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Dynamic and static testing of concrete foundation piles with structure-integrated fiber optic sensors
Schallert, M.GSP mbH, Consulting Engineers for Vibration Analysis and Dynamic Testing Methods Mannheim, Germany
Habel, W. R.Federal Institute for Materials Research and Testing (BAM) Berlin, Germany
Göbel, I. R.Federal Institute for Geosciences and Natural Resources (BGR) Hannover, Germany
Stahlmann, J.Institute for Foundation Engineering and Soil Mechanics, Technical University Braunschweig, Germany
Wardinghus, P.Centrum Pfähle GmbH Hamburg, Germany
Keywords: Concrete-embedded fiber optic sensors, Fabry-Perot sensor, pile integrity testing (PIT), dynamic load testing (DLT), numerical modeling, precast driven piles
ABSTRACT: Assessment of integrity, ultimate bearing capacity, and bearing behavior of concrete piles in existing foundations or during and after installation remains a difficult task. The common dynamic test methods (high-strain load test and low-strain integrity test) are widespread worldwide as instruments of quality assurance. Both test methods use sensors attached to the pile head measuring velocity (low strain) or velocity and strain (high strain) generated by dynamic impact. To obtain reliable signal response over the whole pile length and to gather significant geotechnical information close to the potential location of local faults, high-resolution fiber-optic sensors based on the Fabry-Perot interferometer technique have been developed. They can be integrated into any type of concrete pile (cast-in-place driven or bored piles as well as precast concrete piles) on different levels. Our research is partly motivated by the need for local monitoring of pile deformations during low-strain, high-strain, and static load testing using the same type of sensor. In the laboratory, the fiber-optic sensors have been tested in precast concrete piles with and without local imperfections to determine damage-dependent signal responses. Piles were loaded by static and dynamic tests. All responses from the integrated sensors were compared to signals recorded by conventional methods (i. e. at the pile head). In field tests, the reaction of the integrated sensors in precast driven piles was examined under real environmental conditions. Additionally, the reactions of the integrated sensor systems were researched by numerical modeling to improve the interpretation of the sensor signals based on geophysical methods. The results are promising up to now. The paper presents the most important aspects of the fiberoptic sensor, the results of the laboratory and field tests and the numerical modeling, i. e. design aspects for the sensitive part of the fiber-optic sensor attached to the surface of the sensor-cage, the design of the embedded sensor-cage, testing of small-scale pile models, dynamic low-strain and high-strain testing of driven piles, and strategies of data analysis. Finally, responses from fiber-optic sensors will be compared with those achieved from conventional assessment methods, and the advantages of the multi-sensor chains installed along the pile body will be discussed.
1 INTRODUCTION
Foundations often have to be constructed on soft ground so that e. g. reinforced concrete piles are used to transfer the structural loads into deeper strata with adequate load-bearing capacity. Precise determination of the pile’s behavior is generally difficult and damage of concrete piles still occurs. Therefore since several decades static and dynamic pile testing are widespread worldwide as powerful instruments of quality assessment (Klingmüller and Kirsch 2004, Rausche et al. 1985, Stahlmann et al. 2004).
To improve the analysis of concrete piles by applying established pile-test methods, especially PIT and DLT, a high-resolution fiber-optic sensor has been developed for integration into concrete piles. Strain and strain distribution are measured by a string of sensors embedded at different levels.
Extrinsic Fabry-Perot interferometer (EFPI) sensors have been selected because of their high sensitivity for dynamic measurements compared to other types of fiber-optic sensors already used in static pile tests (Inaudi 2005, Habel 2004, Schallert et al. 2004, Schmidt-Hattenberger et al. 2003, Glisic et al. 2003, 2002 and Oh et al. 2000). The method
described here provides results from deeper pile sections that can be compared with calculations of pile-soil-interactions, e. g. from CAPWAP-analysis using pile-head measurements.
