LCA can be used to determine the environmental and human health impacts of any product or system, over its entire life cycle. Examples of impact categories assessed include global warming, eutrophication, acidification, and human health impacts. LCA is also used to quantify the life cycle water consumption and cumulative energy demand of a product or system. LCAs are typically completed with a “cradle-to-grave” scope, or system boundary, where each of the following life cycle stages is modeled: extraction and processing of raw materials, manufacturing, transportation of materials and equipment, construction, operation and maintenance, and end-of-life. For a geotechnical engineering project, the whole life cycle includes these stages over the entire project life including site investigation, construction, potential remedial measures in the case of a geologic event, and decommissioning.

The International Organization for Standardization (ISO) 14040 and 14044 standards are the most commonly cited standard for LCA (ISO, 2006a; ISO, 2006b). The ISO standard is conceived in four phases. The goal and scope phase includes determining the purpose of the study, the system boundary of analysis, and the application and its function and functional unit, among other criteria. In the life cycle inventory (LCI) analysis phase, all environmental inflows, including energy and material inputs, and outflows, including water and air emissions as well as solid waste outputs, across the modeled system are quantified and aggregated. A life cycle impact assessment (LCIA) is then performed through which characterisation factors are applied to relevant flows to estimate the environmental and human health impacts of the system. Conclusions from the LCI analysis and LCIA are then drawn during the interpretation phase and possible recommendations for technology use or development, or future LCAs are provided.

LCCA accounts for the capital, operation and maintenance, potential replacement, and end-of-life phases of a product or system and can also include external costs associated with the system. In an LCSA, these costs are estimated for the same goal and scope outlined in the LCA. In an LCCA, all future costs are discounted to account for the change in monetary value over time. While less common, LCCA can also be used to estimate the external costs of a system such as the environmental or social cost of environmental emissions. For example, the social cost of carbon (SCC) has been applied to geotechnical projects where the economic damages that would result from 1 ton of carbon emissions are evaluated (Reddy and Kumar, 2018; Raymond et al., 2021b).

Social life cycle assessment is a method to assess the direct and indirect social impacts of a project. Direct impacts refer to those stemming from the project itself, such as the benefits of geologic hazard mitigation on a community, while indirect impacts refer to those that occur further up the supply chain. One method to assess the indirect social impacts of a project are through existing social hotspot databases such as the Product Social Impact Life Cycle Assessment (PSILCA) database and the Social Hotspot Database (SHDB) (Benoit-Norris et al., 2012; Maister et al., 2020). However, these databases currently do not comprehensively cover geotechnical-related construction materials or assess emerging alternative construction methods (e.g., bio-inspired and bio-mediated technologies). Tools such as Design for Freedom have also recently been developed to highlight the resource intensity and equity of building construction material supply chains predominantly across the buildings sector (Grace Farms, 2022). These supply chains are analyzed and evaluated to provide a qualitative assessment of a variety of social sustainability indicators (e.g., labor practices, resource exploitation). While the ground improvement materials can differ from conventional building construction materials, Design for Freedom presents a social and environmental sustainability framework that can be applied to understand the sustainability of materials-intensive industries. Direct impacts such as those on construction workers and the local community should similarly be assessed through qualitative means.

EICP uses plant-derived (e.g., Jack beans) urease enzymes to induce urea hydrolysis, and when combined with calcium chloride calcium carbonate precipitation occurs (Eqs 1–5) (Khodadadi et al., 2017; Almajed et al., 2018). Non-fat milk powder can also be used as an additive in the EICP process to stabilize the enzymes (Martin et al., 2021). EICP can be applied as a treatment to the soil via three mechanisms depending on application purpose and soil properties: mechanical mixing and compaction, solution injection using traditional grouting methods, and surface spraying (Martin et al., 2020; Woolley et al., 2020; Arab et al., 2021). EICP improves the strength of soils, such as the shear strength, by cementing soil grains together (Almajed et al., 2021). C O N H 2 2 a q + 2 H 2 O l → 2 N H 3 a q + H 2 C O 3 a q (1) N H 3 a q + H 2 O l   ↔   N H 4 a q + + O H a q − (2) H 2 C O 3 a q ↔ H C O 3 a q − + H a q + ↔ C O 3 a q − 2 + 2 H a q + (3) C a 2 + + 2 C l − ↔ C a C l 2 a q (4) C a a q + 2 + C O 3 a q − 2 ↔ C a C O 3 s (5)

The reaction rate of EICP is governed by the concentration of inputs, the source of the urease enzyme, which influences enzyme activity, the temperature and pH of the system and the soil type (Saif et al., 2022). These parameters must be optimized to obtain a slow reaction rate to reduce precipitate clogging near the injection points (van Paassen et al., 2010b). Experimental studies have presented varied optimal concentrations for urea and calcium chloride due to differences in urease concentration and enzymatic activity, as well as environmental conditions (Almajed et al., 2019; Ahenkorah et al., 2021).

