I have a job where I need to set a 20x10x10 tank. My problem is the soil is clay and I am in the water table 8'. I need to minimize soil disturbance but need a pit 30x20x10. Just seeing how others do there shoring pits out there. I am looking for the safest most economical way to do this.
This is what our PE said I had to use when I dug and set concrete boxes in heavy clay soil.It is by no means economical but was required and it is the safest.
Sounds like a good job for a slide rail system. Contact a shoring manufacturer in your area. If there arent any, get ahold of Efficiency shoring, or pro tec shoring in Michigan, they will be able to point you in the right direction. We opened a hole for a 30 foot deep manhole today. We were well over 60 feet wide at the top. Our biggest issue is we are in water from 6 feet, to 28 feet where we hit clay. A combination of steel plates, with H columns to pin them in place, benching and sloping, and manhole box was used. It worked decently for use since we had the room to work.
Last time I used the slide rail system I had a hell of a time getting it out of the ground. I had to dig the whole thing up. The slide rail isn't water tight either. I have rented the sheet piling from mabey as well and that wasn't that bad but banging sheets in clay isn't easy. To use the slide rail I need to bury some deadmen 20' back to anchor the middle support column since the pit is 30' long. Got any tips?
can you use the trench boxes? not sure what the tank is for, but more than likely you will be using rock to set the tank on,can you dig a sump in the corner inside the box and pump it out? a 6'' pump can handle a bunch of water if you have some where to pump it to.
Trench Box I think I am going to go with this trench box with 20' steel plates to close up the sides. What do you think? http://northeastshoring.net/10h_x_36l_brute_9_trenchpit_box
should work,how do you plan to dewater? whats the weight of that box? make sure your hoe will handle it.
It will work. We did this last week. Had a 28 foot deep hole float over the weekend on us. We put 24ft spreaders on the box. Set it in place, then placed 3 road plates on each end, and two on the sides of the box to block the lifting eyes. Those two were mainly due to being in silty sand. You cant use plates with pipe spreaders FYI. Just a tip, have a good pile of washed stone on hand.
That box weighs like 35,000 lbs put together. Its 10' tall so I will have about 2' sticking out of the ground. I will most likely get a rough terrain crane for a day to put it together and set into a pilot hole. I could try a double pick with two 345's with the buckets off but the crane is probably the way to go. Then I will use the 345 to work it down to depth. This has box beam spreaders on it which can take the weight of the plates. I want something sturdy as I will most likely have to leave it in the ground for a month. There are piles, pile caps, grade beams and slabs holding this tank up that need to be built first. The tank weighs about 60,000 lbs. I plan on over digging by 1' and putting in 1' of crushed stone with a sump pit in the corner. Hopefully 1-2" pump will handle the water if not Ill have to put a few in.
Why can't you use plates with pipe spreaders?
The engineer for me wouldn't sign off on the excavation design with the pipe spreaders. He was worried they aren't strong enough to take the inward force. The Box beams we are using also give the plates much more surface area to sit on. With the pipe spreaders the plates rest on a very small area of the pipe. Also the box I am using has a spreader close to the bottom and one close to the top. I just haven't decided to use the 16' spreader or the 24' spreader. The tank is only 8' wide on the outside. I just worry the box wont be positioned perfectly in the middle and there wont be enough room.
I worked on a job a few years ago where the hole was 30 ft deep and we had lots of water coming in as we were digging just under the water table. So what was done was a sump pit was excavated three or four feet deeper than the bottom grade of the hole and a piece of galvanized steel culvert was put in the sump pit and it had holes drilled in it all over the place like a piece of weeping tile and a piece of geotextile cloth wrapped around the culvert to keep silt and mud from plugging up the pump and killing all the fishies in the river. lol But the culvert and filter cloth worked really well as the soil was a mixture of silt and sand and finally clay at the bottom.
the sump with clean rock will work,A D S pipe with slots cut in it with gravle placed around it keeps the mud out,just make sure your slots are big enough to keep the water going to the pump quick enough, if it were me i would want the 16' spreaders for an 8foot tank just use your center line for your tank as centerline for the box when you dig the hole. i must of miss understood i thought you were in 8 feet of water,i dont think a 1'' or2'' pump will keep up if the water is not to bad a 3'' trash pump will work great.also when you backfill you can lift the box as you go so it wont get stuck in the ground.hope this helps.