The validation of the fiber optic sensor was first done in the laboratory to determine a proper and reliable method to attach the sensor to the pile structure, followed by laboratory tests on small sizedmodel-piles as well as field tests. In both kinds of tests, precast driven piles with sensors embedded at different levels were loaded dynamically and statically to obtain signal responses over the pile length. The responses from structure-integrated sensors were compared with signals recorded by conventional pile tests as well as with results from numerical modeling.
The paper presents first the design and the calibration of the sensor, displays then the laboratory test facility with the experimental program and its main results. Then, the field tests with full-size piles are described and the test results are demonstrated. Finally, the results of the numerical modelingaccompanying the tests are compared with the measurements.
2 FABRY-PEROT INTERFEROMETRIC SENSOR
2.1 Principle of the sensorExtrinsic Fabry-Perot interferometer (EFPI) sensors have been selected due to their high sensitivity suitable for dynamic measurements (Habel 2000). The sensor consists of two optical fibers inside a small air-filled capillary, Fig. 1.
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Figure 1. EFPI sensor.
Both fibers are fixed at the capillary which defines the gage length. Their ends face each other in a distance of a few micrometers. The injected light is partially reflected by the two surfaces. The EFPI sensor can resolve distance changes of the end faces in the order of nanometers. One of the fibers is connected to the optoelectronic device. The reflected interference signal contains the information about the deformation which can be transferred into strain using the gage length
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2.2 Sensor-cageThe EFPI sensor is glued to the inner surface of a sensor-cage developed for embedding into concrete, Fig. 2. Resistive strain gages (RSG) are attached to the outer surface for the validation of the EFPIsensor. Additionally, a piezoelectric accelerometer sensor (ACC) was placed inside. The sensor-cagetransfers strain from concrete to sensor and protects the sensor against mechanical and chemical disturbances. The size of the sensor-cage was designed for concrete properties and loading conditions of the low-strain method.
Figure 2. Sensor-cage with EFPI sensor, RSG and ACC.
2.3 Investigations of strain transfer behavior
For sensor calibration, the proper application method of the EFPI sensor on steel surfaces had to be determined in laboratory tests because the bonding between sensor and structure influences the uncertainty of measurement considerably.
The strain transfer ratios of different adhesives were determined during axial tensile tests with steel test specimens. The maximum applied strain was about 1000 µm/m. Fig. 3 shows experimental results of surface-attached EFPI sensors with regard to the adhesive thickness.
Maximum strain transfer ratios could be assessed to 0.985 for “adhesive thickness 2” in Figure 3, which was half of the capillary diameter (210 µm). Higher values of adhesive thickness led to lower ratios and unreliable and irreproducible results.
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Figure 3. Strain transfer characteristic for surface-attached EFPI sensors with different adhesive thicknesses.
3 SMALL-SCALE MODEL-PILE TESTS
3.1 Model-piles and test facilityPrecast concrete model-piles, Fig. 4, were tested at a test facility of the Technical University of Braunschweig shown in Fig. 5. The sensors were positioned in the center of the cross-section area at three measuring levels (ML). Pile 1 (without imperfections) has an end-bearing load cell installed at the pile bottom to distinguish between skin friction and toe resistance during SLT. Pile 2 has got a man-made imperfection between ML2 and ML3 with a 34 % reduction of the cross section over a length of 30 cm in the direction of the pile axis.
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Figure 4. Model-piles with embedded sensors: pile 1 without imperfection, pile 2 with imperfection (dimensions in cm).
Figure 5. Test facility for SLT and DLT of model-piles.
The concrete mixture was chosen similarly to that of full-scale precast driven concrete piles. The piles were installed in a sand filled concrete shaft of 3 m height and a diameter of 1.2 m. A sand-water mixture was pumped from the test shaft to a storage shaft and then drained by a dewatering system. Vibro-compaction produced dense compact sand up to the pile toe level. The pile and a second layer of less dense sand were installed afterwards.
