The basic question when building a tower is how to foundation it. This is important for the investor for cost reasons, but also for the time needed to build a given type of foundation. The decision often remains in the hands of the designer, but sometimes it is possible to choose the foundation method under given conditions. So it's worth knowing the basic options.
The foundation method depends on:
- Soil type
- Groundwater depths
- The size/weight of the structure to be built on the foundation
The main selection options are:
Basically, the rule says that the foundation should not extend below the groundwater level. If the water is shallow or very shallow, then a foundation slab weighed down with an earth embankment is most often used (or shallowly buried "Mushrooms" i.e. prefabricated foundations of the F series, e.g. produced by Elbud Gdańsk).
Cast slabs are also often used when there are rocks just below the ground surface in mountainous regions and the cost of crushing them would be inadequate to the price of the alternative.
The great advantage of prefabricated foundations is their attractive cost compared to reinforced concrete slabs, relatively low labor consumption of installation in the field and significant acceleration of investment. The great disadvantage of cast foundations is the time needed to obtain the strength necessary to finalize the investment. Depending on the thermal conditions - it is about a month after pouring the concrete. The construction or installation team must therefore divide the tower construction process into several trips. In the case of prefabricated elements, if everything has been properly prepared for installation - one (though sometimes several days) approach is enough.
Clay and plastic soils always require replacement - regardless of the foundation method. Prefabricated foundations - mushroom foundations - should be covered with a dry concrete screed, leveled with a spirit level (often using a technique used when laying screeds in residential interiors).
Ground screws are an innovative approach to foundations. It works well for smaller facilities - especially for towers up to several meters high on cohesive soil. Their use allows you to eliminate the use of an excavator (an advantage especially in hard-to-reach areas, e.g. mountains). It is very quick to install (a two-meter screw takes about 30 minutes) and well tested - although it must be admitted that most of the experience comes from outside Poland.