One of the major problems in pavement design and construction is encountering with poor subgrade soils. The most common approach to solve this problem is the application of an appropriate soil improvement method. Nevertheless, most of the current soil improvement methods are involved environmental problems. Nowadays the biological methods are considered as a new and eco-friendly for soil improvement. In this study the extent of the effect of two eco-friendly methods including: (1) stabilization with MICP and (2) stabilization with biopolymer, on soil improvement were investigated individually. The efficiency of these methods were then compared with each other. In MICP-treated samples the effects of samples mixing method and relative density were evaluated. The obtained results showed that the mean values of unconfined compressive strength and resilient modulus of the samples made by combined method are respectively 50% and 13% greater than those of the samples made by permeation injection method. On the other hand the strength of the MICP-treated samples does not increase with increasing relative density steadily, unlike their resilient modulus. For samples stabilized with biopolymer, it was found that with increasing relative density, the strength and resilient modulus values increase continuously. The comparison of the results of the strength and resilient modulus tests show that, although the strength of the samples stabilized with biopolymer is about 2 times of the MICP-treated samples, their resilient modulus values do not differ significantly with values of the MICP-treated samples. In general, the resilient modulus test results in all states indicated that the resilient modulus value is dependent on the applied stress level. Also the permanent deformation test results indicated that both of the investigated stabilization method have caused significant reductions in permanent strain values of sandy samples. Keywords: Biological improvement, MICP, Biopolymer, Unconfined compressive strength test, Resilient modulus test, Permanent deformation test
