Trench effects on lateral p-y relations for pipelines embedded in stiff soils and rocks

Existing relationships for lateral backfill pressures on pipelines assume that the trench is adequately wide to contain the failure surface. This condition is commonly violated in design and construction practice, putting at risk the pipeline safety. In this context, size and shape effects for trenches excavated in stiff soils and rocks, are numerically investigated, through experimentally-calibrated parametric analyses. It is shown that, for narrow trenches, ultimate pressures and yield displacements may increase up to an order of magnitude compared to “infinite-trench” values, while excavation of inclined walls reduces the above detrimental effects. Simplified relations are developed to aid pipeline design.     

Subduction zone stresses: Constraints from mechanics and from topographic and geoid anomalies

There are, in principle, direct relations between several important phenomena associated with subduction zones: the depth of oceanic trenches, the magnitude of the net force from trenches acting on subducting plates, the distribution and fault plane orientations of earthquakes, the magnitude of stresses on subduction faults, the bathymetry of back-arc regions, and the magnitudes of gravity and geoid anomalies. These phenomena are related through the stresses transmitted through surface and subducted lithosphere, and are associated with the mass anomaly of the subducted lithosphere. Quantitative estimates suggest that observed trench depths imply a trench pull force on subducting plates which is comparable to the ridge push force but much less than the excess weight of the subducted lithospheric slab. It is further suggested that either the mass anomaly of subducted lithosphere is much less than would be expected on the basis of conventional thermal and compositional models or that (a) a large resistance acts on the upper part of slabs due to high-stress corner flow, and (b) the mass anomaly of the slab is 70–90% compensated either by a broad 1 km-deep back-arc depression or a low density mantle wedge above the slab or both.     

Flexural response of the oceanic lithosphere at an arc—arc junction: implication for the subduction of aseismic ridges

The deformation of the oceanic lithosphere subducting at the junction of two trenches is studied by means of a three-dimensional finite-element analysis. Results show that the existence of a junction (i.e. a change in trend of the trench axis) yields a specific shape of the outer topographic rise. In a convex junction area—such as the Japan and Kuril trenches, the topographic bulge presents a “dome”, whereas in a concave junction area—such as the Java and Sumatra trenches, this bulge is less pronounced. These theoretical results are confirmed by the bathymetry seaward of the junctions of the Japan and Kuril trenches and of the Peru-Chile trench. Moreover, the existence of the abnormal topographic dome in front of a convex junction contributes to the creation of normal faults which help the subduction of seamounts or of other bathymetric features in such areas.     

内支撑结构基坑的空间效应及影响因素分析

具有内支撑结构的围护系统在基坑边角处具有更大的系统刚度,使得基坑边角附近处土体的位移小于距离边角较远处土体的位移,即基坑的变形问题表现出空间特性。为了更好地研究L/He（L为沿基坑纵向方向上的距离;He为开挖深度）、开挖深度等因素对空间效应的影响,量测了两个狭长形地铁车站深基坑不同位置处土体的侧向位移、土体沉降等。通过对现场监测资料的分析发现,边角效应能够减小侧向位移的平面应变比,灌注桩围护结构、SMW工法桩围护结构和地下连续墙在边角附近处的平面应变比(PSR)分别为0.50、0.61和0.72。当平面应变比(PSR)接近于1.00时,对应的L/He值分别为2.50、6.00和4.00。随着L/He值的增大,土体的纵向最大沉降呈先增大后保持稳定的趋势。随开挖深度的增加,边角效应的影响范围呈增大的趋势。在基坑纵向沉降的空间效应中,灌注桩围护结构、SMW工法桩围护结构的土体最大沉降值达到稳定时对应的L/He值分别为2.50和5.20。土体沉降和侧向位移的空间效应有一定的相关性。     

基于改进一次二阶矩法的赵家坡高切坡稳定性分析

为了更客观和科学地评价斜坡的稳定性,采用结构可靠度中的改进一次二阶矩法来分析斜坡的稳定性,同时采用新的、更适于计算机编程的求可靠指标解的迭代方法,对三峡库区赵家坡高切坡第二段稳定性分析标明：（1）潜在滑动面1可靠指标和破坏概率均满足允许风险水平,稳定性程度较高,只需进行表面防护即可;（2）潜在滑动面2可靠指标和破坏概率均不满足允许风险水平,需要进行整体加固并辅以坡面防护;（3）稳定性系数与可靠度分析结果有良好的一致性。     

