Use of Alternative Materials in Sustainable Geotechnics: State of World Knowledge and Some Examples from Poland

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This article presents in a systematic way the origin of alternative materials considered for use in geotechnics, mainly in strengthening weak soils. A concise review can help designers and contractors of earthwork projects in choosing the appropriate soil additive(s) and methods, with this article also highlighting pertinent bibliographic sources regarding the various waste materials.

Abstract

Geotechnical engineering projects carried out within the framework of the low-emission economy and the circular economy are the subject of many publications. Some of these studies present the use of various waste materials, as soil additives, for improving geomechanical behavior/properties. Many of these materials are eagerly used in geoengineering applications, primarily to strengthen weak subsoil or as a base layer in road construction. Information on individual applications and types of these materials is scattered. For this reason, this article briefly discusses most of the major waste materials used for achieving weak-soil improvement in geoengineering applications, and highlights pertinent bibliographic sources where relevant details can be found. The presented list includes waste from mines, thermal processes, end-of-life car tires, chemical processes (artificial/synthetic fibers), and from construction, renovation and demolition works of existing buildings and road infrastructure. The presentation of various applications is supplemented with three dynamically developing innovative technologies based on nanomaterials, microorganisms (MICP, EICP) and lignosulfonate. In addition to the positive impact of using waste (or technologies) instead of natural and raw materials, the paper encourages the reader to ponder whether the waste used really meets the criteria for ecological solutions and what is the economic feasibility of the proposed implementations.

1. Introduction

The title of this article refers to sustainable geotechnics (and in a broader sense, geoengineering), that is, the activities in the field of civil engineering [1,2] that are undertaken in the spirit of the low-emission economy and the circular economy within the European Green Deal [3] resulting from the 2015 international climate agreement in Paris under the leadership of the United Nations. Since then, the International Organization for Standardization (ISO) has been systematically developing appropriate standards (for example, ISO 14040 [4] and ISO 14044 [5]) in established working groups, which are the basis for developing national standards, both in Europe, in the United States, and in other countries. Due to projects supported by the federal government, there is an intensive development in the field of geotechnical and geoenvironmental engineering in the United States. The Subcommittee on Geotechnics of Sustainable Construction under the American Society for Testing and Materials (ASTM) International has developed five active standards related to the recycling and reuse of materials derived from scrap tires (e.g., tire shreds and tire chips), fly ash, foundry sand, asphalt shingles, and recycled aggregate base material. More standards are in preparation. In addition, engineers can use the Industrial Waste Guide [6] published by the USEPA (United States Environmental Protection Agency) updated every few years.

In the context of sustainable geotechnics, engineering activities are related, among others, to the use of waste materials in soil mixtures, soil–cement mixtures, or in the form of independent subbase layers (e.g., a layer of rubber tires) and the development of technology for works with the minimum possible carbon footprint. Many books and articles have been published in this area. Most of them are dedicated to one specific group of waste materials, but not necessarily in geotechnical engineering applications. On the other hand, there are review publications [7], mainly on natural waste materials, whose application in geoengineering in most cases has not gone beyond laboratory investigations [8,9]. In this case, one of the main obstacles is basically the complete lack of legal regulations related to the use of these materials in engineering practice [9]. The work of Jastrzębska et al. [10] is of a slightly different nature, in which the authors synthesize selected waste materials, which they call alternative materials, that is, materials that have lost their status of waste and can be used as substitutes for traditionally used materials. At the same time, the appropriate legal regulations that are in force in Poland are indicated.

The presented article builds on the work of [10] and, at the same time, is an attempt to answer the question of whether the various waste materials used actually meet the criteria for ecological solutions.

2. Legal Conditions in Poland

The use of waste materials is regulated by a number of laws and regulations, and by appropriate standards and quality certificates. In Poland, these include, among others, the following:

• Waste Act [11];
• Regulation of the Minister of Climate on the Waste Catalog [12];
• Act for the Prevention and Repair of Environmental Damage [13];
• Construction Law Act [14];
• The Geological and Mining Law Act [15];
• Documents authorizing products to be placed on the market and used in construction—European Technical Assessment (ETA) or National Technical Assessment (NTA).

