Soil Deformation Measurements Research Articles

Optical fiber displacement sensor based on Stokes Raman backscattering light bending loss

Optical fiber displacement sensor has the advantages of high sensitivity, large dynamic range, long sensing distance and high electromagnetic interference resistance, which has been widely used in many industrial and civil fields. However, multi-parameter simultaneous sensing is the common requirement in many applications. In this paper, we demonstrated a novel and simple optical fiber displacement sensor based on the principle of Stokes Raman backscattering, and the Stokes Raman signal is used to sense the bending loss of the sensing fiber. An optical fiber displacement sensing system and a recoverable displacement sensor is designed and built to measure the optical power loss, which makes it possible to detect the temperature and displacement simultaneously use one optical fiber. The displacement measurement is carried out for five times at 0.25 mm intervals in the range of 0–5 mm, and the experimental results show that there is a good linearity between measured and applied displacement. Its measurement accuracy is 0.1 mm and maximum sensitivity is 0.343 dB/mm. This sensor has the potential to be applied in foundation settlement monitoring, safety pre-warning of subway on-site, soil deformation measurement, and civil structure crack monitoring.

Soil restraint on buried pipelines during oblique relative movements in sand

Soil restraints associated with relative soil-pipe movements is an important consideration for stability assessment of buried pipelines. This paper investigates the effects of pipe surface roughness, inclination angle within the horizontal-vertical plane and embedment ratio on the development of soil restraint on shallow pipes in sand. In total, 24 model tests were performed, using an image-based non-destructive soil deformation measurement technique. The failure and deformation mechanisms during oblique pipe movements were discussed based on image analysis results. The test results were then used to verify the Finite Element Limit Analysis (FELA) model, by which a parametric study on the peak soil restraint in a wider range of soil and pipe parameters was carried out. It is shown that the peak soil restraint increased with greater pipe embedment ratio, inclination angle of movements to the vertical and sand-pipe interface frictional strength because of a larger range of soil was mobilised, and the effects of these factors reduced at deeper depths. To account for the effects of oblique movements on the peak resistance received by pipelines in sand, an analytical relationship is presented and examined based on the test and simulation results.

Measurement of Suction Stress and Soil Deformation at High Suction Range

Abstract Suction stress is the effective stress independent of mechanical boundary conditions but is due to water content variation. It occurs on or near particle contacts and does not directly transfer through the soil skeleton and hence can only be and has been measured indirectly and deduced from soil’s strength or deformation. In this work, a new technology to measure the soil-water retention curve, soil shrinkage curve, and suction stress characteristic curve in a high suction environment is established. A humidity-controlled device to measure suction stress was developed based on a previously established drying cake method. Nitrogen gas and molecular sieves were introduced as desiccant in addition to saturated salt solutions to generate an extreme dry environment up to 850 MPa of matric suction for the first time. A focus on the effect of adsorptive soil-water interaction on hydromechanical properties of expansive soils provides enriched information on elastic modulus variation, soil deformation evolution, and matric suction development of soil in very low water contents, making the suction stress measurement possible for matric suction as high as 850 MPa.

Vision-based measurement of spatio-temporal deformation of excavated soil for the estimation of bucket resistive force

This paper reports a vision-based technique of measuring the spatio-temporal deformation of excavated soil for estimating the bucket resistive force. The proposed measurement technique uses two depth cameras to determine three-dimensional soil-surface displacement. The technique consists of the following two processes: the first is related to image correlation between the two cameras, and the second involves data filtering and smoothing for generating soil deformation as a continuously curved surface. The proposed technique delivers measurement accuracy to the nearest centimeter. Typical experimental results of the three-dimensional measurement of soil deformation using the proposed technique are presented in the paper. Further, this study updates an interaction model for the resistive-force estimation while a bucket excavates soil. The model introduces a correction variable that changes with the bucket wrist angle by exploiting the experimental measurement of soil deformation. The model estimates the resistive force with an error of less than one quarter of the maximum force. These updates also exhibit the effectiveness of the proposed technique.

Mechanisms beneath rectangular shallow foundations on sands: vertical loading

This paper details analysis of deformation behaviour of silica and carbonate sands under a rectangular foundation subject to uniaxial vertical load based on results from a series of centrifuge model tests. A multiscale particle image velocimetry/digital image correlation (PIV/DIC) technique was used to record and analyse the foundation tests with high resolution and measurement precision. Cone penetrometer and pressuremeter tests provide in situ soil characterisation of the tested sand sample in the centrifuge environment. The soil behaviour is analysed through foundation load–settlement response and the observed soil deformation measurements. Different soil deformation mechanisms and strain behaviours were observed in the different sands tested, and the particle shape effect is considered, with data from scanning electron microscopy, to explain the differences. The results and analyses contribute towards better understanding of different soil behaviours under shallow foundations in different sands.

