Pore Pressure Transducers Research Articles

Influence of biocementation treatment extent on dynamic system performance: A centrifuge liquefaction study

Previous centrifuge experiments have shown Microbially Induced Calcite Precipitation (MICP) is a viable ground improvement technique to mitigate liquefaction. However, in these past experiments the entire sample in the model container was MICP treated. This study is an investigation of the effect of treatment extent on the system seismic performance by varying the depth of improvement, while maintaining the same cross-sectional area. Specifically, MICP treated zones (10 cm × 10 cm in model scale in plan view) were varied in depth to 100%, 75%, and 37% of the liquefiable sand layer depth. As a comparison, two additional models were constructed, an untreated and fully treated model. The treated models were initially prepared to a relative density of less than 40% and MICP treatment applied until a change of shear wave velocity (Vs) of 300 m/s was measured. All models were instrumented with accelerometers and pore pressure transducers to capture the dynamic responses during the same sequence of eight shaking events. Cone penetration resistance was measured to verify the uniformity of the untreated zones in the models and track the evolution of penetration resistances at three different times during the shaking sequence. Vs was measured after each shaking event to assess the cementation integrity of the MICP treated zones. Linear potentiometer and pre- and post-surface measurements were obtained to track seismic-induced settlements. Pore pressure generation and settlements were reduced in the MICP treated zones compared to untreated zones, showing the increased liquefaction resistance and system performance of MICP. Amplification of the strong ground motions in the models with treatment through the full liquefiable layer lead to significant degradation of the MICP. While a base isolation effect was observed in the models not treated to the full depth of the liquefiable layer, reducing seismic demands to the MICP treated zoned, and leading to minimal changes in cementation integrity. These findings suggest that there may be a benefit to not improving the entire liquefiable depth for maintaining cementation integrity of MICPtreated soils.

Studies on the performance of water retention type tailings dams subjected to filling and inertial loading in a geotechnical centrifuge

The past decade has witnessed the failure of numerous tailing storage facilities and earthen dams. The most common factors causing failure include seepage and slope instability, apart from seismic activities. In the present study, a series of centrifuge model tests have been conducted to study the performance of water retention type (WRT) tailings dams impounded with fine tailings material (and water) on the upstream side. A custom-designed test setup to simulate upstream reservoir filling and inertial loading is employed in a 4.5 m radius large beam geotechnical centrifuge facility available at the Indian Institute of Technology Bombay, India. The centrifuge models were instrumented with pore pressure and displacement transducers to record porewater pressure (PWP) and surface settlement, respectively, and to ensure long-term safety and integrity of tailings dam. Additionally, an onboard digital camera was used to capture the front elevations of the centrifuge models to perform digital image analysis (DIA) for deducing slope face movements, surface settlements, and failure patterns. The results effectively demonstrate the performance of the WRT dam, delineate the slip surface and failure pattern, and quantify the critical design parameter related to inertial loading, which is the horizontal seismic coefficient (i.e. KHyield and KHmax). Based on these physically observed results, the analytical equations were also deduced for the WRT dams to quantify the stability (in terms of factor of safety, FS) and deformation response. The analytically obtained results agree reasonably well with the performance of WRT tailings dam models in the geotechnical centrifuge.

