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Landfill cover systems should exhibit low gas permeability to minimize the overflow of greenhouse gases and subsequent air pollution. Microbially induced carbonate precipitation (MICP), a biocementation technique, has been applied for subsurface soil stabilization by improving the shear strength of the soil. However, the impact of MICP on the gas permeability of unsaturated soils remains unknown. Considering the role of biocementation in the modification of soil interpores, this study investigated the feasibility of using the MICP technique to reduce the gas permeability of granite residual soils in response to unsaturated conditions. The biocemented soil samples were prepared by mixing soils with MICP chemical solutions at different chemical concentrations. Water retention tests and measurements of gas permeability were performed, in which suction, volumetric water content and gas permeability were continuously monitored. Additionally, energy-dispersive X-ray spectroscopy and X-ray diffraction analyses were performed to investigate the formation of CaCO3 precipitates; scanning electron microscopy was used to study the impact of MICP on the soil interpore structure, and the acid washing method was used to determine the CaCO3 content. The results showed that soils treated with higher concentrations of MICP chemical solutions had higher air entry pressures and residual water contents. This indicates the improvement of water retention due to the presence of MICP, which increases the microstructural porosity and enhances the capillarity, as observed via microscopy. Furthermore, the results revealed that biocementation significantly reduced the gas permeability of the soil and that the change in the maximum gas permeability strongly correlated with the MICP chemical solution concentration and the CaCO3 content. This study highlights the role of MICP in soil–water–air interface studies and the potential application of this biocementation technique to minimizing gas emission issues in landfill cover systems. [ABSTRACT FROM AUTHOR]

A tank test was performed simulating two-dimensional planar flow conditions at a meter scale to evaluate the effectiveness of microbial-induced desaturation and precipitation (MIDP) in stratified soil conditions. The tank setup (116.5 cm tall, 122 cm wide, and 5.25 cm thick) was filled with two layers of fine sand (a target layer of 40 cm and nontarget layer of 21 cm above) that were confined by silt layers above (6 cm), between (9 cm) and below (9 cm) the sand layers. Multiple flushes of substrate solution, containing calcium, nitrate, and acetate, were injected into the lower sand layer to stimulate indigenous nitrate-reducing bacteria to produce biogenic gas, biominerals, and biomass. Embedded sensors were used to measure the changes in electrical conductivity, volumetric water content, and pore pressure in both the target and nontarget sand layers during and between treatment cycles. Time-lapse camera images were used to determine flow velocity distributions in the target layer and identify modes of gas migration. At the end of the test, hydraulic conductivity, calcium carbonate content, and soil–water characteristic curves (SWCCs) were measured on intact samples of the treated material. The results showed that most of the reaction products were formed in the targeted sand layer. During the first treatment cycle, the degree of saturation in the target sand layer decreased to 75% within 5–12 days, at which point it started to migrate upwards until it got trapped and formed a lens underneath the silt layer above. During the second and subsequent treatment cycles, seepage velocity increased due to the entrapment of biogenic gas, the reaction rate increased due to the accumulation of biomass, and the gas formed channels through the silt and migrated further upwards into and through the upper sand and silt layers by irregular venting events. After 4–5 cycles, an equilibrium condition was reached at which the degree of saturation fluctuated from 65 to 80% when gas was being produced and vented to 80–90% when substrates were depleted. The CaCO3 content after 10 cycles over 12 weeks ranged from 1.6% close to the inlet to 0.5% close to the outlet, with an average of 0.68%. The formation of biomass and CaCO3 had a relatively large impact on the saturated hydraulic conductivity but a very limited impact on the SWCC. [ABSTRACT FROM AUTHOR]

