stress. The MICP technique has proved to be efficient to mitigate liquefaction (Sasaki and Kuwano 2016; Xiao et al. 2018; Darby et al. 2019; Riveros and Sadrekarimi 2020b; Mousavi and Ghayoomi 2021; Sharma et al. 2021b; Sun et al. 2021a; Zamani et al. 2021; Lee et al. 2022). The liquefaction characteristics of biotreated soil with various influencing factors have been studied extensively. The dynamic strength and dynamic u are considered the key issues in the studies of resistance to liquefaction (Montoya et al. 2013; Sasaki and Kuwano 2016; Xiao et al. 2019a; Riveros and Sadrekarimi 2020a; Lee et al. 2022). This section will provide an overview of the cyclic strength in biotreated soils during element tests (e.g., cyclic direct simple shear and cyclic triaxial shear tests) and model tests (e.g., shake table and centrifuge model tests), and the major factors that affect the cyclic responses, for example, p_c, m_c, relative density (I_D), and fines content are summarized. Xiao et al. (2019a) systemically investigated the influence of the cyclic stress ratio (CSR) on the development of u in biotreated loose calcareous sands under heavy and light biotreatments, and no treatment as shown in Figs. 5(a–f), where the normalized excess PWP (u/p_a) is defined as the ratio of u to the atmospheric pressure (p_a) (=100 kPa). The number of cycles to liquefaction increased with the decrease in the CSR for all calcareous sands with different biotreatments. As shown in Fig. 5(d), for the same CSR of 0.333, heavily biotreated loose calcareous sand possessed a higher liquefaction resistance than the untreated dense calcareous sand, because the number of cycles to liquefaction for heavily biotreated loose calcareous sand was larger than that of untreated dense calcareous sand. In addition, the number of cycles to liquefaction for the lightly biotreated loose calcareous sand was considerably larger than that for untreated medium dense calcareous sand when the CSR was 0.188. The underlying mechanism was further investigated in Riveros and Sadrekarimi (2020b). The improvement in stress transfer due to the bonding effect led to longer stagnation in the contacts between grain particles and higher dilatancy in the soil matrix. However, when the CSR was 0.25, an opposite phenomenon was found, where the number of cycles to liquefaction for untreated medium dense calcareous sand was larger than that for lightly biotreated loose calcareous sand. In addition, the pattern of the development for uvchanges from heavy biotreatment to untreatment. As shown in Fig. 5(g) for PWP fitting, u/p_a gradually transformed from hyperbolic into transition and then to an S-shaped type with a decrease in biocementation level, where the normalized number of cycles to liquefaction was defined as the ratio of the number of cycles to the number of cycles to liquefaction. Therefore, three PWP development patterns were abstracted for biotreated calcareous sands, as suggested by Xiao (2020). Specifically, the S-shaped type evolution for the untreated loose calcareous sand, which showed a significant increase in u/p_a when approaching a critical value (p_c) could be segmented into four stages: (1) initial; (2) stable developing; (3) rapid developing; and (4) complete liquefaction. In contrast, the hyperbolic-shaped PWP curve in heavily biotreated calcareous sands consisted of three stages: (1) initial rapidly developing; (2) gradually developing; and (3) complete liquefaction. Following the work by Xiao (2020), an empirical equation was proposed to effectively predict the development of u/p_a as: u/p_a = ${\chi }_u(\tan (k_{u}N/N_f){)}^{{\vartheta }_u}$, where the fitting parameters are χ_u, k_u, and ${\vartheta }_u$ are, and the number of cycles in the tests and number of cycles to failure are N and N_f, respectively. The model predictions shown in Fig. 5(g) are in good agreement with the test data on the dynamic PWP with R² > 0.91. Liquefaction under cyclic loading is a failure phenomenon that usually corresponds to several loading cycles when a limiting strain or 100% pore pressure ratio is reached (ASTM 2017). The number of cycles at a double amplitude strain of 5% is frequently adopted for the liquefaction criteria for cyclic triaxial shear tests and the number of cycles at a shear strain of 3% is taken for cyclic direct simple shear tests (Burbank et al. 2013; Han et al. 2016; Simatupang et al. 2018). Another failure criterion was adopted by Riveros and Sadrekarimi (2020b), where the number of cycles at a single amplitude shear strain of 3.75% (that corresponded