Piles

Piles are structural members of timber, concrete, and/or steel, used to transmit surface loads to lower levels in the soil mass. This may be by vertical distribution of the load along the pile shaft or a direct application of load to the lower stratum through the pile point. A vertical distribution of the load is made using a friction, or "floating, " pile and a direct load application is made by a point, or "end-bearing," pile. This distinction of piles is purely one of convenience since all piles function as a combination of side resistance and point bearing except when the pile penetrates an extremely soft soil to a solid base.

桩是木、混凝土或钢制的结构构件，用来把地面上的荷载传递到土体的较深处。荷载传递可以通过沿桩身竖向分布或通过桩尖使荷载直接作用到下面的土层。采用摩擦桩时荷载为竖向分布，采用桩尖时荷载直接作用的是“端承桩”。把桩做这种区别完全是为了方便，因为除了桩是穿过一个极软的士层而到达一个刚性层的情况，所有桩的功能都是由侧阻力和端承力共同构成的。

Piles are commonly used:

(1) To carry the superstructure loads into or through a soil stratum. Both vertical and lateral loads may be involved.

(2) To resist uplift, or overturning, forces as for basement mats below the water table or to support tower legs subjected to overturing.

(3) To compact loose, cohesionless deposits through a combination of pile volume displacement and driving vibrations. These piles may be later pulled.

(4) To control settlements when spread footings or a mat is on a marginal soil or is underlain by a highly compressible stratum.

(5) To stiffen the soil beneath machine foundations to control both amplitudes of vibration and the natural frequency of the system.

(6) As an additional safety factor beneath bridge abutments and/ or piers, particularly if scour is a potential problem.

(7) In offshore construction to transmit loads above the water surface through the water and into the underlying soil. This is a case of partially embedded piling subjected to vertical (and buckling) as well as lateral loads.

桩通常用于下列情况：

(1)把上部结构的荷载传到或传给土层。包括垂直荷载和侧向荷载。

(2)抵抗上拔或倾预，例如在地下水位以下的地下室筏基的受力或支承受倾覆作用的塔架。

(3)在桩体排土和打桩振动的共同作用下，使松散的非粘性土被压实。这些桩以后也可以被拔出。

(4)当扩展式基脚或筏基位于边缘性土层或者其下面有可压缩性较高的地层时，用以控制沉降。

(5)提高机器基础下面士的刚度，以控制该系统的振幅和固有频率。

(6)作为桥台和/或桥墩的一个附加安全系数，特别是当冲刷成为一个潜在的问题时。

(7)在海上工程结构中，把水面以上的荷载经过水传到下面土层内。这是一种部分埋入桩，它既要承受垂直荷载（和压曲）又要承受侧向荷载。

Piles are sometimes used to control earth movements (as landslides) . The reader should note that power poles and many outdoor sign poles may be considered as partially embedded piles subject to lateral loads. Vertical loads may not be significant, although buckling may require investigation for very tall members.

桩有时亦用来控制土体移动（例如滑坡）。读者应该注意到电线杆和许多户外标志杆都可以看作是承受侧向荷载的部分埋入桩。垂直荷载可能不大，但构件很高时可能需要考虑压曲。

A pile foundation is more expensive than spread footings and likely to be more expensive than a mat. In any case great care should be exercised in determining the soil properties at the site for the depth of possible interest so that it can be accurately determined that a pile foundation is needed and, if so, that neither an excessive number nor lengths are specified. A cost analysis should be made to determine whether a mat or piles, in particular the type ( steel, concrete, etc.), are more economical. In those cases where piles are used to control the settlement at marginal soil sites, care should be taken to utilize both the existing ground and the piles in parallel so that a minimum number are required.

