Suction-controlled Oedometer Tests Research Articles

Structural evolutions of MX80 bentonite pellet/powder mixtures under wetting and suction-controlled oedometer loading

The compression behaviour of MX80 bentonite pellet/powder mixture was investigated by performing suction-controlled oedometer tests. The suction from 113 to 4.2 MPa was imposed by vapour equilibrium technique (VET), and zero suction by water circulation (WC). After instantaneously unloaded from various target vertical stresses, the structural observation was conducted by mercury intrusion porosimetry and micro-computed tomography. Results showed that during wetting, the intra-grain micro- and macro-pores increased due to the swelling of pellets and powder grains. During loading, the variation of intra-grain micro-pores was characterised by the loading effect, while that of intra-grain micro-pores was controlled by the suction and loading effects. The inter-grain pores, for the VET samples, were reduced by the swelling of pellets and powder grains during wetting on one hand, and by the movement of pellets and powder grains during loading on the other hand. For the WC samples, most inter-grain pores were closed during wetting, with a few pores in the top closed by further loading. The global compression of bentonite mixture samples during loading was mainly governed by the filling of inter-grain pores for the VET samples, but controlled by the compression of intra-grain pores for the WC samples.

Evaluation of climate change on the collapse potential of unsaturated cement-treated laterite soil for disaster risk reduction

Extreme weather events and intense rainfall may alter the climate, which would probably affect the geotechnical constructions such as unsaturated embankments. Basically, soil moisture content determines the strength of the unsaturated soil, with wetter soils often being weaker. Although it has been proved that unsaturated condition substantially impacts the shear strength and volumetric behaviour of soil, its implications are rarely investigated or taken into account in the design. As a result, changes in temperature and rainfall loads will have an influence on geotechnical constructions and develop long-term seasonal deformations that might severely jeopardize safety and maintenance. Therefore, it is crucial to assess the effects of the climate on soil behaviour for each location through adequate geotechnical laboratory tests. Johor, Malaysia has a large area and abundant tropical soils. Hence, this study aimed to elucidate the influence of climate change on soil behaviour in the tropical regions of Johor. To impose Malaysia's climate, a series of modified suction-controlled oedometer tests are conducted under different matric suctions. The outcomes revealed that the low and high matric suction has significantly impacted the untreated and cementtreated soil. However, the great reduction of soil settlement is mostly from the coupling effect of saturation and stabiliser.

A double structure model for hydro-mechano-chemical behavior of expansive soils based on the surface phenomena of mineral crystals

It is well known that the typical swelling behavior of expansive soils is mainly caused by the nanoscale electrochemical phenomena on the surface of their main constituent mineral (i.e., smectite). In this study, the hydro-mechano-chemical (HMC) model for unsaturated expansive soils, which combines the macroscale soil skeleton behavior and the nanoscale interlaminar behavior of mineral crystals based on the surface phenomena is proposed within a double structure framework. The soil skeleton behavior is modeled using the Cam clay type model based on Bishop's effective stress, which considers the degree of saturation-induced hardening. Thus, it is capable of describing hydraulic collapse. Furthermore, the interlaminar behavior is demonstrated through the interlaminar force equilibrium of clay mineral crystals. To describe the swelling process during wetting, namely the crystalline and osmotic swellings, the hydration and osmotic forces acting on the mineral crystals are related to the degree of saturation. Through simulations of a series of the suction-controlled oedometer tests and the swelling pressure tests with various solutions for FEBEX bentonite, it is indicated that the proposed model demonstrates that it can comprehensively describe the typical HMC behavior of expansive soils.

Effects of Structure on the Compression Behavior of Unsaturated Loess

The influence of structure plays a strong part in hydromechanical properties of soil. A series of suction-controlled oedometer tests have been conducted to explore the compression behavior of intact and compacted loess. The main findings show the following: (1) the yield stress of intact loess is higher than that of compacted loess at a given initial void ratio and under constant suction; (2) the slope of the normal compression line [λ(s)] of compacted loess is larger than that of intact loess; (3) the λ(s) value of intact loess initially increases sharply as suction increases and then trends to a slow decrease as suction increases further, while λ(s) of compacted loess continues to increase; (4) the λ(s) value of compacted loess can be fitted precisely with the Barcelona basic model; and (5) intact loess exhibits anisotropic behavior, in which a vertically trimmed specimen has a higher yield stress and λ(s) value than a horizontally trimmed specimen. The results showed that intact specimens and compacted specimens have different hydromechanical properties. Moreover, anisotropic behavior has also been found in unsaturated silty loess.

