Scaling Laws for Centrifuge Modeling of Capillary Rise in Sandy Soils

The application of geotechnical centrifuge modeling to environmental problems seems promising. In this paper, one aspect of similitude laws concerning the flow of water through soils is investigated. Within the Network of European Centrifuges for Environmental Geotechnic Research, several tests have been carried out to study similitude laws describing the capillary ascension in porous media at different levels of acceleration. The aim of this paper is to present the results obtained at Ruhr-Universit?t Bochum. A fine sand is used in the experiment. For the visualization of capillary height in the soil sample, image processing is used. Different boundary conditions (constant or variable water level) have been investigated and discussed. All these experiments confirm that capillary rise appears scaled by the factor N and time seems to be scaled with N2. Thus, these results support the possibility of extending the use of accelerated small-scale models to the capillary phenomenon in centrifuge and open the way to more complex investigations of flow and pollutant transport in unsaturated soils.     

Centrifuge Modeling of Light Nonaqueous Phase Liquids Transport in Unsaturated Soils

Centrifuge modeling appears useful for studying geo-environmental problems such as pollutant migration in subsurface systems. In this study, centrifuge tests were conducted to simulate a gasoline spill from a leaking underground storage tank (UST) and the subsequent subsurface migration of the gasoline. When the centrifugal acceleration reached the desired g level, the gasoline was released from the UST and then it migrated in the unsaturated soil for a prototype time equivalent to 1 year. After the centrifuge tests, soil samples were collected using sampling tubes and the concentrations of individual constituents in the light nonaqueous phase liquids (LNAPLs) were directly measured by means of gas chromatograph analysis. Two types of unsaturated soils were used to study the migration patterns of LNAPLs in unsaturated porous media. Centrifuge test data show that the migration pattern of LNAPLs is related to the soil type and the physical properties of individual constituents in the LNAPLs.     

Centrifuge Modeling of Nonaqueous Phase Liquid Movement and Entrapment in Unsaturated Layered Soils

Four geotechnical centrifuge tests with different soil layered systems were performed to investigate the movement and entrapment of water and of light nonaqueous phase liquids (LNAPLs) in unsaturated layered soil deposits. The tests were performed at 20 g and a vadose zone condition was created during the centrifuge tests by lowering the water table from the initially water saturated condition. During the water drainage stage, the water distribution within the models and the dynamic air–water capillary pressure saturation relationships of the three sands were obtained using tensiometers and resistivity probes. After achieving the unsaturated condition, a model LNAPL (Soltrol 220? or silicon oil) was injected near the soil surface and the movement and entrapment were monitored during the redistribution stage until the LNAPL reached the top of the water table. Complex LNAPL preferential flow and entrapment patterns were observed in the layered models with different textural interfaces due to the relative movement of all three phases [water, nonaqueous phase liquid (NAPL), and air]. The centrifuge tests data coupled with the numerical analyses show that NAPL properties, subsurface soil structures, initial water saturation, and NAPL infiltration rate affect the variation in entrapment conditions in heterogeneous unsaturated soil deposits.     

Centrifuge Modeling of Slope Instability

This paper demonstrates the use of a centrifuge modeling technique in studying slope instability. The slope models were prepared from sand, and sand mixed with 15 and 30% fines by weight, compacted at optimum water content. The validity of the modeling technique was confirmed using slope models of different heights, inclinations, and soil types. The soil behavior was studied under triaxial and plane strain conditions, and the extended Mohr-Coulomb failure criterion was found relevant for expressing the strength of unsaturated compacted soil based on the angle of internal friction and apparent cohesion. The Bishop’s circular mechanism, together with the extended Mohr-Coulomb failure criterion, was able to simulate the slope failure reasonably well. The rainfall of different intensities was then induced on the 60° stable slopes of sand with 15% fines. It was found that the failure of slope under rainfall may be interpreted as a reduction in apparent cohesion. The centrifuge tests also allowed the rainfall intensity-duration threshold curve (local curve) to be generated for the test slopes, and the accumulated rainfall corresponded well to some of the reported field observations.     

