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.     

海南海花岛软基处理工程中真空预压法的改进与机理分析

疏浚淤泥这种超软土地基目前主要采用排水固结法进行加固,然而,工程实践发现,淤堵是一个重要的问题,新型防淤堵材料和施工工艺是研究的热点。依托恒大海南海花岛真空预压处理工程,引入一种应用于超软土地基处理的新型防淤堵真空预压法。该方法是直排式真空预压方法的进一步改进,将防淤堵排水板取代传统排水板,在连接方式上,将无孔钢丝软管取代了水平波纹管,也将传统的包扎捆绑改进成了三通密封接头,枪钉固定。结合施工动态监测和加固后检测,表明了该方法的加固效果和实用性。在此基础上,结合已有的室内模型试验研究,阐述其抽真空过程中在排水板附近防淤堵的原理和改善机制。     

Experimental study on the clogging effect of dredged fill surrounding the PVD under vacuum preloading

Clogging effect surrounding prefabricated vertical drains (PVDs) is a typical problem when vacuum preloading is applied to a dredged fill foundation. A large-scale model test was designed to clarify the cause and mechanism of the clogging effect, and the basic physical and mechanical parameters of the soil in the clogging zone were tracked during the test. The results demonstrated that a clogging zone was formed around the PVD in the early stage of improvement with conventional vacuum preloading, and the boundary of the clogging zone was approximately 0.2–0.4 of the boundary radius. The clogging zone surrounding the PVD was formed because of the overall movement of the soil toward the PVD under the high vacuum pressure gradient, rather than fine particle migration. The soil in the clogging zone exhibited permeability anisotropy and equivalent ‘smear’ effect. The permeability ratio (k_h/k_v) was less than 1, and the ratio of horizontal permeability coefficients at the test distances of 45 cm and 10 cm were 9.6 at a depth of 20 cm and 8.9 at a depth of 80 cm. An analysis of the microstructure of the soil in the clogging zone demonstrated that the clay particles tended to be vertically oriented. The re-orientation of the clay particles reduced the horizontal permeability coefficient and led to the permeability anisotropy of the soil in the clogging zone. Thus, decrease in the horizontal permeability coefficient and equivalent ‘smear’ effect of the soil in the clogging zone affect the consolidation of dredged fill, which leads to the clogging effect. The permeability anisotropy also slightly affects consolidation.     

Observational method for field performance of prefabricated vertical drains

An exponential formula is used to best-fit theoretical and measured time–settlement (or excess pore pressure) data over the full range of consolidation. The formula fits well theoretical consolidation solutions and measured data regardless of using the incompletely consolidated data, and it is possible to reliably predict the ultimate values. This result has a different trend from those of the hyperbolic and Asaoka (1978) methods. Thus the coefficients of horizontal consolidation and the mobilised discharge capacity q_w(mob) can be expressed in terms of parameters of the exponential formula corresponding to the measured data and the theoretical solutions. The application of the proposed method to six case records on three construction sites (with a maximum drainage path l_m of 7−50 m) indicates that the coefficient of horizontal consolidation for the ideal condition are likely to be used to reconstruct the monitored time–settlement curve and also to adjust the hydraulic and consolidation properties of each monitored point. Based on back-analysis, the mobilised and required discharge capacity for a preliminary design guideline are recommended as: q_w(mob) = (1–5)k_hl_m² and q_w(req) = 19.63k_hl_m², where k_h is the horizontal permeability of soil.     

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.     

PVD improvement combined with surcharge and vacuum preloading including simulations

This paper presents the study of PVD improved reconstituted specimen with and without vacuum preloading on large-scale consolidometer in the laboratory tests. Subsequently, the results of the laboratory tests were analyzed and simulated by 2D (axisymmetric) finite element method (FEM) to back-analyze and confirm the related design parameters which were used further in subsequent numerical experiments. The laboratory test results indicated that the increased hydraulic conductivity in the smear zone of PVD with vacuum preloading (Vacuum-PVD) resulted in the increase in the coefficient of horizontal consolidation (Ch) by 16% as well as the decrease in the ratio between the horizontal hydraulic conductivity of the undisturbed zone (Kh) to the horizontal hydraulic conductivity in the smear zone (Ks) or (Kh/Ks) of about 10%. The Vacuum-PVD and PVD only have the same settlement magnitudes with similar equivalent loads.     

