Comparison of Environmental Impacts of Two Alternative Stabilization Techniques on Expansive Soil Slopes

Zhang, Rui; Long, Mingxu; Zheng, Jianlong

doi:https://doi.org/10.1155/2019/9454929

Advances in Civil Engineering

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Abstract Introduction Materials and Methods Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Advances in Life Cycle Environmental Sustainability of Civil Infrastructure Systems

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Research Article | Open Access

Volume 2019 | Article ID 9454929 | https://doi.org/10.1155/2019/9454929

Comparison of Environmental Impacts of Two Alternative Stabilization Techniques on Expansive Soil Slopes

Rui Zhang,^1,2Mingxu Long,²and Jianlong Zheng^1,2

Guest Editor: Endong Wang

Received01 Aug 2019

Revised29 Sept 2019

Accepted15 Oct 2019

Published31 Oct 2019

Abstract

Two alternative techniques, the lime stabilization technique (LST) and the geogrid reinforcement technique (GRT), are both useful to stabilize expansive soil slopes, but their impacts on the environment need be further evaluated. Based on a case study, two techniques as well as their construction processes were introduced. The energy consumption and carbon dioxide (CO₂) emissions were investigated by the life cycle assessment (LCA). The sensitivity analyses were carried out, including the lime content for LST, the reinforcement spacing for GRT, the embankment height, delivery distance, and treatment width for both techniques. From the LCA results, with the GRT, the energy consumption and CO₂ emissions can be reduced by 7.52% and 57.09%, respectively. The main sources of two techniques are raw material production, soil transportation, and paving stage while the CO₂ emissions of lime production are about 11.68 times of those of geogrid production. From the sensitivity analysis results, as the lime content of LST increases by 1%, the total energy consumption and CO₂ emissions increase by 8.27% and 13.16%, respectively; as the reinforcement spacing of GRT increases by 0.05 m, the total energy consumption and CO₂ emissions increase by 1.63% and 0.69%, respectively; as the embankment height increases by 1 m, the increase rates of energy consumption and CO₂ emissions of LST are 1.68 and 1.61 times of those of GRT, respectively. In this project, when the embankment height is less than 10 m, the geogrid technique has the advantages of energy-saving and emission-reduction. It was found that the GRT is not sensitive to the change of delivery distance and treatment width and significantly reduces the environmental impacts, especially in reducing the impact of global warming.

1. Introduction

Expansive soils are widely distributed in more than 60 countries and regions [1]. They are classified as high plasticity soils that typically contain hydrophilic clay minerals such as montmorillonite and illite. These kinds of clays are characterized by significant swelling and shrinkage properties in the wetting and drying cycles. The deformation and damage of geotechnical infrastructures occur due to the variation in volume and strength of expansive soils with moisture content fluctuation [2–4].

Due to the extensive distribution of expansive soils and the high demand for infrastructure construction in China, it is inevitable to fill and cut the slope in expansive soil regions. In general, the surficial expansive soil on the slope is replaced by plenty of nonexpansive soil to inhibit their significant swelling and shrinkage properties. However, unreasonable treatment method results in significant economic loss and environmental risk due to the disposal of expansive soil and massive consumption of natural material. In order to minimize energy consumption and greenhouse gas emissions in the process of extraction, processing, and transportation, it is important to make full use of expansive soils instead of discarding them [5].

Two types of techniques, chemical treatment and physical treatment, are used for stabilizing surficial layer of expansive soil slope. In the chemical treatment, different additives, including lime or cement, are mixed into expansive soil to improve the physical and mechanical properties (strength, stiffness, California bearing ratio (CBR), durability, etc.) [6, 7]. The sustainable and relatively slow pozzolanic reaction between lime and expansive soils in a high pH environment is the key to the effective and durable stability of lime-soil mixture [8]. Chen et al. [9] used the expansive soil with medium swelling force to construct an embankment slope with a total length of 80 m stabilized by the lime. It is shown that the lime stabilization technique (LST) can ensure the stability of embankment slope, and the combination of LST and edge covering technique can greatly reduce the consumption of lime, which has been popularized and applied in Xiangyang to Jingzhou (Xiangjing) Highway. Li et al. [10] conducted repeated shear tests before and after drying-wetting cycles. It is theoretically proved that the LST combined with the edge covering technique has a good performance. In the physical treatment, the geogrid reinforcement is widely used to stabilize the expansive soil slope, which reduces the soil swelling by tension grids [11]. Several test sections reinforced by different techniques were constructed in Nanning to Youyiguan (Nanyou) Highway. From the results of monitoring and investigation for several years, it is shown that these techniques can significantly reduce the damage of slope [1]. Because of its advantages of good maneuverability, shortening the construction period, and reducing construction cost, the geogrid reinforcement technique (GRT) has been popularized in expansive soil section of highway.