The use of any waste material must meet legal and environmental requirements, and, at the same time, meet the criteria of applicable standards or guidelines for specific use in construction (an important role of accredited laboratories). It is worth noting that most countries in the world develop their own standards and regulations, but always on the basis of ISO (International Organization for Standardization) or ASTM Standards. In addition, some countries are introducing systems of green design, construction, and exploration. Their compliance/implementation is either required (the Estidama Pearl Rating System in Abu Dhabi after 2010), or is associated with receiving an award/points for the project in its full life cycle (Leadership in Energy and Environmental Design in the US) [16].

3. Division of Waste Used in Geoengineering

Alternative materials in the context of geotechnical engineering applications include the following [10]:

• Byproducts generated during the extraction of minerals in the broad sense, in particular, hard coal (burnt and unburnt shales), brown coal (rocks and soils from the overburden and interbeds), metal ores, rock salt, rock raw materials, and natural aggregates.
• Waste from industrial production, especially from thermal processes occurring in power plants (fly ash from conventional or fluidized boilers; slags), steelworks (blast furnace and steel slags), and plants producing mineral binders (fine clinker dust).
• Post-consumer, post-renovation, or dismantling products originating from private farms or the construction industry, including the road construction industry (concrete aggregate and construction rubble; ceramic or glass cullet).
• Used rubber materials, including primarily car tires and their shredded parts (tire derived aggregate, TDA).
• Natural products from agricultural, breeding, and food production: plant fibers from various parts of plants; animal fibers from wool, hair, secretions, feathers; coffee grounds; egg or shell shells; ashes from the combustion of biological substances (e.g., from municipal waste, rice husks, coffee husks, wood, etc.).
• Chemical waste in the form of artificial fibers based on natural biopolymers (cellulose, protein, rubber, etc.) and mineral raw materials, and also synthetic fibers produced from synthetic polymers in the processes of polymerization and polycondensation of organic compounds, such as crude oil or coal (polypropylene (PP), polyester (PET), polyethylene (PE), and polyvinyl (PVA) fibers), or composite materials.
• Other materials that have ceased to be waste.

Figure 1 presents a schematic division of the alternative materials selected for used in geoengineering applications.

4. Industrial Wastes

The group of industrial wastes used in geoengineering includes the following: unburned black coal-shales, burned red coal-shales (primary waste thermally transformed as a result of spontaneous combustion of the black coal-shales), power plant fly ash/bottom ash, slag and ash from biomass combustion, blast furnace and steel slag, and fine clinker dust. A common feature of all these wastes is their very large diversity in terms of mineralogical composition (within the same waste) and their variability of physical and mechanical properties. This fact is influenced by the following [17,18,19]:

• The extraction process and the method of enriching minerals.
• Method and time of waste storage.
• Conditions (temperature and precipitation).
• Combustion technology (including combustion temperature) and exhaust gas purification, as well as the properties of the input materials.

A good example of such influences, not necessarily beneficial, is the fact of a change in the grain size of the material over time (i.e., reduction of the coarse-grained fraction in favor of the sand and fine-grained fractions) due to weathering changes in the landfill, especially in unburned coal-shale. A similar effect is observed during the deliberate compaction of the mining of shales, for example, in the process of incorporation of the shales into the construction of an embankment or substructure [20]. Although in the case of burned shales, the weathering processes and grain crushing are much weaker and the waste material itself becomes resistant to soaking and swelling due to the effect of exposure to high temperature combustion (causing elimination of clay minerals); changes in physical and strength parameters are also observed. Examples of parameter changes are shown in

Table 1. Changes in physical and mechanical parameters of coal shale based on [21,22,23].

