Technical Manual

Inc.

p 190

APPENDIX 2G

GUIDELINES AND RECOMMENDATIONS

Chapter 2

Helical Foundation Systems

friction angle. Laboratory direct shear tests or triaxial tests provide even more accurate estimates

of soil strength which may be warranted for large projects. The additional cost of performing these

tests could be offset by a more economical pier design that would not have been considered

using SPT results alone.

4. SPT N-values may be inconsistent for fine-grained, cohesive soils and may not accurately reflect

the soil shear strength. Tests may also be conducted on intact cohesive soil samples with pocket

penetrometers. These results can vary widely between technicians depending upon the accuracy

of the instrument and how closely the test procedure is followed. Laboratory testing of cohesive

samples collected using undisturbed sampling methods, such as Shelby tube sampling (ASTM

D1587), provides more reliable results. The more common methods for laboratory testing of

undisturbed samples of cohesive soils are the unconfined compression test, the triaxial shear

test and the direct shear test. Undrained shear strengths in excess of 4,000 pounds per square

foot (psf) are typically required to provide end-bearing resistance for push piers.

5. The presence of very loose granular soil and very soft to soft cohesive soil should be identified.

Column buckling may be a concern when very loose or soft soil is present in layer thicknesses

of more than just a few feet, especially for a deep foundation element that derives its capacity

primarily from end-bearing. Column buckling should be considered when SPT N-values are less

than 4 blows/foot. When SPT sampling indicates weight-of-hammer (WOH) or weight-of-rod

(WOR) values, additional laboratory testing for soil strength is recommended to document the

shear strength in the WOH/WOR zone. Cone penetrometer tests (CPT) completed in accordance

with ASTM D3441 may also be considered to measure in situ soil strengths. The CPT is widely

used in lieu of, or as a supplement to, the SPT. The CPT is particularly suited for soft clays, soft

silts and fine to medium sand deposits.

6. The presence of rubble fill, construction debris, or fill soils containing cobbles or boulders should

be identified. Large, hard fractions within fill soils would likely stop advancement of the push pier.

Pre-drilling may be required to allow the push pier to penetrate these soils and reach a suitable

bearing stratum.

7. When project characteristics or site conditions warrant, the Owner and his/her team of Design

Professionals may elect to determine the corrosive characteristics of the soils. Geotechnical or

environmental consultants may classify soils as corrosive based on visual review of soil samples,

from soil survey maps of the area, or from the results of additional testing. At a minimum, pH and

resistivity testing are required for a corrosion analysis. Multi-directional field resistivity testing is

preferred over laboratory resistivity testing. For a more complete corrosion analysis, chemical

analyses may be completed to determine specific concentration levels. Corrosive characteristics

of the soil should be determined from the ground surface to the bearing elevation of the deep

foundation, if practical.

Helical Piles/Anchors

1. Helical piles and anchors are advanced into the ground by the application of torque and crowd.

Crowd (or force) is applied longitudinally with the shaft to initiate penetration into the soil with

the helix bearing plates. Less crowd is typically needed after the helix plates advance a few feet

below the surface. Additional crowd may be needed in order to maintain the proper penetration

rate of about 3 inches per revolution (for a helix plate with a 3-inch pitch) through stiffer/denser

soil layers. Helical piles and anchors are best suited for medium dense sands and stiff to very