Je Norris

Root Reinforcement by Hawthorn and Oak Roots on a Highway Cut-Slope in Southern England

Highway embankments and cutting slopes in the United Kingdom, particularly in the South East of England, are often constructed of or within stiff over-consolidated clays. These clays are prone to softening with time leading to shallow slope failures and costly repairs. Reinforcement by natural vegetation is potentially a cost-effective method of stabilising these types of slopes over the medium–long term. However, there is a lack of information on how natural vegetation reinforces and stabilises clay slopes. To investigate this problem, the potential reinforcement of selected oak (Quercus robur L.) and hawthorn (Crataegus monogyna Jacq.) roots was assessed by conducting in situ root pull-out experiments on a London Clay cutting in south-east England. Pull-out tests were carried out using specifically designed clamps and either a hand pull system with a spring balance and manual recording of force for oak roots or a jacking system with electronic data logging of applied force and displacement for hawthorn roots. Oak roots had a mean pull-out resistance of 7 MPa and that of hawthorn roots was 8 MPa. The electronic data logging of applied force (pull-out resistance) and displacement of the hawthorn roots provided additional data on the failure of branched roots which could be correlated with variations in root morphology. The failure of the roots can be categorised into three modes: Type A: single root failure with rapid rise in pull-out resistance until failure occurs; Type B: double peak failure of a forked or branched root and Type C: stepped failure with multiple branches failing successively. The different types of root–soil bonds are described in relation to root anchorage and soil stability.

How vegetation reinforces soil on slopes

Once the instability process e.g. erosion or landslides has been identified on a slope, the type of vegetation to best reinforce the soil can then be determined. Plants improve slope stability through changes in mechanical and hydrological properties of the root-soil matrix. The architecture of a plants root system will influence strongly these reinforcing properties. We explain how root morphology and biomechanics changes between species. An overview of vegetation effects on slope hydrology is given, along with an update on the use of models to predict the influence of vegetation on mechanical and hydrological properties of soil on slopes. In conclusion, the optimal root system types for improving slope stability are suggested.

Site Investigation for the Effects of Vegetation on Ground Stability

The procedure for geotechnical site investigation is well established but little attention is currently given to investigating the potential of vegetation to assist with ground stability. This paper describes how routine investigation procedures may be adapted to consider the effects of the vegetation. It is recommended that the major part of the vegetation investigation is carried out, at relatively low cost, during the preliminary (desk) study phase of the investigation when there is maximum flexibility to take account of findings in the proposed design and construction. The techniques available for investigation of the effects of vegetation are reviewed and references provided for further consideration. As for general geotechnical investigation work, it is important that a balance of effort is maintained in the vegetation investigation between (a) site characterisation (defining and identifying the existing and proposed vegetation to suit the site and ground conditions), (b) testing (in-situ and laboratory testing of the vegetation and root systems to provide design parameters) and (c) modelling (to analyse the vegetation effects).

Review of in situ shear tests on root reinforced soil

Bioengineering and the quantification of the effects of vegetation in civil engineering are still relatively new concepts in the United Kingdom. This paper reviews the development and use of in situ shear box techniques to measure the shear strength of root reinforced soil. Details of variation in design and the test procedures adapted by researchers are described and problems outlined. The review has led to the development of a 150 × 150 × 100 mm in situ shear box at Nottingham Trent University for testing root reinforced Gault Clay on a bioengineering demonstration site on the M20 motorway at Maidstone, UK.

Hazard Assessment of Vegetated Slopes

The hazard assessment of vegetated slopes are reviewed and discussed in terms of the stability of the slope both with and without vegetation, soil erosion and the stability of the vegetated slope from windthrow and snow loading. Slope stability can be determined by using either limit equilibrium or finite element stability analysis methods. The limit equilibrium methods are extended to incorporate the vegetation parameters that are important for the stability of a vegetated slope. The factors that contribute to soil erosion are reviewed and the techniques for assessing and measuring the rate of soil erosion are presented. The assessment of windthrow hazards are comprehensively discussed and a mechanistic model called ForestGALES is introduced which has flexibility for testing many different forest management scenarios. The hazards presented by snow loading on forested slopes are briefly reviewed.

AN INTRODUCTION TO TYPES OF VEGETATED SLOPES

Many different types of natural and artificial slopes exist throughout the world, those that have the potential and suitability for stabilizing by vegetation