# Resilient Modulus Characterization of Compacted Cohesive Subgrade Soil
## Materials and Methods  
For the tested material, cohesive soil, standard laboratory tests were conducted to classify the soil and determine its physical properties. The tests consisted of particle size analysis, consistency limits, and the Proctor compaction test was conducted. The particle size analysis led to recognising the cohesive soil as a sandy clay (saCl), by performing tests based on sieve and aerometric analysis (the Bouyoucos method using a modification made by Casagrande), in accordance with the EUROCODE 7 [45] standard. Test results are shown in Figure 2.  
In terms of World Reference Base for Soil Resources--WRB--the tested soil can be recognised as albeluvisoil (AB). The soil was deposited by glacial processes. The sandy clay in natural conditions is unconsolidated glacial till. This type of soil shows no stratification. The liquid limit and plasticity limit test were conducted in accordance to [46]. On the basis of six sets of tests using the Casagrande apparatus with varying moisture content the liquid limit LL was established as being equal to 18.9%, classifying this soil as a clay with low plasticity. The plasticity limit PL was equal to 10.3%. The optimum moisture content was conducted using the AASHTO T99 [47] procedure. It was achieved by a compaction in the Proctor mold whose volume is equal to 2.2 dm\({}^{3}\). Standard energy of compaction, equal to 0.59 J/cm\({}^{2}\), was used. Optimum moisture content for sandy clay was equal to 10.2% and the maximum dry density at optimum moisture content reached a value of 2.09 g/cm\({}^{3}\). Table 1 presents the results of physical and mechanical tests conducted on sandy clay. Bender element (BE) and torsional shear (TS) tests in a resonant column apparatus led to the estimation of the shear modulus, Young modulus and, finally, the Poisson ratio. Figure 2: Particle size distribution of the tested soil. After determining the properties of the soil, several series of triaxial tests under cyclic loading conditions were conducted. The tests were performed on compacted sandy clay, with optimal moisture conditions, in accordance with the Proctor method. The maximum dry density of soil samples was equal to 2.09 g/cm\({}^{3}\). In this paper two kinds of tests, one using the RC device and second using the CTRX were ran. However, the tests were conducted with an attempt to maintain similar testing conditions during both measurements. Initial effective confining pressure \(\sigma^{{}^{\prime}}\)3, was set to be 45 kPa, 90 kPa, and 135 kPa during all of the conducted tests. The examined soils were initially saturated, and the B values measured in the triaxial specimens exceeded 0.95, which means full saturation of sandy clay specimens. Subsequently, the tested specimens were consolidated to the set state of stress \(\sigma^{{}^{\prime}}\)3. Repeated loading triaxial (CTRX) tests were carried out with a triaxial apparatus from GDS instruments (GDS, Hampshire, UK). The device is suitable for cylindrical soil specimens of 7 cm in diameter and 14 cm in height. Samples were fully saturated, and a B-value equal to, or greater than, 0.95 was assured at each measurement. Specimens were then subjected to isotropic effective confining pressures of 45 kPa, 90 kPa, and 135 kPa and consolidated. The cyclic-test procedure consisted of applying an average deviator stress value \(q_{m}\) superimposed to a forward-moving pulsating sine wave with constant stress amplitude \(q_{a}\). Details of the experimental design are shown in Table 2. Repeated loading triaxial tests were conducted under the consolidated-undrained (CU) conditions. The frequency used during the tests was equal to 1.0 Hz. The tests were performed in a multistage manner. After the first series of tests (10\({}^{5}\) cycles), further stages were conducted. Each stage was characterized by characteristic deviator stress \(q\) values. The cyclic stresses and initial confining pressure levels were used to define the effects of cyclic loading on soil behavior. The resonant column has a fixed-free configuration.