Influence of site conditions on seismic design parameters for foundations as determined via nonlinear site response analysis

Site conditions, including geotechnical properties and the geological setting, influence the near-surface response of strata subjected to seismic excitation. The geotechnical parameters required for the design of foundations include mass density (ρ), damping ratio (β_s), shear wave velocity (V_s), and soil shear modulus (G_s). The values of the last three parameters are sensitive to the level of nonlinear strain induced in the strata due to seismic ground motion. In this study, the effect of variations in soil properties, such as plasticity index (PI), effective stress (σ′), over consolidation ratio (OCR), impedance contrast ratio (ICR) between the bedrock and the overlying strata, and depth of soil strata over bedrock (H), on seismic design parameters (β_s, V_s, and G_s) was investigated for National Earthquake Hazards Reduction Program (NEHRP) site classes C and D, through 1D nonlinear seismic site response analysis. The Morris one-at-a-time (OAT) sensitivity analysis indicated that β_s, V_s, and G_s were significantly influenced by variations in PI, while ICR affected β_s more than it affected V_s and G_s. However, the influence of H on these parameters was less significant. It was also found that variations in soil properties influenced seismic design parameters in soil type D more significantly than in soil type C. Predictive relationships for β_s, V_s, and G_s were derived based on the 1D seismic site response analysis and sensitivity analysis results. The β_s, V_s, and G_s values obtained from the analysis were compared with the corresponding values in NEHRP to determine the similarities and differences between the two sets of values. The need to incorporate PI and ICR in the metrics for determining β_s, V_s, and G_s for the seismic design of foundations was highlighted.     

Bearing and uplift capacities of under-reamed piles in soft clay underlaid by stiff clay using lower-bound finite element limit analysis

Ensuring a safe foundation design in soft clay is always a challenging task to engineers. In the present study, the effectiveness of under-reamed piles in soft clay underlaid by stiff clay is numerically studied using the lower-bound finite element limit analysis (LB FELA). The bearing and uplift capacities of under-reamed piles are estimated through non-dimensional factors N_cul and F_cul, respectively. These factors increased remarkably and marginally compared to N_cul and F_cul of the piles without bulbs when the bulb is placed in stiff and soft clay, respectively. For a given ratio of undrained cohesion of stiff to soft clay (c₂/c₁), the factors N_cul and F_cul moderately increased with the increase in the length-to-shaft-diameter ratio (L_u/D) and adhesion factors in soft clay (α_s1) and stiff clay (α_s2). The variation of radial stress along the pile–soil interface, distribution of axial force in the under-reamed piles, and state of plastic shear failure in the soil are also studied under axial compression and tension. The results of this study are expected to be useful for the estimation of the bearing and uplift capacities of under-reamed piles in uniform clay and soft clay underlaid by stiff clay.     

基于有限元极限分析的双孔隧道稳定性分析

为进一步探讨在考虑土体内摩擦角情况下,双孔隧道在支护反力及地面超载共同作用下的相互作用规律及稳定性分析,首先,基于有限元极限分析法,采用MATLAB平台编制了相关计算程序;此外,为减少各参数之间的交叉影响,引入无量纲分析方法;然后,将本文模型的计算结果与已有成果进行对比,验证了本文分析方法的合理性;最后,在此基础上求得各影响因素下的隧道稳定性参数(N_s),并绘制了无量纲分析图表。结果表明:(1)N_s随φ的增大逐渐增大,且当φ较大时,在N_s未达到稳定值之前,对其产生显著影响;(2)N_s随S/B的增大而增大并趋于稳定;(3)φ的存在与否对N_s随着γB/c的变化产生相反的规律,且随φ的增大,γB/c的变化对临界间距比(S/B)的值影响减弱;(4)临界间距比随H/B的增大不断增大;(5)S/B和φ是影响隧道破坏模式的关键因素。     

Probabilistic models for the initiation of seismic soil liquefaction

A Bayesian framework for probabilistic assessment of the initiation of seismic soil liquefaction is described. A database, consisting of post-earthquake field observations of soil performance, in conjunction with in situ “index” test results is used for the development of probabilistically-based seismic soil liquefaction initiation correlations. The proposed stochastic model allows full and consistent representation of all relevant uncertainties. including (a) measurement/estimation errors, (b) model imperfection, (c) statistical uncertainty, and (d) inherent variabilities. Different sets of probabilistic liquefaction boundary curves are developed for the seismic soil liquefaction initiation hazard problem, representing various sources of uncertainty that are intrinsic to the problem. The resulting correlations represent a significant improvement over prior efforts, producing predictive relationships with enhanced accuracy and greatly reduced overall model uncertainty.     

