S21-Berlin â€“ Design and Underwater Construction of a Covered Construction Pit without Direct Accessibility from AboveÂ-Paper

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The S21 Berlin project required construction of a 44 m-long covered excavation pit beneath the B 96, whose traffic had to remain undisturbed. The pit had originally been built as a preliminary structure with diaphragm walls, a jet-grouted supporting layer and an anchored jet-grout cutoff, beneath a 2.5 m reinforced-concrete road deck.<br /><br />Design conditions changed, including groundwater levels, geology and tunnel geometry, increasing structural demands. During excavation, a major water inflow raised concerns about the cutoff’s watertightness and stability. The team therefore replaced the planned dry excavation with underwater excavation and designed a concrete base over the existing jet-grouted layer.<br /><br />The deck prevented access from above, so excavation and construction had to be performed through the end diaphragm walls, with divers working in very poor visibility and around numerous obstacles. A remotely operated dredging device, adapted from the Norwegian offshore industry, loosened and pumped soil-water slurry to the surface for processing and dewatering. Divers recovered material that could not be pumped, including hardened grout and debris.<br /><br />Unexpectedly high areas of the existing supporting layer meant the underwater concrete base had to be reduced from the planned 1.5 m to just 1 m thick. The design incorporated steel supports attached to the diaphragm walls, local reinforcement and a massive underwater concrete strut to support the end walls. A flowable-concrete levelling layer, extended anchors and a specially designed rail-guided trolley helped achieve the required placement accuracy. The base was poured in four sections, followed by the concrete strut.<br /><br />The project demonstrated that a covered excavation pit with an underwater concrete base can be built without direct overhead access. Safe execution depended on specialist equipment and underwater methods, careful tolerance control, suitable site conditions, and flexible collaboration among the project teams.

Keywords

S21 Berlin

covered excavation pit

underwater concrete base

diaphragm walls

jet grouting

groundwater inflow

underwater excavation

remotely operated dredging device

diver operations

B 96 traffic