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Technical Manual for Design and Construction of Road Tunnels - Civil Elements

Chapter 12 - Jacked Box Tunneling


Jacked box tunneling is a unique tunneling method for constructing shallow rectangular road tunnels beneath critical facilities such as operating railways, major highways and airport runways without disruption of the services provided by those surface facilities or having to relocate them temporarily to accommodate open excavations for cut and cover construction (Chapter 5). Originally developed from pipe jacking technology, jacked box tunneling is generally used in soft ground at shallow depths and for relatively short lengths of tunnel, where TBM mining would not be economical or cut-and-cover methods would be too disruptive to overlying surface activities.

Jacked box tunneling has mostly been used outside of United States (Taylor et al, 1998) until it was successfully applied to the construction of three short tunnels beneath a network of rail tracks at South Station in downtown Boston . These tunnels were completed and opened in 2003 as a part of the extension of Interstate I-90 for the Central Artery/Tunnel (CA/T) Project. Figure 12-1 shows the opening ceremony for the completed I-90 tunnels. Since CA/T Project represents the most significant application to date of the jacked box tunneling in the US, it will be used to demonstrate the method throughout this Chapter.

Completed I-90 Tunnels

Figure 12-1 Completed I-90 Tunnels

12.2 Basic Principles

Figure 12-2 illustrates the basic jacking sequence of jacked box tunneling under an existing railway. The box structure is constructed on jacking base in a jacking pit located adjacent to one side of an existing railway. A tunneling shield is provided at the front end of the box and hydraulic jacks are provided at the rear. The box is advanced by excavating ground from within the shield and jacking the box forward into the opening created at the tunnel heading. In similar fashion to pipe jacking, lengths of tunnel that would exceed the capacities of jacks situated at the rear of the box structure can be successfully advanced into place by dividing the box structure into sections and establishing intermediate jacking stations. The box structure shown in Figure 12-2 is divided into two sections with an intermediate jacking station set up in between them.

In order to maintain support to the tunnel face, excavation and jacking normally carried out alternately in small increments, typically in the range of 2 to 4 feet. In most cases, the soft ground must be treated by means of ground improvement techniques such as ground freezing, jet grouting, etc. as discussed in Chapter 7 Soft Ground Tunneling to enhance its stand up time. Refer to Chapter 5 for discussions about temporary excavation support systems.

Typical Jacked Box Tunneling Sequence under an Existing Rail Track

Figure 12-2 Typical Jacked Box Tunneling Sequence under an Existing Rail Track

12.3 Central Artery/Tunnel (CA/T) Project Jacked Box Tunnels

The use of the jacked box tunneling method on the CA/T Project in Boston is described by van Dijk et al. (2000) and van Dijk et al., (2001). A major component of the CA/T project was the extension of Interstate I-90 eastward to Boston's Logan International Airport. This extension required three crossings of the network of tracks leading into South Station, a regional transportation hub used by Amtrak and the Massachusetts Bay Transportation Authority (MBTA) for hundreds of train movements daily. The critical surface use of the site, the large spans of the underground openings required to accommodate a multi-lane highway, the relatively shallow cover dictated by the roadway profile, and the poor soils in combination with the high groundwater level at the site led to tunnel jacking being selected as the preferred tunneling method over staged cut-and-cover and conventional tunneling techniques.

The three crossings of the tracks consisted of box structures for the eastbound lanes of I-90, the westbound lanes, and westbound exit ramp that provided access to Interstate I-93. The box structure for the I-90 EB lanes was the longest of the three, at 379 feet. It was constructed in 3 sections, with cross-sectional dimensions of 36 feet high by 79 feet wide, and a total weight of approximately 32,500 tons. The other two box structures were 38 feet high by 78 feet wide and were each constructed in two sections. The I-90 WB tunnel was 258 feet long and weighed approximately 27,000 tons, while the exit ramp tunnel was 167 feet long and weighed 17,000 tons.

