Bestimmung eines Parameters für den Verwitterungsgrad von Tonsteinen

Summary

New parameters to determine quantitatively the degree of weathering and the engineering properties of I-lesozoic mudstones and marls:

The engineering properties of mudstones and marls of Keuper and Jura in Southern Germany have been observed to be also governed by the degree of weathering as it was determined earlier in England (see e.g. CHANDLER 1969). Although,the influence of weathering on the engineering properties is very important for foundation engineering analyses, there has neither been an exact quantification of the degree of weathering nor any quantitative relation between the degree of weathering and the engineering properties so far. Therefore, this paper is concerned with the determination of a quantitative parameter for the degree of weathering, which may be useful to evaluate engineering properties, especially shear strength parameters.

With the research project reported here, a great number of index properties, used in former time to determine the degree of weathering, have been determined such as natural water content, plastic and liquid limit, clay fraction, aggregation ratio, Fe(III)/Fe (II) ratio, FeOOH content, quantitative mineralogical composition, effective shear strength parameters, etc. If there has been a homogenous mineralogical composition in an originally unweathered profile, e.g. Opalinuston, some of these parameters (fig. 2) can be used to deterimine quantitatively five weathering zones and it is also possible to relate characteristic shear strength parameters to each weathering zone in a weathering profile of the same geological history. But these parameters do not apply, if the mineralogical composition is inhomogenous, e.g. Gipskeuper, or if weathering profiles and engineering properties of different geological histories shall be compared.

For this purpose, new parameters are introduced. With the “Quotient PF” the most important variations both in terms of mineralogical composition and gradation are described quantitatively. For this purpose the actual value is

$$ {\text{PF = }}\frac{{{\text{clay fraction}}}}{{{\text{solid compounds - clay fraction}}}}{\text{ }} $$

((1))

and the value for the theoretical end of weathering

$$ {\text{PF}}_{{\text{END}}} {\text{ = }}\frac{{{\text{clay - mineral content}}}}{{{\text{quartz content ( + feldspar)}}{\text{.}}}} $$

((2))

The “Zustandsindex T_W” is a mean value of three figures which are related to either the natural water content, the plastic limit, and the liquid limit. It is standardized such that I_W=0 applies to w_P=w_L and I_w=5 to w_P=w respectively, see fig. 1. The correlation which was determined is given by the following equation:

$$ {\text{I}}_{\text{W}} \;{\text{ = }}\;{\text{1/3}}\;\;{\text{\{ (w}}_{\text{L}} {\text{ - 10)}}\;{\text{/7}}{\text{.9}}\;{\text{ + }}\;{\text{(w}}_{\text{P}} \;{\text{ - }}\;{\text{10)}}\;{\text{/2}}{\text{.4}}\;{\text{ + }}\;{\text{w/4}}{\text{.47\} }} $$

((3))

Then a significant correlation is found between I_W and PF such as

$$ {\text{I}}_{\text{W}} \;{\text{ = }}\;{\text{6}}{\text{.6}}\;{\text{PF}}^{{\text{1/2}}} \;{\text{ - }}\;{\text{0,75}}\quad {\text{and}}\quad {\text{Iw}}_{{\text{END}}} \;{\text{ = }}\;{\text{6}}{\text{.6}}\;{\text{PF}}^{{\text{1/2}}} \;{\text{ - }}\;{\text{0}}{\text{.75}}\;{\text{.}} $$

((4))

By means of these parameters, an “index of weathering V_M” can bee defined as

$$ {\text{V}}_{\text{M}} \;{\text{ = }}\;{\text{I}}_{\text{W}} {\text{/Iw}}_{{\text{END}}} \quad \quad {\text{or}} $$

((5))

$$ {\text{V}}_{\text{M}} \; = \;\frac{{1/3\;\left\{ {({\text{w}}_{\text{L}} {\text{ - 10}})/7.9 + ({\text{w}}_{\text{P}} {\text{ - 10}})/2.4\; + \;{\text{w/4}}{\text{.47}}} \right\}}}{{6.6\quad \quad \quad \frac{{{\text{clay}}\;{\text{mineral}}\;{\text{content}}\quad \quad {\text{1/2}}_{{\text{ - 0,75}}} }}{{{\text{quartz}}\;{\text{content}}\;{\text{( + }}\;{\text{feldspar)}}}}}} $$

((6))

By this index it is possible to compare the engineering properties of diagenetic mudstones and marls having defferent mineralogical compositions and different geological histories.

The actual index of weathering can also be used to define a quantitative parameter of the “degree of weathering V_R” by reference to limit values of V_M:

$$ {\text{V}}_{\text{R}} \;{\text{ = }}\;{\text{100}}\;{\text{(V}}_{\text{M}} {\text{ - V}}_{\text{B}} {\text{)/(V}}_{\text{E}} {\text{ - V}}_{\text{B}} {\text{)}}\quad {\text{\% ,}} $$

((7))

where V_B is the index of unweathered matter and V_E is the index of completely weathered matter. Both values are to be determined for a given weathering profile.

The new parameters are also appropriate to describe quantitatively weathering zones in a homogenous weathering profile (fig. 3), but more over, they are also fitted to calculate correlations between the degree of weathering and the shear streght parameters (tab. 1 and 2) for the actual state and the possible state for the time when the process of weathering is finished.