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Ground Deformation from Multiple Tunnel Openings: Analysis of Queens Bored Tunnels Michael Mooney1, Jacob Grasmick1, April Clemmensen2, Andrew Thompson3, Eric Prantil3, Brett Robinson4 ABSTRACT The East Side Access Queens bored tunnels project involved the construction of four near surface, closely spaced metro transit tunnels beneath the rail yards and mainline railroad tracks. The close proximity of the tunnels provides a unique opportunity to examine the influence of multiple closely spaced tunnel openings on ground deformation, particularly the accumulation of vertical surface deflection due to consecutive tunnels. This paper presents measured transverse vertical deflection profiles from the project, comparing observed with the established theory of Gaussian surface deflection response. Heave behavior prevailed at most cross sections; the transverse heave and settlement profiles caused by individual tunnel excavations exhibited Gaussian form; however, the width of the deflection profiles and the position with respect to tunnel centerline deviates from conventional theory. INTRODUCTION The East Side Access Queens bored tunnels project involved the construction of four near surface, closely spaced metro transit tunnels beneath the rail yards and mainline railroad tracks in Sunnyside yards in Queens, New York (see Figure 1). The tunnels were excavated using two 6.9 m (22.5 ft) diameter Herrenknecht slurry shield TBMs primarily through highly variable glacial till soils and outwash deposits. The close proximity of the four tunnels coupled with their shallow depth provides a unique opportunity to examine the influence of multiple closely spaced tunnel openings on ground deformation. Classical analysis of tunneling-induced ground deformation in a greenfield environment (no buildings) suggests that a Gaussian-shaped longitudinal and transverse deflection profile develops at the ground surface centered above a single tunnel (Peck 1969). With twin tunnels, the resulting surface deflection can be symmetric over the mid-point of two tunnels or asymmetric and shifted towards one tunnel (Cording and Hansmire 1975, Suwansawat and Einstein 2007). Suwansawat (2006) proposed a superposition technique to attribute surface settlement to individual tunnels in a twin tunnel environment is estimated via superposition of individual transverse Gaussian settlement profiles. Suwansawat and Einstein (2007) and Chapman et al. (2007) used this technique to show that the increment of transverse surface settlement caused by each tunnel in twin tunnel configurations (side by side and stacked) is Gaussian in shape and centered above the individual tunnel. This has not been explored extensively and has not been examined for more than two tunnels. The cross-section of four tunnels in the Queens bored tunnels project allows for a more detailed assessment. This paper begins with a brief background to the project and to classical analysis of surface settlement followed by the layout of ground deformation instrumentation used and cross sections analyzed. Cumulative and incremental transverse surface deflection profiles are then presented at select locations along the alignment. The Gaussian shapes of the profiles are analyzed in terms of Gaussian behavior and the applicability of the superposition technique is analyzed. 1 Center for Underground Construction & Tunneling, Colorado School of Mines, mooney@mines.edu 2 U.S. Air Force Academy 3 Hatch Mott MacDonald 4 Traylor Bros., Inc.

