Study on the demand and driving factors of urban underground space use

Tunnelling and Underground Space Technology 55 (2016) 52–58

Contents lists available at ScienceDirect

Tunnelling and Underground Space Technology journal homepage: www.elsevier.com/locate/tust

Study on the demand and driving factors of urban underground space use Li XiaoZhao a,b,⇑, Xu Hui b, Li Congcong c, Sun Liping b, Wang Rui b a

Nanjing University (NJU), Institute for Underground Space and Geoenvironment (IUSG), China School of Earth Sciences and Engineering, Nanjing University, Zhugongshan Building, Xianlin Avenue, Nanjing, Jiangsu Province 210046, China c Department of Science and Technology Studies, University College London, Gower Street, London WC1E 6BT, UK b

a r t i c l e

i n f o

Article history: Received 4 July 2015 Received in revised form 19 February 2016 Accepted 19 February 2016 Available online 8 March 2016 Keywords: Urban underground space City center Demand Driving factor Quantitative characteristics Comparative study

a b s t r a c t For sustainable development of Urban Underground Space (UUS1), determination of the desirable volume and function share of UUS is of vital importance. However, prediction of demand for UUS remains a rather challenging task. Very little quantifiable information, which is a basis for comparative study and improvement of prediction methods, is currently available pertaining to UUS. Xinjiekou, the center of Nanjing city, is called the first commercial center of China. Employing precision mapping on site and inquiring with a variety of organizations concerned, all the developed UUS in the area of Xinjiekou has been investigated in detail. On the basis of the quantitative investigation, the development modes and functional ratios are compared with other Chinese and foreign cases. By quantitative and comparative studies, the demand and driving factors of UUS have been identified. It is also found that the study area has the potential to be further developed and improved. It is hoped that this paper will enhance knowledge about the quantitative characteristics of UUS use, and be helpful for the demand prediction and planning of UUS development. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction In recent decades, many cities have entered into a period of rapid development of Urban Underground Space (UUS) use (Admiraal, 2006). On one hand, it is found that the developments in some areas are not sufficient to address the problems faced. Some of the important potential functions have not been involved. Many underground areas are overcrowded. On the other hand, some of the developed underground spaces are left unused or even abandoned. Flower City Square, the new city center in Guangzhou, has an area of UUS of about 500,000 m2, among which the commercial part of UUS, named the Mall of the World, is about 150,000 m2. After opening, the flow of customers was far lower than expected, so that about two-thirds of the shops were closed. Once an underground opening is created, it is difficult to expand its volume or alter its use (ITA, 1991; He et al., 2012). Because the subsequent development of UUS might be prevented by the need to preserve ground conditions (stress and hydrogeological state

⇑ Corresponding author at: Nanjing University (NJU), Institute for Underground Space and Geoenvironment (IUSG), China. E-mail address: [email protected] (X. Li). 1 UUS means Urban Underground Space. http://dx.doi.org/10.1016/j.tust.2016.02.010 0886-7798/Ó 2016 Elsevier Ltd. All rights reserved.

of geological body) so that existing aboveground and underground structures would not be endangered. Therefore, determination of desirable volume and function share of UUS is of vital importance (Monnikhof et al., 1999). However, prediction of demand for UUS remains a rather challenging task (Chen et al., 2007; Zheng and Dai, 2007). Estimation of urban space needs is an important topic to be studied (Scobotka et al., 1991). Influence factors in UUS need are complex. Little work has been done to gain insight into the intrinsic controls on desirable volume and function share of UUS. A major difficulty in developing prediction method comes from the lack of quantitative information that can be used for comparison. UUS use, its benefits and drawbacks, as well as UUS role in a city, have been very well studied descriptively (e.g. Belanger, 2007; ITA WG 4, 2000; Carmody and Sterling, 1993). However, there is still some lack of the study on the quantitative characteristics of UUS (Bobylev, 2010). This paper analyses the quantitative characteristics of UUS use in the city center of Nanjing, named Xinjiekou, and compares it with other Chinese and foreign cases. It aims to identify the demand and driving factors of UUS. These quantitative data of UUS in Xinjiekou are gained by precision mapping on site and inquiring with a variety of organizations concerned. The case study provided in this paper can also be used as a reference for future comparative studies.

