No_6_June_1969 — Page 36

34

1.0 m. rock blanket

1

existing sea bad

1.2 m. filter layer

upper

ventilation

duct

traffic duct

0.6 m. filter layer

granular fill

bed material

lower ventilation duct

a) steel tunnel section

tremied granular fill

1.5 m. rock blanket

1.0 m. rock blanket

0.6 m. filter layer

traffic duct vent. Straffic duct duct

granular fill

existing sea bed

bed material

b) reinforced concrete section

Fig. 1. Typical tunnel cross-sections

VEISIE

fabrication of shell

bulkhead

Fig. 2. Fabrication

launching

a) steel tunnel units

fitting-out

200

b) reinforced concrete tunnel units

under construction in a dry dock

while. Indications are that prestressing is not economic for any tunnel carry- ing less than six lanes of traffic (cf. the Lafontaine Tunnel, Montreal).

Fabrication (Fig. 2)

In the case of a steel tunnel the steel shell is fabricated, sealed, pro- vided with a concrete keel and launched (probably sideways) rather like a ship or barge. The unit is then completed with the addition of ballast and pro- tective concrete while lying alongside a fitting-out berth. There are instances where reinforced concrete units have been (or proposals where units might be) launched while only partly fabri- cated but common practice is to form a temporary dry dock by excavation or cofferdam within which units are com- pleted in the dry before being sealed and floated out.

Construction can be carried out either in the dry on a firm foundation or partly while afloat (i.e. a flexible foundation). The materials of construc- tion can be used either structurally, as ballast or as protection. The methods and materials can be employed in many different combinations which will be reflected in design criteria and con- struction procedures.

Structural concrete (except in ex- ceptional circumstances) is placed in the dry whereas ballast concrete may be tremied into place. The protection of steel by concrete or bituminous membrane must be carried out in the dry. The order of placing concrete while afloat must be strictly controlled to avoid significant bending stresses be- ing induced in the steel shell.

Placing (Fig. 3)

Placing tunnel units in position is one of the most critical operations dur- ing the construction of a submerged tube tunnel and is usually the time when maximum stresses are induced in the structure.

The selection of this method of tunnelling can depend upon satisfac- tory answers to the problems likely to be encountered at this stage. Can ship- ping be diverted or held up? Are water currents too fast? Is fog a major hazard? Are sudden changes in water density likely? These and many more questions must be answered and solu- tions provided where necessary.

The weight (in air) of a tunnel unit is unlikely to be less than 20,000 tons. By its very nature a unit is unwieldy,

1 A. COUTURE. "The Louis Hippolyte Lafontaine Tunnel, details of design and construction'. The Dock & Harbour Au- thority, Vol. XLVI, No. 535, pp. 10-12, London, May 1965.

Far East BUILDER, June 1969