Industry: Building and Construction Product: RAM and PLAXIS
User: WSP
Country: UK

October 21, 2019

WSP Delivers Optimized Design for Complex Basement Beneath London Landmark Using Bentley Applications

The Admiralty Arch, completed circa 1911, is one of London’s most iconic buildings. The structure is located at the end of The Mall, which connects Buckingham Palace with Trafalgar Square, and incorporates three archways that provide road and pedestrian access with six levels aboveground and two sublevels. WSP Global Inc., one of the world’s leading engineering professional services firms, was contracted to transform the Admiralty Arch into a new Waldorf Astoria hotel, projected to be completed in 2022.

WSP is providing structural engineering for this very challenging project. Belowground, there will be two new sublevels created on either side of the Arch, directly below the existing roadway. Project objectives included overcoming complex engineering and geotechnical requirements, providing both a temporary and permanent structural solution, and driving efficiency through streamlined design and project delivery processes.

The Trafalgar Square basement is 12 meters deep and contains two main levels, including spa and pool facilities at B1 and a ballroom at B2. The sublevels are located directly above two London Underground tunnels, requiring a box-in-box system to provide acoustic isolation for the rooms. Another challenge facing the team was building a new basement under one of the most famous roads in London, while keeping traffic flowing. Only one of the three lanes could be closed at a time, which meant the site had to be divided into three stages, closing one lane, and moving the operation in coordination to traffic needs.

The complex basement structure presented major challenges, such as how it would interact with the soil and two exiting underground tunnels below the site. Without proper planning, the integrity of the tunnels could be compromised, and the upwards movement could deform and permanently damage them. In standard building construction, foundations hold buildings up. However, in this case, without a superstructure above the basement, piles need to be located at either side of the exclusion zones of the tunnels to hold the ground down as the clay begins to expand. Effectively, the foundation of this basement is in tension, holding the tunnels in place as they try to move and deform upwards.

Relying on Bentley’s structural and geotechnical applications, the project team employed a top-down construction methodology to overcome these engineering, modeling, and coordination challenges. As a result, the project team reduced the overall structural design time by approximately 25% and was able to provide timely construction documentation to the site.

WSP used PLAXIS to develop a detailed ground movement assessment (GMA) to fully understand the soil-structure interaction and effectively provide the required tension piles and balance loads. WSP used RAM to prepare the analysis and design model to match the results from the GMA by varying the spring stiffness of the different piles. RAM allowed for effective collaboration within the design team, creating information that is easy to understand and communicate.

The project team also used PLAXIS to predict the movements and forces at a cross-section level and to determine from that analysis the maximum changes in diameter and lining forces and stresses. A two-dimensional finite element analysis (FEA) was undertaken using PLAXIS 2D and a three-dimensional FEA was undertaken using PLAXIS 3D to look at the longitudinal effects along the chain of tunnels.

Using RAM, WSP determined the most efficient solution for all basement slabs and raft foundations for the project. The team was also able to iterate with different thicknesses to find the right balance between slab thickness, reinforcement requirements, and self-weight that was most beneficial against the uplift of the basement. The Bentley open modeling and simulation applications also enabled effective collaboration across the design team and helped create information that was easy to understand and communicate and that was the key to obtaining the proper statutory approvals.

“RAM provides a successful platform for analysis, design, and delivery that exceeds all our expectations,” said Diego Padilla Philipps, associate structural engineer with WSP. “We are committed to using RAM software in our concrete buildings. RAM provides flexibility to coordinate and communicate clearly and efficiently our design process and intent.”

1519_WSP Delivers Optimized Design for Complex Basement Beneath London Landmark Using Bentley Applications

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