Post-occupancy evaluation of the urban underground complex: A case study of Chengdu Tianfu Square in China

ABSTRACT The urban underground complex (UUC) is an important approach in the utilization of urban underground spaces in high-density Asian cities. Presently, UUCs in China are large in scale and in quantity, which makes it necessary to assess the condition of UUCs and evaluate whether they meet users’ requirements. This can serve as feedback and optimize the design and construction of UUCs. This research employs post-occupancy evaluations (POEs) to determine the major problems and concerns among UUC users and assess their satisfaction levels, taking the Chengdu Tianfu Square UUC as an example. Field interviews and online and field questionnaire surveys were used to establish an index set. The results revealed that transport convenience and transfer efficiency were the most frequently mentioned issues by users. They were less satisfied with the UUC’s walking accessibility, internal physical environment, and orientation. The spatial morphology and texture, space proportions, color, decoration, materials, and overall planning had a higher correlation coefficient with the overall user satisfaction results. Eight main factors were identified through factor analysis, which represents the eight key aspects that the design of UUCs should focus on.


Introduction
Utilizing underground space is an important approach to solving the problems of traffic congestion, shortage of land resources in high-density areas, and environmental pollution in the process of rapid urbanization . It is also an effective way for cities to achieve sustainable development (Hunt, Nash, and Rogers 2014). With the rapid development of economy, science, and technology, as well as the diversified demands of users, the connotation of urban underground space continues to expand. The concept of an "underground city" was introduced by Golany and Ojima (1996), laying the theoretical foundation for the development of a modern urban underground space in the direction of integration and systematization. Meanwhile, the concept of the urban underground complex (UUC) emerged (Tong 1994). The UUC is a public, transit-oriented, urban underground architectural structure that has multiple functions, such as commerce, office, culture, and entertainment (Cui, Broere, and Lin 2021). The value of a UUC is to realize the integration of urban resources through the three-dimensional organization of space, comprehensive layout of function, and high-quality environment -with the purpose of mitigating the problems of space capacity shortage for urban centers, high population density, and traffic congestion, and thereby promoting the sustainable development of cities .
In recent years, UUCs in China have been characterized by large-scale development and rapid construction (Cui, Broere, and Lin 2021). Based on earlier data, there were more than 200 UUCs with a total area of more than 10,000 m 2 , and nearly 100 with a total area of over 20,000 m 2 (Liu, Qiao, and Peng 2017). Today, UUCs that have been built or are undergoing planning have already exceeded this number. Among the large-scale UUCs that have successively been built are the culture wave city in Hangzhou Qianjiang New CBD (100,000 m 2 ) and Shenzhen Futian underground transportation hub (137,300 m 2 ) (Qian 2019;Chen et al. 2018).
Although the utilization of UUCs has made a series of developments in China, there are still some issues that need to be solved (Yu and Yan 2021). The development of UUCs lacks overall consideration, and their functions are not comprehensive (Geng and Liu 2013). For example, the entrance of Huachenghui, the underground commercial hub in Zhujiang New CBD, Guangzhou, is not well designed to connect with the ground level -resulting in less demand among passersby (Liu, Qiao, and Peng 2017). The lack of environmental planning research on UUCs has led to insufficient theoretical basis for their design, resulting in a lack of comfort, orientation, and security of the internal environment (Ma 2009).
The above issues occur because of the lack of post-occupancy evaluations (POEs) targeting users' needs, which may lead to the problem that the built environment does not match user requirements. Thus, POE on UUCs is urgently needed to improve the quality of UUCs. The results and evaluations are of practical value to the relevant stakeholders, such as planners, design professionals, and developers.

