Monotonic and Cyclic Behavior of Chiang Mai Sand Under Simple Shear Mode

Record of damages and casualties caused by earthquakes in Thailand reveals that Chiang Mai, the second largest province of Thailand, has faced the great threat among other provinces. Triaxial tests, standard penetration tests, and multichannel analysis of surface wave have been recently performed to understand the dynamic response of Chiang Mai ground. However, the cyclic simple shear test, which could convincingly represent the real seismic ground shaking under repeated horizontal shear force, has not been conducted yet. Therefore, this paper aims to characterize the behaviors of Chiang Mai sand under monotonic and cyclic loadings using a direct simple shear apparatus. Sand specimens taken from the Ping River were prepared by dry deposition technique. The mobilized friction angle at the critical state measured by the direct simple shear test is equal to 37.3° which is 9 % less than that measured by direct shear box test. This paper also provides the appropriate dynamic properties of Chiang Mai sand with the shear strain level <2 % for any practical purpose. Test results showed that the normalized shear modulus agrees well with other investigators while the damping ratio is noticeably smaller than the lower-bound range for sand reported in the past studies.     

Collapse of a Model Strip Footing on Dense Sand Under Vertical Eccentric Loads

The paper focuses on the behaviour of a model strip footing, resting on a saturated dense sandy soil, subjected to centred or eccentric vertical loading. Experimental tests, carried out on a small-scale physical model, are able to reproduce effective stress levels equivalent to those prevailing in prototype problems, thanks to the maintenance of a downward steady-state seepage in the soil. The test program consists of three series of tests, each corresponding to an imposed value of hydraulic gradient, and each involving five load eccentricities; one series, in particular, is carried out with still water. Relevant points of load–settlement curves are related to the evolution of soil-footing collapse mechanism, evidenced by the distortion of some vertical coloured sand strips. The collapse mechanism is formed either by one or two sliding surfaces, depending on both load eccentricities and hydraulic gradient values. Significant differences are shown to occur between centred and eccentric loading footing response. Shear strength parameters obtained from back-analyses carried out on load values recorded at the appearance of each sliding surface on the free soil surface, in both hypotheses of associated and non-associated flow rule validity, are adopted to draw, for each test, a theoretical collapse mechanism consisting, in plane strain, of a log-spiral line with adjacent-end tangents; the obtained theoretical sliding surfaces, in turn, are compared to the experimental ones, showing that these latter are either stress characteristic or zero-extension lines depending mainly on cumulative footing displacements and current effective stress level in the soil.     

饱和砂土中桶形基础承载力的实验研究

桶形基础是近年开发的一种新型的可广泛应用于海洋工程结构的基础形式。由于多种优越性而受到各国石油部门的重视,并引起许多研究人员的关注。通过在饱和砂中的单桶和四桶基础模型实验,研究了桶形基础的静承载特性。分别进行了垂直方向和水平方向的加载实验,其中,四桶基础水平方向加载又分为沿四桶中心构成的正方形的平行边方向和对角线方向施加,得到了载荷位移曲线,对单桶和四桶基础承载力特性,以及加载方向和速率的影响进行了分析和比较。     

Influence of Relative Pile-Soil Stiffness and Load Eccentricity on Single Pile Response in Sand Under Lateral Cyclic Loading

The environment prevalent in ocean necessitates the piles supporting offshore structures to be designed against lateral cyclic loading initiated by wave action, which induces deterioration in the strength and stiffness of the pile-soil system introducing progressive reduction in the bearing capacity associated with increased settlement of the pile foundation. A thorough and detailed review of literature indicates that significant works have already been carried out in the relevant field of investigation. It is a well established phenomenon that the variation of relative pile-soil stiffness (K _rs ) and load eccentricity (e/D) significantly affect the response of piles subjected to lateral static load. However, the influence of lateral cyclic load on axial response of single pile in sand, more specifically the effect of K _rs and e/D on the cyclic behavior, is yet to be investigated. The present work has aimed to bridge up this gap. To carry out numerical analysis (boundary element method), the conventional elastic approach has been used as a guideline with relevant modifications. The model developed has been validated by comparing with available experimental (laboratory model and field tests) results, which indicate the accuracy of the solutions formulated. Thereafter, the methodology is applied successfully to selected parametric studies for understanding the magnitude and pattern of degradation of axial pile capacity induced due to lateral cyclic loading, as well as the influence of K _rs and e/D on such degradation.     

