Combined effects of strain rate and temperature on consolidation behavior of clayey soils

It is considered that the long term settlement of clay deposits, known as secondary consolidation, is caused by clay viscosity. In this paper, the viscous property of clayey soils is examined from two viewpoints: one is temperature and the other is the effect of the strain rate. To investigate these effects, a special constant rate of strain (CRS) loading test, in which the strain rate is changed during the test, was carried out at temperatures of 10 and 50 °C on reconstituted clay samples. Under the normal strain rate, such as the order of 10⁻⁶ s⁻¹, well-known temperature effects on the consolidation behavior were confirmed. That is, the high temperature condition leads to increased hydraulic conductivity due to the reduction in the viscosity of pore water at higher temperatures. It is also observed that the yield consolidation stress decreases with increasing temperature due to the viscous properties of soil skeletons. However, it is found that with higher temperature and smaller strain rates, the clay specimen does not follow conventional viscous behavior, like the Isotache model, but the gradient of stress–strain curve considerably decreases. The reason for different behavior from the Isotache model may be attributed to the creation of a new structure to resist the external deformation, under high temperature and a slow strain rate.     

Unloading behavior of clays measured by CRS test

The unloading behavior of clays was studied by the Constant Rate of Strain (CRS) test, for three clays: two of them are reconstituted and the other was intact. In the conventional CRS test where the stress monotonically increases, the distribution of the pore water pressure in a specimen is assumed to be parabolic, the effective stress is calculated and then the compression behavior is evaluated. However, this assumption cannot be directly applied the unloading condition. In this study, the pore pressure distribution under unloading was simulated by a cubic polynomial under the assumption that hydraulic conductivity does not change in the unloading process. A unique relation in the e–log σ′_v relation was found, irrespective of both the magnitude of stress or strain and the compression index, C_c, at the unloading test, when the consolidation pressure is normalized by σ′_vmax, which is the consolidation pressure before the unloading test. In addition, the creep strain, which is gained by constant loading before the unloading test, was shown to have a great effect on the unloading behavior: that is, the soil behaves stiffly when subjected to a constant load for a prolonged period of time. A strain rate dependency in the unloading process was also observed particularly for heavily unloaded specimens. The unloading behavior was also investigated by the conventional constant load test. The test results show reasonable agreement with those obtained from the CRS test.     

Modified method for predicting lateral displacement of PVD-improved ground under combined vacuum and surcharge loading

A modified method is proposed to predict the lateral displacement (δ) of prefabricated vertical drains (PVDs) improved ground under combined vacuum and surcharge loads, which is derived based on a few modified triaxial tests and a series of finite element analyses of PVD unit cells. It is observed that reducing the surcharge load (p_s) and loading rate (LR) and increasing the vacuum pressure (p_v), pre-vacuum consolidation period (t_v), and initial undrained shear strength (s_u0) could be effective in controlling the outward δ. Variations of the effective stress ratio (K_e) that controlling the δ with p_s, p_v, LR, t_v, and s_u0 are then presented. A synthetic relationship between the normalized horizontal strain (ε_h) by a reference one-dimensional vertical strain (ε_v1) and the normalized K_e by the at-rest earth pressure coefficient (K₀) is proposed for cases with and without t_v. Further, a modified index parameter (β₁) is introduced for quantitatively considering the effect of p_s, p_v, LR, t_v, s_u0, and consolidation properties of the soil, a relationship between K_e and β₁ is then established for evaluating the value of K_e. Combinations of the ε_h/ε_v1–K_e/K₀ and K_e–β₁ relationships enable modified predictions of the δ from basic preloading conditions and soil parameters.     

Increase in Undrained Shear Strength of Clay with Respect to Rate of Consolidation