Settlement of Footings on Compacted and Natural Collapsible Soils upon Loading and Soaking

Collapsible unsaturated soils experience large volumetric compressive deformations upon wetting. Soil collapse is a geotechnical problem that occurs in different regions of the world and profoundly impacts buildings and civil infrastructure constructed on this type of soil, with the associated impact on safety and economy. Compaction is a simple, relatively cheap, and yet effective soil improvement technique that could significantly reduce soil collapsibility. This paper combines laboratory tests, in-situ investigations, and numerical modeling to gain a better understanding of the effect of soil compaction on collapsible-soil settlements upon wetting under field conditions. The finite elements program CODE_BRIGHT was adopted for the numerical simulations of the field tests using model parameters that were previously obtained from independent laboratory experiments. The laboratory campaign comprised suction-controlled oedometer and triaxial tests of the natural and compacted soils considered in this study, as well as permeability and water retention experiments. The numerical simulations show that the proposed approach can model the behavior observed in the in-situ tests involving footing prototypes on collapsible soils subjected to soaking. These analyses also demonstrate that soil compaction is a viable technique to reduce collapsibility in order to meet predefined maximum allowable settlements. Additional analyses were carried out to extend the main findings to other soils and loading conditions. Based on stress-bulb concepts, a method is proposed to reduce settlements in collapsible soils for typical footing operational conditions.

Collapsible Behavior of Lateritic Soil Due to Compacting Conditions

Soil collapse is a major geotechnical problem that causes architectural and structural damage to buildings and engineering infrastructure. Losses due to soil collapsibility have an economical aspect and the study of these soils is very important due to the large occurrence of collapsible soils around the world. In this research, conventional oedometer tests and suction-controlled oedometer tests were conducted to study the benefit of compaction as a method of improving the behavior of a lateritic soil that is particularly susceptible to collapse after soaking. The results offer practical information to minimize problems arising from collapsible soils. The soil water content before the sample is inundated affected the soil collapsible behavior. Soil specimens less dense when compacted at the dry side of the compaction curve exhibited volume variation under loading less than 4% and collapse deformations up to about 11% when exposed to flooding, while soil specimens compacted at optimum moisture content showed volume variation under loading around 10% and a maximum collapse deformation by about 6%. The collapse deformations changed significantly with inundation stress for the soil compacted especially for stresses above 100 kPa. The collapse deformations were almost insignificant when the lateritic soil had a relative compaction of 90% with values almost always less than or equal to 2%, proving that the compaction process is a soil improvement technique that can significantly reduce the collapse induced by inundation.

Constitutive model describing the hydro-mechanical behaviors of compacted bentonite clay based on the crystal surface phenomena

A constitutive model for unsaturated expansive soil based on the crystal surface phenomena is proposed. The behavior of the proposed model is described as a double structure, with the soil skeleton and the interlaminar behaviors. The soil skeleton behavior is modeled by the Cam clay type model based on the Bishop’s effective stress, which can consider the degree of saturation-induced hardening and inelastic behavior in the over-consolidated region and is capable of describing hydraulic collapse. On the other hand, the interlaminar behavior is give as a result of the interlaminar equilibrium of clay minerals. The diffusion double layer repulsive force in the interlaminar equilibrium varies with the degree of saturation, and it mainly causes the hydraulic swelling of expansive soil. The performance of the model is validated through the simulations of the suction-controlled oedometer tests on the heavily compacted bentonite.

