Performance of D-bolts Under Static Loading

D-bolt is a type of energy-absorbing rock bolt. It is made of a smooth steel bar with anchors spaced along the bolt length. A typical section between adjacent anchors is approximately 1-m long, but it can be adjusted to adapt to the rock conditions. The bolt is fully encapsulated with either cement or resin grout in a borehole. The anchors are firmly fixed into the grout, while the smooth bolt sections can freely deform to absorb deformation energy. Full-scale static pull tests were carried out at different testing facilities in two laboratories. The tests show that a smooth bolt section between anchors may elongate by 110–167 mm depending on the section length. Field trials of the D-bolt were conducted in deep metal mines. The measurements showed that the D-bolts were equally loaded within every anchor-between section, avoiding load peaks and premature bolt failure due to stress concentrations caused by fracture/joint opening. The field trials of rebar and D-bolts in a largely deformed mine tunnel showed that the D-bolts behaved satisfactorily, with only a few failed bolts, while a number of the rebar bolts failed at the thread.     

Dynamic Analysis of Subway Structures Under Blast Loading

Public transit systems have become one of the targets of terrorist attacks using explosives, examples of which are the 1995 attack on Paris subway and the 2004 attack on Moscow subway. Considering the intense threats of terrorist attacks on subway systems in metropolitan areas, explicit three-dimensional Finite Element method was used to investigate the dynamic response and damage of subway structures under internal blast loading. The study was motivated by the fact that explosion in subway structure may not only cause direct life loss, but also damage the subway structure and lead to further loss of lives and properties. The study based on the New York subway system, and investigated the influences of various factors on the possible damage of subway tunnel, including weight of explosive, ground media, burial depth and characteristics of blast pressure. A mitigation measure using grouting to improve ground stiffness and strength was also analyzed. Considering the amount of explosive terrorists may use, the present study focused on small-diameter single-track tunnels, which are more vulnerable to internal blast loading and are common in New York City. Blast pressure from explosion was applied to lining surface assuming triangle pressure–time diagram, and the elasto-plasticity of ground and lining as well as their nonlinear interaction was taken into account in the numerical model. It is found from the numerical study that maximum lining stress occurred right after explosion, before the blast air pressure reduced to the atmospheric one, and it was more dependent on the maximum magnitude of air pressure than on the specific impulse, which is the area below the pressure–time curve. Small tunnels embedded in soft soil, with small burial depth, might be permanently damaged even by modest internal explosion that may be perpetuated by terrorists.     

Numerical Study of Behavior of Circular Footing on Geogrid-Reinforced Sand Under Static and Dynamic Loading

A series of axi-symmetry models using finite element analyses were performed to investigate the behavior of circular footings over reinforced sand under static and dynamic loading. Geogrid was modeled as an elastic element and the soil was modeled using hardening soil model which use an elasto-plastic hyperbolic stress–strain relation. Several parameters including number of geogrid layers, depth to the first geogrid layer, spacing between layers and load amplitude of dynamic loading are selected in this paper to investigate the influence of these parameters on the performance of reinforced systems under both static and dynamic loads. The numerical studies demonstrated that the presence of geogrid in sand makes the relationship between contact pressure and settlement of reinforced system nearly linear until reaching the failure stage. The rate of footing settlement decreases as the number of loading cycles increases and the optimum values of the depth of first geogrid layer and spacing between layers is found 20% of the footing diameter. Some significant observations on the performance of footing-geogrid systems with change of the values of parametric study are presented in this paper.     

动荷载作用下辽西冻风积土的损伤演化分析

为了研究辽西冻风积土在动荷载作用下的损伤演化规律,采用动三轴仪进行了室内试验,获得了动本构关系、损伤演化方程以及损伤变量随应变的演化曲线。同时,针对冻风积土损伤的初始阶段,利用扫描电镜观测了微观结构。研究结果表明,冻风积土的损伤演化过程可以采用3个阶段进行描述。损伤的扩展速度在塑性损伤阶段发展较快,并且损伤变量的增长过程具备非线性特征;在微裂纹扩展阶段相对平缓,损伤变量与应变呈近似的线性增长关系。内部微裂纹的产生起于试样的中部,然后由中部向两端呈环状扩展。经过裂纹的交错和汇聚,斜向裂纹逐步形成和扩展。     

基于数值模拟研究岩石材料冲击载荷下的动态抗压强度增强

为了确定岩石材料在SHPB实验中的真实应变率效应,需要去除实验中的横向惯性效应和端面摩擦效应作用引起的动态抗压强度增量。本文假定材料的真实应变率效应、横向惯性效应和端面摩擦效应三者不相关,基于数值模拟的方法确定岩石材料的横向惯性效应和端面摩擦效应。将从SHPB实验中得到的动态抗压强度数据减去分别由横向惯性效应和端面摩擦效应引起的动态压缩强度增强量,即可获得岩石类材料在SHPB实验中的真实应变率效应。     

