Unsaturated Specimens Research Articles

Effects of Soil Clogging and Water Saturation on Freeze–Thaw Durability of Pervious Concrete

The objective of this research was to evaluate the resistance of pervious concrete to degradation during freeze–thaw cycling under different soil clogging and water saturation conditions. Laboratory testing of freeze–thaw durability involved two levels of soil clogging and two water saturation conditions in a full-factorial experimental design, and field testing involved measurements of stiffness and compressive strength on clogged and unclogged locations of an experimental pervious concrete slab placed in northern Utah. Both experimental factors, together with their interaction, were determined to be statistically significant in this research. Specimens that were clogged with soil or completely submerged in water, or both, were damaged at a significantly faster rate than those specimens that remained unclogged and unsaturated. The average number of freeze–thaw cycles to failure was 93 for clogged specimens compared with 180 for unclogged specimens, and 80 for saturated specimens compared with 193 for unsaturated specimens. A comparison of in situ modulus values, core modulus values, and core compressive strengths associated with clogged locations and unclogged locations in the experimental slab indicated no significant differences in structural properties in the clogged and unclogged locations. That was likely attributable to the fact that only the upper 1 to 2 in. of pervious concrete in clogged locations was filled with debris. The remaining depth of the slab seemed to be free draining. These research findings emphasize the importance of ensuring that pervious concrete remains free of debris and well drained during service in cold regions.

Mechanical Performances of Wood Polypropylene Composite due to Extended Moisture Immersion

Mechanical properties of an extruded wood polypropylene composite that was soaked in both osmotic water and artificial seawater were tested and compared in dry, wet, and redried states. Water did influence the material's flexural, compressive, and tensile strength and chord modulus after 13 months immersion. Flexural and compressive properties of the unsaturated specimens soaked in seawater were degraded slower than those of the specimens soaked in osmotic water. There was a significant effect of water type on tensile modulus of elasticity. Both nonreversible damage and recoverable portion of the material from dry to wet and then to redried state were observed for most of the mechanical properties. Results also indicated that impact strength was not affected by water immersion and redrying.

Loss of controllability in unsaturated soils

ABSTRACT A study on the triggering of instability modes during soaking of unsaturated soil specimens is presented. A constitutive model for unsaturated soils is linked in fact with the loss-of-controllability theory. The model is based on the concept of energy conjugated generalised stress variables. A convenient expression for effective stresses for partially saturated soils is adopted. It is shown that the saturation index acts as an extensive variable, the corresponding intensive variable being the modified suction. The controllability of generalised loading tests is then considered, with attention to tests at varying water content. It is shown that instability phenomena, such as collapse at constant axial loading and suction, can be described.

Liquefaction of Unsaturated Sand Considering the Pore Air Pressure and Volume Compressibility of the Soil Particle Skeleton

A series of cyclic triaxial tests of unsaturated soils was conducted to get a better understanding of the general liquefaction state of unsaturated soils. In the tests, cyclic shear strain was applied to fine clean sand with the same dry density but different initial suction states under the undrained condition. During cyclic shear, the volume change of the soil particle skeleton, the pore air pressure and the pore water pressure were measured continuously. Having used the effective stress defined by Bishop (Bishop et al., 1963), where the net stress and suction contribute to the effective stress, our test results showed that unsaturated sand specimens with quite a low degree of saturation lose their effective stress due to cyclic shear. At a zero effective stress state, unsaturated specimens behaved similarly to liquids in much the same way as saturated specimens. From experimental and theoretical considerations, the zero effective stress state (i.e., liquefaction) for unsaturated sand was found to have been established when both the pore air and water pressures build up to the point where it is equal to the initial total pressure. A volume change of pore air under the undrained condition, if a volume change of pore water is negligible, is equal to that of the soil particle skeleton. Therefore, it can be concluded that the liquefaction of unsaturated soil generally depends on the volume compressibility of the soil particle skeleton and the degree of saturation. On the other hand, according to the ideal gas equation of Boyle-Charles law, the volume change required to bring about a zero effective stress state can be calculated from the initial pore air pressure (usually the atmospheric pressure) and the final pore air pressure (the initial confining pressure). Therefore, the liquefaction of unsaturated soils also depends on the initial confining pressure. Based on this concept, the liquefaction potential of unsaturated soil can be evaluated by comparing the volume compressibility of the soil particle skeleton and the volume change of the pore air required to bring about a zero effective stress state.

