# The Mechanical Characterization of Pyroclastic Deposits for Landslide Early Warning Systems

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## Abstract

**:**

## 1. Introduction

## 2. Background

## 3. Materials and Methods

_{max}, the specific unit weight of the soil particles γ

_{s}, the unit weight of soil volume γ, porosity n, and the degree of saturation S

_{r}. The soil is characterized by a low specific unit weight of about 25 kN/m

^{3}, probably due to the presence of internal voids of the soil particles, by porosity ranging between 53% and 55% and a saturation degree ranging between 24% and 46% related to the weather conditions.

- Consolidated isotropically drained and undrained (CID and CIU) triaxial tests on saturated specimens to evaluate soil shear strength and susceptibility to liquefaction;
- Constant head permeability test to determine the saturated hydraulic conductivity;
- Suction-controlled triaxial tests (SCTX) on undisturbed specimens to define hydraulic conductivity function and evaluate the effects of partial saturation on shear strength;
- Infiltration/evaporation method for the evaluation of the soil water retention curve (SWRC).

## 4. Results

#### 4.1. Mechanical Properties in Saturated Conditions

#### 4.1.1. Saturated Shear Strength and Undrained Response

_{a}–q plane in Figure 5b,c), the evolution of volumetric strain during CID test (the ε

_{a}–ε

_{v}plane in Figure 5d), and the development of excess pore pressure during undrained CIU test (the ε

_{a}–∆u plane in Figure 5e).

- A friction angle equal to 38° and cohesion of 15 kPa at peak;
- A friction angle of 38° and cohesion of 0 kPa at the critical state.

#### 4.1.2. Saturated Hydraulic Conductivity

_{sat}, was investigated through constant head permeability tests on natural samples. The results are shown in Figure 7. The values of k

_{sat}are reported as a function of void ratio (Figure 7a) and the mean applied effective stress (Figure 7b). For a low state of stress, close to the in situ stress, the saturated permeability is about 10

^{−5}m/s, which progressively decreases by an order of magnitude for applied effective stress as high as 700 KPa (Figure 7b).

#### 4.2. Mechanical Properties in Unsaturated Conditions

#### 4.2.1. Shear Strength under Unsaturated Conditions (Sr < 1)

_{a}, q) in Figure 8a. In the same Figure, the critical state line corresponding to the saturated condition of the material (dashed line) is also reported. Thus, from the comparison, the mechanical effect of suction on the soil strength can be revealed. All the points representing failure in unsaturated conditions are located well above the failure envelope of the saturated soil, showing that suction in these soils plays a significant role.

#### 4.2.2. Soil Water Retention Curve and Conductivity Function

^{3}), reconstituted with an initial porosity of 0.55, by using two mini-tensiometers (jet-fill, Soil Moisture type) with ceramic tips located at different heights, and a TDR probe inserted horizontally at the mid-height of the specimen. Mini-tensiometers measure suction up to 100 kPa with an accuracy of 1 kPa. The TDR probe was used to measure the volumetric water content of the soil. It is based on the correlation between electric conductivity and water content. Taking advantage of the dielectric properties of the soil, the travel time of the electromagnetic pulse along the metal probes buried into the soil is measured and correlated to the volumetric water content [26]. An estimation of the volumetric water content was also carried out using gravimetric measurements by assuming a constant porosity in the soil sample (no significant volumetric changes and shrinkage occurred during the test).

^{−8}m/s. For suction lower than 30 kPa, the data show a greater dispersion.

## 5. Discussion

#### Assessment of the Post-Failure Evolution of Landslides in Pyroclastic Covers

_{crit}) is introduced instead of the slope angle (α), defined as the slope inclination at which failure occurs under saturated conditions. This critical angle must be determined case by case, eventually assuming a simplified hypothesis. Obviously, for homogeneous cohesionless soils, the critical slope angle is equal to the friction angle of the material.

_{crit}), failure occurs in an almost saturated condition. In the flowchart, the path to be followed (purple line) is the one corresponding to a steep slope (α ≈ α

_{crit}).

_{crit}. Based on the observation of detachment areas, which shows that the sliding surfaces are located at about 1.5 m of depth, α

_{crit}is determined by performing stability analysis at different inclinations of the slope, in the simplified hypothesis of a constant suction profile and by using the peak state parameters (ϕ′ = 38° and c′ = 15 kPa). The analysis results show that it corresponds to a slope inclination of 65°: In the presence of α lower than α

_{crit}, slope failure occurs in saturated conditions; for α higher than α

_{crit}, the mobilization of the soil deposit occurs in soil which is still in unsaturated conditions. Along the Camaldoli hill, slope instabilities mainly involve the upper part of the slope above the vertical tuff cliff where soil deposits present local acclivity higher than 60°. Thus, instability may occur regardless of whether the soil is saturated or not: the potential paths to follow in the flowchart (green lines) correspond to both the last two branches related to the steep slope (α ≈ α

_{crit}) and very steep slope (α ≈ αc

_{rit}).

## 6. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**(

**a**) Geomorphological contexts of the pyroclastic deposits in Campania, Italy; schematic stratigraphy at the location of the study sites: (

**b**) Cervinara; (

**c**) Sarno; (

**d**) Camaldoli.

**Figure 4.**Grain size distribution of Camaldoli ash compared with other liquefiable ash deposits of Campania and with the bounds of liquefiable natural soil deposits.

