Performance of composite foundations with different load transfer platforms and substratum stiffness over silty clay

The semi-rigid pile-supported composite foundation is widely used in highway projects due to its effectiveness in increasing the bearing capacity and stability of foundations.It is crucial to understand the stress distribution across the embankment width and the behaviour of unreinforced foundations.Thus,five centrifuge tests were conducted to examine the bearing and deformation behaviours of NPRS(Non-Connected Piled Raft Systems)and GRPS(GeosyntheticReinforced Pile-Supported systems)with varying substratum stiffness,then a comparative analysis was conducted on embankment settlement,pressures underneath the embankments,and axial forces along the piles.The results indicated that greater substratum stiffness correlates with reduced settlement and deformation at various depths.Deformation occurring 5 meters from the embankment toe includes settlement in NPRS and upward movement in GRPS.The potential sliding surface is primarily located within the embankment in NPRS,whereas it may extend through both the embankment and foundation in GRPS.The pile-soil stress ratio and efficiency in NPRS are higher than in GRPS across the embankment.The axial force borne by end-bearing piles is significantly greater than that by floating piles.As the buried depth increases,the axial force in GRPS initially rises then declines,whereas in NPRS,it remains relatively constant within a certain range before decreasing.This study aids in assessing the applicability of composite foundations in complex railway environments and provides a reference for procedural measures under similar conditions.

Experimental study on seismic reinforcement of bridge foundation on silty clay landslide with inclined interlayer

A shaking table test for a bridge foundation reinforced by anti-slide piles on a silty clay landslide model with an inclined interlayer was performed.The deformation characteristics of the bridge foundation piles and anti-slide piles were analyzed in different loading conditions.The dynamic response law of a silty clay landslide with an inclined interlayer was summarized.The spacing between the rear anti-slide piles and bridge foundation should be reasonably controlled according to the seismic fortification requirements,to avoid the two peaks in the forced deformation of the bridge foundation piles.The“blocking effect”of the bridge foundation piles reduced the deformation of the forward anti-slide piles.The stress-strain response of silty clay was intensified as the vibration wave field appeared on the slope.Since the vibration intensified,the thrust distribution of the landslide underwent a process of shifting from triangle to inverted trapezoid,the difference in the acceleration response between the bearing platform and silty clay landslide tended to decrease,and the spectrum amplitude near the natural vibration frequency increased.The rear anti-slide piles were able to slow down the shear deformation of the soil in front of the piles and avoid excessive acceleration response of the bridge foundation piles.

Vertical Diffusion Coefficient with Stratification Effect for Silty Sediment Suspension Under Waves

To analyze previous experimental data of suspended sediment concentration for silty sediment with different sediment particle sizes due to waves, a new stratification correction coefficient is presented. The suspended sediment concentration gradient and sediment particle diameter are selected as parameters. Furthermore, a diffusion coefficient model with a stratification effect over the whole water depth for silty sediment suspension under waves is developed. The comparison between the suspended sediment concentration calculated by the presented model and several groups of experimental data shows that the model can reasonably reflect the vertical distribution of silty sediment suspension.The stratification effect calculated by the present model decreases with an increase in the sediment particle diameter,which indicates that the model can be extended to describe the suspended sediment concentration of fine to medium sand when the near-bottom sediment concentration is not very high. Although the original model needs to be iteratively solved, the approximate method without iteration is recommended for applications when the near bottom sediment concentration is between 10 and 20 kg/m~3 due to the small difference between the non-iterative and iterative solution for near bed layer suspended sediment concentration, which plays a major role in sediment transport.