A Suction-Control Apparatus for the Measurement of P and S-wave Velocity in Soils

Abstract This paper presents a new apparatus for the measurement of P and S-wave velocities in unsaturated soil specimens under controlled net stress and matric suction conditions. The system consists of a triaxial cell modified to enable an independent control of the pore air and water pressures as well as the confining pressure, and to accommodate P and S-wave sources and receivers. The system also includes three servo-controlled pressure pumps, and a computer control system that drives the pressures and acquires volume changes. Three sets of experiments were conducted to verify the system performance. Specific matric suction values were applied while monitoring the stabilization process of capillary pressures using the P and S-wave velocity measurements. Both P and S-wave velocities increase due to the rise in interparticle forces resulting from the increase in matric suction.

The Axis-Translation and Osmotic Techniques in Shear Testing of Unsaturated Soils: A Comparison

Matric suction in the laboratory testing of an unsaturated specimen is usually controlled by the axis-translation technique or the osmotic technique. To investigate possible differences in the axis-translation and osmotic techniques in shear testing of unsaturated soils, three series of unsaturated triaxial shear tests were conducted on a compacted expansive clay with an initial degree of saturation of 82%. Two of these series were performed using the axis-translation and osmotic techniques at HKUST. Another series was performed using the osmotic technique at ENPC. In the series using the axis-translation technique, the single-stage testing procedure was compared with the multi-stage testing procedure and these two testing methods were found to provide consistent shear strength information. The consistency of the results from the osmotic technique at HKUST and ENPC demonstrated the reliability of the osmotic technique. Based on the extended Mohr-Coulomb shear strength formulation, comparison of the results using the axis-translation technique and the osmotic technique showed that there was no difference in determining the internal friction angle, φ′. However, the values of φ b in the test series using the axis-translation technique were slightly larger than those in the test series using the osmotic technique by 3°-4°.

Stress–strain behaviour of weathered weak rock in middle-sized triaxial tests

A middle-sized triaxial test apparatus for a specimen 20 cm in height and 10 cm in diameter was developed to measure the deformation and strength of weak rock or gravel. High-quality undisturbed samples of a weathered weak rock were taken from a dam site by a core drilling method. To avoid damage to the structure of the weak rock due to saturation of specimens as a result of measuring volume change through the water change in a burette, the lateral deformation of specimens was directly measured in the unsaturated condition using three rings mounted on the specimen. Using the developed triaxial test apparatus, isotropic compression tests and consolidated–drained triaxial compression tests were performed on unsaturated or saturated undisturbed samples under confining pressures of 49, 98, 196, 392, 539, and 683 kPa. The test results show that the stress–strain relationship of the weathered weak rock under both unsaturated and saturated conditions is strongly influenced by the confining pressure when the confining pressure is less than 392 kPa, and the stress–strain behaviour becomes similar to that of normally consolidated clay when the confining pressure is greater than 392 kPa. Comparison of results of triaxial tests on unsaturated and saturated specimens shows that the saturated samples become somewhat weak. The test results also show that the bonding and stress history largely influence the stress–strain relationship at small strain levels.Key words: weathered weak rock, microstructure, undisturbed sample, deformation, strength, triaxial test, unsaturated sample.

