Dynamic Testing of Driven Piles
Dynamic testing of driven piles is a valuable, cost-effective tool for the design and construction control of foundation projects which utilize driven piling. Testing is performed during the pile driving process, and consists of attaching strain gages and accelerometers to a pile near its head. The instrumentation system continuously records measured strains and accelerations in a pile, which are induced downward in the pile by high-strain impacts from each hammer blow, and are reflected upward in the pile by soil or rock resistance, or changes in pile impedance (changes in cross-section, or from the pile toe). The data collected by dynamic testing can be viewed in real time during driving, and for each hammer blow can provide such information as:
- Transferred hammer energy – This provides important confirmation that the hammer is performing at least as well as was assumed when the driving criteria were developed. On larger projects, periodic monitoring during production driving can indicate changes in hammer performance due to hammer-cushion degradation, maintenance issues or other wear, etc.
- Pile-driving stresses – Compression and tension stresses can be monitored to reduce the risk of pile damage, and/or to provide the basis to select a more-economical pile section.
- Estimated capacity – In real time, dynamic monitoring can estimate a pile’s total compression capacity, and provide a rough estimate of capacity distribution between the pile’s shaft and toe.
- Pile integrity – Pile damage is indicated by signature changes in the reflected waveform.
Among other things, subsequent signal-matching analysis of a single recorded blow can yield:
- Capacity distribution estimation – Not only the total pile capacity, but also the shaft and toe resistances, and shaft resistance distribution, can be estimated. This is useful when evaluations require estimating shaft resistance for specific shaft sections (such as when accounting for set-up, scour, downdrag, tension loads, seismic events, etc. during design).
- Estimation of pile driving stresses at any point along the pile – This is especially important for relatively short piles driven to a hard, end-bearing layer, as stresses near the toe can significantly exceed those measured at the gage level, and increase the risk of pile damage.
- Refined estimates of dynamic soil properties – Field-testing-indicated dynamic soil properties can be used in refined wave-equation analyses, providing more-realistic driving criteria.
In addition to scheduled periodic monitoring during project driving, dynamic testing can also be used to troubleshoot and remediate unexpected driving behavior (e.g., pile lengths varying significantly from estimates, or unexpected blow counts). This is done to determine whether the behavior is related to subsurface conditions, hammer performance, construction practices, etc.
Past posts related to driven piles and deep foundations:
- Support Cost: Selecting Cost-Effective Systems for Deep Foundations
- Driven-Pile Economics: Support Cost Based on Available Support
- Improved Pile Design: Load Matching Evaluation (Part 1)
- Improved Pile Design: Load Matching Evaluation (Part 2)