Help me plan my root cellar!

Hi,

I have been thinking about building a root cellar for many years; however, I never had the time or energy to actually build one. Now I have somebody to help me and I may actually starting digging in the winter.

The site can't be accessed by earth moving machines, so we'll basically dig by hand and maybe use a power drill. I haven't got a drawing yet, but the way I'm planning on building is as follows:

- The site: base of a vertical northward hillside, where the sun doesn't reach.
- Interior floor size: about 5 feet wide by 7 feet long.
- First, I intend to dig a drainage channel for drainage.
- Depending on how deep I can make the drainage, I want to bury the cellar at least 4 feet below ground.
- The excavated earth goes on both sides of the cellar, so that about 2 to 3 feet of the cellar will be above ground level.
- The rear of the cellar is build against the vertical hillside.
- I intend to cover the cellar with about 3 feet of soil so that it is flash with the hillside.
- The interior walls are made from large stones with clay in between.
- A metal wire mesh between stones and earth keeps rodents from getting into the cellar.
- The floor is made from flagstones.
- Gravel beneath the flagstones will guarantee drainage.
- A concrete ceiling reinforced with steel bars will be covered with a watertight layer to keep water from percolating into the cellar.
- The wooden door is slanted so that cool air can sink into the cellar when I leave it open during the night. (Even in the summer, nights tend to be cool around here.)
- We have plenty of cork from our cork oaks. I wonder whether it would be a good idea to bury a cork layer in the soil above the cellar??
- I wonder whether it’s a good idea to bury tubes for running cold water either above or below the cellar??

What do you think? Can that work? Do you have any suggestion for improvements?

Cheers,
Dieter

What is the soil type you are working in? I think your plan sounds good - you are suggesting doing a cut-and-cover method with drainage "to daylight" which is great. When building things underground, water intrusion is an issue. If you are at the toe of a slope, there will be more water than if you were higher up. How much area there is upslope, how much rainfall you get, how fast your soil drains, and how deep the water table is will all be important to know.

I am not a structural engineer of any sort, but I would be wary of building my walls out of field stone and clay. A buried structure will have to carry the load over it on the roof, but there can also be "squeezing" forces that will try and buckle the walls at the invert (floor).

You will not be deep enough to develop a zone of arching, but imagine all the dirt on either side trying to slide down a wedge and into your excavation. How much pressure will there be? Without an engineer, youd only be able to guess. Some soils are very cohesive, like heavy clay, so they would resist lateral movement. Some soils are loose, and would be hazardous to even dig in. So while clay will hold its shape, it also does not drain quickly. If water can build up on the back side of your walls, because the backfilled soil is less dense for example, then you will get hydrostatic pressure that will try and push the walls in at the bottom. If you have ever seen concrete bow out the bottom of a form, you will know that it does not take a whole lot of depth to create some very large loads.

I would suggest you make the walls stiff. Concrete block with rebar-reinforced pillars within some or all of the cells would likely be plenty. Also, put some drainage at the base of the walls on the outside, and backfill with something permeable.

Carl Nystrom wrote:What is the soil type you are working in? I think your plan sounds good - you are suggesting doing a cut-and-cover method with drainage "to daylight" which is great. When building things underground, water intrusion is an issue. If you are at the toe of a slope, there will be more water than if you were higher up. How much area there is upslope, how much rainfall you get, how fast your soil drains, and how deep the water table is will all be important to know.  

Hi Carl,

Thanks for your reply.  

We have heavy clay soil that's normally very dry. It's ideal for rammed earth construction. To get to the ground water, I have to dig at least 30 meters (100 feet) into the ground. The hillside never gets water-soaked even with heavy rains. The cellar is a little distance away from the bottom of the valley, where the water runs off during heavy rain falls. The rainwater has dug a bed into the ground. That's where my drainage from the cellar would go. In other words, the cellar floor will in any case be above the rainwater runoff bed.

So I don't worry too much about water-logged soil. The only problem would be if it rained into the cellar, which could turn the cellar into a pool because it takes a long time for water to soak into the soil. That's why I need the drainage.

I try to work with onsite material as much as possible. That means clay, stones and wood. I have done dry-stone walls against a hillside. After 20 years, they haven't caved in. There is only one stone that I have to remove because tree roots are pushing it outward. When building dry-stone walls, I usually fill in the gaps behind the stones with pebbles, so that any humidity will drain behind the stone wall. So, as far as I can tell, the pressure on the walls will be primarily from the concrete ceiling and the soil above, which will strengthen the dry-stone walls.

