Ingenieurgeologische Untersuchungen an Geschiebemergeln im Hamburger Raum

Summary

The ground moraines of the Hamburg area almost exclusively consist of tills. They represent about 40 percent of the soil cover.

The strength behaviour of these tills depends on their petrographic structure and consistency. Yet, the different structures of fabric influence the strength to varying degrees according to the valid consistency:

• with soft to stiff consistency the properties of the grain size fraction 0,06 mm which functions as binding agent in tills, are most important

• with firm consistency an often observed joint structure can be of additional geotechnical significance. Compared with that the marl within the boulder clay is only soil-mechanically active if it occurs as mosaic layers.

• the determined plasticity indices correspond to the granular skeleton/ binding agent ratio. Because of a high binding agent content the Niendorf and Elster moraine show higher plasticity indices and higher liquid limits.

• Density and dry density rise with increasing age of the moraines. The corresponding decrease of the porosity is due to the filling of the pore space with finely distributed clay and calcium carbonate. With comparable granular skeleton and increasing lime content porosity decreases (fig. 4).

• An exponential correlation exists between shear strength and water content: the latter being constant, shear strength will increase according to density resp. age of the marls (fig. 5).

• The behaviour of cohesive soil under influence of water is described in disintegration tests.

The granular skeleton (grain size fraction 0,06 mm) mainly consists of quartz, feldspar, lime and boulder fragments whereas the binding agent consists of swellable clay minerals (smectite), transition – minerals (mixed-layer minerals) and unswellable minerals (kaolinite, chlorite) with a subordinate content of finely distributed calcite, dolomite and quartz.

Quantitative estimates of the clay mineralogical composition based on standardized suspensions show about the same content of kaolinite within the different marls of the Hamburg area (Elsterian moraine, Drenthe moraine, Niendorf moraine, Fuhlsbüttel moraine). This is different from the content of smectite which is highest in the Elster marls and thus helps to identify them (fig. 1). In most cases the lime content supports the stratigraphical Classification of marls as well.

The influence of the varying petrographic structure of the different marls on their soil mechanical behaviour is confirmed by geotechnical investigations:

• the determined plasticity indices correspond to the granular skeleton/ binding agent ratio. Because of a high binding agent content the Niendorf and Elster moraine show higher plasticity indices and higher liquid limits.

• Density and dry density rise with increasing age of the moraines. The corresponding decrease of the porosity is due to the filling of the pore space with finely distributed clay and calcium carbonate. With comparable granular skeleton and increasing lime content porosity decreases (fig. 4).

• An exponential correlation exists between shear strength and water content: the latter being constant, shear strength will increase according to density resp. age of the marls (fig. 5).

• The behaviour of cohesive soil under influence of water is described in disintegration tests.

As the disintegration rate depends on the initial water content of the samples an apparatus was developed to register continuously the disintegration process (figs. 6 and 7).

The disintegration process is determined by the grain skeleton and the kind of cementation (fig. 8). Thus, high lime contents (as they can be found in the Niendorf moraine) have a structure preserving effect because of the increase of fine grains.

Summarizing it can be said that different strength behaviour of tills can be explained geologically and described by mineralogical composition and geotechnical parameters.