creating space underground

0886-7798(96)00052-6

Creating Space Underground

Jeremy Cooper

Abstract--In all major cities, creation of underground spaces to accommodate transportation, communication and utility networks, and complexes for handling, processing and storage of many kinde of materials, is now a high priority in order to protect the living environment at ground level. This paper describes various techniques for creating underground space, including cut-and- cover methods and the New Austrian Tunneling Method, with particular reference to mass transit railway and urban expressway projects undertaken in Hong Kong, Bangkok, Singapore and London. Innovative design techniques for underground spaces and the interrelationship between construction method and structural envelope design are discussed.

Rdsumd--Dans routes les grandes villes du monde, la creation des espaces souterrains pour contenir des reseaux de transportation, communication et des services publics, et aussi les complexes industriels pour la manutention, le traitement et l'entreposage de beaueoup de sortes de mat/~res, est maintenant une priorit~ tr~s pressante pour prot~ger l'environnement vivant en surface. Cet article decrit lea diverses techniques pour la creation des espaces souterrains, y compris les rn~thodes de couper-et couvrir et la NouveUe M#thode Auetriehienne de Creusement, en soulignant sp~cifiquement les projets des m~tros et des voies express darts les zones urbaines dane le Hong Kong, t~ Bangkok, ~ Singapore et d Londres. On discute les techniques innovatrices du plan des espaces souterrains et auesi les relations entre la m~thode de construction et le plan de l' enveloppe structurale.

1. Introduction

T he developing needs and aspirations of hum~nkind for our living environment require increasing provi sion of space of all kinds. This has become a high

priority for most cities in the closing years of this century. Although the twenty-first century is only four years away, the use of extraterrestrial space to alleviate terrestrial overcrowding remains a science fiction concept. So also do the imaginative means of transportation, communication and materials handling which would remove the economic need for agglomeration of populations into ever more con- gested cities.

At the same time, growing public concern for beth con- servation and quality of life is rightly giving pause to unrestrained development of the built environment at ground level. Provision of new urban infrastructure may either coexist or conflict with improvement of the urban environment. In each city, the balance will depend on local priorities and economic circumstances; but unquestion- ably, environmental considerations are now being accorded greater importance everywhere.

Engineers have a responsibility to foster a better environment for living, working and leisure activity at the ground surface, and are therefore turning increasingly to the creation of spaco underground to accommodate new transportation, communication and utility networks, and complexes for handling, processing and storage of many kinds of goods and materials.

Present Address: Jeremy Cooper, MA (Cantab), CEng, MICE, MHKIE, Director, Robert Benaim & Associates (Asia) Ltd., 2102, 21/F Hopewell Centre, 183 Queen's Road East, Wanchai, Hong Kong.

Tunnelling and Underground Space Technology, Vol. 11, No. 1, pp. 51-56, 1996 Elsevier Science Ltd Printed in Great Britain 0886-7798/95 $15.00 + ,00

2. Cut-and-Cover Construction Decisions to build underground are made either because

at-grade or elevated alternatives are totally infeasible or unacceptable; or because the indirect financial, social and environmental costs of such alternatives outweigh the general ly much g rea te r direct costs of underground construction.

Engineers must seek design and construction techniques that minimise costs of underground developments. Cut-and-cover methods are usually cost-effective, espe- da l ly for excavations less than 25 m deep and in areas where little underground infrastructure already exists. More economical underground construction can be obtained from design-and-build contracts. The client's engineer specifies the required finished product and design criteria, leaving the contractor and specialist designers free to propose the best design and construction method, using their combined skills.

Types of Excavation Many systems of retaining temporary excavations are

available to the contractor, each suited to different types of excavation. Minimising costs may dictate use of several systems on the same job site. Examples are Hong Kong Mass Trans i t Rai lway 's Admira l ty s ta t ion and the Singapore's Central Expressway tunnels, both design-and- build projects. In each case, open battered excavation, sheetpiling and diaphragm wails were all used on the same site.

Diaphragm or secant pile walling, often combined with top-down construction, may be the only option if ground movement adjacent to the excavation must be minimised at any cost. However, such solutions rigidly control the sequence and method of working, and are expensive.

Pergamon

At the other end of the scale, the cheapest method will often be open-cut excavation, but its economy depends largely on the slope angle. Geotechnical expertise is needed to define a safe soil slope for the excavation. Open-cut will rarely be feasible in densely developed urban sites.

