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.     

Investigations on secondary compression behaviours of artificial soft sand-clay mixtures

Special soft soils with notable coarse and fine fractions are not only naturally deposited in ravines or interactive marine-terrestrial sedimentation areas but also artificially formed in land reclamation projects via the hydraulic filling method and excavation cuts of different soil layers in open pit mines. The behaviour of these soils is not like that of similar normal sands or clays but rather exhibits the characteristics of transitional mixtures. A sand-clay mixture was prepared in the laboratory to simulate these special soil types, and an incremental loading oedometer test was performed. Thereafter, a secondary compression coefficient (C_α) was determined to investigate the long-term rheological properties of the mixture. The results in the C_α-log σ_v′, C_α-e, C_α-e/e_L and C_α-C_c planes revealed that a threshold e/e_L value (i.e. approximately 0.45) in the case of sands. Note that in a traditional C_α-C_c analysis system, the soils with sand exhibited the same secondary compression behaviour (SCB) as that without sand before reaching the transformation threshold, whereas after this threshold, they appeared to behave much like sandy soils. Hence, a four-phase frame of sand-clay mixture was further introduced to clarify the mechanism of the SCB of soils with and without sand. An updated C_αc/C_cc reflecting the creep mechanism of the clay phase can be effectively adopted to exclude the influence of sand particles. Because the quantitative relationship between C_αc and C_cc for soils with sand was consistent with that without sand, it can be assumed that the SCB of clay-sand mixtures was dominated by the creep of fine fractions. It is porposed that the existence pattern of the sand particles in the mixtures undergoes three stages with an increasing vertical stress, i.e. non-skeleton, partial skeleton and complete skeleton. A further discussion on the mutual relationship between phases on the behaviour of mixtures with sand revealed that isolated sand had no effect on the compressibility and SCB during the initial stage. When a partial skeleton of the sand was established in the second stage, characterized by less stress on the clay matrix, the compressibility and SCB were suppressed. When the vertical stress continuously increased or additional sand content was supplied, a complete skeleton formed. Hence, the behaviour of mixtures (regardless of compressibility and secondary compression) was completely governed by the sand fractions. Generally, the same relationship was observed between C_αc and C_cc during the first and second stages with sand, indicating that the influence of sand fractions on the compressibility and SCB can be counterweighted.     

Colloid straining within saturated heterogeneous porous media

The transport of 0.46 μm, 2.94 μm, 5.1 μm and 6.06 μm latex particles in heterogeneous porous media prepared from the mixing of 0.78 mm, 0.46 mm and 0.23 mm quartz sands was investigated through column transport experiments. It was observed that the 0.46 μm particles traveled conservatively within the heterogeneous porous media, suggesting that under the experimental conditions employed in this research the strong repulsive interactions between the negatively charged latex particles and the clean quartz sands led to minimal colloid immobilization due to physicochemical filtration. The immobilization of the 2.94 μm, 5.1 μm and 6.06 μm latex particles was thus attributed to colloid straining. Experimental results showed that the straining of colloidal particles within heterogeneous sand mixtures increased when the fraction of finer sands increased. The mathematical model that was developed and tested based on results obtained using uniform sands (Xu et al., 2006) was found to be able to describe colloid straining within heterogeneous porous media. Examination of the relationship between the best-fit values of the clean-bed straining rate coefficients (k₀) and the ratio of colloid diameter (d_p) and sand grain size (d_g) indicated that when number-average sizes were used to represent the size of the heterogeneous porous media, there existed a consistent relationship for both uniform sands and heterogeneous sand mixtures. Similarly, the use of the number-averaged sizes for the heterogeneous porous media produced a uniform relationship between the colloid straining capacity term (λ) and the ratio of d_p/d_g for all the sand treatments.     