Characterization of water retention, compressibility and swelling properties of a pellet/powder bentonite mixture

The water retention, compressibility and swelling properties of a pellet/powder bentonite mixture were investigated in the laboratory. The water retention capacity was determined under constant-volume condition using a specially designed cell allowing vapor exchange in all directions; suction controlled oedometer tests were carried out for the compressibility investigation; both constant-volume and swell-consolidation methods were applied for the swelling pressure determination. The vapor equilibrium technique was used for suction control. Comparable results were found for the water retention capacity of the pellet/powder mixture determined under constant-volume condition and one single pellet under free swelling condition for suctions higher than 4 MPa, suggesting that the suction was mainly controlled by the micro-pores inside the pellets/powder grains and the swelling of the clay particles was accommodated by the existing macro-pores. On the contrary, at lower suctions, the constant-volume conditions gave rise to a lower water retention capacity for the mixture, indicating the restricted clay particle swelling after the filling-up of the existing macro-pores. Oedometer compression curves revealed a different behavior for low and high suctions: at high suctions, the volume change behavior was governed by the pellets rearrangement combined with possible pellets crushing upon loading. By contrast, at low suctions, the mixture lost its initial granular structure during suction decrease; thus, the volume change behavior became similar to that of compacted bentonite. Compared to compacted bentonite, the pellet/powder mixture showed a lower yield stress. A lower value of swelling pressure was found with the constant-volume method, confirming the limitation of the swell-consolidation method in determining soil swelling pressure.

Shakedown modeling of unsaturated expansive soils subjected to wetting and drying cycles

It is important to model the behavior of unsaturated expansive soils subjected to wetting and drying cycles because they alter significantly their hydro-mechanical behavior and therefore cause a huge differential settlement on shallow foundations of the structure. A simplified model based on the shakedown theory (Zarka method) has been developed in this study for unsaturated expansive soils subjected to wetting and drying cycles. This method determines directly the stabilized limit state and consequently saves the calculation time. The parameters of the proposed shakedown-based model are calibrated by the suction-controlled oedometer tests obtained for an expansive soil compacted at loose and dense initial states, and then validated for the same soil compacted at intermediate initial state by comparing the model predictions with the experimental results. Finally, the finite element equations for the proposed shakedown model are developed and these equations are implemented in the finite element code CAST3M to carry out the full-scale calculations. A 2D geometry made up of the expansive soil compacted at the intermediate state is subjected to successive extremely dry and wet seasons for the different applied vertical loads. The results show the swelling plastic deformations for the lower vertical stresses and the shrinkage deformations for the higher vertical stresses.

Pellet mixtures in isolation barriers

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper describes the experimental program performed to characterize the hydro-mechanical (HM) behaviour of compacted pellet mixtures. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range from 1.3 to 1.5Mg/m3. Pellets had a very high dry density, close to 2Mg/m3. The outstanding characteristic of these mixtures is their discontinuous porosity. Pore sizes of the compacted pellets varied around 10nm. However, the inter-pellet size of the pores was four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture.Performed tests include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction-controlled oedometer tests and swelling pressure tests. The interpretation of some performed tests required back analysis procedures using a hydro-mechanical (HM) computer code.Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al. (1990) (Barcelona basic model, BBM). Parameters for the model were identified and also a set of hydraulic laws are necessary to perform coupled HM analysis.A large scale in-situ test (the “EB” test in Mont Terri, Switzerland) was described and analyzed. Rock barrier parameters were adjusted on the basis of available tests. The test excavation, barrier emplacement and forced hydration were simulated by means of the CODE_BRIGHT program. The comparison between measurements and computed results include data on relative humidity in the rock and the buffer, swelling pressures and displacements.

Explicit formulation of at-rest coefficient and its role in calibrating elasto-plastic models for unsaturated soils

