# Introducing Cement-Enhanced Clay-Sand Columns under Footings Placed on Expansive Soils
## 4 Results and Discussion  
### Applications of the Cement-Enhanced Clay-Sand Columns  
The targeted application of this cement treatment method is to improve the soil immediately below a foundation to an extended deep level. This is expected to transfer the superstructure loads down to a deep strata where the overburden pressure is high. The stabilized clay-sand column can transfer the contact pressure to a deep level soil within the natural expansive clay formation. The strength of the clay-sand column is lower than that of the lean concrete used to transfer the stresses to the ground. The cement-rich mix makes the soil stiffer and can safely transfer the stresses. The laboratory tests conducted in this research were aimed at investigating the effect of the cement dose, curing time and clay content on the compressibility, swelling potential and the compressive strength. The swelling pressure of the ground needs to be counterbalanced by vertical loads from the super structure and the overburden soil pressure. The study presented here is limited to the assessment of the material suggested to act as a shaft, transferring loads to the ground. Other studies on deep mixing using cement or other stabilizers for expansive soils tend to improve the nature of the supporting ground but lack efficient quality control  
Figure 3: A typical consolidation curve illustrating the key parameters (\(\mathrm{C_{c}}\) and \(\mathrm{C_{r}}\)) and basic steps of the test.  
methods and are faced with practicality issues [32; 33; 34]. Mixing procedures can be manual or mechanical at variable speeds and forces to attain a homogeneous mix. The deep mixing method proved to be successful in enhancing soft soils and increasing the bearing pressure. [PERSON] and [PERSON] [35] showed that deep mixing significantly reduced settlement by average percentages ranging from 33% to 89%, and increased the bearing capacity by up to 2.43 times for soft clay soils treated with cement or lime. Also, [PERSON] and [PERSON] [36] presented deep mixing as an innovation technique to improve the performance of structures placed on expansive soils. They observed that the soil sections treated with lime and cement did not undergo any movement compared to the untreated sections. The process in this proposal is different in that all soils under the foundation need to be excavated and replaced by a controlled mixture of sand, clay and cement at specified ratios and moisture contents, then placed and compacted in layers using tampers and left to cure for at least seven days before the superstructure foundations should be placed. The selection of the stabilized shaft or column below the foundation needs to be based on successful laboratory tests to confirm an adequate strength and compressibility with a low or non-significant swelling potential. Tapering the shaft to increase the contact pressure is an option in the case where the swelling pressure of the soil is high.  
Other applications of the cement-enhanced clay-sand columns include pavements, boundary walls and slab-on-grade systems. The columns can be extended to a depth at which the swelling pressure will not be a risk to the superstructure.  
In Figures 4 and 5, it was assumed that the friction between the clay-sand column and the clay is low and that no uplift can take place. This can be avoided by placing the columns in loose contact with the original soil using a flexible material or forms. Tapering is enhanced by forms which can be designed as needed.  
Figure 4: Cement-treated shaft under foundation: (**a**) Regular, (**b**) Tapered.