Experimental study on deformation characteristics of collapsible loess under isotropic stress

Abstract

In order to study the deformation characteristics of collapsible loess and to construct different vertical stresses, different isotropic stresses are used to represent the vertical stresses, and their corresponding stresses are set to be the hidden stresses in different positions of collapsible loess foundation are analyzed by using 0, 50, 100, 220, 330 and 400?kPa. In this study, various kinds of deformation analysis of collapsible loess are needed. Ground settlement tester, GJY K₀ tester produced by Suzhou Longstan Automation Factory and ADINA software are used. Experiments show that different isotropic stresses are applied to the foundation, and the hidden stresses of the loess are gradually decreasing under the action of different filling heights. At the same time, the higher the height of the fill, the faster the decline of the underlying stress of the foundation loess. With the increase of time at the bottom of the cushion, the corresponding underlying stress at the midline decreases, while with the increase of the height of the fill, the corresponding underlying stress at the midline rises. Study of the underlying stress at the bottom of the pile is set up differently. Under certain time and position of the pile bottom, increasing the isotropic stress can increase the hidden stress at the corresponding pile bottom, and change the filling height at the pile bottom has little effect on the hidden stress at the pile bottom. Under the combined action of water content and isotropic stress, they are positively correlated with the change of lateral stress. With the increase of isotropic stress, the K₀ coefficient rises as a whole. This study can provide a strong reference for the deformation characteristics of collapsible loess itself, and help construction workers to improve the safety of the project in the field of engineering construction.     

消除负摩阻力扩体挤密桩的研制及力学特性分析

针对湿陷性黄土地区等不良土质地区的桩基负摩阻力问题,提出一种既可以消除负摩阻力,又能提高承载力的扩体挤密新型桩,对其结构构成和工作机理进行详细阐述。根据扩体挤密桩的结构特性,建立扩体装置扩张的菱形孔扩张模型,采用复变函数方法,求解了扩体装置扩张的弹塑性解及极限扩张角,推导出该新型桩的单桩极限承载力计算公式。结合算例采用有限元法和提出的理论计算方法对该新型桩的挤土效应和承载特性进行了分析。结果表明:扩体装置对土体应力的影响范围约为扩体装置扩开宽度的2倍~3倍;扩体装置扩张形成的塑性区范围大小在竖直方向上和水平方向上基本相等;各扩体装置承力时具有一定的顺序效应和时间效应,扩体装置的承力能力与周围土体的性质有关;数值模拟结果与理论计算结果基本一致,验证了理论分析方法的正确性;与普通直桩及套管桩相比,扩体挤密桩消除负摩阻力效果良好,提高承载力作用显著,受力机制科学合理,为湿陷性黄土地区桩基工程的设计提供一定的理论依据和参考。     

Study on the height effect of highway embankments in permafrost regions

Thaw settlement is the main embankments distresses of highway in permafrost regions, according to survey data of the Qinghai–Tibet Highway (QTH). It can be effectively mitigated or even controlled by raising the embankment height. In view of this engineering problem, this study proposes the concept of the Height Effect of Embankment in Permafrost (HEEP). The concept represents the deformation and failure rules of embankment resulting from height variations. A thermal-elastic-plastic thaw settlement computational model is used to simulate the settlement processes of embankment, considering scenarios of different mean annual ground temperatures (MAGTs) and different heights. The model is validated by the field monitored data from a specific embankment section along the QTH. It is found that the total deformation of embankment is of considerable value and comes primarily from the thaw settlement of permafrost. Some special structures are recommended to supplement the built embankment, to ensure the stability of embankment in warm permafrost regions. The research results could provide essential theoretical and technological support for the transversal section design of highway embankments in permafrost regions.     

Estimating the stability of unprotected embankment in warm and ice-rich permafrost region

The purpose of this study is to estimate the stability of unprotected embankment in the warm and ice-rich permafrost (WIRP) region along the Qinghai-Tibet Railway. The estimation was based on field observations of the varying thermal regime under the unprotected embankments and of the ongoing settlement of this type of embankment. The observed data shows that on the one hand, the unprotected embankment could mitigate the frost heave of the subgrade by homogenizing the geothermal gradient and by decreasing the temperature fluctuation of the WIRP layers. On the other hand, leaving the embankment unprotected has led to the warming of the underlying WIRP layer. This layer releases a considerable amount of unfrozen water. However, the released water undergoes untimely drainage, leading to the ongoing settlement of the embankment. To stabilize the settlement, two recommendations are provided. For those existing unprotected embankments, using shading boards to cool down the underlying permafrost is recommended. For the future embankments constructed in WIRP regions, the inset trenches which are drilled perpendicularly to traffic are recommended to aid the drainage of the released water.     

