Prepare site for large tanks

I just ordered two 5,100 gallon tanks to collect rainwater.  It will take a couple weeks for them to arrive so I am about to prepare the site where they will sit and I'm looking for recommendations.    

My concern is - when filled with water these things are going to weigh about 22 tons each!    Though I think the surface area is what will save me.   If my math is correct - a 12' diameter tank will only have about 2.6 pounds per square inch applied to the bottom.    

The manufacturer was less than helpful.  They said "pour a concrete slab or put it on grass".    Put it on grass?  REALLY?  I don't have time to plant grass that will just die under the tank anyway.   And I'd rather not incur the expense of a concrete slab if I don't have to (2 tanks * 12' diameter each means I'd need a pretty big slab)

I was thinking about 4" of pea gravel.   Good idea?   Or is there a better medium to put under it?

Your calculations are correct — and 2.6psi isn't all that much and the thermal mass of the water should prevent any crazy frost heaving in the winter. What's the concern with putting it on the ground and just letting it settle a bit? You're going to loose use of that ground with pea gravel or a slab too.

I think when the manufacturer said "or just put it on grass", they meant bare ground versus a slab. As long as the surface is clear of any sharp objects that could puncture the tank it should be fine. Since top soil could settle over time and cause the tanks to shift, I would suggest you remove the top soil and try to get down a little, then compact that subsoil and remove any rocks or roots you find. Then to be extra careful, you could put down a little sand and tamp that level too.

It might add a few bucks but beats possible future damage and the repairs.

Kyle Neath wrote:Your calculations are correct — and 2.6psi isn't all that much and the thermal mass of the water should prevent any crazy frost heaving in the winter. What's the concern with putting it on the ground and just letting it settle a bit? You're going to loose use of that ground with pea gravel or a slab too.

Good point about losing height.  

I live in the mountains of western Montana so my ground is mostly rocks from fist sized to VW sized (with a little soil mixed in there somewhere).   My concern was, with that much weight it could settle over time and bring some sharp rock to the surface.  Piercing the bottom of my tank.

I need to excavate the site anyway to level it.  So maybe I'll just look at the condition of the ground when I'm done and see how threatening it looks.   Who knows?   I could get lucky and be able to drive the excavator back and forth a few times and smooth it out enough to where just going bare ground might work.    

I've just never worked with anything THIS heavy before and didn't know what kind of complications it introduces into the engineering.

what about road base gravel?  This packs down really flat and usually costs less than pea gravel.  It's pretty easy to level off and tamp down and, in theory, won't shift much with the weight of the water.  Well, that's the theory.

The best underlayment for large water tanks is compacted sand and generally it is laid a minimum of 4" thick once compacted.
This gives a nice smooth surface but also allows for drainage under the tank.

Redhawk

Bryant RedHawk wrote:The best underlayment for large water tanks is compacted sand and generally it is laid a minimum of 4" thick once compacted.
This gives a nice smooth surface but also allows for drainage under the tank.

Redhawk

Thanks for the insight.  Any recommendations on how to compact it down?  I have a small tractor, but might be doing the work by hand if it's too close to the house.

I use a hand tamper (pole with a heavy, flat tamper base) They are usually available at hardware stores (home depot, Lowe's, Ace hardware, etc.)

If you have a tool rental place nearby you could probably rent one of the compactors like they use for asphalt laying that is hand directed and motorized for vibration tamping.

Redhawk

Bryant RedHawk wrote:The best underlayment for large water tanks is compacted sand and generally it is laid a minimum of 4" thick once compacted.
This gives a nice smooth surface but also allows for drainage under the tank.

Redhawk

I originally considered sand - but I'm worried about erosion with the heavy spring rains.    Are your tanks covered?

My base is contained by 2x6 boards and we have had torrential rains (7 inches in 3 hours) this spring, none of the sand moved. I do not have my tanks covered so far but they are under trees.
I have plans to build a roof over the tanks and when I do the roof will be another collection point to fill the tanks. (I'll probably need at least one more tank too)

I concur with using sand.

I'd like to add one more thing to consider, which is flexibility in the piping. This could be a currigated flexible pvc pipe like used in swimming pools, or the black rubber type connectors that have a clamp.

Ive had a tank settle enough to break the rigid pvc pipe i plumbed it with .

wayne fajkus wrote:I concur with using sand.

I'd like to add one more thing to consider, which is flexibility in the piping. This could be a currigated flexible pvc pipe like used in swimming pools, or the black rubber type connectors that have a clamp.

Ive had a tank settle enough to break the rigid pvc pipe i plumbed it with .

GREAT TIP!

I had not considered that - but now that you mention it, I can totally see that happening.

I also live in NW Montana, lots of rock that makes digging a real chore. My cistern, 1800 gallons, fills from my well and is underground. The advantage to me is nothing freezes and it is very stable. If you are excavating anyway, why not just bury the thing?

The first thing that needs to be determined is what the tanks are made of....??  If they're fiberglass or some type of plastic then they can sit right on the ground.  Putting them on grass is pointless as the grass will be smothered in a  few weeks - makes me think your tank supplier is about one taco short of a combination plate.  Metal tanks should NOT be sitting right on the ground as the bottom will rust out.  Even a concrete slab can be problematic for metal tanks in wetter climes or regions with lots of freeze/thaw cycles.  The simplest solution for a metal tank is to put it on bed of coarse gravel, at least 3/4"- or larger.  That allows water to flow down - away from the tank - and means only a small amount of the bottom surface of the tank is actually in contact with the edges of the stone, with air in between. I set up a 3,500 gallon metal tank on a bed of 3/4"- gravel in 1985 and its still in good shape.

roberta mccanse wrote:I also live in NW Montana, lots of rock that makes digging a real chore. My cistern, 1800 gallons, fills from my well and is underground. The advantage to me is nothing freezes and it is very stable. If you are excavating anyway, why not just bury the thing?