After the stabilization mechanisms of the above two techniques were studied, the economic benefits of them were also analyzed. However, most studies in this field have focused on the reduction of costs and execution time of the project rather than the environmental impacts [12, 13]. In order to reduce the environmental impacts of engineering construction, life cycle assessment (LCA) is often used for the ecological assessment of environmental impacts of pavement materials, lime, cement, concrete, and geosynthetic materials. In addition, it is also a powerful environmental management tool used for quantifying the use of energy and materials and the waste emissions of products [14, 15] and guiding the in situ sustainable alternative plans [16–20]. In pavement projects, environmental impacts of different materials (warm mix asphalt, hot mix asphalt, steel slag, or natural aggregate) or techniques (asphalt rehabilitation technique or reclaimed asphalt pavement application technique) applied in the projects were analyzed and evaluated by the LCA to determine the optimum material or technique. Therefore, engineers can conduct a sustainable construction plan in the field [21–25]. In addition, the LCA was utilized to assess the environmental impacts of various production processes of building materials, such as lime or cement. Based on the results analyzed by LCA, the emission-reduction effects in the recovery process of cement production and the calcination process in lime production were further evaluated [26–29]. In slope project, compared with the concrete, geosynthetic materials, such as geogrid, are alternative building materials due to their high efficiency in construction. The energy consumption and carbon emission of rigid concrete retaining wall were compared with the geogrid-reinforced retaining wall through the LCA method. It is indicated that the geogrid is far superior to the concrete in terms of economic benefits and environmental protection [30, 31].

In this study, two techniques of lime stabilization and geogrid reinforcement for the expansive soil slope were analyzed by LCA, including the production, transportation, and construction stages of raw materials. The energy consumption and greenhouse gas emissions of different slope reinforcement techniques were determined. Environmental impacts were compared by changing the lime content, reinforcement spacing, and delivery distance. Moreover, based on the above results, the sensitivity analysis of relevant factors was carried out.

2. Materials and Methods

The lime and geogrid are often used to reinforce the expansive soil slope. In order to facilitate the comparison of these two techniques, the expansive soil embankment with the same length of 100 m was selected, and the lime stabilization and geogrid reinforcement techniques were adopted to reinforce the embankment slope.

2.1. Lime Stabilization Technique (LST)

The structures of embankment stabilized by the lime are shown in Figure 1. The width and height of embankment are 24 m and 8 m, respectively. The slope ratio is 1 : 1.5, that is, 1.5 m in length and 1 m in height. The slope angle is 33.7°.

The embankment was divided into three layers. The first part is the bottom layer. It is built with gravel and 0.5 m in height, and 2,362.5 m³ gravels were used as the rising barrier of capillary water.

The second part is the lower embankment. The central part of the embankment was filled with expansive soil for directly reducing the material waste. According to the wet compaction standard, the maximum dry density and the optimum moisture content of lime-soil determined are 1.74 g/cm³ and 16.8%, respectively [10]. And the degree of compaction should reach 90% according to the specification. A total of 18,900 m³ of expansive soils has been utilized. The lime was used to stabilize the edge of expansive soil embankment. Two key design parameters, including treatment width and lime content, need to be determined.

The strength decline of expansive soil embankment is mainly controlled by the repeated changes in temperature and humidity of the natural environment. To ensure higher initial strength and avoid the influence of seasonal wetting-drying cycle and rainfall on the embankment, it is necessary to determine the treatment depth of expansive soil by the edge covering technique [32]. The treatment width can be calculated by combining the depth of active zone with the slope ratio. The depth of active zone or atmosphere refers to the effective depth of the earth’s deformation caused by the precipitation, evaporation, and ground temperature under the action of natural climate. Its value should be determined by observation data of deep deformation or water content and temperature of the soil in each climatic region. If there are no data, it can also be selected according to the relevant regulations. Based on the available data on the depth of the active zone in the Jingmen area [9], considering the vegetation protection and drainage systems, the treatment width was set at 3 m and a total of 3,600 m³ of lime-soil was used.

