The issue of soil compacted state forecasting during the construction of a forest road is considered. The mechanical system formation of contacting particles of the compacted soil are given and substantiated. A model of compaction of a loose soil layer in the system "compaction device - compacted material" has been developed. A new compartment approach in this field of applied research was used, within which the system under study is subdivided into interconnected units, called compartments, the content of a certain substance in each of them is a separate variable of the system. Various technological situations are analyzed with the minimum and maximum allowable boom outreach of the excavator. The modeling of the effect of porosity on the soil layer thickness during its compaction, denser packing of particles and the appearance of residual (irreversible) deformations has been carried out. A method for determining the relationship between the degree of soil compaction and changes in its porosity and deformations has been developed. The complexity of the theoretical study of the compacted soil is substantiated. The practical significance of the developed model of soil layer compaction and the methodology for assessing its deformations has been confirmed with successful implementation at the design and monitoring stages of the construction of a pilot section of a forest road located in the Republic of Karelia on the basis of Petrozavodsk State University.