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- Volume 14, Issue 6, 2016
Near Surface Geophysics - Volume 14, Issue 6, 2016
Volume 14, Issue 6, 2016
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Detecting anomalies in diaphragm walls with electrical resistance measurements
Authors Rodriaan Spruit, Frits Van Tol, Wout Broere and Evert SlobABSTRACTQuality control of diaphragm walls prior to excavation is often difficult. One technique that can be used to detect anomalies in diaphragm walls involves electrical resistance. Electrical resistance measurements across a diaphragm wall can (within a strict framework) be used to verify the presence of leaks in diaphragm walls as a supplement to crosshole sonic logging. From measurements around a test wall conducted in this study, it is concluded that the detectability of anomalies with electrical resistance decreases exponentially with the increasing distance between the measurement electrodes and the wall. Electrical resistance setups with two and four electrodes have been compared. For usable results, a four‐electrode setup must be used in which the potential electrodes need to be placed very close to the wall (less than 0.2 m away). Based upon the test experience, a field setup for verification of a building pit consisting of diaphragm walls is suggested, as well as a setup for determining the quality of the concrete covering the rebar in quay walls constructed with diaphragm walls.
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Crosshole and downhole seismics: a new quality assurance tool for jet grout columns
More LessABSTRACTSealing and strengthening of the subsoil by grout injection is a major issue in the field of geotechnical engineering. One commonly applied method is jet grouting, which allows creating columns of grouted soil by eroding and mixing the in‐situ soil with a thin cement suspension. A general difficulty linked with this method is in predicting the resulting column diameter and its material strength. In this paper, we illustrate the application of a newly developed non‐destructive quality assurance tool used to determine the diameter of jet grout columns. This approach incorporates standard crosshole and downhole seismic measurements. To demonstrate its effectiveness, we tested the new approach within two‐dimensional finite‐difference numerical simulations. Additional field tests showed that this tool is also applicable in real site conditions. For this purpose, three jet grout columns were produced with different process parameters in a depth between 3 and 10 m. The evaluated diameters were within 1 and 1.5 m, slightly deviating from the previously predicted range by the jet grouting contractor. Moreover, we were able to detect the base of the columns at a 10‐m depth with no significant difficulties. On the other hand, unsaturated, less compacted sands between the groundwater level and surface considerably affected the seismic data, hence complicating the detection of the top of the columns.
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Advances in pile integrity testing
Authors Jens-Peter Ertel, Ernst Niederleithinger and Maria GrohmannABSTRACTFor decades, the low‐strain impact integrity testing using a hammer blow is well established as a method of quality assurance for various pile types. However, this method has its limitations. Our research and development focuses on improving the excitation signal using a shaker system in contrast to the standard hammer method. Another approach is to increase the amount of sensors used during testing. The purpose is to identify the direction of wave propagation that gives advantages under difficult conditions, such as piles below structures.
Pile integrity testing using a shaker system was performed on two 11‐m‐long piles of 90 cm in diameter. While one pile was intact, the other one showed a flaw at approximately 3.5 m below pile top, which was confirmed by standard pile integrity testing in 2012. A logarithmic sweep between 500 Hz and 1 KHz of 0.1 s was used as the input signal, being vertically injected into the pile. Prior to that, simulations on similar pile geometries showed that the depth of the pile toe as well as flaws within the pile can be extracted by applying regularised deconvolution. The result is the impulse response in the time domain.
The application of deconvolution on the measured signals shows that it is possible to identify the pile length, but it is more difficult to clearly extract the flaw’s position in the pile. Additional digital signal processing techniques and the improvement of the regularised deconvolution method and the experimental setup need to be investigated.
Another way to improve the pile integrity testing method is to use a multi‐channel sensor arrangement. By arranging several accelerometers vertically along the accessible part of the pile shaft, it is possible to distinguish between downward and upward travelling waves. Furthermore, it is possible to estimate the unknown wave speed, which gives the possibility of more accurate pile length calculations. The method was evaluated successfully during a measurement campaign of a slab foundation with subjacent piles. In 20 of 28 cases, the pile length could be detected accurately.
