Experimental and numerical analysis of large scale pull out tests conducted on clays reinforced with geogrids encapsulated with coarse material

The pullout test is one of the methods commonly used to study pullout behavior of reinforcements. In the current research, large pullout tests (i.e. 100 × 60 × 60 cm) have been conducted to investigate the possibility of pullout resistance enhancement of clays reinforced with HDPE geogrid embedded in thin layers of sand. Pullout tests on clay–geogrid, sand–geogrid and clay–sand–geogrid samples have been conducted at normal pressures of 25, 50 and 100 kPa. Numerical modeling using finite element method has also been used to assess the adequacy of the box and geogrid sizes to minimize boundary and scale effects. Experimental results show that provision of thin sand layers around the reinforcement substantially enhances pullout resistance of clay soil under monotonic loading conditions and the effectiveness increases with increase in normal pressures. The improvement is more pronounced at higher normal pressures and an optimum sand layer thickness of 8 cm has been determined for maximum enhancement. Results of numerical analysis showed the adequacy of the box and geogrid length adopted as well as a relatively good agreement with experimental results.     

Centrifuge and numerical model studies on the behaviour of geogrid reinforced soil walls with marginal backfills with and without geocomposite layers

The aim of this paper is to study the effect of geocomposite layers as internal drainage system on the behaviour of geogrid reinforced soil walls with marginal backfills using centrifuge and numerical modelling. A series of centrifuge model tests were carried out using a 4.5 m radius beam centrifuge facility available at IIT Bombay. A seepage condition was imposed to all models to simulate rising ground water condition. Displacement and pore water pressure transducers were used to monitor the performance of all centrifuge models. A geogrid reinforced soil wall without any geocomposite layer experienced catastrophic failure soon after applying seepage due to the development of excess pore water pressure within the reinforced soil zone of the wall. In comparison, reinforced soil wall with two geocomposite layers at the bottom portion of the wall was found to have a good performance at the onset of seepage and by embedding four geocomposite layers up to the mid-height of the wall from bottom as a result of lowering phreatic surface much more effectively. For analysing further the observed behaviour of centrifuge model tests, stability and seepage analysis were conducted using SLOPE/W and SEEP/W software packages. A good agreement was found between the results of numerical analysis and observation made in centrifuge tests. The effect of number of geocomposite layers as well as its transmissivity was further analysed using parametric study. The results of parametric study revealed that the number of geocomposite layers plays a main role on the good performance of the geogrid reinforced soil walls with marginal backfill.     

Microscale investigation into mechanical behaviors of heat-bonded nonwoven geotextile using DEM

Heat-bonded nonwoven geotextiles (HBNGs) made from synthetic fibers are widely used in engineering practices. One of the challenges on the way is to link the properties of fibers and the fabric's microstructure to the deformation and failure mechanisms of HBNGs. In this study, a random distribution geometry method was developed to reproduce the complex fibrous structure of HBNG. A piecewise linear model was adopted to reproduce the nonlinear stress-strain relationships of single fibers. The present method has been successfully applied in the simulation of uniaxial and biaxial tensile tests and puncture test. The orientation distribution of fibers and the mechanical behaviors (e.g., deformation, strain localization, force-strain relationship) of HBNG specimen were reasonably simulated. Specifically, the hourglass shape during uniaxial tensile test, the axisymmetric deformation pattern during biaxial tensile test and the trumpet shape during puncture test were all well reproduced. The present method provides an applicable tool to study the complicated mechanical behaviors of HBNG and is also helpful to obtain a better understanding of its deformation and failure mechanisms.     