Urban underground complexes
UUCs are often called "underground streets" or "underground shopping malls" in Japan (Mi and Long 2019), while in Europe and the United States, they are often referred to as "underground cities" (Zheng 2012;Jia 2015;Zacharias 2015). Presently, the term "urban underground complex" is mainly used by Chinese scholars. The work that earliest defined a UUC pointed out that it started from the term "building complex" to "urban complex," and then evolved with the three-dimensional redevelopment of the city, which was associated with the urban underground space; this work emphasized the evolution of the UUC and the expansion of its urban functions (Tong 1994). Other scholars have defined a UUC as a threedimensional network that connects, transports, and transforms both the aboveground and underground systems (Li 2006).
There are two main categories of studies related to UUCs. The first category mainly considers the integrated planning and design of internal and external spaces. One of the main characteristics of a UUC is its openness to the city, which makes it necessary to fuse the elements of the underground and aboveground through urban design to realize the integrated design (Zheng 2012). Many scholars have conducted case studies on typical UUCs that have been built, and qualitatively put forward design strategies in terms of functional layout, vertical planning, and traffic flows from the perspective of integrated planning and the design of internal and external spaces (Xie, Jia, and Yang 2012;Liu 2020).
Transportation is the main factor promoting UUC development. In recent years, China has made great efforts to develop rail transit systems based on the concept of transit-oriented development (Calthorpe 2013), which has also accelerated the development of UUCs around rail transit stations. Yu and Yan (2021) have found that the typical problems generally experienced by UUCs are poor user experience -which makes users unwilling to stay -and the lack of necessary functions. They also proposed that more attention should be paid to the overall design of surrounding areas in the early stage of rail transit planning so as to form a network form of UUCs.
The transfer space of a UUC is an important node that connects internal traffic with external traffic. Gao (2019) found that the main factors affecting the design of a transfer space are function, place, and experience. In the context of urban renewal, the public space of UUCs has also played an important role. Chen (2018) conducted a questionnaire survey of typical public spaces and a survey of space usage from three different aspects: the perspective of the city, the interface with the city, and the internal public space. Based on the results, corresponding optimization strategies were proposed.
The other category of UUC research mainly focuses on how to improve the quality of UUCs in order to meet the needs of users. UUCs appear to be less desirable to users as they may be associated with darkness, coldness, dampness, poor air quality due to insufficient ventilation, fear of confinement, and other safety concerns (Nang et al. 2019). Xue et al. (2021) used the semantic differential method to study UUCs and found that atmospheric factors, such as the attractiveness of the decorations, the color of the space, and the quality of the space, were extremely important in shaping users' sense of security. In addition, wayfinding is another important issue when examining UUCs. Chen, Chen, and Zhu (2016) suggested that the reasons for user difficulty in wayfinding were the unreasonable organization of internal space and unclear spatial logic. They also suggested that UUCs' orientation should be enhanced through the creation of spatial morphology, decorations, and characteristics. There are also studies using point of interest data to analyze how to improve the vitality and accessibility of the public space of UUCs; these studies use methods such as space syntax or the establishment of an evaluation system (Xu and Chen 2021) so as to enhance people's willingness to use UUCs and to improve its space efficiency.

