Natural Reinforcement of a Fine Soil for Unpaved Forest Roads—Cyclic CBR Tests

Unpaved roads are essential for transportation infrastructure, particularly for forest industry. Traditionally, unpaved roads are composed of layers using local soils. Poor local soils need to be replaced with gravel, crushed aggregate, or a mixture of materials. Due to traffic and weather conditions, unpaved roads require frequent maintenance and repair. To reduce the amount of quality materials and the frequency of maintenance operations, reinforcements can be used (synthetic or natural). This paper focussed on the behaviour of a fine soil reinforced with natural fibres from the forest value chain (pine needles), to assess their use on unpaved forest roads. Cyclic CBR tests were carried out to assess the resilient response of the soil (unreinforced and reinforced); the tests included initial monotonic loading, followed by cyclic loading. The force–penetration response and CBR value improved with the inclusion of pine needles; the best response corresponded to a percentage of incorporation of 1% (mass). For the cyclic loading phase, the permanent displacement decreased with the number of cycles, approaching a resilient response. The reinforcement with pine needles led to an improved elastic response, represented by an equivalent stiffness modulus. The best behaviour was, again, obtained for a percentage of incorporation of 1% (mass). The addition of fibres led to reduced displacements during the test, relatively to the unreinforced soil. The results showed that for unpaved forest roads, where the investment in soil characterisation is often very limited, cyclic CBR tests can be a promising approach in obtaining design parameters.