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.     

An approach to shorten the construction period of high embankment on soft soil improved with PVD

Prefabricated Vertical Drains (PVDs) are being used to accelerate the consolidation of subsoil for construction of high embankment on soft ground. The construction is carried out in stages and height of the first stage construction depends on in-situ undrained shear strength. Each subsequent stage construction is carried out after completion of either 90% primary consolidation or percent consolidation at inflection point. The height of subsequent stages depends upon the gain in undrained strength of subsoil. In this paper, the authors have advocated an approach to shorten the construction period for high embankments. In this approach, the first stage construction would be carried out based on the in-situ undrained shear strength of subsoil. Instead of waiting for 90% primary consolidation or percent consolidation at inflection point, the embankment is raised in layers of 0.2 m thickness. Based on the time required to gain strength with the construction of the 0.2 m layer, the waiting period is determined for each subsequent layers. The waiting period depends on soil parameters such as subsoil thickness, C_r/C_v ratio and different PVD factors viz. smear, drain spacing and well resistance, pattern of laying of PVD, etc. Using this approach, there would be increase in the consolidation rate and overall reduction in the construction period. A typical practical example has been solved to demonstrate the usefulness of this approach over the two conventional methods. For a 4.5 m high embankment, it is observed that waiting period is reduced by 77% and 43% as compared to the 90% primary consolidation method and inflection point method respectively.     

Effect of FeCl3-conditioning on consolidation property of sewage sludge and vacuum preloading test with integrated PVDs at the Changan landfill,China

Extremely soft sewage sludge lagoon exists in many landfills of municipal solid wastes in China. A two-staged in-situ treatment method, vacuum preloading on the conditioned sludge combined with subsequent cement solidification treatment, was proposed to improve the sludge ground. In this study, vacuum filtration tests and oedometer tests were firstly performed to investigate the effect of FeCl₃-conditioning on the consolidation performance of sewage sludge. Then a pilot test was carried out in the field to examine the effectiveness of vacuum preloading on the FeCl₃-conditioned sludge by using an integrated type of PVD. The test results demonstrated that the coefficient of consolidation increased with more percentage of FeCl₃ added. For the sludge sample conditioned with 10% of FeCl₃ (dry basis), the coefficient of consolidation increased to 4 × 10⁻⁵–1.2 × 10⁻⁴ cm²/s, which was four-nine times greater than that of the non-conditioned sludge. 68 days' vacuum preloading on the 3.2 m thick sludge pit resulted in a magnitude of 70.9% in the average degree of consolidation and a reduction by 47.5% in the total volume of sludge. The water content of the sludge reduced from the initial 860% to 140%–450%. The undrained shear strength of the consolidated sludge with a water content of 140% was about 10 kPa, and decreased sharply with a further increase in water content. The integrated type of PVDs used in the pilot test showed a good performance with respect to the discharge capacity and the resistance against chemical corrosion. Comparative analyses show that the proposed two-staged in situ treatment method has advantages over the sole cement solidification method with respect to cost saving and environment protection.     

Innovative thermal technique for enhancing the performance of prefabricated vertical drain during the preloading process

This paper examines the effect of raising the temperature of soft Bangkok clay, up to 90 °C, on the performance of the prefabricated vertical drain (PVD) during the preloading process. The effect of temperature on the engineering behavior of soft Bangkok clay was first investigated using a modified triaxial test apparatus and flexible wall permeameter which can handle temperatures up to 100 °C. The results of the triaxial tests on clay specimens demonstrate that raising the soil temperature increases its shear strength, under drained heating condition, as well as its hydraulic conductivity. In addition, large oedometer tests were performed to investigate the performance of PVD at elevated temperatures. The response of the soil sample with PVD for the thermal consolidation path which involved increasing the soil temperature at constant vertical effective stress condition and the thermo-mechanical path which involved increasing simultaneously both the soil temperature and the vertical effective stress were investigated. The consequent results indicated that the thermo-mechanical path shows promising results regarding the consolidation rate. For both reconstituted and undisturbed specimens, higher consolidation rate was observed for the soil specimen with PVD loaded under elevated temperature. This behavior can be attributed to the increase in the soil hydraulic conductivity as the soil temperature increases. Therefore, raising the soil temperature during the preloading period can enhance the performance of the PVD, particularly, by reducing the drainage retardation effects due to the smear zone around PVD.     