Post-occupancy evaluation of commercial spaces
The concept of POE was based on environmental psychology in the 1960s. The theory and methods of POE have been developed for over half a century and have formed a mature system. Early POEs mainly focused on case studies, such as the physical environment of student dormitories, office buildings, and users' perceptions (Preiser 2005). In 1988, scholars defined POE as the systematic and rigorous process of evaluation after a building is built and used for a period of time (Preiser, Rabinowitz, and White 1988). In the book Building Evaluation Techniques (Gray et al. 1995), 120 POE concepts, techniques, and tools were introduced. Since then, the research scope and objectives of POEs have gradually diversified. In 2002, the connotation of a POE was expanded to all research carried out regarding the performance of a building (whether the building meets users' expectations), including users' satisfaction with the environment. The objects of POE have been extended to public buildings, urban spaces, and historical blocks, among others. (Li, Froese, and Brager 2018). The process of POE consists of three vital phases: planning, conducting, and applying. These vital phases must be implemented in any survey instrument in order to evaluate the various aspects of building performance (Husin et al. 2015).
One of the main focuses of POE research on commercial spaces is their internal public space. This type of space serves as a framework for connecting various functions. Wang (2017a,b) conducted a POE study on the public space of a large shopping mall in Shenzhen. An evaluation system consisting of five dimensions -i.e., functional layout, spatial organization, signage system, space morphology, and guidance system -was established. Due to the complexity of the internal space, wayfinding is also an important issue considered by scholars. Youm (2013) suggested that users' wayfinding abilities would partially affect user satisfaction and affect revisitation.
With the rapid construction of subways, underground shopping centers have become an important part of life. Han, Jang, and Choi (2017) conducted a survey on seven underground shopping centers in the Busan metropolitan area to understand the factors that determine user satisfaction. The results showed that service satisfaction, underground environment satisfaction, and public facility satisfaction should be taken into consideration. In addition, the connection space between subway stations and commercial spaces has also been studied by scholars. Wang (2017a,b) conducted a POE study on the connecting space between rail transit stations and commercial complexes in downtown Shanghai. Through the data gathered using questionnaires and interviews, the Statistical Package for the Social Sciences (SPSS) program was used to establish an evaluation system consisting of seven categories, such as transport functions, traffic flow, external space, internal space, architectural design, green landscape, and supporting facilities.
However, few POE studies have been conducted on UUCs. Existing studies were mostly carried out from a certain perspective, and the lack of systematic research needs to be enhanced. How effective are existing UUCs in China? Do they match the needs of users? Further research on POE is needed. Therefore, this study aims to use the POE method to discover the major concerns of users, understand their satisfaction level, and identify the factors that have a higher correlation coefficient with overall user satisfaction level, taking the Chengdu Tianfu Square UUC as an example. The results may provide data and suggestions for optimizing the planning and design of UUCs.

Case study: the Chengdu Tianfu Square UUC
The Tianfu Square UUC is located at Tianfu Square in the urban center of Chengdu, Sichuan. It is within an economic, cultural, and commercial center, which also reflects the image of the city (Figure 1). It was built in September 2010 and has been used for more than 10 years. The total area is approximately 100,000 m 2 , with four underground floors. The first and second floors are commercial and underground pedestrian walkways, respectively. The third and fourth floors are the platforms of Chengdu Metro Lines 1 and 2. The ground level is the Tianfu Square and green space. Its main functions include metro stations (transfer stations for Lines 1 and 2), underground retail, catering, leisure, entertainment, and cultural exhibitions ( Figure 2). It is one of the earliest and most typical examples of UUCs in Chengdu. The scope of this research comprises the areas that can be reached and used by occupants, including underground pedestrian systems, underground parking spaces, functional spaces (e.g., those used for commerce, catering, leisure, culture, and entertainment), supporting spaces (e.g., staff offices, bathrooms), and node spaces (e.g., sunken plazas, atriums, entrances, and exits of the ground level).

Post-occupancy evaluation approaches
After distributing online questionnaires (see Sections 2.2.1) and conducting interviews with the staff of the Tianfu Square UUC (see Sections 2.2.2), the frequency and percentage of the collected data were analyzed to identify the major problems, concerns, and factors from multiple perspectives (see Sections 3.1 and 3.2). A factor set for the satisfaction evaluation, which contained 35 variables, was established. The Chengdu Tianfu Square UUC was chosen as a typical case for the satisfaction evaluation (see Sections 2.2.3). The SPSS program was used to perform statistical analyses and explore the main factors that affect the satisfaction evaluation results(see Sections 3.3). Finally, targeted recommendations were proposed.

Online questionnaires
An online questionnaire survey for users of UUCs was conducted(Appendix). The questionnaires were randomly distributed through online survey platforms.
A total of 465 questionnaires were distributed, and 415 valid questionnaires were returned. The efficiency of the questionnaire was 89.2%. The purpose was to obtain information and expectations of users' overall attitude toward UUCs in China; to determine the major problems surrounding UUCs; and to understand users' needs and concerns, which also provided an objective basis for the initial selection of users' satisfaction evaluation index set.

