Saturated soil pressure on Tunnel Portal

[akarimjee]

I have a situation where there is a 10m deep, 2m diameter soakaway on the top of a Tunnel Portal with a water proof membrane capable of taking only 2 bar pressure. The Tunnel is approximately 70m below the Soakaway. The soil is made of Chalk. The question is will the membrane sustain the water pressure? If so what is the method of calculating the actual water pressure on the Tunnel Membrane.

Thanks

Ak

 

[pigdog]

I guess it depends on the permeability of the chalk and the location of the water table. If the tunnel is above the water table, there is no "head" on the membrane. If the tunnel is below the water table and the chalk is highly permeable, (like a unconsolidated sand), you have 70 feet of head. If it is impermeable you have 70' of seal. If the portal and a significant portion of the chalk is below the water table, you could put in a temporary well and perform a slug test, and gain a fair idea of the permeability of the chalk. I think with the permeability and the head you could get to where you're going, but I don't have the equation handy.
roger