Abstract
The design of foundations constitutes a major step for each civil engineering structure. Indeed, the stability of those structures relies on cost-effective and adequately designed foundation solutions. To come up with an optimized design of a foundation, the geotechnical study passes several steps: the geotechnical survey including in situ and laboratory tests, the synthesis of geotechnical parameters to be considered for the design, and the suggestion of foundation solution avoiding over estimated cost and ensuring suitable method of execution. In this chapter, the three currently practiced categories of foundation are briefly introduced. Then, two illustrative Tunisian case histories are analyzed to explain, first, when the practiced foundation solution was inadequately chosen how a non-cost-effective solution can be avoided, and second, why an unsuitable foundation solution can lead to the stopping of the structure functioning and then how to proceed for the design of retrofit solution to be executed for restarting the functioning of the structure.
TopIntroduction
Optimized foundation design (OFD) first passes by an adequately planned geotechnical survey which usually includes boreholes, in situ tests and laboratory tests. Such a program is decided on the basis of the structure dimensions (or area), load intensities and soil conditions. A well planned geotechnical survey (number, location and depth of boreholes and in situ investigation) is followed by a suitable synthesis of geotechnical test results allowing the adoption of realistic geotechnical soil parameters. The latter constitutes the best starting point to think about an optimized design of foundations.
Three categories of foundations are currently practiced for civil engineering structures namely: shallow foundations, deep foundations and intermediate foundations related to reinforced or improved soils (Das, 2014). For each category the adequate type of foundation is decided on the basis of an optimized solution, e.g; cost effective and acceptable time of execution.
Shallow foundations include the following types: isolated footings; strip footings; crossed strip footing (either in one or in two directions) and rafted foundation.
Deep foundations also comprise a big variety of pile types (bored, driven, etc.), the optimized solution rather relies on the installation method of pile to warrant a reliable and cost-effective solution.
Ground improvement techniques represent the third category of foundation which can be considered in between the shallow and deep foundations. Several techniques can be adopted depending on the accorded priority for the project, i.e. to increase the bearing capacity and/or to reduce the settlement, or to accelerate the consolidation of compressible soils: stone columns, rigid inclusions, etc. (Indraratna et al, 2015).
In this chapter it is intended to highlight either the benefits or the disadvantages that can result from well planned or unsuitable geotechnical survey that can also lead to adequate or inadequate foundation solutions. Two Tunisian case histories are presented in detail to capture the learned lessons in regard to unsafe design in terms of non-cost effective or unsuitable foundation solution.
Key Terms in this Chapter
Area Ratio (Substitution Factor): Total cross-sectional area of reinforcing columns divided by the total loaded area (i.e., foundation).
Pressure Meter Test: It is an situ test performed within pre bore hole where the soil is laterally expanded subjected to horizontal stress, up to failure. From the change in volume versus applied horizontal stress two main parameters are determined: EM is the pressure meter modulus and, pl* is the limit net pressure. From those parameters and other correlated soil parameters the verification of bearing capacity and settlement are estimated for the short-term soil behavior.
Optimized Design: It combines the verifications, first of admissible bearing capacity and second of the admissible settlement, to determine the optimized area ratio.