Laboratory characterization and modelling of the thermal-mechanical properties of binary soil mixtures

Binary soil mixtures are extensively used in the construction of geothermal-related earth structures such as geothermal energy piles (GEP), ground source heat pumps (GSHP) and earth air tunnel heat exchangers (EATHE). An evaluation of the binary soil’s thermal-mechanical properties is the key process in determining the final performance of geothermal-related projects. Therefore, the thermal-mechanical properties of binary soil mixtures were systematically investigated in this paper. A series of thermal and mechanical property tests was conducted on five sand-kaolin clay mixtures with sand contents ranging from 0% to 100% by dry weight. The experimental results indicated that the sand-clay mixtures achieved the theoretically densest state when the sand content reached the critical threshold. The further the binary mixture’s sand content was from the critical threshold, the lower the mixture’s density was. As the sand content increased, the shear stress-strain curves gradually shifted from strain-softening behavior to strain-hardening behavior due to the decrease in suction stress. The relationship between the sand content and the shear strength of the mixtures exhibited an “S” shape, which is attributed to the interaction between the sand and clay particles and varied with the sand contents. The shear wave velocity of the sand clay mixtures was found to decrease continuously with the increase in sand content until the sand skeleton had formed. In addition, the thermal conductivity of the binary mixed soil changed linearly with the sand content, and the upper bound of the critical threshold interval (77%) was found to separate the two different heat conduction modes. Finally, an elastic shear modulus (G₀) model, which correlated to the tangent elastic modulus of the binary mixture (E_m), and a more generalized thermal conductivity (K) model were formulated for the binary sand-clay mixtures, and the effectiveness and feasibility of the proposed models were validated by comparing the values predicted with the model and the experimental data.     

Seismic analysis of geosynthetic-reinforced retaining wall in cohesive soils

Although a cohesionless backfill is recommended for geosynthetic reinforced earth retaining walls, cohesive soil have been widely used in many regions across the globe for economic reasons. This type of backfill exposes the soil to the crack formation that leads to reduce the stability of the system. In this paper, to investigate the internal seismic stability of reinforced earth retaining walls with cracks, the discretization method combined with the upper bound theorem of limit analysis are used. The potential failure mechanism is generated using the point-to-point method. Two types of cracks are considered, a pre-existing crack and a crack formation as a part of the failure mechanism. The use of the discretization method allows the consideration of the vertical spatial variability of the soil properties. A pseudo-dynamic approach is implemented which allows the account of the dynamic characteristics of the ground shaking. The presented method is validated using the conventional limit analysis results of an existing study conducted under static conditions. Once the proposed technique to consider the cracks is validated, a parametric study is conducted to highlight the key parameters effects on the lower bound of the required reinforcement strength.     

Nitrifying population densities and inhibition of ammonium oxidation in natural and sewage-enriched cypress swamps

The occurrence of autotrophic nitrifiers in the peat from the floors of a natural cypress dome, a cypress dome receiving deep artesian groundwater, and two cypress domes amended with secondary treated sewage effluent, were assessed by surveying their population densities. The absence of ammonium oxidation in the surface waters of the natural dome was due to the low pH and not to any toxic organic chemical effects present in the humic-colored water. This probably explains the low density (0–56 cells cm⁻³) of autotrophic nitrifiers found associated with the peat from that dome.     

Deformation analysis of geosynthetic-encased stone column using cavity expansion models with emphasis on boundary condition

This paper presents a modified theoretical model to predict the deformation of geosynthetic-encased stone column (GESC) and surrounding soil, using cylindrical cavity expansion model (CEM). The model was distinguished for single GESC and GESC in groups with emphasis on the different boundary conditions. The displacement boundary of CEM was used for GESC in groups, and the stress boundary of CEM was adopted for single GESC. The plasticity development of the soil obeying the Mohr-Coulomb yielding criterion was considered. The stress and settlement of the GESC were analyzed by radial stress and vertical stress equilibrium. This method has been verified via comparison with test data and numerical simulation results. The influences of applied loading, geosynthetic encasement stiffness, and soil stiffness on the mechanical performance of the GESC and the surrounding soil have also been investigated. The proposed theoretical approaches are suitable for predicting the deformation of the GESC, and the surrounding soil. The proposed method in unit cell analysis was more reasonable for GESC in groups.     