Thermo-hydro-mechanical behaviour of a deep plastic clay formation

Several applications which are framed in the energy geotechnics field, particularly the design of energy geo-structures, energy geo-storage, enhanced geothermal energy systems and radioactive waste disposal, require a thorough thermo-hydro-mechanical (THM) characterisation of their host soil/rock formation. The present study comes up from the need to investigate the THM behaviour of deep Ypresian clays (Ycs, 300 to 400 m below ground) since they are one of Belgium’s potential host rock formations for deep geological disposal of heat-emitting and long-lived radioactive waste.
 The impact of temperature on the hydro-mechanical response of deep clays has been extensively tackled for the last four decades. Nevertheless, to the authors’ knowledge, the study of the evolution of radial stresses during drained thermal paths under oedometer conditions has never been addressed. Such information would allow for drawing a better-defined stress state while accurately measuring volume changes. To this aim, the present study used a newly designed and fully instrumented temperature-controlled oedometer cell [3] combined with well-planned test protocols. The local instrumentation consisted of a pore pressure transducer embedded in the soil 3 mm from the bottom boundary, thermocouples at various positions and radial displacement transducers with a localised thin-wall system to estimate radial stress.
 Two well-preserved Ycs core samples retrieved at different depths at Kallo (Belgium) and belonging to distinct lithological units were tested (see Table 1). Both slightly overconsolidated samples (Yield Stress Ratios of Ycs between 1.2 and 1.8, [1, 2, 3]) presented high initial matric suctions induced on deep water-undrained sampling. Consequently, the first stage of the well-planned test protocol consisted of loading at constant water content to bring the samples to the large in situ stresses and diminish the induced matric suction. The remaining matric suction was then reduced by soaking with synthetic water equivalent to in situ. Afterwards, drained loading and unloading paths were followed to attain different vertical effective stresses at varying overconsolidation ratios (OCRs) before performing the main research goal of drained thermal paths. The heating-cooling (H-C) cycles consisted of slowly stepwise increase/decrease in temperature (steps of 10ºC) with intermediate stabilising periods of around 15 hours controlled by the embedded pore pressure transducer.
 Figure 1 depicts the vertical deformation in terms of the mean effective stress (p’) beyond in situ stress. Changes in radial stresses ruled the variations in p’ during heating-cooling cycles since the vertical effective stress was kept constant during non-isothermal paths. At a slightly overconsolidated (OC) state (Core-48), the drained ‘1st H-C’ cycle showed a quasi-reversible response in volume changes and radial stresses. At OC state (OCR=2) on Core-103 (‘2nd H-C’), the response was reversible. However, the induced volume change in normally consolidated (NC) states was not reversible with a contractive response dependent on the stress level. Therefore, the nature of the thermal-induced volume changes aligned with the behaviour observed on other plastic clays. Nevertheless, the stress dependence of the contractive response at NC state differed from the observations made by [4] on NC Boom Clay -the other poorly indurated clay considered as potential host rock in Belgium. As observed in ‘2nd H-C’ (NC conditions) on Core-48, a reduction of the radial effective stress was only recorded at temperatures higher than 70°C. In contrast, the radial effective stress systematically increased on heating at values < 70°C, tending the sample to lower deviatoric stress. This phenomenon might be
 associated with some change in the soil structure on heating that tends to a more isotropic stress state despite the oedometer condition. A clear transition between the elastic and elastoplastic domains was detected during the subsequent loading after each H-C cycle at NC states.
 Finally, the study included a thermo-mechanical elastoplastic interpretation: thermal expansion coefficients dependent on temperature and mean effective stress in the elastic domain and thermal softening function on post-yield drained heating.

Experimental investigation on the efficiency of the phase change materials for enhancing the thermal performance of energy piles in sandy soils

The application of phase change materials (PCMs) is an efficient technique for enhancing the thermal properties of energy piles. In most previously applied studies, paraffin-based materials with low melting temperatures were implemented as PCM to improve energy piles in cold climate conditions. This study explored the efficiency of using lauric acid as a PCM in energy piles by performing small-scale physical model tests under warm climate conditions. Two concrete energy piles with the same diameter of 58 mm and length of 830 mm with PCM and without it were placed in the center of a tank filled with silica sand. The energy piles went through a series of heating tests with the loading temperature (corresponding to the initial temperature) of +10 to +40 °C. To analyze the effect of implementing lauric acid on temperature, and pore water pressure (PWP) distributions in sandy soil, digital thermometers and pore pressure transducers were used in different parts of the inlet side of the model. Results were compared between the two piles with and without PCM in dry and saturated media. The achieved results demonstrate that by adding 1.5% of lauric acid to the energy pile, temperature changes along the pile can be reduced up to 13% in the dry and 10% in the saturated condition. Also, the temperature changes in the soil media could be decreased by up to 5% in dry media. Additionally, the induced excess pore water pressure around the modified energy pile with lauric acid was about 2.14 times lower than the conventional pile. Besides the reduction in the temperature and PWP, the extracted power had an upward trend by increasing the thermal loading. The results revealed that due to the phase transition process, applying a small amount of lauric acid in the energy piles reduces the temperature fluctuations-related responses and could improve the thermal performance of the heat exchange system.

