Modelling of ground deformation control induced by slurry shield tunnelling

Modelling of ground deformation control induced by slurry shield tunnelling Zili Li 1, Jacob Grasmick 1, Mike Mooney 1 1 Center for Underground Construction & Tunnelling, Colorado School of Mines, mooney@mines.edu ABSTRACT Ground deformation induced by soft ground tunnelling can cause consequential damage to existing infrastructure if proper controls are not achieved. Highly controlled pressurized face tunnelling combined with tail-shield segment void grouting has yielded significant reduction in ground surface settlement. In some projects, such as the East Side Access Queens Bored tunnels, near zero ground surface deformation was achieved through proper control of support pressures. To investigate TBM control of ground deformation, 3D soil-fluid coupled finite element analysis was conducted with detailed modelling of the construction sequence (such as face and annulus pressure, as well as grout and segment behavior). The findings derived from finite element modelling and the comparison against field measurements identified critical ground deformation mechanisms during pressurized-faced tunnelling. Results show that both the face pressure applied at the cutterhead and the slurry annulus pressure in the shield skin void control the ground deformation during the passage of TBM. INTRODUCTION Tunnelling in soft ground can induce considerable ground deformation that poses a risk to existing infrastructure. To control ground deformation, pressurized-face tunnel boring machines (TBM) (e.g., slurry shield and earth pressure balance (EPB)) are increasingly employed in urban areas. The effect of support pressures applied at the tunnel face and circumference have an important influence on mitigating ground deformation. Classical analysis of ground deformation induced by open-face tunnelling in a greenfield environment (no buildings) suggests that a Gaussian-shaped longitudinal and transverse deflection profile develops at the ground surface centred above a single tunnel (Peck, 1969). Compared to conventional open-face tunnelling, pressurized-face tunnelling generates a set of support pressures at tunnel face (face pressure), along the shield skin (annulus pressure) and behind the shield tail outside the lining segments (grouting pressure). The applied support pressures act against the total earth pressure (effective stress and pore water pressure) along the TBM and tunnel boundary, and consequently control ground deformation. Although some practical experience has been accumulated from many industry projects (Wongsaroj, 2005, Bezuijen et al., 2005, Dias & Kastner, 2013), the effect of the specific applied face, annulus and grouting pressures on ground behavior has not been fully investigated (Kasper & Meschke, 2006). This paper begins with a brief background to the East Side Access Queens bored tunnels in Sunnyside yards in Queens, New York. Based upon construction records, finite element (FE) analysis was conducted to investigate 3D ground deformation during slurry shield pressurized-face tunnelling and the results compared against field measurements. The FE model will focus on critical TBM parameters face pressure, annulus pressure and grouting pressure, examining their influence on ground settlement. The findings allow us to identify critical factors of pressurized-face tunnelling on ground response, which can be useful for deformation control and TBM operation. GROUND CONDITIONS AND TBM 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. The project is described in detail by

Robinson & Wehrli (2013a,b). Along the tunnel alignment, the ground conditions consist of highly variable glacial till soils and outwash deposits, which include various sandy soil with some small lenses of clay, silt and gravel present. At this site, there is a complex rail network on the ground surface that remained in service throughout construction. To monitor track movement, a sizable array of settlement monitoring points was established on the ground and on the train tracks including over 500 automated motorized total station (AMTS) survey prisms. AMTS data were collected as frequently as ten times per day. The broad array of monitoring locations and measures are described in Mooney et al. (2014) and Grasmick et al. (2015). Similar to conventional open-face tunnelling, the ground deformation induced during pressurized-face shield tunnelling occurs at three sections (e.g., Thewes and Budach, 2009) as shown in Figure 1: 1) Face: at the tunnel face section, slurry forms a filter cake in front of the cutterhead that acts against the lateral effective earth pressure and pore water pressure. The contact force of the cutting tool is assumed insignificant (Festa et al., 2012) such that the face pressure is similar to the slurry pressure measured in the excavation chamber, a reasonable assumption (Bezuijen and Talmon, 2014). In practice, insufficient slurry pressure may cause ground deformation or even face collapse (active failure), whereas excessive pressure may cause blow-out of the soil near the TBM. 2) Shield Annulus: the TBM shield diameter tapers from the face to the tail leaving a gap between excavated ground and the shield skin. For the Queens bored tunnels TBMs, the cutterhead diameter was 6865 mm (assuming no cutterhead tool wear) and tapered to 6814 mm at the tail with a maximum soil-shield gap of 25.5 mm. Bentonite slurry from the working chamber flows into and generally fills the annulus, creating annulus pressure. In addition, grout injected from the tail shield may flow forward into the soil-shield gap, which also generates an annulus pressure around the radial shield gap (Nagel & Meschke, 2011). Provided the annulus pressure is smaller than the surrounding earth pressure, ground around the annulus may deform into the gap and consequently lead to ground surface displacement. 3) Lining Segments: the placement of lining segments inside the tail shield leaves a gap between the excavated ground and the lining extrados. In the Queens bored project, this gap was 130 mm. The soil-liner void is filled with grout during each ring advance. In many projects including the Queens bored tunnels, the grout is often mixed with an accelerant upon exiting the tail shield for rapid curing. Ground deformation may occur if the grout pressure is less than the total applied earth pressure or if grout voids occur. Figure 1: Schematic of Slurry Shield TBM and pressure components (a portion of image courtesy of Herrenknecht)