The lime content refers to the ratio of quicklime mass to dry soil mass. The main purpose of lime stabilization method is to improve the swelling and shrinkage properties and mechanical properties of expansive soil. The optimum lime content should also be determined by laboratory tests, including CBR test, swelling test under the pressure of 50 kPa, and shrinkage test. According to the requirement of Specifications for Design of Highway Subgrades (JTG D30-2015) [33] in China, the CBR value of roadbed filler must be more than 3% before it can be used for the embankment filling. The natural expansive soil often cannot meet this requirement. The expansive soil was taken to test and control the dry bulk density of 18 KN/m³. As shown in Figure 2, when the lime content is more than 3%, the CBR value of lime-soil will be obtained, which is far greater than that of normative requirements. The total rate of swelling-shrinkage (e_ps) is the sum of the expansion rate and shrinkage rate of soil under the pressure of 50 kPa; when the weak and medium expansive soils are used as fillers, the total rate of swelling-shrinkage should not exceed 0.7% after treatment with an inorganic binder. According to the formula of Specifications for Design of Highway Subgrades (JTG D30-1995) [34] in China, when the lime content is 4%, the total rate of swelling-shrinkage is negative, which meets the requirements. Based on the above analysis, it is determined that the best effect can be achieved by adding 3% lime to the expansive soil. However, considering the effect of climate, crushing degree of soil particles, amount, and homogeneity of lime in construction, the actual amount of lime should be increased by 1∼2% appropriately. Therefore, the lime content is determined to be 5%. In order to ensure the construction quality, the lime must meet the requirements of Grade III; that is, the effective content of CaO and MgO in calcareous quicklime should not be less than 70%. In this study, the contents of CaO and MgO in tested lime are 73.28% and 3.87%, respectively [10].

The third part is the roadbed (0∼0.8 m) and the upper embankment (0.8∼1.5 m), commonly known as the working zone. In order to prevent the effect of weathering and drying-wetting cycle on the expansive soil, 3,937.5 m³ of nonexpansive soil with high water stability was used. The construction steps are shown in Figure 3.

2.2. Geogrid Reinforcement Technique (GRT)

The structure of embankment reinforced by geogrid is shown in Figure 4(a). The size and material of the first and third parts of this embankment are the same as those of lime-stabilized embankment. The roadbed and the upper embankment were also covered by nonexpansive soil with a height of 1.5 m. The gravel was used as the bottom layer with a height of 0.5 m. The difference is that the second part uses geogrid to reinforce the edges. The central part of embankment and the reinforced area of geogrid were directly filled with 22,500 m³ expansive soil. Compared with the LST, it is also necessary to determine the reinforcement length and spacing.

(a)

(b)

The detailed layout of the geogrid reinforcement is shown in Figure 4(b). The reinforcement length can be calculated by combining the depth of active zone with the slope ratio. According to the data of depth of the active zone in the Ningming area, the length of reinforcement was determined to be 3 m [1]. To avoid the failure of geogrid caused by the softening of soil material in the fixed part of the geogrid and wetting-drying cycle, a 0.5 m anchorage section was set up with a total reinforcement length of 3.5 m.

Some researchers have deduced the load-deformation relationship of geotextiles and substituted the reinforcement width and the allowable tensile strength of geogrid, thus directly calculating the reinforcement spacing. This method can ensure that the tensile strength mobilized in the reinforcement is consistent with the mobilized deformation [35]. In addition, the friction between soil and geogrid improves the strength and stability of reinforced soil. The spacing of reinforcement is closely related to the stability of slope. The reasonable thickness of each layer can be assumed based on experience, and the spacing can be determined by the minimum safety factor provided. If the reinforcement spacing is too large to provide sufficient tensile strength, it will still lead to the failure of reinforcement. The slope failure and too small spacing can cause waste of materials. Based on the Bishop method, the slope stability was analyzed in this study. The swelling effect of expansive soil after rainfall infiltration and the interaction between soil and geogrid were considered. Due to the establishment of the capillary water barrier, the calculation of soil includes two layers of nonexpansive and expansive soils without considering the effect of groundwater rise. The basic physical properties and shear strength of expansive and nonexpansive soils in the Ningming area of Nanyou Road are shown in Table 1. The tensile strength of geogrid is 35 KN/m. The safety factor (Fs) corresponding to different reinforcement spacing is shown in Figure 5.

