Investigation of Critical Geotechnical, Petrological and Mineralogical Parameters for Landslides in Deeply Weathered Dunite Rock (Medellín, Colombia)

Abstract

The current study site of the project Inform@Risk is located at a landslide prone area at the eastern slopes of the city of Medellín, Colombia, which are composed of the deeply weathered Medellín Dunite, an ultramafic Triassic rock. The dunite rock mass can be characterized by small-scale changes, which influence the landslide exposition to a major extent. Due to the main aim of the project, to establish a low-cost landslide early warning system (EWS) in this area, detailed field studies, drillings, laboratory and mineralogical tests were conducted. The results suggest that the dunite rock mass shows a high degree of serpentinization and is heavily weathered up to 50 m depth. The rock is permeated by pseudokarst, which was already found in other regions of this unit. Within the actual project, a hypothesis has for the first time been established, explaining the generation of the pseudokarst features caused by weathering and dissolution processes. These parameters result in a highly inhomogeneous rock mass and nearly no direct correlation of weathering with depth. In addition, the theory of a secondary, weathering serpentinization was established, explaining the solution weathering creating the pseudokarst structures. This contribution aims to emphasize the role of detailed geological data evaluation in the context of hazard analysis as an indispensable data basis for landslide early warning systems.

Keywords: block-in-matrix structure; dunite; geological investigation; landslide investigation; mineralogical predisposition; pseudokarst; secondary serpentinization.

Figure 1

(a) Location of Medellín within Colombia and the Cordillera Central; (b) The “Comunas” of Medellín (black contours), the location of the study site and the city’s geological units [11].

Figure 2

Idealized column profile of the Medellín Dunite with pseudokarst structures; dimensions are not given, because they vary to an evident extent [12] (after Figure 7.6).

Figure 3

Map of the study site depicting the locations of the drilling and the sample locations (projected coordinate system: MAGNA Medellin Antioquia 2010).

Figure 4

Map of landslide features at the project site, which is marked with the red colored line. The red border represents the project area, the detachment areas are marked in green, the accumulation areas in black. Most of the landslides could be characterized as rotational slides (projected coordinate system: MAGNA Medellin Antioquia 2010).

Figure 5

Geological map of the study site. The yellow colors indicate the bedrock ridges mainly in the north and south of the project site (projected coordinate system: MAGNA Medellin Antioquia 2010).

Figure 6

(a) Picture of the drilling core of drilling A1; (b) Picture of the drilling core of drilling A2. The cores have a length of 1 m each and run from top left to bottom right.

Figure 7

Picture of the drilling core B1, depicted in two rows. The left row shows the drilling core from 0–27.4 m, the right row shows the drilling core from 27.4–50.0 m. The core boxes have a length of 1 m each and run from top left to bottom right.

Figure 8

Results of the uniaxial compressive strength tests plotted vs. the sampling depth.

Figure 9

Results of the tensile strength tests (Brazilian tests) plotted vs. the depth of the sampling.

Figure 10

Grain size distribution of all drillings depicted according to DIN EN ISO 14688-1 (2020) [30].

Figure 11

Shear-displacement curves.

Figure 12

Shear-consolidation curves.

Figure 13

Microscopic photos of the thin sections related to the sample numbers: (a) = D-01; (b) = D-02; (c) = D-03; (d) = D-04; (e) = D-05; (f) = D-06 (ol = olivine, ant = antigorite, chr = chrysotile, pyr = pyroxene, amp = amphibole, chl = chlorite, op = opaque phase) [34].

Figure 14

Microscopic photos of the thin sections related to the sample numbers: (a) + (b) = D-07; (c) = D-08.1; (d) = D-08.2; (e) = D-09; (f) = D-10 (ol = olivine, ant = antigorite, chr = chrysotile, pyr = pyroxene, amp = amphibole, chl = chlorite, op = opaque phase) [34].