Effects of transient temperature gradient on frost heave of saturated silty clay in an open system

Frost heave in seasonally frozen regions is a one-dimensional process that could severely damage infrastructure subgrades.Stress state,temperature and water migration are important factors for frost heave.This work investigated the effects of soil temperature and volumetric water content on the transient frost heave ratio during the freezing of saturated silty clay in an open system and analyzed the relationships between the transient frost heave ratio and freezing rate and between temperature gradient and frost heave rate.The results show that the frost heave ratio,frost heave rate,and freezing rate are positively correlated with the temperature gradient since the temperature gradient drives the water migration during freezing,indicating the transient temperature gradient could be used to evaluate the frost heave of saturated silty clay.The transient freezing rate and transient frost heave ratio are logarithmically related to the transient frost heave ratio and transient temperature gradient,respectively.The effects of transient temperature gradient on frost heave are the principal mechanism responsible for different frost heave characteristics and uneven frost heave along a subgrade of the same soil type.

黄河三角洲潮滩表层沉积物在潮波和波浪作用下的孔压响应及固结机理现场试验研究

为了研究黄河三角洲潮滩表层粉质土在波浪和潮波作用下的孔压响应及对沉积物固结状态影响,在黄河三角洲潮滩上现场取土在试坑内配置了模拟黄河口快速沉积形成的流体状堆积物,利用静力触探试验、孔隙水压力测试手段,对比研究了试坑扰动恢复土体和潮滩原状土体在波浪和潮波作用下的孔压响应,并探讨了黄河三角洲潮滩表层粉质土非均匀固结机理。研究发现:在潮波作用下,潮滩和试坑土体产生了不同的孔压响应,在涨退潮过程中试坑土体超静孔隙水压力经历了负-零-正的变化,相当于经历了一次加卸荷过程,而潮滩土体仅产生了负值超静孔隙水压力。在潮波与波浪相互耦合作用下,潮滩浅部土体产生了明显的孔隙水压力积累,在深度0.3 m附近出现超静孔隙水压力最大值。潮滩表层土体强度沿深度从上到下呈现高-低-次高不均匀固结现象,产生这一现象的机理与潮滩土体在潮波、波浪和潮波相互耦合作用下孔压响应特点有关,特别是潮滩0.3 m深度附近土体受波浪影响显著,长期作用下形成硬壳层。

Thermomechanical and Hydrous Effect of Heavy Fuel Oil in a Building Material Based on Silty Clayey Soil

This study focuses on the use of heavy fuel oil in construction in Burkina Faso.Mixed with silty and/or clay soil,it is used as a coating to reinforce the walls of raw soil constructions which are very sensitive to water.The interest of this paper is to shed light on the thermomechanical and above all water effects of heavy fuel oil on a sample of silty clayey soil.To achieve this,we used heavy fuel oil added in different proportions to silty clayey soil,to make sample of bricks on which tests were carried out.At the end of the experimental tests carried out on materials made(bricks)with our soil sample,it appears that heavy fuel oil moderately reduces the mechanical resistance of bricks and slightly increases thermal diffusion through them.On the contrary,we note a very good water resistance of the bricks thanks to the heavy fuel oil,in particular their water absorption by capillarity.This confirms that the mixture of heavy fuel oil and a silty-clayey soil used as a coating makes it possible to prevent the infiltration of water into the walls of raw soil constructions.However,its use as a construction material does not guarantee very good mechanical resistance,and slightly increases thermal diffusion.

由石蜡基相变材料和煤渣改良的粉砂土的冻融性能

【目的】针对季节冻土区渠道的衬砌结构因冻融作用而受损的现状,同时为了解决石蜡基相变材料(phase change materials, PCM)的泄露,并满足煤渣的再利用需求,研究石蜡基PCM和煤渣对土体冻融性能的影响。【方法】以粉砂土为研究对象,选取石蜡基PCM和煤渣作为改良剂,制备石蜡基PCM改良土和石蜡基PCM-煤渣改良土;通过冻融循环作用下的体积变化率试验、无侧限抗压强度试验,微观结构及潜热研究土体的体积和无侧限抗压强度的变化。【结果】石蜡基PCM能抑制粉砂土的冻融变形,质量分数为8%时2种改良土的体积变化率最小;随着冻融循环次数的增加,土体的无侧限抗压强度均降低,降低速率先大后小;循环次数相同时,加入煤渣的改良土的无侧限抗压强度均大于未加入煤渣的;冻融循环9次后,石蜡基PCM质量分数为10%的石蜡基PCM改良土的无侧限抗压强度最低;2种改良土的结构比粉砂土更密实、稳定;潜热的吸收或释放延缓了土体冻融过程。【结论】将石蜡基PCM和煤渣共同掺入粉砂土,能控制土体的冻融变形且效果稳定,改善土体的冻融性能,减轻季节冻土区输水渠道衬砌结构的冻害,提高煤渣的利用率。

