Original Design Concept

Before the CA/T, the east-west Interstate 1-90 Highway terminated in south Boston forming a very complex surface intersection with the 1-93 which runs north-south through the heart of the city (Figure 7.1). Creating the new I-90/I-93 interchange and extending the 1-90 to Boston’s International Logan Airport required multi-lane highway tunnels to be constructed under the approach to South Station (Figure 7.2). This is a complex network of seven interconnecting rail tracks which carried over 40,000 commuters and 400 train movements daily.

Original design concepts for the CA/T tunnels were based around traditional cut and cover construction techniques. However, for the unique challenges presented in Contract 9A, this required five phased relocations of the railway tracks and associated infrastructure. Such an approach was unacceptable to the railway authorities. Apart from moving the tracks which included complex crossovers and switches, each phase would have involved re-establishing the extensive control systems with their sensitive buried fibre optics. Moreover, it would have been necessary to sequentially construct the elements of each tunnel in deep and narrow isolated trenches between the temporarily relocated tracks (Figure 7.6). This would have been very time consuming with major access and operational challenges to safely manage the multiple interfaces.

Innovations in Tunnel Jacking

The unique combination of challenges that the site presented led to the introduction and development of a wide range of innovation (Powderham et al., 2003, 2004). Some innovations were specific solutions to the various challenges while others created additional benefits in a process of continuous improvement. The temporary ‘parking’ of the first two units of the 1-90 EB jacked tunnel is an example of the latter as discussed in Section 7.4.2.

The key innovative features were as follows:

(a) Scale: The most prominent innovation was the huge leap in scale (Figure 7.7). Each of the three jacked tunnels was well over ten times the size of any constructed in the USA and, at the time of bid, nearly seven times that of any built in the UK (Ropkins, 1998). When this scale was combined with the complex geometry of the site, the adjacent infrastructure and the difficult geology, it led to the development of further innovations that minimised risk and enhanced delivery. As shown in Figure 7.7, with the dramatic juxtaposition of the jacked

Illustrative phase of construction in original design concept

Figure 7.6 Illustrative phase of construction in original design concept.

The 24 m wide, 12 m high 1-90 EB tunnel during jacking. As noted, it had the least clearance between the tunnel and the railway above

Figure 7.7 The 24 m wide, 12 m high 1-90 EB tunnel during jacking. As noted, it had the least clearance between the tunnel and the railway above.

Photo credit Jason Rodwell.

box and the train passing above, the clearance between them for 1-90 EB was minimal, being less than 2 m. The clearance for the other jacked tunnels was in excess of 6 m but which, of course, is still a very low clearance for such large tunnels. There was a trade-off in this context since, with greater depth of the tunnel, the less obstructions were encountered but the ground loading and associated jacking forces were proportionately higher. The structural details of the jacked tunnels are provided in Table 7.1.

(b) Retaining walls: The severe spatial constraints on this site and the geometrical complexity led to the development of an innovative range of retaining wall systems (Figures 7.7, 7.15 and 7.24). The dimensions of the three thrust pits are given in Table 7.2. The headwalls for the three tunnels (Ramp D, and the 1-90 WB and EB) were constructed with soldier pile tremied concrete for ease of control during shield entry to commence tunnel jacking (Figures 7.8 and 7.9). Structurally efficient pre-stressed diaphragm walls were utilised for side walls and which were some of the largest of this type ever constructed. In other locations providing lateral support to the top of the walls was impracticable - for example, the back walls of the 1-90 WB and the sidewalls of

Table 7.1 Structural details of the three jacked tunnels.

Jacked Tunnel Units

Unit

Ramp 0

1-90 W8

1-90 EB

Width

m

23.77

23.77

24.08

Height

m

1 1.58

11.58

10.82

Roof and base slab thickness

m

1.8

1.8

1.8

Wall thickness

m

1.8

1.8

1.5

Tunnel length in final position

m

48.15

75.9

112.78

Shield length

m

2.74

2.74

2.74

Number of tunnel units

No.

2

3

3

Total tunnel concrete volume

m

6,200

9,800

11,200

Total reinforcement weight

t

900

1,470

1,680

Total tunnel weight

t

15,500

24,500

28,000

Maximum jacking capacity

t

25,850

25,850

25,850

Normal jacking capacity

t

15,500

15,500

15,500

Actual jacking load on lead unit

t

9,600

15,400

9,900

Table 7.2 Thrust pit elements and dimensions.

