密松水电站泄洪消能体型优化试验研究

在1:60溢流坝表、中泄水孔断面模型上对密松水电站泄水建筑物的布置方案进行了泄流能力、流速流态、压力分布等相关水力学问题的试验,并通过十多个方案的比较优化,提出了基本可行的泄水建筑物体型和消力池型式。研究成果表明:表孔采用合适的宽尾墩形式,对增加消能率、缩短消力池池长有利,但无法消除宽尾墩形成的水翅击打两侧导墙现象;表孔取消宽尾墩,采用跌坎式消力池方案,可有效降低底流消能时的池底板流速,同时消除宽尾墩形成的水翅击打两侧导墙现象,达到较满意的消能效果。本文中十数个消能方案的研究成果,对类似工程的设计和科研均有参考价值。     

反拱水垫塘底板检修时板块位移的试验研究

按照重力相似准则,设计包括水垫塘、扬压力水箱,拱圈试验段在内的试验模型。对反拱水垫塘在检修时,键槽和直缝的两种底板块接触形式的板块位移分别进行试验研究。研究表明,两种接触方式下,承受扬压力的底板块必然上升,带动相邻的底板块上升,使拱圈形成局部拱或全拱;位移最大处的缝隙为张,其它缝隙则有张有合;在承受扬压力的底板块中,扬压力大的底板块位移最大或次之;相邻两底板块均受扬压力作用,则任一底板块在靠近扬压力大的底板块那一端位移要大;与拱端相邻的底板块,由于受到拱端约束,位移最小。经过对拱圈底板两种接触方案比较,两者底板位移的变化规律是完全相同的,数量级也是一致的,但位移量稍有差别。研究成果可为水垫塘底板结构型式的选择和设计提供参考。     

临江地下结构抗浮计算方法研究

临江地下结构进行浮力计算时,其抗浮设计水位直接采用地面高程或江河水位是不合理的,需要对场地进行渗流分析才能确定。在沿用本奈特假定的前提下,推导出一种简化算法用于临江二元地层的渗流分析,可得到强透水层中的水头分布。在此基础上,提出地下结构物底板水压力的实用计算方法为抗浮设计提供依据。其方法适用于堤前、堤后覆盖层有不同的厚度和渗透系数,以及宽度为有限或无限的边界情况。该法计算简便,结果足够精确。经过算例验证和比较,堤后小型结构物对强透水层中的水头分布影响不大,但大型结构物和堤后宽度收窄将会显著抬高水头。位于覆盖层中的地下室底板浮力呈中部大、四周小的分布形态,但位于强透水层中时则呈线性分布。     

松散地层脉动与稳压灌浆室内试验研究

开展松散地层控制灌浆工艺研究具有重要的工程实践意义。开发了一套室内松散地层灌浆模拟试验装置,可模拟受灌体弹塑性边界,对比进行稳压灌浆与脉动灌浆试验。试验结果表明,在相同围压(0.2 MPa)、水固比(0.6)、不同灌浆压力(0.1,0.2,0.4 MPa)条件下,脉动灌浆灌后结石体7 d强度明显优于稳压灌浆,且强度值分布较均匀。电镜扫描表明,脉动灌浆后的土体呈团状结构,较常规灌浆后的絮状结构更紧密。脉动灌浆参数影响试验表明,采用较大的脉动灌浆压力有利于受灌体挤密,但压力过大更易劈裂扩散,造成浆脉变薄、结石强度降低;采用较小的水固比不利于浆液的扩散,但水固比过大会导致结石体厚度变薄、强度降低。试验论证了松散地层灌浆采用脉动灌浆工艺更加适宜,但需合理控制灌浆压力和水固比。     

