Advanced diagnostics and inspection of common foundation elements in Hong Kong using ultrasonic non-destructive testing and evaluation techniques

Abstract

Non-Destructive Evaluation (NDE) is essential in modern foundation testing. The advantages of NDE are no need to destroy what has been constructed and change the integrity (i.e. material properties) of foundation elements. This study aims to evaluate and inspect common foundation elements in Hong Kong using the ultrasonic NDE techniques. This aim was achieved by the applications of two ultrasonic-based non-destructive testing techniques (i.e. the Ultrasonic Echo Sounding (UES) technique and the Cross-hole Sonic Logging (CSL) technique). The scopes of this study are shown as follows: > Establishment of the in-house developed calibration and verification procedures to verify the real performance and behavior of UES equipment > Investigation of the effects of different sonic access tube materials on the signal strength of ultrasonic wave transmission > Demonstration of the applicability of the UES and CSL testing techniques by performing extensive field investigations > Development of an in-house multi-channel ultrasonic data acquisition prototype to provide an efficient means of performing sonic tests > Assessment of concrete specimens with sonic-tube-installation by the in-house developed prototype > Identification of soil strata by the in-house developed prototype An extensive literature review was performed at the beginning stage of the study to acquire background knowledge about the ultrasonic stress wave propagation principles and the basis (including the theoretical basis, testing procedures and methodologies, data interpretation techniques and limitations) of UES and CSL. Literature review was also performed in the area of ultrasonic wave propagation in soil related to this study. Following the comprehensive review, the in-house calibration and verification procedures were established to ascertain the real performance and behavior of the UES equipment under different testing conditions, as no report regarding the determination of the working performance of UES equipment has yet been published. The calibration work detailed an account of the efforts in the range and depth concerns that shed some lights on the performance of the UES equipment in different working situations. With the expression of calibration uncertainties, the reliability of different measurement techniques was assessed. By the experimental results in the depth calibration, the measuring tape approach was more accurate than the theodolites approach in terms of the value of the calibration uncertainty obtained. In the verification work, the effects of bentonite/slurry solutions, angles of reflection (i.e. simulation of 'bell-out' at a shaft toe) and roughness of reflecting surfaces on the measured ranges (obtained from UES profiles) of a shaft were quantified. By carrying out the extensive tests within the HKSAR territories in this research, UES was proven as a reliable and useful technique to provide an early stage diagnosis and inspection of bentonite/slurry or water-filled shafts (including shaft verticality, shaft diameter, shaft depth, and geometry and dimension of 'bell-out' at a shaft toe) before the concreting stage. Examples of acceptable/slanted-drilled shafts were reported. The application of the UES technique identifies the suspected drilled shafts which can be taken into remediation works at an early stage of construction. The benefits and results are clear improvements in the costs incurred and time taken during the construction project. The need to check deep foundations before and after the concreting stage is equally important. Another focus of this research was placed on the Cross-hole Sonic Logging (CSL) technique. It was developed in the late 1960's and has been used extensively to inspect the integrity of in-situ cast concrete foundation elements after the concreting stage. The applications of UES and CSL provide a better picture regarding to the quality assurance of deep foundation concrete elements since UES and CSL are conducted on foundation elements respectively before and after the concreting stage. A series of tests were conducted to determine the effects of different sonic access tube materials (including polyvinyl chloride (PVC) and steel) on the signal strength of ultrasonic waves. With the development of the in-house developed prototype, the apparent transmission velocity and signal strength of the acquired waveforms were determined by the use of different sonic access tubes. A set of experimental results showed that the signal strength when using PVC sonic access tubes are clearly larger than that when using steel tubes due to the smaller difference in the acoustic impedance value between PVC and water. The success of the applicability of CSL was achieved by the results obtained from the extensive tests carried out (within the HKSAR territories) and examples also shown in this thesis. The results showed that CSL provides the precious information/feedback regarding the construction process of the final finished products (i.e. foundation elements). Questionable/suspected defects (like honeycombing, voids or foreign materials embedded in concrete elements) found were reported and discussed thoroughly and classified into several categories of defects. An adaptive, versatile and relatively low cost prototype was developed and constructed especially for deep foundation concrete elements after installation. This prototype was labelled the 'Multiple Referencing System' (MRS) which was developed based on National Instruments hardware and a LabVIEW (Laboratory Virtual Instrument Engineering Workbench) environment. This prototype is capable of acquiring two or more channels of signals simultaneously in performing a sonic test in a bored pile with sonic-tube-arrangement to save time and cost. A purpose-developed analysis graphical programming (based on the digital virtual instrument) of the in-house prototype MRS was constructed to analyze the signals in different parameters (i.e. signal strength in a time domain and spectral energy content in a frequency domain) in addition to a traditional parameter of first-arrival-time/apparent transmission velocity in order to increase the reliability of the testing method in evaluating test materials. The successful application of the in-house developed MRS in the concrete assessment was demonstrated by carrying out a sonic test in the testing concrete block with artificial defects. The signals acquired in two channels were displayed simultaneously during the measurement. The ultrasonic parameters (including first-arrival-time, transmission velocity, signal strength and spectral energy content of the signals) were investigated and found to be correlated to the properties of the materials. In addition to the concrete assessment, the in-house developed prototype MRS was transferred as a geophysical technique to identify different soil strata. Variation of ultrasonic transmission velocity with degree of saturation and surcharge pressure were analyzed. A 'novel' method, the 'X-factor' technique, was developed to classify soil strata. According to the experimental results, it was found that the 'X-factor' technique has a potential of identifying soil strata and the value of 'X-factor' is correlated to the attenuation of test materials. Summary and conclusions of this research, fulfillment of the research objectives, contributions to the knowledge of NDE foundation testing, and recommendations for future research were presented.