Coupled effects of saline solutions and temperature on the swelling deformation property of GMZ bentonite-sand mixtures

Based on the results of one-dimensional free swelling tests, the coupled effects of NaCl-Na₂SO₄ solutions and temperature on the swelling deformation property of compacted GMZ bentonite-sand mixtures were analyzed. The concentrations of the NaCl-Na₂SO₄ solutions were 0, 0.008, 0.04, 0.08 and 0.16 mol/L, the temperatures were 20 °C, 40 °C, 60 °C, 80 °C and 90 °C and the sand contents were 0, 30 and 50%. X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) tests were also performed to find the influences of the temperature, solution concentration and sand content on the mineral composition and microstructure. The results showed that the maximum swelling strain of the specimens increased with an elevated temperature, while it decreased with increases in the solution concentration and sand content. The swelling process was accelerated by the rising temperature. The XRD tests indicated that the combined effect of a high temperature and a high solution concentration may have caused more Na⁺ from the saline solutions to enter the interlayer, partly displacing the divalent cations. The MIP tests demonstrated that adding sand to the bentonite could reduce the amount of inter-granular pores and increase the intra-aggregate pores. These findings are advantageous to the transfer of heat and pour fluid in the specimens; and thus, the swelling deformation capacity of the bentonite-sand mixtures was higher than that of pure bentonite under the same conditions.     

Relationships between sand and water quality at recreational beaches

Enterococci are used to assess the risk of negative human health impacts from recreational waters. Studies have shown sustained populations of enterococci within sediments of beaches but comprehensive surveys of multiple tidal zones on beaches in a regional area and their relationship to beach management decisions are limited. We sampled three tidal zones on eight South Florida beaches in Miami-Dade and Broward counties and found that enterococci were ubiquitous within South Florida beach sands although their levels varied greatly both among the beaches and between the supratidal, intertidal and subtidal zones. The supratidal sands consistently had significantly higher (p < 0.003) levels of enterococci (average 40 CFU/g dry sand) than the other two zones. Levels of enterococci within the subtidal sand correlated with the average level of enterococci in the water (CFU/100mL) for the season during which samples were collected (r_s = 0.73). The average sand enterococci content over all the zones on each beach correlated with the average water enterococci levels of the year prior to sand samplings (r_s = 0.64) as well as the average water enterococci levels for the month after sand samplings (r_s = 0.54). Results indicate a connection between levels of enterococci in beach water and sands throughout South Florida’s beaches and suggest that the sands are one of the predominant reservoirs of enterococci impacting beach water quality. As a result, beaches with lower levels of enterococci in the sand had fewer exceedences relative to beaches with higher levels of sand enterococci. More research should focus on evaluating beach sand quality as a means to predict and regulate marine recreational water quality.     

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.     

Swelling characteristics of Gaomiaozi bentonite and its prediction

Gaomiaozi （GMZ） bentonite has been chosen as a possible matrix material of buffers/backfills in the deep geological disposal to isolate the high-level radioactive waste （HLRW） in China. In the Gaomiaozi deposit area, calcium bentonite in the near surface zone and sodium bentonite in the deeper zone are observed. The swelling characteristics of GMZ sodium and calcium bentonites and their mixtures with sand wetted with distilled water were studied in the present work. The test results show that the relationship be- tween the void ratio and swelling pressure of compacted GMZ bentonite-sand mixtures at full saturation is independent of the initial conditions such as the initial dry density and water content, hut dependent on the ratio of bentonite to sand. An empirical method was accordingly proposed allowing the prediction of the swelling deformation and swelling pressure with different initial densities and bentonite-sand ratios when in saturated conditions. Finally, the swelling capacities of GMZ Na- and Ca-bentonites and Kunigel Na-bentonite are compared.     

