Driven Concrete Pile Foundation Monitoring with Embedded Data Collector System
Florida Department of Transportation (FDOT) sponsored research performed at the University of Florida resulted in developing a wireless monitoring and real time static capacity estimate technology for driven piles. This new technology, Embedded Data Collector (EDC), uses two levels of instrumentation, embedded in the body of precast prestressed concrete piles near the head and tip. Strain and acceleration measurements obtained at these instrumentation levels during driving are sent wirelessly to a receiver in the field, and analyzed in real time to provide the operator with estimates of static capacity, stresses in the pile, transfer energy, damping factor, stroke height, and other relevant parameters used to evaluate the pile driving process and the driving system. The EDC system is currently undergoing phase one of a two phase field evaluation program planned by the FDOT to determine the level of reliability that can be anticipated by its use. The first phase consists of monitoring piles with EDC instrumentation and concurrently monitoring them with the Pile Driving Analyzer (PDA), given that there is ample data supporting the reliability of PDA. Measurements of strain and particle acceleration converted to force and velocity traces can then be compared between the two systems, along with the corresponding calculated magnitude of downward and upward traveling stress waves as they move along the pile at any point in time. Selected hammer blows recorded by PDA equipment are analyzed by means of signal matching software (CAPWAP) and the estimated static skin, end bearing, and total resistance obtained are compared against EDC static resistance predictions. The second phase of the evaluation will compare EDC estimates of static capacity against instrumented static load test results. The purpose of this paper, is only to present a summary of results obtained thus far in phase one, and compares EDC estimates of static capacity results with those from PDA and CAPWAP.
Determination of Pile Damage in Concrete Piles
Abstract As the number of driven concrete piles with Embedded Data Collectors increases, it has become possible to compare the widely used Beta Method to assess pile damage (using the change of pile impedance) with the so-called Measured Pile Integrity (MPI) method (a combination of two approaches using both the change of pile impedance and the change in pile pre-stress levels). After a brief introduction of both methods, this paper presents the outcome of that comparison based on data from more than 400 concrete piles driven in Florida, which brings into question the reliability of any damage assessment method based solely on the change of pile impedance (such as the Beta Method), especially to detect damage near the pile toe. Moreover the paper demonstrates the need to re-assess the damage classification used for the Beta Method as well as the use of Pile Driving Analysis (PDA) to detect pile toe damage.
Soil Resistance Predictions From Pile Dynamics
The present work extends the application of the force and acceleration records to the calculation of the distribution of soil resistance along the pile. It also shows how the records are used to predict the magnitude of dynamic resistance that the soil applies to the pile, an important factor in choosing efficient hammer characteristics. A method for obtaining a more accurate simplified prediction of total static bearing capacity is also presented. It should be emphasized that the aforementioned predictions are all made from measurements at the pile top only. The work is correlated by presentation of results for 24 pile tests which include construction static load tests as well as specially instrumented load test piles. The application of these results can have considerable impact on foundation costs. Static load tests are very costly and time consuming. In Ohio a single test on a service pile using tension reaction piles (also service piles) typically costs $3,000 to $5,000. This static test provides much useful information about the particular pile which was tested. However, due to variability of soil properties the information may be of less value for other piles in the structure. This is reflected in the large factors of safety commonly used for piles. The proposed dynamic measurements methods can be applied to a substantial number of service piles at less than the cost of a single static load test. The dynamic analysis herein differs from the general dynamics problem in which either the boundary force or acceleration record is given as input and the other record calculated as output. In the present dynamic analysis, both force and acceleration are shown and thus one of the two records can be viewed as redundant information. The second record is, therefore, used in the present analysis to give information on pile resistance effects; e.g., in the absence of soil resistance, the acceleration at the pile top completely determines the force at the top from Newton's and Hooke's laws. The presence of resistance along the pile and at the pile tip affects the force at the top in a precise and predictable manner and makes it possible to compute t e magnitude and location of resistance forces along the pile. A simple soil resistance model is used, which consists of an elastoplastic shear resistance and a linear viscous damper. The dynamic analysis will be reviewed herein to present the basic ideas behind the work. The detailed mathematical expressions are given in Refs. 3 and 9.