Estimating increase in undrained shear strength s_u of clay is an important purpose of consolidation analysis as well as settlement prediction when multi stage loading for construction of earth structures on soft clay is concerned. The present paper investigates the increase in s_u with regard to rate of consolidation. Since values of s_u are usually evaluated as a function of effective vertical stress σˊ_v, rate effect on shear strength increment ratio in normally consolidated state s_un/σˊ_v is first discussed considering the fundamental concept expressed by the equation of s_un/σˊ_v=s_uf/σˊ_y, in which s_uf is in-situ undrained shear strength and σˊ_y is consolidation yielding stress. The paper also describes two case histories where actual increases in s_u were observed in soft clay deposits. The first site is located offshore Osaka-bay where a large-scale seawall was constructed, and soft clay in the site was improved by sand drains. The second is a reclaimed land in Yanai City where a sand fill and a subsequent test embankment were conducted on a soft clay deposit without improvement by vertical drains. It is found from the study that: (1) rate effect on s_un/σ′_v cancels rate effect on σˊ_y, and the equation mentioned above is valid regardless of the rate of consolidation, and (2) the in-situ values of s_un/σˊ_v observed in the two sites vary from 0.27 to 0.37 as consolidation progresses, and they are well related to rate of consolidation. According to the experience at the two construction sites, values of s_un/σˊ_v with regard to rate of consolidation are proposed for design use at field/construction sites.     

Mechanical Behavior of Bentonite-Sand Mixtures as Buffer Materials

For the purpose of establishing the method for estimating in-situ mechanical behavior of artificial buffer materials, stress-deformation behavior of bentonite-sand mixtures were investigated through oedometer test, consolidated undrained triaxial compression test and expansive stress-strain measuring test by changing the clay content as 30, 50, 70 and 100%, and by changing the range of initial dry density of mixture from 1.4 to 1.8 g/cm³. Oedometer test results suggest that the magnitude of consolidation yield stress almost coincides with the maximum expansive stress (p′_s)_max irrespective of bentonite-sand mix proportion, initial density of mixture and the magnitude of molding stress at the specimen making. Strong correlation between consolidation stress and initial tangent modulus during undrained triaxial compression test is observed, and it is found that the reduction rate of rigidity is hardly dependent on the specimen making method, molding stress and the consolidation stress. From the two series of expansive stress-strain measuring tests, it is recommended to perform the measurement of expansive stress by feed back system with the load cell installed at the base of the specimen. A unique relationship is found between the maximum expansive stress (p′_s)_max versus bentonite specific volume v_b, which is defined as the specific volume calculated by excluding the volume of sand particles. The line showing the unique log v_b versus log (p′_s)_max relationship can be recognized as the state boundary line prescribing one-dimensional expansive stress-strain behavior of the bentonite-sand mixtures.     

Laboratory Investigation on Electro-Osmosis Properties of Singapore Marine Clay

In order to be able to design the electro-osmosis scheme, two major properties of clay, electro-osmosis permeability and coefficient of consolidation, need to be known. These properties can be measured in the laboratory by running the electro-osmosis consolidation test. This type of test is carried out with the help of modified triaxial apparatus in which the soil sample has an electric current applied, and its volumetric change accurately measured. It was found that the electro-osmosis permeability (k_e) of Singapore marine clay ranges between 10^–8 and 10^–9 m/v-s, whereas the electro-osmosis coefficient of consolidation (c_ve) varied between 0.17 and 2.81 m²/yr for tests under an electric potential gradient between 2 to 12 volts; c_ve increased with voltage applied. Physical and compressibility properties changes were noted after the electro-osmosis process. Gain in effective stress due to electro-osmosis was significant and magnitude of effective stress gain was directly proportional to the voltage applied. However vertical displacement was found to be smaller and significant magnitude of volume change was contributed from lateral deformation. This paper describes the test apparatus, and procedures and discusses the results from electro-osmosis tests, carried out on Singapore Marine Clay.     

Full scale consolidation test on ultra-soft soil improved by prefabricated vertical drains in MAE MOH mine,Thailand

The reclamation of slurry pond with ultra-soft soil deposit using the prefabricated vertical drains (PVDs) with preloading technique in the Mae Moh mine, Lampang, Thailand is a challenging work and is illustrated in this paper. Geotextile reinforcement was used to strengthen bearing capacity of the soil foundation prior to the installation of sand platform. The delay of excess pore water pressure dissipation at the early loading stage occurred despite the occurrence of large settlements was a distinct behavior of the ultra-soft soil. Within the delayed time, the calculated average degree of consolidation based on measured settlements, U_s increased while the calculated average degree of consolidation based on measured excess pore water pressures, U_e and the undrained shear strength, S_u remained unchanged. Beyond the delayed time, both U_e and S_u increased significantly with time and when U_s > 90%, the difference between U_e and U_s was observed to be small. It was suggested to use U_s for approximation of S_u when U_s > 90% based on the SHANSEP's method. The successful installation and application of PVD to improve ultra-soft soil in this research is applicable for the ground improvement of similar problematic ultra-soft soils in international land reclamation projects.     

