Pore water pressure increment model for saturated Nanjing fine sand subject to cyclic loading

Three groups of dynamic triaxial tests were performed for saturated Nanjing fine sand subjected to uniform cyclic loading. The tested curves of the excess pore water pressure （EPWP） ratio variation with the ratio of the number of cycles are provided. The concept of the EPWP increment ratio is introduced and two new concepts of the effective dynamic shear stress ratio and the log decrement of effective stress are defined. It is found that the development of the EPWP increment ratio can be divided into three stages： descending, stable and ascending. Furthermore, at the stable and ascending stages, a satisfactory linear relationship is obtained between the accumulative EPWP increment ratio and natural logarithm of the effective dynamic shear stress ratio. Accordingly, the EPWP increment ratio at the number of cycles N has been deduced that is proportional to the log decrement of effective stress at the cycle number N-l, but is independent of the cyclic stress amplitude. Based on the analysis, a new EPWP increment model for saturated Nanjing fine sand is developed from tested data fitting, which provides a better prediction of the curves of EPWP generation, the number of cycles required for initial liquefaction and the liquefaction resistance.

Behavior of large post-liquefaction deformation in saturated Nanjing fine sand

Laboratory tests on the large post-liquefaction deformation of saturated Nanjing fine sand were performed by using a hollow cylinder apparatus. The stress-strain responses and the characteristics of excess pore water pressure after liquefaction were studied. It was found that the relationship between deviatoric stress and axial strain presented a sigmoid curve, and there was a good linearity relationship between normalized pore water pressure and deviatoric stress. On this basis, a constitutive model of stress-strain responses and a dissipation model of excess pore water pressure were established. It was found that the results predicted by the two models were in good agreement with the experimental data. The influence of relative densities and confining pressure on the characteristics of liquefied soil were studied, The results showed the relative densities and initial effective confining pressure all had an important influence on the stress-strain responses of liquefied saturated Nanjing fine sand. However, the dissipation model of excess pore water pressure after liquefaction was only affected by the confining pressure.

Self-compacting Concrete-filled Steel Tubes Prepared from Manufactured Sand with a High Content of Limestone Fines

To meet the requirements of construction of concretes filled in the steel tube arches,a C60 grade micro-expansive self-compacting concrete （SCC） was prepared from manufactured sand （MS）.The utilization of MS with a high content of quarry limestone fines was dealed for SCC applications.The workability,compressive and splitting strength,modulus of elasticity,restrained expansion and chloride ion permeability as well as freeze-thaw resistance of three MS-SCC mixes with fines content of 3%,7% and 10% were tested and compared with those of the natural sand （NS）-SCC mix.The experimental results indicate that the performances of the C60 MS-SCC with fines content of 7% are excellent and compared favorably with those of C60 NS-SCC.

Undrained dynamical behavior of Nanjing flake-shaped fine sand under cyclic loading

A series of dynamic behavior tests on Nanjing flake-shaped fine sand were performed by using the WFI cyclic triaxial apparatus made in England. The dynamic behaviors of Nanjing flake-shaped fine sand under different static deviator stress levels and cyclic stress ratios were studied. Through comparing the effective stress path under cyclic loading with static loading, the processes of liquefaction of saturated Nanjing flake-shaped fine sand with development of dynamic pore-water pressure, including the initial compact state, compression state and dilative state, were investigated. The variation of the shear stiffness with the number of cycles and cyclic strain was investigated by analyzing the secant shear modulus in each unload-reload loop of dynamic stress-strain relationship. And by means of the exponential function, the empirical equations of the relationship between secant shear modulus Gsec, shear modulus ratio Gsec/Gmax and cyclic strain ε were established based on series of test results. The results show that according to different combinations of static deviator stress and cyclic stress, two kinds of failure patterns with deviator stress reversal or no deviator stress reversal are observed in the samples tested in this series, including cyclic mobility and the failure of accumulation residual strain. In addition, the degradation of dynamic shear modulus is due to the development of vibration pore-water pressure and it is observed that the shear modulus reduces with the progressive number of cycles.