A Theoretical Model for the Circumferential Strain of Immersed Tunnel Elements Under Tidal Load

In this paper, we assumed that the soil layer back-silted on the surface of the river bottom was impermeable. The tidal load was treated as a large-area overload periodically applied on the silt soil layer. Taking into account the coupling loading of the water and soil, we built a model to calculate the circumferential strain of the immersed tunnel elements. By using MIDAS GTS NX finite element software, we calculated the circumferential strain in the tunnel wall direction under a tidal load. The circumferential strains of the tunnel elements in the Zhoushan immersed tunnel were continuously monitored using fiber Bragg grating sensors for 24 h. The circumferential strain increments at each monitoring position were calculated based on the change of the tide level. The results were compared with the measured increments of the circumferential strains. Based on these data, we summarized the variation pattern of the circumferential strain increments under a periodic tidal load. In addition, in this paper, we calculated the total circumferential strain distribution in the inner and outer walls in different cross-sections of the tunnel elements and identified positions that should be monitored with extra care. The results show that the calculated circumferential strain increments at each measuring position of the tunnel elements are in good agreement with the measured values. This demonstrates that the theoretical calculation model is reliable. In cross-sections of the same tunnel element, the time-dependence curves of the measured circumferential strain increments at the symmetrical measurement points are approximately consistent with each other. Tidal load-induced strain accounts for approximately 12–15% of the total strain. The correlation between circumferential strain over a cross-section and tidal level depends on the constraints on both sides of the cross-section, and stronger constraints result in weaker correlations. It is important to note that tensile strain is large along the inner wall of the middle portion of the top and bottom plates as well as along the outer wall of the sides of the bottom plate.     

Post-cyclic Pullout Response of Geogrid in Compacted Sand and Clayey Sand with Emphasis on Pre-cyclic Load Effect

Geotechnical and Geological Engineering - Soil-geosynthetic interface behavior under cyclic loading will differ from that under static loading. To this end, a series of static and multistep tests...     

多灾种综合风险评估与防范的理论认知:风险防范"五维"范式

在气候变化、经济全球化和快速城市化背景下,人类社会所面临的多灾种巨灾风险不断增加.综合风险防范事关人民群众生命财产安全与社会和谐稳定,是国家公共安全体系建设和可持续发展战略实施的重要内容.在系统梳理世界综合减灾与风险防范行动经验的基础上,客观审视我国防灾减灾救灾工作中存在的问题,从实际国情出发,提出构建中国综合灾害风险防范"五维"范式,即:理论建构引领实践探索、统筹谋划健全"一案三制"、综合思维优化管理模式、有的放矢制定应对策略和多措并举提升防范能力.面对日益复杂严峻的多灾种巨灾风险形势,需不断加强风险形成机制及其多维影响研究和风险普查、评估与监测预警工作,加快推进体制机制改革,完善法律保障和应急预案体系,打造政府主导、社会共治管理模式,依托现代科技实现风险管理信息化、智能化、精细化转型,为扎实提升我国综合灾害风险防范能力、实现安全与发展的有机统筹提供强有力的支撑和全方位的保障.     

Vibratory Influential Zoning for Grade-Separated Tunnels Under the Load of Trains

Grade-separated tunnels are even more vulnerable components in high-speed railway engineering structures. The dynamic characteristics of tunnels under a train load are key indicators that influence the safe operation and durability of the entire line. In this study, with consideration of the major factors influencing grade-separated tunnels for high-speed rails, such as train speed, traffic mode, surrounding rock type, between-tunnel height, and crossing angle, a parameterized finite element model is established based on an orthogonal experimental design. In the proposed model, tensile stress (under which any vibration-induced structural damage is caused) is defined as a main evaluation index, and thus the empirical formula for each influencing factor is obtained. When the index was determined under multiple factors and at different levels, rock height between tunnels is found to be the biggest factor influencing the response of grade-separated tunnels to the vibratory load of the trains, followed by train speed, surrounding rock level, and crossing angle. Finally, based on the relevant standards for concrete stress, the zoning of grade-separated tunnels under the vibratory load of trains is established by multiple factors. Such zoning provides important theoretical references for future structural design, maintenance, and reinforcement of grade-separated tunnels for high-speed rails.     

