Optimization of Impact Pile Driving Using Optical Fiber Bragg-Grating Measurements (original) (raw)
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In recent years, fibre Bragg grating (FBG) sensors have emerged as a relatively new strain sensing technology for civil engineering applications. This paper presents a field trial to assess the feasibility of using FBG sensor arrays to measure strain in driven steel piles. Two FBG arrays were installed in grooves within the wall of an open-ended steel pile such that the finished profile was completely flush with the pile shaft. The pile was then driven into a dense sand deposit using an impact hammer to provide the required installation energy. The FBG gauges were monitored throughout driving in conjunction with accelerometers to quantify the scale of the hammer impacts. The FBG sensors were subjected to hammer blows that yielded pile accelerations between 500 g and 1400 g during installation. The fibre optic sensors were measured throughout driving, where they were observed to respond to the hammer impacts, showing a rapid increase in strain and a return to their initial values bet...
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The XVII European Conference on Soil Mechanics and Geotechnical Engineering, 2019
Improved design of vertically loaded battered pile groups can benefit various geotechnical applications, such as slope stabilisation, earth retention and bridge abutments. Current design methods rely on model pile load tests, while full-scale field tests and numerical modelling based on field results are limited. Questions still exist whether these design methods are accurate enough in predicting the load capacity and settlement behavior of battered piles. To investigate this problem, a steel footing system, termed as a battered mini driven pile group system has been tested. The system comprises of a steel pile cap with varying number of open-ended steel mini piles installed in a battered condition. This paper reports the soil-pile interaction and group effect of battered mini pile based on field pull-out load tests obtained in a site consisted of medium to dense sand deposits. The strain profile along the length of a single battered mini pile was measured with fibre Bragg grating (FBG) sensors during loading. The FBG measurements show that the measured strain profile has good agreement with lateral soil resistance model proposed in literature. From the quick pile load test results of battered mini pile group, it was found that the the group efficiency factor was greater than unity with current configurations but there is a sign of negative group effect with increasing number of piles, indicating the limitation of current design method.
Application Of Fibre Bragg Grating Sensor In Mini Pile Performance And Behaviour Monitoring Final
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Fibre Bragg grating (FBG) sensor technology has been recognised as a reliable strain sensing system in geotechnical engineering due to its certain advantages over traditional electrical resistance strain gauges. However, the uses of FBG sensors in geotechnical applications are still limited. This paper demonstrates an application of FBG sensors for measuring axial strain in a driven mini pile. Installation and calibration methods , and data interpretation of FBG sensors are discussed. After calibration, mini pile was instrumented with FBG sensors and driven into a medium to dense sand deposit using a high-frequency hand-held jackhammer, followed by a quick pull-out pile load test. The results confirm the viability of FBG sensors in measuring stress developed along pile's shaft and demonstrates the robustness of FBG in withstanding the high frequency of pile installation. Distribution of axial strains along the pile shaft was found to be similar to reported patterns in the literature.
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With the advancement in the design procedures from code-based to performance based design, monitoring of structural response has become an integral part of civil engineering expertise. In the past decades, significant increase has been observed in the construction of tall buildings, sky trains, long span bridges, wind turbines and transmission towers etc., which requires pile foundations. Conventional methods of monitoring pile performance, using strain gauges, ultrasonic and acoustic emissions are known to have limitations i.e., not capable of continuous and remote performance predictions, vulnerability to electromagnetic interference, and unreliable monitoring at greater depths, leading to imprecise and inefficient outcomes. Introduction of fiber optic sensors (FOSs) is a good innovated technology for reliable monitoring of performance of spatially distributed subsurface structures such as piles. FOSs offer advantages such as; high accuracy, long life cycle, high temperature durability, resistance to electromagnetic fields and radio frequency intrusion. This paper provides treatise relating to the development of fiber optic sensor technology and its application in monitoring pile performance such as; pile-soil friction distributions, variation of strain distribution with depth, evaluation of load capacity and failure localization, load transfer behavior, temperature monitoring, static and dynamic responses. Also discussed are various types of fiber optic sensors with appropriate protection system and existing shortcomings of fiber optic sensors.