Normally, suction-controlled triaxial tests are used to characterize soil behavior in constitutive modeling of unsaturated soils. However, this type of tests requires sophisticated equipment and is time-consuming. This has been one of the major obstacles to the implementation and dissemination of unsaturated soil mechanics beyond the research context.In contrast to suction-controlled triaxial tests, the suction-controlled oedometer test requires simpler equipment and a shorter testing period. Oedometer tests represent the at-rest earth pressure (K0) condition, which is an important stress state in any simulation. The major disadvantage of the oedometer test is that its lateral stress is controlled by the condition of zero lateral strain and remains unknown during the testing process. At present, no well-established, simple, and objective methods are available that take advantage of oedometer test results for constitutive modeling purposes.This paper derives an explicit formulation of the at-rest coefficient for unsaturated soils and develops an optimization approach for simple and objective identification of material parameters in elasto-plastic models for unsaturated soils using the results from suction-controlled oedometer tests. This is achieved by combining a modified state surface approach (MSSA), recently proposed to model the elasto-plastic behavior of unsaturated soils, with the quasi-Newton method to simultaneously calibrate all parameters governing virgin behavior in elasto-plastic models. The Barcelona Basic Model (BBM) is used to demonstrate the application of the proposed explicit formulation and calibration method. Results predicted using obtained parameters are compared with laboratory test results for the same stress paths in order to evaluate the simplicity and objectivity of the proposed method.

Hydro-mechanical behaviour of bentonite pellet mixtures

Granular mixtures made of high-density pellets of bentonite are being evaluated as an alternative buffer material for waste isolation. Ease of handling is an often-mentioned advantage. The paper described the experimental program performed to characterize the hydro-mechanical behaviour of compacted pellet’s mixtures used in the engineered barrier (EB) experiment. The material tested in the laboratory was based in the pellet’s mixtures actually used for the emplacement of the EB in situ experiment. Grain size distribution was adjusted to a maximum pellet size compatible with the specimen’s dimensions. Dry densities of statically compacted specimens varied in most of the cases in the range: 1.3–1.5 Mg/m 3. Pellets had a very high dry density, close to 2 Mg/m 3. The outstanding characteristic of these mixtures is its discontinuous porosity. Pore sizes of the compacted pellets vary around 10 nm. However the inter-pellet size of the pores is four to five orders of magnitude higher. This double porosity and the highly expansive nature of the pellets controlled all the hydraulic and mechanical properties of the mixture. Tests performed include infiltration tests using different water injection rates and mechanisms of water transfer (in liquid and vapour phases), suction controlled oedometer tests and swelling pressure tests. The interpretation of some of the tests performed required backanalysis procedures using a hydro-mechanical (HM) computer code. Material response was studied within the framework of the elastoplastic constitutive model proposed by Alonso et al. [Alonso, E.E., Gens, A., Josa, A., 1990. A constitutive model for partially saturated soils. Géotechnique 40 (3), 405–430] (Barcelona Basic Model, BBM). Parameters for the model were identified and also a set of hydraulic laws necessary to perform coupled HM analysis.

Testing the Hydromechanical Behavior of a Compacted Swelling Soil

Abstract The paper presents a study of the hydromechanical behavior of a compacted swelling soil in a range of suctions comprised between 0 and around 300 MPa. To perform this study, two kinds of suction controlled oedometers were used, one using the osmotic method and the other using the salt solution method. A detailed review of the conditions for using these two methods is given in the first part of the paper. The second section provides the results of several suction controlled oedometer tests performed in this range of suction. They show that the mechanical behavior of the tested swelling material is strongly affected by the applied suction, even in the high suction range. In addition, it appeared that the samples preparation technique significantly influenced the hydromechanical behavior of the tested soil.

Suction effects on rockfill compressibility

A conceptual mechanism of rockfill deformation was postulated by Oldecop & Alonso (2001) on the basis of the observed rockfill behaviour in suction-controlled oedometer tests. Rockfill mechanical behaviour was linked to water action by means of some crack propagation phenomena, usually known as subcritical crack growth (Atkinson, 1984). For most rocks, the propagation velocity of cracks depends on the applied loads and the chemical action of water contained within the rock particles. Water action is conveniently measured by the relative humidity or by the total suction. It is believed that such phenomena are involved in rockfill particle breakage, an experimental fact that is well recognised as being part of rockfill volumetric deformation under a wide range of stress states (Kjaernsli & Sande, 1963; Sowers et al., 1965; Fumagalli, 1969; Marsal, 1973). A phenomenological constitutive model was proposed (Oldecop & Alonso, 2001) for one-dimensional compression, in which total suction and total stress are the relevant variables. This initial approach was based on a reduced number of laboratory tests, in which the vertical stress was limited to a maximum of 1 MPa. A new experimental programme was performed on the same material, using a newly developed testing device. A large-diameter (300 mm) oedometer, specially designed for the control of relative humidity by means of an air-flow circulation through the specimen, was built. The maximum vertical load was extended to 2·8 MPa in order to investigate the material behaviour in a load range common to rockfill dam structures. New features of rockfill behaviour were observed during this experimental programme. These are described in the paper. Moreover, the previously proposed constitutive model has been reformulated in order to extend its capabilities to cover these new features.

