Comparison between the in situ and laboratory water retention curves for a silty sand

After an extreme rainfall event in May 2002 a series of landslides occurred in Ruedlingen in Canton Schaffhausen, North Switzerland. A 38° steep slope has been chosen in this area beside the river Rhine to carry out an artificial rainfall experiment to investigate the dependence between rainfall, suction, saturation and shear resistance. Two sprinkling experiments were conducted to represent an extreme rainfall event, the second of which resulted in failure of 130 m 3 of the slope. Several cycles of wetting and drying were applied to the soil and suction and volumetric water content were measured at different depths in three locations of the slope, by which in-situ Water Retention Curves (WRC) can be derived. The WRC of an undisturbed sample was also determined from laboratory test. The in situ and laboratory WRCs are compared in this paper and the differences will be discussed

The effects of hydraulic properties of bedrock on the stability of slopes

The transient process of rain infiltration in the soil and the effect of geometry and drainage properties of the bedrock on the pore pressure distribution and the stability of a slope are investigated. The simulated slope is a test field in northern Switzerland, where landslide triggering experiments were carried out. From geological point of view, the experimental site is located in the Swiss Molasse basin. The lithological units in the area are composed of horizontally layered and fractured sandstones intersected by marlstone. The stability of the slope is monitored at different stages of the infiltration using the limit equilibrium method of slices. Several cases were compared to study the effect of the fissures in the shallow bedrock on the stability of the slope. The approximate location and size of the fissures in the bedrock were determined by monitoring of spatial and temporal changes of electrical resistivity during rainfall and also geological investigations of the bedrock before and after the failure

Unsaturated hydraulic conductivity of a silty sand with the instantaneous profile method

The unsaturated hydraulic conductivity of a silty sand at different initial void ratios is measured using the instantaneous profile method. The variation of the suction and volumetric water content is recorded during the infiltration process as a function of time. Accordingly, an infiltration column was developed with a height of 600 mm and an inner diameter of 170 mm. The suction and volumetric water content were measured simultaneously every 100 mm along the column by means of small tensiometers and TDRs, respectively. Hydraulic conductivity is calculated by dividing the water flow velocity by the hydraulic gradient. The soil is reconstituted from Ruedlingen (Canton Schaffhausen, Switzerland), where landslide triggering experiments were carried out in October 2008 and March 2009. The hydraulic conductivity functions are determined and the laboratory values are compared to the in-situ measurements of hydraulic conductivity carried out in the course of the landslide triggering experiments

Mountain Risks: two case histories of landslides induced by artificial rainfall on steep slopes

Mountainous areas tend to be exposed to an enhanced risk of damage caused by natural hazards; most often exacerbated by the topography (leading to gravitational mass movements such as avalanches, landslides, mud and debris flows). This contribution compares landslides induced by artificial rainfall on two different areas located in Switzerland. One field test site was located on slopes above Saas Balen (Gruben glacier, Canton Wallis, Switzerland) and was instrumented. Artificial rainfall tests were carried out in the summers of 1999 and 2000 to investigate hydro-mechanical mechanisms of instability (Teysseire et al., 2000). Shallow failure occurred in the steeper instrumented slope in 2000. The second test field is located near Ruedlingen (Canton Schaffhausen, Switzerland). A landslide triggering experiment was carried out there in autumn 2008 and spring 2009 to replicate the effects of a heavy rainfall event of May 2002, in which 100 mm rain fell in 40 minutes, causing 42 superficial landslides. The slope was subjected to extreme rainfall by artificial means in October 2008 and in March 2009, triggering about 130 m3 of debris. Infiltration of rainfall has led to surface instability slopes in an alpine moraine (Gruben) and in silty sand (Ruedlingen). Both slopes were steeper than the internal angle of friction, having different initial degrees of saturation and suction. The hydromechanical behaviour of these two field full scale landslides will be compared, trying to expose a deeper understanding of the rainfall induced failure mechanisms