射流掺气对水垫塘及坑底压强影响的实验研究

通过对射流掺气与不掺气时,水垫塘底板及冲坑底部脉动压强的对比试验,研究了掺气对水垫塘及冲坑底部压强特性的影响。结果表明:在冲坑未形成前,掺气使水垫塘底板上射流冲击区的脉动压强增加;无论是否掺气,滞点时均压强的量纲一分布遵从相同的规律,底板量纲一时均压强分布存在很好的自模性,且与气体冲击射流的压强分布具有相同的规律。冲坑形成后,掺气不再明显增大冲坑底部的脉动压强,掺气后,坑底脉动压强没有明显的变化。     

板桩加固护岸受力机制的现场试验研究

结合长湖申航道湖州段板桩加固护岸实体工程,在板桩预制过程中将土压力计埋入板桩侧面,底板施工时将土压力计埋入底板下,测试了在板桩混凝土凝固硬化过程中土压力计的受力情况、护岸荷载下板桩桩身两侧土压力的分布规律及护岸底板下土压力分布规律。试验结果表明,混凝土凝固硬化过程对埋入板桩两侧的土压力计会产生较大拉压应力并逐步趋于稳定,板桩两侧土压力及底板下土压力受施工荷载的影响在沉桩及底板施工初期存在较大调整并逐步趋于稳定;板桩靠岸侧和临水侧土压力分布存在明显差别,靠岸侧主动土压力分布呈现先增大后减小、然后增大的非线性分布规律,临水侧被动土压力基本呈线性分布。《板桩码头设计与施工规范》[1]中推荐的土压力计算方法计算靠岸侧主动土压力和临水侧被动土压力标准值均较实测值小,基本呈现随着深度的增加差别逐渐增大的规律;在护岸荷载作用下底板下土压力呈现两端大中间小的抛物线分布规律。     

扶壁式及L式挡土墙的模型土压力试验

汇总分析了扶壁式墙和Ｌ型墙等多种结构模型，在填干砂、湿砂、砂中灌水、水中填砂等不同条件下进行试验，得到扶壁式墙立板和底板土压力减少的分布规律，提出了比较合理的土压力近似计算公式和简易的计算图式，并对有关算例进行了分析     

某水电站表孔溢洪道消力池段结构优化的试验研究

通过对新疆某水电站表孔溢洪道的模型试验研究,发现在原设计消力池通过设计流量(30 a一遇洪水)1 856.2 m³/s时,消力池内水流波动剧烈,表现在池长和边墙高度均不能满足泄洪要求。因此文章通过对多种修改方案的试验研究,提出了在消力池内设置趾墩—消力墩—尾坎的方案四为最优修改方案。试验结果表明：方案四有效的改善了池内水流流态,同时缩短了池长、降低了池深,节省了工程造价,解决了下游冲刷问题,试验成果对于同类工程设计具有借鉴意义。     

砂质土体脉动注浆\

脉动注浆作为深厚覆盖层防渗加固处理的新工艺,尽管已得到一定应用,但脉动压力作用下相应浆液的扩散规律尚不明确。基于脉动注浆理论模型,推导了脉动注浆浆液扩散运移解析方程;采用数值模拟构造脉动周期压力变化模型,依托COMSOL多物理场耦合平台结合MATLAB,开发了考虑渗滤效应的脉动注浆扩散数值模拟程序,模拟脉动注浆的“浆-土”耦合作用过程;并通过自设计的脉动和稳压注浆模拟试验装置,对比脉动注浆与稳压注浆的浆液扩散差异;通过以上方法的相互验证,揭示了颗粒悬浮液流体在脉动压力作用下于全风化花岗岩体的扩散基本规律。研究结果表明：在全风化花岗岩体中,稳压注浆容易劈裂形成浆脉,扩散渗滤范围小,浆液扩散不可控;脉动压力控制下浆液的扩散可以形成有效的加固圈和浆泡,渗滤范围广;理论计算、室内试验、数值模拟所得浆液扩散运移距离一致,上下限误差均在合理范围之内,建立的数值模型和推导的理论公式较好地描述了脉动压力控制下浆液扩散过程和分布。研究成果可为脉动注浆防渗加固、数值模拟及工程实践提供较强的指导意义。     