As shown in Figure 5, when the reinforcement spacing is less than 1 m, the safety factor of slope meets the specification of greater than 1.2. To facilitate the layered filling of expansive soil, the reinforcement spacing was determined to be 0.5 m, and 12 layers of geogrid were laid. Each layer of geogrid was filled with two layers of soil. The compaction thickness of each layer is 25 cm. The geogrid was connected by a rod, and the length of back-wrapping is 1.5 m. The construction steps are shown in Figure 6.

3. Life Cycle Assessment (LCA)

The LCA is a tool for systematically analyzing the environmental performance of a product or process during its life cycle, from the cradle to the grave. It includes the whole life cycle of a product or process, including the extraction and processing of raw materials, manufacturing, transportation and distribution, use, reuse, maintenance, recycling, and final disposal [36]. Based on ISO 14040, its methodology consists of four steps: goal and scope definition, inventory analysis, life cycle impact assessment, and interpretation. As shown in Figure 7, the goal and scope definition include system functions, functional units, system boundaries, environmental impact types, and quality criteria for inventory data. The life cycle inventory analysis is defined as a phase of life cycle assessment involved in the compilation and quantification of inputs and outputs for a product throughout its life cycle. In the life cycle impact assessment, these environmental impacts of various flows of material and energy are assigned to different environmental impact categories. The characterization factor is used to calculate the contribution of each component for different environmental impact categories (climate change, ozone depletion, ecotoxicity, human toxicity, photochemical ozone formation, acidification, eutrophication, resource depletion, and land use). For example, the environmental impact of all greenhouse gases is expressed in terms of carbon dioxide (CO₂) equivalent. The life cycle interpretation deals with the interpretation of results from both the life cycle inventory analysis and life cycle impact assessment [14].

3.1. System Boundaries

A section of the embankment with length, height, and width of 100 m, 8 m, and 24 m, respectively, was considered as the functional unit. The pavement construction, operation, and abandonment of embankments are not considered in this study because these activities are common for two techniques. In the material acquisition stage, only high energy consumption materials, such as lime, geogrid, and soil, were considered. The amount of wood, steel bar, and other materials is not significant, and the corresponding energy consumption is relatively low. The source of natural gravel is similar to that of nonexpansive soil. Hence, to simplify the calculation, the gravel is considered as nonexpansive soil. The dimensions of the first part of gravel and the third part of nonexpansive soil are the same in the two techniques, which has little influence on the research objectives. The same transport standard (i.e., vehicle and fuel types) was used to calculate the environmental impacts during material transportation stage, and the effect of the delivery distance was also taken into account. The energy consumption and emission of construction machinery were mainly considered during the construction stage. For the manual operation, the energy consumption was calculated while the CO₂ emission was neglected. The activities, including the preparations of embankment foundation, construction of drainage facilities, and cultivation layer on the surface of the slope, were not considered by the life cycle analysis.

The lime and geogrid manufacturers are 50 km away from the project site. The nonexpansive soil pit is 20 km away from the project site. The natural bulk density of nonexpansive soil is 18 KN/m³. The effect of delivery distance was taken into account when energy consumption was calculated.

In LST, 5% lime was added. The maximum dry density of lime-soil is 1.80 g/cm³. Then, 324 t of quicklime was used. The expansive soil was premixed with 2% (129.60 t) quicklime in the excavation site, and the remaining 3% (194.40 t) quicklime was mixed with the previous lime-soil mixture before the final compaction in the worksite. After the mixing, bulldozers and graders were used to pave the soil.

In GRT, the width of geogrid is 5.9 m in total, including the reinforcement width of 3.5 m, wrapping length of 0.9 m (calculated from the reinforcement spacing of 0.5 m and slope ratio), and back-wrapping length of 1.5 m. 12-layer grids were used on each side of the slope. Each grid is 4.5 m in length, and the overlapping area between the two grids is about 10 cm. The overlapping area of this part is omitted. The lime, geogrid, and nonexpansive soil were transported by the dump truck with the same transport standard (vehicle and distance). The expansive soil was transported by the dump truck. The bulldozer and grader were used to level the mound. Rolling steps and precautions are the same.