Effect of granulated rubber on shear strength of fine-grained sand

Review of the literature related to the mixture of shredded tire and sand shows that,despite of the increase in shear strength due to addition of tire chips,granulated rubber causes reduction in shear strength of sand.In this study,the shear behavior of mixtures of fine-grained sand and 1-5 mm granulated rubber is investigated.Sixty direct shear tests were conducted on sandegranulated rubber mixtures with various rubber contents(0%,5%,10%,20% and 30%) at different relative densities(50%,70% and 90%) and different normal stresses(34.5 kPa,54.5 kPa,74.5 kPa and 104.5 kPa).The obtained results show that the granulated rubber improves the shear strength of fine-grained sand at medium relative density and low normal stress.The degree of improvement in shear strength is a function of rubber content,relative density and normal stress.The results show that at relative density of 50%,by adding 5% granulated rubber,the internal friction angle of sand increases from 35.1° to 39.2°.However,at relative densities of 70% and 90%,addition of granulated rubber to sand decreases its internal friction angle.The results also indicate that the behavior of sand becomes more ductile with increasing granulated rubber content.Adding granulated rubber leads to greater yielding strain and less tangent stiffness of sand.The maximum dilation angle decreases with the decrease in granulated rubber content.The stress ratio of sample at critical state(ψ= 0°) decreases with increasing granulated rubber content.

Experimental Studies on Modified Suction Caissons in Fine Sand Subject to Uplift Loading

A modified suction caisson (MSC), which was reported by the authors of this paper previously, comprises an external short-skirted structure that is added to a regular suction caisson (RSC). It has been proved that MSCs can improve the lateral bearing capacity and limit the deflection of the caisson compared with RSCs. A series of model tests were conducted to investigate responses of MSCs subject to uplift loading in saturated sand. The effects of external skirt dimensions on the uplift bearing capacity of MSCs were considered. In addition, the influences of the sealed top lid of the skirted structure on the uplift bearing capacity and the resulting passive suction of MSCs were also studied. It was found that the uplift bearing capacities of MSCs are 1.4–1.7 times that of RSCs. Moreover, test results in serviceable conditions show that the sealed external skirted structure of perspex-made suction caissons significantly contributed to the uplift bearing capacity as a result of passive suction. © 2017 Tianjin University and Springer-Verlag GmbH Germany

Effect of Initial Stress Conditions on the Threshold Shear Strain of Nanjing's Saturated Fine Sand

By using GDS dynamic hollow cylinder torsional apparatus, a series of cyclic torsional triaxial tests under complex initial consolidation condition are performed on Nanjing saturated fine sand. The effects of the initial principal stress direction αo, the initial ratio of deviatoric stress η0, the initial average effective principal stress Po and the initial intermediate principal stress parameter b0 on the threshold shear strain γt of Nanjing saturated fine sand are then systematically investigated. The results show that γt increases as η0,p0 and b0 increase respectively, while the other three parameters remain constant. ao has a great influence on γt, which is reduced when ao increases from 0° to 45°and increased when α0 increases from 45° to 90°. The effect of α0 on γt, plays a leading role and the effect of η0 will weaken when ao is approximately 45°.

Cyclic liquefaction screening of sand with non-plastic fines:Critical state approach

There have been significant advances in the application of critical state,CS,in liquefaction potential assessment.This was done by comparing state parameter,j with estimated characteristic cyclic stress ratio,CSR due to an earthquake.A cyclic resistance ratio,CRR curve,which can be determined from cyclic liquefaction tests,separates historical liquefied and non-liquefied data points(j,CSR).On the other hand,the concepts of equivalent granular state parameter,j*,which was developed for sands with fines,can be used in lieu j to provide a unifying framework for characterizing the undrained response of sands with non/low plasticity fines,irrespective of fines content(fc).The present work combines these two propositions,and by merely substituting j*for j into the aforementioned CS approach to capture the influence of fc.A series of static and cyclic triaxial tests were conducted,separately and independently of the concept of j*,for sand with up to fc of 30%.The clean sand was collected from Sabarmati river belt at Ahmedabad city in India which was severely affected during the Bhuj earthquake,2001.The experimental data gave a single relation for CRR and j*which was then used to assess liquefaction potential for a SPT based case study,where fc varies along the depth.The prediction matched with the field observation.

Uptake of Organic Contaminants by Plants from Oil Sands Fine Tailings

The objectives of this experiment were to evaluate the performance of different plant species growing in different kinds of oil sands fine tailings,and to estimate the uptake of organic contaminants by plants from the oil sands fine tailings.In general,total hydrocarbon in the plant could be ranked(beginning with the highest)as:unweathered plant 4 tailings (UWT),Freeze Thawtailings(FT),weathered plant 4 tailings(WT),and consolidated tailings(CT) for the willow,poplar and cattails.For grass,CT amended with tailings sand and muskeg had the highest hydrocarbon level in the field treatment,however,other three kinds of tailings(FT,WT and UWT) had lower but similar to each other hydrocarbon levels.