冻土隧道冻胀力室内模型试验研究

详细地介绍了曲墙式、直墙式和圆形断面隧道衬砌在约束条件、隔热保温层及含水状况等因素变化情况下的相似材料冻胀力室内模型试验,通过分析试验得出不同衬砌断面在各种因素影响下衬砌和围岩间冻胀压力的量值和分布特征,以及由冻胀压力引起的结构内力分布特征。研究表明,直墙式断面受冰胀力最大,曲墙式次之,圆形面最小;曲墙式、直墙式断面冻胀力均呈分布荷载形态,前者拱脚及仰拱脚处冻胀力最大,后者边墙、底板处冻胀力最大。     

A Kinematic Thermal Model for Descending Slabs with Velocity Boundary Layers: A Case Study for the Tonga Subducting Slab

For the purpose of investigating the influence of metastable olivine (MO) phase transformations on both deep seismicity and stagnation of slabs, we constructed a 2-dimensional finite element thermal model for a 120 Ma-old 50o dipping oceanic lithosphere descending at 10 cm/yr with velocity boundary layers, which would mitigate the interference of constant velocity field for the slab. The resulting temperatures show that most of intermediate and deep earthquakes occurring within the Tonga slab are occurring inside the 800oC and 1200oC isotherm, respectively. The elevation of olivine transformation near ~410 km and respective persistence of metastable olivine and spinel within the transition zone and beneath 660 km would thus result in bimodal positive, zonal, negative density anomalies, respectively. These results together with the resulting pressure anomalies may reflect the stress pattern of the Tonga slab: (i) slab pull force exerts above a depth of ~230 km; (ii) MO existence changes the buoyancy force within the transition zone and facilitates slab stagnation at a depth of 660 km; (iii) as the subducting materials accumulated over 660 km, deepest earthquakes occur due to MO transformation; (iv) a flattened ‘slab’ may penetrate into the lower mantle due to the density increment of Sp transformation.     

Three-dimensional P-wave velocity structure beneath the Indonesian region

We present the P-wave seismic tomography image of the mantle to a depth of 1200 km beneath the Indonesian region. The inversion method is applied to a dataset of 118,203 P-wave travel times of local and teleseismic events taken from ISC bulletins. Although the resolution is sufficient for detailed discussion in only a limited part of the study region, the results clarify the general tectonic framework in this region and indicate a possible remnant seismic slab in the lower mantle.

Structures beneath the Philippine Islands and the Molucca Sea region are well resolved and high-velocity zones corresponding to the slabs of the Molucca Sea and Philippine Sea plates are well delineated. Seismic zones beneath the Manila, Negros and Cotabato trenches are characterized by high-velocity anomalies, although shallow structures were not resolved. The Molucca Sea collision zone and volcanic zones of the Sangihe and Philippine arcs are dominated by low-velocity anomalies. The Philippine Sea slab subducts beneath the Philippine Islands at least to a depth of 200 km and may reach depths of 450 km. The southern end of the slab extends at least to about 6°N near southern Mindanao. In the south, the two opposing subducting slabs of the Molucca Sea plate are clearly defined by the two opposing high-velocity zones. The eastward dipping slab can be traced about 400 km beneath the Halmahera arc and may extend as far north as about 5°N. Unfortunately, resolution is not sufficient to reveal detailed structures at the boundary region between the Halmahera and Philippine Sea slabs. The westward dipping slab may subduct to the lower mantle although its extent at depth is not well resolved. This slab trends N-S from about 10°N in the Philippine Islands to northern Sulawesi. A NE-SW-trending high-velocity zone is found in the lower mantle beneath the Molucca Sea region. This high-velocity zone may represent a remnant of the former subduction zone which formed the Sulawesi arc during the Miocene.