3.2 Experimental program
Both piles were tested dynamically by PIT and DLT as well as by SLT.
PIT verified that the embedded sensors measure very small deflections. Impacts were generated at the pile head surface by steel balls with varying diameter and falling height. Signals were also taken at the same time from the pile head by conventional low-strain measurements.
With DLT it was shown that the EFPI sensors can record signals comparable with measurements of conventional gages for DLT attached to the pile skin near the pile head, i. e. axial strain and velocity distribution. A special drop weight, Fig. 5, right, was developed to apply the loads. The bearing capacities calculated by CAPWAP-analysis were compared to results of SLT, and the results of strain measurements of the embedded EFPI sensors were compared to those of RSG at the same location and at the pile top section.
During SLT, loads were increased stepwise by a hydraulic jack. Loads and displacements were measured additionally at the pile head, Fig. 5, left.The embedded strain sensors and the response of the end bearing load cell were recorded simultaneously during the loading cycles.
To read out the optical signals in real-time additionally to the signals of the other sensors, a multi-channel measuring device and appropriate software for controlling the measurements had to be developed.
3.3 Main Results
3.3.1 Integrity testing (PIT)
The responses of the ACC and EFPI sensors verified that the applied wave travels as expected from the pile head, passing the embedded sensors at ML1, ML2 and ML3, to the pile toe and back to the pile head displaying the usual features of wave form and characteristic reflections. The wave velocity distribution was determined from the responses of the embedded sensors. While pile head measurements give only an average value of wave velocity, the measurements from embedded sensors allow a more precise determination, Fig. 6.
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steel ball 30 mm diameter
steel ball 40 mm diameter
steel ball 50 mm diameter
Figure 6: Pile section dependent wave velocity at model-pile 1, calculated from the distances between the embedded sensors and the pile head for different sources of impact.
Close to the impact source at the pile head, the wave velocity decreases faster than later on, probably due to three-dimensional (3D) wave propagation. With the wave velocity determined between the embedded sensors, differences in material properties can be determined more precisely than from pile head measurements only. In the case of model-pile 1, the differences are within a small range representing the variance of the material properties.
Fig. 7 shows responses of ACC (velocity) and EFPI (strain) from model-pile 2 with the artificial geometric imperfection, supplemented by the result of conventional pile head measurement. The wave propagation and additionally the influences from the
geometric defect can clearly be seen by the characteristic lines showing the wave propagation.
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Figure 7: Velocity (above) and strain (below) responses from embedded ACC and EFPI sensors over time from model-pile 2 with imperfection compared to the pile head measurement applying PIT with steel ball impact.
From the pile head measurements, the length of the defect was determined in the time domain from2·L/c = 28.0 cm where L is the pile length and c the average wave velocity (1D-model). Applying the same procedure to the time series of the embedded sensors, a defect length of 30.1 cm follows from ML1, and 31.6 cm from ML3. Hence, results from the embedded sensors are better consistent with the real defect length of 30 cm.
Nevertheless, it must be noted that ML2 did not show significant secondary peaks caused by the superposition of waves reflecting from the defect, probably because the defect was too close to the sensor.
3.3.2 Load testing (DLT and SLT)
Fig. 8 shows as one of the main results responses from all strain gauges (RSG and EFPI) recorded during DLT using the test facility shown in Fig. 5 (right). Responses of conventional DLT sensors attached to the pile skin between pile head and ML1 are also displayed.
It can be seen that the wave propagation is recorded correctly in the time domain. The travel time in the range of 2·L/c is 1.06 ms which corresponds to a wave speed of 4340 m/s and to the difference between two neighboring peaks in Fig. 8.
The comparison of strain from embedded EFPI sensors with embedded RSG (ML1, 2, 3) at the same location at each measuring level reveals that both measuring systems vary in a range of about 5 %. Thereduction in strain from one ML to the next ML is caused by skin friction forces and by radiation damping losses at pile skin and toe.
Figure 8: Strain from embedded and conventional sensors during high-strain testing.
