应力释放效应对抗拔桩承载力影响分析及应用

抗拔桩受力时,桩身径向微量收缩,桩周土体侧向应力释放,桩侧摩阻力降低。文章将桩对土的卸载作用导致摩阻力降低值与不考虑该作用时摩阻力之比,定义为单桩抗拔承载力折减系数J_s。定义土体的回弹模量与压缩模量之比为土的压缩回弹比λ_j。利用极限平衡状态方程,针对刚性桩、钢筋混凝土桩、预应力钢筋混凝土桩与钢管桩,分析了不同的土体压缩回弹比λ_j所反映的应力释放程度对承载力的影响。结合外径1.7 m,长81.3 m,壁厚25 mm的大直径超长抗拔钢管桩静载荷试验进行验证。结果表明压缩回弹比λ_j能很好反映土体应力释放对承载力的影响,是抗拔桩侧阻力低于抗压桩的主要原因,承载力计算值与实测值相符,其精度远高于现行规范方法,可供工程实践与科学研究参考。     

Influence of soil reinforcement on the uplift bearing capacity of a pre-stressed high-strength concrete pile embedded in clayey soil

This paper presents a field study on the uplift bearing capacity of a pre-stressed high-strength concrete (PHC) pile embedded in clayey soil, and on the soil around the PHC pile that was treated with cement paste. The PHC pile was inserted into a pile hole filled with cemented soil by its own weight (by gravity), and the soil compaction effect of a conventionally driven pile induced by the installation process was avoided. The test results showed that: the pile head displacement needed to fully mobilize the uplift bearing capacity of the test piles was about 0.1 D (pile diameter); the ultimate skin friction of the PHC pile–cemented soil interface was much larger than that of the cemented soil–soil interface; the PHC pile and the cemented soil around the pile behaved as an integral pile in the load transfer process; and the measured ultimate bearing capacity of the test piles was 0.91–0.94 times the American Petroleum Institute (API)’s proposed values for piles under compression and 0.79–0.80 times the values calculated with the effective stress method for piles under compression.     

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.     

PHC管桩工程特性分析

以PHC管桩加固某电厂古河道地基为依托,进行了PHC管桩受力特性研究。利用桩身预先埋设的光纤传感器,并借助静载荷试验、高应变和静力触探等现场测试手段,研究了PHC管桩在不同级别荷载作用下桩身沿深度的受力分布特征,发现PHC管桩侧摩阻力不仅与地基土层性质相关,而且其分布特征也与桩埋置深度有很大关系;桩体上段摩阻力较小,中间段发挥度最大,下段桩身轴力衰减较快;根据试验结果提出了PHC管桩单桩承载力修正公式,用该式求得PHC管桩单桩承载力与静载荷试验结果吻合,说明修正公式是合理的。     

Evaluation of pile bearing capacity by dynamic tests

Conclusions 1. In determining the bearing capacity of piles, dynamic resistance should be understood to mean that maximum value of soil reaction which is generated during the hammer blow. However, this concept should not be confused with the resistance obtained from the results of dynamic pile tests.2. Upon the delivery of a dynamic blow, as pile penetration proceeds, an increase in soil reaction occurs which reaches the limiting and then the critical load values as obtained from static tests. Therefore, the results of dynamic tests if the latter are correctly performed should coincide with the critical load obtained from static tests. Experience has shown that for this condition to be fulfilled, the total refusal of piles due to a single blow should be not less than 10–15 mm and the residual part of the refusal should be not less than 2 mm.3. The results of the dynamic tests enable the static pile resistance to be determined for the soil condition at the given instant. As is shown by graphs of the dynamic and static tests, the limiting pile resistance depends not on the type of test but on the condition of the soil and the degree of its thixotropic recovery.4. The determination of pile bearing capacity by the dynamic method, taking their "resting" into account, should be based on the results of the initial blows.Scientific-Research Institute of Foundations. Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 2, pp. 11–13, March–April, 1975.     