Microbially induced calcium carbonate precipitation (MICP) technology is an emerging and environmentally sustainable method for improving the strength and stiffness of soil. Specifically, this innovative approach has gained favor in marine engineering due to the advantaged compatibility between precipitated calcium carbonate induced by MICP and coral sand. Sand containing fines is susceptible to liquefy. Whereas, the impact of fines contents on cyclic behavior of MICP-treated calcareous sand remains uncertain. Consequently, this technical note aims to investigate the liquefaction behavior of biocemented calcareous silty sand by conducting undrained cyclic triaxial shear tests and microscopic analysis. The results revealed the patterns of the excess pore water pressure curves and cyclic deformation characteristics as the fines contents increased. The liquefaction resistance of biocemented sand initially decreases with the addition of fines but subsequently exhibits an increasing trend. Microscopic analysis showed that at the cementation level with the cementation solution concentration of 1 mol/L, the calcium carbonate crystals are mainly attached to the surface of sand grains and this pattern does not directly affect the force chain. [ABSTRACT FROM AUTHOR]

Microbially induced carbonate precipitation (MICP) is a potential method for ground improvement. It can enhance the physical and mechanical properties of soils through soil particle cementation and pore filling. However, the excessive number of injections of cementing solution required for MICP is a major limitation for practical engineering application. To reduce the number of injections and improve the efficiency of the cementing process, an enhancement method was investigated in this study in which an aluminum ion flocculant (AIF) was added to the cementing solution. It can enhance the curing rate and effect of MICP. Experiments were carried out on MICP-treated sand columns, MICP-treated glass beads and aqueous solutions, and the influence of AIF on the production of calcium carbonate and unconfined compressive strength (UCS) was studied. The effect of AIF on the composition and morphology of the deposited calcium carbonate was evaluated from SEM micrographs and other microscopic observations. The results of the study showed that the addition of AIF to the cementing solution significantly reduced the number of injections required and increased the UCS of the reinforcement compared to those of a control group without adding AIF. The proposed method resulted in the experimental sand column being reinforced after 3 treatments of the cementing solution with an UCS of 323.3 kPa. The UCS reaches 1692 kPa after 5 treatments of the cementing solution, compared to 9 treatments using the conventional method. The UCS was 3.2–12.7 times higher than that of the control group for the same calcium carbonate content. [ABSTRACT FROM AUTHOR]

Bio-cementation through microbially induced calcite precipitation (MICP) has the potential to overcome several technical and environmental limitations of conventional cement-based soil improvement techniques. While a significant amount of research has been directed towards better understanding and controlling MICP processes, there is still a lack of multiphysical formulations that can be used for the design of real geotechnical applications in which both the treatment extent and its strength and deformability need to be evaluated. This paper presents the development and application of a comprehensive bio-chemo-hydro-mechanical model that can be used for designing MICP treatments with the finite element method. To overcome the limitations of current approaches based on elasticity, the formulation involves an elastoplastic constitutive model based on Mohr–Coulomb that can predict the strength increase of MICP-improved soils. The model can easily be calibrated with existing experimental results. The scope of model application is demonstrated through the case of a 2D shallow foundation strengthening. Results reveal that the questions of what level of cementation to target and how to distribute cementation efficiently are of equal importance to ultimately serve the needs of specific geotechnical problems, such as those of bearing capacity. [ABSTRACT FROM AUTHOR]

In recent years, microbial mineralization has aroused attention in soil reinforcement. However, most studies focus on soil and coast on land, and the consolidation of seafloor sediments is rarely reported. In this paper, the hydrate reservoir sediments in different sea areas are strengthened, which provides a new method to solve the formation settlement problem in hydrate exploitation. In this study, the microorganisms were cultivated using both fresh and seawater, and hydrate sediments of varying particle sizes (fine sand and silty sand) were consolidated. Triaxial tests and calcium carbonate content tests were conducted to characterize the consolidation effect and detect calcium carbonate content. X-ray diffraction was used to analyze the crystal form and mineral composition of calcium carbonate generated. SEM was employed to observe the microscopic characteristics of the consolidated samples. X-ray computer tomography (X-ray CT) was utilized to analyze changes in pore throat size and the distribution of calcium carbonate in different samples and environments. The experimental results indicate that the consolidated samples exhibit higher strength, particularly in a seawater environment. Fine sand sediment samples primarily demonstrate increased cohesion, from 3.7 kPa initially to 42.3 and 86.8 kPa after consolidation, with the friction angle increasing by less than 2°. While silty sand sediment samples exhibit a greater increase in friction angle after cementation, from 22 to 24.9° and 27.6°, and the cohesion increased by only about 6%. Additionally, it was discovered that the increase in sample strength is not only related to the calcium carbonate content but also to the crystal form and distribution of calcium carbonate within the samples. Under identical sample conditions, those treated in a seawater environment exhibit a more uniform distribution of calcium carbonate and a greater abundance of calcium carbonate crystal forms, such as calcite and vaterite. Furthermore, after consolidation, among the samples treated with fresh water, the porosity of the fine sand sample decreased from 46.38 to 18.26%, and that of the silty sand sample decreased by 25.62%, indicating that fine sand and silty sand samples still possess connecting pores that are not completely obstructed by calcium carbonate. This provides a pathway for improving grouting cycles and gas release following hydrate decomposition. [ABSTRACT FROM AUTHOR]