Modeling monochloramine loss in the presence of natural organic matter

A comprehensive model describing monochloramine loss in the presence of natural organic matter (NOM) is presented. The model incorporates simultaneous monochloramine autodecomposition and reaction pathways resulting in NOM oxidation. These competing pathways were resolved numerically using an iterative process evaluating hypothesized reactions describing NOM oxidation by monochloramine under various experimental conditions. The reaction of monochloramine with NOM was described as biphasic using four NOM specific reaction parameters. NOM pathway 1 involves a direct reaction of monochloramine with NOM (k_doc1=1.05×10⁴-3.45×10⁴ M⁻¹ h⁻¹). NOM pathway 2 is slower in terms of monochloramine loss and attributable to free chorine (HOCl) derived from monochloramine hydrolysis (k_doc2=5.72×10⁵-6.98×10⁵ M⁻¹ h⁻¹), which accounted for the majority of monochloramine loss. Also, the free chlorine reactive site fraction in the NOM structure was found to correlate to specific ultraviolet absorbance at 280 nm (SUVA₂₈₀). Modeling monochloramine loss allowed for insight into disinfectant reaction pathways involving NOM oxidation. This knowledge is of value in assessing monochloramine stability in distribution systems and reaction pathways leading to disinfection by-product (DBP) formation.     

Heat balance and tracer gas technique for airflow rates measurement and gaseous emissions quantification in naturally ventilated livestock buildings

Experiments were performed to study the airflow rates (AFRs) in a naturally ventilated building through four summer seasons and three winter seasons. The AFRs were determined using heat balance (HB), tracer gas technique (TGT) and CO₂-balance as averages of the values of all experiments carried out through the different seasons. The statistical analyses were correlation analysis, regression model and t-test. Continuous measurements of gaseous concentrations (NH₃, CH₄, CO₂ and N₂O) and temperatures inside and outside the building were performed. The HB showed slightly acceptable results through summer seasons and unsatisfactory results through winter seasons. The CO₂-balance showed unexpected high differences to the other methods in some cases. The TGT showed reliable results compared to HB and CO₂-balance. The AFRs, subject to TGT, were 0.12 m³ s⁻¹ m⁻², 1.15 m³ s⁻¹ cow⁻¹, 0.88 m³ s⁻¹ LU⁻¹, 56 h⁻¹, 395 m³ s⁻¹ and 470 kg s⁻¹ through summer seasons, and 0.08 m³ s⁻¹ m⁻², 0.83 m³ s⁻¹ cow⁻¹, 0.64 m³ s⁻¹ LU⁻¹ 39 h⁻¹, 275 m³ s⁻¹ and 328 kg s⁻¹ through winter seasons. The AFRs are not independent values, rather they were estimated for specific reference values, which are: area, cow and LU as well as rates. The emission rates through summer seasons, subject to TGT, were 9.4, 40, 3538 and 2.3 g h⁻¹ cow⁻¹; and through winter seasons were 4.8, 19, 2332 and 2.6 g h⁻¹ cow⁻¹, for NH₃, CH₄, CO₂ and N₂O, respectively.     