福州市永升城基坑深井降水设计与施工技术

永升城基坑深井降水位于福州市内软土地区,面积约10000m2,开挖深度13m,围护桩四周透水,降水总涌水量达19000m3/d,基坑开挖深度及降水规模均列当时福建省同类工程之首。如何合理布置降水工作量及减少对外围其他建筑物的影响是此类工程需要解决的课题。永升城基坑降水解决了这两大课题。     

Propagation of a shear crack in a compressed plane with a circular hole

The problem of the equilibrium of a plane with a circular hole and a shear crack is considered to model failure of an excavation (borehole or circular opening) in rocks weakened by discontinuities (planes of weakness). It is assumed that sliding occurs in a part of the plane of weakness when the Mohr–Coulomb friction criterion is satisfied due to the stress redistribution caused by the excavation. The method of singular integral equations is employed to solve the boundary value problem. Geomechanical problems concerning borehole breakout and rockburst caused by fault-opening interaction are discussed. © 1998 John Wiley & Sons, Ltd.     

Flexural bending curvature and yield zone of subducting plates

ABSTRACT

We quantify flexural deformation of subducting oceanic plates at a global array of 15 ocean trenches, using a new approach of modelling spatial variations in flexural bending shape and curvature. The investigated trenches are chosen to represent a diverse range of subducting plate age of 24–150 Ma, including the Middle America, Peru, Chile, West and East Aleutian, Sumatra, North and South Philippine, Tonga, Kermadec, Kuril, Japan, Izu-Bonin, and South and North Mariana. The studied trenches show systematic intra- and inter-trench variability in the calculated flexural bending curvature, stress distribution, extensional brittle yield zone, and effective elastic plate thickness T_e of the subducting plates. We find that subducting plate age is a critical factor controlling the bending curvature and the corresponding extensional yield zone. The width, depth, and area of the extensional yield zone are all calculated to increase systematically with the subducting plate age. The newly-developed curvature analysis can yield continuously varying apparent T_e(x) from the trench axis to outer rise. The calculated extensional yield zones from the curvature analysis are in general consistent with the observed normal faulting earthquakes of magnitude≥6.0 at the 15 global trenches. Our analyses also reveal that the five deepest regions of the global trenches, i.e. the Challenger, Horizon, Serena, and Scholl Deeps and Galathea Depth, are associated with relatively large flexural bending and calculated yield zones comparing to their respective adjacent trench segments. The Serena and Challenger Deeps of the S. Mariana trench are calculated to have the largest flexural bending among the five deeps.     

考虑拱效应深基坑支护结构土压力分析

针对基坑支护结构土压力的研究以数值模拟为主,而解析理论应用需要完善研究现状,提出一种用于计算基坑支护结构土压力的解析方法。根据填土中土拱效应的作用以及土拱成形的基本理论,结合极限平衡分析方法,应用摩尔圆理论确定了基坑端壁间稳定土拱的轨迹方程,并且得到基于土拱形状所围滑裂土体的体积,确定出基坑支护结构土压力的解析表达式,最后进行了算例分析,研究结果表明土体的物理属性决定了有效滑裂土体的边界。提出的解析方法是深基坑计算土压力的一种新的解析方法。     

双面高陡岩质边坡地震滑坡机制的研究

以国道G213左侧双面高陡岩质边坡为原型,采用新型连续介质离散元计算方法（CDEM）,结合振动台试验结果,对高烈度地震作用下双面高陡岩质边坡的地震滑坡机制进行了研究。结果表明：在地震力和重力作用下,滑体顶部先出现拉应力集中,造成滑体沿滑体结构面后缘产生变形,进而造成该处出现拉伸、剪切破坏点;之后随着地震动的持续,滑体结构面上的剪切破坏点逐渐向滑体中前部的锁固段扩展,同时伴随着滑体表面拉伸破坏点的增加;最终造成锁固段发生渐进性破坏,滑体从剪出口滑出形成滑坡。滑床、滑体间运动的不一致性、地震动能量分布和耗散的差异性以及随地震烈度增大结构面强度的降低是诱发滑坡的3项主控影响因素。在地震动加速度较小时,滑床、滑体内加速度的瞬时频率基本一致,滑体结构面的能量透射系数及滑体内地震动能量的控制性频带均稳定在一定范围内;随着地震烈度的增加,滑体内加速度的瞬时频率逐渐下降,滑体结构面的能量透射系数逐渐减小,最后两者均趋于稳定,同时滑体内地震动能量的控制性频带也逐渐由高频带向低频带转移。     