Behaviour of Track Ballast Under Repeated Loading

Najaf-sea quarry is located in Najaf city about 160 km south west of Baghdad the capital of Iraq. It is the main source that supplies track ballast for maintenance of existing railway network and construction of new railway lines in the middle and southern parts of Iraq. Track ballast experience a complex combination of stresses during its service lifetime, primarily from repeated axial loads of the trains in addition to stresses generated from the environmental conditions. The ideal evaluation of suitability of track ballast must be carried out under real field loading conditions, however such field tests are usually costly and time consuming. On the other hand laboratory model tests simulating field loads under limited boundary conditions can provide satisfactory indication about the suitability of the material. The present paper investigates the deformation characteristics of Najaf-sea track ballast, under repeated loading using model tests simulating ballast conditions under a selected track section. A test setup was designed and manufactured capable of applying both monotonic as well as repeated loading on the track section under different conditions. The repeated model tests which simulate as close as possible the field conditions shed the light on the generated settlement, modulus of deformation and degradation of the ballast particles under different repeated loading levels. Statistical analysis in terms of breakage index and repeated applied load revealed satisfactory correlations that help in understanding the overall performance of the ballast material. The results also demonstrate that 4–5 tamping are capable of controlling both the settlement and modulus of deformation of the ballast material.     

Analysis of the Dynamic Performance of an Underground Excavation in Jointed Rock under Repeated Seismic Loading

Results from field observations of dynamic behaviour of an underground excavation have been compared with numerical studies of the rock deformation history. The field behaviour shows progressive accumulation of rock displacement and excavation deformation under successive episodes of dynamic loading. It is possible to reproduce the modes of rock response quite well using a Distinct Element model of the rock mass, but the way displacements develop is dependent on the joint model used in the analysis. It is suggested that, in rock masses subject to repeated dynamic loading, excavation design may need to take account of the prospect of repeated episodes of transient loading at the excavation site.     

State-of-the-Art Modelling of Soil Behaviour Under Blast Loading

A comprehensive literature review has been carried out on existing models that characterize soil response under the impact of blast shock waves. Various models in the literature are reviewed and discussed in terms of their equations of state that account for the effect of high pressure, failure models that control the yield behaviour, and strength models that represent the effect of high strain-rates, along with a comparison of their advantages and limitations. Then, the application of different soil models to blast-induced liquefaction is elucidated and compared. Consequently, this review provides a comprehensive understanding of the fundamental and unique aspects of modelling soil response subjected to such transient impulsive loading on the grounds of increasing global interest in blast response of soils.     

Behaviour of Backfill Undergoing Cementation Under Cyclic Loading

Geotechnical and Geological Engineering - Cemented paste backfill (CPB), a man-made soil undergoing cementation, is extensively applied to support underground mine openings or spaces and provide...     

Stability of Asymmetric Roof Wedge Under Non-Symmetric Loading

Summary. An analytical method is presented for the calculation of the load carrying capacity of two-dimensional asymmetric rock wedges when the loading on the joint faces is non symmetric, such as the case of an asymmetric wedge formed in the roof of a circular tunnel in an inclined stress field. The pull out force that causes yield at one of the joint faces is evaluated from formulae based on the limiting equilibrium conditions assuming a purely frictional joint resistance. Next, the total pull out force required for the secondary face to yield is calculated. During this step, the wedge is further displaced and while the primary yielding face is plastically deformed, the other face is still in the elastic range until failure. Validation of the analytical procedure is obtained with the UDEC code, which provides an implementation of the Distinct Element Method in two dimensions. When the assumptions made in the analytical procedure are valid, the analytically calculated values for the pull out resistance of the wedge are computed to be close to the numerically obtained ones.     

Behaviour of Cellular Reinforced Sand Under Triaxial Loading Conditions

Cellular reinforcement is a three dimensional reinforcement used for reinforced soil structures. Behaviour of such reinforcement is important for its use in actual practice. Present paper focuses on the behavior of cellular reinforcement in sand under the triaxial loading conditions. Series of triaxial tests are performed on unreinforced and reinforced sand with single layer as well as double layers of cellular reinforcements with 75 mm sample diameter. Six different reinforcement heights of cellular reinforcements (varying from 3 to 50 mm) are used along with one sheet reinforcement of thickness 1 mm. From the experimental failure patterns of the triaxial samples, multiple zones of failure are observed as an effect of cellular reinforcement. Deviator stress–strain curves are studied for single and double layers of cellular reinforcement under three different confining pressures. Peak deviator stress is found increasing with increasing height of cellular reinforcement, which shows the confining effect of cellular reinforcement. Shear strength parameters are evaluated and are found increasing with increase in height of cellular reinforcement, also cellular reinforcement with heights 10 mm and more have showed increased shear strength parameters, as compared to 1 mm thick sheet reinforcement. This assures better behavior performance of cellular reinforcement over the planar one. Failure patterns are also visualized by finite element analysis and found in accord with experimental observations Horizontal displacement for reinforced samples visualized multi-zoned failure pattern. Finite element results for deviator stress–strain relationship are found in reasonably good accord with experimental results.     

