Einflüsse lithologischer Parameter oberflächennaher Sedimente auf Kompressions- und Scherwellengeschwindigkeiten

Summary

Research was concentrated on the measurement of compressional (P) and shear-waves (S) in sands and boulder clay. Results may be summarized as follows.

S-velocities (V_s) in the upper hundred meters of non-consolidated material are generally three to five times smaller than P-velocities (V_p). In saturated sands and clay extreme values of VP /V s up to 15 have been observed (fig. 4b). The records with horizontally polarized shear-waves (SH) are not seriously disturbed by other wave types. No converted waves are created by SH-waves at horizontal boundaries (fig. 2). No interference takes place between Love-waves and SH-reflections because Lovewaves propagate with a similar velocity as the first arrivals do. Because of their shorter wavelength SH-waves have a better resolution than Pwaves. Correlation of abserved velocities V_p and V_s to lithologic and mechanical parameters (fig. 7) indicate that

• (i) V_p and V_s in boulder clay are influenced by its porosity, degree P s of Saturation and the properties of grain boundary contact, which is a function of various parameters like clay-calcite content, size, shape, pressure etc.

• (ii) Porosity in boulder clay decreases with increasing pressure and increasing content of CaCO₃. Increasing percentage of clay results in an increase of porosity. V_p and V_s decrease with increasing porosity in boulder clay as shown in fig. 7d.

• (iii) The influence of the State of Saturation is most different for V_s and V_p: while both values are about constant up to high values of Saturation, V_s shows an abrupt increase at about 99 % Saturation while V_s shows a small decrease which is caused by changes in density and shear modulus (fig. 7a,b). For boulder clay the decrease in V_s may be attributed to the volume change of the water absorbing clay minerals which reduce the shear modulus and show the strength of the material (fig. 7b).

• (iv) The effective pressure (lithologic-pressure minus pore-pressure) has a strong influence on the grain-boundary contacts (fig. 3). The Vg values in dry and saturated sands as well as in boulder clay follow a relationship like

  

  V_p -values follow a similar relationship in dry and partially saturated sands while for V_p in saturated sands and boulder clay a P rather constant value is found for the low pressure conditions of near-surface layers.

• (v) Boulder clay shows a decrease in V_p for an increase of the clay content. This is related to the different elastic properties of the grain boundary contacts of the clay minerals and those of the coarser material. An increasing content in CaCO₃, on the other hand, is correlatable with an increase in V_p and V_s . This correlation is possibly due to the cementation and stabilization of the grain-boundary matrix (see figs. 7e and 7f).

• (vi) A correlation seems to exist between the Vg-values and the resistance against penetration, as shown in the regression lines of fig. 6. Further research on this subject is in progress.

From the above mentioned items the relationships between V_p, V_s and the pressure and Saturation (especially in sands) are the most solid and general ones. Future investigations have to show whether the other correlations have a local or also a more general character.