Constructability Analyses of Vertical Extension Methods for Existing Underground Spaces

: Remodeling underground structures requires careful construction planning, including consideration of costs and scheduling. Therefore, this study aims to analyze and compare the effects of four methods for vertically extending the underground spaces of an existing building under scheduling and cost constraints. The study considers the following extension methods: (1) bottom-up method, (2) normal top-down method after demolition, (3) normal top-down method in parallel with demolition, and (4) top-down method using double beams in parallel with demolition. Twelve illustrative examples are presented to investigate the constructability of these methods in terms of construction scheduling and costs. The construction durations and costs of each example is calculated and compared. We also analyze the structural stability of the examples using MIDAS Gen 2017. We conclude that the top-down method using double beams is the most efﬁcient method in terms of costs and scheduling. The results and analysis process can help practitioners to select appropriate methods to expand underground spaces without demolishing entire existing buildings and efﬁciently manage costs and schedules. In future studies, these extension methods should be applied to real-world projects in various countries to validate and verify their actual effects on construction costs and scheduling.


Introduction
Urban area populations are growing rapidly worldwide and the availability of space in congested urban areas is decreasing [1]. Therefore, underground spaces beneath buildings are increasingly utilized [2][3][4][5]. To make use of such spaces, stable structures are often dismantled and then reconstructed in the same location. However, demolition and reconstruction of buildings incur enormous costs as well as leading to other problems such as environmental pollution and complaints [6]. Therefore, remodeling and renovation processes in the architecture, engineering, and construction (AEC) industry are typically preferred as alternatives. Remodeling underground structures has more significant impacts in terms of the environment, costs, and scheduling than remodeling superstructures [5,7]. Therefore, remodeling work on underground structures requires more careful construction planning, considering both costs and scheduling. The underground space below an existing building can be vertically and horizontally expanded to improve functionality without demolishing the entire building. However, underground vertical extension is more complicated and requires a higher level of technology skill than either underground horizontal extension or new construction in general, and there are few previous studies and example cases of methods for increasing the number of floors in underground spaces. Related studies can be categorized into two main groups: (1) case studies on extending underground spaces below existing buildings [8][9][10][11] and (2) processes to vertically extend underground spaces of existing buildings [12][13][14]. Bing [9] investigated a case to vertically extend underground spaces of residential buildings, which were used for parking lots.
However, specific processes for vertically extending existing underground spaces were not proposed. Park, Lew, Choi, and Lee [12] proposed a process to vertically expand underground space and applied it to a real-world project in South Korea. The detailed application process and pros and cons of the method were introduced. Kim, Lee, Kim, Koo, Jung, and Seo [13] and Jung, Kim, Lee, Hwang, and Seo [14] suggested a new process to vertically extend underground spaces in existing buildings, as well as a process to determine the most appropriate method by considering project characteristics. However, they did not analyze the cost perspective of the proposed processes in depth. Therefore, Seo et al. [15] analyzed the structural stability as well as cost perspective of several vertical underground extension methods without demolishing entire existing buildings. They concluded that the top-down method with multiple posts downward was the most beneficial method, outperforming both the bottom-up and normal top-down methods under the same conditions. However, to expand underground spaces effectively, not only the cost perspective but also the scheduling perspective should be considered. Therefore, this study aims to analyze and compare the constructability, including construction costs and scheduling, of the four vertical underground extension methods in existing structures considering the amount of demolished and used building materials and the required number of laborers on site. The analyzed extension methods are the bottom-up method, normal top-down method after demolishing existing substructures, normal top-down method in parallel with demolition, and top-down method using double beams in parallel with demolition. To analyze impacts on cost and schedule management, we considered 12 illustrative examples. The four vertical underground extension methods in established buildings were applied to these examples for calculation and comparison. used for structural analysis in South Korea because its reliability has been verified in previous studies [16]. The constructability of the 12 illustrative examples was analyzed by considering both cost and scheduling perspectives, as shown in Section 6. Cost and scheduling analysis were calculated based on the quantity of construction materials and equipment used in each work. Vertical underground extension processes typically include preparation, support installation, demolition and backfill work, retaining walls, pile work, percussion rotary drills (PRDs), temporary post work, strut and excavation work, top-down excavation work, foundation work, structural frame construction, finish work, inspection, and miscellaneous work. The durations of each type of work and relevant construction coefficients were determined by experts based on construction plans, calculated quantities, and labor and equipment productivities [17]. The working experience of the experts was approximately 20 years. The processes to analyze the underground vertical extension costs in this paper are similar to those of Seo et al. [15]. To estimate the costs of the four methods, the calculated quantities and itemized unit costs for several types of work in each construction method were determined based on expert opinions and the construction cost calculation ratio standard provided by the Public Procurement Service (PPS) in South Korea. Material and labor costs, miscellaneous expenses, general administrative expenses, profit, and value-added tax (VAT) were also calculated. The details of the assumed and calculated construction durations and costs are described in Section 6. Lastly, discussion and conclusions are provided in Section 7.

