Strength and deformation properties of frozen sand under a true triaxial stress condition

In recent years, the mechanical properties of frozen soils under complex stress states have attracted significant attention; however, limited by the test apparatus, true triaxial tests on frozen soils have rarely been conducted. To study the strength and deformation properties of frozen sand under a true triaxial stress state, a novel frozen soil testing system, i.e., a true triaxial apparatus, was developed. The apparatus is mainly composed of a temperature control system, a servo host system, a hydraulic servo loading system, and a digital control system. Several true triaxial tests were conducted at a constant minor principal stress (σ₃) and constant intermediate principal stress ratio (b) to study the effect of intermediate principal stress (σ₂) on the mechanical properties of frozen sand. The test results showed that the stress–strain curve can be mainly divided into three stages, with evidence of strain hardening characteristics. The strength, elastic modulus, and friction angle increased with the increase in b from 0 to 0.6, but decreased when increasing b from 0.6 to 1, whereas the cohesion varied little with the variation in b. The deformation in the direction of σ₂ changed from dilative to compressive and that in the direction of σ₃ remained dilative throughout.     

A new method for extrapolating the load-displacement response of drilled shaft foundations for interpretation of side capacity

In this study, the side capacity of drilled shaft foundations is estimated from partially-mobilized load–displacement field data using a new method in the extrapolation of load–displacement response. A dataset of 138 bi-directional load tests is used to evaluate the degree of mobilization of unit side resistance. A total of 612 unit side-resistance curves obtained from measured strain gauge recordings are utilized in this study. The proposed extrapolation approach is based on a new technique, the Double Tangent method, characterizing the extent of mobilization for each unit side-resistance curve. Roughly, 12% of the dataset exhibits a fully-mobilized load–displacement response, with the remainder exhibiting varying degrees of a partially-mobilized response. Fully-mobilized records are further characterized using the Double Tangent method over different ranges of mobilization, resulting in four regression models based on predominant soil types. Each model is assessed statistically, and a global regression model is found suitable to predict maximum unit side resistance. The global model is further validated using two independent load test datasets, comparing measured values of unit side resistance against predicted values. The model is then used to predict maximum unit side resistance for all partially-mobilized data within the dataset, and the results are compared to two extrapolation techniques currently used in practice. The corresponding resistance-displacement response is extrapolated using a proposed asymptotic curve-fitting function for side resistance, and an example extrapolation is illustrated to showcase how the proposed method can be used in engineering practice.     

Effect of coral sand powders and seawater salinity on the impact mechanical properties of cemented coral sand

Coral sand is one kind of the important building materials in coral reef engineering practice. The use of cement as a stabilizing agent can significantly improve the mechanical properties of coral sands and is widely applied in the subbase engineering construction in coral reef islands. Cement-stabilized coral sand structures may contain high contents of fine coral particles and salinity because of the high crushability of coral sands and the existence of seawater surrounding them. In this study, the effects of coral sand powders and seawater salinity on the dynamic mechanical properties of cemented coral sand (CCS) were investigated through the split Hopkinson pressure bar (SHPB) tests and Scanning Electron Microscope (SEM) analysis. It was found that the strength (i.e., the peak stress) of CCS specimens increased firstly and then decreased with the increase of powder content. The specimens reached the maximum peak stress when 3% powder content was included. The initial improvement of CCS strength was attributed to the pore-filling effect of coral powders, namely, the micro pores of the CCS specimens could be more effectively filled with higher percentages of coral powders being used in the experiments. However, excessive coral powders resulted in the reduction of specimen strength because these powders could easily be cemented into agglomerates by absorbing water from the specimens. These agglomerates could reduce the cementation strength between the coarse coral particles and the cement. Meanwhile, the peak stress of CCS specimens was found to be negatively correlated with the average strain rate and the ultimate strain. The degree of specimen fracture was found to be correlated with the amount of specific energy absorption during the tests. Furthermore, the “sulfate attack” caused by the inclusion of salinity of water had different influences on the CCS specimens with different coral powder contents. The ettringite and gypsum produced in “sulfate attack” could fill the pores and lead to cracking of the specimens, significantly affecting the specimen strength.     

