Examination of a New Cross-Hole Sonic Logging System for ...

1 Examination of a New Cross-Hole Sonic Logging System for Integrity Testing of Drilled Shafts Samuel G. Paikowsky Geotechnical Engineering Research Laboratory, University of Massachusetts Lowell 1 University Avenue, Lowell, Massachusetts, 01854, USA. Les R. Chernauskas and Leo J. Hart Geosciences Testing and Research Inc. 55 Middlesex Street, Suite 225, North Chelmsford, Massachusetts, 01863, USA. Carl D. Ealy and Albert F. DiMillio Federal Highway Administration 6300 Georgetown Pike, McLean, Virginia, 22101, USA. ABSTRACT: Drilled shafts and other mixed or cast-in-place concrete deep foundation elements can be costly solutions. These foundations usually carry very high design loads, and often serve as a non-redundant, single load-carrying unit. These conditions have created a need for a high-level of quality assurance and con- trol applied to each in-place constructed deep foundation element.

2 The non-destructive testing method, Cross-Hole Sonic Logging (CSL), currently offers the most reliable technique for assessing the integrity of in-place constructed deep foundation elements. Recent years have seen progress in CSL instrumentation, taking advantage of the available computer technology. The software applications, however, have greatly fallen behind, thereby limiting the effectiveness and potential of the CSL. method and deep foundations integrity testing in general. A new, original CSL testing System by the name of PISA (Pile Integrity Sonic Analyzer) makes use of an innovative software and data acquisition System , hence representing the state-of-the-art in deep foundation in- tegrity testing. The PISA has the capability to show real-time graphical information during Logging , includ- ing planar tomography, which can identify the boundaries of a compromised zone within the foundation ele- ment.

3 The equipment operates completely in a Windows graphical environment allowing alphanumeric and graphical reports to be generated directly into word processing software. The real-time graphical representa- tion during Logging and the ease of reporting enables immediate, extensive on-site evaluation and decision- making. The PISA System was evaluated on different construction sites. The case history presented in this paper re- lates to a class A' prediction as tests were carried out on shafts in which defects were intentionally planted. The test results were submitted before the defects locations were known, both presented I the paper. The ob- tained results demonstrate the ease of use, accuracy of measurements and enhanced capabilities of the PISA. The systems' abilities are shown to be superior to any other currently available commercial System .

4 1 INTRODUCTION Cross-Hole Sonic Logging (CSL) is a common testing methods for determining the integrity of in- Deep foundations integrity testing mostly applies place constructed deep foundation elements, such to foundations constructed on-site from concrete or as drilled shafts and caissons. A minor variation grout, such as drilled shafts, drilled mini piles, of this method, called Single-Hole Sonic Logging pressure-injected footings, and pre-cast concrete (SSL) can also be used on smaller diameter drilled piles. Drilled shaft foundations usually carry very mini-piles and augercast piles. These methods are high design loads, and often serve as non- both non-destructive testing (NDT) methods and redundant, single load-carrying units. The integrity involve generating a Sonic pulse with one trans- testing is required for quality control during con- ducer (transmitter) and picking the signal up with struction to detect flaws in the pile ( necking, another transducer (receiver).)

5 The transducers cracking, void, poor quality material, etc.) com- typically consist of a geophone or accelerometer. mon in these cast-in-place concrete piles. As a re- The methods differ only in the number of tests per sult of the increasing design requirements on these pile and the location/orientation of the transducers foundations, a need for a high-level of quality as- within the pile. surance and control has been created. Significant improvements and advances in in- strumentation, data acquisition hardware, and computer technology have been made in recent combinations and thereby the resolution of the years. The software applications, however, have testing zone. greatly fallen behind and have not taken full advantage of the existing technological advances, thereby limiting the effectiveness and potential of the CSL method, as well as other deep foundations integrity testing methods (Chernauskas and Pai- kowsky, 1999).

6 A new state-of-the-art CSL testing System has recently been developed that utilizes unique soft- ware to take advantage of the new hardware (Amir and Amir, 1998a). This System is called the PISA. (Pile Integrity Sonic Analyzer). The PISA is based on a lightweight, portable, pen touch, com- A A'. puter that operates in a Windows graphical envi- ronment. This System is easy to use and efficient with regard to its ability to make the collected data available in a real-time manner. The following paper provides the basic background theory on the CSL integrity testing method, a description of the defect PISA System , and a recent case history including drilled shafts in which defects were intentionally fabricated. A - A'. (b). 2 OVERVIEW OF ULTRASONIC INTEGRITY Transmitter Receiver TESTING METHODS Signal Path Cross-Hole Sonic Logging (a).

7 Cross-Hole Sonic Logging (CSL) is the most common integrity testing method for drilled or cast-in-place foundations. A piezoelectric trans- ducer is used to generate a signal that propagates Figure 1. Typical CSL testing setup showing (a) transmitter as a sound (compression) wave within the con- and receiver at different depths, and (b) plan view of the CSL. crete, while another transducer is used to detect the tubes with possible test combinations. signal. Each transducer is placed into a vertical PVC or steel tube that has been attached to the re- In homogeneous, good quality concrete, the inforcement cage and filled with water prior to the stress/sound wave speed, C, is around 3,800 m/s concrete placement. The water acts as a coupling (12,000 to 13,000 ft/s) and is related to the medium between the transducer and the tube.

8 A modulus, E, and bulk density (unit weight, , and typical tube arrangement and testing principles are gravitational acceleration, g) as follows: presented in Figure 1. The source and receiver transducers are lowered to the bottom of their respective tubes and placed E g C= (1). such that they are in the same horizontal plane.. The emitter transducer generates a Sonic pulse (on the order of 10 pulses per second), which is de- If for any reason the condition of the concrete is tected by the receiver in the adjacent tube. The compromised, the wave speed will be reduced two transducers are simultaneously raised at a rate relative to that of the sound concrete value. Figure of about 300 mm/sec (1 ft/sec) until they reach the 2 presents a typical Sonic signal for which the top of the drilled shaft. Typically this process is propagation time between the transducers is meas- repeated for each possible tube pair combination ured.

9 The vertical axis is the signal amplitude (perimeter and diagonals). Figure 1b shows the six (microvolts) and the horizontal axis is the time tube combinations that can be tested (logged) us- (microseconds). The point where the amplitude ing a configuration of 4 tubes within a drilled begins to rapidly fluctuate indicates the arrival shaft. Increased shaft diameter calls for a larger time of the signal to the receiver ( threshold number of tubes, which increases the number of time). Since the distance between the two tubes is known, the wave speed of the concrete between the tubes can be evaluated by the following rela- tion the transducers in different elevations to cre- tionship: ate more signals, allowing the development of a tomographic presentation of the investigated zone. L The limitations of the method include detection of C= (2) defects only when they exist between the tubes.

10 T The testing can be performed only on drilled shafts for which access tubes were installed. Debonding The wave speed in equation 2 is only an esti- between the tubes and concrete is common if test- mate, as the identification of the arrival time, t, is ing occurs long after the concrete placement. subjective and the distance between the tubes, L, is Testing in fresh concrete is also difficult as certain known only at the top of the shaft. The signal ar- zones may cure at a lower rate, creating difficulties rival times can then be plotted with depth to gen- in the interpretation of the threshold time and en- erate a log for the particular tube combination as ergy. These zones may therefore be interpreted as presented in Figure 3. In addition to the threshold poor quality concrete. times, the energy of each signal may also be plot- ted with depth.