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Soft-ground NATM tunnel designs for the Washington, D.C. Metro
T U N N E L S 8e DEEP SPACE
Soft-Ground NATM Tunnel Designs for the Washington, D.C. Metro Mohammad
I r s h a d a n d Larry H. H e f l i n
Abstract--The state-ol-the-art New Austrian Tunneling Method
R~sum~--Cet article dkcrit les alternatives de conception de la mkthode kvoluke "'NouveUe Mkthode de Tunnels Autrichienne", pour trois parties de contrats sur la route Greenbelt du mktro de Washington (WMATA). L'article dkcrit comment le consultant principal du projet utilise la mkthode N T A M pour arriver ~ une plus grande e[Jicacitk. La dkcision d'utiliser la mkthode est aussi discutke. L'article parle de la conception et de prockdures sures en sols mous, avec par exemple la classification des sols, la sEcuritk, les procedures d"excavation, les systkmes de support initiaux et [inaux, l'ktanchEitE, l'analyse et la conception. On parle aussi des avantages potentiels de la conception innovative de ce tunnel et des mkthodes de contruction.
(NA TM ) design alternatives developed [or three contract sections on the Washington, D.C. metro ( W M A T A ) Greenbelt route are described. The paper discusses the way in which WMA TA, assisted by its general engineering consultant, unqied the diverse NA TM design philosophies in order to achieve efficient, economical and sa[e alternate tunnel designs [or the three contract sections. The process of making the decision to use N A T M alternatives in addition to the conventional designs is discussed. The paper touches on issues relevant to the adoption o[ safe and optimal soft-ground N A T M tunnel designs, including ground classi[ication, safety, excavation procedures, initial and final support systems, waterproo[ing, analysis and design. The potential advantages of this innovative tunnel design and construction method are discussed.
Overview
s a logical extension of its successful experience with the New Austrian Tunneling Method (NATM) construction in rock, The Washington (D.C.) Metropolitan Area Transit Authority (WMATA) included soft-ground NATM alternatives in the bid packages for Sections E-5, E-6e and E-8a of the Greenbelt Route. The decision to include soft-ground NATM designs was made after deliberate consideration of the state-of-the-art in this field, as exemplified by the European experience, and considerations of technical feasibility, safety and economical competitiveness. A total of 23,183 linear ft (7068 m) of single-track tunneling (for a combined route-length of approximately 2.2 miles [3.5 km]) is represented by these projects. This includes 11,760 ft (3587 m) of single-track tunneling for Section E-8a; and 9611 ft and 1812 ft (2931 and 553 m), respectively, for Sections E-6e and E-5. Generalized profiles of Section E-5, E-6e and E-8a are provided in Figs. 1, 2 and 3, respectively.
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Present address: Mohammad Irshad, P.E., Chief Structural Engineer, DeLeuw, Cather Co., 600 Fifth St. N.W., Washington, D.C. 20001; and Larry H. Heflin, P.E., Design Manager, Washington Metropolitan Area Transit Authority (WMATA). This article is adapted from a paper presented at the APTA 1987 Rapid Transit Conference, held in June 1987 in Toronto, Canada.
The designs also include NATM shafts. Section E-5 includes one fan shaft and one vent shaft, both having an elliptical configuration. The E-6e designs call for a circular NATM shaftemergency exit. The Section E-8a design provides two NATM fan shafts, one of which also serves as an emergency exit, and a vent shaft at the p u m p i n g station location. All E-8a shafts have elliptical cross-sections.
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Tunnelling and Underground Space Technology, Vol. 3, No. 4, pp. 385-392, 1 9 8 8 . Printed in Great Britain.
Section E-5 is unique among the three designs. In addition to running tunnels and shafts, it includes an NATM station, Fort Totten, that employs a muhiple-drift approach to excavate the station cross-section. The NATM running-tunnel designs competed against conventional tunneling technology that utilizes WMATA's single-pass, bohed-and-gasketed precast concrete segmental tunnel lining. The
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Figure 3. Generalized profile of Section E-8a of the Washington (D.C.) metro's tunnels. N A T M design for Fort Totten Station in Section E-5 completed against W M A T A ' s conventional cut-and-cover approach, which utilizes cast-in-place reinforced concrete vaulted construction. T h e introduction of soft-ground N A T M designs on the W M A T A project represents a major development in the tunneling industry in the U.S., the potential beneficial effects of which are expected to be felt beyond rail-transit applications. For shallow tunneling in urban environment, the N A T M designs bear the promise of economy, flexibility in terms of construction sequencing and e q u i p m e n t needs, and the potential for realizing dry tunnels in water-bearing grounds. Geotechnical
Background
T h e three soft-ground tunnel sections lie in a coastal plain consisting of a
386
broad belt of gently sloping alluvial sediments overlying crystalline and weathered metamorphic bedrock. The coastal plain province is composed of Cretaceous, Pleistocene, and recent deposits in wide, level terraces. T u n n e l i n g will take place in deposits well over 100 million years old. The deposits are composed of gravel, sand (sandstone and conglomerates in the low portions), and primarily plastic clay interbedded with sand lenses in the upper portions. Occasional boulder beds and ironstone lenses also are found. Repeated erosion and deposition have resulted in the intermixed and lenticular nature of these deposits. The Cretaceous materials in the tunnel area are divided into the following three strata, designated P for Potomac Formation One: Stratum PI: A hard plastic clay with occasional pockets of fine
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
sand. Preconsolidation stresses are high, some 15 to 20 Ions per fie and shear strength avmages apl)Joxima(ely 4 kips per ft 2. Stratum P2: C o l n p a c t If) very compact clay or sand with pockets of silty clay and traces of small gravel. Properly dewatered P2 sand has a long stand-up time and needs little temporary support in opencut excavations. Direct shear tests for these arkosic sands indicate an average effective friction angle of 35 degrees. Stratum P3: Generally a hard silty or sandy clay to fine sand with occasional small gravel, usually near the decomposed bedrock. Standard penetration resistance is high to very high, in the range of about 40 to 100 blows per ft, with shear strengths of 3 to 4 kips per ftL Section E-5 r u n n i n g tunnels and the station are located mainly above the water table. Only in the western portion of the E-5 tunnels does the water table rise to about springline elevation (see Fig. 1). Sections E-6e and E-8a are generally well below the water table (see Figs. 2 and 3). Given proper control of groundwater, it is estimated that standup time can often be expected to be at least several hours and generally more than six, which should allow initial N A T M support to be safely installed.
