focused on the seismic fragility of shallow tunnels based on the work of Wang ... Shield Chamber Over-break in the North River Weehawken New Jersey Shaft ...
10NCEE
Tenth U.S. National Conference on Earthquake Engineering Frontiers of Earthquake Engineering July 21-25, 2014 Anchorage, Alaska
THE VUNERABILITY OF CENTURY OLD CAST IRON LINED TUNNELS TO SEISMIC EVENTS V.TIROLO, JR 1 ABSTRACT
This paper discusses concerns about the seismic vulnerability of cast iron line lined subaqueous tunnels constructed in the early twentieth century. These concerns center on the mining techniques used to advance these tunnels through mixed face ground conditions. In New York City alone more than a dozen tunnels were mined between 1900 and 1920 under both the Hudson and East Rivers. The mining techniques, typical for this period, used to advance the six subaqueous tunnels mined starting in 1904 by Pennsylvania Railroad Company to expand into New York was used as a case history. These six tunnels were mined through highly variable and difficult ground conditions. Among the mining difficulties encountered were mixed facing mining through steeply sloping fractured rock into either loose sands or soft silts. These difficulties were overcome with methodologies conceived at the time. However, after more than a century of use, these methodologies may make these, and similar aging cast iron lined tunnels, more at vulnerable to damage during a seismic event. These methodologies included installing a more rigid tunnel liner, than typically used for the tunnel, in the transition zone between rock and soil in an attempt to reduce segment cracking during; repairing rather than replacing cracked tunnel segments; over-blasting in the shield chambers and transition areas to facilitate advancement of the shield; and using mild steel highly susceptible to corrosion for bolts, reinforcing bars and tie-rods. To adequately assess the seismic vulnerability of aging tunnels we must have a thorough understanding on how these tunnels were constructed and how localized geology may have impacted the as-built condition of the tunnel liner. Using a tunnel portal equivalence is a useful way of categorizing the potential vulnerability of the century old subaqueous tunnel construction through the transition zone from rock into mixed face and soft ground. The significance of these risks on the future performance of these aging tunnels needs to be addressed in future research.
1
CONSULTANT, Skanska USA Inc. East Elmhurst, NY 11370
Tirolo Jr, V. The Vulnerability of Century Old Cast Iron Lined Tunnels to Seismic Events. Proceedings of the 10th National Conference in Earthquake Engineering, Earthquake Engineering Research Institute, Anchorage, AK, 2014.
THE VUNERABILITY OF CENTURY OLD CAST IRON LINED TUNNELS TO SEISMIC EVENTS V.TIROLO, JR1 ABSTRACT This paper discusses concerns about the seismic vulnerability of cast iron line lined subaqueous tunnels constructed in the early twentieth century. These concerns center on the mining techniques used to advance these tunnels through mixed face ground conditions. In New York City alone more than a dozen tunnels were mined between 1900 and 1920 under both the Hudson and East Rivers. The mining techniques, typical for this period, used to advance the six subaqueous tunnels mined starting in 1904 by Pennsylvania Railroad Company to expand into New York was used as a case history. These six tunnels were mined through highly variable and difficult ground conditions. Among the mining difficulties encountered were mixed facing mining through steeply sloping fractured rock into either loose sands or soft silts. These difficulties were overcome with methodologies conceived at the time. However, after more than a century of use, these methodologies may make these, and similar aging cast iron lined tunnels, more at vulnerable to damage during a seismic event. These methodologies included installing a more rigid tunnel liner, than typically used for the tunnel, in the transition zone between rock and soil in an attempt to reduce segment cracking during; repairing rather than replacing cracked tunnel segments; over-blasting in the shield chambers and transition areas to facilitate advancement of the shield; and using mild steel highly susceptible to corrosion for bolts, reinforcing bars and tie-rods. To adequately assess the seismic vulnerability of aging tunnels we must have a thorough understanding on how these tunnels were constructed and how localized geology may have impacted the as-built condition of the tunnel liner. Using a tunnel portal equivalence is a useful way of categorizing the potential vulnerability of the century old subaqueous tunnel construction through the transition zone from rock into mixed face and soft ground. The significance of these risks on the future performance of these aging tunnels needs to be addressed in future research.
