1 The introduction

At present, global warming is an urgent problem for the international community to solve [1,2,3,4]. It has become the consensus of human society that the emission of greenhouse gases leads to global warming. In September 2020, China put forward a major strategic decision of “carbon peak by 2030 and carbon neutrality by 2060 “, which put forward urgent needs for low-carbon transformation in various industries [5,6,7,8]. Many studies have concluded that the construction industry is a major source of CO2 emissions [9,10,11,12,13,14,15]. Data released by the International Energy Agency (IEA) shows that carbon emissions from the construction sector accounts for 40% of total global human carbon emissions [16]. Therefore, the construction industry should be the key research field to achieve low-carbon development at present. Energy conservation and carbon reduction in the construction industry is the basis for realizing the carbon peaking and carbon neutrality goals.

As a temporary measure taken for the construction of the underground part of the main structure, foundation pit support structure is easy to be ignored. The researches related to carbon emission of foundation pit support structure are quite few. However, its huge carbon emissions should not be underestimated. Taking the Kunming comprehensive transportation international hub construction project as an example, the underground diaphragm wall and internal support were considered. The concrete consumption in this project is equivalent to that of 200,000 square meters of residential buildings (8 degrees zone, height of 100 m), whereas the consumption of rebars is equivalent to that of 240,000 square meters of residential buildings (8 degrees area, height of 100 m), leading to the huge consumption of materials and huge emissions of carbon. As a temporary structure, foundation pit structures are generally designed with short service life of 1 ~ 2 years. Moreover, most of the materials such as underground diaphragm wall cannot be recycled, which lead to high intensity of carbon emission.

The national peremptory specification “General Specification for Building Energy Efficiency and Renewable Energy Utilization GB 55015-2021 [17]” forcely requires that “the feasibility study report, construction plan and preliminary design documents of construction projects shall contain the analysis report of building energy consumption, renewable energy utilization and building carbon emission”. However, as a new concept of carbon emission, pit designers do not know how to calculate it, which makes the specifications difficult to be implemented. Difficulties of carbon emission calculation of permanent buildings lie in the operation stage. However, foundation pits do not involve the operation stage and can circumvent this difficulty. The pain spot of foundation pits concentrates in the construction and demolition stage. Generally, the designers are relatively unfamiliar with the two stages, which adds obstacles to the analysis of carbon emission in preliminary design stage. In this paper, Kunming comprehensive transportation international hub foundation pit project was taken as an example to calculate the carbon emission of the whole cycle of the large-scale deep foundation pit and to analyze the carbon reduction measures, which will provide new ideas and references for the designers to carry out carbon emission calculation.

2 Project introduction

2.1 Project overview

Kunming comprehensive transportation international hub construction project is located in the core of Kunming downtown, with a total construction area of 620,000m2, which includes 190,000m2 underground (3 floors underground), and 7 high-rise towers above ground. It is the first urban complex project of TOD international hub in Yunnan, and the cost of the foundation pit project is as high as 450 million RMB. The surrounding environment of the foundation pit is very complex, which is adjacent to subway lines, existing buildings, municipal roads and elevated and river channels, as shown in Fig. 1. Besides, geological conditions are extremely complex, including the soft soil with multiple layers of peaty soil. The mechanical property of soft soil is even worse than that of silt. The foundation pit project with a heterogeneous outline is extremely large in scale, of which the perimeter is 1084 m, the area is 56,800m2 and the pit depth is 18 m. System of underground diaphragm wall with three seven-ring concrete internal support was innovatively adopted as the foundation pit support, which is the overall coupled force internal support project with the largest number of rings around the world. Figure 2 illustrates the arrangement of foundation pit supports system.