Computations of Seismic Passive Resistance and Uplift Capacity of Horizontal Strip Anchors in Sand

In this paper, the limit equilibrium method is used to compute seismic passive earth pressure coefficients and the vertical uplift capacity of horizontal strip anchors in presence of both horizontal and vertical pseudo-static earthquake forces. By considering a simple planar failure surface, distribution of soil reaction is obtained through the use of Kötter’s equation. Presence of pseudo-static seismic forces induces a considerable reduction in the seismic passive earth pressure coefficients. The reduction in seismic passive earth pressure coefficients increases with increase in magnitude of the earthquake accelerations in both horizontal and vertical directions and with increase in wall friction angle. The vertical uplift capacity of horizontal strip anchor is obtained for various values of soil friction angle, embedment ratio and seismic acceleration coefficients in both horizontal and vertical directions by using rigorous computational optimization. Proper justification for selected value of wall friction angle is established. Results are presented in the form of non-dimensional breakout factor for anchor. A significant reduction in breakout factor is observed in presence of both the seismic acceleration coefficients whereas breakout factor increases with increase in soil friction angle and embedment ratio even under the seismic condition. Angles of failure planes keep changing with change in seismic acceleration coefficients and failure zone shifts towards the critical direction of seismic acceleration coefficients. Present results are compared and found in good agreement with some specific available results in literature.     

Effect of Compressive Load on Oblique Pull-out Capacity of Model Piles in Sand

Model tests on steel piles embedded in sand were carried out in the laboratory to study the effects of compressive load (i.e. 0, 25, 50, 75, and 100% of their ultimate capacity in compression) on oblique pull-out capacity of piles. The model piles were of 20 mm × 20 mm cross section, which had an embedded length of 400 and 600 mm. The pull was applied at an inclination of 0°, 30°, 60° and 90° with vertical axis of the piles. The experimental results indicate that the net oblique pull-out capacity of piles decreases with increase in % of compressive load and the decrease depends on the magnitude of the compressive load. Semi-empirical methods, based on experimental results, have been suggested to determine the oblique pull-out resistance of piles subjected to static compressive loads. A comparison of predicted values of the ultimate oblique resistance by proposed methods of analysis with experimental values, and also with those reported by others, showed reasonably good agreement.     

Horizontal Pullout Capacity of a Group of Two Vertical Strip Anchors Plates Embedded in Sand

The horizontal pullout capacity of a group of two vertical strip anchor plates placed along the same vertical plane in sand, has been determined by using the lower bound finite element limit analysis. The effect of vertical spacing (S) between the anchor plates on the magnitude of the total group horizontal failure load (P_uT) has been determined for different combinations of H/B, δ/ϕ and ϕ. The magnitude of P_uT has been obtained in terms of a group efficiency factor, η_γ, with respect to the failure load for a single vertical plate with the same H/B. The magnitude of η_γ becomes maximum corresponding to a certain critical S/B, which has been found to lie between 0.5 and 0.8. The value of η_γ for a given S/B has been found to become larger for greater values of H/B, ϕ, and δ.     

Depth of Fixity of Piles in Clay Under Dynamic Lateral Load

Dynamic experiments were carried out on instrumented model aluminium single piles embedded in clay of different consistencies to study its bending behaviour under lateral loads. Piles with different length to diameter ratios were used. Dynamic lateral load of different magnitudes ranging from 7 to 30 N at wide range of frequencies from 2 to 50 Hz were applied. The load transferred to the pile, pile head displacement and the strain variation along the pile length were measured using a dedicated data acquisition system. Static lateral load tests were also performed to investigate the magnification of dynamic response of piles in clay. It is found that the maximum bending moment due to dynamic load is magnified by about 1.5–4 times in comparison to the static load for short piles but about 9 times for long piles. Depth of fixity and effective pile length is also largely amplified under dynamic loads, thus indicating that a pile which behaves as a flexible pile under static load, may not exhibit flexible behaviour under dynamic load.     

Static Equilibrium of Screw Anchor Pile Under Lateral Load in Sands

The screw anchor piles are installed in ground by screwing which is done with the help of torque motors. In this paper, the lateral load capacity of screw anchor piles is examined through an experimental investigation carried on model piles embedded in dry sand. The tests were carried on screw anchor piles with different number of helices provided in continuation. Lateral loads were applied at different height above the soil surface. The embedment length of screw anchor piles was also varied to study the behaviour of screw anchor piles under lateral loads. Some tests were conducted on plain shaft pile to compare the lateral load capacity of screw anchor piles with that of plain shaft piles. An empirical equation for computation of lateral loads has been developed considering lateral resistance, bearing resistance, uplift resistance and lateral resistance offered by soil in pile on the basis of experimental results. A theoretical model for predicting lateral load capacity of screw anchor piles in dry sand, consistent with the experimental findings has been developed in this study.     