Subsurface Condition and Ground Freezing: As shown in Figure 12-3, the geologic conditions through which the three box tunnel structures were jacked included (at the top of the subsurface profile) a layer of miscellaneous fill 20 to 25 feet thick, primarily a medium dense silty sand. This fill layer contained a number of obstructions related to the more than 150 years use of the site for rail road, industrial and waterfront infrastructure, which included granite block seawalls, rock filled timber cribwalls, brick and masonry structure foundations, a buried trackway, and an abandoned brick-lined sewer. Below the historic fill material was a deposit of weak organic sediments 10 to 15 feet thick, consisting of organic silt with some fine sand and peat. Underlying the organic layer were lenses of alluvial sand and inorganic silt deposits, generally less than 5 feet thick. The remaining part of the profile through which the tunnel boxes were jacked consisted of marine clay, consisting of clay and silt that was soft, except for the upper 15 feet, which was somewhat stronger and less compressible. Groundwater at the site was generally 6 to 10 feet below track level, resulting in the tunneling horizon in each case being completely submerged.

Generalized Subsurface Profile for the I-90 Jacked Box Tunnels

Figure 12-3 Generalized Subsurface Profile for the I-90 Jacked Box Tunnels

The success of the box jacking operation depended critically on maintaining the stability of the soils through which the tunnels passed. For the existing condition of weak soils below the groundwater table, shallow cover over the tunnel boxes, and the large spans required, there were serious concerns about loss of ground at the headings, the potential for significant settlement of the overlying track structures, and loss of alignment control during jacking. Therefore, ground improvement was required to enable the tunneling to be performed effectively and safely. The original design for the tunnels had called for ground treatment consisting of a combination of dewatering and chemical grouting in the miscellaneous fill materials; horizontal jet grouting in the organic sediments; and soil nailing of the marine clay layer. The Contractor was concerned about the potential risks associated with the implementation of this combination of soil stabilization methods, and consequently made a value engineering proposal to substitute ground freezing for all of the methods. This proposal was accepted, and a large scale freezing operation was performed that encompassed all three tunnel alignments, as further discussed below in Section 12.5.

Box Casting Operation: Each tunnel box structure was constructed in a "jacking pit" immediately adjacent to the west side of the South Station track network (Figure 12-4). The jacking pits were constructed by slurry wall methods, with post-tensioning of the sidewalls and the formation of a low-level improved soil "strut" formed by jet grouting below the base slab to reduce the number of bracing levels required so that the boxes could be constructed without interference from a cross-lot bracing system. The concrete base slab of each jacking pit was placed with a tight tolerance on grade, since it served as the casting surface for the box structures and also established the starting profile to ensure that the tunnel sections were jacked to the required alignment.

Tunnel Structure Construction Operation

Figure 12-4 Tunnel Structure Construction Operation

After the base slab of each jacking pit was completed, a series of steel wire ropes were installed longitudinally on the slab and steel plates covering the entire footprint of the box sections were placed on the wire ropes. Shear studs were welded to the base plates to anchor the plates to the concrete slab, so that when jacking started, the frictional resistance that the jacks needed to overcome to move the box structures would result from steel plates sliding over steel wire ropes, rather than concrete sliding on concrete. Figure 12-4 shows the construction of the I-90 WB tunnel box structures.

The structural design of these tunnel sections had to consider not only the long term loads from the overburden and railroad surcharge loads, but also the construction phase jacking loads. Each tunnel was constructed in sections (2 sections for the I-90 WB exit ramp tunnel and the I-90 WB tunnel itself, and 3 sections for the I-90 EB tunnel) to reduce the jacking forces required to move the tunnels into their final positions by using intermediate jacking stations in addition to the jacks positioned at the rear. To prevent soil from entering into the gap between adjacent box sections, a system of transversely continuous sliding overlapping steel "bridge" plates were used. Once jacking was completed, the jacks were removed and the intermediate jacking station areas were filled with concrete.