Fig. 1
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The plan of foundation pit and its surrounding environment

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Fig. 2
figure 2

Arrangement of foundation pit supports system. a Layout of foundation pit. b Profile of foundation pit

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2.2 Overview of foundation pit support

The safety level of grade 1 should be guaranteed in this foundation pit. The underground diaphragm walls with length of 41 m ~ 54 m were divided into 171 slots through H-beam joints. The thickness of the walls is 1000 mm thick in the general area and 1200 mm in the westside close to the existing building. The concrete strength grade is set to be C30 (the concrete grade is increased by one level when it is poured underwater) and the seepage resistance grade is P8. The internal support system comprises three reinforced concrete horizontal supports along the vertical direction: the first horizontal support is also used as a construction bridge and material yard. The thickness of sealing plates is 250 mm and concrete of C40 was adopted for the waist beam, supports and sealing plate. Furthermore, an inclined support of steel pipes is added on the west side near the existing buildings. The vertical support system adopts bored piles with encased angle steel lattice column. The mechanical grade of vertical steel lattice column is Q355B. Column piles are divided into two types: additional column piles and column piles for the main structure. The diameter of the additional column pile is 800 mm, and the corresponding concrete strength grade is C30 (the concrete grade is increased by one level when the concrete is poured underwater). Support replacement is achieved by the help from the main structure. Measures for the support replacement includes: ① the spacing of 5 cm ~ 10 cm between exterior walls of basement and underground diaphragm walls was backfilled by the plain concrete with the same strength grade of foundation floor or basement exterior walls; ② late poured band of both foundation floor and basement floor was supported by H-shaped steel members; ③ temporary steel pipe bracing for support replacement was applied in the missing area of structural floor slab and driveway area.

3 Calculation model for carbon emission of foundation pit support system

The method of calculating carbon emissions for the whole life cycle of buildings was specified in the code of “Standard for building carbon emission calculation” (GB/T 51366-2019) [18]. As a subdivision project of the building construction project, carbon emission calculation model of the foundation pit support system should be firstly defined before calculating the quantity of carbon released by foundation pit construction.

3.1 Calculation boundary

Generally, the calculation boundary is the scope of carbon emission to be measured, namely the carbon emission inventory [19]. The definition of calculation boundary is an important prerequisite for carbon emission calculation. The definition of the calculation boundary mainly includes two important aspects: one is the determination of the accounting subject, and the other is the definition of the accounting period.

For internally supported structures, the accounting subjects are composed of soil retaining members, supporting systems and the materials consumed during the support replacement period, excluding the part used as the main structure.

There are several ways to divide the whole life cycle of a building. According to GB/T 51366-2019, life cycle of a building can be divided into three stages: production and transportation of building materials, construction and demolition and operation. Foundation pit support system, as a temporary project, does not involve the operation stage, so its accounting period includes the production of building materials, transportation of building materials, construction and demolition stages.

3.2 Calculation method

3.2.1 Stages of production of building materials

The formula for calculating carbon emissions in the production stage of building materials is shown in Eq. (1).

$${C}_{sc}=\sum_{i=1}^n{M}_i{F}_i$$
(1)

where Csc are carbon emissions from the production stage of building materials. Mi is the consumption of the ith kind of building materials; Fi is the carbon emission factor of the ith kind of building materials, which can be obtained from Appendix D in the code of GB/T 51366-2019.

3.2.2 Stage of transportation of building materials

Equation (2) is applied to calculate carbon emissions in the stage of transportation of building materials.

$${C}_{ys}=\sum_{\textrm{i}=1}^{\textrm{n}}{M}_i{D}_i{T}_i$$
(2)

where Cys is the carbon emissions in the transportation stage; Di is the average transportation distance of the ith kind of building materials, which is determined by the actual transportation distance (if actual transportation distance is unknown, the default value from GB/T 51366-2019 can be taken); Ti is the carbon emission factor of the ith kind of materials with unit weight and unit transportation distance, which is defined in Appendix E of GB/T 51366-2019.

3.2.3 Stage of construction and demolition

The formula for calculating carbon emissions during the construction phase is shown as Eq. (3)

$${C}_{\textrm{jz}}=\sum_{\textrm{i}=1}^{\textrm{n}}{E}_{\textrm{jz},\textrm{i}}{EF}_i$$
(3)

where Cjz is carbon emissions during the construction stage. Ejz, i is the total consumption of the ith kind of energy in this stage; EFi is the carbon emission factor of the ith kind of energy.

The equation for carbon emission during the demolition stage is:

$${C}_{\textrm{cc}}=\sum_{\textrm{i}=1}^{\textrm{n}}{E}_{\textrm{cc},\textrm{i}}{EF}_{\textrm{i}}$$
(4)

where Ccc is carbon emissions during the dismantling stage; Ecc, i is total consumption of the ith kind of energy in demolition stage.

