Capabilities and Limitations of Cross Hole Ultrasonic Testing of Piles

J.M. Amir F.ASCE 1 & E.I. Amir2

1 Chairman, Piletest.com Ltd. 2 President, Piletest.com Ltd.

ABSTRACT

This paper describes an attempt to quantify flaw detectability in cross hole ultrasonic testing of piles. To this end we constructed a bored pile with access tubes and three pre-fabricated flaws, and then tested it with the CHUM ultrasonic instrument. We analyzed the results and presented them in the conventional one-dimensional format, as well as in two- and three-dimensional tomographic images. We then compared the results with those of a finite element model which showed similar effect of the flaws. We concluded that a flaw is usually detectable by this method only if its size exceeds one-third of the access tube spacing. In addition, the use of advanced tomographic techniques may enhance flaw detectability. INTRODUCTION

Cross hole ultrasonic testing technique is now well-known and has been standardized in several countries (Amir & Amir 2008). Still, there is still scant data regarding its performance in terms of flaw detectability. Sarhan et al. (2002) quoted a number of sources relating to this question, and concluded that flaws occupying up to 15% of the pile's cross section could remain undetected. Amir (2005) showed that flaw detectability is a function of its location in addition to its size.

In order to quantify the performance envelope of the cross hole ultrasonic test we constructed a test pile with three built-in flaws. The pile (or drilled shaft) was bored and cast in situ, with eight 50 mm. access tubes. We tested the pile several times with the Cross Hole Ultrasonic Monitor and then analyzed and presented the results in both one-dimensional presentation and tomography.

For comparison purposes, we modelled and analyzed the flaws by the dynamic module of Plaxis™ finite element program.

As the title of this paper implies, we limited ourselves to flaw detectability and did not address the influence of flaws on either capacity or durability of the piles. This important issue was dealt with by others (Sarhan et al. 2002, Iskander et al. 2003).

Description of the Pile

The 1200 mm dia. pile was bored through dry clayey sand to a depth of 5.50 m from the surface. The reinforcement cage was assembled with eight 50 mm access tubes evenly distributed on the inside. Four of the tubes were of steel (designated N, E, S and W) and the remaining four – of plastic (numbered 1 to 4). In addition, we installed three flaws at three levels. A general view of the setup is given in Figure 1 and details of the flaws are provided in Table 1

After lowering the cage into the hole, the pile was immediately cast with 30 MPa concrete.

FINITE ELEMENT SIMULATION

In order to obtain better understanding of the mechanism governing the cross hole method, we modeled the pile in the dynamic model of Plaxis® finite element program. The concrete parameters we assumed were as follows: Young's Modulus –E = 3.6x109 kN/m2, Poisson's Ratio - ν = 0.1 and specific gravity - γ = 23 kN/m3. The resulting P-Wave speed was 4,001 m/sec. Due to software limitations we performed our analysis in two dimensions, assuming plane strain conditions (Figure 2). The pulse produced by the ultrasonic emitter in this simulation was represented by five stress cycles with a 50 kHz frequency. We studied flaws No. 1 and No. 3 in three different profiles, each with a flaw present or absent, respectively (Table 2). Due to modeling difficulty (and triviality) we did not analyze flaw No. 2.

Figure 1: General view of the reinforcement cage, access tubes and flaws

Table 1: Details of Flaws

No Mean depth

(m)

Description Size

(mm)

% of tube

spacing

% of

cross-section

1 1.60 empty plywood box 200x200x200 32 3.5

2 2.60 bandage

3 3.65 empty plywood box 300x300x300 32 8

Table 2: Simulated Profiles

Run

No.

Flaw

No.

Profile Spacing

(mm)

Flaw

1 1 N-E 710 yes

2 1 N-E 710 no

3 3 N-S 1,000 yes

4 3 N-S 1,000 no

5 3 N-S 1,000 yes

6 3 N-S 1,000 no

Figure 2 : Finite element simulation of

pile with flaws

Results of Finite Element Simulation

The results of all six runs were plotted in the form of particle velocity at the receiver as a function of elapsed time following the emitter triggering. An examination of a typical trace (Figure 3) shows the following properties:

1. The initial part of the trace is flat and featureless. At the First Arrival Time (FAT) a pulse consisting of periodic displacements is observed.

2. Although our input pulse consists of only five cycles, the particle velocity trace at the receiver consists of considerably more cycles. This proves that the pulse recorded is a superposition of numerous wave paths.

3. The pulse amplitude immediately following FAT is relatively small and it grows gradually with time, as more and more wave paths converge at the receiver.

4. The maximum amplitude is located at double to triple FAT value. It is caused by higher energy Rayleigh waves travelling at about half the P-Wave speed along the perimeter of the pile.

5. The existence of a flaw shows influence on both FAT and amplitude.

Table 3 summarizes the FEM simulation results.

