PROBLEMS - Hydrogeology: Objectives, Methods, Applications - page 337

Figure 12 Detailed diagram of the separating wall between the two residences.
Question 1: Based on this data, what can be deduced on the origin of the
disorders?
Question 2: What can be proposed to remedy the situation?
Answer 1: The penetrometer results show that the soil is disorganized in S3,
S4, and particularly S2, below the superfi cial foundations of residence A.
There is no doubt as to the role of the well, and yet the problems appeared
only recently. The existence of an old well downhill indicates the presence
of a superfi cial aquifer. The surface weathering products of the andesite,
when the clayey matrix is minimal, are highly permeable and can, indeed,
become saturated with water.
The examination of the retaining wall cross-section shows that this wall
has neither drainage channels, nor uphill drainages. It therefore plays the
role of a hydraulic barrier. The water infi ltrated on plot A can no longer fl ow
down to feed the aquifer under plot B. They therefore accumulate behind
the retaining wall. Their oscillations, by causing, over several years, the
hydrolysis of the material and a redistribution of fi ne particles, disorganized
the soil below the foundations of residence A.
The observed problems therefore have a hydrogeologic origin.
Answer 2: It will be necessary to revisit the water collection system on plot
A, and to equip the retaining wall with drainage channels and a collection
drain. It will also be necessary to redo residence A's foundations with
underpinning.