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Pile Driving Analyzer (PDA)

Depending on the requirements of the project, dynamic pile testing in today’s high foundation loads require modern construction control. The Pile Driving Analyzer (PDA) helps to quickly evaluate the integrity and bearing capacity of the entire pile installation.  It improves quality control, quality assurance for driven piles.

HRES has monitored several piles dynamically, using a PAK pile driving analyzer to evaluate the actual hammer/piling system performance, pile driving stresses, energy transfer to the pile, pile structural integrity, pile capacity, and soil behavior.  Dynamic measurements of strain and acceleration will be taken near the top of each pile using strain transducers and piezoelectric accelerometers that bolt to the pile and connect to the PDA.  HRES is well-experienced and thoroughly familiar with field connections and monitoring of both pre-stressed concrete and steel pipe piles.

The PDA converts pile strains to forces and accelerations to velocities as a function of time for each hammer blow.  Using wave propagation theory, dynamic variables, such as maximum pile driving forces, maximum pile top hammer transferred energy, and ultimate pile static capacity, are computed and available for display and printing for each hammer blow during driving.  Excessive driving stresses are a frequent cause of pile failure and would typically be corrected by various methods including temporarily reducing hammer energy, increasing pile cushion thickness etc.

If pile damage is suspected during the handling or installation of any test or production piles, HRES can perform low-strain integrity testing using a Pile Integrity Test (PIT) Collector System to further assess the potential damage.

CAPWAP Analyses

The Case Wave Analysis Program version (CAPWAP) is an analytical procedure performed using a micro-computer that was developed by GRL & Associates to compute soil resistance forces and their distributions using pile top force and velocity measurements recorded in the field (Rausche, 1970).

CAPWAP analyses will be performed by HRES using PDA data from selected hammer blows.  Results of these analyses will be used to estimate ultimate static pile capacity, along with soil quake and damping parameters and soil resistance distributions to be used in subsequent final driving wave equation analyses.  If PDA data indicates that time dependent soil strength changes are affecting pile capacity, a re-strike blow will also be chosen for CAPWAP analyses.  It is important to choose a blow from early in the re-strike that exhibits a high-energy blow so as to yield maximum information regarding soil resistance and long-term pile static capacity.

The method of superposition may be used when analyzing end of drive and beginning of re-strike dynamic pile data according to the CAPWAP method.  In cases of very high (more than 15 blows per inch) blow counts, end bearing from end of drive analysis may be added to skin friction from beginning of re-strike analysis to obtain a total and a long-term pile capacity.  This approach, of course, assumes that end bearing does not significantly change with time.

Final Wave Equation Analysis and Pile Driving Criteria

Based on the results of the CAPWAP analysis, final wave equation analysis (WEAP) may be performed.  HRES will provide graphs portraying ultimate pile capacity and compressive and tensile stresses as a function of number of hammer blows.