So-called "Berlin walls" can be of a "permanent" nature, when with the internal lining alone they become a permanent retaining structure, but they can also be temporary, useful only for the excavation, in cases where the definitive resistance to the thrust of the ground is then entrusted to the completion of the underground structures.

Likewise, the tie-rods that are installed to stabilise the retaining walls may be "permanent" if built in ground whose owner does not require their subsequent removal, or temporary if they cannot remain for the duration of the structure's lifespan because they are located beneath public land or private property whose owner does not intend to cede the rights to it.

Variable equilibrium

The design of the retaining walls therefore consists in suitably balancing the thrust of the ground and its state of deformation through various factors such as: the inertia of the wall, its embedment length in the substrate beyond the excavation level, and the type and tension of the tie-rods or struts.

The combined application of these forces makes it possible to progressively create a structure that meets our needs in the particular situation in which it is applied.

The retaining structure we are discussing has certain construction features that make it perfect for these types of operations:

1. It can be built before the excavation;
2. It can be supported so as to limit its lateral deflection during the excavation operations in successive steps as the level descends;
3. When suitably sized, it can halt the settlement of the foundation level of adjacent buildings or infrastructure, thereby preventing damage that would compromise the continuation of the works.

Composition of the structure

The component parts of the retaining wall are generally the following:

• A series of micropiles that descend from the ground surface into the more resistant substrate located below the level of the final excavation;  the distance between the piles is one of the design elements and varies with the diameter of the pile according to the active thrust that the wall receives from the ground in the various excavation phases;
• A reinforced concrete pile-head capping beam, usually rather robust and well reinforced, with the task of making all the piles integral with one another; the capping beam is often the seat of the first row of prestressed tie-rods needed to stabilise the structure and thus make the wall integral;
• Successive rows of active systems, suited to exerting a positive thrust on the wall; this force is distributed among the piles through horizontal courses of paired steel beams. The horizontal force can be exerted through various systems:

1. Classic drilled tie-rods made of steel strands with distributed anchorage, injected with cement grout and subsequently post-tensioned;
2. Tie-rods composed of Dywidag bars that are inserted into the substrate zone, or nails of various lengths if the substrate is rocky and is located immediately behind the piles;
3. Struts to be placed in the excavation in cases where, for various reasons, the tie-rods cannot be built.

Interference with aquifers

Often this type of excavation results in the interception of aquifer horizons, or simple water veins or leaks from the water/sewage system in the vicinity of our site; it is therefore necessary to build counter-walls capable of withstanding the hydraulic thrust and keeping the liquids outside the buildings.

Generally, a new reinforced concrete wall is built after laying a bentonite-type waterproof membrane, to be confined between the casting and the piles, or a thicker counter-wall in concrete admixed with waterproofing substances, which are proving highly reliable.

Examples of excavations in confined urban environments

At this point I would like to show some successfully completed projects, each of which had significant challenges and required an original solution.

Enclosed building

Excavation for the construction of the foundations and the underground levels of a residential building in a small area enclosed between an existing tall building, a major hydraulic structure and the presence of a railway embankment; the site was characterised by the impossibility of carrying out the classic prestressed tie-rods on some of its fronts.

Excavation carried out adjacent to other buildings or heavy infrastructure interferes with the pressure bulb developed in the ground by isolated direct foundations; usually the prestressed tie-rods serve to contain the lateral thrusts of the bulb that is cut into. In this case, it was necessary to resort to the construction of trestle struts to serve the same purpose.

Excavation among the springs

The second example of confined excavation that I bring to your attention concerns a deep excavation carried out in a central area adjacent to a busy municipal road, which had to be kept fully operational and in conditions of complete safety; in this case, the presence of springs in the fractured marly substrate entailed technological choices to withstand the water thrust and keep the building under construction dry.