My first thought was to bury them.   I liked the freeze protection that would offer.  But had two concerns:

1.  They are big.   REALLY big (5,500 gallons each - 12 feet in diameter - 9 feet tall - times two).   That's a pretty big hole - but with a big enough excavator it could be done.  So really this was not the deal breaker.   Just a lot more work.

2.  Crushing.   This is what ultimately scared me away from this idea.  I've read that you have to fill them before burying or the sides may crush from the weight of the soil.    I've also read that you should not drain them once buried for fear of the same thing.   They should be kept at least half full.     Since this is a rain catchment, I can't fill them prior to burying unless I wait a year for the rain to fill them and then come back and bury them next year.   Plus, they will have to get me through the dry months.  Which means - by the end of August they will probably be pretty close to empty each year.

I do intent to set them down a couple feet underground.   To give me more clearance for my roof height.

Wow. Huge. I understand the need for water security. I would like to have a windmill for my well so that I can be more off line, and perhaps a second cistern in tandem with the first. I like the thoughts about gravel for underneath. At some point I may want a small house here constructed with grain silos and the rust issue is important. Thanks for all the input here.

Sand is too instable. Road base is right (or something similar). Put some boards around which you hold with star pickets, level with a rake an hose it a bit in. Our tank is about 20.000litres and we did it this way.

Krofter Young wrote:The first thing that needs to be determined is what the tanks are made of....??  

I thought I was crazy for not seeing this somewhere... I think this is critical.

Without knowing what your tanks are made of, if I was going to make a safe, lasting bed for anything I'd first take as much vegetation away as possible and scour the area for rocks. Then, I'd get sand. Builders sand is the least expensive but white sand, like bedding for above ground pools, seems best. It has the ability to move with the pressure from above such as the pool floor.

Could you consider the expense of excavating and burying only one container and and have the other on  level ground? The buried tank will be protected from the elements and accidents and can supply the house year-round; the above ground tank can channel water to the lower level and hold the overfill/flow before freeze up. I don't know how much water you expect to catch or use but lots of us learn to ration during the "dry" winter.

Hey! I'm down under and have many water tanks. The largest is just shy of 10,000 gallon. It's big.

We use a layer of rock crusher dust underneath water tanks and that stuff compacts amazingly. It is like a really finely ground granite. If you can't get your hands on that, then a layer of sand will do the trick nicely.

When full, they weigh so much that they settle the ground.

The only other trick to remember is do not dig a trench running parallel and close to one side of the water tank. The trench will compact and the water tank can tilt. It happens.

Instead, dig any trench immediately away from the water tank as if you are trying to get away from the water tank as fast as possible (or like the spoke on a bike wheel). Then when you are a short distance away from the water tank then you can change direction.

Chris

I just read some of the other replies.

It is probably not a good idea to put too much material against the side of the water tank - if you bury it for example. The sides are flexible and if the water tank is not full...

Chris

Lots of large tanks around where I live. We use a 70 - 100 mm pad of road base or similar, often covered with a geotextile. My last tank was 32,500 L (~8,500 gal) and it sat on 100 mm (~4") thick pad of road base. You can remove the sod, level then lay your pad or if the site is already level just mow very short then lay your pad. The only compacting I did was with the back of a shovel.
At my new place, the tank is about two thirds the size and I intend to do the same thing except for the addition of some geotextile on which the tank will sit directly. I'm only adding the textile because I have it. It came with the tank.

Re: What about road base gravel?  This packs down really flat and usually costs less than pea gravel.  It's pretty easy to level off and tamp down and, in theory, won't shift much with the weight of the water.  Well, that's the theory.

We put in two 2500 cement gallon water storage tanks last year. The manufacturer said to set them on pea gravel but that was not available. After asking several contractors (and the company doing the install of the electrical panel for the pump) we got the same answer. Use 3/4 inch minus rock. Level and tamp before setting the tank in. Fill in the sides that are underground with soil/rock not containing big sharp things that could push up to it and cause an air space or fill with water and freeze during the winter. But I think a little sand would be fine too. I know yours are plastic but that's what they all recommended for both cement and plastic.

From some experince working civil activities in industrial plants, you need to know what your soil is doing before making a decision on what to add underneath the tanks. Do some research here:

http://extension.psu.edu/plants/crops/soil-management/soil-compaction/diagnosing-soil-compaction-using-a-penetrometer

https://www.certifiedmtp.com/soil-pocket-penetrometer-aluminum/?gclid=CjwKCAjw8IXMBRB8EiwAg9fgMLQ-KDfipU3dnAsNaeNX-MpyvcTMJGp_0ZS6vOusCcHsxLFqmmxujxoCTA4QAvD_BwE

Also keep in mind burying a tank where I'm at (South Texas / Louisiana) can be dangerous in floods, the tanks will float out of the ground without any restraints when the tank is empty and the ground gets saturated for a short period of time. Someone mentioned piping, if your tank moves or your tank/pipes grow due to temperature change you can have issues. Likely not a big deal but some food for thought.

What i would do if i didn't already have a pen test:

1. Remove 8-12" of topsoil
2. Remove debris and compact sub grade to best of ability
3. Add lime stabilized clay/fill or sand, 6" or more, and ideally few inches shallower then excavation
4. Compact to best of ability.
5. Add tank and slowly fill
6. After 7-14 days pipe accordingly.