Oedometric Compression Tests Research Articles

Compression Behavior of Canadian Oil Sands

The compression behavior of Canadian oil sands was investigated by running high-stress oedometer compression tests up to 65 MPa. An X-ray microtomography investigation confirmed the significant disturbance affecting the core samples, showing large pores located between dense intact clusters made up of angular sand grains. Compression tests confirmed the significant effects of disturbance on the com- pression characteristics, providing a much softer response without any memory of the significant load applied by the 3- to 4-km-thick ice sheet during the Quaternary period. High-stress compression allowed us to bring back the samples close to the in situ estimated porosity, providing compression characteristics (estimated fromaloading cycle)close totheintact ones,despiteobviousdifferencesinfabric (includingtheeffects attributable to grain crushing at high stress). Creep behavior was also observed, and an estimate of the order of magnitude of the creep strain corresponding to the Quaternary period is provided. DOI: 10.1061/(ASCE)GT.1943-5606.0000808. © 2013 American Society of Civil Engineers. CE Database subject headings: Oils; Compression; Sand (soil type); X rays; Canada. Author keywords: Heavy oil; Oil sand: High-stress compression; X-ray microtomography; Core disturbance.

A Discrete Element Approach to Model Breakable Railway Ballast

A discrete element approach to assess degradation processes in ballast beds is presented. Firstly, a discrete element model describing strength and failure of strong rock is introduced. For this purpose a granular solid is created by bonding of adjacent particles. A method to define angular ballast stones made from the granular solid is proposed. The strength of these stones is evaluated by compression between parallel platens. Comparing these results to published experimental data yields very good qualitative and reasonable quantitative agreement. Finally, the failure of aggregates of breakable stones is investigated by simulation of oedometric compression tests and indentation of a sleeper into a ballast bed.

Nonlinear Creep and Swelling Behavior of Bentonite Mixed with Different Sand Contents Under Oedometric Condition

Due to the viscous nature, especially the swelling property, bentonite is selected as buffer and backfill material for marine radioactive waste disposal. This article presents experimental results from one-dimensional oedometric compression tests with multi-stage loading and unloading/reloading on specimens of bentonite mixed with different sand contents. The sand contents are 50%, 60%, 70%, 80%, and 90%, respectively, in terms of the dry mass of sand over the dry mass of the mixture. Influences of sand contents on time-dependent stress-strain behavior of the bentonite-sand mixture under 1-D straining are examined with special attention to the nonlinear creep and swelling behavior. It is found that the bentonite-sand mixtures exhibit distinct nonlinear creep and swelling behavior that cannot be neglected. Moreover, a nonlinear creep function proposed by Yin (1999) for Hong Kong Marine Deposits has been modified to describe the nonlinear creep behavior of the bentonite-sand mixture.

Some aspects of the compression and collapse behaviour of an unsaturated natural loess

The compression and collapse behaviour of a natural unsaturated loess from northern France was investigated by running a series of constant rate of strain oedometric compression tests (together with some step loading tests) while monitoring suction by means of a high-capacity tensiometer. The reasonably constant suction measured during constant water compression tests was linked to the collapse of the largest dry pores. Time-dependent behaviour was also observed, with different responses obtained at different strain rates. In a standard fashion, collapse tests appeared to bring the representative point from the constant water content curve to the zero suction compression curve, indicating the existence of a maximum collapse stress. Data were qualitatively interpreted using the loading collapse (LC) yield curve of the Barcelona basic model. Changes in the LC yield curve inclinations could account for the existence of a maximum collapse value.

Studying the stress-suction coupling in soils using an oedometer equipped with a high capacity tensiometer

In the context of research into deep nuclear waste disposal, various works have concerned the hydromechanical behavior of Boom clay, a stiff plastic clay extracted in the SCK-CEN Underground Research Laboratory near the Mol City (Belgium), at a depth of 223 m. Due to some amount of smectite minerals in the clay fraction, Boom clay exhibits swelling properties when hydrated under low stresses. To investigate some aspects of the hydromechanical behavior of Boom clay, oedometer compression tests were carried out on samples of Boom clay close to saturation and submitted to an initial suction. During oedometer compression, the changes in suction with increased vertical stress are monitored by means of a high capacity tensiometer installed at the bottom of the sample. Some aspects related to hydromechanical couplings are examined through the investigation of the changes in suction during oedometer compression, a somewhat delicate and poorly documented experimental approach. A comparison is also made with a completely different soil sample under suction, i.e. a statically compacted unsaturated low plasticity silt. Some technical difficulties typical of this new experimental approach are first described in detail so as to optimize the interpretation of the data obtained. The experiment allows the determination of the point at which suction is changed to positive pressure during compression. Below this point, the ratio between the vertical stress and the change in suction are determined. Above this point, the data show that positive pore pressures are dissipated in a common way. The suction/stress behavior during unloading is also described and discussed. Finally, an interpretation in terms of microstructure effects is provided for both samples. The experimental approach initiated here seems to provide interesting further application to better understand hydromechanical couplings in natural soils in relation with suction increase during stress release.