The blocks along the Sunda and Banda arcs are less well resolved than those in the Philippine Islands and the Molucca Sea region. Nevertheless, overall structures can be inferred. The bowl-shaped distribution of the seismicity of the Banda arc is clearly defined by a horseshoe-shaped high-velocity zone. The tomographic image shows that the Indian oceanic slab subducts to a depth deeper than 300 km i.e., deeper than its seismicity, beneath Andaman Islands and Sumatra and may be discontinuous in northern Sumatra. Along southern Sumatra, Java and the islands to the east, the slab appears to be continuous and can be traced down to at least a depth of the deepest seismicity, where it appears to penetrate into the lower mantle.     

Fracture and surface crust development in a Holocene pahoehoe lava flow on the Island of Tenerife,Canaries

An almost horizontal pahoehoe surface in a Holocene plagioclase basanite lava on Tenerife displays three scales of fracture within the surface crust. An early-formed set of large-scale fractures divides up the surface into an orthogonal set of rectangular slabs with dimensions of several metres and depths of 10–12 cm. The shortest slab dimension is parallel to the flow emplacement direction, inferred from a strong surface lineation. The slabs are domed with the centre an average of 9.6 cm (with range 4–19.6 cm) above the edges of the slabs. Profiles of the slabs normal and perpendicular to the margins and through the crest indicate that they can be described by a power law in which the deflection of the slab, h, is related to the distance from the crest, x, with an exponent between 2 and 3. Analysis of joints within the slabs indicates two smaller scale networks. An intermediate scale joint network bounds blocks with rectilinear to polygonal shapes in plan-view and has a characteristic mean spacing of 24.2 cm (range 10.5–48 cm). The major fractures in this set are normal and parallel to the slab margins. A smaller-scale joint network bounds polygonal equant blocks in plan-view and has characteristic spacing of 6.4 cm (range 3.7–10.5 cm). A model of cooling from the pahoehoe surface is used to constrain the growth of the crust and timing of fracture development. The large-scale slabs are attributed to localised accumulation of gas beneath the growing crust causing buoyant forces. The tensile stresses caused by uplift are sufficient to form the large-scale fractures after 2 or 3 h of cooling. The intermediate scale fracture network is attributed to the flexure of the slab crust. The smaller scale polygonal joint network is related to the build up of isotropic tensile stresses in the cooling slab crust due to thermal contraction with fracture development being promoted by the flexure of the slabs. An analysis of the slab deformation indicates that lava crust is weak. The weakness is explained by division of the crust into three zones: an outer zone with small scale joints that cause negligible strength, a middle zone of elastic behaviour in which stress can build up, and a lower zone of plastic deformation. The crustal slabs display profiles similar to that expected in a bending elastic plate. The deformation of the 10-cm-thick crust can be explained if the elastic zone was about 2-cm thick. This result agrees with an independent calculation of elastic zone thickness based on the position of the brittle–ductile transition being located at the 600°C isotherm at a depth of about 2 cm when the crustal slabs were rifted apart.     

Characteristics of Intraslab Stress Field and Tectonic Implication in the Nankai Trough, Japan

1. IntroductionThe Nankai Trough region (Fig. 1.1) of southwest Japan is one of the most tectonically complex subduction zones in the world. The subduction of the Philippine Sea plate (PH) beneath the Eurasian plate (EU) has caused a series of large and great interplate earthquakes. It is generally accepted that great earthquakes have occurred at intervals of 100-150 years along the Nankai subduction zone since the 684 Hakuho earthquake (Fig. 1.2). However, a large earthquake (M>7.5) has…     

Finite element modeling of slab–foundation interaction on rigid pavement applications

Bending stresses produced in pavement concrete slabs are influenced by the contact condition along the slab–foundation interface. A finite element model for the analysis of frictional contact between unbonded pavement layers is presented. A 2D plate element is proposed for the idealization of composite slabs. Interface elements are used to connect the unbonded layers within the composite slab and to model the interface between the bottom slab layer and the foundation layer. Interface elements have the ability to capture the separation and sliding between pavement layers, due to thermal loads, and to calculate the frictional traction at their interface.     

