Evaluation of Unknown Foundation Lengths

Evaluation of Unknown Foundation Length Services

Occasionally, foundation installation documentation is missing and the need to evaluate the length of an existing foundation element or sheet pile wall may occur. Depending upon the foundation type or accessibility constraints, GRL Engineers may be able to assist in a length assessment for  a foundation element or sheet pile wall using parallel seismic testing, inductance testing, or low strain integrity testing.

Parallel Seismic Testing (PST) is performed by GRL to evaluate unknown foundation lengths. A minimum two-inch diameter borehole must be drilled within 24 inches of the foundation to be tested.  The borehole must also extend well below the expected toe of the deep foundation element. Since the foundation length is unknown, the borehole termination depth must be carefully selected. A PVC pipe is inserted into the borehole. Both the PVC pipe and the surrounding borehole must be filled with water. The pile top is struck with an instrumented hammer while a hydrophone is incrementally lowered down the cased hole. The stress wave travels down the pile and outward through the soil. At each test depth, the wave arrival at the hydrophone is plotted versus time. When the hydrophone is positioned below the foundation termination depth, the wave must travel an increasingly greater distance through soil. The plot of wave arrival time versus depth will have a change in slope corresponding to the termination depth of the foundation element.

Length Inductive Test Equipment (LITE) services are provided by GRL to assess the length of unknown steel foundations . The test can be performed on steel sheet piles, H-piles, pipe piles, cased portions of drilled shafts, and in some instances highly reinforced drilled shafts. The LITE probe is lowered into a PVC cased drilled hole located within 18 inches of the foundation. The LITE detects whether metal is present or not present with the effective radius.  Data interpretation is straightforward. Provided the probe senses the proximity of metal, it will display a high voltage. Once the LITE probe is below the steel or steel reinforced foundation depth, the absence of detected metal will cause a zero or negative value which can be used to ascertain the foundation length.

Pile Integrity Testing (PIT) of concrete piles, timber piles, and drilled shafts is performed by impacting the surface of the deep foundation with a hand-held hammer and measuring the shaft response with a surface mounted accelerometer. The test results can be analyzed using either the pulse echo method or the transient response method. In a pulse-echo test result, significant positive reflections occurring prior to the time of the toe reflection are indicative of significant defects. Pile integrity testing is very economical. Many piles can be tested in a day to provide an assessment of the structural integrity of a deep foundation.