Particle size gradation is an important parameter for small strain stiffness of sand, and it can also affect the biocementation behavior. In this study, a series of isotropic consolidation tests were performed to study the gradation-dependent small strain shear modulus of a glass sand with light biocementation. Shear wave velocities in multi-directions were measured with bender elements. The test results showed that the small strain shear modulus G0 and stiffness anisotropy decrease and increase with the increase in uniformity coefficient Cu, respectively, for the uncemented sand. When sands were biocemented, the decrement of G0 gradually vanishes with biocementation level, showing that G0 increases with Cu. The development of G0 ratios between the biocemented and the uncemented sands generally experiences four stages. The stiffness anisotropy is also changed with biocementation, showing the decrease in stiffness ratio especially for the unloading stages. The changes of stiffness anisotropy are more explicit for the sands with higher Cu and biocementation level. [ABSTRACT FROM AUTHOR]

Granite residual soil is vulnerable to collapse under rainfall-induced erosion. This may in turn lead to the occurrence of landslides or debris flows on slopes. Previous studies on use of microbial-induced carbonate precipitation (MICP) technique for soil stabilization are often conducted on sandy soils and also with rainfall-induced erosion are rarely assessed for a slope treated with MICP. This study investigated the feasibility of using the MICP technique for surface protection of granite residual soil slopes against erosion. The MICP technique was applied to soil samples using the spraying method. Hydraulic conductivity and rainfall erosion tests (using flume) were conducted to assess the coating effects of MICP on granite residual soils. In addition, crust thickness, calcite content and near-surface strength were measured to interpret the results. Tests were repeated to assess any variability in results. Scanning electron microscopy, energy-dispersive spectrometer and X-ray diffraction (XRD) were conducted on treated and untreated samples to interpret the formation of calcite due to MICP. After 7 days of curing, the calcite content increases to 4.3%, whereas mean coating thickness is 4.2 mm. Unconfined compressive strength is increased by 20.3% as compared with the bare soil. MICP treatment reduced the soil hydraulic conductivity and erosion rate by 90.9% and 95.2%, respectively. This was attributed to the bio-cementation process generating a surface coating on granite residual soils, leading to a reduction in pore throats, as observed in the obtained micrographs. Compared to the bare soil, the runoff rate in the MICP-treated soil was increased by 39.4% on average. However, the erosion is found to reduce significantly in MICP-treated soil. Based on ANOVA analysis, it could be concluded that the rainfall intensity found to be a significant factor affecting the erosion rate of granite residual soil slopes. [ABSTRACT FROM AUTHOR]

Microbially induced carbonate precipitation (MICP) involves a series of bio-geochemical reactions whereby microbes alter the surrounding aqueous environment and induce calcium carbonate precipitation. MICP has a broad range of applications, including in situ soil stabilization. However, the reliability of this process is dependent on a number of environmental conditions. In particular, bacterial growth, urease activity and precipitation kinetics all depend on temperature. Batch test and microfluidic chip experiments were performed in this study to investigate the effects of temperature on bacterial density and activity and the MICP processes occurring at different temperatures (4–50 °C). Spatial and temporal variations in the formation and development of calcium carbonate precipitates, including their amount, type, growth rate, formation and deformation characteristics, were monitored. Results show that different types of calcium carbonate precipitates with varying sizes and quantities were produced by varying the temperature. Low temperature (4 °C) did not reduce urease activity, but limited the final amount of cementation; low temperature reduced bacterial growth and attachment ratio, as well as calcium carbonate precipitation rate. High-temperature (50 °C) conditions significantly reduced urease activity within a short period of time, while a repeated injection of bacteria before every two injections of cementation solution increased the final amount of cementation. The findings made from this paper provide insight into how MICP processes vary across a range of temperatures and could be valuable for optimizing the MICP process for different applications. [ABSTRACT FROM AUTHOR]