干湿循环下木质素改良粉土抗剪强度特性

为克服粉土强度低、干湿循环性能差的特点,同时解决固体废弃物木质素的资源化利用的问题,将木质素掺入粉土中,形成木质素改良粉土。为研究木质素改良粉土抗剪强度特性及干湿循环性能,对经历干湿循环后的木质素改良粉土进行直接剪切试验和X射线衍射试验,研究了木质素掺量0、2%、5%、8%、12%、15%和干湿循环0、1、2、3、4次对改良土抗剪强度特性的影响。结果表明:掺入木质素可以显著提升粉土的抗剪强度及干湿循环性能;一定干湿循环次数下,改良土的抗剪强度及黏聚力随着木质素掺量的增加先增大后降低,内摩擦角随着木质素掺量的增加先快速增大后基本不变;当木质素掺量为8%时,改良土的抗剪强度、黏聚力和内摩擦角最高。改良土的抗剪强度、黏聚力和内摩擦角均随着干湿循环次数的增加而降低,当木质素掺量为0时,纯土试样经过1次干湿循环即崩解,强度损失率为100%;当木质素掺量为8%时,改良土的强度损失率最低,黏聚力及内摩擦角的降低幅度均较低。X射线衍射试验表明,木质素掺量为8%时,石英、方解石、白云石含量最高,黏土矿物含量较高,钠长石含量最低,这是造成改良土强度及干湿循环性能提升的直接原因。结果显示,当木质素掺量为8%时,改良土在提高抗剪强度及干湿循环性能方面具有明显优势。研究结果可为实际施工设计提供理论支撑。

冻融对水泥改良路基土力学特性影响研究

以季冻区路基粉砂土为研究对象进行室内三轴压缩试验,分析水泥改良粉砂土与素土的应力-应变关系,探究冻融循环、水泥掺量和围压对土体抗剪强度的影响。试验结果表明:改良土的破坏以脆性破坏为主;围压增长不改变试样的破坏形式,土抗剪峰值随围压的增加而增大;冻融循环作用会削弱土体的黏聚力,内摩擦角的变化规律表现为在一定范围内波动;在水泥掺量相同的条件下,随冻融次数的增多,土的抗剪强度因水泥持续水化反应故呈先减小后增大再减小;当水泥掺量为2%时,土体已获得较大抗剪强度。

井研地区筇竹寺组页岩储层水平井钻井井壁稳定性评价

川西南井研区块筇竹寺组页岩气具有良好的开发前景,但由于对其岩矿组分、力学性质、水化特征等认识程度较低,钻井井壁稳定性认识不清,导致出现掉块、阻卡等复杂情况。为了充分认识该区寒武系筇竹寺组地层的井壁稳定性,明确粉砂质页岩水化作用及其对坍塌压力的影响规律,在岩石力学实验的基础上,研究不同钻井液浸泡时间对岩石力学参数的影响规律,建立考虑水化作用的井壁稳定性预测模型,分析不同井斜方位下坍塌压力的差异。研究结果表明:筇竹寺组伊蒙混层含量高,具有一定膨胀性,水化作用后导致岩石强度降低,对井壁稳定性有较大影响;不同钻井液体系对井壁稳定性的影响有差异,总体上油基钻井液更有利于井壁稳定;考虑了水化作用的井壁稳定预测模型更加符合实际,以此为依据,通过优化钻井液设计可以有效降低井壁失稳风险,提高钻井时效。

