Joao Mendes

Soil suction monitoring for landslides and slopes

Abstract Rainfall is the most frequent triggering factor for landslides and the development of early warning systems has to take account of this. It is suggested that direct measurement of pore pressure gives the most reliable prediction of failure of a slope. The amount of rainfall can be only a crude indicator of failure as the processes that occur between rain falling on a slope and the resulting pore water pressure change are complex, highly non-linear and hysteretic. The paper describes high-capacity tensiometers developed within the EU-funded MUSE Research Training Network that have been used for measuring suctions in slopes. High-capacity tensiometers are capable of direct measurement of pore water pressure down to −2 MPa and are also able to record positive pore water pressures. Two methods of field installation are discussed; one developed by ENPC in France uses a single tensiometer per hole, and the second technique, developed by Durham University in the UK, allows multiple tensiometers to be used at different depths within a single borehole. Continuous monitoring of pore water pressure has been carried out over several months and shows the responses to climatic events.

Strength of an Australian Coal Under Low Confinement

Experimental testing of brittle rocks has shown that both brittle and ductile behaviours can be observed, depending on the level of confinement applied to the specimen. In particular, brittle rocks fail in a brittle mode as long as the confining stress falls below the Mogi line (Mogi 1966). Spalling of rocks is associated with brittle failure and is known to occur under low confinement, i.e. in the vicinity of excavation walls (Stacey 1981; Martin et al. 1999; Cai and Kaiser 2013). Indeed, at low confinement, large tension cracks may develop parallel to the excavation boundary when the stress exceeds the crack initiation threshold, which may lead to rapidly propagating instabilities and formation of thin slabs. Such slabs can represent a significant hazard to the workforce in confined mining excavations. Increasing the level of confinement modifies the nature and propagation mechanism of the cracks that develop upon loading: at high confinement, short shear cracks develop and ultimately join to form a macroscopic shear band. Martin et al. (1999) showed that a single set of Hoek–Brown parameters failed to capture the two mechanisms and they distinguished Hoek–Brown frictional (for high confinement) and brittle (for low confinement) sets of parameters. Their proposed brittle criterion falls below the frictional counterpart reflecting a reduction in strength. Recently, Kaiser and Kim (2008) and Amann et al. (2012) proposed a non-convex criterion to capture the strength under both low and high confining pressures. However, some of the data they used involved a large degree of scatter (in Kaiser and Kim 2008) or not many points were obtained in the low confining range (in Amann et al. 2012). Considering the recent findings by Kaiser et al. and the lack of data in the literature about the strength of coal under low confinement, it has been decided to conduct a series of triaxial tests in order to mitigate this gap. Gaining a better understanding of the behaviour of the coal under low confinement is highly relevant for the stability of coal mine excavations.

Investigating the impacts of climate change on slopes: field measurements

Abstract Climate change has the potential to have significant effects on the stability and serviceability of earthworks slopes, impacting on the performance of transport infrastructures. This paper describes how a unique facility for engineering and biological research was established in NE England through the BIONICS Project (Biological and Engineering Impacts of Climate Change on Slopes). It describes the building and monitoring of a full-scale embankment representative of road/rail embankments in the UK. The paper presents the results of monitoring of pore water pressure carried out between 2007 and 2009. The pore water pressures within the upper 3 m of the embankment have been largely positive, in some case approaching hydrostatic conditions after wetter periods. This is largely due to the wet nature of the summers in both 2007 and 2008 since monitoring began, as well as the application of artificial inundation using a climate control system. Negative pore water pressures (suctions) of the order of −30 kPa were recorded at greater depths below 3 m. The experimental facility provides essential field measurements that can be used to calibrate numerical models of soil responses to climatic changes and gain a better understanding of the response of engineered UK fills to climate events.

Influence of Initial Water Content on the Mechanical Behavior of Unsaturated Sandy Clay Soil

AbstractThe mechanical behavior of a sandy clay soil was investigated by a series of constant water content triaxial tests on unsaturated samples with suction measurements. The tests were carried out in double-cell triaxial cells on compacted samples and also on samples wetted and dried from the as-compacted conditions. A series of tests on saturated samples was also performed to provide a reference state for the unsaturated tests. Because the specimens were at high degrees of saturation (generally >80%), calculations based on effective stress showed a reasonable interpretation of the data for the critical state parameters M and λ. However, the intercept of the critical state line in the ν-axis (Γ) differed and increased with an increase in water content (at compaction). The data were also analyzed using the Bishop stress (p*) approach, and a better fitting was achieved in the stress plane, where it was possible to define a unique critical state line, where M was 0.91. However, in the ν-p* plane, differen...

First Saturation and Resaturation of High Capacity Tensiometers with 1.5 MPa High Air Entry Value (HAEV) Ceramic Filters

High Capacity Tensiometers, or HCTs, are sensors that can measure negative pore water pressures (soil suctions) between -1.5 and -2 MPa. To achieve such measured values, HCTs first need to be fully saturated by water. For the first saturation of an initially dry HCT, the most common procedure involves application of vacuum followed by forced flooding by pressurised water. Instead, for the resaturation of a HCT that has cavitated (and is therefore still water flooded), only application of water pressure is necessary. Typically, the procedures for the first saturation and resaturation of HCTs can last days or weeks, which hinders adoption of these devices by the geotechnical industry. In this paper, faster procedures are presented for both first saturation and resaturation of HCTs built with 1.5 MPa air entry value filters. The duration of the first saturation can be reduced to less than 24 hours if high vacuum is first applied to the ceramic filter followed by water pressurisation at about twice the air entry value of the filter. Even more, resaturation of a cavitated HCT can be achieved in less than 10 minutes by simple water pressurisation of the ceramic filter. This is however true only if the HCT is not left to dry out to the atmosphere completely and is submerged in water just after cavitation.

A Double Cell Triaxial System for Unsaturated Soils Testing

The presence of different pore fluids present in unsaturated soils (water and air) complicates the measurement of sample volume change. Since volume change of the air phase is difficult due to the compressibility of the fluid, it becomes necessary to measure sample volume change by measuring the change in volume of water in the surrounding triaxial cell. This paper introduces and assesses a new double cell triaxial cell developed by Wykeham Farrance. The design of the new triaxial system is similar to the Wheeler modified triaxial cell (Wheeler, 1988) but uses a glass inner cell wall, to avoid problems of absorption of water by Perspex. Furthermore, the cell has been designed to use a high capacity suction probe that can be fitted through the base pedestal. Issues of de-airing the cell and the accuracy of volumetric measurements that can be achieved are discussed in the paper.