基于延缓固结法的地基工后沉降控制的数值分析

延缓固结法(地基延缓固结处理法)是一种通过限制工程运营期间地基的孔隙压力消散来消除后期的地基固结沉降,从而达到控制地基工后沉降目的的新型地基处理方法。通过设置深层水平防渗层完善了延缓固结法,并以此为基础完成该文的研究工作。有限元模拟计算表明,延缓固结法处理的地基工后沉降随帷幕的深度增加而减小,但是当帷幕达到一定深度以后,再增加帷幕的深度,工后沉降不再减小,而竖向防渗帷幕的深度非常小时,延缓固结法反而会使地基工后沉降加大。采用有限元方法比较了分别采用延缓固结法和刚性桩处理地基的超孔压分布、竖向位移场的分布、沉降时程曲线和控制地基工后沉降的效果,且粗略地比较了二者的经济性能。在竖向防渗帷幕的深度适宜的情况下,延缓固结法控制地基工后沉降的作用确实存在。当处理深度在一定范围之内时,延缓固结法和刚性桩相比具有经济和技术双重优势。     

DX桩桩周土应力场分布的模型试验研究

通过室内小比尺的模型试验,可以进一步为确定DX桩沉降计算公式提供必要的依据。在小型模型试验箱中,通过采用杠杆加砝码的装置对22 mm桩径的DX桩在砂土中进行研究,测定单桩的桩顶荷载-桩顶位移曲线,确定承载力,并与相同情况下的直孔桩进行对比;同时,利用微型土压力盒测定土中应力变化,研究荷载在土中的传递规律。试验结果表明,DX桩的承载力及沉降特性明显优于直孔桩;承力盘在上部和下部时,DX桩尽管承载力相差不大,但是盘在下部时会增大桩端附近土体的应力;两个承力盘的DX桩,两盘受力比较一致,且盘受力的影响范围,在竖     

DX桩单桩沉降的理论计算研究

DX桩由于承力盘的存在,改变了桩的荷载传递机理,在提高承载力的同时,沉降变形也显著减小。在桩的现场试验基础上,分析了DX桩与等截面桩的桩顶荷载的分配特性,进而依据Mindlin解和Geddes解求解桩端土的附加应力,编制了相应的程序,计算结果表明在同样的荷载条件下,DX桩的沉降是等截面桩的0.55~0.80倍,并对承力盘的变化引起的DX桩的沉降变化进行了分析。     

Characteristics and mechanisms of embankment deformation along the Qinghai-Tibet Railway in permafrost regions

Based on field monitoring datasets, characteristics of embankment deformation were summarized along the Qinghai-Tibet Railway in four permafrost regions with different mean annual ground temperatures (MAGTs). Then, further analyses were carried out at some typical monitoring profiles to discuss mechanisms of these embankment deformations with consideration of detailed information of thermal and subsurface conditions. The results indicated that in regions with MAGT <− 1.5 °C, embankments only experienced seasonal frost heaves, and of which the magnitudes were not significant. So, the embankments in the regions performed satisfactorily. Whereas in regions with MAGT ≥− 1.5 °C, both traditional embankment and crushed rock embankment experienced settlements, but characteristics and mechanisms of the settlements were different for the two kinds of embankment. For crushed rock embankment, the magnitudes of settlement and differential settlement between right and left embankment shoulders were not significant and increased slowly. In respect that upwards movements of permafrost tables and better thermal stability of permafrost beneath embankment, mechanism of settlements on the embankment was inferred as creep of warm and ice-rich layer often present near permafrost table. While for traditional embankment, particularly in warm and ice-rich permafrost regions, the magnitudes of settlement and differential settlement between right and left embankment shoulders were significant and still increased quickly. Considering underneath permafrost table movements and permafrost temperature rises, mechanisms of settlements on the embankment included not only creep but also thawing consolidation of underlying permafrost. Therefore, some strengthened measures were needed to ensure long-term stability of these traditional embankments, and special attention should be paid on temperature, ice content and applied load within the layer immediately beneath permafrost table since warming and thawing of the layer could give rise to considerable settlement.     