Centrifuge Modeling of Torsionally Loaded Pile Groups

This paper reports a series of centrifuge model tests on torsionally loaded 1×2, 2×2, and 3×3 pile groups in sand. The objectives of the paper are to investigate: (1) the response of the pile groups subjected to torsion; (2) the way in which the applied torque is transferred in the pile groups; (3) the internal forces mobilized in these torsionally loaded pile groups and their contributions to resist the applied torque; and (4) the influence factors that affect the load transfer, such as soil density and pile-cap connection. In these model tests, the group torsional resistances of the pile groups increased monotonically in the test range of twist angles up to 8°. Both torsional and lateral resistances of the individual piles were simultaneously mobilized to resist the applied torque. The torsional resistances were substantially mobilized at small twist angles, while the lateral resistances kept increasing in the whole range of twist angles. Thus, the contribution of the torsional resistances to the applied torque decreased at large twist angles. The piles at different locations in a pile group could develop not only different horizontal displacements, but also different pile–soil–pile interactions and load–deformation coupling effect, hence, the torsional and lateral resistances of the piles are a function of pile location. The soil density had a more significant effect on the torsional resistances than on the lateral resistances of the group piles.     

Centrifuge Modeling of LNAPL Transport in Partially Saturated Sand

Model tests were performed at the Geotechnical Centrifuge Facility of Delft University of Technology, The Netherlands, to examine the mechanics of light nonaqueous phase liquid (LNAPL) movement in a partially saturated porous granular medium. The experiment simulated a 2D spill of LNAPL in an unsaturated sand prepared at two values of porosity. The duration of the centrifuge model tests corresponded to a prototype equivalent of 110 days. The choice of modeling a 2D flow together with the use of a transparent container enabled direct visual observation of the experiments. Scaling laws developed in connection with other centrifuge modeling studies were used to support the test results. Tests were conducted at two different centrifuge accelerations to verify, by means of the “modeling of models technique,” the similitude between the different experiments. The paper presents details of the experimental methodologies and the measuring techniques used to evaluate the final distribution of water and LNAPL content in the soils.     

Liquefaction Centrifuge Modeling of Sands of Different Permeability

This paper presents the results of six centrifuge model tests of liquefaction and earthquake-induced lateral spreading of fine Nevada sand using an inclined laminar box. The centrifuge experiments simulate a gently sloping, 10 m thick stratum of saturated homogeneous sand of infinite lateral extent and relative densities ranging from 45 to 75%. Such idealized models approach some field situations and they provide significant general insight into the basic mechanisms and parameters influencing the lateral spreading phenomenon. The layer was subjected to lateral base shaking with prototype peak acceleration ranging from 0.20 to 0.41 g, a frequency of 2 Hz, and duration of approximately 22 cycles. The simulated field slope angle was 5°. The model deposits were all saturated with a viscous fluid 50 times more viscous than water, so that testing under the increased gravitational field (50 g) produced a deposit with the prototype permeability of the same fine-grained sand saturated with water in the field. Detailed discussions and comparisons of the six centrifuge tests are included. The observed effects of relative density Dr and input peak acceleration amax on the following measured parameters are summarized: thickness of liquefied soil H1, permanent lateral displacement DH, and ground surface settlement S. Comparisons and discussions are also presented on the effect of permeability for a Dr = 45% deposit. This is done by comparing the results reported herein using a viscous pore fluid, with other published centrifuge tests where a similar deposit using the same model soil, also tested at 50 g and shaken with the same input motion, was saturated with water, thus simulating a prototype sand having 50 times the permeability of the fine sand reported in this paper.     

Centrifuge Modeling of Unstable Infiltration and Solute Transport

Unstable flow can result in the formation of fingers during infiltration into dry soil. Centrifuge modeling is a potentially useful tool to study the relationship between finger size, spacing, and velocity. It can also be used to investigate solute transport in such fingers. Physical properties of the fingers are obtained for three tests conducted at elevated acceleration levels. A fourth test was conducted at 1g. The physical parameters compare well with theoretical predictions. To assess solute transport in fingers, a known concentration of solute was introduced after the fingers had formed. The resulting breakthrough curves were analyzed using the two-region model as well as the advection dispersion equation with appropriate initial and boundary conditions. Although the two-region model is physically more plausible, it was found to match the extensive tailing observed in the breakthrough curves only marginally better than the advection-dispersion equation.     

