# Shear Strength Improvement of Clay Soil Stabilized by Coffee Husk Ash
## 3 Result and Discussion  
### Physical Properties Test  
The influence of CHA on the clay consistency limits was investigated by determining the Atterberg limits of the CHA-clay soil. The Atterberg limits give evidence of the number of clay particles present in the soil [56]. Higher PI and LL values mean that the soil has a greater tendency to swell and shrink in response to changes in soil moisture content. The Atterberg limit measurements are displayed in Figure 4. The LL is between 67.0 and 70.9%, while the PL is between 27.77 and 32.42%, resulting in a decrease in the PI of between 34.58 and 43.13%.  
The concentration of CHA improved the PL of the stabilized clay and reduced the LL. Generally, a decrease in the LL of clay indicates a decrease in the clay swelling characteristics [11]. Additionally, an improved plastic limit is probably caused by a reduction in the thickness of the diffused double layer of clay soil particles, which later increases the shearing resistance [57]. The combined alterations in the LL and PL resulted in a decrease in the soil PI with an increase in the CHA concentration. The reduction in the PI was greater with higher concentrations of CHA. The decrease in PI may indicate that the clay was deformed during CHA stabilization [1].  
The plasticity chart in Figure 5 illustrates how the addition of CHA changes the morphology of soil grain. Figure 5a,b illustrate the plot of PI to LL used to classify clay  
Figure 4: Atterberg limit of the clay–CHA mixture.  
samples according to the AASHTO and USCS methods, respectively. As the percentage of stabilizing agent applied to clay increased, the specimens moved downward and to the left from a high compressibility area to a low compressibility. Moreover, the treated samples started to shift from clay to silt soil. The overall variations in the consistency limits of stabilized clay resulted in a shift in soil classification from CH to MH and from A-7-6 to A-7-5 for the USCS and AASHTO methods, respectively. The observed results indicate that CHA stabilization caused the clay particles to form into bigger aggregates, resulting in their change to silt size particles [58]. This will probably result in a decrease in the swelling capacity of clay when exposed to water intrusion due to precipitation [1].  
Figure 6 shows the results of the specific gravity test in which it is found that the soil specific gravity reduces as the CHA concentration increases. The results indicated that the specific gravity of CHA-treated soil decreased from 2.670 to 2.571, 2.531, 2.515, 2.501, and 2.486 for 5%, 10%, 15%, 20%, and 25%, respectively. Specific gravity decreases can be attributed to lightweight material matter in soils as well as soil decomposition. The specific gravity of a particle is proportional to the mass. Moreover, this result is predicted given the low specific gravity of CHA. Materials with a low specific gravity are subject to degradation and property changes over time. Because specific gravity is a ratio of solid density to water density, the denser the substance, the larger the specific gravity [59]. Thus, the trend in the specific gravity indicated that the incorporation of substitute agents such as CHA can lower the soil specific gravity, indicating that it is a decent option for usage as a stabilizing agent in expansive soil.