53

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58

2. Methodologies 2.1. Systematic and detailed investigations in a typical city center In order to obtain accurate first-hand information, understand the actual operational status and internal demand of UUS. Field investigation, observation as well as inquiry in the studied area were needed. 2.1.1. Field measurement and calculation Based on the drawing documents and information provided by the planning authority, field investigations have been carried out in Xinjiekou. For each underground space project, investigating and mapping were conducted floor by floor. In terms of statistics, functions of UUS are divided into following kinds: commerce, transportation and parking functions. In particular cases where a floor is used for commercial development, the whole of the floor would be treated as commercial form including internal facilities such as lifts, stairs, and toilets. Therefore, a certain degree of error exists between the area in calculation and the actual usable area. 2.1.2. Observation and statistic The utilization efficiency of UUS in different time periods has been recorded and analyzed. The types of users were also distinguished in the observation of flow of people. 2.1.3. Inquiring By inquiring at a variety of institutions, some information and drawings were collected, especially some underground structures that cannot be observed directly. The following information was asked of lessors and renters: rents, rental rate and operating situation.

center as well as an administrative and cultural center, which is located in the east coast regions and the most economically developed city of China. The land use types of People’s Square include underground rail transport, ground public transport, underground parking, an underground pedestrian system and underground commerce (Zhu and Tang, 2010). Field investigations have also been carried out in Shanghai People’s Square to verify and supplement the data obtained from references (Table 1). Information on Berlin-Alexanderplatz can be found at www.stadtentwicklung.berlin.de; Bobylev (2010). Many factors have impact on UUS use in a city, including population, economic activity, urban area size, ground conditions, climate, geographical location, etc. (Bobylev, 2016). Table 2 shows some socioeconomic and climatic information of the 3 cities. (For easy comparison, all the GDP data are converted into US$ according to corresponding annual average exchange rates.) It can be seen that the GDP per capita of Nanjing and Shanghai are similar. The GDP of Nanjing is close to Berlin. However, the GDP per capita and GDP per km2 of Berlin are much higher than the two Chinese cities. Besides, as to weather condition, the difference of annual temperature between Nanjing and Shanghai is small. But the temperature of Berlin is a little lower than Nanjing and Shanghai. Based on the above, we can include that the driving force of UUS development between Nanjing and Shanghai is similar and the driving force of Berlin UUS development is stronger than Nanjing and Shanghai. 2.3. Factor analysis combined with comparative cases Data of many other cases have been collected for comparative study on function ratio and driving factors of UUS. 3. Data on the Xinjiekou area, the city center of Nanjing

2.2. Three typical centers selected for comparison 3.1. Study area Berlin-Alexanderplatz and Shanghai People’s Square were selected for the comparison with Nanjing Xinjiekou Area. The three city centers have similar areas, about 0.5 km2, and all are transportation junctions. Alexanderplatz Square is a distinct landmark in the center of Berlin, which is the capital and largest city of Germany. It is also a busy transport hub and highly frequented shopping area (Bobylev, 2010). Shanghai People’s Square is a large public activity

Xinjiekou area is the center of Nanjing city (see Fig. 1), which is also called the first Chinese commercial center. The history of the commercial center dates back to 100 years ago. Today, nearly 700 stores are gathered here, of which about 30 large and mediumsized commercial enterprises are more than 10,000 m2. About 100 Fortune 500 companies have set up branches here. Large public crowds aggregate and disperse every day, especially during

Table 1 UUS by function in Shanghai People’s Square. Category of functional use

Utilities

Parking

Rail transit

Pedestrian

Commerce

Total

Area (m2) Ratio (%)

15,900 7.7

70,200 34

65,960 32

300 0.1

54,080 26.2

206,440 100

Table 2 Information about Nanjing, Shanghai and Berlin.