Rate effects on the interface shear behaviour of normally and overconsolidated clay

This paper presents a study on the effects of shearing velocity, surface roughness and overconsolidation ratio on the strength and deformation behaviour of fine-grained soil–structure interfaces. The results of shear box direct interface shear tests, augmented with soil deformation measurements from particle image velocimetry analyses, indicate that shearing velocity (i.e. velocity at which the continuum surface is displaced) has a controlling effect on the drainage conditions mobilised during shearing. As shearing velocity was increased, undrained conditions were progressively mobilised, characterised by smaller interface strength and volumetric changes for tests on normally consolidated specimens. Increases in shearing velocity resulted in increases in interface strength for overconsolidated specimens. The results presented herein indicate that, as the magnitude of surface roughness was increased, drained shearing conditions were favoured for a given shearing velocity. Soil deformation measurements indicate that the shear zone where localisation occurs becomes thicker as the surface roughness increases, and the magnitude of soil deformations decreases as shearing velocity is increased. The findings presented herein are relevant for prediction and interpretation of soil–structure interface behaviour, as well as for development of constitutive models that capture the combined effect of shear rate, surface roughness and stress history.

Soil creep in a mesotidal salt marsh channel bank: Fast, seasonal, and water table mediated

Muddy banks of marsh channels experience soil creep – a viscous-like slow deformation resulting in a net downslope transport. Here we present the first field evidence of soil creep in a mesotidal salt marsh using both pole displacement and high precision measurements of soil deformation taken with a vibrating-wire extensometer over two years. Soil extended 2–4 mm/m/year in the high marsh platform and 20–40 mm/m/year in the low marsh bank. This was equivalent to a soil diffusivity of about 0.2 m2/year, which is much higher than the diffusivity on hillslopes and also matches the high value predicted by long-term marsh evolution models. High precision (±0.03 mm) soil deformation measurements shed light on the dynamics of the creep process. First, the low bank compressed and extended by ~5 mm/m every tidal cycle, suggesting that the diurnal changes in water level play a role in the bank movements. Second, net soil extension took place mostly during fall. Net soil extension preceded the period of ice formation, suggesting that freezing and ice rafting are not a leading cause of soil creep. Net soil extension was likely triggered by an increase in water table during fall, which decreased the effective stress and thus destabilized the bank. This high water table was caused by a combination of an abiotic factor – the increase in atmospheric precipitation – and a biotic factor – the decrease in evapotranspiration triggered by vegetation senescence. This study confirms that creep is a significant process in marsh morphodynamics and that vegetation can reduce mass wasting in marsh channels.

A novel automatic device to measure deformation inside transparent soil based on digital image correlation technology

In geotechnical engineering, the measurement of soil deformation is one of the most important foundations of soil mechanics research. Transparent soil is a desired choice to obtain information on spatial deformation inside the soil. Based on this, a novel measuring device was developed to automatically record laser speckle images at different laser cross-sections, and the displacement deformation of transparent soil at arbitrary cross-section positions was determined by the planar digital image correlation (DIC) technique. The measuring system was built with two high-precision motorized linear stages. An industrial camera was used in the system to capture the laser speckle images, and synchronization control methods of both the laser transmitter and industrial camera were introduced. The planar DIC technique employed in this paper has been described in detail. The high-precision motorized linear stages proved to have excellent repeatability and the displacement field of a certain laser cross-section in transparent soil can be obtained. Theoretically, the spatial deformation inside the transparent soil can be built based on the displacement fields of multiple laser cross-sections. Finally, the design scheme was confirmed by the model pile experiment. The experiment results show that the automatic measuring system has stable operation, and the deformation of arbitrary laser cross-sections inside transparent soil can be obtained easily. This automatic measurement method can be regarded as an innovation in geotechnical physical model experiments.

