The influence of tree root water uptake on the long term hydrology of a clay fill railway embankment

This paper uses a numerical model to investigate the influence of tree root water uptake and tree removal on pore water pressures and the vertical movement of a clay fill railway embankment. Simulated results of soil wetting and drying are compared with field measurements from an instrumented railway embankment before and after tree removal. A parametric study compares the influence of vegetation on the seasonal movement of the embankment slope. The simulations and field measurements show that while trees cause significant seasonal variations in pore water pressure and water content near the soil surface, they can maintain persistent soil suctions at depth within the tree rooting zone. Demonstration of this result using a numerical model requires a root water uptake function that separates spatially the processes of water infiltration, evaporation and transpiration. When all of the trees are removed, the persistent soil suctions established by the trees are lost as water infiltrates from the soil surface. Leaving the trees in place over the bottom third of the slope can maintain persistent suctions at the slope toe, while potentially also reducing seasonal ground movements at the crest that may adversely affect railway track geometry

Mechanisms of root reinforcement in soils:An experimental methodology using four-dimensional X-ray computed tomography and digital volume correlation

Vegetation on railway or highway slopes can improve slope stability through the generation of soil pore water suctions by plant transpiration and mechanical soil reinforcement by the roots. To incorporate the enhanced shearing resistance and stiffness of root-reinforced soils in stability calculations, it is necessary to understand and quantify its effectiveness. This requires integrated and sophisticated experimental and multiscale modelling approaches to develop an understanding of the processes at different length scales, from individual root-soil interaction through to full soil-profile or slope scale. One of the challenges with multiscale models is ensuring that they sufficiently closely represent real behaviour. This requires calibration against detailed high-quality and data-rich experiments. This study presents a novel experimental methodology, which combines in situ direct shear loading of a willow root reinforced soil with X-ray computed tomography to capture the 3D chronology of soil and root deformation within the shear zone. Digital volume correlation (DVC) analysis was applied to the computed tomography (CT) dataset to obtain full-field 3D displacement and strain information. This paper demonstrates the feasibility and discusses the challenges associated with DVC experiments on root-reinforced soils

Modelling of stress transfer in root-reinforced soils informed by four-dimensional X-ray computed tomography and digital volume correlation data

Vegetation enhances soil shearing resistance through water uptake and root reinforcement. Analytical models for soils reinforced with roots rely on input parameters that are difficult to measure, leading to widely varying predictions of behaviour. The opaque heterogeneous nature of rooted soils results in complex soil-root interaction mechanisms that cannot easily be quantified. The authors measured, for the first time, the shear resistance and deformations of fallow, willow-rooted, and gorse-rooted soils during direct shear using X-ray computed tomography and digital volume correlation. Both species caused an increase in shear zone thickness, both initially and as shear progressed. Shear zone thickness peaked at up to 35 mm, often close to the thickest roots and towards the centre of the column. Root extension during shear was 10-30% less than the tri-linear root profile assumed in a Waldron-type model, owing to root curvature. Root analogues used to explore the root-soil interface behaviour suggested that root lateral branches play an important role in anchoring the roots. The Waldron-type model was modified to incorporate non-uniform shear zone thickness and growth, and accurately predicted the observed, up to seven-fold, increase in shear resistance of root-reinforced soil

Evidence for non-merger co-evolution of galaxies and their supermassive black holes

Recent observational and theoretical studies have suggested that supermassive black holes (SMBHs) grow mostly through non-merger (`secular') processes. Since galaxy mergers lead to dynamical bulge growth, the only way to observationally isolate non-merger growth is to study galaxies with low bulge-to-total mass ratio (e.g. B/T < 10%). However, bulge growth can also occur due to secular processes, such as disk instabilities, making disk-dominated selections a somewhat incomplete way to select merger-free systems. Here we use the Horizon-AGN simulation to select simulated galaxies which have not undergone a merger since z = 2, regardless of bulge mass, and investigate their location on typical black hole-galaxy scaling relations in comparison to galaxies with merger dominated histories. While the existence of these correlations has long been interpreted as co-evolution of galaxies and their SMBHs driven by galaxy mergers, we show here that they persist even in the absence of mergers. We find that the correlations between SMBH mass and both total mass and stellar velocity dispersion are independent of B/T ratio for both merger-free and merger-dominated galaxies. In addition, the bulge mass and SMBH mass correlation is still apparent for merger-free galaxies, the intercept for which is dependent on B/T. Galaxy mergers reduce the scatter around the scaling relations, with merger-free systems showing broader scatter. We show that for merger-free galaxies, the co-evolution is dominated by radio-mode feedback, and suggest that the long periods of time between galaxy mergers make an important contribution to the co-evolution between galaxies and SMBHs in all galaxies.Comment: RJS and RSB are joint first authors. 12 pages, 7 figures, submitted to MNRA

Current and future role of instrumentation and monitoring in the performance of transport infrastructure slopes