矩形钻孔灌注桩

打入桩,桩径较小,水平承载能力很低,人们一般只用它承担垂直荷载,这样可近似按轴向受压构件计算。在轴向受压构件中,用圆形截面也是合理的。钻孔灌注钢筋混疑土桩,桩径一般较大,水平承载能力很大,因此应按在垂直荷载和水平荷载共同作用下的偏心受压构件计算。在偏心受压构件中,圆形截面的力学性能远不如矩形截面好。为改进桩基结构,提高承载能力,应采用矩形钻孔灌注桩。本文论述了矩形钻孔灌注桩的合理性及施工方法。     

软土地基中组合桩水平受荷作用下的试验研究

组合桩为预应力混凝土桩插入水泥搅拌桩中而形成的新桩，为探讨该桩在水平荷载在用下的承载力和破坏模式，通过3根14.0 m长组合桩的水平载荷试验，实测得到该桩在水平荷载作用下桩与土共同作用下的工作性状和破坏特征，分析了组合桩受水平力作用时的临界和极限荷载、破坏模式、地基土的水平抗力比例系数以及桩帽对组合桩抗水平力的有利作用。为合理地评价组合桩的水平承载力，探讨组合桩在水平荷载作用下的计算方法：临界荷载法和转折点法，并给出计算公式。试验和分析结果表明：组合桩具有较好地抵抗水平荷载的能力，有桩帽的组合桩水平受荷性状优于无桩帽的组合桩，桩帽使组合桩由水泥桩和外侧的水泥壳之间相互分开的破坏模式变为外侧的水泥土和土之间分开的模式，保证组合桩在水平荷载下的完整性，用转折点法计算组合桩的水平承载力更为合理。     

超长大直径变截面钢管复合桩竖向承载力算法

变截面钢管复合桩的受力性能较好,在工程中应用较多,但是其理论研究却较为滞后。对该桩型竖向承载力计算方法进行研究,分析了其荷载传递规律,选取桩基受力极限状态,考虑钢管作用与变截面处承载力,利用桩土位移协调关系推导了超长大直径变截面钢管复合桩竖向极限承载力计算公式,并用该方法和现行规范计算方法对鱼山大桥37号桩基进行竖向承载力计算,与试桩自平衡试验实测值进行对比。研究结果表明：试桩自平衡试验曲线平缓无陡降,单桩竖向极限承载力试验值为120 056 kN,利用现行规范公式计算值与实测值相比偏差过大,而本文推导方法计算结果与实测值较为接近,验证了本文方法的可靠性。     

Effect of boring method on bearing capacity of short cast-in-place piles

Conclusions 1. Bored-hole/cast-in-place piles 4–10 m long tend to retain a constant value of the unit bearing capacity, irrespective of soil moisture. Driven piles, and piles cast in punched holes, undergo a loss unit bearing capacity (up to 20%) as their length is increased.2. With the saturation of high-porosity collapsible soils, the unit bearing capacity of bored-hole/cast-in-situ piles up to 10 m long diminishes two to three times, on average. For cast-in-place piles installed in a punched hole, and for driven piles, this reduction ranges up to only 30%. This trend is characteristic of soil layers whose collapsibility properties diminish with depth, from 2–3% to 1–1.5%.3. The unit bearing capacity of cast-in-place piles installed in punched holes is equal to that of driven piles, both for saturated and dry soil conditions.4. Following soil compaction, the unit bearing capacity of concrete cast-in-place piles installed in punched holes is higher than for bored-hole/cast-in-place piles, in the ratio of up to 2 for slightly moistened soils and 3–3.5 for saturated soils.5. All other conditions being equal, the unit bearing capacity is greater for driven piles, and somewhat smaller (by 10–15%) for piles cast in punched holes. The smallest unit bearing capacity is observed in bored-hole/cast-in-place piles (1.5–2 times less than for driven piles).Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 4, pp. 14–15, July–August, 1979.     