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Figure 1. Rendering of Queens bored tunnels (courtesy MTA) BACKGROUND The four tunnels totaling 3207 m in length (refer to table in Figure 1 for individual tunnel lengths) were constructed by the joint venture of Granite Construction Northeast, Inc., Traylor Bros., Inc., and Frontier-Kemper Constructors, Inc. in 2011 and 2012. Two 6.9 m (22.5 ft) diameter Herrenknecht slurry shield TBMs were used. The cross-section at the launch wall (Figure 1) illustrates the four tunnel configuration. At the launch wall, excavation of tunnel YL (yard lead) began at a depth of 22.9 m below the existing ground surface. Tunnel A began 11.9 m deep and tunnels D and BC 11.7 m deep. Tunnel YL was driven first, followed by tunnels A, D and BC. The project is described in detail in Robinson & Wehrli (2013a,b). Geology The ground conditions primarily consisted of highly variable glacial till soils and outwash deposits. 18 different strata ranging from gneiss bedrock to clay lenses were discovered along the alignment during the geotechnical site investigation (67 borings). These strata were highly variable and not uniform across the entire project site. The area mostly consists of well graded sand (SW), poorly graded sand (SP) and silty sand (SM), with some small lenses of clayey sand (SC), clay (C), silt (M) and gravel (G) present. There is a layer of decomposed gneiss resting atop the fractured gneiss bedrock. The first 130 m of tunnel YL was excavated in fractured bedrock while the other three tunnels were excavated in soil. Estimating Ground Deformation Classical analysis of tunneling-induced ground settlement in a greenfield environment (no buildings) suggests that the settlement profile that develop at the ground surface is transversely Gaussian in shape and centered above a single tunnel (Figure 2). The transverse settlement basin or trough is characterized by a width parameter i that is a function of soil type and depth to tunnel center z0 (see Equation 1). The majority of reported K values range from 0.4-0.6 in clay and 0.25-0.45 in sand (see Mair and Taylor 1997). If the strata is layered, New and O’Reilly (1991) proposed a revised relationship for i that is based on a weighted average of layers. It is

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worth mentioning that these values of K and the principle of Gaussian behavior in general has been used to explain settlement behavior. To the authors’ knowledge, Gaussian behavior and these values has not been applied to tunnel-induced surface heave. The volume of the transverse deflection basin Vs is determined from Equation 2 where smax is the maximum deflection. Vs is related to the ground loss VL that occurs at tunnel depth. The settlement at any position y is given by Equation 3.

Figure 2. Classical Gaussian shape settlement trough both longitudinally and transversely where the volume of the transverse settlement profile Vs is typically

equal to the ground loss VL at the tunnel depth (after Mair & Taylor 1997)

0Kzi = (1)

max2 isVs π= (2)

−=

2

2

max2

expiy

ss (3)

A fairly significant body of data indicates that Vs manifested at the surface is approximately equal to VL in cohesive soils and that Vs is less than VL in granular soils (due to dilation of the soil from the surface to the tunnel crown). Conservatively, Vs is often assumed equal to VL in settlement prediction for granular soils. The four most significant contributors to ground loss during tunneling, assuming good practice, include (1) preconvergence and convergence due to stress relief at the face; (2) convergence around the forward shield due to the overcut annulus; (3) convergence at the tail shield due to the annulus outside the segmental concrete liners; and (4) convergence due to liner deformation after grouting (Mair & Taylor 2007). The increased use of pressurized face TBMs to minimize stress relief combined with shield annulus bentonite injection and two-part grouting around the segments from the tail shield has reduced volume loss on tunnel projects from a few percent (of the excavated volume) a decade ago to less than 0.5% on more recent projects. It has been shown that when twin tunnels are excavated, the cumulative surface settlement can be estimated via superposition of individual transverse Gaussian settlement profiles (Suwansawat and Einstein 2007, Chapman et al. 2007). Each surface settlement profile can be related to Vs and VL. Suwansawat and Einstein (2007) showed that for side by side twin tunnels that the subtraction of the first tunnel’s surface settlement profile from the cumulative surface settlement profile yielded a Gaussian profile centered above the second tunnel. They showed a

LV

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similar outcome when analyzing surface deformation for two stacked tunnels. This is a helpful technique as it enables a reasonable estimate of ground deformation in the presence of two tunnels by superposition of the Gaussian profiles. Instrumentation The layout of ground deformation instrumentation for the Queens bored tunnels is shown in Figure 3. Because rail traffic continued throughout construction, a sizable array of settlement monitoring points was established on the ground and on the train tracks throughout the Sunnyside yards. This included approximately 330 surface settlement monitoring points on the ground and over 1500 monitoring points on rail tracks. In addition, over 500 automated motorized total station (AMTS) survey prisms were deployed to monitor track movement and 15 multi-point borehole extensometers (MPBX) were installed, the majority with measurement points immediately above the tunnel crown, 1-2 m below the surface, and at the mid-depth to the crown. Ground and rail deflection data primarily at the northwest end (beginning) of the alignment were collected by manual survey (leveling staff or total station) with a frequency of once per day or less. Not all points were measured each day and the time of day was not recorded; a rolling approach was used depending on where the tunnel headings were. AMTS data were collected as frequently as once per hour.

