## 2 Materials and Methods  
### Design of the Protective Case  
The formation of a suffusion-resistant waterproof layer of soil with increased strength under the tip and along the lateral surface of the pile when installing a bored pile was achieved by silicatization of the borehole during the installation (Figures 2 and 3). For this, a borehole was pre-drilled in saline silty-clayey soils to the required depth from which excess soil was extracted. The inner walls of the borehole were impregnated with a low-concentration sodium silicate solution. The chemical interaction of sodium silicate with carbonates and gypsum contained in saline silty-clayey soils leads to the formation of a viscous silicic acid hydrogel film, which cures on particles and soil aggregates and tightly clogs capillaries and pores. The formation of a silicic acid gel was confirmed by fluoroscopy and microscopy.  
silty clay and on the rapid release of the cementing soil silicic acid gel films due to the reaction of a sodium silicate solution with soil salts [31]:  
\[\text{Na}_{2}\text{O-nSiO}_{2}+\text{CaSO}_{4}+\text{mH}_{2}\text{O}\to\text{nSiO }_{2}\;(\text{m}-1)\text{H}_{2}\text{O}+\text{Ca(OH})_{2}+\text{Na}_{2}\text{SO} _{4} \tag{1}\]  
The coagulation of a silicate solution in contact with salt waste was confirmed by laboratory testing on the interaction with pure salts (CaCO\({}_{3}\), CaSO\({}_{4}\).2H\({}_{2}\)O, etc.) and led to the following conclusions:  
* The rate of coagulation of silicate solutions and the formation of SiO\({}_{2}\) gel films on salts depend on three factors: the concentration of the sodium silicate solution, and the amount and quality of salts present in the salt waste. Film formation during the interaction of a sodium silicate solution with calcium sulfate slows down with an increase in the concentration of the silicate solution, which is explained by a decrease in the solubility of the reacting salt with the increase in the concentration of silicate. The film thicknesses when using sodium silicate of 10%, 20%, and 30% concentrations were, respectively, 40, 20, and 10 microns.
* With regard to the coagulation of silicate solutions in the soil, the sulfuric acid salts of Ca\({}^{+2}\); and Mg\({}^{+2}\) are of the greatest importance as the most soluble compounds, in comparison with carbon dioxide Ca, which is almost inert.  
In addition to the formation of a gel of silicic acid and lime hydrate, exchange reactions occurred in the absorbing complex of the salt waste, and as a result, secondary products formed in an alkaline medium. Therefore, the strength of the fixed salt waste largely depends on the value of the absorption capacity of the salt waste in the natural state, which is expressed in mmol per 100 g of dry salt waste, in an alkaline solution.  
In the course of interaction within the porous media, a silicic acid gel film formed on the soil particles and capillary walls. Pores and capillaries in the nearby soil became clogged and impermeable. As the solution was injected, the thickness of the protective casing increased and the strength of the soil rose. The transition of the solution into the gel proceeded during the first 3 days, and the hardening of the gel and, accordingly, the soil mass around the pile proceeded during the next 15 days and ended after 28 days (R = 1.1-1.8 MPa).  
Then, the borehole was filled with corrosion-resistant concrete, which was achieved through modified additives that also increased the density of the concrete. Further curing of the casted concrete pile took place in a structurally and suffusion-resistant shell with increased strength, water resistance, and waterproofness. The total diameter of a bored pile casted in a protective load-bearing shell including a silicate-fixed soil layer around the pile was about 600-700 mm.