Interviews with the staff of the Tianfu Square urban underground complex
In order to determine the problems surrounding the Tianfu Square UUC and the factors that mainly concerned the users thereof, a field interview with 13 staff members of the UUC was carried out (Appendix). Major problems and factors that may influence user satisfaction were collected and analyzed using open-ended questions. In addition to online questionnaires, this is another way of providing an objective basis for the initial selection of the user satisfaction evaluation index set.

Field questionnaires for user satisfaction
From the perspective of users' needs, a user satisfaction evaluation of the Tianfu Square UUC was likewise carried out. Field questionnaires, which were divided into two sections, were distributed to collect data. The first section was a survey of the respondents' demographic characteristics (e.g., gender, age, occupation, and education level) as well as their travel characteristics (e.g., travel purposes, tools, time consumption, usage frequency, and duration of usage). The second section was the users' satisfaction, which was rated using a 5-point Likert scale. The scores of 5, 4, 3, 2, and 1 indicated that they were very satisfied, satisfied, moderately satisfied, dissatisfied, and very dissatisfied, respectively.

Data collection.
The questionnaires were randomly distributed to respondents at various points in the Tianfu Square UUC. The distribution time was on weekdays and weekends. Sloven's formula was used to determine the minimum number of respondents (n) to render the study statistically valid (Muizz and Mohammad 2016), as follows: The effective population size (N) in this study was 80,000. The sample error (e) was considered as 0.05. This means that there is 95% confidence that the sample size accurately represents the population. This resulted in a sample size (n) of 398 respondents. A total of 450 questionnaires were distributed, and 403 valid questionnaires were returned, with an efficiency rate of 89.6%.

Reliability and validity tests. The reliability
of the questionnaire was tested using Cronbach's alpha, which estimates the internal consistency of the test. As an indicator of reliability, it is often used by scholars in social science research. Cronbach's alpha coefficients range from 0 to 1. The closer the value is to 1, the higher is the consistency. However, the limitation is that the value will increase with an increase in items of the scale. Therefore, the Cronbach's alpha coefficient may be inappropriately increased due to redundant items being included in the scale. The present result was 0.925, which was higher than the acceptable level of 0.70 (Field 2000). SPSS was used to perform the factor analysis. First, the Kaiser-Meyer-Olkin (KMO) Test and Bartlett's Test of Sphericity were performed. When the KMO value is greater than 0.5 and the Bartlett's Test of Sphericity significance level is less than 0.05, it indicates that the questionnaire has structural validity. The results showed that the KMO value was 0.914, and the significance level was 0.000, which was suitable for factor analysis (Table 1). Based on the principal factor and varimax rotation method, a total of eight common factors with characteristic roots greater than 1 were extracted, with their cumulative variance contribution being 60.051%, which was more than 50% (the acceptable level). The load of each common factor was between 0.404 and 0.774, indicating that the questionnaire had good validity and was suitable for further factor analysis.

Online questionnaires for users
Among the respondents who participated in the online questionnaire survey, the percentage of female respondents was 53.98%, which was higher than that of male respondents (46.02%). Users between the ages of 20 and 29 constituted the largest group (41.20%), followed by those between 30 and 39 (39.04%), and middle-aged users between 40 and 49 (11.33%). In terms of occupation, employees accounted for the largest group (43.13%), followed by middle-level cadres or managers (23.13%) ( Table 2).
The respondents were asked: "What is your overall satisfaction with UUCs in China?" The results (Table 3) showed that users' overall impression of UUCs in China is good, meaning that great effort has been put into UUCs in China during planning, design, and construction. UUCs have evolved from having traditional traffic functions or air defense functions to more user-friendly and diversified functions. However, 27.71% of respondents reported being moderately satisfied or unsatisfied with UUCs.
The major problems mentioned by users were sorted by frequency (Table 4), and concentrated on the service facility, the connection between the UUC and its surrounding environment, the scale of the UUC, the internal physical environment, the accessibility of the UUC, and orientation.   The respondents were also asked: "What do you think is the most important factor affecting users' experience of UUCs?" The purpose was to understand the main factors from the users' perspective. The results (Table 5) showed that the users emphasized transport convenience and transfer efficiency, the layout of functions, and service facilities, which are more relevant to their daily use. Orientation and internal psychological environment were also important, both of which influenced their travel efficiency and the perception of UUCs.

