Engineering and Construction

Piles may be made of timber, steel, concrete, and various combinations of these materials (Figure 2.47). Timber piles have been used since Roman times, when they were driven by large mechanical hammers hoisted by muscle power. Their main advantage is that they are economical for lightly loaded foundations. On the minus side, they cannot be spliced during driving and are, therefore, limited to the length of available tree trunks, approximately 65 feet (20 m). Unless pressure treated with a wood pre- servative or completely submerged below the water table, they will decay (the lack of free oxygen in the water prohibits organic growth). Relatively small hammers must be used in driving timber piles to avoid splitting them. Capacities of individual timber piles lie in the range of 10 to 55 tons (9000 to 50,000 kg).

Two forms of steel piles are used, H-piles and pipe piles.  H-piles are special hot-rolled, wide-flange sections, 8 to 14 inches (200 to 355 mm) deep,  which are approximately square in cross section. They are used mostly in end bearing applications. H-piles displace relatively little soil during driving. This minimizes the upward displacement of adjacent soil, called heaving, that sometimes occurs when many piles are driven close together. Heaving can be a particular problem on urban sites, where it can lift adjacent buildings.

Figure 2.47 Cross sections of common types of piles. Precast concrete piles may be square or round instead of the octagonal section shown here and may be hollow in the larger sizes.

 H-piles can be brought to the site in any convenient lengths, welded together as driving progresses to form any necessary length of pile, and cut off with an oxyacetylene torch when the required depth is reached. The cutoff ends can then be welded onto other piles to avoid waste. Corrosion can be a problem in some soils, however, and unlike closed pipe piles and hollow precast concrete piles, H-piles cannot be inspected after driving to be sure they are straight and undamaged.
Allowable loads on H-piles run from 30 to 225 tons (27,000 to 204,000 kg).

Steel pipe piles have diameters of  8 to 16 inches (200 to 400 mm). They  may be driven with the lower end either open or closed with a heavy steel  plate. An open pile is easier to drive  than a closed one, but its interior  must be cleaned of soil and inspected before being filled with concrete,  whereas a closed pile can be inspected  and concreted immediately after driving. Pipe piles are stiff and can carry  loads from 40 to 300 tons (36,000 to 270,000 kg). They displace relatively  large amounts of soil during driving,  which can lead to upward heaving of  nearby soil and buildings. The larger  sizes of pipe piles require a very heavy  hammer for driving.

Minipiles, also called  pin piles or micropiles, are a lightweight form of  steel piles made from steel bar or  pipe 2 to 12 inches (50 to 300 mm) in  diameter. Minipiles are inserted into  holes drilled in the soil and grouted in  place. When installed within existing  buildings, they may also be forced into the soil by hydraulic jacks pushing downward on the pile and upward on the building structure. Since no hammering is required, they are a good  choice for repair or improvement of  existing foundations where vibrations from the hammering of conventional piles could damage the existing structure or disrupt ongoing activities within the building (Figure 2.54).

Where vertical space is limited, such as when working in the basement of an existing building, minipiles can
be installed in individual sections as short as 3 feet (1 m) that are threaded end-to-end as driving progresses.

Minipiles can reach depths as great as 200 feet (60 m) and have working capacities as great as 200 to 300 tons (180,000 to 270,000 kg). 

Precast concrete piles are square, octagonal, or round in section, and in large sizes often have open cores to  allow inspection (Figures 2.47–2.49).

Figure 2.48 Precast, prestressed concrete piles. Lifting loops are cast into the sides of the piles as crane attachments for hoisting them into a vertical position.

Figure 2.49 A driven cluster of six precast concrete piles, ready for cutting off and capping.

Most are prestressed, but some for smaller buildings are merely rein-forced (for an explanation of prestressing, see pages 544–548). Typical cross-sectional dimensions range from 10 to 16 inches (250 to 400 mm) and bearing capacities from 45 to 500 tons (40,000 to 450,000 kg).

Advantages of precast piles include  high load capacity, an absence of corrosion or decay problems, and, in
most situations, a relative economy of cost. Precast piles must be handled carefully to avoid bending and cracking before installation. Splices be-tween lengths of precast piling can be made effectively with mechanical fastening devices that are cast into the ends of the sections.

A  sitecast concrete pile is made by driving a hollow steel shell into the ground and filling it with concrete. The shell is sometimes corru-gated to increase its stiffness; if the corrugations are circumferential, a  heavy steel mandrel (a stiff, tight-fitting liner) is inserted in the shell  during driving to protect the shell  from collapse, then withdrawn before concreting. Some shells with  longitudinal corrugations are stiff  enough that they do not require  mandrels. Some types of mandrel- driven piles are limited in length,  and the larger diameters of sitecast piles (up to 16 inches, or 400 mm)  can cause ground heaving. Load ca- pacities range from 45 to 150 tons  (40,000 to 136,000 kg). The primary  reason to use sitecast concrete piles  is their economy.

There is a variety of proprietary  sitecast concrete pile systems, each  with various advantages and disad- vantages (Figure 2.50). Concrete  pressure-injected footings (Figure 2.51)  share characteristics of piles, piers,  and footings. They are highly resistant to uplift forces, a property that  is useful for tall, slender buildings in  which there is a potential for overturning of the building, and for ten- sile anchors for tent and pneumatic  structures. Rammed aggregate piers and  stone columns  are similar to pressure- injected footings, but are constructed  of crushed rock that has been densely compacted into holes created by  drilling or the action of proprietary  vibrating probes.

Figure 2.50 Some proprietary types of sitecast concrete piles. All are cast into steel casings that have been driven into the ground; the uncased piles are made by withdrawing the casing as the concrete is poured and saving it for subsequent reuse. The numbers refer to the methods of driving that may be used with each: 1. Mandrel driven. 2. Driven from the top of the tube. 3. Driven from the bottom of the tube to avoid buckling it. 4. Jetted. Jetting is accomplished by advancing a high-pressure water nozzle ahead of the pile to wash the soil back alongside the pile to the surface. Jetting has a tendency to disrupt the soil around the pile, so it is not a favored method of driving under most circumstances

Figure 2.51 Steps in the construction of a proprietary pressure-injected, bottom-driven concrete pile footing. (a) A charge of a very low-moisture concrete mix is inserted into the bottom of the steel drive tube at the surface of the ground and compacted into a sealing plug with repeated blows of a drop hammer. (b) As the drop hammer drives the sealing plug into the ground, the drive tube is pulled along by the friction between the plug and the tube. (c) When the desired depth is reached, the tube is held and a bulb of concrete is formed by adding small charges of concrete and driving the concrete out into the soil with the drop hammer. The bulb provides an increased bearing area for the pile and strengthens the bearing stratum by compaction. (d, e) The shaft is formed of additional compacted concrete as the tube is withdrawn. (f) Charges of concrete are dropped into the tube from a special bucket supported on the leads of the driving equipment.