Steel sheetpiling is the commonest type of temporary retaining wall, but its use may be limited by available wall modulus unless composite pile sections are used. On urban sites, noise restraints may impose requirements for vibra- tory or hydraulic driving, which in hard soils may require pre-bering. However, such special enhancements may cancel out cost advantages of this otherwise simple method.

Berlinoise walls, comprising steel H-section or bored- pile klng~osts with timber or precast concrete planks or shotcrete as lagging, can be cost-effective in dry soil. Kingpost modulus and spacing can be tailored to ground conditions, and the use of walings can increase strut spacing.

Strutting is usually cheaper than anchoring retaining walls, except where excavations are very wide, or where interference between struts and permanent works is exces- sive. Design-and-construct can yield savings by integrating temporary and permanent works designs, in terms of construction sequence, s t rut and construction joint locations, and any temporary loads on the permanent works.

Robert BenAim & Associates (RBA) have designed a number of major cut-and-cover s t ructures for urban motorway tunnels and mass transit railway tunnels and stations, using various construction techniques including diaphragm walls, top-down construction and the New Aus- trian Tunneling Method. The designs include alternatives and design-and-build projects for contractors, as well as designs for owners. A number of these projects are de- scribed briefly below.

3. Central Expressway Phase II Tunnels, Singapore (Fig. 1)

Phase II of the Central Expressway (CTE) is a vital link in the network of urban motorways being constructed in the Republic of Singapore. Its route crosses the centre of Singapore City and is 3.7 km long overall, with 2.5 km in two cut-and-cover tunnels for environmental reasons. The tunnels skirt Is tana (the President's Palace), cross under Orchard Read, Stamford Canal, and Singapore River and over the bored tunnels of the Singapore Mass Rapid Transit railway.

A design-and-construct contract was let by the Public Works Department in November 1987 to international joint venture contractor Expressway Construction Pte Ltd, for the sum of SGD $313 million. In January 1988, RBA were

Figure 1. Singapore Central Expressway, Havelock Interchange; MOSS perspective.

awarded a sub-contract for the highway alignment and structural design of the tunnels, together with their foundations and drainage.

The north tunnel is dual four-lane expressway, 735m long, with two slip reads comprising 230 m of tunnel and approach structure. The south tunnel is dual three-lane expressway, 1760 m long, with six slip roads comprising 1100 m of tunnel and 600 m of approach structure. Five slip reads form the underground Havelock interchange, ] i n kin g the expressway to the Central Business District.

Tunnels vary up to 50 m maximum width. Depth of roof below ground varies t o a maximum of 11.5 m, while the deepest excavation is 27.5 m. Structural thicknesses range from 0.75 m to 3.6 m. Each side of the Singapore River, the tunnel is designed to carry future seven-storey buildings.

Structural Modelling and Drafting An important factor in the appointment of RBA to design

the CTE tunnels was their proposal to implement advanced CADD techniques to model the geometry of the tunnel structures, using the MOSS program. Typically, the MOSS program is used for highway alignment design, creating vector strings that define geometrically, with respect to a master string, key points on the highway cross-section at each chainage. On this project, techniques were developed to extend this concept to the geometrical definition of each corner of the cross-section of a tunnel by a vector string related to the highway alignment. A database is created, containing the geometrical model of the entire tunnel.

This database is then interrogated to draw plans, eleva- tions, cross-sections and perspective views of the tunnel structure, to scale, and with offset dimensions, levels and grid coordinates of key points on the structure annotated automatically as required.

The power of this method is that changes in highway alignment automatically regenerate the geometry of the tunnel structure so that revised drawings can be plotted immediately. Also, because numerical data is generated automatically by computer, manual errors of drafting and typography are eliminated. Accurate setting-out data for installation of temporary works was made available to the site very early, and rapid changes to the tunnel structural geometry were implemented as the highway alignment design was finalised.

Horizontal alignment adjustments were made in re- sponse to topographic survey data from site. Vertical alignment adjustments were made to minimise overburden depths, improve surface read geometry, reduce drainage catchments at portals, resolve clashes between main and slip tunnel structures, and to create a route through the complex Havelock interchange for a 1500-mm-dia. sewer, otherwise requiring an undesirable 1.5-kin diversion cost- ing SGD $1 million.

MOSS drawings were exported to an AutoCAD system, where they provided the geometrical basis for drafting and scheduling of tunnel reinforcement, as well as for produc- tion of general arrangement, SEM, and drainage drawings. The design was substantially completed in 17 months, with the great majority of the 600 A0-sizo drawings produced within an eight-month period.

