1,721,012 research outputs found
Mechanical characterisation of lacustrine clay by interpreting spatial variability in CPTU measurements
Full text linkTransportation infrastructure is common in highly populated areas near the major lakes in the Swiss 'Mittelland', where extensive deposits of normally consolidated post glacial clays have formed. Construction on, or in, such soils requires careful consideration during the design process due to their compressibility, low permeability and sensitivity. Achieving a satisfactory engineering characterisation requires a range of field and laboratory tests, which may be evaluated using statistical tools. The Intraclass ratio RI and the modified Bartlett method have been employed for interpreting the variability of the undrained shear strength su from continuous CPTU measurements at the Wauwil site. Finally, comparison between measurements from two cone dimensions, of cross sectional areas of 10 cm2 and 5 cm2, has been undertaken with respect to their capacity of profile detailing based on the normalised cone penetration resistanc
Shear strength of an unsaturated silty sand
Full text linkThis paper presents a series of direct shear tests performed on a silty sand at three different gravimetric water contents. The soil was sampled from test pits south of Ruedlingen in North East Switzerland, where a landslide triggering experiment was carried out on a 37°-40° steep forested slope through infiltration of extreme artificial rainfall. The aim of this work was mainly to establish a correlation between the degree of saturation and slope stability. Direct shear tests were performed on reconstituted samples with a direct shear apparatus under undrained conditions for the water phase. Three of the samples were saturated after the shear phase in order to analyse the effect of wetting on shear strength. The results were interpreted with the assistance of a soil water retention curve (WRC) and an analytical slope stability analysis was performed to apply the laboratory results to the field experimen
Measurement of unsaturated hydraulic conductivity with the instantaneous profile method
Full text linkHydraulic conductivity is one of the most important hydraulic properties, which affects infiltration rate and pore
pressure distribution in saturated and unsaturated soils. The hydraulic conductivity of unsaturated soil is a function
of material variables describing the pore structure (e.g., void ratio and porosity), the pore fluid properties (e.g.
density and viscosity), and the relative amount of pore fluid in the porous system (e.g. water content and degree of
saturation).
The unsaturated hydraulic conductivity of a silty sand is determined in this study. The soil is from Ruedlingen
(Canton Schaffhausen, Switzerland), where landslide triggering experiments were carried out in Autumn 2008
and Spring 2009. The hydraulic conductivity measurements are conducted based on the instantaneous profile
method. This method consists of measuring the variation of the suction and volumetric water content profile
within an infiltration column as a function of time during the infiltration process. Accordingly, an infiltration
column was developed with a height of 600 mm and inner diameter of 170 mm. The suction and volumetric water
content were measured simultaneously every 100 mm in depth by small tensiometers and TDRs, respectively.
These measurements will provide enough data to correlate the hydraulic conductivity with the negative pore water
pressure and/or degree of saturation. Hydraulic conductivity k is calculated by dividing the water flow velocity by
the hydraulic gradient. The water flow velocity v is defined as the volume of water flow passing through the whole
cross-sectional area over a given time increment.
The change in the height of the soil sample during the flow of water inside is monitored using a set of
LVDTs on the upper part of the soil column, enabling the volumetric changes to be tracked during the cycles
of wetting and drying. The tests are performed on statically compacted soils with different initial void ratios.
Several cycles of wetting and drying are applied for each test and the changes in the hydraulic conductivity
function are determined. The saturated conductivity is about 10-6m/s. These values are low compared to the insitu
measurements of hydraulic conductivity carried out in the course of the landslide triggering experiments. The
insitu saturated conductivity is approximately 10-4 m/s. The differences can be explained by the influence of
preferential paths in natural soil. The water passes through macro-pores and along dead roots. The results will be
compared with empirical equations based on water retention curves and grain size distribution
Rainfall induced instabilities: a field experiment on a silty sand slope in northern Switzerland
Full text linkTwo full scale field tests were planned and performed successfully on a steep forested slope located on the east facing
banks of river Rhine in Ruedlingen, in canton Schaffhausen, northern Switzerland. The aim of the experiments was to study the
triggering mechanisms of the landslides due to rainfall. Intensive field investigations were carried out, including in-situ geotechnical
tests, characterisation of hydrological properties of the soil and reinforcing effects of vegetation, geological and hydrogeological
mapping, and subsurface investigations by means of geophysical methods. Additionally, several series of saturated and unsaturated
laboratory tests were conducted on undisturbed and disturbed samples taken from different depths from the vicinity of the selected
slope. The test site was intensively instrumented and monitored over a period of 6 months in the course of artificial rainfall and
natural precipitation. The instrumentation includes conventional and novel methods to measure pore water pressure, volumetric
water content, piezometric height, soil pressure, acoustic emissions, surface and subsurface movements, soil temperature, and
meteorological data. This paper introduces briefly the measurements and findings of this multi-disciplinary project. This paper
focuses on numerical and analytical methods used to explain the behaviour of a marginally stable slope before, and during the failure
induced by rainfall. Simple stability calculations are described that still offer realistic predictions of the status of a slope prone to
failure due to increase of the pore water pressure. The basal and lateral reinforcing effects of vegetation and unsaturated shear
strength of soil are introduced in these two and three dimensional simulations
Mountain Risks: two case histories of landslides induced by artificial rainfall on steep slopes
Full text linkMountainous 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
Modelling the interaction of the bedrock and slope in temrs of drainage and exfiltration
Full text linkWater 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
Infiltration-induced Slope Instability: a multi-scale approach
Full text linkPrecipitation, together with erosion and earthquakes, have been recognized as the main triggering factors of shallow landslides. However, there are relatively few well-documented cases where direct relationships could be established between occurrence and features of shallow landslides, the rainfall characteristics (e.g. intensity, duration) and water retention curves. A field experiment has been performed on a steep forested slope located on the east-facing banks of the river Rhine in Ruedlingen, northern Switzerland. The aim of the experiments was to study the triggering mechanisms of the landslides induced by rainfall. The pore pressure and the degree of saturation, which are linked through the water retention curve, represent two of the main variables affecting the mechanical behaviour of unsaturated soils, and their relationships to rainfall are complex. The difference in the determination of water retention curves at different scales are analysed in this paper for Ruedlingen soil together with their effects on mechanical behaviour at multi-scale
Comparison between the in situ and laboratory water retention curves for a silty sand
Full text linkAfter 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
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