Answer to Question #253576 in Civil and Environmental Engineering for Avee

Question

Formation, characterization and classification of residual soil and dispersive soil?

Answer 1

The thickness of these residual saprolitic horizons may vary between few meters to

more than 20 m with common values of 5–9 m. Although they generally present

strong heterogeneity, it is frequently observed that an average gradual change of

characteristics with depth, especially regarding their mechanical properties. Never-

theless, an accurate mapping of the spatial variability of the mechanical properties,

necessary for geotechnical design, is often very challenging.

The data compiled during the extensive in-situ and laboratorial investigation and

characterization of ISC’2 experimental site, comprising the application of several

geotechnical and geophysical surface and borehole techniques, namely SPT, CPT,

DMT, surface and borehole seismic, electrical resistivity and GPR, offer a valuable

opportunity to compare different methodologies and assess their relative advantages

and limitations.

The schists, gneisses and even migmatites are the metamorphic correspondents of a

former thick sedimentary sequence of graywackes, mudrocks and conglomerate levels

named Schist and Graywacke Complex. The main granitic body at the site is inter-

preted as the result of crustal partial melting during the variscan orogeny. The prox-

imity to a major crustal shear zone – the Porto Tomar Shear Zone, an ancient plate

boundary of Cadomian age (Ribeiro et al., 2003) that separates the NE Central Iberian

Zone from the SW Ossa Morena Zone – and its tectonic activity during the variscan

orogeny favoured the crustal melting and the ascent of granitic magmas.

The upper zone of these basement units are frequently strongly weathered,

forming often a saprolite level with variable thickness, overlaid by a thin layer of soil

and antropic materials, the most common surface and subsurface material in this

region. In this Porto area, the fresh outcroppings of basement rocks are rare. In fact,

the site is geologically formed by a thin upper layer of soil of varying thickness,

overlaying more or less weathered granite (saprolite) contacting an older gneissic

migmatite with dominant sub-vertical foliation. This subvertical structural anisot-

ropy can be considered a determining factor to understand the geophysical response

of the related residual soils resulting from the weathering process