Vegetation effects on the tensile strength of a partially saturated soil

<p>Soil tensile strength plays an important role in the hydro-mechanical behaviour of earth structures and slopes interacting with the atmosphere. Shrinkage-induced cracking may be generated by drying/wetting cycles, with consequent faster water infiltration from the top of slopes and reduction of the safety factor. Vegetation roots were proven to increase soil shear strength, but less is known about their effects on soil tensile strength. For this purpose, new equipment has been designed and used to induce plant growth in compacted soil samples and to perform uniaxial tensile tests on the reinforced material. The equipment is composed of two cylindrical moulds linked by a soil bridge in which the tensile crack is induced due to geometrical restraints.</p><p>For this study, silty sand was chosen and compacted at a low dry density (ρ<sub>d</sub> = 1.60 Mg/m<sup>3</sup>) and at a water content w = 15%. After compaction, samples were gently poured with water up to a high degree of saturation (S<sub>r</sub> ≈ 0.95) and low suction (s ≈ 1 kPa). Then, six of them were seeded with Cynodon dactilon, adopting fixed seeding density and spacing. Plants were irrigated and let to grow for three months: during this period, suction was monitored by a tensiometer. Seven fallow specimens were prepared following the same procedure, for comparison purposes.</p><p>When ready, samples were dried in a temperature/relative humidity-controlled room and left in the darkness for three hours, to attain and equalise the desired value of initial suction. Finally, the tensile stress was induced on the soil by a displacement rate of 0.080 mm/min. For each test, suction was continuously monitored by a tensiometer while the water content was checked at the beginning and at the end. Moreover, the void ratio and the root volume and area ratio were assessed close to the crack generated, at the end of each test.</p><p>The hydraulic state affected the soil mechanical response upon uniaxial extension: an increase of strength and a more brittle behaviour were observed as suction was increasing. At the same suction, a higher strength was systematically observed in the vegetated soil. In fact, even at very low suction (i.e. s = 1 kPa), vegetation roots induced a considerable increase in soil tensile strength (i.e. 10 kPa). The soil hydraulic state also affected the root failure mechanism. In wet soil, the roots subjected to tension were stretched and pulled-out whereas in dry soil they experienced a more immediate breakage (i.e. in concomitance with the cracking of the surrounding soil). Some preliminary PIV (Particle Image Velocimetry) analyses showed differences among dry/wet and fallow/vegetated soils. Indeed, a more diffuse strain field was observed in vegetated samples, thanks to the redistribution of stresses induced by the roots.</p><p>Results were successfully interpreted by a well-established shear strength criterion for partially saturated soils, considering the degree of saturation, suction and soil microstructure. An increase of the soil shear strength was observed and correlated to the presence of roots and to their geometrical and mechanical features. Moreover, good consistency was detected with results coming from other equipment.</p><p> </p>

A Soil Tensile Strength Based Headcut Migration Model of Breach Side Slope

The headcut migration describes the physical process of breach side slope retreat that governs the widening of the breach. Modeling the growth of a breach due to embankment failure is the first step in mapping the resulting inundation in a floodplain. As removal of soil from the toe of the headcut effectively removes physical support for the upper part, the headcut fails on the plane normal to the direction of tensile stress. This process is a typical mode of tensile failure. A numerical model of the headcut migration was established by integrating the effects of the soil tensile strength, soil permeability and embankment geotechnical characteristics. Thus, a simple analytical equation was finally obtained to predict the critical length of the headcut. Furthermore, the presented model was verified by using the limit equilibrium method (LEM) for three typical embankment scales (2, 4 and 6 m high). The comparisons between the present model and the LEM show good agreements. The present model could provide a simple method to predict the critical length of the headcut migration and easily be adopted to breach widening models.

Modelling desiccation crack geometry evolution in clayey soils by analytical and numerical approaches

In the present work, the development and geometry of desiccation cracks are studied by using a finite element code including cohesive joints elements. The numerical results show that cracking occurs sequentially to form different crack families. The propagation of each crack at the onset suddenly reaches an ultimate depth. The cracks in each family appear simultaneously and reach an identical ultimate depth. From the numerical results and additional analytical analysis, empirical correlations are proposed to predict the spacing and crack depth as a function of suction applied on the top surface, the soil parameters, and the desiccation rate. The proposed model shows that higher suction is required to initiate cracks at a higher value of soil tensile strength. In addition, there is a general trend of larger spacing and deeper cracks for a slower desiccation rate. Finally, empirical relations are evaluated by comparing them with in situ experimental observations published previously.

Tensile strength, friability and organic carbon in an oxisol under a crop-livestock system

The crop-livestock system can promote soil compaction in surface layers, mainly due to animal trampling. However, plants and their root growth, in interaction with animal trampling, can decrease the deleterious changes in soil structure caused by this system. Up to the present time, the physical soil modifications in crop-livestock systems, including oat and ryegrass crops for winter animal forages are unknown. The objective of this study was to quantify and to relate tensile strength, friability and soil organic carbon in an Oxisol under a crop-livestock system. The study was conducted in Campo Mourão - Paraná, Brazil. Four forage heights were used for the winter forages: 7, 14, 21 and 28 cm. For each forage height, five soil blocks were randomly collected from each layer of 0 - 0.1, 0.1 - 0.2 and 0.2 - 0.3 m of depth. The increase in carbon content promotes an increase in soil tensile strength at the 0.1 - 0.2 m soil depth, this layer having the highest values for tensile strength. The forage height of 21 cm was found to be the best height for soil friability, and the soil was very friable at this height. Despite a decrease in friability in the upper layers of the soil, the crop-livestock system was not found to be a limiting factor for the subsequent cultivation of annual crops.