Stress Reduction Coefficient and Amplification Factor for Seismic Response of Ground

Incorporating the increase of shear modulus with depth (z) by using a depth exponent p, the response of linear visco-elastic ground under steady state harmonic vibration was examined. The soil mass lying above the rigid base was discretized into a large number of horizontal layers. For a given vertical thickness (Hr) of the upper ground material in the first mode of resonance, it was noted that the resonant frequency (fr) as well as the values of amplification factor (Mf) and the stress reduction coefficient (Cd) at a given z/Hr can be computed as a function of p and D, where D is the damping ratio of the material. An increase in D leads to (1) an increase in Cd; (2) a very little increase in fr; and (3) a decrease in Mf. On the other hand, an increase in p causes: (1) an increase in fr; (2) a marginal increase in Mf; and (3) a decrease in Cd. It was evident that the values of Mf and Cd at a given depth will be affected significantly by changes in the chosen thickness of the soil deposit in resonance.     

Celerity and Amplification of Supercritical Surface Waves

The amplification of supercritical waves in steep channels is examined analytically using a one-dimensional dynamic solution of the Saint-Venant equations. Existing methods were modified to describe the amplification of surface waves over a normalized channel length rather than over a single wavelength. The results are strikingly different, and a generalized graph shows that short waves amplify the most over a fixed channel length. The maximum amplification parameter over a normalized channel length is 0.53 when F = 3.44. Applications to the flood drainage channel F1 in Las Vegas indicate that the amplitude of waves shorter than 100 m would increase by 65% over a channel length of 543 m. These theoretical results await field verification. Supercritical waves could be dampened by increasing channel roughness to reduce the Froude number below 1.5.     

不同地应力下爆破振动效应分析

为研究岩石在不同地应力下的爆破振动效应,选择经验证过的岩石本构,利用ANSYS/LS-DYNA隐式—显式序列求解方法,对不同侧压力系数下岩石爆破过程进行数值分析。分别构建了侧压力系数为0.1、0.5、1.0、2.0、4.0条件下的计算模型,数值模拟结果表明：初始地应力对爆破有较强的抑制作用,岩石破碎区受侧压力系数的影响较小,爆破裂纹优先向较大的地应力方向延伸;岩石质点振动速度峰值（PPV）到达时间不受侧压力系数的影响;当侧压力系数小于1.0时,水平方向的质点振动速度峰值大于竖直方向;当侧压力系数大于1.0时,竖直方向的质点振动速度峰值大于水平方向。     

Saturation Effects of Soils on Ground Motion at Free Surface Due to Incident SV Waves

A study is presented of saturation effects of subsoil on seismic motions at the free surface of a half space due to an inclined (SV) wave. By treating the soil as a partially water-saturated porous medium that is characterized by its degree of saturation, porosity, permeability, viscosity, and compressibility, a theoretical formulation is developed for the computation of free-surface amplitudes in both the horizontal and vertical components, which are defined as a function of the degree of saturation, the angle of incidence, and the frequency. Numerical results are presented using typical sand properties. It is shown that even a slight decrease of full saturation may lead to a substantial influence on the free-surface amplitudes in both the components and the amplitude ratios between them, and this influence is dependent on the angle of incidence. Significant phase shift between the horizontal and vertical components may also occur due to this slight change in saturation. At small incident angles, partial saturation of subsoil generally may cause a greater vertical-to-horizontal ratio compared with a fully saturated model. It is suggested that one may need to carefully take into account the saturation condition in the interpretation of field observations on seismic ground motions.     

Frequency-Dependent Amplification of Unsaturated Surface Soil Layer

This paper presents a study of the amplification of SV waves obliquely incident on a surface soil layer overlying rock formation. Special attention is placed on the influence of the saturation states of the soil layer and the bedrock on the amplification in both horizontal and vertical directions as well as on the amplitude ratios between the two directions at the surface, where the vertical and horizontal amplification and the amplitude ratios are expressed as functions of the frequency of incident waves. The analysis indicates that while the influence of the saturation state of the bedrock is insignificant, a change of the saturation state of the soil layer may have a marked impact on the vertical amplification. For typical seismic frequencies, an unsaturated soil layer can generate greater vertical amplification than a saturated layer; it can also cause larger amplitude ratios between vertical and horizontal components at the surface. The analysis further confirms the potential importance of the saturation condition of near-surface soils in site response analysis.     

