Pile Driving Measurements On The Heather Platform Installation

Abstract The first leg piles of the Heather Platform were instrumented using strain transducers and accelerometers that were attached to the pile just below its top. The signals were processed in real time providing for a hammer blow a printed output of hammer energy in the pile, maximum pile top force and other quantities. The signals were also recorded on analog magnetic tape. A detailed analysis was performed in the laboratory on a digital computer. Using wave equation type pile and soil models and both force and acceleration data, the soil resistance distribution was determined. The Heather Platform piles were drive through two followers connected by gravity. Stress wave effects from the connectors were studied and losses to the impact wave were determined. Introduction The first piles driven at a new installation site provide long awaited data regarding the driveability and pile bearing capacity. Ram stroke and blow count, however, may be insufficient observations when pile driving does not proceed as expected. Then the determination of dynamic quantities such as pile stresses, hammer energy output, and soil resistance may be extremely valuable for decision making. For this reason Union oil Company of Great Britain contracted with the writer's firm for providing both dynamic measurement and data processing services during the initial pile driving operation at the Heather A Platform. The measurements consisted of both pile top force and acceleration. They were field processed by a Pile Driving Analyzer. This unit printed for every blow maxima of pile top force, energy, and velocity among other quantities. Also, the pile bearing capacity was determined by the Case Method (1). The pile driving operation was rather successful; i.e., blow counts were as low as optimistic predictions. In fact, it was surprising that the blow count did not increase with depth, a characteristic behavior of friction piles. Other questions such as the efficiency of gravity connectors called for answers. Thus, a laboratory analysis of the field data was warranted. Experimental and analytical methods, results, and conclusions are described here. Results are presented in the SI unit system except for direct field observations penetrations in feet, blow counts in blows per foot. Background General Remarks Both instrumentation and processing methods were developed at Case Western Reserve University in Cleveland, Ohio under a grant from the Ohio Department of Transportation. The system is described in more detail in References 1 and 2; however, a short description follows. Instrumentation Two each accelerometers and strain transducers were attached on opposite sides of the pile for cancellation of gross bending. The accelerometers were piezoelectric with built-in amplifiers. The strain transducers were diamond-shaped, flexible aluminum frame of 3 inch (7.62 mm) gage length. Resistance strain gages were attached in points of stress concentration on these frames and formed a full Wheatstone bridge. Figure 1 shows schematically the field measurement and processing system. The signals of the two accelerometers (a) and strain transducers (b), all attached to the pile (c) by means of quarter inch bolts, were led through a combination box (d) and signal cable (e) to a Pile Driving Analyzer (f). This unit conditioned, amplified, calibrated, and integrated the signals and then determined the following values in analog form:Maxima of force, velocity, and acceleration.The peak value of the energy transferred into the pile.The pile bearing capacity according to the Case Method (see next section). General Remarks Both instrumentation and processing methods were developed at Case Western Reserve University in Cleveland, Ohio under a grant from the Ohio Department of Transportation. The system is described in more detail in References 1 and 2; however, a short description follows. Instrumentation Two each accelerometers and strain transducers were attached on opposite sides of the pile for cancellation of gross bending. The accelerometers were piezoelectric with built-in amplifiers. The strain transducers were diamond-shaped, flexible aluminum frame of 3 inch (7.62 mm) gage length. Resistance strain gages were attached in points of stress concentration on these frames and formed a full Wheatstone bridge. Figure 1 shows schematically the field measurement and processing system. The signals of the two accelerometers (a) and strain transducers (b), all attached to the pile (c) by means of quarter inch bolts, were led through a combination box (d) and signal cable (e) to a Pile Driving Analyzer (f). This unit conditioned, amplified, calibrated, and integrated the signals and then determined the following values in analog form:Maxima of force, velocity, and acceleration.The peak value of the energy transferred into the pile.The pile bearing capacity according to the Case Method (see next section).