Underground car parks. Detailed Design | The Netherlands

Geotechnical design

On construction sites with sufficient surface availability, the "bottom-up" method is preferred, which involves working underwater until the underwater concrete slab is placed. In contrast, "top-down" construction is used when surface space is extremely limited, despite causing higher costs and longer durations due to hoisting limitations, confined spaces, and air quality concerns. 

This method was applied in the inner-city station boxes of Amsterdam’s North South Line due to significant traffic impact and buoyancy challenges at deep excavation levels. 

Retaining walls

The original design considered an AZ 20-700 sheet pile because the preferred installation method in densely populated urban areas is pressing, and pressing equipment was not available for the AZ-800 range. In the meantime, several presses have been developed for the 800 mm wide piles. Hence the design would nowadays consider an AZ 20-800 which has equivalent section properties to the AZ 20-700, but is barely 8% lighter. 

Generally speaking, a 10 mm thick sheet pile is perceived as the practical lower boundary in terms of stiffness and strength for pressing sheet piles into the ground, but this depends on the soil conditions and length of the pile. 

Fire safety

One of the key topics in designing an underground car park is fire safety. The storage of vehicles in a confined space under the ground level imposes several requirements on the safety measures to be taken in case of a fire than compared to a building. These measures depend on the size of the car park (compartmentation, number of parking spaces,…) and include for instance fire detection and warning systems, possibly the need for ventilation, smoke extraction and/or sprinklers as well as providing fire extinguishers and enough escape facilities. 

As such these measures are independent of the construction method, providing that the structural resistance is maintained for a prescribed fire duration. 

Key indicator - Cost

The impact of the wall type on the overall costs is significant, but within a bandwidth of less than 10 % of the overall cost. The walls themselves have a much bigger bandwidth in costs, but will also bring additional costs later on in the construction process, such as fire safety protection. 

In this cost estimate, the difference in production time is discarded. For this reason, an impact of the planning was determined in order to quantify the potential impact on costs. Additionally, the quality of the watertightness has not been transferred into a cost, where it should be noted that the quality (and therefore cost) has been set relatively high for the sheet pile with the fully seal-welded lock. 

Solely based on these results it can be concluded that the sheet pile wall provides the lowest cost.

Impact of wall type on planning and cashflow

 Here's a brief comparison of different methods based on a structure measuring 30 m by 250 m with a total circumference of 560 m, assuming a 5-day work week: 

• Sheet Piles: Using the pressed-in method, sheet pile installation progresses at 10 m per day, taking 56 days for production and an additional three days for preparation, totaling 59 days or nearly 12 weeks. Seal-welding of sheet pile interlocks runs concurrently and does not affect the critical path. 
• D-Wall: This method achieves a production rate of one panel per day, requiring 112 days for production. With mobilization and support setup, it totals 142 days or about 28 weeks. 
• SoilMix: Achieving four panels per day at 2.5 m each, this method matches the steel sheet piles' speed with 56 days for production and 30 days for preparation, totaling 86 working days or around 17 weeks. 
• Secant Piles: Producing 16 bored piles daily leads to an 8 m daily progress, totaling 70 days of installation with three days for site preparation, resulting in 73 days or approximately 15 weeks.