# Experimental Study on the Mechanical Properties of Xinyang Red Clay Improved by Lime and Fly Ash
## 3 Test Results and Analysis
### Direct Shear Test  
Three samples were installed at a time, and vertical loads of 50 kPa, 100 kPa, and 200 kPa were applied. The curve of shear strength of modified red clay with the addition of lime and fly ash under each vertical load is shown in Figure 4. The shear strength indexes of cohesion and internal friction angle are shown in Table 4. As can be seen from Figure 4, the shear strength of the modified soil is lower than that of the remolded red clay when the lime content is 2% and the fly ash content is 5%, with the vertical loads at 50 kPa and 100 kPa. The shear strength of the modified soil increases with the addition of lime and fly ash. The shear strength of the modified soil with a single lime addition of 5% reaches the maximum under each vertical load. The shear strength of the modified soil with a single lime addition is higher than that of the soil with a single fly ash addition, and then decreases with the addition of lime and fly ash. The shear strength of red clay increased with the increase of lime, fly ash, and dry soil ratio. When the ratio was 1:2:7, the shear strength reached the maximum.  
Figure 4: Curves of direct shear strength with the addition of modified materials: (**a**) single lime, (**b**) single fly ash, and (**c**) mixed lime and fly ash.  
Figure 3: Compaction test results: (**a**) single lime, (**b**) single fly ash, and (**c**) mixed lime and fly ash.  
From Table 3, it can be seen that when only lime is added and the lime content is 5%, the cohesion reaches the maximum value of 43 kPa and the internal friction angle is 14\({}^{\circ}\), which is 0.43 times and 0.16 times higher than the cohesion and internal friction angle of the red clay before improvement, respectively. When only fly ash is mixed and the fly ash content is 15%, the cohesion is 36 kPa at maximum and the internal friction angle is 12\({}^{\circ}\); the cohesion is 0.2 times higher and the internal friction angle is 4% higher than before the improvement. When the ratio of lime: fly ash: dry soil is 1:2:7, the maximum cohesion is 46 kPa and the angle of internal friction is 12\({}^{\circ}\); the cohesion of laterite increases 0.53 times compared to before improvement, and the angle of internal friction increases only 2.4%. It can be seen that the improvement of red clay with lime and fly ash has a significant effect on the shear strength index of cohesion, but has no significant effect on the internal friction angle.  
After the addition of quick lime, the Ca\({}^{2+}\) produced by the hydration reaction exchanges with low-price cations on the soil surface, making the binding water film on the surface of soil particles thinner, shortening the distance between soil particles, increasing the force between particles, and making the overall structure denser, which is shown as an increase in shear strength on a macro scale. However, when the proportion of lime is higher, more free water in the red clay will be consumed, which makes the thickness of the bonded water film on the surface of the soil particles smaller, and weakens the interaction between the particles. After adding a certain amount of fly ash, the generated active substance is wrapped on the surface of fly ash and red clay particles, which promotes the formation of small aggregate structure; small aggregate structure forms the large aggregate structure, cementing each other, reducing pores, increasing the interaction between aggregate structure, enhancing cohesion, and improving the shear strength of the soil to different degrees. However, when lime and fly ash are blended separately and in larger amounts, the shear strength starts to decrease; to reveal the intrinsic mechanism, more research is needed.