Abstract

Geometry is one of the main factors controlling the overall stability of levees, in addition to their hydraulic and mechanical characteristics, while in turn, their material strength strongly depends on their degree of saturation. Given that levees are usually partially saturated (Lo Presti et al., 2020) and that this partial saturation greatly contributes to their stability, any technique aimed at maintaining the levee in this condition is extremely useful. Protection of the levee sides by means of geogrids and biomats reduces the amount of infiltration by water, helps water adsorption by grass vegetation and contributes to mechanical strength thanks to the root apparatus of the grass cover. Vegetation is a good system for controlling slope erosion and instability. It can reduce the meteoric water that infiltrates into the soil and consequently the pore pressure. On the other hand, the root systems not only keep the soil partially saturated but also increase the soil shear strength with their mechanical contribution. The mechanical parameters of the root-soil system are important to evaluate the increase in soil shear strength due to the roots. Mechanically, roots contribute to the stabilization/reinforcement of soil by means of an additional apparent cohesion. It is believed that this apparent cohesion originates mainly from the root tensile strength, as well as capillary forces in partially saturated soils. In terms of root cohesion, an evaluation of this reinforcement can be obtained by means of direct shear tests. The authors set up a systematic experimental activity using a large direct shear box (Vannucci et al. 2019) with the purpose of considering the dual effect of suction and the soil tensile strength. A series of direct shear tests (large and standard shear box) were carried out. This paper presents the results of an experimental laboratory investigation on samples of rooted soil. In particular, the results of direct shear tests with a large shear box (300 × 300 × 100 mm) and with a standard shear box are shown. The results confirm an increase in the soil shear strength, in terms of cohesion, when roots are present. 1

Abstract

Assessing soil properties through geotechnical tests is a crucial activity to understand its behaviour. Field tests serve as a good approach to characterize the geotechnical behaviour of materials according to their in-situ condition. However, the interpretation of these tests often relies on empirical correlations, which can become complex when dealing with soils with notable heterogeneity. This paper objective is to determine the strength parameters and discuss the consolidation effects for a heterogeneous tropical soil deposit using data from field tests with different rates of penetration and laboratory tests. A layer identified as low-strength soil classified as silty-sand to silty-clay exhibited partial drainage during a standard-rate cone penetration. The approach proposed by Randolph and DeJong (2012) was used to determine the penetration rate necessary to mobilize an undrained behaviour of the material, since the cone penetration results shown that the material is mostly contractive and clay-like. CPTu tests were also conducted with penetration rates of 60 mm/s and 100 mm/s, to proper estimate the undrained shear strength (Su). These tests reinforced the presence of a preconsolidated upper layer, enabling the estimation for the overconsolidation ratio (OCR). For the normally consolidated portion of the residual soil, rapid tests that achieved a normalized velocity (V) associated with distinctly undrained penetration were used to derive the undrained shear strength (Su), with a cone factor (Nkt) estimated from standard-rate tests available. The geotechnical parameters estimated were then compared to the laboratory data results showing a convergence with the parameters estimated by the field tests.

Abstract

Volcanic pumice soils are widely distributed in many countries. Mechanical properties of those need to be characterized since seismic ground motions can cause serious hazards such as large-scale slope failures and long-distance debris flows. Since volcanic pumice generally retains extremely high porosity and has a sensitive structure, the natural soil structure is easily broken during sampling or transportation. Therefore, it is preferable to conduct in-situ tests without disturbing the original structure of the soil. In this study, an in-situ direct shear test device was developed, and in-situ cyclic direct shear tests were performed on volcanic pumice (Ta-d). This pumice soil is considered to be the main cause of slope failure in Atsuma, Hokkaido, Japan, due to Easten Iburi earthquake in 2018. Laboratory direct shear tests were also conducted using undisturbed sample taken from the site. The results revealed that soil structure significantly affects shear strength. To evaluate the mechanical behavior of such sensitive soils, it is crucial to use specimens with as less disturbed specimens as possible. The newly developed in-situ direct shear test device was used to determine the cyclic shear strength of sensitive volcanic pumice soils.