to a single amplitude axial strain of 2.5% for cyclic triaxial shear tests) was used to define the initiation of liquefaction. The relationship between the number of cycles to the initial liquefaction (N_L) and that at 5% double amplitude strain (N_ɛa) for biotreated sands was investigated by Xiao et al. (2018). As shown in Fig. 5(h) for N_ɛa versus N_L, the slope of the correlation between N_ɛa and N_L increased from 1 to 1.3, as the I_D of untreated sands increased from 9%–13% to 77%–82% or the biotreatment became heavy, which indicated that more cycles were required to reach 5% N_ɛafor dense sands or heavily biotreated sands, which meant that the biotreated soils had greater strength and stiffness (Maher et al. 1994). The results indicated that loose sands with heavy biocementation exhibited similar behavior to dense sands. After biotreatment, the relationship between N_ɛa and N_L was a remarkable linear correlation. All the test data were fitted by a linear equation with a slope of 1.1 shown in Fig. 5(i) for N_ɛa versus N_L, which indicated that N_ɛa > N_L for biotreated sands. Therefore, taking N_ɛa as the failure criterion was more conservative due to its smaller soil deformation under this condition. The dynamic strength relationship of soil is defined as the relationship between the cyclic stress [for cyclic triaxial shear tests (σ_d) or for cyclic direct simple shear tests ( ${\sigma }_{\tau }$)] and N_f, and their relationship can be expressed as a power function (Rahman et al. 2021) as σ_d/p_a or ${\sigma }_{\tau }/p_a$ = ${\chi }_{\sigma }(N_f{)}^{-{\vartheta }_{\sigma }}$, where the normalized dynamic strengths (σ_d/p_a and ${\sigma }_{\tau }/p_a$) are defined as the ratio of dynamic strength to p_a, and ${\chi }_{\sigma }$ and ${\vartheta }_{\sigma }$ are the fitting parameters. Then, ${\vartheta }_{\sigma }$ was set at the same value (i.e., 0.15) for all sands, for example, calcareous (Xiao 2020), Snake River (Burbank et al. 2013), Ottawa (Montoya et al. 2013; O’Donnell et al. 2017b), Nevada (Zamani and Montoya 2019), and Fraser River sands (Riveros and Sadrekarimi 2020b) with biotreatment or no treatment. Based on a series of undrained cyclic triaxial tests that were performed by Xiao et al. (2019a) for untreated and biotreated calcareous sands with I_D = 9%–13%, 42%–50%, and 77%–82% under different values of p_c, the heavily biotreated calcareous sands exhibited the cyclic mobility failure regime and the untreated calcareous sands with a low density showed the flow failure regime. As shown in Fig. 5(j) for the normalized strength curves of heavily biotreated calcareous sands with I_D = 42%–50%, the dynamic strength of biotreated calcareous sand was greatly influenced by p_c. Based on the values of ${\chi }_{\sigma }$, the dynamic strength of heavily biotreated calcareous sand at p_c = 0.2 MPa was 3.6 times larger than that at p_c = 0.05 MPa, which indicated that an increase in p_c resulted in an increase in liquefaction resistance for biotreated sands. In addition, for heavily biotreated calcareous sands at p_c = 0.2 MPa in Fig. 5(k), the relationship between the normalized strength with different relative densities (9%–50%) was in a narrow band where ${\chi }_{\sigma }$ = 2.9 with I_D = 9%–13% was close to ${\chi }_{\sigma }$ (= 2.7) with I_D = 42%–50%, which indicated that the dynamic strength of the calcareous sands with heavy biotreatment was insensitive to I_D. The reason might be that the effect of heavy biotreatment on the dynamic stress suppressed that of low and moderate density. However, a high density for sand specimens might change this phenomenon, because the effect of high density on the dynamic stress is comparable with that of heavy biotreatment. For I_D = 9%–13% at p_c = 0.2 MPa in Fig. 5(l), the dynamic strength of heavily biotreated sand specimens was 2.5 times larger than that of untreated specimens based on the values of ${\chi }_{\sigma }$. As mentioned previously, a substantial enhancement in liquefaction resistance for biotreated sand is attributed to increases in bonding, density, and particle roughness that results from CaCO₃ precipitation between sand particles. Similar results were found in undrained cyclic triaxial shear tests (Burbank et al. 2013) and undrained cyclic direct simple shear tests (Montoya et al. 2013; O’Donnell et al. 2017b; Zamani and Montoya 2019; Riveros and Sadrekarimi 2020b) for the normalized strength curves of biotreated sands shown in Figs. 5(m–p). Based on the undrained cyclic triaxial shear tests for Snake River sand at p_c = 0.1 MPa (Burbank et al. 2013) in Fig. 5(m), the dynamic strength of heavily biotreated Snake River sand with m_c = 4%–7% was 5.1 times larger than that of untreated Snake River sand when I_D = 33%–39%. According to the undrained cyclic direct simple shear tests performed on Ottawa sands at the vertical stress of 0.1 MPa (Montoya et al. 2013; O’Donnell et al. 2017b) in Fig. 5(n), the improved dynamic strength of moderately biotreated sand with m_c = 2%–4% would be 4.1 times larger than that of untreated sands, which was a considerable enhancement in the dynamic strength for Ottawa sands with moderate biotreatment compared with that for Snake River sand with heavy biotreatment. Therefore, the improvement in the dynamic strength for Ottawa sand that used biotreatment might be much more effective than that for Snake River sand. Sands with nonplastic fines are quite prone to liquefaction compared with clean sands (Hazirbaba and Rathje 2009), which could be improved with biotreatment. Zamani and Montoya (2019) carried out a series of undrained cyclic direct simple shear tests to investigate the cyclic response of untreated and biotreated sands with fines content from 0% to 35%. The dynamic strength of biotreated mixtures was 3.1 times larger than that of untreated mixtures with the same fines content of 35% as shown in Fig. 5(o). However, Sasaki and Kuwano (2016) found that the presence of fines had a negative impact on the microbial process, and fines could hinder efficient precipitation at interparticle contacts. In addition, SEM images showed that the morphology of CaCO₃ was vaterite, which is less stable than calcite. Riveros and Sadrekarimi (2020a) performed a series of undrained cyclic direct simple shear tests on untreated and biotreated Fraser River sands with I_D = 22%, 52%, and 65% in Fig. 5(p). The dynamic strength of the biotreated Fraser River sands was 1.7 times larger than that of untreated Fraser River sands. Furthermore, repeated cyclic loading tests were conducted by Riveros and Sadrekarimi (2020a) to evaluate the persistence of biotreated strengthening. The results revealed that the reliquefaction resistance of biotreated soils reduced due to the fracturing of biobonds regardless of the relative densities. However, the biotreated sands were more resistant to reliquefaction than the untreated sands at similar values of I_D, which was due to densification from CaCO₃ precipitates and residual interparticle bonds. In addition, the measurement of shear wave velocity was adopted by Feng and Montoya (2017) to assess the spatial distribution of CaCO₃ precipitation in biotreated specimens and to evaluate the liquefaction resistance of the biotreated specimens through cyclic triaxial shear tests. The test results confirmed that the dynamic strength of specimens at the same CSR depended on biotreatment levels, and the shear wave velocity could evaluate the distribution pattern of CaCO₃ precipitation. Recent studies on model tests have provided evidence that biotreatment has the potential to effectively enhance the liquefaction resistance of soils. Geotechnical centrifuge tests were conducted by Montoya et al. (2013) to evaluate the seismic performances of biotreated sands. The results indicated that biotreatment increased the dynamic strength and stiffness of sands, which resulted in a reduction in excess pore pressure and settlement. Darby et al. (2019) conducted a series of shake table tests on biotreated sands with three biotreatment levels. The shear wave velocity and cone penetration resistance were adopted to evaluate the MICP cementation level and the degradation of cementation. The test results showed the cone penetration resistance was improved 2.5, 5, and 9 times for the lightly, moderately, and heavily biotreated sand specimens, respectively, compared with that of the untreated sands. Meanwhile, the shear wave velocity increased 1.4, 2.3, and 4.7 times for lightly, moderately, and heavily biotreated sand compared with the untreated sands. In addition, according to the shake table tests conducted by Zhang et al. (2020), the shear wave velocity of the biotreated calcareous sands was improved approximately two and four times for light and heavy biotreatments, respectively, compared with that of the untreated calcareous sands. Zamani et al. (2021) performed a centrifuge model test to study the effect of MICP when mitigating the liquefaction of a sand foundation, and they highlighted that MICP was an effective way to reduce the foundation settlement during earthquakes and to further reduce building damage.

