Consolidation methods

Consolidations are used to stabilize voids and allow the construction of surface structures. They can be carried out using different techniques, such as hedges and tamping, arm pillars, masonry pillars, concrete injections, reinforcements using rebars, etc. These methods make it possible to consolidate weakened areas or those subject to ground movements, thus ensuring the safety of infrastructure and people.

Méthodes de consolidation – Inspection Générale des Carrières

The pillar extraction

A turned pillar is a column of underground material left in place between excavation areas during mining. The turned pillar method is often used in mining operations where the surrounding rock is strong enough to support the loads without requiring artificial reinforcement.

The hedges and stuffings

The oldest and most economical method is the “Hague and Bourrage” method. It was used to organize underground quarries, particularly under Paris and its suburbs. It consisted of extracting all the material, then filling the voids in the quarry with cutting waste and lower quality benches, called “bourrage”, held in place by dry stone walls, the “hagues”. This method made it possible to exploit the quality limestone benches while ensuring the stability of the galleries. Support pillars, called “arm pillars”, were also built to reinforce the hagues and consolidate the ceiling of the quarry.

Arm pillars and masonry pillars

An arm pillar is a pile of massive stone in quarries of varying sizes. You can find both huge pillars and small ones. They are generally distributed all over the place to distribute the load and thus form magnificent forests of pillars.

Masonry pillars are built by stacking stone blocks solidly and fixing them together using lime or more recently concrete. This method makes it possible to locally stabilize the structures under constructions as is the case for the Montsouris reservoir, for the metro or for aqueduct passages.

Anchor rods

In geotechnics, ground anchors (or nails) are reinforcement elements used to stabilize unstable ground or to reinforce geotechnical structures such as rock walls or embankments. In mines and quarries, rock walls can be subject to instabilities, such as rock falls or landslides.

To reinforce these walls, holes are drilled into the rock at regular intervals, at an angle or along faults. Then, geotechnical nails, usually in the form of threaded steel bars, are inserted into the holes and sealed with a cement or resin grout. Once in place, the nails anchor the rock in place, reducing the risk of collapse.

Vaults and arches

Vaults and arches built underground represent architectural feats that combine aesthetics and functionality in often complex environments. These structures are designed to support the load of the soil above while providing a distinctive visual appearance. Vaults reinforce the stability of underground spaces. Arches, on the other hand, play a crucial role in distributing lateral loads, helping to prevent wall subsidence. These architectural elements not only create durable and secure underground spaces, but also environments that captivate the imagination with their ingenuity and beauty.
The use of vaults and arches in underground environments reflects the creativity of architects and engineers, seeking to reconcile structural function with artistic expression, even deep within the earth.

Wooden support pillars

A little in the quarries, but more widely in the mines, from the Middle Ages to the modern era, the means of consolidating the galleries and veins was the timbering. Timbering is the installation of wooden supports to reinforce the galleries or tunnels in order to avoid collapses

Two types of wood are used:

• Oak: the strongest and most resistant to rot, it should be used in warm and humid places.
• Pine and fir are the most used; they are almost as strong as oak and warn before breaking.

The collapses

Sinkholes, also known as ground collapses, are geological phenomena that occur when the ground collapses due to the support pillars being too far apart, or due to the pressure exerted by buildings and the ground on the surface.

These formations appear as circular or elliptical depressions, resulting from the progressive dissolution of underground layers. There are two types:

• Sinkholes, which are sudden, sudden collapses of the ground following the collapse of a pillar and/or the collapse of a gallery.
• Generalized collapse, which occurs when a large number of pillars simultaneously give way under the weight of the covering in a context of undersized pillars.

Special case: gypsum is soluble in water, which causes collapses when water seeps from the surface of the earth.

Underground void removal

The purpose of backfilling is to fill voids with mining waste, quarry waste, backfill materials or materials from other sites (e.g. Grand Paris backfill). The backfilling process can be carried out using heavy equipment such as bulldozers, mechanical shovels and haul trucks, which deposit the backfill materials in the areas to be filled and compact them to ensure adequate stability.

Grouting, on the other hand, involves introducing a material, usually in the form of a slurry, into voids. The most commonly used grouting materials include cement and bentonite suspensions. Grouting can be carried out using special pumps that inject the material at high pressure through wells or boreholes, thus ensuring uniform distribution of the grouting material and effective reinforcement of underground structures.