Subduction, back-arc spreading and global mantle flow

The shapes and orientations of Benioff zones beneath island arcs, interpreted as marking the location of subducted lithosphere, provide the best presently available constraints on the global convective flow pattern associated with plate motions. This global flow influences the dynamics of subduction. Subduction zone phenomena therefore provide powerful tests for models of mantle flow. We compute global flow models which, while simple, include those features which are best constrained, namely the observed plate velocities, applied as boundary conditions, and the density contrasts given by thermal models of the lithosphere and subducted slabs. Two viscosity structures are used; for one, flow is confined to the upper mantle, while for the other, flow extends throughout the mantle.Instantaneous flow velocity vectors match observed Benioff zone dips and shapes for the model which allows mantle-wide flow but not for the upper mantle model, which has a highly contorted flow pattern. The effect of trench migration on particle trajectories is calculated; it is not important if subduction velocities are greater than migration rates. Two-dimensional finite element models show that including a coherent high viscosity slab does not change these conclusions. A coherent high viscosity slab extending deep into the upper mantle would significantly slow subduction if flow were confined to the upper mantle. The maximum earthquake magnitude, M_w, for island arcs correlates well with the age of the subducted slab and pressure gradient between the trench and back-arc region for the whole mantle, but not the upper mantle, flow model. The correlations with orientations of Benioff zones and seismic coupling strongly suggest that the global return flow associated with plate motions extends below 700 km. For both models, regions of back-arc spreading have asthenospheric shear pulling the back-arc toward the trench; regions without back-arc spreading have the opposite sense of shear, suggesting global flow strongly influences back-arc spreading.     

http://www.sciencedirect.com/science/article/pii/S167498711300162X

Seismic observations have shown structural variation near the base of the mantle transition zone(MTZ)where subducted cold slabs,as visualized with high seismic speed anomalies(HSSAs),flatten to form stagnant slabs or sink further into the lower mantle.The different slab behaviors were also accompanied by variation of the "660 km" discontinuity depths and low viscosity layers(LVLs) beneath the MTZ that are suggested by geoid inversion studies.We address that deep water transport by subducted slabs and dehydration from hydrous slabs could affect the physical properties of mantle minerals and govern slab dynamics.A systematic series of three-dimensional numerical simulation has been conducted to examine the effects of viscosity reduction or contrast between slab materials on slab behaviors near the base of the MTZ.We found that the viscosity reduction of subducted crustal material leads to a separation of crustal material from the slab main body and its transient stagnation in the MTZ.The once trapped crustal materials in the MTZ eventually sink into the lower mantle within 20-30 My from the start of the plate subduction.The results suggest crustal material recycle in the whole mantle that is consistent with evidence from mantle geochemistry as opposed to a two-layer mantle convection model.Because of the smaller capacity of water content in lower mantle minerals than in MTZ minerals,dehydration should occur at the phase transformation depth,~660 km.The variation of the discontinuity depths and highly localized low seismic speed anomaly(LSSA) zones observed from seismic P waveforms in a relatively high frequency band(~1 Hz) support the hypothesis of dehydration from hydrous slabs at the phase boundary.The LSSAs which correspond to dehydration induced fluids are likely to be very local,given very small hydrogen(H~+) diffusivity associated with subducted slabs.The image of such local LSSA zones embedded in HSSAs may not be necessarily captured in tomography studies.The high electrical conductivity in the MTZ beneath the northwestern Pacific subduction zone does not necessarily require a broad range of high water content homogeneously.     

