How To Build A Foundation On Clay Soils

2024 Author: Sebastian Paterson | [email protected]. Last modified: 2023-12-16 13:47

So that the house does not "burn"

Clay soils are often found in the practice of summer cottage construction. Sometimes they are water-saturated and heaving. The foundations of houses, as well as sheds and baths in these conditions, are made both tape and columnar.

Strip foundations

Often you can see at the site of future construction deep ditches around the perimeter of the house, into which the formwork is placed. Of course, this is a solid foundation, but very expensive and, I would say, with a very large, excessive margin of safety. Such a foundation is very laborious and material-intensive. In addition, on soils with a close standing groundwater, it is generally difficult to do it. According to the norms, it is assumed that the base of the foundation is below the minimum freezing depth. And in our Northwest region, these depths reach 1.2-1.3 m. Try to dig a ditch under the foundation of such a depth. Often this can only be done in a very dry summer, and the work is hellish.

But there is a way out - this is a "floating" foundation. The ditch is dug shallow, its depth is about 0.5 meters, covered with sand with a layer of 35-40 cm. And formwork is placed on the sand for pouring. Reinforcement of concrete in two tiers with reinforcement thickness of 8-12 mm is required. I have seen strip foundations made without reinforcement. Often in places where ventilation holes are made, vertical cracks occur, the foundation falls into several parts, which is very bad. The "floating" foundation can be low, for example, 40-50 cm high. But it must be reinforced with high quality.

After the rainy autumn, habitual for our climate, the cold comes, and sometimes with very severe frosts. And such a foundation rises by a few millimeters - after freezing water increases its volume by 5 percent, and in the spring it "sits down". In spring, the southern part of the foundation thaws earlier, bending stresses appear in the concrete strip, and these stresses are absorbed by the reinforcement. The concrete itself works well only for compression.

Columnar foundations

Such foundations are made quite often. In a reliable design, as I was explained in one of the companies, a hole with a diameter of 200-250 mm is drilled in the ground below the freezing depth. Gravel is poured there, an asbestos-cement pipe is hammered into the hole and filled with concrete, which is compacted in layers. A very expensive foundation. In the initial period of mass dacha construction in 1983-86, many summer residents made columnar foundations with insufficient depth. Over the years, in many cases, the posts have tilted. I have seen such posts with slopes of 10-20o, and the process of increasing the slope continues there. The columns (50x50 cm) of the foundation of my house on the south side also gradually tilted southward every year. I had to make a strip foundation with the arrangement of reinforcement on the outer and inner sides of the posts.

But there is one more type of columnar foundations - let's call it “prefabricated-backfill”. This is the least laborious and cheapest foundation; it can be used where dense soils (including water-saturated ones) lie at the base of the house (shed, bathhouse). Its technology is as follows: fertile soil with vegetation is removed from the building spot. In those places where there will be supports, pits 50 cm deep and 40x60 cm in size are dug in the clay. The pits are filled with coarse sand or sand and gravel mixture (ASM layer by layer with mandatory full moistening and compaction by hand tamping.

The filling is done up to the ground level, then concrete blocks 30x30x50 cm (or 20x30x50 cm) are placed on the sand. Usually two sides are placed - one on top of the other. Such blocks are commercially available and are relatively inexpensive. The lower strapping of a log house or frame is laid on the blocks through a moisture-insulating gasket (roofing material). It is advisable to treat the lower harness with an antiseptic. A good result is the use of hot waste machine oil. The smallest particles of metal contained in mining clog the pores in the wood, it becomes very resistant to decay. My neighbors have a wooden house built on this foundation, and I have a summer kitchen.

What happens to such a foundation? Water after the autumn rains fills the pores in the sand, but its level is below ground level, the pores of the upper part of the sand cushion are filled with air. When freezing, the water expands and fills some of the pores, but the concrete block remains in the same place. In the spring the ice melts, but this has no effect on the position of the block.

The foundation of such blocks is stable, it has been tested in practice. If there are no blocks, then you can make the suitable blocks yourself by installing the formwork of the required height on the sand. There is no need for reinforcement in the block, but the concrete must be laid in layers, repeatedly piercing each layer with a shovel.

It is advisable to cover the soil under the house or bath with a film (preferably thick - 150 microns), and pour a layer of sand on the film with a layer of 5-7 cm, and attach a "skirt" made of roofing material to the lower crown of the building so that water and snow do not fall under the house. The “skirt” should have ventilation cutouts.