Stress wave transmission and frequency-domain responses of gap-graded cohesionless soils

Gap-graded cohesionless soils are extensively utilized in numerous man-made geotechnical structures such as earthen embankments, man-made fills and used for the prevention of seepage in dams and tailings of mines. However, there is a lack of research available on investigating the transitional behaviour of frequency responses of gap-graded cohesionless soils. Therefore, the present research explores the stress wave transmission in gap-graded silica sand mixtures (having median particle size ratio of 6.4) using disk-shaped piezoelectric transducers (DTs). By employing DTs, shear (V_s) and compression (V_p) wave velocities are measured using more planar waves and eliminate the possibility of fine particle segregation commonly encountered during insertion of bender elements. The experimental results indicate that at an equivalent void ratio, V_s and V_p decrease initially with the increase in fine silica sand content (F_s); however, once a transitional value (F_s,thr) is attained, V_s and V_p start to rise and move towards the values for fine silica sand. Such transitional behaviours of elastic wave velocities and frequency-domain responses are dependent on the state of packing, i.e. denser specimens achieve this at a lower F_s than the looser equivalents. It is observed that for a given F_s, there exists a linear relationship between the maximum transmitted frequency in the packing, i.e. low-pass frequency (f_lp) and the V_s. Furthermore, a novel approach has been described by which the type of gap-graded mixture, i.e. underfilled (internally unstable) or overfilled (stable), can be assessed by experimentalists from the slope of f_lp–V_sresponses. The advantage of adopting f_lp to categorize the gap-graded mixtures is that the same is more sensitive to F_s in comparison to V_s or V_p, and thus a more robust analysis can be achieved.     

高掺砂率膨润土混合土膨胀特性及其膨胀量预测

膨润土与砂混合土中掺砂率的高低会引起混合土膨胀特性的差异。对纯膨润土及其低掺砂率混合土,浸水膨胀完成后蒙脱石孔隙比em与竖向应力σv在双对数坐标内呈唯一线性关系。对高掺砂率混合土,在较小荷载下浸水,不会形成砂骨架,em–σv线性关系仍成立;在较大荷载下浸水,会形成砂骨架,砂骨架形成后,em与σv间不再满足该线性关系。利用砂骨架孔隙比的概念可确定不同掺砂率混合土形成砂骨架时对应的起偏应力及混合土能够形成砂骨架的临界掺砂率。砂骨架形成前,砂颗粒被蒙脱石包围,外力由蒙脱石承担,最终变形量由试样中单位体积蒙脱石的含量决定。砂骨架形成后,竖向应力最终由砂骨架和蒙脱石共同承担,提出了确定不同掺砂率下两者承担比例的方法,进而可确定砂骨架形成后的膨胀变化量,预测结果与膨胀试验结果吻合较好。该方法可以预测高掺砂率混合土的浸水膨胀量。     

Maximum and Minimum Void Ratio Characteristics of Sands

Characteristics of the maximum and minimum void ratios of sands and their possible use for material characterization have been investigated in this study. Data of over 300 natural sandy soils including clean sands, sands with fines and sands containing small amount of clay-size particles have been used to examine the influence of fines, grain-size composition and particle shape on e_max, e_min and void ratio range (e_max - e_min). A set of empirical correlations are presented which clearly demonstrate the link between these void ratios and material properties of sands. The key advantage of (e_max - e_min) over-conventional material parameters such as Fc and D₅0 is that (e_max - e_min) is indicative of the overall grain-size composition and particle characteristics of a given sand and that it shows off the combined influence of relevant material factors. The void ratio range provides a general basis for comparative evaluation of material properties over the entire range of cohesionless soils.Important issues related to the laboratory procedures used for determination of e_max and e_min as well as their applicability to fines-containing sands are also addressed. Three distinct linear correlations were found to exist between e_max and e_min for clean sands, sands with 5-15% fines and sands with 15-30% fines respectively, thus illustrating that the standard JGS procedures for minimum and maximum densities of sands can provide reasonably consistent e_max and e_min values for sands with fines content of up to 30%. The importance of the grain-size distribution and presence of gaps in the grading of composite soils or mixtures of sands with fines produced in the laboratory is also discussed.     

