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.     

Centrifuge modelling of structures with oil dampers under seismic loading

Experimental research into the seismic performance of buildings with passive oil dampers has so far been restricted to large-scale testing of frames erected on laboratory shaking tables that ignore the foundation soil below. This simplification of the problem falls short of replicating dynamic soil-structure interaction that would occur in the field. This paper presents the first experimental attempt at utilising high gravity dynamic centrifuge testing to replicate the response of a damped building at a reduced model scale. The paper compares the dynamic response of two similar two-degree-of-freedom model sway frames, one control (bare) frame and one frame equipped with miniature oil dampers, both structures founded on shallow raft foundations in dry dense sand. The miniature oil dampers successfully mitigate floor accelerations, drifts, and storey shear forces in the damped frame with minor modification to the frame stiffness. For strong, near resonance motions, global rocking of the undamped frame associated with physical uplifting of the foundation from the soil surface and subsequent yielding of sand beneath has led to floor acceleration levels, which are comparable to those obtained in the damped building fitted with miniature oil dampers. Assessment of the instrumentation installed on the miniature oil dampers reveals a viscoelastic damper behaviour with a dependency on stroke magnitude and on velocity.     

砂土自由场地震响应的离心机试验研究

离心机模型试验是研究岩土地震工程问题的有效手段。本文使用层状剪切箱,通过干落法制备了均匀的砂土模型,进行了离心机振动试验;观测了振动过程中孔隙水压力的发展,土体的加速度响应、侧向变形以及竖向沉降。结果表明,土体的运动和变形与孔隙水压力的发展密切相关,但离心机中的试验现象和现场观测的现象存在显著区别。研究结果增强了对振动过程中土-水之间相互作用机理的理解,同时为自由场地震响应分析方法的验证提供了基础数据。     

Analytical and centrifuge studies of pile groups in liquefiable soil before and after site remediation

The objectives of this paper are to show practically: (1) the validation of a proposed three-dimensional effective stress analysis for the pile foundations, and (2) the effectiveness of remedial deposits on pile stresses under liquefaction by making comparisons between the results of centrifuge tests and those of the proposed analysis. Two foundation models supported by end-bending piles were studied with improved and unimproved deposits. There exists a good consistency between the numerical and experimental results for excess pore water-pressure ratios ranging from 0 to about 0·9. From the numerical results, the bending moment at the pile top with the improved deposit is about 50 per cent lower than that with the unimproved deposit. However, it was found that the smaller the bending moment develops in the pile with the improved deposit, the larger the compressive and/or tensional axial stresses in the pile. This is due to the predominant excitation of rocking vibration of the foundation. From the analytical and experimental results, it has been found that the remedial method can be a variable means to protect piles from soil liquefaction hazards. However, both axial stress and bending moment produced in piles should be considered in assessing the liquefied seismic capacity of group pile-foundation–structural systems with improved soil deposits. © 1998 John Wiley & Sons, Ltd.     

青海高原红层滑坡机理的地质综合分析

以西久公路(S101线)河北乡―红土山段改建工程中遇到的龙穆尔沟DH6#滑坡为例,从滑坡变形的强度场变化和应力场变化两条主线出发,综合研究了发育在青海高原红层地区的滑坡的变形及发展机制。研究表明特殊的岩土性质给高原红层滑坡的变形提供了物质基础;强烈的构造运动和特殊的高原气候是该滑坡变形的内部环境条件;河谷下切和强降雨作用是该滑坡变形的外部诱发因素。其中,特殊的岩土性质、河谷下切和强降雨作用对该类滑坡起着控制作用。将高原红层滑坡的发生发展机理归纳为:原始地貌→河谷下切→坡体卸荷松弛→岩土强度衰减→水的作用→岩土强度进一步衰减→坡体整体滑移。     

Dynamic response of a geotechnical rigid model container with absorbing boundaries