Figure 3. Layout of ground deformation instrumentation (R=rail; G=ground)

The uncertainty/error in manual survey stems from both instrumentation uncertainty and systematic or operator error. Instrumentation uncertainty for total station surveys typically falls in the 2-3 mm range. Systematic and operator errors can include poor baseline readings, improper backsighting, incorrect use of survey equipment, etc. These systematic and operator errors can usually be identified when analyzing day-to-day readings, and considerable effort was taken to remove these errors from the data presented herein.

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RESULTS Both longitudinal and transverse deflection profiles were examined; only transverse profiles are presented here given paper length constraints. The day that baseline settlement readings were conducted (May 7, 2011) is established as day 0 for the purposes of data presentation. Tunnel YL excavation began on day 25 (ended day 284), tunnel A began on day 92 (ended day 229), tunnel D began on day 325 (ended day 389), and tunnel BC began on day 363 (ended day 443). To provide a sense for how the sequence of tunneling impacted ground settlement, Table 1 summarizes the time frame when each tunnel heading passed under cross section R7 (see Figure 3 for reference). For each tunnel, the days when the tunnel heading was 50 m behind (denoted -50 m), at the cross section (denoted 0 m) and 50 m ahead are shown. As is conveyed in Table 1, each tunnel passed cross section R7 (and all cross sections) quite independently.

Table 1. Days When Tunnels Crosses R7 Tunnel -50 m 0 m 50 m

YL 25 40 62 A 92 110 116 D 325 334 336

BC 363 369 374 Adjacent transverse vertical deflection profiles from both ground (G) and top of rail (R) measurements are presented for comparison in Figure 4. Deflection profiles are presented for each tunnel excavation after the respective tunnel is approximately 50m past the transverse profile. For all surface deflection data presented in this paper, geotechnical sign convention is used where positive corresponds to settlement and negative to heave. In general, ground and rail transverse deflection profiles exhibit similar behavior both in magnitude and lateral (y) offset of peak deflection and shape of the deflection profile. There was some concern that top of rail deflection would not be representative of greenfield deflections due to possible bridging of the stiffer rail tracks. A comparison of ground and rail deflections illustrate that the rail track did not produce the bridging effect and, therefore, can be assumed to be representative of greenfield deflections. Top of rail deflection data yielded smoother transverse profiles than the ground deflection data. This suggests some uncertainty in analysis of individual ground deflection data, perhaps due to movements of the rebar stakes used to mark the ground deformation points. For these reasons, top of rail deflections will be presented hereafter.

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Figure 4. Comparison of ground and rail deflection profiles

The transverse surface deflection profiles measured at cross sections R9 and R11 (25 m apart) are shown in Figure 5. These data reflect the as-measured cumulative vertical deflections collected when each TBM heading was at least five diameters beyond each cross section to ensure that all effects on immediate settlement/heave had been captured. The R9 and R11 profiles primarily reflect ground heaving rather than settlement. Such a ground response is the result of elevated slurry face support pressure used to mitigate settlement during this project. By following the tunneling sequence, Figure 5a shows 2.5 mm maximum heave due to tunnel YL, followed by a negligible increase in heave due to tunnel A. A slight net settlement occurred during tunnel D construction. Finally, tunnel BC construction induced additional heave (7 mm cumulative). A similar sequence of transverse deflection response is reflected in cross section R11 that is 25 m beyond R9 (Figure 5b).

