# Introducing Cement-Enhanced Clay-Sand Columns under Footings Placed on Expansive Soils
## 4 Results and Discussion
### The Unconfined Compressive Strength  
#### 4.5.1 Effect of Clay and Cement Contents  
Figure 12a-d present the compressive strength versus the clay content for variable cement contents and curing times. A high strength of 2200 to 3500 kPa is observed for the 8% cement additive. Generally, the compressive strength can decrease for clay contents higher than 30%.  
Figure 11: The swelling potential versus the curing time in days for (**a**) 1% cement, (**b**) 2% cement, (**c**) 4% cement, and (**d**) 8% cement additives.  
The variation of the compressive strength of all mixtures with clay contents at all cement content amounts is shown in Figure 12a-d. From these figures, trends of increases in compressive strength with an increase in clay contents were observed up to a clay content of 30%. Beyond this clay content, the trend of compressive strength reverses and the compressive strength decreases with the increase in clay content. This is clearly attributed to the structural arrangements of the sand-expansive clay mixture particles which affect the interaction between different materials. This demonstrates a comparable pattern to the findings of [22, 39, 40]. When the clay content is small, the clay tends to be confined to the voids of the sand grain skeleton, having very limited contribution to the compressive strength response during shear testing; therefore, the compressive strength behavior is dominated by the sand grains. When the clay content is increased, sand grains are moved apart, and the clay particles fill the sand voids, which leads to stress partitioning and neither the sand nor the clay can play its own role, as they do alone. Therefore, the structural changes and set-up results in changes in the compressive strength. On the other hand, a higher percentage of clay content causes the sand grains to disperse in the fine-grained soils and the compressive strength is wholly governed by the clay material. The observed behavior mirrors that seen with [41, 42].  
Regarding the effect of cement content, trends of increases in compressive strength with the increase in cement contents are shown for all mixtures. It is noted that with 8% cement and clays of 30% to 40%, a visible peak in compressive strength appears. This is attributed to the high cement content that leads to stronger bonding and improved cohesion  
Figure 12: Compressive strength versus clay content for (**a**) 1 day of curing, (**b**) 7 days of curing, (**c**) 28 days of curing and (**d**) 90 days of curing at all cement contents.  
between the particles. The increase in cement content results in enhanced hydration reactions, creating a more solidified matrix within the mixture. Similar findings of the increase in compressive strength with the increase in cement content were reported by [43; 44; 45]. The strength gained due to adding cement is basically due to the solidification of lumps and particles as a result of pozzolanic reactions within the mixtures. Figure 13 demonstrates different views of Al-Qatif pure clay (Figure 13a1,a2) and Al-Qatif clay treated with 2% cement (Figure 13b1,b2) as viewed in SEM images with 2000\(\times\) (Figure 13a1,b1) and 15,000\(\times\) (Figure 13a2,b2) magnification. These images of SEM show the particle shape and edge-to-edge contact features of this type of clay [46]. The images show agglomerated particles containing irregularly stacked flat sheets or flakes. These flakes appear in different shapes and sizes under the microscope, with some edges more prominent than others. The most important feature is how flakes are stacked and connected at the edges [47].