Biomineralization and mineralization using microfluidics:A comparison study

Biomineralization through microbial process has attracted great attention in the field of geotechnical engineering due to its ability to bind granular materials,clog pores,and seal fractures.Although minerals formed by biomineralization are generally the same as that by mineralization,their mechanical behaviors show a significant discrepancy.This study aims to figure out the differences between biomineralization and mineralization processes by visualizing and tracking the formation of minerals using microfluidics.Both biomineralization and mineralization processes occurred in the Y-shaped sandcontaining microchip that mimics the underground sand layers.Images from different areas in the reaction microchannel of microchips were captured to directly compare the distribution of minerals.Crystal size and numbers from different reaction times were measured to quantify the differences between biomineralization and mineralization processes in terms of crystal kinetics.Results showed that the crystals were precipitated in a faster and more uncontrollable manner in the mineralization process than that in the biomineralization process,given that those two processes presented similar precipitation stages.In addition,a more heterogeneous distribution of crystals was observed during the biomineralization process.The precipitation behaviors were further explained by the classical nucleation crystal growth theory.The present microfluidic tests could advance the understanding of biomineralization and provide new insight into the optimization of biocementation technology.

State-of-the-art review of soft computing applications in underground excavations

Soft computing techniques are becoming even more popular and particularly amenable to model the complex behaviors of most geotechnical engineering systems since they have demonstrated superior predictive capacity,compared to the traditional methods.This paper presents an overview of some soft computing techniques as well as their applications in underground excavations.A case study is adopted to compare the predictive performances of soft computing techniques including eXtreme Gradient Boosting(XGBoost),Multivariate Adaptive Regression Splines(MARS),Artificial Neural Networks(ANN),and Support Vector Machine(SVM) in estimating the maximum lateral wall deflection induced by braced excavation.This study also discusses the merits and the limitations of some soft computing techniques,compared with the conventional approaches available.

Grain crushing in geoscience materials-Key issues on crushing response,measurement and modeling:Review and preface

Grain crushing is commonly encountered in deep foundation engineering,high rockfill dam engineering,railway engineering,mining engineering,coastal engineering,petroleum engineering,and other geoscience application.Grain crushing is affected by fundamental soil characteristics,such as their mineral strength,grain size and distribution,grain shape,density and specimen size,and also by external factors including stress magnitude and path,loading rate and duration,degree of saturation,temperature and geochemical environment.Crushable material becomes a series of different materials with the change in its grading during grain crushing,resulting in a decrease in strength and dilatancy and an increase in compressibility.Effects of grain crushing on strength,dilatancy,deformation and failure mechanisms have been extensively investigated through laboratory testing,discrete element method(DEM)modelling,Weibull statistics,and constitutive modelling within the framework of the extended crushing-dependent critical state theory or the energy-based theory.Eleven papers summarized in this review article for this special issue addressed the above issues in grain crushing through the advanced testing and modelling.

Determination of minimum overburden depth for underwater shield tunnel in sands:Comparison between circular and rectangular tunnels

With the development of global urbanization,the utilization of underground space is more critical and attractive for civil purposes.Various shapes of shield tunnels have been gradually proposed to cope with different geological conditions and service purposes of underground structures.Generally,reducing the burial depth of shield tunnel is conducive to construction and cost saving.However,extremely small overburden depth cannot provide sufficient uplift resistance to maintain the stability and serviceability of the tunnel.To this end,this paper firstly reviewed the status of deriving the minimum sand over-burden depth of circular shield tunnel using mechanical equilibrium(ME)method.It revealed that the estimated depth is rather conservative.Then,the uplift resistance mechanism of both circular and rectangular tunnels was deduced theoretically and verified with the model tests.The theoretical uplift resistance is consistent with the experimental values,indicating the feasibility of the proposed equations.Furthermore,the determination of the minimum soil overburden depth of rectangular shield tunnel under various working conditions was presented through integrated ME method,which can provide more reasonable estimations of suggested tunnel burial depth for practical construction.Additionally,optimizations were made for calculating the uplift resistance,and the soil thickness providing uplift resistance is suggested to be adjusted according to the testing results.The results can provide reference for the design and construction of various shapes of shield tunnels in urban underground space exploitation.