Groundwater Control and Safety For NATM tunneling to be performed safely in soft-ground conditions, flowing ground must be prevented and even limited r u n n i n g ground situations must be guarded against. Ground stability must be assured by effecting an adequate groundwater drawdown, with grouting used as an additional measure where necessary. Although auxiliary means of initial support, such as spiling of various kinds, p o l i n g plates and similar methodologies, may somewhat reduce the ground movement at and near the face under marginal conditions, they cannot control the flowing ground and may provide only scant help in r u n n i n g ground. European experience spanning some 20 years indicates that soft-ground N A T M tunneling is a reliable and feasible tunneling method; however, in some isolated cases where the basic tenet of ensuring ground stability was violated, problems have occurred. The sophisticated finite-element method (FEM) analysis and design methods employed in the practice of NATM design can be considered valid only if contract provisions and correct construction practices preclude the possibility of ground instability. WMATA designs specify dewatering requirements to ensure the reduction of pore water pressures to tolerable limits. At critical locations, control piezometers have been mandated in sufficient Volume 3, N mnber 4, 1988
numbers to ensure that the design intent will be fulfilled in the field. D u r i n g tunnel excavation, all seeping groundwater will be collected and p u m p e d out using perforated drainage pipes. Where necessary, relief holes will be drilled to collect perched groundwater. Under difficult conditions it may be necessary to seal the excavation face with shotcrete and then use vacuum pipes to relieve the water head. Excavation could continue only after the effectiveness of the dewatering procedure has been demonstrated by the additional exploratory holes.
Design Approach N A T M strives to optimize design by harmonizing structural, geotechnical and construction considerations. T h e ground around the opening is considered both a load-imparting and a loadcarrying ring. N A T M requires a careful sequencing of excavation and installation of support to ensure that surrounding ground movements remain small so as not to significantly detract from the inherent strength of the ground ring. At the same time, the deformations must not be inhibited to the extent that they result in u n d u l y large loads on the
Table 1.
structural support system. T h e tunnel shape is selected to minimize flexure and stress concentration and to ensure stability of the support systems. T h e tunnel l i n i n g is kept relatively thin and flexible and its full contact with the ground is ensured to further mitigate the development of localized excessive bending moments. The ground is subdivided into several categories, usually not exceeding six, in accordance with the geological conditions, and structural support systems are designed accordingly. Soft-ground N A T M design a p p r o a c h emphasizes m i n i m a l ringclosure time in relation to ground stand-up time. N A T M is a versatile, general-purpose method that can be adapted to various g r o u n d conditions and a p p l i e d to tunnels of different sizes and shapes. Its observational approach makes the N A T M a particularly safe method in difficult ground conditions. Unlike conventional tunnel designs, N A T M treats the initial l i n i n g as an integral part of the overall design. T h e finite-element method (FEM) is used extensively to perform structural analysis of both the initial and final lining systems, and to check stresses and deformations in the surrounding ground. Generally, the FEM analysis treats the
ground behavior on an elasto-plastic basis, w h i l e t h e l i n i n g shotcrete and soil anchors are modeled elastically. An a p p r o p r i a t e failure criterion, such as M o h r - C o u l o m b , is used to check for plastification of the ground elements. Typically, calculations are performed to simulate the various excavation sequences. As applicable, hydrostatic pressure is included in the lining design. T h e final lining, preferably of unreinforced cast-in-place concrete, also can be designed using non-FEM analysis techniques. For W M A T A designs, proven FEM analyses were employed to simulate the excavation and lining behaviors. L i n i n g design generally adheres to ACI 318-83, Building Code Requirements for Reinforced Concrete, and the German Code, DIN 1045. These analyses resul ted in economical cross-sections. For example, the m a x i m u m initial shotcrete lining thickness is 8 in. (20 cm), with welded wire fabric reinforcement. T h e cast-inplace unreinforced concrete final lining for the three design contracts is 12 in. (30.5 cm) thick. Considering that the tunnels are subjected to as much as 100 ft (30.5 m) of overburden pressure, plus up to 70 ft (21.4 m) of hydrostatic pressure at the worst location, the
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Undrained shear strength kN/m 2 (KSF) Angle of internal friction, (effective stress) "Elastic" stress-strain modulus E, m N / m 2 (KSI): Initial liner deformation Final liner deformation
Coefficient of horizontal subgrade reaction, KH, kN/m 3 (KCF)
Note: C1 = Chord length of the ARC at springline through which outward deflection is taking place, C1 = O.6R. Volume 3, Number 4, 1988
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 387
W M A T A standard cross-section is an efficient arrangement for most mass transit soft-ground tunneling applications. Table 1 lists the various soil parameters needed for the FEM analysis. If doubt exists about the actual values of soil parameters, parametric studies must be performed to verify the design. Such parametric studies or sensitivity analyses are considered necessary in situations where soil parameters are not totally defined or the applicability of alternate state-of-the-art design approaches is questionable. Empirical checks eventually must validate the final design concepts. The semi-empirical and observational approach of N A T M construction places great emphasis on instrumentation requirements and their implementation. T h e initial support needs are based on the actual geological c o n d i t i o n s encountered during construction and the response of the excavated ground. For the N A T M method to be a p p l i e d correctly a n d efficiently, design assumptions must be verified by in-situ instrumentation. T h e contract designs identify the critical cross-sections where m o n i t o r i n g is required. T h e full range of instrumentation for the three projects includes: shallow settlement indicators, extensometers, sliding micrometers, convergence points within the tunnel to measure movement in all directions, pressure cells to measure earth pressures, load cells to measure stresses in shotcrete lining, and piezometers to monitor variations in pore water pressure and water heads.
WMATA's Standard NATM Design for Running Tunnels T h e r u n n i n g tunnel cross-sections originally proposed by each of the three Section Designers showed considerable diversity in terms of external shape and
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internal configuration. Section E-5 and E-6e designers both proposed an ovoid shape; however, cross-section dimensions and the configuration of the invert differed (see Figs. 4 and 5). T h e Section E-8a designer, on the other hand, employed a circular cross-section (Fig. 6). In view of the obvious advantages associated with standardization of design, W M A T A directed its General Engineering Consultant (DeLeuw, Cather gc Company) to develop a standard tunnel cross-section that would combine the best features of the three preliminary cross-sections and improve other details as necessary to ensure an optimal design. Directive drawings were developed accordingly, showing an ovoid cross-section as illustrated in Fig. 8. T h e geometrics associated with this cross-section are shown in Fig. 9. An overlay of the originally proposed cross sections and the W M A T A standard cross-section is shown in Fig. 7. As can be seen, the initial shotcrete lining has a 6-in. (15-cm) m i n i m u m thickness and is reinforced, usually with wire mesh. T h e final lining is cast-inplace concrete and not usually reinforced. The "knuckle" joint between the final l i n i n g and invert does not have a predetermined location. Each section designer can choose the optimal location, based on site-specific conditions, to develop an optimal invert design. The typical N A T M waterproofing at the interface of initial and final lining is open at the invert level. Tile schematic details of the system are shown in Fig. 10. T h e waterproofing system consists of a layer of porous felt filter fabric (geofabric) approx. 0.25-in. (0.64 cm) thick, and a 60-mil-thick PVC membrane (geomembrane) that acts as a water barrier. T h e geofabric, sometimes referred to as fleece, is secured to the
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Figure 6. Original tunnel cross-section [or Section E-8a (superseded by WMA TA standard, per Fig. 8).
