Integrity Testing of Model Piles with Pile Cap

Transcription

1 Integrity Testing of Model Piles with Pile Cap Shengmin Wu 1, Jiunnren Lai 1, Bo-Huan YANG 1, and Chiung-Fen CHENG 2 1 Dept. of Construction Engineering, Chaoyang Univ. of Tech.; Taichung, Taiwan; Phone: , Fax: ; swu@cyut.edu.tw, jrlai@cyut.edu.tw, bhyang@cyut.edu.tw 2 Dept. of Multimedia and Game Design, Overseas Chinese Univ.; Taichung, Taiwan; Phone: , Fax: ; cfcheng@ocu.edu.tw Abstract In this study, three pile integrity non-destructive testing (NDT) techniques were utilized to assess the structural soundness of six model piles with pile cap. The model piles contain defects of various size, location, and type. The objective of this study is to investigate the feasibility of these NDT techniques in detecting the defects within the model piles. Results of these tests indicate that: with the presence of the pile cap, the three surface reflection pile integrity tests can still distinguish between intact pile and the pile with major defect. However, error in estimated pile length is greater than piles without pile cap. The signal of Ultra Seismic test is less influenced by the pile cap, thus making interpretation of test data easier than the other two tests. Keywords: Integrity, Piles, Stress-wave, Non-destructive testing (NDT) 1. Introduction The ability to assess the structural integrity of piles after a major event such as earthquake or scouring of riverbed plays a key role in evaluating the safety conditions of brides. Several non-destructive testing (NDT) methods based on wave propagation theory have been used to assess the integrity of drilled shafts or cast-in-place piles. These tests are also call pile integrity test (PIT) and can be classified into two groups: direct transmission method and surface reflection method. The most common direct transmission PIT is the Cross-hole Sonic Logging (CSL) [1] or the Cross-hole Tomography (CT) [2]. A CSL/CT test normally requires two pairs of steel or PVC access tubes installed in the shafts and tied to the rebar cage. The cage is then lowered into the bore hole and the concrete is placed. A sound source and receiver are lowered into the tubes, maintaining a consistent elevation between source and sensor. A signal generator generates a sonic pulse from the emitter which is recorded by the receiver. Relative energy, waveform and differential time are recorded and logged. This procedure is repeated at regular intervals throughout the pile. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the structural soundness of the concrete throughout the pile can be gleaned. The CSL/CT method is considered to be more accurate than the surface reflection PITS in the determination of structural soundness of concrete within the drilled shaft [3]. However, for existing piles with pile cap, access tubes are often not available, thus the CSL was not investigated in this study.

2 In a surface reflection PIT, the pile top is struck by a hand-held hammer which generates a stress wave that travels down the pile, reflects off the pile toe or other material/cross-section changes and then back to the pile top. An accelerometer or geophone is attached to the pile top by a thin coupling compound and records both the incident wave and reflective waves. Depends on how the signal were analyzed, the surface reflection PITs are further classified into Sonic Echo (SE) method [4], Impulse Response (IR) method [5,6], and Ultra Seismic (US) method [7]. These surface reflection PITs have been successfully applied to assess the structural integrity of newly-built individual piles. However, relative little experiences were on existing group piles with pile cap. Therefore, the objective of this study is to investigate the feasibility of the three surface reflection PIT techniques in detecting the defects for piles with pile cap. 2. Theoretical Background A typical surface reflection PIT setup is shown schematically in Fig. 1. These tests involve impacting the top of a pile with a hammer to introduce a downward traveling transient stress wave. When the wave encounters a change of the impedance within the pile, such as a defect or pile toe, it will reflect back to the pile head and recorded by an accelerometer or a geophone. (IR Test) P(t) Hammer (SE Test) Signal Analyzer (US Test) Soil Pile Figure 1. Schematic drawing of surface reflection PIT setup The SE test requires only the particle velocity response history to perform integrity analysis. A velocity waveform of a pile containing a necking defect is illustrated in Fig. 2, the location of the defect or the length of the pile can be calculated from the travel time (dt) of stress waves reflected from the defect or the toe of the pile using the following equation: L = c dd 2..(1)