Embedded fiber optic sensors display lowerstrains than conventional sensors. This could be
explained by different locations and different gagelengths. While conventional gages have a gage length of about 75 mm, the embedded sensors are of shorter length of about 30 mm.
Fig. 9 shows time-dependent curves of force and velocity times impedance calculated from measured strain and acceleration values.
Except for the differences in magnitude discussed above, a very good accordance between both measuring systems in the ML located very close to each other can be noticed.
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Figure 9: Results of DLT at model-pile 1 (drop weight of 60 kg and falling height of 70 cm): force over time and impedance multiplied with velocity over time, comparison of conventional and embedded sensors (ML1).
Although the concrete-embedded fiber-optic strain sensor was developed predominantly for dynamic loading, SLT were carried out as well at both model piles. Loads were applied with the test facility shown in Fig. 5 (left) in three cycles and at defined load steps. The maximum applied load was 110 kN which led to a permanent pile head settlement of 50 mm after the third loading cycle.
Comparing the shapes of the strain curves, it can be concluded that the concrete transfers the strain properly to the sensor-cage. The results of embedded RSG at the three ML of pile 1 are shown in Fig. 10. Comparison of both embedded sensor systems shows good agreement in general. The results achieved from EFPI and RSG differ in a range from5 % to 10 % (Fig. 11).
Geotechnical parameters can now be derived from measured strain values. Assuming a homogeneous and constant modulus of elasticity along the pile length, the axial force at each ML (i = 1, 2, 3) can be calculated using Hooke’s law.
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Figure 10: Results from SLT at model-pile 1: strain distribution over time at the three ML from embedded RSG.
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Figure 11: Comparison of embedded RSG and EFPI during SLT at model-pile 1.
3.4 Conclusion from model-pile testsThe main outcome is that embedded EFPI sensors can be used for both dynamic pile tests as well as for static load tests. Calculations can be made on the basis of measurements at different locations which provide more precise information about the pile’s behavior. Occurring discrepancies between both measuring systems, i. e. EFPI sensors and conventional RSG, were further investigated in field tests.
4 FULL-SCALE PILE TESTS
Based on the results of the model-pile tests, the concrete-embedded transducer has been verified for field tests dependent on the properties of the concrete mixture (aggregate size) and real environmental and dynamic loading conditions. The predominant variation was the geometric dimension
of the sensor-cage. EFPI and RSG were attached to the new sensor-cage in the well proven way developed for the model-piles.
4.1 Piles and ground conditions
Field tests were carried out in northern Germany with two reinforced concrete driven piles of 19 m length and cross sections of 35 cm x 35 cm and 40 cm x 40 cm, respectively. The ground stratification at the test site was determined by one boring and two cone penetration tests (CPT). Accordingly, in each pile, eight transducers were installed at five different measuring levels (ML). In ML1, ML 3 and ML 5, two of the transducers were located at different positions in the cross section, and a conventional accelerometer (ACC) was installed inside one of the sensor-cages. In ML2 and ML4, just one of the transducers was installed without ACC. Fig. 12 shows the reinforcement cage with sensors attached to the lower pile section before concreting.
Figure 12. Sensors fixed at the reinforcement cage (above right: sensor cage for measurements under field test conditions).
Since two EFPI sensors and three RSG were attached to each sensor-cage, a total number of 40 strain gages had to be controlled. Therefore, aspecial measuring device was developed for EFPI signals. A conventional multi-channel data acquisition system was used for the other sensors. Asampling rate of 100 kHz per channel could be realized for dynamic loading conditions (PIT and DLT).
Fig. 13 gives an overview of the soil stratification in terms of CPT results and the situation of piles in the ground (right) with positions of the sensors.
There is a very week layer of clay, silt and peat with low bearing capacity between 2 m and 14 m depth.The sandy soil underneath 14 m is sufficiently load-bearing.
