LOAD TRANSFER CURVES FROM PILE MONITORING WITH DISTRIBUTED

FIBRE OPTIC SENSORS

Francine Tchamaleu* (francinechantal.tchamaleupangop@studenti.unipd.it), Simonetta Cola*, Silvia Bersan **,

Luca Schenato***

* Università degli studi di Padova, Dipartimento ICEA

** CRUX Engineering, Phoenixstraat 28c, Delft, Netherlands

*** Research Institute for Geo-Hydrological Protection, National Research Council, Italy

ABSTRACT. This study presents the results from a static load test on a continuous flight auger (CFA) pile

instrumented with distributed fiber optic sensors (DFOS) for strain measurements. It aims to evaluate the capability

of DFOS based on Optical Frequency Domain Reflectometry (OFDR) technology to provide reliable strain data

necessary to get the real load transfer curves at the pile-soil interface and compare them with load transfer functions

available in the literature. A direct approach methodology using MATLAB algorithms combined with extrapolation

functions such as the one proposed by Chin and a ratio function is used to find out the corresponding load transfer

curves developed during the load test for each soil layer around the pile shaft at the pile tip. As a result, DFOSs

have been demonstrated to be sufficient good as monitoring devices of strain and analyzing the actual performance

of such pile during the static load test.

1. INTRODUCTION

The monitoring of foundation piles is essential to control their bearing capacity and quality. It is also a valuable

tool for design optimization and construction practices. However, instrumentation traditionally used in monitoring

piles has been demonstrated to be insufficient in giving an accurate description of their ultimate shaft resistance.

The progress in technology and computer science, with the use of DFOS, is expected to improve the monitoring of

civil engineering structures and reduce the gap between theory and practice. This innovative sensor system provides

thousands of “strain gauges” along a single cable connected to the structures, embedded in the soil, or inserted into

a borehole (Soga, 2014).

The distributed sensing technology

used in this study consisted of Optical

Frequency Domain Reflectometry

(OFDR) based on Rayleigh backscattering

utilizing a commercial interrogator

(Optical Backscatter Reflectometer

OBR4600 from Luna Innovation Inc.). In

this case study, the fiber is interrogated

only through one end, and the intensity of

the backscattered signal in single-mode

fiber can be sampled every 20 μm along

with the fiber up to 70m. This technique

allows sampling the longitudinal strain

exerted to the fiber with a spatial

resolution of less than 1 cm. Rayleigh

DFOSs have been chosen in this case

because of the high spatial resolution and

thus to obtain a detailed view of the

infrastructure characteristics (Bersan et

al., 2018).

Battista et al. (2016) and Soga

(2014) presented several cases in which the analysis of the strain and temperature data from DFOS based on

Brillouin time domain reflectometry (BOTDA), in which results from DFOS data is always interpreted with FEM

model based on hyperbolic load transfer function coupled with optimization methods.

In this paper, a static load test on a CFA pile instrumented with Rayleigh-based DFOSs is performed to

verify the design of the foundation system of new steel tied arched bridge in Treviso (Italy). The load transfer

method is used to describe the stress-strain relationship at the pile-soil interface by using a 1D FEM spring-mass

model for both pile and soil response to conduct the load settlement analysis. Many authors proposed different load

(a) CPT profile (b) Pile load test equipment

Figure 1: Site characterization and pile load test equipment

X Incontro Annuale dei Giovani Ingegneri Geotecnici. Atti del Convegno ‒ F. Ceccato, M. Rosone e S. Stacul © 2021 Associazione Geotecnica Italiana, Roma, Italia, ISBN 978-88-97517-16-0

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transfer functions in literature, such as Reese et al. (1969) and Chin-Kondner (1970), generally used in pile design.

According to these load transfer functions, the unit shaft resistance and base resistance are expressed as a function

of pile settlement and other specific parameters related to some geotechnical properties of soil and pile typology.

However, using these functions may lead to many issues and errors in evaluating the actual bearing capacity of the

pile.

