OBSERVATIONAL METHOD: Construction of New or Underpinned Foundations

The observational method is an important tool that can be used for the design and construction of new or underpinned foundations. Concerning the observational method, Terzaghi and Peck (1967) state:

Design on the basis of the most unfavorable assumptions is inevitably uneconomical, but no other pro- cedure provides the designer in advance of construction with the assurance that the soil-supported structure will not develop unanticipated defects. However, if the project permits modifications of the design during construction, important savings can be made by designing on the basis of the most probable rather than the most unfavorable possibilities. The gaps in the available information are filled by observations during construction, and the design is modified in accordance with the findings. This basis of design may be called the observational procedure.

. . . In order to use the observational procedure in earthwork engineering, two requirements must be satisfied. First of all, the presence and general characteristics of the weak zones must be disclosed by the results of the subsoil exploration in advance of construction. Secondly, special provisions must be made to secure quantitative information concerning the undesirable characteristics of these zones during construction before it is too late to modify the design in accordance with the findings.

As mentioned earlier, the observational method is used during the construction of the foundation.

It is a valuable technique because it allows the geotechnical engineer, based on observations and testing during construction, to revise the design and provide a more economical foundation or earth structure. The method is often used during the installation of deep foundations, where field perfor- mance testing or observations are essential in confirming that the foundation is bearing on the appro- priate strata.

In some cases, the observational method can be misunderstood or misused. For example, at one project the geotechnical engineer discovered the presence of a shallow groundwater table and indicated in the feasibility report that the best approach would be to use the observational method, where the available information on the groundwater table would be supplemented by observations during construction. When the excavation for the underground garage was made, extensive groundwater control was required and an expensive dewatering system was installed. But the client had not anticipated the cost of the expensive dewatering system. The client was very upset at the high cost of the dewatering system because a simple design change of using the first floor of the building as the garage (i.e., above-grade garage) and adding an extra floor to the building would have been much less expensive than dealing with the groundwater. As the client stated: “if I had known that the below-grade garage was going to cost this much, I would never have attempted to construct it.”

As this case illustrates, the observational method must not be used in place of a plan of action, but rather to make the plan more economical based on observed subsurface conditions during construction. The client must understand that the savings associated with the observational method may be offset by construction delays due to the redesign of the foundation.

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