Transparent Soil Model Research Articles

Anti-sliding mechanism of soil–rock slope in transparent soil and discrete-element method

Double-row piles are an effective means to control landslides. The purpose of this research study was to explore internal deformation of the anti-sliding mechanism of double-row piles by using a transparent soil model test. Furthermore, the influence of the section shape, the space between the front and rear rows and the layout of piles on the formation, development and failure process of the soil-arching effect were also studied. The PFC2D numerical simulation method is used to simulate the model tests. The results show that the deformation process of a soil–rock composite slope with double-row piles can be divided into the following three stages: the initial stage, the uniform stage and the acceleration stage. The soil-arching effect would be obvious when the space between the front and back piles is four times the diameter of the pile. The spacing between the front and rear piles is four times the diameter of the pile, and the quincunx arrangement is more advantageous to delay a landslide of the soil–rock composite slope than is a rectangular section pile arrangement. This finding is a significant contribution to reveal the anti-slide mechanism of the double-row piles from the visualisation of the internal slipping.

Bending Properties of Short‐Cut Basalt Fiber Shotcrete in Deep Soft Rock Roadway

To study the effect of short‐cut basalt fiber (BF) on the bending and toughening properties of shotcrete, bending toughness tests of short‐cut BF shotcrete slabs with different volume fractions were carried out. A transparent soil model and Scanning Electron Microscopy (SEM) were used to observe the distribution of BF with different volume fractions and analyze the toughness enhancement mechanism of basalt fiber shotcrete. The supporting effect of basalt fiber shotcrete with different volume fractions was verified by an underground engineering test. The test results show the following: (1) when the fiber content is within 3∼4.5 kg/m3, the distribution of BF in transparent soil is uniform, and it does not easily agglomerate, which is beneficial to improving the bending toughness of shotcrete. (2) The shotcrete slab with 4.5 kg/m3 fiber content had the best reinforcing effect. Compared with the control group, the peak load and absorption capacity increased by 56.67% and 636.96%, respectively, and the maximum crack width decreased by 32.10%. (3) The SEM analysis indicated that the basalt fibers distributed randomly and evenly in the concrete, which can form a three‐dimensional spatial skeleton with a stable structure. Excessive fiber incorporation can increase fiber agglomeration during stirring and spraying. (4) The support results of an underground engineering test show that, in 35 days, short‐cut basalt fiber shotcrete with 4.5 kg/m3 fiber content is better at restraining the surrounding rock than shotcrete with other fiber contents. BF has the properties of crack resistance, bridging, and toughening for shotcrete and can significantly improve the ability of shotcrete to restrain surrounding rock deformation. As a new type of fiber, BF has great significance in deep soft rock underground engineering.

Mining-induced ground movements and displacements using transparent soil model test

Surface subsidence induced by mining subsidence affects the overlying buildings and nearby utilities. Therefore, an investigation of the internal displacement and surface settlement is very important. A small-scale transparent soil model was developed to study the surface settlement profile and displacement field induced by mining subsidence. An analysis of the continuous change of the displacement field inside the transparent soil models indicates that the subsurface settlement at different subsidence stages can be approximated by a normal probability curve. Additionally, the observed surface settlements are consistent with settlement predictions of both the probability integral method and the Gaussian distribution method. The results indicate that it is a good way to study further the underground structure–soil interaction.

Application of transparent soil model tests to study the soil-rock interfacial sliding mechanism

When transparent soil technology is used to study the displacement of a slope, the internal deformation of the slope can be visualized. We studied the sliding mechanism of the soil-rock slope by using transparent soil technology and considering the influence of the rock mass Barton joint roughness coefficient, angle of the soil mass, angle of the rock mass and soil thickness factors on slope stability. We obtained the deformation characteristics of the soil and rock slope with particle image velocimetry and the laser speckle technique. The test analysis shows that the slope sliding can be divided into three parts: displacements at the top, the middle, and the bottom of the slope; the decrease in the rock mass Barton joint roughness coefficient, and the increase in soil thickness, angles of the rock mass and soil mass lead to larger sliding displacements. Furthermore, we analyzed the different angles between the rock mass and soil thickness. The test result shows that the displacement of slope increases with larger angle of the rock mass. Conclusively, all these results can help to explain the soil-rock interfacial sliding mechanism.

