Underground room

[steveo275]

Hello, I am thinking about tunneling an underground shelter off the side of my house. The entrance would be 5 feet off of the back of the foundation. I have planned for a 25'L 10'W 8'H room. I have worked out the logistical problems with dirt removal. However, I am wondering about proper depth for this type of structure. If the ceiling was at a depth of 5 feet, would that be enough to leave the excavation face unsupported at 2 foot increments? The soil is a dry clay with few rocks. Water is not an issue as the site is 30 feet above a river. Also, what are some recommendations for a ceiling. The walls will be cinder block, on top of a concrete footer with a 4" concrete floor. If I pre cast a 4" reinforced concrete ceiling in 2 foot sections, would it be enough to support the weight of the earth with the walls alone, no beams?

 

[civilperson]

No, an arch shape or a thicker section is required for the ten foot span since you have 5 feet of overburden and possible live loading on top of the soil. The walls are needing engineering also, reinforcing and grouting cells full with steel down into the footing.

 

[steveo275]

For the walls I plan on using 8' vertical #4 rebar cast into the footing. The walls will be made of 8x8x16 concrete block, filled with concrete to create a reinforced wall. If I use the same ceiling but with colums placed in the center of the span with footings, would it eliminate the need for an arch? I need the ceiling to be square, no colums would be nice. Also, can I leave the excavation face temporarily unsupported while I dig? For the shoring I am using three 8' 4x4 colums and one 10' 2x12 beam to support the 10' span.