利用原位测试试验确定黄河口潮滩粉质土固结状态研究

黄河口泥沙快速堆积在河口一带,在波浪和潮波作用下,表层沉积物处于超固结状态,但试验中发现采用Casagrande作图法求取的黄河口粉质土先期固结压力往往偏大。为了了解黄河口粉质土固结状态、合理估算先期固结压力,在黄河口刁口流路三角洲叶瓣潮坪上,现场取土在试坑内配置了模拟黄河口快速沉积形成的流体状堆积物,利用原位测试手段(静力触探、十字板剪切试验和孔隙水压力测试),并在长期观测基础上,对比研究了1.0 m深度范围内试坑和潮滩原状土体固结过程及固结状态。研究发现:黄河口快速沉积粉质土在自重作用下固结很快,固结完成后,土体强度随时间发展呈现不均匀增长,沿深度方向从上到下出现高-低-次高不均匀固结特征;历经16个月后,试坑和潮滩原状土体先期固结压力进一步提高,固结不均匀性和结构性不断增强。从试坑土体自重固结完成后的实际固结状态及原状土体物理性质指标来看,Casagrande作图法结果偏大,采用静力触探比贯入阻力法、十字板剪切试验不排水抗剪强度经验公式法估算的试坑和潮滩原状土体先期固结压力数值更为可靠;同时该方法为土体固结状态研究提供了新途径。

粉砂质泥岩蠕变特性与非线性黏弹塑性本构模型

针对边坡、隧道工程中粉砂质泥岩长期稳定性问题,采用DZSZ-150型多场耦合岩石三轴压力试验机,利用粉砂质泥岩相似材料试样,开展不同围压下粉砂质泥岩蠕变试验。根据蠕变试验结果建立可描述粉砂质泥岩蠕变全过程的非线性黏弹塑性本构模型,以7 MPa围压下蠕变试验结果为例,对所提出的蠕变本构模型进行合理性和可行性验证。结果表明:试样的稳态蠕变速率与偏应力大小有关,随着偏应力的增加,试样稳态蠕变速率与偏应力之间符合指数函数关系;在加速蠕变阶段的蠕变速率相对低偏应力水平下减速蠕变和稳态蠕变速率明显要增大很多,且非常短时间内就发生了蠕变破坏;提出一种描述粉砂质泥岩全蠕变过程的非线性黏弹塑性本构模型,该模型能描述粉砂质泥岩蠕变-破坏全过程的蠕变变形规律,反映了粉砂质泥岩在不同偏应力水平作用下的蠕变特性,克服了经典模型中无法准确描述加速蠕变过程的难题。

碱激发地聚物固化海相淤泥质软土抗压强度及固化机制研究

海相淤泥质软土富含各类可溶性盐和有机质,使用水泥进行固化时易出现劣化效应。为了解决传统硅酸盐水泥固化常出现加固失效的工程问题,以NaOH为碱激发剂,使用地聚物-水泥联合固化海相淤泥质软土,研究水泥与地聚物掺量比、碱激发剂含量和养护龄期对固化土无侧限抗压强度的影响,并采用扫描电镜和X射线衍射试验分析碱激发地聚物固化海相淤泥质软土的微观形态和水化产物,进一步揭示其固化机制。研究结果表明:单独加入NaOH对水泥土强度的提升并不明显,水泥土强度并不会随着碱激发剂含量的增加而显著提高。同时不同钙系地聚物的固化效果差异显著:矿渣地聚物固化土早期强度增长迅速,后期强度增长缓慢,其无侧限抗压强度随NaOH含量的增加先增加后降低,随矿渣的掺量增加而提高。粉煤灰地聚物固化土强度增长较慢,NaOH含量越高其无侧限抗压强度越大,且强度随粉煤灰的掺量增加而降低。将1/2的水泥以矿渣替代,固化土28 d强度最高可达6.19 MPa,相较于水泥土提升了107.5%。而使用同样比例的粉煤灰替代,其28 d强度最高仅为3.012 MPa。微观试验分析表明碱激发剂和地聚物的加入,可促进可溶盐水化结晶和有机质碱性环境降解,有效增加有效水化产物,增强颗粒间的凝聚力使得土体结构更加紧密,宏观上表现为固化土强度的提升。研究结果可为解决水泥加固海相淤泥软土所引起的实际技术问题提供理论指导。

