Thursday 3 June 2010

MARINE CIVIL ENGINEERING

Caisson types and forms of construction

There are four main types of caisson:

Box caissons

Open caissons

Compressed air caissons

Monoliths.


Box Caissons

· Box caissons are prefabricated boxes, usually in concrete, with sides and a bottom, which are set down on a prepared base.

· Filled with concrete to form a massive foundation for a pier or similar structure

· Excavation of the site is carried out by dredger or grab in normal conditions, i.e. gravel or mud bed, and a layer of crushed rock is levelled on the bed of the sea or river to receive the caisson.

· The box caisson may be concreted by one of three methods: tremie pipe, pump, or bottom-opening skip

· Box caissons must be anchored or ballasted to prevent flotation before the concrete fill has been placed.


Open Caissons

· Open caissons are structures which are open at both top and bottom and are suitable for foundations in waterways where the sub-stratum is soft clay or silt and therefore easily excavated by grab or air lift.

· They are also suitable for deep foundations in water where compressed air working would require air pressures above 340 kN/m2 (3.4 bar): men cannot work in pressures greater than this.

· Open caissons are not suitable for sinking through ground containing obstructions, e.g. large boulders, unless the depth permits their removal by divers or by compressed air working.


Compressed Air Caissons

· Compressed air caissons are suitable for sinking foundations in troublesome ground, e.g. ground containing obstacles that require moving by hand.

· Very suitable for foundations which by other methods might result in the settlement of adjacent structures.

· Advantages include dry working conditions and accurate levelling and testing of the foundation bed together with ideal conditions for the placing of concrete.

· Rate of sinking is very slow

· Air locks should be located at a height that will not descend below the highest water level during sinking.


Monoliths

· Similar to open caissons with kentledge but differ in that they are much heavier.

· Consists of reinforced concrete walls of substantial thickness to provide sufficient weight to prevent overturning

· Used for quay walls which have to resist great impact forces from ships coming in to berth.

· Unsuitable for sinking through very soft deposits because it would be difficult to control the verticality of the structure.

· Air decks can be fitted to monoliths and open caissons to permit compressed air working.


Positioning and sinking of caissons

· May be partly constructed in dry dock and towed to the site.

· An alternative method of launching the caisson is to winch down a slipway into the water.

· Box caissons are designed to be floated to the site and sunk in place, so normal dry dock construction is most suited.

· The caissons are towed out to the site by tugs and are positioned by one of the following methods:

· Piling enclosure or dolphins

· Wire cables to submerged anchors

· Anchored pontoons or barges

· Radio buoys and beacons

· The sinking of caissons can be achieved by one of the following methods:

· Free sinking or jacking down between anchored pontoons

· Lowering by block and tackle or cranes from anchored pontoons

· Winching from submerged anchors

· Controlled sinking, using air domes, and ballasting within lateral control, usually radio beacons.

· Rate of sinking will depend on the amount of insitu walling to be completed.

· The top of the caisson should maintain a minimum of 10 metres (in very rough conditions 25 metres) above high water.


Sealing and filling caissons

· All loose material should be removed from the bottom of the excavation and the excavation should be level or concave.

When sealing open caissons and monoliths the first layer of concrete will have to be placed under water by tremie or bottom opening skip.

No comments:

Post a Comment