Introduction In the United States and other developed countries aging infrastructure is becoming an area of concern. This is particularly true for aging urban transportation infrastructure. These transportation systems carry millions of commuters daily. Many of these structures are vulnerable to seismic events. Naturally attention has focused on upgrading above ground transportation infrastructure. Tunnels are considered of less concern except possibly at their portals. It is generally assumed that tunnels are constructed in intimate contact with the surrounding ground and therefore no free response of the liner is possible Dowding [1]. However, Dowding and Rozen [2] did indicate that besides tunnel damage occurring at portals, damage also occurred in poor ground conditions that made construction difficult and from unsymmetrical loadings. More recently, Hashash [3] and others have also stressed the importance of evaluating contact between the liner and the ground. Hashash [3] references Kuesel’s [4] concerns about tunnels in soil to rock transition zones. Argyrioudis and Pitilakis [5] focused on the seismic fragility of shallow tunnels based on the work of Wang [6]. Modern tunnels are designed and constructed to account for seismic loading [6-8].
Subaqueous Tunneling with Shields and Compressed Air The end of the 19th and beginning of the 20th centuries in Europe and the United States began an unprecedented period of subaqueous tunnel construction. The successes of this period were made possible by three fundamental advances in tunnel technology. These were the introduction of the circular tunnel shield by Peter Barlow and James Henry Greathead in 1869 on the Tower Subway project in London and also in 1869 by A.E. Beach on a Broadway tunnel project in New York City; the use of cast iron segmental liner again by Barlow and Greathead on the Tower Subway; and finally the first successful use of compressed air in tunneling again by Greathead on the City and South London Railway in 1886 [9-10]. Tunnels designed and constructed at the end of the 19th and beginning of the 20th centuries were not designed to account for seismic loadings. These tunnels were built at different time when the feasibility and safety of mining tunnels beneath waterways was still questionable. Therefore the focus of these early tunnel engineers was to combine tunnel shields and compressed air to erect permanent relatively watertight segmental cast iron tunnel liners, on line and grade that would safely support ground loadings and rail live loads without excessive settlement. In New York City alone more than a dozen tunnels were mined between 1900 and 1920 under both the Hudson (North River) and East Rivers. Typical of tunnels constructed during this period were the six subaqueous tunnels built by Pennsylvania Railroad Company (PRR) to expand their system into New York and New England. The total length of these 7 meter (23 ft.) diameter subaqueous tunnels was 11,250 meters (7 miles). These six tunnels were mined with 12 shields using compressed air through highly variable and difficult ground conditions. The maximum depth of the tunnel invert was 30 meters (98 ft.) below mean high water [9]. Average compressed air pressures exceeded 200 kPa (2 bar). The maximum pressure was 300 kPa (3 bar). These subaqueous tunnels were mined through rock, soft soils, and mixed face conditions (rock and soil). The tunnels often encountered obstructions near the shorelines including concrete bulkheads, nests of timber piles and riprap. The tunnels were shallow often with less than one tunnel diameter below the mudline. Even with the use of temporary clay blankets in the river, blows of compressed air through the tunnel face were common [11-13]. Compressed air sickness was common as was injury and death (more than 30 miners died in these tunnels). Within this environment, the engineers and miners (sandhogs) focused on expeditiously erecting the liner and completing the tunnel (holing though). In the following paragraphs we will review three construction methodologies used to complete these tunnels and how these practices could contribute to the current seismic vulnerability of these aging tunnels.
Advancing the Tunnel Shield Through Rock and Mixed Face
Excavation in Full Face Rock Shield Chamber Even today in our modern era of advanced Tunnel Boring Machines (TBMs), machines have difficulty mining through mixed face conditions. Tunnel shields in the 19th and beginning of the 20th centuries were designed for soft ground mining only. All the shield experience developed by Greathead and others in London was in soft ground. The six PRR tunnels would be mined through soft ground, mixed face and rock. Due to their size and complexity, tunnel shields are normally disassembled at the fabrication yard, lowered in pieces and re-assembled below ground in a shaft or chamber. Whenever possible, shields are assembled in free air. Since the land portions of all six PRR tunnels originated in rock, all shields were re-assembled in rock chambers. Cover was shallow in these portal areas. Typically, cover was less than one tunnel diameter. Rock caverns were constructed by drill and blast methods. Figure 1 Forgie [11] shows the extent of rock over-break that was typical in these rock caverns. Sometimes these chambers did not require timbering but over-break was always extensive. Figure 2 from Forgie [11] shows the erection of cast iron or cast steel segments horseshoe bents within the rock chamber. These bents provide a reaction system for the shield propelling jacks.