Effects of Particle Stratification on Fixed Bed Absorber Performance

The effects of adsorbent particle size distribution (PSD) and the layering of particles in stratified and reverse stratified modes on the performance of fixed bed adsorber were investigated. Using trichloroethylene as the adsorbate and granular activated carbon as the adsorbent, experimental studies were conducted in stratified beds for different flow rates and influent concentrations. The homogeneous solid diffusion model was modified to take into account PSD and was used to simulate breakthrough curves. The PSD-based model was validated using experimental data and was found to be more accurate in predicting the breakthrough curves than the non-PSD-based model. The validated model was used to conduct simulations to examine the effects of key variables on performance in the stratified and reverse stratified modes. In the reverse stratified mode, the adsorbent particle size decreases gradually in the direction of flow. Model simulations indicate that this mode of operation increases breakthrough time, decreases the time to reach saturation, and thereby increases the overall adsorbent capacity utilization. The mass transfer zone for the reverse stratified bed was found to be narrower and sharper than that for the stratified bed. These model predictions have important ramifications to the water and wastewater industry in terms of reducing the overall cost of treatment using granular activated carbon adsorption.     

Density Stratification Effects in Sand-Bed Rivers

In this paper the effects of density stratification in sand-bed rivers are studied by the application of a model of vertical velocity and concentration profiles, coupled through the use of a turbulence closure that retains the buoyancy terms. By making the governing equations dimensionless, it is revealed that the slope is the additional dimensionless parameter introduced by inclusion of the buoyancy terms. The primary new finding is that in general density stratification effects tend to be greater in large, low-slope rivers than in their smaller, steeper brethren. Under high flow conditions the total suspended load and size distribution of suspended sediment can be significantly affected by density stratification, and should be accounted for in any general theory of suspended transport.     

浅述夯扩灰渣土桩地基处理

采用由拆房土组成的碎石三合土为桩体材料的夯扩灰渣土桩进行软弱地基的地基处理,与同类型工程相比,不仅极大地降低工程费用,而且技术可行、施工简便。更主要地是能消耗掉大量的建筑垃圾,对城市环境保护起到了一定的作用。     

Effective Soil Density for Propagation of Small Strain Shear Waves in Saturated Soil

This technical note defines an “effective soil density” that controls the velocity of small strain shear waves in saturated soil. Biot theory indicates that the ratio of effective density to saturated density will generally range from 0.75 to 1.0 and is a function of specific gravity of solids, porosity, hydraulic conductivity, and shear wave frequency. For many geotechnical applications, effective density will be equal to saturated density for low hydraulic conductivity soils (clays and silts) and may be less than saturated density for high hydraulic conductivity soils (clean sands and gravels). The findings are relevant to applications involving the propagation of small strain shear waves through saturated soil, and in particular for laboratory and field tests in which shear modulus is back-calculated from measured shear-wave velocity.     

Effects of Near-Fault Ground Shaking on Sliding Systems

A numerical study is presented for a rigid block supported through a frictional contact surface on a horizontal or an inclined plane, and subjected to horizontal or slope-parallel excitation. The latter is described with idealized pulses and near-fault seismic records strongly influenced by forward-directivity or fling-step effects (from Northridge, Kobe, Kocaeli, Chi-Chi, Aegion). In addition to the well known dependence of the resulting block slippage on variables such as the peak base velocity, the peak base acceleration, and the critical acceleration ratio, our study has consistently and repeatedly revealed a profound sensitivity of both maximum and residual slippage: (1) on the sequence and even the details of the pulses contained in the excitation and (2) on the direction (+ or ?) in which the shaking of the inclined plane is imposed. By contrast, the slippage is not affected to any measurable degree by even the strongest vertical components of the accelerograms. Moreover, the slippage from a specific record may often be poorly correlated with its Arias intensity. These findings may contradict some of the prevailing beliefs that emanate from statistical correlation studies. The upper-bound sliding displacements from near-fault excitations may substantially exceed the values obtained from some of the currently available design charts.     

Torsional Surface Waves in Nonhomogeneous Anisotropic Medium under Initial Stress

This paper studies the possibilities of propagation of torsional surface waves in nonhomogeneous anisotropic half-space under compressive initial stress. Two types of variations in nonhomogeneity, mainly hyperbolic and quadratic, have been discussed, and it is observed that in both cases the torsional surface wave will propagate in the media under consideration. The velocities of propagation have been computed and are presented in graphs. The study shows that for hyperbolic variation of nonhomogeneity, the increase of the anisotropic factor, increases the velocity of propagation, always keeping it more than that of shear wave in homogeneous medium. The presence of initial stress increases the velocity of propagation. In the case of quadratic variation it is found that the presence of initial stresses increases the velocity of propagation. The increase of anisotropy decreases the velocity, and the decrease in the nonhomogeneity factor increases the velocity, always keeping the velocity of torsional surface waves less than that of shear wave in the homogeneous medium.     

