Geotechnical surveys and structures

• the integration of your project into its environment, including in particular the earthworks, dewatering of excavations, stability of the land,  retaining structures and underpinning,
• construction of bases with conventional foundations and special foundations, slabs,
• preliminary works such as ground improvement,

• management of groundwater and rainwater,
• integration of underground utilities, roadways and car parks,
• taking into account of the specific constraints of underground structures or those in aquatic sites.

To ensure the project can fit into its environment, the geotechnical investigation defines the Geotechnical Influence Zone  (GIZ), which is conditioned by all the interactions between the structures to be built, the natural environment and any existing structures. 

Adaptation to the topography of the site involves general earthworks. Cut and fill, following the contours of the land: the geotechnical earthworks survey defines the conditions relating to the movements of earth, the stability of the structures (embankment slopes, induced settlement, ground improvement, etc.) and the particular conditions of the construction, including those relating to drainage.

The management of excavated material and the dewatering of the excavations also require the integration of certain regulatory and environmental constraints, such as water, soil pollution, pumping and stability of excavations, the rates at which water can be discharged into the drains, etc.

These issues are taken care of by our dedicated Environment or Hydrogeology departments.

Linear infrastructure and/or large-scale surveys are taken care of by our Earthworks unit.

Our intervention also includes checking the general stability of the site, before, during and after the works. The ground stability survey therefore looks at potential slip surfaces, the influence of groundwater, the risks to third parties, the methods of stabilisation by earthworks, drainage, retaining structures and the conditions of performance of the works, in synergy with any other departments concerned (Geology and natural risks, Hydrogeology, etc.).

Gravity retaining walls (RC, gabion, modular, reinforced earth walls, etc.), self-supporting, braced or tieback walls (sheet piles, diaphragm wall, Berliner wall, etc.), soil nail walls are all types of retaining wall whose stability needs to be ensured.

In some cases, the project will require underpinning of the foundations of surrounding buildings. This involves transferring the load of the adjoining structures to a deeper level, to allow for the excavations for the new project. In particular this implies analysing the sensitivity of existing buildings  (loads carried to the ground, admissible displacement, vibrations due to the works, etc.), so as to guarantee the bearing capacity of the structures and their compatibility with the stresses generated by the works.

We can support you with the instrumentation of these adjacent structures with the help of our specialist departments.  Find out more

Once the conditions are met for the construction of the structures, a geotechnical survey will define the subgrade conditions for the structures.

Footings, foundation pits, raft foundations, piles and micropiles: designing foundations consists of finding the best solutions from a technical and economic point of view, whilst guaranteeing the bearing capacity of the structures, their resistance to lateral loads, the compatibility of settlement and deformations with the structures they are bearing and the conditions of performance of the works.

The geotechnical survey therefore provides the characteristics and hypotheses necessary to justify these choices, including the specific parameters of the behaviour of the soils (swelling shrinkage, seismic context and liquefaction, soil aggressiveness towards materials, etc.).

The geotechnical survey also considers the support for the lower levels of the project: supported slabs, whether or not there is a ventilation space, traditional slabbing, prior ground improvement works.

The geotechnical survey for the slabbing estimates the expected settlement under the operating loads and defines the ground improvement techniques to as to limit its amplitude, where applicable, defines the dimensions of the base course necessary and establishes the earthworks needed to lay it (trafficability, bearing capacity, drainage, etc.).

Finally, it provides the slab contractor with the geotechnical hypotheses to be taken into account in the design calculations for the slabbing itself.

Meeting the bearing capacity and deformation property criteria for the structures (foundations, slabs, backfill, etc.) may require prior soil improvement work.

Substitution, vibro stone columns, rigid inclusions, compaction grouting, vibroflotation, pre-loading, dynamic compaction, dynamic replacement…, the geotechnical ground improvement survey  defines the amount of soil to be improved, the best techniques to achieve the objectives set in terms of bearing capacity and deformation properties as well as the conditions of performance of the works.

These are advantageous solutions both technically and economically for structures with significant distributed loads, which can also overcome certain problems, such as the treatment of the risk of liquefaction or the stability of slopes and structures made of excavated material.

Dewatering of excavations, groundwater lowering and reinjection, sealing injections, protection of underground layers: the geotechnical design of groundwater management structures defines precisely how the project will be adapted to the hydrogeological context.

This encompasses the lithology, the presence and fluctuations of the water table(s), the runoffs and piezometry, the permeabilities, the definition of the characteristic levels, etc.

Our hydrogeological engineers provide the design assumptions and study in detail the structures to be built, in both the provisional and the final phases: nature of the retaining walls (waterproof or not), dewatering or groundwater lowering by pumping and related dewatering flow rates, specific works such as sealing injections (plugs, walls, regeneration, etc.), impacts on surrounding structures, protection of basements (drainage and/or tanking), as well as the conditions of performance of the works.

Integrating rainwater management is becoming a major concern for structural designers, both from the point of view of the interaction with the groundwater and the structures to be built and from a regulatory point of view.

The hydrogeological investigation of the site identifies the potential solutions for plot-based rainwater management (infiltration structures such as swales, trenches, basins, soakaways, etc.) or, if such solutions are not possible, suggests alternatives (retention/discharge into the drains) or hybrid solutions (retention/infiltration and partial discharge into the drains).

Our Hydrogeology department deals with all these problems.

Roadways and car parks, roads and motorways, railway lines: the geotechnical investigation for this type of surface structure defines, first and foremost, the earthworks and drainage necessary to obtain a suitable platform, but also the dimensioning of the base courses.

As regards the underground  utilities, based on a prior survey of the existing situation, where appropriate, the geotechnical investigation defines the conditions of the excavations, the stability of the trenches, the management of the water extracted and the backfilling.

For roads and car parks, the geotechnical investigation may include the dimensioning of the pavement structures: nature and thicknesses of the foundation layers and the surface course including, including frost resistance checking.

In this area, we call in our dedicated Earthworks unit for surveys concerning special types and/or large-scale structures.

Underground works can mean  placing utilities  underneath structures (shaft sinking, directional drilling, micro-tunnelling machines), large-scale structures (traditional tunnel excavation, tunnelling machines, etc.), or inlet and outlet structures, galleries, covered trenches, etc.

In addition to covering all the issues already mentioned, these geotechnical structures also require a high level of mastery of the deformations generated in the zone of influence of the structures, which is strongly dependent on correctly matching the techniques employed to the geological, geotechnical and hydrogeological context.  Finally, the management of the excavated material is also particularly important.

Quay (on piles, sheet piles, "Danish quays", block or caisson), dolphins, bank works, dykes, port platforms: these structures involve the same geotechnical issues as already discussed.

In addition to the usual constraints presented by land sites, there are constraints specific to structures in aquatic environments: scouring, tidal amplitudes, currents, fast emptying, boat impacts, etc.

Find out more about our comprehensive approach