|
The construction of breakwaters is not like any other massive wall or rock-fill bund. Nor it is like other marine works taken up in the sheltered waters of harbours. They pose unique problems and use unique technology. The technology involved in building dams and bridges is not of much use, except for minor aspects. Essentially, it had to tackle the destructive actions of dynamic sea waves, storm surges, cyclonic wind loads, tidal fluctuations of sea level and attacks of marine life. Let us begin our discussion exactly on this note.
Unfavourable factors and forces acting on breakwaters
The following factors are detrimental to the functioning of breakwaters.
- Buoyancy
This force reduces the apparent weight of structure and hence, the marine structures need to have more weight than normal to remain stable, unless the foundations are absolutely impervious.
- External forces
The break waters indeed face very rough external forces, predominantly, wind, wave action and incidental quay loads. The wind forces generally produce vibrations in the masonry structures and tend to weaken the different courses of masonry. Whereas, the receding waves induces enough suction pressure at the bottom to cause erosion of the foundation. Foundation and upper foundation strata must be adequately protected.
- Chemical action of sea water
The aggressive chemical nature of sea water causes damage to the materials of construction by solution and leaching as well as by formation of compounds, which reduce the strength and durability of cement concrete.
- Marine life
Sea insects and other organisms, which tend to make burrows may undermine the structure’s integrity. Hence, the breakwaters need to be made particularly strong and hard.
Breakwater failures
- Failures of breakwaters are not uncommon and such incidents have usually occurred during the unexpected and unusually fierce storms.
- With the rubble mound or heap type breakwaters, the settlement is gradual and the slope tends to flatten commensurate with the wave action. Evidently, the chances of spectacular failures are less for heap type breakwaters whereas, the failure of wall type breakwaters will be sudden and destructive.
- All in all, it is not to be overstated that the design and construction of maritime structures demand skill and experience of a very high order. Isn’t it true, given the fact that sea is the ultimate epitome of nature’s power and that its fury is often incalculable and unpredictable?
Constructional Features
As the problems posed by breakwaters themselves, the solutions to them were also unique. Let us sample a few of them, first from the construction features of breakwaters:
- As the maximum wave effect on the structures is felt between high tide level and low tide level, heavier and bigger blocks of up to 33 tons each are deposited on the slopes in this zone as well as top.
- The batter on the front or seaward face abruptly changes from 2.8:1 to 1:1 to provide a sharp edge to break the oncoming waves.
- Wall type breakwater construction must be limited to shallow and relatively calmer sea areas. They are unsuitable for great depths and rough waters.
- Strong foundation bed resistant to erosion must be available at site.
- Wherever, bearing capacity of soil on sea floor is less, rubble may be deposited to improve the distribution of load over a larger area.
- Alternatively, sheet piles can also be driven whenever possible.
- In certain places, where the top soil consists of soft clay or silt or other undependable soils, trenching may be adopted to remove the top soil. The breakwaters of Ennore Coal Port on East Coast of India, which has one of the longest breakwaters in world, used this technique.
|