嵌岩桩破坏模式有限元极限分析

以某嵌岩桩现场静荷载试验为背景,基于有限元分析软件ANSYS这一平台,利用有限元极限分析法中的荷载增量法,探讨了不同情况下的嵌岩桩破坏模式。由数值模拟的分析结果可看出,当桩身承载力比基岩承载力低时,破坏一般发生在桩体;当桩身承载力比基岩承载力高时,破坏多发生在桩底基岩及桩与周围岩土界面上;当基岩中存在结构面时,尤其是软弱结构面,破坏多发生在桩周基岩与软弱结构面上。基岩中倾斜的软弱结构面对嵌岩桩的极限承载力影响较大,建立无结构面情况下嵌岩桩桩基模型进行对比,可以看出无结构面情况下的嵌岩桩桩基承载力是有结构面的嵌岩桩桩基承载力的4倍,当软弱结构面与嵌岩桩的距离较近时,嵌岩桩的承载力会急剧降低。     

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.     

考虑桩土变形协调的软土刚性桩复合地基设计计算

 按照规范设计,软土刚性桩复合地基桩间土的承载力往往被过高使用,其设计承载力对应的沉降大于复合地基沉降,而实际中是不可能出现这种情况,因此,桩间土承载力并不能按照设计发挥,主要原因是没有考虑桩、土变形协调的关系。为揭示桩、土变形协调的内在本质以及软土承载力被过高使用的问题,采用双曲线切线模量法,分别计算桩、土的p-s曲线,然后叠加成复合地基的p-s曲线,建立桩、土变形协调关系,以此分析桩、土荷载分担的情况。分析后发现,按照规范方法设计时,通常软土的承载力使用过高和桩的承载力设计偏低,如单桩承载力或其强度不足,易导致桩及复合地基破坏。针对规范方法的不足,提出合理的软土刚性桩复合地基设计应考虑桩、土变形协调的关系,应根据其p-s曲线按照相应沉降量所对应的荷载确定桩间土承载力,建立的考虑桩、土变形协调的软土刚性桩复合地基设计计算方法,可为今后设计提供指导作用。     

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.     

竖向及水平荷载加载水平、顺序对单桩承载力的影响

在桩基础设计中通常分别确定单桩的竖向和水平承载力,而不考虑竖向荷载与水平荷载之间的相互作用对桩承载力的影响,这显然不能反映桩的真实受力状况。软土地基上建造超高层建筑有时需要采用长度达80 m甚至更长的超长桩,目前考虑超长桩的竖向荷载和水平荷载二者之间相互作用的研究还很少。利用有限元软件ABAQUS模拟天津市某工程的79 m长桩载荷试验结果,通过参数调整使有限元计算结果与载荷试验的结果一致,在此基础上进行仿真模拟,研究竖向及水平荷载作用下超长桩的荷载传递与变形特点。分析结果表明超长桩表现出与已有文献中的刚性短桩不同的变形特性:对于承受水平荷载与竖向荷载的桩,竖向荷载对水平荷载作用下桩的性状的影响与竖向荷载相对于水平荷载的施加顺序、竖向荷载的大小以及土质条件等均有关。就本文算例,先施加竖向荷载再施加水平荷载时,存在一个最优的竖向荷载,在最优竖向荷载作用下,竖向荷载减小桩顶侧移的有利作用最明显,且竖向荷载减小桩顶侧移的作用主要表现在水平荷载加载后期,即水平荷载较大时。在土质条件较好的情况下竖向荷载对桩顶侧移的有利影响不如土质条件较差时那样明显。先施加水平荷载再施加竖向荷载时,竖向荷载不再起到减小水平荷载作用下桩顶侧移的作用。加载顺序变化对桩水平承载力的影响要大于对桩竖向承载力的影响。     

Effectiveness of using pile foundations in housing construction in Tyumen

Conclusions 1. Pile foundations of residential buildings in Tyumen under the usual soil conditions with the use of 30×30 cm prismatic piles and low monolithic grillage are, as a rule, ineffective.2. The use of pile foundations with a high grillage does not produce perceptible advantages and their technicoeconomic indices are close to those of foundations with a low grillage.3. The use of grillage-free pile foundations permits obtaining indices comparable with strip foundations at a pile length up to 8 m (working length 6 m).4. The economic effectiveness of pile foundations under conditions of Tyumen can be increased by using short piles of rational designs having an increased bearing capacity under the given soil conditions.Translated from Osnovaniya, Fundamenty i Mekhanika Gruntov, No. 3, pp. 4–6, May–June, 1977.     

