## 9 Other Innovative Technologies in Geotechnical Engineering  
In addition to alternative materials, innovative technologies for soil modification, using nanomaterials, microorganisms (MICP--Microbially Induced Calcium-Carbonate Precipitation method, and EICP--Enzymatic Induced Calcium-Carbonate Precipitation method based on urease enzymatic acceleration), or biopolymers (e.g., lignosulfonate) are developed in geotechnical engineering. Nanomaterials include carbon nanotubes, commonly called graphene, and nanoparticles: colloidal silica (produced from saturated solutions of silicic acid), nanobentonite (a processed clay mineral formed by the weathering of volcanic ash [119]; it is characterized by a higher purity than natural bentonite [120] and higher moisture absorbency and swelling properties), and laponite (made from a synthetic sheet of silicate nanoparticles). The microscopic properties of nanomaterials, including the size, microstructure, surface effect, and rheology of the nanoparticle suspension, change the soil composition, its structure, and particle interactions. This ultimately improves the mechanical properties of the soil, including compressive strength, and mitigates or prevents soil liquefaction. Instead of traditional injection using cement and chemical grouts such as sodium silicate, acrylate, and epoxy resin, a better solution is to use nanomaterial suspensions. Carbon nanotubes, nanobentonite, and nanosilica used for soil improvement have been proven to be non-toxic to soil and groundwater [121; 122; 123], and their production process does not consume as much coal, energy, water, and aggregates, as is the case with cement. In addition, nanomaterials have a favorable price-performance ratio. Despite nanomaterials having a relatively high unit price, only a small amount of them are required to effectively reinforce soil. Appropriate stabilization is provided by 3% of carbon nanotubes by weight of soil and 5%, 10%, and 3% by weight of the pore water of colloidal silica, nanobentonite, and laponite, respectively [124]. In the last century, many results of multidisciplinary research on nanomaterials have been published, indicating their potential applications, including subsoil improvement. Currently, more quantitative studies are indicated to be needed to reliably conduct the ecological risk assessment [125; 126], improve the production process [127], and implement commercial practices [128] in order to reduce the cost of nanomaterials. The methods of soil stabilization through bioccementation include the microbially induced calcium-carbonate precipitation (MICP) method and the enzymatic induced calcium-carbonate precipitation (EICP) method based on urease enzymatic acceleration. In the MICP method, calcite precipitation occurs through the reaction of microorganisms (most often _Sporosarcina_/_Bacillus pasteurii_ bacteria) in the presence of urea and calcium chloride (or seashells dissolving in diluted glacial acetic acid instead of calcium chloride [129]), and it takes quite a long time before the natural reaction of the bioccmentation process occurs (even several weeks) [130]. In the EICP method, the same process occurs muchfaster because of the additional presence of the urease enzyme. In most cases, biocementation techniques are dedicated to noncohesive soils due to the ease of application of the suspension and its penetration through the subsoil. However, there are already studies related to cohesive soils and fly ashes [131; 132; 133; 134; 135]. Both of these methods (MICP and EICP), as an ecological alternative to cement-based soil strengthening techniques, have been the focus of research in the last decade. However, the opinions of researchers are divided on their effectiveness in the context of the following:  
* Improving the strength of the subsoil. EICP gives slightly worse final parameters than MICP. For this reason, intensive research work is ongoing on adding natural fibers to reinforced soil [136; 137]. * Time needed to achieve the reinforcement effect.