Predicting Residual Strength in Unsaturated Concrete Exposed to Sulfate Attack

Deleterious damage that manifests in cracking and strength loss due to expansion and exfoliation often occurs when hardened concrete is exposed to sulfates. For a degradation-free service life, modeling sulfate attack is as important as considerations for strength and stability in concrete. This paper focuses on modeling the impact of gypsum as a constitutive durability product responsible for strength loss in concrete due to sulfate attack. The model blends the calculation of coupled moisture-sulfate transport processes with quantitative simulation of chemical reactions involving sulfates, moisture, and an incipient portlandite phase in concrete. Account is taken of variations in sulfate and moisture diffusivities to reflect microstructural changes when a portlandite matrix is stoichiometrically transformed to gypsum. Implementation of the model to both water-saturated and unsaturated concretes is numerically achieved according to standard Galerkin procedure in the finite-element sense. Contrary to expectations, simulation results for the case of initially unsaturated specimens before immersion in a sulfate solution are not significantly different from the initially saturated condition. Calculations of global relative residual strength predicated on degradation profiles generated by the model showed good agreement with postsulfate-immersion-test residual strength data.

X-ray Fluorescence Determination of Heavy Metals in Humic Acids

Sources of errors due to the chemical composition and nonuniform surface density (Ps) of unsaturated specimens in the x-ray fluorescence analysis of heavy metals in humic acids have been assessed quantitatively. Two methods are proposed to take into account these errors: a constraint equation with respect to 1/Ps treated as a variable and the use of a combined internal standard for which a variable in the calibration function is line intensity of the reference element. The dependence of correction coefficients in the constraint equation on Ps was studied. Performance characteristics are reported a procedure developed for the x-ray determination of heavy metals in humic acids.

Deformation and strength characteristics of weathered soft rock using triaxial tests

In order to study the evolution of a lime-treated silty soil under in-situ conditions of mixing, laying out, compaction and environment, an experimental dike was built near Rouen, in northwest France; this dike is 25 m long, 4 m wide at the crest and 1.8 m high. While special care was taken to mix the soil and lime very thoroughly, to precisely measure the quantity of water, to compact the soil using a vibrating sheepsfoot roller, the building conditions were those usually used to construct embankments in all other respects. Samples were taken from the dike approximately 1 month, 6 months and 12 months after the completion of the work and underwent triaxial and oedometric testing. To compare the properties of treated soil with those of untreated soil, another smaller “reference” embankment was built nearby under similar conditions with silty soil.Small-strain triaxial tests were performed on unsaturated specimens of treated and untreated soil using an original strain measurement device. The change in axial secant moduli can be described by a power law E = βσ1m, where β is a parameter that depends on curing time and m is nearly constant (m ≈ 0.45).CU and CD large-strain triaxial tests carried out on saturated treated and untreated specimens, consolidated under confining stresses ranging from 10 to 300 kPa, showed a definite increase in cohesion as a function of curing time, with nearly unchanged friction angles. The possibility to use a non-linear failure criterion was examined. The compressibility coefficient Ciso, slope of the critical state line (CSL) in the [log(p′), e] coordinate system, is significantly lower in treated than untreated specimens, but its value remains constant whatever the curing time. At the same time, the CSL of the treated specimens is above that of the untreated silt. The brittleness of the material, derived from the peak to residual deviator stress ratio qpeak/qres is the same for treated and untreated specimens and is independent of curing time and confining stress. However, the axial strain corresponding to the peak of CD curves is much higher in untreated than in treated specimens. The permeability of the saturated lime-treated soil (10− 9 m/s) is the same as that of the untreated soil. The value measured one month after the embankment was constructed, does not change with time or with the confining stress. The results of the present study confirm those of previous studies on laboratory-prepared unsaturated specimens: they have demonstrated the effectiveness of lime-treatment in the much more difficult environment of a real working site and shown that lime-treated samples present a significant gain in their engineering properties, even when saturated, provided that the dosage of lime and water, mixing and compaction conditions are carefully controlled. The new material thus obtained presents lower plasticity, improved workability, increased stiffness and cohesion without any loss of performance in terms of brittleness or permeability.