3.2. Data Inventory

Assuming that the production process of China’s lime rotary kiln is used, 153.00 kg standard coal and 42.97 kW·h power are consumed per ton of lime. According to the National Bureau of Statistics’ China Energy Statistical Yearbook 2013, the average calorific value of the raw coal is 20,908 kJ/kg and that of the standard coal is 29,271 kJ/kg. The standard coal consumption rate for thermal power generation is 321 g/kW·h, which is converted to 9,396 KJ/kW·h. It is obtained that the energy consumption per ton of lime is 4,882.50 MJ/t. The emission in the lime reaction process is 0.683 t/t. The emission of fossil fuel combustion is 1.85 t/t, and the indirect emission of power consumption is 0.9402 t/t [37]. It is obtained that the CO₂ equivalent emission per ton of lime is 1,006.49 kg/t. Assuming that the production process of German geogrids is adopted [31], the mass of grids per square meter is 0.58 kg/m². The production energy consumption of grids per square meter is 78.70 MJ/m² and the CO₂ equivalent emission of grids per kilogram is 3.4 kg/kg [30].

Due to the constraints of construction level, both techniques need to adopt artificial cooperation to calculate the total working hours. 170 calories are consumed for moderate labor per hour, and each calorie is equivalent to the energy of 4.186 kJ for calculating the artificial energy consumption. 270 h and 210 h are consumed by the lime-stabilized soil per 1000 m³ and laying artificial geogrid per 1000 m², respectively.

2.423 MJ is consumed by highway transportation per t·km [38]. The energy consumption can be obtained from the total amount and distance of transportation of lime, geogrid, and soil. The default value of diesel emission factor is shown in Table 2. Diesel fuel trucks can calculate various greenhouse gas emissions according to the total energy consumption and then be expressed by the CO₂ equivalent. For a given greenhouse gas, the impacts on the environment can be measured over a period of time (usually 100 years), the environmental impacts of these gases vary, and the mass of other greenhouse gases is converted to the CO₂ equivalent by the global warming potential (GWP). The global warming potential of various greenhouse gases is shown in Table 3.

The type of energy was determined by the type of construction machinery. According to the Highway Construction Quota and Highway Construction Machinery Cost Quota (JTG/TB06-03-2007) [39] in China, the number of shifts of excavators, bulldozers, rollers, and dump trucks per 100 m³ and their diesel consumption per shift were counted, as shown in Table 4. The net calorific value of diesel is 43 MJ/kg, as shown in Table 2. The energy consumption and equivalent emissions of CO₂ were calculated by the diesel combustion.

4. Results and Discussion

4.1. Comparison of Two Alternative Stabilization Techniques

4.1.1. Energy Consumption

The specific energy consumptions of LST and GRT are shown in Tables 5 and 6, respectively. Among them, the energy consumptions of lime production and geogrid production are 1,581,930.00 MJ and 1,114,392.00 MJ, respectively. The calculation results show that these two materials in the production process with the same order of magnitude of energy consumption are the main sources of energy consumption in the whole LCA analysis process. The energy consumption of material production accounts for a large proportion in both techniques. The lime production process includes quarrying, crushing and screening, calcination, hydration and classification, packaging, and other steps. The geogrid production process includes plastic sheet extrusion, hole-making, heating, horizontal and vertical tension, directional cutting, and other steps. Both of them have the heating process. However, due to the lime production process, the specific heat capacity of stone is much higher than that of plastics, and the energy consumption of decomposition of calcium carbonate is much more than that of melting of plastics. Therefore, the energy consumption of lime production is higher than that of geogrid production.

In LST, the lime production (40.34%), expansive soil transportation (17.34%), nonexpansive soil transportation (14.30%), and expansive soil paving (9.41%) are the major energy-consuming processes. In GRT, the geogrid production (30.73%), expansive soil transportation (22.32%), nonexpansive soil transportation (15.46%), and expansive soil paving (12.11%) are the major energy-consuming processes. Because of the same amount, transportation standard, and paving technique of expansive soil and nonexpansive soil, the main source of energy consumption of the two techniques are also the same. Considering the balance of filling and excavation, the source of expansive soil is relatively close, but the amount of expansive soil is relatively large. Due to the heavy workload of the dump truck, the transportation energy consumption of expansive soil is higher. Nonexpansive soil needs to be transported for a long distance. The long-distance road transportation increases fuel consumption and energy consumption. Expansive soil paving requires bulldozers and graders to work successively, which also causes more fuel consumption.