DX旋挖挤扩灌注桩群桩数值分析研究

通过数值计算软件FLAC3D对DX旋挖挤扩灌注桩（简称DX桩）群桩的承载性状和桩周土体的沉降规律进行了研究。分析了桩间距、承力盘布置对群桩极限承载力和沉降控制能力的影响,并比较了6根基桩的DX群桩和8根基桩的直桩群桩的极限承载力。研究结果表明：DX群桩的基桩具有DX单桩的受力性状。承力盘下方土体位移大于承力盘上方土体位移,盘腔上部出现临空面。3D(D为盘径)桩间距的承载力和沉降控制能力最佳。同一地层内,承力盘布置方式（平行、错开布置）对DX群桩的承载力影响甚微。6根基桩的DX群桩的极限承载力是8根基桩的直     

管桩承载性状的数学描述

基于5根桩现场试验资料的分析,对表征管桩承载性状的桩顶荷载-桩顶沉降曲线、桩身压缩-桩顶荷载曲线进行了数学描述。结果表明:1)用Boltzmann数学模型拟合5根桩的桩顶荷载-桩顶沉降曲线,效果很好,相关系数都达到了0.996以上。2)从综合系数-桩顶荷载曲线图上可发现5根桩的曲线形状有3种类型。3根短桩的综合系数大,2根超长桩的综合系数小。这些反映出综合系数是一个变量而非常数,要由它来确定桩身压缩难度较大。3)用Boltzmann数学模型对桩身压缩-桩顶荷载关系曲线进行回归拟合,效果也很好,各单桩的相关系数都达到了0.996以上,5根桩的综合相关系数为0.9871。这样就可根据已有试桩资料的回归拟合曲线及其方程式来预测和计算该场地及其类似场地中钢管桩的桩身压缩。4)Boltzmann数学模型是适合描述钢管桩桩顶荷载-桩顶沉降、桩身压缩-桩顶荷载这两类曲线的,它为桩身压缩及其极限值的预测、为桩顶极限承载力的判定提供了新方法。5)极限荷载与长径比成大致的负线性关系。     

双向增强体复合地基桩土应力比计算

针对竖向增强体-水平增强体-桩间土的整体相互作用特点,将水平向加筋垫层视为具有一定刚度的板,竖向桩体及桩间土简化为刚度不同的弹簧系列。以单个竖向桩体影响范围内的复合地基作为典型单元体进行分析,基于Winkler弹性地基圆板理论推导出水平加筋体的挠曲函数表达式,导出双向增强体复合地基桩土应力比计算式。最后对某工程算例进行计算分析,对路堤荷载大小、工后沉降值、面积置换率、桩土刚度比进行了参数研究,并探讨了桩土应力比随其主要影响因素的变化规律,计算与实测结果吻合较好。     

湿陷性黄土中基桩浸水前后刚度变化的试验研究

建立桩土体系的力学模型,并求得静刚度的表达式,通过在湿陷性黄土地区的试桩,得到了浸水前后的桩体导纳曲线,在导纳曲线上找到刚度计算的相关参数,算出浸水前后刚度值,并作出比较,得到刚度变化的幅度。     

减沉桩基础作用特性的有限元分析

合理地控制沉降是减沉桩基础设计的指导思想和目标,由于这种基础形式形成和发展较晚,实际工程中大都以以往的经验为依据进行设计。采用弹塑性有限元法模拟减沉桩基础,分析桩土共同作用的机理,结合典型工程,重点分析影响沉降量的各个要素,并充分发挥有限元分析方法的特长比较了沉降量对各个影响因素的敏感程度,为减沉桩的设计提供理论依据和有效途径。有限元分析结果与工程实测结果吻合较好,表明减沉桩基础中桩和桩间土协同工作,共同承担上部荷载,能很好地控制建筑物的沉降量;调整桩长是控制沉降量的最有效手段。     

Experimental study for the compressible behavior of warm and ice-rich frozen soil under the embankment of Qinghai–Tibet Railroad

The permafrost with ice-rich content under the embankment of Qinghai–Tibet Railroad (QTR) is interestingly and increasingly warming, thus potentially posing an adverse effect on the operation of the railroad. This study first shows in-situ data to illustrate the essence of the warming permafrost. Experimental study for the compressible behavior of frozen soil remoulded in the laboratory from the permafrost sample in situ was constructed to characterize the permafrost's engineering properties. The results show that the settlement incurred by the additional load from the built embankment will be of considerable magnitude and that, when suffering from an incremental rise in temperature, the settlement will be greatly magnified. This experiment shows that this warm and ice-rich frozen soil is essentially sensitive to both load and temperature warming.     