The failure mechanism of biocemented sand has been largely speculated based on scanning electron microscopy images of biocemented sand after mechanical tests. However, some breakage patterns cannot be directly observed, such as the separation of CaCO3 crystals and the separation of CaCO3 crystals and sand surfaces. A study of the breakage process of biocemented sand at microscale is essential for understanding the mechanism of biocementation. In this work for the first time, a series of in situ splitting tensile tests were conducted using a tensile & compression module installed inside an SEM machine so that the breakage process of a biocemented sample under loading could be observed directly. The results showed that no breakage was observed before peak stress indicating an elastic–plastic deformation stage. Once the peak stress was reached, the stress fluctuated around the peak stress. At the end of the tests, two kinds of breakage of biocement were observed, i.e., calcite-calcite breakage and calcite-silica breakage. The detailed breakage seems also affected by the form of CaCO3. Furthermore, besides increasing active precipitates, the improvements in the strength of biocement and the bonding strength between biocement and the sand surface were suggested to improve the performance of MICP for stabilizing sand. The findings of microscale in situ observation during mechanical tests can also provide basic evidence for modeling debonding process of biotreated soils using constitutive models or numerical simulation methods. [ABSTRACT FROM AUTHOR]

A coupled bio-chemo-hydro-mechanical model (BCHM) is developed to investigate the permeability reduction and stiffness improvement in soil by microbially induced calcite precipitation (MICP). Specifically, in our model based on the geometric method a link between the micro- and macroscopic features is generated. This allows the model to capture the macroscopic material property changes caused by variations in the microstructure during MICP. The developed model was calibrated and validated with the experimental data from different literature sources. Besides, the model was applied in a scenario simulation to predict the hydro-mechanical response of MICP-soil under continuous biochemical, hydraulic and mechanical treatments. Our modelling study indicates that for a reasonable prediction of the permeability reduction and stiffness improvement by MICP in both space and time, the coupled BCHM processes and the influences from the microstructural aspects should be considered. Due to its capability to capture the dynamic BCHM interactions in flexible settings, this model could potentially be adopted as a designing tool for real MICP applications. [ABSTRACT FROM AUTHOR]

The use of biopolymer to improve the performance of microbially induced carbonate precipitation (MICP)-treated sands is a novel and eco-friendly concept. This work found an anionic biopolymer, γ-polyglutamate (γ-PGA), could significantly improve the performance of MICP-treated sands. Comparing the control with absence of γ-PGA, the concentration of 0.1–9 g/L γ-PGA increased the compressive strength of MICP-treated sands by 1.54–3.96 times and significantly reduced the brittleness. The MICP process analysis and microstructural detection demonstrated that γ-PGA in the specimens provided many nucleation sites and templates for calcite generation, partially kept the bacterial urease activity by replacement of the bacteria as nucleation sites, thereby improving the calcite generation. The γ-PGA also cemented sand grains with calcite through the hydrogen bond-type intermolecular interactions. Both the calcite generation and the hydrogen bond-type intermolecular interactions by γ-PGA played vital roles in enhancing MICP for soil improvement. Additionally, γ-PGA, as a viscoelastic admixture between the crystals and sand grains, effectively dissipated the energy of stress and thus reduced the brittleness of MICP-treated sands. This is the first report on the application of anionic biopolymer to MICP technology. It provides a novel concept in promoting the efficiency and sustainability of MICP. [ABSTRACT FROM AUTHOR]