Ideally, I would like to do an arched ceiling with stones; however, I think that will be for another life as it is too much of a challenge right now. That's why I compromise by using a reinforced concrete ceiling. I'm not sure if I could use wooden beams. There are some Eucalyptus trees I want to cut down, which could serve to cover the cellar, but I'm not sure how well Eucalyptus will preserve underground.

Many thanks for your advice,
Dieter

Edit: since we'll be digging by hand, we'll dig vertically into the ground just big enough for the cellar and the stone walls. In other words, there won't be the sloping space with loose earth on the outside of the stone walls (shown in your drawing) that could put lateral pressure onto the walls.

Heavy clay should be advantageous to what you are planning. I dug a tunnel into clay in the woods behind my parents place when I was a kid, and it is still standing 20 years later. Also, your stone wall will not really hold pressure like a masonry wall would, so hydrostatic pressure would likely just push out the clay instead of moving the stones. If you extend the waterproof membrane out over the walls like an umbrella, that will help a bit with keeping the moisture at bay. Sounds like a fun project, post some pictures when you get underway!

"Do you have any suggestion for improvements?"

I don't know much about building techniques, if that's what you are asking.

I would suggest to place stairs inside the cellar. This way the cool air near the ground doesn't leave the room every time you open the door.
Also, the higher the building, the more stratification you get, with warmer air near the ceiling, cooler air in the ground. To encourage stratification, use shelves made out of isolating materials (not metal) and drawer-like stands (hold temperature better whenever you open the door).
Gravel in the drainage may prevent losing cool air by the drainage while allowing water to move out.

Here's a prefab root cellar: https://www.wired.com/2016/04/store-wine-underground-inside-pre-fab-food-cellar/
It costs 'just' 10 K$. Not suggesting to buy this, but to copy the design.

Since your roof is not exposed to sun, you only have to worry about outdoors temperature for isolation. You didn't say where you live, but if your cool/hot cycle is fast (guaranteed cool summer nights), then you don't need to add isolation to the walls, provided they are thick enough. You may add the extra insulation layer later if you think you need it.

Do you plan on using an insulated/airtight door? If so, maybe look for one with a small glazed area, so you get some dim light inside, saving the electric installation, wished you to close the door while browsing for your stuff. Torchlights/candles can be used at night.

Abraham Palma wrote:"I would suggest to place stairs inside the cellar. This way the cool air near the ground doesn't leave the room every time you open the door.
Also, the higher the building, the more stratification you get, with warmer air near the ceiling, cooler air in the ground. To encourage stratification, use shelves made out of isolating materials (not metal) and drawer-like stands (hold temperature better whenever you open the door).

...
Since your roof is not exposed to sun, you only have to worry about outdoors temperature for isolation. You didn't say where you live, but if your cool/hot cycle is fast (guaranteed cool summer nights), then you don't need to add isolation to the walls, provided they are thick enough. You may add the extra insulation layer later if you think you need it.

Do you plan on using an insulated/airtight door? If so, maybe look for one with a small glazed area, so you get some dim light inside, saving the electric installation, wished you to close the door while browsing for your stuff. Torchlights/candles can be used at night.

Thanks for the advice.

I was planning on a thick wooden door above the stairs leading to the below-ground part of the cellar. I don't expect to spend much time in the cellar, so I rather avoid a window in the door.  If need be, I could even lay an electric cable into the cellar.

We are in the Alentejo region of Portugal a few miles inland from the Ocean.  When the sky is clear, the temperature difference between day and night is often 20 or more degrees Celsius. In other words, even with daytime temperatures of 30 degrees, the thermometer can fall below 10 degrees at night.

As I said, we have plenty of cork. So throwing in some cork while adding soil to the top of the cellar wouldn't present any additional effort or cost. I just wonder what effect it would have.

Another thing I was wondering about is whether I should provide ventilation. At the cellar front, I could open the door for ventilation. Do you think it's a good idea to have a ventilation shaft at the rear of the cellar to get air circulating from front to rear if necessary? Ventilation may be useful when the air humidity is very high, which happens at night or during rain season. High air humidity could cause mold.

