Bio-cementation for tidal erosion resistance improvement of foreshore slopes based on microbially induced magnesium and calcium precipitation

In most coastal and estuarine areas,tides easily cause surface erosion and even slope failure,resulting in severe land losses,deterioration of coastal infrastructure,and increased floods.The bio-cementation technique has been previously demonstrated to effectively improve the erosion resistance of slopes.Seawater contains magnesium ions(Mg^(2+))with a higher concentration than calcium ions(Ca^(2+));therefore,Mg^(2+)and Ca^(2+)were used together for bio-cementation in this study at various Mg^(2+)/Ca^(2+)ratios as the microbially induced magnesium and calcium precipitation(MIMCP)treatment.Slope angles,surface strengths,precipitation contents,major phases,and microscopic characteristics of precipitation were used to evaluate the treatment effects.Results showed that the MIMCP treatment markedly enhanced the erosion resistance of slopes.Decreased Mg^(2+)/Ca^(2+)ratios resulted in a smaller change in angles and fewer soil losses,especially the Mg^(2+)concentration below 0.2 M.The decreased Mg^(2+)/Ca^(2+)ratio achieved increased precipitation contents,which contributed to better erosion resistance and higher surface strengths.Additionally,the production of aragonite would benefit from elevated Mg^(2+)concentrations and a higher Ca^(2+)concentration led to more nesquehonite in magnesium precipitation crystals.The slopes with an initial angle of 53°had worse erosion resistance than the slopes with an initial angle of 35°,but the Mg^(2+)/Ca^(2+)ratios of 0.2:0.8,0.1:0.9,and 0:1.0 were effective for both slope stabilization and erosion mitigation to a great extent.The results are of great significance for the application of MIMCP to improve erosion resistance of foreshore slopes and the MIMCP technique has promising application potential in marine engineering.

Recent development on optimization of bio-cementation for soil stabilization and wind erosion control

This paper reviews and analyzes recent research development on bio-cementation for soil stabilization and wind erosion control.Bio-cement is a type of cementitious materials by adopting natural biological processes for geotechnical and construction applications.Bio-cementation is usually achieved through microbially-or en-zymeinduced carbonate precipitation(MICP or EICP).The use of soybean urease can be a cost-effective solution for carbonate precipitation and bio-cementation,which is named SICP.The produced calcium carbonate can cement soil particles and bring considerable strength improvement to soils.In this paper,the mechanisms and recent development on the technology optimization are reviewed first.The optimization of bio-cementation involves 1)altering the treatment materials and procedures such as using lysed cells,low pH,the salting-out technique;and 2)using cheap and waste materials for bio-cement treatment and bacterial cultivation.The objectives are to improve treatment uniformity and efficiency,use bio-cement in more scenarios such as finegrain soils,and reduce costs and environmental impacts,etc.Studies on the mechanical behaviour and wind erosion performances of bio-cemented soil show that the wind erosion resistance can be improved significantly through the bio-cement treatment.In addition,the use of optimized method and additives such as xanthan gum and fibers can further enhance the strength,treatment uniformity or ductility of the bio-cemented soils.Attention should be paid to wind forces with saltating particles which have much stronger destructive effect than pure wind,which should be considered in laboratory tests.Field studies indicate that bio-cement can improve soil surface strength and wind erosion resistances effectively.Besides,local plants can germinate and grow on bio-cemented soil ground with low-concentration treatments.

Compressibility characteristics of bio-cemented calcareous sand treated through the bio-stimulation approach

