Ramakant writes: We are in discussion with a builder to buy a lot to build our new home. Currently it’s a wooded area. However we discovered that this particular lot had a pond at one time covering almost 50% of the lot.
Is it safe to build a new home on this lot? If so what are the things we need to consider?
The builder says that we just need to stabilize the foundation with more concrete and other things. He says that this is nothing unusual and that they do very often.
Steve Bliss, of BuildingAdvisor.com, responds: I would proceed with caution because a wet building site could bring you headaches for years to come. If any part of your foundation or septic system (if needed), is below the water table for part of the year, then you will have problems. It is nearly impossible to keep water out of a structure built below the water table. Also, building on wet ground can raise moisture levels inside the house, leading to mold and mildew.
If you are building on a part of the site that is “high and dry” and well above the surface level of the former pond, then your house should be fine. In fact, it’s possible that the pond could be restored and become a positive feature of your land. However, you still need to address the pond area and understand why there was a pond there and whether it will return – or possibly remain a wet and swampy portion of your land.
Whether natural, man-made, or a little of both – that is, a natural spring or pond that was enlarged by a previous owner – any pond needs a water source. The source is either subsurface water or surface water, or a combination of the two. Subsurface water is either the water table or a spring, while surface water is either a stream or rain and snow runoff from a large area. In either case, you can expect that the water source will increase and the water table will rise during the rainy season or after the snow melts in cold climates.
Since there was once a sizable pond here, there are still water sources and the water could certainly end up in your basement or crawlspace. A wet site can even cause problems with a slab foundation as the excess moisture in the soil can pass through the concrete and into your home, resulting in high moisture levels and possibly mold and mildew.
Homes can be safely built on wet sites, but special drainage around the house and foundation and an effective moisture barrier below the foundation will be required. As for the type of foundation needed, that depends on the type of soil and the weight of the structure. Again, a house can be built on any type of soil, but the weaker the soil, the more difficult and expensive it is to build a suitable foundation.
Just because something is possible, it does not mean it’s a good idea. Before proceeding, I would get the opinion of an engineer who specializes in this area – called a soils, geotechnical, or civil engineer. In an hour’s time, they could tell you whether it is feasible and wise to build on this site. Also be aware that special zoning rules may apply if the land is legally considered a regulated wetlands.
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Rick says
Discovered Wet, Loose Soil When Excavating For Foundation
Hope all is well. We’ve started our family house construction project in Toronto, Canada using a local builder. We demolished and started excavation and have hit our first big surprise already by hitting wet/loose soil conditions. As a result, geotechnical soil test was conducted. Below is a summary of the current solution which definitely adds a significant cost to the project. I would appreciate your commentary on the solution to see if there is any opportunities to improve or save costs or other considerations. If you have any questions, please let me know as well.
1. Dig Drywell in back yard
2. Excavate down past the under side of footing by roughly 8″
3. Fill 6″ up with clean gravel and weeping tile
4. 2″ lean slab on top of the 6″ gravel
5. On lean slab, footings will be installed. Around the exterior of the footings, weepers will be installed and covered with 1-2′ of clear gravel
6. Inside the footings/house weepers will also be installed and covered in the typical 6″ of clear gravel with a 4″ basement slab on top
7. Exterior foundation walls will get upgrading water proofing, and interior foundation walls will also get a waterproofing system
8. There will be 2 sump pumps
a. Lower Sump pump (1)- will have the weepers under the lean slab and the soak away pit on it
b. Upper Sump Pump (2) Will have everything that is above the lean slab, footing weepers and interior house weepers.
Also, in general the approach is we will start digging with a hope to find solid ground and improvise from there and hopefully not hit a water table issue.
buildingadvisor says
Sorry to hear about your unpleasant surprise underground. When I get over my head with structural or foundation issues, I do what you did — hire an engineer and almost always follow their advice.
I would not want to second guess his recommendations as he has seen and tested the site and has the background and professional credentials. (He’s a specialist; I’m a generalist.) “Wet, loose soil conditions” could mean a lot of different things.
Engineers do tend to be conservative in their recommendations (for example using a 2.5 safety factor in structural sizing), but that makes sense given the many uncertainties of real-world performance of buildings. This includes variability in materials, workmanship, loading, and environmental factors.
