Sizing foundations quickly is an important skill for the exam. Ideally, your first guess for foundation size in part 1a should be about right, especially if poor ground conditions are a key constraint.
Soil parameters
The exam question typically gives the following design parameters, which can be used to size the foundations:
- SPT 'N' for granular soil
- Undrained shear strength cu for cohesive soil
- Safe bearing pressure or allowable load for rock
Pad foundations
For pad foundations such as bankseats, a safe bearing pressure can be calculated quickly based on the undrained shear strength or SPT 'N'. This is fast enough to size the pad in part 1a (design appraisal).
Piled foundations
For piled foundations, the approximate cross sectional area of piles required under a pier can be calculated based on the area of bridge deck supported. This is a very quick number that doesn't require any consideration of loading, so it is also suitable for part 1a. The rule is based on achieving an efficient structural design of the pile, i.e. 5 or 6 MPa stress at SLS; if the ground conditions are very poor then this may result in piles that are too long to be economical, in which case more piles would be provided. In most cases it is a good starting point.
Pilecap geometry (pile spacing, pilecap thickness) can be determined from the pile diameter. A spacing of 3 pile diameters between the centrelines of piles is used to reduce interaction effects between adjacent piles (i.e. to ensure each pile can reach its full capacity). Base thicknesses are driven by the desire to avoid shear reinforcement in the pile cap, which is expensive to fix.
The geotechnical capacity of a pile can be determined using the formulae for skin friction and end bearing resistance. These calculations are fairly quick, but probably not something you want to get into in part 1a unless it is critical to the viability of one or both of your options. They could be used in part 2c (calculations) if time is short.
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