Pile Response to Lateral Spreads: Centrifuge Modeling

The paper presents results of eight centrifuge models of vertical single piles and pile groups subjected to earthquake-induced liquefaction and lateral spreading. The centrifuge experiments, conducted in a slightly inclined laminar box subjected to strong in-flight base shaking, simulate a mild, submerged, infinite ground slope containing a 6-m-thick prototype layer of liquefiable Nevada sand having a relative density of 40%. Two- and three-layer soil profiles were used in the models, with a 2-m-thick nonliquefiable stratum placed below, and in some cases also above the liquefiable Nevada sand. The model piles had an effective prototype diameter, d, of 0.6 m. The eight pile models simulated single end-bearing and floating reinforced concrete piles with and without a reinforced concrete pile cap, and two 2×2 end-bearing pile groups. Bending moments were measured by strain gauges placed along the pile models. The base shaking liquefied the sand layer and induced free field permanent lateral ground surface displacements between 0.7 and 0.9 m. In all experiments, the maximum permanent bending moments, Mmax occurred at the boundaries between liquefied and nonliquefied layers; the prototype measured values of Mmax ranged between about 10 and 300 kN?m. In most cases the bending moments first increased and then decreased during the shaking, despite the continued increase in free field displacement, indicating strain softening of the soil around the deep foundation. The largest values of Mmax were associated with single end-bearing piles in the three-layer profile, and the smallest values of Mmax were measured in the end-bearing pile groups in the two-layer profile. The companion paper further analyzes the Mmax measured in the single pile models, and uses them to calibrate two limit equilibrium methods for engineering evaluation of bending moments in the field. These two methods correspond to cases controlled, respectively, by the pressure of liquefied soil, and by the passive pressure of nonliquefied layers on the pile foundation.     

Centrifuge Modeling of Large-Diameter Bored Pile Groups with Defects

In this research, centrifuge model pile-load tests were carried out to failure to investigate the behavior of large-diameter bored pile groups with defects. The model piles represented cast-in-place concrete piles 2.0?m in diameter and 15?m in length. Two series of static loading tests were performed. The first series of tests simulated the performance of a pile founded on rock and a pile with a soft toe. The second series of tests simulated the performance of three 2×2 pile groups: One reference group without defects, one group containing soft toes, and one group with two shorter piles not founded on rock. The presence of soft toes and shorter piles in the defective pile groups considerably reduced the pile group stiffness and capacity. As the defective piles were less stiff than the piles without defects, the settlements of the individual piles in the two defective pile groups were different. As a result, the applied load was largely shared by the piles without defects, and the defective pile groups tilted significantly. The rotation of the defective pile groups caused large bending moments to develop in the group piles and the pile caps. When the applied load was large, bending failure mechanisms were induced even though the applied load was vertical and concentric. The test results confirm findings from numerical analyses in the literature.     

Pore Pressure Generation Models for Sands and Silty Soils Subjected to Cyclic Loading

This paper discusses the applicability of two simple models for predicting pore water pressure generation in nonplastic silty soil during cyclic loading. The first model was developed by Seed et al. in the 1970s and relates the pore pressure generated to the cycle ratio, which is the ratio of the number of applied cycles of loading to the number of cycles required to cause liquefaction. The second model is the Green-Mitchell-Polito model proposed by Green et al. in 2000, which relates pore pressure generation to the energy dissipated within the soil. Based upon the results of approximately 150 cyclic triaxial tests, the writers show that both models are applicable to silty soils. A nonlinear mixed effects model was used for regression analyses to develop correlations for the necessary calibration parameters. The results show that the trends in both α and pseudoenergy capacity calibration parameters for the Seed et al. and Green et al. pore pressure generation models, respectively, differ significantly for soils containing less than and greater than ～ 35% fines, consistent with the limiting fines content concept.     

Centrifuge Modeling of Ship Impact Loads on Bridge Pile Foundations

Bridges that cross navigable waterways may be affected by accidental ship impacts. To better characterize ship impact loads on bridge pier structures, a comprehensive centrifuge model test program involving 48 ship impact tests was performed on a 2×3 pile group and a 3×3 pile group founded in saturated silty sand. These model tests simulated groups of 2-m-diameter by 31.5-m-long pipe piles. The effects of three factors related to the ship (tonnage, speed, and bow structure) and two factors related to the bridge pier structure (superstructure mass and pile-group size) were investigated through these impact tests. The characteristics of the ship impact load were identified and the mechanism of the ship-bridge collision was analyzed. The test results show that the ship impact load was highly dependent on the ship bow structure and the ship impact speed. The test results were compared with other published data and the AASHTO loads. An empirical equation was suggested to relate the ship impact load to the five influencing factors.     