Testing and modeling of rockfill materials:A review

The research development of rockfill materials （RFM） was investigated by a series of large-scale triaxial tests. It is observed that confining pressure and particle breakage play important roles in the mechanical property, dilatancy relation and constitutive model of RFM. In addition, it is observed that the conven- tional dilatancy relation and constitutive model are not suitable for RFM due to the complex mechanical behavior. Hence, it needs to propose a unified constitutive model of RFM, considering the statedependent and particle breakage behavior.

Laboratory Model Tests and DEM Simulations of Unloading-Induced Tunnel Failure Mechanism

Tunnel excavation is a complicated loading-unloading-reloading process characterized by decreased radial stresses and increased axial stresses.An approach that considers only loading,is generally used in tunnel model testing.However,this approach is incapable of characterizing the unloading effects induced by excavation on surrounding rocks and hence presents radial and tangential stress paths during the failure process that are different from the actual stress state of tunnels.This paper carried out a comparative analysis using laboratory model testing and particle flow code(PFC2D)-based numerical simulation,and shed light upon the crack propagation process and,microscopic stress and force chain variations during the loading-unloading process.The failure mode observed in the unloading model test is shear failure.The force chains are strongly correlated with the concrete fracture propagation.In addition,the change patterns of the radial and tangential stresses of surrounding rocks in the broken region,as well as the influence of the initial stress on failure loads are revealed.The surrounding soil of tunnel failure evolution as well as extent and shape of the damage zone during the excavation-induced unloading were also studied.

Liquefaction evaluation of dam foundation soils considering overlying structure

The liquefaction analysis procedure conducted at a dam foundation associated with a layer of liquefiablesand is presented. In this case, the effects of the overlying dam and an embedded diaphragm wall onliquefaction potential of foundation soils are considered. The analysis follows the stress-based approachwhich compares the earthquake-induced cyclic stresses with the cyclic resistance of the soil, and thecyclic resistance of the sand under complex stress condition is the key issue. Comprehensive laboratorymonotonic and cyclic triaxial tests are conducted to evaluate the static characteristics, dynamic characteristicsand the cyclic resistance against liquefaction of the foundation soils. The distribution of thefactor of safety considering liquefaction is given. It is found that the zones beneath the dam edges andnear the upstream of the diaphragm wall are more susceptible to liquefaction than in free field, whereasthe zone beneath the center of the dam is less susceptible to liquefaction than in free field. According tothe results, the strategies of ground improvement are proposed to mitigate the liquefaction hazards. 2015 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences. Production and hosting byElsevier B.V. All rights reserved.

Prediction of lining response for twin tunnels constructed in anisotropic clay using machine learning techniques

Excessive structural forces generated inside tunnel linings could affect the safety and serviceability of tunnels,emphasizing the need to accurately predict the forces acting on tunnel linings during the preliminary design phase.In this study,an anisotropic soil model devel-oped by Norwegian Geotechnical Institute(NGI)based on the Active-Direct shear-Passive concept(NGI-ADP model)was adopted to conduct finite element(FE)analyses.A total of 682 cases were modeled to analyze the effects of five key parameters on twin-tunnel struc-tural forces;these parameters included twin-tunnel arrangements and subsurface soil properties:burial depth H,tunnel center-to-center distance D,soil strength s_(u)^(A),stiffness ratio G_(u)=s_(u)^(A),and degree of anisotropy ss_(u)^(P)=s_(u)^(A).The significant factors contributing to the bending moment and thrust force of the linings were the tunnel distance and overlying soil depth,respectively.The degree of anisotropy of the surrounding soil was found to be extremely important in simulating the twin-tunnel construction,and severe design errors could be made if the soil anisotropy is ignored.A cutting-edge application of machine learning in the construction of twin tunnels is presented;multivariate adaptive regression splines and decision tree regressor methods are developed to predict the maximum bending moment within the first tunnel’s linings based on the constructed FE cases.The developed prediction model can enable engineers to estimate the structural response of twin tunnels more accurately in order to meet the specific target reliability indices of projects.

