Environmental Engineering Reference
In-Depth Information
- a test at a constant initial water content, corresponding to the initial natural
state of the soil;
- a test in which the unsaturated sample is wetted under a small load, and
subsequently loaded in a zero-suction condition, close to saturation.
constant water content
saturated
void ratio e
Figure 6.5.
The double oedometer method [JEN 73]
The amount of collapse as a function of applied stress is taken from the
difference in void ratio, taken from the two curves under the corresponding stress.
Note that the two experimental curves may have to be fitted at low stresses initially
to account for the variability in natural loess.
6.2.2.
Geotechnical characterization of the samples
In order to characterize the collapse behavior of the loess from Northern France,
soil samples were manually extracted in September 2002 at various depths at a site
located 140 km north of Paris, at a 25 m distance from the TGV line, not far from
the site of Croisilles (CR in Figure 6.4). Four block samples were trimmed and cut
using a shovel at different depths (1.2, 2.2, 3.5 and 4.9 m) in a 1.5 m wide and 9 m
long trench that was excavated step by step. Visual observation showed that samples
were brown (1.2 and 4.9 m) or light brown (2.2 and 3.5 m). X-ray diffractometry
showed that the samples at 1.2, 2.2 and 3.5m were mainly composed of quartz and
feldspar, with some clay. Some carbonates (calcite and dolomite) were observed in
the 4.9 m sample. Analysis carried out on the clay fractions (<2 µm) showed that in
all cases the clay fraction was made up of kaolinite, illite and interstratified illite-
smectite. Figure 6.6 shows that the grain size distribution curves of the four soils are
comparable and typical of loess, with a slightly higher clay fraction (18 and 20%) in
the 1.2 and 4.9 m samples respectively, as compared to 16% in the two other
samples.