Microbially induced calcium carbonate precipitation (MICP) is a novel and potentially sustainable bio-mediated soil stabilization technology for geotechnical engineering applications, which can be promoted by many processes, including denitrification and urea hydrolysis. Compared to urea hydrolysis-based MICP, no ammonium is produced in denitrification-based MICP, but the amount of precipitation induced by the conventionally adopted methods (semi-stagnant column and continuous flow) reported in the early studies is small. This paper presented a large-volume circulation method to induce calcium carbonate precipitation into the sands via denitrification. The sands were treated with denitrification to different levels, and a series of consolidated drained (CD) triaxial tests were conducted after the treated sands were fully saturated to investigate the mechanical response. The results show that the peak drained strength and dilatancy are all improved by the denitrification-based MICP for samples treated. The cohesion and peak strength effective friction angle can be improved by 22 kPa and 3° with 4.48% calcium carbonate contents for denitrification-based MICP. However, the critical state line of soil cannot be shifted. The resulting calcium carbonate precipitation amount and rate can be as high as 4.61% by weight and 0.129% per day. The precipitation rate is increased by at least 5 times compared to the conventional adopted methods for denitrification-based MICP. In addition to CaCO3 precipitation, biofilms were also produced during denitrification. The high conversion ratio and precipitation-substrates ratio also indicate the high efficiency of this method. [ABSTRACT FROM AUTHOR]

Bio-mediated techniques have the potential to be an eco-friendly and sustainable solution for engineering problems in the presence of unfavorable soil conditions. During the microbial induced carbonate precipitation (MICP) process, calcium carbonate is the byproduct of a series of biological and chemical reactions in the soil media. Although the shear response of MICP-treated sands with different calcium carbonate content has been extensively investigated, the behavior of this material subjected to varying stress paths with different levels of cementation and particle sizes is still unknown. In this study, the material behavior of MICP-treated sands under axisymmetric compression, radial extension, constant p', and constant q stress paths at moderate and heavy level of cementation is evaluated by conducting drained triaxial tests on specimens with relative density of about 40%. Shear wave velocity was measured during the course of treatment and shearing to monitor cementation and degradation processes. In addition, the effect of stress relaxation and compression after bio-treatment on shear response is evaluated. A previously proposed nonlinear failure envelope for MICP-treated sands is also verified by comparing the shear and normal stresses at failure with those predicted by the nonlinear failure envelope. [ABSTRACT FROM AUTHOR]

A homogeneous microfluidic chip was used to investigate the pore-scale characteristics during the process of microbially induced calcium carbonate precipitation (MICP). An image-processing scheme was developed to measure the projecting areas of the precipitated calcium carbonate. Calcium carbonate first precipitated on the bacterium side before spreading to the rest of the chip. The distribution of calcium carbonate was more uniform along the length of the microchip than along the width. Raman back-scattering spectroscopy was used to examine the chemical composition of the precipitate, identifying calcite and vaterite as the main mineral phases. Bacterium traces were noted on crystal surfaces in SEM images, suggesting a higher adsorptive capacity for irregular precipitates than well-shaped crystals. [ABSTRACT FROM AUTHOR]

Microbially induced carbonate precipitation (MICP) has been actively investigated as a promising method to improve soil properties. A burning issue impeding its wide application is the severe spatial inhomogeneity of the CaCO3 distribution. Inspiring by the temperature sensitivity of the bacteria activity, a temperature-controlled one-phase MICP method is proposed consisting of two major steps: (1) grouting the specimen with the mixture of cementation and bacteria solutions in a low temperature; (2) inducing CaCO3 precipitation by exposing the specimen to room temperature. A series of experiments are conducted to demonstrate the advantages of the proposed method over the normal two-phase MICP method. Specimens treated with the proposed temperature-controlled method present higher CaCO3 contents with a roughly uniform distribution along the height of the specimen; the strength of those specimens are substantially improved with apparent dilatancy due to the effective bond network formed by the homogeneously distributed CaCO3 precipitation. SEM images indicate that the temperature-controlled method tends to form small crystals distributing uniformly on the grain surface, which may increase the roughness of the grain and the residual stress more effectively. [ABSTRACT FROM AUTHOR]