Field interview with staff of the Tianfu Square UUC
After conducting field interviews and open questionnaires with the staff of the Tianfu Square UUC, major problems were summarized (Table 6), and the factors mentioned at a higher frequency were recorded ( Table 7). The results showed that transport convenience and transfer efficiency are the most concerning factors, because solving traffic problems is the key motivation of UUC utilization ). However, the connection between the UUC and its surroundings is weak. The second most concerning factor is the internal environment. With no external façade, it is the internal space that directly affects users' perceptions of an underground space (Durmisevic and Sariyildiz 2001). The staff was not satisfied with the physical environment, such as temperature and lighting. Psychological aspects, such as space proportion and cultural elements, were also unsatisfactory. The safety operation and management of the UUC are also important factors. Due to the enclosed features of underground buildings, once an emergency occurs, users must be evacuated quickly. Hence, it is crucial to design proper entrances, exits, and signage systems to improve their safety. In addition, the staff expressed that their work efficiency is directly related to the stability and efficiency of the equipment; therefore, it was frequently mentioned that more advanced equipment was needed. In terms of orientation, owing to the enclosed feature of UUCs, occupants often get lost -especially the elderly and users travel for consumption. Therefore, a clear and uniform signage system and a space design that enhances the identity of the space are needed.
Comparing the results between the online questionnaires and the staff interviews, both the visitors and staff thought transport convenience and transfer efficiency were the most important factors. They both mentioned that the connection between the UUC and its surroundings is weak and inconvenient. In addition, because the staff had been in an underground space for a long time, they highlighted the quality of the internal environment (e.g., the comfort of the physical environment, such as the air, light, and sound) as well as factors related to the psychological environment (e.g., the proportion of space and cultural elements). Visitors, such as subway riders, paid more attention to functional factors, such as retail and catering. The service facility was a factor that both groups focused on. The difference is that visitors are more concerned about public service facilities that are related to their daily use, such as seats, and the staff focused on the stability and capabilities of equipment from a management perspective. Orientation was a key factor mentioned by both groups, and directly affects UUC user efficiency. Safety operation and management factors were more important to the staff. These results provide an objective basis for building an evaluation index set. 3.3.1.1. Profiles and trip patterns. Based on the survey results, the users of the Tianfu Square UUC comprised 50.12% male respondents and 49.88% female respondents. Users between the ages of 20 and 29 constituted the largest group (34.74%), followed by those aged below 20 (17.37%), and middle-aged users between 40 and 49 (12.41%). In terms of occupation, the proportion of students and employees was relatively high (26.55%) ( Table 8). The statistics indicated that the users were mainly young people, bringing vitality to the UUC. Meanwhile, the UUC is also attractive to middle-aged and elderly people. Thus, it is crucial to plan and design based on the various needs of different user groups.

