EH4及地质雷达在隧道事故调查中的应用

引水隧道施工中屡次发生坍塌,用探地雷达对衬砌质量及地质情况进行探测。经数据反演,清晰勾画出衬砌背后空洞、松散堆积体分布范围,地下水的赋存特征,为治理事故提供切实可靠依据。     

Geotechnical Investigation of the Titanic Wreck Site

Abstract

Recent marine forensic investigations have largely unravelled the sequence of events concerning the sinking of the R.M.S. Titanic and its descent through nearly 3800 m of water to the seafloor on the morning of 15 April 1912. In particular, the velocity and attitude of the Titanic's bow section (at present lying upright, reasonably intact, and embedded by ～12 m at the prow) as it hit the bottom are of general interest to marine accident investigators. During the 1998 Titanic Science Expedition, a single sediment sample was retrieved from the seafloor (depth 20–30 cm) near the wreck by the deep water submersible, Nautile. Published geological studies suggest the seafloor in this area has remained largely undisturbed since 1912. Geotechnical analysis of the sediment sample reveals that the impact was probably a substantially undrained event and that the characteristic undrained shear strength of the sediment is ～25kPa within 10–16 m below the seafloor. A simple analytical model was used to calculate the embedment of a cuboid with dimensions and mass of the water-filled bow as a function of impact velocity, impact angle, and the undrained shear strength of the sediment. The results indicate the impossibility of a steep angle of impact and fast velocity. The most likely scenario is an impact velocity of 5–10 m/s at a fairly shallow angle (<40°), which corroborates the results of hydrodynamic investigations.     

Geotechnical evaluation of slope and ground failures during the 8 October 2005 Muzaffarabad earthquake, Pakistan

A large devastating earthquake with a magnitude of 7.6 struck in Kashmir on Oct. 8, 2005. The largest city influenced by the earthquake was Muzaffarabad. Balakot town was the nearest settlement to the epicenter, and it was the most heavily damaged. The earthquake caused extensive damage to housing and structures founded on loose deposits or weathered/sheared rock masses. Furthermore, extensive slope failures occurred along Neelum and Jhelum valleys, which obstructed both river flow and roadways. In this article, failures of natural and cut slopes as well as other ground failures induced by the earthquake and their geotechnical evaluation are presented, and their implications on civil infrastructures and site selection for reconstruction and rehabilitation are discussed. It is suggested that if housing and constructions on soil slopes containing boulders as observed in Balakot and Muzaffarabad are allowed, there should be a safety zone between the slope crest and allowable construction boundary.     

Geotechnical properties of the Cassino Beach mud

Knowledge of the marine soils properties, together with hydrodynamic and climatic data, plays an important role for a better understanding of the dynamic behavior of sandy and muddy coasts. This paper deals with reporting and basic interpretation of two campaigns of exploration and characterization of the mud of Cassino Beach, southern Brazil, carried out during the years of 2004 and 2005. Samples were obtained by means of cores collected at some locations offshore, and were submitted to various laboratory geotechnical tests, including determination of the physical index, grain size distribution, Atterberg limits, and shear resistance by both triaxial and shear vane tests. Results confirm the existence of a very soft soil deposit offshore Cassino Beach, highly plastic, compressible, and viscous, forming an important database for further studies.     

Experimental and conceptual evidence about the limitations of shear wave velocity to predict liquefaction

Based on the liquefaction performance of sites with seismic activity, the normalized shear wave velocity, V_s1, has been proposed as a field parameter for liquefaction prediction. Because shear wave velocity, V_s, can be measured in the field with less effort and difficulty than other field tests, its use by practitioners is highly attractive. However, considering that its measurement is associated with small strain levels, of the order of 10⁻⁴–10⁻³%, V_s reflects the elastic stiffness of a granular material, hence, it is mainly affected by soil type, confining pressure and soil density, but it is insensitive to factors such as overconsolidation and pre-shaking, which have a strong influence on the liquefaction resistance. Therefore, without taking account of the important factors mentioned above, the correlation between shear wave velocity and liquefaction resistance is weak.In this paper, laboratory test results are presented in order to demonstrate the significant way in which OCR (overconsolidation ratio) affects both shear wave velocity and liquefaction resistance. While V_s is insensitive to OCR, the liquefaction resistance increases significantly with OCR. In addition, the experimental results also confirm that V_s correlates linearly with void ratio, regardless of the maximum and minimum void ratios, which means that V_s is unable to give information about the relative density. Therefore, if shear wave velocity is used to predict liquefaction potential, it is recommended that the limitations presented in this paper be taken into account.     

Centrifuge modeling of rocking‐isolated inelastic RC bridge piers

Experimental proof is provided of an unconventional seismic design concept, which is based on deliberately underdesigning shallow foundations to promote intense rocking oscillations and thereby to dramatically improve the seismic resilience of structures. Termed rocking isolation, this new seismic design philosophy is investigated through a series of dynamic centrifuge experiments on properly scaled models of a modern reinforced concrete (RC) bridge pier. The experimental method reproduces the nonlinear and inelastic response of both the soil‐footing interface and the structure. To this end, a novel scale model RC (1:50 scale) that simulates reasonably well the elastic response and the failure of prototype RC elements is utilized, along with realistic representation of the soil behavior in a geotechnical centrifuge. A variety of seismic ground motions are considered as excitations. They result in consistent demonstrably beneficial performance of the rocking‐isolated pier in comparison with the one designed conventionally. Seismic demand is reduced in terms of both inertial load and deck drift. Furthermore, foundation uplifting has a self‐centering potential, whereas soil yielding is shown to provide a particularly effective energy dissipation mechanism, exhibiting significant resistance to cumulative damage. Thanks to such mechanisms, the rocking pier survived, with no signs of structural distress, a deleterious sequence of seismic motions that caused collapse of the conventionally designed pier. © 2014 The Authors Earthquake Engineering & Structural Dynamics Published by John Wiley & Sons Ltd.     

