Small-scale Centrifuge Research Articles

Development of a small-scale geotechnical centrifuge

Oil sand fluid fine tailings in northern Alberta have very poor water-release characteristics and require many decades to fully consolidate under their own weight. Generally, physical and/or chemical treatments are necessary to improve the consolidation behaviour and manage the tailings in an economical and environmentally acceptable manner. The effect of each treatment method is commonly evaluated, in part, by measuring the short- and long-term consolidation behaviour of treated samples. Geotechnical beam centrifuges, large-strain consolidation apparatus and geocolumns are typically used to measure the consolidation properties. However, these methods require many months to complete or are expensive and difficult to deploy in the field. A small-scale geotechnical centrifuge suitable for use in both research environments and laboratories at industrial sites was developed to study and/or monitor various treatment and disposal options in a short time. This paper presents the motivation for developing a small-scale centrifuge, the components and features of the centrifuge and the assortment of tests completed to validate the apparatus and testing methods. The validation tests proved that the small-scale centrifuge produces comparable results with existing methods while reducing the cost and time required to evaluate the consolidation performance of different tailings treatment options.

Novel experimental technique to model impression piles in centrifuge testing

A novel experimental technology for small-scale centrifuge tests on piled foundations has been investigated. The technology is suitable for bored piles where the pile shaft has been profiled to improve bearing capacity, such as with the impression pile which has an enhanced shaft capacity owing to the small impressions created along the shaft. In previous centrifuge testing, impression piles have been created by pouring resin into a profiled bore. However, in the technique described, a pile made from three-dimensionally (3D)-printed rigid plastic components with a mandrel mechanism is used to create a nodular shaft surface during model making. Once assembled the pile has the same geometry as the cast-in-situ impression pile. 3D-printed plastic piles allow for fast model making and demonstrate excellent repeatability. Because of the ductile behaviour of the soil–plastic interface the impressions improve the performance of a pile over the entire load settlement curve, not just at ultimate capacity. In addition, a greater increase in ultimate capacity was registered for the 3D-printed plastic impression piles compared to similar resin impression piles. At working load, the 3D-printed plastic impression piles outperformed traditional straight-shafted piles by 90%.

Experimental study on water transfer mechanism of quicklime modified centrifugal dewatering clay

This study investigates the water transfer mechanism of quicklime modified centrifugal dewatering clay. A small-scale centrifuge device was used to investigate the change of bound water content in modified clay under different laboratory test conditions including quicklime content and curing age, and analyze the water transfer mechanism in the process of quicklime treatment, and its impact on clay properties. The results show that the content of pore water decreased by 6.2% ∼ 16.9%, the content of bound water increased by 4.2% ∼ 12.9%, and the content of hydration water increased by 6.1% ∼ 14.1% due to hydration reaction caused by the quicklime. The increase of quicklime content facilitates the conversion of free water into bound water and hydration water in a relatively short time. The pozzolanic reaction occurs on the surface of the clay particles to absorb free water and convert it into bound water, while part of Ca (OH)2 crystals generated by hydration reaction adhere to the surface of clay particles, grow into needle-shape and fibrous structures, and absorb free pore water and convert it into hydration water. For field applications, the mixing percentage of quicklime is recommended to be below 9% by dry weight. From the cost perspective, 7-day curing age is recommended in the field. The essence of quicklime modification is to transform the free water with relatively low pF potential energy into the bound water and hydration water with relatively high pF potential energy. A schematic diagram of water transfer mechanism was also proposed in this study for quicklime modified clay and validated using the aforementioned experimental data.

