Effects of initial particle gradation and rock content on crushing behaviors of weathered phyllite fills——A case of eastern Ankang section of Shiyan—Tianshui highway,China

The objective of this paper is to investigate the effects of initial particle gradation and rock content on the crushing behavior(i.e.grain size before and after crushing) of weathered phyllite fills.Compaction tests were conducted on weathered phyllite fills with rock contents of 35%,45%,55%,65% and 75%(by weight).First,the particle size distributions(PSDs) were observed before and after compaction,and then the particle breakage of weathered phyllite fills was analyzed by fractal dimension.Relative fractal dimension was proposed to evaluate the effects of initial rock content and initial gradation on the particle breakage.It was found that the fractal dimension method can well characterize the crushing behaviors of the weathered phyllite fills.The finer the fills were,the more they were compacted.That is,after the first compaction,the relative fractal dimension of the weathered phyllite fills increased as the rock content increased,reaching the values of 0.013,0.016,0.024,0.037 and 0.08,respectively.After the second compaction,these relative fractal dimension values,dominated by the initial particle gradation,became 0.059,0.072,0.052,0.095 and 0.118,respectively.In conclusion,the weathered phyllite fills with 55% rock content exhibited the least breakage and were most suitable for filling the subgrade.Findings in this paper will provide significant guidance for the construction of weathered phyllite filling subgrade in future projects.     

Behavior of raft on settlement reducing piles:Experimental model study

An experimental program is conducted on model piled rafts in sand soil.The experimental program is aimed to investigate the behavior of raft on settlement reducing piles.The testing program includes tests on models of single pile,unpiled rafts and rafts on 1,4,9,or 16 piles.The model piles beneath the rafts are closed ended displacement piles installed by driving.Three lengths of piles are used in the experiments to represent slenderness ratio,L/D,of 20,30 and 50,respectively.The dimensions of the model rafts are 30 cm×30 cm with different thickness of 0.5 cm,1.0 cm or 1.5 cm.The raft-soil stiffness ratios of the model rafts ranging from 0.39 to 10.56 cover flexible to very stiff rafts.The improvement in the ultimate bearing capacity is represented by the load improvement ratio,LIR,and the reductions in average settlement and differential settlement are represented by the settlement ratio,SR,and the differential settlement ratio,DSR,respectively.The effects of the number of settlement reducing piles,raft relative stiffness,and the slenderness ratio of piles on the load improvement ratio,settlement ratio and differential settlement ratio are presented and discussed.The results of the tests show the effectiveness of using piles as settlement reduction measure with the rafts.As the number of settlement reducing piles increases,the load improvement ratio increases and the differential settlement ratio decreases.     

In Situ Tests--Predicted vs. Observed Settlements： A Comparative Case Study

This paper aims to deal with the comparison of the estimated settlements derived by in situ tests with the observed settlements in site, in order to evaluate the accuracy of settlement prediction by in situ tests, in comparison not only with site observation by topographic means, but also with the values of settlements derived by numerical analysis by means of PLAXIS 2D and 3 D. The site where are carried out the tests and periodically are observed the settlements since the beginning of construction process, is located in the Oil Product Terminal, at the industrial park of Porto Romano, Durres, Albania. The main purpose of this project was the ground improvement by using preloading method in order to prevent liquefaction process and settlements. The data used to conduct this study are taken by the site investigation done after inserting into the soil vertical drains made of columns of free--draining gravel （gravel pile drains） until 14 m depth and center-to-center spacing of 2 m, and wick drains （premanufactured） until 25 m depth and center-to-center spacing of 1.8 m. The observed settlements are periodically measured by topographic equipments. This paper will present the conclusions derived by settlement analyzes from in situ tests and site observations.     

Durability problems of lining structures for Xiamen Xiang’an subsea tunnel in China

Durability problem of reinforced concrete for underground structures is a hot issue in the field of structural engineering.For underground structures,the prediction of structural service life and methodology for durability design are needed to estimate structural durability.Taking the case of Xiamen Xiang’an subsea tunnel as background,which is designed to meet the requirement of 100-year service life,the influential factors on tunnel lining durability are analyzed.Under the criteria of crack controlling and bearing capacity of lining structures,the theoretical service life of Xiamen Xiang’an subsea tunnel lining is studied.The regulations,which are needed for the diffusion capability of chloride ions in concrete by the relevant diffusion tests,are proposed.After a quick corrosion test,the bearing capacity test on eccentric short columns is implemented to investigate the variation rules in the bearing capacity of models with time.Influence of the corrosion degree of steel bars on the bearing capacity of models is also investigated.Based on the results of model tests,the acceleration ratio between the quick corrosion in laboratory test and the natural corrosion environments is established.Thus,the natural service life of subsea tunnel lining structures can be obtained by means of laboratory tests.Then,the proposed method using this modified model is employed to predict the service life of tunnel lining structures.Finally,the design and construction measures for improving the durability of lining structures of subsea tunnel are introduced.The proposed method in the present study based on a real engineering project is superior to those with only theoretical assumptions,and would be more suitable for similar projects.     