One-dimensional compression tests on quartz sands treated by microbially induced carbonate precipitation (MICP) were carried out to evaluate the effects of gradation and calcium carbonate (CaCO3) content on compression behaviors. The experimental results reveal that the compressibility of specimens increases with increasing coefficient of uniformity or decreasing CaCO3 content. The evolution of void ratio with vertical stress could be generally characterized into three stages based on the underlying mechanisms. The initiation of bond breakage occurs around vertical stress of 0.036 MPa, and the dominating mechanism transits to particle breakage around vertical stress of 8.3 MPa. Scanning electron microscope analyses demonstrate that bonding effect and coating effect of CaCO3 precipitation are responsible for the lower compressibility of MICP-treated specimen. The presence of small particles leads to more interparticle CaCO3 bonds whose breakage would still allow the small particles to fill the intercoarse-grain voids. [ABSTRACT FROM AUTHOR]

Animal fibers with α-keratin had obvious advantages of mechanical strength and durability on reinforced microbially induced carbonate precipitation (MICP)-cemented loose sands. Herein, wool fiber, α-keratin-rich animal fiber with high strength, was applied to reinforcement of MICP-cemented loose sands. The mechanical properties and underlying mechanism were experimentally explored. Results showed that adding 0.1 wt% wool fiber enhanced up to 3.34-fold the compressive strength and significantly improved the flexure resistance performance. The microstructural results revealed that wool fibers providing many nucleation sites for calcite precipitation not only significantly decreased the porosity of the specimen, but also effectively generated calcite and distributed fiber-bridging microstructure uniformly, resulting in a marked reinforcement of MICP performance. Further studies indicated that sand grains were cemented with calcite through the intermolecular hydrogen bonds HN–H...O–C(Si). This study first reports the potential and the underlying mechanism of animal fiber on improvement of MICP performance and provides new insight into enhancing mechanical behavior of MICP-cemented loose sands with fibrous proteins. [ABSTRACT FROM AUTHOR]

Due to more extreme weather events and accelerating sea-level rise, coastal sand dunes are subjected to more frequent storm wave inundation and surge impacts, which contribute to widespread coastal erosion problems. In this study, two novel bio-mediated methods, microbial-induced carbonate precipitation (MICP) and enzymatic-induced carbonate precipitation (EICP), were investigated and compared for their effectiveness in mitigating sand dune erosion under wave attack. Small-scale laboratory model tests were performed on MICP-treated, EICP-treated, and untreated sand dunes at dune slope angles and two wave intensities for up to 2 h. The cross-shore profile was captured continuously during the course of the erosion test. The erosion volume above the still water level (SWL) and landward retreat distance at the SWL were calculated based on the captured bed profiles. The results show that both EICP and MICP could substantially reduce sand dune erosion at mild-to-moderate wave and dune slope conditions. However, the effectiveness of MICP treatment deteriorated at steeper dune slopes with longer period of wave attack. Under the most adverse condition (i.e., steepest dune slope, biggest wave, and longest period of wave attack), neither EICP nor MICP could effectively mitigate erosion. Fundamentally, the variable effectiveness of MICP and EICP treatment for sand dune erosion control was attributed to the spatial distribution pattern of formed calcite precipitation, which was determined by the way how EICP and MICP were applied. The calcite precipitation was relatively uniform in EICP-treated sand dunes. In MICP-treated ones, however, substantial calcite precipitation concentrated in the shallow surface layer as confirmed by the surface penetration test and SEM observation. [ABSTRACT FROM AUTHOR]