Structural Analysis and Reinforcement Design The tunnels were designed in reinforced concrete, al-

though in one area the roof was par t ia l ly prestressed to reduce reinforcement and control crack widths. External surfaces below the water table were specified to be designed to BS5337 (Water Retaining Structures) with crack widths limited to 0.1 ram. Elsewhere, design was to BS8110, with crack widths limited to 0.3 mm.

When the SLS value of M / b d 2 exceeds approximately 1.2MPa, the reinforcement requirement increases rapidly and the structure becomes uneconomical. Slab and wall

52 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 1, 1996

thicknesses were optimised, using tapered thickness slabs with haunches a t all roof/wall junctions. The central haunch of the base slab reduced reinforcement at the centre and, by concentrating ground bearing pressures under the walls, allowed the rest of the base slab to be more flexible, reducing bending moments.

Computers were used extensively for analysis and design of tunnel structures. The STRESS-3 plane frame analysis program, together with RBA's purpose-written pre- and post-processing programs, were used to automate the structural design process. The pre-precessor automatically assembles structural models from basic geometrical data, and subsequently analyses these models under a set of standard loadcases. The post-processor envelopes analysis results and calculates flexural and shear reinforcement areas at SLS and ULS to code requirements.

The beneficial effect of coexistent axial force is taken into account in all flexural and shear calculations, as is enhance- ment of shear capacity near supports. Automatic calcula- tion of required reinforcement areas at closely spaced node points permits careful curtailment of reinforcement.

Construction Methods Construction was generally bottom-up in open excava-

tion, except for one narrow section of the south tunnel site, where diaphragm walls and top-down construction were used to provide maximum support to adjacent buildings, and to allow early reinstatement of the read over.

In a few areas, open battered excavation was possible. Elsewhere, Berlinoise walls with steel king-posts and timber laggings, and both s tandard and composite steel sheetpiles were used. Struts and walings were used to support temporary walls in all normal width sections of the tnnnel.

The widest parts of the excavation, in the area of the Havelock interchange, were up to 70 m across and more than 20 m deep. In this area, contiguous bored pile walls or composite sheetpile walls were used with support provided by ground anchors.

One section of the north tunnel was built in two stages. First, half the tunnel, including the central wall, was constructed and backfilled. The existing surface read was diverted onto the roof of the half-tunnel while the remainder was built. Steel preps inside the half-tunnel supported the roof under backfill and traffic loads, avoiding extra reinforcement for this temporary case.

The project used more than 400,000 m 3 of concrete and 35,000 tonnes of reinforcement in total.

4. Ratchadamnoen Road Tunnel, Bangkok (Fig. 2) This tunnel, ]ecated in the historic centre of Rmxgkok

adjacent to Sanam I,nang, was intended to carry t!~ough- traffic travelling from Ratchadamnoen Klang Road towards the Phra Pinklao Bridge, to ease peak hour congestion at this busy principal read intersection.

A study by the Japanese International Co-operation Agency (JICA) had identified a number of similarly con- gested intersections, and recommended construction of flyovers for through traffic. However, for the RatchadAm noen intersection, the JICA report recommended building a tunnel because of the sensitive environmental, aesthetic, pro- tocol and security aspocts of the site, which is located not only close to historic and cultural buildings and government offices, but also on the route of the Annual royal procession from Chitralada Palace to the national parade at Sanam Luang.

STS Engineering Consultants Ltd, together with RBA as specialist structural design consultant, were appointed by the Bangkok Metropolitan Administration (BMA) in 1988 to design this tunnel.

Figure2. Ratchadamnoen Tunnel; MOSS perspective and section.

The final proposed alignment of the tunnel and de- pressed approach ramps was approx. 440 m long and curved in plan, following the lines of the existing at-grade roads above. The proposed tunnel width provided three full traffic lanes. However, to minimise the length and cost of the tunnel, final design called for a limited headroom of just 3.5 m.

The proposed design used diaphragm walls for the permanent walls of the tunnel and approaches, and also to provide the foundations and flotation resistance of the tunnel. The general ground level in Bangkok is sinking, due to consolidation of clay s t ra ta as water is pumped out of underlying aquifers. The diaphragm walls are therefore specified to be founded as high as possible, consistent with lateral soil pressure resistance, so that the tnnnel will settle with the surrounding soils and not rise out of the ground.