A FBG based displacement transducer for small soil deformation measurement

A small soil deformation measurement system based on fiber Bragg grating (FBG) has been developed for underground displacement monitoring. This new displacement transducer consists of two anchorage plates for the fixation of displacement sensor in soil, a single FBG sensor for sensing the occurred deformation, and a PolyVinyl Chloride (PVC) tube for the protection of inner FBG sensor. The two anchorage plates were fabricated and fixed on the PVC tube surface by FDM process. Gauge length, maximum measurement displacement, and displacement resolution of the proposed FBG displacement transducer were 90 mm, 0.9 mm, and 0.0747 mm, respectively. Raw material of the anchorage plates was Polylactic Acid (PLA) material. An embankment model was established and installed with multiplexed four FBG displacement transducers for the measurement of internal horizontal displacement in laboratory. Both static load and movable load were applied on the top surface of the embankment model. It is found that maximum error of the four sensors under different static load was less than 6%. The monitored strains of four different FBG displacement transducers under movable load were highly consistent, indicating that the smart FBG based small deformation monitoring system can be used to monitor vehicle flow in future. All monitored displacement values were less than 0.03 mm. Therefore the FBG displacement transducer characterized by high resolution can be used to monitor small displacement occurred underground.

Measurement of micro-scale soil deformation around roots using four-dimensional synchrotron tomography and image correlation

This study applied time lapse (four-dimensional) synchrotron X-ray computed tomography to observe micro-scale interactions between plant roots and soil. Functionally contrasting maize root tips were repeatedly imaged during ingress into soil columns of varying water content and compaction. This yielded sequences of three-dimensional densiometric data, representing time-resolved geometric soil and root configurations at the micronmetre scale. These data were used as inputs for two full-field kinematic quantification methods, which enabled the analysis of three-dimensional soil deformation around elongating roots. Discrete object tracking was used to track rigid mineral grains, while continuum digital volume correlation was used to track grey-level patterns within local sub-volumes. These techniques both allowed full-field soil displacements to be quantified at an intra-rhizosphere spatial sampling scale of less than 300 µm. Significant differences in deformation mechanisms were identified around different phenotypes under different soil conditions. A uniquely strong contrast was observed between intact and de-capped roots grown in dry, compacted soil. This provides evidence that functional traits of the root cap significantly reduce the amount of soil disturbance per unit of root elongation, with this effect being particularly significant in drier soil.

Synchronised multi-scale image analysis of soil deformations

New apparatus and techniques for performing synchronised multi-scale particle image velocimetry or digital image correlation (PIV/DIC) soil deformation measurements have been developed. A central camera records a full field of view (FoV) of the model capturing the ‘macro’ deformation mechanism and the boundaries of the model. Simultaneously, an adjacent slave camera records a subset of the full FoV capturing the ‘micro’ soil response in a region of special interest, such as under the corner of footing. The ‘micro’ FoV images have higher resolution in terms of particle/pixel size ratio (d/p), resulting in the ability to measure localised deformations that are invisible to lower resolution images. Recommendations are made with respect to appropriate subset size and spacing for high-resolution images. A photogrammetric correction process requiring a small number of static control points is proposed and the performance is validated against a conventional photogrammetric calibration utilising a large array of static control points. Lastly, results from a validation experiment are presented comparing the PIV/DIC output from the ‘macro’ and ‘micro’ FoV, illustrating that: (a) the photogrammetric correction method proposed is robust and (b) that there has been an improvement in spatial resolution of the strain measurements that can be obtained through the ‘micro’ FoV camera.

Leveling vs. InSAR in urban underground construction monitoring: Pros and cons. Case of la sagrera railway station (Barcelona, Spain)

Monitoring is required when dewatering underground construction sites to anticipate unexpected events and preserve nearby existing structures and/or buildings. The most accurate and widespread monitoring method to measure displacements is leveling, a point-like surveying technique that typically allows for tens of discrete in situ sub-millimeter measures per squared kilometer. Another emerging technique for mapping soil deformation is the interferometric synthetic aperture radar (InSAR) method, which is based on SAR images acquired from orbiting satellites. This remote sensing technique can provide better spatial point density than leveling, more extensive spatial coverage and lower-cost acquisitions. This paper analyses, compares and discusses leveling and advanced multi-temporal InSAR measurements when they are used to measure the soil deformation induced by the dewatering associated with underground constructions in urban areas. This comparison, which has not been considered in previous works, is of paramount importance to ascertain the most suitable technique (or combination of techniques) in these contexts. To do so, an experiment was performed in the future railway station of La Sagrera, Barcelona (Spain), in which leveling and advanced multi-temporal InSAR were used to quantify ground deformation by dewatering. The results showed that soil displacements measured by leveling and InSAR were not always consistent. In the context of soil deformation measurements produced by dewatering in urban areas, InSAR measurements appear to be more accurate for investigating soil deformation, whereas leveling was more appropriate for quantifying the real impact on the nearby buildings.