Shear behaviour of methane hydrate bearing sand with various particle characteristics and fines

It is recognized that methane hydrate (hereafter referred to as MH) is trapped in the sand sediments of alternating sand and mud layers in the turbidite of the Nankai Trough, Japan. The existence of fines within the marine sediment significantly affects its mechanical and physical properties. A series of plane strain compression tests at high pressures were performed in order to investigate the effect of the particle characteristics and fines content of the host sands on the shear behaviour of MH bearing sands. MH bearing sands were artificially produced using rounded glass beads and three other silica sands with different fines contents. A high-pressure low-temperature testing apparatus was equipped with a camera to observe deformation of the specimens during shearing and particle image velocimetry analysis was conducted on pictures taken during the experiments. The experimental results show that strength enhancement due to the bonding effect in MH bearing sand increases with the level of fines content. Values for both the cohesion and friction angle of MH bearing sand composed of Toyoura sand increased along with increasing MH saturation. However, in the case of MH bearing glass beads, only the value for cohesion increased when MH was formed. The maximum shear strain of MH bearing glass beads was mostly concentrated near the shear band. While the maximum shear strains of the three other MH bearing sands were concentrated within the shear band, some was widely distributed in the region outside of the shear band. A rise in the degree of MH saturation increased the angle and narrowed the width of the shear band, regardless of the fines content.     

Ultimate bearing capacity of rigid footing under eccentric vertical load

In geotechnical engineering, the stability of rigid footings under eccentric vertical loads is an important issue. This is because the number of superstructure buildings has increased and the situation of structures being subjected to eccentric vertical loading is occurring more and more frequently. In this study, focus is placed on the ultimate bearing capacity of a footing against the eccentric load placed on two types of soil, namely, sandy soil and clayey soil, using a finite element analysis. For the sandy soil, the study newly introduces an interface element into the footing-soil system in order to properly evaluate the interaction between the footing and the soil, which greatly affects the failure mechanism of the footing-soil system. For the clayey soil, the study improves the analysis procedure by introducing a zero-tension analysis into the footing-soil system. Two friction conditions between the footing and the soils are considered; one models a perfectly rough condition and the other models a perfectly smooth condition. For a two-dimensional analysis of the footing-soil system, the rigid plastic finite element method (RPFEM) is applied to calculate the ultimate bearing capacity of the eccentrically loaded footing. The RPFEM is extended in this work to calculate not only the ultimate bearing capacity, but also the distribution of contact stress along the footing base. The study thoroughly investigates the effect of the eccentric vertical load on the ultimate bearing capacity in the normalized form of V/V_ult and e/B where e is the length of the eccentricity and B is the width of the footing. V_ult indicates the ultimate bearing capacity of the centric vertical load. The failure envelope in the plane of V/V_ult and M/BV_ult is further investigated under various conditions for the sandy and clayey soils. M is the moment load induced by the eccentric vertical load. This study examines the applicability of the failure envelope obtained for the eccentric vertical load to the cases where two variables, V and M, are independently prescribed. The obtained results are coincident and indicate the wide applicability of the failure envelope in the normalized V-M plane in practice. Finally, in a comparison with previous researches, the numerical data in the present study lead to the derivation of new equations for the failure envelopes of both sandy and clayey soils.     

Interacting limits to algal growth: Light, phosphorus, and carbon dioxide availability

Most studies of limiting factors to algal growth seek to quantify the effect of individual factors. However, rates and amounts of photosynthetic carbon fixation in laboratory cultures of Anacystis nidulans show simultaneous dependence on the availability of phosphorus, carbon dioxide, and light. Thus, not all limiting factors may act independently to control algal growth in aquatic systems, and further research into the extent of such interactions is needed.     

Theoretical and experimental study of factors affecting capillary suction time (CST)

Factors affecting capillary suction time (CST) were studied theoretically and experimentally. The factors were elucidated from a linearly measured capillary suction time (LCST), which was obtained with a simpler device than conventional radial equipment. The LCST should be correlated with the specific resistance to filtration of sludge under the condition that a quantity, i.e. the product of specific resistance and cake mass deposited per unit filtrate volume, is greater than some critical value. If this quantity is greater than the critical value, the LCST is not always correlated with the filterability, but the LCST can always be correlated well with the solid concentration of suspension.     