Assessment of Bholari River Sand for Its Geotechnical Characterization as Fine Aggregate

Present study is aimed at assessment of Bholari river sand for its geo­technical characterization and suitability as fine aggregate which is being quarried in Jamshoro district,Sindh,Pakistan.For this purpose,sand samples(n=11)were collected from quarry sites and main river channel.Physical properties reveal that Bholari sand is mainly coarse to fine in size(0.2 mm-5 mm).Average values of fineness modulus,specific gravity,bulk density and void content of collected samples are 2.58,2.56,1659.90 kg/m^(3) and 35.12%respectively which varied within the corresponding permissible ranges of ACI(American Concrete Institute).Carbonate content of about 57.59%is also in agreement with corresponding range for fine aggregate.Petrographic examination revealed that Bholari River sand mainly compris­es of quartz and rock fragments with subordinate limestone fragments.As per classification of Dott(1964),Bholari sand is lithic arenite where quartz(50%)occurs as main mineral followed by rock fragments(30%),feldspars(15%)and other opaque minerals(5%).Texturally,the sediments are angu­lar(77%)to sub-round(33%).All these physical properties lie within the range set by National Highway Authority(NHA)and American Standard of Testing Material(ASTM).It is concluded that Bholari River sand is suita­ble for use in concrete mixed with cement and asphalt.

Basic Properties of Concrete Incorporating Recycled Ceramic Aggregate and Ultra-fine Sand

Recycled ceramic mixed sand（RCMS） was obtained by partially replacing ultra-fine sand with recycled ceramic coarse sand（RCCS）. The effects of RCCS replacement rate on the apparent density, workability, compressive strength and splitting tensile strength of recycled ceramic concrete（RCC） were investigated. In addition, the relationship between the water-cement ratio and compressive strength of RCC was also studied. The experimental results indicate that the reusing of recycled ceramic aggregate can improve the cohesiveness and water retentiveness of fresh concrete and benefit the mechanical properties development. When the RCCS replacement rate is not less than 40%, the mechanical properties of RCC are superior to those of the reference concrete. Moreover, when recycled ceramic medium sand was completely used as fine aggregate, the maximum increase in both compressive strength and splitting tensile strength were obtained, comparing with those of reference concrete, the increment ratio was 19.85% and 32.73%, respectively. The microscopic analysis shows that the using of recycled ceramic aggregate can meliorate distinctly the structure of the interfacial transition zone（ITZ） and increase the compaction degree of cement paste. Furthermore, an expression of the compressive strength of RCC and the cement-water ratio is regressed and gains a good linear relativity. It is an effective way to recycle waste ceramic, and the consumption of recycled ceramic aggregate could reach from 26.9% to 47.6% of the total weight of aggregate in producing concrete.

嵌岩低桩承台锁口钢管桩围堰设计与施工技术

昌九高铁扬子洲赣江公铁大桥西支主桥为(48+144+320+144+48)m无砟轨道钢箱桁组合梁斜拉桥。桥塔墩位于通航河道内,桥位处河床覆盖层浅,基岩强度高,基础由大直径钻孔桩和矩形嵌岩低桩承台组成,承台采用锁口钢管桩围堰施工方案。G33号主墩围堰平面设计尺寸54.56 m×28.52 m,锁口钢管桩采用Q345B材质∅1020 mm螺旋钢管,长28 m,钢管桩之间采用C-T形锁扣连接;围堰设置4层内支撑,单层内支撑设3道对撑,内支撑四角设型钢斜撑;基底设置混凝土垫层参与围堰结构受力。围堰采用XR360旋挖钻机在岩层中引孔,孔内换填细砂后插打钢管桩,钢管桩壁内、外两侧换填砂采用高压旋喷注浆加固。围堰设置智能化监测系统,对围堰受力、变形等进行实时动态监控。实践证明,该桥围堰结构安全可靠、止水效果良好、施工快捷高效。

砂-粉混合料颗粒接触状态的临界条件确定

砂-粉混合料被广泛应用于高速铁路路基、人工筑岛及海底沉管隧道垫层等重大工程,如何科学地表征砂-粉混合料复杂的颗粒接触状态及连续演变的力学行为特征,并深入揭示其对混合料静/动力学特性的影响已是一项紧迫的基础性科学研究任务。本文基于代表性砂-粉混合料的基本物理性能指标及力学特性指标试验数据,验证Rahman等提出的区分“细粒填充砂粒”和“砂粒悬浮细粒”的阈值细粒含量FCth半经验公式预测能力。基于理想二元介质材料的理论最小孔隙比计算方法,提出确定中间性态土颗粒接触状态临界条件参量FC_(in-min)和FC_(in-max)的方法,分析各参数对FC_(in-min)和FC_(in-max)的影响规律,建立基于基本物理性能指标的FC_(in-min)和FC_(in-max)预测方法,最终提出全FC范围(FC=0~100%)的砂-粉混合料各颗粒接触状态临界条件参量的有效评价方法。