Stability analysis of bank slope under conditions of reservoir impounding and rapid drawdown

Stability of an ancient landslide in a reservoir area is analyzed by using centrifugal model tests, soil laboratory tests and numerical analysis. Special attention is paid to variation in water level, simulation of large-scale heterogeneous prototype slope, and strength reduction of sliding zone soils after slope sliding. The results of centrifugal model test show that reservoir impounding can reduce sliding resistance at the slope toe, followed by toe collapsing and front cracking of slope. Rapid drawdown can produce hydrodynamic pressure towards reservoir at the front of slope. Deformation is observed in the middle and upper slope, which reduces the slope stability further and forms the pull-typed landslide trend. Reinforcement of slope toe is effective for preventing the progressive failure. The results of laboratory test show that slope toe sliding will lead to the redistribution of soil density and moisture content, which will reduce the shear strength of soil in sliding zone, and the cohesion of immersed soil is reduced gradually and finally vanishes with time. The numerical results show that the strength reduction method used in finite element method (FEM) is very effective in capturing the progressive failure induced by reservoir water level fluctuations, and the evolution of failure surface derived from numerical simulation is very similar to that observed in centrifugal model test.     

Optimization of dewatering schemes for a deep foundation pit near the Yangtze River,China

A deep foundation pit constructed for an underground transportation hub was excavated near the Yangtze River. Among the strata, there are two confined aquifers, between which lies an aquiclude that is partially missing. To guarantee the safety of pit excavation, the piezometric head of the upper confined aquifer, where the pit bottom is located, should be 1 m below the pit bottom, while that of the lower confined aquifer should be dewatered down to a safe water level to avoid uplift problem. The Yangtze River levee is notably close to the pit, and its deformation caused by dewatering should be controlled. A pumping test was performed to obtain the hydraulic conductivity of the upper confined aquifer. The average value of the hydraulic conductivity obtained from analytical calculation is 20.45 m/d, which is larger than the values from numerical simulation(horizontal hydraulic conductivity K_H = 16 m/d and vertical hydraulic conductivity K_V = S m/d). The difference between K_H and K_V indicates the anisotropy of the aquifer. Two dewatering schemes were designed for the construction and simulated by the numerical models for comparison purposes. The results show that though the first scheme could meet the dewatering requirements, the largest accumulated settlement and differential settlement would be94.64 mm and 3.3‰, respectively, greatly exceeding the limited values. Meanwhile, the second scheme,in which the bottoms of the waterproof curtains in ramp B and the river side of ramp A are installed at a deeper elevation of-28 m above sea level, and 27 recharge wells are set along the levee, can control the deformation of the levee significantly.     

Finite element analysis of anchor plates using non-coaxial models

The non-coaxial model simulating the non-coincidence between the principal stresses and the principal plastic strain rates is employed within the framework of finite element method（FEM） to predict the behaviors of anchors embedded in granular material.The non-coaxial model is developed based on the non-coaxial yield vertex theory,and the elastic and conventional coaxial plastic deformations are simulated by using elasto-perfectly plastic Drucker-Prager yield function according to the original yield vertex theory.Both the horizontal and vertical anchors with various embedment depths are considered.Different anchor shapes and soil friction and dilation angles are also taken into account.The predictions indicate that the use of non-coaxial models leads to softer responses,compared with those using conventional coaxial models.Besides,the predicted ultimate pulling capacities are the same for both coaxial and non-coaxial models.The non-coaxial influences increase with the increasing embedment depths,and circular anchors lead to larger non-coaxial influences than strip anchors.In view of the fact that the design of anchors is mainly determined by their displacements,ignoring the non-coaxiality in finite element numerical analysis can lead to unsafe results.     