Compression behaviour of minerogenic low energy intertidal sediments

Existing geotechnical approaches that describe volumetric changes in intertidal sediments in response to applied vertical effective stresses are limited by a lack of empirical research into their one-dimensional compression behaviour. In this paper we address this deficiency by presenting the results of an investigation into the compression behaviour of minerogenic low marsh and tidal flat sediments. We have tested samples of these sediment types obtained from Greatham Creek (Cowpen Marsh, Tees Estuary, UK). Analysis of physical properties and oedometer compression tests demonstrates that, contrary to the implicit assumptions of existing models, the surface sediments studied are overconsolidated. Structural variability between samples arises due to sedimentological factors, notably variations in organic content. We attribute overconsolidation to tidal exposure and falls in groundwater level that permit desiccation and cause capillary suction stresses. Greater rates of compression with respect to effective stress occur in sediments with higher initial voids ratios and more open, unstable initial structures. Variability in structure decreases with application of higher effective stresses due to the destructuration of the sediments, which also creates increased homogeneity of compression behaviour under higher effective stresses. We subsequently develop a new conceptual framework to describe compression behaviour in minerogenic intertidal sediments that incorporates overconsolidation. We advocate a statistical approach that accounts for structural variability and variations in compression behaviour at effective stresses less than and greater than the yield stress. We argue that our conceptual framework is broadly applicable to minerogenic intertidal sediments at different locations and burial depths within Holocene stratigraphic sequences providing site-specific compression data are collected. Inter-site transfer and application of measured material properties should not be undertaken due to local variations in compression behaviour resulting from varying ecological, sedimentological, geochemical, climatic, geomorphic and hydrographic conditions. The individual characteristics of different field locations should be carefully considered before the suggested framework is routinely applied.

Quantifying the effect of matric suction on the compressive properties of two agricultural soils using an osmotic oedometer

The compaction of cultivated soils by agricultural machines considerably affects both the structure and physical properties of soil, thus having a major impact on crop production and the environment. Soil mechanical resistance is highly variable both in time and space because it depends on soil texture, soil porosity and soil water state (matric suction). This paper is devoted to the effect of soil matric suction on the compression index C c, which is one of the mechanical parameters that describes the soil mechanical resistance to compression. We performed oedometer compression tests to study the compression index of a loamy soil and a sandy clay, with matric suction control, by using the osmotic technique. Soil samples were prepared by compacting soil passed through a 2 mm sieve at dry bulk densities of 1.1 and 1.45 Mg m − 3. The mechanical stress and matric suction ranges considered corresponded to field condition, with vertical stress less than 800 kPa and matric suction less than 200 kPa. The results show that the compression index C c was constant or changed little with matric suctions for the two soils at different initial densities. For the loamy soil, the average compression index C c equals 0.73 for an initial bulk density of 1.1 Mg m − 3 and 0.41 for 1.45 Mg m − 3 for matric suction higher than 20 kPa. For the sandy clay, the average compression index C c is 0.35. From a practical point of view, our study shows that the mathematical determination of mechanical parameters should be standardized due to the S-shaped compression curves that may significantly impact the compression index estimation. The observation of constant compression index with matric suction is a useful result for modelling soil strain due to traffic and predicting the compaction of cultivated soils.