The external surfaces of the box structures could not be waterproofed because the waterproofing material would have been torn away during jacking. Water seepage control was achieved by using low permeability concrete mixes to construct the boxes and grouting the interface between the boxes and the surrounding ground through grout ports cast into the walls and roof slab after tunneling and jacking were completed.

A cellular concrete shield was constructed at the front of each lead box section to support the excavation operation by establishing multiple access points to the face that could be closed off if stability problems developed. A beveled steel knife edge was provided at the perimeter of the shield that was flared a small amount to ensure that the opening into which the tunnel box structures would be jacked could be closely controlled, but also excavated large enough to prevent the boxes from getting stuck as they were pushed forward.

Tunnel Excavation: Mining of the frozen soils at the tunnel face, which had estimated uniaxial compressive strengths in the range of 700 to 1400 psi, was done primarily with roadheaders, working at two levels within the shield. Figure 12-5 shows a typical view of the roadheader mining operation.

Excavation of the Frozen Ground at the Front of the Tunnel Shield by Roadheader

Figure 12-5 Excavation of the Frozen Ground at the Front of the Tunnel Shield by Roadheader

The roadheaders also proved to be effective at removing the numerous timber piles that were encountered. For removing masonry obstructions, which were firmly bound in place in the frozen soil mass, hydraulic hammers were used. The excavated material dropped to the bottom of the shield during the mining operation, where it was collected using a Gradall machine and a loader. A wheel-mounted scoop tram was used to shuttle the material to the rear of the tunnel box structure and dump it into a skip bucket, which was lifted out of the pit by crane and stockpiled for loading onto haul trucks. Figure 12-6 shows the scoop tram loading the skip bucket.

Scoop Tram Loading Excavated Material into Skip Bucket for Removal

Figure 12-6 Scoop Tram Loading Excavated Material into Skip Bucket for Removal

Based on typical mining production rates, stand-up time for the unsupported frozen ground, the volume of excavated material to be handled, the design of the jacking system, and the shift schedule, the Contractor determined that incremental excavation advance for efficient, consistent progression of the jacking operation was approximately 3 feet. Depending on the amount of obstructions encountered in a particular round, the advance rate achieved was generally one to two rounds per day, or 3 to 6 feet. At the completion of each excavation increment, the Contractor had to check the shield perimeter to ensure that all obstructions, including abandoned freeze pipes, were cut back sufficiently to be clear of the tunnel box.

Anti-Drag System As discussed previously, an anti-drag system was installed above and below the tunnel box structure to reduce the frictional resistance between the box structure and the surrounding ground. The system worked to even out the friction acting over the roof and bottom surface areas of the box, which contributed to alignment control during jacking, and also reduced the potential for surface settlement and lateral movement of the shallow overburden over the tunnel by separating the interface between the box concrete and the soil. This was achieved by installing a series of greased 3/4-inch diameter wire ropes that were anchored to the jacking pit and threaded through slots in the shield into the interior of the tunnel box structure, where they were stored on slings mounted on the soffit of the roof slab and on reels on the base slab located inside the tunnel. The system was configured so that as the tunnel moved forward, the wire ropes were run out from the storage units to cover the portion of the top and bottom surfaces of the box structure that was embedded in the ground beyond the thrust pit. More discussions of the Anti-Drag System (ADS) are provided in Section 12.4.1.

