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 methods are frequently used in practice, where tools such as concrete pulverizers or hydraulic rock and concrete splitters 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 a 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). In practice, primary and secondary piles are often constructed in sequence to achieve the required geometry and watertightness. 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, including the need for temporary casing, support fluids, and controlled spoil management.

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. Stabilization of the borehole by temporary casing or support fluids where required by ground conditions.
4. Installation of the reinforcement cage, with embedded items where required for anchors or monitoring systems.
5. Concreting, usually by the tremie method, with continuous delivery.
6. Construction of subsequent piles in alternation; overlap as required for sealing.
7. Follow-on works: pile head removal to design elevation and installation of bracing or tie-back anchoring.
8. Documentation, initial monitoring, and release for excavation in accordance with the construction stage concept.

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. Deformation targets and serviceability criteria are defined project-specifically and verified by numerical or simplified methods with staged construction. 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.
• Concrete placement: tremie concreting with verified slump, temperature control, and continuity at overlaps.
• Reinforcement: single or double cages as required for bending and shear, with couplers for staged connections.

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. Typical fields of application include deep basements along property lines, metro and shaft structures, and protection of adjacent infrastructure. 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 pulverizers are frequently used for selective concrete reduction, and hydraulic splitters for low-vibration 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. Close coordination with the temporary works design ensures that residual load paths and construction stages remain robust.

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 splitters 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 pulverizers enable material removal with good visual control, especially at edges, in corners, and in confined excavation pits. The method selection depends on section thickness, reinforcement density, allowable vibrations, and accessibility.

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 shears or Multi Cutters.
• Openings and penetrations: Local weakening using splitting technology followed by breakout with concrete pulverizers 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 shears.

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. Power demand, hose routing, and work envelopes should be coordinated with excavation stages and site safety zones.

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.

• Implement water mist or local extraction at the tool to reduce airborne dust.
• Use acoustic screens and time windows to manage noise in sensitive areas.
• Define a monitoring plan with trigger values for vibration, settlement, and groundwater level.

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. Predetermined breaking points, sleeves, or soft concrete zones can facilitate later interventions. This allows subsequent interventions – such as with concrete pulverizers or hydraulic splitters – 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.
• As-built capture and updates to digital models to preserve geometry, overlaps, and joint locations for later phases.

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. Hybrid solutions are possible where sections of a project demand different performance levels. 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 pulverizers, hydraulic splitters, and supplementary tools such as steel shears 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. A dismantling plan, material flow documentation, and verification of disposal or reuse routes support compliance and sustainability targets.