Remediation against floatation of underground structures

Underground structures located in liquefiable soil deposits are susceptible to floatation following an earthquake event due to their lower unit weight relative to the surrounding saturated soil. Centrifuge tests have been carried out to assess the effectiveness of existing remediation techniques in reducing the uplift of underground structures, namely in situ densification and the use of coarse sand backfill. The centrifuge test results showed that these methods do reduce the uplift displacement of buoyant structures. Their performance was thereafter linked to the theoretical mechanism of floatation of underground structures. Based on the understanding from preceding tests, a further improvement on the use of the coarse sand backfill was carried out, which produced a greater reduction in the uplift displacement of the structure. Each of these techniques, however, does pose issues when applied in the field, such as possible damage to surrounding structures, construction issues and maintenance problems.

Boundary conditions in physical model tests-the influence of deck pinning on the response of piled bridge abutments in laterally spreading soils

Physical models are widely used in the study of geotechnical earthquake engineering phenomena, and the comparison of modelling results to observations from field reconnaissance provides a transparent means of evaluating the design of our physical models. This paper compares centrifuge tests of pile groups in laterally spreading slopes with the response of piled bridge abutments in the 2011 Christchurch earthquake. We show that the model foundation's fixity conditions strongly affect the success with which the mechanism of response of the real abutments is replicated in the tests. © 2012 American Society of Civil Engineers.

Isolating shallow foundations from seismic loading

Most modern design codes do not allow for movement between a shallow foundation and the underlying soil during seismic loading. Consequently, the full magnitude of seismic energy is transmitted from the soil to the foundation during an earthquake. This energy either has to be dissipated before reaching the superstructure via engineering solutions such as base isolation systems, or the structure itself must withstand the full impact of the earthquake resulting in high material usage and expensive design. However, the inherent hysteric behaviour of soil can be used to isolate a foundation from the underlying soil. As part of a study into the soil-structure-interaction of shallow foundations, methods to optimise foundation isolation were investigated. In this paper the results from centrifuge tests investigating two of these methods are compared to results when no special foundation layout was implemented and the impact of the proposed isolation methods is discussed. © 2010 Taylor & Francis Group, London.

Centrifuge modeling of seismic liquefaction effects on adjacent shallow foundations

Shallow foundations built on saturated deposits of granular soils in seismically active areas are, regardless of their static bearing capacity, critical structures during seismic events. A single centrifuge experiment involving shallow foundations situated atop a liquefiable soil deposit has been performed to identify the mechanisms involved in the interaction between liquefaction-induced effects on neighboring shallow foundations. Centrifuge test results indicate that liquefaction causes significant settlements of footings, which are affected by the presence of neighboring foundations and can be extremely damaging to the superstructure. The understanding of these interaction effects is very important, mainly in densely populated urban areas. The development of high excess pore-pressures, localized drainage in response to the high transient hydraulic gradients, and earthquake-induced vertical motions to the footings are also important effects that are discussed to assist in enhancing current understanding and ability to predict liquefaction effects on shallow foundations. © 2014 Taylor & Francis Group.

Imaging the failure of a rock-fill dam following liquefaction

Rock-fill dams are popular in developing countries due to their ease of construction and use of local materials. They are used to store water and to provide flood defences. The presence of such dams in earthquake-prone regions poses risks, particularly from ground liquefaction. In this paper, results from physical model tests on dams with different configurations are presented. Model dams with impermeable cores including sheet pile walls and clay cores were tested and the effect of reservoir water was investigated. High-speed photography was used to capture the response of the model dams allowing the movement of foundation soil below the dam to be established. It is concluded that the stiffness of the impermeable core has a significant influence on the ultimate deformation of the dam. The presence of reservoir water led to increased downstream movements of the dam and differential settlements between the upstream and downstream sides.