Parameter			Black Shale
Name	Symbol	Unit	Directly from the Mine	Fresh from the Dump	Aged from the Dump	After Burning (Red Shale)
Coefficient of uniformity	CU	-	4–160	22–170	14–2740	25–420
Optimum moisture content	wopt	%	7–12	9–16	8–20	8–12
Maximum dry density	ρds	Mg/m3	1.7–1.9	1.6–1.9	1.2–2.0	1.6–1.8
Permeability coefficient at Is = 0.95 1	k	m/s	10−4–10−5	10−4–10−6	10−4–10−8	10−5–10−6
Internal friction angle at Is = 0.95	φ	°	38–47	36–42	30–46	26–42
Cohesion at Is = 0.95	c	kPa	4–35	21–33	10–50	5–12

¹ I_S—a compaction coefficient, [-].

.

The basic application of coal shales is their use as a material for levelling land depressions and sinkholes caused by mining/opencast mining; for the construction of embankments of rivers, streams, and water reservoirs; settling tanks and landfills for industrial and municipal waste; construction of railway and road embankments; as well as road construction layers. Polish and world examples of the use of coal shales include the modernization of flood embankments on the Vistula River near Oświęcim, Poland (2015) [24], the construction of an artificial island in the Beaufort Sea, Canada (1974–1980) [25], an artificial reef in the Atlantic Ocean off Long Island (1980) [26], and the ski slope ‘Alpin Centre’ all year in Bottrop, Germany (2001) [27].

It should be realized that such a large variability in the geoengineering parameters characterizing a given waste requires a number of basic tests to be performed before its reuse, which include, among others, determination of the following: composition and grain size parameters, compaction parameters, permeability coefficient, swelling parameters, frost resistance, strength parameters, organic matter content, sand equivalent value of soils and fine aggregate, California Bearing Ratio (CBR), and abrasion resistance. In turn, during the use of geoengineering structures containing shales, it is recommended, especially in the first years, to monitor the quality of water in contact with the waste material.

Power plant waste generally does not contain hazardous substances in quantities that could pose a threat to the natural environment [28]. Some of them can be used successfully in geoengineering applications to strengthen, stabilize or modify the soil and in road/railway works [29,30]. Polish examples of the use of ash–slag mixtures include the construction of municipal roads in Wola Rzędzińska (2016), the provincial road DW 869 Rzeszów-Jesionka (2017/2018), and the Strzyżów bypass along the provincial road no. 988 (2018/2019), as described in [31]. The decks and piers of Tampa Bay’s Sunshine Skyway Bridge in the U.S. are another example of the use of fly ash as an additive to concrete [32]. However, there are situations where the insufficiently recognized properties of the waste used on the road base result in serviceability failures of the constructed pavements (Figure 2).

Slags are used mainly in road construction, mainly as aggregates for the construction of all layers of road structures, subgrades, slopes, and embankments [33,34]. Blast furnace slags can be used for soil stabilization if they meet the requirements of the Standard PN-EN 14227-15:2015-12 [35]. Researchers focus on the risk of releasing heavy metals from steel slags [36], the variability of the compaction parameters of blast furnace slags from heaps [37], and the risk of damage to the structure in which the slags are used with an unfinished decomposition process [38,39]. In turn, to reduce soil swelling, it is recommended to combine the slag with fly ash and lime [40].

A side effect of the production of mineral binders, such as cement/lime, is fine clinker dust [41]. It can be used to stabilize fine-grained soils and sands, increasing the CBR index, increasing compressive strength, increasing optimal moisture content for compaction, and reducing swelling [42]. It can also be used to solidify soils while binding heavy metals [43]. The use of fine clinker dust in the construction of road layers and in earthworks is regulated by the standard [44].

