Interaction of cast-in-place piles with soil under type II collapsibility conditions

Conclusions 1. Under the given and similar hydrogeologic conditions, the settlements of piles fully cutting through the collapsible mass and sunk 2–12 m into the noncollapsible soils, when the pile bearing capacity under the external load is satisfied, are practically equal to the ground surface settlements in the pile site.2. For long-term soaking of the soils from the top, the low position of the depth at which soil compression starts under the action of the dead weight of the mass (it is in the range 14–16 m), and the development of compression deformations in the noncollapsible layers, which prevents detachment of the upper soil zone from the piles, are essential under the given conditions.3. For increase in the pile length from 24 to 31 m and support on denser soil layers (alluvial clays), the soil base resistance and the additional load caused by the soil mass action increase correspondingly, but the pile settlements remain practically unchanged.For substantial reduction of the possible absolute pile settlements with prolonged soaking of the soils under the given hydrogeologic conditions, the pile length should be increased by about 20 m. However, for establishment of the foundation dimensions it is extremely important to compare the allowable and possible differential settlements of adjacent foundations, which are comparatively small under the given conditions.Scientific-Research Institute of Bases and Underground Structures. TISI. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 14–17, March–April, 1986.     

Experience with construction on long underreamed piles in collapsible soils

Conclusions 1. The noneffectiveness of construction of underreamings in long piles resting on insufficiently dense soils is due to high resistance on their lateral surfaces and small settlements until rupture occurs, as a result of which the tip resistance is not realized.2. The increase in the settlements of bored-cast-in-place piles for long-term soaking under the above-mentioned soil conditions does not depend on the external load applied to them and on the use of underreamings, for all practical purposes, but it is detemined by the laws of development of collapse depressions.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 2–5, July–August, 1990.     

Effective types of piles for farm buildings

Conclusions 1. Tee-section driven piles, intended for construction of framed farm buildings, transmitting a thrust of 100–200 kN to the foundations, behave as absolutely rigid elements for all practical purposes.2. For analysis of such piles, it is necessary to use a soil base model with a modulus of subgrade reaction increasing linearly as the depth increases and decreasing as the thrust increases.3. Under the thrust action on the lateral surface of the pile additional friction forces are developed, as a result of which the load-carrying capacity of the pile under the vertical load is increased.4. Weakening of the soil base by engineering canals passing alongside the pile brackets results in increased horizontal displacements. In wetted collapsible soil, the effect of weakening of the base is manifested to a greater extent than in soil having the natural water content.TsNIIÉ Psel' stroi. Siborggazstroi. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 11–15, July–August, 1981.     

Investigation of the bearing capacity of piles driven in plastic frozen soil

Conclusions The bearing capacity of a pile lowered into a borehole whose cross section exceeds the cross section of the pile for the possibility of filling in with slurry (frozen-in, friction, bored-sunk piles) is close to the calculated, but has a greater settlement under a standard load in comparison with a bored-drive pile.The bearing capacity of piles driven by the VMS-1 vibratory hammer into plastic frozen soil (bored-drive piles) is on the average 1.6 times greater than the calculated. Such an excess of the bearing capacity of the piles can be explained by the formation of a new structure and frost texture of the soil within 30–50 mm around the pile skin. A characteristic feature of the newly formed zone of soil is a considerably greater homogeneity and density of the soil than under natural conditions and absence of ice lenses and interlayers directly contacting the pile.The settlements of the pile tested under a standard load were less than the allowable deformations for the bases of the majority of buildings and structures.The cross-sectional area of the pilot hole should be equal to 0.65–0.75 of the pile section in the case of driving it at the time of the maximum depth of thawing of the soil and 0.95 at the time of seasonal freezing of the soil.Restoration of the contact bonds between the soil and pile during its freezing-in after driving into soil with a temperature of -0.5°C and higher can continue for several months.Deceased.Central Scientific-Research Institute of Transport Construction (TsNIIS). Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 19–20, January–February, 1979.     

湿陷性黄土地基桩的负摩擦力问题

本文对我国湿陷性黄土地区桩侧负摩擦力进行分析研究。叙述湿陷性黄土自重湿陷与桩基负摩擦力的关系,对于非自重湿陷性黄土是否考虑桩基负摩擦力问题作了初步探讨。根据湿陷性黄土地基的不同受水方式,提出计算“中性点”的近似方法。并根据自重湿陷性黄土地基上的试验资料,认为在工程中应重视桩基出现第二次负摩擦力峰值问题。为了防止桩基因黄土地基湿陷而产生负摩擦力,介绍了克服桩基负摩擦力的措施。最后提出五点建议,供研究分析湿陷性黄土地基桩基负摩擦力问题时参考。     