A state-dependent elasto-plastic model for saturated and unsaturated soils

A state-dependent elasto-plastic constitutive model for both saturated and unsaturated soils is presented in this paper. The model, which is developed under an extended critical-state framework, uses two independent stress state variables: net normal stress and matric suction. In addition, the influence of a simplified non-linear soil-water characteristic curve on mechanical behaviour is incorporated in the model by using a bi-linear function. Most importantly, a state-dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio, internal state (density) and soil suction. In this paper, details of the model formulation and the determination of model parameters are described and reported. By using the new model, numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine-grained (sandy silt) and coarse-grained (gravelly sand) weathered soils. The simulated triaxial tests include undrained shear tests on saturated specimens, constant water content tests and wetting tests under constant deviator stress on unsaturated specimens. The computed results show that the stress–strain and volumetric-shear–strain relationships at both loose and dense and saturated and unsaturated states can be modelled properly with a single set of parameters. Moreover, the model is capable of capturing some key observed features, such as a sudden increase in shear strain and volumetric strain as suction is reduced under a constant deviator stress.

A state-dependent elasto-plastic model for saturated and unsaturated soils

A state-dependent elasto-plastic constitutive model for both saturated and unsaturated soils is presented in this paper. The model, which is developed under an extended critical-state framework, uses two independent stress state variables: net normal stress and matric suction. In addition, the influence of a simplified non-linear soil-water characteristic curve on mechanical behaviour is incorporated in the model by using a bi-linear function. Most importantly, a state-dependent dilatancy formulation is introduced to account for the effects of stress level or stress ratio, internal state (density) and soil suction. In this paper, details of the model formulation and the determination of model parameters are described and reported. By using the new model, numerical simulations of triaxial tests have been carried out on both saturated and unsaturated fine-grained (sandy silt) and coarse-grained (gravelly sand) weathered soils. The simulated triaxial tests include undrained shear tests on saturated specimens, constant water content tests and wetting tests under constant deviator stress on unsaturated specimens. The computed results show that the stress–strain and volumetric-shear–strain relationships at both loose and dense and saturated and unsaturated states can be modelled properly with a single set of parameters. Moreover, the model is capable of capturing some key observed features, such as a sudden increase in shear strain and volumetric strain as suction is reduced under a constant deviator stress.

A Double Cell Triaxial System for Continuous Measurement of Volume Changes of an Unsaturated or Saturated Soil Specimen in Triaxial Testing

Abstract This paper presents an improved Double Cell Triaxial System (DCTS) for continuous measurement of the volume change of an unsaturated or saturated soil specimen in triaxial testing. The DCTS developed by the author overcomes the two critical shortcomings of the conventional triaxial system and has advantages over the existing modified triaxial cells for measuring the volume changes of unsaturated/saturated soils during water flushing, consolidation, and compression. The calibration of the DCTS has been done using both a dummy solid copper specimen and a fully saturated marine clay specimen. The water volume changes of the inner cell and the outer cell are measured and compared on the copper specimen under compression of the inner/outer cell pressure up to 400 kPa. It is found that the volume changes of the outer cell are significant, while the volume changes of the inner cell are negligible. On the saturated marine clay, volume changes of the clay specimen under isotropic consolidation and drained axial compression are measured using two methods: the conventional method (Method A) by measuring the volume of water coming out from inside the specimen and the method (Method B) by using the DCTS to measure the internal cell water volume changes. The volume changes using the two methods are compared. It is found that the differences of the volume changes using the two methods are within 5.6%. The calibration results using both the solid copper specimen and the saturated marine clay specimen show that the DCTS is accurate and reliable for continuously measuring the volume changes of unsaturated or saturated soil specimens during consolidation and compression.

