In temperate European climates, the season of peak water demand by vegetation (summer) is out of phase with the season of greatest rainfall (winter). This results in seasonal fluctuations in soil water content and, in clay soils, associated problems of shrinking and swelling that can in turn contribute to strain-softening and progressive slope failure. This paper presents field measurements of seasonal moisture content and pore water pressure changes within the surface drying zone of a cut slope in the London Clay at Newbury, Berkshire, UK. A climate station was installed at the site to measure the parameters needed to determine specific plant evapotranspiration. This information was used to carry out a water balance calculation to estimate the year-round soil moisture deficit caused by the vegetation. The calculated soil moisture deficit matches reasonably closely the field measurements of soil drying. The field measurements of seasonal changes in pore water pressure and suction are linked quantitatively to the measured changes in water content using the soil water characteristic curve for the London Clay. The suctions generated by the light vegetation cover at Newbury were found not to persist into the winter and early spring.KEYWORDS: clays; field instrumentation; monitoring; pore pressures; slopes; suction Dans les zones européennes de climat tempéré, la période de demande d'approvisionnement en eau la plus importante pour la végétation ne correspond pas à la saison de pluviosité la plus forte (hiver). Il en résulte des fluctuations du taux d'humidité dans le sol, et, pour les terrains argileux, des problèmes associés de gonflement et rétré-cissement qui peuvent, à leur tour, contribuer à un écrouissage négatif et une fracture progressive du versant. Cet article présente des expérimentations réalisées in situ pour mesurer le taux d'humidité et les variations de pression hydrique interstitielle saisonniers à l'intérieur de la zone d'assèchement de la surface d'un versant découpé dans le London Clay à Newbury, dans le Berkshire, au Royaume-Uni. Une station climatologique a été installée sur le site afin de relever les paramètres néces-saires pour déterminer l'évapotranspiration végétale spé-cifique. Ces informations ont été utilisées pour calculer le bilan hydrique et estimer ainsi, sur une année, le déficit hydrique que la végétation provoque au niveau du sol. Les résultats de ces calculs en matière de déficit hydrique montrent une corrélation raisonnablement étroite avec les mesures de l'assèchement du sol in situ. Pour la succion et la pression interstitielle de l'eau, les mesures des variations saisonnières, relevées in situ, sont qualitativement liées aux variations de teneur en eau obtenues à l'aide de la courbe caractéristique d'humidité du sol pour le London Clay. Nous avons pu observer que les succions produites par la légère couverture végétale à Newbury ne persistent pas en hiver et au début du printemps.

Seasonal cycles of soil water content cause shrinking and swelling in clay soils, which can in turn contribute to strain-softening and progressive slope failure. This paper presents and analyses six years of field measurements of soil water content and pore water pressures in the upper layers of a lightly vegetated London Clay slope near Newbury, UK, and shows how they can be related quantitatively to the climate using a water balance model. The field observations are set in the context of a 40-year run of rainfall data for the site. Moderately extreme rainfall and drought events were experienced over the period [2003][2004][2005][2006][2007][2008], allowing almost the full variation in likely pore water pressures to be characterised. Pore water pressures were found to return to near hydrostatic during most winters. Variations in summer rainfall, particularly during June-August, are shown to have a large influence on the magnitude of the cycles of pore water pressure and effective stress. The 40-year rainfall dataset is used to calculate approximate return periods for the observed soil conditions, and provides a benchmark for calculating the impacts of expected climate change on similar sites.

Terrestrial laser scanning (TLS) has been used widely for various applications, such as measurement of movement caused by natural hazards and Earth surface processes. In TLS surveying, registration and georeferencing are two essential steps, and their accuracy often determines the usefulness of TLS surveys. So far, evaluation of registration and georeferencing errors has been based on statistics obtained from the data processing software provided by scanner manufacturers. This paper demonstrates that these statistics are incompetent measures of the actual registration and georeferencing errors in TLS data and, thus, should no longer be used in practice. To seek a suitable replacement, an investigation of the spatial pattern and the magnitude of the actual registration and georeferencing errors in TLS data points was undertaken. This led to the development of a quantitative means of estimating the registrationor georeferencing-induced positional error in point clouds. The solutions proposed can aid in the planning of TLS surveys where a minimum accuracy requirement is known, and are of use for subsequent analysis of the uncertainty in TLS datasets.

Discrete piles are used to stabilise infrastructure slopes, especially where there is insufficient additional land to allow construction of large toe berms or regrading of the slope. Compared with more conventional structures such as retaining walls, there are few field data on how discrete piles typically bend and displace under slope loading. This paper presents the results from monitoring a number of discrete piles used to stabilise a railway embankment at Hildenborough, Kent, UK. Bending deflections deduced from strain gauges are compared with the displacements and rotations measured by inclinometer tubes cast into the piles. Four years after pile installation, the piles were bending downslope over their lower halves, with little bending measured in the upper sections. Regrading of the rockfill piling platform shortly after pile construction caused some of the pile loading, with further loading caused by the continued tendency for slope movement. Analysis of the piles using a simple elastic analysis gives bending moments and displacements close to those measured.

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Terrestrial laser scanning (TLS) can record a large amount of accurate topographical information with a high spatial accuracy over a relatively short period of time. These features suggest it is a useful tool for topographical survey and surface deformation detection. However, the use of TLS to survey a terrain surface is still challenging in the presence of dense ground vegetation. The bare ground surface may not be illuminated due to signal occlusion caused by vegetation. This paper investigates vegetation-induced elevation error in TLS surveys at a local scale and its spatial pattern. An open, relatively flat area vegetated with dense grass was surveyed repeatedly under several scan conditions. A total station was used to establish an accurate representation of the bare ground surface. Localhighest-point and local-lowest-point filters were applied to the point clouds acquired for deriving vegetation height and vegetation-induced elevation error, respectively. The effects of various factors (for example, vegetation height, edge effects, incidence angle, scan resolution and location) on the error caused by vegetation are discussed. The results are of use in the planning and interpretation of TLS surveys of vegetated areas.

Seasonal shrinkage and swelling of clay fill railway embankments can disturb the track geometry, resulting in train speed restrictions that disrupt normal operations. Such movements are exacerbated by vegetation, but reliable analytical descriptions of the effects of trees on embankment behaviour are not yet established. This paper presents and analyses the results of a field experiment, carried out on a heavily vegetated clay railway embankment to investigate quantitatively the influence of trees. After the first year of monitoring, the mature trees initially present on the upper two-thirds of the embankment slopes were removed. The field monitoring data are used to assess and understand the mechanisms of soil water content and pore water pressure changes before and after tree removal, and their influence on the vertical and lateral displacements of the embankment slopes. Removal of the vegetation stopped seasonal volume changes in the clay fill at the crest of the earthwork, but also resulted in the loss of the deep-seated suctions generally beneficial to embankment stability. The wider implications for the management of vegetation on embankment slopes are discussed.

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