How we choose the foundation for structures and bridges?

 Thank you my dear friend about your invitation ,,  I,agree with the answer,of Mr. Ameen and Mr. Wolf, where the choice of the foundations and kind and its relevance to the structure of the fundamental duties of the designer therefore the designer is responsible for selecting the appropriate foundation. Consider the following factors in that selection: 

- Design load. The magnitude of the design load dictates the required size of the foundation from a structural standpoint. 

-Subsurface formations. The depth and strength of subsurface formations determine the type of foundation chosen. In general, drilled shafts are well suited to areas with competent soil and rock. While drilled shafts have been successfully installed in soft soil, they may be less efficient than piling. In general, use piling where softer soil is present. Very hard material at or near the surface makes driven pile installation difficult. 

- Corrosive conditions. Salts, chlorides, and sulfates are detrimental to foundations. Where these conditions exist, take preventive measures. Use sulfate-resistant concrete as defined in  Standard Specification for construction in seawater or soils with high sulfate content. Consult the  list of recommended corrosion protection areas for specific areas of Texas that may have structures with possible corrosion due to sulfate soil or salt water. The use of steel piling in corrosive environments is not recommended. If steel piling must be used, an appropriate protective coating must be selected, additional steel section provided or a combination of these methods utilized to ensure proper performance of the foundation elements. 

-Economic considerations.. Consider economics in the final selection. Compare the foundation types. The cost of a drilled shaft foundation, for instance, may be less than piling. It may be feasible to use fewer piles at higher design loads, or fewer drilled shafts with larger diameters to maximize economy. If no clear economic difference exists between piling and drilled shafts, you may choose to include both and offer the contractor alternate designs in the contract plans. 

-Superstructure type. The type of superstructure chosen for the bridges may dictate or eliminate certain foundation types. For instance, short-span structures over streams may work well with trestle piling, but tall, single column flyovers justify footings with multiple shafts or piling. 

-Special design requirement. Special designs are sometimes necessary to straddle another structure or utilities and may require a different type of foundation than the rest of the structure. 

Design foundations as either drilled shafts or piling. Study all the available soil data, and choose the type of foundation most suitable to the existing soil conditions and the particular structure

Basics: First off all do a well prepared soil investigation, based on all the types of foundations you expect based on the site. Then the decision for a type of foundation is as simple as it can be and also correct.

I see a lot of engineers choose a foundation type without any knowledge of the soil, thats impossible and will lead to high costs later in the design phase.

First we must test the soil(General soil tests ) the results of these tests will show  to the designers all characteristics of the deep soil layers and their grains and the level of the ground water with these results the designers will calculate the bearing capacity of the soil which the structure built on and finally from these data calculated  they prepare report consists of determining  the type of a suitable footing as well as  the weak soil layers treatment criterias .

Agreed with most colleagues answers,specially Mr. Fouad & Mr. Yakoob

Basically, it depends on the type of soil and the structure, keeping in mind the loads.

The foundation for structures and bridges preliminarily depend on the type of structure or bridge to be built.(eg: load, shape, type, e.t.c)

The type of soil and cost considerations are to be secondary.

Availability of resources and man power to be considered next.

Depending on place of bridge or structure in the water or valley or earth >

depending on materials of structures or bridges - Rc or metal or wood

depending on soil of foundations .

thanks

The soil test is very essential to determine the bearing capacity of the soil and get knowlege about under ground strata. That will able the designer to select type of foundation. Public safety is most important than cost and other things and that will have the designer to be consider first of all.

I agree with the colleagues.

All Answers are very comprehensive and I agreed with my all friends specially Mr. Fouad , Mr. Yakoob & Mr. Kinsbergen