9.5 Deep Foundation Installation

Deep Foundation Installation

Why This Topic Matters for the PE Construction Exam

When near-surface soils cannot support the loads of a structure, deep foundations are required to transfer loads to deeper, stronger strata (like bedrock or dense sand). The PE exam covers the means and methods of installing deep foundations, the equipment used, and the methods used to determine if the installed foundation meets the design capacity. Understanding the differences between driven piles and drilled shafts, and how to evaluate pile driving logs, is crucial.

Driven Piles

Driven piles are prefabricated elements pushed into the ground using high-energy impact hammers. They displace the soil, which can increase the density of surrounding granular soils but may cause heave in cohesive soils.

Pile Types

• Steel H-Piles: Excellent for driving through hard layers or driving to bedrock for end-bearing capacity. They cause minimal soil displacement.
• Steel Pipe Piles: Can be driven open-ended or closed-ended. Often filled with concrete after driving.
• Precast Concrete Piles: Durable and cost-effective, but heavy and susceptible to damage (cracking or spalling) during hard driving.
• Timber Piles: Traditional and economical for lighter loads, usually treated with preservatives.

Pile Hammers

• Drop Hammers: A heavy weight lifted by a cable and dropped. Simple but slow.
• Single-Acting Diesel/Air Hammers: Energy is used to lift the ram, and it falls by gravity.
• Double-Acting Hammers: Energy is used to lift the ram and drive it downward, increasing the blow rate and energy transfer.
• Vibratory Hammers: Uses rotating eccentric weights to vibrate the pile into the ground. Highly effective in loose sands but ineffective in stiff clays.

Pile Capacity and the ENR Formula

Historically, contractors and engineers used dynamic formulas to estimate the safe bearing capacity of a pile based on its resistance to driving (the "set" or penetration per blow). The most famous is the Engineering News Record (ENR) Formula.

While modern geotechnical engineering relies on wave equation analysis (WEAP), the ENR formula still appears in academic contexts and exams to demonstrate the relationship between hammer energy and pile capacity.

ENR Formula for Safe Bearing Capacity ($Q_a$):

$Q_a = \frac{2 \cdot E}{S + C}$

Where:

• $Q_a$ = Safe load capacity (in lbs).
• $E$ = Energy per blow of the hammer (in ft-lbs). For a drop hammer, $E = W \cdot H$ (Weight of ram $\times$ Height of fall).
• $S$ = Average penetration (set) per blow for the last few blows (in inches).
• $C$ = Empirical constant (typically 1.0 for drop hammers and 0.1 for single-acting steam/air hammers).

Note: This formula already includes an implicit Factor of Safety of 6. The result is the safe (allowable) capacity, not the ultimate capacity.

Drilled Shafts (Caissons)

Drilled shafts (also known as bored piles or caissons) are installed by excavating a cylindrical hole, placing a rebar cage, and pouring concrete. Unlike driven piles, they do not displace the soil.

Construction Methods

1. Dry Method: Used in cohesive soils above the water table where the hole will stay open on its own without caving in.
2. Casing Method: A temporary steel pipe (casing) is driven or vibrated into the ground to hold back unstable soils or water. The hole is drilled inside the casing, concrete is poured, and the casing is extracted as the concrete rises.
3. Slurry Method (Wet Method): Used when the soil is highly unstable (e.g., loose sands below the water table). A viscous fluid (bentonite or polymer slurry) is pumped into the hole during drilling. The hydrostatic pressure of the slurry keeps the hole from collapsing. Concrete is pumped to the bottom of the hole using a tremie pipe, displacing the lighter slurry upwards.

Foundation Testing

Verifying that deep foundations can support the design loads is a critical quality control step.

• Static Load Test: The most reliable method. A massive reaction frame or dead weights are placed over the pile, and hydraulic jacks apply a measured static load. Settlement is recorded. It is expensive and time-consuming.
• Pile Driving Analyzer (PDA): A dynamic testing method. Sensors (strain gauges and accelerometers) are bolted to the pile during driving. They measure force and velocity during hammer impacts. The data is analyzed (often using CAPWAP software) to determine soil resistance, hammer efficiency, and pile integrity. It is much faster and cheaper than static tests.
• Crosshole Sonic Logging (CSL): Used for drilled shafts. Tubes are cast into the concrete. A sonic transmitter is lowered down one tube and a receiver down another. The time it takes for sound to travel between the tubes indicates the quality of the concrete and detects voids or soil inclusions.