Stress-Deformation Behavior Under Anisotropic Drained Triaxial Consolidation of Cement-Treated Soft Bangkok Clay

In this study the compression behavior of high water content cement-treated soft Bangkok clay is further investigated by conducting a constant stress ratio (CSR) test at various stress ratios (η). The test utilized cement-treated clay specimens with cement contents (A_w) of 10% and 15%, each of which was in combination with 100% and 130% total clay water contents. The test results confirmed that the ratio of after-curing void ratio to cement content (e_ot/A_w) can effectively characterize the compression behavior of cement-treated clay. The specimens with higher values of e_ot/A_w yielded higher volumetric and shear strains at the same stress ratio. While those with lower values of e_ot/A_w resulted in lower shear strains, with consequent higher values of strain increment ratios (dɛ_v/dɛ_s) both before and after transitional yield points. Significantly, the e_ot/A_w ratio has described the relationship of the compression yield loci of cement-treated clay at various stress ratios and mixing conditions.     

Shrinkage and desaturation properties during desiccation of reconstituted cohesive soil

A vacuum evaporation method, proposed by the authors to reduce the water content more quickly than by air drying, was applied to six saturated reconstituted cohesive soil samples to investigate shrinkage and desaturation properties during desiccation. The test conditions were a vacuum pressure of p_v=?93.9 to ?97.5 kPa, a consolidation pressure of σ_v=68.6–392 kPa, an initial water content of w₀=0.59–0.92 w_L, and an initial surface area of the specimen of A_s0=20–205 cm², where w_L is the liquid limit. The results obtained for these restricted conditions are as follows. The vacuum evaporation of pore water from the soil occurs at a vacuum pressure higher than about ?93 kPa (|p_v|>93 kPa), but the evaporation process is very slow. The minimum void ratio, e_min, at the no-shrinkage phase of the soil subjected to the vacuum pressure, becomes a constant value. The relations e_min≈1.15 e_s and w_s≈87(e_min/G_s) are obtained, where e_s is the void ratio corresponding to the shrinkage limit, w_s, and G_s is the specific gravity of the soil particles. Using the vacuum evaporation method, the continuous relations for w?e, w?V/V₀, and w?S_r are more easily and more rapidly obtainable than with the conventional method by air drying. These three relations were formulated using two parameters, namely, an experimental parameter that is simply obtained using vacuum evaporation tests and a parameter that can be assumed and determined easily. The three formulated relations show a good agreement with the experimentally obtained results. Furthermore, if the basic physical parameter, w_s, has already been obtained, then the three relations can be estimated roughly without the performance of any tests.     

Undrained Shear Characteristics of Saturated Sand Under Anisotropic Consolidation

A series of undrained triaxial compression tests was performed on Toyoura sand in order to investigate the behavior of sand under large deformation. The present study focuses on the effects of anisotropic consolidation on the undrained behavior of sand. A wide range of initial states of sand is covered and taken into account with the behavior of sand varying from contractive to dilative. Different states of consolidation stress were shown to affect the stress-strain behavior of sand and the development of excess pore water pressure up to an axial strain of 5%. Beyond a strain in excess of 10%, the behavior of sand was shown to become independent of the stress state at consolidation. Consequently, the relation between void ratio and confining stress at steady state and quasi-steady state are independent of the extent of anisotropic consolidation. Moreover, the initial dividing curve between dilative and contractive behaviors in an e ~ p' diagram was shown to move down as the sand is more anisotropically consolidated.     

