Several dispersive soil dams in Northeast Thailand have failed through surface and piping erosion. These dispersive soils are inorganic sandy, silty clay or clayey silt of low plasticity with a high content of dissolved sodium in the pore water. The addition of bentonite has no effect on the dispersivity. On the other hand, the addition of from 20% to 30% Bangkok clay transforms the soil into non-dispersive. The crumb test initially indicates the dispersivity of the soil. However, pinhole test and chemical tests are necessary to confirm the dispersivity. The liquid and plastic limits increase linearly with increasing percentage of Bangkok clay. The maximum dry density decreases and the optimum moisture content increases with increasing percentage of Bangkok clay.

This study examines the bearing performance and load-transfer mechanisms of triaxial geogrid-reinforced unpaved roads over soft clay subgrades using large-scale testing and three-dimensional finite-element modeling. Eleven plate-bearing tests were performed on geogrid-reinforced unpaved roads over compacted high-plasticity (CH) and low-plasticity (CL) clays, considering granular base thicknesses of 0.05-0.20 m. Numerical simulations using the Hardening Soil model reproduced load-settlement response, stress redistribution, failure mechanisms, and geogrid strain development. Unreinforced granular bases increased bearing stress at a normalized settlement of 0.1B by approximately 2.0-3.5 times compared with soft clay alone. With triaxial geogrid reinforcement, bearing stress at 0.1B increased to about 520-860 kPa for CH clay and 650-1000 kPa for CL clay. Granular bases, with and without geogrid reinforcement, reduced vertical stress transmitted to the subgrade by approximately 23-77%, with consistently higher stress reductions observed for the CL subgrade due to its greater stiffness and lower plasticity, which promote more effective stress diffusion within the granular base. Failure modes shifted from localized punching to distributed deformation, while geogrid strains remained low at serviceability settlements, typically below 200 mu epsilon. Numerical results closely matched experimental observations, supporting performance-based design of geogrid-reinforced unpaved roads over soft clay.

This book outlines the problems commonly encountered during infrastructure construction on soft and subsiding ground in lowland environments and their solutions in terms of soil/ground improvement techniques. For soft and cohesive soils on subsiding environments, ground improvement can be done by reinforcement, by addition of chemical additives, and by drainage or dewatering. The soil fills above ground can also be improved by reinforcing with geogrids, steel grids and/or geotextiles. The book is divided into the following sections: (1) ground improvement by vertical drains; (2) ground improvement using granular piles; (3) lime/cement deep mixing method; (4) mechanically stabilized earth (mse) embankments/walls.

This paper applies a modified vacuum-PVD system combined with the embankment surcharge and multiple field distributed air-water separation sub-tanks to improve soft Bangkok clay. The system efficiently generated and maintained vacuum pressures from − 80 to -90 kPa at the top of the embankment surcharge. The case study involved the construction of the third runway of Suvarnabhumi International Airport Thailand, which was built on soft Bangkok clay deposits. The back-calculation results of monitored data from the second improvement of the third runway extension were compared with the corresponding results of the first improvement of the taxiway extension, including the evolutions of settlements, excess pore pressures, and flow parameters. The second improvement with 29 kPa surcharge had lower settlements but recorded higher pore pressures and flow parameters. The first improvement with 34 kPa surcharge registered higher settlements but indicated lower pore pressures and flow parameters. The unit weights, water contents, liquid limits, compression indices, and void ratios were reduced. The undrained shear strengths, maximum past pressures, and over-consolidation ratio were increased. Inward lateral movements perpendicular to the surcharge embankment due to higher vacuum pressures than the lower embankment surcharge load were observed. The surcharge embankment combined with the modified vacuum-PVD system using an airtight membrane and horizontal prefabricated drain has been successfully applied to improve soft Bangkok clay.