The heterogeneity in the precipitation of CaCO₃ was observed in small columns (Al Qabany and Soga 2013), meter-scale specimens (Martinez et al. 2013), and large-scale specimens (van Paassen et al. 2010b). The nonuniformity of CaCO₃ precipitation could be alleviated by adopting regulatory treatment procedures, for example, low pH or low-temperature one-phase injection methods (Cheng et al. 2019; Xiao et al. 2019e), and using optimized bacterial activity and concentrations of cementation solution for small column specimens (Mujah et al. 2019). Low-rate biostimulation was suggested to improve the uniformity of CaCO₃ distribution in large-scale tests (San Pablo et al. 2020); however, the efficiency of this method is affected by soil type. Considering the complexity and diversity of soils in the field, the uniformity of CaCO₃ precipitation would be a great challenge in industrial applications. In addition, one of the disadvantages of using MICP is the production of ammonia, which can be toxic with long-term exposure (Mujah et al. 2016; Cheng et al. 2019). Several methods have been proposed to mitigate or eliminate the impact of ammonia during and after MICP. The low pH one-phase injection strategy that was proposed by Cheng et al. (2019) meant that more ammonium remained in the ionized ( ${\mathrm{NH}}_4^{+}$) instead of atmospheric form (NH₃). With this method, the ammonia gas released can be reduced by ≥90%. As suggested by Yu et al. (2016), dipotassium phosphate could be introduced to immobilize the ammonia by forming environmentally friendly struvite minerals during MICP. According to their study, 88.52% of ammonia could be fixed by applying an optimum formula (Yu et al. 2019). In addition, the struvite could act as a bonding material, which enhances the cementation among sands. Asparaginase activity could be used to mitigate ammonia release, as proposed by Li et al. (2015). Approximately 93% of the ammonia release was removed in the presence of asparaginase. In addition, the postprocessing method was proposed by extracting solutions from soils and then repairing the extracted solutions, for instance, turning ammonia into fertilizer as proposed by Mujah et al. (2016). However, the previous methods require further validations in the field, where their ammonia removal efficiency can be further confirmed.

There are two methods to predict the mechanical behaviors (e.g., strength mobility, dilatancy, and bonding fracture) in biotreated sands: constitutive models (Gai and Sánchez 2019; Xiao et al. 2021e) and the discrete element method (DEM) (Feng et al. 2017; Yang et al. 2017, 2019; Kashizadeh et al. 2021; Zhang et al. 2021a). To date, a few studies have been carried out on constitutive models of biotreated soils, although constitutive models of cement-reinforced soils and structured soils could provide valuable references in this regard (Suebsuk et al. 2011; Nguyen et al. 2014; Rahimi et al. 2016; Ouria 2017; Xiao et al. 2017). For the prediction of monotonic shearing behavior in biotreated sands, Gai and Sánchez (2019) considered CaCO₃ precipitation as a cementation factor in the yield surface, and further proposed a subloading surface-based constitutive model for biotreated sands in the critical state soil mechanics framework by incorporating biocementation degradation during shearing. Their predictions were in good agreement with the observations on the stress–strain relationships for biotreated sands. Xiao et al. (2021e) proposed a thermodynamics-based model for biotreated calcareous sands to capture their monotonic and cyclic shearing behaviors. The true c, stress-induced anisotropy, and bond structure were considered in a hyperelastic relation that was further coupled with thermodynamic plasticity by incorporating the concepts of granular configuration entropy. In addition, the increased density, c strength, and bond structure due to biotreatment and their evolution with the breakage of biobonds were considered in this model. Therefore, the stress path evolution, stress–strain relationship, and development in biotreated calcareous sand were predicted well using this model. As mentioned previously, the traditional grouting method in biotreatment will lead to a nonuniform distribution of CaCO₃ (Feng and Montoya 2016; Lin et al. 2016a; Nafisi et al. 2019, 2020), which might affect the shearing responses of the biotreated sand specimens during triaxial tests. For example, the shear–band appeared at a small value of axial strain, which led to difficulty in determining the critical state of biotreated sands (Feng and Montoya 2016), and the stress–strain curve of the biotreated specimens might show step-type declines or irregular fluctuations (Nafisi et al. 2020). In addition, the critical state theory for cemented soil assumed that its critical state is the same as that of the untreated soil, which indicated that the cementation was completely lost at the critical state. However, the cementation in a biotreated specimen was not completely broken when it was sheared monotonically to a large strain. Some CaCO₃ precipitates still adhered to the surface of the sand particles after shearing, which led to a higher critical state strength than that of the host sand (O’Donnell and Kavazanjian 2015; Feng and Montoya 2016; Lin et al. 2016a; Cui et al. 2017; O’Donnell et al. 2017b; Xiao et al. 2019e). Of note, the crushed or abraded CaCO₃ precipitates during triaxial testing generated fine particles that could fill the void between sand particles or make a certain contribution to support the soil skeleton. Different shapes of these fine CaCO₃ fragments have different effects on the overall mechanical properties, for example, the changes in critical state lines in the stress and compression planes, according to previous studies (Yang and Wei 2012; Wei and Yang 2014; Xiao et al. 2019c; Nguyen et al. 2021). These phenomena, which were not considered in the previous models, led to a great challenge when establishing a more comprehensive constitutive model to predict the mechanical behaviors of biotreated soils. Yang et al. (2017) conducted three-dimensional DEM simulations to analyze the drained triaxial shear responses in biotreated sands. The effects of five microscale parameters for the DEM model, that is, two elastic parameters and three rupture parameters, were evaluated by parametric analyses. The interparticle ϕ had the greatest influence on the mechanical responses. Feng et al. (2017) explored the bond breakage pattern and the formation of shear bands through microstructure analyses when the macroscopic responses of their DEM simulations were in good agreement with the test results of biotreated sands that were performed by Feng and Montoya (2016). However, heterogeneous distributions of CaCO₃ in sand specimens can lead to heterogeneous bonds in sand grains, which was not considered in the previous DEM models.

