LINING OF TUNNEL

GET EMBED WITH CONCEPT OF  TUNNEL LINING AND THEN MOVE TOWARDS OBJECTIVE

LINING OF TUNNEL

The objects of providing the tunnel lining are:

1)It withstand soil pressure when driven in soil soft.

2)It gives correct section to the tunnel

3)It reduces losses in friction and erosive action and ensures the stream line motion when the tunnel as to carry water by providing a smooth passage  at good velocity free from turbulence

4)It forms a protective covering to certain types of rock prone to air slaking

5)It keeps  the inside of the tunnel free from water percolation

6)It supports large slabs of rock which might have become loosened during blasting.

 Design of thickness of concrete Tunnel lining:

Stresses in tunnel lining primary originate in earth pressure.The laws of action of earth pressure involve many factors.Therefore the designer is therefore well advised to primarily

depend on judgement which may indicate the nature  of stresses.

Firm soil is usually considered to exert  only a downward pressure on arched roof and though the sides are lined no great consideration is attached to side or horizontal pressure in designing.As a long tunnel generally passes through varied geological strata no hard rule is prescribed.The nature  and geological structure of soil met  with should be primary guide in designing the lining and method of construction.  

The empirical formula commonly applied to obtain a working section is T  ⇨  D 

Thickness of concrete lining can be kept 2.5cm per 30 cm bore diameter of tunnel 

T⇨0.083D ,

T - lining thickness in m , D - diameter  in m 

But this thickness restricted to 230mm 

Allowance for movement of side walls - 5 to 7.5 cm

Allowance for movement for roof arch - 22 to 60 cm 

Types of  Tunnel Lining:

1)Timber lining:Used for both temporary and permanent  support

Timber lining

2)Brick lining : Used in case of underground sewer

3)Stone masonry lining: Used for sides lining

4)Cast iron lining:Used in subaqueos region

5)Plain & RCC lining ; Used in hard and soft rock

6)Cast steel lining:Used for primary lining for shield

Timber Lining Packing :

When timber arch is erected it is blocked from the roof for purpose.Then wedges at foot of posts are driven tight.Dry packing with muck or lean cement concrete 1:3:6 shot into the space fills the gap between the lagging and the roof .This type of lining last for 10 to 15 yrs.

Concrete Lining:

Concrete lining is done using proper formwork. The form should show the true outline of finished tunnel section.The tunnel lining work is divided into three operation 

1)Ground mould is used for floor lining or invert concreting

Ground Mould

2)Leading frame is name applied to side wall form

Leading frame

3)Trusses are used as centers for roof arch.

Steel Forms and their use for tunnel lining:

There are two principal types 1)Separate side wall and arch form

                                                2)Telescopic form

1) Separate side wall and arch form:

• This is standard form of tunnel arrangement for large tunnels lining.
• In this method the invert is first laid and then after that track is laid for carrying units for the sides and the arch one following the other.Concrete is poured down into side forms. As this form unit is moved forward it is followed by arch form..The arch forms are filled by concrete placers.It is of non telescopic form.    

Separate wall system

2)Telescoping forms:

The main ribs are made up of sections hinged together so that a back unit could be collapsed and move  forward under form units.This types is very suitable for tunnels up to 20ft to 25ft.

Telescopic steel forms

 As the tunnel size increases the hinged section become too heavy for handling and erecting.The traveler carries Jacks for erecting and collapsing the rib section.