# Resilient Modulus Characterization of Compacted Cohesive Subgrade Soil
## 4 Conclusions  
The geotechnical design of pavement constructions need to also take into account the deformation properties of soils. The fundamental geotechnical concepts, like the strain level or deviator stress quantity, should be taken into consideration. The results presented in this paper illustrate the cohesive soil resilient modulus \(M_{r}\) and maximum resilient modulus \(M_{r\ max}\). The test results lead to the following conclusions:  
1. The strain observed during this test indicates three possible ways of soil the respond to cyclic loading. Under the low deviator stress amplitude, low strain accumulation is observed. In intermediate deviator stress amplitude levels, the strain accumulation was higher, the growth of strain accumulation can be observed. The plastic strain accumulation presents a characteristic growth tendency, which is caused by pore pressure development.  
Figure 12: Modulus value versus axial strain of tested sandy clay for different conditions: radial stress \(\sigma^{\prime}{}_{3}\) equal to (**a**) 45 kPa and (**b**) 90 kPa, as defined in Table 2.  
During the cyclic triaxial tests the stress path moves towards the deviator stress axis in the first few stages. When critical state of soil is achieved, the stress path starts to move in the opposite direction. The critical state did not last during the entire time of the test, which can be observed as plastic strain development during cycling. The strain rate decreases and, after numerous cycles, the purely resilient state can be observed.
3. The pore water pressure develops in a similar scenario for three radial stress test conditions. At the first stages of the tests the pore water pressure rises due to the first load cycles, which cause the greatest pore pressure build up. Later, the pore water pressure develops through further stages of cyclic loading until the critical state is achieved. When pore water pressure reaches the equilibrium state, a hardening process begins.
4. The maximum value of the [PERSON]'s modulus from RCA and TS tests for radial stress \(\sigma^{{}^{\prime}}_{3}\) equal to 45 kPa was 135.5 MPa, and for \(\sigma^{{}^{\prime}}_{3}\) equal to 90 kPa, \(E_{max}\) was 218.2 MPa.
5. The recoverable strains characterized by the resilient modulus \(M_{r}\) value in the first cycle was between 44 and 59 MPa for confining pressure \(\sigma^{{}^{\prime}}_{3}\) equal to 45 kPa, between 48 and 78 MPa for \(\sigma^{{}^{\prime}}_{3}\) equal to 90 kPa, and from 44 to 81 MPa for \(\sigma^{{}^{\prime}}_{3}\) equal to 135 kPa. The resilient modulus \(M_{r}\) decrease was caused by plastic strain development.
6. The analytical model presented in this article takes into consideration the actual values of effective stress \(p^{\prime}\) (parameter \(T\)), actual excess pore water pressure in reference to initial conditions before cyclic loading \(\Delta u_{1}\), the loading characteristics (\(q_{max}\) and \(\chi\)), and the position of the effective stress path (the \(T\) parameter).
7. The maximum resilient modulus for soil material was characterized. During cyclic loading, soil first degrades, then when \(M_{r\,max}\) reaches the plateau stage by around \(1\times 10^{3}\) to \(2\times 10^{3}\) cycles, pore pressure reaches equilibrium, and the maximal resilient modulus value starts to increase.  
[PERSON] and [PERSON] prepared the manuscript and performed the cyclic triaxial tests. [PERSON] and [PERSON] performed the TS tests. [PERSON] and [PERSON] consulted the research program and proofread the manuscript.  
The authors declare no conflict of interest.