The total energy consumptions of the two techniques are shown in Tables 5 and 6. The total energy consumptions of LST and GRT are 3,921,051.46 MJ and 3,626,208.38 MJ, respectively. The energy consumption of LST is about 1.08 times of that of GRT. Compared with the energy consumption of LST, the energy consumption of GRT is reduced by 7.52%. There is no obvious energy-saving advantage. Through comparison, it can be found that the total energy consumption of LST is higher than that of GRT because of the higher energy consumption of lime production and the increase of process steps of lime-stabilized expansive soil. The energy consumption of geogrid per unit mass is obviously less than that of lime, but the total content of geogrid used as the main material is larger than that of lime. In the case of large contents of geogrids, the GRT has no obvious energy-saving advantages.

4.1.2. Carbon Dioxide Emissions

The specific CO₂ emissions of LST and GRT are shown in Tables 7 and 8, respectively. Among them, the emissions of lime production and geogrid production are 326,102.76 kg and 27,923.52 kg, respectively. The results show that the energy consumption of the two materials in the production process is not the same order of magnitude. Lime production process is the main source of carbon emissions in the whole life cycle analysis process, while the carbon emissions in the grille production process account for a small proportion in the whole life cycle analysis process. The CO₂ emission of lime production is about 11.68 times of that of geogrid production. A large number of CO₂ emissions were produced by the decomposition of calcium carbonate in the lime production process. It is the main source of production emissions. However, no corresponding chemical reaction occurs in the geogrid production process. The CO₂ emissions mostly are produced from other energy consumptions. Therefore, the CO₂ emissions of geogrid production are far lower than those of lime production.

The major processes of CO₂ emissions in LST are the lime production (65.22%), expansive soil transportation (10.11%), nonexpansive soil transportation (8.34%), and expansive soil paving (5.49%). The major processes of CO₂ emissions in GRT are the geogrid production (14.34%), expansive soil transportation (30.90%), nonexpansive soil transportation (21.41%), and expansive soil paving (16.77%). The main sources of CO₂ emissions and energy consumption of the two techniques are the same. The proportion of lime production in LST is much higher than that of other sources while the CO₂ emissions of geogrid production in GRT are less and even much lower than those of fuel combustion in soil transportation and paving process.

The total CO₂ emissions of the two techniques are shown in Tables 7 and 8. The total emissions of LST and GRT are 499,973.90 kg and 214,529.27 kg, respectively. The emissions of LST are about 2.33 times of those of GRT. Compared with the CO₂ emissions of LST, the CO₂ emissions of GRT are reduced by 57.09%. It can be seen that GRT has obvious emission-reduction advantages. From the above analysis, it can be seen that the emission-reduction mainly results from the low emission of geogrid production and the simplification of construction steps. The carbon emission of geogrid per unit mass is much lower than that of lime. Although the total content of geogrid used as the main material is larger than that of lime, the GRT still has obvious emission-reduction advantages.

4.2. Sensitivity Analysis

The LCA results of the two techniques vary with some parameters. If the material is determined, the analysis results depend on the amount of material and the delivery distance. For the LST, the lime consumption is directly determined by the lime content. For the GRT, the geogrid consumption is directly determined by the reinforcement spacing. The treatment scope of the slope is directly determined by the height of embankment and the width of overlapping, which indirectly determine the amount of lime and geogrid. Also, the results of the two techniques are also affected by the distance between the manufacturers of lime and geogrid.

4.2.1. Sensitivity to Lime Content of LST

From the previous analysis, when the lime content is more than 3%, it has better treatment effect on expansive soil roadbed. According to the construction conditions, the lime content should increase appropriately. The current lime content is 4%, 5%, 6%, 7%, and 8%, respectively. Energy consumption and CO₂ emissions were calculated. Other conditions remained unchanged, and the calculation results are shown in Figure 8.

As shown in Figure 8, as the lime content increases by 1%, the total energy consumption increases by 324,236.52 MJ and the effect on the resource consumption increases by 8.27%; the total emissions increase by 65,804.27 kg and the effect on the global warming increases by 13.16%. It can be seen that as the lime content increases, the energy consumption and emissions of the production process also increase. As shown in Tables 5 and 7, the CO₂ emissions and energy consumption in the production process accounted for 65.22% and 40.34%, respectively, in the whole LCA process. As a result, for every 1% increase in lime content, the growth rate of total emissions is greater than that of total energy consumption. Therefore, the global warming is more sensitive to the change of lime content.

