As increasing amounts of non-plastic silt are added to a sand, its classification transitions from sand to silty sand to sandy silt and eventually to silt. This transition leads to a fundamental change in the soil’s behavior from sand-like to silt-like with a corresponding increase in compressibility and decrease in both shear strength and resistance to liquefaction. Numerous studies have shown that this change in behavior occurs over a relatively narrow range of silt contents. This range is referred to by several names in the literature, including threshold fines content (TFC). The TFC represents the silt content at which the soil begins to transform from a sand matrix, in which the silt particles are entirely contained in the voids between the sand grains, to a silt matrix that contains isolated sand grains. Below the TFC, the soil behaves essentially as a sand; above the TFC the soil behaves essentially as a silt.
While the concept and importance of the TFC has been increasingly recognized over the last 20 years, several aspects of it have not been widely discussed in the literature. This paper will focus on four of these aspects: the existence of both an upper-bound and a lower-bound TFC for a given soil, the range and distribution of TFC upper- and lower-bound values for natural soils, the effect of relative density on the TFC, and the behavior of soils with fines contents between the upper-bound and lower -bound TFC.

As part of a foundation design for a structure supporting a new fire protection system at the Norcan Oil Terminal, a geotechnical investigation campaign satisfying the needs of the project was carried out. Following the campaign that included different sampling and testing methods (drilling, field vane test and CPT), the interpretation of field and laboratory data results was undertaken and allowed the design of deep foundations. The dimensioning of the piles supporting the fire protection structure was carried out and optimized based on the structural loading cases and ground conditions. Pile loading tests were also carried out which validated the design. All the steps taken, validation of the calculations using in situ tests and the dimensioning methods used are presented.

Excavation support systems are often located above the ground water table (GWT) where the soil is typically in an unsaturated state. The presence of capillary suction within the vadose zone (i.e. above the GWT) contributes towards an increase in the soil shear strength and stiffness. However, current design procedures ignore the contribution of capillary suction and assume saturated soil conditions. This is because reliable prediction of the mechanical behavior of unsaturated soils is challenging. However, extending saturated soil mechanics principles result in an erroneous design for excavation support systems, especially in semi-arid and arid regions, where the GWT is typically at a greater depth. In this paper, a simple numerical technique is proposed to investigate the performance of cantilever diaphragm walls in unsaturated soils. The only information required in addition to the conventional saturated soil properties is the soil water characteristic curve. Numerical analysis is carried out with SIGMA/W to determine deformations and wall straining actions. The results of the present study suggest that ignoring capillary suction results in a conservative design. In addition, estimates of deformations, forces and moments are erroneous. This study is of interest for practicing engineers as it provides a simple yet reliable approach for the rational design of excavation support systems extending mechanics of unsaturated soils.

A concrete plate supported directly by the soil continuum is a very common construction form. The response of the plate when it carries external load is influenced by the soil, and the response of the soil is also influenced by the action of the plate under the load. Thus, developing a
sub-grade model for soil-structure interaction problem is essential in order to predict the response of both components of the system and arrive at an optimum design.

Many sub-grade models have been developed in order to improve on the inherent lack of shear interaction among the individual springs found on the long-enduring model of Winkler. These models still have shortcomings with the nature of simplifying assumptions they make to ease the mathematical equation. Recently a generalized model is presented for a sub-grade idealized as an elastic layer overlying a rigid base. In contrast to previous works no stresses, strains or displacements are neglected a priori.

The objective of this work is implementing, verifying and calibrating this improved continuum-based generalized sub-grade model in the analysis of strip plates on an elastic foundation. The governing differential equation of a strip plate on elastic foundation is formulated. Then, closed form particular solutions, when using Winkler type and Kerr equivalent Pasternak models, are obtained by considering different boundary conditions of long and short length of a strip plate, under different loading conditions. Microsoft excel programs are written for the computation of deflection, moment and shear force. The sub-grade models will be calibrated using Finite Element based Plaxis 2D software. Lastly, numerical illustration is provided using these models in comparison with Plaxis 2D model for long and short strip plates subjected to selected symmetrical loading conditions. Thus, the findings of the calibrated models can be used in routine analysis of strip plate on elastic foundation