By: Don Carter, MS, P.E.
AUGER OR CONCRETE PIERS -- A hole is drilled into the ground, and then the cavity is filled with concrete.
FOR
1. Materials are cheaper.
2. Design flexibility - can be made any diameter, any spacing.
AGAINST
1. Labor and equipment intensive.
2. Requires big, cumbersome motorized equipment that damages landscaping.
3. Large soil loss with disposal.
4. Limited access - trees, shrubs, lamps, soft soil, etc.
5. Requires geotechnical evaluation to be done right.
6. Pier’s own weight can oppose benefit - forces down cancel forces up unless bedded on rock.
7. Can’t preload the existing structure.
HELIX PIERS -- A screw anchor in the ground, much like a lag bolt is into wood.
FOR
1. Works well in confined space.
2. Proof tested during installation via torque.
3. Design material availability – solid square shaft, solid round, tubular etc.
4. Driven independent of structure therefore ¬won’t harm a fragile foundation during install.
AGAINST
1. High material costs.
2. Specifying engineers often not familiar with product or concept.
3. Industry standard shafts don’t accommodate head eccentricity well in heavier loads.
4. Slenderness ratio acceptability.
5. Eccentric load transfer
RESISTANCE (PUSH) PIERS -- A steel shaft pressed into ground to end bearing strata.
FOR
1. Works well in confined space.
2. Measurable capacity during installation.
3. Design flexibility.
4. Preloaded by structure that it will ultimately support, including overburden.
AGAINST
1. Most copied, therefore most abused by amateurs.
2. Might lift a light load before hitting end bearing strata.
SEGMENTED CONCRETE CYLINDER PIERS – Cable connected concrete cylinders pressed into the ground to refusal.
FOR
1. Low cost.
AGAINST
1. Skin friction produces high resistance that is mistaken for end bearing.
2. Pier is topped with bearing blocks that are under reinforced and often fail.
3. Installers do not use manifolded jacks thus lifting is thus uneven.
4. Lifting is not targeted to a predefined elevation.
5. Installation history is based on Sunbelt soils and houses with no basements.
6. Uses Bottle Jacks for driving and lifting.
GENERAL COMMENTS
• Of those piers that fail, most fail at the bracket connection.
• Some soils are stiff and can trick the installer into believing he has hit end bearing when it’s actually skin friction. Better piers use a friction collar.
• Best warranty is two-part. One from the manufacturer one from the installer.
• Warranty only as good as ability to invoke. Check time in business and experience with local soils. Many foundation companies in today’s yellow pages were not there last year and many will not be next year.
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