One of the major challenges encountered in earthquake geotechnical physical modelling is to determine the effects induced by the artificial boundaries of the soil container on the dynamic response of the soil deposit. Over the past years, the use of absorbing material for minimising boundaries effects has become an increasing alternative solution, yet little systematic research has been carried out to quantify the dynamic performance of the absorbing material and the amount of energy dissipated by it. This paper aims to examine the effects induced by the absorbing material on the dynamic response of the soil, and estimate the amount of energy reduced by the absorbing boundaries. The absorbent material consisted of panels made of commercially available foams, which were placed on both inner sides of end-walls of the soil container. These walls are perpendicular to the shaking direction. Three types of foam with different mechanical properties were used in this study. The results were obtained from tests carried out using a shaking table and Redhill 110 sand for the soil deposit. It was found that a considerably amount of energy was dissipated, in particular within the frequency range close to the resonance of the soil deposit. This feature suggests that the presence of foams provides a significant influence to the dynamic response of the soil. The energy absorbed by the boundaries was also quantified from integrals of the Power Spectral Density of the accelerations. It was found that the absorbed energy ranged between a minimum of 41% to a maximum of 92% of the input levels, depending mainly on the foam used in the test. The effects provided by the acceleration levels and depth at which the energy was evaluated were practically negligible. Finally, practical guidelines for the selection of the absorbing material are provided.     

Modelling of Progressive and Instantaneous Failures of Foliated Rock Slopes

Summary Mechanisms of flexural toppling failure of slopes in foliated rock masses are investigated both experimentally by testing small scaled models in a centrifuge and theoretically by using a limiting equilibrium model and a finite element model based on the Cosserat theory. Both theoretical models include bending moments of rock layers in their formulation. Two main failure mechanisms are observed: (1) instantaneous failure involving a large volume of failed material and (2) progressive failure where the fracture initially localises near the toe region and then progresses further into the rock mass with increasing load. These two mechanisms of slope failure are observed to be controlled by the magnitude of the joint friction angle. The joint friction angles of about 20° and above are most likely to cause instantaneous failure while the lesser friction angles result in a progressive failure. Joint cohesion is not found to have a similar effect on the failure mechanisms, provided that it is low enough to allow sliding between the rock layers – a prerequisite for flexural toppling.     

垂向动载荷下桶形基础响应实验研究

对垂向动载荷作用下吸力式桶形基础(简称桶基)响应进行离心机实验模拟.结果表明,在垂向动载荷作用下,当载荷幅值超过一定值时,桶基周围砂土软化甚至液化,发生明显的沉降.桶基周围土体的沉降随着载荷幅值的增加而增加.由于液化区的滤波和对动载的衰减作用,发生沉降的范围有限,离桶壁约一倍桶高距离.超孔隙水压从桶基边沿水平向逐渐衰减,从土面开始往下逐渐衰减到零.桶基周围砂土完全液化的厚度随载荷幅值的增加而增加,最大值约为桶高的40%.     

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.     

防波堤椭圆形桶式基础结构的贯入受力特性实验研究

某工程拟在深水软土地基上修筑防波堤,为了尽量减少地基处理充分利用天然地基,创新性设计出一种轻型薄壁的预制防波堤结构,其挡浪部分为直立薄壁圆筒,基础部分则为倒扣的薄壁椭圆形桶,并且椭圆形下桶为外壁和内隔板分成9个格室,防波堤结构浮运至指定位置后,拟采用负压工法施工安装就位。这种新型防波堤结构为国内外首次提出,其下沉施工设计尚无规范可循,为此开展了土工离心模型试验,在模型加速过程中模拟了椭圆形下桶基础在淤泥层中的自重下沉,之后利用新研发了一种大行程作动加载装置给椭圆形下桶施加下推力,让其继续向下贯入直至穿越整个淤泥层,以模拟负压工法的贯入下沉。试验测量了下桶贯入下沉过程中的推力与贯入位移,还尝试测量了桶壁和内隔板断面的压应变,由此分析了下桶基础的下沉总阻力、桶壁摩擦力以及截面压应变随贯入位移的变化。结果发现,这些曲线均出现了转折点,根据转折点对应的下沉总阻力确定了椭圆形下桶基础贯入过程所遭遇的临界下沉总阻力值,据此估算了负压工法中所需施加的压力差。