Numerical study of piezoball dissipation test with penetration under partially drained conditions

The piezoball, a full-flow penetrometer equipped with a pore pressure transducer, is increasingly being used in in-situ investigations due to its advantages of characterizing soft soils relative to the piezocone. The operative coefficient of consolidation, ch, can be estimated by monitoring the dissipation of excess pore pressures after piezoball penetration. However, the determination of ch from the dissipation test becomes complicated if the piezoball is penetrated under partially drained conditions. The performances of piezoball penetration and subsequent dissipation are investigated using a coupled large deformation finite-element approach. The numerical results are verified by comparing with existing centrifuge tests, against both undrained and partially drained conditions. A comprehensive interpretation method is then presented for inferring ch from piezoball tests against various drainage conditions. It is found that the variation of ch with drainage conditions is essentially dependent on the distribution of pore pressures before dissipation. Two backbone curves determining the nominal initial excess pore pressure at the equator of the probe and the coefficient of consolidation are obtained to assess the effect of partial drainage, which capture the influences of the piezoball geometry, stress level and over-consolidation ratio.

Analysis of transient seepage through a river embankment by means of centrifuge modelling

Earthen river embankments are typically in unsaturated conditions during their lifetime and the degree of saturation within their bodies may vary significantly throughout the year, due to seasonalfluctuations of the river stage, as well as infiltrations of meteoric precipitation and evapotranspiration phenomena. Given the significant effects of partial saturation on the hydro-mechanical behaviour of soils, realistic assumptions on the actual water content distribution inside the embankments are essential forproperly modelling their response to hydraulic loadings. In this framework, centrifuge modelling is a useful tool to get insights into the evolution of saturation conditions of a water retaining structure during flood events. It allows for the direct observation of the groundwater flow process, which is hardly detectable at the prototype scale, enabling, at the same time, the validation and calibration of predictive numerical tools.In this paper, the results of a centrifuge test carried out on small-scale physical model of a compacted silty clayey sand embankment subjected to a simulated high-water event, at the enhanced gravity of 50-g, are presented and discussed. The physical model was carefully instrumented with potentiometers, miniaturized pore pressure transducers and tensiometers. Pore pressures and suctions measured during the experiment showed that the stationary flow conditions were reached only after an unrealistic hydrometric peak persistence. It therefore emerges that, for the design and/or the assessment of the safety conditions of a river embankment similar to the one tested, the simplified hypothesis of a steady-state seepage, in equilibrium with the maximum river stage expected could result, in many cases, an excessively conservative assumption.

Experimental analysis of embankment on soft soil improved with bottom ash columns

Bottom ash is a byproduct formed during the combustion of coal and can be used as an alternative to aggregate and sand as a green material in columns to achieve sustainability in ground improvement. In this study, the bearing capacity performance of highway embankment models constructed on bottom ash columns reinforced soil was investigated. Model tests were carried out with various area replacement ratios (Ar) of 10.91 %, 16.37 % and 21.83 % and column length to diameter ratios (L/D) of 6 and 8. Load cell and three miniature pressure transducers were used for the measurement of vertical stress on the embankment surface, top and base of the column and adjacent soil surrounding the columns. A Pore pressure transducer was employed to record the excess-pore water pressure during the loading test. The stress concentration ratio and column failure modes were investigated for the stress transfer mechanism. The tests results revealed that bottom ash columns, whether floating or end bearing, was most effective in enhancing the ultimate bearing capacity (qult) of improved soil at almost 1.24, 1.39 and 1.63 times and 1.27, 1.42 and 1.83 times that of unreinforced soil for Ar of 10.91 %, 16.37 % and 21.83 % with L/D of 6 and 8, respectively. The stress concentration ratio (SCR) increased with higher Ar and L/D values. The highest improvement ratio was obtained for the model improved with bottom ash columns at Ar = 21.83 % and L/D = 8. The end-bearing bottom ash columns fail by bulging and floating bottom ash columns fail by bulging along with punching at the base. Furthermore, a mathematical expression with R2 = 0.9913 was developed for calculating the approximate qult. The approximate qult are in good agreement with actual qult. The findings of this study resulted in the utilization of bottom ash as a sustainable granular material in the column for the ground improvement underneath embankments.

