This work studies the effect of binder and fibre quantity on the mechanical behaviour of "Baixo Mondego" soft soil, chemically stabilised with binders and reinforced/non-reinforced with short polypropylene fibres. The experimental programme was comprised by four types of tests, one to evaluate compressive strength (unconfined compressive strength test) and three to evaluate tensile strength (direct tensile strength test, split tensile strength test and flexural strength test). The results indicate that increases in binder content cause an increment in the stiffness, the compressive and tensile strength, but have a lower impact on the specimens reinforced with fibres. In general, the addition of a low quantity of fibres to the stabilised soft soil originates a decrease in the stiffness, compressive and direct tensile strength, a reduction of the loss of strength after peak and a change in behaviour, from brittle to more ductile. The results also reveal that the impact of the addition of fibres on the strength depends on the strain mechanism used in each test. Therefore, in flexural strength tests the impact of the fibres is significant, while in the direct tensile strength tests the inclusion of fibres has a negligible effect. Relationships between the compressive and tensile strength are presented at the end of this study, as well as between the tensile strengths evaluated from the direct tensile strength test, split tensile strength test and flexural strength test.

The effect of curing time, content of grout solution and concentration of urea, calcium chloride and urease enzyme on the process of enzymatic calcium carbonate precipitation was analysed. Initially, the process was studied in test-tube experiments by evaluating the amount of calcium carbonate precipitated using X-ray diffraction tests. The method was then applied to stabilise a sandy soil to examine the strengthening effect using unconfined compressive strength tests, X-ray diffraction and scanning electron microscopy. The results showed the effectiveness of the method for improving the mechanical properties of a sandy soil. The soil strength and stiffness increased with increases in content of the grout solution and curing time, while an increase in the concentration of urease and urea–calcium chloride only had a positive effect for lower concentrations. The results also showed a relationship between unconfined compressive strength and calcium carbonate content and pH value.

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