Tunnel Jacking Operation: At the completion of each excavation round, the tunnels structures were jacked into the space created at the face. This was accomplished by a group of 25 hydraulic jacks positioned at base slab level at the rear of the tunnel box, and additional groups of 26 to 32 jacks situated in the intermediate jacking stations. Each jack had a working capacity of 533 tons at a working pressure of 6100 psi, and could deliver a maximum thrust of 889 tons at a pressure of 10,200 psi. The maximum stroke of the rear jacks was 42 inches, while the stroke of the intermediate station jacks was limited to 16.5 inches. At each jacking station, the individual jacks were connected in nine clusters of 2 to 4 jacks each. This simplified the hydraulic control, and also enabled some horizontal steerage capability through variable operation of the clusters. The required thrust reaction for the jacks was transferred to a heavily reinforced concrete block wall at the rear of the jacking pit through a series of steel pipe sections referred to on the project as "packers." The loads exerted on the reaction block wall were in turn transferred into the surrounding ground through the pit base slab and rear wall. More discussions about the jacking operations are included in Section 12.4.2.

12.4 Load and Structural Considerations

In most aspects, the structural loading and design considerations for jacked box tunnels are similar to those for cast-in-place cut and cover tunnels as discussed in Chapter 5. Readers are referred to Sections 5.3 for detailed discussions about structural framing, design, buoyancy, waterproofing, etc., and Sections 5.4 about loads and load combination. Section 5.5 provides discussions about structural design procedures and considerations for a box tunnel.

However, in addition to the typical design loads discussed in Section 5.4, jacked box design can be dominated by two unique loads during construction: jacking thrust loads and interface drag loads.

12.4.1 Ground Drag Load and Anti-Drag System (ADS)

Ground drag, resulted from the contact pressures between soil and box structure is calculated and multiplied by appropriate friction factors, and is used to estimate drag loads at frictional interfaces; an appropriate adhesion value is used at the interface between the box and cohesive ground. Simplifying assumptions are made in developing ADS loads and modeling box/ADS/soi1 interaction, the validity of which is done by back-analyses of loads and other historical data. To reduce such an enormous drag load, an anti-drag system (ADS) is used to separate the external surface of the box from the adjacent ground during tunnel jacking.

As described in Section 12.3, the CA/T tunnels utilized an ADS consisting of an array of closely-spaced wire ropes which are initially stored within the box with one end of each rope anchored at the jacking pit. As the box advances, the ropes are progressively drawn out through guide holes in the shield and form a stationary separation layer between the moving box and the adjacent ground. The drag forces are absorbed by the ADS and transferred back to the jacking pit. In this manner the ground is isolated from drag forces and remains largely undisturbed. Readers are also referred to Ropkins 1998 for more discussions for other ADS applications.

12.4.2 Jacking Load

The ultimate bearing pressures on the face supports and on the shield perimeter are used to calculate the jacking load required to advance the shield. Note that the face pressure must be analyzed using the treated soil properties. In addition, jacking load also includes the ADS loads as discussed above.

Jacking thrust is provided by means of specially built high capacity hydraulic jacking equipment. Jacks of 500 tons (4,448 kN) or more can be utilized on large tunnels. As discussed in Section 12.3, jacks with a capacity of 533 tons at a working pressure of 6100 psi (42 MPa) were used in the I-90 tunnels (Figure 12-7). For jacking a large size road tunnel structure, multiple jacks are required to provide sufficient jacking thrust to counter the face pressure. In addition, using multiple jacks offers some steerage control redundant capacity in the event of possible underestimates of the required jacking loads.

Reaction to the jacking thrust developed is provided by either a jacking base or a thrust wall, depending on the site topography and the relative elevation of the tunnel. An example of a heavily reinforced thrust block wall is also shown in Figure 12-7. These temporary structures must in turn transmit the thrust into a stable mass of adjacent ground. A thrust wall is normally stabilized by passive ground pressure. In developing this reaction, the wall may move into the soil and this movement must be taken into account when designing the jacking system. When a thrust wall is used in a vertical sided jacking pit, care is required to ensure that movement of the thrust wall under load does not cause any lack of stability elsewhere in the pit.

