Urban Mass Transit Stations

INTRODUCTION

Public transit agencies are developing and extending their service throughout the US. Construction of such systems is difficult due to the seemingly unpredictable costs and years of disruptions of normal life. In urban areas these systems are constructed underground either due to lack of available space or the preference for above ground commercial development. While underground construction has a long history in the US, the traditional cut-and-cover method still prevails, even though newer and arguably better technologies are available.

This paper evaluates and compares open-cut (cut-and-cover) construction methods with a mined technology for the construction of underground stations. Open-cut methods have been developed and refined for over 100 years and have been improved with new technologies for support of excavation and the use of temporary traffic decking to mitigate some of the detrimental impacts of the method. In contrast, mined tunneling with shotcrete support is rather new in this country, being applied only for about 20 years. “Hand Mined” (HM) tunneling, also known as “New Austrian Tunneling Method” (NATM), “Sequential Excavation Method” (SEM), “Drill and Blast” (in hard rock) or “Shotcrete Method” utilizes a highly developed “Toolbox” enabling it to handle any ground condition. Transit agencies in the US and around the world recognize the advantage of the mined approach, in terms of limited disruption but also in terms of reduced risk and costs.

CONSTRUCTION METHODS

There are three general construction methods available for the construction of underground station structures, open-cut, semi-cut-and-cover (“Mailänder Bauweise”) and mined methods. Local conditions may dictate the method, but often there is a choice to be made.

Open-cut and Top-down Tunneling
Cut-and-cover is described as the construction of a usually rectangular tunnel structure built in a trench and backfilled with suitable material. The trench can be sloped or supported with a range of walls including interlocking sheet piles, slurry, secant piles, or soldier pile. Alternatively, the open-cut method utilizes slurry or soldier pile and tremmie concrete (SPTC) walls as support of excavation which forms the final structure. After excavation the invert and roof slabs are poured, and the structure is backfilled with suitable material. For ease of discussion both of these methods will be referred to as cut-and-cover method.

Historically the cut-and-cover technique was considered the most cost effective construction method for shallow tunnels. Today in urban areas this technique has numerous disadvantages due to its environmental impact as well as the cost and construction delays due to utility and traffic relocations.

Mined Tunneling
Mined tunneling for large cross sections in soft ground, such as station tunnels can only be achieved with a sequential excavation method, e.g. hand mining (HM). This technique utilizes a “toolbox” for pre- and final support and requires design competence, quality control and experienced supervision during construction. HM or shotcrete method uses the open-face tunneling technique with ground support provided by shotcrete, with lattice girders, welded wire fabric or steel fibers as reinforcement where needed. When the ground is self-supporting many of the support elements can be omitted. The success of this method depends on the early closure of the ring (invert) with installation of the support system immediately after each excavation step to minimize settlement and induce ground arching. Combined with comprehensive monitoring systems this method is recognized as the most flexible and adaptable system, but also one of the safest and most controllable tunneling techniques.

Comparison between Open-cut and Mined Tunneling Methods
Costs are influenced by tunnel depth and construction method.  The capital investment for station structures increases sharply for construction depth in excess of 1 ½ times the tunnel height, due to difficulties with pile driving and horizontal stiffening of open pits. Construction cost for hand mined tunnels, conversely is less dependent on the vertical position of the structures.

Cut-and-cover constructions require a considerable amount of surface space, traffic diversion and redirection, and road excavation and reinstatement.  With HM only areas around the access shafts are impacted, which are rather flexible in the location and may be used for permanent access to the station. Significant utility relocations required for cut-and-cover construction are limited or not necessary for HM.

HM provides more flexibility for the alignment and the shape of the cross section and therefore minimizes required excavation volume, which causes high volume of traffic and limited landfill space.  Furthermore, backfilling activity occurs to a far higher extent for cut-and-cover methods than for HM.

HM in soft ground requires readily available construction machinery, e.g. backhoes, excavators, drill rig, etc.  For mixed faces and hard rock roadheader or blasting equipment may be required.  Cut-and-cover constructions often require heavy machinery with long lead times, depending on the nature of support wall (e.g. hydro mill).

As mentioned previously hand mined tunnels can be built in almost any ground; however, prior soil improvement may be necessary.  Ground improvement means and methods like jet grouting, ground freezing, dewatering, grouted pipe spiling or the use of compressed air can increase the construction cost considerably.