Bersan et al. (2018) presented a first analysis of the strain data results from DFOS of this case study. The

authors highlight the advantages of using the high spatial resolution offered by the OFDR technique to identify

local anomalies and changes of strain with great accuracy. However, the estimation of the load transfer curve is

based on the calibration of DFOS strain results only for the sand layer (from 7m-12m) by using the 1D FE model

coupled with the gradient-based optimization technique developed in COMSOL Multiphysics. In this paper, we

present all the possible sources of strain measured by DFOS during load application, and a direct approach to

analyze the continuous strain data along the pile shaft for each soil layer at each load step is adopted. This

interpretation method improves the understanding of the actual load transfer mechanism and allows to better

evaluate the efficiency of DFOS as a standard monitoring instrument for geotechnical structures.

2. SITE CHARACTERIZATION AND PILE LOAD TEST

A new steel tied-arch bridge, with a span of 65m, is part of the “Terraglio Est” road, which connects the

highway A4 to the city of Treviso in the Venice area. The bridge foundation system comprises 20 and 18 CFA

piles with a length of 22m and 23.5 m at North and South abutments, respectively. The piles have a diameter of

0.64m and are made of micro concrete C25/30 reinforced with B450C steel bars connected with spirals.

A static cone penetration test (CPT) and boreholes were performed at each side of the bridge abutment to

identify the soil profile and relevant geotechnical soil properties. The ground condition is quite similar under both

bridge abutments, and it is characterized by a silty clay deposit in the first 7m, overlying relatively dense sand,

which extends until 18m depth below ground level. From 18m to 22m, the profile shows a stiffer clay soil overlain

a thick deposit of a very dense sandy gravel soil (Figure1a).

A sacrificial pile was built at the edge of the pile group of the South abutment with a total length of 24.5 m

because the ground level at the moment of the test was higher than the design level of the raft. It had 3 sections of

steel reinforcement cages and was instrumented with 3 lengths of single-mode fiber-optic Brugg BRUsens V9

cables for strain measurements. The 3 Fiber-Optic (FO) cables were regularly spaced at 120° and fastened for the

first time at the top of the cage with stainless steel cable ties. Then, the cage was lifted in a vertical position with

two cranes; the FO cables were tensioned with a weight of 15 kg at the lower end and then fastened again with

those stainless cable ties at 1.30 m above the end cage. Finally, plastic cable ties and adhesive tape were used at

each 2-3m along the bar to clamp the cable and ensure the connection with the steel reinforcement cage.

Unfortunately, cable 2 broke during the installation procedure (figure 1b). Further details about the instrumentation

of the test pile can be found in Bersan et al. (2018).

3. RESULTS AND INTERPRETATION

Pile load test equipment is composed of a reaction beam anchored to the ground utilizing four tensile piles,

among which two were an integral part of the south abutment foundation, and two were only built for loading test

purposes (Figure1b). Three Linear Variable Displacement Transducers (LVDTs) fixed at the side of the pile cap

were used to monitor pile head settlement. During the maintained static

load test, the pile was subjected to a vertical compression load using 3

hydraulic jacks resting on the concrete cap for two load stages until the

maximum load of 3080kN. At each load stage, the load was maintained for

a duration of 20min, the entire load test lasts for about 10h, and strain

measurements are obtained with a spatial resolution of 10mm by

interrogating the FO cable with the OFDR technique.

3.1 DFOS strain results

The pile top settlement monitored using the 3 LVDTs during the

load test is plotted in Figure 2 as a function of the applied load. The

Figure 2 Load-displacement curves from

pile load test (Bersan et al., 2018).

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different values among them reflect the

presence of a slight eccentricity of the

load applied at the pile cap. This

eccentricity can also be observed on the

strain measured along each FO cable

(Figure 3). We can deduce that the load

is applied more on the plane of F1-F3

cable because of the high value of strain

up to 350με whereas maximum strain at

F2 cable is only around 180με.

Significant differences in strain are

observed in the first 4 m, where the pile

crosses a soil layer with low resistance

according to the CPT profile (Figure1a).

From this observation, it was decided to

neglect the important strain variations

measured by the DFOS in the first layer

because they are not justified by possible

resistance of the soil in that layer and,

indeed, they could be an effect of the

bending moment.