Strength-increase mechanism and microstructural characteristics of a biotreated geomaterial

Microbially induced calcite precipitation(MICP)is a recently proposed method that is environmentally friendly and has considerable potential applications in artificial biotreated geomaterials.New artificial biotreated geomaterials are produced based on the MICP technology for different parent soils.The purpose of this study is to explore the strength-increase mechanism and microstmctural characteristics of the biotreated geomaterial through a series of experiments.The results show that longer mineralization time results in higher-strength biotreated geomaterial.The strength growth rate rapidly increases in the beginning and remains stable afterwards.The calcium ion content significantly increases with the extended mineralization time.When standard sand was used as a parent soil,the calcium ion content increased to a factor of 39 after 7 days.The bacterial cells with attached calcium ions serve as the nucleus of crystallization and fill the pore space.When fine sand was used as a parent soil,the calcium ion content increased to only a factor of 7 after 7 days of mineralization.The nucleus of crystallization could not normally grow because of the limited pore space.The porosity and variation in porosity are clearly affected by the parent soil.Therefore,the strength of the biotreated geomaterial is affected by the parent soil properties,mineralization time,and granular material pore space.This paper provides a basis for theory and experiments for biotreated geomaterials in future engineering practice.

Seismic behaviors of utility tunnel-soil system: With and without joint connections

Seismic responses of utility tunnel-soil system were studied via shaking table model tests with considerations of two kinds of double box utility tunnels:with and without joint connections.These two testing utility tunnel models were made of galvanized iron wire and micro-concrete,and the ground was simulated by the dry standard sand through layered tamping treatment.The utility tunnel-soil system was subjected to horizontal vibration in uniaxial direction perpendicular to the longitudinal direction of tunnel model.Via instrumentations of earth pressure gauges,accelerometers and strain gauges,the earth pressure response,acceleration response and bending moment response were measured.The testing results show that the joint connections in the utility tunnel along the longitudinal direction play an important role in determining the characteristic of earth pressure response and bending moment response,whereas the effect of joint connections on acceleration response is less significant.In addition,the partition wall exhibits the consistent acceleration response with the side-wall of double box utility tunnel model under seismic condition.Based on the testing results,it is suggested that the joint connection should be taken reasonably into consideration during design and construction for engineering practice.

Tunneling and deep excavations in spatially variable soil and rock masses:A short review

In an urbanization process,infrastructure elements such as tunnels and deep excavations are widely used to service the development of cities.Owing to the lengthy geological processes of geomaterials and the limited availability of site-specific test data,soil and rock properties exhibiting spatial variability are frequently encountered in geological and geotechnical engineering.This paper presents a comprehensive review of the application of spatial variability in tunneling and deep excavation over the past 20 years.It is found that the spatial variability is generally modeled as a random field(RF)in finite element software,based on random field theory(RFT).This model has been widely used in the design,stability evaluation,and probabilistic analysis of tunnels and excavations.Previous works have proven that the performance of tunnels and deep excavations can be better captured by considering the spatial variability,as compared with conventional deterministic analysis methods.Nonetheless,current research still faces many factual scientific problems.Therefore,this paper also identifies some research gaps,as well as recommendations for further investigations.

Propagation characteristics of transient waves in low-strain integrity testing on cast-in-situ concrete thin-wall pipe piles

The three-dimensional effects of pile head and the applicability of plane-section assumption are main problems in low-strain dynamic tests on cast-in-situ concrete thin-wall pipe piles.The velocity and displacement responses were calculated by a theoretical formula deduced by the authors.The frequency and influencing factor of high-frequency interference were analyzed.A numerical method was established to calculate the peak value and arrival time of incoming waves on top of the piles.The regularity along circumferential and the influence of radius or impulse width were studied.The applicability of plane-section assumption was investigated by comparison of velocity responses at different points in the sections at different depths.The waveform of velocity response at different points forked after the first peak,indicating that the propagation of stress waves did not well meet the plane-section assumption.