Close Up of High Capacity Hydraulic Jacks, Reaction Blocks, and Packers

Figure 12-7 Close Up of High Capacity Hydraulic Jacks, Reaction Blocks, and Packers

As the tunnel box and the jacks were gradually advanced away from the thrust wall, the Contractor needed to come up with a method to continue to transfer the jacking reaction force back into the thrust block wall. This was done by installing a series of 3-ft diameter structural steel pipe sections (i.e., packers) to bridge the gap between the jack pistons and the thrust block wall. Figure 12-7 also shows Initial short packer sections installed once the tunnel box structure had been jacked away from the rear reaction block a distance exceeding the maximum stroke of the jacks. The packers were connected together with 1-inch thick diaphragm plates that were anchored to the base slab in the thrust pit. Three views of the packer installations are shown in Figure 12-7 , Figure 12-8, and Figure 12-9 .

A jacking base is normally stabilized by shear interaction with the ground below and on each side. Where the interface is frictional, the interaction may be enhanced by surcharging the jacking base by means of pre-stressed ground anchors or compacted tunnel spoil. The jacking base is also stabilized by both the top and bottom ADS which are anchored to it.

Installation of Packer Sections and Connecting Diaphragm Plates

Figure 12-8 Installation of Packer Sections and Connecting Diaphragm Plates

Progressive Installation of Packer Sections and Connecting Diaphragm Plates

Figure 12-9 Progressive Installation of Packer Sections and Connecting Diaphragm Plates

12.5 Ground Control

As discussed previously, the soft ground most likely will need to be pre-treated to provide sufficient stand-up time during jack tunneling. In addition, ground may need to be stabilized in advance to control surface settlement must be controlled when tunnel jacking at such a shallow depth.

Techniques for stabilizing ground for jacked box tunneling include: grouting, well point dewatering, and freezing which are presented in Sections 7.6.5 "Grouting Methods", 7.6.6 "Ground Freezing", and 7.6.7 "Dewatering". Ground freezing is discussed hereafter to demonstrate how a ground control measure is used for jacked box tunneling.

12.5.1 Ground Freezing for CA/T Project Jacked Tunnels

As discussed in Section 12.3 above the Contractor made a value engineering proposal to replace the various soil stabilization methods indicated in the Contract with ground freezing. This alternative approach offered several advantages, including the ability to completely stabilize the soil mass through which the tunnel box structures were jacked. In contrast, the horizontal jet grouting and soil nailing methods in the original design, would have required tunnel jacking to be interrupted periodically to permit installation of the ground improvement measures from the heading. Ground freezing also offered: 1), the advantages of improved face stability, which made breasting of shield compartments unnecessary, 2), better encapsulation of obstructions which otherwise had the potential to suddenly ravel into the heading when exposed, and 3) the avoidance of windows of untreated ground.

The freezing system was installed entirely from the ground surface overlying each tunnel alignment, within the track network. The Contactor selected a conventional brine freezing system, with an ammonia plant providing the refrigeration. In the freeze plant, ammonia gas was compressed, condensing it to a liquid, then evaporated to chill the brine to an average temperature range of -25°C to -30°C. The brine used to cool and eventually freeze the ground was circulated through circuits of vertical freeze pipes as shown schematically in Figure 12-10.

Schematic Arrangement of Freeze Pipes to Freeze Ground Mass Prior to Tunnel Jacking

Figure 12-10 Schematic Arrangement of Freeze Pipes to Freeze Ground Mass Prior to Tunnel Jacking.

Each individual freeze pipe consisted of a 4.5-inch diameter steel pipe closed at the end, with a 2-inch diameter plastic pipe inserted in it that was open at the bottom. As shown in Figure 12-11, the chilled brine was pumped from a supply header line into the inner pipe, where it exited at the bottom and rose up in the annulus between the inner and outer pipe, cooling the surrounding ground in the process. At the top of the pipe, the brine was sent to the next freeze pipe for cooling circulation, as part of a circuit of 4 to 7 pipes. After passing through all of the pipes in the circuit, the brine was pumped back to the freeze plant for re-chilling through a return header pipe. The brine was circulated continuously in this manner through all of the circuits comprising the freeze zone in what was a closed system. The temperature of the ground mass was gradually lowered over a period of 4 to 5 months until the soil froze and an average target temperature of -10°C was reached.