DFOS installed in construction material can measure strain from different sources; structural, thermal,

rheological, and others. In this case, only the contribution of structural strain sources is considered since the

variation of concrete temperature during the load test can be negligible. We can identify axial and bending strain

along the FO cable among structural strains due to load eccentricity.

3.2 Load transfer curve and extrapolation method.

Two hypotheses were considered to obtain the load transfer curves, t-z, and q-z. The vertical displacement

(z) was determined by integrating strain profile from top to bottom, whereas the unit shaft resistance (t) and the

unit base resistance (q) were determined by differentiating the load with respect to pile depth. The elastic modulus

of the reinforced concrete pile is considered constant along with the depth, E=37 GPa, and we also assume a

constant cross-section along with pile depth.

By selecting some “reference” sensing points along the FO cable based on the stratigraphy of the soil, we

can estimate the average axial strain contribution at each load step due to only the axial load effect (Figure 4a). An

algorithm implemented in a MATLAB code, adopting the same procedure proposed in Mohamad & Tee (2015),

(a) (b) (c)

Figure 4: (a) average measured and approximated DFOS strain profile; (b) displacement along pile

length; (c) unit shear resistance at pile shaft.

Figure 3 Results from Pile load test (Bersan et al., 2018)

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was employed, using only the approximated axial strain instead of the total strain measured by DFOS. From direct

integration and differentiation of the approximated axial strain along with pile depth, the displacement and unit

shaft friction profiles along with the pile at each load step were obtained (Figures 4b and 4c). These data are the

fundamental parameters for defining the load transfer curve for each soil layer at the pile shaft and pile tip. Figure

5 shows the load transfer data obtained from DFOS and their interpolation/extrapolation using a load transfer

function of Chin-type for clayed soils and a ratio function for sandy soils. Good agreement can be observed among

the DFOS load transfer data and those proposed in the literature.

4. CONCLUSION

A static load test is conducted on a CFA pile instrumented with Rayleigh DFOSs interrogated with OFDR

technology to provide strain measurements with a spatial resolution of 10mm. Continuous strain profile along the

FO cables combined with pile head settlement from LVDTs at each load step helped separate strain from axial load

and bending moment generated by load eccentricity from the hydraulic jacks.

DFOS have been demonstrated to be a reliable monitoring instrument to infer load transfer curves for piles.

These can be combined with both theoretical approaches and FEM to reduce the uncertainties during the evaluation

of load transfer curves. Moreover, these load transfer curves from direct integration and differentiation of DFOS

strain measurement agreed with load transfer functions proposed in the literature for pile design. However, despite

DFOS offers many advantages in monitoring geotechnical structures, they are mechanically fragile, so they need

to be handled properly during the installation process.

5. BIBLIOGRAPHY

De Battista, N., Kechavarzi, C., & Soga, K. (2016). Distributed fiber optic sensors for monitoring reinforced concrete piles

using Brillouin scattering. https://doi.org/10.1117/12.2236633.

Schenato, L. (2017). A review of distributed fibre optic sensors for geo-hydrological applications. Appl. Sci. 2017, 7(9),

896. https://doi.org/10.3390/app7090896.

Bersan S., Bergamo O., Palmieri L., Schenato L., Simonini P. (2018). Distributed strain measurement in a CFA pile using high

spatial resolution fibre optic sensors. Engineering structure, 160, 554-565. https://doi.org/10.1016/j.engstruct.2018.01.046.

Mohamad H, Tee BP. (2015). Instrumented pile load testing with distributed optical fibre strain sensor. Journal Teknologi,

77(11):186–93. http://dx.doi.org/10.11113/jt.v77.6381.

Soga K. (2014) Understanding the real performance of geotechnical structures using an innovative fibre optic distributed strain

measurement technology. Rivista Italiana di Geotecnica. 48: 7-48.

Reese, L. C., Hudson, W. R., & Vijayvergiya, V. N. (1969). An investigation of the interaction between bored piles and soil.

In Soil Mech & Fdn Eng Conf Proc/Mexico/.

Castelli, F., Maugeri, M., & Motta, E. (1992). Analisi non lineare del cedimento di un Palo Singolo. RIG, 2, 92.

Figure 5: Load transfer data from DFOS and the extrapolated curves at each soil layer and at pile tip.

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