桩底压浆灌注桩承载力计算

对桩底压浆灌注桩在各种地质条件下的工作机理进行了分析,利用静载试桩结果,统计出单桩承载力压浆较未压浆其承载力提高的范围,给出了该桩承载力计算建议公式.     

钢管桩逆作复合基础竖向承载性能试验研究

为了研究砂性土中钢管桩复合基础的承载特性,设计了模型桩系列试验。结果表明:对于单桩复合基础,封桩前阶段的荷载全部直接由承台底砂土承担,封桩后阶段桩体达到极限承载力之前,桩体分担的荷载占荷载增量的73.5%～92%,当桩的承载能力达到极限后,随着荷载的继续增加,桩分担的荷载比例较之前衰减明显,而土体分担的荷载比例增加。钢管桩八桩复合基础中,桩身轴力在桩身上部的衰减梯度明显小于中下部,角桩衰减速率最大,边桩次之;桩身摩阻力自上而下逐渐发挥,桩顶以下1.55 m处摩阻力达到最大;每级荷载作用下,角桩的侧阻力大于边桩。钢管桩群桩复合基础的竖向承载力远大于相对应的高承台群桩承载力。     

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.     

五星形桩与圆桩水平承载性能对比模型试验研究

五星形桩是一种横截面异形桩,是在圆桩的基础上向内切割5个圆弧,形成截面类似五星形的异形桩。按其截面性质分为周长最大化五星形桩F1、周长面积比最大化五星形桩F_2两种桩型,为掌握五星形桩的水平承载性能进行了与圆桩的对比模型试验研究。试验用土为干砂,砂雨法土样制作,模型桩为预制钢筋混凝土桩,相似比为1∶8。模型试验桩包括:五星形桩F1、五星形桩F_2、与五星形桩F_2截面周长相同的圆桩C1、与五星形桩F_2截面面积相同的圆桩C_2。由于五星形桩水平承载性能具有方向性,试验采用理论计算中水平承载力最大的方向施加水平荷载,试验结果表明:F1、C1和F_2的水平极限承载力相当,但F_2的截面面积最小,仅为C1的0.44倍;与C_2相比,F_2的水平极限承载力是其1.63倍,可见,合理截面形式的五星形桩可以提供更大的水平承载能力;五星形桩与圆桩弯矩分布规律基本一致,都在4倍直径左右(五星形桩为外接圆直径)达到最大,但五星形桩截面面积小,抗弯刚度不足,容易折断,总体水平承载性能不及截面周长相同的圆形桩,但优于截面面积相同的圆形桩。     

软土地区大直径中掘扩底法管桩承载性能现场试验分析

基于软土地区大直径中掘扩底法管桩静载荷试验,分析软土地区大直径中掘扩底法（简称中掘法）管桩承载特性,研究成果表明：在等桩径等桩长条件下,中掘扩底法管桩的单桩极限承载力较常规锤击法管桩低10%左右,但较钻孔灌注桩可提高30%以上;中掘法管桩桩顶荷载沉降曲线均呈现缓降型,单桩竖向桩基极限承载力应根据桩顶沉降量确定;在桩顶达到极限承载力时,中掘法管桩桩端阻力有明显提高,桩端阻力可占桩顶荷载40%~50%,呈现为端承摩擦桩,竖向承载能力得到明显提高。文中研究成果可为软土地区大直径中掘扩底法管桩的理论研究和工程应用提供参考价值。