开挖与降雨作用下边坡失稳机理及模拟分析

边坡开挖和降雨通常是导致边坡失稳的重要原因。本文以湖南湘西山区某国道扩建开挖边坡为研究对象,基于现场边坡监测结果和数值模拟分析,研究了在边坡开挖和降雨条件下坡体变形位移的过程。结果表明:边坡的破坏是一个渐变的过程,不同的影响因素对边坡的影响不同。开挖切方是浅层坡体失稳的诱发因素,开挖切方破坏了坡体的应力平衡,使坡体的应力重新分布,并在坡体中产生浅层的滑动面。雨水的入渗是坡体深层滑动面的诱发因素,雨水沿着裂缝渗入坡体,使浅部滑动面上下土体的变形差进一步加大,进而产生浅层牵引式滑动破坏。同时雨水的入渗使碎石土和强风化页岩交界附近产生高孔隙水压力,在水-岩土共同作用逐渐形成软化的滑带土,从而形成深层滑动面。     

Stormwater harvesting in ephemeral streams: how to bypass clogging and unsaturated layers

To cope with water scarcity in drylands, stormwater is often collected in surface basins and subsequently stored in shallow aquifers via infiltration. These stormwater harvesting systems are often accompanied by high evaporation rates and hygiene problems. This is commonly a consequence of low infiltration rates, which are caused by clogging layers that form on top of the soil profile and the presence of a thick vadose zone. The present study aims to develop a conceptual solution to increase groundwater recharge rates in stormwater harvesting systems. The efficiency of vadose-zone wells and infiltration trenches is tested using analytical equations, numerical models, and sensitivity analyses. Dams built in the channel of ephemeral streams (wadis) are selected as a study case to construct the numerical simulations. The modelling demonstrated that vadose-zone wells and infiltration trenches contribute to effective bypassing of the clogging layer. By implementing these solutions, recharge begins 2250–8100% faster than via infiltration from the bed surface of the wadi reservoir. The sensitivity analysis showed that the recharge rates are especially responsive to well length and trench depth. In terms of recharge quantity, the well had the best performance; it can infiltrate up to 1642% more water than the reservoir, and between 336 and 825% more than the trench. Moreover, the well can yield the highest cumulative recharge per dollar and high recharge rates when there are limitations to the available area. The methods investigated here significantly increased recharge rates, providing practical solutions to enhance aquifer water storage in drylands.

     

基于动力学和材料软化特性的边坡渐进破坏特征研究

边坡的失稳是一个从量变到质变的动态渐进破坏过程,此问题也是边坡领域研究的重点与难点之一。在考虑岩土材料软化特性和动力学求解的基础上,建立了边坡渐进破坏仿真的理论框架;利用ABAQUS软件的动力显式求解模块实现了边坡的渐进破坏仿真;根据塑性应变揭露了剪切带的扩展过程,由软化本构确定了滑面材料的分区演化规律,根据等效塑性应变确定了边坡的滑面,通过滑面位置将边坡分为滑体、滑带、滑床,并分别研究了边坡各分区内部特征点运动学变量的发展过程,从而揭示了边坡的渐进破坏过程;基于材料参数沿滑面的时空分布,利用矢量和法得到了边坡不同演化阶段的安全系数。对比该方法与Bishop法确定的滑面位置与安全系数,发现两种方法峰值和残余强度对应的安全系数比较接近,该方法搜索所得滑面位于Bishop法自动搜索的滑面之间,验证了此方法的合理性及可靠性。最后分析了材料软化特征对边坡稳定性的影响,在保持其他参数不变的条件下,增大残余黏聚力,边坡的滑面位置加深,安全系数的初始值减小,安全系数的快速减小阶段有所推迟,并且快速减小阶段经历的时间有所延长,稳定后的安全系数有所增大。保持其他参数不变,增大残余黏聚力对应的等效塑性应变阈值,边坡的滑面位置加深,安全系数的初始值减小,安全系数的快速减小阶段有所推迟,但快速减小阶段经历的时间基本不变,达到稳定的时间有所推迟,同时稳定后的安全系数略微有所增大。     

