Civil Engineering Reference
In-Depth Information
Besides, the Boom clay is a 'rhythmical' deposit with a banded structure.
Clayish stratums were formed when wave motion did not reach the bed
of the sea, whereas coarser fractions were deposited in more turbulent
periods when the wave motions did reach the sea bed.
The largest section of the original Boom clay stratum was eroded at the top
at the end of the Oligocene epoch. The Boom clay still present in the trajec-
tory of the Westerschelde Tunnel is over-consolidated and is thus far more
rigid than the original clay stratum.
Fig. 7.3
A sample
of Boom clay
60
60
5
5
70
70
5
5
11
80
80
10
5
5
90
90
9
5
5
10 0
10 0
18.00
19.00
20.00
21.00
22.00
23.00
Layering and structural characteristics of Boom clay
The Boom clay can be subdivided into five different zones. Generally, the
transitions between the layers are gradual. For the design and the construc-
tion of the Westerschelde Tunnel, the (geotechnical) subdivisions of the
Boom clay was further restricted to 2 layers. This appeared to be adequate
for the engineering and implementation.
As a result of the geological manner of coming about, the Boom clay shows
a number of structural characteristics:
- fine cracks (fissures), parallel to the stratification;
- (vertical) cracks;
- sandy layers;
- clay structures which have been pressed through the upper-lying material
vertically (diapires);
- lime concretions (septaria);
- concretions formed by ferric sulphides (pyrite).
The local presence of these characteristics and the degree in which they
occur are influential to the geotechnical properties such as strength, rigidity
and permeability.
Geology of the glauconitic sands
Glauconitic sand occurs in the so-called Berg sands, a formation which lies
below the Boom clay, and in the Formations of Oosterhout and Breda which
lie above the Boom clay. Glauconite is coloured green to black and is a relatively
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