# Effect of Mineral Additives on the Behavior of an Expansive Soil for Use in Highway Subgrade Soils
## 4 Results and Discussion
### Field Tests and Observations  
The effectiveness of replacing BCS with BCS stabilized with lime and natural pozzolans to control the heave of a highway embankment was investigated. The test road was constructed at the Southern Bypass of Nairobi where the soil profile is about 3 m thick black cotton soil followed by gravel soil. Figure 10 shows the construction of the highway subgrade. The excavation depth of the BCS foundation was 0.8 m; after embedding a moisture content sensor in the BCS, the 3% lime + 15% VA-stabilized BCS was backfilled into the foundation ditch. In contrast, a test road was constructed replacing the gravel soil with BCS. Before the construction of the highway embankment, settlement plates located at the edge and center of the pavement were installed.  
The moisture migration and soil movement of the foundation BCS were measured in a dry-wet cycle from 13 July 2016 to 20 November 2016. Figure 11 presents the moisture change of the foundation BCS below the treated soil cushion. There was a small change in moisture content of the BCS at the different monitoring points. When the BCS was replaced with soil stabilized with lime and natural pozzolans, the maximum decrease of moisture content located at the slope to  
Figure 10: Schematic diagram of the field test replacing BCS with 3% lime + 15% VA-stabilized BCS/gravel soil.  
Figure 9: TEM images of stabilized soil samples.  
maximum increase of moisture content located 7.5 m away from the slope toe was 2.67%. When the BCS was replaced with gravel soil, the maximum variation of moisture content located at the slope toe was 4.44 %. The moisture change in that BCS has no evident regularity and appears to be little affected by soil surface evaporation and rainfall infiltration. Therefore, moisture migration in the soil is ascribed to the uneven distribution of initial matric suction in the foundation BCS.  
The vertical displacement of the BCS foundation from July 13, 2016 to November 20, 2016 is presented in Figure 12. The settlement and heave of the BCS foundation appear alternately during the following dry season and rainy season, and the settlement shows an increasing trend. The small vertical deformation of the BCS foundation was measured on the edge of the pavement and in the center of the road. The maximum and minimum vertical deformations of the BCS foundation replacing BCS with soil stabilized with lime and natural pozzolans, both measured on the edge of the pavement, were 4 mm and 10 mm, respectively. The difference in vertical deformation of the BCS foundation increased with increasing observation time. However, the measured vertical deformation differential was not large, and the maximum value was only 12 mm. When BCS was replaced with gravel soil, a smaller vertical deformation differential of only 5 mm was measured in the BCS foundation. Small vertical deformation differentials were measured on the two test roads; one interpretation for this is that the test roads were not constructed on the main line. The vertical deformation in the embankment was not affected by traffic loading. Only rainfall and surface evaporation affected the test roads. Either soil stabilized by lime and natural pozzolans or gravel soil can prevent rainfall from infiltrating into BCS layers. Meanwhile, there was a small difference when replacing BCS with soil stabilized with lime and natural pozzolans or gravel soil. Hence, soil stabilized with lime and natural pozzolans could become one option for treating BCS.  
Figure 11: Moisture change in foundation BCS with time during the period July 2016–November 2016. Replacement of BCS with (**a**) soil stabilized with lime and natural pozzolans and (**b**) gravel soil.