Centre National d’Etudes Spatiales (CNES) appointed Fugro to carry out a geophysical assessment for the Ariane 6 project after successfully delivering seismicity surveys and vibration monitoring to predict the ground motion effects from the Ariane 5 launch vehicle.
We were asked to undertake investigations for all civil engineering elements for the new launch vehicle, Ariane 6, and to confirm the suitability of the planned launch pad position.
Tropical locations are conducive to space launches and CNES site lies just 5° north of the equator where a peak in rotational velocity forces assists space vehicle propulsion through the earth’s atmosphere.
However the underlying pre-Cambrian geology and deeply weathered soils mean ground conditions are highly variable and, in turn, the location of rockhead for foundations highly unpredictable.
We focused the initial geophysical investigations for Ariane 6 on characterising an area of virgin ground for the launch pad of a 1.2-tonne rocket.
Our objectives were to map changes in thickness of the tropically weathered soils and fractures in the bedrock. This work would help the design team optimise the site layout and manage subsurface risk.
Using an integrated approach proven for other infrastructure projects, including nuclear energy sites and bridge foundations, we designed a suite of investigations that would derive high-quality data to give a continuous and detailed view of the subsurface.
We combined three techniques – seismic refraction, resistivity (ERT) and electromagnetic – to a target depth of 50m.
Our geophysics team also needed to distinguish between bedrock and large boulders to securely anchor foundations in competent rock as the deeply weathered geology could have hidden boulders as large as 20-tonnes.
Based on the success of the integrated geophysics for the first phases, CNES asked us to undertake investigations for all civil engineering elements for Ariane 6. This work included screening and identifying potential areas for the booster building, the rocket and satellite assembly facility, and connecting transport links. The selected area will see 1 million cubic metres of material moved across a 170-hectare site, and support the 600-tonne booster and the heavy rocket.
We were also asked to confirm the suitability of the envisaged launch pad position or use the data gathered to support its relocation to an alternative site.
Ariane 6 is subject to particularly challenging constraints, integrating the project within existing structures and programmes - Soyuz and Ariane 5 among them. As such, changes in footprint must consider the safety radius of other programmes as well as the permitted curvature of rail track infrastructure. It means that any major positional change is restricted to the launch pad, requiring our teams to also screen for potential alternative sites based on boulder locations, overburden thickness of no more than 15 metres and a flat rockhead.
In forming the required understanding of ground structures, we closely considered sites geology and geomorphology in tandem with the field work and interpretation of geophysical profiles. Our findings indicated that the initial proposal for the launch pad site needed reviewing, and we could propose three possible alternatives.
The selection of the site exceeded the expectations of the client, project constructors and suppliers, Spaceport employees, as well as private and public investors. As a result, the earthworks started quickly after.
Although our surveyors worked in a very logistically demanding tropical forest environment, tackled undulating ground and thickly covered forest ﬂoor, the careful project management and quality control in the field allowed our geophysicists to refine their approach to maximising efficiency and to minimising environmental impact. We successfully delivered detailed ground investigation and managed the construction risks for the new infrastructure.