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Re: Expansion Joint in Tunnel

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At 05:00 PM 20/11/2007, you wrote:
Dear Sefiant,
We are having 50m long tunnel connected between two buildings at raft foundation level. Building sizes are approximately 50m x 50m and building has one basement hence tunnel is connected between basements and entirely buried in soil.   And water table is 3m above bottom of tunnel as per geotechnical report.   We are not providing any Expansion Joint, is it acceptable or not?

The 2 rafts and the connecting tunnel would have to be analysed as a single raft to determine the uplift stresses and moments in the tunnel. Presumably only the base of the tunnel is connecting to the rafts with the sides and top only receiving stress as you progress along the tunnel so there is not a full connection between the rafts and the tunnel. This would need to be allowed for in the analysis. As well there will be significant restraint to shortening due to shrinkage and temperature change. This will have to be allowed for in the design.

Even if an expansion joint is provided, if dowels or keys are used to maintain the level of the tunnel and the rafts where they connect, then a combined analysis will be needed with a pin at the join as load will still transfer across the joint putting stress into the tunnel. 

Also We are having underground car parking structure approximately 500m long, 60m wide and top of roof slab is 1.5m below ground level. We have not provided any expansion joint in raft but expansion joint is provided every 50m c/c above raft. Request you to comment on the proposal whether it is ok or not?

500m is technically possible (though I have never attempted it, prefer expansion joints) if you provide sufficient reinforcement to resist full restraint stresses (due to shrinkage and temperature change). I would think at least .6% reinforcement would be needed purely for the restraint problem, but possibly more. Prestress will not help as you will not get any into the centre of the slab. It all depends on what level of crack control you need for the water pressure from below.

Can anybody having the design experiences share some knowledge?
Thanks in advance.

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