# Underground structures detection



## zorro (May 6, 2009)

Does anyone knows a way to detect underground structures? Let me explain my problem.

I own a very old house. The _kind of_ skeptic tank is tough by most elder neighbors to consist of two cavities dug in the ground. I do see two quite big spot of lawn (a few feet wide each) that are "falling down" since I bough the house. Each year, the two depressions seem to lower even more. I think these are kind of covers (probably wood) and I'm afraid they will break at one point as they are obviously very old. As I have no clue of the size of the cavities, it's hard to figure out how close I can safely get. They must be quite big since they are receiving poop and greywater for 90 years now. 

Any advice on how I should proceed to determine the size of the cavities so I can establish a safety perimeter and then fill the cavities or build new covers?


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## ZoomZoom (Dec 18, 2009)

Have you considered digging down to find the cover? Putting in a new sewer system can get pretty pricey. I'd remove the dirt to examine the cover. I'd then have a honey-wagon come in and pump the tanks out. I'd then make new lids if needed.


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## NaeKid (Oct 17, 2008)

You don't describe where you are living, but, here in Alberta, we have many companies that do ground-scanning using a sonar-based system. The sonar-systems are used to map underground caverns that could possibly hold water, oil, natural gas, etc prior to the drilling-crew moving onto the land and tapping the resources.

Some more information about how that kind of technology works, you may wish to peak at: GPRS --Ground Penetrating Sonar


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## northernontario (Oct 29, 2008)

This is why you should always make the sale of a house conditional on having the septic system pumped (or inspected). 

It's very likely that you'll be investing a lot of money to fix this...

I would do what bczoom suggests, and start digging. Of course, the first thing to check is where all the piping leads in your house. (I read in another thread that you've got a crawlspace on rock)... find the piping coming from your kitchen/shower/toilet... locate where it leaves the house through the foundation. If those sinking spots are nowhere near "in line" with where the drainpipe leaves your house, then dig anyways, but likely they aren't septic. 

And when you do dig... when you do find your septic system... make a detailed drawing so that someone else can find it later! 

Be careful when digging; have someone nearby if things do collapse. Old septic systems are not something to mess around with.


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## zorro (May 6, 2009)

dtompsett said:


> This is why you should always make the sale of a house conditional on having the septic system pumped (or inspected). It's very likely that you'll be investing a lot of money to fix this...


You're right, that's why I paid my house 1/4 of it's otherwise normal value  - and why the previous owner was not able to sell for years. The law here says one cannot fix such an old non regulatory system. One is required to build a new system. One cannot even remove the cover to have the content pumped or inspected.  But you have the right to continue using it unless it smells so much that someones complains to the authorities or it contaminates someone else well (which is not likely to happen in my case).

I'll check if there are sonar companies around here. The two ground depressions are not in line with the pipe exiting the house, I've already checked that.

I know it's dangerous to fall into that and you can die if you smell too much the gas that forms inside it.

As I don't plan buying a new system (I plan building a composting toilet and a greywater treatment system), I would just like to determine the size of the cavities, secure the whole thing so nobody dies and I don't have trouble with local authorities.


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## JeepHammer (Oct 10, 2008)

zorro said:


> Does anyone knows a way to detect underground structures? Let me explain my problem.
> 
> I own a very old house. The _kind of_ skeptic tank is tough by most elder neighbors to consist of two cavities dug in the ground. I do see two quite big spot of lawn (a few feet wide each) that are "falling down" since I bough the house. Each year, the two depressions seem to lower even more. I think these are kind of covers (probably wood) and I'm afraid they will break at one point as they are obviously very old. As I have no clue of the size of the cavities, it's hard to figure out how close I can safely get. They must be quite big since they are receiving poop and greywater for 90 years now.
> 
> Any advice on how I should proceed to determine the size of the cavities so I can establish a safety perimeter and then fill the cavities or build new covers?


Long, steel rod with 'T' handle at top.
Wait until the 'Rainy' season, go out and probe.
The same way we find old cisterns, septic tanks, root cellars when I worked on the house moving crew before the heavy equipment moved in to work.

You will rarely find anything deeper than 4' underground.

Work towards the 'Sink' to find the spot where there is actually something there...

Gutter drains are usually just a foot or two under ground,
While most septic systems will be about a foot to the lid.
You will find the edges of a septic tank before you reach the lid...

If you find it's a cistern or septic tank you don't use anymore (City Sewage system) then you can just backfill the tanks without issue.

If its an ACTIVE septic system...
Then you are in for some REAL FUN!
Nothing like having to clean out or replace a septic tank and leach field!
More fun than I ever care to have again!...
--------------------------

Most of the time when you have a 'Depression' in the yard, 
It's from some idiot digging through a drainage tile,
OR,
One of the old clay/ceramic tiles that were used for everything has collapsed and the yard is draining away via the drain system...

The idiots in the gas company dug through my rain water gutter drains when they put the gas pipeline in,
And now my yard has 'Sink Holes' in it from them...

I'm sure the city sewage treatment plant is suffering from all the dirt coming through also...
(about 85% of 'Solid Waste' processed by waste treatment is dirt that has crept into the system)

Anyway, a simple rod called a 'Probe' if you buy one, will do the trick for finding what ever the culprit is.

I've even seen old tree stumps rot out and cause depressions as they rot down, so be prepared for anything!


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## zorro (May 6, 2009)

I'm really happy to have had all these suggestions. I've checked, there are no sonar companies around here. But I'll try JeepHammer ideas as soon as snow is melted (there is still about 4 feet deep of snow everywhere around the house). I'll let you know if this works or else how I will find out about the mysterious cavity. 

We will also definitely build a compost toilet this summer (with ventilation + solar rays to help desiccate things faster). That way, we hope we may extend somewhat the life of the existing septic system.


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