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Looking for Ideas for an excavation

5037 Views 26 Replies 13 Participants Last post by  Upchuck
I am getting ready to bury a 10,500 gal water tank for a fire suppression system. The tank is going in the ground 9' from an existing building, the top of the footing is 5'8" below grade(1 foot footing). The dimension of the tank is 13'2" top to bottom, with either 12" of stone under it or on undisturbed soil, there is a 4' round sump station located 5-10' away from it/building that is 4'8" deeper than the bottom of the tank.
My question is this: the tank was supposed to go in back in the early part of sept when the water table was much lower, but the engineers and precast co have been redesigning if for the last month and a half and the water table is up substantially, finish grade on top of the tank is 410' and the water table is at 394.
I need to make sure that the building does not shift but with the water up I am worried about the 45* slope leading up to the existing footing is going to undermine. Driving sheet piling will cost 22K, I was going to underpin the corner originally, but with the water up that doesn't seem like a good idea. I obviously will de water and drop the table some, but am nervous about the proximity of the building. It is up to me for BMP to design the support.
Any ideas?
to add insult to injury the engineers wont answer the questin how much ground cover over the tank.:laughing: I guess it is not that important

Thanks in advance.
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Tank dimensions

To answer a previous question the tank is 16' long 8' wide, 13' 2" high. the soils are cobbly gravel as it is uphill from a river. I dont think the ground water comes into play up there until the very bottom of the excavation for the tank. The sump chamber is another story as it is almost 5' lower, but it is detached and 5-10ft away from the far end of the tank.

existing -
- O ----------sump
----------- * Gah! this drawing worked before.
[ * * musta got reformatted
[ * *
[ * *
[ * *
[ * < ------Tank
New addition -

that is the proposed layout, like I said the redesign is not even done yet.
I like the ideas so far, one thing I thought of asking was if I could replace any of the load bearing soil that gets undermined with flowable fill. It is really the 45* angle of load bearing soil that I am most worried about.

thanks for all of the input
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Just got the redesign bak and it looks like the new tank 11x15x9 will keep me up out of the water and away from the load bearing soil. I guess that they didnt like the number I gave to shore up the excavation.:laughing:


This sucker is FINALLY going in the ground today, after a 2 month fight over the change order to build the tank to code and who was going to pay it(I won) I get to put this thing to rest today.:clap: This is the last bit of UG on the project from hell. I will post pictures later of the dig and the tank. Water was only an issue on the last 2 feet. I ended up using a 10-16ft trench box for the deepest portion, and placing a sump in the low corner of the hole with a 2" submersible pump going 24/day. The tank got reconfigured to be done in two parts: one 8'x16'x11' and one 6ft round wet well 17' deep, 12" blue brute connecting the two. Had to pull 3 ft of frost over the weekend to start the dig on monday, used a Thawsall and as Dayexco would say it worked slicker then snot on a doorknob.
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