An improved experimental medium for freshwater toxicity studies using Daphnia magna

Some of the difficulties involved in formulating experimental media for use in toxicity studies are pointed out, with particular reference to heavy metals and their possible chelation by agents in the media.

A medium suitable for Daphnia culture with the green alga, Chlorella sp. was chosen from the literature to investigate physiological responses by Daphnia to sublethal cadmium poisoning. Like most synthetic media this included the labile Fe-EDTA complex as a source of soluble iron which is shown to be essential for growth of both Daphnia and Chlorella. Attention is drawn to the almost universal complexing ability of EDTA with metal ions, and to the mechanisms whereby EDTA may be released from the Fe-EDTA complex under experimental conditions. Synthesis of a ferri-gluconate complex as an alternative source of soluble iron is described; this complex being theoretically more iron-specific than Fe-EDTA, whilst providing equally well for Chlorella growth and Daphnia growth and fecundity. Restriction of pH variation in the experimental medium, within its chemically stable range of 6.0–9.0, is shown to be difficult over 10 days: most buffers with a suitable pK_a value (ca. 7.5) being toxic or having some complexing ability. TES buffer (pK_a 7.5) and HEPES buffer (pK_a 7.55) were used at various concentrations, and 0.001 M HEPES is shown to restrict pH variation to within 0.5 pH units over 10 days without adversely affecting Chlorella growth or Daphnia growth and fecundity.

Chemical estimation of the degree of cadmium-complexing by each iron source and each buffer under investigation was not possible since the presence of chelating agents imposes restrictions on the methods available, particularly at low cadmium concentrations. Instead, a bioassay of the reduction in 48 h acute toxicity of cadmium to Daphnia in media containing each potential chelating agent in turn is used. These data indicate that 0.001 M HEPES buffer does not complex cadmium at 0.1 ppm, and that ferri-gluconate is less likely to do so than ferri-EDTA.     

考虑多重不确定性的我国多层#br# 冷成型钢结构地震风险评估

准确评估建筑结构的地震风险是实现其性能化抗震设计的重要组成部分。冷成型钢结构体系作为绿色和工业化建筑的典型代表,却鲜有针对于我国的地震风险评估研究。文章旨在分析和评估多层冷成型钢结构抗震分析所涉及的多重不确定性因素,进而定量评估该类结构在我国的地震风险。文章设计1栋八度抗震设防的6层冷成型钢结构,基于经过振动台验证的简化数值分析方法建立结构的简化数值模型,定量求解结构的多重不确定性因素和在使用周期内(50年)的地震风险。研究结果表明：由于我国一些地区的地震危险性形状系数较大,在考虑多重不确定性后,结构的破坏概率显著提高,50年使用周期内倒塌概率的增幅达到10倍以上;算例结构在50年使用周期内基本满足“小震不坏、中震可修、大震不倒”的抗震设防要求,对于个别特殊区域应适当控制结构发生轻微破坏的概率。此外,文章建议应全面考虑多重不确定对冷成型钢结构地震风险的影响,对不确定性参数的取值和地震风险的计算方法可提供借鉴。     

Predicting lateral displacement caused by seismic liquefaction and performing parametric sensitivity analysis: Considering cumulative absolute velocity and fine content

Lateral displacement due to liquefaction (D_H) is the most destructive effect of earthquakes in saturated loose or semi-loose sandy soil. Among all earthquake parameters, the standardized cumulative absolute velocity (CAV₅) exhibits the largest correlation with increasing pore water pressure and liquefaction. Furthermore, the complex effect of fine content (FC) at different values has been studied and demonstrated. Nevertheless, these two contexts have not been entered into empirical and semi-empirical models to predict D_H. This study bridges this gap by adding CAV₅ to the data set and developing two artificial neural network (ANN) models. The first model is based on the entire range of the parameters, whereas the second model is based on the samples with FC values that are less than the 28% critical value. The results demonstrate the higher accuracy of the second model that is developed even with less data. Additionally, according to the uncertainties in the geotechnical and earthquake parameters, sensitivity analysis was performed via Monte Carlo simulation (MCS) using the second developed ANN model that exhibited higher accuracy. The results demonstrated the significant influence of the uncertainties of earthquake parameters on predicting D_H.     

