Substructure

Introduction

The Mollstraße/Otto-Braun-Straße development phasing plan and site layout are devised to ensure safe, effective, and compliant construction of the 9-storey mixed-use development comprising commercial, residential, and underground parking. Meeting RIBA Stage 5 and German BaustellV/DGUV compliance, the plan optimizes zoning for material storage, access routes, welfare facilities, and crane operations within CDM 2015 safety requirements.

Phasing is a structured five-stage process that allows parallel workstreams and early completions:

Enabling works (utilities, hoarding, site setup)
Groundworks & substructure (piling foundations, excavation)
Superstructure & envelope (zone-based construction)
Internal fit-out & services (staged commissioning)
Soft Landings handover (performance monitoring, user training).

This approach balances logistical efficiency, risk management, and sustainability supported by 3D visualizations, HSE-compliant measures for a robust, client-ready proposal.

Site Overview

The proposed development is located at the corner of Mollstraße and Otto-Braun-Straße in Berlin. The planned structure is a 10-storey mixed-use building consisting of:

3 floors for commercial use (medical center, shops, cafes, and offices)
7 floors for residential apartments
1 level of underground parking

The site occupies an approximate area of 5150 m², with a building footprint (built-up area) smaller than the total plot. The internal floor space is consistent across all levels, and the gross internal floor area (GIFA) reflects the cumulative footprint over the 9-storeys. This supports the building’s mixed-use purpose effectively, balancing residential and commercial functions.

Figure 1: Site area boundary showing total plot size of 5150 m²

Figure 2: Built-up area footprint indicating 3736 m² building coverage

Foundation Recommendation: Bored Cast-In-Place Pile Foundation

A bored cast-in-place pile foundation system is recommended due to several technical and contextual factors:

Technical Specifications

Foundation Type: Bored cast-in-place reinforced concrete piles

Pile Diameter: 500–600 mm

Depth: 18–25 m, depending on borehole conditions
Concrete Grade: C30/37 (DIN EN 1992)
Steel Reinforcement: B500B

Pile Cap: Cast in-situ concrete caps to transfer column loads
Soil Profile Assumption:
1. 0–7 m: Urban fill and silty clay
2. 7–25 m: Dense sandy gravel (bearing stratum)

Pile Design Considerations and Methodology

The pile design process begins with an initial geotechnical investigation, including borehole drilling and soil sampling, to determine the properties of subsoil layers. These investigations help identify the allowable bearing pressure and the depth at which competent strata, such as dense sand or gravel, are located. Based on these parameters, pile lengths are established to ensure adequate load transfer. The selection of pile diameter (500–600 mm) and depth (18–25 m) for this project is consistent with standard practice for mid-rise buildings in Berlin (Tomlinson & Woodward, 2014). The use of cast-in-place piles allows for flexibility in adjusting lengths on-site depending on real-time drilling feedback and soil resistance. Reinforcement cages are prefabricated and lowered into position before concrete is poured using the tremie method to prevent segregation and ensure full encasement.

Load transfer Mechanism and Structural Integration

The following structural layout diagrams illustrate the spatial organization of critical load-bearing components for the basement and above-ground structure. The Retaining Wall Plan shows strong concrete walls built around the edge of the basement. These walls are about 102.47 m wide and 54.2 m long. They help hold back soil and water from outside the basement.

The Column Plan shows a grid of concrete columns spread across the building area, which is about 85m by 43m. These columns carry the weight of the building down to the foundations. The columns are spaced evenly, with some starting about 3 m from the outer edge, so the structure fits well with both the foundation and the building layout.

Figure 3: A column grid layout for load distribution across built-up area

Figure 3: Column grid layout for load distribution across built-up area (85 m × 43 m) and Boundary columns are sized at 500 mm x 500 mm, while internal columns measure 400mm x 500 mm. The distance between each column is uniformly 4 meters, supporting a consistent structural framework.

Figure 4: A Parking Plan

Figure 4: The drawing titled Parking Plan, labelled as Level B1, illustrates a clearly defined layout of the parking area. The overall dimensions of the parking area are 43.00 meters in width and 85.00 meters in depth. Each car parking bay has a standard width of 2.60 meters, with a bay depth ranging approximately from 5.00 to 5.50 meters, as implied by the layout. The aisles between parking bays are approximately 6.00 meters wide, which allows for convenient two-way vehicle movement. The structural grid system follows a modular pattern, where most rows maintain an equal spacing. Entry and exit ramps are clearly indicated, ensuring a logical and efficient vehicle circulation flow throughout the plan. Additionally, all measurement values are accurately marked, and essential annotations such as Entry and Exit are clearly labelled to enhance readability and navigation within the plan.