5. Rubber Wastes—Used Car Tires

Rubber waste in geoengineering applications mainly concerns used rubber tires, or rather their crushed parts (Figure 3): pieces ≥ 300 mm, shreds 20–400 mm, chips 10–50 mm, granulate 0.8–20 mm, and powder < 0.8 mm. The physical and mechanical parameters of the crushed tires depend on the grain size (fraction), the crushing method, and the impurity content (of steel and textile cord). Due to the fact that TDAs are characterized by low particle density (compared to soils), they are a desirable material for the construction of lightweight road embankments on weak-bearing soils [45]. Other applications in geotechnics [46] include as a backfill material behind retaining structures [47], backfills for culverts and underground pipelines [48], drainage and seepage layers (also in landfills), frost protection layers, thermal insulation layers, and vibration insulation layers [49].

There are many examples of TDA applications in the literature, although the specific investment location is not often indicated. Applications described in more detail include the following:

• Retaining wall (car tires) at the Lane Cove Tunnel Pacific Highway Exit project in Sydney, Ecoflex International [50].
• Lightweight road embankment (compressed car tires) at the Tampere Western Ring Road project in Tampere, Finland [51].
• Road embankment (shreds) at the Dixon Landing I-880 Project in California (2000) [52].
• Lightweight backfill behind retaining walls (shreds) at the Caltrans 207 Project in Riverside, California [52].
• Insulating layer (geosynthetic with TDA) as a subbase of the BART (Bay Area Rapid Transit) railway embankment at the Warm Springs Extension Light-Rail Project in Fremont, California [52].
• Drainage layer (TDA) in municipal waste landfills—Kiefer in Sacramento, California [52], Tri-Community in Fort Fairfield, Maine [47].
• Drainage layer (shreds) of the East Nippon Expressway in Hokkaido, Japan [49].
• Recultivation (20,000 waste tires) of the Nan-Liao municipal waste landfill in Taiwan [53].

However, each of the mentioned applications requires a series of tests on soil–rubber mixtures (mainly in terms of optimum moisture content for compaction, swelling parameters, and mechanical properties), especially since the results do not show a constant trend, but are variable depending on numerous factors [54,55].

It should be noted that the results of environmental studies [56] did not show significant hazards resulting from the use of TDA in construction work. According to the Regulation of the Climate Minister on the Waste Catalog [12], neither used tires nor products made of them are considered hazardous. However, it is recommended to use worn out tire materials only in an environment with neutral pH and above the level of the groundwater table.

6. Natural Wastes

Within broadly understood sustainable environmental geotechnics, a wide range of natural waste materials are used in the form of, for instance, plant fibers or ashes after combustion, animal fibers, and secondary waste from industrial production, e.g., agricultural, breeding, and food production (eggshells, chicken/bird feathers, and coffee grounds) (Figure 4). Regardless of the origin of the above-mentioned wastes, their task in a geoengineering context is, among others, to strengthen weak subsoil, reduce swelling and shrinkage of expansive soils, reduce bulk density, prevent the formation of tensile cracks, increase hydraulic conductivity, increase liquefaction resistance, reduce thermal conductivity, control surface erosion, etc. Therefore, the use of natural wastes in geotechnical engineering can take place in the following areas [57,58,59,60,61,62,63,64,65,66,67,68,69]:

• For railway and road construction in stabilizing the substructure of the railway track and the subgrade of temporary/access roads with low traffic intensity, construction sites, parking lots.
• In retaining walls, combining soil stabilization with short fibers or geotextiles with geogrids.
• For the protection of railway embankment slopes, as patches in the local repair of damaged slopes, or to increase the slope inclination angle to reduce the width of the embankment footprint.
• Enhancing the bearing resistance of weak soil deposits (with the addition of cementing agents) to support shallow foundations (thereby avoiding the need for deep or indirect foundations).
• As structural/non-structural fill material in road embankment construction.
• Strengthening weak soils in flood-prone conditions and under landfills.
• Stabilization of expansive soils.
• As a filler material in bricks, plasters, mortar, and compacted substrate.
• Production of hybrid composites.

