Septic tanks in 18" ground water and running sand

I have 2, 2000 gallon holding tanks to install in 18" ground water and running sand. To top off house is existing along with trees and garage so not lots of room. Was thinking trench box 20x20 with steel plates on each end. Sounds good but rental guy says no. 20' spreader not available so I priced new pipe 3k he still saying not good idea I will get the box stuck. I have a cat 320 & 315 to work with. I'm thinking of buying 4 used steel plates and bolt the corners together should weigh about 26000# hopefully be able to lift out with both excavators. Am I crazy to think this will work? I've been doing septics and excavation my whole life but never had this much water or running sand. Any other suggestions? It is tidal water so dewatering is not really a option.

Dewatering is the only way to go even if it's tidal. Bash a well in and hook it to a good sized pump, 6" or bigger then leave it running for 24/7. Just make sure the well is below invert level. Iv'e been on heaps of sites like what you have discribed and they always have turned into a $hit fight because they wouldn't dewater and in all cases they did their dough or bailed out. Others came in and dewatered then made a squillion. An outfit I worked for got a job putting a raising sewer main across a tidal river in the days before directional boring. They sheet piled half way across the river making it like a large box and filled it with river gravel, put in well points/spears and a well and pumped it for 10 days. Result, a dry hole, layed the pipe, blanked the end then moved everything across the river and did it over.

The only problem with dewatering is where I send the water. It's a residential lot less than a acre in critical area and have been told it's ok to dewatering on day of excavating but must use filter bags and straw berms to contain. This is just going to saturate the area and run back to the wellpoint I think but maybe Im wrong. I got a price on sheet pile and that is way out of line for the budget. 50k for a 20'x 20' hole 7' deep I have to dig inside there sheet pile. House is a rebuild after fire for 96 year old woman so it's a rush to get her back in but also has limit on funds. Originally figured 20 k ballpark 7 for tanks 4/5 for sureing and rest labor and profit. But now I can't find someone to rent a box wide enough to set both tanks and counterweight. So priced out making my spreader bars that's about 3k but people are telling me 20' wide spreder is to much then rent plates for the box end. I'm worried I won't be able to get them back up though with a 320 & 315 excavator. That's why I'm thinking plates bolted together as I could backfill the tanks and if I can't pull it I can dig on outside hold with the excavator and burn the bolt off with long torches far enough away that it won't hit me when I cut. I know not best plan but it's residential and won't have eyes on us. This way I could pull one plate at a time. No one will need to get in the hole at any time.

Have you thought about doing one tank at a time, that would drop your hole size considerably... yeah you got to do it twice but sometimes that's just quicker than a bigger cluster.

Pete, one other problem you face is that the tanks will float out of the ground if you can't control the ground water. Which ever method you end up doing ensure that you have enough water on site to fill the tanks before backfilling the hole. I've seen swimming pools that were poured insitue float because the builders wouldn't listen. They had to start from scratch and redo the job on their dime.

I guess it is not your problem but this made me wonder how they expect to pump and clean out such a tank in the future, to keep it from floating.

Run a fire hose into it at the same rate the vacuum truck is removing it and hope you get most of the insolubles?

He mentions counterweight 4 posts up.

So what you really need is some sort of aeration device fastened to the bottom of the tank and a good sized air compressor and just let it 'quicksand' into the desired depth... when it gets there just fill it with water, shut off the air and you're done

Just thinking outside the box

Ronsii, obviously you'd have to fill them with water first. Maybe a cheatah style reservoir for water and air to blast it underneath? Or steel plate bolted tight to the tank and projecting 1' below, then a suction hose to remove water and sand from under it, once it reaches the desired level, pump sand back till it stops.

@Delmer LOL, yeah I thought of that after I posted it

The counter weight is supposed to stop it from floating as these are holding tanks meant to be pumped completely empty.

