6讲浅基础承载能力

1、footing column pile slab pier abutmentmat foundation shallow foundationisolated footing continuous footing raft foundation box foundationpile foundation footing foundationstrip footing spread footingfriction pile timber piledriven pile displacement pile point bearing pile 地下水位 管涌 隆起 井点 降水 不均匀沉降 预应力

2、预制 砂浆 承载能力 weak strong hard soft loose stiff rigid firm sound steady stable brittleuniform 均匀得homogeneous 均质的、均匀的isotropic 各向同性的anisotropic 各向异性的viscouse 粘性的cohesive 粘性的noncohesive 非粘性的cohesionless 无粘性的、无内聚力的ductile 延性的、塑性的Unit 7 Bearing Capacity of Shallow FoundationsIntroductionThe designer of a s

3、hallow foundation must ensure that the foundation meets basic considerations of safety, dependability, functional utility(实用、有用), and economy. Specifically, the foremost(最主要的) of these are the requirements of adequate（足够的） depth, tolerable （容许的）settlements, and safety against failure.The requirement

4、 of adequate depth centers around environmental influences which could affect adversely（有害的） the foundation performance（工作, 履行）.The foundation must be deep enough with respect to（相对于、关于、就而论） the depth of frost penetration（侵入） and depth of seasonal（季节性的） volume changes in the soil to prevent excessiv

5、e movements resulting from these influences. The foundation must be deep enough to exclude（排除） the possibility of erosion（冲蚀） and undermining of the supporting soil by water and wind currents. The foundation should also be adequately（适当地）placed with respect to adjacent structures, existing or antici

6、pated（预期的）,to minimize the possibility of mutual（相互的） damage by construction operations or by transmission of additional loads to the supporting soils.The requirement of tolerable settlements is concerned with total and differential settlements of all foundations under the planned structure. The dif

7、ferential settlements must be limited in order not to cause structural distress（开裂） or excessive tilting of the superstructure and they are also often limited by the serviceability（使用性） requirements of the superstructure；for example, crane tracks（吊车轨道） and many machines have limits of tolerable tilt

8、. The total settlements must be limited because they invariably（不变地、总是） induce（引起） differential settlements, even in apparently（明显的） homogeneous soil conditions. They are also often limited by considerations of such factors as access（出入口）to adjacent buildings（楼房之间的过道）, water and sewage（下水道）connectio

9、ns, etc.The requirement of safety against failure is centered around two principal kinds of failure that may be of concern in design：the structural failure of the foundation and the bearing capacity failure of the supporting soils.The structural failure of the foundation may occur if the foundation

10、itself is not properly designed to sustain(承受) the imposed（施加的）stresses. For example, an improperly（不当的）, proportioned（失调的）or inadequately（不足的）reinforced footing or mat（片筏基础）may fail in tension, compression, or shear, as any other reinforced concrete structural member. An improperly cast of inadequa

11、tely reinforced concrete pile may be broken during handling（人工操作，处理，搬运） and driving（打桩）; it may also be broken by excessive lateral loads for which it was not designed. A free-standing（独立的）steel or wooden pile may buckle(弯曲) just as any other column, particularly if it is subjected to combined axial

12、 forces and moments. All foundations must possess（具有）a conventional（常规的，惯例的）safety against such failures.This section is devoted primarily to（致力于） the discussion of the problem of bearing capacity failure of the soil. Consider the simplest case of a shallow footing (Fig. 11. la) subjected to a verti

13、cal static or transient（短暂的、瞬时的）load Q. If the vertical displacement w of the foundation is recorded（记录）as the load Q increases, a load-settlement curve similar in shape to a stress-strain curve may be obtained (Fig. 11. lb). The shape of this curve depends generally on the size and shape of the foo

14、ting, the composition（成分、组成）of the supporting soil, and the character, rate, and frequency of the loading. Normally, the curve will indicate the ultimate load Q0 that the foundation can support. This can be either a peak load, as shown by curves 1 and 2 in Fig.11. lb, or a load at which a constant f

15、inal（收尾的，末） rate of penetration is achieved, as shown by curve 3. 图11.1 Modes of Failure It is known from observation of behavior of foundations subjected to load that bearing capacity failure occurs usually as a shear failure of the soil supporting the footing. The three principal modes of shear fa

16、ilure under foundations have been described in the literature as general（整体的） shear failure, local（局部） shear failure, and punching shear（冲剪） failure.General shear failure is characterized by the existence of a well-defined failure pattern consisting of a continuous slip surface from one edge of the

17、footing to the ground surface (Fig. 11. 2a). In stress-controlled conditions, under which most foundations operate（运行，引起）(and, perhaps, fail) failure is sudden and catastrophic（灾难性的）. Unless the structure prevents the footings from rotating, the failure is also accompanied （伴随）by substantial（大的、实际的）

18、tilting of the foundation. In strain-controlled conditions (occuring, for example, when the load is transmitted by jacking) a visible decrease of load necessary to produce footing movement after failure may be observed (Fig. 11. 2a) 破坏之后使基础发生移动所需的荷载明显减小（图11.2a）。 A tendency（趋势） for bulging（隆起） of adj

19、acent soil can be recorded through most of the loading process（加载过程） on both sides of the footing, although the final soil collapse occurs only on one side. In contrast with the above-described failure mode, punching shear failure is characterized by a failure pattern which is not easy to observe (F

20、ig. 11. 2c). As the load increases, the vertical movement of the footing is accompanied by compression of the soil immediately underneath. Continued penetration of the footing is made possible by vertical shear around the footing perimeter（基础周边的垂直剪力有可能使基础连续下沉）. The soil outside the loaded area remai

21、ns relatively uninvolved（不受影响） and there is practically no movement of the soil on the sides of the footing. Both the vertical and the horizontal equilibrium of the footing are maintained. Except for（除了，只有）sudden small movements of the foundation in the vertical direction, there is neither visible c

22、ollapse（破裂）nor substantial（大的）tilting. Continuous increase in vertical load is needed to maintain footing movement in the vertical direction.Finally, local shear failure is also characterized by a failure pattern which is clearly defined only immediately below the foundation (Fig. 11.2b). This patte

23、rn consists of a wedge and slip surfaces, which start at the edges of the footing just as in the case of general shear failure. There is visible tendency（趋势）of soil bulging（隆起） on the sides of the footing. However, the vertical compression under the reacting is significant（有效的，值得注意的） and the slip su

24、rfaces end somewhere（某处） in the soil mass. Only after a considerable vertical displacement of the footing (say up to one-half the width or diameter of the footing) may the slip surfaces appear at the ground surface. Even then（即使那样，指具有很大的垂直位移） there is no catastrophic（灾难性的） collapse or tilting of the footing which remains deeply embedded（埋）, mobilizing（利用） the resistance of deeper soil strata. Thus, local shear failure retains some characteristics of both general shear and punching modes of failure, representing（表示） truly