Sérgio D. N. Lourenço

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.

A new procedure for the determination of soil-water retention curves by continuous drying using high-suction tensiometers

Soil-water retention curves (SWRCs) can be determined using high-suction tensiometers (HSTs) following two different procedures that involve either continuous or discrete measurement of suction. In the former case, suction measurements are taken while the sample is permanently exposed to the atmosphere and the soil is continuously drying. In the latter case, the drying or wetting process is halted at different stages to ensure equalization within the sample before measuring suction. Continuous drying has the advantage of being faster; however, it has the disadvantage that the accuracy of mass measurements (necessary for the determination of water content) is affected by the weight and stiffness of the cable connecting the HST to the logger. To overcome this problem, an alternative continuous drying procedure is presented in this paper in which two separate but nominally identical samples are used to obtain a single SWRC; one sample is used for the mass measurements, while a second sample is used for suction measurements. It is demonstrated that the new continuous drying procedure gives SWRCs that are similar to those obtained by discrete drying.

Determination of the soil water retention curve with tensiometers

An alternative technique for the determination of the soil water retention curve has recently been proposed whereby a tensiometer is used to measure soil suction and a balance to record the water content variations. The soil water retention curve is obtained by drying the soil either continuously or by stages (i.e. each drying stage is followed by an equalization period). Initial results from tests on compacted soil suggest that the relatively fast evaporation rate during continuous drying affects the water retention curve whereas the stage drying procedure provides more accurate results. Factors such as sample geometry and tensiometer position (relative to the sample) are also likely to affect the response obtained during continuous drying. These are the object of future investigation.

Atmosphere–vegetation–soil interactions in a climate change context; impact of changing conditions on engineered transport infrastructure slopes in Europe

In assessing the impact of climate change on infrastructure, it is essential to consider the interactions between the atmosphere, vegetation and the near-surface soil. This paper presents an overview of these processes, focusing on recent advances from the literature and those made by members of COST Action TU1202 – Impacts of climate change on engineered slopes for infrastructure. Climate- and vegetation-driven processes (suction generation, erosion, desiccation cracking, freeze–thaw effects) are expected to change in incidence and severity, which will affect the stability of new and existing infrastructure slopes. This paper identifies the climate- and vegetation-driven processes that are of greatest concern, the suite of known unknowns that require further research, and lists key aspect that should be considered for the design of engineered transport infrastructure slopes in the context of climate change.

Towards a Tensiometer Based Suction Control System for Laboratory Testing of Unsaturated Soils

This paper presents the development of an automated tensiometer based suction control system for testing unsaturated soil samples. The system is able to dry and wet soil, while measuring suction (pore water pressure – air pressure) and water content. The system uses air circulation within a closed loop to dry the soil, or water injection to wet the soil, to achieve the required pore water pressure while the air pressure is kept atmospheric. Pore water pressure is controlled by using a feedback computer system that dries or wets soil samples according to measurements obtained from sample-mounted high suction tensiometers. Excess moisture in the air circulation loop is captured by an in-line moisture trap consisting of a sealed cell containing a desiccant (silica gel), which is placed on an electronic balance to give continuous measurements of retained water. Changes of the sample water content are measured as the difference between the amount of water injected and that retained by the moisture trap. The system is fully automated and runs controlled by software with minimum assistance. The proposed suction control system presents advantages over the conventional axis translation technique as it avoids the need for elevated pore air pressures and hence better replicates the natural processes of wetting and drying of soils. The system was developed for use in a triaxial cell, but could also be used with other instruments and for the determination of water retention curves.

On the Measurement of Water Pressure in Soils with High Suction Tensiometers

Past studies on the use of high suction tensiometers for measuring negative pore water pressure have focused on three different aspects, namely, initial saturation of the probe, calibration over both positive and negative pressure ranges, and measurement procedures. Among these three areas, the one focusing on measurement procedures has undeservedly received less attention. Aspects related to measurement conditions during laboratory or field testing are as important as the initial pre-conditioning or calibration of the probe. According to the particular type of measurement, different aspects of the testing procedure have to be considered in order to obtain accurate readings of pore water pressure. This note presents preliminary data to highlight the importance of factors, such as measurement time, soil-probe contact, and material type, when measuring suction by means of high suction tensiometer.

Synthetic Water Repellent Soils for Slope Stabilization

Open image in new window Water repellent soils, or non-wettable soils, are described as having delayed wetting of the soil surface and water infiltration, and have been studied by soil scientists and agriculturists for decades. Soil water repellency induced by wildfire is believed to be a major trigger of post-fire debris flows, by changing the hydrological characteristics of the slopes, the rainfall infiltration is delayed, leading to the increased surface runoff and eventual soil mass movement. On the other hand, the potential applications of water repellent soils in the field of slope engineering have also been recognized recently. Due to their ability to inhibit water infiltration while remaining gas permeable, water repellent soils are considered to be promising fill materials and impermeable barriers. Soil water repellency is widely observed to occur in nature because of wildfire and organic matter, while in the laboratory, it can be induced by coating the soil particles with low surface energy substances such as silane compounds. An advantage of synthetic water repellent soils is that the level of water repellency is adjustable (from very wettable to extremely water repellent), and therefore the rate of infiltration can be controlled in various scenarios. Since intense rainfall and subsequent infiltration significantly contribute to fill and natural slope failures, water repellent soils have proved to be effective in hindering the infiltration and generation of excess pore pressure and therefore could increase the overall factor of safety during rainstorms. Landfill cover is another potential application of water repellent soils.

Wettability Assessment of an Oil Coated Soil

Soil wettability depends on the nature of the grain’s surface, existence of hydrophobic substances and can vary both spatially and temporarily. Low wettability conditions occur in both the natural and the built environment, including waste and contaminated soils. Low wettability in soils influences water infiltration, evaporation, and soil water retention. This contribution presents an assessment of wettability of oil contaminated samples recovered from South Quay, Barry Docks, UK. The Water Drop Penetration Test and the Molarity of an Ethanol Droplet tests were used to assess soil wettability. Measurements were conducted on statically consolidated samples at decreasing water contents. The results are discussed within the context of unsaturated soil mechanics and have applications in geo-environmental engineering.