Underground Vertical Extension Methods
The constructability of the four underground vertical extension methods in existing buildings was analyzed in this paper. They were the bottom-up method, normal top-down method after demolition, normal top-down method in parallel with demolition, and top-  (1) Bottom-up method: all of the excavation of soil under an existing structure should be completed before vertical underground space extension is performed. (2) Normal top-down method after demolition of existing underground structures: prior to the construction of structural frames of each basement floor in the substructure, soil with volume corresponding to height of one basement level should be excavated below the floor under the existing structure. These demolition and construction processes are repeated until reaching the desired number of floors. (3) Normal top-down method in parallel with the demolition of existing underground structures: the construction of structural frames of each extended floor is carried out while supporting and demolishing existing underground structures and the other processes are the same as those in the normal top-down method after demolition. (4) Top-down method using double beams in parallel with the demolition of existing underground structures: the construction process of this method is very similar to the top-down method in parallel with demolition, but this method uses double beams to support the existing building and installs structural frames for extended underground structures.
The process of vertically extending the underground spaces without demolishing entire existing buildings is complicated because we must consider construction costs, scheduling, and structural stability issues simultaneously. Section 3 discusses the detailed processes of the four extension methods listed above. To analyze the effects of these four methods on construction costs and durations, 12 illustrative examples were created in this study with the number of extended basement floors and underground vertical extension methods as variables in Section 4. Prior to conducting cost and scheduling analyses of the methods, we analyzed the structural stability of examples using MIDAS Gen 2017 in Section 5. This software has been widely used for structural analysis in South Korea because its reliability has been verified in previous studies [16].
The constructability of the 12 illustrative examples was analyzed by considering both cost and scheduling perspectives, as shown in Section 6. Cost and scheduling analysis were calculated based on the quantity of construction materials and equipment used in each work. Vertical underground extension processes typically include preparation, support installation, demolition and backfill work, retaining walls, pile work, percussion rotary drills (PRDs), temporary post work, strut and excavation work, top-down excavation work, foundation work, structural frame construction, finish work, inspection, and miscellaneous work. The durations of each type of work and relevant construction coefficients were determined by experts based on construction plans, calculated quantities, and labor and equipment productivities [17]. The working experience of the experts was approximately 20 years. The processes to analyze the underground vertical extension costs in this paper are similar to those of Seo et al. [15]. To estimate the costs of the four methods, the calculated quantities and itemized unit costs for several types of work in each construction method were determined based on expert opinions and the construction cost calculation ratio standard provided by the Public Procurement Service (PPS) in South Korea. Material and labor costs, miscellaneous expenses, general administrative expenses, profit, and value-added tax (VAT) were also calculated. The details of the assumed and calculated construction durations and costs are described in Section 6. Lastly, discussion and conclusions are provided in Section 7.

