9.3. More information on subsidence prevention and protection

Methods and techniques of prevention and repairing of the damages due to slow subsidence are the same of rapid subsidence.

Prevention of subsidence, as for landslides in general, corresponds to a step comprising several successive stages sometimes requiring adapted techniques to seek cavities (Côte et al., 2005 ; Pothérat, 2005). The following stages are classically retained:

1. Preliminary recognition allows an identification of the sectors prone to subsidence. This step comprises:

(a) a preliminary stage consisting of a geological expertise to identify the grounds prone to dissolution (natural cavities) or exploitation (anthropogenic cavities). Information is then completed by an investigation of archives, ancient maps and plans, interviews (population and organizations), etc. In France, there exists a national database on underground cavities “BDCavité”;

(b) a research of surface indices by using remote sensing techniques such as photo-interpretation of vertical or oblique photographs, infrared thermic radiometry, accomplished with ground validation (research of morphological indices, etc.)

2. Detection of the voids starting from a selection of specific hazard zones (achieved in stage 1), to precisely locate voids and decompressing zones (surfaces and volume) by:

(a) sub surface geophysical methods: microgravimetry to obtain a map of geophysical anomalies, and other seismic or electromagnetic methods, ,.

(b) drillings with instantaneous or differed diagraphs (??) and videoscopy in drillings,

(c) exploration, if accessible, and cartography.

3. Interpretation, monitoring and hazard assessment.

4. Identification of elements at risks and assessment of their vulnerability

5. Risk mapping (in 3-4 levels).

Main techniques of protection are classified in two distinct categories (MEDD, 2004):

A. ‘active techniques’ to avoid the triggering of the movement. They consist in supporting and consolidating the cavities to avoid subsidence. For instance, it is possible to reinforce existing pillars or to reduce the distance between the pillars:

– by arranging additional pillars, by the construction of new pillars in masonry in the accessible cavities or a grouting (mixture of concrete and additives) forming studs in the inaccessible cavities (with 1: pile; 2: natural or artificial caves; 3: grouting forming stud; 4: pillar in masonry).

– by filling the cavity by thorough fill (if it is accessible) or by gravitating injection of liquid mortar (generally, mixture of sand, bentonite and additives) after installation of containment walls (with 1: pile; 2: natural or artificial caves; 5: liquid mortar forming containment wall; 6: filling liquid mortar; 7: injection drill).

If the cavity is close to the surface, it is essential to control the water infiltrations which could accentuate the phenomenon

B ‘passive techniques’ which seek to control the consequences of subsidence. They aim at reinforcing the structures of threatened constructions (by peripheral chaining, etc.) so that they do not undergo the consequences of the relative settlements. The realization of deep foundations, crossing the cavity, can be a means of protecting. Lastly, the buried networks must be flexible to adapt to the deformations (regular controls of their sealing must be envisaged).

Realization of deep foundations (i.e. concrete piles), crossing the cavity, can be a means of protecting (with 1: pile; 2: natural or artificial caves; 8: casing for protection against alteration)

Côte, PH., fauchard, C., Pothérat, P. (2005). Méthodes géophysiques pour la localisation de cavités souterraines : potentialités et limites. In Evaluation et gestion des risques liés aux carrières souterraines abandonnées. Actes des journées scientifiques du LCPC, pp. 8-17.
Embleton, C., and Embleton C. (eds.) (1997), Geomorphological Hazards of Europe. Developments in Earth Surface Processes 5. Amsterdam : Elsevier, 524p.
Flageollet, J. C. (1988), Les mouvements de terrain et leur prévention, Paris : Masson, 224p.
LCPC (2000). Guide technique pour la caractérisation et cartographie de l’aléa dû aux mouvements de terrain. Collection ‘les risques naturels’. Laboratoire Central des Ponts et Chaussées, 91 p.
Maquaire, O., (2005). Geomorphic hazards and natural risks, In: Koster, E., A. (ed.), The Physical Geography of Western Europe, Oxford Regional Environments, Oxford University Press, Chapter 18, 354-377.
Ministère de l’Environnement, 1997, Plans de prévention des risques naturels (PPR) : guide général.. La Documentation Française, Paris, 76p.
Ministère de l’Environnement, 1999, Plans de prévention des risques naturels (PPR) : risques de mouvements de terrain. Guide méthodologique.. La Documentation Française, Paris, 71p.
Ministère de l’Ecologie et du Développement Durable, 2004. Dossier d’information sur le risque Mouvement de terrains, 20 p. (à télécharger sur site du MEDD).
Pothérat, P. (2005). L’opération de recherche « Carrières souterraines abandonnées ». Localisation, dignostic de stabilité, gestion. Rapport de synthèse. Géotechnique et risques naturels, GT 77. LCPC, 132 p.