Respondents' profiles and satisfaction
(1) Respondents' travel purposes, time, and mode The results showed that the majority of users travelled to the UUC for leisure (25.06%), followed by hanging out (17.87%) and for trips (15.88%) (Figure 3). This showed that the UUC is gradually transforming from having the single transport function and civil air defense function to a dynamic and diverse place that carries multiple urban functions besides transport. The users mostly accessed the UUC between 10 and 30 min (36.48%) (Figure 4). The major travel mode was the subway (73.95%) ( Figure 5). The Tianfu Square Metro Station is located in the urban center of the • For tourists, the connection between the UUC and surrounding business districts, park and hotels is weak, which causes inconvenience in travel.
Planning and site selection • Located in the urban center, the UUC is given high value both on tourism and consumption, especially on weekends. It would be better if more space is considered in the early planning stage. Layout of functions • The commercial freight elevator and the passenger elevator in the underground parking space are set at the same position, causing traffic cut.
• Some new metro stations include functions such as convenience stores in the transfer lobby. Due to space shortage in the Tianfu Square UUC, few were set.
Organization of traffic flow • The entrances to the underground commercial street and to the above ground business district look the same. Passengers usually go to the wrong entrance and get lost. The signage systems for each function are not uniform.
• During holidays or exhibitions, the crowds in the commercial area will occupy the transfer space. It is recommended to consider separate lobby and exits to evacuate the commercial crowds. Orientation there are not many passengers asking for directions, because most of them are commuters and familiar with the environment. But on holidays, there are always people asking for directions, who travel for consumption and are not familiar with the environment. So, the signage system is very important. Service facilities • The entrances and exits are equipped with escalators (mainly going up) and stairs. Since there are old communities and schools nearby, the convenience of the elderly and children should be considered.
• There are not enough elevators and they can't go directly from the bottom to the top level because the lower part is in the paid area and the upper part is in the non-paid area, which is inconvenient for passengers, especially for the elderly.
• A large number of intelligent equipment, such as 3D tactile maps are needed.

Internal environment
• Feel hot in the indoor environment during summer, maybe due to the large glass ceiling designed in the centre of the Tianfu Square UUC.
• There are noise affecting their work.
• The interior is not bright enough.
• The height of the space is slightly insufficient.
• The design of the escalator and stairs only considers evacuation requirements, and does not make full use of space between the columns, resulting in dead ends and waste of space. It is also inconvenient to renovate.
• The cultural elements need to be improved. (Continued)

Safety operation and management
• the requirements of employees are ignored, because more space is reserved for commercial functions.
Once there is an emergency, the evacuation of passengers will be affected.
• Need to pay more attention to the stability of the equipment, and replace old equipment in time.      city. It was the first transfer station to be completed and operated in Chengdu, and has been used for more than 10 years. According to the station master, the average daily transfer volume was approximately 80,000. Therefore, most visitors prefer to travel by subway, which reduces the use of private cars to a certain extent and relieves traffic pressure in the area.
(1) Respondents' travel frequency and duration of usage When asked about travel frequency, 48.39% of users chose "only occasionally," followed by "two to three times a week" (16.63%) ( Figure 6). With regard to the duration of usage, 34.74% of users stay for 1-2 hours, and 33.50% of users stay for less than 30 minutes (Figure 7). The results showed that most of the users did not go to the UUC very often, but many of them were willing to stay for a significant period of time for consumption, such as dining and shopping. UUCs should be optimized in terms of planning and design in order to improve the comfort of users and enhance their willingness to stay.

Respondent satisfaction. The users were
generally satisfied with the Tianfu Square UUC (4.09) ( Table 9). The results revealed that public transport accessibility (by metro), overall planning, security service, and internal traffic and transfer flow organization were found by the respondents to be relatively satisfactory. However, the scores related to the internal physical environment, walking accessibility, and orientation were relatively lower -which were also mentioned in the online questionnaire survey and staff interview.