Underground Vertical Extension Methods
The constructability of the four underground vertical extension methods in existing buildings was analyzed in this paper. They were the bottom-up method, normal top-down method after demolition, normal top-down method in parallel with demolition, and topdown method using double beams in parallel with demolition. When carried out in parallel with demolishing existing substructures, top-down methods can also be used for vertical underground extension. In this paper, the normal top-down method and top-down method using double beams were considered as top-down methods in parallel with demolition. In congested urban areas, the top-down method is generally used to improve structural stability of retaining walls, to create available work spaces on basement floors, and to reduce construction durations [2,18]. Temporary or permanent columns should be constructed in the ground ahead of the excavation process to utilize the top-down method for vertically extending existing underground structures. Generally, after demolishment of the existing substructures, the columns are installed to improve their constructability. PRDs are typically used to install columns. However, PRDs have large diameters and their construction costs are relatively large. In contrast, the top-down method using double beams utilizes supports to reinforce existing underground structures and then installs retaining walls and temporary posts. Double-beam systems can reduce the section sizes of steel beams, the amount of steel used, and the heights of basement floors. In particular, the top-down method using double beams might be more efficient for downward vertical expansion in underground spaces because it can reduce the height of extended basement floors. Additionally, the dismantled temporary posts are reusable. The bottom-up method is the most common method. An open-cut method using temporary struts is widely used for retaining walls if the bottom-up method is applied on site. However, this method has several challenges, such as collapse of retaining walls and negative impacts on nearby buildings. Therefore, a thorough approach considering structural safety of the structures should be established when dismantling the installed temporary struts. In general, the top-down method is regarded as a safe option for constructing substructures with excavation.
To determine an appropriate underground vertical extension method by reviewing geological conditions on site and a number of drawings, the following project characteristics should be considered [19]: the number of basement floors in an existing building, the number of basement floors to be extended, whether workspaces on the ground floor are available during construction, whether retaining walls can be constructed, retaining walls type, soil conditions, and foundation type of existing substructures. The process to select an appropriate method for vertically extending underground spaces is illustrated in Figure 2.
Because the net distance between the outer walls of an existing underground structure and adjacent buildings should be sufficient for constructing retaining walls, the distance should be at least 1.2 m in South Korea [20]. Therefore, it is assumed in this paper that the net distance is greater than 1.2 m and that retaining walls are installed inside the existing substructure.

Bottom-Up Method
Underground extension by the bottom-up method requires the completion of excavation work up to the foundation depth with reinforcement of the struts prior to the installation of structural frames of substructures. The underground extension process of the bottom-up method is illustrated in Figure 3. After demolishing entire existing basement floors and backfilling (Figure 3a,b, respectively), the substructure can be reconstructed, thereby vertically expanding the underground space. Retaining walls are installed, and then temporary struts and H-piles are set up to guarantee the structural safety of the substructures during excavation (Figure 3c). Struts are installed at 2.5 m vertical interval. Structural frames of extended substructures are constructed from the foundation to the ground level repeatedly (Figure 3d). If all of the structural frames are constructed, H-piles and struts are finally dismantled (Figure 3e). Figure 3f represents the underground structure that has been extended by the bottom-up method. Figure 4 illustrates the process of demolishing existing substructures and then vertically extending underground spaces applying the top-down method. The method requires the process to construct structural frames in underground spaces from basement first floor (B1F) level to basement third floor (B3F) and the foundation level [21] (Figure 4d-f). Retaining walls are supported by the constructed structural frames, including slabs and beams, without additional temporary struts [13]. This method helps to efficiently manage construction costs and scheduling since available work spaces are provided for laborers on the constructed slabs [13], as shown in Figure 4c,d. In contrast, vertical underground extension, in which the bottom-up method is applied, requires the reinforcement of existing substructures with H-piles and struts to tolerate the weight of used construction equipment.

Bottom-Up Method
Underground extension by the bottom-up method requires the completion of excavation work up to the foundation depth with reinforcement of the struts prior to the installation of structural frames of substructures. The underground extension process of the bottom-up method is illustrated in Figure 3. After demolishing entire existing basement floors and backfilling (Figure 3a,b, respectively), the substructure can be reconstructed, thereby vertically expanding the underground space. Retaining walls are installed, and then temporary struts and H-piles are set up to guarantee the structural safety of the substructures during excavation (Figure 3c). Struts are installed at 2.5 m vertical interval. Structural frames of extended substructures are constructed from the foundation to the ground level repeatedly (Figure 3d). If all of the structural frames are constructed, H-piles and struts are finally dismantled ( Figure  3e). Figure 3f represents the underground structure that has been extended by the bottom-up method.