Arrangement of an Individual Freeze Pipe showing Brine Circulation

Figure 12-11 Arrangement of an Individual Freeze Pipe showing Brine Circulation.

The freeze pipes were installed within the track area using a sonic type drill rig, which used a vibratory coring bit to advance a starter hole through the miscellaneous fill material and any obstructions contained within it, and then vibrated the outer steel freeze pipe into place in a dry drilling process that displaced the underlying organic sediments and marine clay deposits. The drill rig was mounted on a turntable on the back of a high-rail truck vehicle, which provided flexibility for locating the pipes between rails and outside of the timber ties and switching and signal equipment. Most of the drilling work was done at night by using a series of carefully coordinated track outages with the Railroad, and the sonic drilling method proved to be very effective for installing the freeze pipes quickly with relatively little drilling spoils being generated. Figure 12-12 shows the system in operation while commuter trains continued to run through the freezing area. A total of nearly 1800 freeze pipes were used on the project to freeze over 3.5 million cubic feet of soil.

Ground Freezing System in Operation while Commuter Trains Run Through the Area

Figure 12-12 Ground Freezing System in Operation while Commuter Trains Run Through the Area

The ground freezing method was very effective at providing a stable face over the entire tunnel cross-sectional face area, as shown in Figure 12-13. The one significant disadvantage of the method was the expansion of water when it freezes caused the overlying track area to heave. The amount of heave varied considerably over the alignment of each tunnel, depending on the variation in moisture content of the underlying soil profile. Typically the maximum deformation, which was monitored daily by detailed surveys of rail elevations, was in the range of 4 to 7 inches. The heave tapered to the original ground elevation over distances that extended laterally from tunnel centerline to approximate distances of about 50 to 70 feet beyond the edge of the tunnel box structure. The magnitude of this deformation required periodic re-profiling of the tracks by the Railroad to ensure that their rail geometry requirements for safe operation of their trains were maintained.

The temperature of the frozen soil mass was monitored by a series of temperature probes installed at each freeze site. After the target temperature was reached, the freeze system was adjusted to maintain that temperature, which controlled the stability of the soils at the tunnel face. As the excavation progressed for each tunnel, the freeze circuits were shutdown and the brine and inner pipes removed from the outer steel pipes, which were left in place. This progressive shut-down and dismantling of the freeze system was timed to avoid any significant warming of a section of the soil mass prior to it being exposed in the tunnel heading. When the abandoned steel freeze pipes were encountered, they were removed by cutting them out with a torch.

The Tunnel Designer should ensure that ground treatment measures do not in themselves cause an unacceptable degree of ground disturbance and surface movement.

Frozen face seen from shield at front of jacked box structure, showing soft soils stabilized so they stand vertically, and the encapsulation of an old brick-lined sewer obstruction.

Figure 12-13 Frozen Face Seen from Shield at Front of Jacked Box Structure

12.5.2 Face Loss

Design should also include provisions for controlling face loss which occurs when the ground ahead of the shield moves towards the tunnel as a result of reduction in lateral pressure in the ground at the tunnel face. With face loss, as the tunnel advances, a greater volume of ground is excavated than that represented by the theoretical volume displaced by the tunnel advance.

In cohesive ground, face loss is controlled by supporting the face at all times by means of a specifically-designed tunneling shield and by careful control of both face excavation and box advance. The shield is normally divided into cells by internal walls and shelves which are pushed firmly into the face. Typically 0.5 ft (150mm) of soil is trimmed from the face following which the box is jacked forward 0.5 ft (150mm). This sequence is repeated until the tunneling operation is complete, thus maintaining the necessary support to the face.