Simulation of soil–foundation–structure interaction of Hualien large-scale seismic test using dynamic centrifuge test

Understanding the soil–structure interaction (SSI) mechanism is crucial in the seismic design of nuclear power plant (NPP) containment systems. Although the numerical analysis method is generally used in seismic design, there is a need for experimental verification for the reliable estimation of SSI behavior. In this study a dynamic centrifuge test was performed to simulate the SSI behavior of a Hualien large-scale seismic test (LSST) during the Chi-Chi earthquake. To simulate the soil profile and dynamic soil properties of the Hualien site, a series of resonant column (RC) tests was performed to determine the model soil preparation conditions, such as the compaction density and the ratio of soil–gravel contents. The variations in the shear wave velocity (V_S) profiles of the sand, gravel, and backfill layers in the model were estimated using the RC test results. During the centrifuge test, the V_S profiles of the model were evaluated using in-flight bender element tests and compared with the in-situ V_S profile at Hualien. The containment building model was modeled using aluminum and the proper scaling laws. A series of dynamic centrifuge tests was performed with a 1/50 scale model using the base motion recorded during the Chi-Chi-earthquake. In the soil layer and foundation level, the centrifuge test results were similar to the LSST data in both the time and frequency domains, but there were differences in the structure owing to the complex structural response as well as the material damping difference between the concrete in the prototype and aluminum in the model. In addition, as the input base motion amplitude was increased to a maximum value of 0.4g (prototype scale), the responses of the soil and containment model were measured. This study shows the potential of utilizing dynamic centrifuge tests as an experimental modeling tool for site specific SSI analyses of soil–foundation–NPP containment system.     

河南省主要城市浅层地温能特征及开发利用建议

河南省浅层地温能资源具有分布广泛、资源量丰富、开发利用方便等特点,发展前景广阔。通过对河南省主要城市的地质、水文地质条件和岩土体结构特征等浅层地温能赋存特征的分析,采用数理统计和对比分析等综合研究的方法,对比研究了不同地貌类型城市200m以浅的岩土体热物性特征、浅层地温场分布特征和岩土热响应特征等的异同及其分布与变化规律。结果显示:河南省主要城市浅层地温能的赋存层位主要为第四系及新近系上部的各类松散堆积物,这些松散的堆积物和储存于其孔隙内的地下水为浅层地温能的载体;岩土导热系数会随着孔隙率的增加而减小,随岩性颗粒变粗而增大;城市地层综合导热系数和换热能力与地下水径流条件呈正相关;位于盆地和山前地带水文地质条件优越的城市或地段,浅层地温能开发利用宜采用地下水换热方式,松散堆积物厚度较大且以细粒相沉积为主的城市或地段,宜采用竖直地埋管换热方式。     

正断层诱发砂土中群桩基础破坏及避让距离研究

地震中发震断层诱发桩基失效,导致上部结构破坏甚至坍塌,相关破坏机制和避让距离缺乏系统研究。通过离心机试验和数值模拟,针对基岩正断层活动诱发上覆砂土中群桩基础的静力破坏展开研究,考察不同群桩断层相对位置下群桩的破坏特征。试验与计算结果均表明,当群桩跨越断层时,正断层活动使群桩向上盘一侧倾斜,并使基桩弯向上盘一侧。基桩桩顶荷载的重分布进一步使基桩形成受拉和受压两种破坏模式。数值参数分析表明,在不同桩位上,群桩的变形响应可划分为5个特征区域。对于埋深为20.0 m的基岩正断层,群桩在上盘和下盘一侧的安全避让距离分别为23.5 m和15.9 m,其中下盘一侧离开断层7.9 m至上盘一侧离开断层4.1 m的区域需要进行重点避让。     

砂土地基和粉质黏土地基基坑悬臂开挖离心模型试验

分别开展砂土和粉质黏土两种典型土质条件下基坑悬臂式开挖离心模型试验,详细叙述试验过程中所要解决的关键问题,并提出合理的解决方案。通过对比分析两组试验结果,得到以下结论：非饱和土地基制备中参数控制困难,分层夯实法有待进一步改进,而砂雨法制备的砂土地基参数可控性更好;两组试验的结果有差异,砂土地基试验所呈现的土压力、地基变形、支护弯矩的变化规律更好,因此,岩土离心试验可适当考虑以砂土代替非饱和土;对于采用悬臂式支护结构的基坑,开挖引起的地表沉降曲线在砂土中呈指数型,而在粉质黏土中呈直线型;开挖引起的粉质黏土地基土体位移范围较砂土地基更大;开挖引起的砂土中挡墙弯矩较粉质黏土更大,砂土和粉质黏土中最大弯矩位置都随开挖逐渐下移;在砂土试验中开挖引起主动区土压力各处均减小,而在粉质黏土试验中开挖引起土压力在挡墙底有增大趋势。该基坑工程离心模型试验过程及数据处理方法可为进一步试验提供参考。