Normal Top-Down Method in Parallel with Demolition
The normal top-down method can also be applied for vertically extending underground spaces in parallel with demolition of existing substructure. Figure

Top-Down Method Using Double Beams in Parallel with Demolition
A process for vertically extending underground spaces of existing buildings using the top-down method with double beams in parallel with demolition is illustrated in Figure 6. When the top-down method using double beams is applied, the processes for installing supports and retaining walls are the same as those in the normal top-down method in parallel with demolition ( Figure 6a). To avoid the collapse of columns in Sustainability 2021, 13, 3647 6 of 20 existing underground structures during excavation, it is required to install temporary posts instead of PRDs to support loads during construction, as shown in Figure 6b. Two or four temporary posts are installed around one column. After the extension process is completed, these posts can be removed and reused in other projects. Slabs, beams, and columns on the first basement floor of existing underground structures are partially demolished and girders for supporting double beams are installed (Figure 6c). The support girders of double-beam systems consist of two rows of steel beams. The details of double-beam systems are explained in the paragraph above Figure 7. Deck plates are placed on top of the installed double beams to construct slabs on the ground floor ( Figure 6d).  Figure 4 illustrates the process of demolishing existing substructures and then vertically extending underground spaces applying the top-down method. The method requires the process to construct structural frames in underground spaces from basement first floor (B1F) level to basement third floor (B3F) and the foundation level [21] (Figure 4d-f). Retaining walls are supported by the constructed structural frames, including slabs and beams, without additional temporary struts [13]. This method helps to efficiently manage construction costs and scheduling since available work spaces are provided for laborers on the constructed slabs [13], as shown in Figure 4c,d. In contrast, vertical underground extension, in which the bottom-up method is applied, requires the reinforcement of existing substructures with H-piles and struts to tolerate the weight of used construction equipment.     Figure 4 illustrates the process of demolishing existing substructures and then vertically extending underground spaces applying the top-down method. The method requires the process to construct structural frames in underground spaces from basement first floor (B1F) level to basement third floor (B3F) and the foundation level [21] (Figure 4d-f). Retaining walls are supported by the constructed structural frames, including slabs and beams, without additional temporary struts [13]. This method helps to efficiently manage construction costs and scheduling since available work spaces are provided for laborers on the constructed slabs [13], as shown in Figure 4c,d. In contrast, vertical underground extension, in which the bottom-up method is applied, requires the reinforcement of existing substructures with H-piles and struts to tolerate the weight of used construction equipment.  spaces in parallel with demolition of existing substructure. Figure 5 illustrates the vertical underground extension process of the normal top-down method in parallel with demolition. To utilize the normal top-down method in parallel with demolition, one should install supports to reinforce existing basement floors (Figure 5a) before constructing retaining walls and PRDs (Figure 5b), unlike the normal top-down method after demolition. The other construction processes of the normal top-down method in parallel with demolition are similar to the normal top-down method after demolition as shown in Figure 5c-f.

