Drilled shaft foundations are broadly described as cast-in-place deep foundation elements constructed in a drilled hole that is stabilized to allow controlled placement of reinforcing and concrete. 

Construction of 12-foot-diameter drilled shafts for the main pylon foundation of the Christopher S. Bond Bridge, in Kansas City (photo by Gary Naugle, MoDOT).

Drilled shaft foundations are broadly described as cast-in-place deep foundation elements constructed in a drilled hole that is stabilized to allow controlled placement of reinforcing and concrete. Several other types of deep foundations are employed in transportation works, as described below with distinctions from drilled shafts.
  • Driven piles are prefabricated structural elements that are installed into the ground with a pile-driving hammer. Driven piles have been used to support structures for thousands of years, and, in present times, steel H, pipe and prestressed concrete piles are commonly used for transportation structures. Driven piles typically are 12 inches to 36 inches in diameter or width, and, thus, smaller in size than drilled shafts. Driven piles displace the soil into which they are driven, and cannot penetrate hard materials or rock. In soft or caving soils, there is no concern for stability of a hole.

  • Micropiles are drilled piles that typically are less than 12 inches in diameter, and are constructed using a high-strength steel rod or pipe, which is grouted into the bearing formation. These piles can be drilled into even hard rock and achieve very high axial resistance for a very small structural member. Micropiles are favored in conditions where the small size is an advantage, and where lightweight, mobile drilling equipment must be employed.

  • Continuous-flight auger piles and drilled displacement piles are drilled pile foundations that typically are 12 inches to 30 inches in diameter. These piles are distinguished from drilled shafts in that the pile is formed by screwing the continuous-auger or displacement tool into the ground, and then grouting or concreting through the hollow center of the auger; thus, there is not an open hole at any time during the construction process.
All of the types of piles described above most often are used in groups connected at the pile top with a reinforced concrete pilecap. Drilled shafts are distinguished from other types of piles in that drilled shafts often are substantially larger in size, frequently used as a single-shaft support for a single column without a cap, and often installed into hard bearing strata to achieve very high load resistance in a single shaft.

Description and History

Drilled shaft foundations are formed by excavating a hole, typically 3 feet to 12 feet in diameter, inspecting the soil or rock into which the foundation is formed, and constructing a cast-in-place reinforced concrete foundation within the hole. The foundation as con structed supports axial forces through a combination of side-shearing and end-bearing resistance. The large-diameter reinforced concrete member also is capable of providing substantial resistance to lateral and overturning forces as illustrated on Figure 1 (p. 30). Drilled shafts for transportation structures are fairly commonly used to depths of up to 200 feet in the United States, but can extend to depths of as much as 300 feet or more.

Drilled shafts also are referred to by other names, including drilled piers, caissons, cast-in-drilled-hole piles, and bored piles. The common reference to these foundations as “caissons” reflects the history of development of drilled shaft foundations.

The term “caisson” is more accurately used to reference very large footings that are sunk into position by excavation through or beneath the caisson structure, and the use of drilled shafts evolved in many respects from caisson construction. Caisson construction has been used for hundreds of years, and was pioneered in the U.S. bridge construction in 1869 by James Eads in St. Louis. A diagram of caisson construction is shown on Figure 2 (p. 30) from one of the world’s most famous bridges, the Firth of Forth crossing in Scotland. These caissons were constructed as “pneumatic caissons,” in which air pressure was maintained below the caisson as it sunk to prevent water inflow into the chamber below where workers excavated beneath the caisson cutting edge to sink the caisson to the required bearing stratum. Pneumatic caissons are rare today because of safety issues, but open-well caissons still are occasionally used for bridges in deep water environments.

Open-well caissons typically consist of a box open at both top and bottom, with dredge wells for excavating the soil through the caisson to sink it into place. Several large bridges recently have been constructed on large rectangular open-well caissons including the new Tacoma Narrows Bridge and the Mississippi River crossing at Greenville, Miss. (See figure 3 on p.30).

Smaller, circular caissons or shafts were used to support building structures and some transportation structures in the early 1900s in several large cities including Kansas City, Chicago, Boston and New York. These early forms of drilled shafts usually were excavated by hand. The first known building supported on caissons of this type is the City Hall in Kansas City, which was constructed in 1890. Because of concern that timber piles might rot, the city building superintendent, S.E. Chamberlain, designed the foundations to consist of 92 caissons, 41⁄2 feet in diameter, placed to bear on limestone at a depth of around 50 feet. The excavation was supported by cylindrical sections of 3⁄16-inch boilerplate to prevent the collapse of earth surrounding the excavation, and backfilled, not with concrete, but with vitrified brick laid in hydraulic cement. Chamberlain’s description of this approach at the annual convention of the American Institute of Architects in Chicago in the fall of 1890 may have contributed to the adoption of this technique for several structures in that city soon afterward.

Several notable buildings in Chicago that had been founded on spread footings had suffered damaging settlement. The use of timber piles caused such heaving of the surrounding area that the owners of the Chicago Herald got a court injunction to stop construction of the pile foundations at the Chicago Stock Exchange building because of structural damage to their building. The diagram at left in Figure 4 (above) illustrates a foundation of the type designed by William Sooy Smith for one wall of the Chicago Stock Exchange building in 1893. The shafts were constructed as circular excavations with tongue-and-grooved timber lagging, which was driven ahead of the excavation and braced with iron hoops. This method of excavation with timber lagging in a circular form became known as the “Chicago Method.” These types of foundations are not actually caissons in the true sense of the word, but the term stuck and still is used today even for modern drilled-shaft construction.

The diagram in the right of Figure 4 illustrates a “Gow caisson” of the type pioneered by Col. Charles Gow of Boston, who founded the Gow Construction Co. in 1899. The telescoping casing forms could be recovered during concrete placement. In the 1920s, the Gow Co. built and used a bucket-type auger machine that was electrically powered and mounted on the turntable frame of the crawler tractor of a crane, thus promoting the development of machine-drilled shafts.

Although there has been a significant evolution of the drilled shaft industry over the past 40 years to the type of construction and design that is prevalent today, machine-drilled shafts became more widespread during the 1930s, and became increasingly used during the building boom after World War II. The A.H. Beck Co. began using drilled shafts in 1932 (see Figure 5 left}) and, along with McKinney Drilling (founded 1937), were some of the pioneers of the drilled-shaft industry in Texas. Augered uncased holes smaller than 30-inch diameter were common, and sometimes tools were employed to rapidly cut an underream or bell. In California, “bucket-auger” machines were more common, using a bottom-dumping digging bucket to dig and lift soils rather than an auger.

The photo in Figure 7 (p. 34) shows construction of the main pylon foundation for a new cable-stayed Missouri River bridge in Kansas City, 108 years after the pioneering first use of drilled shafts for the City Hall. The equipment and construction methods have advanced far beyond the original concepts proposed in 1890, but the basic idea is the same – to support the structure on bedrock below weak soils using small, economically constructed caisson foundations. The history of drilled shafts thus is seen to have come full circle. The large caisson construction techniques used for bridges were adapted to construct small-diameter caissons to support buildings that led back to the use of large drilled shafts for bridges and other transportation structures.

The development of improved equipment, materials and methods for design and testing have allowed the cost-effective use of drilled shafts in a greater variety of applications and with greater reliability than was ever before possible. 

This article is provided through the courtesy of the National Highway Institute. It is excerpted from the institute’s publication, “Drilled Shafts: Construction Procedures and LRFD Design Methods.”