4.2.2. Sensitivity to Reinforcement Spacing of GRT

The dimension of reinforcement includes the reinforcement length, reinforcement spacing, and back-wrapping length. For ensuring the slope stability, the reinforcement width is 3.5 m and the back-wrapping length is 1.5 m. The energy consumption and CO₂ emission were calculated when the reinforcement spacing was 0.5, 0.55, 0.6, 0.65, 0.7, 0.75, and 0.8 m, respectively. The calculation results are shown in Figure 9.

As the reinforcement spacing increases by 0.05 m, the total energy consumption decreases by 59,094.96 MJ, and the effect on energy consumption decreases by 1.63%. The total emission decreases by 1,480.54 kg and the effect on global warming decreases by 0.69%. When the embankment height is constant, the increase of reinforcement spacing will change the number of wrapping layers of geogrid, thus reducing the consumption of geogrid materials. However, it is shown from the data that the environmental impact is not sensitive to the change of reinforcement spacing, mainly due to the low energy consumption and emission of production and transportation of geogrid, and its low proportion in the whole LCA process.

4.2.3. Sensitivity to Embankment Height of Two Techniques

The original embankment height is 8 m. The energy consumption and CO₂ emission of the two techniques were calculated when the embankment height is 10 m, 12 m, 14 m, and 16 m, respectively. The heights of bottom and top (0.5 m and 1.5 m, respectively) remained unchanged; that is, the height of the treated slope increased from 6 m to 8 m, 10 m, 12 m, and 14 m, respectively. Other conditions remained unchanged, and the calculation results are shown in Figures 10 and 11.

As shown in Figure 10, the total energy consumptions of LST and GRT increase by 389,675.46 MJ and 655,011.69 MJ, respectively. The increase rate of energy consumption of GRT is about 1.68 times of that of LST’s. As the embankment height increases by 1 m from 8 m, the energy consumptions of both techniques increase with the increase of subgrade workload. The increase rate of energy consumption of LST is slower than that of GRT’s. There is an intersection point where energy consumption is equal. It is shown that the GRT is more sensitive to the change of embankment height. When the embankment height exceeds 10 m, the LST is more sensitive to the change of embankment height and has more energy-saving advantages.

As shown in Figure 11, as the embankment height increases by 1 m from 8 m, the total CO₂ emissions of LST and GRT increase by 63,711.99 MJ and 39,520.43 MJ, respectively. The increase rate of CO₂ emissions of LST is 1.61 times of that of GRT’s. The LST has an energy-saving advantage when the embankment height is greater than 10 m. However, because of a large number of emissions in the lime production process, with the increase of embankment height, the emissions of LST are still far higher than those of GRT, and there is no emission-reduction advantage.

4.2.4. Sensitivity Analysis to Delivery Distance between the Manufacturer and the Worksite of Two Techniques

The delivery distance between the lime and geogrid manufacturers is 50 km. Assuming that the delivery distance between the manufacturer and the worksite is 100, 150, 200, and 250 km, respectively, other conditions remained unchanged, and the energy consumption and CO₂ emissions of the two techniques were calculated. The calculation results are shown in Figures 12 and 13.

As shown in Figure 12, as the delivery distance between the lime or geogrid manufacturer and the worksite increases by 1 km, the energy consumptions of LST and GRT increase by 785.05 MJ and 19.90 MJ, respectively. The increase rate of energy consumption of LST is about 39.45 times of that of GRT’s. In treating the expansive soil slope with the same size, the mass of lime required is much higher than that of geogrid. With the same delivery distance, transportation volume and fuel consumption increase significantly. With the increase of delivery distance between the manufacturer and the worksite, the energy consumption of geogrid does not increase significantly.

As shown in Figure 13, as the delivery distance between the lime or geogrid manufacturer and the worksite increases by 1 km, the CO₂ emission of LST increases by 58.37 MJ and the energy consumption of GRT increases by 1.48 MJ. The increase rate of energy consumption of LST is about 39.44 times of that of GRT’s. With the increase of lime delivery distance, the energy consumption increases significantly, but transportation emission accounts for a small proportion in the whole LCA process. Therefore, the CO₂ emissions of the two techniques do not show a significant increasing trend.

