How to build on a plot with complex soil conditions?

Office buildings, blocks and shopping malls are emerging in English cities, and thousands of single-family houses or smaller commercial and service facilities outside cities. There are fewer and fewer plots with simple ground conditions. Investors often struggle with obstacles in the form of wet, loamy soils, they are limited by uncontrolled embankments or other obstacles. Thanks to specialized geotechnical solutions, today you can forget about such inconveniences and build virtually everywhere, regardless of whether the investment is carried out by a great developer or, for example, the owner of a small plot, wanting to build a house for himself.

The type of soil has a direct impact on the cost of foundation works, and thus also the cost of the entire construction. Ground conditions, depending on their complexity, are divided into: simple, complex and complex. Simple conditions, as we read in the ordinance of the Minister of Transport, Construction London and Maritime Economy of April 25, 2012, occur in the case of genetically homogeneous and lithological homogeneous layers of land, horizontally lying, not including low-bearing mineral soils, organic soils and uncontrolled embankments, at the water level below the planned foundation level and absence of adverse geological phenomena.

Complex soil conditions occur in the case of heterogeneous, discontinuous, genetically and lithologically variable soil layers, including low-bearing mineral soils, organic soils and uncontrolled embankments, with the groundwater table at the level of the proposed foundation and above this level, and in the absence of adverse geological phenomena.

In turn, complicated conditions determine the layers of land covered by the occurrence of unfavorable geological phenomena, especially karst, landslide, suffusion, duststone, glacitectonic and expansive and sinking soils, in mining damage areas, with possible discontinuous rock mass deformations, in the areas of river valleys and deltas and in marine areas.

Translating this into a language accessible to everyone, it is best if the plot on which we want to build, e.g. a single-family house, has a sandy and cohesive soil: this means the need for shallow excavations, the costs of foundation walls or labor are also lower. A house built on such land settles evenly. Worse, however, if we are dealing, for example, with uncontrolled embankments. However, the most common problem is high groundwater. Improper foundation of the building on the ground with low load capacity can mean intensive settlement, which in the best case will result in cracks in the floor, and in extreme situations the entire structure will be at risk.

Let’s explore the ground

If we have a plot, but we are not sure about its soil conditions, we should opt for geotechnical surveys. They, before we start construction, will allow to determine the type of soil (its load capacity) and ground water level. These are two key pieces of information when it comes to designing foundations. Anyway, this type of examination can also be ordered before buying a plot. Soil testing consists of drilling several holes in the ground (preferably if we already know where the house will be erected): if we are going to build basements, wells must be made to a depth of 4 to 6 m, in the case of a house without a basement – up to 3 m. In this way, a geotechnician is able to determine the height of the water table and analyze each layer of soil.

In the case of single-family housing, such tests are not mandatory. The area of ​​Poland is mostly occupied by land with adequate load-bearing capacity, which is why many investors give up such operations, which often results in smaller or larger breakdowns at the construction site, which could be prevented at a low cost. A much larger expense awaits us than £ 1000 or £ 2000, if it turns out during the construction works that we have problems, e.g. with groundwater. In practice, very little importance is attached to proper ground recognition. What we can save on geotechnical surveys will later, unfortunately, result in an increase in uncontrolled investment costs when we encounter unexpected geotechnical situations. If we don’t anticipate them, we’ll be in big trouble. Therefore, by increasing the expenditure in the first phase of implementation, in the second phase we will be able to sleep peacefully and save some money.

There are no grounds for construction

Meanwhile, if we know the ground conditions at an early stage of the investment, the rented office will immediately design the appropriate foundation method. Even the “most difficult” soil conditions do not mean that there is no possibility of building in the dream place. Currently, thanks to the development of geotechnics, foundations are easily made in a subsoil with low bearing capacity – in principle, you can build anywhere. We, geotechnicians, try to adapt the technology so that the investor’s idea can be realized. There are no more lands, and the location and attractiveness of the plot most often determine the choice and construction decision.

In recent years, the use of prefabricated piles has become very popular in residential construction, including multi-family housing. Thanks to this method, you can build a building even where the load-bearing layers of the soil are at a depth of, e.g., 5-6 m. Besides, prefabricated piles are used to make foundations for all types of constructions, in virtually any soil conditions.

It is geotechnics that makes such complex investments as construction of tunnels, underground garages or multi-level buildings with several or even several floors deep. In Southeast Asia, buildings are erected on stilts longer than 100 m, in Poland most often the foundation does not exceed 15 m deep. But already in Western European countries, e.g. in Paris, underground parking lots with six floors are being built. Our standard is still one underground floor, although there are also examples of buildings with four or five levels – these are not yet very popular yet, although they are increasingly needed, taking into account the huge parking problems in most Polish cities.

You can learn everything about the construction of facilities in even the most complex soil conditions by taking part in the 9th Conference “Geoengineering in Construction”. The meeting will be held on 3-5 December this year. in Cracow. At Wawel, you will be able to listen to professional speeches and talk to experts in this field.