The behaviour of containment bund using stacked geotextile tubes filled with lightly cemented soft soil

This land reclamation project used stacked geotextile tubes to construct a 1 km long containment bund. In the past, sand is the preferred infilling material of geotextile tubes due to its ease of construction. However, due to sand scarcity in Singapore, soft soil becomes a potential alternative infilling material. One of the challenges of using soft soil as infilling material is that there will be an excessive settlement and concerns about the overall stability of the stacked tubes. Hence, the soft soil is lightly cemented before infilling into the geotextile tubes. This paper presents the design consideration and the construction sequence of the construction of a stacked geotextile tubes containment bund. Extensive instrumentation was installed at a section of the containment bund to study the behaviour of this geotextile tube. The instrumentation includes strain gauges, pore pressure transducer and total pressure cell. After two years of construction was completed, Standard Penetration Test (SPT) and laboratory triaxial test were used to evaluate the strength of the infilling material. All the instrument data and strength tests results were used to ascertain the performance of the stacked geotextile tubes containment bund.

Centrifuge modelling of wave-induced seabed response in clay

This paper describes a programme of centrifuge tests investigating the behaviour of clayey seabed under wave loading using an in-flight wave loading system. Three model seabeds of kaolin clay capturing typical unconsolidated, normally consolidated and overconsolidated soil responses were considered, with each seabed experiencing several episodes of wave loading and resting. Data acquisition measures included pore pressure transducers, accelerometer, bender elements and T-bar penetrometers. The depth-wise distribution of excess pore pressure, soil strength and modulus, as well as the motion of the liquefied layer of the seabed, was monitored throughout to enable a thorough investigation into the liquefaction and reconsolidation features of the soil. For the unconsolidated and normally consolidated soils, remarkable development of residual pore pressure was observed, and there was evidence that the strength/modulus recovery cannot be achieved by the surficial soil within a prototype time of 15 days. Within a certain depth below this surficial layer, there was a drastic increase in undrained strength, and this phenomenon was carefully examined by a modified moving-boundary model. For the overconsolidated soil, the build-up of residual pore pressure was rather limited, but discernible amplification of oscillatory pore pressure amplitude was observed. Implications for practice in offshore engineering are discussed based on the experimental findings.

LEAP-UCD-2017 and additional tests at NCU

In this study, three centrifuge modeling tests related to the Liquefaction Experiment and Analysis Projects were conducted. Test models were prepared in a rigid-wall container with a length, central height, and sand slope of 20 m, 4 m, and 5°, respectively, in the prototype scale. Two additional centrifuge modeling tests with different sand materials were carried out to obtain more information. The results of these two tests showed that the properties of the two sand materials were similar; however, different grain sizes of sand particles with the same relative density or void ratio affected the behavior of the saturated sand slope during seismic shaking. Furthermore, a series of experiments were conducted to evaluate the applicability and performance of a fiber Bragg grating pore pressure transducer as an alternative method for pore pressure measurement. • Different particle size sands with the same relative density have different excess pore water pressure generation behavior. • The sample with a larger soil particle grain size but the same void ratio is less compressive when shearing. • The FBG-pore-pressure-transducer can effectively measure static pressures and is insensitive to total stresses.