Scaling the failure of concrete gravity dam subjected to underwater explosion — Part I: Suggestion of a dimensionless number

The small-scale model test is the major feasible approach to experimentally investigate concrete gravity dams subjected to underwater explosions. However, speculating results of the dam prototype requires knowledge of scaling law. Our previous research finds that the dimensional analysis-based scaling law is violated since the strain-rate effects between small-scale dam models and prototypes are dissimilar, resulting in distinct tensile failures between them. The non-scalability of the strain-rate effect is one of the most troublesome issues in similarity analysis, plaguing the research community for years. This two-part paper attempts to address this challenge by proposing a correction method to the dimensional analysis-based scaling law for scaling the failure of concrete gravity dams subjected to underwater explosions. The strategy contains two steps. The first step is to derive dimensionless damage numbers through the dimensional analysis method. The second step is then to propose the correction method based on the verified dimensionless damage number. Part I of this work deals with the first step, while Part II addresses the second step.In Part I of this work, a dimensionless damage number is suggested to reflect the dynamic bending-induced tensile response mechanism of concrete gravity dams against underwater explosions. Three aspects can be reflected in this dimensionless damage number: the inertia force of the loading, the resistance ability of the material to deformation, and the resistance ability of the dam structure to deformation. For validation, a numerical scheme comprising 12 geometrically similar dam models with geometrical scaling factors β = 1, 1/2, 1/5, 1/10, 1/20, 1/30, 1/40, 1/50, 1/60, 1/70, 1/80, and 1/100 is devised based on small-scale centrifuge tests. Results have demonstrated that when strain-rate effects are close, such as for β = 1/40–1/100 dam models, the suggested dimensionless damage number can well predict the dam dynamic responses and failures, and shows an excellent linear relation with the dam dimensionless displacement. However, when strain-rate effects are dissimilar, such as for β = 1–1/30 dam models, the suggested dimensionless damage number fails, and correction methods are required that will be proposed in Part II of this work.

Scaling the failure of concrete gravity dam subjected to underwater explosion shock loading

The small-scale model test is the major affordable approach to experimentally investigate concrete gravity dams against underwater explosions. However, results of the small-scale model should be properly converted to the prototype, which requires the knowledge of scaling law. This study presented the scaling of failures of concrete gravity dams against underwater explosion shock loadings through small-scale centrifuge tests and numerical simulations. The dimensional analysis-based scaling law, which was essentially the geometrical scaling law for dams and the well-known Hopkinson scaling law for underwater explosion shock loadings, was employed. According to the scaling law, a numerical scheme comprising six numerical simulations with geometrical scaling factors β = 1, 1/10, 1/30, 1/50, 1/80, and 1/100 was devised. Results demonstrated that dams with different scaling factors exhibited essentially the same failure mode, i.e., the structural bending-induced tensile failure mode. On this basis, it was discovered that most dam dynamic responses followed the scaling law. It was also found that when the strain-rate effect was pronounced, such as in dam tensile stresses and tensile failures, the scaling law was violated. Despite the incomplete scaling law, the small-scale model tests can be used to identify the most vulnerable positions of the dam prototype.

Failure modes of concrete gravity dam subjected to near-field underwater explosion: Centrifuge test and numerical simulation

Concrete gravity dams are often referred to as vertical cantilever structures with approximately fixed bottoms due to their massive gravity, frictions with the foundation, and additional anti-sliding techniques. Given this, when modeling concrete gravity dams subjected to underwater explosions, should the dam bottom be considered as the fixed boundary or the frictional boundary? In this study, two small-scale centrifuge tests and numerical simulations were used to investigate the dam failure modes due to near-field underwater explosions. Emphasis was placed on the effect of dam bottom boundary condition (DB-BC) to improve understanding of dam failure mechanisms and provide references for constructing experimental and numerical dam models. To this end, the current research began with a combined experimental and numerical investigation on the failure modes of dams with fixed bottoms. Then, a numerical study was conducted to evaluate the effect of DB-BC. Results revealed that dams with frictional bottoms demonstrated essentially the same failure mode as those with fixed bottoms. Both exhibited mainly the structural bending-induced tensile failure mode. To the authors’ knowledge, this is the first time the effect of DB-BC has been explored. In particular, by centrifuge tests, this paper presented the urgently needed experimental data in this field including dam failure patterns and strain histories as well as test videos.