近海黏性土中桩土界面剪切特性与含水率效应

为了研究粉质黏土-混凝土截面的抗剪强度在不同物理条件下的影响特征,本文针对粗糙度、含水率对其的影响展开研究,根据前人经验总结自制出了型直剪试验仪,研究了混凝土在不同粗糙度以及不同含水率的条件下,粉质黏土-混凝土界面的剪切特性受到的物理影响。结果表明:混凝土表明粗糙度对试验结果产生直接影响,并且随着粗糙度的增大,界面抗剪强度、界面摩擦角、剪切刚度系数以及界面残余剪切应力均相应增大,最大增幅可达37.0;而界面附着力随粗糙度的增大呈先减小后增大的趋势,增大幅度达23.7%。随着黏性土含水率的增大,界面抗剪强度、剪切刚度系数和界面残余剪切应力均呈先增大后减小趋势,含水率在20%~28%之间存在一个最优含水率使得界面抗剪特性最优;粉质黏土-混凝土界面的抗剪强度受粗糙度和含水率的影响较大。试验结果可为静压桩的工程实践提供借鉴与参考。

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。

盾构废弃粉质黏土泥浆底部真空脱水试验研究

盾构施工中产生的废弃粉质黏土泥浆具有含水率高、强度低、颗粒粒径较小及难以快速固结的特点,脱水处置难度大;如何对其进行有效的脱水固化处理,是减少其在运输或存储过程中污染环境的关键。基于自行研制的废弃泥浆真空脱水装置,开展自重、自重+真空、自重稳定+真空、自重稳定+分级真空等4种不同形式下的室内脱水模型试验研究,分析废弃粉质黏土泥浆在不同的加载方式下泥—水界面沉降、孔隙水压力、脱水量以及脱水后残余泥浆含水率的变化规律,并对不同加载方式下对废弃粉质黏土泥浆的脱水效果进行对比。试验结果表明:4种脱水方式中,先对废弃粉质黏土泥浆进行自重脱水处理、再对其底部分级施加真空作用的加载方式对废弃粉质黏土泥浆的脱水效果最好,可有效降低废弃粉质黏土泥浆的含水率;采用该方法对初始含水率为97.50%的废弃粉质黏土泥浆进行脱水处理后,其含水率分布范围介于28.21%~34.25%,其内部孔隙水压力最小可达-72.92 kPa。同时,基于分段线性化的思想建立了废弃泥浆一维脱水理论分析模型,对废弃粉质黏土泥浆在脱水过程中的泥—水界面沉降进行数值模拟,并将数值模拟与试验结果进行了对比分析;随后通过对不同加载方式下脱水效率进行模拟分析探究了最佳加载方式。该研究可为盾构废弃粉质黏土泥浆快速脱水处理提供理论基础和实践依据。

冻融循环对季冻土区粉质黏土-混凝土界面剪切性能的影响

为探究季冻土区粉质黏土-混凝土界面剪切性能,进行了不同冻融循环次数、土体含水率和法向应力的粉质黏土-混凝土二元体冻融循环试验和直剪试验,探讨了界面抗剪强度、抗剪强度参数和抗剪强度损伤度的变化规律。结果表明:直剪试验得到的应力-应变曲线均发生应变硬化现象,可分为弹性变形阶段(剪切位移为0~3 mm)和弹塑性变形阶段(剪切位移为4~15 mm);冻融循环对界面抗剪强度有劣化作用,即通过对土体造成损伤,导致界面内摩擦角和黏聚力下降,从而降低界面抗剪强度;随着冻融循环次数增加,界面抗剪强度损伤度增加,当冻融循环进行0、4次,抗剪强度损伤迅速,冻融循环进行12~20次,抗剪强度损伤较缓,最大界面抗剪强度损伤度为25%;土体含水率的增加对抗剪强度有削弱作用,随着土体含水率的增加,界面内摩擦角降低,但黏聚力先增加后减小,当土体含水率为20.7%时,黏聚力达到最大值;法向应力的增加对抗剪强度有增强作用。

