Finding bedrock depth to plan septic and foundation types
My partner and I have access to a potential building lot located in an area where the limestone bedrock is sometimes very close to the surface. The specific piece of land is well drained throughout the year. The depth of till above the bedrock will influence decisions regarding whether a conventional septic leach field can be installed vs mound, whether a basement, crawlspace or slab is appropriate etc. I’m considering attempting to use a soil probe as an inexpensive way to begin to an get some idea of the usability of the site and wonder what might be the best way to go from there. Ground penetrating radar? Digging test pits? Any other ideas?
Thanks for any thoughts on the subject.
Hugh
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Ground penetrating radar would probably work but it's often expensive and finicky. Digging test pits is the most straightforward and might be cheapest too. You should reach out to a local geotechnical engineer to discuss options. Depending on access, a pickup truck mounted drilling rig might also be appropriate. Note that all of this will be affected by frozen ground and snow cover. Of course you have to have the current owner's permission before any of this work, and they might insist on restoration.
Easy peasy. A small back hoe can do the whole thing in an hour or two.
Hugh,
I faced a similar situation a few years ago. I needed to assess a lot before making an offer to see if a "normal" septic field and utility trenches could be placed. Before committing to the expense of test pits and soil analysis, I asked if I could probe with steel bar. I used 1/2" rebar sections cut to 5' to test for the required 4' trench depth and the required septic field depth. A 12 lb sledge and a long afternoon driving in rebar confirmed that I had sufficient overburden to make trenching and a septic field possible. Plan on hitting rocks that give false depths and expect to poke in small areas to verify if you are really on the limestone. There are still many other considerations to take into account before digging test pits.
Randomly poking about with a backhoe might not yield as much useful info as a first run with steel rebar. I spent $1000 on combined trailering charges and excavation time to verify my siting choices. I also had a the septic engineer out for a review of the soils before back-filling everything. Plan on having a fence post jack or some way of extracting the rods. Leaving them is not an option unless you have permission and cap them with rebar caps to prevent impalement (plus ever finding them again).
As you note, the info gained will help site the house and septic with the required separations. Depending on the size of the lot you are looking at, you may find that there are competing needs that will make it a poor choice. The size of the necessary septic field typically varies according to the soil percolation rates and the number of bedrooms in the house. In my state, the number of bathrooms is oddly not a factor. You seem to be aware that septic fields need certain in situ soil characteristics or you will need to excavate or do a mound type.
The soil here varies wildly and one critical requirement for placing a traditional septic leaching field is the soil depth UNDER the pipes or chambers. A confounding factor may be the underlying rock structure. It may well tip in a direction that puts your neighbors well, house or septic field at risk. The link below may help in defining what you might be faced with.
https://www.epa.gov/septic/types-septic-systems
I would caution about googling costs of septic systems. The numbers I have seen are quite fanciful in my opinion. I spent over $10,000 on mine over 12 years ago thanks to the tricky but do-able "normal" chamber style leach field. The poor soil percolation rates and the 5 bedroom design parameter combined to push the field size to approx 2500 sq ft of area. This sounds like I have a large mansion, but the local permitting process categorized our hobby work spaces as bedrooms despite not having closets. (another odd way of defining bedrooms) I had to comply and I guess I am future proofed should someone with a huge family with no hobbies be a potential buyer. Currently, I think my system would be $15k now and a specialty one could go $30k in a blink. I think one shown on this old house went to $60k, but it was a steep lot.
If you are going to be placing a well, be sure to check for trenching from well to house and the required distance from septic field to the well head. The bedrock here varies wildly, along with the soil types. Placing utility cables and water lines can be very costly if rock busting a trench to depth over any distance.
And just a thought on the lot being well drained. Think about the surrounding area and what will be draining toward you and away from you. The spring run off here tracks the sandstone bedrock I am built on and I did have to plan carefully for my basement.
Onslow, thanks so much for such a detailed and thoughtful answer. The seller is a close family friend so it's not a typical real estate situation. They're not out to get top dollar, rather due a favor if possible. They're just as concerned with the suitability of the proposed lot as we are.
The lot rests at the high point of a very gently sloped area. I've already got some rebar on hand plus a sledge. I assume grinding somewhat of a rounded point on the rebar might make it drive much more easily. It's been cold recently so there may well be some significant frost in the ground that might make things difficult if not impossible until we see some warming. It's a sweet location if we can make it work.
Not a building lot until it has a septic design and well.
That would be the sellers responsibility if they want money for the land.
Your risk, your money.
This squares with my experience. And land that doesn't "perc" is nearly worthless.
It would be unusual for a seller to allow a buyer to do perc testing.
DC and Gus,
As the Real Estate agents say, "location, location, location". I could show you a 3 acre lot for $750K that is almost entirely a 40 degree slope. All rock. Definitely going to be an interesting challenge placing a septic field. I am pretty sure the seller has no intention of putting in a well or providing a septic plan.
In real estate sales it is not at all unusual to have contingencies written into a proposed sales contract. Many times purchase may be contingent on sale of a prior home, but one can try to make a sale contingent on a number of factors. The response of the seller will depend on how much they want to sell or for how much. How many or what kind of contingencies the buyer makes will be driven by how desperately they want the lot.
In my area, well permits are not easily obtained and there are no guarantees as to what kind of water you will get. Salty, arsenic laden, sulfur, etc. Despite risking a 700' dry well or a $60K septic system, lots do sell.
I am guessing the OP's lot has some desirable features that will make the effort worth while.
60k septic system, a bargain
They get much worse
There is, in fact, a sucker born every minute
Land indeed sells.
It is not a building lot until the prerequisites are met
I almost bought an industrial lot some years back that would indeed involve running a perc before owning it. You have to have it under contract, because the moment it percs, it is worth more money. Now this was a city owned lot, so wasn't real worried, but the 'partner' I would have been tied to didn't answer the phone, so nope, walked away. That lot sat for more than a decade.
There is a difference in price, and there is a difference in buyer
The problem from the seller's perspective is that if they accept that contingency, and the lot fails, now it's a known defect that they have to disclose to future buyers. If you're trying to transfer risk to the buyer that's not how you want to do it.
That said, people who deal in raw land are not always rational.