On the use of the Kozeny–Carman equation to predict the hydraulic conductivity of soils

The saturated hydraulic conductivity of a soil can be predicted using empirical relationships, capillary models, statistical models, and hydraulic radius theories. A well-known relationship between permeability and the properties of pores was proposed by Kozeny and later modified by Carman. The resulting equation is largely known as the Kozeny–Carman (KC) equation, although the two authors never published together. In the geotechnical literature, there is a large consensus that the KC equation applies to sands but not to clays. This view, however, is supported only by partial demonstration. This paper evaluates the background and the validity of the KC equation using laboratory permeability tests. Test results were taken from publications that provided all of the information needed to make a prediction: void ratio, and, either the measured specific surface for cohesive soils, or the gradation curve for noncohesive soils. The paper shows how to estimate the specific surface of a noncohesive soil from its gradation curve. The results presented here show that, as a general rule, the KC equation predicts fairly well the saturated hydraulic conductivity of most soils. Many of the observed discrepancies can be related to either practical reasons (e.g., inaccurate specific surface value; steady flow not reached; unsaturated specimens, etc.) or theoretical reasons (some water is motionless; hydraulic conductivity of soils is anisotropic). These issues are discussed in relation to the predictive capabilities of the KC equation.Key words: permeability, prediction, gradation curve, specific surface.

Laboratory Study of Loose Saturated and Unsaturated Decomposed Granitic Soil

Weathered soils are used extensively as fill materials in slope construction in tropical and subtropical cities such as Hong Kong. The mechanical behavior of loose decomposed fill materials, particularly in the unsaturated state, has not often been investigated and is not yet fully understood. The objective of this study was to understand the mechanical behavior of loose unsaturated decomposed granitic soil and to study the effects of the stress state, the stress path and the soil suction on the stress-strain relationship, shear strength, volume change, and dilatancy via three series of stress path triaxial tests on both saturated and unsaturated specimens. It was found that loose and saturated decomposed granitic soil behaves like clean sands during undrained shearing. Strain-softening behavior is observed in loose saturated specimens. In unsaturated specimens sheared at a constant water content, a hardening stress-strain relationship and volumetric contractions are observed in the considered range of net mean stresses. The suction of the soil contributed little to the apparent cohesion. The angle of friction appeared to be independent of the suction. In unsaturated specimens subjected to continuous wetting (suction reduction) at a constant deviator stress, the volumetric behavior changed from dilative to contractive with increasing net mean stress and the specimen failed at a degree of saturation far below full saturation. It was revealed that the dilatancy of the unsaturated soil depends on the suction, the state, and the stress path.

Performance of pfa concrete in a marine environment––10-year results

Abstract Steel reinforced 100×100×300 mm concrete prisms, with nominal strength grades C25, C35 and C45 and different pfa levels (0–50%), were exposed to various curing treatments during the first 28 days prior to exposure in the tidal zone of the BRE marine exposure site. Chloride concentration profiles and rebar weight losses had previously been measured on specimens exposed for 1, 2 and 4 years and this paper reports the results of similar measurements after 10 years exposure. Chloride profiles were also measured for specimens after 1 and 28 days immersion in seawater under laboratory conditions. Pfa concretes showed substantially increased resistance to the penetration of chlorides compared with control Portland cement (PC) concrete specimens. The improved resistance of the pfa concrete to the penetration of chlorides resulted in reduced corrosion of steel bars imbedded in the concrete. Threshold chloride levels for corrosion, estimated from relationships between steel weight loss and chloride content at the location of the steel, were found to decrease with increasing pfa content. Chloride concentration profiles after 28 days of immersion in seawater showed that considerable chloride penetration occurred during this period due to sorption (capillary suction) of the seawater into the unsaturated specimens. This results in a significant error in diffusion coefficients calculated from the concentration profile using the standard solution to Fick’s second law. The error may be substantial for pfa concretes where chlorides penetrating due to sorption immediately after exposure may outweigh subsequent diffusion during continued seawater exposure. The performance of the concretes, particularly the PC concretes, in this programme is considered in the light of current and new British Standard recommendations for concrete exposed to marine tidal conditions. The adequacy of these recommendations is, however, difficult to assess because the highest concrete quality tested fell a little short of the minimum quality required in the recommendations and cover to reinforcement was also less than required. Nevertheless, the superior performance of concretes containing 30% or more pfa was clearly demonstrated.

