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.     

Water and soil particle movement in unsaturated bentonite with constrained and free swelling boundaries

Bentonite is considered a barrier material in the geological disposal of high-level radioactive waste. Depending on the construction method, it is necessary to know what the behavior of the bentonite barrier will be in the wetting process during the absorption of underground water under different boundary conditions. In this study, water and soil particle movement during the wetting process of a compacted bentonite under constrained and free swelling conditions was studied experimentally and numerically. For the constrained swelling boundary condition, in which the swelling deformation was fully constrained, the distributions of the gravimetric water content (w) were measured for specimens with different dry densities (ρ_d), and then the water diffusivity (D_w) was obtained. It was found that D_w showed a slightly decreasing trend with an increase in ρ_d, while the numerical simulations showed that the difference in D_w induced by ρ_d was minor in terms of the evolving degree of saturation in the tested range. For the free swelling boundary condition, the distributions of w and ρ_d were measured for specimens with an initial ρ_d of 1.6 Mg/m³. To obtain D_w and the soil particle diffusivity (D_s), an existing theoretical framework, to which new concrete calculation equations had been added, was presented. Then, the performance of the framework was examined by numerical simulations to illustrate the water and soil particle movement under conditions similar to those of the experiment. It was found that the framework can describe the experimental results well, but that the accuracy of the results largely depends on the accuracy of the experimental data.     

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.     

An Improved Model of Soil Response to Load,Unload and Re-Load Cycles in an Oedometer

There are a number of advantages to be gained by representing the oedometric compression of a soil skeleton, during virgin loading, unloading and subsequent reloading by ‘log (v) versus log (p′)’ relationships rather than the conventional ‘e versus log₁₀ (p′)’ expression. The paper presents an augmented version of the basic, two parameter (C′_c, C′_s), ‘log (v) versus log (p′)’ model in which the addition of two further parameters (C′_r, C′_o) enables a complete, non-linear response for any load-unload-reload cycle to be reproduced. All four parameters, deduced from a single such cycle in an oedometer, can then be used to predict the response of the sample in any other unload-reload cycles. Results are presented from tests of this kind on a range of fine-grained soils to demonstrate the key attributes of the model. These include the generation of ‘log (m_v) versus log (p′)’ diagrams that furnish practically useful m_v values applicable throughout any unload-reload cycle. The model also provides a simple means of assessing the overconsolidation ratio of an undisturbed soil sample.     

Experimental study for temperature effect on swelling pressures during saturation of bentonites

Swelling pressure tests were conducted at 25, 50, and 80°C to elucidate the temperature effect on swelling pressures at peak, valley, re-peak and equilibrium points (p_peak, p_valley, p_re-peak, and p_eq) in swelling pressure evolution curve. Three powder bentonites (Kunigel-V1, Volclay, and Kunibond) and two granular bentonites (MX-80 and Kunigel-GX) were used to investigate the grain size effect on p_peak, p_valley, p_re-peak, and p_eq at different temperatures. Experimentally obtained results were the followings. 1) For the powder bentonites, p_peak, p_valley, p_re-peak, and p_eq increased with the increase of dry density and rose with increasing temperature; 2) For the granular bentonites, p_peak, p_valley, p_re-peak, and p_eq decreased as temperature rose and increased as dry density rose; 3) All five bentonites were found to have larger values for (p_eq-p_valley) at higher dry density and lower temperature conditions. Swellings and montmorillonite fabric unit interactions during saturation were discussed with swelling pressure evolution curve. Temperature effect on p_eq was carefully explained.     

Small Strain Stiffness and Non-Linear Stress-Strain Behaviour of Cement-Mixed Gravelly Soil

The stiffness at small strains and non-linear stress-strain relation of compacted cement-mixed well-graded gravelly soil as well as the ageing effects were evaluated by drained triaxial compression tests on compacted moist specimens cured for different periods at isotropic and different anisotropic stress states. In all the tests, the initial stress-strain relation at small strains less than about 0.001% was essentially elastic and the initial Young's modulus, E₀, was essentially the same as the E_eq value evaluated by applying unload/reload cycles under otherwise the same conditions. The E_eq values were rather independent of strain rate. The E_eq value from the first unload/reload cycle applied during otherwise continuous ML became noticeably lower than the elastic modulus evaluated at the same stress state, more as approaching the peak stress state. After a number of small unload/reload cycles and long sustained loading, the E_eq value became closer to the elastic modulus due to a decrease in the viscous effects. The ratio of E₀ to the compressive strength (q_max) was similar to that of concrete but noticeably larger than those of uncompacted cement-mixed soil, sedimentary softrock and unbound gravelly soil. Both E₀ and q_max increased with time by ageing, while the E₀/q_max ratio decreased with time. When ML was restarted at a constant strain rate after ageing with a shear stress, the tangent stiffness became very high for a large stress range with a substantial change in the non-linearity of stress-strain relation.     

