Mass Wasting - Hydrogeology: Objectives, Methods, Applications - page 260

a hillside retaining wall (Gilli & Mangan, unpublished). Over a Liassic
limestone substratum, an ancient closed depression revealed a clayey fi ll
of glacial origin with a maximum thickness of 9 meters, the whole being
leveled to the average elevation of 1750 m by silty-sandy colluvium with
rocky patches, over a thickness ranging from 1 to 3 meters. Studies revealed
that the disorders could not be attributed to a deep karst withdrawal, but
were instead the result of the high permeability of the surface colluvium
and the establishment of temporary water circulations at the roof of the
glacial clay. Only the channeling of the outfl ow along sandier zones and
the progressive removal of silt were responsible for the creation of ablation
funnels, which, in fact, perfectly followed the identifi ed drainage axes. The
erection of the desired construction posed no particular problems, given
the size of the excavations in the middle of the rock mass, which largely
exceeded the very superfi cial fringe concerned by the suffosion process.
3.3 Impact of human action
Settling and collapsing can be generated or aggravated by human
interventions that are responsible for prompting the dissolution of soluble
horizons, the reactivation of ancient karst circulations, or the washing out
of unconsolidated fi ll.
3.3.1 Dissolution
This is a problem essentially affecting deep deposits of highly soluble
substances (rock salt or potassium salts) which, after the injection of water
into wells, are exploited through pumping in the form of brine, creating
sizeable residual cavities.
Letourneur & Michel (1971) cite the formation of a settling depression
30 cm deep and 200 m in diameter in Hutchinson, Kansas (United States),
in only 1.5 months, after the extraction of 6,700 m 3 of brine (after Ries and
Watson).
The surface consequences can sometimes be much more serious. Such
was the case, at the exploitation site of Permian evaporites in Saxony
(Germany), where the opening in Viennenburg in May, 1930 of a funnel
60 m in diameter and 40 m deep swallowed a car and caused numerous
problems in the surrounding houses (Cramer, 1942 in Nicod, 1976). Similarly,
the spectacular Haraucourt collapse followed the extraction of Lorraine
brine (Nicod, 1993), as did the damage to building in the village of Miery
(Jura).