The Loading Test on the Singe Pile with Pile Cap in Transparent Soil Model

Pile-cap-soil interaction is important to understand the load transfer mechanisms of pile foundations. However, studies on pile-cap-soil interactions are quite limited. This article reports a series of small-scale model tests in laboratory that was carried out on the single piles with and without pile caps. Transparent soils made of fused quartz and refractive index–matching fluids are employed to represent natural angular sand. Pile cap effects, diameter effects, and pile length effects are investigated. Elastic load limits are found to increase linearly with pile cap area, pile length, and pile diameter for the tested pile models. Soil deformation around the pile head is greatly influenced by the pile cap size, pile diameter, and pile length. Soil deformation at the pile head can be described by Terzaghi’s bearing capacity failure theory. The soil particles below the pile cap next to the pile body move downward, which might reduce the shaft friction or induce the negative shaft friction. All the loading tests show that the pile cap can increase the pile’s bearing capacity. For small-diameter model piles, soil deformation is centered below the pile base, no sliding surfaces are formed at the pile base when settlement S = 4 mm. For large-diameter piles, it is possible that a bearing capacity failure forms two sliding surfaces on the left and right of the pile base. This study indicates that the appropriate usage of the bearing resistance of soils below the pile cap can increase the bearing capacity of pile foundations. This study reveals the soil deformation below the pile cap, which benefits the utilization of the bearing resistance of pile caps. The soil deformation zone at the pile head and around the pile base is also investigated.

Application of transparent soil model test and DEM simulation in study of tunnel failure mechanism

With fast growing demand for modern transportation, more shallow tunnels are being constructed and planned. Understanding the deformation and failure mechanism of a shallow underground tunnel has been a topic of research. The influences of the surrounding material (rock and soil) strengths and buried depths on the deformation and failure mechanism are investigated through the transparent soil model test technique and PFC3D numerical simulation in this study. It can be observed from model tests that the failure mode of test 1 (relative density of 30%, buried depth of 60 mm) is similar with that of test 2 (relative density of 70%, buried depth of 60 mm), both showing the funnel shape. The difference lies in that the tunnel stability in test 1 is lower. On the other hand, tests 2 and 3 (relative density of 70%, buried depth of 120 mm) are of different failure shapes as the latter displays a chimney shape. PFC3D simulation is carried out to numerically examine the failure modes under different testing conditions. The stress field and the displacement field derived from the numerical results can help to interpret the tunnel failure mechanism. In addition, it is obvious from this study that the Peck formula (1969) is less applicable for the surrounding materials with low strength. Therefore, it should be amended according to the surrounding materials, especially for the granular soils.

Transparent Sand Experimental Method for Geotechnical Physical Modeling Using Digital Imaging of Particle Image Velocimetry

By means of the technology of particle image velocimetry (PIV), the physical modelings, using transparent sand and natural sand, were set up to simulate the shallow foundation, respectively. The transparent soil is composed of fused quartz and correspondingly refractive index-matched flammable liquid. The flammable liquid is blended by two white mineral oils, Krystol40 and Puretol7. Transparent sand model was projected by a laser light sheet; meanwhile, the digital images of soil deformations caused by shallow foundation were captured. The PIV was employed to obtain the generated displacement fields from the digital images. The experimental results indicate that the deformation trends in transparent sand model are consistent with that in natural Ottawa sand model; however, the influential region is expanded by about 1.5 times, and the zone and magnitude of down-drag subsidence are extremely enlarged in transparent soil model, compared with those in Ottawa sand model. The verification of test results shows that the shallow strain path method is not suitable for the small-scale transparent sand and natural sand models.