Experimental Study on the Shearing Behavior of Saturated Silty Soils Based on Ring-Shear Tests

The shearing behavior of saturated silty soils has been examined extensively by performing undrained and partially drained (the upper drainage valve of the shear box was open during shearing) ring-shear tests on mixtures of a sandy silt with different loess contents. By performing tests at different initial void ratios, the shear behavior of these silty soils at different initial void ratios is presented and discussed. Undrained-shear-test results showed that the liquefaction phenomena in ring-shear tests were limited within the shear zone; for a given void ratio or interfine void ratio, both the peak and steady-state shear strengths decreased with increase of loess content. The partially drained shear tests revealed that a great reduction in the shear strength could result after the shear failure, due to the buildup of excess pore-water pressure within the shear zone; the magnitude of reduction in shear strength after failure was affected by the initial void ratio, the shear speed after failure, as well as the loess content in the sample. For a given void ratio or interfine void ratio, with increase of loess content, the drained peak shear strength became smaller, while the brittleness index became greater. It was also found that due to localized shearing, the permeability of the soil within the shear box after drained shearing could be three orders of magnitude smaller than before shearing.     

Modeling Transport of Gaseous Ozone in Unsaturated Soils

The use of in-situ ozone venting is an effective and economic technology to remediate soils contaminated with organic chemicals. A model was developed in this study to describe the transport of gaseous ozone in unsaturated soils. Mass balance equations for soil organic matter, phenanthrene, and ozone were incorporated into the model. The model was found to fit the experimental data obtained from one-dimensional columns, using the previously published rate expressions for the reactions of ozone with soil organic matter and phenanthrene. However, it was observed that the initial unsteady-state conditions for the gas pressure resulted in minor deviations between the simulated and experimental results. Based on the simulated results, the reactions of ozone with phenanthrene and organic matter can be modeled as either parallel or serial reactions. As the initial distribution of contaminant would be nonuniform and some contaminant would be sorbed or trapped in immobile regions, it is recommended that in situ ozonation be ceased no sooner than when the effluent ozone concentrations begins to stabilize.     

Centrifuge Permeameter for Unsaturated Soils. I: Theoretical Basis and Experimental Developments

A new centrifuge permeameter was developed with the specific objective of expediting the measurement of the hydraulic characteristics of unsaturated soils. The development, theoretical basis, and typical results associated with using the centrifuge permeameter for concurrent determination of the soil-water retention curve (SWRC) and hydraulic conductivity function (K function) of unsaturated soils are presented in this paper. Components developed for the centrifuge permeameter are described, including the centrifuge, permeameter, water flow control system, and instrumentation used to concurrently and nondestructively measure the infiltration rate (flow pump and outflow transducer), volumetric water content (time domain reflectometry), and matric suction (tensiometers) in flight during steady-state infiltration. A companion paper focuses on definition of the SWRC and K function for a clay soil using the procedures described in this paper. While conventional geotechnical centrifuges are used to reproduce the response of earth structure prototypes, the centrifuge developed in this study is used to accelerate flow processes. Accordingly, it required a comparatively small radius (0.7 m) but high angular velocity (up to 875 rpm or 600 g’s) to impart a wide range of hydraulic gradients to an unsaturated soil specimen. Analytical solutions to Richards’ equation in the centrifuge indicate that steady-state infiltration allows direct determination of the relationships between suction, volumetric water content, and hydraulic conductivity from the instrumentation results. Typical instrumentation results during a drying stage are presented to illustrate determination of data points on the SWRC and K function at steady state. These results were found to be consistent with analytical flow solutions.     