Top-down construction was proposed. After installation of diaphragm walls, the roof slab would be constructed in a shallow excavation, with a monolithic connection to the walls. The roof slab would then be backfilled and traffic diverted back over the t-nnel . Excavation beneath the roof, without temporary preps, would be followed by construction of the base slab and l~nislllng works.

The existing Phra Pinklao Bridge approach read ~m~ on a reinforced concrete deck structure, built over the Klong Lot drainage canal in the early 1970's. For about a third of its length, the proposed tunnel alignment ~]n~ along this road, intersecting the deck structure and the Klong below. The deck structure must be modified to accommodate the tunnel, while the proposed tunnel design incorporates a siphon beneath its base slab to maintain the drainage function of the Klong.

5. Umehouse Link Tunnel, London (Fig. 3) The Limehouse Link is a key element in the plan by

London Docklands Development Corporation (LDDC) to provide road access from the Isle of Dogs and Royal Docks enterprise zones to the City of London. The link was planned as a dual two-lane tunnel following a serpentine route through largely derelict or underused land, to avoid the major environmental consequences to residential com- munities which would have been associated with any at- grade solution. The tunnel is 1.55 kin long, plus portal approaches and two short slip road tunnels diverging near the midpoint.

The tender design minimised ground movements and the width of the construction corridor, by using top-down construction and d iaphrag~ walls as the permanent external walls of the twin-cell tunnel, throughout. Iustallation of temporary props was specified prior to casting the roof slab

Volume 11, Number 1, 1996 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 53

Original Value design Engineering

design Diaphragm - - walls

- Skin wall

Prop

• Base slab

Drainage

l ~ Profiled roof

Sk in c o l u m n s

..• NO props

.J o Road base and I . . . . drainage cast wi lh slab

. . . . . . . ~ B l i n d i n g strut

Service -- french

Figure 3. Limehouse Link Tunnel, London--Value Engineering changes.

and backfilling, and again prior to casting the base slab. The central wall and two continuous " s k i n walls" inside the diaphragm walls completed the permanent structure.

LDDC awarded the GBP 170 million contract to the Balfour Beatty-Fairclough Joint Venture in 1989, accept- ing the JV's proposed alternative for the 450m Limehouse Basin section. With no adjacent residential properties in this section, the JV proposed to build bettom-up in cofferdam, offering a GBP 4 million saving and a 6-month reduc- tion from the 48-month contract period.

Alternative Tunnel Design RBA were appointed by the JV to design the permanent

works for the alternative t, mnel section and to carry out the independent check of the contractor's temporary works designs.

Construction within an open cofferdam enabled the tunnel cross-section to be tailored closely to the structural requirements, making the most efficient use of each structural element. The roof slab was haunched at wMljunctious and thicknesses of both base and roof slab were reduced from centre to edge. The secondary road construction concrete was integrated with the structural base slab. An external toe was provided at the base to mobilise overburden weight for anti-flotation.

Close cooperation between all part ies enabled the buildability of the tunnel to be mAxdmised, by fully integrating permanent works design, temporary works design and construction methods.

Value Engineering The studies made to optimise permanent and temporary

works designs for the alternative tunnel section suggested

that, while retaining the diaphragm walls and top-down construction, a review of the conforming design and construction method would save time and materials. The JV and LDDC negotiated a variation agreement which included Value Engineering of the conforming design, with cost savings achieved to be shared between client and contractor.

RBA reviewed the permanent works design. Redesign- ing roof and base slabs, with haunches and tapering thickness to complement applied bending moments, made sig- nificant savings in concrete and reinforcement quantities and reductions in congestion. Integrating the road construction concrete with the base slab, as in the alternative tunnel, made the structure more efficient and removed a major construction activity from the programme. Replacing continuous =skin walls ~ with discrete "skin columns ~ provided the same function with 60% less materials. Redesign- ing base slab, temporary roof propping, and construction sequence, allowed casting of the central wall in longer bays.

Together with Mort MacDonald, the JV reviewed the imposed construction method and developed an "observa- tional method ~. First "hard" props were installed, then "soft" props, and then no props at all, carefully measuring and recording effects. Wall displacements were shown to be much less than the design predicted, and temporary props were dispensed with.

The savings in time achieved by these Value Engineering changes, particularly avoidance of handling and working round vast quantities of temporary steelwork, made it possible to recover earlier programme delays and complete construction five months early. Materials savings were: 9000 m 3 of concrete; 12,000 t of reinforcement; 50,000 couplers; 11,000m 2 of skin wall; and 20,000 dowels.