Rational permeability coefficient of a permeable lining for composite tunnel lining structures

 Various factors influence the tunneling are simplified considering the axial symmetry of the composite lining structure. The analytical expressions for the seepage field,stress,and displacement in the surrounding rock and those for the grouting circle,permeable lining,and ordinary lining are obtained according to the principle of effective stress. The seepage discharge,plastic zone,stress,and displacement around a tunnel affected by the different permeability coefficient ratios between the permeable lining and surrounding rock are studied. The reasonable value of permeability coefficient for the permeable lining are discussed. The results show that the grouting circle controls the tunnel seepage discharge well and has a reasonable permeability coefficient and an optimal radius. The grouting circle controls the plastic zone development in the surrounding rock,i.e.,the thicker the grouting circle,the smaller the plastic zone in the surrounding rock. The tunnel seepage discharge and the effective stress in the grouting circle and lining increase gradually with the increasing of permeability coefficient ratio and tend to be stable after the ratio reaches 0.1. Comprehensive analysis on the factors such as the stability of tunnel seepage discharge,the plastic zone of the surrounding rock and the stress around the tunnel indicates that it is relatively reasonable for the permeability coefficient ratio to be greater than or equal to 0.1.     

Laboratory study on the mechanical behaviors of an anisotropic shale rock

Shale gas is becoming an important energy source worldwide.The geomechanical properties of shale rocks can have a major impact on the ef fi ciency of shale gas exploration.This paper studied the mineralogical and mechanical characteristics of a typical gas shale in Ohio,USA.Scanning electron microscope(SEM)with energy dispersive X-ray(EDX)analyses was employed to measure the microstructure and material composition of the shale rock.The anisotropic behaviors of shale rock,including compressive and tensile strengths,were experimentally measured.The characteristics of shale rock were also studied by nondestructive wave speed measurements.The shale demonstrated strong anisotropic behaviors with the tensile strengths perpendicular to the bedding plane around 300e360 times of that parallel to bedding plane.Results of ultrasonic tests indicated that both compression and shear wave velocities show strong anisotropic patterns.The compression wave speed was the smallest in the direction perpendicular to the bedding plane;while the shear wave speed was the smallest in the direction parallel to the bedding plane.The ratio of wave speed anisotropy is around 1.3e1.4 for compression wave;the ratio of shear wave speed anisotropy is larger and more diverse compared with the compression wave anisotropy.This might be related to the larger variability in the frictional adhesive strength along bedding plane than the compressive adhesive strength.     

强夯置换滨海软土成墩效果模型试验研究

强夯置换的碎石墩深度和直径是表征墩型的重要指标,而强夯参数、施工方式是影响墩型的重要因素。研究强夯置换机制对优化强夯置换工艺具有重要意义。依据模型与原型的相似关系比0.05设计了室内模型试验,通过重塑软土形成的模型地基,研究了不同夯击能(0.018、0.027、0.036、0.045kN·m)、不同填料方式(1/4填料、1/3填料、1/2填料、满填料)、不同夯锤锥体角度(0°、30°、60°、70°、80°)3个独立影响因素及其组合变化对成墩形状的影响。结果表明：墩径随置换深度的变化曲线以最大墩径埋深为分界点,其上满足二次多项式分布规律,其下满足线性递减规律。夯击能与置换深度总体上呈正相关关系;锥体角度与置换深度呈负相关关系;锥体角度与夯击能对墩体直径的影响较小;夯坑填料方式对墩体直径的影响显著,与墩体扩径系数呈正相关关系。     

饱和盐渍土碎石排水桩加强夯试验研究

在青海察尔汗盐湖地区的饱和盐渍土上进行碎石排水桩加强夯试验研究。对不同的碎石排水桩参数、不同的强夯能量进行对比试验。通过静载荷试验、孔隙水压力测试、变形测试、静力触探试验、重型圆锥动力触探试验、波速测试等大量的原位测试试验,明确了饱和盐渍土碎石排水桩加强夯处理的碎石桩优化设计参数、强夯单点夯击能量的选择、强夯处理效果、强夯的影响深度等。给出了强夯处理后地基承载力、压缩模量等土性指标的确定方法。     

强夯的物质点法模拟及其能量转化规律分析

提出大应力范围内的密度相关土体本构模型,适用于分析强夯等作用下的土体变形问题。对刚柔接触算法进行了修正,用物质点法结合提出的本构模拟了强夯过程。与其他数值模拟将荷载假设为三角形应力波作为输入荷载不同,通过输入夯锤与土体的碰撞速度实现加载。模拟结果与承德机场4标段某处试验数据进行了对照,吻合较好。提出强夯过程中的能量转化率的概念,对能量转化的规律进行了研究,为研究强夯问题提供了新视角。模拟分析表明,能量转化率的提高不总意味着每击夯沉量的提高,因为能量在较大范围的扩散可能导致高能量转化率下的低夯沉量;剪切变形过程中吸收较多的塑性应变能,可能会使体积压缩变形吸收的塑性应变能向局部集中。通过数值模拟还发现,重锤低落时的能量转化率高于轻锤高落,一般可产生更大的夯沉量。     

