# Permeability and Stiffness Assessment of Paved and Unpaved Roads with Geocomposite Drainage Layers
## 3 Test Methods
### Laboratory Large-Scale Horizontal Permeameter Test (HPT)  
The large-scale horizontal permeameter test (HPT) was developed to measure the saturated hydraulic conductivity of the aggregate materials under horizontal flow conditions because most permeability tests employ vertical flow, which does not accurately represent how water typically drains or flows through the pavement base layers horizontally in the field. The device can simulate direct horizontal flow situations under different pressure heads and is large enough (soil tank dimensions are 1 m \(\times\) 0.46 m \(\times\) 0.33 m) to effectively test multiple material layers, as shown in Figure 3. The inside wall of the HPT soil tank has several ribs (1 cm in height) installed perpendicular to the flow direction to prevent water flowing through the interface of the soil specimen and the inside wall of the soil tank.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Parameter** & **Crushed Limestone Base** & \begin{tabular}{c} **Subgrade of** \\ **Paved Sections** \\ \end{tabular} & \
\begin{tabular}{c} **Unpaved Road** \\ **Surface Aggregate** \\ \end{tabular} &
\begin{tabular}{c} **Subgrade of** \\ **Unpaved Sections** \\ \end{tabular} \\ \hline \hline Gravel content (\%) (\(\circ\)-75 mm) & 65.2 & 5.3 & 57.9 & 0.9 \\ Sand content (\%) (4.75–0.075 mm) & 27.7 & 39.7 & 30.3 & 39.8 \\ Silt content (\%) (4.75–0.005 mm) & 3.9 & 29.3 & 9.2 & 30.6 \\ Clay content (\%) (\(\circ\)-0.005 mm) & 3.2 & 25.7 & 2.6 & 28.7 \\ D\({}_{0}\) (mm) & 0.3 & – & 0.049 & – \\ D\({}_{30}\) (mm) & 3.6 & 0.08 & 1.731 & 0.003 \\ D\({}_{00}\) (mm) & 10.1 & 0.12 & 9.720 & 0.081 \\ Coefficient of uniformity, c\({}_{a}\) & 33.7 & – & 198.96 & – \\ Coefficient of curvature, c\({}_{c}\) & 4.36 & – & 6.31 & – \\ Liquid limit (\%) & \multicolumn{1}{c}{Non-plastic} & 33 & \multicolumn{1}{c}{Non-plastic} & 43 \\ Plastic limit (\%) & \multicolumn{1}{c}{Non-plastic} & 15 & \multicolumn{1}{c}{} & 22 \\ AASHTO classification & A-1-a & A-6(5) & A-1-a & A-7-6(10) \\ Unified soil classification system & GP-GM & CL & GP-GM & CL \\ (USCS) group symbol & Poorly graded gravel with & Sandy lean clay & Poorly graded gravel with silt and sand & Sandy lean clay \\ \hline \hline \end{tabular}
\end{table}
Table 2: Summary of the gradation, plasticity, and classifications of the geomaterials used in this study.  
To quantify the improvement in horizontal drainage offered by the geocomposite layer, representative base material (Unified soil classification system (USCS) group symbol: GP-GM) were collected from the field and compacted in the HPT soil tank with and without a layer of geocomposite. Three HPTs were conducted; one specimen consisted of the base material only, and the other two had one of the two types of geocomposite drainage materials embedded (i.e., GC-1 and GC-2). The testing specimens were compacted to the standard Proctor maximum dry unit weight of 22.0 kN/m\({}^{3}\), determined in accordance with ASTM D698 [15]. The saturated horizontal hydraulic conductivities of the three specimens were measured under four different constant water heads of 50, 100, 150 and 200 mm. In the field, the geocomposite materials are placed at the interface of material layers. However, for the HPT test, the geocomposite layer was embedded at the middle of the test specimen to prevent any boundary issues such as the water flowing through the gap between the geocomposite layer and the bottom of the soil tank.