12.5.3 Over Cut

Design should also include provisions for controlling overcut in soft ground, by ensuring that the shield perimeter is kept buried and cuts the ground to the required profile. However, a degree of over-cut at the roof and sides beyond the nominal dimensions of the box is required for three reasons:

1. The hole through which the box travels must be large enough to accommodate irregularities in

the external surfaces of the box.

2. It is desirable to reduce contact pressures between the ground and the box, to reduce drag.

3. Overcutting may be required to fully remove obstructions at the perimeter of the shield.

The amount of over-cut required should be minimized if unnecessary ground disturbance and surface settlement is to be avoided. This demands that the external surfaces of the box be formed as accurately as possible. Typical forming tolerances are: ± 0.4 in (10mm) at the bottom and ± 0.6 in (15mm) at the walls and roof.

12.6 Other Considerations

12.6.1 Monitoring

The jacked box tunneling operation must be carefully monitored and controlled to ensure proper performance and safety. Throughout the tunneling operation, movements at the ground surface over the area affected by the tunneling operation, jacking forces and vertical and horizontal box alignment are all regularly monitored and compared to predicted or specified values.

Chapter 15 presents a variety of available instrument for monitoring ground surface movement (Section 15.2). Section 15.7 discusses overall instrumentation management considerations. Daugherty (1998) also provides detail discussions about the instrumentation design for the CA/T C09A4 tunnels.

12.6.2 Vertical Alignment

Design should also include provisions for controlling vertical alignment. A long box has directional stability by virtue of its large length to depth ratio. The box is guided during the early stages of installation by its self weight acting on the jacking base. Beyond the jacking base, the bottom ADS 'tracks' maintain the box on a correct vertical alignment. As the pressure on the ground under the 'tracks' is normally less than or similar to the pre-existing pressure in the ground and as localized disturbance of the ground is eliminated, no settlement of the tracks can occur. Any tendency for the box to dive is thereby prevented.

In the case of a short box or series of short boxes, it is necessary to steer each box by varying the elevation of the jacking thrust. This is done by arranging groups of jacks at each jacking station at different elevations within the height of the box and by selectively isolating individual groups. The jacking process is complicated by the need to check, at each stage of the operation, the alignment of all box units and if necessary to employ a suitable steering response at all jacking stations.

12.6.3 Horizontal Alignment

Design should also include provisions for controlling horizontal alignment. As discussed previously under vertical alignment, a long box has a degree of directional stability by virtue of its length to width ratio, and is normally guided during the early stages of installation by fixed guide walls located on the jacking base along both sides of the box. Where appropriate, steerage may also be used and is normally provided by selectively isolating one or more groups of thrust jacks located across the rear of the box. Depending on the ground conditions, some adjustment in horizontal position can also be obtained by controlling the amount of undercut/overcut of the excavation on one side of the heading relative to the other.

In the case of a short box or series of short boxes, fixed side guides are also appropriate but more reliance has to be placed on steerage.

12.7 References

Daugherty, C.W., (1998), "Monitoring of Movements Above Large Shallow Jacked Tunnels," Proc. GeoCongress 98, Boston, ASCE Jacked Tunnel Design & Construction, pp 39-60.

Ropkins J.W.T. (1998). "Jacked Box Tunnel Design," Proc Geo-Congress 98, Boston, ASCE Jacked Tunnel Design & Construction " pp 21-38.

Taylor, S. and Winsor, (1998). "Developments in Tunnel Jacking," Proc Geo-Congress 98, Boston , ASCE Jacked Tunnel Design & Construction " pp 1-20.

Van Dijk, P.A., Taylor, S., and Rice P.M., "Box Jacking in Boston," Proc. North American Tunneling 00, Boston , June 2000, pp. XX-YY.

Van Dijk, P., Almeraris, G. and Rice, P., "Construction of I-90 Highway Tunnels Under Boston's South Station Rail Yard by Box Jacking," Proc., RETC, San Diego , June 2000, pp. 221-239.

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Updated: 06/19/2013
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