Torsional response of base-isolated structures due to asymmetries in the superstructure

The torsional response of base-isolated structures when eccentricities are set in the superstructure is presented. Nonlinear dynamic analyses were used to study peak responses for different ratios of static eccentricities (e_s) between the center of mass and the center of rigidity for the superstructure. Unidirectional and bidirectional actions of selected ground motions typical of firm soils of the Mexican Pacific Coast were used in the study. An effective period range between 1.5 and 3.0 s (1.5 s≤T_I≤3.0 s) for the base-isolated structures was considered in the present study. Bilinear isolator systems with yield forces of 5% and 10% the weight of the complete structure (V_y/W=0.05 and V_y/W=0.10) and a postyield stiffness of 10% their elastic stiffness (k₂/k₁=0.10) were considered. Peak dynamic responses such as maximum isolator displacements and peak displacement ductility demands were studied and compared to the ones obtained for symmetric systems of reference for the different ground motions under consideration, assessing the importance of the relative value of e_s on those response quantities.     

Toxicity of chlorine to juvenile spot, Leiostomus Xanthurus

The sensitivity of juvenile spot, Leiostomus xanthurus, to total residual chlorine (TRC) in flowing sea-water was investigated. Incipient LC₅₀ bioassays, histopathology, avoidance tests and the combined effect of thermal stress and TRC were used to assess sensitivity.

Estimated incipient LC₅₀ values were 0.12 mg 1⁻¹ TRC at 10°C and 0.06 mg 1⁻¹ TRC at 15°C. Histological examination of spot used in the incipient LC₅₀ bioassay at 15°C and sacrificed while alive indicated pseudobranch and gill damage occurred in individuals exposed to a measured TRC concentration of 1.57 mg 1⁻¹. Spot exposed to lower concentrations of TRC, 0.02 0.06 mg 1⁻¹ at 15°C and sacrificed alive showed no consistent tissue damage.

Spot demonstrated temperature dependent avoidance responses to TRC. At 10°C, a concentration of 0.18 mg 1⁻¹ was required for significant (X²; P < 0.05) avoidance; at 15 and 20°C, spot showed significant avoidance of TRC concentrations as low as 0.05 mg 1⁻¹.

Simultaneous exposure of spot to thermal stress (5, 10 or 13°C above the acclimation temperature of 15°C) at measured TRC concentrations of 0.05 0.07 and 0.34–0.52 mg 1⁻¹ demonstrated a significant, (Z²) with Yates correction, P < 0.05) increase in sensitivity to TRC with increased temperature and exposure times for some of the groups tested.     

Ammonia oxidation in nitrosomonas at NH3 concentrations near km: Effects of pH and temperature

Nitrosomonas europaea from continuous pure cultures was incubated with 26.4 μ M NH₃(= 0.37 mg NH₃-N l⁻¹) at various NH₄⁺ concentrations, pH values and temperatures. Measured rates of nitrite formation were significantly influenced by pH. Likewise unexpectedly, the maximum ammonia oxidation rate occurred between pH 6.7 and 7.0. Temperature had an even stronger effect on the rate of ammonia oxidation than the availability of NH₃. It is concluded that the assumption of a strict dependence of the rate of ammonia oxidation on substrate concentration is an unjustified oversimplification. Among the mechanisms which could explain ammonium uptake and oxidation near or below pH 7.0, the formation of NO from HNO₂ is considered.     

Evaluation of kinetic parameters of a sulfur-limestone autotrophic denitrification biofilm process

In this study, four kinetic parameters of autotrophic denitrifiers in fixed-bed sulfur–limestone autotrophic denitrification (SLAD) columns were evaluated. The curve-matching method was used by conducting 22 non-steady-state tests for estimation of half-velocity constant, K_s and maximum specific substrate utilization rate, k. To estimate the bacteria yield coefficient, Y and the decay coefficient, k_d, two short term batch tests (before and after the starvation of the autotrophic denitrifiers) were conducted using a fixed-bed SLAD column where the biofilm was fully penetrated by nitrate-N. It was found that K_s=0.398 mg NO₃⁻–N/l, k=0.15 d⁻¹, k_d=0.09–0.12 d⁻¹, and Y=0.85–1.11 g VSS/g NO₃⁻–N. Our results are consistent with those obtained from SLAD biofilm processes, but different from those obtained from suspended-growth systems with thiosulfate or sulfur powders as the S source. The method developed in this study might be useful for estimation of four Monod-type kinetic parameters in other biofilm processes. However, cautions must be given when the estimated parameters are used because the measurements of the biomass and the biofilm thickness could be further improved, and the assumption of sulfur being a non-limiting substrate needs to be proved.     