The experimental program contained PIT before and after driving, monitoring of the entire driving process, DLT one day and a couple of months after driving as well as SLT a few weeks after driving. One of the most important measurements was the monitoring of the driving process to control how the fiber optic EFPI sensors behave during high energy loading conditions.
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Figure 13. Results of cone penetration tests and location of piles and 5 measuring levels depended on soil stratification.
4.2 Main results
4.2.1 Integrity testing (PIT)
Fig. 14 shows the time-dependent responses of the embedded sensors with time t > 4·L/c which can be used to prove of the functionality of the EFPIsensors during PIT. The results of ACC sensors are drawn in the uppermost diagram. For comparison,the signal of conventional PIT from the pile head is given (dotted line). In the middle diagram, the results of RSG are shown which are located at the same place as the EFPI sensors (below). As it is known from theory, due to the alternation of compression and tension, velocity and strain will have opposite signs when they are reflected at the
free end of a pile (in Fig. 14, compression has a positive sign).
The results of RSG and EFPI sensor are in very good agreement with only small deviations in the amplitude. That means that the EFPI sensor is able to detect the very small deformations caused by PIT of about a few µm/m.
It can be noticed as one conclusion of this result that PIT with the embedded sensors can help to find variations in material properties in terms of section-wise changes in wave speed, as already shown for the model-pile tests (Fig. 6). Furthermore, additional geotechnical analysis can be made with the results from PIT after driving, with regard to the detection of defects or the determination of soil damping influences.
Velocity over time (P1: 40cm x 40cm)
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Figure 14. PIT-responses of ACC (uppermost diagram) and strain sensors (RSG in the middle and EFPI below) for pile 1: pile is lying on the ground.
4.2.2 Load testing (DLT and SLT)
One day after pile driving, DLT was applied to both piles. All EFPI sensors survived the load application. In order to show the original EFPI output signal, Fig. 15 displays the responses from one blow of DLT. RSG signals are given in µm/m and the interferometric signals are given in electrical voltage. Whenever a change in strain appears, a change in the EFPI signal output takes place.
These fiber optic sensor outputs have to be converted into strain. Fig. 16 shows the result of this mathematical procedure with the best agreement between RSG and EFPI from DLT. Fig. 17 shows the result of the same ML 2 from the SLT.
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RSG_ML3 EFPI_ML3
ML 4
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML4 EFPI_ML4
ML 5
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10E
FP
I-signal(V)
RSG_ML5 EFPI_ML5
6,2
m6,
2m
1,2
m1,
2m
1,2
m3
m
ML 1
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML1 EFPI_ML1
ML 2
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML2 EFPI_ML2
ML 3
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EFP
I-signal(V)
RSG_ML3 EFPI_ML3
ML 4
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML4 EFPI_ML4
ML 5
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10E
FP
I-signal(V)
RSG_ML5 EFPI_ML5
ML 1
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML1 EFPI_ML1
ML 2
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML2 EFPI_ML2
ML 3
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EFP
I-signal(V)
RSG_ML3 EFPI_ML3
ML 4
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10
EF
PI-signal(V
)
RSG_ML4 EFPI_ML4
ML 5
-1000
100200300400500600700800900
29 31 33 35 37 39 41 43time (ms)
Stra
inR
SG
(µm
/m)
-20
-15
-10
-5
0
5
10E
FP
I-signal(V)
RSG_ML5 EFPI_ML5
6,2
m6,
2m
1,2
m1,
2m
1,2
m3
m
Figure 15. DLT responses of strain-sensors (RSG, lower curves and EFPI, upper curves); pile is embedded in the ground.
-200
0
200
400
600
800
1000
31 33 35 37 39 41 43 45 47 49 51 53 55time (ms)
Stra
in(µ
m/m
)
RSG_ML2
EFPI_ML2
Figure 16. DLT: comparison of strain from embedded RSG and EFPI at ML2.