Instrumentation is often used to monitor the performance of engineered infrastructure slopes. This paper looks at the current role of instrumentation and monitoring, including the reasons for monitoring infrastructure slopes, the instrumentation typically installed and parameters measured. The paper then investigates recent developments in technology and considers how these may change the way that monitoring is used in the future, and tries to summarize the barriers and challenges to greater use of instrumentation in slope engineering. The challenges relate to economics of instrumentation within a wider risk management system, a better understanding of the way in which slopes perform and/or lose performance, and the complexities of managing and making decisions from greater quantities of data

Supermassive black holes in merger-free galaxies have higher spins which are preferentially aligned with their host galaxy

Here we use the Horizon-AGN simulation to test whether the spins of SMBHs in merger-free galaxies are higher. We select samples using an observationally motivated bulge-to-total mass ratio of < 0.1, along with two simulation motivated thresholds selecting galaxies which have not undergone a galaxy merger since z = 2, and those SMBHs with < 10% of their mass due to SMBH mergers. We find higher spins (> 5{\sigma} ) in all three samples compared to the rest of the population. In addition, we find that SMBHs with their growth dominated by BH mergers following galaxy mergers, are less likely to be aligned with their galaxy spin than those that have grown through accretion in the absence of galaxy mergers (3.4{\sigma} ). We discuss the implications this has for the impact of active galactic nuclei (AGN) feedback, finding that merger-free SMBHs spend on average 91% of their lifetimes since z = 2 in a radio mode of feedback (88% for merger-dominated galaxies). Given that previous observational and theoretical works have concluded that merger-free processes dominate SMBH-galaxy co-evolution, our results suggest that this co-evolution could be regulated by radio mode AGN feedback.Comment: RSB and RJS are joint first authors. Submitted to MNRAS, 10 page

Black-wattle growth in reponse to application of nitrogen, phosphorus and potassium

Due to the lack of information about Black-wattle fertilization, this study evaluated black-wattle plants growth in response to different fertilization levels of nitrogen, phosphorus and potassium six years after implantation. The statistical design used was a randomized blocks with trifatorial distribution. Total height (m), diameter at breast height (DBH) (cm) and stem volume with bark (m³ ha-1) were evaluated. Black-wattle showed a positive and significant growth response to N and P (interaction) fertilizations and absence for K. m To obtain the maximum development of black-wattle, for the soil and climate condition studied, it is required the use of the maximum dose of nitrogen (40.0 kg ha-1 N) and 78.9 kg ha-1 phosphorus, not requiring the addition of potassium

Galaxy Zoo: evidence for diverse star formation histories through the green valley

Does galaxy evolution proceed through the green valley via multiple pathways or as a single population? Motivated by recent results highlighting radically different evolutionary pathways between early- and late-type galaxies, we present results from a simple Bayesian approach to this problem wherein we model the star formation history (SFH) of a galaxy with two parameters, [t, τ] and compare the predicted and observed optical and near-ultraviolet colours. We use a novel method to investigate the morphological differences between the most probable SFHs for both disc-like and smooth-like populations of galaxies, by using a sample of 126 316 galaxies (0.01 < z < 0.25) with probabilistic estimates of morphology from Galaxy Zoo. We find a clear difference between the quenching time-scales preferred by smooth- and disc-like galaxies, with three possible routes through the green valley dominated by smooth- (rapid time-scales, attributed to major mergers), intermediate- (intermediate time-scales, attributed to minor mergers and galaxy interactions) and disc-like (slow time-scales, attributed to secular evolution) galaxies. We hypothesize that morphological changes occur in systems which have undergone quenching with an exponential time-scale τ < 1.5 Gyr, in order for the evolution of galaxies in the green valley to match the ratio of smooth to disc galaxies observed in the red sequence. These rapid time-scales are instrumental in the formation of the red sequence at earlier times; however, we find that galaxies currently passing through the green valley typically do so at intermediate time-scales.

In situ measurements of near-surface hydraulic conductivity in engineered clay slopes

In situ measurements of near-saturated hydraulic conductivity in fine grained soils have been made at six exemplar UK transport earthwork sites: three embankment and three cutting slopes. This paper reports 143 individual measurements and considers the factors that influence the spatial and temporal variability obtained. The test methods employed produce near-saturated conditions and flow under constant head. Full saturation is probably not achieved due to preferential and by-pass flow occurring in these desiccated soils. For an embankment, hydraulic conductivity was found to vary by five orders of magnitude in the slope near-surface (0 to 0.3 metres depth), decreasing by four orders of magnitude between 0.3 and 1.2 metres depth. This extremely high variability is in part due to seasonal temporal changes controlled by soil moisture content, which can account for up to 1.5 orders of magnitude of this variability. Measurements of hydraulic conductivity at a cutting also indicated a four orders of magnitude range of hydraulic conductivity for the near-surface, with strong depth dependency of a two orders of magnitude decrease from 0.2 to 0.6 metres depth. The main factor controlling the large range is found to be spatial variability in the soil macro structure generated by wetting/drying cycle driven desiccation and roots. The measurements of hydraulic conductivity reported in this paper were undertaken to inform and provide a benchmark for the hydraulic parameters used in numerical models of groundwater flow. This is an influential parameter in simulations incorporating the combined weather/vegetation/infiltration/soil interaction mechanisms that are required to assess the performance and deterioration of earthwork slopes in a changing climate