having all the vices and more of a dead ship, and yet it must be sunk into posi- tion with great accuracy. Every opera- tion must be planned in great detail and carried out with care and yet with expedition.
One method of placing a unit is to bring it to a stage where it is only just buoyant after which it is floated to above its final position and sufficient ballast then added to sink it. While be- ing sunk the unit hangs from a floating rig and negative buoyancy is taken by cables as the unit is lowered. Through- out this operation each cable must be carefully controlled by winch and jack.
An alternative method would be to retain slight positive buoyancy through- out the entire operation until the unit is placed. Thus the unit would have to be pulled down against temporary an- chorages (concrete block kentledge or similar) on to the bed of the waterway and would not have to be ballasted through the unstable state of zero buo- yancy until after it is held down in its final position.
Water is a convenient ballast in that it can easily be moved by pipe and pump but otherwise is not satisfactory unless divided into numerous small ballast tanks. It is an unstable material liable to slop about and tends to ac- centuate out of balance forces in the event of a unit tilting or yawing.
Ballast materials often used are tre- mie concrete or crushed stone. The latter can be used afterwards within the unit as concrete aggregate. The pro- blem with ballast is one of logistics the movement of large quantities of material quickly and accurately under difficult conditions.
The variations in type and layout of plant that can be used for placing units are unlimited but typical exam- ples are shown diagrammatically in fig. 3.
Bedding (Fig. 4. )
Although bedding is discussed as a separate operation it is part of the con- tinuous placing-bedding-jointing opera- tion and choice of method is dependent upon the methods of placing and jointing employed.
The American type unit is generally less sensitive to deformation than the European type and consequently does not require to be quite so carefully
bedded. It is usual for the unit to be placed direct on to a bed of granular material which has been accurately screeded. This must be done with great care if unacceptable stresses and mis- alignment are to be avoided.
As a consequence of the European
warping winch
lowering winch
cable bracing
PLAN
crown block
double drum lowering winch
travel block
lowering winches
are not shown
Fig. 3. Examples of placing rigs
steel tunnel unit
pontoon
dam plate
SECTION
a) floating rig
hydraulic jacks
reinforced concrete tunnel unit
b) self-elevating platform
type unit being relatively sensitive to deformation and having a sophisticated inter-unit joint, care has to be taken that adjacent units register and that they are thoroughly and uniformly bedded. To meet these requirements it is common for the units to first sit on prepared point supports which can be jacked against to facilitate accurate alignment and for sand then to be jetted under the unit to a uniform density.
In certain circumstances it has been considered necessary and has been pos- sible to bear the units on piles as suspended structures. Jointing (Fig. 5)
As soon as possible after a unit is placed in its correct position (either placed in its correct position (either before or after bedding is completed) it must be joined to the preceding unit Generally the American type unit has a tremie-concreted joint whereas the
TTTT
European type unit has a gasketed joint.
In the concreted joint a hood plate closes the joint between adjacent units which is then sealed with an in-situ concrete collar tremied into place with- in dam plates.
The gasketed joint requires greater care in placing but once made is more positive in action. The unit is placed to register with the preceding unit, the gasket around the former being brought up against the steel bearing plate around the latter. At this stage water is trapped between adjacent bulkheads and fur ther movement is not possible.
However, as soon as water is tapped off from between these bulkheads the forces due to water pressure acting on the two ends of the unit are no longer balanced and the large compressive force ensures that the gasketed joint is sealed: Permanent connection is sub- sequently made inside the tunnel while
Far East BUILDER, June 1969
35
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