Effect of bubble pulse on concrete gravity dam subjected to underwater explosion: Centrifuge test and numerical simulation

The purpose of this study is to use small-scale centrifuge tests and numerical simulations to explore the effect of bubble pulse on concrete gravity dams subjected to underwater explosions. To the best of the authors’ knowledge, there is still no systematic research focusing on this topic. In this study, five small-scale centrifuge tests previously performed were used. A numerical model was established based on the Coupled Lagrangian-Eulerian method and was validated by the centrifuge tests. Results revealed that the bubble pulse can introduce a secondary attack that can further destroy the dam structures on the base of the damage induced by the shock wave. Local responses of the dam were relevant to peak pressures of the shock wave and the bubble pulse, whereas global structural responses were dependent on their impulses. For a standoff distance greater than the maximum bubble radius, their impulses were close and thereby the induced global structural responses were comparable. For a standoff distance less than the maximum bubble radius, the impulse of the shock wave was remarkably higher than that of the bubble pulse, and thus the dam structures were destroyed by the shock wave before the bubble pulse arrived.

Development of a novel lateral resistance measurement penetrometer for pipe–soil interaction centrifuge model test

Offshore pipelines are exposed to potential submarine landslides. It is difficult to measure the impact forces exerted on a pipeline induced by submarine landslides because the upper seabed is very soft and an impact angle exists in practice. Current laboratory experiments are limited on measuring the normal force exerted on pipeline by the slides. This article describes the feasibility and performance of a novel penetrometer, which is designed to measure impact forces of submarine landslides on offshore pipeline including both the normal and frictional forces from variable impact angle in centrifuge test, termed as the lateral resistance measurement penetrometer. The calibration procedure for this particular version is illustrated in detail and the reliability and feasibility of the lateral resistance measurement penetrometer are preliminarily confirmed by an experimental study. Pipe–soil interaction centrifuge tests were subsequently performed on a normally consolidated kaolin sample with 40 g level conducted in Dalian University of Technology drum centrifuge to investigate the effect of incident angles on the impact forces exerted by submarine landslides on a pipeline. The impact forces deduced from the lateral resistance measurement penetrometer are analysed in comparison with published results. The lateral resistance measurement penetrometer is a potentially valuable device to measure the interaction force between submarine landslides and offshore pipeline, cables and other subsea facilities in small-scale centrifuge models.

Input of fully 3D FE soil-structure modelling to the operational analysis of jack-up structures

Jack-ups are mobile structures widely employed in the offshore industry as drilling rigs or installation/maintenance vessels (e.g. for offshore wind farms). To assure safety at each location, site-specific assessment is required to predict the performance of the unit during installation and operations. The response of jack-ups to environmental loads is highly affected by the interaction between all footings (spudcans) and the underlying soil, an interaction still challenging to describe under general 3D loading.This work emphasises the potential of 3D continuum simulations to capture non-linear soil-structural interaction in jack-up units. An integrated jack-up–spudcans-soil 3D finite element (FE) model is set up by including strain-hardening soil plasticity and geometrical non-linearity (P−Δ effects). After preliminary calibration of soil parameters, the FE model is successfully validated against literature results, namely obtained through (i) small-scale centrifuge experiments and (ii) numerical simulations based on macroelement foundation modelling. The validated FE model is then used to inspect several implications of soil modelling assumptions, as well as the response of the jack-up to relevant 3D loading combinations.The results presented support 3D continuum modelling as a suitable approach to analyse spudcan fixity and, overall, the operational performance of jack-ups. Despite higher conceptual/computational difficulties, fully 3D simulations can valuably complement the insight from (rare) integrated physical modelling, and contribute to the improvement of soil-spudcan macroelement models.