Evaluation of hydro-mechano-chemical behaviour of bentonite-sand mixtures

Bentonite-sand mixtures are widely used in engineering barrier of deep geological disposal of high-level radioactive nuclear waste and anti-seepage barrier of civil geotechnical engineering. Under the action of groundwater solution infiltration and external stress, the hydro-mechanical (HM) behaviour of bentonite-sand mixtures, i.e. the swelling characteristics and permeability, will change. Once the anti-seepage and filtration effect is weakened or lost, the pollutants will spread to the biosphere. Therefore, it is necessary to study the swelling characteristics and permeability of bentonite-sand mixtures under coupled mechano-chemical (MC) effect and to establish corresponding prediction model. For this reason, swelling tests under salt solution with different concentrations are conducted on pure bentonite and its mixtures with 30%, 70% and 90% sand contents, the compression tests are carried out on saturated samples, and the saturated permeability coefficient k of the sample under each load is calculated by Terzaghi's one-dimensional consolidation theory. The concepts of true effective stress p_e, montmorillonite void ratio e_m and critical sand content α_s are introduced to determine the e_m-p_e relationship and finally the k-e_m relationship of bentonite-sand mixtures. It is found that when the sand content α ≤ α_s, the e_m-p_e relationship of the mixture is linear and independent of the salt solution concentration, and when α > α_s, the e_m-p_e relationship of bentonite-sand mixture is bi-linear with the true effective deviatoric stress p_esα as the intersection. In addition, the e_m-k relationship also shows the linear trend when α ≤ α_s, and the slope of the line increases with the increase of the salt solution concentration. When α > α_s, the k-e_m relationship will deviate from the linear relationship. Moreover, the larger the sand content is, the farther the deviation is. On the basis of summing the regularity, a model for predicting the HM behaviour of bentonite-sand mixture under the coupled MC effect is proposed. By comparing the swelling and permeability test results with model prediction results of different types of bentonite and its sand mixtures, the predictive model is verified. The study on the HM behaviour of bentonite-sand mixtures under salt solution infiltration and the model establishment can provide experimental and theoretical basis for the design and construction of anti-seepage engineering by bentonite-sand mixtures.     

Gmax of Fine-Grained Soils at Wide Void Ratio Range,Focusing on Time-Dependent Behavior

Effects of creep as well as initial water content at deposition on the pseudo-elastic shear modulus, G_max, of fine-grained soil are studied in a consolidometer equipped with bender elements. The state boundary (SB) concept is discussed in void ratio (e)-G_max-Effective Vertical Stress (σ′_v) space and the condition of metastability associated with short-term creep and long-term natural creep are exemplified from laboratory creep tests and in-situ soil test results, respectively. In addition to the conventional laboratory reconstitution method adopted for almost all clay samples that use an initial water content of twice the liquid limit, the soil samples from Minato-Mirai (MM) site were also prepared by directly consolidating the slurry with a water content ranging from 1.5 to 5.0 times the liquid limit. The larger primary metastable region or higher degree of on-depositional structuration associated with the samples at higher initial void ratio is confirmed. However, no appreciable difference was found in the increase in G_max with time, represented by N_g, between the samples prepared by the two different methods. In addition to the highly plastic MM clay, the study of N_g also covered other clayey soils of different origin and plasticity index range from 29 to 78. Metastability index, MI(G_max)^e that manifested the aspect of structuration and déstructuration reaches a maximum value at the end of sustained loading and vanishes slowly with any increase in stress level. By using liquidity index instead of void ratio in the e-log G_max plot, the metastability index, MI(G_max)^LI, is found to represent a wider variety of soils with minimum scatter. A slight stress level dependency of metastability increase was observed yielding smaller values at higher stresses. For the present test conditions and duration, subsequent stressing of 1.5-2.0 times the creep stress brought about the complete déstructuration of the creep-added soil-structure formed in the previous creep step.     

The removal of colour from effluent using various adsorbents—III. Silica: Rate processes

The rate of adsorption of Astrazone Blue, a basic dye, on Sorbsil Silica has been studied. The parameters studied include particle size, initial dye concentration, agitation and dye solution temperature. The rate controlling step is mainly intraparticle diffusion, although a small boundary layer resistance is experienced. The activation energy for the adsorption of Astrazone Blue on silica is 13.2 ± 0.6 kJ kg^?1. The diffusion coefficients vary from 9 × 10^?9 cm² s^?1 at 20°C to 10 × 10^?8 cm² s^?1 at 82°C.     