6. Two Stations for Jubilee Line Extension, London (Figs. 4, 5, 6)

The Jubilee Line Extension (JLE), running from Green Park to Stratford, was planned by London Underground (LU) to serve the rapidly redeveloping areas in London's docklands, and connect to a number of existing underground and mainline railways, beth in central London and in Essex.

RBA, in jo in t ven tu re with Works In te rna t iona l Consultancy of New Zealand, was appointed by LU in 1991 as technical contractor for the civil and structural engineering design of the Canada Water (Contract 106) and North Greenwich (Contract 110) stations.

Canada Water Station Canada water station is made into a very complex

structure by the requirement to provide an interchange with the existing East London Line (ELL) which cresses the JLE at 90 ° in an old brick tunnel on a horizontal spiral and

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Stub ¢ ~ w n for

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Figure 4. Canada Water JLE.

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i t ~ i '" i t ~ i . * Tem#o~ W I I t i * . shle t ' , " ',~, p,,,.=

Figure 5. Canada Water ELL. Figure 6. North Greenwich.

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5 4 T U N N E L L I N G A N D U N D E R G R O U N D S P A C E T E C H N O L O G Y Volume 11, Number 1, 1 9 9 6

Figure 7. Hong Kong Mass Transit Railway; Kowloon to Tai Koh Tsui Tunnel~:.

a vertical curve. Design work included devising structures and methods for bnihling under, over and around the live ELL railway, to complete all the preparatory work prior to final transfer of the ELL onto its new structure in a single 30-day closure.

The station site is partially inside a housing estate, with two tower blocks i m m , ~ a t e l y adjacent to the site boundary. To avoid damage to these buildings, for which foundation records could not be traced, and to allow earliest reinstatement of the estate land, the top-down method was adopted for the station structure in this area. Avery stiffcontiguous bored pile wall is strutted by the station roof, intermediate floors and base slab. The remainder of the station is constructed more conventionally bottom-up in retained temporary excavation. The typical station cross-section is approx. 22 m wide and 22 m deep. A segmented arch base slab is used to reduce bending moments. Provision is made for future developments to be founded on stub columns on the station roof.

North Greenwich Station North Greenwich station occupies an empty site, vacated

a l ~ r demolition of a gasworks, with no adjoining structures. The highest grade of sulphate-resistant concrete was specified, together with a thick membrane, to keep out contaminants from the extremely polluted surrounding soil.

The station envelope includes a crossover at one end and a siding at the other. At 358 m long, it is three times the length of the platforms. The structure is a 150,000-tonne concrete "submarine" approx. 28 m wide and 20 m deep. The roof and base slabs are s t rut ted apar t by two lines of inclined oval columns, precast in 80 MPa concrete. To counter the huge flotation forces, every internal space not required for trains, passengers or services is filled with ballast.

7. Airport Railway Tunnels, Hong Kong (Fig. 7) Hong Kong Mass Transit Railway Corporation (MTRC)

planned the Airport l~ i lway to incorporate two lines: the Airport Express Line (AEL), a limited stop express service from Hong Kong Island to the new Chek Lap Kok airport, and the Lantau Line (LAL), a full stopping service between Hong Kong and Tung Chung new town on Lantau Island, adjacent to the airport.

RBA, in joint venture with Meinhardt of Hong Kong and Works International Consultancy, was appointed by MTRC in 1992 to carry oul~ civil and structural design of the Contract C504 tunnels.

The C504 tunnels comprise 950 m of four-cell running tunnel and approach structure, between Kowloon station and Tai Kok Tsui station. The tunnels are being built by cut- and-cover techniques in freshly reclaimed land. A major grade-separated interchange for the new West Kowloon Expressway is being constructed over the tunnels by the

same contractor. The cost-saving design dispensed with any need for piled foundations to the tunnels.

8. Paddington Station for Crossrail, London (Fig. 8) CrossRail, a joint project by British Rail (BR) and Lon-

don Underground (LU), will provide a new East-West underground rail link through central London from Bethnal Green to Paddington.

RBA, in joint ven tu re wi th Works In te rna t iona l Consultancy of New Zealand, were appointed in 1992 as technical contractors to design the civil and structural works for the Paddington station. A particular feature of this commission was the client's specific requirement that consideration should be given to the use of the New Austrian Tunneling Method (NATM) for any underground par t of the project. Provided the ground conditions are appropriate, and the design is made correctly, NATM can be a very economical technique for underground construction, particularly in stiff clays such as those found in London.