**Figure 5.**Triaxial test results: (

**a**) stress plane; (

**b**) results of CID tests in Ɛa–q plane; (

**c**) results of CIU tests in Ɛa–q plane; (

**d**) results of CID tests in Ɛa–Ɛv plane; (

**e**) results of CIU tests in Ɛa–∆u plane.

**Figure 8.**Unsaturated shear strength: (

**a**) stress plane; (

**b**) intercept of cohesion as a function of suction.

**Figure 13.**Comparison of the mechanical properties of three investigated pyroclastic deposits: (

**a**) SWRCs; (

**b**) HCFs; (

**c**) apparent cohesion trends; and (

**d**) undrained paths in the compression plane.

**Figure 14.**Flowchart for simplified assessment of the post-failure evolution of landslides in granular soils (red box highlights the potential post-failure evolution in flowslide; green boxes denote evolution in slide or debris avalanches).

d_{max} (mm) | γ_{s} (kN/m^{3}) | γ (kN/m^{3}) | n (%) | S_{r} (%) |
---|---|---|---|---|

6 | 24.9 | 12.0 | 53–55 | 24–46 |

**Table 2.**Triaxial tests in saturated conditions: state parameters at the end of the consolidation stage and the applied mean effective stress [21].

Test ID | γ (kN/m^{3}) | γ_{d} (kN/m^{3}) | w (%) | n (%) | e | p′_{0} (kPa) |
---|---|---|---|---|---|---|

C_Cid_1 | 16.9 | 11.5 | 46.5 | 53.6 | 1.16 | 73.4 |

C_Cid_2 | 17.0 | 11.7 | 45.4 | 53.1 | 1.13 | 99.38 |

C_Cid_3 | 17.0 | 11.8 | 44.4 | 52.7 | 1.11 | 147.66 |

C_Cid_4 | 16.7 | 11.6 | 42.9 | 53.3 | 1.14 | 149.86 |

C_Cid_5 | 16.7 | 11.4 | 47.2 | 54.4 | 1.19 | 25.85 |

C_Cid_6 | 16.9 | 11.7 | 45 | 53.1 | 1.13 | 229.5 |

C_Cid_7 | 16.6 | 11.2 | 48 | 54.9 | 1.22 | 49.56 |

C_Cid_8 | 16.9 | 11.5 | 46.8 | 53.7 | 1.16 | 125.67 |

C_Ciu_1 | 17.1 | 11.9 | 43.1 | 52.1 | 1.09 | 297.46 |

C_Ciu_2 | 16.8 | 11.6 | 44.2 | 53.2 | 1.14 | 34.69 |

C_Ciu_3 | 17.0 | 11.7 | 44.7 | 52.8 | 1.12 | 69.97 |

C_Ciu_4 | 17.1 | 11.9 | 44 | 52.3 | 1.10 | 220.11 |

C_Ciu_5 | 16.9 | 11.8 | 43.5 | 52.7 | 1.12 | 126.69 |

C_Ciu_6 | 17.0 | 11.8 | 44.2 | 52.7 | 1.12 | 99.83 |

C_Ciu_7 | 17.0 | 11.9 | 42.8 | 52.3 | 1.10 | 148.01 |

C_Ciu_8 | 17.0 | 11.9 | 42.6 | 52 | 1.09 | 199.99 |

SCTX | γ (kN/m^{3}) | γ_{d} (kN/m^{3}) | n (%) | e | S_{r,f} | p–u_{a} (kPa) | u_{a}–u_{w} (kPa) |
---|---|---|---|---|---|---|---|

C_USP_1 | 15.2 | 11.8 | 52.50 | 1.104 | 0.64 | 50 | 5 |

C_USP_2 | 14.4 | 11.8 | 52.45 | 1.104 | 0.48 | 50 | 15 |

C_USP_3 | 13.6 | 11.8 | 52.47 | 1.105 | 0.34 | 50 | 35 |

C_USP_4 | 13.5 | 11.6 | 53.61 | 1.154 | 0.36 | 50 | 50 |

C_USP_5 | 14.2 | 11.8 | 52.46 | 1.105 | 0.44 | 100 | 5 |

C_USP_6 | 12.8 | 11.1 | 55.48 | 1.247 | 0.30 | 100 | 15 |

C_USP_7 | 15.4 | 11.4 | 54.96 | 1.192 | 0.74 | 100 | 35 |

C_USP_8 | 13.4 | 11.7 | 52.87 | 1.123 | 0.33 | 100 | 50 |

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**MDPI and ACS Style**

Damiano, E.; de Cristofaro, M.; Brunzo, A.; Carrieri, G.; Iavazzo, L.; Netti, N.; Olivares, L.
The Mechanical Characterization of Pyroclastic Deposits for Landslide Early Warning Systems. *Geosciences* **2023**, *13*, 291.
https://doi.org/10.3390/geosciences13100291

**AMA Style**

Damiano E, de Cristofaro M, Brunzo A, Carrieri G, Iavazzo L, Netti N, Olivares L.
The Mechanical Characterization of Pyroclastic Deposits for Landslide Early Warning Systems. *Geosciences*. 2023; 13(10):291.
https://doi.org/10.3390/geosciences13100291

**Chicago/Turabian Style**

Damiano, Emilia, Martina de Cristofaro, Antonia Brunzo, Goffredo Carrieri, Luisa Iavazzo, Nadia Netti, and Lucio Olivares.
2023. "The Mechanical Characterization of Pyroclastic Deposits for Landslide Early Warning Systems" *Geosciences* 13, no. 10: 291.
https://doi.org/10.3390/geosciences13100291