A new fluorescent oligonucleotide probe for in situ detection of s-triazine-degrading Rhodococcus wratislaviensis in contaminated groundwater and soil samples

A bacterial strain (FPA1) capable of using terbuthylazine, simazine, atrazine, 2-hydroxysimazine, deethylatrazine, isopropylamine or ethylamine as its sole carbon source was isolated from a shallow aquifer chronically contaminated with s-triazine herbicides. Based on its 16S rDNA sequence analysis, the strain FPA1 was identified as Rhodococcus wratislaviensis. The disappearance time of 50% of the initial terbuthylazine concentration in the presence of this strain (DT₅₀) was 62 days. This strain was also able to mineralise the [U-ring ¹⁴C] triazine-ring, albeit at a slow rate. A 16S rRNA target oligonucleotide probe (RhLu) was designed, and the FISH protocol was optimised, in order to detect R. wratislaviensis in s-triazine-contaminated sites. The RhLu probe gave a positive signal (expressed as % of total DAPI-positive cells) in both the groundwater (2.19 ± 0.41%) and soil (2.10 ± 0.96%) samples analysed. Using the RhLu probe, R. wratislaviensis can be readily detected, and its population dynamics can be easily monitored, in soil and in water ecosystems contaminated with s-triazine. To the best of our knowledge, this is the first report showing the isolation, from groundwater, of a bacterial strain able to degrade s-triazines.     

Time-dependent tests on intact rocks in uniaxial compression

The influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine. An experimental data processing method for such creep tests is suggested. The correction formulas to determine the rheological model parameters are derived for the case when load relaxation is considered. Creep and relaxation tests under uniaxial compression on four types of rocks are conducted using an electronic hydraulic servo-controlled stiff testing machine. The creep and relaxation laws of the different rocks are compared. The complete stress–strain curves for red sandstone specimens are obtained at nine strain rates from 2.43×10⁻⁶ to 4.38×10⁻³/s. The effects of strain rates on rock strength and limit strain are discussed. Empirical equations to evaluate the strain rate dependence of rock mechanical properties are presented.     

Stress-Dilation of Undisturbed Sand Samples in Drained and Undrained Triaxial Shear

Stress-dilation behavior of undisturbed sand samples tested in both drained and undrained triaxial shear has been studied. The stress-dilation relation is recognized as being a basic component of the stress-strain behavior of granular materials. The dilation angle, ψ commonly used to represent the dilation characteristics of the soil, is defined clearly for plane strain conditions. However, the paper discusses confusion regarding its definition under triaxial loading conditions, and adopts the definition sin ψ=-dε_v^p/dγ_max^p. Drained triaxial tests performed on specimens obtained from undisturbed block samples of sand indicated that the undisturbed material exhibits a well defined stress-dilation relation. By referring to plastic (irrecoverable) components of strain, it was found that this relation was also compatible to results of tests in undrained triaxial shear. Demonstration of this compatibility required that the small membrane penetration effects in the undrained triaxial shear tests, resulting from changing effective confining stress, be taken into account. From the results of the present investigation, and of other studies reported in the literature, it was found that the relation between friction angle, φ′ and dilation angle, ψ, under axi-symmetric conditions, as defined above, can be reasonably expressed by the empirical expression: φ′≍0.4ψ+φ′_cv.     

Strength and Deformation Characteristics of Scoria in Triaxial Compression at Low Confining Stress

Scoria deposit can be found around many volcanoes of the Quarternary epoch in the world. In Japan, Mount Fuji is one of volcanoes, where surface area is widely layered by scoria. From a geotechnical engineering point of view, scoria has many problems because of its physical characteristics by means of its collapsible properties. Since scoria is a non-cohesive material, it is vulnerable to water-induced change and fails easily due to rainfall or snow melting. These are the primary causes of debris flow which frequently occurs in the Mount Fuji area. Typically, this type of debris flow is called “Yukishiro” in Japanese. For better understanding of the shear behavior of scoria within low stress level condition, drained triaxial compression tests were performed on loose dry scoria under low confining pressures (10-80 kPa) which are assumed as representative of the field surface stress conditions. The effects of grain size, dry density ρ_d and effective confining stress σ′_c upon stress-strain behaviour and angle of internal friction φ_d were investigated. Attention was also paid to level of grain breakage B_g and reproducibility of the test. Grain breakage investigations were carried out on the particle breakage phenomenon, which occurs during testing. Additionally, initial Young's modulus of scoria was identified. As a result, reproducibility of the test is good in terms of stress-strain curves, however volumetric strain versus axial strain exhibits less reproducibility than stress-strain curves. At stress level of 10-80 kPa, it was found that the dependency of φ_d on σ′_c is almost negligible, while there is noticeable dependency of φ_d on grain size. The effect of grain size on stress-strain relationships was observed. It was also found that grain breakage was noticed at stress level of 10~80 kPa, which describes the dependency scoria parameters on confining stress and the effect of grain size upon B_g-σ′_c relationships was also observed.     

