Finished Detail

This is a cross-section of the part where the tunnel goes through a building at 1:1 scale. The detail is meant to be horizontal, as it shows the side of the tunnel. 

Overall Detail- 420Lx90W 
The concrete (at bottom of picture) is part of the building- as such it is reinforced with iron.
The metal plate "I" shape in the middle of the detail is the clamp that holds the tunnel in place and can be found on three sides (top, left, right) of the tunnel. It is bolted into the concrete.
The top section of the detail is the tunnel and includes the two concentric curved lengths of metal as well as the insulation between. 

Close-up look at the tunnel section
The concentric curved lengths were shaped by hand with a hammer, wood and the thick curved metal plate as a guide- held apart by the frame of the tunnel as the metal is plated to a criss-crossing structure. It is only a slight curve as the full structure is 4m in diameter.

The struts were electrically welded to the lengths in the workshop (note: thin metal is not good for welding. The two struts are connected to the length in the corners and metal has been melted away because it couldn't withstand the extreme heat). 

Concrete had to be re-cast as the first one cracked under the pressure of the hammer drill the day before crits (also the holes to be drilled lined up exactly where the reinforcing was). New mold made had two holes on either side of the length for doweling to pass through as placeholders for the bolts (M12x100mm) that would go through the concrete so that drilling would no longer be needed.

Lesson learnt: concrete has got to be mixed outside of the mold, best on the ground, then shoveled into the oiled frame, tapping all sides to get air out and concrete into the corners. Consistency- when fully in mold, some "liquefaction" occurs with water coating the surface while curing. 

Tunnel Wall
Made to detach from the concrete and metal clamp as a design solution to easily bringing the structure on site.  The tunnel had to be two walls thick; to prevent echo, control temperatures by installing insulation and for the purpose of wiring (electricity and internet).

The wiring is shown with the pipe in the corner embedded in the insulation with a cross-section of the wiring. The versatile and flexible rubber skin (interior/exterior) prevents glare off the metal, prevents slipping of the tunnel against clamp, helps insulate the cold metal (colour black: collects heat, inside it's off-white/grey to disperse harsh glare and brighten interior) and also translates the "cable" type material/idea. 

The thick metal place bolted in place into the concrete- keeps the clamp steady. The  U-shaped metal was welded in the workshop onto the metal plate, then the two ends welded to the larger (machine bent) curved plate that conforms to the curve of the tunnel.

Tip: buy mild steel. Bowmac is coated in zinc (which doesn't melt under electric welding- becomes powder) and had to be sanded off the surface before starting any welding.

Bolt going through the concrete- the tension of both bolts at either end snapped the concrete partially through the middle but the metal place keeps both sides in place.