Futuristic Paris landmark stands on geotechnical know-how

Challenge

The 180 m high Duo 1 (39 storeys) and 122 m high Duo 2 (29 storeys) include over 100 000 m2 of mixed-use space and 9 basement levels, rising out of a busy urban landscape. As a key partner of the project team, Fugro defined the requirements for and delivered geotechnical investigations, geotechnical engineering and stability monitoring that:

• Delivered safe and cost-effective deep foundations, diaphragm walls and basement levels

• Minimised settlement to safeguard surrounding infrastructure

Solution

The highly relevant Geo-data acquired by our specialists were crucial to the feasibility assessment, ground evaluation, and design for the foundations of this statement building. Fugro also provided post-construction monitoring.

Geotechnical investigations

To inform the foundation and basement-level designs, our industry-leading geoconsultants and engineers created the workscopes and performed geotechnical investigations, including:

• Advanced monochamber pressuremeter testing to acquire precise ground strength data at > 80 m depth in the complex geology

• Core sampling down to 70 m

• Investigation of existing structures using hand and mechanical excavation

• A range of laboratory testing particularly relevant to the design of tall buildings, such as resonant column and cyclic triaxial

Geotechnical engineering tasks

We also provided a full range of geotechnical engineering tasks according to France’s national standard NF P 94-500. This covered the preliminary design stage AVP (2013-2014), detailed design stage PRO (2014-2015), and DCE/ACT stages (2015-2016), with the following engineering data acquired for the foundations and retaining walls:

• Calculations for completed components in 2D (Plaxis 2D® software) with stability assessment related to a circular break (TALREN® software)

• Calculations for completed components in 3D (Plaxis 3D® software), which integrated inputs from the structural models to establish load distributions and evaluate potential foundation deformation and settlement

• Calculations for retaining walls through nine basement levels, using the elastoplastic method on reaction modules (RIDO® software)

Our experts were later appointed to oversee the ongoing construction work from a geotechnical perspective to verify the integrity of nearby pre-existing structures (flyover, tram/railway, roads, buildings). This supervision work supports the planning of nearby future infrastructure, specifically intersections with the Number 10 underground line and TGV trainline, plus an observation point overlooking the SNCF railway lines.

Innovation highlight

To better determine the behaviour of the complex geological layers in a high-pressure context, we used our innovative monochamber pressuremeter cell to achieve accurate results; these were required to depths of 80 m and at high pressures of up to 120 bars in the limestone.

With enhanced test capacity and the benefit of our expert interpretation, the monochamber accurately measured soil engineering properties down to 80 m below ground level. This provided certainty of the design safety ratio and confidence in the scope so we could optimise foundation design to reduce our client’s construction budget.

Impact

We provided a uniquely detailed and highly accurate geotechnical design and ground model, based on Geo-data from soil tests and incorporating stratigraphy and geological risk to deliver integrated insight on the integrity of existing structures. As well as enabling the timely and compliant completion of our geotechnical design and geoconsulting tasks, our ground model also helped structural engineers, foundation contractors and other project partners to make expedient and robust design and construction decisions.

Our efficient data delivery and high bar for accuracy provided certainty of the design safety ratio and timely opportunities to optimise piling design, while ensuring the smooth progress of groundworks and foundation phases.

By informing foundation design, ground risk and many critical construction decisions, Fugro’s ground model provided the client with a powerful tool for avoiding unforeseen problems and minimising whole-life costs.