Anisotropy of fibre reinforced sands under generalised loading conditions

Ground improvement technologies are extensively applied within the infrastructure asset (e.g. retaining walls, embankments, foundations support, pavements and slopes stabilisation among others) and they have a strategic importance for the current geotechnical practice. Among them, mixing soils with tensile resistant fibres is a relatively new soil reinforcing technique which can represent a cost-effective solution for a large number of applications. The reinforcing effect of the fibres is highly anisotropic as a result of the preferential bedding induced by the mixing and compaction processes employed. This anisotropy certainly dictates the effectiveness of the technique and its disregard can be critical, if not catastrophic, in real applications where rotation of the principal stresses and strains axis almost invariably occurs within a soil mass. However, the anisotropy of this material has never been investigated and a model which is able to predict its behaviour under loading modes representative of field conditions is still not available. It follows that the field application of this reinforcing technique is currently limited to geotechnical systems of very minor importance.

This research will fill the gap between current knowledge and the needs for more extensive and important application of the technique. The anisotropy of the composite material will be experimentally investigated by using the Hollow Cylinder Torsional Apparatus (HCTA), which possesses a unique freedom to impose a variety of loading conditions in the generalised multiaxial stress space and simulate those of real geotechnical systems. A new anisotropic constitutive model for fibre reinforced soils in the multiaxial stress space will be developed based on the experimental findings. The new model will allow the use of numerical analyses for the safe design of geotechnical systems involving fibre reinforced soils. Indeed, the design of any fibre reinforced soil systems will be invariably governed by the anisotropy of the material.

The fulfilment of the objectives is crucial for the safe and reliable application of the fibre reinforced technique in important geotechnical systems. Considering the expanding market of ground improvement technologies driven by the growing urbanisation of populations, the present research is timely for boosting and encouraging the field implementation of this technique. Ground improvement technologies are undoubtedly beneficial for reducing the carbon footprint and the required energy of the construction processes: they encourage the treatment and reuse of readily available material (reducing the volume of earth movements).
Fibre reinforced soils can be a very cost-effective option for applications with particular site constraints (e.g. site accessibility or variable geometry) or where natural and/or inorganic sources of fibres are readily available. Moreover, with climate change having a major impact on the magnitude and frequency of cutting and embankment slope failures, fibre soil reinforcement is one of the most cost effective remedial and prevention measure. If compared with traditional planar reinforcement (e.g. geogrids) fibres are uniformly diffused throughout the soil and they prevent the formation of weak planes; therefore, coupling fibres with traditional planar inclusions may also be an attractive solution. Fibres can be also easily produced from either natural or recycled resources.

The direct beneficiaries from this research proposal will be practising geotechnical engineers, geotechnical contractors and their clients willing to exploit an alternative ground improvement technology. The following research impact can be foreseen:

1. Attract R&D investments for technique exploitation
The research will address the crucial issue of strength anisotropy of fibre reinforced soils in view of their field application. Such understanding will raise engineers' confidence for the safe application of this technique and thus it is expected to result in considerable industrial investment for exploiting the potential of this technique and establishing the in situ technological processes. As a witness of the increasing industrial interest on this technique, the fibre reinforced soil technique has been tried for the remediation and prevention of slope desiccation and failure in short sections of the M11 motorway and the A27. It is known that the railway embankment assets is facing the same type of problems.

2. New market for geotechnical practioners, contractors and fibre producers
There is a very wide range of potential applications for the use of fibres as reinforcement of geotechnical systems. These comprise embankments, dams or retaining walls which represent a new and vast market opportunity for civil engineering practioners and contractors. Moreover, industries involved with production of fibres and mechanical industries, which can produce machinery for cost-effectively mixing soil and fibres, will benefit from the exploitation of this technique.

3. New opportunities for geotechnical software companies
The application of fibre reinforcing technique will represent a new commercial opportunity for suppliers of geotechnical numerical software which will be required for the design of the structure serviceability.

4. Training of skilled people for non-academic professions
The present project will create a skilled researcher (the RA) who can then be employed or provide assistance for the field implementation of this technique. Since this research is pioneering this ground improvement technique, there is an obvious shortage of skills in the field and the RA can help in filling this gap by sharing his or her knowledge. The RA will be also a suitable candidate for guiding any future exploitation of the technique implementation in practical application.