## 4 Results
### Hypothetical Genes  
The above results show that all three genes can stabilise soil foundations to various extents; however, in the engineering world, the tested genes are inefficient because usually only the specific area in the foundation needs stabilisation. Hence, targeted stabilisation would need to be realised using bespoke gene promotors to suit specific geotechnical contexts. Such hypothetical genes can be simulated and tested as part of the synthetic biology design process. Therefore, the model is envisaged as part of the synthetic biology design cycle. Using the data above, we can, therefore, begin to design such a system. In the proposed system, the implementation of a biological logic gate is suggested. In our natural pressure-sensitive genes, a varying relationship to pressure is observed. By engineering the cells, one can distinguish between different logical operations in response to a stimulus. In this case, a gene response is designed which is proportionally responsible to the stimulus such that as pressure subsides, gene activity does as well. In Figure 14, the expression of a hypothetical gene called D is shown; it can be seen that the gene does not respond to pressure until it increases to a threshold value of 200 kPa, after which the gene remains \"switched on\" and a constant rate of expression of 1.03 \(\times\) 10\({}^{-6}\) mM/CFU/hour is employed. We also test two further hypothetical constructs, Genes F and G, with threshold values of 400 and 600 kPa. The calcium carbonate precipitation after 7 days due to Genes D, E and F in different soil profiles are shown in Figure 15.  
This system has several advantages, which are explained as follows. Firstly, the pressure switch is activated at a threshold value and its status will not switch off after pressure within the soils diminishes leading to the maintenance of high levels of urease activity. We know, for example, that in sandy soils, water pressure drops quickly as water escapes the open pores between sand grains. However, sandy soils are also likely to be in the most need of soil improvement. Assuming that the genetic switch can operate fast enough, the water pressure beneath the applied load, therefore, does not need to stay high for long to trigger the cementation process [51; 52]. Figure 14: Hypothetical gene relationships in this study. Secondly, when a certain reinforced area is needed in multi-layer conditions, the stabilisation in this method can be more accurate by controlling the threshold value of gene expression and injection time. For example, for the soil profile with drained materials (e.g., sandy soil) at the upper layers and firm soil (e.g., silty clay) as the bearing stratum at the lower layers, the latter soil which holds the piles needs to be stabilised, but the former does not. Soon after the soil consolidation, the pore water pressure dissipates in the upper layers for a short period but remains for a period (depending on the soil condition) in the lower layers. Consequently, one can use the designed gene with a threshold value relative to the kept pore water pressure and inject it after a short period of consolidation to make the CaCO\({}_{3}\) precipitation occur only in the bearing stratum. Finally, for the complex engineering projects with different positions to be stabilised to different degrees, two or more types of synthetic genes with different threshold values can be utilised and injected into the different matched positions, respectively; this can ensure the needed positions are stabilised with different rates and to the target degrees eventually. Based on the synthetic biology method, the soil stabilisation patterns in the three hypothetical cases mentioned above are efficient and economical, compared with the experimental genes, so it can be concluded that the synthetic genes have the potential to be used in different working conditions with a better stabilisation effect.