-200
20406080
100120140160180200220
0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000time (s)
Stra
in(µ
m/m
)
RSG_ML2 EFPI_ML2
Figure 17. SLT: comparison of strain from embedded RSG and EFPI at ML2.
The comparison of both embedded measuring systems shows again a very good agreement. Generally, the EFPI-sensors show lower values in strain than the RSG at the same location, respectively.
Further data analysis is still in process, especially from the geotechnical point of view.
5 NUMERICAL MODELING
Accompanying the experimental program, numerical calculations were carried out. The objective is to verify that the measurements show plausible results which can be reproduced by calculation. Starting with relatively simple models, this goal has beenrealized so far for the low-strain method (PIT). Furthermore, since the pile and soil models become more complex and realistic, the calculations can give additional information and further experimental investigations can planned, e. g. detecting defects in concrete piles. In the following sections, some of the modeling results are described.
5.1 Unconstrained pileThe integrity tests at the unconstrained piles were measured at the pile head and on 3 (accelerometers) or 5 (EFPI) additional sensor locations in the pile during the field tests. The mathematical model of the test is simulated by a beam realized by the Finite Difference Method (1-D-FDM) as well as by a three-dimensional Finite Element Model (3-D-FEM). The input load had to be estimated by comparison with the pile reaction measurement at the pile head. Measured and modeled results match remarkably well for the particle velocity, Fig. 18, as well as for the strain, Fig. 19 and Fig. 20. The measured results are taken from Fig. 14 (upper and lower part). Fig. 21 and Fig. 22 show the comparison of both mathematical models: The friction-free 1-D-FDM model shows numerical errors increasing with time whereas the 3-D-FEM model displays the results of friction in the concrete.
-0.004
0
0.004
0.008
0.012
0 2 4 6 8 10 12 14
17.86 m15.39 m3.04 m0 mVEL_ME1VEL_HEADVEL_ME3VEL_ME5
Time t [ms]
Parti
cle
velo
city
v[m
/s]
Figure 18. Integrity test of the unconstrained 19m long pile: comparison between accelerometer measurements (VEL_MEn with sensor in ML1 at 3 m, ML3 at 15.4 m, and ML5 at 17.8 m from the pile head) and calculation results by the 1D-FDM model at appropriate levels.
-2.5
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
2.5
0 2 4 6 8 10
17.86 m16.53 m15.39 m9.12 m3.04 mEFPI_ME51EFPI_ME4EFPI_ME32EFPI_ME2EFPI_ME12
Time t [ms]
Stra
in�
[�m
/m]
Figure 19. Integrity test of the unconstrained 19m long pile: comparison between strain measurements (EFPI_MEn with sensor in ML1 at 3 m, ML2 at 9.2 m, ML3 at 15.4 m, ML4 at 16.6 m, and ML5 at 17.8 m from pile head) and calculation results by the 1D-FDM model at appropriate lengths.
-2.5
-2.0
-1.5
-1.0
-0.5
0
0.5
1.0
1.5
2.0
2.5
0 2 4 6 8 10
EFPI_ME 1.2EFPI_ME 2EFPI_ME 3.2EFPI_ME 4EFPI_ME 5.1FEM_ME1.2FEM_ME2FEM_ME3.2FEM_ME4FEM_ME5.1
Time t [ms]
Stra
in�
[�m
/m]
Figure 20. Integrity test of the unconstrained 19m long pile: comparison between the EFPI strain measurement and the 3-D-FEM model.
-0 .00 50
-0 .00 25
0
0 .00 25
0 .00 50
0 .00 75
0 .01 00
0 .01 25
0 2 4 6 8 10 1 2 14
F E M _H E A DF E M _M E 1F E M _M E 3F E M _M E 5F D M _ H EA DF D M _ M E 1F D M _ M E 3F D M _ M E 5
T im e t [m s ]
Parti
cle
velo
city
v[m
/s]
Figure 21. Integrity test of the unconstrained 19m long pile: comparison between the 1-D-FDM model and the 3-D-FEM model: particle velocity of the nodes on the symmetry axis.