Mixed-Mode Fracturing of Rocks Under Static and Cyclic Loading

Static diametrical compression tests conducted on inclined cracked chevron notched Brazilian disc (CCNBD) Brisbane tuff specimens showed that the notched cracks at the centre of the specimens opened (Mode I) up to 30° crack inclination angle (β), whereas crack closure (Mode II) started for β > 33°, and closure became more pronounced at even higher β of 45° and 70°. Both the experimental and numerical results showed that the crack initiation angle (θ) was a function of the β. Scanning electron microscope (SEM) images showed that fatigue damage on cyclic loading of Brisbane tuff is strongly influenced by the failure of the matrix due to both intergranular and transgranular fracturing.     

Coalescence of Fractures Under Uni-axial Compression and Fatigue Loading

In this paper, white Portland cement was used as an experimental material. Prismatic specimens with pre-existing flaws at different angles of inclination (α) varying through 0°, 30°, 45°, 60°, 75° to 90° and cylindrical specimens with different numbers of pre-existing flaws (n) varying through 0, 1, 2 to 3 were tested under uni-axial compression tests. Crack initiation, propagation, coalescence, and failure were observed. The corresponding analytical expression for the stress intensity factor under uni-axial compression was derived, the coefficient of friction and the stress intensity factor of the specimens on the surfaces of the crack were analysed, and the corrective coefficient for the stress intensity factor was introduced. Fatigue tests with a loading frequency of f = 100 Hz were carried out on cylindrical specimens with constant amplitude of the cyclic load which is a proportion of the compressive load at failure (F _f) obtained from the uni-axial compression tests. The fatigue property of the specimens was analysed and the relationship (S _max − lg N _f) between the maximum stress and the number of loading cycles at failure for specimens with pre-existing flaws was proposed. The effect of pre-existing flaws on the fatigue life (N _f) and dynamic load (S _D) which can be applied was investigated.     

加荷与卸荷过程中的冻土强度特性

模拟冻土与构筑物相互作用的实际过程，即加荷与卸荷过程，在－５℃条件下对冻结原状粉土进行了试验，发现两种受力过程中冻土的抗剪强度不一致，卸荷时冻土的强度值要高于加荷时的，加荷时其粘聚力ｃ＝０，内摩擦角ψ＝１７．５°；卸荷时ｃ＝０．５８，ＭＰａ，ψ＝１４．５°，当０．５≤σ≤４ＭＰａ时，卸荷时冻土的抗剪强度比加荷时的高１．３－３倍。纠正了长期以来认为两种受力过程中冻土的强度值是一致的看法。     

动荷载作用下冻结兰州黄土的动应变幅变化特征研究

采用分级循环正弦荷载模拟车辆振动, 通过动三轴试验对冻土的动应变幅进行了研究. 结果表明: 对于频率为0.1 Hz的前5级加载和频率≥0.5 Hz的各级加载作用下的动应变幅, 可以采用等价应变幅描述每一级动荷载作用下的动应变幅. 存在一个临界频率(约为7 Hz), 当加载频率小于该临界频率时, 等价应变幅变化复杂; 当加载频率大于该临界频率时, 等价应变幅随加载级数基本保持不变. 当加载频率≥0.5 Hz时,不同加载级数下等价应变幅随加载频率的增大而减小. 围压对等价应变幅影响较小; 当频率≤0.5 Hz时, -0.5 ℃条件下的等价应变幅略大于-1.0 ℃条件下的等价应变幅; 当频率>0.5 Hz时, 不同温度下的等价应变幅近似相等.     

冻结砂土在动荷载下的蠕变特征

通过分析不同试验条件下的蠕变过程曲线,探讨了冻结砂土在动荷载下的蠕变模型,分析了最大加载应力,温度及加载频率对冻土蠕变破坏应变,破坏时间和最小蠕变速率的影响.结果表明,当最大加载应力变大时,破坏应变增加,破坏时间缩短,最小蠕变速率变快;加载条件相同时,温度越低,破坏应变越小,破坏时间越长,最小蠕变速率越小;加载频率变化时,最小蠕变速率的变化无明显规律,都在一个量级范围内,当频率变大时,最小蠕变速率略有变小的趋势.频率增加,破坏时间缩短,破坏应变减小.频率小于7Hz时,频率对破坏应变和破坏时间影响较大,而当频率大于7Hz时,随频率加快,破坏时间和破坏应变只略有减小.