Multicycle expansion tests in natural soils

This paper presents an application of multicyclic expansion tests carried out with a pressuremeter at two experimentation sites to evaluate soil susceptibility to this type of solicitation. These unique pressuremeter tests, interpreted in terms of deformation (volume change or radial strain) and number of cycles, offer relationships as a function of cyclic shear ratio. New test procedures and probe enhancements were also implemented to develop additional data and to improve the quality of these tests. These cyclic tests were performed using a pre-bored Ménard pressuremeter as well as a new pressuremeter probe equipped with a miniature pore pressure transducer. Two sites were studied, one located in French Antilles and the second one located in Brittany (France), both consisting of normally consolidated sandy and silty soils profiles. An estimation of the relationship between cyclic stress ratio applied during the tests and the number of cycles to reach failure are presented and discussed in this paper. These results were compared to traditional cyclic laboratory test results and showed great potential for this in situ testing method. The results were used to develop preliminary charts for liquefaction prediction. This article presents a summary of the analysis and application of these cyclic pressuremeter tests.

Instrumented flume tests on the failure and fluidization of tailings dams induced by rainfall infiltration

Although numerous studies focus on the mechanical behaviour and instability of tailings dam materials, a framework that adequately describes all the processes involved in their failure and fluidization is still needed. Therefore, we used a series of instrumented flume tests, with pore pressure transducers, micro-seismic accelerometers, internal displacement transducers and laser scanning, to investigate the mechanisms of tailings-dam failure and fluidization. The artificial rainfall induced water infiltration was adopted to induce the failure. The test results showed that continuous rainfall expanded macro-voids and led to particle rearrangement and local collapse, producing a gradual buildup of pore pressure. Furthermore, the increasing porosity enhanced seepage forces and thus reduced the shear strength of the tailing's materials so that the dam models were close to liquefaction. However, a dam model with a higher hydraulic conductivity would not be totally damaged, even if failure and fluidization occurred. The amount of antecedent rainfall likely significantly contributed to dam failure because it raised the moisture content of the tailing's materials, which promoted internal erosion; thus, the rainfall might have been sufficient to generate local fluidization. Rainfall decreased the peak pore pressures required to initiate failure and shortened the time to failure but hardly influenced the fluidization movement distance.

Seismic site response of layered saturated sand: comparison of finite element simulations with centrifuge test results

A numerical model based on the finite element framework was developed to predict the seismic response of saturated sand under free-field conditions. The finite element framework used a non-linear coupled hypoplastic model based on the u-p formulation to simulate the behaviour of the saturated sand. The u-p coupled constitutive model was implemented as a user-defined routine in commercial ABAQUS explicit 6.14. Results of centrifuge experiments simulating seismic site response of a layered saturated sand system were used to validate the numerical results. The centrifuge test consisted of a three-layered saturated sand system subjected to one-dimensional seismic shaking at the base. The test set-up was equipped with accelerometers, pore pressure transducers, and LVDTs at various levels. Most of the constitutive models used to date for predicting the seismic response of saturated sands have underestimated volumetric strains even after choosing material parameters subjected to rigorous calibration measures. The hypoplastic model with intergranular strains calibrated against monotonic triaxial test results was able to effectively capture the volumetric strains, reasons for which are discussed in this paper. The comparison of the numerical results to centrifuge test data illustrates the capabilities of the developed u-p hypoplastic formulation to perform pore fluid analysis of saturated sand in ABAQUS explicit, which inherently lacks this feature.

Deep, In Situ Nonlinear Dynamic Testing of Soil with Controlled Blasting: Instrumentation, Calibration, and Application to a Plastic Silt Deposit

Abstract This study presents a novel test method to obtain dynamic properties of soil in situ using controlled blasting. Experimental and calibration protocols have been developed to conduct such investigations for any soil at any depth to obtain the coupled response between nonlinear shear modulus and pore pressure generation in soil. Controlled blasting using buried explosives with different charge weights at various depths were used as an active seismic source and the soil response measured. Micro electro mechanical system (MEMS) accelerometers, geophones, pore pressure transducers, and the MEMS-based inclinometers were calibrated to develop unique excitation voltage-to-unit relationships. Procedures to determine the post-installation sensor locations and orientations were developed to quantify the appropriate body wave velocity and shear strain in the soil. The conversion of calculated strains to a constant-volume direct simple shear-equivalent shear strain facilitated direct comparison to previously reported shear modulus reduction curves. Although this manuscript primarily focuses on instrumentation and calibration protocols, an example of the results of a test blast program is provided to demonstrate observation of the in situ, coupled, nonlinear fluid-mechanical response of an instrumented plastic silt deposit at a depth of 9 to 11.5 m below ground surface.

