Foundation Investigations

Foundation investigation

Test Methods

Armor Tower, Inc. engineers are highly experienced at performing foundation investigations. Here's a summary of the test methods used when Armor Tower conducts a foundation investigation:

Sonic Echo / Impulse Response Test

The Sonic Echo/Impulse Response (SE/IR) test is typically used on caisson type foundations for caisson length determination. This method requires that some portion of the top or upper sides of foundation be exposed and smooth so that the receiver can be attached. This requirement is for source and receiver attachments, which are used to generate and measure the wave energy that travels down the foundation and reflects back from the bottom depth of the foundation and or any discontinuity along the length of the foundation.

Clear echoes from the bottom depth with no shallower reflections would indicate the foundation length based on the reflection time, and also indicate that the foundation is sound with no defects and voids. The maximum testable length for foundations is typically 30 times length of the foundation diameter (i.e. a 2-foot diameter foundation is testable out to about 60 feet of embedded length maximum).

A wave velocity of 12,500 ft/sec, which is an average velocity of waves in concrete, is used to determine the depth of the caisson foundation. The SE/IR test method provides a depth accuracy of about 5-10% and relies on reflections from a change of impedance between two interfaces.

If there are significant impedance changes in a pier foundation, they are manifested as shallower echoes in the SE/IR results, otherwise the reflections that are present in the records would be from the bottom of the pier (caisson). Depending on the velocity contrast between the contact and the bedrock and the penetrations into bedrock and bedrock-like soils, the bottom echo reflections may be weak in the SE/IR records.

Parallel Seismic Test

The Parallel Seismic (PS) method requires the use of a cased borehole located within a few feet of the structure to be tested. The ideal case is a borehole that is parallel to the structure and located only 2-3 feet away from it.

The borehole is drilled to a depth of approximately 10-15 feet deeper than the expected depth of the structure. The borehole must be cased, and the internal diameter of the casing should be no smaller than 2.0 inches. The casing then is grouted in accordance to the appropriate portions of ASTM 4428. The bottom of the casing is capped, and the casing is filled with water. (Typically a 2 inch ID PVC pipe is used for this purpose).

The testing is performed using 24-channel seismograph (Geometrics Geode) coupled to a 24 component hydrophone string. The seismograph is microprocessor-controlled and records data digitally.

The PS test requires hitting the exposed portion of the structure (it could be a helical pier or an embedded pole) while monitoring response at many locations in a borehole. The time of transit of a sound wave is the basic measurement - and the path of the sound wave is from the top of the pile cap, thorough the pile cap, along the pile, through the soil between the pile and the hydrophone, to the hydrophone. The source of the sound wave remains at the top of the pile cap, and the hydrophone assembly is set at successively greater depths. The measurement resolution for this method is typically about one to two feet.

Foundation Estimates

When no construction information is available for review, reasonable estimates of as-built conditions may be developed. Soil boring and downhole metal detectors are used to locate the size of the footing and to estimate for reinforcing steel.

For more information on Foundation Investigation or to get a quote, please contact us.