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  • Writer's pictureSi Shen

Selection of foundation type

Updated: Nov 28, 2020

As a rule of thumb, all attempts are made to construct a foundation as small and shallow as possible, for obvious cost-saving reasons. Whether the small and shallow foundation gives the design a ‘pass’ or not depends mainly on two criteria: resistance and settlement. The optimal solution that the designer needs to find is usually the cheapest foundation that satisfies both criteria. The design process can get very complex in difficult ground conditions, but this post provides some rules of thumb that are applicable most of the time.

Resistance and settlement

There are generally speaking two sources of resistance capacity for any given foundation

  • Bearing resistance, which comes from the vertical compressive contact between the bottom of the foundation and the ground that lies underneath

  • Frictional resistance, which comes from the fact that the soil squeezed tight against the side of the foundation generates friction (shear capacity)

Even if the ground does not fail under pressure, it sometimes is too ‘spongy’ to act as a competent founding material, and result in excessive settlement, which never a good thing for foundations. Differential settlement, as in one part of the foundation settles quicker and/or larger than the others, is nastier than a uniform settlement.

Archetypes of foundations

There are very loosely speaking two archetypes of foundation, with no clear demarcations in between:

  • Shallow foundation, usually in the form of pad, strip, matt and raft. Whatever names they are given, these foundations are typically just a few meters deep, almost solely relying on bearing resistance. The reason for choosing this type of foundation is either because the formation is stiff enough (such as presence of competent rock at shallow level), or the structure above isn’t very heavy thus does not demand much resistance capacity from the foundation

  • Deep foundation, commonly exceeding 10m in depth, taking the form of:

o Pile foundations. Piles are essentially slender, solid and deep-reaching columns buried in the ground. If the ground is relatively loose, the piles can be precast concrete piles driven into the ground with a (huge) hammer; if the ground is too stiff for driven piles, or if the piles needed are too large too long, then the piles are made in-situ by drilling a hole in the ground and filled with concrete

o Caissons/shafts – these are essentially stocky hollow piles. Whilst piles normally come in groups, caissons are normally singular because they are large enough to handle the loadings by one alone.

Why a deep foundation?

There are numerous benefits of having a deep foundation:

  • Deeper foundations have larger dimensions to offer resistance – size matters.

  • The effective stress in the ground is higher at deeper levels (increasing roughly linearly with depth), resulting in higher friction per unit area, thus a higher effectiveness per unit length

  • More competent (as in higher stiffness) soil layers, and perhaps even rock layers, resulting are normally present at lower levels, in higher bearing resistance. Rocks typically have a stiffness a few orders of magnitude higher than that of soil. The purpose of having a deep foundation sometimes is to reach a stiff layer present down below.

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