Estimation of elastic wave velocity through unsaturated soil slope as function of water content and shear deformation

The objective of this study was to estimate the elastic wave velocity of an unsaturated soil slope, and to verify its applicability. The elastic wave velocity in a silty sand was measured. The individual influence of the volumetric water content and the tilt angle on the normalized wave velocity through unsaturated soil were investigated through a series of varied slope model tests. The relationship function of the normalized wave velocity-volumetric water content-tilt angle was established. To verify the proposed estimation function, a series of fixed slope model tests was carried out. The relationship functions were used to estimate the behaviors of the wave velocity in rainfall-induced slope failure model tests. The applicability of the proposed relationship functions for the wave velocity behaviors was also presented. It was found that the estimation function is highly consistent with the measurements for the wave velocity behaviors through unsaturated soil slope in the presented test conditions. In addition, the effects of the rainfall duration/initial water content, density, slope angle and surface layer thickness on the decrease rate of the normalized wave velocity with the volumetric water content and the tilt angle within the test conditions in this study were seen to be small.     

Laboratory study and interpretation of mechanical behavior of frozen clay through state concept

Towards the development of a mechanical model that can be part of multi-physical analysis of frozen soils, a program of systematic frozen-unfrozen parallel triaxial tests at different temperatures and strain rates was conducted. The mechanical behavior of the reconstituted high-plasticity clay samples was investigated and interpreted through a state concept based on Ladanyi and Morel’s (1990) postulate on the unique relationship between the inter-particle “effective” stress and the strain path. The Critical State Lines (CSLs) for clay specimens frozen undrained were mapped by referring to the shear behavior of unfrozen specimens sharing the same strain history. With other conditions set identical, the shear strength linearly increased with a decrease in the temperature for the range from ?10 °C to ?2 °C, and log-linearly increased with an increase in the strain rate for the range from 0.001%/min to 0.1%/min. Direct comparison of the strain-rate effects between frozen and unfrozen specimens with identical strain paths and states in the soil skeleton clearly indicates that the viscoplasticity derives from that of pore ice. A conceptual interpretative framework invoking temperature- and strain rate-dependent state bounding surfaces and CSLs was proposed to describe the behavior of frozen soils under steady and non-steady temperature and strain rate. The above observations of the behavioral features of frozen and unfrozen soils, with further experimental work, are expected to lead to the construction of a unified framework for describing the behavior under both states and the transition between them.     

Hydraulic performance and chemical compatibility of a powdered Na-bentonite geosynthetic clay liner permeated with mine drainage

The hydraulic and chemical compatibility of a geosynthetic clay liner (GCL), containing powdered Na-bentonite, was evaluated against artificial acid rock drainage (ARD) in terms of the swell index, hydraulic conductivity and heavy metal retention. Six artificial ARDs with an approximate pH of 3 and different metal concentrations (electrical conductivity, EC, ranging between 75 and 1000 mS/m; ionic strength ranging between 8 and 400 mM) were used in the experiments. The results of free swelling tests showed that high metal concentrations (EC higher than 70 mS/m) negatively impact the swell volume by lowering it. The hydraulic conductivity of the GCL permeated with distilled water was 1.2 × 10^?11 m/s, falling in the range of 7.9 × 10^?12 to 1.1 × 10^?10 m/s when prehydrated with distilled water and permeated with ARDs. The ion exchange and metal precipitation appeared to be the main mechanisms controlling the metal attenuation on the bentonite. The ion exchange mechanism starts with the release of Na from the bentonite and the sorption of the bi- and tri-metals present in the ARDs onto the bentonite. After the depletion of Na, the ion exchange reaction proceeds with the desorption of Ca and Mg from the bentonite and the sorption of cations present in the ARDs onto the bentonite layers. The depletion of Na from the bentonite and the subsequent release of Ca and Mg correlate to the sudden drop in pH and a gradual increase or equilibration of the hydraulic conductivity. It is possible to say that, after this point, hydraulic and chemical equilibrium is reached. From the overall results, the tested GCL showed acceptably low hydraulic conductivity and the potential to attenuate heavy metals present in ARDs.     