Monitoring and Analysis of Pore and Earth Pressure at the Pile Surface Using Piezoresistive Silicon Pressure Transducers

The piezoresistive silicon pressure transducers based on the piezoresistive effect have demonstrated their potential in the accurate monitoring of pressure. However, their usage in the pore and earth pressure monitoring at the pile surface under hydraulic jacking has not yet been explored. In this study, two self-made model piles (one is a closed-ended pile and the other is an open-ended pile) were instrumented with piezoresistive silicon earth transducers and pore pressure transducers and then driven into the soil using a hydraulic jack. A comprehensive investigation was first carried out for the structure of the model piles, the installation procedure of the transducers, and the composition of the test system. The pore and earth pressure measurements of the transducers were used for the evaluation of the distribution of the pore pressure, excess pore pressure, radial earth pressure, and radial effective earth pressure. The model test results indicate that the piezoresistive silicon pressure transducers are suitable for monitoring the pore and earth pressure at the pile surface during jacking. In addition, the pore pressure, excess pore pressure, radial earth pressure, and effective radial earth pressure along the test piles were affected by the penetration depth and the pile end form.

Suction measurement in freezing soils using pore pressure transducers

Frost heave is major problem for infrastructures build in cold regions. Frost heave occurs due to suction (negative pore water pressure) generated due to the freezing process close to the frost line, i.e., at the frozen fringe. To understand and predict these negative pore water pressures is a key factor to accurately calculate the segregation heave, i.e. heave related to the formation of ice lenses. Segregation heave is the major part of the total heave and also the most challenging to predict. Many attempts have been presented in literature where the generated suction during freezing is related to temperatures, temperature gradients, grain size of the freezing soil etc. Very few laboratory tests have been presented in which the actual suction is measured during the ice lens formation process and compared with theoretical estimations. One reason is that these measurements are challenging. This paper presents results from laboratory measurements of generated suction during freezing. Laboratory tests were conducted on a silty soil sample and suction was measured at the frozen fringe using small pore pressure transducers (PPT’s). The samples were subjected to one-dimensional freezing from top to bottom in an open water system at a constant temperature gradient. Temperatures were measured at various points along the height of the soil sample while suction was measured at middle of the sample. Test results have shown that PPTs do not show pressure change in long-term static pressure test under sub-freezing temperature. For suction measurement at the frozen fringe, pore pressure readings should be measured at various points along the sample height.

Centrifuge model study on the influence of desiccation cracks on the seepage behavior of upstream clay anti-seepage system subjected to abrupt flood

The influence of desiccation cracks on the performance of upstream anti-seepage systems subjected to abrupt flood was investigated using centrifuge experiments of scaled-down embankment dam models. Desiccation cracks were introduced in the sloping core and the upstream blanket by simulating solar radiation. Both intact and cracked models were subject to 100×g centrifuge acceleration. Pore water pressure was monitored using pore pressure transducers placed at preselected locations within the model, and the opening width and length of the dominant cracks were manually measured. The crack number was lower on the blanket surface than on the sloping core, but the crack opening width and length developed rapidly. The intact anti-seepage system, which lowered the infiltration surface and effectively dissipated the pore water pressure, showed good performance, whereas the cracked anti-seepage system suffered seepage failure owing to high pore water pressure at the crack tip under the abrupt flood condition. The results indicate that desiccation cracks on the blanket surface provide the essential material condition for hydraulic fracturing, and abrupt flood provides the favorable mechanical condition. Desiccation cracks on upstream anti-seepage systems pose a major hidden danger and should be excavated and backfilled in a timely manner to prevent dam accidents.

