1. Work Area 1: Railway foundations / sub-base
Professor William Powrie and Dr Jeffrey Priest

2. Current work Influence of PSR on sub-base performance
Testing using Hollow Cylinder Apparatus (HCA)
Work being undertaken by Anna Mamou
Pore pressure response of embankments
Testing using advanced tri-axial tests
Work undertaken by Aingaa Sellaiya
Influence of suction on soil stiffness
Testing using modified Resonant column
Work undertaken by Louise Otter

3. Background: Rotation of principal stresses as train passes over
Influence of PSR on sub-base performance
Background: Rotation of principal stresses as train passes over
Adapted from Brown, S.F., “Soil Mechanics in Pavement Engineering”, Géotechnique 46, No. 3, 393.

4. Background
Modelling of track behaviour has allowed us to quantify variation in direction of major principal stress at different depths below the centre line (Ko = 1.0).
Representative stress paths can be applied to soil specimens to determine the influence of PSR on material behaviour

5. Advanced cyclic testing on soil specimens using Hollow Cylinder Apparatus

6. HCA allows both axial and shear stress to be applied to specimen independently

7. Effect of PSR at different stress ratios
Research will investigate how material properties (Clay/Sand), magnitude of stress ratios, etc. influence the effect of PSR

8. Climate change impacts on embankment behaviour
Many of the UK’s railway embankments were built >100 years ago from loosely compacted clay fill. The clay fill is highly heterogeneous and has high volume change potential.
During dry summers, embankment shrinkage causes track deformation requiring train speed restrictions. During wet winters, embankment swelling and loss of shear strength can cause major slope failures and network disruption. This has been linked to weather influencing pore water pressures within the fill.

9. Finite element modelling using Vadose/w of a site at Charing, near Ashford showed saturated and unsaturated water flow in response to a climate boundary condition.
Changes in pore pressure lead to shrink/swell behaviour of embankment

10. Laboratory testing using advanced triaxial apparatus will be conducted to investigate changes in soil behaviour as a result of cycling pore pressure

11. Influence of suction on soil stiffness
Near surface soils are likely to be unsaturated at some point during the year. In future climate scenarios, unsaturation is likely to be greater in degree and persist for longer periods
When unsaturated, pore water within the soil will be in suction. What influence will this have on soil stiffness? What factors, such as soil density, soil fabric, etc, affect it?
Brief intro to problem

12. Unsaturated Soil

13. Matric Suction

14. Degree of suction related to the water content in the soil
Degree of suction related to the water content in the soil. However, although suction increases its influence of soil behaviour will reduce
Swcc curve showing suction ranges and likely suction that may be predicted in-situ

15. Resonant Column adapted to allow both internal air pressure and water pressure to be controlled. Resonant column allows non-destructive testing of soil specimens
Modified apparatus to allow pore air pressure, and pore water to be controlled (inset shows high air entry porous stone), and made sample on pedastal. Drive head highlighted to show dynamic exciatation

16. Water content changes seem to influence G, at 8-10% water content the clay mixes are nearly saturated and so high suctions occur at small contact areas and does not influence global behaviour

17. Recent publications
Quinn D, Hayward M, Baker CJ, Schmid F, Priest J. A., Powrie W., (2010). "A full-scale experimental and modelling study of ballast flight under high speed trains." Proc. IMechE: Part F: J. Rail and Rapid Transport, 224:2, 61-74 DOI:  / JRRT294
Priest J. A., Powrie W., Yang L. A., Grabe P.J., Clayton C. R. I., (2010). "Measurements of transient ground movements below a ballasted railway line." Geotechnique, 60(9),  DOI: 0.1680/geot
Priest J. A., Powrie W., (2009). "Determination of dynamic track modulus from measurement of track velocity during train passage." J. Geotech. and Geoenvir. Engrg. Volume 135, Issue 11, pp  DOI:  /(ASCE)GT
Yang L. A., Powrie W., Priest J. A., (2009). "Dynamic stress analysis of a ballasted railway track bed during train passage." J. Geotech. and Geoenvir. Engrg. Volume 135, Issue 5,  DOI:  /(ASCE)GT
Holscher P, Werweij A, Coelho B, Kremer A, Priest J. A., Powrie W., (2009). "Field test for reduction of maintenance of railway transition zones." 17th International Conference on Soil Mechanics & Geotechnical Engineering, Alexandria, Egypt 5-9 October 2009