Technical Manual

Inc.

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