# Innovative Quality Assessment of Pavement Subgrades Using the Glegg Impact Soil Tester
## 3 Verification of the Suitability of CIST Device for Quality Control of Cohesive Soils  
The CIST device, according to the original methodology [11], can be used to evaluate the compaction of cohesive soils for penetrations of less than 20 mm. To verify this condition and the hypothesis that the device measures CBR values identical to or correlated with those determined according to STN 72 1016 [13], laboratory measurements were carried out on the following soils: sandy clay and clayey gravel. Measurements were performed on large-scale rings with clayey soils and on the soil samples used for the CBR test (Figure 1) according to STN 72 1016. The moisture content of the clayey soil test samples ranged from  
Figure 4: Schematic of LDD 100 and views of CIST and LDD 100 measurements at the Scientific Research Workplace SvF UNIZA. Figure 3: Construction of the experimental field for research on unbonded pavement structures. 4 to 19%. For the moisture range indicated, the clayey soil samples reached a bulk density level of 1710 to 1960 kg\(\cdot\)m\({}^{-3}\). Measurements were taken for each sample on both the upper and lower surfaces each time. Figure 5 illustrates the correlation dependence of the Clegg hammer push \(y_{CIegg}\) [mm] into the clay soil samples (Figure 1) measured after its fourth fall from the CIST device and from the Clegg impact value (CIV) read from the device after the fourth fall. Figure 5 shows that a compaction of 20 mm corresponds to a CIV value of 7. According to the equipment manual, the fourth compaction value can be converted into a CBR equivalent using the following formula:  
\[CBR_{Clegg}=\left(0.24\text{-}\text{CIV}+1\right)^{2} \tag{2}\]  
where \(CBR_{Clegg}\) is the CBR value [%] evaluated according to Formula (2). For the identified threshold value of CIV = 7, we obtained the following value:  
\[CBR_{Clegg}=\left(0.24\text{-}\text{CIV}+1\right)^{2}=\left(0.24\text{-}7+1 \right)^{2}\]  
\[CBR_{Clegg}\doteq 7.2\%\]  
The dependencies of the 14 values of the correlated variables were evaluated using several forms of correlation dependencies (CD): linear, power, exponential, and polynomial. The linear (\(R\) = 0.9819) and exponential (\(R\) = 0.9890) correlation coefficients demonstrated the highest correlation coefficient value. Based on the logical premise that CIV values can take only positive values, it was possible to consider the exponential correlation dependence only. When assessing the quality of earth structures made of fine-grained soils (clays, sands, loamy soils, etc. ), we can use the measured CIV value [-] to calculate the compression \(y_{CIST}\) [mm] according to Formula (3). \[y_{CIST}=53.717\text{-}e^{-0.148\text{-}\text{CIV}} \tag{3}\]  
From Formula (3), we can determine the value of CIV from the measured value of \(y_{CIST}\) [mm] according to Formula (4). \[\text{CIV}=-\frac{ln(y_{CIST}/53.717)}{0.148} \tag{4}\]  
Another verified premise was to validate the ability of the CIST to accurately detect the effect of clay soil moisture on their CBR values. The laboratory-observed dependencies of the Clegg values on the moisture content of the tested samples are presented in Figure 6.  
Figure 5: Laboratory-determined dependence of the CIV hammer push into the samples of clay and loam subgrade of pavements. A credible assessment of objectified correlation dependencies necessitates an overview of the most commonly utilized correlation characteristics. In a broader sense, correlation is any statistical relationship between two random variables in bivariate data. It is crucial to note that correlation does not always imply causation. The degree of causal dependency is expressed through the coefficient of determination, which is a fundamental output in the regression analysis. The correlation coefficient is a statistical measure gauging the intensity of the relationship between the relative movements of the two variables. Its values range from \(-1\) to \(+1\).