Permeation grouting using cement is widely used for ground improvement. However, this method can only be used for coarse sand or gravel. To overcome this problem, permeation grouting using biogrout through a microbially induced calcium carbonate precipitation method has also been developed. Biogrout is finer and thus can permeate through fine sand. However, biogrouting method is not efficient for medium or coarse sand. For this reason, different grouting methods or materials may have to be used when the ground conditions are highly variable. It will be more efficient and highly desirable to apply only one grouting method to ground with soils of different grain sizes. In this study, a new biogrouting method using biogrout containing bioslurry is developed to allow soil with grain sizes ranging from fine to coarse sand to be treated efficiently. Bioslurry is a slurry containing preformed urease active calcium carbonate crystals, and biogrout is made of mainly calcium ions, urea and urease-producing bacteria. The testing results on sands with grain sizes ranging from 0.30 to 2.36 mm have shown that the proposed method could be applied to sand of different sizes by varying the solid content in the bioslurry. For medium or coarse sand, biogrout with a high solid content (i.e., 20–40%) can be used, whereas for fine sand, biogrout with a low or zero solid content will work. Furthermore, the effect of grain size, the type of biogrout, and CaCO3 content on permeability and uniaxial compression strength of grouted sand were also investigated. [ABSTRACT FROM AUTHOR]

The mechanical efficiency of the biocementation process is directly related to the microstructural properties of the biocemented sand, such as the volume fraction of calcite, its distribution within the pore space (localized at the contact between grains, over the grain surfaces) and the contact properties: coordination number, contact surface area, contacts orientation and types of contact. In the present work, all these micromechanical properties are computed, for the first time, from 3D images obtained by X-ray tomography of intact biocemented sand samples. The evolution of all these properties with respect to the volume fraction of calcite is analyzed and compared between each other (from untreated sand to highly cemented sand). Whatever the volume fraction of calcite, it is shown that the precipitation of the calcite is localized at the contacts between grains. These results are confirmed by comparing our numerical results with analytical estimates assuming that the granular medium is made of periodic simple cubic arrangements of grains and by considering two extreme cases of precipitation: (1) The calcite is localized at the contact, and (2) the grains are covered by a uniform layer of calcite. In overall, the obtained results show that a small percentage of calcite is sufficient to get a large amount of cohesive contacts. [ABSTRACT FROM AUTHOR]

The effect of grain size distribution on the unconfined compressive strength (UCS) of bio-cemented granular columns is examined. Fine and coarse aggregates were mixed in various percentages to obtain five different grain size distributions. A four-phase percolation strategy was adopted where a bacterial suspension and a cementation solution (urea and calcium chloride) were percolated sequentially. The results show that a gap-graded particle size distribution can improve the UCS of bio-cemented coarser granular materials. A maximum UCS of approximately 575 kPa was achieved with a particle size distribution containing 75% coarse aggregate and 25% fine aggregate. Furthermore, the minimum UCS obtained has applications where mitigation of excessive bulging of stone/sand columns, and possible slumping that might occur during their installation, is needed. The finding also implies that the amount of biochemical treatments can be reduced by adding fine aggregate to coarse aggregate resulting in effective bio-cementation within the pore matrix of the coarse aggregate column as it could substantially reduce the cost associated with bio-cementation process. Scanning electron microscopy results confirm that adding fine aggregate to coarse aggregate provides more bridging contacts (connected by calcium carbonate precipitation) between coarse aggregate particles, and hence, the maximum UCS achieved was not necessarily associated with the maximum calcium carbonate precipitation. [ABSTRACT FROM AUTHOR]

An experimental study has been performed to investigate the effect of the biocalcification process on the microstructural and the physical properties of biocemented Fontainebleau sand samples. The microstructural properties (porosity, volume fraction of calcite, total specific surface area, specific surface area of calcite, etc.) and the physical properties (permeability, effective diffusion) of the biocemented samples were computed for the first time from 3D images with a high-resolution images obtained by X-ray synchrotron microtomography. The evolution of all these properties with respect to the volume fraction of calcite is analysed and compared with success to experimental data, when it is possible. In general, our results point out that all the properties are strongly affected by the biocalcification process. Finally, all these numerical results from 3D images and experimental data were compared to numerical values or analytical estimates computed on idealized microstructures constituted of periodic overlapping and random non-overlapping arrangements of coated spheres. These comparisons show that these simple microstructures are sufficient to capture and to predict the main evolution of both microstructural and physical properties of biocemented sands for the whole range of volume fraction of calcite investigated. [ABSTRACT FROM AUTHOR]