Figure 1. Shield Chamber Over-break in the North River Weehawken New Jersey Shaft
Of course any timbering used for initial support of the rock was not removed. Once the segmental bents are constructed, attempts were made to fill the larger voids with large stones or sacks filled with rock debris. This was particularly hazardous for large overhead voids. Therefore, it is likely that the primary means of filling voids was erecting bulkheads at selected intervals in the chamber and gravity feeding a sand cement grout behind the bulkheads. It is current tunneling practice to compare the volume of grout pumped to the measured volume of voids. In the PRR tunnels the volume of voids was not measured, thus the probability that all the voids were filled is small. The only field check that the voids were filled was the sand cement grout appearing at the top of the temporary bulkhead. This check was crude at best and did not account for air pockets and obstructions to the flow of grout. It is likely that some voids in the shield chamber were not filled so contact between the liner and the ground is uncertain.
Figure 2. Erecting Segments in the North River Manhattan Shield Chamber Launching Shield Once a sufficient number of segments were erected in the chamber, a concrete cradle was cast directly onto the rock subgrade and up to the springline of the tunnel. Rails were embedded in lower portion of the cradle to provide a smooth riding surface for the skin of the shield. The tunnel shield was erected on this cradle and rail system. The shield was then launched. Full face
drill and blast excavation continues as long as possible in front of the shield. The concrete cradle provided contact between the cast iron liner and the rock. However, the cast iron liner was not structurally connected to the cradle. Therefore, the lower portion of cast iron liner was restrained transversely (no-slip) but longitudinally only by friction between the cast iron liner and the castin-place cradle (interface between no-slip and full-slip). The restraint of upper portion of the cast iron liner, above the springline, is questionable because of the difficulties in grouting. Forgie [12] described the tendency of the tunnel liner to sag under its own weight when the segments were not immediately fully grouted to rock. Figure 3 from Forgie [11] shows tie-rods and turnbuckles that were used in these transition zones rock sections to maintain the liner’s shape until grouting was completed. The shield was moved forward on the concrete cradle until it was anticipated that the rock line would drop off. A concrete bulkhead and air locks were then constructed behind the shield. Excavation continued as additional tunnel rings (assemblies of segments) were erected within the shield’s tail section and the shield jacked forward. Once the bulkhead was constructed, the remaining tunneling would utilize compressed air to maintain face stability during mining.
Figure 3. Tie-rods used to maintain liner circularity. Excavation in Mixed Face (Face Partly in Rock with Soft Ground Above)
Mixed face conditions are among the most difficult that can be encountered in tunneling. As was mentioned previously, in the late experience 19th Century there was no experience using a shield in either full face rock conditions or in mixed face. Therefore a variety of techniques, all with advantages and disadvantages were attempted. Brace et al [13] described the mining procedures used in the East River Tunnels. They illustrated these procedures in Figure 4. Generally this technique involved supporting the soft ground in front of the shield with poling boards and breasting prior to removing the rock in the lower portion of the face. After the rock was excavated, the concrete cradle was extended, cast iron rings erected and the shield shoved forward.
Figure 4
Tunneling Through Mixed Face Ground Conditions [13]
In these transition zones (mixed face into soft ground) backfilling with a sand cement grout continues behind the liner. However, except in sections of the tunnel immediately below existing structures, e.g. buildings or rail yards, back grouting was often delayed until a sufficient number of rings were erected and exited the tail of the shield. Again tie-rods and turnbuckles were used to keep the cast iron liner from ovaling. Compressed air blows were common during mixed face tunneling. Clay blankets placed above the mudline helped eliminate the loss of compressed air at the tunnel face. Rock and mixed face conditions occurred in the two North River tunnels only at the shorelines. However, the eight shields used to mine the four East River tunnels were in and out of rock, mixed face and soft
