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Foundation Design - Choosing Building Foundation

In this section we will concentrate mainly on How to Choose a Building Foundation, as this is the most common problem faced by engineers when designing buildings, be it single storey or multi-storey. What is applicable to building is also applicable to other structures such as water tower, retaining wall foundations, the same principles apply.

View Choosing Building Foundations 

As discussed in Types of Foundations, building foundations can be divided into Two Groups - Piled or Unpiled. View Sketch for Types of Foundations

When do you choose a Piled Foundation

A Piled Foundation is used when the soil or ground conditions near to ground surface cannot permanently support the building structure, and the load of the building has to be taken to deeper soil stratum or to bed rock.

A footing which is piled is very "rigid" which simply means it will not be subjected to much settlement. A building foundation should thus be piled if you do not want any settlement to occur or that the amount of settlement is so small that it will not case any damage to the building structure or finishes.

You also need to choose a piled foundation if the building is subjected to large uplift and horizontal forces due to wind, over-turning, or horizontal traction forces from vehicles. The piles will provide resistance against uplift or horizontal forces via raking piles.

Where the building is of irregular in shape and has a high possibility of differential movements, then use of piles in the foundation would provide uniform rigidity.

When do you choose a Bearing Foundation / Slab Foundation

If the soil and ground conditions near to the ground surface is hard enough to support the weight of the structure without causing excessive permanent settlements then a Bearing Concrete Foundation is suitable, which can be in the form of a concrete slab foundation, pad footing, a strip footing, a raft foundation. Choosing a Bearing Foundation (unpiled foundation) would need more care as compared to piled foundation. A proper study and evaluation of the present and future soil conditions would be required. Such issues as whether the soil will weaken over the years due to weathering, when surrounding future development would cause ground water to be raised or lowered, or mining, tunneling would cause soil subsidence.

As a general guide, if the soil is from a Cut Excavation from a hill, or exposed rock, or firm consistency of soil strata, like firm clayey or sandy soils, horizontally and  to depths exceeding 3 times the plan dimensions of the building then bearing foundation can be considered.

A basement foundation, slab foundation are unique in that the uplift by floatation due to ground water may have positive or negative impact to the carrying capacity of the foundation.

A bearing concrete foundation should not be used in cases where there are significant uplift and overturning forces such as in tall and narrow buildings, structures subjected to sway due to wind or earthquakes, or when there are uncertainties in the future surrounding areas development. A strip or raft foundation should also not be used where the length of the building or structure is particularly long as differential settlements will have undesirable consequences.

How Ground Conditions affect Foundations

Whether it is pile foundations or slab foundations, the engineer need to assess the soil and ground or terrain carefully. The following give some guidelines on choosing a suitable foundation :-

  • Soil Conditions - a detail soil investigation would need to be carried out to determine all soil and sub-surface conditions, such as type of soils, strata characteristics, ground water, rocks, caves, mining, chemicals, etc.
  • Terrain conditions - existing terrain and contours, and the platform level by which the building foundation will sit. This is crucial as it will determine whether total or differential settlement would occur in the long term.
  • Surrounding development - how the present and future of the surrounding areas will be developed. Knowing this will determine whether the design criteria used for foundation design could change during the life span of the buildings, usually 100 years. This could be harder to predict unless there is a stringent development plan in place. If this is not available, then the engineer should assume the worst case situation. i.e, if the foundation is designed as a Raft with floatation effect, then assessment need be made for situations in the future when ground water is being removed, unless such occurrence could not ever happen. In situations of uncertainty piled foundations should be used. 

Next we will discuss on Soil & Geotechnical Investigations --->

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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng)
MEC Engineers, Civil & Structural Engineer


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Authur- Philip Goh (B.Sc, MIEM, P.Eng. MICE, C.Eng) MEC Engineers, Civil & Structural Engineer HP: 016-8672189, Email: Widget: email cloaker