Experimental investigation of the distribution of stresses in large bored situ-cast piles

Conclusions 1. Diagrams were obtained of the distribution of specific skin friction of bored situ-cast piles with a diameter of about 1 m and length of 18 m and of their transformation as a result of flooding the soils and successive loading of the pile with gradual involvement of shearing resistance forces in a downward direction.2. Before the long bored situ-cast piles broke loose, the specific skin friction in homogeneous soil is practically constant over the length of the pile, which confirms the correctness of the concept used in [3].3. On flooding soils at the base of the piles, when the external load is equal to zero and there is still no slumping of the soil layers under their own weight, substantial normal stresses occur in the cross sections of long bored situ-cast piles, as a result of which they are additionally loaded by the soil hanging on them.4. The foot of long bored situ-cast piles penetrating loose layers of loam soils and supported on a layer of loam with =0.662 carried no more than 10% of the total load applied to the pile prior to breaking loose. Thus it behaves as a friction pile before breaking loose.5. After the pile breaks loose, the normal contact stresses on its foot increase in direct proportion to settlement. It follows from this that to increase the point resistance before breaking loose it is necessary to preliminarily compact the bottom of the hole of the bored situ-cast pile.Scientific-Research Institute of Bases and Underground Structures (NII Osnovanii). Research Department, All-Union Planning, Surveying, and Scientific-Research Institute (Gidroproekt). Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 11–13, May–June, 1980.     

Bored/cast-in-place piles employing soil compaction of hole bottom,for the "Atommash" Plant

Conclusions 1. Compaction of the soil at the bottom of deep holes of 1-m diameter as described, raised the bearing capacity of piles at their ends, under the Volga-Don ground conditions, by three to five times.2. In view of the fact that the toe of long (exceeding 15 m) floating bored/cast-in-place piles of usual construction contributed only to a weak extent (up to 10% of the total resistance, under Volga-Don conditions) to the performance of the pile, right up to its failure, it is expedient and economically efficient to install such piles only in holes previously rammed at their ends. Under ground conditions of type II in regard to collapsibility, this measure is particularly important in order to avoid inadmissible pile settlements resulting from negative friction following soil collapse under the weight of its own thickness. Cast-in-place piles placed in prerammed holes were introduced in the construction of Section 5 of the Volga-Don "Atommash" Plant, and are recommended for buildings of a substantial number of stories in Volga-Don.Scientific-Research Institute of Bases and Underground Structures. (NII Osnovanii) Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 6, pp. 10–13, November–December, 1980.     

基于减小筏板差异沉降的刚性桩复合地基试验研究

针对高层建筑筏板荷载分布特点,采用内密外疏布桩和仅在筏板中心布桩方式,通过现场缩尺(1:10)模型试验,完成了带上部结构无桩筏板和刚性桩复合地基筏板静载荷试验。分析了筏板沉降、桩端平面以下地基沉降和筏板外侧地面沉降、筏板下桩土反力分布、桩土荷载传递及桩土荷载分担比。研究了工作荷载下不同布桩方式降低筏板差异沉降的效果。     

垂直受荷桩负摩阻力时间效应的试验研究

 在地面堆载作用下,对3根施加不同荷载桩的负摩阻力性状的时间效应进行现场测试。试验结果表明,负摩阻力随堆载作用时间的延续而变化;堆载使土体沉降,土体沉降就会引起负摩阻力。在堆载初期,负摩阻力引起的桩附加沉降速率随着桩顶荷载的增大而加快,且稳定的附加沉降随着桩顶荷载的增大而增加。在有无桩顶荷载的2种情况下,下拉力随着堆载作用的时间延续而增大,稳定的下拉力随着桩顶荷载的增加而减小。在堆载作用下,桩顶荷载推迟了负摩阻力出现的时间。桩顶荷载越大,中性点初次出现的时间越晚、位置越浅。中性点出现的时间随着桩顶荷载的增加而延后,桩顶荷载的增大会加剧这些现象。上述研究结果对于桩的负摩阻力性状的研究有一定参考价值。     

Actual and calculated loads on piles and results of observing settlements of a large-panel building

Conclusions 1. During the construction period the loads on extreme piles along the axes of transverse bearing walls are considerably greater than on the middle ones.2. Under the effect of rigidity of the building a redistribution of the loads on the piles during its operation occurs even in the absence of a grillage, i.e., the more loaded piles are unloaded and the less loaded ones are additionally loaded.3. The actual loads on the piles in the operating period are considerably less than the calculated, which indicates the need to refine the recommendations of SNiP II-6-74 for the purpose of taking into account the useful loads in calculating the foundations of residential buildings.4. The factor =0.2, concerning conversion from settlements of single piles in static tests to settlements of piles in foundations under long-term load, given in the draft of the new edition of SNiP II-17-77, is acceptable.5. To accumulate data on the actual loads on pile foundations it is necessary to continue their measurements under various soil conditions.Deceased.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 17–19, May–June, 1977.     

