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 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 shallow 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 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 excessive 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 anticipated（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 differential 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. 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）connections, 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 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 inadequately 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 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 vertical 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 footing, 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 final（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 failure 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