Stabilisation with cement is an effective way to increase the stiffness of base and subbase layers and to improve the rutting of subgrade. The aim of the study is to investigate the effect of different percentages of cement additives (1.5%, 3.0%, 4.5% and 6.0%) on the resilient modulus of coarse-grained soil used on road foundations. The influence of the compaction method, the standard Proctor and the modified Proctor, as well as the sample curing time is analysed. The cement addition significantly increases the resilient modulus and reduces the resilient axial strain. Extending the curing time from 7 to 28 days also improves the resilient modulus. The change in the compaction energy from standard to modified does not increase the resilient modulus of the stabilised gravelly sand due to its compaction characteristics. The test results of the resilient modulus of the gravelly sand stabilised with cement indicate the possibility of using it as a material for the road base and subbase due to meeting the AASHTO requirements. However, the non-stabilised gravelly sand does not meet the above requirements. It has been sheared during cyclic tests at the first load sequence, regardless of the compaction method.

The aim of this study was to compare the stiffness of gravelly sand under various load conditions—static conditions using the CBR test and cyclic conditions using the resilient modulus test. The tests were conducted on natural soil and soil improved by the addition of polypropylene fibers and/or 1.5% cement. The impacts of the compaction and curing time of the stabilized samples were also determined. The soil was sheared during the $M_r$ tests, even after fiber reinforcement, so the resilient modulus value for the unbound sand could not be obtained. The cement addition improved $M_r$, and the curing time also had an impact on this parameter. The fiber addition increased the value of the resilient modulus. The CBR value of the compacted gravelly sand was relatively high. It increased after adding 0.1% fibers in the case of the standard compacted samples. The greater fiber addition lowered the CBR value. For the modified compacted samples, each addition of fibers reduced the CBR value reduced the CBR value. The addition of cement influenced the CBR increase, which was also affected by the compaction method and the curing time. The addition of fibers to the stabilized sample improved the CBR value. The relationship $M_r=f\left(CBR\right)$ obtained for all data sets was statistically significant but characterized by a large error of estimate.

The study aimed to present the differences in the mechanical characteristics of gravelly sand after adding a small amount of cement and/or fibre reinforcement. The tests were conducted on compacted non-cohesive soil – the gravelly sand or gravelly sand improved with 1.5% cement addition. Polypropylene fibres of 18 mm length were used, added in amounts of 0.1%, 0.2% and 0.3% to the dry mass of the soil. Gravelly sand was compacted at the optimum water content to the maximum dry density using the standard Proctor energy. The soil samples were sheared under triaxial compression conditions. Adding fibres changed the behaviour of the sample towards plasticisation of the material. The fibre content increases the maximum shear strength, which is obtained under a greater relative deformation. The failure mode of the lightly cement-bonded soil is characterized as a semi-brittle material, whereas that of the uncemented gravelly sand is the ductile mode. The failure plane of the soil with added fibres is close to horizontal, which means that the fibre-reinforced material has entered the plastic zone.