Preliminary results of in situ corkscrew tests in coarse-grained debris with vegetation roots

This short paper presents preliminary results ofstudy aimed at evaluating the effects of tree cutting as a predisposing factor of debris-flow triggering (Lepri et al., 2024). The study area (Nottoria, Perugia, Italy) was affected by debris flow events in 2012 and in 2015.

The material of the debris flow source area is classified as calcareous pebbles in a marly-clayey matrix, with very angular grains.

Woody beeches and oaks’ roots, with diameters varying between 0.5 and 2 mm were found in the retrieved soil samples.

In-situ investigations on the material involved in the debris flows, consisting of corkscrew tests, water content and suction monitoring, lidar drone is in progress, jointly with geotechnical laboratory experiments.

In this abstract we present the results of corkscrew tests.

The equipment presents a rotating arm at the end of which there is a load cell and a steel screw (Figure 1).

 

Figure 1. Corkscrew equipment.

The screw has a height H₄ = 125 mm, a diameter d_cs = 40 mm, a helix diameter = 6 mm and an helix pitch of 28 mm.

The peak strength was recorded using a 300 kg load cell (Steinberg systems – SBK-KW-300KG).

The corkscrew was driven into the ground by manual rotation, after which the load cell is connected, and the soil sample is pulled out by using a lever system. The load cell provides the pullout force T_max.

The shear stress along the lateral surface of the soil sample is then calculated following equation (1) provided by Meijer et al. (2018):

                                                                                         (1)

Corkscrew tests were performed at increasing depths (0–125, 125–250, 250–375 mm). Once the soil sample was extracted, the roots content was assessed and the water content and suction measured.

Figure 2 shows the location where corkscrew tests were performed, while the results are plotted in Figure 3 in terms of peak shear stress against the horizontal effective stress.

Figure 2. Corkscrew tests location

 

• a)    b)

Figure 3. a) Extracted rooted sample; b) Results from corkscrew tests: shear stress vs vertical effective stress

 

References

Lepri, A., Fraccica, A., Cencetti, C., and Cecconi, M. (2024a). A preliminary study on the possible effect of deforestation in debris flows deposits, EGU24-15726, Vienna, Austria, 14–19 Apr 2024.

Lepri A., Fraccica A., Cecconi M., Pane V. (2024b). Effetti del taglio di vegetazione sull'innesco di una colata detritica a Nottoria (PG): caratterizzazione geotecnica preliminare. Incontro Annuale dei Ricercatori di Geotecnica 2024- IARG 2024 - Gaeta, 4-6 Settembre 2024.

Meijer, G.J., Bengough, A.G., Knappett, J.A., Loades, K.W., Nicoll, B.C. (2018). In situ measurement of root-reinforcement using the corkscrew extraction method. Can. Geotech. J. 55 (10), 1372–1390. (https://doi.org/10.1139/cgj-2017-0344).