Surface settlements occur in both, mined and cut-and-cover method.  The maximum settlement is about the same for both methods, but the HM method features a smaller area of influence.  Moreover, there are more methods to control and influence the surface settlement, using the “Toolbox”.  The design of mined tunnels requires qualified and experienced engineers, while the cut-and-cover constructions have been carried out in most parts of the world and local engineers have experience using it.

The characteristics of the two different construction methods are summarized in Table 1.

Table 1:Comparison of Cut-and-Cover and Mined Tunnels

RISK COMPARISON

A common misconception is that mined tunneling involves greater risk during the construction stage than cut-and-cover.  There have been several collapses or other stability failures of mined tunneling projects around the world including, Turkey, Japan, UK and the US.  The Heathrow Airport collapse in October 1994 triggered a thorough review of the NATM method by the British Health and Safety Executive (HSE). According to the report the method was never the cause, but the misapplication of the method compounded by management failure.

Cut-and-cover construction methods also involve high risks, if not applied correctly.  Emergency evacuation of houses along badly constructed slurry walls have been reported throughout the industry.

Extensive and independent scientific risk analyses and comparisons for tunnel construction methods have become increasingly popular worldwide. One example of a comparison between cut-and-cover and NATM tunneling can be provided. The Massachusetts Highway Department evaluated the risks for the construction of one section of a project, which was originally designed as a cut-and-cover excavation. The contractor submitted a Value Engineering Change Proposal (VECP) [5] featuring the New Austrian Tunneling Method (NATM).  The section included a tunnel located beneath Atlantic Avenue and the MBTA Red Line South Station.  Significant features of the contract included a three to five-lane highway tunnel for northbound traffic, two highway ramps, a two-lane MBTA transit way tunnel with a turnaround, reconstruction of a large portion of the MBTA South Station, and utility relocation.

In the risk analysis for the project (cut-and-cover) and the contractor’s proposal (NATM), two different sections were analyzed, whereas Section 2 was the underpinning of the existing Metro line and Section 1 comprised of the tunnels outside the underpinning area.

Table 2: Findings of Risk Analysis for Boston Central Artery Project, Section C11A1. Numbers shown are Risk Numbers – Higher Numbers represent Higher Risk (Source [5]).

The results as shown in Table 2 indicate that the risks for all sections are higher for cut-and-cover than for NATM.  During construction and after construction risk levels for Section 1 are similar (within 29%) albeit slightly greater for the cut-and-cover method.  For Section 2 (underpinning), the cut-and-cover project has safety and serviceability risks between three to four times greater than those for the NATM.  In particular, the risk for the drift was found to be nearly four times greater with cut-an-cover.  Similarly, the main excavation for the cut-and-cover project is four times as risky as the main excavation for the NATM.

COST COMPARISON

Construction costs for underground structures depend on country specifics like experience with the proposed method, the local labor market with all its regulations, material prices, price volatility, and the experience and knowledge of the engineers designing the project, but also geology, size of tunnel, and surface structures and their sensitivity and are, therefore, not easy to compare. While traditional cut-and-cover construction costs are rather well known due to the extensive and long experience with this method, costs for mined tunnels are often questioned.

The progress with mined tunneling methods made in the late 1970s allowed the contractors to offer mined design alternatives cheaper than conventional construction methods.  The tendencies may be well described based on the examples of the subway construction in Munich and Bochum (both in Germany). The general development of the bid price of traditional tunnel construction methods and HM during its introduction and development for the subway construction in Munich is shown in Figure 1.  The closeness of the bid prices at its introduction in the mid 1970s combined with the above mentioned advantages of the HM induced the early adoption of the technique in places such as Munich.

Figure 1: Cost development of Cut-and-Cover, TBM and NATM (Shotcrete) Running Tunnels
 (Construction Cost per Meter) of the Subway Construction in Munich.

As the city council of Bochum, Germany started its city railway construction in 1970 it was common knowledge that cut-and-cover tunneling was at least 50 % cheaper than mined tunneling.  A value engineering design alternative, which was carried out in 1973 for a certain allotment disproved this assumption.  The NATM alternative was more than 20% cheaper than the second cheapest offer, which featured the cut-and-cover method.  After scrutinizing the proposal from the technical, static, constructive and geologic points of view, the offer was finally accepted.  Beside the cost advantage, the prospects of fewer interruptions and annoyances on the surface did not harm the choice.  Moreover it was known that settlements would be negligible and that relatively little surface space would be required.

In the study for the city of Bochum three tunnel sections with varying numbers of parallel tracks were compared for NATM and cut-and-cover method, which is shown on Table 3.  Due to the cost impact a differentiation is made concerning the requirement of a temporary traffic decking at the cut-and-cover method. The decking reduces the disturbance but increases the cost due to more coordination, sequencing, and difficulties during construction.