Behavior of cantilever and geosynthetic-reinforced walls on deformable foundations

A series of plane-strain model tests was performed to investigate the behavior of cantilever soil retaining walls (CWs) and geosynthetic-reinforced soil retaining walls with a rigid facing (GRS-RWs) placed on non-deformable and deformable foundations with various subgrade reaction moduli (k_v). The walls were designed to have configurations similar to those used in practice, with similar controlling safety factors against sliding. Screw jacks and springs were used to simulate undeformable and deformable grounds, respectively, with various maximum foundation settlements of S_max ≒ 0, 2.5, 5, and 10% of the backfill height (H). Test results show that the GRS-RW has better settlement-tolerating performances, in terms of the tilting angle (θ), the horizontal displacement (D_h), and the settlement of the crest of the backfill (D_v1), than those of the CW. For both CWs and GRS-BWs, the worst scenario of the wall performance, in terms of D_h, θ and D_v1, occurred at a moderate foundation settlement of S_max/H ≒ 5% (or k_v = 1.8 kPa/mm), rather than at a greater foundation settlement of S_max/H ≒ 10%, which facilitates a tilting-backward displacement mode. Experimental results also indicate that local lateral pressure coefficients against facing (K_f,z) for CWs may reach the at-rest (or K_o) state at the central portion; values of K_f,z may reach the passive (or K_p) state at the lower portion of the wall. In the case of CWs, the measured values of local and global lateral pressure coefficients (K_f,z and K_f) tend to increase with increasing maximum foundation settlement. This is not the case for GRS-RWs, which exhibited a relatively settlement-independent response, in terms of K_f and K_f,z, against facing. To develop relevant limit-equilibrium-based design methods for CWs and GRS-RWs placed on deformable foundations, knowledge of lateral pressure coefficients associated with various displacement and tilting induced by the foundation settlement are required.     

Back-analyses of flow parameters of PVD improved soft Bangkok clay with and without vacuum preloading from settlement data and numerical simulations

Prefabricated vertical drains (PVDs) with embankment preloading (conventional PVDs) and with embankment combined with vacuum preloading (Vacuum-PVDs) are examined using the field data obtained from the site of the Suvarnabhumi Airport, Thailand. The flow parameters were back-analyzed by comparison of measured and predicted or simulated data. The flow parameters were illustrated in terms of the horizontal coefficient of consolidation (C_h) and the ratio between the horizontal hydraulic conductivity in undisturbed zone (k_h) and the horizontal hydraulic conductivity in smear zone (k_s) or (k_h/k_s). Numerical simulations using one-dimensional FEM PVDCON software with equivalent vertical permeability, k_ev, to determine the appropriate C_h and k_h/k_s of PVDs with conventional embankment preloading and with embankment combined with vacuum preloading schemes were made. Furthermore, numerical simulations using axisymmetric FEM by ABAQUS software, incorporating horizontal (k_h) and vertical (k_v) permeabilities, to determine the appropriate k_h/k_s based on back-calculated C_h of conventional PVD and Vacuum-PVD schemes were also done. The Vacuum-PVD scheme indicated faster rate of settlement than conventional PVD scheme by about 1.7–1.8 times with slight reduction of the k_h/k_s ratios. For conventional PVD, it was demonstrated that the increase in k_h/k_s ratios reduced the simulated rate of settlement.     

Effect of shearing history on stress wave velocities of sands observed in triaxial compression tests

For the accurate design of geotechnical structures subjected to static and dynamic loadings, precise estimation of elastic wave velocities and hence, small strain stiffness of soil is essential. However, the interpretation of elastic wave velocities propagating in deformed/sheared soil has not been comprehensively explored. In this research, shear (V_s) and compression wave velocity (V_p) measurements have been undertaken on three kinds of uniformly graded sands during drained triaxial compression by keeping minor principal stress constant. Planar piezoelectric transducers have been used to overcome the limitations associated with the more commonly used bender elements, such as distortion of transducers during specimen shearing. This technical note reveals that the increase of major principal stress in the wave propagation direction has a more significant influence on V_p in comparison to V_s. The axial strain (ε_a) at which peak V_s is noted is comparable to the ε_a at which specimen dilation or phase transformation takes place. The V_s values show a substantial drop after phase transformation, although there is an increase in the mean stress level. However, V_p increases even after specimen dilation takes place, and it is the major principal stress that dictates its evolution during triaxial compression. Moreover, for a given material and initial stress level, elastic wave velocities of specimens prepared at different initial densities approach one another during shearing and later merge at a large ε_a.     

