# Prediction of Soil-Water Retention Curves of Road Base Aggregate with Various Clay Fine Contents
## 2 Materials and Methods  
The studied material is a kind of well-graded gravel (crushed tuff material, CTA), with a minimum void ratio (\(e_{\text{min}}\)) of 0.220 and maximum void ratio (\(e_{\text{max}}\)) of 0.386 [32]. The commercial Kaolin was used to mix the coarse-grain soil at different \(F\)Cs (\(FC=m_{f}/m_{c}\), where \(m_{f}\) and \(m_{c}\) are the dry mass of fines and coarse-grain soil) to mimic the coarse grain fouled by subgrade soil. The gradation curves of CTA and Kaolin clay are shown in Figure 1. The specimens with various \(F\)Cs were prepared, having the identical dry densities of \(\rho_{d}=2.12\) Mg/m\({}^{3}\), with a compaction degree \(D_{\text{c}}=95.1\%\) for the specimen with \(FC=0\%\). This study aims to reveal the effects of \(FC\) on SWRC when soil is the coarse grain type. Therefore, the threshold fine content (\(FC\)th), which distinguishes the ranges for the soil characterized by being coarse grain and fines, needed to be determined first. According to the method proposed by [PERSON] [33], \(FC_{\text{th}}=\frac{(1+e_{\text{max}})\cdot G_{\text{t}}}{\rho_{d}}-1\) (where \(e_{\text{max}}\) refers to the maximum void ratio of the CTA; \(G_{\text{s}}\) refers to the specific density of CTA; and \(\rho_{d}\) refers to the total dry density of the specimen). For the tested soil, \(FC\)th is \(8\%\). In the tests, the CTA was mixed with fine contents of \(0\%\), \(2\%\), \(4\%\), \(6\%\), and \(8\%\). For specimens with \(FC=0\%\), \(4\%\) and \(8\%\), the test results were used to study the evolution of PSD (including pores formed by coarse-grain soil and fines) when \(FC\) increases. Using three \(F\)Cs allowed us to see the evolution of PSD with \(FC\), since we include the minimum amount of fines (\(FC=0\%\)), medium amount of fines (\(FC=4\%\)) and maximum amount of fines (\(FC=8\%\)). In addition, the specimens with \(F\)Cs of \(2\%\) and \(6\%\) were used to manifest the validation of the proposed SWRC model. In the water retention tests, a large-scale SWRC test apparatus (LSSTA), which was designed especially for coarse-grain soils, was employed to test the SWRC of soils with different \(F\)Cs. A sketch diagram of the LSSTA is shown in Figure 2. The LSSTA is composed of the following parts: pneumatic controllers, a back pressure controller and a ceramic plate (with an air entry value of 300 kPa), which are used to initialize the matric suction in specimens; an axial displacement transducer, axial stress transducer and air pressure transducer, used to measure the variation of volumetric strain, air pressure and axial stress  
Figure 1: The gradation curve of CTA and Kaolin clay. during the test; an air flushing device, used to flush the pore air generated during the test and improve the accuracy of the measurements of the water volume variation in soil; and a data logger and computer, used to record the data generated during the test. Detailed descriptions of the LSSTA can be found in the work of [PERSON] et al. [34]. The cylindrical specimen for water retention tests was 150 mm in diameter and \(75\) mm in height, prepared by the moisture tamping method in three layers. For specimens with different fine contents, the dry density of the soil was kept constant, \(\rho_{d}=2.12\) Mg/m\({}^{3}\). After transferring the specimens into the LSSTA, the specimens were submerged in de-air water for 48 h to achieve a saturated state (\(s=0\) kPa). A net axial stress of 3 kPa was applied to the top of the specimen to ensure a good contact between the loading cap and specimen. This was carried out to accurately measure the volumetric strain in the test. The net axial stress was kept constant during the test. Then, we increased the matric suction to the prescribed value based on the axis-translation method, \(s=u_{a}-u_{w}\). \(u_{a}\) is the pore air pressure which is initialized by the pneumatic controller; \(u_{w}\) is the pore water pressure which is initialized by the back pressure controller.