initial shotcreted tunnel surface with plastic-headed nails. The fleece performs the dual function of facilitating drainage and preventing damage to the geomembrane from the rough surface of the initial shotcrete. The geomembrane is then applied over the fleece and is secured by heat-welding it to the previously installed plastic discs. The longitudinal seams between the 4-ft. (1.2-m) wide geomembrane strips also are heat-welded to ensure an integral waterproof " u m b r e l l a " for the final lining, and are checked for watertightness by means of compressed air. T h e standard design requires that protection of the initial shotcrete lining from construction equipment traffic be facilitated by means of a temporary concrete slab or gravel bed. T h e drainage system takes into consideration the percolation of groundwater through the shotcrete lining; it is submerged to mitigate the likelihood of clogging up of the drainage pipes due to calcification from exposure to air.
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Figure 4. Original tunnel cross-section [or Section E-5, Fort Totten Station (superseded by WMATA standard, per Fig. 8). 388
Figure 5. Original tunnel cross-section for Section E-6e (superseded by WMA TA standard, per Fig. 8).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Figure 7. Comparative diagram showing the originally proposed tunnel crosssection and the implemented WMA TA standard cross-section. Volume 3, Number 4, 1988
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/ Other portions of the drainage system that cannot be sealed off from free air are designed to be cleaned easily by mechanical means when necessary. T h e m a x i m u m construction tolerance i m p l i c i t in the cross-section geometry is about 2 in. (5 cm), based on satisfactory past experience. T h e standard configuration strikes an o p t i m a l compromise between the requirements of dynamic clearance for all possible situations and a minimization of the cross-sectional area, which is directly related to the excavation volumes. T h e ovoid shape also ensures an efficient structural behavior of the lining. T h e geometry of the W M A T A standard cross-section can be used u p to a m a x i m u m superelevation of 4 in. (10 cm), associated with the tightest radius of curvature on the W M A T A system (755 ft [230 ml]). W i t h m i n o r adjustments, it can accommodate floating slabs (see Fig. 11) and the safety walk can be placed on either side of the curve center. T h e initial l i n i n g is designed to withstand all the g r o u n d and superimposed surface loads. T h e final lining is designed to sustain the interface loads between it and the initial lining, as well as an equivalent hydrostatic pressure determined by the designers to provide a sufficient safety factor should any problem develop in the drainage system. Little or no hydrostatic pressure is expected to develop over the long term because of the expected long life of the groundwater drainage system.
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Excavation Sequence and Initial Support Systems for Running Tunnels T h e excavation procedure and the initial support are varied to achieve o p t i m a l accommodation of the different g r o u n d conditions to take advantage of cost savings. T h e expected ground conditions are categorized to reflect the Volume 3, Number 4, 1988
Figure 10. Typical NA TM waterproo[ing detail. TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 389
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anticipated ease or difficulty of the tunnelling operation. A menu of initial support systems is provided to cater to the needs of the different ground types actually encountered. Based on the actual ground conditions observed, the owner's engineers in the field decide, jointly with the contractor, which initial s u p p o r t is to be used and whether additional support is necessary. However, where additional initial support is required by the contractor to protect adjacent structures or address local adverse geological conditions in the vicinity of the tunnel, such support must be installed. Four basic ground categories--Excellent, Good, Fair and P o o r - - g e n e r a l l y are used for the three design sections. Only Section E-da employs an additional category, known as Condition-5, which reflects the required additional support needed at seven critical locations to protect adjacent structures and facilities and to allow for localized adverse geological conditions. T h e specified excavation procedure minimizes ground disturbance in order to preserve the inherent strength of the ground. T h e excavated area is kept small and timely installation of initial support is required. Typically, the full range of excavation sequence consists of crown excavation followed by bench and invert excavation. A typical detail of the basic excavation procedure is shown in Fig. 12. Timely ring closure is mandated by the contract documents. Generally, the separation between adjacent tunnel headings is not allowed to be less than the greater of five tunnel diameters or three days of tunnel advance. Depending on the ground conditions and a need to protect surface installation, where applicable, the following range of initial support systems has been incorporated in the design: (1) Sealing shotcrete. (2) Lattice girders.
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Figure 12. Typical NA TM excavation procedure.
(3) Soil anchors. (4) Welded wire fabric. (5) Rebar splices for initial shotcrete lining. (6) Structural shotcrete in layers as specified. (7) Reinforcing rod spiling. (8) Perforated p i p e spiling to be used for p r e g r o u t i n g short distances ahead of the face, generally 10-15 ft (3-4.5 m). (9) Plastic sleeve pipes to be used for pregrouting for longer distances ahead of the face, generally 50-70 ft (15-21 m). (I0) Steel p o l i n g plates. Items (I)-(6) are considered standard means of initial support when installed in a timely manner in good-to-excellent ground conditions. Items (7)-(10) are considered supplementary support systems in fair-to-poor ground conditions or where additional support is needed to protect adjacent structures and facilities.
NATM Shafts On the basis of separate studies performed for Sections E-5, E-6e and E8a, it was established that in each case it was highly desirable to utilize N A T M shaft designs with the N A T M r u n n i n g tunnel alternates. Apart from the significant cost savings, the N A T M shafts offer the following advantages: (1) T h e waterproofing system used for r u n n i n g tunnels can be economically extended to include those portions of the shafts that lie below the water table, thereby effectively preventing water intrusion through the shafts. (2) Excessive post-construction settlements of the ground around the shafts can be avoided because ground movements associated with N A T M shaft construction are minimal. (3) N A T M shafts utilize relatively flexible construction that is structurally more compatible with the N A T M tunnel behavior than the more rigid conventional shaft construction. Con-
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
struction details at the junction of tunnel and shaft, or tunnel and cross adit, are simplified. The smooth transition at the shaft-tunnel junction reduces the possibility of any undesirable stress concentration. (4) N A T M shaft designs make it possible to sink the shafts prior to proceeding with the tunnel excavation. This process offers the flexibility of working several tunnel headings simultaneously to speed u p the advance rate and, in turn, reduce construction time and costs. The optimal cross-section configuration for each shaft is determined individually by striking the best compromise between the functional requirements and the structural-geotechnical considerations. The N A T M shaft design philosophy is similar to NATM tunnel design approach described above. T h e N A T M shaft design obviates the need for steel ring beams and lagging utilized in the conventional construction for initial support. In N A T M design, sealing shotcrete is first a p p l i e d to the excavated ground, followed by installation of the reinforced shotcrete liner. Where needed, lattice girders and soil anchors are used as a part of the initial support. T h e final liner is p l a i n or reinforced concrete, as necessary.