Three-dimensional deformations in transparent soil using fluorescent markers

This paper presents details of a new, simple and low-cost technique that allows three-dimensional (3D) movements of individual particles, and particle aggregations, to be observed in geotechnical physical model experiments. This has been achieved by seeding a transparent granular mass formed from sand-sized fused silica particles and a pore fluid with matching refractive index with crushed uranium glass particles, which do not significantly impact transparency. When illuminated by low-power lasers, the particles fluoresce and appear bright green, which greatly increases the sample's texture, and can be easily detected by photography. These tracer particles have been used to obtain two-dimensional soil deformations using both digital image correlation and particle tracking; and by using stereo-photogrammetry, 3D positions and displacements can also be analysed. Displacements were measured through 85 mm of the transparent model, and the technique should allow a single plane to be illuminated and measured at much greater depths. Fluorescent uranium glass tracer particles enable both individual glass particles and particle aggregations to be tracked. The stereo-photogrammetric system developed is shown to be a cost-effective and viable method of quantifying 3D deformations within a transparent soil model.

Experimental and analytical study of X-section cast-in-place concrete pile installation effect

A new type of displacement pile, the X-section cast-in-place concrete (XCC) pile, has been developed recently in China. Extensive field and laboratory experiments are being undertaken in order to evaluate the soil displacements during pile installation. The paper describes laboratory arrangements and results of transparent soil model tests aimed at capturing the non-axisymmetric soil displacement field. Results from model tests using the XCC pile are presented to reveal the complete soil displacement patterns (a) in the horizontal plane, and (b) in vertical planes of symmetry. Based on results from the transparent soil tests, a modified cavity expansion model (MCEM) is proposed to predict the radial displacements in any vertical plane induced by XCC pile installation. The radial displacements predicted by the MCEM are compared with data from the transparent soil test and the field experiment. The results show reasonable agreement for all cases, with the proposed MCEM proving more suitable than the conventional (axisymmetric) CEM for predicting the radial displacements induced by XCC pile installation. It may therefore provide a basis in the future for developing predictions of stress changes and, ultimately, shaft capacity of the XCC pile in comparison to displacement piles of circular cross-section.

Soil Deformations During Casing Jacking and Extraction of Expanded-Shoe Piles, Using Model Tests

A small-scale 1-g transparent soil model was employed to investigate soil deformations caused by casing-assisted pile jacking of plastic tube cast-in-place concrete piles. The transparent soil was made of fused quartz and a refractive index matched blended oil. Models were sliced with a laser light sheet and digital images were employed to record jacking and extraction of the casing, allowing the use of digital image correlation to derive the generated displacement fields, non-intrusively. The effects of the shoe shape as well as use of a casing to jack the pile were investigated. It was found that under unconfined conditions, soil response to jacking is significantly affected by the pile shoe shape, with displacements adjacent to a conical shoe markedly smaller compared to a flat shoe. Moreover, casing extraction results in recovery of some of the soil deformation that takes place during pile jacking.

Shear modulus and damping ratios of transparent soil manufactured by fused quartz

The mixture of fused quartz and pore fluid with the same refractive index has been used as a synthetic transparent soil to mimic the behavior of natural soil. Previous research has focused on its static properties. In this paper, the dynamic properties of fused quartz (dry samples and oil saturated samples), including shear modulus and damping ratio, were examined through a series of resonant column tests and dynamic torsional shear tests. It was found that fused quartz has similar dynamic behavior to that of natural soil. With the test results, fused quartz shows a great potential as a substitute for natural soil and is expected to be widely used in dynamic transparent soil model tests.

Modelling of chemical grout column permeated by water in transparent soil

The process of injecting chemical grout into cohesionless soil and its permeation by water are simulated by using a transparent soil model and a finite element model (FEM). The urea-formaldehyde resin is used as the grouting material in the transparent soil modelling. Two black and white charge-coupled device cameras are mounted in an orthogonal position to provide a three dimensional view of the groundwater-grout interface. The groundwater-grout interface is primarily influenced by the advection of the flowing water with increasing distance from the injection point owing to the reduction in the grouting pressure. An equation based on fluid dynamics in porous media and Darcy's law is then proposed to predict the displacement of the groundwater-grout interface in flowing water. A numerical model based on the FEM is also developed to simulate this process. The numerical simulation results are found to match the physical modelling well.