Comparison of Soil–Water Characteristic Curves from Conventional Testing and Combination of Small-Scale Centrifuge and Dew Point Methods

Soil–water characteristic curve (SWCC) is an important unsaturated soil property relating the water content of a soil to soil suction and it is conventionally measured using Tempe cell, pressure plate and salt solution methods. However, these tests are tedious and time consuming. The SWCC measurements using fast and efficient methods are required for engineering designs such as excavation, slope protection, retaining wall and landfill cover designs. This paper describes the testing procedures and apparatuses associated with rapid measurements of a complete SWCC of a residual soil as obtained from combined measurements using a small-scale centrifuge and dew point methods. The SWCC test results obtained using these alternative methods were compared with the SWCC data from Tempe cell, pressure plate and salt solution methods. Shrinkage tests were carried out in this study to incorporate the volume change of soil into SWCC. The experimental data from all SWCC tests were evaluated using first order analysis with 95% confidence interval for determination of upper and lower bounds of SWCC. The analysis results showed that the SWCC data obtained from tests using small-scale centrifuge and dew point methods were in good agreement with those obtained from Tempe cell, pressure plate and salt solution methods. This indicates that the combination of small-scale centrifuge and dew point methods can be used to generate a complete curve of SWCC for the residual soil. In addition, the time required to perform SWCC tests using the alternative methods is shorter than the SWCC tests using the conventional methods.

Physical Modeling of Soil Liquefaction: Repeatability of Centrifuge Experimentation at RPI

Numerical models are commonly used in practice to predict the occurrence and propagation of liquefaction and lateral spreading. Nevertheless, there are still only a few good-quality data sets to validate these models. An international collaboration between physical and numerical modelers has been initiated through the Liquefaction Experiments and Analysis Projects (LEAP) to establish a standard method for validation of numerical models. One of the main goals of LEAP is to provide numerical modelers with high-quality experimental data that can be used in a series of prediction exercises. In this framework, a small-scale centrifuge test of a sloping deposit was conducted at Rensselaer Polytechnic Institute (RPI) in January 2015 and provided comprehensive data sets for evaluation of soil liquefaction numerical tools. The same experiment was repeated in September 2015 to assess the robustness and repeatability of the physical modeling methods at RPI. This paper serves a double objective: (1) to study the response and liquefaction potential of sloped deposits experimentally and (2) to establish the repeatability of experimental data to be used in numerical simulations.

General evaluation of sand column models by X-ray MicroCT

Assessment of the structural and geotechnical conditions of embedded structures in small-scale models is challenging when undisturbed samples are required. Modelling of stone column reinforcements of soft clay deposits are typical examples of this situation. In this case, a non-intrusive inspection is very important for gathering additional information that can explain the test results. Herein, the use of X-ray computed tomography (MicroCT) scans for assessing the quality of stone column reinforcements in a small-scale centrifuge model after tests are performed is presented. The results indicate that this technique can provide crucial information in three-dimensional (3D) space for interpreting the test results regarding the structural conditions of the columns. Some features as granular grading distribution, clogging, deformation and general aspects that may interfere with the results are shown for some columns. Sampling techniques are described in detail, and the difficulties encountered in the study are discussed, mainly those related to image treatment. The final product (3D image) is quite attractive in terms of visual details of the model enabling, thus, a more qualified conclusion.

Laboratory development of a vertically oriented penetrometer for shallow seabed characterization

Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1–2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field.