Nanjing Shanghai Berlin

Area (km2)

Population (’000)

Population density (person/km2)

GDP (billion $)

GDP per capita ($/ person)

GDP per km2 (million $/km2)

Weather condition Annual temperature (°C)

Average highest– lowest (°C)

6587.0 6340.5 891.8

8187.8 24151.5 3517.4

1243.0 3809.1 3944.1

129.364 348.804 145.015

15799.6 14442.3 41227.6

19.6 55.0 162.6

15.4 17.05 9.67

20.2–11.6 20.58–14.14 13.36–5.88

⁄ Berlin: The data of city area and population (2013) come from ‘‘Bevölkerungsstand in Berlin am 31. Dezember 2013 nach Bezirken".Amt für Statistik Berlin-Brandenburg (in German). 18 February 2014. Retrieved 20 August 2014. The GDP data (2013) come from Federal Statistical Office of Germany. The climate data (1876–2015) come from en. wikipedia.org. ⁄ Shanghai: The data of city area, population and GDP (2013) are from Shanghai Statistical Yearbook; the climate data (1991–2010) are from China Meteorological Administration. ⁄ Nanjing: The data of city area, population and GDP (2013) are from Nanjing Statistical Yearbook; the climate data (1971–2010) are from China Meteorological Administration.

54

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58

China

Jiangsu province

Jiangsu province

Nanjing Shanghai

Nanjing city

Xiejiekou

Nanjing: capital of Jiangsu province Area: 6587 km2 (built-up 637 km2) GDP per capita: 76,263 RMB in 2011 Population: 8 million at the end of 2011 Xinjiekou: center of Nanjing, 0.51km2. Fig. 1. Location of the study area. (The 3D map of the Xinjiekou area can be found: http://nanjing.edushi.com/.)

Table 3 Land use categories in Xinjiekou, the case study area. Source: Wang and Sun (2009) Land use categories

Commerce and office

Services

Culture and entertainments

Residence

Utilities

Roads

Relics and military facilities

Green space and square

Total

Area (ha) Ratio (%)

23 46.8

1.9 3.9

1.6 3.2

1.9 3.9

0.5 1

12.5 25.4

2.7 5.4

5.1 10.4

49.2 100

holidays, when the average daily passenger flow is more than 1 million people. The large volume of visitors has brought business opportunities as well as heavy traffic pressure. Xinjiekou area is the Central Business District (CBD) of the city of Nanjing. This paper studies the core area, about 0.51 km2, which is a quadrangle block surrounded by Changjiang Road, Huaqiao Road, Wangfu Street, Huaihai Road and Hongwu Road (Fig. 1).

transport. The specific land use categories are shown in Table 3. The land use intensity and building density is very high, with commercial use and office sites as two main functions. The overground building area is about 1,950,000 m2, in which the office building area is about 895,000 m2, constituting the highest proportion 45.9%; the commercial building area is the second largest area, which is about 813,000 m2, constituting 41.7% of the total land use.

3.2. Types of land uses and buildings on ground

3.3. Data on UUS

As the city center and the first Chinese commercial center, there are two main categories of land use in Xinjiekou – commercial and

All the underground spaces in the study area have been investigated, including precise mapping and inquiring with institutions

55

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58 Table 4 UUS use by function in Xinjiekou. Category of functional use

Area (m2) Ratio (%)

Dynamic traffic Subway station

Pedestrian

47,673 15.2

2113 0.7

Parking

Commerce

Utilities

Others

Total

132,279 42.2

87,845 28

8400 2.7

35,186 11.2

313,496 100

ing and field investigation, it is found that the depths of pipelines are mainly 2–3 m. According to the Nanjing Statistical Yearbook 2012, the density of drainage pipes in built up area is about 8.31 km/km2. The area of utilities is estimated to be 8400 m2, which is formed by pipelines with a total length of 4.2 km and diameter of 2 m. There are other small amounts of underground spaces, which are spare or under construction. The development of underground space in the studied area is classified according to functions as shown in Table 4. 3.3.2. UUS use by depth (layer) The utilization of UUS in Xinjiekou mainly gives priority to the 1st basement and the 2nd basement (see Fig. 2). The 1st basement, with a floor depth of about 7 m, a storey height of about 4 m, is mainly used for commercial functions especially in the traffic nodes close to rail transit where the development intensity is very high. The 2nd basement is mainly used for parking that provides service for high-rise buildings on ground. In some areas, the parking lot takes up from 2nd to 5th basement because of huge demand.

4. Analysis of factors in UUS development 4.1. Influence of land use type and ground buildings FAR2 on development intensity of UUS Fig. 2. Layout of UUS by layer in Xinjiekou area, Nanjing.