Visualization of Chemical Grout Permeation in Transparent Soil

This paper presents an experiment that visualizes the permeation of chemical grout in transparent soil. The low initial viscosity (5 mPa · s) urea-formaldehyde resin (UFR) was chosen as grouting material in this experiment. The transparent soil used in this study is made of fused silica and a calcium bromide solution with the same refractive index. Triaxial and permeability tests are carried out to demonstrate that the geotechnical properties and hydraulic permeability of this transparent soil are typical of granular soils and suitable for modeling natural sand in permeation problem. A combined grouting and optical measurement system is developed for this study, which consists of an air pressure driven grout injection station to inject grout into a transparent soil model, laser to illuminate the cross-section of interest inside the model, and a charge-coupled device (CCD) camera to capture a series of images during the whole grout injection and permeation process. Image processing techniques including digital image correlation (DIC) are applied to sequence of images to detect the edges of the grout bulb and displacement distribution. The relationship of the grouting radius and grouting time corresponds to Maag's formula on permeation grouting. As a result, the validity of this innovative experiment has been verified.

Reconstruction of three dimensional convex zones using images at model boundaries

This paper presents a method for predicting positions of color cubes inside a square transparent solid object from images taken at the orthogonal boundary surfaces. The work is developed for use in mapping flow of non-aqueous phase liquids (NAPL) in transparent soils. Transparent soil models have been developed to study the flow of contaminants through porous media, in bench scale tests. Yellow transparent cubes are used to represent NAPL plumes and clear transparent cubes are used as representations of transparent soil in order to definitively validate the algorithm. Color space information is used to relate concentration and image intensity. The new algorithm employs a so-called 3D carving method to iteratively reconstruct a 3D model using images taken at three orthogonal boundaries. The methodology presented in this paper is a fast, relatively accurate, non-intrusive and inexpensive method for quantifying NAPL zones in transparent soil models.

Methodology for Optical Imaging of NAPL 3D Distribution in Transparent Porous Media

Three-dimensional mapping of non-aqueous phase liquid (NAPL) distribution within saturated porous media is an important issue in bench-scale geo-environmental studies. In this paper, a methodology for optical imaging of 3D multiphase liquid distribution is presented. The natural aquifer is simulated using a transparent soil surrogate that represents the macroscopic behavior of natural sand. To achieve transparency transparent fused quartz grains were saturated with a matched refractive index mineral oil solution that represents the natural aquifer and injected with a green-dyed sucrose solution to simulate dense NAPL contamination. The spatial volume was first estimated and then reconstructed using orthogonal images acquired at the model boundaries at various stages of contamination and remediation. Color space analysis was employed to segment the NAPL zone and transform pixel information into integrated concentration values using a previously published calibration model. The chromatic components CR and a of YCBCR and Lab, respectively, were combined to render the spatial concentration profile. A novel iterative reconstruction algorithm named 3D carving was used to resolve three 2D projections into a 3D model. The results show that the proposed methodology provides better efficacy for NAPL zone reconstruction in comparison with conventional image analysis routines. The technology presented in this paper is a sustainable, fast, accurate, non-intrusive, and inexpensive method for spatial mapping of contamination zones using bench-scale transparent soil models.

Modelling of projectile penetration using transparent soils

This paper presents a novel physical modelling method to study projectile penetration problems. The method employs a recently developed transparent synthetic soil made of oil-saturated fused quartz, which represents the macroscopic behaviour of silica sand. Digital image correlation (DIC, also known as particle image velocimetry (PIV)) techniques were employed to quantify the response of granular soils to high-speed penetration, non-intrusively. A conical nose projectile made of steel was accelerated into a transparent soil target at an impact velocity of 13.6 m/s. Visualisation of the soil deformation was made possible by means of a novel seeded plane within the transparent soil model. The complete penetration event was recorded using a high-speed digital camera. The captured images were analysed using DIC, and further analysis of displacement trajectories and strains were also carried out using in-house algorithms.