Mined Station Design: Fort Totten Fort Totten Station in Section E-5 represents a unique hybrid design concept: half of the 600-ft (183-m) station is underground and the other half is at-grade. T h e decision to adopt this scheme was the culmination of an exhaustive study undertaken to resolve environmental concerns and engineering issues related to the siting of this facility. T h e original General Plans called for a standard cut-and-cover Volume 3, Number 4, 1988
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W M A T A vault for the underground portion of the station. After deciding to develop N A T M alternate designs for r u n n i n g tunnels, W M A T A undertook a detailed study to investigate the feasibility of a mined station vis-fi-vis the conventional cutand-cover construction. Based on the positive findings of this study, General Plans for a mined station design alternate were prepared, and a detailed design utilizing N A T M construction techniques was commissioned. T h e typical N A T M design crosssection is shown in Figure 13. T h e initial s u p p o r t is provided by 10-in. (25cm) reinforced shotcrete. T h e final liner is 16-in. (40.6-cm) cast-in-place reinforced concrete. A typical membrane waterproofing system, similar to that used for r u n n i n g tunnels, is provided. T h e architectural liner consists of precast concrete coffered panels. Unlike the conventional design, which provided an unobstructed space, the N A T M design uses 4-ft x 7.5ft (1.2-m x 2.3-m) columns, placed at 33.3 [t (10 m) on center, a l o n g the center line of the station. Although the N A T M design maintains the width of the trainroom, which has a central platform, the ceiling height is significantly less for the N A T M alternate than for the conventional design. However, the N A T M alternate has a clear advantage in terms of significantly lessening the adverse environmental impact associated with the cut-and-cover construction. T o minimize ground disturbance, the N A T M excavation will use the multiple drift approach indicated in Fig. 14. A Volume 3, Number 4, 1988
total of five separate steps are required to accomplish the excavation of the entire cross-section. For the first 100 ft (31 m) of excavation in the vicinity of the portal, where the ground cover is shallow, an innovative horizontal jet g r o u t i n g technique will be used to form a structural umbrella before proceeding with the excavation (see Fig. 15).
alongside the conventional designs. In rail-transit applications, N A T M designs are capable of offering cost competitive and technically desirable solutions to many tunneling challenges routinely encountered in the urban and suburban environment. T h e European experience indicates that soft-ground NATM tunneling technology is based on sound and practicable principles. T h e work performed so far on the W M A T A project bears out the potential promise of N A T M construction. Under favorable conditions, not only
Conclusion
Where feasible, soft-ground N A T M tunnel designs deserve due consideration
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tunnels but larger openings such as stations are also feasible. WMATA's design experience indicates that NATM shafts should be used in c o n j u n c t i o n with NATM tunnels to extract the m a x i m u m benefits from this construction method. T h o u g h essentially a free-air tunneling method, NATM construction has also been used, in Europe, in conjunction with compressed air. Such cases include situations where dewatering was either n o t permissible because of concerns about settlements of old structures, or impractical because of lack of surface space for well installation in congested urban areas. All WMATA designs employ the standard free-air construction method. Adequate drawdown of groundwater is mandated by the design documents to ensure sufficient reduction of pore water pressures prior to excavation. Competent design and correct construction procedures are a prerequisite for safe and successful application of NATM in soft ground. The contract documents for NATM designs customarily are more detailed and explicit than documents for conventional designs. Construction techniques must be adapted to be compatible with the NATM philosophy. The special role of instrumentation as a tool for verifying design assumptions must be fully recognized and addressed. No hard-and-fast rules can be cited in
392
comparing NATM with shield-driven tunneling. In some situations, shielddriven t u n n e l i n g may be considered more desirable or even indispensable. For example, in relatively long tunnel construction, a tunnel boring machine (TBM) may prove more effective in terms of advance rate and, hence, may result in lower construction time and cost. Where groundwater drawdown is not permissible, an appropriately designed TBM may be the only feasible method in free air. In terms of the WMATA designs, NATM construction offers the following advantages: (1) NATM construction in soft ground is expected to result in smaller settlements than construction by conventional t u n n e l i n g methods. Not only are the absolute values of settlement less, but differential settlements also are significantly reduced. (2) The use of membrane waterproofing results in watertight tunnels, shafts, and stations. (3) E l i m i n a t i n g heavy steel sets for initial lining, and eliminating rebar in the final l i n i n g result in considerable economies. (4) The availability of construction shafts generally enables the contractor to advance tunnels simultaneously from multiple headings. Estimated average advance rates of 10 ft (3 m) per day per heading were feasible for the WMATA tunnels. Where an intermediate shaft is
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY
available to perform excavalion of tt+w two tunnels, four headings ~au be advanced simultaneously. This implies an advance rate of 40 fl pet day, based on rather conservative assumptions. (5) Unlike conventional tunneling, NATM construction does not rely on expensive tunnel boring ,nachines. Excavation equipment cati be changed with relative ease to best suit tile ground conditions. The need for heavy equipment investment up from and the risk of total dependency on one major piece of machinery are eliminated. NATM t u n n e l i n g requires special expertise on the part of the contractor, and a cooperative working relationship between the contractor and the owner. Construction management techniques must also be adapted to best suit NATM construction needs. WMATA's successful experience with two rock NATM projects indicates that these challenges can be met successfully in tile U.S. The three contract sections were separately bid during late 1987 and early 1988. For Section E-5, the NATM option emerged as the lowest bid. The bid is based on performing all the construction--mined tunnels, ventilation shafts and the underground position of Fort Totten Station--using the NATM technique. Section E-6e bidding also resulted in a contract award based on the NATM alternate. However, WMATA currently is reviewing a value engineering proposal from the contractor which, if successful, will result in conventional construction in lieu of NATM. In Section E-Sa, the longest section, NATM alternate was not successful against tile conventional construction. T h e lowest bid in this case is based on WMATA's segmental concrete lining. However, in this case it can be argued that the inclusion of the NATM alternative resulted in very competitive bidding. The construction of Section E-5 is scheduled to begin during the third quarter of 1988 and its progress will, no doubt, be closely watched by the t u n n e l i n g industry. This first largescale application of soft-ground NATM in the U.S. is expected to infuse innovative t h i n k i n g into the design and construction of future similar underground structures and facilities. []
Volume 3, Number 4, 1988
Soft-Ground NATM Tunnel Designs for the Washington, D.C. Metro Mohammad
I r s h a d a n d Larry H. H e f l i n
Abstract--The state-ol-the-art New Austrian Tunneling Method
R~sum~--Cet article dkcrit les alternatives de conception de la mkthode kvoluke "'NouveUe Mkthode de Tunnels Autrichienne", pour trois parties de contrats sur la route Greenbelt du mktro de Washington (WMATA). L'article dkcrit comment le consultant principal du projet utilise la mkthode N T A M pour arriver ~ une plus grande e[Jicacitk. La dkcision d'utiliser la mkthode est aussi discutke. L'article parle de la conception et de prockdures sures en sols mous, avec par exemple la classification des sols, la sEcuritk, les procedures d"excavation, les systkmes de support initiaux et [inaux, l'ktanchEitE, l'analyse et la conception. On parle aussi des avantages potentiels de la conception innovative de ce tunnel et des mkthodes de contruction.