Use of Centrifuge-Based Physical Modelling for Understanding the Performance of Geostructures

Centrifuge-based physical modelling is widely adopted for understanding the performance of geostructures, like levees subjected to flooding and drawdown and geogrid-reinforced soil walls subjected to seepage. In this paper, an attempt has been made to bring out the advantage of centrifuge-based physical modelling to understand (i) the performance of levees subjected to flooding using a custom-designed and developed in-flight flood simulator at 30 gravities with and without chimney drain and (ii) the performance of geogrid-reinforced soil walls with and without chimney drain subjected to seepage at 40 gravities. In both the cases, silty sand was used to model soil and fine sand was used in chimney drain. All centrifuge model tests were performed using the 4.5-m-radius large-beam centrifuge facility available at IIT Bombay. Models were instrumented with linearly variable differential transformers for measuring surface settlements and pore pressure transducers to measure raise in pore water pressure within the soil at the onset of flooding for levees and at the onset of seepage for geogrid-reinforced soil walls. Additionally, digital image analyses of photographs of front elevation of levee models and geogrid-reinforced soil wall models were carried out to obtain face movements, movements of markers embedded within the levee, markers stuck to geogrid layers of reinforced soil walls at the onset of flooding and seepage. The developed in-flight flood simulator was found to be capable of generating the flood rate ranging from 2.2 to 7 m/day. Further, results of centrifuge model tests conducted on levees without any chimney drain were noticed to undergo catastrophic failure within 4.25 days of flooding-induced seepage, whereas a levee with chimney drain was found to sustain flooding-induced seepage of 37.5 days. Geogrid-reinforced soil wall was constructed with silty sand as a structural as well as backfill, without any drainage system experienced catastrophic failure. Contrary to this, geogrid-reinforced soil walls with chimney drain as an external drainage system helped in averting catastrophic failure. However, probability of piping failure near the toe region of the wall cannot be ruled out. Further, the use of geocomposite layers as an internal drainage system within the reinforced zone also explored and the placement of geocomposite layers at one-third portion of height from bottom was found to be effective.

Pressure infiltration characteristics of foam for EPB shield tunnelling in saturated sand – part 1: ‘clean’ foam

Infiltration of pressurised foam into saturated sand was investigated in a laboratory set-up that provided an hydraulic gradient comparable to real tunnels. Four different stages were identified in the infiltration: ‘foam spurt’ (the foam infiltration into the pores of the sand); water flow with reduced permeability; water flow with increased permeability; and a residual stage. It appears that the infiltration depth of foam into sand decreases with increasing foam expansion ratio (FER) of the foam until a limit FER (approximately 15 for the foaming agent and sand used in this experiment). For higher FERs the permeability remains more or less constant. It was also found that a ‘dry’ foam (FER = 20) is not essential for formation of a low-permeability layer at the sand surface; even a ‘wet’ foam (FER = 5) can form a low-permeability layer. Furthermore, a foam-infiltrated zone is essential to achieve a low-permeability layer. In these tests, the flow velocities were lower than the normal excavation velocity of a tunnel-boring machine. Thus there was no foam-infiltrated zone and consequently no zone with low permeability during drilling. By comparing the pore pressures measured using pore pressure transducers with and without hydrophilic filter papers, the capillary pressure in the foam-infiltrated sand was found.

Centrifuge Modeling of Soft Soil Reinforced with Granular Columns

This paper presents a geotechnical centrifuge model of an embankment built in stages, over a 6.0 m layer of soft soil reinforced with nine floating granular columns arranged in a 3 × 3 square grid. The container was divided in two compartments where reinforced and unreinforced soil were tested simultaneously for comparison purposes. The load was applied by means of a 4 m high sand embankment (prototype dimensions) built in three steps. For each step, the centrifuge was stopped to allow the placement of the embankment layer and then re-accelerated up to 26 g and the excess of pore pressure was allowed to dissipate after each step. The instrumentation, comprised of total stress cells, pore pressure transducers and displacement transducers, was configured to collect information about the load transfer mechanism between the soil and the instrumented central column for each loading step during pore pressure dissipation. Results showed that despite representing only 30% of the total load, the first step provoked almost 70% of total vertical displacement. Finally, the columns were exhumed for a detailed and comprehensive visual analysis aiming to support and explain the results obtained.