Most indications of possible applications of natural waste result from laboratory tests. Unfortunately, there are no examples of actual reinforcement of the subsoil in this way. In Poland, such investments have not been recorded at all. It can be said that this direction, especially in the field of plant fibers, is not interesting to authorities or engineers. This is probably related to the climate zone, which significantly determines the life cycle of plants. The situation is completely different in countries with a warm climate, where research on soil–plant fiber mixtures is developing very intensively. Based on the available literature, it is clearly visible that in this respect, India, China, Bangladesh, Australia, Nigeria, Indonesia, Turkey, and South Africa dominate.

Natural waste is a group of waste materials that is, on the one hand, the most neutral to the environment (with the exception of coffee grounds, whose use requires confirmation of the absence of toxic compounds using a leachate test [66]), but, on the other hand, is difficult to precisely categorize due to its diverse biochemical properties related to its microstructure (e.g., see Figure 5) [70], which in turn is influenced by numerous environmental and climatic factors; i.e., those related to plant cultivation and factors related to species (origin of feathers or shells).

In the case of plant fibers, important parameters influencing the properties of green composite include the following:

• Cellulose and lignin contents, and their mutual proportions [70,71].
• Microfibrillar angle, i.e., the average angle between the fiber axis and the microfibrils (see Figure 5). The smaller the microfibrillar angle, the higher the strength and stiffness of the fiber, while a larger microfibrillar angle provides higher plasticity [72,73].
• Crystallinity index, which determines the relative percentage of crystalline material in cellulose [74].
• Shape factor, i.e., the ratio of the length of the fiber to its diameter [70,75].
• Water absorption capacity, which affects the adhesion and friction properties at the fiber–soil interface [76,77].
• Arrangement of the fibers (random or oriented), and the percentage of fibers in the mixture [78].

The issue of fiber adhesion to the soil matrix is a topic of discussion by many researchers. It should be noted that to prevent the negative effects of emerging interphase gaps and the lack of fiber adhesion to the soil, various waterproof coatings have been used since the 1980s such as asphalt emulsion, rosin and alcohol mixture, paints, bituminous materials, water-soluble acrylic, and polystyrene coatings [79,80]. These coatings change the surface properties of the fibers, such as surface energy, polarization, surface area, cleanliness, and wettability [71]. An additional effect of such a procedure is the protection of plant fibers against biological degradation in the substrate. Mercerization, which is one of the methods of chemical treatment of natural fibers (i.e., so-called alkaline treatment), plays a similar role [81,82]. In addition, mercerization generally increases the roughness of the fiber surface, which is a desired effect. Among the substances used to treat and coat natural fibers to reduce their biodegradability, the most commonly used are boric acid, borax, carbon chloride, and sodium hydroxide [70]. It is worth remembering that chemical modification is intended to ensure dimensional stability, improve adhesion, reduce water absorption, and increase resistance to biological factors [83]. Despite the possible use of such chemical treatments, we can still consider the benefits of solutions based on green geocomposites [9].

Other natural wastes also require special treatments before they can be used, as additives, to strengthen the soil, including the following:

• Spent coffee grounds require drying and hardening by geopolymerization at 50 °C [58,84,85].
• The preparation of eggshells or shells for further applications in geoengineering consists of washing and cleaning them with fresh water, heating at a temperature of 100 °C to 250–500 °C, followed by crushing or grinding them. In this form, they are suitable for stabilizing the soil, thereby reducing the consumption of natural lime from limestone, for instance. It is worth mentioning that the use of eggshells in geoengineering applications has been extended to using shells from crustaceans (crabs, lobsters, and shrimps) and mollusks (snails, oysters, clams, mussels, and scallops), which can pose a serious problem as natural wastes [86,87].
• The chicken (poultry)/bird feather fibers require particularly careful repeated washing and drying at 50 °C [57,88,89].

On the basis of the above examples, and considering the diversity of natural waste materials, there is a need for ongoing testing of new composite mixtures, development of technologies for the production of soil–fiber mixtures on a large scale, along with reasonable use of chemical agents necessary for preventing the biodegradation of the composites.

