Dynamic Pile Analysis Using CAPWAP and Multiple Sensors

Abstract

A new system of multiple dynamic strain sensors and accelerometers embedded in test piles provides direct synchronous measurement of force and acceleration at various locations within a pile during high strain dynamic pile testing. This instrumentation system creates an opportunity to explore side and end bearing pile resistance distribution with a higher level of confidence and reliability than from top measurements alone. Dynamic testing results using embedded transducers are presented for a 16-inch Auger Pressure Grouted Displacement (APGD) pile. The procedure is explained to estimate static side and end bearing components by matching force and/or acceleration data at the pile top and tip using the CAPWAP program. Static load and displacement characteristics at the pile top predicted by CAPWAP are compared to static pile load test results. A Pile Driving Analyzer (PDA) collected pile top strain and acceleration measurements during the high strain dynamic testing. The PDA immediately converts these strain and acceleration signals to force and velocity records which yield the dynamic compression stresses and the tension along the shaft, to determine if a higher energy drop can be performed without exceeding the recommended material stress levels. In this manner, dynamic testing was performed utilizing GRL's APPLE 15 ton drop hammer system at multiple drops heights varying between 23 inches and 36 inches, until a significant set (0.25 in) was obtained. In addition to PDA instrumentation, strain and acceleration signals were captured by two digital transducer modules embedded in the test pile near the pile top and toe. Each transducer module included a microprocessor, signal conditioning circuitry, a 16-bit analog-to-digital conversion circuit, and Controller Area Network (CAN) hardware. Each analog signal was digitized by the transducer module circuitry at a rate of 7,750 samples per second. The total number of strain and acceleration points is defined by the user and can range up to 8,000 samples per signal. Prior to conducting the high strain dynamic test, both transducers were placed into a dynamic testing mode by issuing a command via a field laptop computer. Samples were then acquired continuously by each transducer module, and every sample was compared to a user defined trigger level. Once a sample exceeded the specified threshold value, each transducer module reserved a user specified number of pre-trigger samples in SRAM and continued sampling until the user specified total number of samples was obtained. At the completion of sampling, data from each transducer module was transferred to a central controller via the CAN data bus where it was then sent to a laptop computer and stored to disk. Figure 1 presents a picture of the high strain testing setup and instrumentation.