Top-Down Method Using Double Beams in Parallel with Demolition
A process for vertically extending underground spaces of existing buildings using the top-down method with double beams in parallel with demolition is illustrated in Figure 6. When the top-down method using double beams is applied, the processes for installing supports and retaining walls are the same as those in the normal top-down method in parallel with demolition ( Figure 6a). To avoid the collapse of columns in existing underground structures during excavation, it is required to install temporary posts instead of PRDs to support loads during construction, as shown in Figure 6b. Two or four temporary posts are installed around one column. After the extension process is completed, these posts can be removed and reused in other projects. Slabs, beams, and columns on the first basement floor of existing underground structures are partially demolished and girders for supporting double beams are installed ( Figure 6c). The support girders of double-beam systems consist of two rows of steel beams. The details of double-beam systems are explained in the paragraph above Figure 7. Deck plates are placed on top of the installed double beams to construct slabs on the ground floor ( Figure 6d).
Prior to dismantling temporary posts and girders (Figure 6f), new foundations and columns in the extended substructures should be constructed (Figure 6e). The top-down method using double beams helps reduce the amount of steel and height of basement floors because the steel beams are smaller than those used in the other methods.  Figure 7 illustrates the detailed construction process for applying the double-beam system. As shown in Figure 6b,c, temporary supports and girders for supporting double beams should be installed before installing double beams on the constructed girders and brackets (Figure 7c). Slabs are then constructed on the installed double beams and brackets (Figure 7d). Additional structural frames, such as reinforced concrete (RC) columns, are constructed on the basement floor and the installed temporary elements, including posts and girders, are dismantled. Prior to dismantling temporary posts and girders (Figure 6f), new foundations and columns in the extended substructures should be constructed (Figure 6e). The top-down method using double beams helps reduce the amount of steel and height of basement floors because the steel beams are smaller than those used in the other methods. Figure 7 illustrates the detailed construction process for applying the double-beam system. As shown in Figure 6b,c, temporary supports and girders for supporting double beams should be installed before installing double beams on the constructed girders and brackets (Figure 7c). Slabs are then constructed on the installed double beams and brackets (Figure 7d). Additional structural frames, such as reinforced concrete (RC) columns, are constructed on the basement floor and the installed temporary elements, including posts and girders, are dismantled. Figure 7 illustrates the detailed construction process for applying the double-beam system. As shown in Figure 6b,c, temporary supports and girders for supporting double beams should be installed before installing double beams on the constructed girders and brackets (Figure 7c). Slabs are then constructed on the installed double beams and brackets (Figure 7d). Additional structural frames, such as reinforced concrete (RC) columns, are constructed on the basement floor and the installed temporary elements, including posts and girders, are dismantled.

Overview of Illustrative Examples
To analyze differences in cost and scheduling among the four extension methods, structural analysis, design, and process analyses were conducted for illustrative example building. An actual residential building in South Korea that would be remodeled and vertically expanded was determined as a case to create illustrative examples.

Overview of Illustrative Examples
To analyze differences in cost and scheduling among the four extension methods, structural analysis, design, and process analyses were conducted for illustrative example building. An actual residential building in South Korea that would be remodeled and vertically expanded was determined as a case to create illustrative examples.
The building has one basement floor with 75 car parking spaces with dimensions of 5.5 × 5.2 m, as shown in Table 1. The example building was considered for expansion of underground spaces from B1F to B3F, basement fourth floor (B4F), or basement fifth floor (B5F). The remodeled building includes 59 car parking spaces with dimensions of 7.8 × 9.0 m in each extended basement floor. The module size of the parking spaces was updated according to the revised enforcement regulations for parking lots in South Korea. Every example has one basement floor before extending underground spaces. As shown in Table 2, 12 illustrative examples were created through considering types of vertical underground extension methods, structure types, and the number of extended basement floors (from B3F to B5F). Each vertical extension method has three illustrative examples (extending to B3F, B4F, and B5F). The structure type of the bottom-up method is RC and those of the normal top-down method after demolition, in parallel with demolition, and using double beams are steel frames.

Structural Analysis
This study also analyzed the structural stability of the 12 illustrative examples. We used MIDAS Gen 2017 to analyze the structural stability in phases during the demolishing of the existing basement floor, excavating to the lowest level, and constructing structural frames of the extended substructure. The designed structural members can be classified into two types. The first one consists of members for securing the safety of the entire structures after completing extension of the substructures. The other type consists of members for guaranteeing structural safety in the demolition, excavation, and construction stages. If the bottom-up method is applied, excavation to the lowest floor level of extended underground spaces should be finished prior to construction of structural frames of the extended underground structure. Therefore, the structural stability of the extended RC substructure was checked.
This paper assumed the ground condition to be sandy soil in the 12 illustrative examples. We designed the examples so that there were installed temporary supports with 1.5 m intervals in the empty underground spaces. This ensures structural safety of the existing underground space when construction equipment is placed on the ground floor. Since building materials can be placed and stored on the ground floor, the live loads acting on the ground floor and basement floors were assumed to be 20.0 kN/m 2 and 1.5 kN/m 2 during construction, respectively. On the other hand, after completion, the live load on the ground floor and basement floor were 5.0 kN/m 2 and 3.0 kN/m 2 , respectively. Based on the Korean Building Code of 2016, the structures of the examples were designed and analyzed.
When the four methods are applied to expand underground spaces (from B1F to B3F) of an example case, the structural analysis results and the size of the required main structural members for each method are explained in this section. In consideration of the roles of temporary members and structural members after extending substructure, column and beam sizes were designed. By contrast, the sizes of strut and H-piles were designed based on the requirements of temporary members only. Figure 8 shows a structural plan for the B1F. Temporary struts were required to be installed at a depth of 2.5 m for cases applying the bottom-up method. On the other hand, because the structural frames of extended underground spaces, including beams and slabs, take the place of temporary struts, they are not considered for the structural analysis in these examples.