Effect of geotextile ageing and geomembrane surface roughness on the geomembrane-geotextile interfaces for heap leaching applications

A series of large scale direct shear experiments is used to investigate the effect of the geomembrane (GMB) surface roughness, geotextile (GTX) properties, and GTX ageing, on the GMB-GTX interface shear behaviour. Interfaces involving smooth, coextruded textured, and structured surface GMBs underlying four different nonwoven needle-punched staple fibres (GTXs) with mass per unit areas between 200 and 2400 g/m², and a geocomposite drain (GCD) are examined at normal stresses between 250 and 1000 kPa. The results showed that the interlocking between the GMB and GTX increased with increasing the GMB asperity height and/or decreasing the mass per unit area of the GTX. For the interfaces that involved GTXs preaged prior to the shear box experiments for up to 2 years at 85 °C, it was found that the 2400 g/m² heat bonded two-layered GTX exhibited internal shear failure at low shear displacements. However, all the highly aged single layered GTXs showed an increase in the peak interface friction angles with the increase in their ageing. For these single layered GTX, the results suggest that assessing the interface friction angles using unaged GTXs for the stability analysis is conservative as long as the GTX remains intact in the field.     

Stress-strain behavior of geotextile: A proposed new indirect calculation using the static puncture test (CBR test)

Some of the main applications of geosynthetics include use as a hydraulic barrier in sanitary landfills, as a reinforcement element and in pavement engineering. In most cases, these materials are subject to the overlapping effects of tensile strength and puncture. This paper presents a review of indirect methods for calculation of stress and strain averages by means of the California Bearing Ratio (CBR) puncture strength test. In addition, a new calculation method is proposed based on the Kirchhoff plate theory, which interprets the behavior of thin circular plates subjected to a uniform normal loading. This new method enables analysis of the stress-strain in each stretch of the geosynthetic. The methodology is applied to four woven geotextiles of different weights. The results of the new calculation method yielded a better stress-strain correlation with direct tensile strength tests, presenting the smallest relative errors compared to the other indirect calculations reviewed. With the aid of a disk and pins, vertical displacement values at different points in the geotextiles were measured and showed good agreement with analytical predictions. Therefore, the static puncture test combined with the new proposed calculation method is a good alternative for determining the stress-strain parameters of geotextile.     

Structural damage and shear performance degradation of fiber–lime–soil under freeze–thaw cycling

The freeze–thaw cycling damages the soil structure, and the shear performance of soil are degraded. A series of tests on lime–soil(L–S) and fiber–lime–soil(F–L–S), including freeze–thaw test, the triaxial compression test, nuclear magnetic resonance (NMR) test and scanning electron microscope (SEM) test, were completed. The test results showed that fiber reinforcement changed the stress–strain behavior and failure pattern of soil. The cohesion and internal friction angle of soil gradually decreased with the increase of freeze–thaw cycles (F–T cycles). The pore radius and porosity of soil increased, while the micro pore volume decreased, and the small pore volume, medium pore volume and large pore volume increased, and the large pore volume had a little variation after 10 F–T cycles. The number of pores of F–L–S was less than L–S, demonstrating that the addition of fiber helped to reduce the pore volume. The interweaved fibers limited the development and the connection of cracks. By means of the spatial restraint effect of fiber on the soil and the friction action between fiber and soil, the shear performances and freeze–thaw durability of F–L–S better were than that of L–S.     