Effect of exposure time and copper concentration on reproduction of the fathead minnow (Pimephales Promelas)

Three concurrent studies were conducted to determine the chronic effect of prespawning exposure to various concentrations of copper on fathead minnow reproduction. Copper was introduced into the three exposure systems to give 6-, 3-, and 0-months exposure prior to spawning. Prespawning exposure time had no significant effect on reproduction. Number of eggs produced per female decreased, however, with increase in copper concentrations. Egg production at copper concentrations of 37μg 1⁻¹ and higher was significantly lower (P = 0.05) than in the control, but at concentrations of 24μg 1⁻¹ and lower it was not different. The maximum acceptable toxicant concentration (MATC) was estimated to be 32μg Cu 1⁻¹, which is 0.07 of the 96h LC₅₀. This application factor for copper is similar to those found in other studies.     

Effect of silica fume and steel fibers on some properties of high-strength concrete

The main disadvantage of high-strength concrete is its highly brittle behavior and this can beovercome by adding fibers to the concrete. This would also improve some other mechanical properties of high-strength concrete such as tensile strength and compressive strength. These properties are not very well established for high-strength steel-fiber reinforced concrete (HSFRC) yet. In this study the influence of silica fume on the properties of HSFRC were investigated by using silica fume of two different percentages and three different hooked-end fibers namely, 30/0.50, 60/0.80 and 50/0.60 length/diameter (mm/mm). Fibers were added to concrete in three different volume percentages of 0.5, 1.0 and 2.0 by volume of concrete. The results indicated that there is a linear function between splitting tensile strength (F_splt) and volume percentage of fibers (V_f) [i.e. F_plt = A(V_f) + B, where A and B are correlation coefficients] as well as between splitting tensile strength (F_splt) and compressive strength (F_c) of plain series A concrete [i.e. F_splt = C (√F_c) + D, where C and D are correlation coefficients]. These relations can describe the development of splitting tensile strength of HSFRC containing no silica fume, 5% silica fume and 10% silica fume by weight of cement. On the other hand, although silica fume has an effect on compressive strength, volume percentage and aspect ratio of steel fibers has little effect.     

SV波入射下均匀饱和地层渗透系数对深埋隧道的影响机制

对于饱和场地中地下结构的抗震分析问题,地层渗透系数决定了孔隙水与土骨架耦合作用的强弱,是研究饱和地层–结构动力相互作用的关键参数之一。然而,目前研究通常基于渗透系数为零或无穷大的假定,忽略了实际地层的渗透特性且对其影响机制尚不清晰。针对不同类型均匀饱和地层对应的渗透系数分布范围,基于Biot两相介质理论,首先建立了饱和地层不同渗透系数情况下的u-w格式模型,推导渗透系数分别为有限值、零和无穷大3种情况下P₁,P₂,S波的波速解析表达式,分析了地层渗透系数变化对自由场波速的影响规律;进而,建立了不同类型饱和地层与深埋隧道动力作用分析模型,并与典型算例精确解对比验证了本模型的有效性;在此基础上,分别研究了砂岩、卵石土、砂土、粉土4种类型饱和地层渗透系数变化及两种极端值情况下隧道结构及周围地层孔隙水压力的响应机制。结果表明:对于不同类型饱和地层,3种体波在地层中的传播速度均随渗透系数k增加呈上升趋势,当k<10^-4 m/s时,波速为最小值且与k→0极端情况一致,当k>10~2 m/s时,波速为最大值且与k→∞极端情...     

Empirical rate equation for trihalomethane formation with chlorination of humic substances in water

Trihalomethane (THM) in drinking water is formed by chlorination of humic substances. In this study, the rates of THM formation in aqueous solution of humic acid were examined under various conditions. The following rate equation was obtained empirically. [THM] = k (pH − a)[TOC][Cl₂]₀^mtⁿ.

Here, [THM] is the concentration of total THM after t h, [TOC] and [Cl₂]₀ are the concentrations of total organic carbon and chlorine dose, k is the rate constant and a, m and n are parameters. The values of k, a, m and n for humic acid as reagent were obtained as 8.2 × 10⁻⁴ (l^mmg^−mh⁻ⁿ), 2.8, 0.25 and 0.36, respectively. The activation energy was obtained as 37 kJ mol⁻¹. Further, it was proved that the above equation could be applied to the rates of THM formation from precursors in actual river and lake waters.     