Settlement Characteristic of Beach Sands and Its Evaluation

Compression characteristics in sands under working load are an important design consideration because they are directly related to settlement response. The compressibility of sand materials is significantly dependent on soil characteristics. Sands distributed in Jeju island's coastal areas, Korea, were derived from volcanic rock or from shells. Compression tests were conducted to assess the compression characteristics of these sands. From the test results, each sand showed different behaviors due to its composition origin. The measured settlement of Jeju harbor area shows that in addition to the initial elastic compression, a considerable additional compression occurred with time. The prediction with the oedometer compression test method was well matched with the measured data. However, the empirical methods (based on elastic approach) predicted a smaller than that actually occurred because they did not consider the crushing, shattering, and rearrangement of particles, which were real phenomena in sandy soils under loading condition. Based on these studies, we recommend that the settlement of Jeju coastal area sandy soils should be estimated by conducting compression tests.

An evaluation of the osmotic method of controlling suction*

Experimental techniques of testing the mechanical properties of unsaturated soils are complex and difficult to conduct. As a consequence, complete sets of parameters that characterise the behaviour of unsaturated soils remain scarce and necessary. In this context, it has been found useful to gather the information obtained after some years of practice of the osmotic technique of controlling suction. As compared to the more documented axis-translation technique, the osmotic technique has its own advantages and drawbacks that are discussed in this paper, together with some potential future developments. The osmotic method has been developed by soil scientists in the 1960s and adapted to geotechnical testing in the early 1970s. This paper presents the osmotic technique and comments on its advantages (including suction condition close to reality and higher suctions easily attained) and drawbacks (including some concern with the membrane resistance and some membrane effects in the suction/concentration calibration). Various applications to geotechnical testing are presented such as the determination of the water retention curve, oedometer and triaxial testing procedures and the determination of the permeability of unsaturated soils. Recent developments, that include the extension of the method up to high suctions (10 MPa) are also described, together with some recent and novel applications such as data from high controlled suction oedometer compression test and the determination of the oil/water retention properties of oil reservoir chalks.

New basis for the constitutive modelling of aggregated soils

Natural and compacted soils are usually characterized by aggregation of particles. The mechanical behaviour of these materials depends on soil structure. The oedometric compression tests performed on aggregated samples presented here showed that these materials exhibit a yield limit depending not only on stress history and stress state but also on soil structure. Evidence is provided using the neutron tomography technique. These results revealed that soil structure modification occurs together with plastic deformations. The experimental results are used to propose a new state parameter to quantify the soil structure. Based on pore-scale experimental observations, an evolution law for this parameter is proposed as a function of associated plastic strains. Considering both soil fabric and inter-particle bonding effects, a new yield limit depending on stress state, stress history and soil structure is introduced for the aggregated soils. Accordingly, a new constitutive framework consistent with strain hardening plasticity is proposed to consider soil structure effects in the modelling of aggregated soils.

Identification of material parameters of a thermo-mechanical model for pebble beds in fusion blankets

In this paper, a thermo-mechanical model of pebble beds is adopted [D. Hofer, M. Kamlah, Drucker-Prager-Cap creep modelling of pebble beds in fusion blankets, Fusion Eng. Des. 73 (2005) 105–117] and developed based on experiments by Dr. Reimann at Forschungszentrum Karlsruhe (FZK). The framework of the present material model is composed of a non-linear elastic law [O. Coube, Modelling and numerical simulation of powder die compaction with consideration of cracking, PhD Thesis, University Pierre et Marie, Paris VI, 1998], the Drucker-Prager-Cap theory [ABAQUS, Analysis User's Manual, Version 6.5, 2004], a modified creep law [D. Hofer, M. Kamlah, Drucker-Prager-Cap creep modelling of pebble beds in fusion blankets, Fusion Eng. Des. 73 (2005) 105–117]. Furthermore, the volumetric inelastic strain dependent thermal conductivity of beryllium pebble beds [J. Reimann, G. Piazza, Z. Xu, A. Goraieb, H. Harsch, Measurements of the thermal conductivity of compressed beryllium pebble beds, FZKA 7096 (2005)] is taken into account and full thermo-mechanical coupling is considered. By analyzing the deformation mechanism of the oedometric experiments, a new method is developed to determine the set of material parameters, including the temperature dependent hardening law. With the new method, the material parameters can be derived directly from the empirical equations (the so-called “Reimann fits” for pebble beds) including the thermoplastic behaviour. All these thermo-mechanical constitutive laws are implemented in ABAQUS by user defined field (USDFLD) and CREEP subroutines. The oedometric compression tests and creep tests under different temperatures are simulated by the present model, and the results show that the model gives a good description of experimental results. The analysis on the precompaction procedure of pebble beds assembly (PBA) is applied to show the feasibility of the present material model.