Effects of specimen thickness on apparent swelling pressure evolution of compacted bentonite

With the interests to see specimen thickness (h_sp) effect on apparent swelling pressure (p_s) of compacted bentonite and capture characteristics of p_s development during wetting bentonite specimens (p_s evolution, hereafter), a series of tests to measure p_s of compacted bentonite specimens with a dry density (ρ_d) range of 0.99–1.76 Mg/m³ and a specimen thickness (h_sp) range of 2–10 mm were conducted. Test results suggest: (1) p_s evolution generally has common characteristics with four feature points chronologically: peak (t_p, p_p), valley (t_v, p_v), initial equilibrium (t_ei, p_ei) and an equilibrium (t_eq, p_eq), where, t_p, t_v, t_ei and t_eq represent time (t) and p_p, p_v, p_ei, and p_eq denote p_s of the four feature points, respectively; (2) within the tested range, h_sp effect is generally minor on p_s at feature points, and p_p, p_v, p_ei, and p_eq have well correlations with specimen final dry density (ρ_df); (3) time scale for specimens with different h_sp can be well unified by a time coefficient χ_sp (=h_sp/√t).     

Swelling ability and behaviour of bentonite-based materials for deep repository engineered barrier systems: Influence of physical,chemical and thermal factors

Compacted bentonite–sand (B/S) mixtures have been used as a barrier material in engineered barrier systems (EBSs) of deep geological repositories (DGR) to store nuclear wastes. This study investigates the individual and combined effects of different chemical compositions of deep groundwaters (chemical factor) at potential repository sites in Canada (the Trenton and Guelph regions in Ontario), heat generated in DGRs (thermal factor), dry densities and mass ratios of bentonite and sand mixtures (physical factors) on the swelling behavior and ability of bentonite-based materials. In this study, swelling tests are conducted on B/S mixtures with different B/S mix ratios (20/80 to 70/30), compacted at different dry densities (ρ_d = 1.6–2 g/cm³), saturated with different types of water (distilled water and simulated deep groundwater of Trenton and Guelph) and exposed to different temperatures (20 °C–80 °C). Moreover, scanning electron microscopy (SEM) analyses, mercury intrusion porosimetry (MIP) tests and X-ray diffractometry (XRD) analyses are carried out to evaluate the morphological, microstructural and mineralogical characteristics of the B/S mixtures. The test results indicate that the swelling potential of the B/S mixtures is significantly affected by these physical and chemical factors as well as the combined effects of the chemical and thermal factors. A significant decrease in the swelling capacity is observed when the B/S materials are exposed to the aforementioned groundwaters. A large decrease in the swelling capacity is observed for higher bentonite content in the mixtures. Moreover, higher temperatures intensify the chemically-induced reduction of the swelling capacity of the B/S barrier materials. This decrease in the swelling capacity is caused by the chemical and/or microstructural changes of the materials. The results from this research will help engineers to design and build EBSs for DGRs with similar groundwater and thermal conditions.     

Sorption-desorption column tests to evaluate the attenuation layer using soil amended with a stabilising agent

Sorption-desorption column tests using acrylic columns (? 5 cm × h 10 cm) were employed to evaluate the sorption performance of an attenuation layer against geogenic contamination. The attenuation layer material was silica sand amended with 1, 5, or 10% of a stabilising agent. The main component of the agent was magnesium oxide. The sorption behaviour of the materials was determined by a fluoride solution (C₀ = 80 mg/L F^-), while the desorption behaviour was determined by distilled water. Breakthroughs (C/C₀ > 0.05) occurred after approximately 1, 20, and 50 PVF for stabilising agent contents of 1, 5, and 10%, respectively. The one-dimensional advection-dispersion equation modelled the breakthrough curves obtained from the tests. The predictions gave unrealistic estimates, especially for the breakthrough point where C/C₀ = 0.05. For the 1% agent content, approximately 20% of the sorbed mass, S_s, was desorbed, but the percentage of desorbed mass, S_d, was much smaller for the higher agent contents. The difference between the sorbed and desorbed masses was defined as the immobilised fraction, S_s - S_d. For the 5% agent content, S_s - S_d = 4.0 mg/g. The results suggest that when silica sand is amended with magnesium oxide as an agent, the mixture can immobilise the fluoride in the attenuation layer.     

