Drilled Shafts, Piers, and Caissons

Drilled Shafts, Piers, and Caissons

As-built condition controls: A clean, sound 4-ft-diameter shaft and a nominal 4-ft hole with soft base debris, contaminated sidewall, or defective concrete do not have the same usable resistance.

Terminology and Load Transfer

A drilled shaft is a cast-in-place deep foundation constructed in an excavated hole, often with a reinforcement cage. “Drilled pier” commonly describes a similar large-diameter element, sometimes with a smaller depth-to-diameter ratio. “Caisson” has regional and historical meanings. On the exam, use the geometry and construction stated rather than inferring capacity from the name.

Axial compression resistance is organized into side and tip components:

Q_n = Q_s + Q_p,

subject to the applicable method, limiting values, compatibility, and construction assumptions. Side resistance depends on soil/rock strength, effective stress, roughness, socket quality, and load direction. Tip resistance depends on bearing material, base area, cleanliness, and the movement needed to mobilize it. Structural shaft capacity and geotechnical resistance remain separate.

For a 2026 exam, use the active PE Civil handbook, AASHTO LRFD 8th edition with May 2018 errata, and ACI 318-14 where structural concrete provisions apply.

Worked Side-and-Tip Resistance

A drilled shaft has diameter D = 4.0 ft and 30 ft of usable side-resistance length. The problem provides uniform nominal side resistance q_s = 1.5 ksf and nominal toe resistance q_p = 35 ksf. Side and base resistance factors are stated as φ_s = 0.55 and φ_p = 0.45.

Shaft perimeter and base area are

p = πD = 12.566 ft

and

A_p = πD²/4 = 12.566 ft².

Nominal side resistance is

Q_s = q_s pL = 1.5(12.566)(30) = 565.5 kips.

Nominal tip resistance is

Q_p = q_p A_p = 35(12.566) = 439.8 kips.

Using the separately supplied factors, total factored geotechnical resistance is

R_r = φ_sQ_s + φ_pQ_p

R_r = 0.55(565.5) + 0.45(439.8) = 508.9 kips.

For factored axial demand P_u = 480 kips, utilization is 480/508.9 = 0.943, so this axial geotechnical screen passes. A separate problem-given structural design strength must also exceed 480 kips, and settlement must be acceptable. If base cleanliness or sidewall quality invalidates the provided unit resistances, nominal diameter cannot rescue the calculation.

Rock Sockets

Rock sockets can transfer load through side shear, base bearing, or both. Characterize rock mass, discontinuities, weathering, strength, socket roughness, diameter, and cleaning. Core recovery or intact compressive strength alone may not represent jointed rock-mass behavior.

Full side and tip resistances may require different displacements to mobilize. Follow the applicable AASHTO compatibility and combination provisions rather than automatically summing both maxima. Soft seams below a socket can govern settlement or punching. Uplift reverses the side-transfer direction and requires cage/development and rock/soil checks under the tension model.

Construction Method and Groundwater

A dry method requires a stable excavation and controlled water. Casing supports unstable ground but removal can neck the shaft or disturb concrete if poorly sequenced. Slurry supports the hole through fluid pressure, but its density, viscosity, sand content, and level must be controlled; a filter cake or contaminated interface can reduce side transfer. Under-water concrete placement normally requires a controlled tremie or pump procedure that avoids mixing fresh concrete with fluid.

Overbreak changes concrete volume and possibly stiffness, but it does not automatically increase design diameter or resistance. Underbreak, caving, inclusions, necking, or segregated concrete can reduce both structural section and geotechnical contact. Construction specifications and inspection are part of the resistance model.

Structural Cage Design

Treat the shaft as a reinforced-concrete member under axial load, bending, and shear. Lateral soil response can move peak moment below grade. Use ACI 318-14 for strength, reinforcement, splice/development, transverse reinforcement, and cover under the stated exposure. Check minimum longitudinal reinforcement, bar spacing, confinement, and cage termination.

The cage must also survive lifting, splicing, lowering, centralizing, casing extraction, and concrete flow without distortion. Reinforcement must develop into the pile cap or column. A cage adequate in the final analysis may be unbuildable if bars and cross ties block aggregate and tremie flow.

Inspection and Integrity

Maintain a shaft log documenting tools, elevations, strata, groundwater, casing/slurry observations, final depth and diameter, socket, base cleaning, cage, and concrete placement. Compare theoretical with actual concrete volume to flag anomalies; volume alone cannot locate or classify a defect. Applicable integrity tests can supplement records, while load tests measure load–movement response.

Complete Workflow

1. Define axial, lateral, uplift, and construction demands.
2. Establish subsurface profile and construction method.
3. Calculate side/tip resistance using verified usable geometry and compatibility.
4. Apply the stated factors and compare with structural strength and settlement.
5. Design cage, cover, splices, confinement, and cap development.
6. Specify inspection, cleanout, slurry/casing, concrete placement, and anomaly evaluation.

Geotechnical equations assume a construction quality that must actually be achieved and documented.