Factor analysis and naming
By building up a correlation matrix between each factor, and each factor with the overall satisfaction result, it was found at the 95% and 99% confidence levels that almost all variables had a significant relationship with the overall satisfaction results, except for the X5 variable (Table 9). The spatial morphology and texture, space proportions, color, decoration, and materials, and overall planning had a higher correlation coefficient with the user satisfaction results. Meanwhile, if there was multicollinearity between variables, the significance of the result would be reduced, the confidence interval would be wider, and the probability of accepting the hypothesis would be higher. A commonly used method for checking multicollinearity is the variance inflation factor (VIF) method. A high VIF indicates that the associated independent variable is highly collinear with the other variables. When VIF≥5 or VIF≥10, there is serious collinearity among the independent variables (Ma 2008). Taking the overall satisfaction results as the dependent variable, all variables were imported to construct a multiple linear regression model for the multicollinearity test, and the results showed that VIF<10 -indicating that there was no serious multicollinearity between variables (Table 10).
The KMO value of this questionnaire was 0.914 (Table 1), which was acceptable for factor analysis. Therefore, eight common factors were extracted with a characteristic root greater than 1 through dimension reduction, and the cumulative variance contribution was 60.051% (Table 11), which was more than 50% (the acceptable level). The results were ideal. By applying the varimax rotation method to the load matrix of the questionnaire data, the eight groups of factors can be named and explained separately (Table 12).
(1) Visual and atmosphere factor Factor 1 contains variables such as the morphology and proportion of the internal environment, color and decoration, and artificial and natural light, among others. Given that the UUC is located underground and has no external facade, its internal design is very important (Fan and Xiao 2019); it should be designed with users' visual comfort in mind and aim for a rich cultural atmosphere. In addition, a lack of natural light causes anxiety in enclosed, windowless spaces (Yao, Deng, and Xue 2021). Therefore, natural elements should be considered to reduce the feeling of being underground. This factor is referred to as the "visual and atmosphere factor." Site selection factor Factor 2 contained four variables related to site selection. Its location should be convenient, accessible, and coordinated with the surroundings. It should also be consistent with the city master plan (Shu et al. 2009). This makes it an important factor in assessing the sustainable development of the UUC and the city. This factor is referred to as the "site selection factor." Orientation factor The variables in Factor 3 represent the orientation aspect of UUCs. As an underground architectural structure, a UUC is obviously different from aboveground buildings. It has an enclosed space that lacks a connection to the ground. This feature makes it difficult for users to locate themselves and may cause psychological panic (Fan and Xiao 2019). Therefore, it is very important to quickly locate oneself and one's orientation for this type of building. This factor is referred to as the "orientation factor." Functional factor The four variables contained in Factor 4 represent the rationality of the internal functions of a UUC. One of the main features of a UUC is the comprehensiveness of the functions. The diversity and reasonable organization of functions can form a complementary relationship, creating more value for a UUC (Wang 2018). This factor is referred to as the "functional factor." Physical environment factor Factor 5 mainly includes variables that are closely related to the physical environment, such as acoustic and thermal environment, landscape and greenery, air quality, and sanitary conditions. These variables are closely related to user experience and comfort (Durmisevic and Sariyildiz 2001; Fan and Xiao 2019),  which affects their willingness to stay and duration of usage in a UUC. This factor is referred to as the "physical environment factor." Safety management factor Factor 6 contains variables that are closely related to safety operations and management, such as safety signs and broadcast effectiveness, security duties, consultation, and guidance. Although a UUC already has natural energy-saving and disaster-prevention advantages, there is still a potential security liability. Safety management must be strengthened (Zhao et al. 2017). This was also repeatedly mentioned in an interview with the staff and users in earlier research. This factor is referred to as the "safety management factor." Traffic organization factor A UUC is constructed with the motivation of public transport. Thus, meeting users' travel needs is a priority; a UUC should be accessible through public transport (Shu et al. 2009). Meanwhile, the internal functions of a UUC are connected to each other with    transport as the skeleton, forming a three-dimensional and compact structure for user efficiency. This factor is referred to as the "traffic organization factor." Service facility factor Factor 8 includes access facilities, such as stairs, escalators, elevators, and barrier-free facilities, among others. It also includes public service facilities, such as relaxing, guide facilities, and service desks. These variables are key elements in improving the quality of integrated services for the diverse requirements of users (Shu et al. 2009). This factor is referred to as the "service facility factor."