The rocking response of large flexible structures to earthquakes

The rocking response of structures subjected to strong ground motions is a problem of 'several scales'. While small structures are sensitive to acceleration pulses acting successively, large structures are more significantly affected by coherent low frequency components of ground motion. As a result, the rocking response of large structures is more stable and orderly, allowing effective isolation from the ground without imminent danger of overturning. This paper aims to characterize and predict the maximum rocking response of large and flexible structures to earthquakes using an idealized structural model. To achieve this, the maximum rocking demand caused by different earthquake records was evaluated using several ground motion intensity measures. Pulse-type records which typically have high peak ground velocity and lower frequency content caused large rocking amplitudes, whereas non-pulse type records caused random rocking motion confined to small rocking amplitudes. Coherent velocity pulses were therefore identified as the primary cause of significant rocking motion. Using a suite of pulse-type ground motions, it was observed that idealized wavelets fitted to velocity pulses can adequately describe the rocking response of large structures. Further, a parametric analysis demonstrates that pulse shape parameters affect the maximum rocking response significantly. Based on these two findings, a probabilistic analysis method is proposed for estimating the maximum rocking demand to pulse-type earthquakes. The dimensionless demand maps, produced using these methods, have predictive power in the near-field provided that pulse period and amplitude can be estimated a priori. Use of this method within a probabilistic seismic demand analysis framework is briefly discussed. © 2013 Springer Science+Business Media Dordrecht.

Towards a unified description of rocking structures

Numerous studies on the rigid rocking block have generated a wealth of knowledge about rocking behavior. However, evaluation of more complex rocking systems requires the derivation and solution of complicated equations of motion. This paper investigates the possibility of a unified description of several rocking systems through investigation of rocking mechanisms which describe the masonry wall and the masonry arch. Effective rocking parameters are derived for each of these structures, and the similarity of the rocking behavior is discussed. The error of the proposed approximation, which defines the limitations for this approach, is quantified for the example structures considered. Where appropriate, a unified description of rocking would allow the use of rocking spectra, which would be useful to readily predict the response of a wide array of rocking structures.

Experimental and numerical investigation of the seismic behavior of rectangular tunnels in soft soils

Underground structures constitute crucial components of the transportation networks. Considering their significance for modern societies, their proper seismic design is of great importance. However, this design may become very tricky, accounting of the lack of knowledge regarding their seismic behavior. Several issues that are significantly affecting this behavior (i.e. earth pressures on the structure, seismic shear stresses around the structure, complex deformation modes for rectangular structures during shaking etc.) are still open. The problem is wider for the non-circular (i.e. rectangular) structures, were the soilstructure interaction effects are expected to be maximized. The paper presents representative experimental results from a test case of a series of dynamic centrifuge tests that were performed on rectangular tunnels embedded in dry sand. The tests were carried out at the centrifuge facility of the University of Cambridge, within the Transnational Task of the SERIES EU research program. The presented test case is also numerically simulated and studied. Preliminary full dynamic time history analyses of the coupled soil-tunnel system are performed, using ABAQUS. Soil non-linearity and soil-structure interaction are modeled, following relevant specifications for underground structures and tunnels. Numerical predictions are compared to experimental results and discussed. Based on this comprehensive experimental and numerical study, the seismic behavior of rectangular embedded structures is better understood and modeled, consisting an important step in the development of appropriate specifications for the seismic design of rectangular shallow tunnels.

Determination of 266 pesticide residues in apple juice by matrix solid-phase dispersion and gas chromatography-mass selective detection

A macro matrix solid-phase dispersion (MSPD) method was developed to extract 266 pesticides from apple juice samples prior to gas chromatography-mass selective detection (GC-MSD) determination. A 10 g samples was mixed with 20 g diatomaceous earth. The mixture was transferred into a glass column. Pesticide residues were leached with a 160 mL hexane-dichloromethane (1:1) at 5 mL/min. Two hundred and sixty-six pesticides were divided into three groups and detected by GC-MSD under selective ion monitoring. The proposed method takes advantage of both liquid-liquid extraction and conventional MSPD methods. Application was illustrated by the analysis of 236 apple juice samples produced in Shaanxi province China mainland this year. (C) 2004 Elsevier B.V. All rights reserved.

Mass chains with passive interconnection: Complex iterative maps and scalability

This paper introduces the problem of passive control of a chain of N identical masses in which there is an identical passive connection between neighbouring masses and a similar connection to a movable point. The problem arises in the design of multi-storey buildings which are subjected to earthquake disturbances, but applies in other situations, for example vehicle platoons. The paper will study the scalar transfer functions from the disturbance to a given intermass displacement. It will be shown that these transfer functions can be conveniently represented in the form of complex iterative maps and that these maps provide a method to establish boundedness in N of the H ∞-norm of these transfer functions for certain choices of interconnection impedance. © 2013 IEEE.

