1. Introduction
Soft soils are widely distributed in coastal areas and are characterized by a high natural water content, low bearing capacity, and high compressibility, and cannot be used directly in engineering [
1]. However, the treatment of the soft soil foundation is a very challenging task, which requires certain methods such as preloading, chemical stabilization, and electro-osmosis [
2,
3]. The chemical stabilization method is widely used to achieve the stabilization of soft soil by mixing some cementitious materials (such as cement, lime, water glass, ion-exchange-class, and polymer-class stabilized materials, etc.) into the soft soil [
4,
5,
6].
Cement is widely used in the field of soft soil stabilization due to its wide range of sources and stable properties. However, cement has problems such as high energy consumption, high carbon emission, consumption of non-renewable resources, and environmental pollution [
7,
8]. Replacing cement with low-carbon supplementary cementitious materials (SCMs) such as steel slag, fly ash, and silica fume is considered as the most promising strategy for sustainable soft soil stabilization design [
9,
10,
11].
Steel slag is the solid waste the metallurgical industry produces, accounting for about 12–15% of steel output [
12,
13]. Steel slag is a potential SCM due to its similar composition to cement [
14,
15]. The low reactivity and poor volume stability of steel slag can be improved by grinding and carbonation [
16,
17]. Most of the existing studies on the partial replacement of cement by steel slag for soft soil stabilization have focused on the effect of steel slag on the mechanical properties of stabilized soil [
18,
19]. However, there is a lack of scientific quantitative research on the environmental and economic impact of cement–steel-slag-stabilized soils (SCSs).
Life cycle assessment (LCA) is a scientific method to qualitatively and quantitatively assess the potential environmental impact of a product or process during its life cycle [
20,
21], and LCA is considered one of the essential tools for sustainability assessment. Ghasemi et al. [
22] performed the LCA study to compare the environmental impact using slurry and wet carbonation processes for converter slag and found that the net avoided global warming potential (GWP) of the slurry and wet routes were 525.56 and 426.67 kgCO
2eq/MWh, respectively. Li et al. [
23] evaluated the environmental impact of steel slag aggregates and steel slag blocks. The GWP results showed that steel slag blocks could achieve negative carbon emissions. However, LCA involves complex logical relationships between products, activities, and the environment, the results of which are often difficult to understand and to use to share inventory information between different disciplines [
24].
Ontology can standardize concepts, terms, and their relationships, provide methodologies for building knowledge frameworks, and is widely used for information retrieval, integration, decision making, and knowledge sharing between different domains by combining knowledge terms and predefined rules [
25,
26]. Zhang et al. [
27] proposed an ontology-based semantic representation method for the product life cycle, which implemented a formalized and shared product life cycle design. Hou et al. [
28] developed the concrete structures design ontology with embodied energy and carbon as sustainable evaluation indices. Meng et al. [
29] proposed a multi-objective design method for an energy pile system based on ontology from the perspective of technology, economy, and sustainability. Based on the Monte Carlo simulation approach, Cui et al. [
30] established a comprehensive seismic risk assessment ontology framework for the subway station. Ontology has obvious advantages in solving multi-domain, multi-objective problems due to its shareability, interoperability, and reusability. However, little research has been carried out to develop ontology frameworks for soft soil stabilization.
This study intends to propose a sustainable evaluation framework for SCSs based on LCA and ontology. The framework takes unconfined compressive strength (UCS), GWP, and cost as indicators, and conducts a multi-objective decision-making study on stabilized soils by combining a knowledge base with semantic web rules, with a view to obtaining a design method for soft soil stabilization with optimal overall benefits. This paper is organized as follows: In
Section 2, the LCA method is applied to quantify the GWP and costs at each stage of the life cycle and to propose sustainable environmental and economic indicators.
Section 3 presents the development of the ontology framework for the evaluation of stabilized soils (OntoESS).
Section 4 conducts a case study of marine soft soil stabilization to verify the practicality of the OntoESS framework and to investigate the effects of steel slag fineness, carbonation degree, and substitution ratio on the sustainability of stabilized soils and to compare it with the sustainability of pure-cement-stabilized soils (S-C). The main conclusions and next steps of this study are presented in
Section 5.
5. Discussion
Cement is the most commonly used soft soil stabilization material for foundation treatment in coastal engineering, but the environmental problems caused by the high carbon emission of its production have gradually attracted the attention of all countries. The use of steel slag to partially replace cement to stabilize soft soil has caused widespread concern among researchers. The ontology framework proposed in this study fills the gap in the research on the sustainability evaluation of cement–steel-slag-stabilized soils, because most of the current evaluation studies on stabilized soils only focus on one or two aspects of engineering performance, the environment, and the economy [
50,
51], failing to integrate the indicators from the perspective of sustainability, and the results of their evaluations are difficult to use for providing references for designers.
From the study results, the steel slag treated by grinding and carbonation can obtain better sustainability than S-C by replacing the cement to stabilize soft soil with a lower content, demonstrating the feasibility of steel slag for soft soil stabilization. However, long transport distances for steel slag should be avoided so that the cost advantages of SCSs are not masked. It should be noted that the preparation of stabilizers in this study was carried out at the laboratory scale. In the future, the influence of the equipment and labor factors required for the mass production and on-site construction of stabilizers should be considered. In addition, durability is also a focus of future research on the sustainability of stabilized soils. Based on the flexibility and reusability of the ontology framework, it can be improved and extended according to new materials and processes, and tools with unified interfaces can be developed for different projects and phases, so as to realize the efficient and collaborative design, operation, and maintenance management of stabilized soils in the whole life cycle.
6. Conclusions
In this study, an ontology framework was developed to evaluate the sustainability of stabilized soil with a cement–steel slag blend (SCSs). Firstly, a quantitative approach for sustainability evaluation indicators of SCSs based on LCA was proposed. Then, an SCS ontology model was developed, and related domain knowledge and basic data are integrated into the knowledge base. According to the semantic web rules, the reasoning and query of evaluation indices were further realized. The ontology framework proposed in this study can clearly describe the logical relationship between production activities and the environment during the life cycle of SCSs, which helps designers to clarify the influence of materials and processes on the sustainability of stabilized soils, and then obtain the optimal design and optimization direction from a macro perspective.
The practicability of the proposed ontology framework was verified by a case of marine soft soil stabilization. The case study found that the four types of steel slag can achieve better sustainability than pure-cement-stabilized soil (S-C) at a lower content. The stabilization of soft soils with FSS-C-18h with 10% and 20% substitution rates represented the best stabilization scheme, achieving similar strengths to S-C while significantly reducing carbon emissions and costs. From the sensitivity analysis of the transport distance of steel slag, even if the transport distance of steel slag is significant, SCSs are still favorable to the environment. However, the transport distance of steel slag greatly affects the cost, which must be considered when selecting suppliers.