While EICP eliminates the need for large volumes of cement, cost and environmental concerns still exist. For example, the cost of the urease enzyme has been highlighted as a concern (Khodadadi et al., 2017). Environmental concerns of EICP are the production of ammonium, which contributes to eutrophication, and chloride ions which contribute to salinisation and can corrode steel (Eqs 2, 4). If the overall reaction does not occur to completion, ammonia may also be introduced as an intermediary product to the groundwater (Eq. 1). When applied to the surface layer of the soil, the introduction of urea may result in nitrous oxide and ammonia emissions to the air (Raymond et al., 2021b). However, at depth, it is unlikely for these emissions to occur.

MICP via urea hydrolysis has been the most widely researched MICP pathway. MICP relies on introducing cultivated microorganisms, most often Sporoscarcina pasteurii which carry the urease enzyme, urea, and calcium chloride to the soil that is to be treated through augmentation (van Paassen et al., 2010b). Stimulation of microorganisms native to the soil to be treated can also be conducted to achieve MICP (Burbank et al., 2011; Burbank et al., 2012; Gomez et al., 2014; Gomez et al., 2017). MICP can be applied below the surface through injection of the microorganisms and substrates. MICP can improve the load-bearing capacity of soils and provide liquefaction mitigation (Whiffin et al., 2007; Montoya et al., 2013).

The MICP process via urea hydrolysis is as follows: C O N H 2 2 a q + 2 H 2 O l   →   2 N H 3 a q + H 2 C O 3 a q (6) N H 3 a q + H 2 O l   ↔   N H 4 a q + + O H a q − (7) H 2 C O 3 a q ↔ H C O 3 a q − + H a q + ↔ C O 3 a q − 2 + 2 H a q + (8) C a 2 + + 2 C l − ↔ C a C l 2 a q (9) C a a q + 2 + C O 3 a q − 2 ↔ C a C O 3 s (10)

Similar to EICP, the rate of substrate injection must be controlled to avoid precipitate clogging at the injection points (Mortensen et al., 2011). The efficiency of the MICP process is dependent on the rate of ureolysis and treatment formula used (Martinez et al., 2013). This influences the applicability of MICP as a method to produce uniform treatment over large areas (van Paassen et al., 2010a).

Among the environmental concerns of MICP, ammonium chloride is produced as a by-product of the MICP process and must be removed or treated to reduce the risk of eutrophication and salinisation of groundwater. Rinsing processes have been introduced to remove ammonium from the treated soil, however, this produces wastewater which must be treated (Lee et al., 2019; San Pablo et al., 2020). Regarding costs, bio-stimulation of native organisms may be favored over bio-augmentation as this requires the construction and maintenance of on-site bioreactors (Sharma et al., 2022).

MIDP is a two-mechanism hazard mitigation process that relies on microbial denitrification to produce biogenic gas for desaturation and calcium carbonate precipitation for biocementation. MIDP uses the user-supplied substrate recipe of calcium acetate and calcium nitrate. Acetate acts as the electron donor (provides energy to the microbes) to facilitate microbial reduction of the nitrate to inert nitrogen gas through denitrification. Dissolved inorganic carbon (i.e., CO₂, CO₃ ²⁻, HCO₃ ⁻, and H₂CO₃) is also produced during denitrification, which when combined with the provided calcium leads to calcium carbonate precipitation. Biogenic gasses desaturate the soil, thereby reducing the liquefaction potential of soil by dampening pore pressure build-up during a cyclic shaking event (Eqs 11–16) (Dejong et al., 2014; O’Donnell et al., 2017). The precipitation of calcium carbonate mitigates liquefaction through the same hazard mitigation method as MICP, by improving the strength and dilatancy of soil (Eq. 17) (O’Donnell et al., 2017; Hall, 2021).

When aiming to mitigate liquefaction, MID may be preferred over MIDP since treatment via desaturation is reached before sufficient carbonate precipitation occurs to provide liquefaction mitigation (Hall et al., 2022b). Additionally, a greater amount of calcium acetate and calcium nitrate are required for carbonate precipitation, increasing costs and environmental impacts.

An advantage of MID and MIDP is that microorganisms do not need to be cultivated and injected into the soil since denitrifying microorganisms are widely found across various soil types, reducing costs (Hall et al., 2018). Another advantage of the processes is that users extract groundwater from the treatment site to dissolve and mix the substrates then, this water is re-introduced to the ground in a closed system with no external input of water. Existing groundwater technology, including injection and extraction wells, can be used to implement MID and MIDP (Moug et al., 2022).

Ongoing field trials of MID have demonstrated a desaturation persistence of at least 92 days, the length of the experiment when monitoring ceased (Moug et al., 2022). While longer-term trials of MID have not been conducted yet and the durability of desaturation has not been robustly tested, previous studies have indicated that abiotic desaturation can persist for up to 26 years (Okumura, 2006).

Benefits of MID and MIDP are the lack of waste products generated. For example, when the MID and MIDP reactions occur to completion, the products, nitrogen gas and carbon dioxide, do not need to be removed as is with EICP and MICP. It is unlikely that a significant amount of carbon dioxide will be released to the atmosphere through this process because of the typical pH levels of application sites and the high solubility of CO₂, leading the majority of produced CO₂ to remain in solution (Hall, 2021).