田间含碎石土壤水分入渗及再分布试验研究

为探明碎石存在于田间土壤中如何改变土壤水分运移通道、影响土壤水分运动,揭示田间含碎石土壤水分运动的复杂过程,采用便携式针头降雨器在铜川崾岘梁修建的临时小区进行模拟降雨实验,测定含碎石浅层土体的入渗和水分再分布过程,采用土壤水分运动通量法计算浅层土体不同深度各截面面积的水量在雨后不同时间段的变化,分析含碎石土壤再分布特征及碎石含量对浅层土体入渗和水分再分布的影响。结果表明:碎石有利于田间浅层土壤入渗和蓄存;室内较田间测定的含碎石土壤入渗率小一个数量级以上;降雨后较短时段,碎石含量相对高的林地小区的浅层土体水分在降雨停止后呈增加趋势,表现了蓄水过程,碎石含量较低的撂荒地则表现了释水过程;降雨后较长时段,多数小区浅层土体(0-30 cm)水量变化呈现了"排水-存储交换-排水"过程,少数小区仅呈现了"排水"过程;雨后16 h左右的土体各截面面积的水量与碎石含量呈现了以直线y=0.4873为轴对称的递增和递减的指数为1/2的幂函数关系,说明碎石对黄土区典型土石区土体水分运动具有促进和阻滞双面影响。本文的研究结果可为含碎石土壤的水分利用及水分循环提供参考。

基于GIS和CSLE的陕西省土壤侵蚀定量评价方法研究

通过对陕西省土壤侵蚀的定量评估,探索省域尺度上土壤侵蚀定量评价制图方法,以期为第四次全国土壤侵蚀普查提供技术支撑。利用陕西省日降雨数据、土壤类型图、土地利用图、DEM、植被覆盖图,选择CSLE模型,在ArcGIS平台上计算研究区2006年土壤侵蚀量,并与水利部标准评价结果进行对比分析。探索了坡度坡长变换对土壤侵蚀的影响。定量化的评价结果能较好的反映气候、土壤及水保措施对土壤侵蚀的影响,比水利部标准评价结果更能反映实际侵蚀。坡度坡长变换后对土壤侵蚀有明显影响。在GIS支持下,通过土壤侵蚀模型进行定量评价,可高效、客观反映土壤侵蚀情况及其主要影响因子,对区域治理和有关决策有极其重要参考价值。为了准确真实的反映土壤侵蚀程度,必须进行坡度坡长变换。

陕西省米脂县耕地动态变化与粮食安全研究

根据米脂县1990—2007年粮食生产的相关数据,对该县耕地、人口和粮食生产动态变化以及粮食总产与其影响因素进行了灰色关联度分析。结果表明,退耕还林还草工程是耕地总面积迅速减少的主要原因;提高粮食单产保障粮食总产是实现粮食安全的最关键因子;持续增长的人口造成耕地的承载压力越来越大,并对该区实现粮食安全提出了更大的挑战。最后,根据分析结果提出了增加科技投入,提高单产,保护耕地,挖掘耕地潜力等措施以保障该区域的粮食安全,为政府部门制定相关粮食安全政策提供科学的理论依据。

70年来陕西省纸坊沟流域农业生态经济系统耦合态势

"农业生态经济系统耦合"的研究与实践对于实现农业产业与资源相一致,建立持续、高效的农业生态经济系统具有重要的意义。耦合度可以阐明农业经济系统与农业生态系统互动关系,判定农业生态经济系统耦合态势。在分析陕西省纸坊沟流域农业生态经济系统耦合关系的基础上,参照系统科学等理论及相关研究结果建立了农业经济系统与农业生态系统耦合度模型,并计算和分析了该流域70a来的耦合度。结果表明,农业生态经济系统的耦合过程可以划分为4个阶段:Ⅰ.经济系统依赖生态资源进行原始化农业生产阶段;Ⅱ.农业生产掠夺式利用生态资源,生态系统供给能力不断减少阶段;Ⅲ.农业经济系统与生态系统协调化发展阶段;Ⅳ.降低农业发展速度,促使生态系统重建阶段。纸坊沟流域从1938～2008年先后经历了第Ⅰ阶段、第Ⅱ阶段和第Ⅲ阶段。目前处于第Ⅲ阶段,但在"系统发展"过程中已潜伏了越来越大的危机,到2018年系统耦合突破"协调"界限,"相悖态势"将明显表现出来。为此,纸坊沟流域必须调整产业布局,发展草畜产业,进行产业升级,优化产业结构,实现农业生态经济系统协调、持续发展。