## 4 Geotechnical Investigation  
The geotechnical investigation was carried out to evaluate the properties of the soil material in the foundations of the old and new roads. Considering the pavement problems, the road sections from Km 9+000 to Km 38+000 were selected for detailed field and laboratory investigations. Both undisturbed and disturbed soil samples were collected from eleven test pits and one borehole. Three main test pits were identified for the foundation, old, and new roads with loose soils. The bulk density, specific gravity, void ratio, and permeability of the collected samples were determined as per ASTM standards. In addition, the moisture content was noted using a speedy meter, and then, by laboratory testing. Moreover, to calculate the shear strength properties, direct shear tests were performed as per ASTM D3080 [54]. The direct shear tests were carried out at varying normal stresses under controlled conditions. The test was performed by deforming a specimen at a controlled strain rate on or near a single shear plane determined by the configuration of the apparatus. At least three or more specimens were tested, each under a different normal load, to determine the effects upon shear resistance and displacement [54]. The bearing capacity values were also derived from the shear strength parameters using [PERSON]'s equation. It was observed from the field observations and laboratory test results that the foundation soil consists of well-distributed gray-colored cohesive clayey soils up to twelve meters below the ground surface. The locations of the three primary samples of the soil from the pavement surface are shown in Figure 7, and the soil condition along the road is shown in Figure 8. Some of the soil testing results are shown in Table 1. The soil properties of the foundation were the same throughout the selected weak road sections. Also, the groundwater level was measured about two meters below the excavated ground surface, but some locations were waterlogged. It was further observed that natural ground and new subgrade showed a cohesion intercept. Strain hardening behavior was observed in all the soil tests, possibly due to high soil moisture content and a relatively loose state. Table 1 shows that the bearing capacities of the foundation and old road are higher than the bearing capacity of the new road. Figure 8: Road section before widening. Figure 7: Locations of soil samples. The moisture content was measured at ten locations near Km 9+000, Km 16+200, Km 23+050, Km 28+200, Km 34+000, and Km 38+000, and the results are shown in Table 2. The type of soil is mainly clayey at all the locations. At Km 38+000, the moisture content is highest for the foundation soil on the left side. On the other hand, at Km 28+200, the moisture content is lowest for the foundation soil on the left side. At Km 38+000, the moisture content for the new road is lowest compared to the moisture content measured at the same location, and this is because the soil sample has been borrowed to use as subgrade material. The direct shear test results are shown in Figures 9 and 10. The shear stress, shear displacement, and vertical height curves are plotted for the foundation, new road, and old road in Figure 9. Three soil samples were tested at three normal stresses: 104 kPa, 212 kPa, and 300 kPa. It is shown in Figure 9a that in the soil specimen of the foundation soil, the shear stress attains a peak value of approximately 219 kPa at the normal stress of 300 kPa. With further increases in normal stress the shear stress becomes constant. In contrast, the shear stress attains peak values of 92 kPa and 167 kPa at normal stresses of 104 kPa and 212 kPa, respectively. The maximum stress increases by 70% when the normal stress is increased from 104 kPa to 300 kPa. Figure 9a also shows the shear stress and shear displacement behavior of the soil sample at three normal stresses: 104 kPa, 212 kPa, and 300 kPa.