House foundation made of natural stone: rubble base

Recommendation points

• The main differences between rubble foundation
• Geological setting
• Choosing a stone for the foundation
• Trench preparation
• Laying rubble foundation

The base of the house does not have to be made of monolithic concrete only. Today we will talk about the stone foundation, the conditions under which it is possible to build it, suitable rocks and types of stones, and also describe in detail the technology of rubble masonry.

The main differences between rubble foundation

As you know, reinforced concrete almost ideally combines such properties of building materials as compressive strength and resistance to bending effects. This makes it possible to erect buildings of considerable mass on a concrete base, while neglecting the heterogeneity of the underlying soil layers. But in some cases there is no need to provide both of these conditions..

The rubble foundation is a masonry of relatively large stones. Due to the strength of the stone rocks, the base perceives even very heavy loads, and the relative cheapness of the base material allows the foundation to be made more massive. Rubble masonry has no reinforcement and the concentration of loads is critical for it, however, due to its massiveness, increased support area and bedding at great depths, where the soil layers are much denser, such a foundation is also very stable.

Separately, it is worth mentioning the decorative and operational qualities of the rubble foundation. It is less susceptible to the forces of frost heaving, does not require additional protection, and the above-ground part of the rubble masonry is an almost finished base with a unique appearance. But for the possibility of building a stone foundation, a number of conditions must be met:

1. Favorable geological setting.
2. Lack of seismic activity.
3. Availability of natural stone in the construction region.
4. The ability of the wall material to withstand concentrated impacts or the device of an armored belt.

Geological setting

Before laying the foundation of the house and thereby finally approving the project, it is required to agree on the possibility of constructing a foundation from rubble masonry. To do this, it is required to conduct hydrogeological surveys – to carry out punctures in the soil below the freezing depth, to determine the dynamics of groundwater twice a season and to hand over samples taken from the reference depth to check the physical and mechanical characteristics of the soil.

The ideal operating conditions for the rubble foundation are those in which the groundwater level does not rise closer than 20–30 cm to the foundation plane. Exceptions are possible, but at the same time the structure of the base changes: the submerged part is laid without a binder with a solution, but with careful ramming and splitting. Any rubble foundation must lie below the freezing depth. The exception to the rule is non-porous soils – rocky and detrital, as well as coarse and medium sands.

The requirements for the supporting soil are the same as in the construction of prefabricated block foundations, they are described in detail in SP 50-101-2004. The main requirements are uniform soil density at all points, absence of subsidence properties and bearing capacity corresponding to the mass of the building and the support area.

Choosing a stone for the foundation

It is necessary to distinguish between the concepts of rubble concrete and rubble masonry. The first is a method of reducing the consumption of concrete mixture by filling unreinforced sections of the massif with stone or construction (concrete) battle. Rubble masonry is an array that does not have reinforcement and consists of at least 80% natural stone.

The stone for rubble concrete can be used completely different. Of course, preference is given to rock and igneous rocks, which, due to their high density, ensure the massiveness and stability of the base. But limestone and sandy shale can also be used, provided that they are mined in open pit mines. Waste rock from underground workings is not suitable: falling under atmospheric pressure, it stratifies and acquires a loose structure.

Usually, the origin of rubble stone does not have to be chosen, the raw materials that are available in the construction region are used. However, if you still have a choice, you need to consider the trade-off between strength and ease of processing. Firstly, the average mass of the main elements of rubble masonry is 30-50 kg, larger and heavier stones can be stacked exclusively at corners and dressings. In view of the above, it may be necessary to split stones that are too large..

In addition, stones may need to be processed to eliminate sharp edges and edges. Ideally, stones should have the shape of an irregular polygon that fits into a parallelepiped with a length of 50 cm, a width of 20-30 cm and a height of up to 20-25 cm. For this, the largest elements are subjected to plinting, during which the main surfaces are cut, which should be like as flat as possible. All stones used in rubble masonry must be washed with water and a metal brush, with special care on those edges that were not chipped during processing.

Trench preparation

The rubble foundation in the lower part is 20–30% wider than the strip foundation with the design supporting capacity. Thus, in the cross section, the tape acquires the shape of a trapezoid, the upper base of which should be at least 20 cm greater than the wall thickness, taking into account the finish. It is thanks to this form that the susceptibility to lateral heaving forces is lost..

Rubble stone foundation scheme

The trench is dug 20 cm below the freezing depth with a width equal to the bottom of the foundation. The bottom is covered with one layer of crushed stone of fraction 20–40, and then very carefully rammed with a wooden block with a metal penny. After that, the walls of the trench are covered with plastic wrap, the edges of which are turned over the sides and pressed down with stones. The task of the film is to prevent soil particles from entering the foundation during the laying process.

When the walls of the trench are closed, its bottom is covered with 10–20 crushed stone in thin layers, each of which is carefully compacted with a manual rammer. Layer-by-layer compaction is carried out until the total thickness of the crushed stone cushion is 20 cm.

Laying rubble foundation

The first row of rubble masonry is made with the largest stones, which are located with the long side along the direction of the trench. For the bottom row, stones should be selected that have two parallel faces and are approximately equal in height. If necessary, the elements of the masonry are skirted, and all the voids formed are filled with cleaved.

The next row of stones is laid in the transverse direction. In this case, the dressing with a solution between the first and second row is not performed, the gaps between the elements of the masonry are also covered with fine gravel, which is compacted with a hammer. Dressing with a solution is carried out when laying the third and subsequent rows. In this case, you can choose one of the ways of rubble masonry – under a bay or a paddle.

The first one involves laying large stones about 70-80 cm high, fitted as tightly as possible, but with the formation of vertical gaps. When the stones are laid, the foundation is poured with liquid concrete of a class of 7.5 or higher with a fine filler, as a rule, stone chips, granite or slag screening are used.

Laying under the scapula is a classic method for constructing a rubble foundation. The laying is done in layers, while the direction of the main stones changes from longitudinal to transverse for additional dressing. Each large stone is always installed with the widest and flattest side down, while a small amount of cement mortar should be applied to the previous layer of masonry.

When a whole row is laid out, a solution is thrown on top of it with a shovel, and then – the debris left after plinting, or crushed stone of a large fraction. With the help of metal scrap, pinning is carried out, ensuring that the fine filler fills all the free space as tightly as possible. When the top layer of cement has set a little, the next row is laid.

Both types of masonry are carried out in such a way that its width becomes slightly smaller with each layer. After laying the next row, the width of which is less than the previous one, the film is lifted and the sinuses are covered with a small amount of soil. When the foundation has reached ground level, the masonry technique either changes to decorative with careful adjustment of stones and jointing, or formwork is erected and a concrete belt is poured with upper and lower reinforcement about 20-25 cm high.