Calcareous sand is widely present in coastal areas around the world and is usually considered as a weak and unstable material due to its high compressibility and low strength.Microbial-induced calcium carbonate precipitation(MICP)is a promising technique for soil improvement.However,the commonly adopted bio-augmented MICP approach is in general less compatible with the natural soil environment.Thus,this study focuses on the bio-stimulated MICP approach,which is likely to enhance the dominance of ureolytic bacteria for longer period and thus is deemed more efficient.The main objective of this paper is to investigate the compressibility of calcareous sand treated by bio-stimulated MICP approach.In the current study,a series of one-dimension compression tests was conducted on bio-cemented sand pre-pared via bio-stimulation with different initial relative densities(D r).Based on the obtained compression curves and particle size distribution(PSD)curves,the parameters including cementation content,the coefficient of compressibility(a v),PSD,relative breakage(B r),and relative agglomeration(A r)were discussed.The results showed that a v decreased with the increasing cementation content.The bio-cemented sand prepared with higher initial D r had smaller(approximately 20%e70%)a v values than that with lower initial D r.The specimen with higher initial D r and higher cementation content resulted in smaller B r but larger A r.Finally,a conceptual framework featuring multiple contact and damage modes was proposed.

Cementation of Loose Sand Particles based on Bio-cement

Loose sand particles could be cemented to sandstone by bio-cement（microbial induced magnesium carbonate）. The bio-sandstone was firstly prepared, and then the compressive strength and the porosity of the sandstone cemented by microbial induced magnesium carbonate were tested to characterize the cementation effectiveness. In addition, the formed mineral composition and the microstructure of bio-sandstone were analyzed by X-ray diffraction（XRD） and scanning electron microscopy（SEM）, respectively. The experimental results show that the feasibility of binding loose sand particles using microbial induced magnesium carbonate precipitation is available and the acquired compressive strength of bio-sandstone can be excellent at certain ages. Moreover, the compressive strength and the porosity could be improved with the increase of microbial induced magnesium carbonate content. XRD results indicate that the morphology of magnesium carbonate induced by microbe appears as needles and SEM results show that the cementation of loose sand particles to sandstone mainly relies on the microbial induced formation of magnesium carbonate precipitation around individual particles and at particle-particle contacts.

Unconfined compressive strength of MICP and EICP treated sands subjected to cycles of wetting-drying,freezing-thawing and elevated temperature:Experimental and EPR modelling

Microbial-induced carbonate precipitation(MICP)and enzyme-induced carbonate precipitation(EICP)are two bio-cementation techniques,which are relatively new methods of ground improvement.While both techniques share some similarities,they can exhibit different overall behaviours due to the differences in urease enzyme sources and treatment methods.This paper presented 40 unconfined compressive strength(UCS)tests of MICP and EICP treated sand specimens with similar average calcium carbonate(CaCO3)content subjected to cycles of wetting-drying(WD),freezing-thawing(FT)and elevated temperature(fire resistance test e FR and thermogravimetric analysis e TG).The average CaCO3 content after a certain number of WD or FT cycles(ACn)and their corresponding UCS(qn)reduced while the mass loss increased.The EICP treated sand specimens appeared to exhibit a lower resistance to WD and FT cycles than MICP treated specimens possibly due to the presence of unbonded or loosely bonded CaCO3 within the soil matrix,which was subsequently removed during the wetting(during WD)or thawing(during FT)process.FR test and TG analysis showed a significant loss of mass and reduction in CaCO3 content with increased temperatures,possibly due to the thermal decomposition of CaCO3.A complete deterioration of the MICP and EICP treated sand specimens was observed for temperatures above 600C.The observed behaviours are complex and theoretical understanding is far behind to develop a constitutive model to predict qn.Therefore,a multi-objective evolutionary genetic algorithm(GA)that deals with pseudo-polynomial structures,known as evolutionary polynomial regression(EPR),was used to seek three choices from millions of polynomial models.The best EPR model produced an excellent prediction of qn with a minimum sum of squares error(SSE)of 2.392,mean squared error(MSE)of 0.075,root mean square error(RMSE)of 0.273 and a maximum coefficient of determination of 0.939.