Complications of MID and MIDP include the impact that the primary and intermediary products can have on technical feasibility and environmental impacts. For example, the hydraulic conductivity of the soil may be reduced by the production of nitrogen gas and carbon dioxide gas and this may impact flow of the substrates through the soil (Stallings Young et al., 2021). Further, if the reaction is incomplete, process intermediates (primarily nitrite, but potentially nitrous and nitric oxide) may accumulate (Hall, 2021). The accumulation of these intermediates are potentially toxic and harmful to the environment. Intermediates also inhibit denitrification and can lead to reduced precipitation and desaturation (Eqs 13, 15) (van Paassen et al., 2010a; Hall, 2021)). C a C H 3 C O O 2 a q ↔ C a a q + 2 + 2 C H 3 C O O a q − (11) C a N O 3 2 a q ↔ C a a q + 2 + 2 N O 3 a q − (12) 1 2 N O 3 a q − + 1 8 C H 3 C O O a q − + 1 8 H a q + → 1 2 N O 2 a q − + 1 4 H 2 O l + 1 4 C O 2 a q (13) N O 2 a q − + 1 8 C H 3 C O O a q − + H a q + → N O a q + 5 8 H 2 O l + 1 8 C O 2 a q + 1 8 H C O 3 a q − (14) N O a q + 1 8 C H 3 C O O a q − + 1 8 H a q + → 1 2 N 2 O a q + 1 4 H 2 O l + 1 4 C O 2 a q (15) 1 2 N 2 O a q + 1 8 C H 3 C O O a q − + 1 8 H a q + → 1 2 N 2 g + 1 4 H 2 O l + 1 4 C O 2 a q (16) C a a q + 2 + C O 3 a q − 2 ↔ C a C O 3 s (17)

We conducted a systematic literature review of published ground improvement studies, including journal articles, conference papers, book chapters and industry reports. We completed a literature search following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework (Page et al., 2021). The databases utilized for the literature search were Web of Science, Scopus, and the American Society of Civil Engineers (ASCE) databases. We identified articles by searching abstracts using the search term: (“sustainability” OR “life cycle” OR “cost” OR “economic” OR “carbon” OR “energy”) AND (“EICP” OR “MICP” OR “carbonate precipitation” OR “microbially induced desaturation” OR “biocement” OR “biogrout” OR “bio-grout” OR “bio-mediated” OR “bio-mediated” OR “bioinspired” OR “bio-inspired”) AND (“assessment” OR “analysis” OR “footprint”). The resulting studies were then reviewed for relevance and duplication to arrive at a final set of studies that met our review criteria. We further sought to identify studies that may have been overlooked in the database searches by screening the references cited in the relevant studies that were found via database.

To respond to the research goals of this study, which include an assessment of the relative frequency of sustainability assessment of bio-mediated and bio-inspired ground improvement technologies compared to conventional methods, we also conducted a literature review of quantitative sustainability assessments applied to conventional ground improvement techniques. The flow chart for this literature search is presented in Supplementary Figure S1.

We reviewed each of the identified sustainability assessment studies with specific focus on the following parameters:

Application (e.g., EICP, MICP, MIDP)

The goal of examining these parameters is to examine variability across the methods and modeling choices in existing studies, which determines the comparability and comprehensiveness of studies that reflect the current state of practice. The greater the variability in methods and key assumptions, the less comparable study findings are likely to be.

We also document the findings of the reviewed studies, with the goal of making recommendations for reducing the environmental impacts and cost of bio-mediated applications.

Eight studies published between 2009 and 2023 were identified, three of which evaluate EICP, three evaluate MICP, one evaluates MID and MIDP, and one evaluates MID only. A summary of these studies is provided in Table 2. Six of these studies assess either one or multiple bio-mediated technologies or pathways and compare them to a traditional, cement-based ground improvement technique. Meanwhile, Martin et al. (2020) only provides a hotspot analysis of EICP. Each study is a process-based LCA considering environmental impacts or cost. None of the identified studies includes a social life cycle assessment.

The life cycle studies considered ground improvement applications, including soil stabilization for roadways and railways, foundation reinforcement, and liquefaction mitigation. Porter et al. (2021) did not provide a specific application for MICP, instead focusing on the production of calcium carbonate only. Each EICP and MICP life cycle study considered a urea hydrolysis pathway. Porter et al. (2021) also evaluated another five pathways for MICP including denitrification (MIDP).

The frequency of LCSA applied to biotechnologies compared to conventional technologies has been uneven over time, but biotechnologies are over-represented in the literature relative to their frequency of application in real world projects. Figure 2 shows the results of literature reviews for both biotechnologies and conventional based on the number of studies published per year between 2009 (the first year an LCSA of a ground improvement technology was published) and 2023. What is evident, is that there has been uneven growth in LCSA-related studies since 2009, but that biotechnology LCSAs are growing in popularity alongside conventional ground improvement LCSAs.