Underpinning

Underpinning - Underpinning is a construction method used to retroactively reinforce and deepen the foundations of existing buildings. It becomes necessary when excavation takes place next to an existing building at a depth greater than the bottom of its foundation (e.g., for a new building with a basement) or when the existing foundation needs to be reinforced due to settlement, scouring, or bearing capacity issues.

Methods and Techniques

There are various underpinning techniques: Classic underpinning is performed in sections: The existing foundation is exposed in short sections (max. 1.0-1.25 m), and a new, deeper concrete foundation is installed below the old one. Jet grouting - Cement mortar is injected into the subsoil under high pressure, forming soil columns that support the foundation. Micropiles - slender piles are drilled next to the foundation into the load-bearing subsoil and connected to the foundation via a header beam. The choice of method depends on the soil conditions, the condition of the existing building, and the required depth.

Before a underpinning method is determined, a site investigation (drilling, probing) is usually required to determine the bearing capacity of the subsoil, the groundwater level, and the condition of existing foundations. The results of the site investigation are incorporated into the structural engineer’s structural analysis. In densely built-up urban areas such as Nuremberg’s Old Town or the Wilhelminian-style neighborhoods, historic foundations made of quarry stone or brick are frequently encountered-these are more sensitive to vibrations and require particularly careful planning.

Legal Aspects

Underpinning a neighboring building is often the subject of disputes under neighbor law. Anyone digging deeper next to an existing building must, pursuant to § 909 BGB, ensure the structural stability of the neighboring property. The costs of underpinning are generally borne by the party responsible-that is, the building owner who is excavating the deeper foundation pit. In Bavaria, Art. 9 BayNachbG further regulates the obligation to secure neighboring structures. The neighbor must tolerate the underpinning but is entitled to compensation if damage occurs to their building.

For buyers of land or buildings in inner-city locations, we recommend clarifying before the deed is executed whether a construction project is planned on the neighboring property that could require underpinning measures. Such a project can involve construction activity, vibrations, and temporary disruptions-and, in the worst case, cause damage to one’s own building that is difficult to recover for later. The Building Authority provides information on approved projects in the neighborhood.

Practical Tip for Property Owners in Nuremberg

We recommend that owners in the Nuremberg metropolitan area who are planning a new building next to an existing structure-or whose neighbor is doing so-clarify the issue of underpinning early on with a structural engineer. In Nuremberg’s Old Town and the densely built-up Wilhelminian-style neighborhoods (Südstadt, Gostenhof, Johannis), underpinning is often required for basement expansions and infill development. The costs range from approximately 500-1,500 euros per running meter of foundation length-depending on depth and method.

Document the condition of the neighboring building before construction begins through a preservation-of-evidence report to avoid future disputes. The report documents the initial structural condition (existing cracks, settlement damage, masonry quality) both photographically and in writing. If new damage occurs during the construction project, the report can clearly prove that it was caused by the construction work. The cost of a preservation of evidence report (approx. €500-2,000) is a sound investment given the potential liability risks.

Frequently Asked Questions

Who bears the costs of the underpinning?

The costs are borne by the building owner who is responsible for the deeper excavation. This also applies if the underpinning affects the neighboring building. The neighbor must tolerate the underpinning but is entitled to proper execution and compensation for any damage. In practice, the costs are calculated as part of the construction costs for the new building and can be substantial-for a typical row house in Nuremberg, €10,000-30,000.

Do I need a permit for underpinning?

Underpinning itself is generally not a project requiring a separate permit, but rather part of the building permit for the new construction or renovation. The structural stability analysis for the underpinning must be submitted to the building authority as part of the structural calculations. In Bavaria, the structural engineer reviews the underpinning plans and approves them. Underpinning work may not begin without approved structural calculations.

How long does underpinning take?

The duration depends on the length of the foundation to be underpinned and the method used. With traditional section-by-section underpinning, one can expect approximately 1-2 running meters per day. For a foundation length of 15 meters, the underpinning takes approximately 2-3 weeks. Jet grouting and micropiles are faster (approx. 3-5 running meters/day) but more expensive. In addition, there are curing times for the concrete-the excavation pit may only be deepened further once the target compressive strength has been reached.

What role do soil conditions in Nuremberg play in underpinning?

In downtown Nuremberg and the historic districts, one frequently encounters natural sandstone, sandy soils, and, in deeper layers, Keuper clay. These soils can change their bearing capacity when exposed to water or vibrations. Keuper clay, in particular, tends to swell and shrink with fluctuations in moisture, which can trigger settlement movements in the foundation. Before planning any underpinning work, we therefore recommend obtaining a geotechnical report that documents the soil conditions at the specific site. The Nuremberg City Planning Office and the State Building Authority have historical soil investigations on file that can serve as a preliminary guide-but they do not replace a project-specific investigation. When planning new basement constructions or underground parking garages in existing neighborhoods in Nuremberg, the groundwater situation should be taken into account early on: An unexpectedly high groundwater table can significantly increase the cost and duration of the underpinning process because water-bearing layers must be sealed or diverted during construction work.

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