Cementor:A toolbox to generate bio-cemented soils with specific microstructures

Bio-cemented soils can exhibit various types of microstructure depending on the relative position of the carbonate crystals with respect to the host granular skeleton.Different microstructures can have different effects on the mechanical and hydraulic responses of the material,hence it is important to develop the capacity to model these microstructures.The discrete element method(DEM)is a powerful numerical method for studying the mechanical behaviour of granular materials considering grain-scale features.This paper presents a toolbox that can be used to generate 3D DEM samples of bio-cemented soils with specific microstructures.It provides the flexibility of modelling bio-cemented soils with precipitates in the form of contact cementing,grain bridging and coating,and combinations of these distribution patterns.The algorithm is described in detail in this paper,and the impact of the precipitated carbonates on the soil microstructure is evaluated.The results indicate that carbonates precipitated in different distribution patterns affect the soil microstructure differently,suggesting the importance of modelling the microstructure of bio-cemented soils.

Experimental Study on Urease Activity and Cementation Characteristics of Soybean

A new and more ecologically sound cementing material known as“bio-cement”has been found to have the capacity to consolidate loose gravel into sand columns offering a certain degree of strength,and to fill and repair cracks in concrete to restore resilience.The typical representative is the microbial induced calcium carbonate deposition technology(MICP)and enzyme induced calcite precipitation(EICP).As part of this research,EICP with soybean urease as the core was studied.The test results show that soybean urease activity is significantly affected by pH and urea concentration values,while the external nickel source is not found to impair a stimulating effect on activity.When the concrete specimens were immersed in the composite solution of soybean urease,urea,and calcium chloride after having been subjected to a high temperature,a continuous layer of white precipitations quickly appeared on the surface of the specimens.Measured using a metalloscope,the thickness of the precipitations was found to reach up to 2.0 mm,while the surface water absorption rate was reduced by 70%.The effects of this combined outcome are believed to significantly protect and improve the durability of the concrete specimens previously subjected to a high temperature.At the same time,the composite solution is shown to be capable of cementing fly ash,with the cubic strength of the finished samples reaching 4.0 MPa after 3 days.Results from the use of a scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),and X-ray diffraction(XRD),reveal that both the white precipitations on the surface of the concrete specimens and the cement binding the fly ash particles are calcite crystals.It is concluded from these preliminary study results that the use of soybean urease as a bio-cement had proved successful.

Mitigation of soil liquefaction using microbial technology:An overview

Soil liquefaction is a major geo-hazard.As liquefaction could occur anywhere in a sand layer and result in large-scale lateral spreading,treatment for liquefaction needs to be carried out over a large extent.The cost-effectiveness of the treatment then becomes a major consideration.With the development of microbial geotechnologies,some new approaches for liquefaction mitigation have been developed.Some of the methods offer more advantages over the existing methods.This paper gives an overview of the recent progress in bio related soil liquefaction mitigation methods.These include both bio-cementation and biogas desaturation.The mechanisms of bio-cementation and biogas desaturation are discussed.Recent up-scaled model tests and field trials are also reviewed.The studies so far have demonstrated that there is a great potential for some of liquefaction mitigation methods to be adopted in practice,although there are still challenges that need to be studied further.These include treatment efficiency,long-term sustainability,and biosafety.A brief introduction to some emerging technologies for liquefaction mitigation such as bio-gelation and use of fungi are also introduced.

Microbially induced carbonate precipitation(MICP)for soil strengthening:A comprehensive review

Geotechnical research has been yearning for revolutionary innovations that could bring breakthroughs to conventional practices,especially at a time when energy efficiency and environmental sustainability are of unprecedented importance in the field.Recently,exciting opportunities emerged utilising microorganisms,the ubiquitous soil dwellers,to provide solutions to many geotechnical problems,prompting the development of the new,multidisciplinary subject of biogeotechnics.Research interest has been centred on the use of microbially induced carbonate precipitation(MICP)to improve the engineering properties of soils.The present work aims to comprehensively review the progress of more than a decade of research on the application of MICP in soil strengthening.Through elucidation of underlying mechanisms,compilation and interpretation of experimental findings,and in-depth discussion on pivotal aspects,with reference made to key published studies,a holistic picture of the state of the art of MICP-based soil strengthening is drawn.Current knowledge gaps are identified,and suggestions for future research are given,along with the opportunities and challenges that lie ahead of practically implementing this technique in real-world geotechnical applications.