Thrust Pits

Unit

Ramp D

1-90 WBD/l-90 EBD

Combined thrust pit

Average width

m

30

80

Average length

m

60

80

Retained height

m

25

25 (max)

Diaphragm wall

m

1.2

1.2

Cantilever T-section rear wall

m

Not used

0.9 (with 4 m deep web section)

Base slab thickness - front

m

0.9

0.9

Base slab thickness - rear 6 m

m

1.5

1.5

Jet grouted strut below base slab

m

6

6

the 1-90 EB thrust pits. For these, 3.5 m deep T-panel cantilever diaphragm walls were used (Figures 7.15). While the ground freezing (described below) robustly stabilised the ground beneath the railway, the ground movements it generated (locally up to 300 mm both laterally and vertically) had major implications for the railway alignment and for retaining the walls of the thrust pits (e.g. see Figures 7.25 and 7.26).

(c) Ground freezing: The challenging and variable ground conditions demanded close attention to stability and comprehensive ground movement control. While ground freezing beneath an operating railway

Long section showing overall system for tunnel jacking

Figure 7.8 Long section showing overall system for tunnel jacking.

Thrust pit section showing support from jet grout and headwall beams

Figure 7.9 Thrust pit section showing support from jet grout and headwall beams.

system was not unique, its application here was on an unprecedented scale when viewed in context with the ground conditions and, in particular, the organic deposits and the clay. The low permeability of these deposits leads to high expansion during the freezing process with the potential to cause large ground movements both laterally and vertically. The organics would also create high volume changes during the thawing process. Some 1,800 freeze pipes to depths of 20 m were installed using a track mounted sonic drilling rig mostly during night-time possessions. The ground freezing was phased over a period of two years in sequence with the jacked tunnel construction. Liquid ammonia was used as the primary refrigerant to cool the calcium chloride brine to an entry temperature of -30°C. Apart from stabilising the ground and creating a much safer working environment at the tunnel face, ground freezing greatly simplified the shield entry through the headwalls of the thrust pits. It also allowed the unique temporary ‘parking’ of two 10,000 tonne tunnel box units (the first two elements of the 1-90 EB jacked tunnel) beneath the operating railway. This was another ‘first’ that successfully addressed the spatial constraints imposed on the combined thrust pits and delivered extra programme savings by allowing the optimum sequence of tunnel installation (Figure 7.15). The numerous obstructions encountered in the tunnel face were also considerably easier and safer to deal with in the frozen ground.

  • (d) Shield design and excavation: Another key benefit enabled by the ground freezing was the simplification of the shield design. The stable faces allowed much larger cells to be used so that the tunnel could be fully excavated using roadheaders (Figure 7.10). All of these factors contributed to mitigating the risk of interruption either to or from railway operations. However, in creating these major benefits, the ground freezing also brought some additional challenges. While the excavation at the tunnel face was safer, the ground had been converted from a soft deposit to a rock - and a rock that was essentially without joints. Efficient excavation required development of enhanced performance for the British Webster roadheaders. Further innovations were introduced to cope with the large ground movements and potentially high pressures created by the freezing. These included pressure relief systems and a heating control system in the ground and tunnel walls to prevent the tunnels becoming frozen in the ground - particularly during the 5 months of temporary parking of the 1-90 EB units (See Sections 7.4.3 and 7.7.3).
  • (e) Anti-drag system: The special anti-drag system (ADS) developed for this project had the highest capacity and the most extensive ever installed. A total of 900 closely spaced 19 mm steel cables were used on the roof (Figures 7.8, 7.11 and 7.12). Steel cables were also used under the base slabs of each tunnel. As shown in Figure 7.8, the cables were fed from reels housed in the roof and base of the lead section of each jacked tunnel. The cables were continuous over the full length of each tunnel running from the anchorages in the head- wall and thrust base for the roof and base slab sets, respectively. They were sacrificial being cut on completion of the tunnel jacking and left in place. The primary role of the ADS was to decouple the
I 0 Breakthrough of 1-90 WB tunnel

Figure 7. I 0 Breakthrough of 1-90 WB tunnel.

tunnel boxes from the ground and infrastructure above. This prevents the ground moving laterally with the tunnel as it is jacked forward. The ADS also helps to reduce jacking loads and, when used with a lower set of cables, provides improved alignment control. Trains safely moved uninterrupted overhead as the tunnel sections were installed below the tracks at a rate between 1 and 2 m per day.

 
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