The key to calculate carbon emission in construction and demolition stage is to determine the energy consumption. There exist the following three methods to calculate carbon emission in these two stages:

The first method is the estimation of energy consumption in each construction process, which is also the recommended method in GB/T 51366-2019. According to the engineering quantities of each construction process and measures, the consumption of machine-team of unit construction quantity in each project (defined in the “consumption quota of housing and decoration construction”) and the energy consumption of corresponding unit machine-team (defined in Appendix C of GB/T 51366-2019), the total energy consumption can be obtained by summarizing one by one. This method is relatively complicated and suitable for computing by computers. However, the carbon emission software in the market has not yet developed this function.

The second method is the statistical method of construction energy consumption list, i.e., the construction energy consumption ledger is retrieved to count the physical amount of various types of energy consumption in the construction process and calculate the corresponding carbon emissions. This method is suitable for projects where the construction organization has a good awareness of energy saving and management and has established a full process energy consumption ledger. However, this method cannot be used before construction, which can only to be applied after construction. Meanwhile, the energy consumption from different sub projects is not easily defined statistically.

The third method is the empirical formula method or proportional estimation method, which is the most crude estimation method. This method is often used to estimate the carbon emission of the whole construction stage of the building project. Until now, there is no relatively reliable empirical formula or proportional coefficient for the foundation pit project.

For the Kunming comprehensive transportation international hub construction project, the first method was adopted in this paper for calculating carbon emission. Manual statistical calculation is feasible because only the carbon emission of foundation pit support is calculated. Three types of energy sources are usually used, i.e. diesel, gasoline and electricity. The carbon emission factor of fossil energy is basically fixed [20], which can be calculated by combining the average low-level heat generation and the discounted standard coal factor of fossil energy from “General Rules for Calculating Comprehensive Energy Consumption (GB/T 2589-2020)” and carbon content per unit calorific value and carbon oxidation rate from “Provincial Greenhouse Gas Inventory (NDRC Climate [2011] No. 1041)”. The carbon emission factors for diesel and gasoline are 3.0959 kgCO2/kg and 2.9251 kgCO2/kg, respectively. The carbon emission factor for electricity is calculated using the carbon emission factor published by the Ministry of Ecology and Environment of the People’s Republic of China, and in this paper 0.581 kgCO2/kWh published in the year of 2022 was adopted.

4 Calculation and analysis of carbon emission

Taking the foundation pit project of Kunming Comprehensive Transportation International Hub as an example, carbon emission calculation and data analysis of each sub-project of supporting structure in different stages were carried out based on the Code of GB/T 51366-2019.

4.1 Carbon emission calculation

4.1.1 Carbon emission during the production stage of building materials

Considering the large scale and huge consumptions of materials in this foundation pit project, resource recovery measures are taken to achieve energy saving and emission reduction. According to GB/T 51366-2019, carbon emission due to the renewable construction waste generated by demolition, which can be evaluated by 50% of the carbon emission of the primary raw materials, shall be deducted from the total carbon emission. After demolition, RC internal supports and breast beams were recycled and transported to the recycle station to be crushed and reused. The recycling rate of concrete is 30%, whereas that of steel is 100%. That’s to say, the recycling coefficient of concrete and steel are 0.15 and 0.5, respectively. The recycling coefficient of steel columns is 0.5, because the steel columns can be reprocessed completely after demolition. As for steel pipes for inclined bracing and support replacement, they are reusable for 10 years. Generally, total of ten use times are applied to considering the recycle effect. Thus, the consumption coefficient of each use time can be calculated as 0.05. The corresponding recycling coefficient is 0.95. The calculating results of carbon emissions during the production stage of building materials are summarized in Table 1.

Table 1 Calculation of carbon emission in production stage of building materials
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4.1.2 Carbon emissions of transportation stage

Carbon emissions in the transportation stage of incorporate all the consumed energy caused by the transportation of building materials from production site to the construction site. In this project, the building materials were purchased according to the principle of proximity. The mode of transportation is heavy-duty diesel trucks (30 t) and the actual transportation distance is used. The calculating results of carbon emissions during the transportation stage of building materials are summarized in Table 2.