ground conditions continuously throughout the drive. East River contains shallow submerged ridges of Inwood marble, Hellgate dolomite and Fordham gneiss. The Manhattan shields mining east encountered, 23 % full face rock, 37% mixed face and 40% soft ground. Often the mixed stratum of boulders and cobbles covered the top of rock surface. The shields drive west from Long Island City encountered somewhat less ground variability with a higher percentage of soft ground. Broken Liner Plates At a number of locations with both the East and North River Tunnels cast iron liner segments cracked. Cracked occurred for a number of reasons including faulty erection, eccentric shoving pressures, tunnel distortion (ovaling and sagging), inadequate grouting, foreign objects such as bolts or small tools caught between the segment and the skin of the shield, contact of liner with rock projections and movement of the shield against the liner (“iron bound” conditions). Shields were often damaged and distorted not only when they were mined through rock, particularly rock ledges, but also when they encountered obstructions such as rip rap protected and pile supported concrete, stone and timber bulkheads near the shorelines. Additionally, liners cracked due being overstress by the point loads of piles encountered when the tunnel passed under bulkhead and pier structures. In some locations, it was decided to use cast steel liners at a number of potentially high stress concentration areas. In a letter dated July 29, 1908 from H.C Booz, Principal Assistant Engineer of the Pennsylvania Railroad (15) he reports on a letter written to Rea by Alfred Noble, Chief Engineer, East River Division concerning cracked liner plates. In his letter, Noble lists more than thirty reasons that liner plates crack. These include jacking pressures improperly applied, foreign materials getting between the tail of the shield and the liner, defects in the fabrication of the plates, careless handling, vertical distortion of the liner, liner struck by flying rock during blasting, liner binding in tail of shield, excessive tightening of bolts. Repair Rather Than Replacement of Broken Tunnel Liner Plates Of 304 broken liner segments reported by Noble 50 percent were not removed or replaced. In most cases it was decided to repair rather than replace cracked tunnel liner segments. This repair consisted of placing repaired steel tie-bars looped between the flanges of the cracked segment as shown in Fig 5. The tie-bars were eventually encased with the concrete invert or secondary concrete liner. In addition to repairing cracked tunnel segments, in areas containing numerous cracked segments, spiral bent reinforcing steel was passed to reinforce the normally unreinforced secondary concrete tunnel liner. Figure 5 from Forgie [14] shows, cracked segments and thus segment repairs often occurred in the tunnel’s invert. Figure 6 from the Pennsylvania Railroad archives shown spiral reinforcement in the tunnel above the springline.[15]
Figure 5. Steel Tie Rods Used to Repair Cracked Tunnel Segments [14]
Figure 6. Spiral Reinforcement in Tunnel [14]
Impact of These Tunneling Practices on Seismic Vulnerability The PRR tunnels went into operation in 1910. They have performed successfully for over 100 years. In addition to the construction issues described above, PRR had concerns about the construction of the PRR subaqueous tunnels. Among others the most important were concerns included excessive tunnel settlement in the soft North River silts during train live loading; or excessive tunnel buoyancy and cyclical movement of the tunnels with the tide; drift of the tunnels downstream with the river flow; and increased water infiltration into the tunnel over time. Fortunately it was only the intervention of the PRR Vice President in charge of the project, Samuel Rea, an engineer by training, which prevented the entire tunnel structure from being supported on piles. The dispute Rea had with the majority of the Board of Engineers on the issue of supporting the North River tunnels on screw piles is chronicled in detail within the minutes of the Board’s meetings [16]. In this his only support on the Board was its chair General Charles W. Raymond. Some settlement has occurred but has had no impact of train operations. Similar subaqueous tunnels constructed during that era have also performed admirably. However, construction practices in subaqueous tunnel developed during the early 1900s have increased the seismic vulnerability of these tunnels. Using the PRR tunnels as an example: Shaft –Tunnel Interface There is no physical separation between the tunnels and the shafts. All shafts are supported on sound rock. Shaft walls are typically one to two meters (3 to 6 ft.) thick. The tunnel portals all began in rock chambers immediately outboard of the shafts. The rock in both the East and North Rivers drop off precipitously often with 45° the rock slopes. There is no soft transition between these structures the flexible tunnel structure and the rigid heavy wall shafts. Rock-Soil Interface There is no transition between the rock tunnel sections and soft ground. Grouting was problematic. In Rock and mixed face sections, segments often cracked in the cast iron segments located below the tunnel springline. Heavier cast iron segments, increased weight of each lineal foot from 4,205 kilograms (9,272 lbs) to 5,260 kilogram (11, 594 lbs), or cast steel liners were used in a number of locations of high stress such as under pile supported bulkheads to reduce the number of cracked liners. These practices further may make the potential movements of the liners in the rock and mixed face sections of the tunnel during a seismic event incompatible with those in the soft ground river sections.