Root reinforcement of soils under compression

It is well recognized that roots reinforce soils and that the distribution of roots within vegetated hillslopes strongly influences the spatial distribution of soil strength. Previous studies have focussed on the contribution of root reinforcement under conditions of tension or shear. However, no systematic investigation into the contribution of root reinforcement to soils experiencing compression, such as the passive Earth forces at the toe of a landslide, is found in the literature. An empirical-analytical model (CoRoS) for the quantification of root reinforcement in soils under compression is presented and tested against experimental data. The CoRoS model describes the force-displacement behavior of compressed, rooted soils and can be used to provide a framework for improving slope stability calculations. Laboratory results showed that the presence of 10 roots with diameters ranging from 6 to 28 mm in a rectangular soil profile 0.72 m by 0.25 m increased the compressive strength of the soil by about 40% (2.5 kN) at a displacement of 0.05 m, while the apparent stiffness of the rooted soil was 38% higher than for root-free soil. The CoRoS model yields good agreement with experimentally determined values of maximum reinforcement force and compression force as a function of displacement. These results indicate that root reinforcement under compression has a major influence on the mechanical behavior of soil and that the force-displacement behavior of roots should be included in analysis of the compressive regimes that commonly are present in the toe of landslides

Landslide triggering experiment in a steep forested slope in Switzerland

A landslide triggering experiment was carried out on a 37°- 40° steep forest slope in North-East Switzerland by sprinkling water artificially to represent an extreme rainfall event. This project is part of a multidisciplinary collaboration, which includes geotechnical engineering, hydrology, hydrogeology, forest engineering, geophysics and photogrammetry. A three dimensional model of the ground was developed from non-invasive geophysical surveys, insitu probing, combined sprinkling and dye tracer tests and shallow test pits. Laboratory tests were carried out on undisturbed samples under various degrees of saturation This paper mainly focuses on the characterisation of the site

Effects of cement-polymer interface properties on mechanical response of fiber-reinforced cement composites

Cement is one of the most consumed materials in the world. The cement industry is responsible for a large portion of global carbon dioxide emissions. Cement production and therefore carbon dioxide emissions can be decreased by increasing the durability and enhancing the mechanical properties of cement-based materials. On the other hand, an important weakness of concrete is its weak tensile properties, which are the main reasons for its failure and low durability. Therefore, over the past 30 years, many studies have focused on improving tensile properties using a variety of physical and chemical methods. One of the most successful attempts is to use polymer fibers in the structure of concrete to obtain a composite with high tensile strength and ductility. However, a thorough understanding of the mechanical behavior of fiber-reinforced concrete requires knowledge of fiber cement interfaces at the nano scale. In this study, a combination of experimental and molecular dynamics (MD) techniques is used to study the nanostructure of fiber cement interfaces. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) analysis are used to obtain a better understanding of the C-S-H fiber (in cement chemistry notation, C = CaO, S=SiO2, and H=H2O) interfaces. The results show that the C/S ratio changes in the interface of cement and polymeric fibers and is largely affected by the functional group of the polymers. The results are then used to propose a more realistic molecular dynamics model for C-S-H in the vicinity of the three most used polymeric fibers: polypropylene, polyvinyl alcohol, and nylon-6. The full atomistic simulations show that the molecular structure of C-S-H at the interface depends on the properties of the polymer functional group. The adhesion energy between the polymeric fibers and the relevant C-S-H structure is then computed using atomistic simulations. The adhesion energy between C-S-H and polymers increases with the polarity of the fiber. The mechanical response of cement paste with added polymeric fibers is then experimentally studied using the split-cylinder test. The experimental results further show that the adhesion energies between the fibers and cement increase as a function of the polarity of the fibers

Modelling the interaction of the bedrock and slope in temrs of drainage and exfiltration