Table 3: Simulation results

Flaw No. Profile FAT increase

(%)

Attenuation

increase

1 NE 11 Marked

3 NS 10 Considerable

3 EW 0 Negligible

0 200 400 600 800 1000

Time (microsec)

Par

ticl

e ve

loci

ty

No defect Defect

Figure 3: Profile N-S – influence of flaw No. 3

MODEL PILE TESTING

We performed the cross hole tests following ASTM Standard D6760-08 (ASTM 2008). We then analyzed the results and presented them using the following methods:

1. One-dimensional (1D) line plots of FAT and attenuation overlay on "waterfall" background.

2. Two-dimensional tomography (2DT) – fuzzy logic (Amir & Amir 1998)

3. Two-dimensional tomography - parametric (Amir & Amir 1998)

4. Three-dimensional tomography using straight-line wave propagation - 3DT (Haramy & and Mekic-Stall 2000)

Results of Model Pile Testing

The results are presented in Figures 4 to 7.

0m

1.0

2.0

3.0

4.0

4.9

0

0.0

24

0.2

48

0.4

72

0.6 Arrival time [ms]

Attenuation [db]

0

0.0

24

0.2

48

0.4

72

0.6 Arrival time [ms]

Attenuation [db]

0

0.0

24

0.2

48

0.4

72

0.6 Arrival time [ms]

Attenuation [db] Profile N-E Profile N-S Profile E-W

Figure 4: 1D results of cross hole logging

Figure 5: Results of attenuation-based

3DT

Figure 6: Results of fuzzy-logic 2DT Figure 7 : Results of parametric 2DT Discussion of the Results

One-dimensional presentation

The results of the one-dimensional presentation (Figure 4) were summarized in Table 4. Evidently, flaws No.1 and No. 3 hardly qualify for the Caltrans category of "significant anomalies" (severe signal distortion, much lower signal amplitude, FAT increase of more than 20%). In many real-life situations such flaws would escape detection altogether. Flaw No. 2, geometrically the smallest, made the strongest mark on the FAT, thus confirming that detectability is a function of location no less than of size.

Table 4: Summary of 1-D results

Flaw No. Profile FAT increase

(%)

Attenuation increase

(dB)

1 NE 20 6

2 EW 38 5

3 NS 20 6

3 EW 0 4

3DT

Although we took much care in collecting the data, regular FAT-based 3DT failed to reveal the flaws. At this point we decided to replace FAT with attenuation in the analysis. Apparently, the results we obtained (Figure 5) were quite good for flaws No. 1 and No. 3, while No. 2 again appeared much larger than its real size.

2DT

All three flaws show reasonably well in Fuzzy-logic tomography (Figure 6) and in parametric tomography (Figure 7). We managed to arrive at this result, however, only after some manipulation with the parameters. This means that while we were able to confirm the location (and to a certain degree the shape and size) of the flaw, we would probably fail to perform a class A prediction without prior knowledge.

CONCLUSIONS

The main conclusions we drew from this exercise are as follows:

1. The anomaly created by a flaw depends not only on the flaw's size, but also on its location. The closer a flaw is to an access tube the larger it appears in both 2DT and 3DT.

2. A flaw located half way between two access tubes is detectable only if its size exceeds about one third of tube spacing or about 10 percent of the pile's cross section.

3. Piles used for validation purposes should preferably be constructed with at least some of the flaws not smaller than 40 percent of the tube spacing.

4. Flaw detectability can be enhanced by using tomographic, and specifically 3DT techniques. Attenuation-based 3DT appears to be an especially effective technique and merits further investigation.

5. Plane strain finite element modelling is a useful tool for predicting the results of the cross hole ultrasonic test. Due, however, to modelling imperfections, the results are more qualitative than quantitative. The main discrepancies are the absence of the third dimension, the representation of the emitted pulse as unidirectional instead of radial and disregarding the access tubes and the water.

REFERENCES:

Amir, E.I & Amir J.M. (1998). "Recent advances in ultrasonic pile testing." Proc. 3rd Intl Geotechnical Seminar on Deep Foundation On Bored and Auger Piles, Ghent: 181-185

Amir, J.M. (2005). "Discussion on “Low Strain Integrity Testing of Piles: Three–Dimensional Effects” by Y.K. Chow, K.K. Phoon, W.F. Chow and K.Y. Wong, , J. Geotechnical and Geoenvironmental Engrg. 131 (2): 342-343

Amir, J.M. & Amir E.I. (2008). "Critical comparison of ultrasonic pile testing standards." Proc. Intl. Conf on Application of Stress Wave Theory to Piling, Lisbon (accepted for publication)

ASTM (2008). "Standard test Method for integrity testing of concrete deep foundations by ultrasonic crosshole testing, Designation D 6760-08," West Conshohocken PA: 3

Haramy, K.Y, and N. Mekic-Stall, 2000. "Evaluation of drilled shafts at Piney Creek Bridge using cross-hole sonic logging data and 3D tomographic imaging method." Proc. 1st Intl. Conf. on the Application of Geophysical Methodologies & NDT to Transportation Facilities and Infrastructure, St. Louis, MO:

Iskander, M., Roy, D., Kelley, S. and Ealy, C. 2003. "Drilled Shaft Defects: Detection, and Effects on Capacity in Varved Clay." J. Geotech. and Geoenvir. Engrg, Vol. 129, No. 12, pp. 1128-1137

Sarhan, H., Tabsh, S.W., O'Neill M., Ata, A. & Ealy, C. (2002): "Flexural behavior

of drilled shafts with minor flaws, in Stiff Clay" Proc. Intl. Deep Foundations Congress, GSP No. 116, ASCE, Reston/VA: 1136-1150.