64.2%

70.0%

55.0%

60.0%

45.0%

50.0% 40.0% 30.0%

25.4%

28.9%

24.0%

26.1%

21.0%

20.0%

10.4%

10.0% 0.0%

Roads

Xinjiekou

Construction land

People Square

Green space & square

Alexanderplatz

Fig. 3. Comparison of land use types.

concerned, on the basis of some collected drawings. Further information is listed in the Appendix A. Table 4 summarizes the functional categories of UUS in the study area. Fig. 3 shows the layout of UUS by layer.

3.3.1. UUS use by function As seen from Table 4 and Fig. 2, the functions of UUS within the studied area are mainly parking lot, commercial district and traffic road. Parking is listed as a function distinct from dynamic traffic; the latter includes the subway station and underground pedestrian system. Underground commercial uses include retailing, catering, clothing, entertainment and other forms of business. The underground pipeline data in the studied area are insufficient. By enquir-

Underground space planning is an important part of urban planning (Chen and Liu, 2011). Urban infrastructure planning should be coordinated with land use planning for the purpose of the right resource being allocated to the right place in an economical and reasonable way (Kivell, 1993; Jenks et al., 1996; Jenks and Jones, 2010). The different types of land use create imparity of demands of UUS (Edelenbos et al., 1998). It can be observed form Fig. 3 that the three city centers studied all have the characteristic of transport junction. The proportions of roads which are used for traffic are similar, which are around 25%. The influence of land use type on UUS development is mainly shown in the differences of ratios of construction land and green space (see Table 5). Both Shanghai People’s Square and Berlin Alexanderplatz Square belong to the category of Green Space. The ratios of Construction land to Green square land (C/G) are only 0.38 and 0.58 respectively. On the other hand, Nanjing Xinjiekou is a typical Business Center, the ratio of C/G is up to 6.17. As a business center, Xinjiekou can attract more people, which would in turn lead to stronger demand and higher value of UUS development. Therefore, the intensity of UUS development in Xinjiekou is nearly double compared to the other two city centers. UUS is the product of the surface space needs being extended downwards. The surface FAR (Floor Area Ratio) may influence the development of UUS. The FAR of Xinjiekou is the highest, where the use of UUS reaches 626,000 m2/km2. However, the FAR in Alexanderplatz is more than 6 times that of the Shanghai People’s Square, though the scale and the intensity of UUS in the two areas are equivalent. It seems that, compared to surface FAR, the land use 2

FAR means Floor Area Ratio.

56

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58

Table 6 UUS by function (%). Dynamic transport Rail transit Xinjiekou People’s Square Alexanderplatz

15.2 32 30

Driveway

Parking lot

Commerce

Utilities

Others

42.2 34 31

28 26.2 6

2.7 7.7 3

11.2

Pedestrian passage 0.7 0.1

3

27

Fig. 4. UUS use by function (%).

type plays a more significant role in affecting underground space development scale and intensity. 4.2. Driving factors on UUS of different functions

Fig. 5. Two types of traffic pattern.

Workers

15% 40%

15%

Shoppers Visitors from other places Others

30% Fig. 6. Users of underground parking lot.

A reasonable function ratio is an essential condition for the achievement of efficient utilization of underground space. Various demands and driving forces create different functions of underground space. Early Chinese underground spaces are mainly used for civil air defense works, however, diversification of usage has appeared in today’s cities. The function ratio of underground spaces in Chinese cities exhibit different characteristics compared to western cities. Despite objective factors such as population density and vehicle numbers, the influence of the tradition of culture has contributed to the difference of city pattern. Table 6 presents the different function ratios of underground space in the three city centers studied. The major functions include commerce, dynamic traffic, parking lot and municipal facility. Dynamic traffic could be divided further into rail transit, driveway and pedestrian passage. The commercial function of underground space includes shopping center, retailing, catering, clothing, entertainment and other forms of business. The rest of the spaces include office sites, spare spaces, spaces under construction and historical sites. 4.2.1. Dynamic traffic The major driving factor of the development of underground space is traffic demand. Underground transportation system is a way of extending transportation when the overground facilities could not satisfy the traffic demand and would result in congestion.

Table 5 UUS use by function in Xinjiekou.

