Water-Rock Interaction Mechanisms and Ageing Processes in Chalk - Advances in Data, Methods, Models and Their Applications in Geoscience - page 164

microstructural changes in geomaterials in their natural state, under controlled conditions of
temperature and pressure.
Some other aspects of this technology are discussed, along with suggestions for potential
development of this tool for further geomechanical applications and analyses of the Estreux
chalk.

2. Site scale: Stability analysis

The study was carried out on Estreux abandoned underground chalk mine in Northern
France, which is located 10 km east of the city of Valenciennes. The Estreux chalk formation
is of the Late Cretaceous geological period (89 to 94 Ma years ago).
The Estreux mine, which was principally dug for blocks for dimension stone (for buildings
and masonry) at the end of the 18 th century, covers an area of 10 hectares. The chalk was
excavated at a depth of 20 metres using the pillar-and-stall method, where networks of
galleries were dug from access shafts, leaving large pillars in place to support the weight of
the roof. This prevents the openings caving in, at least during excavation. The width of the
galleries in the Estreux mine vary from 2 to 3 metres, and the pillars measure on average
between 1.5 and 4.5 metres square. The excavation ratio (relation between the area exploited
and the total area) is around 78%.

2.1 Lithology
The lithostratigraphic section of the Estreux mine overburden comprises, from top to bottom:
- a few tens of centimetres of soil
- one metre of clay
- around eleven metres of tuffeau
- three metres of white chalk
- two metres of grey glauconitic clay
- impermeable argillaceous marls
In 1980, Raffoux and Ervel conducted geomechanics laboratory tests on over one hundred
core samples from this mine. The mechanical properties obtained are shown in Table 1.
From these characteristics and the mining geometry, they estimated a stability factor close to
1 in several zones of the mine. As the mine is not stable in the long term, site monitoring was
deemed necessary, and four convergence meters were subsequently installed in a weak zone
of the mine.

Elastic Limit,
Mpa

Breaking limit,
Mpa

Young's Modulus,
GPa

Poisson's
Ratio

State

Dry 9 10.3 2.83 0.3

Saturated 4.6 5.3 1.61 0.3

Table 1. Mechanical characteristics of the Estreux chalk (Raffoux & Ervel, 1980)

Since August 2003, new instrumentation has been installed (Fig. 1) to analyse geomechanics
phenomena of the rock mass over time. The galleries were monitored and a pillar of 1.4 m
square and 1.8 m high was instrumented. The instrumentation comprises:
-

an ambient hygrometer

- a water-level sensor