-1.6
-0.8
0
0.8
1.6
0 2 4 6 8 10
FEM_ME 1.2FEM_ME 2FEM_ME 3.2FEM_ME 4FEM_ME 5.1FDM 3.04 mFDM 9.12 mFDM 15.39 mFDM 16.53 mFDM 17.86 m
Time t [ms]
Stra
in�
[�m
/m]
Figure 22. Integrity test of the unconstrained 19m long pile: comparison between the 1-D-FDM model and the 3-D-FEM model; strain of the nodes on the symmetry axis.
5.1.1 In situ pile
The character of the foundation soil is displayed by means of the cone penetration test (CPT) results in Fig. 13, where the location of the pile is shown as well. CPT results were processed by a correlation method of Andrus et al. (2002, 2003) to estimate a velocity-depth profile of the subsoil so that dynamic soil parameters could be determined.
The 1-D-FDM model of the unconstrained pile was embedded according to Wakisaka et al. (2004). The spring and damper constants were calculated as given by Wakisaka et al. (2004).
Fig. 23 shows the model’s behavior for a pile with elastic footing at the pile head location. For the sake of comparison, the unconstrained pile’s reaction is shown, too. In Fig. 24, measured and calculated particle velocities at the pile head can be compared. The clay layer with the low bearing capacity has been modeled with constant and with real values of spring (C) and damper (D) parameters. Fig. 25 shows, that the damping element controls the answer of the model so that it coincides with the measurement.
Fig. 26 displays displacement and particle velocity as a function of time at different depths of the pile.
-0.001
0
0.001
0.002
0.003
0.004
0.005
0 2 4 6 8 10 12 14 16 18 20
FDM/elastic footingFDM/free pileRecord ID: 1
Time t [ms]
Par
ticle
velo
city
v[m
/s]
Pile head
Figure 23. Integrity test of the 19m long pile in situ: comparison between the accelerometer measurement and the answer of the 1-D-FDM model.
-0.001
0
0.001
0.002
0.003
0 2 4 6 8 10 12 14 16 18 20
Record ID: 1FDM/Klei layer = const.FDM/Klei layer = variable
Time t [ms]
Par
ticle
velo
city
v[m
/s]
Pile head
Figure 24. Integrity test of the 19m long pile in situ: comparison between the integrated accelerometer measurement while the pile is embedded in the ground and the answer of the 1-D-FDM model for a clay layer with constant soil properties and a clay layer modeled according to the CPT measurements.
-0.004
-0.002
0
0.002
0.004
0.006
0 2 4 6 8 10 12 14 16 18 20
R ecord ID: 1FD M /Klei layer = const.FD M /D=0 in Klei layerFD M /C=0 in Klei layerFD M /C=0, D =0 in K lei layer
Time t [ms]
Par
ticle
velo
city
v[m
/s]
Pile head
Figure 25. Results of modeling the integrity test of the 19m long pile in situ: Parameter studies of the 1-D-FDM model where the influence of the spring (C) and damper (D) components in the clay layer are examined.
0
0.5x10-6
1.0x10-6
0 2 4 6 8 10 12 14
FDM: 17.86 mFDM: 9.12 mFDM: 3.04 mFDM: 0 m
Time t [ms]
Dis
plac
emne
tw[m
]
-0.0005
0
0.0005
0.0010
0.0015
0.0020
0 2 4 6 8 10 12 14
FDM: 17.86 mFDM: 9.12 mFDM: 3.04 mFDM: 0 m
Time t [ms]
Par
ticle
velo
city
v[m
/s]
Figure 26. Results of modeling the integrity test of the 19m long pile in situ: Displacements (above) and particle velocities (below) as a function of time, derived from the 1-D-FDM model.