(NA TM ) design alternatives developed [or three contract sections on the Washington, D.C. metro ( W M A T A ) Greenbelt route are described. The paper discusses the way in which WMA TA, assisted by its general engineering consultant, unqied the diverse NA TM design philosophies in order to achieve efficient, economical and sa[e alternate tunnel designs [or the three contract sections. The process of making the decision to use N A T M alternatives in addition to the conventional designs is discussed. The paper touches on issues relevant to the adoption o[ safe and optimal soft-ground N A T M tunnel designs, including ground classi[ication, safety, excavation procedures, initial and final support systems, waterproo[ing, analysis and design. The potential advantages of this innovative tunnel design and construction method are discussed.
Overview
s a logical extension of its successful experience with the New Austrian Tunneling Method (NATM) construction in rock, The Washington (D.C.) Metropolitan Area Transit Authority (WMATA) included soft-ground NATM alternatives in the bid packages for Sections E-5, E-6e and E-8a of the Greenbelt Route. The decision to include soft-ground NATM designs was made after deliberate consideration of the state-of-the-art in this field, as exemplified by the European experience, and considerations of technical feasibility, safety and economical competitiveness. A total of 23,183 linear ft (7068 m) of single-track tunneling (for a combined route-length of approximately 2.2 miles [3.5 km]) is represented by these projects. This includes 11,760 ft (3587 m) of single-track tunneling for Section E-8a; and 9611 ft and 1812 ft (2931 and 553 m), respectively, for Sections E-6e and E-5. Generalized profiles of Section E-5, E-6e and E-8a are provided in Figs. 1, 2 and 3, respectively.
A
Present address: Mohammad Irshad, P.E., Chief Structural Engineer, DeLeuw, Cather Co., 600 Fifth St. N.W., Washington, D.C. 20001; and Larry H. Heflin, P.E., Design Manager, Washington Metropolitan Area Transit Authority (WMATA). This article is adapted from a paper presented at the APTA 1987 Rapid Transit Conference, held in June 1987 in Toronto, Canada.
The designs also include NATM shafts. Section E-5 includes one fan shaft and one vent shaft, both having an elliptical configuration. The E-6e designs call for a circular NATM shaftemergency exit. The Section E-8a design provides two NATM fan shafts, one of which also serves as an emergency exit, and a vent shaft at the p u m p i n g station location. All E-8a shafts have elliptical cross-sections.
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Tunnelling and Underground Space Technology, Vol. 3, No. 4, pp. 385-392, 1 9 8 8 . Printed in Great Britain.
Section E-5 is unique among the three designs. In addition to running tunnels and shafts, it includes an NATM station, Fort Totten, that employs a muhiple-drift approach to excavate the station cross-section. The NATM running-tunnel designs competed against conventional tunneling technology that utilizes WMATA's single-pass, bohed-and-gasketed precast concrete segmental tunnel lining. The
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Figure 3. Generalized profile of Section E-8a of the Washington (D.C.) metro's tunnels. N A T M design for Fort Totten Station in Section E-5 completed against W M A T A ' s conventional cut-and-cover approach, which utilizes cast-in-place reinforced concrete vaulted construction. T h e introduction of soft-ground N A T M designs on the W M A T A project represents a major development in the tunneling industry in the U.S., the potential beneficial effects of which are expected to be felt beyond rail-transit applications. For shallow tunneling in urban environment, the N A T M designs bear the promise of economy, flexibility in terms of construction sequencing and e q u i p m e n t needs, and the potential for realizing dry tunnels in water-bearing grounds. Geotechnical
Background
T h e three soft-ground tunnel sections lie in a coastal plain consisting of a
386
broad belt of gently sloping alluvial sediments overlying crystalline and weathered metamorphic bedrock. The coastal plain province is composed of Cretaceous, Pleistocene, and recent deposits in wide, level terraces. T u n n e l i n g will take place in deposits well over 100 million years old. The deposits are composed of gravel, sand (sandstone and conglomerates in the low portions), and primarily plastic clay interbedded with sand lenses in the upper portions. Occasional boulder beds and ironstone lenses also are found. Repeated erosion and deposition have resulted in the intermixed and lenticular nature of these deposits. The Cretaceous materials in the tunnel area are divided into the following three strata, designated P for Potomac Formation One: Stratum PI: A hard plastic clay with occasional pockets of fine
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
sand. Preconsolidation stresses are high, some 15 to 20 Ions per fie and shear strength avmages apl)Joxima(ely 4 kips per ft 2. Stratum P2: C o l n p a c t If) very compact clay or sand with pockets of silty clay and traces of small gravel. Properly dewatered P2 sand has a long stand-up time and needs little temporary support in opencut excavations. Direct shear tests for these arkosic sands indicate an average effective friction angle of 35 degrees. Stratum P3: Generally a hard silty or sandy clay to fine sand with occasional small gravel, usually near the decomposed bedrock. Standard penetration resistance is high to very high, in the range of about 40 to 100 blows per ft, with shear strengths of 3 to 4 kips per ftL Section E-5 r u n n i n g tunnels and the station are located mainly above the water table. Only in the western portion of the E-5 tunnels does the water table rise to about springline elevation (see Fig. 1). Sections E-6e and E-8a are generally well below the water table (see Figs. 2 and 3). Given proper control of groundwater, it is estimated that standup time can often be expected to be at least several hours and generally more than six, which should allow initial N A T M support to be safely installed.
