The mode of failure in weathered/altered volcanic rocks is influenced prevalently by its intrinsic properties and the degree of weathering. The strength of samples depends also on the strength of constituting grains and the nature of cement supporting the grains. In turn, grain mineral composition plays an important role in failure processes due to differences in shape, sizes, and chemical stability. Simple visual inspection strongly suggests a correlation between strain rate and particle size of the fragments resulting from triaxial tests. Figure 4.26 and Figure 4.27 illustrate this perception. Based on this, pore structure and clast-size in pyroclastic rocks (SPRA), seem to be crucial in the mode of failure, as cracks concentration is localized around clasts in failed specimens (Figure 4.23; Figure 4.24; Figure 4.25). Sizes and alignments of crystals seem to have influence just in fresh and slightly weathered lavas (SLA). The primary controls on ignimbrite (TGT) are the groundmass, composed prevalently by altered pumice, which in some cases; has been replaced by porosity. In unwelded ignimbrite, the mode of failure is controlled by the large content of pores (≈50 %).
In order to follow failure patterns of each sample, series of x-ray tomographie images were prepared. Images were acquired in post-failure phases of triaxial tests. The acquisition of the images of fresh and slightly weathered lava was not possible because of their violent failure mode. In this way, schematic illustrations of failure are presented in Figure 4.26 and Figure 4.27. They could be described as follow:
SLA1: Failure mode of fresh sample is characterized by conical shearing. In this sample, intrinsic properties of material as crystals arrangement and pore structure seem to have little influence on mode of failure (Figure 4.19).
Figure 4.19 a) 3D Schematic illustration of structures resulting from failure modes in SLA1 sample. b) Post- failure photo of the specimen
77 SLA2: Failure mode of slightly weathered sample is characterized by vertical fracturing, parallel to the direction of loading. In this case, the size of the crystals and its arrangement seem to strongly influence the mode of failure. Moreover microfractures aligned sub- perpendicular with respect to axial load seems to influence some samples (Figure 4.20).
Figure 4.20 a) 3D Schematic illustration of structures resulting from failure modes in SLA2 sample. b) Post- failure photo of the specimen
SLA3: Failure mode of this specimen is characterized by localized deformation along a fracture plane inclined ≈ 60° with respect to the horizontal direction. The pattern of this fracture suggests that weathered/altered stains and crystals arrangement are the primary characteristics in controlling failure (Figure 4.21 and Figure 4.27a).
SLA4: Again, failure mode of this specimen is characterized by localized deformation along a fracture plane inclined ≈ 60° with respect to the horizontal direction. Plane is localized at the bottom of the sample, where rock material is highly fractured and a large proportion of cavities is localized. Perpendicular to the deformation plane there are some fractures; they follow the crystals alignment (Figure 4.21b and Figure 4.27b).
78 Figure 4.21 a) 3D Schematic illustration of structures resulting from failure modes in SLA3 and SLA4 samples.
b) Illustration is reconstructed with series of x-ray tomography images
SLA5: Failure mode of this sample is characterized by large cracks, parallel to its longitudinal axis. Complete fracture occurs along a possible pre-existing plane of weakness oriented ≥ 60° with respect to the horizontal direction (Figure 4.22c and Figure 4.27c). Porosity in this sample is reduced and seems to have little influence in mode of failure.
Figure 4.22 a) 3D Schematic illustration of structures resulting from failure modes in SLA5 samples. b) Illustration is reconstructed with series of x-ray tomography images
SPRA1: Failure mode is controlled by a variable texture and grain sizes characteristics. This specimen is composed by a density grading sequence of four different poorly stratified layers, from bottom to top they are described as follow: first layer is made up of fine coarse and sub- angular pumice lapilli with scattered lithic fragments. Second layer is made up of very fine sand; texture is closed or very dense with interlocked clasts. Third layer is made up of well- sorted coarse sand fragments of pumice and lithics. Top layer is made up of fine to coarse
79 sand fragments of sub-angular pumice lapilli prevalently. Failure mode is characterized by stress differentiation along layers. Sub-verticals and conical fractures are localized at the bottom of the specimen. On the contrary, in the middle of the sample, there are two sub- horizontal compaction bands, mainly controlled by stratification and grains arrangement. Finally, the top of the specimen is characterized by localized deformation along a curved fracture (Figure 4.23e and Figure 4.27d).
SPRA3: Failure mode of this specimen is controlled by variable texture and grain sizes characteristics. Failure pattern is sub-horizontal (compaction bands) and it is mainly controlled by stratification and grains arrangement (Figure 4.23 and Figure 4.27f).
Figure 4.23 a) 3D Schematic illustration of structures resulting from failure modes in SPRA1 and SPRA3 samples. b) Illustration is reconstructed with series of x-ray tomography images
SPRA2: Failure mode of this specimen is characterized by a fracture performed through the rock matrix, controlled by lithic fragments (≈1.5 cm), which influence the concentration of stresses (Figure 4.24f and Figure 4.27e).
80 Figure 4.24 a) 3D Schematic illustration of structures resulting from failure modes in SPRA2 sample. b)
Illustration is reconstructed with series of x-ray tomography images
IGTF: Large proportion of pumice clasts with open structures plays an important role in the failure process of this specimen. Failure mode involves changes in shape and size of pumice clasts, moreover fragmentation of their structures could control the strength of the rock (Figure 4.25 and Figure 4.27h).
Figure 4.25 a) 3D Schematic illustration of structures resulting from failure modes in IGTF sample. b) Illustration is reconstructed with series of x-ray tomography images
IGTA: Failure mode of this completely weathered specimen is characterized by a fracture developed through the rock matrix. Fracture is associated to the proportion, arrangement, sizes and geometry of the grains (crystals and pumice clasts) contained within the groundmass. Post-failure observations, suggest pore structure as a minor characteristic in controlling failure (Figure 4.26d and Figure 4.27g).
81 Figure 4.26 a) 3D Schematic illustration of structures resulting from failure modes in IGTA sample. b)
82 Figure 4.27 2D Schematic illustration of structures resulting from various failure modes in weathered/altered
volcanic rocks. Fracture mode is represented by four different profiles. Base of each sample has 54 mm of diameter. Fracture pattern in IGTF is highlighted by a dash line.