Golder Associa tes were appo in ted as spec ia l i s t geotechnical subconsultants, and carried out the complex three-dimensional finite element analysis of the excavation and temporary NATM lining, with assistance from Profes- sor Strobl of Austria, who provided expert advice on NATM techniques. RBA provided the structural analysis and design for the permanent lining.

Together, the design team developed the final scheme which, by maximising the use of arched structural elements constructed by NATM techniques, has created a cost-effective design for the whole station. The architect's objective of natural light penetration to platform level has also been achieved in a station structure with minimal ground-level footprint. Without the use ofNATM, the station would have been designed in a full-width open excavation, causing much greater disruption during construction; and its structural elements would have been generally much thicker than their arched equivalents.

The typical station cross-section comprises a three-bore NATM tunnel at a depth of 20 m below ground level, containing two tracks and a 288-m-long platform. The outer tunnel bores are approx 10m diameter, at 18.75-m centres. The central bore is opened up to the surface via a walled vertical slot. The slot walls, also formed using NATM techniques, arch in plan between large-diameter bored piles at 12 m c/c longitudinally. The piles are spaced

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Figure 8. CrossRail Paddington Station; Typical Section.

Volume 11, Number 1, 1996 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 5 5

apart at 10 m c]c laterally and are propped laterally by struts at two levels. Facilities such as ticket halls, M&E plant rooms, ventilation and emergency escape shafts, are housed in end boxes 48 m and 36 m long. At depth, these boxes are extensions of the typical platform cross-section, while upper parts are formed by secant bored pile walls. Other NATM access tunnels and cut-and-cover boxes complete the works.

Method of Construction The large diameter bored piles are constructed first, to

the specified positional and verticality tolerances. Next, the outer t-nnel bores are excavated and tempo-

rarily lined with shotcrete following the NATM technique. Each bore is excavated in two passes, a heading followed by a bench drift. 25 m separation is maintained between the excavation fronts in the eastbound and westbound bores, and between the excavation fronts for the heading and bench drift in each bore.

On completion of excavation and NATM lining of the bores, the trackside halves of longitudinal beams spanning between the piles below platform level are constructed. Waterproofing membrane is applied to the NATM lining and the permanent insitu concrete lining to invert, outer wall and roof of the NATM bores is constructed. Temporary struts at the inner wall prop the tunnel roof.

Construction of the slot, by excavation and NATM lining of arched walls between piles, is done in lm stages, installing permanent lateral concrete struts between piles, plus temporary struts at intermediate levels as required, down to the lower permanent strut level. Waterproofing membrane is applied to the NATM lining of the arched walls and the permanent concrete lining constructed, connecting to the previously constructed tunnel roof linlng and to the piles.

The remaining soil between the tunnels is excavated and NATM lining applied to the arched invert to the

central slot. Waterproofing membrane is applied and the permanent concrete invert constructed, together with the inner halves of the longitudinal beams between piles. Temporary struts and props are removed and the platform structure completed.

9. Conclusion By creating space underground, urban infrastructure

can be developed with minimal permanent impact on the environment at ground level. Economic design requires careful consideration of the form of structure at the pre- liminary design stage; cut-and-cover designs can offer economic and rapid construction for transportation and other facilities to about 25 m depth. The best solution will often be obtained from some form of design-and-construct contract where, within criteria laid down by the client, contractors and their specialist designers are free to de- velop fully integrated solutions to permanent works, temporary works and construction techniques, with benefits in time and cost.

In creating space underground, the designer's task is to provide the required functions within robust and buildable structures, utilising minimum materials compatible with the design criteria. Using purpose-adapted CADD soft- ware, engineers can determine rapidly the geometry, dimensions and reinforcement of underground structures. The very nature of the forces to be resisted by such structures can lead to innovative, elegant and economic designs in the hands of skilled engineers.

The percentage area of a city covered by reads is a statistic often used to compare Bangkok with other major cities, to justify the need for more roads in Bangkok. Next century, we may use a similar three-dimensional statistic, comparing cities by percentage volume of underground space utilised. Perhaps "2001--An Underground Space Odyssey" would be a more appropriate title! [ ]

56 TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY Volume 11, Number 1, 1996

creating space underground

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