These facts are also reported by Fookes et al. (2000), who, commenting on their geomorphological model for hot wet climate, state that the depth of weathering can be many tens of metres, controlled by the distribution of discontinuities in the rock mass. Also, duricrusts are for- med where groundwater concentrates soluble weathering products (Thomas, 1994). Deeply weathered colluvial soils are often difficult to distinguish from in-situ weathe- ring profiles. Laterites are hard subsurface deposit of oxides of aluminum and iron found in tropical soils where the water table fluctuates with seasonal changes in preci- pitation. This process is called “‘laterization”, and when Fig. 1. The “Serra do Mar” region in Brazil. Ab’ Saber (2003) describes the main characteristics of saprolitic and colluvial soils in Southeastern Brazil: Fig. 2. Formation or colluvium under a rock outcrop (Fookes, 1997). Fig. 3. Normalized shear stress vs. displacement of residual (left column) and colluvial soils (right column) (Lacerda and Silveira, 1992). It is difficult to obtain field da prior to a failure, since the exact lo is often unpredictable, but a simu a of pore pressures cation of the rupture ation of this mech- anism can be made in the laboratory. Drained triaxial tests with cyclic pore pressure (CPP tests) were per- formed (Fig. 7). Tests were carried samples of a saprolitic soil from the in Rio de Janeiro. The tests were out on undisturbed Soberbo Road Slide performed using the following procedure: (1) saturation by back pressure; (i1) consolidation under isotropic condition; (iii) increase of deviator stress with controlled stress (anisotropic pressure, with the total stresses maintained constant; Fig. 6 defines pore pressures uo, Umax and ur. Pore pressure wu, is that corresponding to an equilibrium friction angle, such as the residual effective friction angle. Pore pressure wr is that necessary to fail the soil by a monotonic increase in pore pressure, and wax 1S he maximum value of the applied cyclic pore pres- sure in the cyclic triaxial test. The ratio U.= (Umax — Uo)/ (up—Uy) gives an idea of the degree of intensity of pore pressure cyclic loading. U.=1 means rupture in just one cycle. Fig. 5. Rupture by monotonic increase in pore water pressure (Brand, 1981). Fig. 4. Typical variation of piezometric levels in residual soils from gneissic—granitic rocks (Lacerda, 1989) Fig. 6. Definition of pore pressures Ug, Umax and up Fig. 8. Yield surface of granitic residual soil. Fig. 9. U, vs. number of cycles to failure. Fig. 11. The Santa Genoveva Nursing Home accident: cross section (Barros et al., 1989). completely saturate the 5 m thick colluvium, composed of more than 30% of rock blocks, some of which more than 4 m in diameter, at a 20° inclination. Sometimes geologists in Brazil use the term “talus/colluvium” to distinguish this particular type of colluvium. This amount of rain was divided in three distinct periods by Santos Barros and Brandao (1992); the accumulated rainfall in the last 3 days was 441 mm, with a peak hourly rainfall of 56 mm/hr in February 22, the day of the debris flow. The slab fell from elevation 525 m and was broken on its first impact on the rock scarp just below, at elevation 400 m; the resulting flying blocks (with an energy that cut some high trees in half) impacted the colluvium at elevation 280 m. Figs. 11 Fig. 12. The Santa Genoveva Nursing Home accident: plan view (Barros et al., 1989). Fig. 13. Aspect of the nursing home | day after the landslide (Photo by Ary Maciel). and 12 show the section and the plan view of the site. The colluvium liq declivity of the s home, ate evation in an almost paral which usually carried superficial water storms. T he two uefied, and ran until a 180 m. Another debris s nursing home, indicated by the black arro seen in Fi accident a break in the ope, in which there was a nursing ide occurred el paleo thalweg, also saturated, and flows during debris flows joined exactly at the w, as can be g. 12. About 53 people were killed in this one. Figs. 13 and 14 show res pectively the Saprolitic soils in the saturated state are often sepa- rated from the upper colluvial cover by a clayey soil, which can be an old mature residual soil that was covered by