© 2014 Foundation Supportworks
®
,
Inc.
All Rights Reserved
p 189
APPENDIX 2G
GUIDELINES AND RECOMMENDATIONS
Chapter 2
Helical Foundation Systems
GEOTECHNICAL INVESTIGATION GUIDELINES FOR
HELICAL PILE, HELICAL ANCHOR AND PUSH PIER DESIGN
Design professionals rely on site-specific geotechnical investigations to provide soil strength
parameters for use in foundation design. However, when these investigations do not properly identify
a suitable bearing stratum, the project may be impacted with additional costs or delays until such
adequate information is obtained. At the very least, contractors left to bid on a project with little to
no usable soil information will do so conservatively. These bid proposals are then often filled with
clauses outlining potential change order items; e.g., additional footage, revisions to the shaft section,
revisions to the helix plate configuration, costs and responsibility due to failed load tests, etc. These
extras often add up to many times the cost to simply complete deep soil borings and obtain the
necessary soil information.
This document is not intended for incorporation into bid specifications, proposals or requests for
proposals. Ultimately, it is the role of the Owner and his/her team of Design Professionals and
Contractors to determine the scope of geotechnical investigation necessary based on considerations
that may include structure size and type, structure design category, anticipated design loads,
acceptable levels of risk and/or available funding.
This document is provided as a guide to assist Design Professionals, Contractors and Owners when
helical piles, helical anchors, or push piers are planned. Due to differences in design, fabrication,
and performance of seemingly equivalent products between manufacturers, these guidelines are
intended exclusively for products designed and manufactured by Foundation Supportworks
®
, Inc.
(FSI).
Hydraulically-Driven Push Piers
1. Push piers are advanced into the ground with hydraulic equipment exerting a steady but high
downward force at the top of the pier. With most push pier systems using 4-inch O.D. shafts or
smaller, this translates to high contact pressures with the soil. In order to provide competent
bearing and to also minimize the potential for the push pier to punch through a seemingly
competent but thin layer of soil, a minimum thickness of 10 to 15 feet of hard/dense material
should be identified or be known to exist. This material may consist of hard clays, dense to very
dense sands, or competent bedrock. The required thickness and strength of this layer would
increase with an increase in the required pier capacity.
2. The Standard Penetration Test (SPT) completed in accordance with ASTM D1586 is a common
method of retrieving disturbed soil samples in the field while also providing correlations to several
soil strength parameters. The SPT is performed by driving a 2-inch O.D. split barrel sampler 18
inches with a 140 pound hammer falling a distance of 30 inches. The number of blows required to
drive the sampler the final 12 inches is recorded as the standard penetration number, or N-value.
Typically, N-values of 35 to 40 blows per foot for clay soils and 30 to 35 blows per foot for sand
are required to provide end-bearing resistance for push piers. If the loading is extremely light or
the piers are long enough to develop significant skin friction, lower end-bearing resistance may
be required. However, these guidelines are good rules of thumb for most installations.
3. The standard penetration test provides a reasonable indication of strength and density of granular
soils with correlations available to relate SPT N-values to relative density, unit weight and internal