循环温度作用下饱和黏土中摩擦型桩变形特性研究

能量桩兼具支承上部荷载与能量交换的双重功能,循环温度作用下端承型桩的承载性能、摩擦型桩的变形问题是两大主要问题;目前针对长期循环温度作用下饱和黏土中摩擦型桩变形特性与机理的研究仍相对较少。该文针对饱和黏土中的摩擦型桩,开展了长期循环温度作用下桩基热响应特性模型试验研究,实测了桩/土温度分布、温度引起的桩周土体孔隙水压力以及桩顶变形等发展规律,初步探讨了桩顶累积沉降的产生机理与变化规律。研究结果表明:单次温度循环过程中,桩顶位移变化率在制热时略小于制冷时,桩顶位移变化率的差值随着循环次数的增加而逐渐减小,从而累积沉降也逐渐趋于稳定;该文试验条件下,经过长期（20次）温度循环,摩擦型桩的桩顶累积沉降逐渐稳定在2%D（D为桩径）。     

基于现场试验的超长桩端阻力承载性状研究

为弄清大直径超长桩端阻力承载特性,基于15个场地中共32根桩的现场试验资料,采用作图法、归一化处理法、数学模型拟合法研究了大直径超长桩端阻力随桩端沉降的发挥性状。发现,桩端阻力随桩端沉降的变化主要有3种类型,即类型一(近似直线增加型)、类型二(转折点后增幅大为变缓的双折线增加型)、类型三(转折点后增幅几乎为0的双折线增加型),3种类型代表着大直径超长桩的破坏型式大致分为类型三代表的桩端刺入破坏和类型一、类型二代表的桩端整体剪切或局部剪切破坏;玻耳兹曼数学模型拟合桩端阻力-桩端沉降归一化曲线的效果好,相关系数R2达0.95以上;桩端阻力达到极限状态时的桩端沉降值至少在0.444d―123.433d之间,显示超长桩端阻力极限状态时的桩端沉降基本上无定值,但大部分在1.00mm―40.00mm之间;注浆后的端阻力为注浆前的2.5倍以上。结果表明,超长桩的端阻力不能被忽略,应加强研究,应充分利用它。     

Assessment of the effectiveness of two heat removal techniques for permafrost protection

Two mitigation techniques, an air convection embankment and an embankment of a granular material with an integrated heat drain, have been tested for the implementation in the shoulders of road and airfield embankments in permafrost regions. Both techniques will allow cold air to penetrate the embankment from the bottom, while warm air is dissipated at the top. The techniques have been tested in the laboratory, where a small-scale embankment (SSE) was build and placed in a cold room to measure the embankment temperatures during winter conditions. A numerical modeling has been developed and calibrated on the SSE to verify the effects on the thermal regime of full-scale embankments. The results have shown that both techniques will cause a decrease in temperature, which will minimize or even possibly avoid permafrost degradation underneath the embankments. The laboratory results have also shown that the effectiveness of the air convection embankment technique can be increased during winter conditions by ventilating the top and the bottom of the embankment shoulders. Installation of air intakes along the shoulders will facilitate air flow into the system during winter and will trap the cold air in the bottom of the embankment through the summer period. This solution has been verified using the numerical model.     

群桩(土)-承台-结构的动力相互作用分析

基于共同作用理论,在导出了时域中上、下部相互作用动力平衡方程的基础上,利用桩-桩动力相互作用因子法获取了群桩的动力阻抗,对水平地震作用下群桩-土-上部结构的反应特性进行了较全面的分析。着重讨论并估计了群桩抽桩布置于体系动力反应的影响,通过数值计算分析,取得了一些有价值的认识,并认为受水平地震力作用桩承结构也可借鉴考虑竖向上、下部共同作用沉降优化的理念和思想来进行抽桩设计。     

Evaluation of base grouting for the bearing behavior of barrette piles socketed in a gravel formation

Abstract

A comprehensive analysis for the effects of base grouting on barrette piles was carried out based on six compressive load tests on piles located in a newly developed residential area of the Taipei basin. All piles were socketed for a length of 3 m into a gravel formation at a depth of more than 45 m below the ground surface; four of these were base-grouted using the high-pressure cleaning and grouting method. Based on the strains measured at several depths along the pile shaft, side friction t–z curves for various soil strata and end bearing q–w curves in gravels are deduced. Comparing the test results of piles with and without base grouting, the effects of base grouting can be quantitatively evaluated. It is concluded that the base grouting has almost no influence on the initial stiffness of the pile head response curve for such a long piles, but significantly increases the ultimate bearing capacity and plastic stiffness of the pile head response curve for such long piles when they experience a large pile head displacement.     

高层主裙楼变厚度桩筏基础数值试验

 提供了一种桩-土-筏相互作用的计算模型，将筏板下的桩、土看成弹簧作用在筏板上，简化得到了软弱地基群桩应力和沉降等效分析模型，提出了一种群桩中单桩沉降的计算方法，建立了群桩中单桩非线性刚度的计算模型，并采用三维退化层合单元法对高层建筑主裙楼桩筏基础进行分析．