The authors present some experiences, partly unfavorable, obtained in the course of exploratory works, designing, construction and maintenance of the hydrotechnical underground structures in the Eocene clastic sediments (flysch) in Yugoslavia, Algeria and Iraq. Because of the considerable presence of these sediments, especially in the Mediterranean area, many specific properties have been noted:

  • Variability of flysch content, formation, state and properties. For that reason, more detailed exploratory works must be done before design elaboration.

  • Geotechnical properties depend, to a great extent, on the. local mineralogical and petrographical properties of the flysch layers.

  • Possible swelling and additional influences (stresses) on the retaining structures.

  • Erosive instability of the flysch layers surrounding the underground structures.


The term flysch in Mediterranean area denotes complex clastic Eocene sediments that are characterized by Spatial and lithological heterogeneity with the beds of well and loosely jointed members.

The geotechnical properties de- Pend greatly on mineralogical com- Position. Thus in weakly cemented (geological term: loose) samples of flysch (clayey silt, silty clay), the part of phyllosilicates, typical minerals of clay, is represented in the order of 50% or more. In the cemented (geological term: well jointed) sediments ~sandstone), the part of phyllosil- ~cates does not exceed 25%. The most represented phyllosilicates are micaceous minerals chloritevermiculite and allophane.

Dependance between mineralogic- Petrographic composition and geotechnical properties has been observed. Generally, the part of the clayey particles increases with the increase of the montmorillonite mineral part which carries unfavorable properties (swelling of flysch formations). As the amount of silty and clayey particles increases, it is observed that, besides the appearance of water, during execution and support work, the difficulties arise in ensuring temporary and permanent stability of underground structures, particularly hydrotechnical structures.


The radiographic, microscopic and granulometric investigations have been carried out on the flysch samples taken at the defined locations so as to define their mineralogical and petrographical composition. The table n 1 represents some mineralogical and geotechnical properties of the flysch samples.

* The Eocene clastic complex is characterized by lithological heterogeneity in vertical and lateral sense with changes of firm and loosely jointed members. On the basis of a relatively small number of samples submitted to mineralogical and petrographical analyses, the following types of sediments have been determined:

  • coarse sand and clayey silt with a great percentage of sand fractions (about 30%) and with some gravel fractions,

  • clayey silt with predominantly fine particles,

  • fine-grained clay with predominantly clayey particles,

  • silt with predominantly silty particles.

* As siltstones are rocks with a low degree of diagenesis and lithification and as they take a predominant part in the flysch complex structure, it can be said that they exercise the most important influence on the flysch behavior. It has been noted that in weakly cemented (geologically loose) samples of flysch (clayey silt, silty clay) the part of phyllosilicates, typical minerals of clay, is represented in the order of 50% or more.

This content is only available via PDF.
You can access this article if you purchase or spend a download.