Static Load Testing of Driven Piles
Driven pile test programs may include static load testing, which involves incrementally loading a pile while measuring (at a minimum) pile deflection. Static loads may be applied in axial compression (from the top down, or from the bottom up), in axial tension, or laterally. The most-common type of static load test is top-down axial compression. It is worth emphasizing that the main objective of static testing is to characterize soil behavior. In pile design, this is where the majority of uncertainty exists, as pile behavior can be predicted reasonable well. Static compression and tension load test piles may be instrumented to obtain load-transfer information (i.e., shaft resistance distribution) to correlate with static analyses and/or dynamic testing results, characterize set-up, estimate shaft resistance in scour, down force, or liquefiable zones, etc. Static lateral load test piles may be instrumented to obtain deflected-shape information to aid in refining lateral pile design software input parameters.
Frequently, static load testing is performed to obtain information pertinent to design and/or construction, and as such, can be requisite for good engineering practice. Since static testing carries a quantifiable up-front cost (of which all interested parties seem painfully aware), potential cost-saving aspects of testing can be overlooked. This includes static testing during design. Design-phase (pre-construction) static testing can confirm the driveability and load-carrying ability of several pile types, sizes, or lengths. It can also characterize soil/pile set-up to aid in producing the most-economical foundation design. Running static load tests to geotechnical plunging failure to determine the soil profile’s ultimate capacity (as opposed to running a proof test to some multiple of the design load) can result in a more-economical design. This is because all of the soil’s available resistance is utilized, as opposed to only part of it.
Appreciating another cost-effective aspect of static testing requires recognizing the inter-related design aspects of construction-control method, safety factor, allowable pile load, the resulting required number of piles and pile-cap size, and how all these factors affect total foundation cost. Because of their increased certainty of capacity determination, static load tests often allow the use of a reduced safety factor. This means that a pile with a given ultimate capacity can be assigned a higher allowable load, with fewer piles required, a reduction in pile-cap sizes, and a decrease in total foundation costs (including testing costs).
This concept is illustrated in the table attached. A review of the shaded portions of the table indicates that although a static load test may be the most-expensive construction-control method, it can result in the lowest overall foundation cost.
For more information about cost-effective design of deep foundations (including webinars and technical papers), visit wkg2.com, or email Van E. Komurka at komurka@wkg2.com.
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