7. Chemical Wastes—Synthetic and Artificial Fibers

The difference between synthetic and artificial fibers can be explained as follows. Artificial fibers are made from biopolymers (cellulose, protein, rubber, etc.) and mineral raw materials found in nature, and are subjected to chemical treatment. On the other hand, synthetic fibers are made from polymers (not found in nature) in the process of polymerization and polycondensation of organic compounds, such as crude oil or coal (Figure 1). Most waste synthetic fibers are derived from materials commonly called “plastics” and from the clothing industry, especially sportswear [90,91].

In geoengineering, chemical waste includes materials occurring in the form of synthetic and artificial fibers of different lengths (i.e., as strips and granules from water bottles, woven polypropylene bags, plastic sheets, etc.), which represent an interesting potential alternative to soil reinforcement employing traditional soil stabilization methods [92]. This is due to their sufficient tensile strength, hydrophobicity, low density, chemical resistance, lack of toxicity (according to the author of this study, this aspect is a potentially controversial issue), low cost, and easy availability [68,90]. Similarly to natural fibers, synthetic fibers can be used effectively to improve the geomechanical properties of weak soils [93,94], including, among others, for achieving increases in the CBR index and the modulus of elasticity of clay soils that make up the lower layer of the road or railway subbase [95,96,97,98,99,100,101]. They also reportedly have a beneficial effect in reducing the swelling of expansive soils and increasing the strength of noncohesive soils, often being used in combination with other additives, such as cement or lime [102,103,104,105,106].

It should be noted here that although the proposed methods of soil reinforcement using synthetic fibers are beneficial from an engineering perspective, the authors reporting on such investigations in various publications do not present any discussion or consideration of the potentially significant long-term environmental impacts that may arise from the presence of plastic fibers randomly mixed into the subsoil [107]. Some researchers [107,108] have drawn attention to this important topic, pointing out (i) the impossibility of removing the myriad of embedded plastic fibers from the soil, and recycling them, at the end of the useful life of the fiber-reinforced earth structure, and (ii) to the potential long-term pollution of the in-soil and groundwater due to the disintegration of the plastic material into increasingly finer-sized particles (i.e., micro- and nano-plastics) [108].

8. Wastes from Construction, Renovation and Demolition of Buildings and Road Infrastructure

This group of waste materials is relatively well recognized in the civil engineering profession and is widely used in local urban investments [109,110,111,112,113,114]. Waste from the construction, renovation, and demolition of end-of-life buildings and road infrastructure is primarily associated with concrete aggregate and construction rubble (in geotechnical applications, they can be used alone or as a mixture with soil), asphalt, and with ceramic and glass cullet (mainly to strengthen expansive soils) [115]; relatively, most of the research has been devoted to glass fibers [90,116]. For instance, cullet is most often used in combination with other soil additives, for example, cement [117,118]. The wastes mentioned can originate from both industry and households.

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 biocementation 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 biocementation process occurs (even several weeks) [130]. In the EICP method, the same process occurs much faster 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. For example, for the same research case, the EICP process required 1 week, while the MICP process required 2 months [130].
• Effect of the temperature of the soil on the biocementation process [130].
• Effect of long-term efficiency of the biocementation process under conditions of variable subsoil water content and type of this water (e.g., seawater), especially in the case of using plant fibers [137].
• Environmental impact and cost-effective technology. For example, Rahman et al. [138] and Deng et al. [139] questioned the MICP method as a method that fulfils the criteria of sustainable development.