Repair of Cracked Liner Segments and Spiral Reinforcement. Inspections of cast iron lined tunnels have revealed the little to no corrosion degradation of the cast iron structure. These liners were originally designed empirically. The thicknesses of the liner’s webs and flange vary from 40 to 60 mm (1.5 to 2 inches) [11]. Over a period of 100 years, these cast iron liners have lost less than 5 mm (0.2 in.) of thickness due to corrosion. What is much more likely to have corroded significantly over this same period of are the cast steel segments, the steel bolts connecting both the cast iron and cast steel segments, and the steel tierods and steel reinforcement used in secondary concrete liner at the cracked segments[13-14]. Evidence of mild steel corrosion is limited [17]. However, data on corrosion does exist in tunnels without a secondary concrete liner such as sections Steinway Tunnel, East River, New York City, 1907; Tunnel F, of the H&M Tunnels (now the PATH Tunnels), Hudson River, 1909, and the Joralemon Street Tunnel, East River, New York City, 1907, where spalling concrete has exposed the cast iron liner and spiral reinforced concrete secondary liner. In all these tunnels, mild steel components such as bolts, reinforcing steel and tie-rods were significantly corrosion. Therefore, the shaft, rock chamber and mixed face tunnel sections the structural integrity of the segmental liners to all but compressive loads is compromised by corrosion. This corrosion would also increase the fragility of these cast iron liners to seismic ground deformations. The corrosion potential of cast steel liners relative to cast iron liners is unknown at this time but deserves further investigation. Conclusions Understanding how older structures were constructed assists in understanding how they perform over time. The construction of subaqueous tunnels has advanced significantly since the late 19th and beginning of the 20th centuries. Today Earth Pressure Balance Machines and Slurry Shields have replaced shield tunneling with compressed air for subaqueous tunnel mining. Liners are no longer cast iron but precast concrete. Seismic analysis is part of liner design [6-7]. As time passes and tunneling design and construction practices technology advance, there is a tendency to neglect or ignore past construction practices. For aging tunnel infrastructure this neglect should be avoided. Studying the details of how tunnels were constructed gives not only give us insight on how they will perform in the future but also provided valuable guidance on where they are vulnerable to future seismic loadings. A useful analogy when studying the potential vulnerability of the century old subaqueous tunnel construction in rock and mixed face conditions ground is the shaft/tunnel interface and the transition between rock and soft ground as tunnel portal not part of a continuous tunnel structure. Dowding and Rozen [2] have indicated that tunnel damage occurred at portals, in poor ground conditions that made construction difficult and from unsymmetrical loadings. Hashash [3] and Kuesel [4] expressed concerns about the tunnel soil to rock transition zone. Rock cover at these transition zones were also less than one tunnel diameter. Finally, Hashash [3] and others have also stressed the importance of evaluating contact between the liner and the ground. Construction practices in the early 1900s make contact between the cast
iron liner and the ground in rock and mixed face ground are uncertain. These portal/transition areas are also more likely to contained significantly corroded mild steel structural element and cracked cast iron segments. Corrosion would also increase the fragility of the cast iron liners in these areas to seismic ground deformations. When we develop our models to study the reaction of tunnels to seismic events it is more convenient for us to assume that the assumptions we use for modern tunnels studies also apply for aging tunnels. Often this is not the case. In this brief paper we have tried to provide some insight into the potential vulnerability of an aging subaqueous tunnel structures by examining the practices used more than100 years ago to construct these structures. It is the intent of this paper to re-kindle historic construction practices in general and tunnel mining practices in particular in the when we are developing our aging tunnel infrastructure seismic risk assessment models.
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10. Hewett, B. and Johannesson, S. Shield and Compressed Air Tunneling: McGraw Hill, New York 1922. 11. Forgie, James, The Construction of the Pennsylvania R.R. Tunnels Under The Hudson River at New York City. Engineering News; 56, 24, December 13, 1906, pp. 603-614. 12. Forgie, J, The Construction of the Pennsylvania R.R. Tunnels Under the Hudson River at New York City. Engineering News; 57 9: February 28, 1907, pp. 223- 234 13. Brace J, Mason F, Woodward S. The New York Tunnel Extension of the Pennsylvania Railroad. The East River Tunnels, Transactions of the American Society of Civil Engineers, Sept. 1910 XXVII: Paper No. 1159. 14. Forgie, James. James Forgie Papers, 1890-1946, 1949, Smithsonian Institution, National Museum of American History, Archives Center 15. Pennsylvania Railroad Company, Executive Department, General Correspondence, Archives of the Hagley
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