Occupants’ interactions with windows in 8 residential apartments in Beijing and Nanjing,China

Occupants’ interactions with windows influence both building energy consumption and exposure to airborne pollutants indoors. Occupants’ window opening behavior varies from region to region due to physical environmental factors and social reasons. China is now confronting severe atmospheric pollution, which may affect occupants’ window opening behaviors. A field study was conducted in 8 naturally ventilated residential apartments in Beijing and Nanjing, China. This involved periodically monitoring window states of eight residential apartments within each season from October 2013 to December 2014 by magnetic induction devices (TJHY, CKJM-1). Relationships between the probability of window opening (p) and explanatory variables, including outdoor air temperature (t _o), outdoor relative humidity (RH), outdoor wind speed (V _s), and ambient PM2.5 (particles with aerodynamic diameter less than 2.5 microns) concentrations (C _p), were analyzed. Stochastic models of occupants’ interactions with windows in monitored residences were established via univariate and multivariate linear logistic regression for both cities. According to the results, t _o is the most important explanatory variable affecting occupants’ interactions with windows in monitored residences. The best multivariate linear logistic model result from the “backward selection” procedure based on “Akaike Information Criterion” (AIC) includes t _o, RH, V _s and C _p as explanatory variables, which implied that outdoor air quality, represented by C _p, has become a concern affecting Chinese residents’ interactions with windows.     

Predicting deformation of PVD improved deposit under vacuum and surcharge loads

A series of modified triaxial tests was conducted to investigate the deformation characteristics of mini-prefabricated vertical drain (PVD) unit cells. The factors considered are the (1) magnitudes of surcharge load (p_s) and vacuum pressure (p_vac); (2) pre-vacuum consolidation period (t_va) before applying surcharge load; (3) surcharge loading rate (SLR); and (4) initial effective stress state in the specimens. Based on the test results, relationships between the coefficient of earth pressure (K_es) at the end of surcharge load application and the normalized horizontal and vertical specimen strains are established. Further, a method is proposed for estimating the value of K_es, and therefore the horizontal and vertical strains of the PVD improved soil layer subjected to combined vacuum pressure and surcharge load using loading conditions and basic soil properties. Finally, the proposed method was applied to a case history reported in the literature and good agreement between the field-measured and calculated lateral displacement and settlement was obtained, which suggesting that the proposed method can be a useful tool for designing preloading projects involving combined vacuum and surcharge loads.     

Effects of specimen density and initial cyclic loading history on correlation between shear wave velocity and liquefaction resistance of Toyoura sand

Some previous studies have shown a good correlation between the shear wave velocity, V_s, and the cyclic resistance ratio, CRR. Recently, however, a V_s-based liquefaction assessment method has become an alternative and supplementary method to the conventional N_SPT-based method. It is known that the CRR is influenced not only by the specimen density, but also by the soil fabric. Unfortunately, there are concerns that different combinations of the effects of the specimen density and the soil fabric may generate different relations between V_s and the CRR even if the tested specimens are of the same soil material. In the current study, a series of V_s measurements and undrained cyclic triaxial tests is performed on Toyoura sand specimens with different soil fabrics for three different specimen densities. The fabric of the specimens is varied by applying initial cyclic loading. The results of the V_s measurements indicate that the V_s of the specimen is affected by the initial cyclic loading histories, and the results of the undrained cyclic triaxial tests show that there is a good correlation between V_s and the CRR. However, the correlation varies depending on the specimen density even when the tested material is Toyoura sand only. In other words, the soil-type specific correlation between V_s and the CRR depends on the specimen density. Therefore, the results indicate that both V_s and the specimen density are necessary parameters for an accurate assessment of the CRR.     