Bottom-Up Method
The structural system of substructure extended by the bottom-up method was the conventional RC frame system. Table 3 presents the sizes of main structural members and ratios of acting stress to the corresponding holding strengths of each member by force type. The highest values can be observed in the girders on the ground floor, where the ratio of force to holding strength reaches 0.992. These results indicate that the members are optimally designed. Column C1 (B5F) and strut ST7 have axial force ratios of 0.987 and 0.773 during construction, respectively. Therefore, it could be concluded that the designed structural members are safe for the applied load.
of 2016, the structures of the examples were designed and analyzed.
When the four methods are applied to expand underground spaces (from B1F to B3F) of an example case, the structural analysis results and the size of the required main structural members for each method are explained in this section. In consideration of the roles of temporary members and structural members after extending substructure, column and beam sizes were designed. By contrast, the sizes of strut and H-piles were designed based on the requirements of temporary members only. Figure 8 shows a structural plan for the B1F. Temporary struts were required to be installed at a depth of 2.5 m for cases applying the bottom-up method. On the other hand, because the structural frames of extended underground spaces, including beams and slabs, take the place of temporary struts, they are not considered for the structural analysis in these examples.

Bottom-Up Method
The structural system of substructure extended by the bottom-up method was the conventional RC frame system. Table 3 presents the sizes of main structural members and ratios of acting stress to the corresponding holding strengths of each member by force type. The highest values can be observed in the girders on the ground floor, where the ratio of force to holding strength reaches 0.992. These results indicate that the members are optimally designed. Column C1 (B5F) and strut ST7 have axial force ratios of 0.987 and 0.773 during construction, respectively. Therefore, it could be concluded that the designed structural members are safe for the applied load.

Normal Top-Down Method after Demolition
For the structural members of underground spaces extended by the normal top-down method after demolition, steel girders and steel-reinforced concrete columns were used. Table 4 lists the sizes of used structural members and the ratios of acting stress to their holding strengths. The ratios of the holding strengths of the girders reach 0.810 and 0.867 at G1 and G3, respectively. For the columns, the ratio reaches 0.357 at C3. The maximum ratios of the columns are smaller than those of other members because the top-down method requires steel reinforced concrete (SRC) columns that are composed of PRD-installed steel columns and concrete. Therefore, all examples that apply the normal top-down method after demolition are structurally stable and the structural members are designed optimally.

Normal Top-Down Method in Parallel with Demolition
The demolition process of existing substructures using the normal top-down method in parallel with demolition is different from that of the normal top-down method after demolition. However, the process for constructing structural frames of substructures extended by the normal top-down method in parallel with demolition is very similar to that by the normal top-down method after demolition. Therefore, the results of structural analyses for these two methods are similar. Table 5 shows the sizes and ratios of structural members in applying the top-down method using double-beam systems. Structurally, high strength ratios of 0.965 and 0.910 are observed at C1 in B5F and B4F, respectively. Because ratios of actual stress to holding strength of every structural member are less than 1.0, the examples applying the top-down method using double-beam systems are structurally stable. As shown by the results of the structural analysis of substructures extended by the four methods, all of the examples were structurally stable.

Results and Analysis
The constructability issues in the four extension methods were analyzed and compared from the perspectives of construction cost and scheduling. The construction cost of each method was calculated based on actual quantities of materials and laborers. Construction scheduling was predicted using general construction durations from sites in South Korea based on a schedule planning method for top-down methods proposed by Lee et al. [22].

