Veränderung bodenmechanischer Parameter bei überkonsolidierten Peliten durch Verwitterung — Versuch einer Deutung anhand des Opalinustones

Summary

A quantitative method for the determination of the degree of weathering for overconsolidated claystones is presented. Plastic and variable hard claystone particles are separated in a specially designed rotation cylinder (fig. 2) by suspension. Since the degree of weathering is defined by the ratio of plastic particles to the sum of plastic and variable hard particles the determination can be easily done (fig. 1). After applying a different number of rotations the “degree of fine-fraction” is determined by evaluation of the grain-size distribution in a semilog plot. As shown in fig. 3 the latter degree is defined by the ratio of the area under the grain-size curve to the area over the grain-size curve in the ränge 0,002 mm to 0,04 mm. The plot of the so determined “degree of fine-fraction” versus the number of rotations is called “fineness-line”, which is cut by an empirically determined straight line in the ränge of total Separation of components. The intersection of “fineness-line” with the straight-line as shown in fig. 4 leads to the degree of weathering.

For a certain weathering profile in a claystone (Aalenium) the relationship of soil mechanical parameters to the degree of weathering was examined. The conditions observed are described by a general qualitative model of weathering, basing on a paper of OSIPOV (1975) with the theoretical background of different types of bondings between particles in clay (fig. 9). OSIPOV distinguishes bet-ween 3 types (fig. 8) : coagulation- type of bonding with plastic behaviour, condensation type of bonding, and cementation type of bonding with a brittle behaviour. Düring Sedimentation the coagulation type of bonding is predominant, because it is the most stabil bonding type under normal conditions with respect to thermodynamics. Düring the diagenetic process the hydrated zone of the clay particles are thinned out and break at this points of contact. Thus, the bondings of coagulation at these points are converted into bondings of condensation or cementation resp. After removing the geological loading the bondings of condensation remain in a metastable condition. Weathering is to be understood as a process, which makes the phenomenon regressive. The bondings of condensation and cementation are destroyed and reversely converted into bondings of coagulation.The mechanical properties of the material at each stageof weathering are determined by the amount and the spatial arrangement of the Single bonding modes.

Düring the first stage of the weathering process (W 1 - W 2) a mechanical destruction of the bondings of condensation takes place caused by deloading forces. Widening of fissures and horizontal micro-structure are the consequences. Shear strength determinations by means of large scale in situ shear tests and laboratory triaxial tests show in this stage a more remarkable decrease of shear strength parallel to Sedimentation than vertical because of the dominating widening of horizontal structure in relation to widening of vertical fissures (fig. 7).

Düring the second stage (W 2 — W 4) the destruction of the bondings of condensation is spatially spread by stresses, which are produced by processes of swelling and shrinking. At the same time the reverse acting formation of coagulation bondings is starting.

The third stage is characterized by an intensive formation of coagulation bondings connected to a plastification of the material. An additional increase of the water content and a decrease of the density are the consequences (fig. 5). The increased influence of oxydation and a permanent forming of acid maintain the plastifying process. The increase of base exchange capacity shows that the reactivation of clay minerals has taken place as indicated* by growing portion of coagulation bondings (fig. 6). As could be shown yet by microscopic and X-ray-refraction measurements only in this stage a spatial net of the brittly type condensation bondings is so much weakened that a decisive destruction of the micro parallel structure takes place (fig. 10). By means of the conversion of the weathering process a reverse of the originally overconsolidated claystone into an unconsolidated plastic clay takes place.

früher Arbeitsgruppe Geotechnik am Zentrum für InfrakStrukturplanung der Universität Stuttgart