Canada has two national civil codes of practice that include geotechnical design provisions: the National Building Code of Canada and the Canadian Highway Bridge Design Code. For structural designs, both of these codes have been employing a Load and Resistance Factor format embedded within a Limit States Design framework since the mid-1970's. Unfortunately, Limit States Design in geotechnical engineering has been lagging well behind that in structural engineering for the simple fact that the ground is by far the most variable (and hence uncertain) of engineering materials. Although the first implementation of a geotechnical limit states design code appeared in Denmark in 1956, it was not until 1979 that the concept began to appear in Canadian design codes, i.e., in the Ontario Highway Bridge Design Code which later became the Canadian Highway Bridge Design Code (CHBDC). The geotechnical design provisions in the CHBDC have evolved significantly since their inception in 1979.2 advances appearing in the CHBDC along with the steps taken to calibrate its recent geotechnical resistance and consequence factors.

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Current foundation design practice for serviceability limit states involves proportioning the foundation to achieve an acceptably small probability that the foundation settlement exceeds some target maximum total settlement. However, it is usually differential settlement that leads to problems in the supported structure. The design question, then, is how should the target maximum total settlement of an individual foundation be selected so that differential settlement is not excessive? Evidently, if the target maximum total settlement is increased, the differential settlement between foundations will also tend to increase, so that there is a relationship between the two, although not necessarily a simple one. This paper investigates how the target maximum total settlement specified in the design of an individual foundation relates to the distribution of the differential settlement between two identical foundation elements, as a function of the ground statistics and the distance between the two foundations. A probabilistic theory is developed, and validated by simulation, which is used to prescribe target maximum settlements employed in the design process to avoid excessive differential settlements to some acceptable probability.

Although the settlement of deep foundations (piles) is not generally a concern if the piles are driven to refusal, settlement can become a design issue if no stiff substratum is encountered. This paper investigates the reliability-based design factors required for the serviceability limit state design of deep foundations. The goals of the paper are first to develop a probabilistic deep foundation model, which includes the effects of spatial variability and which is validated by simulation, and second to recommend the geotechnical resistance factors required to achieve specified target reliability indices against excessive settlement of deep foundations.