Pile Testing Verification – an Alternative Approach

Dynamic pile testing is undertaken for a number of reasons including: 1. To confirm that the pile meets serviceability and geotechnical capacity requirements; 2. To assess pile integrity, either during installation (driven piles) or after construction (cast-insitu piles) and 3. To verify that the piling hammer delivers the energy required to satisfy the design criteria and that stresses during testing are kept within acceptable limits. In addition, testing allows us to establish and calibrate acceptance criteria – relationships such as resistance vs set curves, and/or correlated pile driving formulas. These relationships are premised on the interrelationship of capacity (C), transferred energy (E) and pile movement (M) which is represented primarily by pile set. These ECM relationships allow capacity to be inferred from measurement of transferred energy and pile movement and are used to infer the capacity of untested piles. However, for a variety of reasons, transferred energy can vary significantly between piles which, being undetected, undermines the reliability of ECM relationships. An alternative approach to using ECM relationships is proposed based on pile set and pile force (F). We demonstrate through parametric studies and review of project data, that these FCM relationships are reliable alternatives which bypass the problems with variable energy transfer. Of course, impact force will also vary with hammer performance, but impact force can generally be accurately determined from the measured impact velocity as a proxy. Pile velocity can be measured by attaching a single accelerometer to the pile, or by using a high frequency displacement monitoring device. FCM-based acceptance criteria have the significant advantage that both the necessary force (F) and displacement (M) inputs can be verified by simple measurements on all untested piles.