Effects of jet grouting slabs on responses for deep braced excavations

Jet grouting slabs are widely used in deep excavations owing to their effectiveness for reducing the deflection of the diaphragm wall and the prop forces acting on the struts and improving the basal-heave stability.In this paper,according to case histories in Singapore,a series of finite-element numerical simulations are performed to evaluate the effects of jet grouting slabs on responses to deep braced excavations.On the basis of a parametric sensitivity study,a reasonable thickness of jet grouting slabs is proposed.The effects of the wall depth,wall stiffness,soft-clay thickness,and stiffness on the performance of the jet grouting slabs are assessed by comparing and analyzing a series of simulation results.It is found that the soft-clay thickness significantly affects the wall deflection and basal heave in deep excavation.During the design of support structures,soil profiles should be considered first.The findings of this study provide a reference and guidance for the support system design of similar projects.

Experimental analysis on the interaction between underground structures and sand layer under groundwater level change

Groundwater level change stands a momentous role in affecting the geotechnical construction stability and safety of underground structures.Global warming and active underground construction cause conspicuous changes in the groundwater level,which further leaves an impact on the underground structures’serviceability.To reveal the interaction between underground structure and soil under groundwater level change in the sand layer,model tests of circular transportation tunnel and rectangular utility tunnel were carried out.With the self-designed experimental equipment and innovative experimental methods,the changes in tunnel stress,bending moment,buoyancy,and vertical displacement of the rise and drawdown of the groundwater level in the sand layer were studied.The results revealed the developments of concentrated structural forces during the rising and falling process of the groundwater water level,indi-cating critical locations that should be strengthened.Meanwhile,both tunnels showed the same movement trend:settling first,floating afterwards and settling at last.And it is concluded that no reduction is required when calculating buoyancy in sands using measured pore pressure.Conclusions can provide a notable reference for future related research and engineering designs.

Determination of groundwater buoyancy reduction coefficient in clay:Model tests,numerical simulations and machine learning methods

Groundwater plays an essential role in stabilizing underground structures.However,hydrostatic uplift forces from groundwater can create safety hazards.This paper obtained the groundwater buoyancy reduction coefficients of 36 types of clays through model tests and conducted a finite element simulation to obtain the buoyancy reduction coefficients of additional clays with varying soil properties.Machine learning methods,including extreme gradient boosting(XGBoost)and random forest(RF)algorithms,were used to analyze and identify the soil parameters that have a significant impact on the reduction of groundwater buoyancy.It was found that the permeability coefficient and saturation are the primary factors that influence the reduction of groundwater buoyancy.Additionally,the prediction models developed by XGBoost and RF were compared,and their accuracy was evaluated.These research findings can serve as a reference for designing underground structures that can withstand the potential risk of buoyancy in clay.

Numerical investigation of pile responses induced by adjacent tunnel excavation in spatially variable clays

The pile responses induced by adjacent tunnel excavation have been a hot research topic in geotechnical engineering.Tunnel excavation may exert disturbance to the surrounding soil mass and then influence the adjacent pile foundations.In this paper,the random finite difference analysis considering the spatial variations of soil properties is conducted to explore the effect of tunnel excavation on the adjacent pile response by varying the distance of pile away from the tunnel centerline(D),the pile length(L),the pile diameter(d_(p)),the tunnel depth(h),and the anisotropic ratio between the horizontal and the vertical scales of fluctuation(δ_(x)/δ_(y)).A set of pile response curves are developed to assess the influence of tunnel excavation on the adjacent passive pile foundations in spatially variable clays and provide guidelines for the tunnel excavation in the complex constructed environment.