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Choosing a foundation for problematic soil: strip, pile, or slab?

A significant part of Europe is occupied by lowlands with water-saturated soil, subject to frost heaving and often weak-bearing. For such conditions, several types of foundations have been developed. We suggest which one will be optimal in each specific case.

When choosing a foundation design, the composition of the soil, groundwater level, terrain features, mass and configuration of the building, and even its operating mode are taken into account. Without going into the technological details of the construction of strip, pile, slab, and combined foundations, let’s talk about the specifics of their “behavior”, the advantages and disadvantages of certain design solutions.

Read more: Foundation base and types of soils

Strip foundation

The simplest foundation design for a low-rise building is a strip foundation. Dominated, including in swampy areas, during the 1980–90s.

Shallow tape was extremely popular due to its relatively low cost; at the same time, it was recommended by some territorial construction standards. The total height of such a tape does not exceed 1 m, the width is usually 30–40 cm, and the laying depth is 30–50 cm (often they do without a trench at all, but only remove the fertile layer on the building area).

As a result of seasonal movements of the soil, such a foundation is elastically deformed, as a result of which cracks may appear in the walls of the house. Even reinforced reinforcement (at least two horizontal reinforcement belts with a diameter of 10–16 mm) does not always help. The practice has shown that a floating strip foundation is not the best option for a building with block (block-brick) walls, especially if it has a complex configuration, terraces, and other extensions. Meanwhile, it is quite suitable for rectangular frames, logs, and timber buildings.

A deep strip foundation is optimal for a heavy building being built on a site with fairly dense soil – clay or loam with a minimal admixture of silt, without quicksand. (In general, the most cost-effective slab-tape construction with a basement or ground floor, but it is only suitable for dry areas and is not discussed in the article.)

The depth of soil freezing depends on its type and local climatic conditions. 

The strip foundation can be either monolithic or block. A monolithic reinforced concrete structure better resists the effects of lateral forces of frost heaving, is generally more rigid, and can be erected without the help of vehicles – however, then the time costs will increase significantly.

A simplified technology allows the volume of excavation work and construction time to be reduced, in which formwork is constructed only for the above-ground part of the foundation, and the underground part is poured into a trench lined with rolled waterproofing material. However, due to inevitable unevenness, the wall of such a foundation adheres quite firmly to the ground and is more susceptible to the tangential forces of frost heaving. However, the load created by the building usually compensates for them. 

If the dense layer of soil is located significantly below the freezing depth, the tape turns out to be unreliable or too expensive.

Another popular method involves installing permanent formwork from EPS boards. This formwork is easy to install (special adjustable ties are used to fasten the walls, they also serve as support for reinforcing belts) and neutralizes the tangential forces of frost heaving: the adhesion of smooth EPS to the ground is low. 

Read more: How to pour a strip foundation: step-by-step instructions

Relatively recently, a technology has been developed for the construction of strip foundations of low-rise buildings using a vibratory grab (soil collector) and block vibro-submersible formwork. The essence of the method is as follows: first, using a vibrating grab, a fragment (“grab”) of a trench approximately 2 m long is developed, into which small (conventionally 0.3 × 0.7 × 1.5 m) thick-walled metal containers (profiling blocks) are immersed in turn using a truck crane. with a vibration unit rigidly attached to them. The blocks are equipped with a locking connection, thanks to which they line up precisely in a row. Next, a concrete mixture is poured into them and so, gradually lengthening the trench and rearranging the blocks, a monolithic strip is formed. The advantage of this method is speed, and also that the concrete mixture is well compacted by vibration. But the disadvantages are the need to use powerful equipment, pre-mixed concrete and the inability to provide continuous reinforcement and pouring, which negatively affects the strength of the tape. 

What does georeconnaissance give?

Without accurate data on the composition of the soil on the site, it is undesirable to begin construction of even a light frame house. The granulometric company must drill at least four wells 3–5 m deep, take soil samples from different depths, and make their granulometric analysis. This information will allow you to correctly select the type of foundation and calculate the structure based on its load-bearing capacity. In the case of a pile-driven or pile-screw foundation, the required parameters can also be determined by test-driving or screwing several piles. 

Read more: Three main mistakes when building a foundation

Pile foundation

Pile foundations are practically not subject to frost extrusion, but have a relatively low load-bearing capacity (or rather, increasing it sometimes requires too much expense). They are preferable for construction on slopes and flooded areas and are more reliable than strip ones if the top layer of soil is subsident, but underneath it, at a depth of no more than 2–5 m, there are dense rocks on which stand piles can be supported. On silted soils, hanging piles are sometimes used, the load-bearing capacity of which is ensured by friction forces between the side surfaces and the soil, however, for such unfavorable “geology”, slab and combined structures, which will be discussed below, are better suited. 

The pitch, required length, and cross-section of piles are determined.

Driven and vibrating piles are manufactured at the factory. In terms of strength, they are superior to reinforced concrete products made on-site (including using tamping devices). Another advantage of a driven (vibro-submersible) foundation is that the piles compact the soil during installation, thereby increasing their load-bearing capacity. However, it is necessary to provide access to the site for a pile-driving machine or a truck crane with a vibrating installation. The cost of the structure is quite high.