The influence of the initial improvement through prefabricated vertical drains (PVD) and surcharge embankment on the second improvement by vacuum PVD has not been comprehensively explored in the existing literature. The expansion of the Second Bangkok International Airport (Suvarnabhumi Airport) underwent initial treatment with PVD and surcharge, followed by further soft ground improvement with vacuum PVD, which serves as a valuable case study for understanding the impact of the initial PVD improvement. This study highlights the effects of the initial PVD improvement through a comparison of two case studies: the first improvement by vacuum PVD on Taxiway Extension D at Zone 27 and the second vacuum PVD improvement on the Third Extension North at Zone 7. Extensive back analysis results reveal significantly higher flow parameters and reduced settlements for the second improvement at Zone 7 compared to the first improvement at Zone 27. The water contents, void ratios, and compression indices decreased while the undrained shear strengths and maximum past pressures increased. These results demonstrated the effectiveness of the vacuum PVD improvement with a modified field-distributed air-water separation system. Numerical studies further corroborated these findings and confirmed the observed crack patterns at the surface resulting from inward lateral movement.

This paper presents a case study in which wet and top feed vibro replacement stone columns are used to support a 20-m-high fuel tank built on loose to medium sand deposits. The design considerations, installations, applications, field tests, correlations, and subsequent three-dimensional finite element analyses are presented. The soil profile of the case study comprised approximately 8 to 12 m thick loose to medium-dense silty sand layers with a high groundwater table. The 1-m-diameter stone columns were installed in a triangular pattern with a spacing of 2.1 m. The improvement depths were 8, 11, and 12 m. After construction was completed, quality assurance of the stone column installation was performed via various field tests. Based on the hydro test results, the improved ground could carry an allowable bearing capacity of 200 kPa transferred from the 20-m-high fuel tank and exhibited a maximum settlement of less than 50 mm, which met the design criteria of this project. Owing to reasonable quality control during stone column installation, the tilt of the tank caused by the nonuniformity of the improved sands was less than 10 mm. The enhanced strength ratio of the surrounding sands was 2 the densification effect. Furthermore, three-dimensional finite element simulations were performed. The results were consistent with the field measurements, verifying the effectiveness of the ground improvement technique. This case study demonstrates the practical application and benefits of vibro-replacement stone columns in enhancing bearing capacity, minimizing settlement, and improving overall foundation stability in challenging geotechnical environments, contributing to future engineering practices for similar projects.

This manuscript presents the case study of soft Bangkok clay improvement and simulations using combined vacuum pressures with pre-existing surcharge embankment preloading, including installations of air-tight membranes and horizontal prefabricated drains connected to the top of the PVDs as well as to the vacuum system with a modified air-water separation system. The 17-m-long PVDs were installed from the top of the embankment in a triangular pattern at a spacing of 0.9 m. Monitoring instruments were installed to measure surface settlements, lateral movements, and pore water pressures in the soft clay layer. The subsequent analyses included settlement calculations, settlement predictions using observational methods, flow parameter back-calculations, soil property comparisons before and after improvement, and FEM simulations. The very soft to soft clay was transformed to medium-stiff clay because its undrained shear strengths and maximum past pressures increased, and its water contents, void ratios, and compression indices decreased. The analyses of the degree of consolidation utilized one-dimensional and observational methods as well as a numerical simulation for the prediction of consolidation settlements and comparison to the field data. The results illustrated the effectiveness of the modified vacuum-PVD system. The findings could be used for guidance of similar soft clay improvement projects.

Soil properties are known to have high spatial variability and often fluctuate with depth. The objective of this study was to investigate the effects of using different models to simulate the spatial variability of undrained shear strength (su) to calculate the failure probability of an embankment on soft ground. Two-dimensional random fields of su were generated based on one Gaussian and two non-Gaussian copulas, with stationary and nonstationary assumptions. Statistical parameters of su variation—mean, coefficient of variance, and scale fluctuation (correlation length)—were estimated from simulated and field data. Monte Carlo probabilistic analyses were performed on embankment stability based on both stationary and nonstationary random fields and all copula approaches; results showed more frequent embankment failures at low water levels in the embankment ditch. In particular, the nonstationary random field (su increases with depth) simulations more closely reflected real observed data, with higher probabilities of slope failure and lower mean factor of safety than the stationary random field simulations. Additionally, the non-Gaussian copulas provided simulated data that more accurately reflected observed field data, highlighting the importance of copula selection when characterizing soil parameter random fields.