1.6 Establishing the Geotechnical Model of a site

Establishing the geotechnical model of a site is a challenge for the Geotechnical Engineer. It requires profound knowledge of geomechanics theory, understanding of in situ and laboratory test procedures, experience and Engineering Judgement.

As engineering judgement plays perhaps the most important role in interpreting geotechnical site investigation results, every Geotechnical Engineer called to establish a geotechnical model of a specific area by interpreting the same test results, may end up with a different model. This is absolutely acceptable, provided that the model will lead to the safe and economical design of the structure under consideration.

Establishing a geotechnical model consists of identifying soil layers (called “geotechnical layers” or “geotechnical units”) that, for engineering purposes, can be assumed to behave uniformly when subjected to different actions. Each borehole log (Figure 1.8) will perhaps include several, even dozens of different descriptions of soil layers, with thickness ranging from a few meters to a few centimetres. Using the USCS (Unified Soil Classification System) descriptions, examining the borehole cores (and/or their photos) and the in situ/laboratory test results, the Geotechnical Engineer will try to unify these layers, towards creating a simple yet realistic model. Keep in mind that, we do not want to end up neither with a model that includes too many geotechnical layers, for which we do not have enough data to quantify their geotechnical behaviour, nor with too few geotechnical layers, as too crude layers will not behave uniformly. Balancing between a too detailed and a too crude layer discretisation is simply a matter of experience, and good understanding of the behaviour of geomaterials.

The above concept is further explained in the following example, which presents the establishment of the geotechnical model in an area where a bridge is to be constructed.