Often, biocementation technologies are combined with the use of plant fibers, which reduce the brittle fracture of soils subjected to only biocementation. Kannan and Sujatha [140] showed that nanosilica and banana fibers beneficially complement each other in improving the geotechnical properties of low-plastic organic silt. The nanosilica increased the strength of the fiber-reinforced soil, and banana fibers prevented the brittle failure nature imposed by the nanosilica. Also, they improved drainage characteristics through aggregation action. Salimi et al. [141] confirmed improving the mechanical properties and durability of the marl clayey soil mixture with glass fibers (0.75%), lime (6%), and nanoclay (1%), in freeze–thaw cycles. In turn, Arabani et al. [142] showed that the use of rice plant fibers as a highly renewable and environmentally friendly material in combination with nanoclay as a natural material can significantly improve the mechanical parameters of clayey soils. Rawat and Satyam [143] confirmed the effectiveness and durability under cyclic wetting and drying conditions of coastal sand reinforced with carbon fibers (0.40%), basalt (0.40%), and polypropylene (0.20%) and biotreated using the MICP method.

Another technology that has gained significant interest in recent years is the use of lignosulfonate in soil stabilization as a naturally derived biopolymer sourced from lignin. The attractiveness of lignosulfonate is determined by its intermolecular interactions, hydrophobic and hydrophilic effects, adhesive and binding properties, erosion control capabilities, compatibility with different soil types, and environmental sustainability [144,145,146].

Lignosulfonates offer several advantages, such as carbon neutrality and renewability, and are non-hazardous and non-corrosive [147,148]. They improve the physical properties of the soil without changing its pH or causing harm to the vegetation of the soil. Lignosulfonate also works well in soils with the addition of polypropylene fibers [149]. Furthermore, considering the abundance of lignosulfonate as a plant polymer, it is considered cost effective [150]. In addition, the utilization of lignosulfonates as industrial waste from paper and wood helps minimize waste generation and promotes the concept of circular economy.

Apart from the obvious advantages of using lignosulfonate, researchers point to the necessity of recognizing such issues as the following: solubility of lignosulfonate in water, electrical and hydraulic conductivity of soils treated with lignosulfonate [151], response of soil–lignosulfonate mixtures to cyclic, dynamic, and critical loads, the effects of water content and temperature changes on the mechanical parameters of mixtures containing lignosulfonate, durability of solutions using lignosulfonate, and at the same time their biodegradability [152].

10. Economic Feasibility, Durability, and Biodegradability in Sustainable Projects

An economic feasibility study of a sustainable project should contain the economic and financial analysis [153]. The financial feasibility refers to the income/profit derived from the project, while the economic feasibility focusses on the socioeconomic impact of this project at the scale of a given country/city/area.

The building sector has a particular impact on the environment through the vast demand for building materials, as well as resource-intensive processes (natural, energy, chemical resources, etc.) and generating significant carbon emissions. As one of the few, Abera [154] showed the next stages of an economic feasibility study of alternative materials, including the following:

• Extensive literature review of recycled materials, biobased composites, and low-carbon options in the context of manufacturing processes, materials properties, alternative applications, and the environmental benefits and the challenges they might pose in real-world applications.
• Thorough experimental analysis to assess the mechanical, thermal, and durability properties of selected materials in terms of their performance and suitability for various building applications.
• Full life cycle assessment (LCA) of sustainable construction materials, from raw material extraction to manufacture, transport, consumption, and final disposal.

However, it should be noted that in the context of the title of this study, such considerations about the economic feasibility of alternative materials are rare and most often concern one specific material and its purpose. The worst situation is with natural materials, which appear in detailed LCA analyses only when they are combined with another material, e.g., cement, in a composite. For example, such life cycle analyses (LCAs) for a composite was presented by the following:

• Radhakrishnan et al. [155] for novel cementitious composites where conventional synthetic microfibers were replaced by biobased fibers
• Taufik et al. [156] for a composite material of rice husk and a tin–lead alloy for engineering applications.

Few researchers present more detailed economic feasibility. Huang and Wang [124], based on their own calculations and observed trends, concluded that although the nanomaterials used in soil improvement are still in the experimental phase, they will be successful in the final stage of commercial practice. These predictions are confirmed by Arabani et al. [157] who found that global nanoclay production will increase from approximately 106 million tons (production by 2020) to 139 million tons by 2030, indicating a 30% increase in the production and abundance of these materials for engineering applications.

