What is Rostwerk? How does he work?

Grillage (it. Rostwerk, from Rost - lattice and Werk - structure)
A grillage is the upper part of a pile or columnar foundation that distributes the load from the load-bearing elements of a building (structure). As a rule, this is a reinforced concrete monolithic structure under the load-bearing walls of the building and standing on columns dug into the ground (piles or pillars).

Installation of pile-grillage foundation: piles + grillage

A grillage is called high if it is located significantly above the ground or water level. Pile and columnar foundations consist of piles and grillage.

The material for the grillage in construction is mainly concrete and reinforced concrete (precast or monolithic), less often - wood and metal. The grillage is cast below the freezing level or at a certain distance from the ground.

How does he work? The load from the house is transferred to the belt. Due to the presence of longitudinal reinforcement, it is redistributed over the entire area. Since the tape also rests on the ground, part of the load is transferred to it, the rest falls on the piles. In this case, the load and shrinkage are uniform: they are "aligned" by the tape.

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In winter, when heaving forces begin to affect the foundation, all the advantages of the pile-strip foundation are manifested. If the house stands on heaving soils, their depth is below the freezing level, it is very difficult to imagine the conditions under which the house will warp or it will give uneven shrinkage. When heaving forces act on the tape, the "heels" of the piles, and they themselves, do not allow the soil to move the foundation. Therefore, strip-pile foundations are an excellent choice on highly heaving soils.

An example of a pile-grillage foundation in conjunction with a floor slab

At the same time, the costs are much higher than in the construction of a conventional pile foundation, but much lower than in the construction of a tape below the freezing depth.

Pile-grillage foundation. The grillage is located in close proximity to the surface of the earth

I recommend installing pile-grillage foundations in the following cases:

When unstable and weakly bearing soils go to a sufficiently large depth. These are karst, forest, peat soils, quicksands, plant and fertile soils of great thickness (more than 1.4-1.5 meters). In this case, the load must be transferred to dense soils located below with normal bearing capacity. It is not always possible to get to the bottom of them, and if it is possible, then the foundation turns out to be too expensive. Therefore, transferring the load using piles is the best choice.
In areas with large elevation differences. In this case, it is often much cheaper to use piles of different heights than to carry out work on leveling the ground or pouring a deep tape that can compensate for the difference in height.