Role of the interface between distributed fibre optic strain sensor and soil in ground deformation measurement.

Recently the distributed fibre optic strain sensing (DFOSS) technique has been applied to monitor deformations of various earth structures. However, the reliability of soil deformation measurements remains unclear. Here we present an integrated DFOSS- and photogrammetry-based test study on the deformation behaviour of a soil foundation model to highlight the role of strain sensing fibre–soil interface in DFOSS-based geotechnical monitoring. Then we investigate how the fibre–soil interfacial behaviour is influenced by environmental changes, and how the strain distribution along the fibre evolves during progressive interface failure. We observe that the fibre–soil interfacial bond is tightened and the measurement range of the fibre is extended under high densities or low water contents of soil. The plastic zone gradually occupies the whole fibre length when the soil deformation accumulates. Consequently, we derive a theoretical model to simulate the fibre–soil interfacial behaviour throughout the progressive failure process, which accords well with the experimental results. On this basis, we further propose that the reliability of measured strain can be determined by estimating the stress state of the fibre–soil interface. These findings may have important implications for interpreting and evaluating fibre optic strain measurements, and implementing reliable DFOSS-based geotechnical instrumentation.

Measuring unsaturated soil deformations during triaxial testing using a photogrammetry-based method

When characterizing an unsaturated soil using the triaxial test apparatus, it is required to measure the soil deformation during loading. Recently, a photogrammetry-based method has been developed for total and localized volume change measurements on unsaturated soils during triaxial testing. In this study, more in-depth discussions on the photogrammetry-based method are addressed, such as system setup, the measurement procedure, accuracy self-check, data post-processing, and differences from conventional image-based methods. Also, an application of the photogrammetry-based method on unsaturated soil deformation measurements is presented through a series of undrained triaxial tests with different loading paths. After testing, three-dimensional (3D) models of the tested soils at different loading steps were constructed based on the 3D coordinates of measurement targets on the soil surface. Clear barreling processes for soils during deviatoric loading were observed through the constructed 3D models at different axial strain levels. Soil volume changes and volumetric strain nonuniformities during isotropic and deviatoric loadings were extracted based upon detailed analyses of different soil layers. Through a full-field strain distribution analysis, a shear band evolution process was captured for the soil during deviatoric loading at a low confining stress. The photogrammetry-based method proved to be very powerful for in-depth soil deformation characteristics investigation.

Experimental study on pullout performance of sensing optical fibers in compacted sand

The distributed optical fiber sensing systems have played an increasingly important role in monitoring civil infrastructures over the past few years. One of the main challenges of their applications to geotechnical monitoring is to increase the reliability of strain sensing optical fibers in measuring the deformation of surrounding soil masses. In this paper, a pullout test method is proposed to characterize the deformation compatibility between an optical fiber and soil. A series of pullout tests on three types of sand-embedded optical fibers are conducted to investigate the performance of the fiber–sand interface. Based on the test results, an explicit tri-linear pullout force–displacement relationship is proposed to describe the mechanical behavior of the fiber–sand interface. The performance of the three fibers regarding fiber–sand interaction mechanism is evaluated in terms of ratio of effective pullout displacement to diameter, ratio of residual pullout displacement to diameter, peak shear strength and residual shear strength. All four parameters of the three fibers are found to have approximately linear relationships with the applied confining pressure, which reveals that the deformation compatibility of the fiber–sand interface is utterly dependent on the confining pressure. For all the three fibers, the first shear stiffness coefficient is about 8N/mm and the ratio of residual to peak shear strength is about 0.5. Furthermore, the Mohr–Coulomb failure criterion is used to get the cohesions and friction angles of the three fiber–sand interfaces. Through a comparison of the pullout performance, one out of three types of fibers tested is found to be more preferable for soil deformation measurement in laboratory-scale tests. The conclusions can provide valuable references for predicting the fiber–soil interface behavior and evaluating the reliability of strain monitoring data.