Visualization of dyed NAPL concentration in transparent porous media using color space components

Finding a correlation between image pixel information and non-aqueous phase liquid (NAPL) saturation is an important issue in bench-scale geo-environmental model studies that employ optical imaging techniques. Another concern is determining the best dye color and its optimum concentration as a tracer for use in mapping NAPL zones. Most bench scale flow studies employ monochromatic gray-scale imaging to analyze the concentration of mostly red dyed NAPL tracers in porous media. However, the use of grayscale utilizes a third of the available information in color images, which typically contain three color-space components. In this study, eight color spaces consisting of 24 color-space components were calibrated against dye concentration for three color-dyes. Additionally, multiple color space components were combined to increase the correlation between color-space data and dyed NAPL concentration. This work is performed to support imaging of NAPL migration in transparent synthetic soils representing the macroscopic behavior of natural soils. The transparent soil used in this study consists of fused quartz and a matched refractive index mineral-oil solution that represents the natural aquifer. The objective is to determine the best color dye concentration and ideal color space components for rendering dyed sucrose-saturated fused quartz that represents contamination of the natural aquifer by a dense NAPL (DNAPL). Calibration was achieved for six NAPL zone lengths using 3456 images (24 color space components×3 dyes×48 NAPL combinations) of contaminants within a defined criteria expressed as peak signal to noise ratio. The effect of data filtering was also considered and a convolution average filter is recommended for image conditioning. The technology presented in this paper is fast, accurate, non-intrusive and inexpensive method for quantifying contamination zones using transparent soil models.

Investigation on the shear moduli and damping ratios of silica gel

Silica gel, a typical desiccant widely used in industry to absorb moisture, is a porous inert colloid with different sizes in beaded or angular shape. The mixture of silica gel and pore fluid of matched refractive index has been used as transparent media to mimic the behavior of sand. Previous studies have been focused on the static properties of transparent soil. In the current study, the dynamic properties of silica gel, including small-strain shear modulus and damping ratio, were examined through a series of resonant column tests. Four different gradations of silica gel were tested under confining pressures of 50, 100, 200, 300, and 400 kPa. The test results fully displayed the dynamic behavior of the silica gel. The test data also revealed that silica gel has a certain similar dynamic behavior as those of natural soils. With the test findings, silica gel could be used as a surrogate for natural soils in dynamic transparent soil model tests.

Evaluation of tunnel face stability by transparent soil models

Accurate estimation of tunnel face support pressure is necessary for economical and safe shield tunneling in cohesionless soils. This paper presents measurements of tunnel face support pressure and associated soil movements obtained using a transparent soil model that simulates shield tunneling in medium dense saturated sand. The use of a transparent soil surrogate permits measuring the internal soil deformations within the model soil. Soil deformations associated with various face support pressures are presented for 4 cover-to-diameter ( C/ D) ratios. Failure is found to be sudden with sand flowing into the tunnel leading to a prismatic wedge in front of the tunnel face and a vertical chimney of soil above. A minimum support pressure was achieved with support pressures as low as 10 ± 1% of the effective vertical stress at the tunnel axis. The stability of the tunnel face was related to the coefficient of active earth pressure with C/ D ratio having a small effect on the magnitude of required pressure at collapse.

Analysis of Tunneling-Induced Ground Movements Using Transparent Soil Models

Ground movements induced by shallow tunnels affect the safety of nearby underground and aboveground structures. Therefore, the reliable prediction of these movements is important. A transparent soil model is used to investigate not only the surface settlement profile induced by shield tunneling, but also the distribution of soil deformation within the soil mass near the tunnel. The observed surface settlements are consistent with the normal probability curve commonly used for predicting settlement, with only the inflection points or trough width parameters somewhat different. The measured data are consistent with field measurements in that the trough width parameter is independent of the volume loss and linearly proportional to the tunnel depth. An analysis of the displacement field inside the transparent soil models indicates that the subsurface settlement trough at different depths can be approximated by a normal probability curve; and the horizontal displacement can be expressed by the trough width parameter and the volume loss, at the point at which maximum horizontal displacement occurs at the point of inflection. Additionally, the measurements indicate that subsurface ground movements can be in excess of the observed surface settlement, which can adversely affect underground utilities.