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An instrumented pile load test was conducted for a 1.2m diameter bored pile at Putrajaya to verify pile performance towards geotechnical design. This test pile was instrumented with new monitoring technique using distributed strain sensing known as Brillouin Optical Time Domain Analysis (BOTDA) and compared with conventional sensors, i.e. vibrating wire strain gauge, LVDT (linear variable differential transformer) and dial gauge. This manuscript includes the description of subsurface conditions consisting of weathered granitic residual soils, test pile installation and instrumentation setup of Maintain Load Test (MLT). Field measurement results such as the load transfer response and average unit shaft resistance using the distributed fibre optic strain sensor were well matched with the results using the conventional sensors. However, the distributed fibre optic strain sensor has the added advantage of detecting the localized defect such as pile necking, bending, and overall behaviou...
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Recent technological advancement of optical fibre sensing has led new ways in measuring the performance of geotechnical structures. The distributed sensing, namely Brillouin Optical Time Domain Analysis (BOTDA) is a novel technique of measuring strains in a continuous manner which has inherent distinct advantages over conventional point-based sensors. In bored pile instrumentation particularly, obtaining distributed strain profile is important when analysing the load-transfer and shaft friction of a pile, as well as detecting any anomalies in the strain regime. Features such as defective pile shaft necking, discontinuity of concrete, intrusion of foreign matter and improper toe formation due to contamination of concrete at base with soil particles, among others, may reduce the pile capacity. The objective of this article is to present one of the earliest deployed BOTDA optical fibre sensors in rock-socketing bored piles in Malaysia and share invaluable lessons learned from the instr...
Static Pile Load Tests: Contribution of the Measurement of Strains by Optical Fiber
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The use of optical fiber for measuring strain and or temperature is increasing in civil engineering. Optical fiber is known to be relatively cheap, compared to the more traditional sensors commonly used, and can, depending on the chosen technology, give a complete distribution along the instrumented element, which can be very interesting in case of an element with variable dimensions along the length of said element. Therefore, for static pile loading tests, the use of optical fiber should present many advantages, compared to more conventional methods of strain measurements. The aim of this paper is to shed new light on the advantages and disadvantages of using optical fiber for the measurement of deformations along a foundation element. After a review of the state of the art of the most commonly used methods, the contribution of the optical fiber for the instrumentation of the foundation elements is evaluated, by comparing the results obtained by the use of the fiber with those obt...
Interpretation Method of Distributed Fibre Optic Strain Sensor in Instrumented Static Pile Load Test
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Interest on the instrumented pile load test using Distributed Fibre Optic Strain Sensor (DFOSS) has become a main discussion topic among engineers in the geotechnical field. Limitation of conventional instruments like Vibrating Wire Strain Gauge (VWSG) have led to the need of improvement in the sensing technology. DFOSS technology offers a continuous strain profile in a single sensing cable, and it differs from the conventional sensing technology, that only provides a point-wise measurement. Interpretation of the pile load results to establish concrete modulus and load transfer curves are critically analysed in this paper. This paper aims to propose the best methodology of unit shaft friction curve fitting method based on several points selection methods to provide a reliable and presentable of design parameter.
Distributed Fiber Optic Sensing of Axially Loaded Bored Piles
Journal of Geotechnical and Geoenvironmental Engineering, 2018
Instrumented pile tests are vital to establish the performance of a pile and validate the assumptions made during initial design. Conventional instrumentation includes vibrating wire strain gauges and extensometers to measure the change in strain or displacements within a pile. While these strain and displacement gauges are very accurate, they only provide strain/displacement readings at discrete locations at which they are installed. It is therefore common to interpolate between two consecutive points to obtain the values corresponding to the data gaps in between; in practice, these discrete instrumented points could be tens of Manuscript Click here to download Manuscript Pelecanos_et_al_Manuscript_7.2.docx 2 meters apart, at depths corresponding to different soil layers, and hence simple interpolation between the measurement points remains questionable. The Brillouin Optical Time Domain Reflectometry fibre optic strain sensing system however is able to provide distributed strain sensing along the entire length of the cable, enabling the full strain profile to be measured during a maintained pile load test. The strain data can also be integrated to obtain the displacement profile. In this paper, three case studies are presented where the performance of three concrete bored piles in London is investigated using both conventional vibrating wire strain gauges and distributed fibre optic strain sensing during maintained pile load tests which enabled comparisons to be made between the two instrumentation systems. In addition, finite element analyses were conducted for the three piles and it was found that the ability to measure the full strain profiles for each pile is highly advantageous in understanding the performance of the pile and in detecting any abnormalities in the pile behaviour.