Drilled Piers Design.

Drilled piers (also drilled shafts or drilled caissons) are often more economical than piles where equipment capable of rapid drilling is readily available, because of the large capacity of a pier as compared with a pile.

     a. Pier dimension and capacities.  Drilled piers can support large axial loads,  up to 4, 000 kips or more, although typical design loads are on the order of 600 to 1, 000 kips.  In addition, drilled piers are used under
lightly loaded structures where subsoils might cause building heaving.  Shaft diameters for high-capacity piers are available as follows:

From 2- 1/2 feet by 6-inch increments
From 5 feet by 1-foot increments

Also available are 15- and 2-foot-diameter shafts.
Commonly,  the maximum diameter of drilled piers is under 10 feet with a 3- to 5-foot diameter very common.

Drilled piers can be belled to a maximum bell size of three times the shaft diameter.  The bells may be
hemispherical or sloped.  Drilled piers can be formed to a maximum depth of about 200 feet.  Low capacity drilled piers may have shafts only 12 to 18 inches in diameter and may not be underreamed.

     b. Installation.  The drilled pier is constructed by drilling the hole to the desired depth, belling if
increased bearing capacity or uplift resistance is required, placing necessary reinforcement, and filling the
cavity with concrete as soon as possible after the hole is drilled.  The quantity of concrete should be measured to ensure that the hole has been completely filled.

Reinforcement may not be necessary for vertical loads; however, it will always be required if the pier carries
lateral loads.  A minimum number of dowels will be required for unreinforced piers to tie the superstructure to the pier.  Reinforcement should be used only if necessary since it is a construction obstruction.

Consideration should be given to an increased shaft diameter or higher strength concrete in lieu of reinforcement.  In caving soils and depending on local experience, the shaft is advanced by:

          (1) Drilling a somewhat oversize hole and advancing the casing with shaft advance.  Casing may be used to prevent groundwater from entering the shaft.

When drilling and underreaming is completed, the reinforcing steel is placed, and concrete is placed
immediately.  The casing may be left in place or withdrawn while simultaneously maintaining a head of concrete.  If the casing is withdrawn,  the potential exists for voids to be formed in the concrete, and special attention should be given to the volume of concrete poured.

          (2) Use of drilling mud to maintain the shaft cavity.  Drilling mud may be used also to prevent water from entering the shaft by maintaining a positive head differential in the shaft,  since the drilling fluid has a higher density than water.  The reinforcing steel can be placed in the slurry-filled hole.  Place concrete by tremie.

          (3) Use of drilling mud and casing.   The shaft is drilled using drilling mud, the casing is placed, and the drilling mud is bailed.  Core barrels and other special drilling tools are available to socket the pier shaft into bedrock.  With a good operator and a drill in good shape, it is possible to place 30- to 36-inch cores into solid rock at a rate of 2 to 3 feet per hour.  Underreams are either hemispherical or 30- or 45-degree bell slopes.

Underreaming is possible only in cohesive soils such that the underslope can stand without casing support, as no practical means currently exists to case the bell.

     c. Estimating the load capacity of a drilled pier.
Estimate the ultimate capacity, Q, of a drilled pier as follows:

The design load based on an estimated 1-inch settlement is:

          (1) Drilled piers in cohesive soil.  The skin resistance can be computed from the following: H

Use table 11-2 for the length of shaft to be considered in computing H and for limiting values of side shear.  The base resistance can be computed from the following:


          (2) Drilled piers in sand.   Compute skin resistance from the following:


Arching develops at the base of piers in sand similar to piers in clay; thus, the bottom 5 feet of shaft should not be included in the integration limits of the above equations.  The base resistance for a settlement of about 1 inch can be computed from the following:


Design Parameters for Drilled Piers in Clay
Table 11-2.  Design Parameters for Drilled Piers in Clay

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