@Carl, a dry stone wall can have greater strength than a brick wall built with mortar. If the mortar were to deteriorate underground, the brick wall risks collapsing. That can't happen with a dry stone wall, which is built to have stability in itself. The secrete is to place each stone so that the upper side slants slightly towards the earth. That means the pressure applied from above will primarily be downward, while a minor force will press the stones against the earth. The greater the pressure from above, the greater the strength of a well-built dry-stone wall.

Cheers,
Dieter

We are almost neighbours!
If Alentejo is anything like Lisboa, then a thick wall suffices. About the effect on extra insulation... it depends. I am assuming you will not insulate underground walls, since it will ruin any positive effect of being underground. When you have a thermal mass wall, you have to count it on its average temperature, which is the temperature that will effectively transmit to the interior. If the average temperature of the above ground wall is greater than the underground average temperature, then an extra insulation could lower the temperature, which I suspect it might be the case.

You are absolutely right about ventilation.
A residential basement is reccommended to have at least 3 air changes per hour (the whole volume of the room must be renovated every 20 minutes), maybe a cellar has lower requirements.
I think your drainage already counts for intake ventilation, so it's the outtake air which will limit the flow. This can be your door if it is not airtight, or it can be a small chimney on the roof. I guess you can use a hygrometer once the cellar is completed, and if you see consistently high humidity (over 80%), then install the chimney.

Unfortunately I see heaps of problems.
I am a Civil Engineer, and I believe you need a rethink about these points;
- earth walls have high levels of lateral pressure
-I doubt if a drystone wall will prevent collapse in this case, unless its circular which improves its strength or very thick.
- I have been learning drywalling and its a great medium.
-

but I'm not sure how well Eucalyptus will preserve underground.

I live in Australia, that timber is bad in the soil.

- Reinforced concrete roof is fabulous, it will need support back to the ground to stay in place.
- soil is very heavy

Please send pictures of the works in progress.

- earth walls have high levels of lateral pressure  

I would say that earth walls CAN have high levels of lateral pressure. If your engineering firms reputation was on the line, this building method would never fly. I suspect that this design COULD work. I imagine that underground storage was probably done for eons in your area, right? Are there extant buildings that use this building method that would give you some ideas about what methods were used in the past and have stood the test of time?

I have stayed in some old buildings in Italy that were built using the same technique that you are suggesting to use. As we were hanging up salamis in the basement, I asked the farmer, "What happens if there is an earthquake?" Matter-of-factly he replied: "Siamo tutti morti."

Knowing that something might kill you is not necessarily enough reason not to do it. Those old houses were beautiful. There are safer alternatives, though.

I am with you on risk, I race sidecars with speeds over 200kph.
The issue with earth involves a saying from tunnellers I have worked with over the years.
"you only get killed in good looking earth"
Meaning sand trenching would always be boarded up to prevent collapse.
Good looking stuff is not always good and those trenches do the killing.

Dieter, if you already have a hill, why not dig horizontally into the hill rather than dig vertically into the ground? This would leave three walls covered with earth and the forth wall (where your access door is) could be timber-framed. You could still do the poured concrete roof and cover it with sod. I think this would greatly reduce the lateral pressure on your drystack walls, and make drainage easier as well. Just run a drainage pipe around the base of the walls and let it come out to the front.

Yes, plan to install the chimney for cross ventilation. Our building codes require 1 square foot (.0929 Sq. Meters) per 150 Sq. ft (14 sq. Meters) of floor space. You can always size the chimney larger and incorporate a valve of some sort to change the size of the opening. On the building code side of things, our codes allow for rubble stone foundation walls, providing they are 16 inches (40,64 cm) thick and do not retain more than 8 feet (2,44 M) of unbalanced fill dirt.

Basically, if you build those stone walls thick enough, and don't have more than 2,44 M of dirt up against them, you shouldn't have a structural issue.

I have been learning to drystone wall, its an interesting and time consuming task, but the outcomes are great,

Umm, guys, root cellars are not residential.  They have requirements (actually the items stored do) for both ventilation AND humidity. Specifically, depending on the contents, you may NEED humidity in excess of 90%. If you are planning to store canned goods, you need a cool, DRY area so no rust, unless using the new reusable plastic lids. Before doing plans, decide what you want to store, list THOSE requirements and plan to meet them with the engineering. Don't put the cart before the horse.

The ventilation meets several needs; aid in humidity control, temperature control, and odor control (stored veggies need fresh air, and give off gases as they age). I hope I am preaching to the choir here, as in you know all this and just haven't mentioned it.