M. F. Bransby

Resistance of simple plant root systems to uplift loads

Plant root systems frequently permeate both natural and engineered soil slopes, influencing slope stability via mechanical reinforcement and soil drying. These root systems are often loaded by external forces during slope movements and when plant stems are subject to animal foraging or wind gusts. A series of physical model tests were conducted to examine how root geometries, root properties, and soil effective stress states affect the pullout capacity of simple unbranched model roots. Lengths of wood, rubber, and real roots were pulled from dry and partially saturated sand. The tests revealed the importance of the root to soil stiffness ratio during progressive failure, the mechanical properties of soil (and interfaces) at low effective stresses, the root diameter, and the tortuosity of the root material. Scaling issues due to shear banding are more important, and effective stresses under wet conditions are smaller than in conventional geotechnical practice because roots have a relatively small diameter ...

Pipeline plough performance in sand waves. Part 1: model testing

Offshore pipelines are often buried in the seabed by ploughing a trench, placing the pipe at the base, and then backfilling. The ploughing operation is critical in terms of cost and project time, with increased risk due to uncertain soil conditions or geohazards. One problem that can be encountered is the presence of sand waves or megaripples on the seabed surface. This may affect the progress of the plough, prevent the plough from generating a level trench or modify the size of the spoil heaps for backfilling. These aspects have been investigated by conducting a series of small-scale model tests in the laboratory. These have revealed information about the plough kinematics and the resulting trench conditions when ploughing in sand waves with different wavelengths and amplitudes. It is shown that it may be possible to plough through regions of sand waves and estimate likely plough performance by knowing the sand wavelength and amplitude relative to the plough size.

Mechanics of root-pullout from soil: A novel image and stress analysis procedure

When plants are loaded by external forces, whether they be above ground e.g. wind or canopy weight, or from within the soil e.g. soil displacement on slopes, the roots will be mechanically loaded. Exactly how the plant roots carry loads during these events is unknown because of their complex morphology and the heterogeneity of the root properties. To gain greater insight into plant root–soil mechanical interactions, a series of tests have been carried out to investigate the mechanical behaviour of roots and rubber root-analogues under tension during pull-out from soil. The results of the mechanical tests are augmented by a novel use of image analysis (specifically Particle Image Velocimetry) of sequential digital photographs taken during loading. This allows root and soil movements to be measured during the tests so that more can be learned about the effects of root morphology on the load distribution and deformation behaviour. The testing methodology and philosophy are presented here together with preliminary results.

Caisson Foundations Subjected to Reverse Fault Rupture: Centrifuge Testing and Numerical Analysis

Recent large-magnitude (M>7) earthquakes have caused numerous failures induced by surface faulting, demonstrating the need to account for tectonic deformation in seismic design. Thanks to their usually high rigidity, embedded (e.g., caisson) foundations may divert the fault rupture and lead to favorable performance, whereas surface or piled foundations may fail. We present a series of centrifuge model tests to investigate the response of caisson foundations embedded in a cohesionless soil stratum, the base of which is subjected to reverse faulting. We elucidate the interplay between the propagating fault rupture and the caisson, focusing on the role of the location of the outcropping rupture relative to the caisson. The rigid-body of the caisson causes diversion and/or bifurcation of the shear localization, which is forced to develop preferentially around the edges of the caisson. The observed failure pattern and the consequent caisson response depend strongly on the exact caisson position relative to the...

Novel biomechanical analysis of plant roots

The mechanical behaviour of individual roots and their interaction with soil controls plant anchorage and slope stabilisation, and this is controlled by plant genotype. Tensile tests were performed on roots of tobacco (Nicotiana tabacum ‘Samsun’) plants with lignin biosynthesis pathways affected by down-regulating cinnamyl-alcohol dehydrogenase (CAD) enzyme production. Altering this pathway resulted in root stiffness <50% of the unmodified control, although failure stress was not different. Like most biological tissues, the roots had non-linear mechanical behaviour, were irregular in shape, and heterogeneous. Particle image velocimetry (PIV), applied for the first time to the tensile testing of materials, identified the localised strain fields that developed in roots under tension. PIV uses a cross correlation technique to measure localised displacements on the surface of the root between sequential digital images taken at successive strain intervals during tensile loading. Further analysis of root sections showed that non-linear mechanical behaviour is affected by cellular rupture, with a clear step-wise rupture from cortex to stele in some younger roots. This will affect slip planes that develop under pull-out at the root–soil interface. By assessing localised axial and radial strain along a root section with PIV, we have been able to determine the true stress that controls ultimate failure and the true stress–strain behaviour along the root length. The techniques used have clear potential to enhance our understanding of mechanical interactions at the root–soil interface. Abbreviations: CAD, cinnamyl-alcohol dehydrogenase; PIV, particle image velocimetry

Pipeline plough performance in sand waves. Part 2: kinematic calculation method

Offshore pipelines can be buried in the seabed by ploughing a trench, placing the pipe at its base, and then backfilling. The presence of sand waves or megaripples on the seabed surface can affect the progress of the plough and prevent the plough from generating a level trench with a uniform trench depth. A calculation method has been presented that makes assumptions about the motion of the plough to predict the kinematics of ploughs through regions of nonuniform seabeds. Results from the calculation methodology are compared with those from small-scale model tests with good agreement, and the detailed kinematics of ploughs are then examined. The calculation method suggests that as a plough moves through a sand-wave field, the oscillation of the plough about the skids results in the trench base being formed alternately by the share tip and heel. The new method allows prediction of likely offshore plough performance given known plough geometry, sand wavelength, and wave amplitude and may be used as a tool f...