## 4 Numerical Model of Bedding Slope
### Analysis of seepage Results
#### 4.3.2 Natural Slope Seepage Analysis  
Figure 10 shows the unit flow and pore water pressure for the natural slope on 27 July, with a significant change in the way rainwater infiltrates. Different from the highway slope, in the case of the natural slope, since the roadbed area is not excavated, the strongly weathered silty mudstone and the weakly weathered silty mudstone below the surface overburden are not exposed, and the rainwater has no channel for rapid infiltration from the former roadbed area. After a small amount of the rainwater recharges the surface overburden, most of the rest of the rainwater forms surface runoff that flows from above to below to the Danqing River at the foot of the slope. The change of pore water pressure inside the slope is limited, the maximum pore water pressure remains at 311.22 kPa, and no local high pressure is formed. In summary, the effect of heavy rainfall on the internal moisture distribution of the slope is not significant under natural slope conditions.  
Eight monitoring points (nodes 2341, 2347, 2344, 2343, 2342, 2349, 2346, 2348) distributed from the top to the bottom are set up at the foot of the slope, and the pore water pressure versus time curves are shown in Figure 11. At the early stages of rainfall, the pore water pressure at surface node 2341 at the foot of the highway slope and natural slope rapidly goes up to zero, reflecting the rapid saturation of the soil at the surface and the formation of surface runoff. In the middle of the rainfall, the pore water pressure at the foot of the highway slope basically remains unchanged. In the late rainfall period, the pore water pressure at the bottom of the first soft interlayer, as well as at the top and bottom of the second and third soft interlayers, in the weakly weathered zones of the highway slope rises rapidly. The pore water pressure at node 2348, which has the largest increase, grows from 130 kPa to 210 kPa. The pore water pressure at node 2344 at the top of the first soft interlayer is similarly trending upward, but by less than 10 kPa. At the end of the rainfall period, the pore water pressure difference between the top and bottom of the soft interlayers is at its maximum. However, the pore water pressure at all nodes of the natural slope, excluding the surface node 2341, is essentially stable during all rainfall periods.  
The change mechanism of pore water pressure in the highway slope is analyzed as follows: the soft interlayer in the weakly weathered zone is relatively impermeable as a water-resisting layer, and the weakly weathered silty mudstone is relatively permeable as a water-permeable layer, and the combination of the rock layers and the groundwater level reaches the conditions for the composition of typical pressurized water. Rainwater enters the slope from the fast infiltration channel in the roadbed area and then seeps downward along the permeable layer to the foot of the slope, and the infiltration and recharge of rainwater causes the pore water pressure at the bottom of the soft interlayer to increase rapidly. Although the combination of rock layers in the natural slope meets the conditions for the formation of pressurized water, the lack of water source results in constant water pressure.