西山煤田煤层气井水力压裂效果剖析及启示

水力压裂是改善煤层渗透率的常用方法,其改造效果直接影响煤层气井的产能。详细观测了西山煤田屯兰区块5口煤层气井的井下揭露煤层及裂隙展布情况,并联合体视镜、扫描电镜和显微CT等研究煤体结构、微裂隙、石英砂和煤粉的分布特征,结合区域地应力和水力压裂施工参数,剖析煤层气井的压裂效果。研究结果表明:水力压裂产生的宏观裂隙形态复杂多样,包含水平型、垂直型、X型和T型。距离煤层气井筒越近,煤体越破碎,以碎裂煤和碎粒煤为主,水力压裂裂隙发育,远离煤层气井筒的煤体主要是原生结构煤,以原生裂隙为主。石英砂主要铺置在水平裂隙内,仅有少量分布在T型裂隙内。石英砂与煤的裂隙面强烈摩擦、碰撞和嵌入,与压裂液破裂煤体叠加形成大量煤粉,造成煤粉裹挟石英砂堵塞裂隙。宏观裂隙内的煤粉主要受水力压裂时煤体破裂、压裂液冲刷煤体及石英砂与裂隙面摩擦而产生,微观裂隙内的煤粉在煤体破断时产生。煤粉与石英砂混合堆积在裂隙内,对携砂液产生巨大阻力,造成石英砂无法向煤层气井的远端运移。地应力的大小和方向、煤层及顶底板的强度是影响水力压裂裂隙张开与延展方向的重要因素,携砂液黏度低无法悬浮石英砂,容易造成石英砂与煤粉聚集堵塞裂隙。

考虑饱和砂土液化阶段性特征的触变性流体本构模型

合理评价循环荷载作用下饱和砂土的液化过程及性质演变规律,是解决液化砂土大变形问题的关键。通过饱和砂土不排水循环三轴试验,分析了饱和中密南京细砂液化阶段性特征,引入Gompertz函数来描述表观黏度与孔压比之间的关系,提出了一种修正的孔压触变性流体模型,并验证了模型的合理性。①饱和砂土液化过程具有明显的阶段性特征,根据孔压比增长速率可分为固态土体阶段、固液相变阶段、触变性流体阶段和稳定性流体阶段,并基于其他学者的试验结果验证了四阶段特性及阶段划分方法的适用性。②采用Gompertz函数代替线性函数来描述表观黏度与孔压比之间的关系,并利用不同的破坏速率参数c来表征循环荷载作用下饱和砂土的不同阶段,提出了考虑液化阶段性特征的修正孔压触变性流体模型,为解决地震液化问题提供了一种新的统一方法。

粉细砂岩油藏硅烷基改性控砂剂性能评价与应用

微粒运移是影响粉细砂岩油藏开发效果的主要原因,常规抑砂剂难以达到有效控制微粒运移的要求。为进一步提高药剂稳定储层微粒的性能,文章在计算机分子模拟基础上,设计引入硅烷基改性的新型控砂剂,对合成的控砂剂进行了结构表征和性能测试。新型控砂剂体系含有酰胺基、硅烷基等功能性官能团,与储层微粒通过氢键、共价键作用相互吸附,形成稳定均匀的分子膜,原位固定地层粉细砂或黏土颗粒。性能评价表明:新型控砂剂体系耐冲刷排量达8 500 mL/h,远高于常用抑砂剂900 mL/h的指标,对不同粒径微粒均具有良好的控砂作用,储层原位控砂作用显著。截至目前,现场应用60井次,措施有效率95%,延长稳产期1年以上,取得了显著效果。该技术在全国疏松砂岩油藏具有推广示范意义。

