haywardbaker.com

Aggregate Pier Design &

ConstructionWentworth Institute of Technology | Beatty Hall | Boston, MA

Kevin Dawson, PE, Vice President, New England Area Manager

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Presentation Outline

What

Why

WhenHow

•Design

•Installation

Wrap Up

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What

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Vibro Piers = Aggregate Piers

• Also known by names such as GeoPier, Rammed

Aggregate Pier, Short Aggregate Pier, Compacted

Aggregate Piers, Stone Column and others

• Piers or columns of dense aggregate installed as

foundation elements to support light to medium

loads

• Often referred to as an intermediate foundation

system, i.e. not shallow but also not deep

• Usually about 24 to 42 inches in diameter and

about 10 to 30 feet deep

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Aggregate Piers: How they work

• Aggregate piers form a composite

system with the surrounding soil

• When confined laterally, the

aggregate pier is much stronger

and stiffer than the surrounding

soil

• Higher modulus columns attract

load and dissipate it gradually

with depth

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This Not This

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Why

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reduce total & differential settlement

Increase allowable bearing capacity

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When

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Selection of Ground Improvement Techniques

Soil Type Governs the Ground Improvement Technique

Mine Spoils

Loose Sand

Undocumented

Fill

Low Strength

Cohesive Soils

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Commercial & Industrial Historic Land Use

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Filled Land & Former Industrial &

Commercial Land Use

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Private Industrial Site Redevelopment

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Public Redevelopment – Filled Industrial Land

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When Summary

• Undocumented fill or loose soil less than 30’ thick

• Appropriate loads (deep foundations or alternate ground improvement technique not required)

• Less expensive alternate ground improvement techniques not viable

• Cost/schedule impact of excavation and replacement not viable

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How; Design

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Aggregate Pier Design Methodology

Design based on spring analogy:

– Rigid footing: aggregate pier deflection equals

matrix soil deflection

– Stiff spring (aggregate pier) takes more load than

the soft spring (matrix soil)

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Total Settlement = Settlement from the Improved Zone +

Settlement from the Unimproved Zone

IMPROVED ZONE

UNIMPROVED

ZONE

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Pier Testing: Modulus Verification Test

Testing done either with a load frame and

anchors or utilizing the vibratory rig as a

counterweight

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How; Installation

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Wrap Up

• Cost effective intermediate foundation solution for fill & loose native soil sites

• Know the limits of the system

• Alternate ground improvement systems or deep foundations may be best match for thick soft soil deposits or if improvement depth > 30’

• Market is extending the use of aggregate piers

• Understand the risk/reward relationship