考虑板土相互作用的排水板通水特性试验研究

为研究排水板在实际工况下的通水特性,研制排水板纵向通水量测试新仪器,采用室内真空预压模型试验、堆载预压模型试验和直接充灌淤泥等方法来制作板土单元体(试样),并开展板土单元体(试样)通水能力测试。试验结果表明:无论是直接充灌淤泥法、堆载预压法还是真空预压法,高性能排水板通水量均大于现行规程试验结果,而普通排水板通水量均小于现行规程试验结果,现行规程方法高估了真空预压后普通排水板的通水能力。对比结果表明:高性能排水板在堆载预压后通水能力与真空预压后通水能力接近,而普通排水板在堆载预压后通水能力明显高于真空预压后通水能力。因此,对于变形大且固结时间长的新近吹填淤泥地基加固工程应优先选用高性能排水板。     

Influence of biofilm on the transport of fullerene (C60) nanoparticles in porous media

The significance of biofilm on fullerene C₆₀ nanoparticles transport and deposition were examined both in porous media and quartz crystal microbalance with dissipation (QCM-D) systems under a variety of environmentally relevant ionic strength (1-25 mM in NaCl and 0.1-5 mM in CaCl₂) and flow conditions (4-8 m day⁻¹). The magnitudes of deposition rate coefficients (k_d) were compared between porous media with and without biofilm extracellular polymeric substances (EPS) coating under equivalent fluid velocities and solution chemistries. The observed k_d were greater in porous media with biofilm EPS coating relative to those without biofilm EPS coating across the entire solution ionic strengths and fluid velocities examined, demonstrating that the enhancement of C₆₀ deposition by the biofilm EPS coating is relevant to a wide range of environmental conditions. This greater deposition was also observed on silica surfaces with biofilm EPS coating in QCM-D system. The results clearly showed that biofilm EPS have a great influence on C₆₀ deposition. Derjaguin-Landau-Verwey-Overbeek (DLVO) theory could not explain the enhanced C₆₀ deposition by biofilm EPS. Biochemical and physical characteristics of biofilm EPS were responsible for the increased C₆₀ deposition.     

Removal of Pharmaceutical and Personal Care Products (PPCPs) under nitrifying and denitrifying conditions

The contribution of volatilization, sorption and transformation to the removal of 16 Pharmaceutical and Personal Care Products (PPCPs) in two lab-scale conventional activated sludge reactors, working under nitrifying (aerobic) and denitrifying (anoxic) conditions for more than 1.5 years, have been assessed. Pseudo-first order biological degradation rate constants (k_biol) were calculated for the selected compounds in both reactors. Faster degradation kinetics were measured in the nitrifying reactor compared to the denitrifying system for the majority of PPCPs. Compounds could be classified according to their k_biol into very highly (k_biol > 5 L g_SS⁻¹ d⁻¹), highly (1 < k_biol < 5 L g_SS⁻¹ d⁻¹), moderately (0.5 < k_biol < 1 L g_SS⁻¹ d⁻¹) and hardly (k_biol < 0.5 L g_SS⁻¹ d⁻¹) biodegradable.Results indicated that fluoxetine (FLX), natural estrogens (E1 + E2) and musk fragrances (HHCB, AHTN and ADBI) were transformed to a large extent under aerobic (>75%) and anoxic (>65%) conditions, whereas naproxen (NPX), ethinylestradiol (EE2), roxithromycin (ROX) and erythromycin (ERY) were only significantly transformed in the aerobic reactor (>80%). The anti-depressant citalopram (CTL) was moderately biotransformed under both, aerobic and anoxic conditions (>60% and >40%, respectively). Some compounds, as carbamazepine (CBZ), diazepam (DZP), sulfamethoxazole (SMX) and trimethoprim (TMP), manifested high resistance to biological transformation.Solids Retention Time (SRT_aerobic >50 d and <50 d; SRT_anoxic >20 d and <20 d) had a slightly positive effect on the removal of FLX, NPX, CTL, EE2 and natural estrogens (increase in removal efficiencies <10%). Removal of diclofenac (DCF) in the aerobic reactor was positively affected by the development of nitrifying biomass and increased from 0% up to 74%. Similarly, efficient anoxic transformation of ibuprofen (75%) was observed after an adaptation period of 340 d. Temperature (16-26 °C) only had a slight effect on the removal of CTL which increased in 4%.     