Gas transport parameters of differently compacted granulated bentonite mixtures (GBMs) under air-dried conditions

Granulated bentonite mixtures (GBMs) have been regarded as effective buffer materials in the deep geological disposal of radioactive waste due to their operational advantages, such as ease of transportation and in-situ placement/backfilling. Many studies have been done to characterize the hydraulic and thermal properties of GBMs as well as their swelling properties. Only limited studies, however, have investigated their gas transport properties, even though these properties affect their compactness during in-situ placement/backfilling and subsequent gas diffusion and advection in the buffer zone. The aim of this study is to understand the gas transport parameters, i.e., air permeability (k_a) and gas diffusivity (D_p/D_o), of tested samples compacted at different dry densities (DDs) under air-dried conditions, linking them with the measured density distribution characteristics determined by microfocus X-ray computed tomography (MFXCT) analysis. Two types of GBMs were used in this study: 1) FE-GBM (prepared from National Standard® bentonite, Wyoming, USA): this material was used in the Full-scale Emplacement (FE) experiment at the Mont Terri rock laboratory, Switzerland) and 2) OK-GBM (prepared from a bentonite, originating from Japan, with the trade name of OK bentonite, Kunimine Industries). The tested samples were firstly packed in a 100-cm³ acrylic core with different DDs, ranging from loose to dense (1.09 to 1.75 g/cm³), and scanned by MFXCT. The weighting factors, w_f (fine fraction; lower density) and w_c (coarse fraction; higher density) (w_f + w_c = 1), were determined after the peak separation of the measured CT brightness histograms from the reconstructed three-dimensional multiplanar reconstruction (MPR) images of the MFXCT analysis. The measured k_a and D_p/D_o were highly dependent on the DDs, the k_a (?) values fitted well with a power law model, and the D_p/D_o (?) was predicted accurately by several previously proposed models. For both FE-GBM and OK-GBM, there were good linear relationships between the gas transport parameters and w_c × DD, implying that the weight of the coarse fraction controlled k_a and D_p/D_o. Moreover, the Kozeny-Carman model, incorporating the measured volumetric surfaces from the MFXCT analysis, was able to predict the k_a values well.     

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.     

Electrochemical disinfection of Escherichia coli in the presence and absence of primary sludge particulates

Electrochemical (EC) residual disinfection of Escherichia coli (E. coli) in the presence and absence of primary sludge particulates (PSPs) was studied. The kinetics followed a first-order rate law. When PSPs were absent, the EC residual disinfection rate coefficient (k) increased linearly with EC pretreatment energy (E_C, 0–0.63 kWh/m³). However, with 143 mg PSPs/L, k first increased linearly with E_C (0–0.28 kWh/m³) and then decreased linearly with E_C (0.28–0.42 kWh/m³). H₂O₂ was detected during EC pretreatment in PSPs-free samples and the H₂O₂ concentration (C_H) increased with E_C (0–0.83 kWh/m³) linearly. Chloride was detected in PSPs aqueous samples (143 mg PSPs/L) and its concentration (C_C) changed during EC pretreatment: initially, a decrease of C_C was observed when E_C increased from 0 to 0.28 kWh/m³, followed by an increase of C_C when E_C increased 0.28–0.42 kWh/m³. In both cases, k correlated to the initial post-EC chloride concentration (C_CI) in an inverse linear relationship. This two-stage change of C_C and k was caused by a combination of two reactions: anodic oxidation of chloride and the reaction of chloramines with excess chlorine. This paper explains the mechanisms underlying EC residual disinfection in the presence and absence of PSPs, and proposes a feasible strategy for EC disinfection when PSPs are present, an approach that could be useful in the treatment of combined sewage overflow (CSO).     