Development of a New Experimental Apparatus for the Study of the Mechanical Behaviour of a Rock Discontinuity Under Monotonic and Cyclic Loads

The shear behaviour of rock discontinuities in seismic condition is still not fully understood although earthquakes can be an important triggering cause of instability phenomena of rock blocks. For this purpose, a special apparatus was designed and developed at the University of Parma (Italy), which is to be placed inside the MTS press available in the Laboratory of Materials Testing. The press allows the application of monotonic and cyclic loads, in load or in deformation control. Quantitative evaluation of the rock-joint damage is realized by a photogrammetric survey of the discontinuity before and after the tests. The surface comparison enables the identification of the damaged areas. Finally, theoretical and experimental results are interpreted in the light of the damage model developed by Belem et al. (Rock Mech Rock Eng 33(4):217–242, 2001) for a quantitative evaluation of joint roughness and resistance.     

床面附近泥沙交换率在悬移质输沙计算中的应用

总结并分析了关于悬移质运动方程的源/汇项或底部边界条件的已有的处理方法.认为在悬移质输沙计算中,这些方法因引入了平衡输沙概念、忽略了推移质颗粒对悬移质运动的影响或因缺乏床面附近泥沙质量交换率的理论表达式,均存在着某些不足.运用床面附近泥沙质量交换率理沦,对一般的悬移质输沙方程的底部边界条件进行了新的探讨,并提供了关于一维和垂向二维悬移质输沙的计算实例.     

Modeling Clam-inspired Burrowing in Dry Sand using Cavity Expansion Theory and DEM

Acta Geotechnica - The Atlantic razor clam exhibits exceptional penetration performance in wet sands by periodically expanding and contracting its shell and foot during burrowing. Essentially, this...     

偏心荷载作用下软弱下卧层承载力验算方法研究

以均布荷载作用下软弱下卧层竖向附加应力的简化计算方法为基础,导出了偏心荷载作用下软弱下卧层竖向附加应力的简化计算公式,并提出偏心荷载作用下软弱下卧层承载力验算的基本原则。     

近接隧道列车运行时地表振动响应数值模拟

鉴于目前城市地下铁路隧道平行、交叠的情况逐年增加,且对多列车耦合荷载作用下地表振动规律尚不清楚,本文建立了大直径盾构隧道下穿双线地铁隧道三维数值分析模型,并通过现有运行列车参数建立了刚体列车模型以及实体轨道模型。采用Hertz接触模拟了运动列车轮与钢轨的接触,通过罚接触与硬接触模拟了隧道与围岩的非线性接触,利用无限元人工边界模拟了无限半空间。使用时域显式整体分析方法模拟了编组列车在隧道内的运行并与实测地表振动加速度结果进行了对比,结果表明本文建立的数值模型能够较好地反映出真实地铁列车运行时地表的振动响应。在此基础上,分析了多线隧道交汇段不同列车运行工况下地表竖向加速度的时频特性以及分布规律。     

Einstein床沙质输移理论的再研究——关于细颗粒床沙质输沙率的计算

本文对Einstein输沙模式进行了简化处理.在此基础上,通过严格的数学推导,求出了Einstein床沙质挟沙能力公式中的J₁、J₂及p积分,从而给出了Einstein床沙质输沙函数的解析关系.通过与数值解的比较,表明本文的结论是正确的.由于本文所给的解析结果较简单,所以使Einstein输沙公式有可能在实际中,特别是在河床变形计算中得到广泛应用.但是本文是在悬浮指数z<1的情况讨论的,因此结论只适用于以悬移质运动为主的细沙河流.     

高应力下砂土与钢材界面单剪试验研究

疏排水沉降引起的竖直附加力导致了华东地区多个立井井筒破裂,高应力下土与结构界面剪切特性研究是解决这一工程问题的关键。在自行加工、改造的高应力单剪仪试验系统上进行了一系列单剪试验。试验数据分析表明高应力下砂土同钢材界面剪应力-剪切位移关系可用非线性弹性-理想、塑性本构关系描述;高应力下界面的强度准则符合无粘聚力的莫尔-库仑公式表示。