Underground Car Parks | Germany | ArcelorMittal Sheet Piling

Introduction

Typically, the construction of an underground car park involves the creation of an excavation supported by retaining walls. The choice of retaining wall system usually depends on the specific boundary conditions, such as soil characteristics, groundwater level, excavation depth, logistical constraints, and imposed loads. Within the excavation, the underground car park is then constructed using reinforced concrete walls. The selected retaining wall either remains in the ground permanently or is removed if feasible (possible only with sheet pile walls or soldier pile walls).

This approach represents a common and proven construction method for underground car parks under certain conditions. From the perspectives of sustainability, resource conservation, space efficiency, and cost-effectiveness, retaining walls used as permanent wall systems offer further opportunities for optimisation.

The following wall systems will be analysed and compared based on the points described:

V1: Permanent sheet pile wall;
V2: Temporary sheet pile wall (used only as an excavation wall);
V3: Permanent secant pile wall;
V4: Permanent diaphragm wall.

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Underground Car Parks | Germany

Boundary Conditions

Geometry/Structural Components

The designated construction site has a specified area of 28 x 50 metres. The new building is to be constructed on a 28 x 28 metre footprint. The underground car park will consist of two levels, each with a clear ceiling height of 2.70 metres.

In this example, it is assumed that the groundwater table is located just below the surface level. As a result, the construction of the excavation and the underground car park will require a base slab made of underwater concrete. This base slab prevents water intrusion into the excavation from below during construction.

Additionally, the slab, along with the micropiles embedded within it, will resist the uplift pressure caused by dewatering the excavation. For the structural design, a 1.40-metre-thick concrete base slab is assumed, including a 30 cm levelling/drainage layer. To account for excavation tolerances, an additional 30 cm is subtracted.

It should be noted that the underwater slab and the micropiles have no influence on the results of the system calculations, as the boundary conditions remain consistent across all systems.

Construction Phases

The following construction sequence is specified for the execution of the works:

1. Construction of the retaining wall, depending on the chosen construction method:

a. V1 - Permanent sheet piling:

Drilling of loosening holes for the driving track
Driving/ vibrating of the sheet piles

b. V2 - Temporary sheet piling:

Drilling of loosening holes for the driving track
Driving/ vibrating of the sheet piles

c. V3 - Bored pile wall:

Installation of the bored piles, including concreting and reinforcement (further steps only after the concrete has cured)

d. V4 - Slurry wall:

Construction of the slurry wall, including concreting and reinforcement (further steps only after the concrete has cured)

2. Installation of the stiffening layer at +59.00 m NHN (preceded by: excavation to +58.50 m NHN + parallel lowering of the groundwater level to +58.00 m NHN (BZ I))

3. Underwater excavation to +50.50 m NHN (BZ II)

4. Installation of UW concrete, including the levelling layer, buoyancy piles, and drainage layer, and dewatering of the construction pit to +50.50 m NHN (BZ III)

5. Concreting of the basement floor slab (U2), columns, and ceiling above U2, temporary bracing of the ceiling above U2 to the retaining wall (→ not applicable for "V2 - Temporary sheet piling"), and removal of the stiffening layer (BZ IV)

6. Concreting of columns, ceiling above U1, including the reinforced concrete connection to the retaining wall (→ not applicable for "V2 - Temporary sheet piling") (EZ), removal of the temporary sheet piles (V2)

Calculation Basis

Loads

Upon completion of the underground garage, the superstructure will be constructed with 4 floors. The vertical loads resulting from this will be transferred to the ground through the external walls and columns. As a result, the retaining structures will also carry vertical loads in the final state, except for the temporary sheet piling. The vertical load is defined as a line load with Nk = 350 kN/m.

Due to the proximity of neighbouring buildings, the retaining wall is directly adjacent to the existing structures. The neighbouring buildings are founded on strip foundations.

The upper edge of the strip foundation is set at +58.00 m NHN, with a width of 1.0 m. The floor level of the neighbouring building is at +60.00 m NHN. The foundation load has been calculated, assuming the dimensions and materials of the existing building, with Gk = 350 kN/m.

Additionally, a load of pk = 5 kN/m² is applied at the ground level from the rear edge of the existing foundation. This covers the load from the self-weight of the floor slab and additional imposed loads.

Technical Regulations

The standards, recommendations, and guidelines used can be found in Chapter 7, “References.”

Partial Safety Factors

The partial safety factors for the design of the retaining walls are taken from EC7 (DIN EN 1997-1), Tables A 2.1 and A 2.3, and the EAB, Tables A 6.1 and A 6.2.

According to EC7-1 and DIN 1054 (2010), construction measures for temporary purposes are classified into the design situation BS-T, and permanent structures are classified into BS-P. For the present study, the retaining wall types, slurry wall and bored pile wall, are examined as permanent structures. The sheet piling is examined both as a permanent structure and as a temporary structure.

**Summary of geometrical results for retaining wall variants**
Variant	Designation	Wall Type	Chosen (L_Wall)
[-]	[-]	[-]	[m]
V1	Sheet piling, braced, permanent	AZ 32-750; S 355 GP	14.50
V2	Temporary sheet piling as excavation support	AZ 32-750; S 355 GP	14.50
V3	Overlapping bored pile wall, braced, permanent	Ø 1,18 m	14.50
V4	Slurry wall, braced, permanent	t = 1,00 m	16.00

The sheet piling is the most cost-effective solution. The difference is approximately 18% compared to the bored pile wall and about 24% compared to the slurry wall. Additionally, a temporary sheet piling solution was considered.

Conclusion

The permanent sheet piling is identified as the preferred option according to the evaluation matrix. It achieves excellent results in most criteria, particularly in terms of sustainability. The permanent sheet piling is planned as a long-term structure, eliminating the need for an additional outer wall. Furthermore, steel, compared to reinforced concrete, is lighter and more easily recyclable when it comes to future dismantling. Only with regard to execution risks does the sheet piling perform slightly worse, as it may encounter difficulties when dealing with obstacles in the ground, compared to bored pile walls and slurry walls.

In conclusion, it can be said that this study lays important foundations for future projects. Looking ahead, there is a growing demand in the construction industry for new and more efficient solutions for conventional constructions. This investigation has demonstrated that a shift in thinking is possible, and when it comes to the construction of underground garages, other designs may offer a better option in the future.

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