Estimating Undrained Shear Strength of Soft Clay Ground Improved by Pre-Loading with PVD — Case History in Bangkok—

The installment of pre-fabricated vertical drain (PVD) combined with pre-loading is an efficient way to gain the prescribed strength and compressibility of very soft clay over a short period of time. This note describes case histories of such improvement performed in Bangkok. The change in undrained shear strength, s_u, before and after the ground improvement was investigated at two sites in Bangkok area by means of field vane test (FVT) and flat dilatometer test (DMT). The results of FVT showed that the s_u value of soft Bangkok clay increased over nearly the full depth down to the tip of the PVDs. It is also demonstrated that DMT may be a useful tool for promptly assessing the change of s_u in soft clay due to pre-loading. Applicability of the SHANSEP method in estimating the profile of s_u with depth in recently overconsolidated clay ground after pre-loading is also discussed.     

Full-Scale Embankment Consolidation Test using Prefabricated Vertical Thermal Drains

In this study the field feasibility of an innovative thermal technique to improve the performance of prefabricated vertical drains (PVD) used in conjunction with the preloading ground improvement method is investigated. For this purpose, two identical 6.0 m high full-scale test embankments for preloading were constructed over the soft Bangkok clay where a conventional PVD system was installed underneath one embankment and a novel prefabricated vertical thermo-drain (PVTD) system was utilized for the other. The PVTD unit consists of a U-tube made of cross-linked polyethylene plastic (PEX) that is attached to a conventional PVD unit. Preheated water at about 90°C is circulated through the attached U-tube to raise the soil temperature underneath the PVTD embankment. The behavior of the two test embankments were compared in terms of excess pore water pressure and consolidation results. The comparison shows the advantage of a PVTD system over a conventional PVD system. The rate of consolidation increases significantly in the PVTD system due to the temperature effect on the hydraulic conductivity. Moreover, the embankment with the PVTD system generates more settlement due to the thermally induced irreversible contraction of saturated normally consolidated soft Bangkok clay.     

Bio-reaction of nitrobenzene with Microcystis aeruginosa: characteristics, kinetics and application

The bio-reaction of nitrobenzene (NB) with Microcystis aeruginosa was investigated at different initial algal densities and NB concentrations by performing static experiments. The results showed that the elimination of NB was enhanced by the bio-reaction, and the reaction rate varied as a function of the reaction time. Moreover, the reaction rate was significantly affected by the algal density and NB concentration. A kinetic analysis showed that the elimination of NB in a solution without algae appeared to be pseudo-first-order with respect to the NB concentration, whereas a first-order model was too oversimplified to describe the elimination of NB in a solution with algae. Assuming that different algal cells have the same effect on the bio-reaction under the same conditions, the bio-reaction can be described as dC_NB = −k₀C_A^mA_NBⁿdt (where k₀ is the reaction rate constant, C_A is the algae density and C_NB is the concentration of NB). When the growth of algae was not considered, the value of k₀, m and n were 8.170 × 10⁻⁴, 0.5887 and 1.692, respectively. Alternatively, when algae were in the exponential growth phase, the value of k₀, m and n were 1.6871 × 10⁻⁵, 0.7248 and 2.5407, respectively, according to a nonlinear fitting analysis. The kinetic model was also used to elucidate the effect of nutritional limitation on the bio-reaction.     

Performance of Band Shaped Vertical Drain for Soft Hai Phong Clay

Performance of band shaped prefabricated vertical drain (PVD) installed into soft Hai Phong clay with a 110 cm triangle arrangement is reported together with the engineering properties of the clay investigated by field and laboratory tests. Stationary piston sampling was carried out to obtain high quality undisturbed soil samples for laboratory tests and reliable engineering characteristics of the clay. It was assumed for the design of PVD spacing and preloading that the ratio of apparent value of horizontal coefficient of consolidation c_h(ap) to vertical coefficient of consolidation c_v is equal to 1.0. The settlement monitored in the field, which clearly showed that the actual settlement was faster than expected, resulted in the c_h(ap) value 1.5 times as much as c_v determined by the laboratory test.