4.2.5. Sensitivity Analysis to Treatment Width of Two Techniques

The original treatment widths of LST and GRT are 3 m and 3.5 m, respectively. For ensuring the stability of the slope and not wasting materials, the treatment width increases from 3 m to 5 m, and it increases by 0.5 m each time. The energy consumption and CO₂ emissions of two techniques were calculated, respectively, as shown in Figures 14 and 15.

As shown in Figure 14, as the embankment treatment width increases by 0.1 m, the energy consumptions of LST and GRT increase by 56,068.41 MJ and 198.99 MJ, respectively. The increase rate of energy consumption of LST is 281.76 times of that of GRT’s. With the increase of embankment treatment width, the volume of treated expansive soil increases, and the amount of lime and geogrid increases accordingly. On the one hand, because the lime needs to be evenly mixed in expansive soil, the amount of lime will be significantly increased while the geogrid is wrapped around expansive soil and the increased reinforcing area is limited. On the other hand, because of lime with the secondary mixing process, the volume of lime-stabilized soil will increase and cause additional workload of construction machinery. Therefore, the energy consumption of LST is more sensitive to the treatment width.

As shown in Figure 15, as the embankment treatment width increases by 0.1 m, the CO₂ emissions of LST and GRT increase by 11,116.53 MJ and 14.80 MJ, respectively. The increase rate of CO₂ emissions of LST is 751.12 times of that of GRT’s. With the increase of embankment treatment width, the energy consumption of LST increases significantly due to the increase of lime consumption and extra workload, and the CO₂ emissions of lime production are much higher than those of geogrid production, resulting in a significant increase of CO₂ emission. However, the production emission caused by the increase of geogrid consumption is relatively low. Therefore, CO₂ emissions of LST are more sensitive to the treatment width.

5. Conclusions

Based on the LCA results and sensitivity analyses of two techniques, the conclusions are summarized as follows:(1)The case study on an expansive soil embankment of 8 m high and 24 m wide shows that compared with the LST, the GRT can reduce the energy consumption by 7.52% and CO₂ emissions by 57.09%.(2)The main sources are raw material production, soil transportation, and paving stage. Because much energy (1,581,930 MJ) is consumed in the lime production process, which is 1.42 times that of the geogrid production process (1,114,392 MJ), a large amount of CO₂ (326,102.76 kg) is released during the reaction process, which is 11.68 times that of the geogrid production process (27,923.52 kg). Therefore, in this study, under the same other conditions, the selection of geogrid materials for the slope reinforcement can significantly reduce the environmental impacts, especially in reducing the impact of global warming. In addition, there is no potential of groundwater pollution in the GRT.(3)For the LST, as the lime content of LST increases by 1%, the total energy consumption and CO₂ emissions increase by 8.27% and 13.16%, respectively; as the reinforcement spacing of GRT increases by 0.05 m, the total energy consumption and CO₂ emissions increase by 1.63% and 0.69%, respectively. When the geogrid reinforcement spacing decreases, more materials will be consumed, but the environment impact is not obvious.(4)As the embankment height increases by 1 m, the increase rates of energy consumption and CO₂ emissions of LST are 1.68 and 1.61 times of those of GRT, respectively. The energy consumption of two techniques intersects. When the embankment height is less than 10 m, the GRT has the advantages of energy-saving and emission-reduction.(5)With the increase of delivery distance between the manufacturer and the worksite, the energy consumption of LST increases obviously in transportation, but the increase of CO₂ emissions is not obvious in transportation. The GRT is not sensitive to the change of delivery distance between the manufacturer and the worksite.(6)Increasing the width of embankment treatment will increase the amount of lime and geogrid. The LST with larger increase of lime and the additional consumption caused by construction technique is more sensitive to the change of treatment width while the GRT with smaller increase of geogrid and lower production is insensitive to it.

Data Availability

The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

This research was funded by China Scholarship Council, the National Natural Science Foundation of China (grant no. 51978085), Highway Industry Standard Compilation Project of Ministry of Transportation (grant no. JTG-201507), and Open Fund of National Engineering Laboratory of Highway Maintenance Technology (Changsha University of Science & Technology) (grant no. kfj180102).

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Copyright © 2019 Rui Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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