5.2 Modeling in situ piles with defects
Fig. 27 to Fig. 29 show the consequences of necks in the 1-D-FDM model. The presence of a defect is displayed by a sharp local drop in the strain-length curve, Fig. 29. Image plots, Fig. 27, give a more
thorough impression of the time and space gap in the strain display where defects of a particular size can be detected. In Fig. 28, the display of a conventional integrity test for the defect constellations given in Fig. 27 can be seen.
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1.0
Relative length x/L
Tim
et[
ms]
Strain amplitude
-6
-4
-2
0
0 0.2 0.4 0.6 0.8 1.0
Relative Length x/L
Tim
et[
ms]
Strain amplitude
Figure 27. Results of modeling the integrity test of the 19m long pile in situ: image plots of the time and space dependent strain amplitude. There are 3 cross-section reductions of 1 element’s length (0.19 m) at x/L = 0.2, 0.3, and 0.7 by 10% (left) and 20% (right).
Hence, we assume that with the implementation of a sufficient number of sensors in a pile the assessment of defects can be made easier and more accurate.
-0.0010-0.0005
00.00050.00100.00150.00200.0025
0 2 4 6 8 10 12 14 16 18 20
Record ID: 1FDM/Klei layer = variableFDM/pile with faults (10%)FDM/pile with faults (20%)
Time t [ms]
Par
ticle
velo
city
v[m
/s]
Pile head
Figure 28. Integrity test of the 19m long pile in situ: comparison between the accelerometer measurement and the answer of the 1-D-FDM model for the intact pile and the pile with 3 cross-section reductions of 10% and 20% (cf. Figure 11).
-5x10-7
-3x10-7
-1x10-7
1x10-7
0 0.2 0.4 0.6 0.8 1.0
time = 0.88 mstime = 1.32 mstime = 1.76 mstime = 4.07 ms
Relative length x/L
Stra
in[m
/m]
Figure 29. Integrity test of the 19m long pile in situ: Strain as a function of space, modeled by the 1-D-FDM model.
6 CONCLUSIONS
Low-strain and high-strain dynamic pile tests as well as static load tests were performed with small-scale model piles and full-scale precast concrete driven piles to proof that fiber-optic extrinsic Fabry-Perot interferometer sensors can be used in the field of geotechnique. The results of the pile tests show that the developed concrete-embedded fiber-optic sensor with its sensor-cage allows the measurement of three different tests with a single measuring system. Especially during PIT, the Fabry-Perot sensors record very small deformations inside the concrete structure, and using a number of embedded sensors distributed along the pile length at defined locations, PIT allows the determination of material properties by calculating the wave velocity of different pile sections separately.
Results from fiber optic sensors were compared with measurement results obtained from conventional resistive strain gages and accelerometer sensors. Especially in the field test,both measuring systems showed good agreement.
7 OUTLOOK
Future work includes the optimization of the sensor-cage, the development of an improved processing of the EFPI raw data and the continuation of the accompanying numerical modeling to improve the quality assessment and estimate of the bearing capacity of concrete piles by using the signals from embedded sensors. The ultimate goal is a complete fiber optic measuring and interpretation system.
ACKNOWLEDGEMENTS
The work has been supported by the German Federal Ministry of Economics and Technology and German industrial partners (Astro- und Feinwerktechnik Adlershof GmbH Berlin, Bilfinger Berger AG Mannheim, Gesellschaft für Schwingungsunter-suchungen und dynamische Prüfmethoden mbH Mannheim, Glötzl GmbH Rheinstetten and Centrum Pfähle GmbH Hamburg). It has been carried out under the leadership of the German Federal Institute for Materials Research and Testing (BAM) Berlin together with the Federal Institute for Geosciences and Natural Resources (BGR) Hannover and the Institute for Foundation Engineering and Soil Mechanics (IGB), Technical University of Braunschweig, Germany. The authors thank all partners involved for the very helpful discussions,contributions and support. Ms. Dr. I.R. Göbel is especially indebted to Ms. R. Pfeiffer (BGR) for contributions to the numerical modeling.
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