The removal of colour from effluent using various adsorbents—IV. Silica: Equilibria and column studies

The rate of adsorption of Astrazone Blue, a basic dye, on Sorbsil Silica has been studied. The parameters studied include particle size, initial dye concentration, agitation and dye solution temperature. The rate controlling step is mainly intraparticle diffusion, although a small boundary layer resistance is experienced. The activation energy for the adsorption of Astrazone Blue on silica is 13.2 ± 0.6 kJ kg⁻¹. The diffusion coefficients vary from 9 × 10⁻⁹ cm² s⁻¹ at 20°C to 10 × 10⁻⁸ cm² s⁻¹ at 82°C.     

Time-Dependent Shear Deformation Characteristics of Geomaterials and their Simulation

The viscous aspects of the stress-strain behaviour of saturated and air-dried clean sands in drained plane strain compression (PSC) and saturated clean sand and soft clays in undrained triaxial compression (TC) are presented. Common as well as different viscous features among the different geomaterials are addressed. The general three-component model is used as the framework for constitutive modelling, in which the total strain rate ε is decomposed into elastic and irreversible components ε^e and ε^ir while the stress σ is decomposed into inviscid (non-viscous) and viscous components σ^f and σ^v. In the simplest model (called the new isotach model) among those described in the paper, σ^f is a nonlinear function of ε^ir, while σ^v is a non-linear function of ε^ir and always proportional to σ^f for primary loading. This model is relevant to kaolin for the full pre-peak range and a reconstituted low-plasticity clay (Fujinomori clay) at small strains, both in undrained TC. The model is modified to simulate the viscous effect that decays with ε^ir, as observed with clean sands and a natural soft clay. It is shown that the second type of model (called the viscous evanescent model and the TESRA model) simulates well the above-mentioned behaviour, not only during primary loading, but also at unloaded conditions. The model is further modified to simulate the behaviour of Fujinomori clay whereby the rate at which the viscous effect decays gradually increases with ε^ir (the general TESRA model). The viscous components σ^v of the three models can be represented by a set of common equations, and the other models are specifically simplified versions of the general TESRA model.     

Measured and predicted performance of prefabricated vertical drains (PVDs) with and without vacuum preloading

This paper presents the effectiveness of vacuum preloading in accelerating the consolidation of PVD improved soft Bangkok clay by comparing with the corresponding results without vacuum preloading. Laboratory tests were conducted using a large scale consolidometer having diameter of 300 mm and height of 500 mm with reconstituted specimens installed with prefabricated vertical drains (PVD) with and without vacuum preloading. In addition, field data were collected from Second Bangkok International Airport (SBIA) site improved by PVD with and without vacuum pressures. Analyses were carried out to compare the compressibility parameters (C_h and k_h/k_s) by back-calculation of laboratory and field settlements using Hansbo (1979) method. From the laboratory tests, the horizontal coefficient of consolidation (C_h) values from reconstituted specimens were 1.08 and 1.87 m²/yr for PVD without and with vacuum pressure, respectively and the k_h/k_s values were 2.7 for PVD only and 2.5 for vacuum-PVD. After the improvement, the water contents of the soft clay were reduced, thereby, increasing its undrained shear strengths. Similarly, the field data analysis based on the back-calculated results showed that the k_h/k_s were 7.2 and 6.6 for PVD without and with vacuum, respectively. The C_h values increased slightly from 2.17 m²/yr for PVD only to 3.51 m²/yr for vacuum-PVD. The time to reach 90% degree of consolidation for soils with vacuum-PVD was one-third shorter than that for soils with PVD only because of higher C_h values. Thus, the addition of vacuum pressure leads to increase horizontal coefficient of consolidation which shortened the time of preloading. The PVDCON software was found to be useful to predict the settlements of the PVD improved ground with and without vacuum preloading.