Modeling of Moisture Loss in Cementitiously Stabilized Pavement Materials

The paper presents a theoretical and experimental approach for the modeling of moisture loss during the drying of cementitiously stabilized pavement materials containing varying contents of fine-grained soil. The process of moisture loss was characterized by the isotropic nonlinear diffusion theory. Laboratory tests were undertaken using general purpose Portland cement and two binders comprising industrial waste products. Measurement of material characteristics included the coefficient of moisture diffusivity and the humidity isotherm. Locally available basaltic crushed rocks and clay were respectively used as the host pavement material and fine-grained soil. Independent laboratory tests were undertaken to validate the adopted theoretical approach, which showed close agreement between the experimental and predicted results. The laboratory results indicated that moisture loss decreased with the inclusion of clay soil within the mix. As the drying progressed, the rate of moisture loss became slower, which can be explained by the reduction in the coefficient of moisture diffusivity with the decrease of moisture content.     

Modeling of Moisture Diffusion in FRP Strengthened Concrete Specimens

Modeling the movement and distribution of moisture in the fiber-reinforced polymer (FRP) composites strengthened concrete structure is important because the interfacial adhesion between FRP and concrete is susceptible to moisture attack. Using relative humidity as the global variable, the moisture diffusion governing equation was derived for the multilayered system in this study. The moisture diffusivity (diffusion coefficient) and the isotherm curve, which correlates the moisture content to environmental relative humidity, of each constitutive material (concrete, epoxy, and FRP) were experimentally determined. A multilinear diffusivity model was developed for concrete based on desorption test, and a linear diffusivity model was proposed for epoxy adhesive based on absorption test. A simple method was developed to directly measure the FRP/concrete interface region relative humidity (IRRH). Finite-element analysis was performed to study the moisture diffusion in the FRP-adhesive-concrete system. The IRRH values were obtained for different environmental relative humidity in the numerical study. The error between the experimental and numerical results of IRRH at test locations was less than 5% RH. The good agreement between experimental and numerical results indicates that the approach developed in this study worked well.     

Centrifuge Modeling of Seismically Induced Uplift for the BART Transbay Tube

The BART Transbay Tube (TBT) is an immersed cut-and-cover subway tunnel that runs from Oakland to San Francisco, California. The loose sand and gravel backfills placed around the tunnel are considered to be liquefiable, and the clays under the backfill are soft in some zones along the alignment. These conditions could potentially result in uplift of the tunnel during strong earthquake shaking. This paper describes centrifuge model tests performed to verify numerical methods used to assess the stability and to evaluate the potential uplift mechanisms of the TBT. The observed mechanisms of uplift were a ratcheting mechanism (sand migrating under the tunnel with each cycle of relative movement), a pore water migration mechanism (water flowing under the tunnel), and a bottom heave mechanism, involving soft soils below the base of the trench. A fourth potential mechanism, viscous flow of liquefied soil, was not observed. The volume of the tunnel relative to the volume of the trench and the densities and permeabilities of the nonhomogeneous backfill were important parameters affecting the uplift of the tunnel. From the experiments reported here and analyses reported by the designers, it was concluded that the magnitude of uplift is limited and, hence, that an expensive ground improvement project to densify the backfill was unwarranted.     

Experimental Study on Moisture Migration in Unsaturated Loess under Effect of Temperature

In this paper, moisture migration in loess considering temperature effect is studied by tests on unsaturated loess samples with different densities and initial moisture contents. Test results reveal that obvious changes in moisture content distribution in a loess sample can be observed after temperature difference is exerted on the two ends of the sample. Moisture content at the cold end increases and that at the hot end decreases. Under the effect of temperature difference, moisture content difference at the two ends of a soil sample is related to the initial moisture content, soil density, and magnitude of the temperature difference. Generally speaking, larger temperature differences and smaller soil densities result in more obvious moisture migration and larger moisture content differences at the two ends of the soil sample. When the initial moisture content is large, the moisture content difference caused by a temperature difference is small; when the initial moisture content is small, the moisture content difference caused by a temperature difference is also small; when the initial moisture content is moderate, the moisture content difference caused by a temperature difference is large. After the analysis of test results, taking the soil density and moisture content into account, a formula is obtained to determine the moisture content gradient resulting from the temperature gradient. Reliability of the formula is verified by comparing the measured and calculated data. Because of the reverse migration of liquid water and water vapor at the end of the experiment, it is difficult to determine the thermal potential and matrix potential. Based on the experimental data, this paper probes into the water potential equation that can be used for stability analysis. The equation considers the comprehensive impact of soil density, temperature gradient, moisture content, and moisture content gradient on water potential. It only applies to analyze stable distributions of temperature and does not apply to unstable temperature distributions.