New methods for in situ measurement of mechanical root-reinforcement on slopes

  • Gerrit Johannes Meijer

Student thesis: Doctoral ThesisDoctor of Philosophy


Vegetation can increase the resistance of slopes against landsliding. The mechanical contribution of roots to the shear strength of the soil is however difficult to measure in situ. Existing methodologies are time-consuming and therefore not suitable to quantify spatial variability on the slope. Furthermore, some existing methods, for example large in situ shear box testing, can be difficult to apply on remote sites with difficult access, e.g. steep slopes. Therefore in this thesis novel, simple and portable methods to quantify mechanical root-reinforcement in the field were developed.The ‘blade penetrometer method’, one of these new methods, was based on standard penetrometer testing but used an adapted tip shape to increase sensitivity to roots. Root depths and diameters could be quantified based on characteristics of the depth–resistance trace, both in the laboratory and in the field. Several new analytical interpretive models were developed to predict the force–displacement behaviour of roots loaded under various conditions: one assuming roots broke in tension and another assuming roots broke in pure bending. Both methods did take root–soil interaction into account. Based on these models, some roots were shown to have broken in bending and others in tension, depending on plant species and root diameter.Two new methods were developed to measure the root-reinforced soil strength directly. The ‘pin vane’ was an adaptation of a standard field shear vane, replacing the cruciform blades of the latter by prongs to minimise the effects of soil disturbance and root breakage during installation. This was one of the main problems encountered when using standard vanes in rooted soil. This ‘pin vane’ method was qualitatively shown to be able to measure the reinforcing effects of both fine and thick roots (or root analogues), both in the laboratory and the field. This method will be most useful when the strength of densely rooted surface layers is to be analysed, e.g. for erosion resistance purposes.Another newly developed shear device was the ‘corkscrew’. Rotational installation of the screw ensured minimal soil and root disturbance. During vertical extraction the root-reinforced shear strength was mobilised along the interface of the soil plug caught within the screw. The measured extraction force could be related to the reinforced soil strength. This method underestimated the strength in surface layers (especially at 0–125 mm and less so at 125–250 mm depth) but functioned well in deeper soil layers important for landsliding. Although laboratory results were promising, during in situ testing in deeper layers (>125 mm) local variation in soil stress, gravel content and water content, combined with low root volumes, made it difficult to accurately quantify the effect of the roots. Where the effect of roots was pronounced, e.g. in more heavily rooted surface layers (0–125 mm), significant positive trends between the measured soil strength and root strength and quantity were found. Measured reinforcements were small compared with various root-reinforcement model predictions but comparable to direct shear tests on rooted soil reported by others.These new methods, although still in the early stages of development, showed promising results for practical use in field conditions. The equipment was simple to use and portable, enabling measurements on sites with difficult accessibility. However, more work is required to validate the interpretive models developed and to calibrate these methods for a wider range of soil and root conditions.
Date of Award2016
Original languageEnglish
SupervisorGlyn Bengough (Supervisor), Jonathan Knappett (Supervisor), Kenneth W. Loades (Supervisor) & Bruce C. Nicoll (Supervisor)


  • Mechanical root-reinforcement
  • In situ testing
  • Soil bio-engineering
  • Slope stability
  • Corkscrew
  • Pin vane
  • Blade penetrometer
  • Vegetation

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