Evaluation of back-calculated elastic moduli of unreinforced and geocell-reinforced unbound granular material from full-scale field tests

This paper presents a field-scale experimental track over a poor subgrade with an unreinforced section and a geocell-reinforced section subjected to in-situ performance tests. Plate load tests and Benkelman beam tests were carried out distributed in several unreinforced and reinforced layers. The objective was to: (1) examine the variability of the elastic modulus of unbound granular material (UGM) due the influence of its thickness and the presence of poor subgrade in its base, (2) evaluate the modulus improvement factor (MIF) generated by the geocell reinforcement in the UGM and (3) verify the most appropriate condition to apply the MIF to transport infrastructure design. The results showed that there is a significant influence of the thickness of the UGM layer on its elastic modulus when the layer is supported directly over a soft subgrade. The MIF values obtained in field suggest that its determination is mostly related to the UGM maximum elastic modulus rather than its decreased values (by virtue of poor subgrade or reduced thicknesses), and that the analytical formulation presented for MIF calculation has good predictive capability to be applied to pavement design.     

Influence of thermal runaway in styrene–acrylonitrile bulk copolymerization revealed by computational fluid dynamics modeling

The computational fluid dynamics (CFD) and kinetic-based moment methods coupled approach is adopted to simulate the bulk copolymerization of styrene–acrylonitrile (SAN) in a stirred tank reactor. Numerical simulations are carried out to investigate the impacts of impeller speed, monomer ratio, initiator ratio, and initial reaction temperature on the copolymerization process and product properties. Particularly, the Chaos theory is selected as a criterion for evaluating the occurrence of the thermal runaway. The Flory's and Stockmayer's distributions are employed to calculate chain length distribution and copolymer composition distribution of copolymer. The simulation results highlight that the appearance of thermal runaway can be postponed by properly increasing the rotation speed, decreasing the initiator loadings, initial acrylonitrile contents and initial reactor temperature. Furthermore, significant differences exist in the product properties that predicted by the ideal and non-ideal models, which demonstrates that the temperature heterogeneity plays a crucial role in SAN copolymerization. This study could offer references for the safe operation and design of polymerization processes.     

倾斜入射下的单截面平面度绝对检验

针对大尺寸光学平面的直线度的纳米级测量精度需求，提出了倾斜入射下单截面平面度绝对检验方法，实现了对超过相移干涉仪口径的长平晶绝对检验。利用棱镜转向实现倾斜入射角度的精密预标定，棱镜标定角度的精度高于圆光栅和图像分析等方法，可提高测量不确定度到0.0042 μm。对比了常规三面互检绝对检验结果与本方法的差异，在相同尺寸下，直线度误差仅为1.2 nm。在确认标定反射镜位置后，整个倾斜入射的干涉图调整过程将被完全集中到待测长平晶的工作面上，不需要再对反射平晶进行操作。调整长平晶时各个维度的操作互不干涉，可快速简便地得到测量结果。     

Lanthanide doped fluorosilicate glass-ceramics:A review on experimental and theoretical progresses

Significant developments have been made in the past few decades for lanthanide(Ln)ions doped fluorosilicate glass-ceramics(Flusi-GCs).As novel generation of luminescence materials with a wide range of applications,Flusi-GCs as a single host combine the advantages of glass and ceramics/crystals as well as fluorides and silicates.In this review,the chemical design principles and experimental procedures of Flusi-GCs are summarized in detail.Flusi-GCs are categorized as those containing PbxCd_1-xF₂,RF₃(R=Y,La,Gd),MF₂(M=Ca,Sr,Ba),xMF₂-yRF₃(R=Y,La-Lu),mAF-nRF₃(A=Li,Na,K),KTF₃(T=Zn,Mn)and K2 SiF6 nanocrystals(NCs).Theoretical breakthroughs mainly by molecular dynamic(MD)simulation have been recapitulated as efficient routes for composition-design,nano-crystallization-prediction,and performance-optimization of Flusi-GCs containing target fluoride NCs.Essential research progresses pertaining photonic applications have been made in random lasers,communication amplifiers,optical fibers,spectral converters,white light-emitting-diodes(WLEDs),and thermal sensors.In the end,we propose three future research directions for Flusi-GCs.