Visualization of tunnelling-induced ground movement in transparent sand

This paper presents an experimental investigation of internal soil deformation ahead of a tunnel boring machine by using transparent sand and digital image correlation (DIC) techniques. Soil deformation and its control are often very critical issues for protecting adjacent properties and services during tunnel construction. Currently, most of soil deformation measurements are limited to ground surface settlement since natural soil is not transparent. The visualization of spatial deformation inside soil masses will improve the understanding on the influence in tunnelling. Transparent sand is used in this study, which is made of fused silica and a calcium bromide solution with a matching refractive index. An optical set-up is developed that consists of a laser, camera, and computer. The laser is used to illuminate the targeted traverse section ahead of a scaled shield machine. The images of laser speckles generated through interaction between the laser light and silica are captured by the camera and then transferred onto a computer. DIC is used to calculate the soil displacement between two images obtained before and after the machine movement. Two model tests are performed with an overburden cover that varies by one to two times the tunnel diameter. The results show that soil deformation changes with increases in tunnel depth. The settlement troughs at various soil depths are similar to Gaussian curves. As expected, the trough becomes narrower as the soil depth increases. The influence zone changes from a rectangle over a reversed trapezoid shape in the shallower tunnel to a bell over a trapezoid shape restrained within the soil mass in the deeper case. The limitations of this study are also discussed in the paper.

근접 사진측량에 의한 모형말뚝 선단부 주변의 지반 변형 측정

In this paper, we studied on measurement of soil deformation around the tip of model pile by close-range photogrammetry. The rigorous bundle adjustment method was utilized to monitor the soil deformation in the laboratory model pile-load test as function of incremental penetration of the pile. Control points were installed on the frame of the laboratory model box case and more than 150 target points were inserted inside the soil around the model pile and on the surface. Four overlapping images including three horizontal and one vertical image were acquired by a non-metric camera for each penetration step. The images were processed to automatically locate the control and target points in the images for the self-calibration and the bundle adjustment. During the bundle adjustment, the refraction index of the acrylic case of the laboratory model was accounted for accurate measurement. The experiment showed the proposed approach enabled the automated photogrammetric monitoring of soil deformation around the tip of model pile.

Stereo particle image velocimetry measurement of 3D soil deformation around laterally loaded pile in sand

A developed stereo particle image velocimetry (stereo-PIV) system was proposed to measure three-dimensional (3D) soil deformation around a laterally loaded pile in sand. The stereo-PIV technique extended 2D measurement to 3D based on a binocular vision model, where two cameras with a well geometrical setting were utilized to image the same object simultaneously. This system utilized two open software packages and some simple programs in MATLAB, which can easily be adjusted to meet user needs at a low cost. The failure planes form an angle with the horizontal line, which are measured at 27°–29°, approximately three-fourths of the frictional angle of soil. The edge of the strain wedge formed in front of the pile is an arc, which is slightly different from the straight line reported in the literature. The active and passive influence zones are about twice and six times of the diameter of the pile, respectively. The test demonstrates the good performance and feasibility of this stereo-PIV system for more advanced geotechnical testing.

A New Image-Based Soil Deformation Measurement System

A Soil Deformation Measurement System using OPENCV library and FFTW library in C++ was developed in this paper. The system applied camera calibration based on neural network and Fasst Fourier Transform (FFT) cross-correlation algorithm for Particle Image Velocimetry (PIV). It is used to obtain soil deformation data, such as displacements, velocity and strain, and visualize the deformation. Experiments show that this system could acquire deformation data from soil images accurately, efficiently and continuously, which provides a strong proof that image processing technology has practical significance and application value in the research field of geotechnical engineering.

Development of a Robust Stereo-PIV System for 3-D Soil Deformation Measurement

Abstract This paper presents a robust stereo-particle image velocimetry (Stereo-PIV) system developed for three-dimensional (3-D) soil deformation measurement in geotechnical engineering. Also known as digital image correlation, PIV is a popular image processing technique to measure two-dimensional (2-D) fluid velocity in fluid dynamics. The Stereo-PIV technique extends 2-D deformation measurement to 3-D based on a binocular vision model, where two cameras with a well posed geometrical setting, are utilized to image the same object. Although commercial Stereo-PIV systems are available in the market, the applications of this technique are still limited by their high cost and special hardware requirements. This study presents a robust Stereo-PIV system, which utilizes two standard complementary metal–oxide–semiconductor (CMOS) cameras and two open software packages to apply this technique to geotechnical engineering testing. In order to apply this technique, three steps have to be performed: the first step is the calibration for the system to get the parameters of the two cameras with respect to the region of interest (ROI); the second step is to obtain a series of paired images of the ROI during deformation and calculate the corresponding 2-D displacement fields from these images; and the last step is to construct a 3-D displacement field based on the camera parameters calculated in the first step and the corresponding 2-D displacement vector pairs from the second step. The developed system is applied in a model test to measure 3-D sand deformation around a laterally loaded pile. The results demonstrate that the system can obtain a reasonable result and can be readily adjusted for more advanced 3-D deformation measurement in geotechnical engineering research.