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Due to the effect of structure, Bangkok clay is stable in a metastable state. Its void ratio, e, is the summation of the void ratio sustained by the intrinsic fabric, eR, and the additional void ratio due to the structure, es. The intrinsic state line (eR versus log σ′v, where σ′v is the effective vertical stress) is developed in terms of the void ratio at the liquid limit, eL. At the post-yield state, es is inversely proportional to σ′v. The residual additional void ratio, esr, which cannot be eliminated by the increase in effective vertical stress, is constant at about 0.20 for soft Bangkok clay and 0.12 for medium stiff Bangkok clay. From these findings and the ideal condition of zero compression at the pre-yield state, the field yield stress and field compression curve can be assessed. The undrained shear strength is directly related to the field yield stress, since both reflect the structure. The soil structure does not significantly influence the permeability. The permeability of the clay in structured and destructured states is identical under the same void ratio and can be determined from the generalized state parameter, e/eL. These observations result in a simple and practical method for assessment of the engineering properties of natural Bangkok clay.Key words: Bangkok clay, destructured state, compression, intrinsic state line, permeability, structured state, vane shear strength.

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This paper presents a case history on the failure of Suranivet 9, a student dormitory in the campus of Suranaree University of Technology (SUT), Thailand. The dormitory encountered excessive diŠerential settlement due to the variation in soil proˆle. Part of the building was underlain by very stiŠ to hard SUT silty clay and part by loose clayey sand. Underpinning to extend the foundations down to stable stratum was employed to strengthen bearing capacity and minimize settlement. The underpinning design and procedure were summarized. In practice, the static formula was used for the preliminary micro-pile design (selection of pile section and length for diŠerent loads and soil proˆles). The undrained shear strength (S u ) of SUT silty clay was approximated using the Stress History and Normalized Soil Engineering Properties (SHANSEP) technique and standard penetration number (N ). Theˆnite element method was employed to predict the load-settlement curve of the micro-pile. Modiˆed cam clay model was proved as a suitable model for this prediction. The measured settlements of the underpinned foundations after one year service were less than 0.5 mm. This small settlement guarantees the stability of the underpinned structure. It is also found that the settlement ratio (ratio of the measured settlement of underpinned foundations to the predicted settlement of single micro-pile) varied from 0.7 to 3.0.

Compaction curves from 16 coarse- and 9 fine-grained soils, which cover all soil types classified by the Unified Soil Classification System are analyzed to develop the Modified Ohio’s curves. For all soils, the relationships between water content and degree of saturation on both the dry and the wet sides of optimum are represented by power functions. Their compaction curves under standard Proctor energy follow the Ohio’s curves. Optimum degree of saturation, ODS, of coarse-grained soils is lower than that of fine-grained soils. However, for a given soil, the ODS is practically the same for different compaction energies, E. Even though compaction characteristics (optimum water content, OWC, and maximum dry unit weight, γd max) are different for different soils, their relationship between normalized OWC/OWCst and E is practically the same, where OWCst is the OWC at standard Proctor energy. Based on this finding, the Modified Ohio’s curves are introduced under compaction energy levels of the half standard, half modified, and modified Proctor energies. The verification of the Modified Ohio’s curve is also illustrated in this paper. These curves are useful in rapid estimation of laboratory compaction curves from a single set data of dry unit weight and water content.