Xinjiekou People’s Square Alexanderplatz ⁄

Study area (km2)

Construction area/green space

Floor area of ground buildings (m2)

FAR of ground buildings

Area of UUS (m2)

Development intensity of UUS

0.500 0.560

6.170 0.580

1,950,000 200,000

3.90 0.357

313,000 206,000

0.626 0.368

0.570

0.380

1,300,000

2.28

210,000

0.368

Study area = the area of study area; Construction area/green space = (the area of construction land)/(the area of green space); FAR (Floor Area Ratio) of ground buildings = (the total covered area on all floors of all ground buildings on a certain plot)/(the area of the plot); Area of UUS = the area of underground space in study area; Development intensity of UUS = (the total area of UUS developed on a certain plot)/(the area of the plot). ⁄ A typical construction area means a piece of land for buildings and structures; A typical green space means a piece of land for public green space which including public park, road and square, etc.

57

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58 Table 7 Reasons to choose underground commerce. Reasons to choose underground commerce

Convenient nearby subway station

Attractive goods

Comfortable in extreme weather

Cost effective goods

Others

Total

Sample number Ratio (%)

33 45.8

19 26.4

8 11.1

5 6.9

7 9.8

72 100

A sophisticated subway system could assume about 50% of the traffic volume of the city. The development of underground transportation would cut congestion above ground and at the same time play a vital role in promoting regional development. From Fig. 4, it could be observed that the ratio of rail transit in Xinjiekou is significantly less than that of the other two cities. In Shanghai People’s Square, there are three subway lines that pass through the site. The stations that have multiple connecting lines are not common in China. However, this is not an indication that underground railway development is not valued in China. Indeed, most Chinese cities with population over a million make an underground railway system the backbone of their transportation system. In China, every subway station generally has one or two lines passing through the station and this is connected with the geographical layout of the streets and the traditional culture. Chinese people have a preference for regular and balanced architectures, in other words, a square style. Reflecting this in city planning, the layouts of the streets usually appear as in a chessboard shape. Consequently, the traffic patterns in Chinese cities take the form of a multicenter grid, which is different from the annular and radial patterns of western cities (see Fig. 5). Owing to the multicenter pattern, the number of transfer lines at each station is relatively less. The proportion of underground railway area in Xinjiekou is lower than for the comparison cities. However, there is no need to increase this type of underground space. 4.2.2. Underground parking The proportion of underground parking lot in Nanjing Xinjiekou is about 42.2%, in contrast, the corresponding figure in Berlin Alexanderplatz and Shanghai People’s Square is only about 30%. Through field investigation, it is found that the parking price in Xinjiekou is two to three times of that of other regions. However, the utilization rate of the parking lot has exceeded 80%. Although the proportion of parking lot in Xinjiekou is far more than that of the central area of the other two cities, the demands for building underground parking lot are still seen. The demand for parking in a particular region is determined not only by the visitor flow rate, but also is influenced by the ground building type, urban location, and the type of visitors. The increase in visitor flow rate does not always lead to the increase in parking demand. Indeed, after the construction of subway in some areas of Paris, the parking demand decreased sharply and the parking lots were transformed into shopping malls. Nanjing Xinjiekou is a commercial center as well as an office and business center. Shoppers and office workers accounted for 70% of the whole number of visitors (see Fig. 6). The average parking time is quite different for different types of people. Among all the visitors in this area, a higher proportion of demand for parking is shown on office workers and their average parking time is longer than other groups. 4.2.3. Commercial use From Fig. 4, it can be seen that the proportion of commercial use of underground space in Xinjiekou is the biggest in the three places (28%). In Alexanderplatz Square, this figure is only 6% (Table 6). Through investigation, it is found that the commerce use in underground space development in most developed cities of China