哈雄文和他的建筑人生路——纪念哈雄文先生诞辰110周年

哈雄文1927年在清华学校毕业后赴美国留学,1932年毕业于宾夕法尼亚大学美术学院建筑系和艺术系,1958年由同济大学来到哈尔滨工业大学,筹建哈尔滨建筑工程学院。他是我国第一代建筑与城市规划的留学生和前辈建筑师之一,是我国近现代建筑与城市规划领域的设计大师、学术大家、政府高级行政官员、职业团体领导人、教育家。本文谨从富国利民心、建筑教育情等方面来叙述哈雄文先生所走过的建筑与城市规划人生路,谨以此来纪念其诞辰110周年。     

Expansive soil modified by waste steel slag and its application in subbase layer of highways

Steel slag is a waste by-product of the steel industry. The recycling usage of steel slag is limited due to the mutative chemical compositions it contains and its low cementation. In this investigation, the composition adjustment and activation of steel slag were studied to produce an optimal slag-based composite with improved cementation efficiency. The controlling moduli of cement clinker were introduced to standardise the composite. Subsequently, the composite was used to modify Hefei expansive soil (a kind of engineering waste for swelling properties) in embankment construction. The basic physical properties including free swelling ratio, California bearing ratio, unconfined compressive strength, microstructure, and mineral evolution were evaluated to understand the engineering performance and mechanism of modified expansive soils. The results show that the cementation of the slag was significantly improved after the composition adjustment and activation. Furthermore, the treated soil can satisfy the requirement of the Chinese standard for first-class road/highway when the composite incorporation ratio is more than 5%. The microstructural and mineralogical analysis shows that the component adjustment and activation enrich the cementation of the slag, resulting in the suppression of the swelling potential and improved strength. The above findings improve the reuse efficiency of steel slag, especially in expansive soil modifications.     

Geotechnical properties of steel slag aggregates: Shear strength and stiffness

Steel slag is an industrial by-product formed in the furnace during the steelmaking process. Electric arc furnace slag (EAFS) and ladle furnace slag (LFS) are the primary by-products of steelmaking from steel scraps. This research evaluates the physical, geotechnical and engineering properties of LFS, EAFS and a blend comprising 50% LFS and 50% EAFS (LFS50 + EAFS50) through laboratory testing. The specialized laboratory tests undertaken in this study include California bearing ratio (CBR) and unconfined compressive strength (UCS) tests, direct shear tests (DSTs), consolidated drained (CD) triaxial tests and repeated load triaxial (RLT) tests. The shear strength responses of the steel slag were found to vary with the dilatancy-induced peak strength of the LFS and the LFS50 + EAFS50 mixture and the dilatancy-associated strain-hardening behavior of the EAFS. Based on the high shear strength parameters and the adequate stiffness that were attained, the steel slag was found to have the potential for usage as a geo-material. LFS and LFS50 + EAFS50 were well-graded and had high CBR values, which would deem them suitable for roadwork applications. EAFS, however, was found to be poorly graded and to have relatively lower CBR values, which would deem it suitable for less stringent applications such as engineering fill and pipe bedding. The viability of using these by-products as geo-materials in civil infrastructures can transform these current waste by-products, particularly LFS, from being stockpiled at steel company plants to being used as alternative green material.     