Effect of interface adhesion factor on the bearing capacity of strip footing placed on cohesive soil overlying rock mass

The problem related to bearing capacity of footing either on pure soil or on pure rock mass has been investigated over the years. Currently, no study deals with the bearing capacity of strip footing on a cohesive soil layer overlying rock mass. Therefore, by implementing the lower bound finite element limit analysis in conjunction with the second-order cone programming and the power cone programming, the ultimate bearing capacity of a strip footing located on a cohesive soil overlying rock mass is determined in this study. By considering the different values of interface adhesion factor (α_cr) between the cohesive soil and rock mass, the ultimate bearing capacity of strip footing is expressed in terms of influence factor (I_f) for different values of cohesive soil layer cover ratio (T_cs/B). The failure of cohesive soil is modeled by using Mohr−Coulomb yield criterion, whereas Generalized Hoek−Brown yield criterion is utilized to model the rock mass at failure. The variations ofI_f with different magnitudes of α_cr are studied by considering the influence of the rock mass strength parameters of beneath rock mass layer. To examine stress distribution at different depths, failure patterns are also plotted.     

基于Copula理论岩体质量Q值及波速与变形模量多变量相关性研究

 以锦屏I级水电站坝址大理岩岩体原位变形试验和勘察结果为例,在分析岩体变形模量 、纵波波速 以及岩体质量指标Q值间的两两相关关系的基础上,利用Copula函数能够描述多变量相关性的优点,构造了拟合 , 和Q值间关系的最优Copula函数,并在岩体变形模量估值中进行运用。研究结果表明,三维Clayton Copula函数是拟合岩体波速 ,Q值和变形模量 间相关关系最优Copula函数。利用构造的 - -Q间关系的最优Copula函数求条件概率,可在已知岩体 和Q值条件下计算其他方法对变形模量估值的保证率,或者计算具有一定保证率的岩体变形模量估值,得到具有最优拟合的变形模量估值。该方法建立多因素间的相关关系,使现场试验和勘察的数据得到充分利用,为建立岩体变形模量与岩体参数间相关关系及估值提供一种新的途径。     

A miniature triaxial apparatus for investigating the micromechanics of granular soils with in situ X-ray micro-tomography scanning

The development of a miniature triaxial apparatus is presented. In conjunction with an X-ray micro-tomography (termed as X-ray μCT hereafter) facility and advanced image processing techniques, this apparatus can be used for in situ investigation of the micro-scale mechanical behavior of granular soils under shear. The apparatus allows for triaxial testing of a miniature dry sample with a size of \mathrm{8}\hspace{0.17em}\mathrm{mm}\times \mathrm{16}\hspace{0.17em}\mathrm{mm} (diameter \times height). In situ triaxial testing of a 0.4–0.8 mm Leighton Buzzard sand (LBS) under a constant confining pressure of 500 kPa is presented. The evolutions of local porosities (i.e., the porosities of regions associated with individual particles), particle kinematics (i.e., particle translation and particle rotation) of the sample during the shear are quantitatively studied using image processing and analysis techniques. Meanwhile, a novel method is presented to quantify the volumetric strain distribution of the sample based on the results of local porosities and particle tracking. It is found that the sample, with nearly homogenous initial local porosities, starts to exhibit obvious inhomogeneity of local porosities and localization of particle kinematics and volumetric strain around the peak of deviatoric stress. In the post-peak shear stage, large local porosities and volumetric dilation mainly occur in a localized band. The developed triaxial apparatus, in its combined use of X-ray μCT imaging techniques, is a powerful tool to investigate the micro-scale mechanical behavior of granular soils.     

Prediction on CO2 uptake of recycled aggregate concrete

Carbonation of concrete is a process which absorbs carbon dioxide (CO₂). Recycled aggregate concrete (RAC) may own greater potential in CO₂ uptake due to the faster carbonation rate than natural aggregate concrete (NAC). A quantitative model was employed to predict the CO₂ uptake of RAC in this study. The carbonation of RAC and the specific surface area of recycled coarse aggregates (RCAs) were tested to verify accuracy of the quantitative model. Based on the verified model, results show that the CO₂ uptake capacity increases with the increase of RCA replacement percentage. The CO₂ uptake amount of 1 m³ C30 RAC within 50 years is 10.6, 13.8, 17.2, and 22.4 kg when the RCA replacement percentage is 30%, 50%, 70%, and 100%, respectively. The CO₂ uptake by RCAs is remarkable and reaches 35.8%–64.3% of the total CO₂ uptake by RAC when the RCA storage time being 30 days. Considering the fact that the amount of old hardened cement paste in RCAs is limited, there is an upper limit for the CO₂ uptake of RCAs.