Soil Bearing Conditions

By David B. South

People often ask about the type of soil bearing a Monolithic Dome needs. The answer is simple: a Monolithic Dome just needs to have a reasonable amount of soil bearing. If you can put any other building on the soil, the Monolithic Dome will do fine.

There are several reasons for this: The dome is extremely strong. I tell people to think of it as a gigantic plastic bowl. You turn the bowl upside-down and it has a tremendous amount of strength inherent in it, and it will sit down evenly.

When calculating bearing pressure, we take the total weight of the structure, plus the total weight of the load which is to be imposed on the structure, and divide that by the perimeter footage to get a pressure per lineal foot around the circumference. Then we take the soil bearing pressure and divide it into that number and that gives us the width of the footing.

For instance, if we are building a 100-foot diameter by 35 feet high, dome we can expect it to take 180 yards of concrete. If we use two ton per yard and an average thickness of five inches, we come up with 360 tons or 720,000 pounds.

Next, we add to that the live load of 50 pounds per square foot of floor area. This will add another 400,000 pounds. When we combine the two, we have about 1,100,000 pounds. We divide that by the circumference of the building to get 3500 pounds per foot around the perimeter of the building.

Most soil bearing in the United States for undisturbed soil is 3000 pounds per square foot. We divide that into the lineal pounds per foot and come up with a footing width of 1.2 feet. That seems like a pretty small footing width, and we might want to make it a little wider just on account of cuz -- no other reason.

If we are in lousy soil conditions, such as along a river, we may want to use 1000 pounds per square foot for the soil bearing criteria. We would then divide by 1000 to come up with a footing width of 3.5 feet. We once built a dome where the soil bearing was 300 pounds per square foot. Using that as a criteria, we would need a footing 12-feet wide. Such an extremely rare condition only exists where soil is very plastic.

If we go back to our plastic bowl analogy, we can see that the shell and the footing act as one piece. The footing is actually a shoe, if you will, on the bottom of the dome; it contains tendons that hold the bottom of the dome together and hold the entire dome up. The dome becomes a beam that's much taller than a grade beam and can span great distances. Because of this super strength of the shell/beam, soft spots in the footing are of little concern. In fact the dome will not have much problem with settlement. The ground allows settlement, and the dome will generally settle very evenly.

If a cut-and-fill is necessary under a Monolithic Dome, it's a good idea to over-excavate the cut area and refill it to provide a more even soil bearing throughout the entire structure. By doing this, settlement that does occur will be even throughout.

Because the Monolithic Dome can be put on poor soil bearing, we find it is used for that purpose. But if built on poor soil, it must be remembered that the dome acts as one piece and anything attached to it has to fend for itself.

In other words, a porch attached to a Monolithic Dome on poor soil conditions tends to move separately. This can also be true of the floor, which will tend to move separately. Therefore, attachments to the dome should be provided with flexible joints, and large floors should be allowed to float. Floors with a 40- foot diameter or smaller should become part of the footing/floor combination, forcing them to move together.

Unless soil conditions are very poor, the floor/dome differential settlement is not usually much of a problem. The Monolithic Dome is obviously the best choice for poor soil loading conditions as it tends to float like boat.

Soil Bearing Conditions

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