Geotechnical characteristics of claystones of Aveiro (Portugal) are presented. These formations generally have mechanical properties corresponding to the borderline between Soil Mechanics and Rock Mechanics. Some problems arise when foundations are excavated in those sediments.
Dans ce travail sont presentees les caracteristiques geotechniques des argilolithes de la region de Aveiro (Portugal). Ces formations ont generalement, proprietes mecaniques qui correspondent à la frontiere entre la Mecanique des Sols et la Mecanique des Roches. Quelques problemes qui resultent des excavations pour les foundations dans ces sediments sont presente.
Der Text behandelt geotechnische Eigenschaften der Tonstein der Gegend von Aveiro (Portugal). Diese besitzen mechanische Eigenschaften die im Grenzbereich zwischen Bodenmechanik und Felsmechanik Iiegen. Einige Probleme bei der Ausschachtung von Fundamenten in diesen Sedimenten werden dargestellt.
This work was done in the Cretaceous claystones of Aveiro on the portuguese coast (Central Region of Portugal, SW Europe). This lithostratigraphic unit is designated by "sandstones and clays of Aveiro" (Teixeira and Zbyszewski, 1976) and "Vagos Clays" (Barbosa, 1981) and "Aveiro clays" (Ferreira Gomes, 1992). It occupies an area of about 100 Km2, with about 5 Km average width and 20 Km length; only 30 per cent outcrops and the rest is covered by recent sediments. The thickness of this geological unit ranges from 10 metres to the north to about 150 metres to the south. The sub-horizontal layers have 0.5 to 2.0 meters of thickness. The colour of these sediments is generally greenish, and sometimes reddish, greyish, dark, yellowish, bluish and whitish. This unit usually remains below the water table.
These claystones turn into residual soils after being immersed in water for some hours or days. The residual soils are classified as CH, CL and MH (Unified Soil Classification System, ASTM 1987). The "in situ" physical parameters of these sediments are presented in Table 1; the highest values of Wn and en correspond to the shallow unstressed sediments and the lowest values correspond to the massif not decompressed. Relationship for several physical parameters "in situ" are presented in figure 1. Geophysical studies such as electrical sounding and seismic refraction tests have been done. The values of resistivity (R) varied between 7 and 23 Ohm.m and more frequently between 13 and 18 Ohm.m. The seismic velocities (Vp), varied between 250 and 3280 mls.
To determine the strength characteristics were done several tens of Windsor tests (Ladeira, 1988) in several points of the massif and in profiles of recent excavations. Using the average of penetration depths (P) in each place, values of P between 16 and 70 cm were obtained which correspond to uniaxial compressive strength (qu) between 67 and 2081 kPa (qu=1.33 × 106 p-2.33; Ladeira, 1988); the lowest values correspond to depths until 1 metre and the highest values to depths greather than 4 metres. Carbonate layers (0.3 to 0.5 metres of thickness) occur in some places associated with the claystones and they can reach 6000 kPa of uniaxial compressive strength. Samples were collected from some selected places which allowed the achievement of specimens with a height/diameter ratio (HID) of about 2.5; the specimens were submitted to uniaxial compressive strength tests (ISRM, 1978) are in concordance with the Windsor test results, varying approximately in the same limits. The natural moisture content (Wn) was determined in each specimen; the relationship between this parameter and the uniaxial compressive strength is presented in the figure 2.