Table 2 Carbon emissions at the stage of transportation of building materials
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4.1.3 Carbon emission during construction stage

For the foundation pit support structure, the construction stage comprises the construction process of support structure and support replacement construction process. The carbon emission in this stage includes the carbon emission generated by the completion of each subproject construction and that generated by the implementation process of each construction measure. According to GB/T 51366-2019, the energy consumption of temporary facilities constructed in the construction process can be estimated by 5% of the energy consumed by the sub-projects.

Similarly, method of estimation of energy consumption in each construction process is adopted to calculate the carbon emission. It is difficult to list every construction machinery one by one because hundreds of different kinds of machines were used in multiple construction processes. In this paper, the energy consumptions in the process of grooving of underground diaphragm walls were calculated in detail and exhibited in Table 3 as an example. Similarly, energy consumptions corresponding to the other construction processes can be obtained. For simplicity, the final calculating results of energy consumptions and carbon emissions in the construction stage were collected in Table 4.

Table 3 Energy consumptions from grooving of underground diaphragm wall
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Table 4 Carbon emission during the construction stage
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4.1.4 Carbon emissions at the stage of demolition

The carbon emissions calculated at the stage of demolition starts from the beginning of demolition to the ending of disassembled materials transported to the designated location. Carbon emissions originate from energy consumption from mechanical equipment, small machines and temporary facilities, together with energy needed to transport disassembled waste materials. The energy consumption of construction temporary facilities in the demolition stage is estimated at 5% of the energy consumed by construction machinery. The energy consumption of construction machinery in the demolition stage is calculated using the method of estimation of energy consumption in each construction process. The removed supports and steel columns were carefully recycled. The recycling station is 20 km away from the construction site. Therefore, heavy-duty diesel truck (18 t load) is used for transportation. The carbon emission of waste transportation is calculated according to Eq. (2). Carbon emissions caused at the demolition stage are shown in Table 5.

Table 5 Carbon emissions at the stage of demolition
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4.1.5 Full-cycle carbon emission

The full-cycle carbon emission of foundation pit project is the sum of carbon emissions from all aforementioned stages. The synthetical results of the whole cycle are shown in Table 6. Referring to the evaluation indicator of carbon emission of unit building area in building engineering, evaluation indicator of carbon emission of unit foundation pit volume, i.e. the ratio of carbon emissions to pit excavation volume, was proposed in this paper for better comparisons among different foundation pit projects. The total carbon emissions of the full cycle of foundation pit project in this paper are 73,252.51 tCO2e, and the carbon emissions of unit foundation pit volume are 71.65 kgCO2e/m3.

Table 6 Summary of carbon emissions from different working stages
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4.2 Discussions of carbon emissions results

4.2.1 Analysis of carbon emissions in each stage

Figure 3 illustrates the proportion of carbon emissions occupied by each working stage. As is shown, stage of production of building materials contribute 89.3% of the total emissions for the concrete support used in this project, which is dominant, followed by 8.9% in the construction stage and 1.3% in transportation stage, 0.5% in demolition stage. It is revealed that the amount of building materials consumed in the support structure is the decisive factor influencing carbon emissions. Consequently, the design scheme of support structure has significant impact on the carbon emissions. When designing foundation pit support structure, purpose of carbon reduction and materials saving are consistent. It is a challenge work to make balance among carbon reduction, economical efficiency and requirements of construction deployment and overall construction efficiency.

Fig. 3
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Proportion of carbon emissions at each stage

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4.2.2 Carbon emission analysis of each sub-project

The carbon emission calculation results of each sub-project in each stage are shown in Table 7. As is exhibited, the proportion of carbon emissions in each stage is basically the same as the overall proportion for each sub-project. Carbon emission of production stage has the largest proportion leading to becoming the decisive influencing factor. Except for the demolition stage, subproject of underground diaphragm wall produced the largest carbon emissions (as much as 87% of the total carbon emissions in each stage) among all the subprojects, especially in the construction stage. The reason is that the subproject of underground diaphragm wall involves the extremely huge amount of engineering work, complex construction procedures and large energy consumption of big machinery.

Table 7 Carbon emission of each subproject (tCO2e)
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The proportion of carbon emissions of each subproject in each stage is shown in Fig. 4. Underground diaphragm walls contribute more than half of the CO2 emissions, accounting for 58.6%, followed by internal support accounting for 26.5% and column piles, columns, support replacement accounting for 6.7%, 5.0%, and 3.2%, respectively. The main reason is that the underground diaphragm wall has the largest consumption of building materials and is not recyclable, while both internal supports and columns can be recycled and only half of the carbon emissions are counted. The column piles save lots of building materials by using the main structure piles achieving energy saving and emission reduction. The average carbon emission of each subproject is shown in Table 8.