the colluvium, or a layer with a concentration of elluviated, fine material from the more impermeable colluvium. In this case, the aquifer of the saprolitic soil can be confined, and semi-artesian or even artesian pore pressures can develop in the lower stratum if the rate of infiltration from higher elevations is high. The example of a slide due to this cause is that of Urubu’s Hill in the city of Rio de Janeiro, which occurred on April 8, 1966. Several previous slides had been reported by the Fig. 15. Large fallen rock block, with more than 10 m in maximum dimension, in a forested slope (Tijuca forest, Rio de Janeiro). Photo by the Author. residents of the densely occupied area. These slides happened 30 years before, and it is logical to assume that even when the area was uninhabited they may have also occurred, but had been unnoticed. The slide involved the Fig. 14. Aspect of the region washed through by the landslide. Gneissic rock appears where a 5 m thick colluvium/talus existed. At right and far lef note the intact colluvium (Photo by the Author in 1989). Fig. 16. Urubu’s Hill plan (adapted from Moreira, 1974). subsidence of the upper part of the slope, forming a scarp with 4.5 m, cutting a house in half. At the foot of the slope the ground suffered a heave of | m, and the soil and the walls of the houses at this region were cracked. All residents had time to evacuate their houses, because the slide was not sudden. This accident is reported by Moreira (1974). Fig. 16 shows the plan of the site. Rotary drill borings with Denison sampling, piezometers and 4 inclinometers were installed. Sounding S-54, at the side of inclinometer I-4, showed that the subsoil consisted of a colluvium/talus up to 17 m thick resting on a clayey Average from several classification tests in each layer (Moreira, 1974) Table 1 Shear strength tests (Moreira, 1974) *DST — direct shear test; submerged; **CIU — isotropically consolidated triaxial test, saturated. Table 2 Fig. 17. Cross section A—A’ showing failure surface and piezometric lines (adapted from Moreira, 1974) Fig. 18. 25-day accumulated rainfall in the summer of 1966. artesianism. Local artesianism can initiate landslides in upper colluvial layers, as the Author has seen in Brazil. Fig. 19. Influence of a hidden spring on the flow pattern of a slope (Lacerda, 1999). The remnants of the of Soberbo Road debris flow in the city of Rio de Janeiro, in 1967, have been extensively studied (Soares et al., 1988; Lacerda and Schilling, 1992). The material deposited after the flow constitutes a colluvium, which moves continuously (in reality, intermittently, according to the elevation of the ground water table). Fig. 20 shows a partial plan of the slide. Section C—Cl along zones C, B and D is shown in Fig. 21. The two upper branches of this complex slide present a colluvial layer on top of the residual soil. The thickness of the colluvium lies between 6 and 10 m. The water table is close to the surface, except at two zones, when it appears actually at the surface. At these points impermeable diabase Fig. 20. Map of the Soberbo Road slide (Lacerda and Schilling, 1992). Fig. 21. Section along the Soberbo Road Landslide showing two families of diabase dykes (Lacerda, 1999). Fig. 22. Displacements measured by inclinometers in the region between the diabase dykes (Lacerda and Avelar, 2003) the deepest piezometer just at the upper contact with the dyke rising more than | m above the ground elevation. Of course, the ground was very wet, with rivulets of water springing at the surface. more impermeable dikes influences the flow lines. Thus, artesian pressures can be observed just before the dykes, as shown. In the Soberbo Road case artesianism was indeed observed, the water level of Fig. 23. Flume set up (Lacerda and Avelar, 2003). Fig. 23 shows the Flume set up (Lacerda and Avelar, 2003). Water reservoirs at the top and bottom of the box are maintained with a constant water level, and observations are made during the steady-state seepage pattern established. Electric and standpipe piezometers Fig. 25. Slide of the soil mass above the barrier (dyke) (Lacerda and Avelar, 2003). Fig. 24. Sequence of elevation of the water level inside the flume (Lacerda and Avelar, 2003).