Drilled piles will cost 30–40% less than driven ones and are ideal for houses with not too heavy (timber, log, and aerated concrete blocks). To build such a foundation, you don’t need any vehicles—it’s enough to rent a mechanized drill and a concrete mixer. The essence of the technology is that holes with a diameter of 150–250 mm are drilled in the ground, they are built on with formwork, water is pumped out if necessary, and then a reinforcement cage is lowered into each well, and concrete is poured and compacted with a submersible vibrating device. If the soil crumbles or floats, asbestos-cement casing pipes are first inserted into the holes. 

When using a drill with a folding plow, it is possible to form a widening at the bottom of the piles, which will increase the load-bearing capacity of the structure and at the same time eliminate the possibility of frost extrusion.

Screw piles. This is the cheapest of the pile foundations but its service life when using conventional welded products does not exceed 50 years. Products with cast tips are much more reliable (but also one and a half to two times more expensive). Reinforced concrete screw piles are even more durable, but this recent development has not yet been put into mass production. 

A sand and gravel cushion under a strip foundation reduces the forces of frost heaving only if there is deep drainage. In other cases, it serves to level and increase the bearing capacity of the base.

Slab foundation

The slab foundation has a significant reserve of bearing capacity and ensures the most uniform distribution of the load from the building to the ground. It is this type of foundation that is often optimal on weak-bearing and mobile soils, including silty and silty sands, loess and peat bogs. In addition, the design greatly simplifies the installation of the first floor (there is no need to build columnar supports for beams or arrange backfill with a screed). But its cost is quite high – usually even more than that of a pile-driven foundation. This is due to the high consumption of materials (sand gravel, concrete, and in the Swedish version, also moisture-resistant insulation) and the need to order a factory-made concrete mixture. 

An uninsulated slab foundation is used for heavy buildings with brick walls. The structure is a flat and smooth monolithic reinforced concrete slab, poured into formwork on top of a sand-gravel cushion. It is simple and reliable but requires a lot of reinforcement and concrete (2–3 times more than a full strip foundation, and 4–6 times more than a pile foundation). The fact is that the load from the walls and the forces of frost heaving are concentrated mainly at the edge of the slab, and to prevent the deformation and cracking of the monolith, it is necessary to increase its thickness to 250 mm or more. 

A conventional (non-insulated) slab foundation is one of the best solutions for constructing a heavy brick or concrete building on a weak-bearing soil of homogeneous composition.

A ribbed plate is somewhat less material-intensive (but more labor-intensive). The ribs can be arranged both from below (by pouring into trenches) and from above (using formwork). From the point of view of strength, the first option is preferable, but the second is somewhat cheaper, since the upper ribs simultaneously serve as a base and support for the floor of the first floor.

An uninsulated slab, like a shallow-buried strip, is a floating foundation and can undergo seasonal movements (especially if the house is not heated in winter), which must be taken into account when constructing extensions.

You can overcome the forces of frost heaving by increasing the depth of the foundation, draining the area, or protecting the soil from freezing. The last method is the simplest and cheapest.  

The insulated Swedish slab protects the base soil from freezing, so it is not affected by the forces of frost heaving. However, in terms of rigidity, the structure is inferior to a conventional monolith and is mainly used in the construction of buildings with frame or aerated block walls. This type of foundation typically serves as a subfloor and is equipped with a hydronic heating system used to heat the entire building. And since the stove has significant thermal inertia, it is easier to maintain a constant temperature in the house – both in winter and summer. Perhaps its only drawback is the relatively low base. Sand and gravel bedding helps overcome this disadvantage.

An insulated slab foundation is optimal for a house in which people live permanently. For a dacha in the same geological conditions, piles or a floating tape are better suited

Reinforced concrete foundation

The design of any reinforced concrete foundation on problematic soil places increased demands on the quality of concrete. It is advisable to purchase the ready-made mixture from a trusted company. When preparing it yourself, the mixture must be thoroughly mixed, which means you can’t do without a mini-concrete mixer. It is also advisable to buy washed sand since clay inclusions weaken concrete, and use medium-fraction crushed stone (20–40 mm), rather than river gravel. Cold working seams are extremely undesirable (in the case of a slab, they are unacceptable), and if they cannot be avoided, then additional reinforcement of the weakened area with pieces of rod no less than 1 m long is necessary. 

Read more: How to properly reinforce a strip foundation with your own hands

Hybrid foundation options

The desire to combine the advantages of different types of foundations has led to the emergence of combined structures. 

  • A pile-strip foundation is a combination of a shallowly buried strip and drilled or driven piles. The bearing capacity of such a foundation is mainly provided by the tape, and the piles prevent it from being pushed out. But when the soil freezes, the structure experiences increased loads (especially if the belt is underloaded) – that is why it is not worth using screw piles, whose blades can come off. This foundation goes well with small block-brick buildings that are simple in plan.
  • Pile-slab foundations are extremely rare, but are still used in private housing construction for the construction of large brick houses. It is a pile field (piles are placed in increments of 1.5–2 m throughout the entire building area), combined with a thick monolithic slab. 
  • A pile-drill foundation with a grillage and screed is a good alternative to a Swedish slab, providing a high base at no additional cost, which means better protection of the walls from moisture. A screed 100–150 mm thick with single-level reinforcement is installed on top of a grillage and carefully compacted sand bedding. In this case, the base is insulated from the outside with EPS slabs to reduce the likelihood of freezing of the soil under the foundation. 

Read more: Finnish-type foundation: what is it and how to build it

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