This is a question for the pub if ever - you guys are always a great resource for this sort of thing.
We’re looking at buying 1.5 acres next to a river (flood history: severe flooding in 1513, 1626, 1635 and 1840) and the agent says that the land will most likely need a micropile foundation. What I’m wondering is, what are these things, how do they work, and can I have a finished basement with them? Is it just for foundation stabilization, i.e., “all other ground is shifting sand” or something else? I know every case is specific when it comes to land, but something general would be helpful for figuring out how we can proceed.
The terrain is 6,440 square meters with 300 square meters of allowed habitable construction surface, which is almost unheard of within an hour’s drive of Paris, which is why we’re so interested in jumping on it. I really need to know what specifically is questionable about this. A 300m2 house with more than half a hectare of land will be worth upwards of 600,000 euros - once built it’s more than 100% increase in value of the property. I’m not looking for affirmation, but I just need more than “don’t do it.”
for this type of investment it is worth getting a LOCAL professional civil engineer that would be able to do the design. pay a consult fee and ask what would really be involved. Then, like i say so often, get a second opinion. it is worth spending a little dough boy (notice how i worked the brewery in to this) to get the details
There is no way anyone on this site can give you solid advice without seeing the property and even then, that person(s) need to be trained in that area. I would guess that a core sample will need to be taken to really know what you’re dealing with. I doubt you’d be happy having your house float down the river.
Also, be sure to check into insurance and make sure you can get it in that area.
I’m not looking for super specifics, just an idea of what this work entails. I had no idea until today what micropiles were, and at that it’s not very clear. People build houses on crappy soil all the time (see: city of london) but I just don’t know how these things work.
Because nobody does DIY in France, and because most people who live in the country (this is considered the sticks here) don’t want to go through the trouble of building a house, or have inherited the house they own.
Your general contractor should be able to recommend a civil engineer. Like others have said, get a couple opinions before moving forward. Being on a river, the shallower layers of soil are most likely silt and sand, so the micropiles would get driven below those layers into something more solid for your house to stand on.
I found this:
Micropiles are small diameter piles that can be installed in a variety of soils from non-cohesive, poorly-graded granular soils, to cohesive plastic clays. Also known as minipiles, pin piles, needle piles or root piles, micropiles can offer a viable alternative to conventional piling techniques, particularly in restricted access or low headroom situations. A micropile foundation system may be advantageous in areas where large boulders are sporadic in the subsurface, as the small diameter micropiles may be able to be installed around such boulders. Micropiles are installed using water flush rotary drilling or rotary percussion drilling techniques. Measuring between 6 and 12 inches in diameter, micropiles consistently achieve capacities of 20 to 100 tons, with special installations up to 200 tons. Micropile drilling methods generate minimal disturbance or vibration to adjacent structures, making micropiles an excellent underpinning alternative.
I’d say that you can forget about a basement. The property probably has a high watertable and at best you’d wind up with an indoor swimming pool. Also have the contractor or civil engineer look at upstream development. A little more drainage from upstream properties can increase downstream flooding. I’m in the NW suburbs of Chicago and flooding happens more frequently to people living near the local Des Plaines river due to upstrem urbanization.
Unfortunately, yes. Typically farmland, but also whole towns are built in the flood plains. And people hope the Army Corps keeps the damns working.
Annual flooding here in the Midwest is a BIG deal. Last year they blew a dyke in downstate Illinois to save one town and washed away everything on the other side of the dyke, which was mostly farmland.
Construction on pilings is a common practice, though I don’t know how common for a single family home but I would guess uncommon. What it basically means is that the soil won’t support what you’re doing.
Get a local professional. Someone there has dealt with this. How to get a good reference for who to deal with is an important question.
Sounds like the land has two concerns; flooding and unstable subsurface conditions caused by river sediment. Looking at the flood history, it’s been over 170 since the last major flood. Also, there are long periods between floods. This could mean the next major flood may not happen in your lifetime, or the next major flood is way past due and could happen in the next rainy season.
How do you feel about building a basement in a flood zone? You have to decide what your tollerance for risk is. Will the worry cause you to lose sleep?
Building a basement over piles should not be a problem. Building a basement in an area with a high water table could mean constant headaches.
You need to consult with a geotechnical engineer who is familiar with local conditions and construction techniques. I am certain the engineer will request test borings on the site to determine the specific subsurface conditions. The engineer will know who to recommend to do the foundation work and can act on your behalf to make sure the design is properly constructed.
The people I’ve talked to today say I need to request an ‘etude du sol’ - soil study. They do exactly what Puna mentions, take ground penetrating radar scans of the whole lot, then take at a minimum of three core samples. It’s expensive, but I’d rather pay a bit more for some place where I won’t have problems later on. More cost now = less cost later, etc.
I know it’s not ideal to do this, but the thing to remember is that the entire country is less than the size of Texas, but with three times the population. All of the ‘really good’ land already has a house on it, and demo costs are crazy crazy high because of anti-pollution and antiquity regulations. Which means to get anything with some property, you have to buy swampland or something slightly better…
When I was a little one I lived with a pack of wild lesbians who decided to build their own house… on a flood plane… from reclaimed timber… octagonal. It actually worked really well and is still standing 30+ years later. What they did was build pilings out of 24"x24" beams that were salvaged from an OLD factory. They tied several together and put them in 6’ holes so when the river flooded (which is did every year) the pilings would float up slightly and then settle back as the water level dropped. Now this was in VT in the 70’s so zoning was not much of a problem and you would probably have a hell of a time getting past zoning in france what with the antiquity rules etc. but it can be done in even the most unstable land. It just takes a lot of foresight.