Effects of lime addition on geotechnical properties of sedimentary soil in Curitiba,Brazil

The soil of the Guabirotuba geological formation(Paraná Basin, Brazil) has physico-mechanical properties which are not suitable for its utilization in pavement construction, in protection of hillsides and slopes, or as shallow foundation support. Treatment of this soil by lime addition would improve its usability. The present context intends to determine the ratio between the splitting tensile strength(q_t)and the unconfined compressive strength(q_u) of clayey soil in the metropolitan region of Curitiba City,which has been treated with different lime contents and curing times. The control parameters evaluated include lime content(L), curing time(t), moisture content(w), and ratio of porosity to volumetric lime content(η/L_v). It was observed that the q_t/q_u ratio is between 0.17 and 0.2 in relation to the curing time,and an exponential relation exists between them. Meanwhile, the unconfined compressive strength of lime-treated soil was found to be approximately four times the initial value.     

Swelling pressure of compacted MX80 bentonite/sand mixture prepared by different methods

Compacted bentonite/sand mixture have been considered as possible sealing/backfilling material in the deep geological disposal of high-level radioactive nuclear waste. The swelling pressure of the compacted bentonite/sand mixture is of significance in the design of the geological repository, which requires good consistency between data obtained by laboratory and field measurement. In this work, a series of swelling pressure tests were performed on compacted MX80 bentonite/sand specimens prepared by methods commonly adopted in laboratory (As-compacted, Transferred) and those mimicking the real block manufacturing process (Trimmed, Inserted). Results shown that with identical dry density (especially when the dry density was larger than 1.70 Mg/m³), largest swelling pressure was obtained in specimens prepared by method Inserted, followed by method As-compacted, Transferred and Trimmed. The distinct difference between the swelling pressure could be largely attributed to the effects of residual post-compaction lateral stress. More interestingly, specimens prepared by methods Trimmed and Transferred followed a similar swelling pressure-dry density relationship. From this point of view, method Transferred without causing mass loss, change of bentonite content and possible technological voids effect was recommended in lieu of method Trimmed for specimen preparation in laboratory.     

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.     

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.     

Hydromechanical behavior of unsaturated expansive clay under repetitive loading

Compacted layers of expansive soils are used in different engineering projects,such as subgrades,engineered clay barriers,and buffers for radioactive waste disposal.These layers are exposed to a variety of stresses and wetting conditions during field serviceability.Coupling between hydraulic and mechanical repeated loading provides insight understanding to the induced progressive deformation of expansive clay.This study was conducted to investigate the hydromechanical behavior of unsaturated compacted expansive clay under repeated loadingeunloading (RLU) conditions.Two series of onedimensional (1D) oedometer tests were conducted under controlled matric suction up to 1500 k Pa using the axis translation technique (Fredlund soil-water characteristic curve device,SWC-150).The first test series was carried out at different levels of controlled matric suction for non-repeated loading eunloading (NRLU) cycles.RLU cycles were applied in the second test series at different repetitivestress levels and under different levels of matric suction.The results indicated increasing axial wetting strain ε_a(s),axial swell pressure s_s(s),compression index C_c(s),and swell index C_s(s) with suction reduction.The estimated loadecollapse (LC) curves obtained from NRLU series (LCN) and RLU series (LCR)indicated increasing yield stress s_y(s) with increasing suction.This is attributed to the developed apparent cohesion between soil particles,which in turn rigidifies the material response.Applying repetitive loading induced a notable reduction of compression index C_c(s) at the same level of suction,whereas swell index C_s(s) seems to be independent of repetitive loading.Finally,repetitive loading exceeding initial yield stresses results in plastic hardening and,hence,enlargement of yield stress locus(i.e.LC_R curve).     

Internal stability analysis of geocell-reinforced slopes subjected to seismic loading based on pseudo-static approach

Seismic stability analysis of geocell-reinforced slopes (GRSs), considering shear and moment strength in addition to tensile resistance for geocells, is a novel topic for which scarce studies are found in the literature. Despite few available studies, an analytical approach is presented in this study to investigate the seismic internal stability of GRSs, employing the pseudo-static method based on a limit state approach. Results are given in terms of conventional design charts representing the required total strength and critical length of geocells. The results show that with increasing the horizontal seismic acceleration (k_h), the internal stability degenerates since the required strength and critical length of geocells increase. For GRSs subjected to greater k_h, the effect of increasing the vertical seismic component (k_v) on increasing the required strength and length of geocells is more considerable than those subjected to lower k_h values. Parametric analyses are conducted, under various seismic conditions, to investigate the effect of increasing the geocell height and raising the number of geocell layers, leading to the reduction in the required strength and length of geocells. Such effects are found to be dependent on the parameters such as the intensity of seismic excitation, material properties and geometry of slope.