Robert Jansen
Technical Sales Manager
A construction pit requires a retaining wall that stands securely during excavation and execution. For greater retaining heights, a propped excavation construction is often more efficient than a braced excavation, offering more workspace and better control of deformations.
A cofferdam is a temporary earth-retaining structure that creates space for underground construction. The cofferdam keeps soil and groundwater out of the work area during excavation, foundation work, basement construction, or the installation of underground infrastructure.
The wall usually consists of a sheet pile wall, combi wall, secant pile wall or CSM wall. The choice depends on excavation depth, soil pressure, watertightness, environmental requirements and the desired finishing. With limited retaining height, a free-standing wall may suffice; for greater depths or stricter deformation requirements, the wall is fitted with support.
For this support, there are two main variations: a bracing frame that props the walls against each other, or a cellular cofferdam that deflects the horizontal forces into a stable ground layer behind the wall. Strand anchors are therefore an efficient solution: high capacity, slim profile and a clear construction pit without internal bracing. This keeps the execution logistically more manageable, even in urban or phased projects.
Other solutions and applications
In the design and execution of cofferdams, several factors jointly determine the functioning of the structure. The interaction between the wall, support, and subsoil determines both the failure safety and the serviceability limit state:
Especially in urban areas or with sensitive adjacent properties, vibration control, phasing, and managing deformations are critical. Early coordination between the structural engineer, contractor, and anchor supplier prevents surprises during execution and secures the schedule. A well-considered choice between a strutted excavation support system and a anchored design also determines the lead time and feasibility within the project budget.
A braced excavation construction is made up of a diaphragm wall, walings, and an anchoring system that transfers horizontal forces into the subsoil.
The wall is typically a diaphragm wall or a combi-wall. The wall absorbs earth pressure, water pressure, and loads from the surroundings and transfers them via bending and clamping at the base. The load on the wall changes with each excavation phase, which determines the choice of one or more anchoring layers.
The purlin or distribution beam is placed just above the excavation line of the relevant phase. This distributes the concentrated anchor forces evenly over the wall and prevents local deformations at the anchor head.
The strand anchor consists of an anchor head with wedges, a free length in a duct, and an anchored length which transfers force to the ground via grouting. The tensile force from the wall is thus transferred to a stable soil layer behind the active slip surface.
The rod, purlin, anchor head, and anchored length function as a single system. For temporary cofferdams, consideration is also given to deactivating or removing the anchor after completion.
Every situation is different. In a consultation, we will clearly outline your project and possibilities. You will immediately know where you stand.
Robert Jansen
Technical Sales Manager
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A coffer dam is a temporary earth-retaining structure that creates space for safe underground construction. The dam retains soil and groundwater during excavation, foundation work, cellar construction, or the creation of underground infrastructure.
A diaphragm wall construction pit is common for narrow pits where walls can brace each other. For wider or deeper pits, or where working space and crane usage are critical, an anchored construction pit offers more advantages: clear working space, simpler logistics, and often better control of deformations.
A construction pit anchor is a ground anchor that transfers the horizontal load from the sheet pile wall or combi-wall to a stable soil layer behind the active failure plane. Strand anchors are an efficient solution for this purpose due to their high capacity and slender design.
Both. For most coffer dams, a temporary anchor (PL0/PL1, lifespan ≤ 2 years) is sufficient. When the coffer dam becomes part of the final structure, or when anchors continue to function during the operational phase, permanent anchors (PL2/PL3) are used.
In construction pits near installations with cathodic protection or stray currents, a metallic path via the anchor can cause undesirable corrosion or current leakage. An electrically insulated strand anchor (PL3) interrupts this path. Deepro is currently the only supplier in Europe to offer this with ETA certification.
Yes. For temporary cofferdams, it is determined in advance whether the anchor is fully removed, only relaxed, or left in the ground. This depends on requirements from the environment, the permit, and the end-user, and is taken into account during the design phase.
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