**1. Introduction**

Growing interest in sustainable methods of soil improvement has led to increased interest and research in the application of microbially induced calcium carbonate precipitation (MICP). MICP is a biogeochemical process which can be used to improve the mechanical properties of loose, saturated sand, by increasing its strength and sti ffness and reducing its tendency to dilate [1]. MICP may be used as an alternative to the traditional Portland cement-based method of soil cementation [2]. MICP may occur through a variety of metabolic pathways, including photosynthesis, ureolysis, ammonification, denitrification and methane oxidation [3], in addition to sulphate reduction and iron reduction [4]. Ureolysis was selected for this study. This process is dependent upon suitable bacteria and cementation media. Ureolysis increases the alkalinity of fluid in soil pore spaces as a result of the degradation of urea to carbonate and ammonium (Equation (1)), and induces calcium carbonate precipitation (Equation (2)) [5].

$$\text{CO(NH}\_2\text{)}\_2 + 2\text{H}\_2\text{O} \rightarrow 2\text{NH}\_4^+ + \text{CO}\_3^{2-} \tag{1}$$

$$\text{Ca}^{2+} + \text{CO}\_3^{2-} \rightarrow \text{CaCO}\_{3(S)}\tag{2}$$

Precipitation of calcium carbonate (CaCO3) leads to pore-filling, inter-particle binding and particle roughening; resulting in improved soil strength and sti ffness and also reduced permeability [6].

Urease positive *Sporosarcina pasteurii* (formerly *Bacillus pasteurii*) has commonly been used for studies on cementation of granular soil via MICP [3,5,7–9]. *S. pasteurii* is tolerant to a wide pH range. Stocks-fischer et al. [8] reported urease activity between pH 6 and 10, with the optimum environment being slightly alkaline. *Sporosarcina ureae* has also been shown to promote MICP via ureolysis, as demonstrated in preliminary work by Spencer and Sass [10], and by Botusharova [11]; however, the process is much slower when compared to using *S. pasteurii*. Both *S. pasteurii* and *S. ureae* will express urease, regardless of ammonia compound concentrations [12] and are spore-forming.

Recent studies have explored the enhancement of biocemented soils with additional materials such as fibres, these have focussed on mostly synthetic fibres. Fibres utilised to improve strength characteristics have included polyvinyl alcohol (PVA) [13], polypropylene (Fibermesh 150) [14], basalt fibres [15] and carbon fibres [16]. Gao et al. [17] used polypropylene fibres in an MICP based surficial treatment of sand for seepage control. Of these examples only basalt fibres are a natural material, these are however a finite resource.

This study investigates the use of jute fibres, as an innovative approach to enhance the properties of MICP treated sand using sustainable natural fibres. The use of fibres to reinforce soil is an established technique, first proposed by Vidal [18] in 1969. Jute fibres are widely used in building materials, textiles and packaging. Jute is the most common natural fibre cultivated in the world, it is biodegradable and has good tensile strength [19]. Natural fibres, such as jute, are affordable and recyclable. For structural applications, fibres may be premixed with soil during construction or incorporated *in situ* using deep mixing techniques. Fibres can be mixed with soil to construct embankment dams and other water-retaining structures to improve resistance to piping erosion, with fibre content determined by suitable piping tests [20].

This current study, which combines jute fibres and MICP, has been completed as part of an investigation into the feasibility of the use of jute and other absorbent materials to enable autonomous self-healing of biocemented sand via MICP [10]. Self-healing, in the context of construction materials, can be defined as, 'the partial or total recovery of at least one property of a material' [21]. As a further enhancement of a biocemented sand material self-healing is of interest to improve the sustainability of biocemented soil structures and reduce future maintenance and repair costs. Under loading (shear, tension, compression) the calcium carbonate binding between the silica sand particles may fail. The likely failure mechanism being a fracture within the precipitated calcium carbonate or between the precipitate and the silica sand particles [22].

Previous studies by Montoya and Dejong [1] and Botusharova [11] demonstrated that, by injecting the nutrients and precursor chemicals (cementation medium) required for the MICP process into degraded biocemented sand, strength regain could be achieved. To enable biocemented sand to self-heal, provided viable spores of *S. pasteurii* are present, would require a store of this cementation medium within the biocemented sand matrix. The development of a biocemented sand material that can autonomously self-heal is novel and has been inspired by previous studies on self-healing of cementitious materials using immobilisation, as summarised by Spencer and Sass [10]. Having identified jute as a potentially suitable carrier material for immobilisation during preliminary studies, further investigation was required on the effectiveness of jute to store cementation medium and release this to achieve self-healing.

Laboratory experiments have been undertaken to (i) determine the effect of jute fibres on the process of MICP, (ii) quantify effect of jute fibres on strength properties when incorporated into a biocemented sand material, (iii) further investigate potential for use of jute fibres in self-healing geotechnical systems.