桩基础要比扩展式基脚多花很多钱，也可能比筏基更加昂贵。在任何情况下，都应该对现场可能有影响的深度内在的参数非常仔细地加以确定，以便决定是否需要采用桩基础，如果需要，则不要采用过多的桩和过长的桩。应该先分析造价，然后决定采用筏基还是桩基，特别是要决定采用哪一种桩（钢、混凝土等）更为经济。在那些边缘性土层中为控制沉降而采用桩基时，应该注意要利用原有的地基和桩，以使所需要的桩数为最少。

Piles are inserted into the soil via a number of methods:

(1) Driving with a steady succession of blows on the top of the pile using a pile hammer. This produces both considerable noise and local vibrations which may be disallowed by local codes or environmental agencies and, of course, may damage adjacent property.

(2) Driving using a vibratory device attached to the top of the pile. This is usually a relatively quiet method and driving vibrations may not be excessive. The method is more applicable in soil deposits with little cohesion.

(3) Jacking the pile. This is more applicable for short stiff members.

(4) Drilling a hole and either inserting a pile into it or, more common, filling the cavity with concrete which produces a pile upon hardening.

可以采用许多方法把桩置于土中：

(1)用桩锤对桩顶均匀地连续锤击把桩打入，这种方法产生相当大的噪声和现场振动。这两项都可能是当地法规和环保机构禁止的，当然也可能损坏附近的财产。

(2)采用在桩顶上安装振动器的方式使桩下沉。通常这是一种比较安静的方法，而且沉桩振动也不太大。这种方法适用于黏聚力小的士层。

(3)用千斤顶压桩，这种方法对刚度较大的短桩比较适用。

(4)钻孔后将桩插入，或者更为常见的是向钻出的孔洞内浇筑混凝土，混凝土硬化后就形成桩。

When a pile foundation is decided upon, it is necessary to compute the required pile cross section and length based on the load from the superstructure, allowable stress in the pile material ( code value), and the in situ soil properties. This is so that the necessary number and lengths of piles can be ordered by the foundation contractor. Dynamic formulas, pile-load tests, or a combination are used on site to determine if the piles are adequately designed and placed. It is generally conceded that a load test is the most reliable means of determining the actual pile capacity.

在决定采用桩基时，必须根据上部结构传来的荷载、桩体材料的允许应力（规范值）和现场土质，来计算桩的截面和长度。这样基础承包商就能据此来订购所需要的桩的数量和桩长。在现场使用动力学公式，荷载试验或两种方法结合来确定桩的设计和设置是否妥当。通常认为，荷载试验是确定桩的实际承载力的最可靠方式。

Pile-capacity determinations are very difficult. A large number of different equations are used, and seldom will any two give the same computed capacity. Organizations which have been using a particular equation tend to stick with it -particularly if a successful data base has been established. In a design situation one might compute the pile capacity by several equations using the required empirical factors suitably adjusted (or estimated) and observe the computed capacity. From a number of these computations some "feel" for the probable capacity will develop so that a design recommendation/ proposal can be made.

确定桩的承载力非常困难，虽然已经有许多公式可以供使用，但很难有任何两个公式会给出相同的计算结果。有些机构使用过某一特定的公式后便一直采用它，特别是在已经成功地建立起数据库以后。设计时要通过若干个公式计算桩的承载力并采用所需的经验系数加以适当调整（或估算），然后观察计算得到的承载力。根据若干次这种计算，对桩可能有的承载力便有了“概念”，据此就能作出设计建议。

We should note that although all the pile-capacity equations are for a single pile, rarely is a single pile used, rather two or three or more piles are used in a group. We should further note that the soil properties which are used in the design are those from the initial soil-exploration program, and the soil properties which exist when the foundation is in service may be a little or considerably different depending on how the piles have been placed and the number of piles in the group.

应当注意虽然所有的桩承载力公式都是针对单桩的，但是在实际中很少使用单桩，而是采用两根或三根（或更多）的桩成群使用。应该注意设计中采用的土质性能是由最初土地勘察时得到的，而土的性质在基础使用时会发生一些或较大的变化，这与桩是如何设置的和桩群中的桩数有关。