A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
8. Hudson’s formula and Van der Meer formula are commonly used in
the design of armour. Which one is a better choice? (A2)
Hudson’s formula is commonly adopted in preliminary design to obtain
rough initial estimate of rock size. The formula is derived from the results of
regular wave tests. However, this formula does not take into account the
following elements which Van der Meer formula does: wave period,
damage level, permeability of structure and storm duration. Moreover,
Hudson’s formula deals with the use of regular waves only.
Compared with Hudson’s formula, Van der Meer formula is more
complicated and it is derived from results of a series of physical model
tests. They include the consideration of wave period, storm duration,
clearly-defined damage level and permeability of structure. The choice of
the appropriate formula is dependent on the design purpose (i.e.
preliminary design or detailed design).
9. Should the Morison equation or diffraction analysis be adopted in
determining wave force on piles? (P1)
The choice between the Morison’s equation and diffraction analysis in
determining the wave forces on piles depends on the ratio between the
diameters of piles to wavelength. If the ratio between the diameter of piles
to wavelength is less than 0.2, the Morison equation is usually
recommended. The reason behind this is that the effect of viscosity and
separation is significant below this ratio. On the contrary, if the ratio
between the diameter of piles to wavelength exceeds 0.2, the waves are
scattered with negligible occurrence of separation. As such, diffraction
analysis is adopted to calculate the wave forces on piles.
10. Why is larger concrete cover (e.g. 75mm) normally used in marine
concrete? (MC1)
In marine environment, the cover to reinforced concrete in exposure zones
is specified to be much larger than normal concrete (e.g. 75mm). Based on
past experimental studies, the concrete cover is identified to be directly
related to the corrosion failure of marine structures.
In Eurocode 1 under Ultimate Limit State the probability of failure is less
than 10 -4 while under Serviceability Limit State the probability of failure is
less than 10 -2 . For corrosion aspects, it is normally classified under SLS
state. For OPC concrete with 50mm cover, the corrosion probability is
195