I like the idea of pumping them down had thought about it but thought maybe to far outside of the box. The only issue is they have to go where drawn on the plan which is side by side so doing one at a time is probably not going to work as it would disturb the first one set. Also need to bury the counter weights in the hole. On top of all this they are tar coated then once set needs to be vacuum tested in place so if I crack it I still don't have anything. The counter weights are just over size parking stops cabled over the tank.
I do have a 4000gal vac truck and a 185 cfm air compressor if anyone thinks it will work I'd like to hear more thought on it.

I do stuff like this all the time, just not on this scale, so to speak. There's no way I'd try it first in the location you describe, only in an open field with similar conditions and plenty of room to work.

Almost certainly not worth the effort. If you are desperate for work and don't mind playing around, I'd clamp the steel right to the side of the tank, and have a pipe going under the tank to the center then projecting down maybe 2'. 1/4" plate 24" wide as a collar on the tank, with 6-10" projecting below the bottom of the tank. I can think of a lot of ways to make it removeable and reusable for the second tank, or leave it in place.

06Pete said:

The counter weight is supposed to stop it from floating as these are holding tanks meant to be pumped completely empty.

I like the idea of pumping them down had thought about it but thought maybe to far outside of the box. The only issue is they have to go where drawn on the plan which is side by side so doing one at a time is probably not going to work as it would disturb the first one set. Also need to bury the counter weights in the hole. On top of all this they are tar coated then once set needs to be vacuum tested in place so if I crack it I still don't have anything. The counter weights are just over size parking stops cabled over the tank.
I do have a 4000gal vac truck and a 185 cfm air compressor if anyone thinks it will work I'd like to hear more thought on it.

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Maybe using the vac truck to control the water and the 320 flatout digging with the 315 at the ready to lower the tank you may just get away with it. Your'e going to have to go like hell though. It's a pity the tanks are tared, any other way of putting them in the hole will take the tar off I'm guessing.

First thing I want to ask: Is this a hard money job? If it is, I am glad it is not be doing the work.
If it is T&M, does the customer have the money to pay you? You are talking about a lot smaller than an acre, It sounds like the cost will be more than the lot is worth.

I have worked for this builder for 17 years he always pays might be slow at times but it comes. I used to just do everything to had rate for all machines but lately he has been getting budget# first this one the budget # I gave blew his mind so I'm trying to figure out a way to get it done in theory cheaper but will make sure he knows if that doesn't work it's his dime to cover whatever it takes.
The lot is on the water so lot value is high and it has sentimental value to the owner to.
I don't have a field with like condition to test this in.

Well, you can stand with the big numbers and use it as a not to exceed. I have used liquid nitrogen pumped into the ground to freeze a 14'x14'x12'deep space to dig, mighty pricey and you gotta be quick. We were setting a small vault that "had to be exactly there." There are numbers you can find that will tell you how much you need to do what you want to do. I had a dirt contractor digging for me and he found the numbers. It worked and we send some nitrogen back in the truck, so it didn't cost as much as we had anticipated.

I once watched a 25,000 gallon fuel tank erupt from the ground after being in place less than a week. It lifted the slab it was anchored to. In Seattle, there is a lot of dredge fill on the waterfront and the engineers didn't figure a slab heavy enough to match the tides we get once or twice a year. Asphalt paving contractor was just starting to lay down gravel. We all stood looking amazed as it came out of the ground. "It was supposed to be able to stay down empty...."

no ideas
sounds like a bad job to do
sounds like it is going to pop out of the ground some time
what about a concrete vault instead of plastic

They are concrete and still need the weight to not float. 13.5x6.5 x 5.25 weighing 20000# each plus counter weight of 5000#

Drive steel pilings and dewater it. I have seen it done for bridge piers many times. There are piling extractors available to pull them when you are done.

If you've got an 18" ground water problem are custom tanks an option? Might take more square footage/and an additional tank but we've used low profile tanks in similar situations, creativity in the tanks you install might alleviate the issue