Scheduling Perspective
The vertical underground extension methods involve several types of work, including preparation, support installation, demolition and backfill work, retaining walls, pile work, PRDs, temporary post work, strut and excavation work, top-down and excavation work, foundation work, structural frame construction, finish work, inspection, and miscellaneous work. The detailed construction durations of each type of work were assessed by experts based on calculated quantities and productivities for each type of work, as shown in Table 6. To calculate the amount of excavated soil, the daily excavation workload and construction coefficient should be considered. In this paper, the daily excavation workload was assumed to be 300 m 3 per day, which is consistent with the average daily excavation workload (300 to 400 m 3 ) in Seoul, South Korea. The construction coefficients for the strut and top-down methods are 1.0 and 0.9, respectively. Table 7 presents the predicted construction durations of each work for vertically expanding underground spaces using the four methods. From the scheduling perspective, the top-down method using double beams in parallel with demolition is the most efficient method for vertically expanding underground spaces of existing buildings. In addition, it is more efficient in the order of the top-down method in parallel with demolition, top-down method after demolition, and bottom-up method.
As the number of basement floors to be expanded increases, the differences among the total construction durations of each extension method generally increase. The increase rates of the construction durations for constructing structural frames of underground spaces using the bottom-up method are greater than those for performing finishing work using the top-down methods. Therefore, from the scheduling perspective, the greater the number of basement floors to be extended, the more advantageous the top-down method using double beams.
The total construction durations of the three top-down methods for extending existing underground spaces are approximately 9% to 25% shorter than those of the bottom-up method in the illustrative examples because the top-down methods do not include the construction duration required for installing structural frames of extended underground spaces. However, the differences among the total construction durations of the top-down methods and bottom-up method for vertically extending underground spaces are smaller than those related to installing structural frames for extended underground spaces because the top-down methods should additionally consider the construction duration of finishing work, unlike the bottom-up method.
Top-down methods in parallel with demolition processes are more effective than the top-down method after demolition from the scheduling perspective. Top-down methods in parallel with demolition should consider construction durations for installing supports for existing underground spaces, but they do not consider the construction duration for demolishing entire existing substructures and backfilling prior to installation of retaining walls, unlike the top-down method after demolition. In South Korea, the construction duration for support installation (20 working days) is generally shorter than that for demolition and backfilling work (40 days). If the top-down method using double beams in parallel with the demolition process is applied, we should also consider the construction duration for temporary post-work (13 days) but should not consider the construction duration for PRDs (31 days). Therefore, the top-down method using double beams might eliminate 18 working days.    The construction durations per extended basement floor of the four construction methods were also compared in this study (Figure 9). The construction duration per extended basement floor of the top-down method using double beams in parallel with demolition was generally the shortest, followed by the normal top-down method in parallel with demolition, top-down method after demolition, and bottom-up method. The construction duration per extended basement floor when applying the top-down method using double beams in parallel with demolition was approximately 75% to 78% of that of the bottom-up method. Interestingly, the decrease rate in construction duration per extended basement floor when applying the normal top-down method after demolition (35%) was slightly greater than those of the normal top-down methods in parallel with demolition and using double beams (34% and 32%, respectively).