is around 25%. For less developed regions, where UUS development is still at a primitive stage, commerce generally holds a larger share. For example, in Nanbei Street of Xian (the capital of Shanxi Province) and in the Central Area of Changsha (the capital of Hunan Province), the figures are 31.6% and 31.9% respectively. This is because commercial developments are relatively easier for achieving economic benefits in the short term. As a city develops and the traffic demand increases, the demand for underground transportation increases correspondingly. The development of underground transportation, especially the rail transit, would in turn stimulate the underground commercial development in adjacent areas. For Xinjiekou station, the size of shopping area is around 7909 m2 and the average rents in 2010 were 800–1000 RMB/ Month m2. Other derivative business resources include: advertisements, newspaper stands, self-service equipment, DM magazines, square activities, etc. In 2010, the commercial gains of MTR Corporation from all of these channels was around 20,000,000 RMB (Wang, 2011). Hence, underground transportation and commercial developments could be mutually improved. A survey was conducted on a random sample for the reason to choose underground commerce (see Table 7). A questionnaire survey about the reason to choose underground commerce was conducted in Nanjing Xinjiekou underground space. The dates of the survey are March 28 (Friday), March 30 (Sunday) and April 14 (Monday) in 2014. A total of 72 pedestrians randomly met was involved in these surveys. The result is shown in Table 7. It is found that the most important reason for choosing underground commerce is that it is convenient (nearby subway station, especially within 100 m). The corresponding ratio is 45.8%. The second important reason is the attractiveness of goods in the commercial sites. The next most important reason is that underground commercial spaces are comfortable in extreme weather. The factors of visitors’ psychology should be taken into account in the development of underground commerce. 4.2.4. Utilities The ratio of UUS used for utilities is quite low in the three study cases, because they are all old regions. In addition, because of the large investment required for the construction of utilities and the difficulty of obtaining economic gains in short terms, in less developed regions and the initial stage of UUS development, they are valued less than the development of underground commerce. In contrast, in more developed cities and the later stage of UUS development, once the importance of urban sustainable development is realized (Cano-Hurtado and Canto-Perello, 1999; Curiel-Esparza et al., 2004), the ratio of utilities is relatively higher. The corresponding figures in Paris, Stockholm and Tokyo are 13%, 41% and 8% respectively (Bobylev, 2009). In Beijing Zhongguancun Square, the ratio of municipal infrastructure is as high as 20%. 5. Investigation on current utilization and forecast of future demands for UUS in Xinjiekou The demand level, which is different from district to district, largely determines the stage of UUS development (Li et al., 2013). In order to assess the current utilization and identify aspects for future improvements, field investigation, observation and inquiry into specific areas are carried out.

58

X. Li et al. / Tunnelling and Underground Space Technology 55 (2016) 52–58

The current volume of UUS in Xinjiekou is around 310,000 m2, which has exceeded the level of some advanced western countries. But from the perspective of the demand, there is still room for further development of underground space:  The traffic is very congested during rush hours. The average speed of vehicles is below 10 km/h.  Parking utilization rate in Xinjiekou is up to 80%. It is very difficult for parking on weekends and holidays. The parking rate increased from previous 3, 6 and 12 RMB/h to current 20 RMB/h.  The markets located in the central node have the most customers, consequently, the rents are higher. Among all the businesses, catering has the biggest share of profits. Underground catering is a complement to the lack of small restaurants over ground. However, not every demand needs to be satisfied. For example, an excessive level of parking would surpass the carrying capacity of a specific area and aggravate the traffic congestion over ground. Hence, Xinjiekou has taken the strategy of setting the parking charging at a level above other regions rather than increasing parking areas. Underground business forms need to be featured, capable of change according to the changes in demand and highlight the complementary quality in order to enhance the aggregation effect of a particular region. Currently, the connectivity of channels between underground shopping malls and parking lots is poor in Xinjiekou. The most commonly adopted form of UUS is the utilization of basements of high-rise buildings. In the future, the construction of connections could be considered which would create a genuine underground city. 6. Conclusion Through statistical and comparative analysis, following conclusions are found: (1) Compared to building volumetric fraction, land use type plays a more significant role in affecting underground space development scale and intensity. (2) The modes of UUS utilization in business center and green square city center are quite different. (3) Location, ground building type as well as the type of visitors should be taken into account to predict the demand of UUS. (4) Field investigation also reveals that the quality of underground commercial space needs to be improved to increase connectivity to form a mutually complementary aggregation effect. (5) In different locations, various development modes, scale and function shares of UUS should be adopted to meet different demands. A reasonable, planned and sustainable way in the development of UUS should be sought. There is a clear need to continue case investigation, quantitative data collection, and further comparative study. With more and more refined quantitative data to be added, a better understanding of driving factors and a predictive tool on demand of UUS could be expected. Acknowledgements The authors are grateful to Mr. Fensheng Gao of Nanjing Planning Bureau and everyone who providing assistance in the field