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Figure 5. Transverse surface deflection profiles (a) R9 and (b) R11

after passage of each of the four TBM headings (+ is settlement; - is heave)

According to the Suwansawat (2006) superposition technique, the incremental surface settlement incurred from any single tunnel should be transversely Gaussian in shape and centered over the respective tunnel. Here we apply this technique to both heave and settlement behavior, recognizing that the application to heaving is beyond what Suwansawat (2006) intended. The R9 data from tunnel YL and the increments thereafter are presented in Figure 6. ∆A was determined by subtracting tunnel YL induced deflection from tunnel A deflection, ∆D was determined by subtracting the cumulative deflection after tunnel A from the cumulative deflection after tunnel D, etc. Each profile was then fit with a Gaussian curve (per Equation 3). Figure 6a shows that the heaving induced during tunnel YL construction exhibits Gaussian behavior, suggesting that the Gaussian response applies both to settlement (per the literature) and heaving as shown here. The transverse heave profile is centered above tunnel YL with a width parameter i = 16 m, and combined with a depth to tunnel center = 23 m, the resulting K = 0.7. This is slightly greater than the 0.25 – 0.6 range of K values published in the literature for settlement behavior, but within reason. The increment of transverse heave due to tunnel A construction was also found to be Gaussian in shape with a smaller width factor (i = 9 m). Greenfield settlement theory, however, would suggest a narrower deflection profile with i closer to 4-5 m for a depth to 7 m. This difference may be due to the influence of tunnel YL and/or as a result of heave vs. settlement. Further, a more significant finding is that the symmetry axis of the settlement profile is offset approximately 10 m from the centerline of tunnel A (see Figure 6a). A number of factors likely contribute to the higher i and the offset. The excavation of tunnel YL redistributes the ground stress in the vicinity of YL, e.g., arching sheds vertical stress above YL to the YL spring lines and influences spring line horizontal stresses. This creates a non-homogeneous stress field through which tunnel A is excavated. Strain follows stress, and therefore the deformation will be asymmetric. This effect is likely amplified by the non-uniform geology at this location (see Figure 7).

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Figure 6. (a) Incremental R9 surface deflections and their Gaussian fits; (b) a summation

of the fitted Gaussian increments and their match with experimental data

Figure 7. Estimated geological profile at cross section R9

The increment of transverse settlement that occurred during tunnel D construction does exhibit Gaussian behavior but is not centered over tunnel D’s center line. The fitted width of the Gaussian response (i = 12 m) is greater than what greenfield Gaussian theory predicts (i = 4-5 m assuming Equation 1 and granular soil). The increment of transverse heave due to tunnel BC construction also exhibits Gaussian response with a much greater width (i = 27 m) than greenfield theory would suggest (i = 4-5 m). This increment of transverse settlement is not aligned with the centerline of tunnel BC. Finally, when the fitted Gaussian distributions are added as shown in Figure 6b, the cumulative transverse settlement profiles match well with the observed deformations in Figure 5 (RMSE < 1 mm), supporting the notion that transverse heave and settlement profiles due to individual tunnels are well represented by Gaussian curves.

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Cross section R11 deflection response due to tunnel YL and the increments thereafter are presented in Figure 8 with their respective Gaussian curve fits. Figure 8a shows that all tunnels induce settlement or heave that clearly exhibits Gaussian behavior, further supporting the notion that Gaussian behavior applies for both settlement and heave. For tunnel YL, the width parameter i = 8 m combined with a depth to tunnel = 23 m results in an average K = 0.4. This is a reasonable value within the range of published literature for settlement behavior. The increment of transverse heave due to tunnel A construction was similar to tunnel YL. Its response is Gaussian in shape but with a larger width factor (i = 18 m). Greenfield theory would suggest tunnel A would cause a narrower deflection profile (width factor closer to 4-5 m) because the depth to tunnel is much smaller (9.5 m). The increment of transverse settlement that occurred during tunnel D also exhibits Gaussian behavior but is not centered over tunnel D’s location. The fitted Gaussian width factor (i = 20 m) does not match greenfield theory using published data (i = 4-5 m). The increment of transverse heave due to tunnel BC construction also exhibits Gaussian response with an even greater width (i = 23 m) than classical approach would suggest (i = 4-5 m). This increment of transverse heave is more aligned with the centerline of tunnel BC than increments in settlement/heave due to tunnels A and D. When the fitted Gaussian distributions are added as shown in Figure 8b, the cumulative transverse deflection profiles match well with the observed deformations (RMSE < 1 mm). The geological cross section for R11 presented in Figure 9 once again reflects the non-homogeneous geology in which the TBMs were driven.