The potential influence of subduction zone polarity on overriding plate deformation, trench migration and slab dip angle

A geodynamic model exists, the westward lithospheric drift model, in which the variety of overriding plate deformation, trench migration and slab dip angles is explained by the polarity of subduction zones. The model predicts overriding plate extension, a fixed trench and a steep slab dip for westward-dipping subduction zones (e.g. Mariana) and predicts overriding plate shortening, oceanward trench retreat and a gentle slab dip for east to northeastward-dipping subduction zones (e.g. Chile). This paper investigates these predictions quantitatively with a global subduction zone analysis. The results show overriding plate extension for all dip directions (azimuth α = − 180° to 180°) and overriding plate shortening for dip directions with α = − 90° to 110°. The wide scatter in data negate any obvious trend and only local mean values in overriding plate deformation rate indicate that overriding plate extension is somewhat more prevalent for west-dipping slabs. West-dipping subduction zones are never fixed, irrespective of the choice of reference frame, while east to northeast-dipping subduction zones are both retreating and advancing in five out of seven global reference frames. In addition, westward-dipping subduction zones have a range in trench-migration velocities that is twice the magnitude of that for east to northeastward-dipping slabs. Finally, there is no recognizable correlation between slab dip direction and slab dip angle. East to northeast-dipping slabs (α = 30° to 120°) have shallow (0–125 km) slab dip angles in the range 10–60° and deep (125–670 km) slab dip angles in the range 40–82°, while west-dipping slabs (α = − 60° to − 120°) have shallow slab dip angles in the range 19–50° and deep slab dip angles in the range 25–86°. Local mean deep slab dip angles are nearly identical for east and west-dipping slabs, while local mean shallow slab dip angles are lower by only 4.7–8.1° for east to northeast-dipping slabs. It is thus concluded that overall, there is no observational basis to support the three predictions made by the westward drift model, and for some sub-predictions the observational basis is very weak at most. Alternative models, which incorporate and underline the importance of slab buoyancy-driven trench migration, slab width and overriding plate motion, are better candidates to explain the complexity of subduction zones, including the variety in trench-migration velocities, overriding plate deformation and slab dip angles.     

增压式真空预压处理软基的加固机理

常规的真空预压往往由于竖向排水板堵塞引起排水固结效果差、工期长。对此,笔者采用增压式真空预压处理某一站场软土地基弥补上述不足。通过分析对比增压式和常规真空预压处理方式下地基在加固期内的沉降、孔隙水压力变化特征,评价增压条件下的处理效果。结果表明：采用增压式真空预压三、四区地表沉降量明显大于常规处理的一区,且孔隙水压力消散速度也快于一区;而1.0 m间距的三区产生的预压沉降量较1.2 m间距的四区增加约30%;增压式真空预压利用增压管的增压作用能加速软基排水过程并改良排水板易淤积的不良性能。     

秦岭造山带勉略缝合带构造变形与造山过程

秦岭晚古生代以来造山过程中的构造变形研究对建立华北板块与华南板块之间的最终拼合过程尤为重要。为此，选择勉略带进行了详细的野外调查，对其物质组成，几何学结构，变形序列，运动学和动力学作了系统解剖，认为：勉略带是有一定宽度，由一定实体组成的蛇构造混杂岩带，它包括不同时代，不同构造背景，不同起源的一系列构造岩片，如：基底岩片组，洋壳岩片组，岛弧岩片组，碰撞构造沉积楔形体，大陆边缘岩片组，它们被一系列的北倾南冲的断裂分割。勉略带到－高川段的北倾南冲逆冲断裂和勉略带北部南倾北冲的逆冲断裂组合成为现今不对称的正花状几何学结构，但勉略带的北界状元碑断裂和南界 到－略阳断裂走滑特征明显。结合勉略带邻区的构造特征，分别对带内重要断裂及岩片的构造解析表明，除主造山的大地构造演化阶段外，可将勉略带构造变形序列及演化历程总体归纳为三大阶段：俯冲变形阶段，主造山碰撞变形阶段的陆内造山调整变形阶段，并对不同变表阶段的时限，变形特征，运动学和动力学分别作了描述；最后针对该区复杂的楔入，挤出（逃逸）走滑，逆冲过程，提出了一个统一的动力学演化模式。