Modelling the failure mechanisms of Michelangelo's David through small-scale centrifuge experiments

It has been noted since the mid 1800s that the Michelangelo's David, the standing marble male nude representing a masterpiece of the Italian Renaissance, is affected by small cracks on both legs that threaten its stability. Understanding the characteristics and the conditions under which these lesions developed is thus critical for the preservation of this universal masterpiece. In this study, we use an analogue modelling approach to test the conditions that led to the development of fractures in the David's legs and to get insights into its stability. Small-scale (10 cm-high) gypsum replicas of the statue were deformed in a centrifuge, where the models were affected by a body force stronger than gravity but otherwise playing the same role. Analysis of the model results suggests that both the stability and the resulting deformation of the statue are highly sensitive to its attitude. A forward inclination promotes destabilization: the higher the angle of inclination ( α ), the more unstable the statue becomes under its own weight, confirming existing FEM modelling. In a vertical position, rupture of the statue typically occurs in the lower portions of the legs, but ruptures tend to develop progressively higher along the legs as α increases. Comparison of these results with the lesions detected on the actual David suggests that a long-lasting, small forward inclination (likely close to ∼ 5°) of the statue may have represented a critical driving factor for the development of the observed damages.

Geosynthetic reinforcement of a granular load transfer platform above rigid inclusions: comparison between centrifuge testing and analytical modelling

ABSTRACT: The soil reinforcement method, which consists of placing a granular mattress above a set of rigid inclusions, is used to transfer part of the surface load to the piles through arching. The addition of an extensible geosynthetic layer at the base of the mattress increases load transfer. A small-scale centrifuge model, the mobile tray device, is used to simulate the foundation behaviour by modelling soft soil settlement between the inclusions. A parametric study was conducted to examine the influence of the mattress thickness and of the pile spacing. The different test configurations are compared with regard to the stress transmitted to soft soil, to differential settlement and to the efficiency of load transfer. With thicker mattresses and/or closer pile meshing, load transfer increases and surface settlements are reduced. Geosynthetic maximum experimental and analytical deflections are also examined. For the analytical study, conducted using the EBGEO German standard, three different distributions of the vertical stress applied to the geosynthetic layer are compared: uniform, triangular and inverse triangular. All three scenario results fall within the range of the experimental results. The inverse triangular distribution reveals some deflection values that were lower than the uniform one. The triangular distribution gives the highest deflection values.

Centrifuge observations on combined loading of a strip footing on dense sand

Experimental observations on the behaviour of a small-scale model strip footing on dense sand subjected to combined loading under an enhanced gravitational field are presented. These observations are interpreted within the framework of plasticity-based force-resultant models, established earlier under 1g conditions, with the aim of describing changes of behaviour between small-scale 1g and centrifuge models. The comparison reflects the tendency of 1g-based observations to overestimate, on the unsafe side, the normalised deviatoric load capacity for low levels of V0/Vpeak. On the other hand, the 1g-based response is found to be conservative compared to centrifuge-obtained responses for high levels of V0/Vpeak.

Centrifuge modelling of solifluction processes: displacement profiles associated with one‐sided and two‐sided active layer freezing

AbstractTwo sets of small‐scale centrifuge experiments were undertaken to simulate solifluction processes associated with one‐sided freezing (with no permafrost) and two‐sided freezing (with permafrost) within an active layer 0.8–0.9 m thick at the prototype scale. Models were frozen on the laboratory floor under normal acceleration due to gravity and thawed in the centrifuge under an acceleration of 10 gravities. In the one‐sided freezing experiment, ice segregation was concentrated near the surface, whereas in the two‐sided model it also caused an ice‐rich transition zone to form near the base of the active layer. After repeated freeze‐thaw cycles the one‐sided experiment revealed concave downslope profiles of movement with shear strain decreasing with depth, whereas in the two‐sided experiment, convex downslope profiles indicated the greatest shear strain in the basal part of the active layer. Although smaller surface velocities were measured in the two‐sided experiment, higher volumetric velocities developed. The centrifuge experiments showed displacement rates similar to field measurements and analogous to full‐scale laboratory experiments. Copyright © 2008 John Wiley & Sons, Ltd.

