# Innovative Quality Assessment of Pavement Subgrades Using the Glegg Impact Soil Tester
## 6 Discussion  
We further consider the most important outcomes of this research to be the following:  
* As part of the verification of the suitability of the CIST device, its ability to evaluate the deformation modulus of fine-grained soils even at compressions greater than 20 mm, which is the recommended value according to the original 1980 manual for the device, was identified. The high value of the correlation (Figure 5) of the CIST hammer compression with the Clegg Impact Value (CIV) \(R\) = 0.9890 (very strong correlation--Table 1) induces the assumption that the CIST can be used to assess the quality of cohesive soils up to a compression value of 40 mm, which corresponds to a CBR of 2.2% in the case of homogeneous cohesive soils. This fact allows its applicability for subgrades with a bearing capacity characterized by a modulus of deformation from 20 MPa, which is the lower value recommended by the authors for non-motorized roads;
* Objectification of Formula (4), allowing for the back-calculation of CIV [-] from the measured value of \(y_{CIST}\) [mm];
* The dependence of \(CBR_{Clgg}\) [%] on the moisture of clay soils ranged from 5 to 19% (Figure 6) according to the following relationship: the correlation showed a very strong correlation (Table 1);
* Converting CBR values evaluated from CIST measurements to CBR values evaluated according to STN 72 1016 [13]; \[CBR_{STN72\ 1016}=0.44\cdot CBR_{Clgg}+2.53\] (8)
* In general, the Clegg methodology for CBR determination shows significantly higher values (Figure 10) than the authors' observations and the results of foreign authors [12; 20; 21; 22; 23];
* The average \(CBR_{Clgg}/CBR_{STN721016}\) ratio was found to be 1.8-fold.  
The detailed measurements of the bearing capacity of the clay subsoil at the Scientific Research Workplace were carried out using the LDD 100 and CIST devices. The detailed results of the measurements are presented in the 3D plots in Figures 12-14. The figures demonstrate that the isomorphic model of the natural bedrock with dimensions of 3.1 by 3.8 m was considerably inhomogeneous concerning the evaluated parameters. In total, 99 measurements were performed using the LDD 100 device and the CIST at the same measurement points for each series of measurements. Based on the observed values (Figures 15-17), the following correlations of the dynamic modulus of deformation \(E_{\text{vd}}\) with the Clegg Impact Value (CIV) were determined for different clay subsoil moisture \(w\).  
\[E_{\text{vd},w=17.54} =2.15\cdot\text{CIV}-0.97 \tag{9}\] \[E_{\text{vd},w=18.63} =1.97\cdot\text{CIV}-0.32\] (10) \[E_{\text{vd},w=19.08} =1.75\cdot\text{CIV}+0.64 \tag{11}\]  
All correlations from (8) to (11) demonstrated very strong correlations (Table 1). For the credibility comparison of the linear correlation dependence directions, Formulas (9)-(11) were transformed into a normalized form, i.e., the correlation dependencies passing through the number 0 (Figure 18).  
When assessing the quality of the pavement subbase, as with other construction components of the transport infrastructure, it is necessary to apply a systematic approach considering legitimate economic and environmental aspects. It is also the requirements of the users, the inhabitants of the road environment, and the economic level of society. The significant output of the authors for the systematic approach of interest was the presentation of the credibility correlation dependence of \(E_{\text{drf},2}\) determined by the static plate load test according to STN 73 6190 from the modulus of deformation \(E_{\text{vd}}\) measured by LDD 100 (Figure 19) [24].