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Behaviour and frequency characteristics of acoustic emissions from sandy ground under model pile penetration
Authors Wuwei Mao, Shogo Aoyama, Shigeru Goto and Ikuo TowhataABSTRACTAcoustic emission (AE) testing is regarded as an effective non‐destructive technique and is capable of detecting micro‐level defects inside a material. In the field of civil engineering, the acoustic emission technique has been widely applied to the studies of steelwork, concrete, and composite materials. However, the geophysical investigations of the application of acoustic emission, dealing with porous granular media, are comparatively limited. In this study, the acoustic emission testing is implemented in a model pile system to investigate subsoil behaviour subjected to pile penetration. The results reveal that the tendency of acoustic emission settlement and load settlement shows high similarity. In addition, the detected acoustic emission signals are studied in the frequency domain using fast Fourier transformation. Higher frequency acoustic emission signals (>100 kHz) are interpreted to be associated with sand particle crushing, which provides a new insight to evaluate the feature of sand grain crushing. Furthermore, the distinction of acoustic emission characteristics observed among different pile penetration sequences demonstrates the effect of ground density on the subsoil behaviour. The results obtained in this paper are beneficial to further clarify the bearing mechanism of pile foundations and also to provide useful information on the fundamental characteristics of acoustic emission signals originating from stressed granular soils, that can be extended to other acoustic emission‐based field investigations.
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Using machine learning algorithms to link volumetric water content to complex dielectric permittivity in a wide (33–2000 MHz) frequency band for hydraulic concretes
More LessABSTRACTThis paper focuses on the development and validation of an innovative method for estimating volumetric water content in concrete mixtures. A supervised learning method (support vector machine) has been used to resolve the inverse problem, i.e., generate in‐laboratory calibration curves correlating the controlled water content in various concrete mixtures with the frequency‐dependent complex dielectric permittivity originating from the coaxial electromagnetic transition line. An extrapolation procedure using a frequency‐power‐law model has been developed and validated for estimating the complex permittivity over a broad frequency bandwidth. Implementation of this extrapolation method allows considering various physical phenomena (i.e., polarisation versus water content) that typically affect the dielectric behaviour of concrete as a function of frequency. The two‐step estimation procedure (involving extrapolation and support vector regression methods) proposed in this paper has been validated on a wide array of moisture‐controlled concrete specimens in the laboratory. The procedure helps building calibration curves that rely on both complex effective permittivity and volumetric water content, taking into consideration the frequency dependence.
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A comparison of modified free‐space (MFS), GPR, and TDR techniques for permittivity characterisation of unbound granular pavement materials
Authors Wayne B. Muller, Habibullah Bhuyan and Alexander ScheuermannABSTRACTThis paper reports on a laboratory experiment comparing permittivity measurements using a modified free‐space approach to results using common‐offset ground‐penetrating radar and time‐domain reflectometry on moist and compacted samples of unbound granular road pavement materials. In the first part of the experiment, unbound granular samples from the same source were prepared to varying moisture contents and a fixed target density. Separate samples were prepared for modified free‐space and time‐domain reflectometry testing, all of which were also measured using ground‐penetrating radar. In the second part of the experiment, samples were mixed to a consistent gravimetric moisture content and varying densities before undertaking the modified free‐space, time‐domain reflectometry, and ground‐penetrating radar measurements. Reasonably good agreement was found between modified free‐space and ground‐penetrating radar measurements, which also compared well with literature relations for crushed rock pavement materials. The time‐domain reflectometry results were relatively consistent with those literature relations, although they appeared to deviate from the ground‐penetrating radar and trend of modified free‐space results for lower density and drier samples.
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Rock permittivity characterization and application of electromagnetic mixing models for density/compactness assessment of HMA by means of step‐frequency radar
ABSTRACTThis work aims to determine the compactness/density of hot mix asphalt by measuring its permittivity by means of step‐frequency radar. As hot mix asphalt (HMA) is mainly made of rocks; their dielectric properties are measured in the frequency range of 0.5 ‐ 4 Ghz with step‐frequency radar, using cylindrical cavities. The results show that the rocks can be considered as low‐loss dielectric. As electromagnetic mixing models are required to translate measured permittivity to the compactness, power law models and unified mixing rules are needed for laboratory experimental data. The slab permittivity of various compactness is determined with the help of the step‐frequency radar system. This study shows that: (i) the selection of the electromagnetic mixing model has a critical impact on the accuracy of the calculated compactness; (ii) the choice of the host matrix for a family of unified mixing rules has huge consequences; and (iii) the best assessment of compactness/density is given by the complex refractive index model and Rayleigh and Böttcher models with an aggregate matrix.
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Volumes & issues
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Volume 22 (2024)
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Volume 21 (2023)
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Volume 20 (2022)
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Volume 19 (2021)
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Volume 18 (2020)
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Volume 17 (2019)
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Volume 16 (2018)
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Volume 15 (2017)
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Volume 14 (2015 - 2016)
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Volume 13 (2015)
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Volume 12 (2013 - 2014)
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Volume 11 (2013)
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Volume 10 (2012)
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Volume 9 (2011)
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Volume 8 (2010)
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Volume 7 (2009)
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Volume 6 (2008)
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Volume 5 (2007)
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Volume 4 (2006)
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Volume 3 (2005)
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Volume 2 (2004)
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Volume 1 (2003)