高填方土石混合料强夯振动响应及加固机理研究

基于土石混填料强夯加固的大型现场振动速度测试的试验成果,深入地分析了质点振速和波形在强夯过程中的传播及演化,以此研究了1000 kN·m夯击能作用下土石料填筑体的内部质点的振动响应及强夯加固机理。结果表明：土石料在强夯作用下土体内部仅有冲击波和振动波两种振动形态;提出了强夯起始加固临界振速的概念和确定方法,并给出相应的振速值;通过分析填筑体空间振速演化规律,阐述了粗粒土石混合料的强夯加固机理,将强夯过程分为加固发展阶段和加固退化阶段。为工程条件下强夯施工层厚给出指导性建议。     

煤矸石地基在强夯冲击荷载作用下的物理模型试验研究

 为了研究强夯法加固煤矸石地基的加固效果,了解强夯过程中不同深度处动应力分布规律,测定不同夯击能的有效加固深度,进行室内模型试验研究。用DH5939动态应变仪采集不同夯击能、不同击数、不同测点位置煤矸石地基中的动应力。试验结果表明：单击夯沉量随夯击次数的增加而减小。在夯击次数相同情况下,单击夯击能越大,夯沉量也越大。在强夯作用下,动应力主要为单一的波峰,没有明显的第二波峰,作用时间极短,动应力达到峰值所需的时间明显小于衰减时间。沿夯锤不同深度的动应力达到峰值具有明显的时滞性,在同一深度,随着夯击能、夯击次数的增加,动应力也相应增加。另外,强夯后煤矸石地基的物理力学特性指标如压实度、黏聚力等较夯前有较大提高,夯击能越大,提高幅度越明显,夯击能相同时,距夯点位置越近,提高幅度越明显。满足实际工程需要的最佳夯击能约为3 000 kN•m,最佳夯击击数为7～9击。该成果不仅适用于强夯法处理煤矸石地基,对其他松散易碎介质如建筑渣土的强夯地基加固也有一定的参考价值。     

强夯法施工锤重参数的合理选用

通过对常规方法确定强夯施工参数及下落锤体对地基土体冲力作用的研究，指出在同一单击能量情况下，采用重锤低落距处理地基工程优于轻锤高落距强夯方案。     

高真空击密法加固滨海吹填土地基试验研究

采用高真空击密法加固某滨海饱和吹填砂性土地基。试验结果表明：1）井点强降水能迅速降低夯前地下水位和静孔隙水压力,有效抑制强夯时产生的超静孔隙水压力的增长;2）不同夯点产生的动应力对土体沿深度产生叠加效应,深度为（2～3）倍夯锤直径处的土体强度提高最显著,且夯点间距越小加固效果越好,但对有效加固深度没有明显影响;3）加固...     

不同形状夯锤在灰土挤密桩施工中的应用研究

灰土挤密桩被广泛应用于处理黄土地基的湿陷性,其处理效果主要依赖于灰土的夯填质量,实践证明夯填质量除了和夯击能量有关外,还与夯锤的形状有很大关系。文章对三种高度和重量相同但锤头形状不同的夯锤(半球型、椭球型、锥型)的夯填效果进行了对比分析。采用LSDYNA动力分析程序分析了三种落锤以相同高度自由下落时对土体的动力响应。对三种夯锤施工的试桩区进行了复合地基载荷试验,对桩间土和桩体土样的基本物理性质进行了分析。综合动力有限元分析结果和现场试验分析结果发现:椭球型锤贯入效率最高;半球型夯锤夯填效果最好,承载力提高率最大且挤密效果最均匀。     

十堰500kV变电站碎石土强夯地基的检测

十堰500kV变电站工程地基采用强夯法进行处理,夯填料为挖方区云母片岩破碎料。规范中对于根据动探方法评价碎石土的指标有明确的规定,但其适用于天然碎石土地基。对于强夯处理形成的人工碎石土地基能否采用该标准,借助于该强夯工程根据静载试验和超重型动力触探对比试验对其进行了研究。     

高铁路基动载沉降现场监测分析

高铁路基的工后沉降控制是一大难题,其中,动荷载影响的研究尚处于数值计算与试验阶段。依托沪宁城际铁路建设、运营监测数据,基于分离动、静荷载引起沉降的思路,通过静载作用沉降与实测运营沉降取差值,获取动载作用下的沉降测试值,分析了路基由动载引起的沉降特性。结果表明:动载引起的沉降对运营后实测沉降的贡献有限,并不是引起部分断面总体沉降偏大的主要原因;不同地质条件及路堤填筑高度对动载引起沉降的影响较小;动载引起沉降的速率在运营前期发展较快并达到最大,之后沉降速率减小,沉降趋于稳定,运营后的第4年累积沉降约为2~3mm。