There are general opinions in the literature, not supported by any calculations, according to which the use of natural materials is more energy efficient and less polluting to the environment in contrast to traditional methods based on inorganic and artificial materials, which are toxic to the soil [158]. This approach is valid as long as no chemical or other modifications of natural fibers are used. However, there are opinions that even when natural materials are modified, technologies using these materials are in many ways less burdensome for the environment and still economically profitable [159].

The real challenge for researchers is the decomposition of artificial/synthetic materials over time in the aquatic and terrestrial environment (mainly soil) to the size of microplastics (particles from 0.1 to 5000 nm). The impact of microplastics on the ecosystem, including human organisms, is very widely discussed. In light of this discussion, the choice of natural material for geoengineering applications is a great indication. Extensive studies on this topic were presented by O’Kelly et al. [108], O’Kelly and Soltani [107], and Hasheminezhad et al. [160].

The issues of durability/biodegradability of solutions based on natural materials, and assessment of biocomposite performance in engineering applications are also the subject of many discussions [161,162,163]. On the one hand, it is written about of the need to replace environmentally harmful synthetic materials with natural, renewable, and biodegradable materials, even if this is associated with the deliberate protection of natural materials against degradation [159]. On the other hand, it is pointed out that in the chemical treatment methods and coatings developed (to enhance tensile strength, hydrophobicity, and degradation resistance) hazardous/expensive chemicals are used, or during such treatments, byproducts are generated that are difficult to dispose of [161]. This divergence of opinions shows the great complexity of the problem. To date, no laboratory-based standard protocol has been developed that could be used as an objective indicator of the long-term stability of natural materials subjected to biological or chemical degradation [161]. According to the author of this paper, this is mainly due to the fact that soil–natural material mixtures are tested primarily in the laboratory, not under in situ conditions. However, the author is convinced that this will soon change with increasing awareness of society and the depletion of natural resources.

11. Summary

This article presented an introduction to the use of various waste materials in geoengineering, mainly to improve the geomechanical behavior/properties of weak soils. The most well-known of them are wastes originating from the mining industry, power plants, steelworks, and the construction, renovation, and demolition of end-of-life buildings and roads, with many publications on them reported in the literature. The situation is similar regarding chemical (natural and synthetic) fibers, with their development progressing very dynamically. At the same time, there is growing discussion about the long-term impact of plastic, even biodegradable plastic, on the biosphere [108,164,165].

In turn, interest in natural wastes, especially those of fibrous nature, has become very popular in recent years, although this research area has generally not moved beyond the experimental sphere, with still a considerable way to go before reaching wide-scale application in the field. Obstacles include, among others, (i) the need to develop an effective technology to produce soil–fibrous mixtures, along with methods for preventing the biodegradation of the composites (which, depending on the viewpoint, is an advantage or disadvantage of the solution); and (ii) the lack of specific legal regulations, including the formulation of practical guidelines/standards, for the use of natural fibers in geotechnical engineering projects and construction, and quality control [9]. At the same time, the long-term impacts of such applications on the natural environment is generally not widely discussed and researched, mainly due to the lack of pertinent field test studies. Another aspect is that when recycling waste materials, the impact of this process on the environment should be reduced, especially in terms of energy and water demand [87] using, for example, renewable energy sources. Moreover, each of these actions should be evaluated, not only in terms of environmental footprint, but also in terms of economic value (considering costs of collection, cleaning, storage, transport, and disposal).

Summarizing the above considerations, the author would like to express that the main motivation for this article is not to criticize the various proposed soil improvement methods, which seem smart/beneficial from a geotechnical engineering point of view and in achieving waste reutilization. Rather, as also expressed in [107], the aim of this article is to broaden the discussion and increase awareness among the geotechnical engineering community about the potential uses of various so-called waste materials, as well as their possible threats to local environmental contamination, and potential additional energy and water demand.