Measurements of Unsaturated Hydraulic Conductivity Functions of Two Fine-Grained Materials

Water unsaturated hydraulic conductivity (k_w) functions of two remolded fine-grained materials were measured over a wide range of degrees of saturation (S_r) with two methods. The instantaneous profile method (IPM) was used for S_r>50%. An original vapor equilibration method (also known as the vapor equilibrium technique, VET) was used for S_r<50%. Both materials compacted at the standard Proctor optimum water content and maximum density, have saturated hydraulic conductivities (k_sat)<10^-7 m/s. The VET couples the total soil suction (s) control from desiccators with saturated salt solutions with water mass measurements from a digital laboratory balance. The k_w measurements of the two techniques are consistent and complementary. The effect of hysteresis on the k_w functions at higher s values was also investigated. The experimental results suggest that the hysteretic effect on the k_w-S_r and k_w-s relationships cannot be neglected, and that the measured k_w are significantly dependent on the initial S_r. The VET tests on the specimens that were initially dried give the lowest values of k_w and the tests on the specimens that were initially saturated give the highest values of k_w. The relative hydraulic conductivity values are very small (k_rw<2×10^-5) in this saturation range (S_r<50%) for the tested materials.     

Barrier properties of a geosynthetic clay liner using polymerized sodium bentonite

The hydraulic and swelling properties of a polymerized bentonite (PB), and the self-healing capacity of a geosynthetic clay liner (GCL) using the PB as the core material (PB-GCL) were investigated experimentally. Five different test liquids included of deionized water, NaCl solutions (0.1 M and 0.6 M) and CaCl₂ solutions (0.1 M and 0.6 M) were used in this study. The PB exhibited a higher free swelling index (FSI) than that of the untreated bentonite (UB) for all test liquids. For permeability test, under a given void ratio (e), the value of k of the PB is much lower than that of the UB in NaCl and CaCl₂ solutions. The PB-GCL specimens demonstrated a higher self-healing capacity than that of the corresponding GCL specimens using the UB (UB-GCL). Specifically, when using a 0.6 M CaCl₂ solution for a 20-mm-diameter damage hole, the UB-GCL specimen provided a zero healing ratio (healed damage area/total damage area), but the PB-GCL specimen demonstrated an approximately 76% healing ratio. The results from this study indicate that the PB-GCL provided better barrier performance against cationic liquids with higher cation valence and concentrations compared to that of the UB-GCL.     

Transition of gap-graded soil fabric – shear wave measurements and dispersion relation

The dynamic response of gap-graded soils is complex; it requires massive experimental and theoretical investigation in order for a sound understanding to be developed. The complexity of the behaviour of such soils arises partially due to the presence of many influential parameters, such as the finer sand/silt content (F_s), size ratio (R_d), and void ratio (e). In this contribution, experiments are conducted on a series of gap-graded soil mixtures, where the coarser sand is mixed with four different finer sands or non-plastic silt with different sizes, namely various R_d, and one well-graded material is also tested. The S-wave velocity (V_s) and low-pass threshold frequency (f_lp) are measured in the isotropic stress state using a triaxial apparatus equipped with planar piezoelectric transducers. The experimental results reveal that, for a given e or relative density (D_r), both f_lp and V_s decrease at a low F_s, but increase at a high F_s. The slope of the linear regression between f_lp and V_s appears to capture this transitional behaviour, depending on F_s and R_d, which can then be used to correlate the physical state of the gap-graded soil fabric. The transitional behaviour is more noticeable at a larger R_d, which is in line with the existing literature. To understand the effect of the R_d on the shear wave propagation at a low F_s, the particle-scale dispersion relation for binary chains is newly introduced and correlated with accompanying experimental observations.     

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.