Tuesday, January 8, 2013

Selection of the Appropriate Foundation.

The selection of the appropriate foundation solution is perhaps the most important part of the design process and most difficult to define. The engineer should not confuse structural calculation and analysis with design.
Calculation usually involves analysing, from certain parameters, the forces and stresses involved in a particular structural element.

Structural design is the process of exploiting engineering knowledge in an attempt to produce the most suitable and economic structure. The foundation selection is governed by many factors which include: sub-soil conditions, past site usage, adjacent construction, size/scale of development proposals, timescale/cost limitations. While this is not a comprehensive list it can be appreciated that the subsoil or any one factor is only part of the overall equation.

The selection of the foundation type to be adopted to accommodate the various criteria is a design process which evolves. It is necessary to approach this process on a broad front taking account of all the relevant information and  balancing the factors which can vary as decisions are made.

Foundation design should therefore be carried out using  a careful blend of geology, soil mechanics, theory of structures, design of materials, experience, engineering judgement, logic and down-to-earth engineering. The designer must understand that sampling and testing soils, while being by no means an accurate science, is, when corrected by logic and practical experience, an excellent guide for use in the design of foundations. Foundation design is one of the most challenging aspects of engineering and no two foundation conditions are the same. The need for the engineer to see and feel the subject cannot be overemphasized.

Foundation design like other structural design requires  a good sound basic approach in order to achieve a truly  successful result. There is a tendency in engineering to use test information and theory in a rigid uncompromising way when preparing designs or, alternatively, where experience has shown such theories and tests to be suspect, to revert to an attitude that testing and theory is of no use in practical design. Both approaches are wrong, design should be the process of using all the available tools and information in an attempt to produce the most suitable solution. It should be appreciated that theory is the process of simplifying by assumption the things which actually happen in order to make it possible for a human brain to understand and analyse. Very few assumptions are correct and hence the errors produce a variation from reality. Part of the process of design is to understand such theories, remember the assumptions made and their likely effect on the answer, and to make due allowance for such errors in the choice of the solution.

The sampling and testing of sub-soil materials involves numerous practical site problems, such as disturbance of samples and errors in testing. If the test results are used in isolation such testing is often unreliable. Part of the design process is therefore to understand the practical difficulties, seek indications of unreliability in the results, assess the implication and magnitude of such errors, and make suitable allowance in the design.

The most demanding and exciting part of the engineering process must be approached next, that of making use of  all this information in the design of foundations. However, the designer must first make sure that all the necessary information has been gathered and its reliability assessed.

Information collection/assessment   Previously have discussed the assessment of information from ground investigation and the collection of details of the soil, groundwater, chemicals, etc., together with the...

General approach to choice of foundations   Having collected the information about the site including that noted in check-list 1 and obtained from the client answers to the queries including those noted in check-list 2, the...

Questioning the information and proposals   Having gathered the information together for the design, the engineer’s first considerations should take account of the following: (1) Is the investigation sufficient to design a safe...

Exploitation of foundation stiffness and resulting ground pressure    In addition to the basic knowledge of structural theory there is also a need to keep in mind the basic pressures which develop in various ground conditions and the effect of the...

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