Measurement of soil-water characteristic curves for fine-grained soils using a small-scale centrifuge

Considerably long periods of time are required to measure soil-water characteristic curves using conventional equipment such as pressure plate apparatus or a Tempe cell. A commercially available, small-scale medical centrifuge with a swinging type rotor assembly was used to measure the soil-water characteristic curves on statically compacted, fine-grained soil specimens. A specimen holder was specially designed to obtain multiple sets of water content versus suction data for measuring the soil-water characteristic curve at a single speed of rotation of the centrifuge. The soil-water characteristic curves were measured for three different types of fine-grained soils. The three soils used in the study were processed silt (liquid limit, wL = 24%; plasticity index, Ip = 0; and clay = 7%), Indian Head till (wL = 35.5%, Ip = 17%, and clay = 30%), and Regina clay (wL = 75.5%, Ip = 21%, and clay = 70%). The soil-water characteristic curves for the above soils were measured in 0.5, 1, and 2 days, respectively, using the centrifuge technique for suction ranges from 0 to 600 kPa. Time periods of 2, 4–6, and 16 weeks were required for measuring the soil-water characteristic curves for the same soils using a conventional pressure plate apparatus. There is reasonably good agreement between the experimental results obtained by the centrifuge and the pressure plate methods. The results of this study are encouraging as soil-water characteristic curves can be measured in a reduced time period when using a small-scale centrifuge.Key words: unsaturated soils, soil-water characteristic curve, centrifuge technique, soil suction, matric suction, water content.

An examination of rapid, centrifuge physical modeling studies of contaminant movement in freezing soil

AbstractThis paper reviews the complex factors interacting in the movement of contaminants in soil subject to seasonal freezing. This includes those relevant to the soil itself, the contaminant itself, and environmental factors, all of which must be understood for prediction and effective design of remediation. Numerical modelers, as well as laboratory researchers examining behavior of small elements of the soil system, require reliable information on the range of full-scale system responses, but it is not feasible to acquire this by full-scale tests. Even field workers benefit from this information in planning data collection. Small physical models of contaminants moving through soil have routinely been limited in their usefulness because of differences in model fluid pressures and soil stresses, compared to full-scale conditions. However, small-scale centrifuge modeling presents the opportunity to produce correct and rapid physical simulation of full-scale system response using field soil and real contaminants, under the range of different boundary conditions. This paper discusses the existing recent centrifuge modeling work that supports the thesis that the technique can be applied to understanding and analyzing this complex problem. Five studies are reviewed: one on simulation of soil freezing effects in the absence of contaminants; three on the simulation of contaminant movement through saturated and unsaturated unfrozen soil, and heat transport effects through the fluid phase of unfrozen soils; and one that simulates the combination of contaminant movement in freezing soil.

Centrifuge Model Tests of Nailed Soil Slopes

This paper presents the results of 24 centrifuge model tests of nailed soil slopes and vertical walls, constructed out of dry Leighton buzzard sand. The walls were initially supported by fluid pressure from flexible rubber bags against the face, and excavation was modelled by gradually draining the fluid from the bags. The finished model walls were 200 mm high and were initially tested at 30 g acceleration to correspond to a prototype structure 6.0 m high. If failure was not obtained, the acceleration was increased progressively to a maximum of 80 g. No surcharges were applied. The main parameters varied in the tests were the wall slope, nail length, nail surface roughness, nail inclination, facing stiffness and facing roughness. Observations were made of the mechanism of failure when it occurred, of soil pressures on the facing, and of pre-failure deformations. Failure was always by pull-out rather than breakage of the nails, and a series of pull-out tests of the model nails was conducted to aid interpretation of the results. Failure surfaces were seen to have the shape of logarithmic spirals, and limit equilibrium analyses based on these surfaces agreed well with experimental observations. Prior to failure, earth pressures on the facing compared reasonably well with those calculated by Coulomb’s method, except at the base of the wall. One test set out to model a full-scale trial wall, and although the construction process could not be exactly modelled in the small-scale centrifuge tests, comparisons were sufficiently good to give confidence in the model test results.