矿山巷道地压形成机理及分布规律的研究

通过对国内外学者研究巷道地压问题论述的分析,总结出矿山巷道地压形成的一般机理,并运用数值分析软件和有限差分的基本原理,对半圆拱形巷道在一定的侧压系数条件下开挖前后围岩应力状态进行数值模拟与分析,总结出半圆拱形巷道开挖后围岩应力的分布规律。     

Effect of Gravity and Initial Stress on Torsional Surface Waves in Dry Sandy Medium

This problem studies the effect of gravity and initial stress on the propagation of torsional surface waves in dry sandy medium. The mathematical analysis of the problem has been dealt with the Whittaker function. Assuming the expansion of the Whittaker function up to linear term, it is concluded that the gravity field will always allow torsional waves to propagate. The expansion of the Whittaker function up to quadratic terms shows that two such wave fronts may exist in the medium. Finally, it is concluded that the sandy medium without support of a gravity field cannot allow the propagation of torsional surface waves, where as the presence of a gravity field always supports the propagation of torsional surface waves regardless of whether the medium is elastic or dry sandy.     

Analysis of Tunneling-Induced Ground Movements Using Transparent Soil Models

Ground movements induced by shallow tunnels affect the safety of nearby underground and aboveground structures. Therefore, the reliable prediction of these movements is important. A transparent soil model is used to investigate not only the surface settlement profile induced by shield tunneling, but also the distribution of soil deformation within the soil mass near the tunnel. The observed surface settlements are consistent with the normal probability curve commonly used for predicting settlement, with only the inflection points or trough width parameters somewhat different. The measured data are consistent with field measurements in that the trough width parameter is independent of the volume loss and linearly proportional to the tunnel depth. An analysis of the displacement field inside the transparent soil models indicates that the subsurface settlement trough at different depths can be approximated by a normal probability curve; and the horizontal displacement can be expressed by the trough width parameter and the volume loss, at the point at which maximum horizontal displacement occurs at the point of inflection. Additionally, the measurements indicate that subsurface ground movements can be in excess of the observed surface settlement, which can adversely affect underground utilities.     

预裂-缓冲爆破中柱状孔间应力波作用分析

利用新型全息动光弹,以南芬矿高台阶靠帮控制爆破的现场参数为实验模拟基础,研究了两预裂孔间应力波迭加的动态变化过程。在两炮孔均实行孔底和孔口同时起爆(四起爆点),并对孔底进行加强装药条件下,获得了不同时刻的等差条纹图。发现沿炮孔方向应力波分布有很大不同,提出了初始P波与后出现的S波迭加形成的波是所有波中最重要的部分。研究表明,孔底加强装药是深孔孔底产生预裂效果的关键因素,孔底连线上应力场强度不受端部效应影响。     

Effects of Jet Grouting on Adjacent Ground and Structures

The basement excavation of the Singapore Post Center involved extensive jet grouting to improve the soft marine clay present within the excavation. The treated soil mass, with much improved strength and deformation characteristics, was intended to act as an internal strut below the bottom of the excavation level, reducing movements caused by the basement excavation. This paper presents the performance of production grouting carried out during the construction of the building's basement. Results of monitoring suggest that the jet grouting caused the retaining diaphragm walls to move between 9.7 and 36.4 mm. The soils behind the walls also moved away from the excavation. Movements ranged from 35.3 to 53.6 mm within 1–2 m from the wall to 13.5 to 32.8 mm at 4.5–20.5 m away from the wall. The recorded soil heave ranged from 2 to 24 mm, with the majority of the measurements being less than 10 mm. The backward movements induced by the production grouting are similar to those induced by preloading a strut of braced excavation system. Provided the backward movements do not exceed the allowable limits, they would help in minimizing the ultimate positive movements induced by the subsequent basement excavation. The jet grouting also induced some bending moments on the diaphragm walls and caused the adjacent structures to tilt and move away from the jet grout area.     

X射线法测试Be应力的影响因素分析

根据X射线法测量应力原理，结合Be力学参数特点，分析了衍射晶面选择、应力梯度以及覆盖物等对应力测量结果的影响，同时还分析了铍材性能和处理工艺对应力的影响。结果表明，衍射晶面、应力梯度和处理工艺对应力值变化影响很大，测试结果不确定度相对较大。