3 Where c is the stress wave traveling speed and is about 3000~4500 m/s for concrete. For the IR method, the impact force and the particle velocity response must be both measured versus time on the impacted surface. These two time histories are then transformed to the frequency domain using the Fast Fourier Transform (FFT). The mechanical admittance (or mobility) of the pile is defined as the ratio of the amplitudes of the particle velocity and the force in the frequency domain. From the frequency (df) of repeated pattern in the mobility curve (Fig. 3), the location of the defect or the length of the pile can be calculated by Eq. 2: c L =..(2) 2 dd Figure 2. Velocity waveform from a SE test [8] Figure 3. Mobility curve from an IR test [8] Ultra seismic test works on similar principles as SE/IR tests except the location of receiver. In an SE/IR test, the receiver is placed on the top surface of the plie while the receiver is placed on the exposed side surface in an US test. Multiple receivers spaced at mm to record waveforms from a single impact or single receiver repositioned between each impact to obtain multiple perspectives. An example of waveforms of the same pile containing a necking defect is shown in Fig. 4, the defect and the toe of the pile can also be identified. 4 Elapsed Time (x10-3 Sec) necking tip Depth Below Pile Head (m) -3 Figure 4. Waveforms recorded at multiple locations from an US test [8]

4 3. Model Piles Tested In order to investigate the feasibility of the three surface reflection PIT techniques in detecting the defects within piles with cap. Six model piles were constructed. Each pile has a diameter of 0.3m and is 3m-long. Three piles were placed in a raw and jointed by a cap with dimensions of 2.1m/0.5m/0.3m (length/width/high). A photograph of these piles is shown in Fig. 5. Pile P1 is an intact pile. Piles P2 and P4 contain a crack made by 1mm thin Styrofoam at different locations. Pile P3 contains a necking and pile P4 contains a weak zone of 100mm in length. There is about 35% reduction in impedance for these two defects. Pile P6 contains a major defect (damaged) made by 20mm-thick Styrofoam. A schematic drawing of the defect within each pile is shown in schematically Fig m 1.2m 1.2m 1.2m 1.0m P1 3.0m 1mm Styrofoam 100mm Weak Zone 100mm 20mm Necking P2 P3 P4 P5 P6 1mm Styrofoam 20mm Styrofoam Figure 5. Picture of the constructed model piles Figure 6. Schematic drawing of prefabricated defects 4. Test Results 4.1 Sonic Echo test Sonic Echo tests were performed on the six model piles with and without cap. The velocity waveforms of these piles without cap are shown in Fig. 7. For the intact pile P1, only incident wave and reflective wave from toe appear in the waveform. The pile length calculated from the travel time of these two waves using Eq. (1) is 2.9m, which is very close to the actual pile length. Piles P2~P5 contain minor defects. For these four piles, reflective waves from both defect and toe can all be identified. The estimated locations of defect and pile lengths are all very close to the actual values. Since pile P6 contains a major defect (damaged), only the reflective wave from defect can be identified. The estimated location of defect is also comparable with the actual vale. After the caps were added to these piles, SE tests were performed again. Results of these tests are shown in Fig. 8. The effect of pile cap on the waveform is significant. For the intact pile and the four piles with minor defects, reflective wave from the toe can still be identified. However, the estimated lengths (pile length plus cap thickness) are about 15% higher than the actual value. Furthermore, reflective wave from the defects can no longer be identified. For the pile with major defect (P6), only the reflective

