A laboratory investigation on an undisturbed silty sand from a slope prone to landsliding

Abstract

A laboratory investigation is presented for undisturbed samples of a silty sand under saturated conditions. The soil was sampled from test pits south of Rüdlingen in North-East Switzerland, where a landslide triggering experiment was carried out on a steep forest slope. The aim of the work was to characterise the behaviour of the soil in triaxial tests, in the light of the possible failure mechanisms of the slope. Conventional drained and undrained triaxial tests were conducted to detect critical state conditions as well as peak shear strength as a function of confining pressure. Soil specimens were also exposed to stress paths simulating in situ water pressure increase to study the stress-strain response and to enhance the ability to predict failure conditions more accurately in the future. Possible unstable response along the stress paths analysed was investigated by means of second order work and strain acceleration. The results show that temporary unstable conditions may be encountered for this soil at stress ratios below ultimate failure and even below critical state line, depending on void ratio, drainage conditions and time dependent compressibility. A modified state parameter is explored as a potentially useful tool to discriminate conditions leading to eventual collapse.

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

“…The behaviour of soil was investigated under saturated and unsaturated conditions (Springman et al 2009, Casini et al 2010a. The average saturated permeability of soil was determined to be k sat = 1 * 10 −7 m/s.…”

Infiltration-induced Slope Instability: a multi-scale approach

“…In order to understand the in situ triggering experiment, an extensive soil investigation was conducted to understand how the strength is mobilised in the soil and how this may be related to change in water content and void ratio in the slope [10,12]. The investigation includes tests on both undisturbed and reconstituted soil samples, in saturated and unsaturated conditions.…”

“…Mobilised shear strength has been investigated at multi-scale, with standard and modified shear box apparatuses in saturated and unsaturated conditions [13,14,15]. Unsaturated and saturated statically compacted and natural specimens have been tested under triaxial stress conditions to characterise the mechanical behaviour of the soil at different gravimetric water contents in a triaxial stress path apparatuses and to link the mechanical behaviour with the soil-water retention curve obtained under suctioncontrolled conditions with different void ratios [7,10].…”

“…The clay fraction increases with depth from 4% at shallow depths to 10% at about 2 m. The silt fraction also increases with depth from 25% to 32%, while the sand fraction decreases from 67% to 56%. The increase of the fine fraction with depth may be due to internal erosion, sedimentological and morphological reasons, and downward transport promoted by infiltrating water [10].…”

“…Some surface movements were detected during this extreme event, although failure did not occur. Subsequently, a range of measures was implemented, such as relocating the distribution of the sprinklers to provide more rainfall to the upper part of the slope, so that a failure mechanism was triggered in March 2009, incorporating about 150 m 3 of debris [1,4,10,11].…”