DHARMCAISSONS AND WELL FOUNDATIONS 769
Sand fill SandislandDredge wellCompleted Caisson
Fig. 19.6 Sand island method of placing open caisson
An artificial island of sand is made for temporarily raising the ground surface above the
water level. Thus a relatively dry area is obtained for sinking the caisson. The size of the sand
island should be sufficient to provide adequate working space around the caisson (Fig. 19.6).
For the construction of sand island, a woven willow mattress is first sunk to the river
bed to provide protection against scour. A timber staging is then constructed around the pe-
riphery of the proposed island. Sheet piles are driven to enclose the area, the mattress cut
along the inside of the sheet piling, and the inside mattress removed. The area is filled with
sand upto the required level.
In case it is not possible to create dry conditions, the caisson is constructed in slipways
or barges and towed to its final position by floating. Guide piles are used for sinking the first
few lifts. Sinking is done through open water and then penetrating it into the soil.
As the soil is excavated from the dredge wells, the caisson sinks by its own weight. The
excavation is done by dredging with grab buckets. The soil near the cutting edge is removed by
hand, if necessary. During the period of casting a lift and curing, the sinking process is neces-
sarily stopped. Water jets are occasionally used on the exterior to facilitate sinking.
When the caisson reaches the desired final depth, its bottom is plugged by providing a
concrete seal, which is more commonly done by the ‘Tremie Method’. After the concrete has
fully matured, the water inside the caisson is pumped out. The top surface of the concrete seal
is cleaned and further concreting is done, depending upon the need.
Although it is desirable to sink the caisson perfectly vertical, it is extremely difficult to
sink it true to position. Corrective measures are to be used when tilts occur, as indicated in a
later section.