滩海吹填粉细砂地基长期承载力特性试验研究

对某运行10年以上出现沉陷冒水现象的滩海人工岛吹填粉细砂地基承载力进行分析,基于螺旋板载荷试验结果,研究了标贯击数、动探击数与地基承载力的关系,拟合了经验公式,在此基础上综合分析粉细砂地基承载力现状,探究地基长期承载力特性。研究结果表明,吹填粉细砂形成的滩海人工岛地基在长期反复的水力作用下可进一步密实,承载力有所提高,有利于场地的运行安全。但鉴于其松散特性,水力作用下易流失,防渗措施不到位可能导致沉陷脱空、冒水等工程问题,影响场地安全运行。因此,针对本工程沉陷冒水现象,需采取原位固化、帷幕封闭等防渗措施进行处理。

穿越层状多孔介质的非达西渗流特征研究

穿越层状多孔介质的非达西渗流多见于人类活动影响下的土壤-地下水系统中。采用自行开发设计的室内渗流实验装置,设置不同粒径的单层多孔介质和相应粒径组合的双层多孔介质,从水力坡降与渗流流速关系、非达西渗流参数和临界雷诺数等方面,探讨了平均粒径、粗细颗粒界面对穿越层状多孔介质非达西渗流特征的影响规律。结果表明:当水流穿越层状多孔介质时,粗细颗粒界面对非达西流渗流特征具有重要影响;作为判断达西流态到非达西流态转换的临界雷诺数,其在单层多孔介质中随多孔介质平均粒径的增大而减小,而当水流穿越层状多孔介质时,临界雷诺数不仅随平均粒径的增大而增大,而且随粗/细颗粒粒径差的减小而增大;受粗细颗粒界面的影响,单层多孔介质中临界雷诺数均低于粗颗粒层与单层多孔介质相同相应粒径组合的双层多孔介质;通过引入非线性分量指数发现,粗细颗粒界面对水流惯性力分量的影响具有一定抑制作用,其抑制性影响程度与粗细颗粒差别大小呈负相关。研究结果对研究穿越层状岩土体的非达西渗流问题具有较好的指导意义。

陕北油田黄土工区道路冲蚀灾害模型试验研究

为探究陕北油田黄土工区道路冲蚀灾害特征及路面细沟冲蚀影响因素,采用物理模型试验,对道路细沟冲蚀发育过程,不同工况下细沟长度、平均宽度以及平均深度的发展特征和产沙速率变化规律进行了研究。结果表明:细沟的发育过程可分为面蚀阶段,跌坑形成阶段,细沟发育阶段,切沟侵蚀、崩塌阶段4个阶段。细沟长度、平均宽度和平均深度与放水时间成幂函数关系,随着放水时间的增大而增大。其中细沟长度随着试验时长的增加先急剧增加,然后逐渐减少;细沟平均宽度随试验时长的增加总体呈现先快速增加后增加速度减慢;平均深度变化速度均在某一时刻达到拐点,流量越大达到拐点的时间越早。冲蚀灾害受流量和坡度影响显著,总体上随着坡度和流量的增大而增大;产沙速率总体上与流量呈线性关系,在其他条件一致的情况下,产沙速率随着流量的增大而增大;与南部的粉黄土和黏黄土相比,陕北油田工区的砂黄土产沙速率明显偏大,冲蚀灾害更为严重。

含泥量对砂类硫酸盐渍土工程特性的影响分析

为了明确含泥量对砂类硫酸盐渍土的盐胀和力学特性的影响,人工配制了不同细粒土含量的砂类硫酸盐渍土,在1%和3%(质量分数,下同)含盐量单向冻结盐胀试验的基础上,选取了细粒土含量为5%、15%、30%和40%的砂样进行常温、低温三轴剪切试验。研究结果表明:此试验条件下,不同级配砂类硫酸盐渍土的冻结温度为-0.7~-0.1℃,当砂样孔隙溶液浓度在冻结温度之上达到饱和时,降温过程中会首先生成盐结晶;1%含盐量条件下,高细粒土含量(≥30%)砂样的起胀温度在4~9℃之内,而低细粒土含量砂样的起胀温度在0℃附近,3%含盐量砂样的起胀温度为20~23℃;试验含水率和细粒土含量通过影响土体中自由水的含量对盐冻胀产生显著影响。在力学特性方面,随着细粒土掺量的增加,砂类硫酸盐渍土的抗剪强度表现出先增大后减小的趋势,细粒土由增强摩擦转变为颗粒间的“润滑”作用;此外,冻结后砂土转变为承载能力更强的“土-盐-冰骨架结构”,抗剪强度大幅提高,并呈现出明显的脆性破坏特征,由于冻结砂土受相对温度的影响,随着含盐量的增加,破坏应力呈先减小后增大的趋势。