Fig. 4
figure 4

Proportion of Carbon emissions produced by different subprojects

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Table 8 Average carbon emissions of subprojects
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5 Measures for carbon reduction

5.1 Selection of internal support scheme

As shown in Fig. 5, the original design scheme of the inner support is a huge plan circle with pairs of bracings and corner bracings set up in the northwest corner to form an independent area, and with 60 steel pipes set up in the southeast corner as an independent area. Obviously, most of the main structure is located under the inner support. Thus, main construction cannot be carried out until the finish of removal of the inner support, greatly affecting the construction progress of main working nodes. The design scheme of support structure was optimized by pioneeringly adopting seven circles of support structure, which reduced the supporting members and column piles and improved the efficiency of sioil excavation and transportation. The most important thing is the six towers can be early constructed before the demolition of internal support, shortening the main structure construction period by 3 months.

Fig. 5
figure 5

Original design scheme of support structure

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Carbon emissions from the two design schemes were compared in Table 9. Considering the decisive proportion occupied by carbon emissions from production stage, only production stage was discussed. The same calculation method and process proposed in above sections were applied to calculate the carbon emission of original design scheme. As is depicted in Table 9, the total carbon emission of original design scheme with one huge circle is 74,715.19 tCO2e. The carbon emission increased by 9323.74 tCO2e compared with the optimized design scheme. An increase of 4353 tCO2e, 4381 tCO2e, 577 tCO2e were produced by the production of materials used for internal support, column piles and columns, respectively. As a result, the minimization of the consumption of building materials was achieved by optimizing design of support structures, accomplishing carbon reduction of 9323.74 tCO2e, which accounting for 12.7% of the total carbon emissions.

Table 9 Calculated carbon emission of original design scheme at production stage
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5.2 Recycle of internal support

The RC support members and steel columns in this project were collected as recycling materials. The crushed concrete from internal support is mainly used as raw material for producing masonry products (shown in Fig. 6). the constitution of masonry products is: coarse aggregate: cement: water: admixture = 3:1:0.2:0.004, in which the proportion of crushed concrete in coarse aggregate is about 80%. Steel bars from internal support and steel columns are melted and reprocessed to achieve energy saving and emission reduction. As calculated in Table 1, recycling of concrete from inner support reduced carbon emission by 1536.30 tCO2e, the recycling of steel bars from inner support steel reduced carbon emission by 8953.96 tCO2e, and the recycling of steel columns reduced carbon emission by 3417.86 tCO2e. Total of 13,908.12 tCO2e was reduced, accounting for 19.0% of the total emission of foundation pit support structure.

Fig. 6
figure 6

Recycle of concrete material

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6 Estimation method of carbon emission factor per unit project and general carbon reduction measures

6.1 Estimation method of carbon emission factor per unit project

According to the calculation of carbon emissions from the foundation pit support of Kunming Comprehensive Transportation International Hub, the carbon emissions from the support structure are huge. As a special design, the foundation pit support should conduct comparison of carbon emissions indicators when comparing and selecting different design schemes. However, the calculation process of carbon emissions is complex, especially in the construction and demolition stage, which involves the statistics of machine shifts and energy consumption during the construction process.

In order to estimate the carbon emission of the support structures in the foundation pit quickly, this paper proposes a quota-based fast calculation method for the carbon emission factor of the unit work volume, which includes the following steps: firstly, calculate the carbon emission factor of the construction and demolition stage of the unit work volume of each sub-project through the quota; secondly determine the carbon emission factor of the production and transportation phase of building materials used in the unit work volume; finally, the sun of the above factors leads to the carbon emission factor of the unit work volume, as is listed in Table 10. The total carbon emission of the project will be calculated by multiplying the total amount of work volumes in each sub-project by the carbon emission factor of the unit work volume. It needs to be emphasized that the carbon emission of recycled building materials should be deducted. The calculation formula is shown as:

$$G\textrm{HG}=\sum_{\textrm{i}=1}^{\textrm{n}}{M}_{\textrm{i}}\left({C}_{\textrm{i}}-{\beta}_{\textrm{i}}{F}_{\textrm{i}}\right)$$
(5)

where GHG is total carbon emissions in the whole life cycle; Mi is consumption of the ith material of each sub-project; Ci is the carbon emission factor of unit quantities of each sub-project, which is defined by Table 10; βi is the recovery factor of demolition, which is taken by 50% of recycling ratio; Fi is the carbon emission factor at the production stage of the ith building material.