survey and data collection, and wish to acknowledge the support of the Sino-Swiss Science and Technology Cooperation Program (2009DFA22680), National Natural Science Foundation of China (41272314), China Urban Geological Investigation Program (1212011) and the European Commission Marie Curie IRSES project (FP7-IRSES-612665). Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.tust.2016.02.010. References Admiraal, J.B.M., 2006. A bottom-up approach to the planning of underground space. J. Tunn. Underground Space Technol. 21 (3–4), 464–465. Belanger, P., 2007. Underground landscape: the urbanism and infrastructure of Toronto’s downtown pedestrian network. J. Tunn. Underground Space Technol. 22, 272–292. Bobylev, N., 2009. Mainstreaming sustainable development into a city’s master plan: a case of urban underground space use. J. Land Use Policy 26 (4), 1128– 1137. Bobylev, N., 2010. Underground space in the Alexanderplatz area, Berlin: research into the quantification of urban underground space use. J. Tunn. Underground Space Technol. 25 (5), 495–507. Bobylev, N., 2016. Underground space as an urban indicator: measuring use of subsurface. J. Tunn. Underground Space Technol. 55, 40–51. Cano-Hurtado, J.J., Canto-Perello, J., 1999. Sustainable development of urban underground space for utilities. J. Tunn. Underground Space Technol. 14 (3), 335–340. Carmody, J., Sterling, R., 1993. Underground Space Design: A Guide to Subsurface Utilisation and Design for People in Underground Spaces. Van Nostrand Reinhold, New York, 325 pp. Chen, Zhilong, Liu, Hong, 2011. The Master Planning of Urban Underground Space. Southeast University Press, Nanjing, 310 pp. Chen, Zhilong, Wang, Yubei, Liu, Hong, Xiao, Qiufeng, 2007. Prediction of underground space needs. J. Plan. 10 (23), 9–13. Curiel-Esparza, J., Canto-Perello, J., Calvo, M.A., 2004. Establishing sustainable strategies in urban underground engineering. J. Sci. Eng. Ethics 10 (3), 523–530. Edelenbos, J., Monnikhof, R., Haasnoot, J., et al., 1998. Strategic study on the utilization of underground space in the Netherlands. J. Tunn. Underground Space Technol. 13 (2), 159–165. He, L., Song, Y., Dai, S.Z., et al., 2012. Quantitative research on the capacity of urban underground space – the case of Shanghai, China. J. Tunn. Underground Space Technol. 32, 168–179. ITA Working Group No. 4, 2000. Planning and mapping of underground space – an overview. J. Tunn. Underground Space Technol. 15(3), 271–286. ITA Working Group on Subsurface Planning, 1991. Policy statement on legal and administrative issues in underground space use. J. Tunn. Underground Space Technol. 6(4), 471. Jenks, M., Burton, E., et al., 1996. The Compact City: A Sustainable Urban Form? E & FN Spon. Jenks, M., Jones, C., 2010. Dimensions of the Sustainable City. Springer. Kivell, P., 1993. Land and the City: Patterns and Processes of Urban Change. Routledge. Li, HuanQing, Li, XiaoZhao, Parriaux, Aurèle, Thalmann, Philippe, 2013. An integrated planning concept for the emerging underground urbanism: Deep City Method Part 2 case study for resource supply and project valuation. J. Tunn. Underground Space Technol. 38 (6), 569–580. Monnikhof, R.A.H., Edelenbos, J., van der Hoeven, F., et al., 1999. The new underground planning map of the Netherlands: a feasibility study of the possibilities of the use of underground space. J. Tunn. Underground Space Technol. 14 (3), 341–347. Scobotka, P. et al., 1991. Methodology for evaluating a nation’s urban space needs: a case study of Slovakia. J. Tunn. Underground Space Technol. 6 (1), 103–112. Wang, Yujue, Sun, Shijie, 2009. Study on the microscopic characteristics of land use in Central Business District: a case of Nanjing Xinjiekou core area. In: Urban Planning and Scientific Development: Proceedings of China Urban Planning Conference in 2009. China Urban Planning Society, Tianjin, pp. 1237–1249. Wang, Xinmin, 2011. Underground space utilization and development of a Nanjing metro transfer station. J. Urban Rapid Rail Transit. 24 (1), 11–14 (in Chinese). Zheng, Yonglai, Dai, Sheng, 2007. Prediction of ecological underground spatial scale in central business district of a city. Chin. J. Underground Space Eng. 3 (5), 796– 800. Zhu, Min, Tang, Yongjing, 2010. Optimization of Shanghai People’s Square underground space net. Chin. J. Underground Space Eng. 6 (6), 1112–1117.