Drilled Shafts and Caissons - Theory & Concepts

Drilled Shafts and Caissons

Learning Objectives

Understand the applications and advantages of drilled shafts over driven piles.
Describe the different construction methods (dry, casing, and wet/slurry).
Calculate the bearing capacity of drilled shafts in clay and sand using FHWA methods.
Understand lateral load behavior and settlement characteristics of drilled shafts.
Describe types of caissons and rigorous integrity testing methods.

Construction methods and bearing capacity calculations for drilled shafts.

Drilled Shaft

A cast-in-place deep foundation constructed by excavating a large-diameter cylindrical hole and filling it with reinforcing steel and concrete to support massive loads.

Overview

Drilled shafts (also referred to as bored piles, drilled piers, or cast-in-drilled-hole (CIDH) piles) are a highly versatile type of deep foundation. Unlike driven piles, they are constructed by excavating a large-diameter cylindrical hole in the ground and subsequently filling it with reinforcing steel and concrete. They are designed to support massive axial and lateral loads, often eliminating the need for complex pile groups and pile caps.

Construction Methods

STEP-BY-STEP

The primary challenge in drilled shaft construction is maintaining the stability of the excavated hole, particularly in loose, granular soils or below the groundwater table. Three main construction methods are employed:

Dry Method: Applicable in firm, cohesive soils (stiff clays) above the groundwater table that can remain stable without support during excavation and concrete placement.
Casing Method: Used when the excavation passes through caving soils (loose sands or gravels) or when artesian groundwater is present. A steel casing (pipe) is driven or vibrated into the ground to provide structural support to the hole walls. The casing may be permanent or temporarily extracted as the concrete is placed.
Wet (Slurry) Method: The preferred method for deep excavations in caving soils or below the water table. The hole is kept filled with a specialized drilling fluid (usually a bentonite or polymer slurry). The hydrostatic pressure of the slurry, combined with a filter cake that forms on the borehole walls, prevents collapse. The concrete is then pumped to the bottom of the hole using a tremie pipe, displacing the lighter slurry upwards.

Belled Piers

In stable cohesive soils, the base of the drilled shaft can be enlarged using a special underreaming tool to form a "bell." This significantly increases the end-bearing area ( $A_p$ ), enhancing the total capacity ( $Q_u$ ) without needing to increase the shaft diameter over its entire length.

Bearing Capacity of Drilled Shafts

The ultimate load capacity ( $Q_u$ ) is the sum of the end-bearing (point resistance) and skin friction (shaft resistance), identical to the principle for driven piles. However, the analytical methods for estimating $Q_p$ and $Q_s$ differ from driven piles due to the construction process (which relieves initial horizontal stress rather than increasing it via driving) and the larger diameter (which requires more settlement to fully mobilize end bearing).

Ultimate Load Capacity

Calculates the total capacity of a drilled shaft from point resistance and shaft resistance.

Q_u = Q_p + Q_s

Variables

Symbol	Description	Unit
$Q_u$	Ultimate load capacity	kN or kips
$Q_p$	End-bearing capacity	kN or kips
$Q_s$	Skin friction capacity	kN or kips

Drilled Shafts in Cohesive Soil (Clay)

The FHWA (Federal Highway Administration) guidelines, primarily based on the work of Reese and O'Neill (1988), are standard practice in the US.

Skin Friction in Clay (Alpha Method)

Estimates shaft resistance using the empirical adhesion factor.

Q_s = \sum_{i=1}^{n} (\alpha \cdot s_u \cdot p \cdot \Delta L)

Variables

Symbol	Description	Unit
$Q_s$	Total skin friction capacity	kN or kips
$\alpha$	Empirical adhesion factor. For drilled shafts, $\alpha$ is typically 0.55 for $s_u \le 1.5 \text{ tsf}$ . For $s_u > 1.5 \text{ tsf}$ , $\alpha = 0.55 - 0.1(s_u - 1.5)$ .	unitless
$s_u$	Undrained shear strength of the clay layer	kPa or tsf
$p$	Perimeter of the shaft ( $\pi D$ )	m or ft
$\Delta L$	Length of the shaft segment	m or ft

Crucial Exclusion Zones for Skin Friction

The FHWA method mandates excluding the top 1.5 m (5 ft) of the shaft and the bottom 1 diameter ( $1D$ ) from the skin friction calculation due to seasonal moisture changes, soil disturbance, and stress relief. If a bell is used, the excluded zone at the bottom is extended to one shaft diameter above the top of the bell.

End Bearing in Clay

Estimates the point resistance for a shaft in cohesive soil.

Q_p = A_p \cdot N_c^* \cdot s_u

Variables

Symbol	Description	Unit
$Q_p$	End-bearing capacity	kN or kips
$A_p$	Area of the shaft base	m² or ft²
$N_c^*$	Bearing capacity factor. $N_c^* = 9.0$ for $s_u \ge 0.5 \text{ tsf}$ . For $s_u < 0.5 \text{ tsf}$ , $N_c^* = \frac{2}{3} \cdot \left[1 + 0.2\left(\frac{L}{D}\right)\right] \cdot 9.0 \le 9.0$ .	unitless
$s_u$	Undrained shear strength	kPa or tsf

Drilled Shafts in Cohesionless Soil (Sand)

For sands, the FHWA method utilizes the $\beta$ method for skin friction and empirical correlations with SPT $N$ -values for end bearing, as proposed by Reese and O'Neill.

Skin Friction in Sand (Beta Method)

Estimates the shaft resistance based on effective vertical stress.