Table 3: Bid Price Relationships of Main Items of Cut-and-Cover and NATM Tunnels
 For City Railways in Bochum

Twin-track Tunnels (Side-Platform Tunnels)
Based on the results, twin-track tunnels with a cross sectional area size of 50 m2 (mined) and 75 m2 (cut-and-cover) are the most cost effective sections.  However, it also has to be mentioned, that in soft grounds and silty clays large cross sections can not always be driven without ground improvements and therefore the choice is not totally unrestricted.

The excavation volume of a mined tunnel is only 28 % (in average) of a cut-and-cover tunnel and changes with depth.  Savings on concrete volume significantly affects this cost. The cost of the excavation for mined tunneling is higher (about 2.5 times), but is compensated by the lower volume. 

Triple-track tunnels
Triple-track tunnels are necessary when an additional track for stabling or reversing of a train is required.  The cross section size is approximately 100 m2 for NATM and 85 m2 for cut-and-cover.  The NATM excavated volume in average amounts to 41 % of that of cut-and-cover due to the overburden. 

Stations with Central Platform
Extensive experience was gathered with the construction of underground stations in Bochum. Despite the fact that hauling costs were higher and the primary lining was 13 % more expensive, the overall total costs were still lower for the mined option.  This is due to the lower concrete consumption, being only two thirds of the cut-and-cover tunnel.

In Bochum some experience was gained also regarding the cost of utility relocation.  For mined constructions the cost for utility relocation was about 20% lower than the cost for cut-and-cover constructions.  The magnitude depends highly on tunnel location and may be as much as one third of the total construction cost of an underground station.  Since mined tunnels are usually situated lower and are more flexible in its alignment, many utility lines can simply be avoided.

It can be seen, that NATM is usually cost competitive even if extensive additional measures, like ground improvements, have to be applied. The free market model and economy of size also work in a rather small and confined industry. When a market is created for a new, competitive method the price will adjust very quickly.

By the end of the year 2002, 56% of all constructed underground, urban and rapid transit system tunnels in Germany were built using mining means, 19% of which utilized EPBS and 81% HM. Figure 2  shows the development of the construction method quotas in the recent years.

Figure 2:  Percentage of Used Construction Methods for Underground, Urban and Rapid Transit Systems
 at Turn-of-the-Year in Germany.

Example: Mined Tunnel Cost Comparison in the US
A detailed cost comparison between cut-and-cover and NATM construction methods was conducted for a pedestrian tunnel at Dulles International Airport in Virginia (USA).  This tunnel, which serves as an underground corridor for pedestrians between two of the airport terminals, is approximately 235 m long.  The mined option has a cross sectional area of 85 m2, which is comparable to a typical subway station cross section.  The crown and the invert levels of the tunnel are approximately 4.7 m and 13.1 m respectively below the ground surface. A significant portion of the tunnel’s crown is located in a mixed face (i.e. soil and rock interface).  Also several existing utility lines traverse the alignment of the tunnel (e.g. fuel, storm sewer, fiber optic). The construction time was estimated to be nearly one year.

The preliminary engineering envisioned open-cut construction with typical support of excavation consisting of shotcrete and tie-backs, soldier piles and lagging, with a cross section 10.8 m wide and 5.9 m high with minimal overburden.

NATM was estimated to be 25% cheaper, with a shorter construction schedule and lower surface impacts and thus was selected to be the construction method.

SUMMARY AND CONCLUSION

For the construction of underground transit stations two options are available, cut-and-cover and mined methods. While the American market promotes the use of the first, international evidence exists that mined tunneling is competitive in terms of cost and schedule, especially once the technology has been adopted as an accepted technique.

Mined tunneling also has advantages over cut-and-cover methods in terms of limited disruptions, greater flexibility, reduced soil removal and decreased risk. Case histories have shown substantial direct savings once the method is accepted and used widely.

Many cities around the world, e.g. Santiago de Chile recognized the advantages of mined tunneling and changed the prevailing construction method from cut-and-cover to HM. In the US an increasing number of projects have utilized the benefits of hand mined tunneling, after a slow initial adoption dating back to shotcrete applications in the middle of the 1980s. Further usage of mined tunnel options in the US will expand the base of designers, contractors and owners familiar with the technique, thereby further accelerating its cost effectiveness. Accepting the benefits is certainly a first step.

REFERENCES

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