Cost Perspective
The quantities and itemized unit costs for several types of work for each construction method were calculated by experts in South Korea based on the construction cost calculation ratio standards published by the PPS in South Korea. Table 8 lists the used construction cost calculation ratio standards. Material and labor costs, miscellaneous expenses, general administrative expenses, profits, and value-added tax (VAT) were considered in this study. Table 9 lists the calculated total extension costs of the illustrative examples. From the cost perspective, the top-down method using double beams in parallel with demolition is the most efficient method for vertically expanding existing underground spaces. In addition, it is more economically advantageous in order of the normal top-down method after demolition, normal top-down method in parallel with demolition, and bottom-up method. In all examples, as the number of basement floors to be expanded increases, the differences among the total underground extension costs of each construction method generally increase. Therefore, the greater the number of basement floors to be extended, the lower the relative vertical underground extension cost of the top-down method using double beams.   --172  172  --172  172  --172  172   Demolition  505  505  505  505  505  505  505  505  505  505  505  505   Backfill  53  53  --53  53  --53  53  --Retaining wall  2013  2013  2013  2013  2704  2704  2704  2704  3349  3349  3349  3349   Pile  1104  730  730  730  1480  978  978  978  1856  1227  1227  1227   PRD  -180  180  --240  240  --299  299  -Temporary post  ---275  ---387  ---479   Strut  290  ---363  --- The vertical underground extension costs of the top-down methods are approximately 7% to 9% lower than that of the bottom-up method because the construction costs of structural frames for vertically extending underground spaces and piling processes using the top-down methods are lower than those of the bottom-up method. However, the costs of top-down construction using steel frames should be considered when applying top-down methods to increase the number of floors in existing underground structures. Unlike the trends in the scheduling perspective, the normal top-down method after demolition is more effective than that in parallel with demolition from the cost perspective because the cost associated with supporting existing underground spaces during top-down construction processes ($172,000) is greater than the backfilling costs ($54,000) of the top-down method after demolition. The most economical method is the top-down method using double beams in parallel with the demolition process because this method might reduce the costs of PRDs, top-down construction, and structural frame construction simultaneously, unlike the other methods.
The extension costs per basement floor of the four methods were also compared in this study ( Figure 10). The cost per extended floor for the top-down method using double beams in parallel with demolition is generally the lowest, followed by the top-down method after demolition, top-down method in parallel with demolition, and bottom-up method. The total extension cost per basement floor when applying the top-down method using double beams in parallel with demolition is approximately 90% of that of the bottom-up method. The decrease rates in the extension cost per basement floor of the four methods as the number of floors increases are similar at 18% to 19%.

Discussion and Conclusions
In this study, we analyzed the constructability of the four methods for vertically extending existing underground spaces of buildings, namely the bottom-up method, normal top-down method after demolition, normal top-down method in parallel with demolition, and top-down method using double beams in parallel with demolition. To analyze constructability of each method, their cost and scheduling perspectives were considered. The vertical underground extension costs and construction durations of 12 illustrative examples were calculated and compared in this study. Considering the number of basement floors extended vertically by the four methods, 12 illustrative examples were created. Their structural stabilities were also verified in advance.
Generally, the top-down methods were more effective than the bottom-up method from both the cost and scheduling perspectives. Among the four vertical underground extension methods, the top-down method using double beams was the most beneficial method. It was more beneficial in order of other top-down methods and the bottom-up method. From the financial perspective, the normal top-down method after demolition was more economical than that in parallel with demolition, whereas the opposite trend appeared from the scheduling perspective. Because the construction processes of the two top-down methods are different, different work types and workloads are required to apply different methods. Therefore, the cost savings of both methods differ from the time savings. The larger the number of basement floors to be expanded, the more financially and periodically beneficial the top-down method using double beams in parallel with demolition is in comparison to other methods. Interestingly, the benefits of schedule management for the top-down method using double beams are greater than those for the other methods.
Comparative analysis of the constructability issue of the vertical underground extension methods can help determine adequate methods to expand underground spaces without demolishing entire existing buildings and with efficient management of costs and schedules. Although examples were presented and analyzed from the two perspectives in this study based on calculated quantities, regulations, realistic assumptions, etc., our findings have not yet been applied to real-world projects. In South Korea, the government and private sector have selected pilot projects for applying vertical underground extension methods, such as large-scale residential complexes, to improve such environments and increase the number of parking lots in underground spaces. However, these projects have been delayed based on safety concerns and the absence of relevant regulations.
Furthermore, this paper focused on building systems in South Korea only. As the unit costs of construction materials, productivity indexes, and levels of construction technologies used vary across different countries, the impacts of the four vertical underground extension methods on time and cost reduction in other countries may differ from those in South Korea. Therefore, the methodology for calculating costs and scheduling of extending underground spaces can be applied to other countries with minor adjustments to such variables. However, we expect that the quantity of construction materials that would be used and dismantled for extending underground spaces without demolishing entire existing buildings would be similar, regardless of the aforementioned regional differences. Therefore, various vertical underground extension methods should be applied to realworld projects in other countries to calculate, validate, and verify their actual effects on construction costs and scheduling in the future.