No_6_June_1969 — Page 38

external protective concrete is tremied into place between the joint and dam plates.

Tunnel units require protection against various types of damage. Some of the causes of damage are: floating craft and other objects; mishandling during placing operations; abrasion; dragging anchors; and sunken vessels.

Waterproofing

They also require protection against the ingress of water. With steel con- struction it is usual for the steel tube to be considered the waterproof mem- brane but with a reinforced concrete section it is usual for an external waterproof membrane to be provided.

Temporary protection (either even- tually removed or left in place) usually consists of timber rubbing strips and fenders strategically placed to prevent damage.

Permanent protection often consists of an external bitumen membrane as waterproofing which in turn is itself protected by steel sheet where abrasion is likely. Top protection, against an- chors and the like, can be a rock blanket 2 or 3 metres thick or per- haps a reinforced concrete slab ad- ditional to the main structure. One of the advantages of a rock blanket several metres thick is that it helps to spread

pontoon

pontoon

the weight of any vessel that might sink across the line of the tunnel.

Ventilation

Experiment has shown that tunnels up to 300 metres in length can be self- ventilating even when traffic densities rise above 2000 v.p.h. This self-venti- lation is due to the 'piston effect' of traffic (assumed to be uni-directional) moving through a tunnel.

It does not occur when there are traffic hold-ups and in this situation a tunnel may become seriously con- taminated with carbon monoxide. Therefore it is normal to provide at least emergency artificial ventilation which can be put into operation im- mediately CO contamination reaches a specified level. Artificial ventilation must always be provided in long tunnels.

There appears to be no universal rule relating type and volume of ven- tilation to length of tunnel and traffic density even though there must be a broad relationship between traffic den- sity and ventilation plant capacity. What is certain is that underwater tunnels always seem to have artificial ventilation however short they may be.

The four types of ventilation are: Natural and Artificial (longitudinal, semi-transverse and transverse).

hold-down weight

travelling screed

hold-down weight

ELEVATION

Fig. 4. Bedding tunnel units

a) example of screeding rig

b) sand jetting gantry

pontoon

SECTION

Longitudinal ventilation is when air is blown in at some point (or points) along a tunnel and then passes along the tunnel to be exhausted at the portals.

Semi-transverse ventilation is when air enters through ports throughout the length of a tunnel and then passes along the tunnel to be exhausted at the portals.

Transverse ventilation is when air enters and leaves through ports through- out the length of a tunnel.

Lighting

The lighting of a tunnel deserves to be carefully designed for it can make a great difference to driver com- fort and so have a great bearing on road safety. The problem is that it is not possible, by any reasonable means, to light a tunnel as brightly as the sunlit exterior and all that can be done is to attempt to prepare the eye at the entrance to a great change in brightness.

This is usually done by means of sun louvres at the entrance followed, immediately inside the tunnel, by as high a level of illumination as is pos- sible. Once the eye is adjusted inside the tunnel, the level of illumination can be at a lower level until the exit is approached, whereupon it must be graded up again to as high a level as possible before exit. Again, sun louvres are often used in an attempt to graduate the sudden increase in brightness.

The eye can adapt more quickly at exit than at entrance. At exit it is subjected to the additional hazard of glare but this can be minimised by the provision of a high standard of lighting as the exit is approached and by care in the siting and design of the exit to avoid sight lines pointing straight into the sun.

Lighting at night poses similar prob- lems but these are not so acute and can be more easily remedied.

Lighting fittings must be positioned and detailed with care to avoid flicker and the casting of shadows. The avoi dance of disturbing effects such as these will contribute greatly to the convenience and safety of tunnel users.

Traffic control and toll collection

Heavily trafficked tunnels of con- siderable length are provided nowadays with sophisticated methods of traffic control. This is understandable because as traffic density increases, level of service drops and a facility becomes increasingly sensitive to delays. Tunnels of any length are almost certain to be main traffic arteries, are frequently toll collecting and are physically

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Far East BUILDER, June 1969