## 3 Results  
### Oedometer Test Results  
We conducted the oedometer tests on compacted soil specimens in order to find the possible impact of the compaction process on the compressibility properties of the tested soil. The compaction might be responsible for the built-in vertical stress. The compressibility curves of the performed oedometer tests are shown in Figure 3. The test results showed a similar run of the stress-strain curve except for Test No. 3, where the possible inconsistency of side contact between the soil sample and steel ring occurred. The compressibility curves were recalculated to find the soil compressibility modulus \(M\) from the oedometer tests. The modulus \(M\) changed when applied stress increased as well (Figure 4). Soil samples in optimum moisture content were less stiff to the applied load than the soil specimens compacted in the dry site. This phenomenon became important when higher applied vertical stress  
Figure 4: Compressibility modulus from the laboratory oedometer test results for compacted soil samples with different moisture contents and with constant compactive effort. Figure 3: Laboratory oedometer test results on compacted soil samples with different moisture contents and with constant compaction effort. was imposed on the compacted sandy subgrade (for example, shallow foundation, or embankment, or leveee construction). The possible reason for such a response of soil specimens was the overconsolidation phenomena due to compaction. For the investigation of the overconsolidation impact on soil response to vertical applied stress, the determination of preconsolidation pressure was performed. In Figure 5, the compressibility characteristics are presented. The method of work was employed to calculate the preconsolidation pressure. The reason for that was the gentle curved \(e\)-\(\sigma^{\prime}\) characteristics where the graphic methods like the Casagrande method could not be applied with much confidence. The method of work proposed by [PERSON] et al. [34] was based on the work input in the test. The sum of work (\(\sum\)W) was the sum of finite work \(\Delta W_{i}\) in the \(i^{\text{th}}\) load step during the oedometric test. \(\Delta W_{i}\) was the product of average stress \(\sigma^{\prime}_{ang}\) on the sample (\(\sigma^{\prime}_{ang}=(\sigma^{\prime}_{i}+\sigma^{\prime}_{i+1})\)/2) and the strain difference \(\Delta\varepsilon\) (\(\Delta\varepsilon=(\varepsilon_{i+1}-\varepsilon_{i})\)). Figure 5: _Cont._  
The preconsolidation pressure \(\sigma^{\prime}{}_{c}\) differed when different moisture content test results were compared. We estimated the highest preconsolidation pressure for peak moisture content (\(\sigma^{\prime}{}_{c}\) - 460 kPa). The lowest one was observed for samples where the moisture content was equal to 8.1%, which was equal to 210 kPa. Other investigators also have concluded that compacted non-cohesive soils behave as if they were overconsolidated [35; 36; 37]. Some of them stated that the observed overconsolidation must be a function of remolding water content [38; 39]. Figure 6 presents a three-dimensional view of consolidation from the oedometer test results under constant moisture conditions during the sample compaction process equal to 4.6%, 6.1%, 9.4%, 10.8%, and 12.9%. The influence of moisture content on soil consolidation properties could be seen by analyzing the fit to the mathematical data surface. The graph surface led to the derivation of the mathematical model, which allowed calculating the void ratio in specified loading conditions and moisture content. Equation (1) presents the formula as mentioned above for calculating the void ratio \(e\), as a function of effective consolidation stress \(\sigma^{\prime}{}_{3}\) (kPa) and moisture content during the compaction process \(m\) (%) (6):  
\[e=\frac{1}{2-\frac{0.3m^{2}}{100\%}+\frac{2.\sigma^{\prime}{}_{3}^{3}0.5}{100 kPa}} \tag{6}\]  
where R\({}^{2}\) is equal to 0.956.