This study summarized the strength characteristic of biotreated soils under monotonic and cyclic loadings from previous publications. Based on the results from these publications, biotreatment shows great promise when improving the mechanical behaviors of grained soils, and fine and medium-sized grained soils could be more effectively treated with MICP. In general, the UCS and STS of grained soils could be enhanced with biotreatment, and they increased with an increase in the CaCO₃ precipitation. The UCS and STS were affected by the distribution of CaCO₃ in grained soil specimens, and the addition of fiber or polymer could effectively improve the UCS and STS of biotreated soils. However, adding more fiber could not further improve the strength when the fiber content was larger than a critical value. In addition, biotreatment could reduce the compressibility of sands, which could be attributed to the restraint of particle breakage that was caused by the precipitation of CaCO₃ between sand particles. The compression of biotreated sand can be divided into three stages: (1) abrasion or attrition at the interface between CaCO₃ and the sand grains; (2) breakage of CaCO₃ bonding; and (3) breakage of sand particles. In addition, the shear behaviors of biotreated soils were investigated by undrained and drained triaxial tests and direct shear tests. The common finding from these publications was that the stress–strain relationships changed from strain hardening to strain softening as m_c increased. The shear strength of biotreated specimens was much higher than that of the untreated ones. The effective c of the specimen showed an increase with an increase in m_c, and the peak state ϕ might increase, remain unchanged, or even decrease as m_c increased, where the role of CaCO₃ precipitation changed from interparticle bonding to particle roughing and densifying due to biobond breakage during shearing. Furthermore, another common finding was that the MICP technology showed great potential for mitigating liquefaction. The development of uin biotreated sands is different from that of untreated sands with the same density, and the heavily biotreated loose sands resembled the undrained cyclic characteristics of untreated dense sands. In addition, the improvement in the dynamic strength of loose sands was more effective than that of dense sands, and the biotreated sands were more resistant to reliquefaction than natural sands; however, sands with nonplastic fines could hinder the formation of effective precipitates between soil grains. Of interest, the shear wave velocity and cone penetration resistance were adopted in shake table tests and centrifuge tests, which showed the potential to evaluate the biotreatment effect. Finally, the challenges and further perspectives for biocementation were proposed in this study. The problems in biotreated soils, such as the nonuniformity of CaCO₃ precipitation, the not ecofriendly by-products, and the low economic efficiency, cannot be fully solved currently, although efforts have been made on these issues. In addition, heterogeneous CaCO₃ precipitation and localized shear bands led to more difficulties in constitutive modeling and numerical simulations for the complex stress–strain relationships in biotreated grained soils. More large-scale field applications of MICP are expected in the future, and a key concern from the biological and geotechnical perspectives is to improve the cost and cementation efficiencies in these applications of MICP.

All data, models, and codes generated or used during this study that support the main findings and further perspectives appear in the published article.

The authors would like to acknowledge the financial support from the National Science Foundation of China (Grant Nos. 52078085, 41831282, and 51922024), and the Natural Science Foundation of Chongqing, China (Grant No. cstc2019jcyjjqX0014). The authors would like to thank Hanlong Liu, Huanran Wu, Jinquan Shi, Jinxuan Zhang, Hao Cui, Jian Hu, Yue Sun, Wentao Xiao, Ninghao Wang, Jun Li, Peng Zhou, Haotian Guo, and Yu Zhang for their great help and support when the authors prepared the paper.