Table 10 Carbon emission factor of unit work volume of each subproject
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In Table 10, the carbon emission factor at the production stage are taken from Appendix D of the Standard for Calculation of Building Carbon Emissions (GB/T 51366-2019). The carbon emission factor at the transportation stage is calculated according to Appendix E of the Standard for Calculation of Carbon Emissions from Buildings (GB/T 51366-2019), where the transportation distance is taken by the default value and the transportation mode is considered as the common heavy-duty diesel truck (30 t). The carbon emission factor at the construction and demolition stage is calculated according to the energy consumption estimation method, which means the carbon emission corresponding to the unit material consumption of each sub-project would be calculated by the consumption of machine-shift of each sub-project queried from Consumption Quota of Housing Construction and Decoration Engineering TY 01-31-2015. Considering the fact that the carbon emissions at the construction and demolition stage account for a relatively small proportion of the total carbon emissions and the energy consumptions of similar subprojects have little difference, the same carbon emission factor can be adopted in similar subprojects. The errors are acceptable.

6.2 General carbon reduction measures for foundation pit projects

On the basis of carbon emission calculation and analysis of the implemented carbon reduction measures related to the foundation pit of Kunming Comprehensive Transportation International Hub, the following approaches for carbon reduction in foundation pit projects are summarized as:

  1. (1)

    The type of support materials with low carbon emission or reusable are preferred.

  2. (2)

    The integral design scheme of foundation pit project is preferred. In the premise of meeting the use requirements, as far as possible to reduce the quantity of materials and to shorten the construction period.

  3. (3)

    The combination of permanent and temporary measures such as comprehensive use of column piles and structural piles is preferred.

  4. (4)

    It is advisable to use green, energy-saving, environmental-friendly construction techniques and methods. Furthermore, mechanical machineries with high efficiency of reducing carbon emissions are preferred.

  5. (5)

    After the removal of the support, it is appropriate to take measures to recycle the supporting members and materials.

7 Conclusions

This paper establishes a model for calculating carbon emission of foundation pit project. The calculation boundary and calculation method are illustrated and applied in calculation and analysis of foundation pit project of Kunming Comprehensive Transportation International Hub. Besides, the application of two types of carbon reduction measures are introduced and applied in this project. The following conclusions can be drawn:

  1. (1)

    The way to realize the full-cycle carbon emission calculation of foundation pit project is explored based on the actual applications. All calculating steps and parameter definitions are clearly exhibited giving references for the engineers of foundation pit projects. The achievements in this paper are helpful for the compulsory popularization and application of carbon emission calculation.

  2. (2)

    The total carbon emission of the foundation pit project of Kunming Comprehensive Transportation International Hub is 73,252.51 tCO2e. The carbon emission corresponding to unit pit volume is 71.65 kgCO2e/m3.

  3. (3)

    For the analyzed project, concrete internal support was applied. The carbon emissions at the stage of production of building materials account for 89.3% of the total emission, which means the amount of consumed building materials is the decisive factor influencing carbon emission.

  4. (4)

    Analysis of carbon emissions of each subproject revealed that carbon emissions from underground diaphragm walls account for 58.6% of total emission, followed by internal support with proportion of 26.5%, column pile of 6.7%, column of 5.0% and support replacement of 3.2%.

  5. (5)

    Design scheme of support structures has significant impact on the carbon emission. The optimization of internal support system from huge circle scheme to seven circles scheme achieved the carbon deduction of 9323.74 tCO2e, which occupied 12.7% of total emission of foundation pit project.

  6. (6)

    A carbon reduction of 13,908.12 tCO2e was accomplished in this project by the way of recycling measures of waste materials.

  7. (7)

    The calculation and analysis of carbon emission of the actual project conducted in this paper demonstrated the fast calculation method of carbon emission factor of unit work volume and general carbon reduction measures, which will provide reference for the upcoming overall applications of carbon emission calculation.