雄安新区粉质黏土的动力特性试验研究

为分析雄安新区粉质黏土的动力特性,采用GDS多功能动态循环三轴试验系统,对雄安新区粉质黏土进行14种工况(不同围压、固结比和加载频率)下的动三轴试验,深入分析了动骨干曲线、动弹性模量、动剪切模量、阻尼比等在不同工况下的变化规律及相关参数。试验结果表明:随动应力幅值的增大,动应变呈非线性增长,且存在某一临界动应力值;随动应变的增大,动模量逐渐减小,且减小速率先快后慢、最后趋于稳定;随动剪应变增长,阻尼比呈非线性增长,且阻尼比最大值不超过0.12;随围压与固结比增大,动强度和动模量均明显增大;围压与固结比对阻尼比影响较小,具有归一性特征;随频率增大,动应力幅值、动弹性模量增大,但增长幅度并不明显。综合分析三变量对雄安新区粉质黏土动力特性的影响程度得出:围压影响最大、固结比次之、加载频率最小。

非饱和粉质黏土中毛细作用对X70钢电化学腐蚀行为的影响

【目的】探究毛细作用对X70钢在土壤中腐蚀的规律。【方法】基于非饱和土与电化学阻抗谱理论,结合极化曲线技术(PC)、电化学阻抗技术(EIS)、扫描电子显微镜(SEM)等测试方法,在室内条件下通过一维土柱仪的模拟试验研究了粉质黏土中毛细作用对X70钢的电化学腐蚀行为。【结果】试验结果表明,毛细作用对X70钢的电化学腐蚀行为具有一定的影响,即在土柱12 cm、27 cm高度位置处的钢片腐蚀速率随埋置时间先增大后减小,在土柱42 cm、57 cm高度位置处的钢片腐蚀速率随埋置时间先减小后增大;毛细水在粉质黏土中的上升速率满足先快后慢的规律。【结论】X70钢片表面的腐蚀类型以点蚀为主,且腐蚀产物表面伴有较深较宽的裂缝,随着土样高度的增加,腐蚀程度降低,整体上腐蚀坑的大小和数目均呈现出减小趋势;毛细水上升过程中,当X70钢片处于固-液-气三相界面(TPB)时,其腐蚀速率最快,腐蚀程度最严重。

掺加粉质黏土的泡沫轻质土材料性能试验研究

为解决工程弃土再利用难题,在泡沫轻质土中掺加了粉质黏土,通过试验分析粉质黏土掺量、湿密度、养护龄期、泡沫掺量对泡沫轻质土材料流值、抗压强度的影响。采用扫描电镜试验观察泡沫轻质土微观结构,分析宏观性能与微观结构的关系。结果表明:水固比相同时,泡沫轻质土的流值随粉质黏土掺量的增加而减小;相同土掺量下,流值随初始湿密度的增大而增大;合理控制粉质黏土掺量和初始湿密度以便施工具有良好的流动性和稳定性;水固比为0.75时,粉质黏土掺量增大,泡沫轻质土的流值减小,在50%掺量下湿密度为700 kg/m^(3)和800 kg/m^(3)轻质土仍表现出较好的流动性;抗压强度随粉质黏土掺量增加而逐渐降低,随初始湿密度增大而增大;当粉质黏土掺量为33%、水固比为0.75时,湿密度为700 kg/m^(3)和800 kg/m^(3)轻质土7 d抗压强度均超过1.0 MPa,满足路基路床部位施工要求;泡沫掺量的增加引起泡沫轻质土湿密度、流值及抗压强度的降低,增加了内部孔隙率;观察微观结构发现,气孔完整独立、分布均匀、孔壁厚度大、形状为球形等良好气孔结构会增强泡沫轻质土结构密实度;粉质黏土掺量和泡沫掺量增加影响气孔完整性,导致泡沫轻质土宏观性能变差。