The structural behavior of the pile foundation involves both end bearing and skin friction. In layered soil profiles like those in Berlin, where silty clay overlay dense granular soils, the piles rely primarily on end bearing at the lower strata while also benefiting from skin friction along the embedded shaft. The pile caps, designed with reinforced concrete, distribute axial and lateral loads from the superstructure into the piles. Coordination with structural engineers is essential to align the foundation layout with column positions, shear core zones, and the basement retaining walls. The integration of the substructure with the reinforced concrete basement ensures load continuity and resistance to hydro static uplift pressures. Structural waterproofing using membrane or crystalline admixtures may be considered for durability.

Typical Pile Foundation For A 9-Storey Mixed-Used Building

The diagram above shows a typical pile system with:

Pile caps below ground
Multiple bored piles
Connection to load-bearing soil

Construction Considerations

The installation of bored piles will require:

Temporary casing or slurry support to prevent borehole collapse.
Groundwater control during excavation.
Testing of excavated spoil for potential contamination due to prior use (Tomlinson & Woodward, 2014).
Health & safety risks deep excavation, rig stability, spoil management.
Equipment needed Kelly bar rigs, rotary heads.
Quality control pile integrity testing, concrete cube testing.

Pile layout will align with the architectural column grid and allow phasing for the basement parking excavation.

The installation of bored piles will require the use of specialist equipment such as rotary drilling rigs with Kelly bars, temporary casing systems, and tremie pipes for concrete placement. During boring, compliance with DIN EN 1536 standards is essential, including proper borehole cleaning, reinforcement placement, and concrete pouring under controlled conditions. Site safety is critical, with risks related to deep excavation, machinery operation, and spoil removal requiring method statements and health and safety assessments. Quality control measures, such as cross hole sonic logging (CSL) or low-strain integrity testing, will be applied to ensure structural soundness. Concrete cubes will be tested regularly to validate compressive strength in accordance with EN 12390-3 standards.

Underground Utility Coordination

A full underground utility survey will be carried out prior to any excavation. This will identify sewer mains, water supply, drainage pipes, and electrical conduits within the basement footprint. The survey data will be used to adjust pile positions and retaining wall design, ensuring that no existing utilities are damaged during construction. Where relocation is necessary, coordination with Berlin utility providers will ensure compliance with local regulations (Survey2Plan, 2024; Utility location, 2025).

Construction Sequencing and Site logistics

Efficient sequencing is vital for the substructure phase. The process begins with site clearance and setting out, followed by excavation for pile caps and installation of bored piles. Temporary access roads, material storage zones, and equipment staging areas must be clearly defined to optimist crane and rig operations. During construction, spoil from boreholes must be removed promptly to avoid contamination and logistical clutter. After piles are cast and cured, pile cap form-work and reinforcement are installed, followed by concrete placement. Waterproofing membranes and drainage systems are then applied before progressing to the basement slab and walls. This sequencing aligns with RIBA Stage 5 guidance and ensures minimal interruption to the critical path schedule. Regular coordination meetings with contractors, planners, and site engineers will be crucial to maintain progress and resolve interface issues proactively.

Sustainability Measures

The following sustainable practices are recommended:

Low-carbon concrete incorporating GGBS or fly ash to reduce embodied CO₂ (Turner, 2022).
Prefabricated steel reinforcement cages to improve on-site efficiency and reduce waste.
Use of recycled demolition materials in back-fill or slab-on-grade concrete.
Efficient phasing of excavation and pile works to minimize machine runtime and energy use.

This substructure solution was developed in collaboration with the Quantity Surveying team to align with cost planning, and with the Real Estate team to ensure structural feasibility supported the proposed use mix and market expectations.

Real-World Case Study: Europacity, Berlin

A relevant local precedent is the City Harbour District (Europacity) near Berlin Hauptbahnhof. There, 607 bored full-displacement piles with diameters of 460–560 mm and depths of 9–18 m were used to support a large mixed-use development. The project, completed by PORR Spezialtiefbau GmbH in 2017, included phased pile construction in similar subsoil conditions (PORR Spezialtiefbau GmbH, 2017).

This case confirms that pile foundations of this scale and depth are standard for mid-rise urban developments in Berlin. It also demonstrates the suitability of pile solutions in constrained sites with mixed-use requirements.

Presented By: Lotavath, Ganesh

(34055017)

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