Behavior of a Loosely Compacted Unsaturated Volcanic Soil

In this paper, the stress-strain relationsihp and volumetric behavior of a loosely compacted unsaturated decomposed volcanic soil (fill) were studied by conducting three series of triaxial stress path tests: (1) consolidated undrained on the saturated fill; (2) constant water content; and (3) a reducing suction under constant deviator stress on the unsaturated fill. The last two series of tests were designed to simulate the effects of undrained response and rainfall infiltration in initially unsaturated slopes, respectively. It was found that the saturated loose volcanic soil behaves like clay under isotropic compression but it resembles sand behavior when it was subjected to undrained shear. For isotropically consolidated unsaturated specimens sheared under a constant water content, a hardening stress-strain and a nonlinear shear strength–suction relationship are observed. At relatively high suctions, both angle of friction and apparent cohesion appear to be independent of suction. Volumetric contraction during shear is observed in this series of tests. On the other hand, anisotropically consolidated loose unsaturated specimens subjected to a reducing suction change from contractive to dilative behavior as the net mean stress increases. This observed volumetric behavior, unlike the shear strength, is stress path–dependent and cannot be explained by using the existing elastoplastic critical state theoretical framework extended for unsaturated soils.

Effects of Stress History Due to Unsaturation and Drainage Condition on Shear Properties of Unsaturated Cohesive Soil

Several studies have concluded that the effect of matric suction on the shear strength of unsaturated soil is very important. However, little attention is given to the stress history due to unsaturation and drainage condition in the arrangement of these experimental results. This paper attempts to show the relationship between shear properties and matric suction under a specified stress history and drainage condition. In this study, an unsaturated specimen is set up by dehydrating the normally consolidated soil with controlled matric suction to a specified stress history. Triaxial apparatus and hollow cylindrical torsional shear apparatus are employed to examine the effects of stress history and drainage condition, respectively. Through systematic experiments, the effects of stress history and drainage condition on the stress-strain relationship are clarified. The increase in shear strength by matric suction which is measured at the point of maximum shear stress is observed to be independent of stress history and drainage condition. However, in strain softening behavior it was found that the ultimate strength is independent of matric suction and traces a unique line which is parallel to the critical state line on the pnet-q plane.

Laboratory Studies on the Volume Change Characteristics of Kaolinite Contaminated with Sodium Phosphate/Sulfate

Abstract Reported distress to an industrial structure from phosphate/sulfate contamination of kaolinitic foundation soil at an industrial location in Southern India prompted this laboratory study. The study examines the short-term effect of sodium sulfate/phosphate contamination on the swell/compression characteristics of a commercial kaolinite. Experimental results showed that the unsaturated contaminated kaolinite specimens exhibited slightly higher swell potentials and lower compressions than the unsaturated uncontaminated kaolinite specimens. It is suggested that the larger double layer promoted by the increased exchangeable sodium ion concentration is responsible for the slightly higher swell potentials and lower compressions of the unsaturated contaminated kaolinite specimens.

Equipment for One-Dimensional Compression and Triaxial Testing of Unsaturated Granular Soils at Low Stress Levels

Abstract This paper describes the design of equipment for examining the behavior of unsaturated granular soils at low stress levels. The one-dimensional compressibility is examined in a 254-mm-diameter compression cell whose unique features include the measurement and control of low-level stresses using load cells incorporated in the loading platen and the ability to determine suctions at the base and top of the specimen. Stiffness and strength behavior is examined in a new double wall triaxial cell that allows accurate determination of the volume change of the 150-mm-diameter unsaturated specimen. Facilities are incorporated in the cell for monitoring axial and radial strains on the central portion of the specimen. A technique for preparing compacted specimens without damaging the brittle high-entry ceramics in the base plates of the cells is described. The performance of the new equipment is discussed, and results of repeatability tests are reported.