Figure 8. (a) Incremental R11 surface deflections and their Gaussian fits; (b) a summation

of the fitted Gaussian increments and their match with experimental data

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Figure 9. Estimated geological profile at cross section R11

Figure 10a illustrates the measured deflection at transverse cross section R20 after passage of each TBM. R20 experienced nearly zero deflection due to tunnel YL, followed by subtle heaving due to tunnel A, a reduction in heave after tunnel D and a subtle increase in heave after tunnel BC. The incremental deflections attributed to each tunnel are shown in Figure 10b. Each incremental profile exhibits a Gaussian shape. The profiles induced by tunnels YL and D are offset from center while the profiles induced by tunnels A and BC are centered above their respective tunnels. Further, the profiles induced by tunnels A and BC exhibit width factors (i = 5 m and 8 m, respectively) that are consistent with and slightly greater than classic approach would predict. However, the settlement profile due to tunnel D exhibits a width factor i = 10 m that is greater by a factor of 2 than that predicted by greenfield theory. When the fitted Gaussian curves are added as shown in Figure 10c, the cumulative transverse deflection profiles do not match as well with the observed deflections as compared to sections R9 and R11 (RMSE < 2 mm).

Figure 10. (a) Incremental R20 surface deflections and their Gaussian fits; (b) a

summation of the fitted Gaussian increments and their match with experimental data

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A summary of the observed width parameters i and K estimated by fitting Gaussian responses to 112 transverse deflection profiles (28 cross sections x 4 tunnel advances at each) is presented in Table 2. The deflection profiles in both settlement and heave situations consistently exhibited Gaussian behavior. The measured width parameters i and K for tunnel YL were found to be greater than the range and mean reported in the literature for predominantly granular soils (K is typically 0.35-0.45). Incremental heave and settlement profiles for tunnels A, BC and D exhibited considerably greater widths than reported in the literature for single tunnels (3x-4x greater) while tunnel YL yielded a slightly higher width parameter (1.5x-2.0x greater).

Table 2. Summary of width parameters for Gaussian curve YL ∆A ∆D ∆BC i K i K i K i K

mean 16.1 0.7 14.5 1.5 15.8 1.7 14.2 1.5 min 3.9 0.2 3.0 0.3 3.3 0.4 3.1 0.3 max 36.6 1.7 31.5 3.6 43.8 4.9 29.8 2.9

std dev 8.3 0.4 8.7 1.0 10.7 1.2 8.6 0.8 Furthermore, the peak of the incremental transverse deflection profiles did not coincide with centerlines of the tunnels, specifically for A, D and BC. A summary of the y-offset (- left, + right of tunnel centerline) from the tunnel center to the centerline of the Gaussian curves is presented in Table 3. The average y-offset for tunnel YL of 5.1 m is reasonable in comparison to average y-offsets of tunnels A, D and BC. A likely cause is a combination of the altered stress field created by each tunnel (leads to a heterogeneous stress field) combined with heterogeneity in the geological cross section. This phenomenon will be further explored through finite element modeling of the geological conditions combined with staged adjacent tunnel openings (beyond the scope of this paper).