Water may infiltrate in the soil mass and reach the soil-bedrock interface during and after a rainfall event. Depending on the permeability of the bedrock, infiltrated water can generate a perched water table. This water table may either rise, increasing the degree of saturation, or decrease, causing negative pore pressures (suctions) to develop in any capillary zone. Decrease in the suction decreases the shear strength mobilised in the material, which may trigger failure. A test field (average slope of 38◦ and ∼250 m2 area) was selected near Ruedlingen (Canton Schaffhausen, Switzerland) where landslide triggering experiments were carried out in autumn 2008 and spring 2009. The experimental site is located in the Swiss Molasse basin. The lithological units in the area are composed of horizontally layered sandstones intersected by coloured marlstones both of the Lower Freshwater Type. Above the test site is the transition to the layered sandstones of the Upper Marine Molasse (OMM) (Brönnimann, 2010). The slope was subjected to extreme artificial rainfall in October 2008 over a period of 4 days. Some surface movements were detected during this extreme event, although failure did not occur (Springman et al., 2010). Subsequently, a range of measures were implemented, such as relocating the distribution of the sprinklers to provide more rainfall to the upper part of the slope, so that a failure was triggered in March 2009, incorporating about 130 m3 of debris. In order to compare the experiments of 2008 and 2009, the transient process of rain water infiltration in the soil and the effect of the topography and drainage properties of the bedrock at the lower part of the slope on the pore pressure distribution are investigated. The finite element method is used to simulate the percolation process of infiltrated water into the soil. The stability of the slope is monitored at different stages of the infiltration using the limit equilibrium method of slices. Several cases were compared to study the effect of the fissure geometry and hydraulic properties. The approximate location and size of the fissures in the bedrock were determined by monitoring of spatial and temporal changes of electrical resistivity during rainfall and also visual investigations of the bedrock after the failure. According to these simulations, the slope might have failed during the first experiment if there had been no “drainage fissures” in the lower part. Also, the interconnected fissures and the horizontal intrusion of a very permeable sandy layer in the upper part of the slope was found to affect the slope stability. Nevertheless, the impact of the drainage and horizontal fissures is different. The latter have a storage function that can delay reaching and accumulating the water at the interface of the soi

Mechanics of bioinspired lamellar structured ceramic/polymer composites: Experiments and models

Creation of super-tough ceramics is one of the main goals of materials science. Bioinspired design is shown to be the most effective method to achieve this goal. Previous studies on the mechanical performance of biological multilayered materials such as nacre have shown that their outstanding mechanical properties are direct results of the small-scale features and optimized arrangement of the elements in their microstructure. Hence, the freeze casting technique has been recently introduced as a novel method to create a new class of bioinspired polymer/ceramic composites. However, the method is cumbersome and the mechanics that controls the overall performance of these composites is not well-known. In this study, the mechanical performance of bioinspired alumina/polydimethylsiloxane (Al 2 O 3 /PDMS) and alumina/polyurethane (Al 2 O 3 /PU) composite samples with lamellar structure are experimentally and analytically investigated. Bioinspired multilayered samples with micron-size layers are fabricated using the challenging freeze casting technique. Different parameters such as solution concentration, freezing rate, and sintering temperature affect the structure, and subsequently, the mechanical performance of these multilayered materials. Moreover, in order to fully understand the underlying toughening and deformation mechanisms, a micromechanics model of the mechanical response of lamellar composites is presented. The closed-form solutions for the displacements of the layers as a function of constituent properties are derived to calculate the mechanical response of lamellar structured composites such as elastic modulus, strength, and tensile toughness. The experimental results agree well with the proposed analytical models. Fracture mechanics tests are also used to study the Resistance-Curve (R- Curve) behavior of the samples. Furthermore, important toughening mechanisms in these samples are discussed and governing equations for fiber bridging and fiber pull-out in lamellar ceramic/polymer composites are presented. Finally, detailed material design relationships are derived to identify future directions in the design of next generation structural composites

Toughening mechanisms in bioinspired multilayered materials

Outstanding mechanical properties of biological multilayered materials are strongly influenced by nanoscale features in their structure. In this study, mechanical behaviour and toughening mechanisms of abalone nacre-inspired multilayered materials are explored. In nacre's structure, the organic matrix, pillars and the roughness of the aragonite platelets play important roles in its overall mechanical performance. A micromechanical model for multilayered biological materials is proposed to simulate their mechanical deformation and toughening mechanisms. The fundamental hypothesis of the model is the inclusion of nanoscale pillars with near theoretical strength ( σth ∼ E/30). It is also assumed that pillars and asperities confine the organic matrix to the proximity of the platelets, and, hence, increase their stiffness, since it has been previously shown that the organic matrix behaves more stiffly in the proximity of mineral platelets. The modelling results are in excellent agreement with the available experimental data for abalone nacre. The results demonstrate that the aragonite platelets, pillars and organic matrix synergistically affect the stiffness of nacre, and the pillars significantly contribute to the mechanical performance of nacre. It is also shown that the roughness induced interactions between the organic matrix and aragonite platelet, represented in the model by asperity elements, play a key role in strength and toughness of abalone nacre. The highly nonlinear behaviour of the proposed multilayered material is the result of distributed deformation in the nacre-like structure due to the existence of nano-asperities and nanopillars with near theoretical strength. Finally, tensile toughness is studied as a function of the components in the microstructure of nacre