Secant pile wall

The secant pile wall is a central construction method for modern excavation pit support and slope stabilization. It enables safe excavation pits in confined inner-city locations, underpinning of existing structures, and permanent or temporary retaining walls. Over its life cycle, in addition to planning and construction, processing and deconstruction tasks also occur—such as pile head removal, openings for penetrations, or complete removal of temporary walls. In these phases, selective, low vibration levels methods are frequently used in practice, where tools such as concrete pulverizer or hydraulic rock and concrete splitters from Darda GmbH in combination with mobile hydraulic power units enable precise and controlled processing without taking on the character of a pure demolition project.

Definition: What is meant by secant pile wall

A secant pile wall is a support structure made of cylindrical piles drilled into the subsoil and constructed with reinforcement and concrete. These piles are arranged in a line and resist earth pressure, groundwater loads, and, where applicable, traffic loads. Depending on the arrangement and overlap of the piles, a distinction is made between tangent (touching), secant (overlapping), and discontinuous bored pile walls (pile rows with shotcrete between). Secant pile walls can be built as temporary (only for the construction period) or permanent systems. Watertightness against groundwater is achieved structurally by pile overlap, joint sealing systems, or supplementary cut-off base slabs.

Construction methods, types, and configuration of the secant pile wall

Secant pile walls are produced step by step: first, drilling is carried out along the planned wall alignment, the reinforcement cage is installed, and the pile is concreted. Construction usually proceeds in alternating-pile sequence to maintain ground stability. The method is selected based on geological and hydrological boundary conditions as well as excavation pit geometry.

Types and characteristics

• Tangent bored pile wall: Piles touch at the shaft surface. Suitable where groundwater inflow is low; watertightness often limited.
• Secant pile wall: Piles overlap. Increased watertightness, better composite cross-section; common where groundwater is present.
• Discontinuous bored pile wall: Piles with intermediate fields of shotcrete. Economical in self-supporting soils; watertightness depends on the shotcrete and any integrated sealing systems.
• Interlocked variants: Piles keyed into rock or competent subsoil to dissipate high horizontal loads.

Execution steps at a glance

1. Surveying, setting out of axes, and fixing the drilling points.
2. Drilling (e.g., by Kelly drilling method or continuous flight auger) down to load-bearing strata.
3. Installation of the reinforcement cage, with embedded items where required for anchors or monitoring systems.
4. Concreting, usually by the tremie method, with continuous delivery.
5. Construction of subsequent piles in alternation; overlap as required for sealing.
6. Follow-on works: pile head removal to design elevation and installation of bracing or tie-back anchoring.

Design, anchoring, and sealing concepts

The structural analysis considers redistribution of earth pressures, construction stages, groundwater, and surcharge loads from adjacent development. Secant pile walls are designed as cantilevered, tie-back anchored, or braced systems. Where groundwater is present, a sealing concept is required: in secant pile walls, the overlap improves watertightness; additionally, waterstops, injection joints, or cut-off base slabs may be used. Selection is project-specific and follows the applicable standards.

Typical parameters

• Pile diameter: approximately 60–150 cm, larger in special cases.
• Pile center-to-center spacing: dependent on diameter and type; for tangent walls typically about the pile diameter; for secant walls smaller.
• Embedment depth: dependent on subsoil and loads, often into competent strata or rock.
• Tie-back anchoring: temporary or permanent anchor rows coordinated with construction stages.

Applications of the secant pile wall in the construction sequence

Secant pile walls are used in inner-city excavation pit projects, for slope stabilization, underpinning of existing structures, and in tunnel construction and special foundation engineering. They offer low vibration levels during construction and adaptability to complex geometries. Over the life cycle, tasks arise that require targeted material removal, such as recesses, service routes, openings, or the removal of temporary wall sections.

Interfaces to concrete demolition and specialized deconstruction

Controlled methods are particularly effective for pile head removal to foundation elevation, exposing reinforcement for bracing connections, or creating penetrations. In practice, concrete pulverizer are frequently used for selective concrete reduction, and hydraulic splitter for low vibration levels splitting of massive sections. These activities typically fall within the fields of concrete demolition and special demolition, building gutting and concrete cutting, and—depending on the project—special operations and rock excavation and tunnel construction.

Selective processing and deconstruction of secant pile walls

Precision is crucial when adapting or deconstructing secant pile walls: material should be removed only where it is structurally and logistically permitted. Hydraulic splitter use controlled wedge pressure in pre-drilled holes to separate concrete or rock sections in a predictable way—suitable for massive pile heads, foundations, or protruding pile remnants. Concrete pulverizer enable material removal with good visual control, especially at edges, in corners, and in confined excavation pits.

Typical work steps

• Pile head removal: Reduced concrete removal down to the specified elevation; once the reinforcement is exposed, it can be cut to length with steel shear or Multi Cutters.
• Openings and penetrations: Local weakening using splitting technology followed by breakout with concrete pulverizer for services, pump sumps, or control shafts.
• Deconstruction of temporary walls: Selective removal where the excavation support is not needed permanently; depending on boundary conditions, a combination of splitting and shearing.
• Exposing connection reinforcement: Gentle concrete removal to provide reinforcement for bracing, corbels, or anchor heads; cutting reinforcement with steel shear.

Hydraulically driven tools are supplied via a hydraulic power pack and can be deployed with flexibility in construction logistics. Combination shears and Multi Cutters can, depending on the site situation, combine cutting reinforcement with crushing concrete. Rock splitting cylinders are considered particularly when boreholes are available or can be drilled.

Emissions, occupational safety, and environmental influences

In urban projects, vibrations, noise, and dust must be minimized. Splitting and shearing methods are considered low vibration levels and can offer advantages compared to percussive methods. Dust suppression (e.g., by water mist), organized material logistics, and protected areas for third parties must be planned. Safety briefings, machine setup coordination, and clear interface coordination between special foundation engineering and deconstruction trades increase operational safety. Legal requirements must be checked project-specifically; construction-phase monitoring and measurements (e.g., settlement monitoring or ground vibration monitoring) are common practice.

Planning notes for secant pile walls

The choice of wall type depends on subsoil, groundwater inflow, space constraints, service life, and acceptable emissions. Openings, future penetrations, or temporary deconstruction zones should be considered early to coordinate reinforcement, joints, and construction sequence accordingly. This allows subsequent interventions—such as with concrete pulverizer or hydraulic splitter—to be executed in a planned and safe manner.

Quality assurance and documentation

• Production records: drilling and concreting logs, reinforcement documentation, and anchor logs where applicable.
• Checks: position and verticality tolerances, concrete quality, embedment depth, sealing joints.
• Construction stages: verifications for bracing or tie-back anchoring, monitoring concepts, and observational methods.

Distinction from alternative systems

Compared to diaphragm walls, secant pile walls offer flexible construction and lower emissions; diaphragm walls are often advantageous where very high watertightness and great depths are required. Soldier pile walls and shotcrete shoring are economical in stable soils and for lower loads but offer less watertightness. Sheet pile wall systems are suitable for temporary excavation pits but often require more space and cause higher vibrations. The decision is project-specific and integrates subsoil, groundwater, loads, and construction phases.

Lifecycle: construction, use, and deconstruction

Secant pile walls can remain permanently as part of the excavation or site works, or be deconstructed after the construction phase. During deconstruction, selective methods with concrete pulverizer, hydraulic splitter, and supplementary tools such as steel shear are used. This facilitates separation of concrete and steel for recycling and reduces emissions in sensitive environments. Procedures must be planned for the specific project and take into account structural analysis, adjacent structures, and environmental aspects.