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Breakwaters are important shore and harbour protection works. They are classified into three types based on their form. They are 1) Heap or Mound Breakwater, 2) Mound with Superstructure and 3) Upright Wall breakwater.
Heap or Mound Breakwater
This simplest type is a heterogeneous assemblage of natural rubble, undressed stone blocks, rip rap supplemented in many cases with artificial blocks of huge bulk and weight, the whole being deposited without any regard to bond or bedding.
Construction
This is constructed by dumping of rubble and stones into the sea, till the heap or mound emerges out of the water. The mound is then allowed to consolidate and its side slopes will be stabilized by the wave action. The quantity of material required depends upon the depth of sea, tidal range, height and intensity of waves and exposure. On exposed faces, the waves tend to flatten the slope on seaward face and drag down the mound. Protection must be given to such slope and top surface. Obviously, the erosive action of the waves is the more between the high and low tide levels.
Protection
- Natural stones and rubble themselves are not always competent enough to withstand sea wave action. Heavy blocks of concrete weighing 25 to 30 tons, pre-cast and cured on shore under controlled conditions, are extensively used on top, and on front or seaward face or on both faces for protection.
- These are either dumped at random or laid in courses as in masonry and resist the flattening action of the sea waves by their sheer weight alone to a great extent. These blocks are prepared as rectangular cuboids with their ends facing the waves, so as to be stable against impact or overturning.
- Paving the upper parts and exposed slopes up to low tide level with heavy blocks of granite set in cement mortar is another method of protection against waves.
- Tetrapod method: The old method of using heavy cubes or cuboids of concrete or stone is based the principle that the weight of the block counters the force of waves and when laid at random, the sharp edges and corners help breaking the waves and lessening their ferocity. Of late, the tetrapod blocks of cement concrete are widely used.
- Tetrapods: The tetrapod is a pre-cast concrete block, with four prongs radiating at 12O0 to each other from the centre of block. The prongs are cylindrical in shape, slightly truncated and look like the old wooden barrels or casks of wine. Tetrapods of required weight, depending on the height of waves, are simply dumped at random on the exposed face of the breakwater at a slope of 1.5 to 1.
Mound with superstructure
There are two sub-types in this type depending on whether the superstructure is founded at low water level or below low water level.
- Mound with superstructure founded at low water level: A solid superstructure consisting of a quay on the top towards the inner or harbour side and protected by a parapet wall on the sea face is constructed on top of a rubble mound. Unlike the ordinary mound breakwater, this type of construction makes it possible for ships to come close to the breakwater wall on harbour side for cargo handling. Heavy concrete blocks are used on the sea face for protection at about low water level between parapet and mound.
- Mound with superstructure founded below low water level: This type of construction has the added advantage of founding the superstructure well below the level of disturbance by waves.
Upright Wall Breakwater
This is wall constructed of uniform shaped heavy rectangular blocks on concrete with a considerable portion of wall to be laid under water. Obviously, it is stronger, stable and durable, unlike the mound construction, requires less volume of concrete, easy to maintain, expensive and difficult to construct.
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