Impact of pore structure and morphology on flow and transport characteristics in randomly repacked grains with different angularities

The flow and transport behaviors in porous media are closely linked to the structure and morphology of the pore space. A fundamental objective of most studies of porous media is to link the pore structure to the hydraulic functions, such as the permeability, capillary pressure and diffusivity, which are necessary for engineering applications. In this paper, an attempt is made to build a direct link between the hydraulic functions and the morphological measures of diverse porous media. Porous columns with different structures and morphologies are generated by randomly packing grains with different shapes and sizes. The pore structure of the repacked porous media is visualized through X-ray computed tomography and quantified by a series of parameters, including the set of Minkowski functionals, diverse characteristic pore sizes, geometric tortuosity and fractal dimension. The intrinsic permeability, molecular diffusivity and apparent thermal conductivity of the repacked porous media are simulated numerically. The Minkowski functionals have the capacity to characterize the microscale complex pore domain of the porous media in a macroscale way. A good linear relationship is shown among the effective pore size, nominal opening dimension and critical pore neck size obtained from the morphological analysis regardless of the shapes and sizes of the grains. The three different pores may serve as the characteristic pore correlation to the intrinsic permeability. The Kozeny-Carman equation can be used to mimic the intrinsic permeability and to serve as a quality-control tool for porous media with different grain angularities. A topologically based model can generally provide a single relationship for porous media randomly repacked with grains of different angularities. The molecular diffusivity of angular grains is found to be larger than that of round ones. The molecular diffusivity is linearly related to the porosity and fractal dimension. Porous media repacked with round grains tend to attain denser packing, a higher number of contacts per unit volume and higher thermal conductivity than media packed with angular particles. The apparent thermal conductivity has a negative linear correlation to the porosity and fractal dimension of porous media with different grain morphologies.     

Investigation for the key technologies of ultra-high asphalt concrete core rockfill dams

The mechanical characteristics of ultra-high asphalt concrete core rockfill dams (UACCRDs) at different periods is investigated via Rankine’s earth pressure theory, and a shear safety control standard for UACCRDs is proposed. The reasonable material parameters of the asphalt concrete core (ACC) and transition material that independently and comprehensively satisfy the shear safety control standard are back-calculated. The engineering measures that reduce the stress level (shear stress) of the ACC are given. Moreover, the engineering measures (straight asphalt concrete core rockfill dams (SACCRDs) are designed as curved asphalt concrete core rockfill dams (CACCRDs)) that reduce the tensile stress of the ACC are proposed. Based on the theory of the straight beam and curved beam on Winkler elastic foundation, the simplified mechanical models of straight asphalt concrete core (SACC) and curved asphalt concrete core (CACC) are established. The improvement effect of CACC that reduces tensile stress is also investigated. The results show that the following value ranges of the internal friction angle, cohesion of ACC and the internal friction angle of transition material for the suitable construction of UACCRDs are recommended: φ_a ≥ 30.5°, C_a ≥ 0.25 MPa and φ_t ≤ 43.5° (h = 200 m), with the growth gradient adjusted by 0.5%, 1.5% and ?0.5%/25 m. The stress level of ACC can be obviously reduced by increasing the internal friction angle and cohesion of ACC, and reducing the internal friction angle of transition material. The simplified mechanical models of SACC and CACC can estimate the force and deformation characteristic of the ACC (SACC and CACC) well. The CACC can significantly reduce tensile stress to a level approximately 42.8% lower than that of SACC.     

Beyond the code: Energy, carbon, and cost savings using conventional technologies

As states in the U.S. adopt new energy codes, it is important to understand the benefits for each state and its building owners. This paper estimates life-cycle energy savings, carbon emission reduction, and cost-effectiveness of conventional energy efficiency measures in new commercial buildings using an integrated design approach. Results are based on 8208 energy simulations for 12 prototypical buildings in 228 cities, with 3 building designs evaluated for each building-location combination. Results are represented by easy-to-understand mappings that allow for regional and state comparisons. The results show that the use of conventional energy efficiency technologies in an integrated design framework can decrease energy use by 15-20% on average in new commercial buildings, and over 35% for some building types and locations. These energy reductions can often be accomplished at negative incremental life-cycle costs and reduce a building's energy-related carbon footprint by 9-33%. However, generalizing these results on energy use, life-cycle costs, and carbon emissions misses exceptions in the results that show the importance of location-specific characteristics. Also, states do not appear to base energy code adoption decisions on either potential energy savings or life-cycle cost savings.