5 wave from defect can be identified. The estimated location of defect (including cap thickness) is 2% smaller than the actual value. P1 P2 Velocity P3 P4 P5 P ( -incedent wave; -reflective wave from defect; -reflective wave from toe) Figure 7. Waveforms of SE test on piles without cap Figure 8. Waveforms of SE test on piles with cap 4.2 Impulse Response test The mobility curves from IR tests on the six piles with cap are shown in Fig. 9. As expected, it is not possible to obtain any useful length information due to the effect of pile cap. However, for the pile (P6) with major defect, the amplitude (mobility) of this pile at frequency below 500Hz is significantly higher than those of the other piles, and maybe used as an indication of damage. Mobility (10-7 ) P1 P2 P3 P4 P5 P Frequency (Hz) Figure 9. Mobility curves from IR tests on the six piles with cap

6 4.3 Ultra Seismic test Ultra Seismic tests were also performed on the six model piles with pile cap. Three receivers were used to record the waveforms simultaneously from the same one impact. The receivers were spaced at 0.2m while the 1st receiver was placed 0.2m beneath the lower surface of the pile cap. Results of these US tests are shown in Fig. 10. Similar to the SE tests, only the reflective wave from toe can be identified for the intact pile and the four piles with minor defects. However, the effect of pile cap on the waveform is less significant thus making the reflections easier to identify, especially when waveforms from multiple receivers are compared together. Furthermore, the error in estimated pile length is a little less than SE test. Similar to SE test, only the reflective wave from the major defect can be identified for the damaged pile P6. (a) P1 (b) P2 (c) P3 (d) P4 (e) P5 (f) P6 Figure 10. Waveforms of US test on piles with cap

7 5. Conclusions Six model piles with various defects were constructed to study the feasibility of three surface reflection PIT (Sonic Echo, Impulse Response, and Ultra Seismic) techniques in detecting the defects of piles with pile cap. Results of these tests indicate that the signals are influenced significantly by the presence of pile cap. These PITs are not capable of detecting minor defects of the piles with pile cap. However, they can still detect major defects within these piles. For the US test, the effect of pile cap is less significant thus making the reflections easier to identify, especially when waveforms from multiple receivers are compared together. Acknowledgements The work presented in this paper is sponsored by the Ministry of Science and Technology (formerly National Science Council) of Taiwan, R.O.C. under Grant No. NSC E MY3. Their support is deeply appreciated. Reference 1. R.T. Stain and H.T. Williams, Interpretation of Sonic Coring Results: A Research Project, Proceedings, the 4 th Int. DFI Conference, Balkema, pp , B.W. Han and D.G. Wang, Application of Sonic Tomography in the Integrity Testing of Concrete Piles, Proceedings, the 4 th Int. Conference on the Application of Stress-Wave Theory to piles, Hauge, Netherlands, pp , D.A. Hollema and L.D. Olson, Crosshole Sonic Logging and Velocity Tomography Imaging of Drilled Shaft Foundations, Proceedings, NDT-CE 2003, Berlin, Germany, Vol. 8, No.10, Sept., J. Steinbach and E. Vey, Caisson Evaluation by Stress Wave Propagation Method, Journal of the Geotechnical Engineering Division, ASCE, Vol. 101, No. GT4, pp , A. Davis and C. Dunn, From Theory to Field Experience with the Nondestructive Testing of Piles, Proceeding, the Institute of Civil Engineers, Vol. 57, Part 2, pp , R.J. Finno and S.L. Gassman, Impulse Response Evaluation of Drilled shafts, J. of Geotechnical and Geoenviromental Engineering, ASCE, Vol.124, No. 10, Oct., pp , L.D. Olson, F. Jalinoos, and M.F. Aouad, Determination of Unknown Subsurface Bridge Foundations, the NCHRP 21-5 Project Report, Transportation Research Board, p. 71, 1998.

8 8. M.H. Hsieh, A Study on the Signal Analysis from Integrity Testing on Individual Piles, M.S. Thesis, Dept. of Construction Engrg., Chaoyang Univ. of Tech., 2004.