Groundwater Control and Safety For NATM tunneling to be performed safely in soft-ground conditions, flowing ground must be prevented and even limited r u n n i n g ground situations must be guarded against. Ground stability must be assured by effecting an adequate groundwater drawdown, with grouting used as an additional measure where necessary. Although auxiliary means of initial support, such as spiling of various kinds, p o l i n g plates and similar methodologies, may somewhat reduce the ground movement at and near the face under marginal conditions, they cannot control the flowing ground and may provide only scant help in r u n n i n g ground. European experience spanning some 20 years indicates that soft-ground N A T M tunneling is a reliable and feasible tunneling method; however, in some isolated cases where the basic tenet of ensuring ground stability was violated, problems have occurred. The sophisticated finite-element method (FEM) analysis and design methods employed in the practice of NATM design can be considered valid only if contract provisions and correct construction practices preclude the possibility of ground instability. WMATA designs specify dewatering requirements to ensure the reduction of pore water pressures to tolerable limits. At critical locations, control piezometers have been mandated in sufficient Volume 3, N mnber 4, 1988
numbers to ensure that the design intent will be fulfilled in the field. D u r i n g tunnel excavation, all seeping groundwater will be collected and p u m p e d out using perforated drainage pipes. Where necessary, relief holes will be drilled to collect perched groundwater. Under difficult conditions it may be necessary to seal the excavation face with shotcrete and then use vacuum pipes to relieve the water head. Excavation could continue only after the effectiveness of the dewatering procedure has been demonstrated by the additional exploratory holes.
Design Approach N A T M strives to optimize design by harmonizing structural, geotechnical and construction considerations. T h e ground around the opening is considered both a load-imparting and a loadcarrying ring. N A T M requires a careful sequencing of excavation and installation of support to ensure that surrounding ground movements remain small so as not to significantly detract from the inherent strength of the ground ring. At the same time, the deformations must not be inhibited to the extent that they result in u n d u l y large loads on the
Table 1.
structural support system. T h e tunnel shape is selected to minimize flexure and stress concentration and to ensure stability of the support systems. T h e tunnel l i n i n g is kept relatively thin and flexible and its full contact with the ground is ensured to further mitigate the development of localized excessive bending moments. The ground is subdivided into several categories, usually not exceeding six, in accordance with the geological conditions, and structural support systems are designed accordingly. Soft-ground N A T M design a p p r o a c h emphasizes m i n i m a l ringclosure time in relation to ground stand-up time. N A T M is a versatile, general-purpose method that can be adapted to various g r o u n d conditions and a p p l i e d to tunnels of different sizes and shapes. Its observational approach makes the N A T M a particularly safe method in difficult ground conditions. Unlike conventional tunnel designs, N A T M treats the initial l i n i n g as an integral part of the overall design. T h e finite-element method (FEM) is used extensively to perform structural analysis of both the initial and final lining systems, and to check stresses and deformations in the surrounding ground. Generally, the FEM analysis treats the
ground behavior on an elasto-plastic basis, w h i l e t h e l i n i n g shotcrete and soil anchors are modeled elastically. An a p p r o p r i a t e failure criterion, such as M o h r - C o u l o m b , is used to check for plastification of the ground elements. Typically, calculations are performed to simulate the various excavation sequences. As applicable, hydrostatic pressure is included in the lining design. T h e final lining, preferably of unreinforced cast-in-place concrete, also can be designed using non-FEM analysis techniques. For W M A T A designs, proven FEM analyses were employed to simulate the excavation and lining behaviors. L i n i n g design generally adheres to ACI 318-83, Building Code Requirements for Reinforced Concrete, and the German Code, DIN 1045. These analyses resul ted in economical cross-sections. For example, the m a x i m u m initial shotcrete lining thickness is 8 in. (20 cm), with welded wire fabric reinforcement. T h e cast-inplace unreinforced concrete final lining for the three design contracts is 12 in. (30.5 cm) thick. Considering that the tunnels are subjected to as much as 100 ft (30.5 m) of overburden pressure, plus up to 70 ft (21.4 m) of hydrostatic pressure at the worst location, the
Soil parameters recommended ]or NA TM design.
Stratum Soil parameters
P1 clays
P2 sands
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20.42 (130)
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0.4
0.3
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Probably 1 approx.
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0.45
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143.64 to 167.58 (3 to 3.5)
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27.58 (4) Least stable ground P2, (SP-SM) 41.37 (6) least stable ground P2, (SP-SM)
157.09 E/2C1 (E/2C1)
157.09 E/2C1 (E/2C1)
Undrained shear strength kN/m 2 (KSF) Angle of internal friction, (effective stress) "Elastic" stress-strain modulus E, m N / m 2 (KSI): Initial liner deformation Final liner deformation
Coefficient of horizontal subgrade reaction, KH, kN/m 3 (KCF)
Note: C1 = Chord length of the ARC at springline through which outward deflection is taking place, C1 = O.6R. Volume 3, Number 4, 1988
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 387
W M A T A standard cross-section is an efficient arrangement for most mass transit soft-ground tunneling applications. Table 1 lists the various soil parameters needed for the FEM analysis. If doubt exists about the actual values of soil parameters, parametric studies must be performed to verify the design. Such parametric studies or sensitivity analyses are considered necessary in situations where soil parameters are not totally defined or the applicability of alternate state-of-the-art design approaches is questionable. Empirical checks eventually must validate the final design concepts. The semi-empirical and observational approach of N A T M construction places great emphasis on instrumentation requirements and their implementation. T h e initial support needs are based on the actual geological c o n d i t i o n s encountered during construction and the response of the excavated ground. For the N A T M method to be a p p l i e d correctly a n d efficiently, design assumptions must be verified by in-situ instrumentation. T h e contract designs identify the critical cross-sections where m o n i t o r i n g is required. T h e full range of instrumentation for the three projects includes: shallow settlement indicators, extensometers, sliding micrometers, convergence points within the tunnel to measure movement in all directions, pressure cells to measure earth pressures, load cells to measure stresses in shotcrete lining, and piezometers to monitor variations in pore water pressure and water heads.
WMATA's Standard NATM Design for Running Tunnels T h e r u n n i n g tunnel cross-sections originally proposed by each of the three Section Designers showed considerable diversity in terms of external shape and
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internal configuration. Section E-5 and E-6e designers both proposed an ovoid shape; however, cross-section dimensions and the configuration of the invert differed (see Figs. 4 and 5). T h e Section E-8a designer, on the other hand, employed a circular cross-section (Fig. 6). In view of the obvious advantages associated with standardization of design, W M A T A directed its General Engineering Consultant (DeLeuw, Cather gc Company) to develop a standard tunnel cross-section that would combine the best features of the three preliminary cross-sections and improve other details as necessary to ensure an optimal design. Directive drawings were developed accordingly, showing an ovoid cross-section as illustrated in Fig. 8. T h e geometrics associated with this cross-section are shown in Fig. 9. An overlay of the originally proposed cross sections and the W M A T A standard cross-section is shown in Fig. 7. As can be seen, the initial shotcrete lining has a 6-in. (15-cm) m i n i m u m thickness and is reinforced, usually with wire mesh. T h e final lining is cast-inplace concrete and not usually reinforced. The "knuckle" joint between the final l i n i n g and invert does not have a predetermined location. Each section designer can choose the optimal location, based on site-specific conditions, to develop an optimal invert design. The typical N A T M waterproofing at the interface of initial and final lining is open at the invert level. Tile schematic details of the system are shown in Fig. 10. T h e waterproofing system consists of a layer of porous felt filter fabric (geofabric) approx. 0.25-in. (0.64 cm) thick, and a 60-mil-thick PVC membrane (geomembrane) that acts as a water barrier. T h e geofabric, sometimes referred to as fleece, is secured to the
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Figure 6. Original tunnel cross-section [or Section E-8a (superseded by WMA TA standard, per Fig. 8).