Discussion
Transport convenience and transfer efficiency are the issues that users care the most about. This may be related to the fact that the development of a UUC is driven by transportation. Wang, Li, and Wang (2019) previously conducted a study with a sample size of 507 workers, and found that because the respondents stayed in an underground space for a long time, they paid more attention to physical environmental factors such as light, air quality, and noise -which is similar to the results of this study. Meanwhile, users pay more attention to factors related to convenience and comfort, such as functions, service facilities, orientation, visual effects, and atmosphere. This result is similar to that of a study conducted in an underground shopping center in South Korea by Han, Jang, and Choi (2017).
The index set established by factor analysis in this study contains eight common factors, including functional factors, traffic organization factors, external and internal environment factors, and service facility factors. Wang (2017a,b) conducted a POE study on the connecting space between rail transit stations and commercial complexes in the central urban area of Shanghai (N = 373), and established an index set with the following dimensions through SPSS: functions, circulation, external and internal spaces, architectural design, landscape, and supporting facilities. The index set of seven dimensions is similar to that established in this research, indicating that the index set of this research is positively feasible.
In terms of orientation, Wang (2017a,b) conducted a POE study on the spatial orientation of large shopping malls in Shenzhen (N = 340), and established an evaluation index set containing 13 factors, including the identity of the atrium, identity of ground level entrances and exits, signage system, and the difference between internal spaces. These factors are consistent with the four orientation factors in this study. In previous relevant POE studies, factors such as site selection were rarely considered. The site selection factors included in this study are in line with the evaluation system established by Cui et al. (2017) in the POE study of an urban transportation complex. The difference was the "overall scale" factor. This may be related to the fact that a UUC is underground, and users are more sensitive to the scale of space.
To improve the quality of UUCs through planning and design, and to meet the diverse needs of users and increase their overall satisfaction, the following recommendations were formulated based on the results of this research: Walking accessibility between UUCs and their surroundings should be enhanced. These were specifically mentioned in staff interviews and online questionnaires. This variable also scored relatively lower in user satisfaction evaluations. Since the advantage of a UUC is a more integrated walking system that is relatively weather-proof, planners and designers should take advantage of this unique feature. This strategy may start with a careful design of the entrances and exits in order to increase the connectivity and identity of UUCs.
The orientation should be optimized through an internal space design. The results of the staff interview and online questionnaire survey showed orientation problems, such as unclear identification systems and it being easy to get lost. The users' evaluation scores for variables related to orientation were relatively low. Orientation not only includes the recognizability and unified design of the signage system, but also the understandability and identity of different internal spaces. Users can locate and find their way much easier by creating spatial sequences with different themes and morphological features, which improves their travel efficiency. In addition, the visibility of node spaces such as atriums will build a visual connection between the underground and the aboveground -reducing the feeling of being underground.
More natural elements are needed. Among the eight factors extracted, two included natural elements (visual and atmospheric factors, and physical environment factors). The staff interviews emphasized the problem of poor indoor lighting and air quality. The online questionnaire survey revealed that 27.71% of users thought that the quality of air and ventilation was not good, and 15.18% thought that there was not enough natural light and greenery. Correlation analysis results showed that natural light and air had a significant relationship with the satisfaction evaluation results. Natural elements can create visual and perception connections between underground space and aboveground space, thus reducing the negative experience of being underground.
The quantity and quality of service facilities must be improved. The UUC staff expressed that the quality of the facilities should be improved in terms of stability and capabilities. The results of the online survey showed that 36.87% of users thought that public service facilities, such as seats, toilets, trash cans, and consultation desks, were insufficient and inconvenient to use. This factor also had a significant relationship with the satisfaction evaluation results. When observing the Tianfu Square UUC, it was also observed that the number of recreational seats was not enoughespecially during peak hours such as lunch time -and could not satisfy the middle-aged and elderly people with children, who are one of the major user groups.

Conclusions
The UUC is still a relatively young building type in China and has great potential in the future. The results of this study revealed that the users' overall impression of UUCs in China was positive. However, UUCs have certain limitations. Natural light, air, and green elements that are easily available aboveground are difficult to obtain underground. These elements also had a significant relationship with user satisfaction results, which needs more attention. The internal design of the space, such as morphology and texture, proportion, color, decoration, and materials, were also significantly related to user satisfaction results. More effort should be made in these areas to increase user comfort.
However, it should be noted that the systematic index set was established using one case in this study. More case studies are needed to further testify and optimize the index set. There are also different types of UUCs in different regions. The concerns of users may vary, and the planning and design strategies may lay emphasis on different aspects, which are worth focusing on in future studies. It is hoped that this study will provide value for the planning, design, construction, management, and operation of UUCs.