Builders have told me on more than one occasion that the engineer is “overbuilding,” but I’ve never seen a failure in their work. Consider it an inexpensive insurance policy.
Given the complexity of this system, it would probably be worthwhile to have the engineer inspect the work before it is buried under backfill.
Not sure why he is asking for waterproofing of the interior foundation walls. Does he mean the interior of the foundation walls or are there foundations walls inside the foundation perimeter?
I would also want a little more detail from the engineer on the size of the drywell along with the drywell design. Also what dimension and type of drain tile and drain piping (perforated vs. solid) does he want Also, I assume he wants landscape fabric protecting the gravel from getting clogged with silt. The devil is always in the details.
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R M says
This solution was proposed by builder using the geotech engineer report.
A quick update – To get a more definitive solution and risk assessment, the engineers suggested boring test holes and then providing their evaluation. So, we are proceeding with these boring holes – even though this approach comes with some additional cost ~7.5% of the total solution described below, it will provide a more reliable plan and approach instead of excavating and then reacting.
Thanks again for your guidance – it has already helped me guide toward leaning towards getting an official solution/approval from geotech engineer.
Katy Wenderly says
Considering a Wet Site? Just Say No!
If you are considering buIlding a new home on wet ground, here is my advice. JUST SAY NO! I have lived in a home built on wet ground for 30 years. We bought the land in 1988, which happened to be a record-setting drought year. No water problems were evident. The house was already there. We did not build it. That’s the only part of the story that I stress. We did not build the home. But we thought that it would suffice until we could build on a high and dry spot (which was also part of our land). For many reasons, that never happened. We have battled mold, rust, mildew, fungi, and bugs of all kinds, including their larva. physically, we have suffered from rashes, allergies, and other problems that I won’t describe. We have put thousands of dollars into drainage systems in an effort to stop rain water from streaming into our house. For the most part, that problem seems to be solved. But our concrete slab still “floats” on wet ground. During spring when the snow and ice melt, we often have water percolating up through cracks in the concrete slab foundation. This happens in the living room.. I am constantly on the internet looking for a new home. We we young, and just HAD to have this property. And it is beautiful here, esoecially up on the hill where the land is dry.! Best of luck to you.
RICHARD says
How to Build a Foundation on Swampy Land
I am considering building a home on a swampy site. How do you deal with a foundation on soil that is wet and marshy? Any suggestions would be appreciated.
buildingadvisor says
Where I live in the Northeast, swampy areas are called “wetlands” and building is generally not allowed. However, it is possible to build a home or commercial building on wet and swampy land, but difficult to get it right. Usually a geotechnical of civil engineer gets involved in designing the foundation and site drainage.
Often the soil is first improved by replacement in the foundation area with sand and gravel or hardening with soil stabilizers. You may want to raise the area where the house sits to keep the structure and surrounding area high and dry. The foundation is typically either a “raft” foundation or pile foundation.
A raft foundation is a thick structural slab with thickened edges that act as footings. Sometimes additional thickened or “grade beams” are added to the interior of the slab as well. The slab is usually done in a single pour as a monolithic slab. The subgrade is typically a layer or sand over a layer of crushed stone, which has been properly placed and compacted. A raft foundation spreads the structural load over a large area and, if properly designed, can accommodate some soil movement without cracking or shifting.
The second approach is a pile foundation. This can be an end-bearing pile or friction pile. An end-bearing pile is driven until it reaches hard soil or stone. A friction pile is driven until it is deep enough to support the intended load. Helical piers are a type of friction pile that is gaining acceptance for exterior decks, but also can be used for homes and commercial buildings. The building can be supported on wood beams or concrete grade beams that rest on the piles.
Whatever type of foundation you choose, you will want an effective moisture barrier below the house or slab to keep unwanted moisture out of the structure. You will also want to create a drainage system on the site to keep the house site and surrounding area as dry as possible. This would include the road and driveway leading to the house. This might involve the use of swales, curtain drains , drainage diches, and culverts to direct water away from the house area.
As you can see, this is a challenging project – possible to do successfully, but requiring professionals with a proven track record of building dry homes on wet and unstable soils.