Table 3. Summary of y-offset (m) for fitted Gaussian curves

YL ∆A ∆D ∆BC avg 5.1 16.3 14.8 12.2 max 13.8 38.9 13.6 35.7 min -15.9 -13.3 -47.0 -31.4

std dev 7.3 14.4 14.0 16.6

CONCLUSIONS A progressive analysis of transverse surface deflection profiles was carried out at 28 cross sections as each of the four tunnels was constructed (112 profiles). While settlement did occur in some areas, the majority of deflection profiles observed revealed heaving behavior. An analysis of the incremental profiles, i.e., deflections due to individual tunnels, showed that both settlement and heave profiles exhibited Gaussian response. The majority of the fitted Gaussian surface deflection profiles induced by tunnel YL excavation were centered above YL and exhibited trough width parameters i and K that are consistent to slightly greater than those reported in the literature. The observed width parameters i and K for incremental deflection caused by tunnels A, D and BC deviated significantly from those reported in the literature for single tunnels in greenfield conditions. Specifically, trough widths were found to be 3-4 times greater than expected for single tunnels at these depths in granular soil. In addition, the incremental deflection profiles for the subsequent tunnels A, D and BC were not aligned directly

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above the respective tunnel. The results show that one cannot assume superposition of deflection profiles from individual tunnels driven in undisturbed ground as a method to estimate cumulative deflection profiles. These findings illustrate that the prediction of deflections due to the second, third and fourth tunnels in a closely-spaced multi-tunnel environment is not simple. The stress field in the ground is altered by each tunnel excavation, and excavation of a subsequent tunnel through an asymmetric stress field will yield asymmetric deformation. In this project, the heterogeneous geological conditions at many cross sections likely contributed to the results. ACKNOWLEDGEMENTS The authors thank the New York City MTA for the opportunity to work with the project data. The authors also thank a number of individuals for their help, namely Chris Fagan (Geocomp) and Robert Duran, Robert Godinez, and Richard Moore (all from Colorado School of Mines). REFERENCES Chapman, D.N., Ahn, S.K. and Hunt, D.V.L. (2007) “Investigating ground movements caused by the construction of multiple tunnels in soft ground using laboratory model tests,” Canadian Geotechnical Journal, 44, 631-643. Cording, E.J. and Hansmire, W.H. (1975) “Deflections around soft ground tunnels,” General Rep. 5th Pan American Conf. Soil Mech. and Found. Engineering, Session IV, 571-632. Ercelebi, S.G., Copur, H. and Ocak, I. (2011) “Surface settlement predictions for Istanbul Metro tunnels excavated by EPB-TBM,’ Environ. Earth Sci., 62, 357-365. Mair, R.J. and Taylor, R.N. (1997) “Bore tunneling in the urban environment,” Proc. 14th Intl. Conf. Soil Mechanics and Foundation Engineering, 4, 2353-2385. New, B.M. and O’Reilly, M.P. (1991) “Tunnelling induced ground movements; predicting their magnitude and effects,” Proc. 4th Intl. Conf. on Ground Movements and Structures, Cardiff, invited review paper, Pentech Press, 671-697. Peck, R.B. (1969) “Deep excavations and tunneling in soft ground,” Proc. 7th Int. Conf. Soil Mechanics & Foundation Engineering, State of the Art Volume, Mexico City, 225-290. Robinson, B. and Wehrli, J.M. (2013a) “East Side Access – Queens bored tunnels case study,’ Proc. 21st Rapid Excavation and Tunneling Conference, Washington, D.C., 1014-1041. Robinson, B. and Wehrli, J.M. (2013a) “East Side Access – Queens bored tunnels engineering challenges,’ Proc. 21st Rapid Excavation and Tunneling Conference, Washington, D.C., 1086-1118. Suwansawat, S. (2006) “Superposition technique for mapping surface settlement troughs over twin tunnels,” Proc. Intl. Symp. on Underground Excavation and Tunnelling, Bangkok, Thailand. Suwansawat, S. and Einstein, H.H. (2007) “Describing settlement troughs over twin tunnels using a superposition technique,” J. Geotechnical & Geoenvironmental Engineering, ASCE, 133(4), 445-468.