Possibility of reducing the duration of static pile tests

Conclusions 1. The experiments, carried out under different conditions, did not show substantial differences in the limit strengths of prismatic piles tested at the standazd loading rate and by the accelerated method when it is considered that attenuation of the settlement has been reached when it has a rate of 0.1 mm in 15 min. The tests performed by continuous increase in the load over a period of 10–15 min led to overestimation of the limit strengths by 20–30%.2. Under conditions in which the settlements are not the object of special studies, accelerated pile tests (with a settlement stabilization criterion of 0.1 mm in 15 min), in place of standard prismatic piles applicable under the most widespread soil soil conditions (for example, in alluvial, deluvial, and similar deposits). In this case, it is advisable to use the pile limit strength for settlements which are smaller by a factor of two than for the standard tests.NIIpromstroi Institute. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 9–12, January–February, 1987.     

湿陷性黄土地区单桩负摩阻力计算研究

湿陷性黄土地区地基土浸水湿陷后,桩侧表面可能产生负摩阻力,将导致桩基承载力降低。对湿陷性黄土地区桩基负摩阻力的产生机理及变化规律进行了分析。通过对实测曲线的分析及借鉴前人的研究成果,提出了一种适用于湿陷性黄土地区的双折线形桩侧负摩阻力分布模式。在有效应力法的基础上提出了湿陷性黄土地区负摩阻力的计算方法。     

湿陷性黄土特性对桥梁桩基础的影响

针对湿陷性黄土的工程特性,分析了其对桩荷载传递规律的影响,并探讨了黄土地区桥梁桩基浸水前后的受力特性,总结了黄土地层中桩基设计需注意的问题,提出了消除负摩阻力的技术措施,避免湿陷性黄土给工程造成破坏。     

大直径钻孔灌注桩负摩阻力试验研究

 针对大面积堆载情况下,周边土体的沉降使桩基产生负摩阻力从而导致桩基承载力特性变化的问题,以宁海电厂工程2组冲孔灌注桩的现场负摩阻力试验为例进行讨论。通过对原位试验结果的全面分析,探讨桩周土体固结沉降对桩身所受下拉荷载和中性点位置的影响。根据实测桩土沉降曲线确定的中性点与根据桩身轴力沿深度变化曲线确定的中性点位置大体相一致,位于可压缩土层下部,桩身最大轴力随固结时间而增大,中性点位置也随时间略有上移;分析桩侧摩阻力系数的大致范围以及施工工艺对负摩阻力的影响,现场试验得到的桩侧摩阻力系数为0.3～0.4,由于桩基施工的影响导致该值与规范相比略大,工程中应充分考虑成桩工艺对负摩阻力的影响;指出负摩阻力桩基的设计分析中沉降计算至关重要。得出的结论可指导同类工程的设计和施工。     

Construction on loess soils

Conclusions 1. Inadmissible settlements and tilts of several multistory buildings in Volgodonsk are the result of collapse of the soils under external loads owing to disregard of the requirements for elimination of the collapsibility within the limits of the entire deformable zone under shallow foundations.2. The cause of the possible settlements of structures on pile foundations fully cutting through the collapsible mass lies in development, under soaking from the top, of deformation phenomena with compaction not only of the collapsible layers but also of the underlying non-collapsible soils, which was first mentioned in [9].The concept that the most hazardous situation for a pile foundation, in this regard, occurs when collapse takes place under the weight of the overlying mass with the GWL rising from bottom to top is erroneous and is not confirmed by construction experience.3. In the development of deformation phenomena, it is important to distinguish between the active period and the settlement attenuation period, since differential settlements occur only during the active period for all practical purposes.4. It is possible to eliminate completely the possibility of substantial settlements of structures under severe Type II collapsibility conditions either by construction of deep founations carried to 40–50 m so that they will rest on solid material, or by elimination of the possibility of soaking of the soils. These highly uneconomical solutions can be used only in especially important cases. On the other hand, in the massive construction practice it is justifiable to correctly fulfill the SNiP requirements, taking into account the possible additional settlements caused by compression of the underlying noncollapsible soils.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 1, pp. 24–27, January–February, 1991.     