Q_s = \sum_{i=1}^{n} (\beta \cdot \sigma_v' \cdot p \cdot \Delta L)

Variables

Symbol	Description	Unit
$Q_s$	Total skin friction capacity	kN or kips
$\beta$	Coefficient relating skin friction to effective vertical stress ( $\beta = K \tan \delta$ ). According to Reese and O'Neill, for depths $z$ from 0 to 115 ft: $\beta = 1.5 - 0.135\sqrt{z}$ (where $z$ is in feet), limits $0.25 \le \beta \le 1.2$ .	unitless
$\sigma_v'$	Vertical effective stress at the midpoint of the soil layer	kPa or tsf
$p$	Perimeter of the shaft	m or ft
$\Delta L$	Length of the soil layer segment	m or ft

End Bearing in Sand

Empirical correlation based on uncorrected SPT N-value.

q_p = 0.6 \cdot N \le 30 \text{ tsf}

Variables

Symbol	Description	Unit
$q_p$	Unit end bearing resistance	tsf
$N$	Uncorrected SPT blow count near the base of the shaft	blows/ft

Lateral Load Behavior (P-y Curve Analysis)

Because of their massive diameter, drilled shafts are frequently used to resist extreme lateral loads (e.g., wind, earthquakes, ship impacts on bridge piers) without battering. The soil-structure interaction is complex and non-linear. The standard method of analysis uses $p-y$ curves, where the shaft is modeled as a beam on non-linear elastic springs. The $p-y$ curve represents the non-linear relationship between lateral soil pressure ( $p$ ) and lateral deflection ( $y$ ) at any depth. This requires specialized software (like LPILE) to solve the governing differential equation.

Settlement of Drilled Shafts

The settlement of a drilled shaft at working loads is typically dominated by the elastic compression of the massive concrete shaft itself, rather than soil compression.

Skin Friction Dominance: Under typical design loads, almost the entire applied load is transferred to the soil via skin friction along the upper portion of the shaft. Very little load reaches the base. Therefore, settlement is small and elastic.
End Bearing Mobilization: Large settlements (often $> 5\%$ of the shaft diameter) are required to fully mobilize end-bearing resistance at the base. This magnitude of settlement is generally unacceptable for structural serviceability.
The total settlement ( $S_{total}$ ) is the sum of: 1) Elastic compression of the shaft ( $S_e(1)$ ), 2) Settlement due to load transmitted at the tip ( $S_e(2)$ ), and 3) Settlement due to load transmitted along the shaft ( $S_e(3)$ ).

Types of Caissons

STEP-BY-STEP

Caissons are massive, hollow substructures constructed at or near the surface and then sunk into the ground to the required depth to form a foundation. They are often used for bridge piers over water or huge industrial pump houses.

Open Caissons: Open at both the top and bottom. Soil is excavated through the open shafts inside the caisson (using clamshell buckets), and the heavy concrete structure sinks under its own weight. The bottom is usually sealed with concrete underwater once the target depth is reached.
Pneumatic Caissons: Used when the excavation must be kept dry but is below the water table. The bottom of the caisson is sealed off to form a "working chamber." Compressed air is pumped in to precisely balance the external hydrostatic water pressure, preventing water (and fluid soil) from entering. Workers (sandhogs) excavate the soil manually or with small machinery in this high-pressure environment. Highly hazardous and expensive.
Box Caissons: Closed at the bottom and open at the top. They are typically cast on land, floated to the site, and then sunk onto a prepared level bed of rubble or concrete by filling them with water, sand, or gravel. Used primarily for breakwaters or shallow bridge piers where deep excavation is not required.

Quality Control and Integrity Testing

STEP-BY-STEP

Because drilled shafts are constructed entirely underground (often blind, especially in wet conditions), rigorous quality control is critical to verify the structural integrity of the final concrete element.

Cross-Hole Sonic Logging (CSL): The most common method. Several parallel steel access tubes are tied to the reinforcing cage before concrete placement. After the concrete cures, a sonic transmitter is lowered down one tube and a receiver down another. The speed of the ultrasonic pulse is measured; significant delays indicate anomalies (e.g., soil inclusions, necking, or poor quality concrete).
Thermal Integrity Profiling (TIP): Measures the heat of hydration generated by the curing concrete. Because a larger volume of concrete generates more heat, temperature profiles can map the actual "as-built" diameter of the shaft and detect bulges or necking.
Gamma-Gamma Logging: A radioactive source and sensor are lowered into access tubes to measure the bulk density of the concrete, effectively identifying voids or weak zones.

Key Takeaways

Drilled shafts are high-capacity, cast-in-place deep foundations that often eliminate the need for pile caps.
Construction methods (Dry, Casing, Wet/Slurry) are selected based on the stability of the soil and the presence of groundwater.
Drilled shafts are highly effective at resisting massive lateral loads, requiring non-linear $p-y$ curve analysis for design.
At working loads, drilled shaft settlement is generally small and dominated by skin friction; massive settlement is needed to engage end bearing.
Like driven piles, capacity is the sum of skin friction ( $Q_s$ ) and end bearing ( $Q_p$ ), but the analytical methods (e.g., Reese and O'Neill) differ due to construction effects and scale.
For drilled shafts in clay (Alpha method), the top 1.5m and bottom 1 diameter are typically excluded from skin friction calculations due to disturbance and stress relief.
Belled piers can significantly increase end-bearing capacity in firm, stable cohesive soils without increasing the diameter of the entire shaft.
Caissons (Open, Pneumatic, Box) are massive sunk structures used for huge loads or deep water applications, with pneumatic caissons requiring pressurized work environments.
Rigorous integrity testing (CSL, TIP) is mandatory for drilled shafts to detect underground construction defects like voids or necking.