Suction effects on rockfill compressibility

A conceptual mechanism of rockfill deformation was postulated by Oldecop & Alonso (2001) on the basis of the observed rockfill behaviour in suction-controlled oedometer tests. Rockfill mechanical behaviour was linked to water action by means of some crack propagation phenomena, usually known as subcritical crack growth (Atkinson, 1984). For most rocks, the propagation velocity of cracks depends on the applied loads and the chemical action of water contained within the rock particles. Water action is conveniently measured by the relative humidity or by the total suction. It is believed that such phenomena are involved in rockfill particle breakage, an experimental fact that is well recognised as being part of rockfill volumetric deformation under a wide range of stress states (Kjaernsli & Sande, 1963; Sowers et al., 1965; Fumagalli, 1969; Marsal, 1973). A phenomenological constitutive model was proposed (Oldecop & Alonso, 2001) for one-dimensional compression, in which total suction and total stress are the relevant variables. This initial approach was based on a reduced number of laboratory tests, in which the vertical stress was limited to a maximum of 1 MPa. A new experimental programme was performed on the same material, using a newly developed testing device. A large-diameter (300 mm) oedometer, specially designed for the control of relative humidity by means of an air-flow circulation through the specimen, was built. The maximum vertical load was extended to 2·8 MPa in order to investigate the material behaviour in a load range common to rockfill dam structures. New features of rockfill behaviour were observed during this experimental programme. These are described in the paper. Moreover, the previously proposed constitutive model has been reformulated in order to extend its capabilities to cover these new features.

Investigation of the behaviour of bentonite by means of suction-controlled oedometer tests

In the context of an investigation of the near-field as a repository of high-level radioactive waste, the FEBEX Project, a set of laboratory tests has been designed to give a better understanding of the thermo-hydro-mechanical behaviour of unsaturated, expansive clays and to achieve an experimental corroboration of the constitutive models for these clays. Among these tests, several suction-controlled oedometer tests have been performed on a compacted, highly-expansive clay. Seven of them, in which combined paths of loading, wetting and drying have been followed, are presented in this paper. Suctions employed range from 0 to 500 MPa, and vertical loads from 0.1 to 9 MPa. The point of air entry has been identified for suctions around 120 MPa, beyond which the sample becomes very stiff. The influence of the stress path on the subsequent reversibility of the deformations has been highlighted. The swelling properties seem to be kept after intense drying. The behaviour observed, including collapse, agrees with models that relate the effects of the deformations in the microstructural level to those in the macrostructure.

Suction-controlled oedometer tests in montmorillonite clay: preliminary results : M. V. Villar & P. L. Martin, in: Engineering geology of waste disposal, ed S.P. Bentley, (Geological Society of London; Engineering Geology Special Publication, 11), 1996, pp 309–312

Suction-controlled oedometer tests in montmorillonite clay: preliminary results : M. V. Villar & P. L. Martin, in: Engineering geology of waste disposal, ed S.P. Bentley, (Geological Society of London; Engineering Geology Special Publication, 11), 1996, pp 309–312

Suction-controlled oedometer tests in montmorillonite clay: preliminary results

Abstract A Spanish montmorillonite clay is being characterized in its unsaturated state for use as a sealing material in high-level radioactive waste repositories. The ‘water content versus suction’ curve has been determined and oedometer tests are being performed under suction pressures, ranging from 0.1 to 100 MPa, controlled by air pressure or sulphuric acid solutions. The methodology of the tests is described and the first results are presented.