空沟对列车运行引起的地基振动隔振效果研究

为研究空沟对高速列车引起地基振动的隔振效果,在已有研究基础上改进出一种新的解析研究模型。模型中首次运用了饱和半空间模型来研究地基上隔振沟对高速列车的隔振效果;隔振沟则通过在饱和半空间土体上设置3个合适宽度、截面为矩形的弹性层来模拟;中间矩形弹性层为路堤,路堤上方放置了枕木与轨道。枕木与轨道分别通过纵向异性Kirchhoff薄板与Euler梁来模拟,饱和土地基采用Biot多孔饱和介质理论来描述。控制方程通过傅立叶变换与傅立叶级数展开,在变换域中进行求解。研究表明,随着列车运行速度的提高,空沟的隔振效果明显提高;饱和土体固-液相的耦合作用对隔振沟的隔振效果的影响明显,尤其当列车运行速度超过土体表面Rayleigh波速时,随着土体渗透系数的增加,空沟隔振效果显著降低。此外,列车运行速度超过土体表面波速时,饱和土地基上空沟的隔振效果明显优于相应单相弹性地基上空沟的隔振效果。     

对基坑抗隆起稳定安全系数的改进

基坑的平面形状、尺寸以及围护结构的入土深度都会影响基坑的抗隆起稳定性,但现行规范推荐的抗隆起稳定安全系数算法均不能反映这些因素的综合影响。为解决这一问题,区分不同情况,对基坑底隆起破坏可能的模式做了必要的分类,分别给出了不同破坏模式情况下抗隆起稳定安全系数的计算方法。基于此方法,基坑围护结构的强度、刚度和入土深度都会影响基坑的抗隆起稳定性。计算分析表明,无论是对于排水情况还是不排水情况,狭窄基坑都具有更好的稳定性,而围护结构的入土深度可提高基坑的稳定性。改造后的基坑安全系数使传统算法可以考虑更多因素,为狭窄基坑缩小围护结构入土深度提供了一个理论手段。     

In-situ Rock Spalling Strength near Excavation Boundaries

It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ _max = 3σ ₁ ?σ ₃) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ _max at the excavation boundary. The ratio of σ _max /UCS is related to the observed depth of failure and failure initiation occurs when σ _max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.     

地震作用下岩质边坡倾倒破坏分析

倾倒破坏是陡倾层状岩体边坡一类主要的变形破坏形式,地震作用下岩质边坡块体倾倒破坏分析具有重要的工程指导意义。基于块体极限平衡,针对岩块长细比较大的情况,推导出了地震作用下岩质边坡倾倒破坏的一般解析解。对于简单的反倾边坡,给出了地震作用下倾倒破坏的显式解析解,并推导了岩块间法向力、破坏模式转变点和坡脚剩余下滑力计算公式。通过4个典型算例研究了地震作用对反倾边坡破坏模式、倾倒破坏时所允许的最大陡倾节理视倾角、破坏模式转变点和坡脚剩余下滑力的影响,为反倾边坡的抗震支护设计提供了理论基础。最后,根据地震影响系数与破坏模式转变点和坡脚剩余下滑力的定量关系对比分析了解析法与传递系数法,结果显示,当岩块长细比大于20时,解析法具有足够的计算精度。     

构造控制风化岩边坡侧旋滑移及监测结果映现——以高陂水利枢纽右岸边坡为例

本文以梅州市大埔韩江高陂水利枢纽右岸边坡为例,据现场工程地质勘察、钻探开挖揭露和坡体变形监测数据,分析该风化花岗岩边坡地质特点及不同开挖阶段监测点的位移。发现该边坡的滑移受到地质构造控制:断裂控制其后缘拉裂缝;节理密集带控制东边界侧缘沟谷;与斑状花岗岩相伴生的辉绿岩脉控制其西边界。工程上采取关闭滑体中部通过的公路、补充地质勘探、加强变形体监测、滑体中上部快速削方卸载、保护好西缘辉绿岩脉之下起抗剪支撑作用的块状斑状花岗岩体、减少西侧缘坡脚开挖范围并及早回填砼挡土墙、滑坡前缘跳槽式开挖等工程抢险和加固处理措施,取得了预期效果,保证了坡脚尾水房挡土墙下挖期间的施工安全。     

软土深基坑中的角落效应

在对软土深基坑档墙与土体的变形、破坏等基坑支护失事原因分析的基础上,提出基坑角落效应的概念,指出角落效应为当前软土深基坑工程设计与施工所忽视,分析了角落效应的形成机理及其综合特性,讨论了角落效应对基坑稳定性的影响及其治理建议。