initial shotcreted tunnel surface with plastic-headed nails. The fleece performs the dual function of facilitating drainage and preventing damage to the geomembrane from the rough surface of the initial shotcrete. The geomembrane is then applied over the fleece and is secured by heat-welding it to the previously installed plastic discs. The longitudinal seams between the 4-ft. (1.2-m) wide geomembrane strips also are heat-welded to ensure an integral waterproof " u m b r e l l a " for the final lining, and are checked for watertightness by means of compressed air. T h e standard design requires that protection of the initial shotcrete lining from construction equipment traffic be facilitated by means of a temporary concrete slab or gravel bed. T h e drainage system takes into consideration the percolation of groundwater through the shotcrete lining; it is submerged to mitigate the likelihood of clogging up of the drainage pipes due to calcification from exposure to air.
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Figure 4. Original tunnel cross-section [or Section E-5, Fort Totten Station (superseded by WMATA standard, per Fig. 8). 388
Figure 5. Original tunnel cross-section for Section E-6e (superseded by WMA TA standard, per Fig. 8).
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
Figure 7. Comparative diagram showing the originally proposed tunnel crosssection and the implemented WMA TA standard cross-section. Volume 3, Number 4, 1988
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/ Other portions of the drainage system that cannot be sealed off from free air are designed to be cleaned easily by mechanical means when necessary. T h e m a x i m u m construction tolerance i m p l i c i t in the cross-section geometry is about 2 in. (5 cm), based on satisfactory past experience. T h e standard configuration strikes an o p t i m a l compromise between the requirements of dynamic clearance for all possible situations and a minimization of the cross-sectional area, which is directly related to the excavation volumes. T h e ovoid shape also ensures an efficient structural behavior of the lining. T h e geometry of the W M A T A standard cross-section can be used u p to a m a x i m u m superelevation of 4 in. (10 cm), associated with the tightest radius of curvature on the W M A T A system (755 ft [230 ml]). W i t h m i n o r adjustments, it can accommodate floating slabs (see Fig. 11) and the safety walk can be placed on either side of the curve center. T h e initial l i n i n g is designed to withstand all the g r o u n d and superimposed surface loads. T h e final lining is designed to sustain the interface loads between it and the initial lining, as well as an equivalent hydrostatic pressure determined by the designers to provide a sufficient safety factor should any problem develop in the drainage system. Little or no hydrostatic pressure is expected to develop over the long term because of the expected long life of the groundwater drainage system.
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Excavation Sequence and Initial Support Systems for Running Tunnels T h e excavation procedure and the initial support are varied to achieve o p t i m a l accommodation of the different g r o u n d conditions to take advantage of cost savings. T h e expected ground conditions are categorized to reflect the Volume 3, Number 4, 1988
Figure 10. Typical NA TM waterproo[ing detail. TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY 389
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anticipated ease or difficulty of the tunnelling operation. A menu of initial support systems is provided to cater to the needs of the different ground types actually encountered. Based on the actual ground conditions observed, the owner's engineers in the field decide, jointly with the contractor, which initial s u p p o r t is to be used and whether additional support is necessary. However, where additional initial support is required by the contractor to protect adjacent structures or address local adverse geological conditions in the vicinity of the tunnel, such support must be installed. Four basic ground categories--Excellent, Good, Fair and P o o r - - g e n e r a l l y are used for the three design sections. Only Section E-da employs an additional category, known as Condition-5, which reflects the required additional support needed at seven critical locations to protect adjacent structures and facilities and to allow for localized adverse geological conditions. T h e specified excavation procedure minimizes ground disturbance in order to preserve the inherent strength of the ground. T h e excavated area is kept small and timely installation of initial support is required. Typically, the full range of excavation sequence consists of crown excavation followed by bench and invert excavation. A typical detail of the basic excavation procedure is shown in Fig. 12. Timely ring closure is mandated by the contract documents. Generally, the separation between adjacent tunnel headings is not allowed to be less than the greater of five tunnel diameters or three days of tunnel advance. Depending on the ground conditions and a need to protect surface installation, where applicable, the following range of initial support systems has been incorporated in the design: (1) Sealing shotcrete. (2) Lattice girders.
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(3) Soil anchors. (4) Welded wire fabric. (5) Rebar splices for initial shotcrete lining. (6) Structural shotcrete in layers as specified. (7) Reinforcing rod spiling. (8) Perforated p i p e spiling to be used for p r e g r o u t i n g short distances ahead of the face, generally 10-15 ft (3-4.5 m). (9) Plastic sleeve pipes to be used for pregrouting for longer distances ahead of the face, generally 50-70 ft (15-21 m). (I0) Steel p o l i n g plates. Items (I)-(6) are considered standard means of initial support when installed in a timely manner in good-to-excellent ground conditions. Items (7)-(10) are considered supplementary support systems in fair-to-poor ground conditions or where additional support is needed to protect adjacent structures and facilities.
NATM Shafts On the basis of separate studies performed for Sections E-5, E-6e and E8a, it was established that in each case it was highly desirable to utilize N A T M shaft designs with the N A T M r u n n i n g tunnel alternates. Apart from the significant cost savings, the N A T M shafts offer the following advantages: (1) T h e waterproofing system used for r u n n i n g tunnels can be economically extended to include those portions of the shafts that lie below the water table, thereby effectively preventing water intrusion through the shafts. (2) Excessive post-construction settlements of the ground around the shafts can be avoided because ground movements associated with N A T M shaft construction are minimal. (3) N A T M shafts utilize relatively flexible construction that is structurally more compatible with the N A T M tunnel behavior than the more rigid conventional shaft construction. Con-
TUNNELLING AND UNDERGROUND SPACE TECHNOLOGY
struction details at the junction of tunnel and shaft, or tunnel and cross adit, are simplified. The smooth transition at the shaft-tunnel junction reduces the possibility of any undesirable stress concentration. (4) N A T M shaft designs make it possible to sink the shafts prior to proceeding with the tunnel excavation. This process offers the flexibility of working several tunnel headings simultaneously to speed u p the advance rate and, in turn, reduce construction time and costs. The optimal cross-section configuration for each shaft is determined individually by striking the best compromise between the functional requirements and the structural-geotechnical considerations. The N A T M shaft design philosophy is similar to NATM tunnel design approach described above. T h e N A T M shaft design obviates the need for steel ring beams and lagging utilized in the conventional construction for initial support. In N A T M design, sealing shotcrete is first a p p l i e d to the excavated ground, followed by installation of the reinforced shotcrete liner. Where needed, lattice girders and soil anchors are used as a part of the initial support. T h e final liner is p l a i n or reinforced concrete, as necessary.
