Infilling of underground quarries for the construction of 7 floor buildings with shallow foundations.
A large part of the Ile de France area is undermined by old underground quarries that require the use of specialist ground treatment to allow development of these plots of land. As part of the Concorde zone project in Vitry sur Seine (94), Solétanche Bachy has carried out underground consolidation works which have allowed developers to build a shopping centre and blocks of flats comprising a ground floor and 7 upper floors on this 15,000m2 site using noly shallow foundations.
The works consisted in treating between two and three levels of old limestone mine workings found at depths of between 18 and 26m below ground level. Precisely targetted treatment of collapsed rock zones and superficial material eliminated the need for deep foundations.
The contract specified:
- Gravity placement of infill mortar.
- Pressure grouting to consolidate the top of the in-filled voids.
- Pressure grouting of previously backfilled mine voids, areas of surface settlement and any crown pillar failure.
For the treatment to meet its objectives the drilling grid was reduced from 5m x 5m to 4m x 4m beneath the planned buildings, hence increasing drilling quanities by almost 60%. These boreholes were then fitted with two grouting tubes; one, a simply perforated liner, for the gravity filling of the void, the other a tube à manchette for the pressure grouting at the top of the void. After completion of these works, a systematic pressure grouting treatment was carried out in the areas of, crown pillar failures, sink holes and superficial soils that had been loosened.
The contract included a performance requirement in terms of the engineering properties of the soil on completion of the works. These properties were substantiated by 110 test boreholes sunk after the lapse of four weeks from completion of the grouting works, as required by the specifications.
Grouting involves the injection of a pumpable product (slurry), which will subsequently stiffen, into the soil or into man-made material (masonry), in order to consolidate the soil or structure or make it impermeable, through filling all the voids it contains. The slurry can fill the voids in the ground, the cracks within rock, solution cavities (it is then referred to as fissure and permeation grouting) and/or displace the surrounding soils through a bottom-up process or by fracturing (compaction grouting or solid injection - see the section on the subject - and strain injection). Grouting with soil displacement may be used to prevent potential damage to the structure brought about by excavations (galleries and tunnels, major urban excavations, etc.) and this is called compensation grouting (see the relevant section).
Suitable foundations are required for all building and civil engineering structures to ensure that they perform within the settlement criteria established in the design of the structure. Special foundations are used where shallow footings do not provide adequate support for a structure.
A number of grouting techniques have been developed to allow structures to be built on karstic cavities or on quarries.
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