湿陷性黄土中大直径扩底桩垂直承载性状的试验研究

本文根据湿陷性黄土中桩侧和桩底均埋有测试元件的４根大直径扩底桩的现场实测数据，分析了浸水湿陷对桩垂直承载力的影响，得出了负摩阻力的变化规律，最后提出浸水湿陷全过程中大直径桩荷载传递机理的《三阶段分析法》。     

Method and results of testing piles tamped into holes drilled in soils classed as type II in terms of proneness to slump-type settlement (in order of discussion)

Conclusions 1. Piles that are tamped into predrilled holes cutting through soils classed as type II in terms of proneness to slump-type settlement, which have expended tips supported on clayey soils and clays that are not prone to slump-type settlement and that are situated above the water table under conditions prevalent in Zaporozhe, have low (with respect to type-I soils), but, in many cases, completely adequate bearing capacity, and can be used to build many structures.2. The load friction in soil classed as type II in terms of proneness to slump-type settlement amounts to 30–35 kN/m² along the lateral surface of a pile in Zaporozhe.3. The allowable design load on a pile in soils classed as type II in terms of proneness to slump-type settlement should be determined, as a rule, from pile tests in an experimental trench where the soil develops slump-type settlement under its own weight.4. A smaller load friction acts on a pile whose lateral surface is covered with three to five layers of elastic sheathing than on the unprotected piles.Scientific-Research Institute of Bases and Underground Structures. Dnepropetrovsk Civil-Engineering Institute. Ukrainian State Design Institute for Special Construction. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 7–10, May–June 1984.     

黄土塬地区大直径长桩承载性状与优化设计研究

 黄土塬地区桩基问题研究匮乏,依托陇东首栋超高层建筑,在试验桩身上布置混凝土应变计、钢筋应力计,承台底板下和桩端布置土压力盒,对原地基土、单桩基础和单桩承台基础分别进行现场原位载荷试验;利用ANSYS有限元软件对全短桩基、全长桩基及长短桩组合桩基在竖向荷载作用下的筏板沉降变形、地基土应力场与沉降变化进行分析。结果发现：(1) 黄土塬场地地基土夹层交互分布、湿陷性不连续,存在由非湿陷性黄土变成湿陷性黄土的可能,桩周土层对桩基内力传递与分布影响显著,桩身出现多个中性点,湿陷性土层下限深度确定更加复杂;(2) 各级荷载作用下,桩基Q-S曲线呈缓变型发展,表现为典型的摩擦型桩,桩身内力发挥具有异步性;试验加载至8 000 kN时,桩顶最大沉降为8.15 mm,单桩和单桩承台端阻力分别仅占桩顶荷载的4.8%和2.1%;(3) 单桩承台基础中承台底部实测反力呈倒“盆”形分布、边缘应力较大,桩–土–承台体系的承载性能优于单桩基础;桩基础设计时,可结合经验以承载力和最大允许变形量进行控制,提高桩身线刚度抵抗自身压缩变形,减小桩基上部沉降;(4) 长短桩组合桩基础充分利用与发挥了长桩控制沉降的作用与地基土浅层承载的能力,减少了长桩数量,节省了桩基造价,值得进一步深入研究。     

人工制备湿陷性黄土地基地下连续墙浸水试验研究

为研究湿陷性黄土地基地下连续墙基础竖向极限承载特性及浸水后负摩阻力分布特征,选用石英粉、砂、膨润土、石膏和工业盐制备了人工湿陷性黄土,对人工制备湿陷性黄土的物理力学特性进行分析;采用人工制备湿陷性黄土填筑模型试验,进行地下连续墙基础承载特性试验研究。研究结果表明:人工制备湿陷性黄土的物理力学参数与天然黄土基本一致,可用于湿陷性黄土与构筑物相互作用模型试验相似材料。地下连续墙竖向承载力达到其极限时,外墙和内墙总侧摩阻力荷载分担比为67%,确定地下连续墙为端承摩擦型基础。地基浸水湿陷后,中性点深度比为0.64~0.73,试验结果与桩基浸水试验测试结果较为一致。由于地下连续墙基础具有良好的整体性和防渗性,芯土不受水的影响,内墙侧摩阻力与承台土反力能够得以发挥,有效减小地下连续墙基础的沉降。     

地面水对黄土地区桥梁桩基承载力影响试验研究

黄土因其特有的工程性质,使位于黄土地区的桥梁桩基础在桩周土受地面水浸湿后产生湿陷变形,该变形引起桩的负摩阻力,降低了桩的承载力。通过陕西芝川河特大桥桩周浸水前、后的荷载试验,对黄土区域桩基浸水前、后的承载性状进行了分析研究,揭示了地面水对黄土区域桥梁桩基承载力性状具有较大的影响;分析了桩及桩周土浸水期间的沉降变化规律。研究成果对黄土区域公路桥梁桩基础的设计与施工具有重要理论价值与指导意义。