Mined Station Design: Fort Totten Fort Totten Station in Section E-5 represents a unique hybrid design concept: half of the 600-ft (183-m) station is underground and the other half is at-grade. T h e decision to adopt this scheme was the culmination of an exhaustive study undertaken to resolve environmental concerns and engineering issues related to the siting of this facility. T h e original General Plans called for a standard cut-and-cover Volume 3, Number 4, 1988
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Figure 13. Fort Totten Station cross-section [or Section E-5.
W M A T A vault for the underground portion of the station. After deciding to develop N A T M alternate designs for r u n n i n g tunnels, W M A T A undertook a detailed study to investigate the feasibility of a mined station vis-fi-vis the conventional cutand-cover construction. Based on the positive findings of this study, General Plans for a mined station design alternate were prepared, and a detailed design utilizing N A T M construction techniques was commissioned. T h e typical N A T M design crosssection is shown in Figure 13. T h e initial s u p p o r t is provided by 10-in. (25cm) reinforced shotcrete. T h e final liner is 16-in. (40.6-cm) cast-in-place reinforced concrete. A typical membrane waterproofing system, similar to that used for r u n n i n g tunnels, is provided. T h e architectural liner consists of precast concrete coffered panels. Unlike the conventional design, which provided an unobstructed space, the N A T M design uses 4-ft x 7.5ft (1.2-m x 2.3-m) columns, placed at 33.3 [t (10 m) on center, a l o n g the center line of the station. Although the N A T M design maintains the width of the trainroom, which has a central platform, the ceiling height is significantly less for the N A T M alternate than for the conventional design. However, the N A T M alternate has a clear advantage in terms of significantly lessening the adverse environmental impact associated with the cut-and-cover construction. T o minimize ground disturbance, the N A T M excavation will use the multiple drift approach indicated in Fig. 14. A Volume 3, Number 4, 1988
total of five separate steps are required to accomplish the excavation of the entire cross-section. For the first 100 ft (31 m) of excavation in the vicinity of the portal, where the ground cover is shallow, an innovative horizontal jet g r o u t i n g technique will be used to form a structural umbrella before proceeding with the excavation (see Fig. 15).
alongside the conventional designs. In rail-transit applications, N A T M designs are capable of offering cost competitive and technically desirable solutions to many tunneling challenges routinely encountered in the urban and suburban environment. T h e European experience indicates that soft-ground NATM tunneling technology is based on sound and practicable principles. T h e work performed so far on the W M A T A project bears out the potential promise of N A T M construction. Under favorable conditions, not only
Conclusion
Where feasible, soft-ground N A T M tunnel designs deserve due consideration
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tunnels but larger openings such as stations are also feasible. WMATA's design experience indicates that NATM shafts should be used in c o n j u n c t i o n with NATM tunnels to extract the m a x i m u m benefits from this construction method. T h o u g h essentially a free-air tunneling method, NATM construction has also been used, in Europe, in conjunction with compressed air. Such cases include situations where dewatering was either n o t permissible because of concerns about settlements of old structures, or impractical because of lack of surface space for well installation in congested urban areas. All WMATA designs employ the standard free-air construction method. Adequate drawdown of groundwater is mandated by the design documents to ensure sufficient reduction of pore water pressures prior to excavation. Competent design and correct construction procedures are a prerequisite for safe and successful application of NATM in soft ground. The contract documents for NATM designs customarily are more detailed and explicit than documents for conventional designs. Construction techniques must be adapted to be compatible with the NATM philosophy. The special role of instrumentation as a tool for verifying design assumptions must be fully recognized and addressed. No hard-and-fast rules can be cited in
392
comparing NATM with shield-driven tunneling. In some situations, shielddriven t u n n e l i n g may be considered more desirable or even indispensable. For example, in relatively long tunnel construction, a tunnel boring machine (TBM) may prove more effective in terms of advance rate and, hence, may result in lower construction time and cost. Where groundwater drawdown is not permissible, an appropriately designed TBM may be the only feasible method in free air. In terms of the WMATA designs, NATM construction offers the following advantages: (1) NATM construction in soft ground is expected to result in smaller settlements than construction by conventional t u n n e l i n g methods. Not only are the absolute values of settlement less, but differential settlements also are significantly reduced. (2) The use of membrane waterproofing results in watertight tunnels, shafts, and stations. (3) E l i m i n a t i n g heavy steel sets for initial lining, and eliminating rebar in the final l i n i n g result in considerable economies. (4) The availability of construction shafts generally enables the contractor to advance tunnels simultaneously from multiple headings. Estimated average advance rates of 10 ft (3 m) per day per heading were feasible for the WMATA tunnels. Where an intermediate shaft is
TUNNELLINGAND UNDERGROUNDSPACETECHNOLOGY
available to perform excavalion of tt+w two tunnels, four headings ~au be advanced simultaneously. This implies an advance rate of 40 fl pet day, based on rather conservative assumptions. (5) Unlike conventional tunneling, NATM construction does not rely on expensive tunnel boring ,nachines. Excavation equipment cati be changed with relative ease to best suit tile ground conditions. The need for heavy equipment investment up from and the risk of total dependency on one major piece of machinery are eliminated. NATM t u n n e l i n g requires special expertise on the part of the contractor, and a cooperative working relationship between the contractor and the owner. Construction management techniques must also be adapted to best suit NATM construction needs. WMATA's successful experience with two rock NATM projects indicates that these challenges can be met successfully in tile U.S. The three contract sections were separately bid during late 1987 and early 1988. For Section E-5, the NATM option emerged as the lowest bid. The bid is based on performing all the construction--mined tunnels, ventilation shafts and the underground position of Fort Totten Station--using the NATM technique. Section E-6e bidding also resulted in a contract award based on the NATM alternate. However, WMATA currently is reviewing a value engineering proposal from the contractor which, if successful, will result in conventional construction in lieu of NATM. In Section E-Sa, the longest section, NATM alternate was not successful against tile conventional construction. T h e lowest bid in this case is based on WMATA's segmental concrete lining. However, in this case it can be argued that the inclusion of the NATM alternative resulted in very competitive bidding. The construction of Section E-5 is scheduled to begin during the third quarter of 1988 and its progress will, no doubt, be closely watched by the t u n n e l i n g industry. This first largescale application of soft-ground NATM in the U.S. is expected to infuse innovative t h i n k i n g into the design and construction of future similar underground structures and facilities. []
Volume 3, Number 4, 1988