The interpretation of CPT data from hydraulically placed PFA

Cone penetration test (CPT) results on an ∼ 20 year old, hydraulically placed pulverised fuel ash (PFA) deposit are reported, along with the results of oedometer compression tests on ‘undisturbed’ specimens recovered during the site investigation and resedimented PFA samples. The latter showed that aged PFA can undergo significant secondary compression. The correlation between volume compressibility and cone resistance, m v = 1 / ( αq c), is fitted to compressibility data from undisturbed samples to determine α. The resulting CPT compressibility profile shows good agreement with compressibility trends for aged PFA estimated from the tests on resedimented ash. It is therefore recommended that a value of α = 11 should be used for normally consolidated, aged PFA. The danger of biased sampling in very loose non-cohesive materials and the need for depth profiling by in situ measurement are highlighted.

Thermo-Mechanical Modelling of Pebble Beds in Fusion Blankets and its Implementation by a Return-Mapping Algorithm

AbstractIn this investigation, a thermo-mechanical model of pebble beds is adopted and developed based on experiments by Dr. Reimann at Forschungszentrum Karlsruhe (FZK). The framework of the present material model is composed of a non-linear elastic law, the Drucker-Prager-Cap theory, and a modified creep law. Furthermore, the volumetric inelastic strain dependent thermal conductivity of beryllium pebble beds is taken into account and full thermo-mechanical coupling is considered.Investigation showed that the Drucker-Prager-Cap model implemented in ABAQUS can not fulfill the requirements of both the prediction of large creep strains and the hardening behaviour caused by creep, which are of importance with respect to the application of pebble beds in fusion blankets [1]. Therefore, UMAT (user defined material's mechanical behaviour) and UMATHT (user defined material's thermal behaviour) routines are used to re-implement the present thermo-mechanical model in ABAQUS. An elastic predictor radial return mapping algorithm is used to solve the non-associated plasticity iteratively, and a proper tangent stiffness matrix is obtained for cost-efficiency in the calculation. An explicit creep mechanism is adopted for the prediction of time-dependent behaviour in order to represent large creep strains in high temperature. Finally, the thermo-mechanical interactions are implemented in a UMATHT routine for the coupled analysis.The oedometric compression tests and creep tests of pebble beds at different temperatures are simulated with the help of the present UMAT and UMATHT routines, and the comparison between the simulation and the experiments is made.

A bounding surface plasticity model for sands exhibiting particle crushing

A new bounding surface constitutive model for sands is presented and is suited to a wide range of stresses, including those sufficient to cause particle crushing. The basic concepts of critical state soil mechanics are shown to be valid, and a uniquely shaped critical state line is defined to capture the three modes of plastic deformation observed across a wide range of stresses, including particle rearrangement, particle crushing, and pseudoelastic deformation. A limiting isotropic compression line is separated from the critical state line in the υ – In p′ plane by a constant shift along an elastic unload–reload line. In the deviator stress – mean effective stress (q–p′) plane, the loading and bounding surfaces are homologous about the origin and defined by a simple and versatile function. Isotropic hardening and softening of the loading and bounding surfaces are controlled by plastic volumetric strains. A commonly used non associative flow rule is adopted. Experimental results of monotonically loaded drained and undrained triaxial tests, isotropic compression tests, and oedometric compression tests are presented for a quartz sand and used to calibrate the model. Membrane penetration is accounted for in the model simulations of the test results. A single set of material parameters is introduced enabling rigorous and accurate predictions of stress–strain behaviour in sands.Key words: sand, bounding surface, plasticity, particle crushing.

Compression of a soft sphere packing

Mechanical properties of packings of deformable spheres of polyelectrolyte gel are studied experimentally. These particles are plunged into a brine. They have the property to swell and shrink when the concentration of salt of the solution is varied. An oedometric compression is performed imposing cycles of deformation at constant speed and constant salinity Cs. Under many different conditions, we study the laws of deformation relating the macroscopic compression force F, to the macroscopic strain . We find empirical non linear relations of the type . The values of this exponent m are discussed and compared to the results of measurements on a single sphere compressed on a plane as well as to the results of experiments and simulations on dry model granular assemblies. The swelling and deswelling properties of the spheres are used to perform isotropic compression tests. In this situation we determine the relation between the force at equilibrium and the macroscopic strain . The results are compared with those obtained in the oedometric compression tests.