MICROPILES

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Recent Advances in the Selection and Use of Drilled Foundations

Dan Brown1, M.ASCE, Ph.D., P.E., DGE

1President, Dan Brown and Associates, PLLC, 300 Woodland Road, Sequatchie, TN 37374;

Phone: [phone]; [e-mail]

ABSTRACT: The construction and design of drilled foundations in recent years has

been most significantly affected by developments in drilling techniques related to

materials, equipment, and generally improved capabilities in construction. In

addition, advancements in technology for testing and QC/QA have resulted in

improvements in performance, reliability and design. This paper describes some of

the most significant developments affecting drilled foundations, including large

diameter drilled shafts, continuous flight auger piles, drilled displacement piles, and

small diameter micropiles.

INTRODUCTION

The drilled foundation options available in current practice include an incredible

range of available technology, from very small diameter micropiles only a few inches

in diameter to large drilled shafts that may be as large as 4m (13ft) in diameter.

These foundations share a common feature in that the foundation is constructed by

drilling a hole into the bearing formation and constructing the foundation into that

hole by placing a cementitious material such as grout or concrete. This critical part of

the structure is thus cast in-situ rather than prefabricated and installed into the ground

as with a driven pile. (note: although helical anchors might also be considered

“drilled” foundations, these will not be included as this paper already covers enough

ground just dealing with cast-in-place drilled foundations!).

The differences between types of drilled foundations relate mostly to the method

of installation and how the casting operation for the pile is completed, although this

relates directly to the equipment used to construct the foundation. This paper will

discuss the state of practice of drilled foundations in a way that is consistent with

each particular drilled foundation type, as follows.

Micropiles are most often 30cm (12in) or less in diameter and often selected for

use because of the advantages provided by the lightweight and maneuverable

equipment available to install these piles. The distinguishing feature from a design

perspective is that the pile itself is typically designed as a steel member such as a bar

or tube which is bonded to the bearing stratum with a cement grout. Micropiles are

most effectively used where the bearing materials allow effective utilization of the

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high strength of the steel. Granular soils or rock often provide suitable bearing

formations and micropiles are often used in rock or in highly variable conditions

where difficult drilling may be encountered.

Continuous Flight Auger Piles (CFA piles) are typically 30 to 100 cm (12 to 40

in) diameter and most often selected for use because of the advantages provided by

the speed and cost-effectiveness of the installation method and equipment. Often

called “augered cast-in-place (ACIP)” or “augercast” piles in U.S. practice, the

distinguishing feature of the construction of these piles is the fact that the concrete

(sometimes a sand-cement mix) is placed through the hollow center of the continuous

flight auger drill string as the augers are withdrawn and then the reinforcement is

placed into the wet fluid mix after the casting operation is complete. The pile is thus

a reinforced concrete structural element and designed accordingly. CFA piles are

usually most cost effective when used at lengths of 10 to 30 m (30 to 100 ft) and

constructed entirely in soils, although occasionally these piles are used in weak rocks.

Because of the speed with which the pile can be drilled and completed, it is not

uncommon for a constructor to install several piles within a single hour of work.

Drilled Displacement Piles are constructed using a technique similar to CFA piles,

but using tooling and more powerful equipment such that the drill tool is advanced

while displacing the soil to form the hole rather than extracting the soil. These piles

provide the obvious advantages that ground improvement is achieved during

installation in some types of soils, and the handling and removal of spoils (which may

include contaminants in some situations) is avoided. With the controls and

monitoring equipment available on modern drill rigs used for these piles, there has

also been progress in relating the torque and crowd pressures to the stratigraphy so

that the performance of a specific pile can be related to installation measurements.

Drilled Shafts are most often 1 m (3ft) or more in diameter and constructed by

excavating