FINITE ELEMENT MODEL To simulate ground deformation during slurry shield tunnelling, a soil-fluid coupled FE model was developed using ABAQUS TM 6.12 (ABAQUS Inc., 2012). In this study, one area along the first tunnel excavated (tunnel YL) is modelled in the FE analysis (Sta. 192+80 1960+75); future research will consider the effect of multiple tunnel interaction on the ground deformation. Considering symmetry about the x-z plane at y = 0, a one-half FE model was developed with zero transverse displacements (y axis) at y = 0 and y = 180 m, and zero longitudinal movements (x axis) at x = 0 and x = 180 m, respectively (Figure 2). The top model boundary (z = 0) was set to be free, whereas the vertical movement at the bottom boundary (z = 45 m) was fixed. For brevity, the ground profile was idealised as a homogeneous, highly permeable granular soil. The dense sand was firmly packed by glacial till and has a high friction angle of 40. For this model, average soil parameters determined from the geotechnical data report at this section of the alignment were used (Table 1) while the initial pore water pressure was assumed hydrostatic with the water table at 6 m below the ground surface (Robinson & Wehrli, 2013a,b). The soil was modelled using 20-node triquadratic displacements, trilinear pore pressures and reduced integration solid elements that produce a realistic soil stress-strain profile (Wongsaroj, 2005). A linear elastic, perfectly plastic Mohr-Coulomb constitutive model was employed with non-associative flow (dilation angle = 10 o ). The cover was 19.05 m, rendering the cover to diameter ratio (C/D) equal to 2.8.

Figure 2: Finite element model of slurry shield TBM tunnelling Table 1: Assumed homogenous soil properties Stratum Sandy soil Dry unit weight, γ (kn/m³) 16 Saturated unit weight, γ (kn/m³) 20 Young s modulus, E (MPa) 200 Poisson s ratio 0.3 Friction angle, ( ) 40 Dilation angle, ψ ( ) 10 Coefficient of lateral earth pressure at rest, K0 0.5 Hydraulic conductivity, k(m/s) 3 10-2 In the longitudinal direction of the tunnel alignment, model element lengths equal the ring width of 1.524 m within the tunnel excavation zone, whereas a coarse mesh was employed at the far boundary. The soil along the tunnel alignment is indicated by the light shaded (yellow) elements, the segment liner by the dark shaded (blue) elements and the annulus grout along the tunnel circumference is represented by the slight lighter shaded (brown) elements. The excavation process was modelled as a repeated sequence of (Ⅰ) deactivation of ring elements to be excavated that are within the TBM diameter (D = 6.9 m) at ring i and creates a shield length section (P = 9.14 m, a length of six rings), and (Ⅱ)

placement of 1.524 m long tunnel linings next to the existing tunnel lining at ring i - 6 surrounded by backfill grouting pressure (see Figure 2.d). The support pressures at the face, shield annular gap and liner gap were modelled based on measured tunnel YL pressures. Figure 3 presents the slurry pressure p SL and grout pressure p G (interpreted at the springline). The box corresponds to the portion of the alignment where the construction sequence was modelled and the FE deflection results compared to the observed surface deformation. As evidence by Figure 3, the slurry pressure p SL remains relatively constant between chainage 1925+00 and 1960+00 with an average pressure of 220 kpa. Assuming slurry flows from the tunnel face into the soil-shield gap, an annulus pressure resulting from slurry pressure is distributed at the shield circumference (Nagel et al., 2011). The grout pressure varies significantly between 150 and 350 kpa (with spikes to 500 kpa). For computational simplicity, an average grout pressure of 328 kpa at the springline is applied along the tunnel boundary within one ring behind the TBM and hardens at the next excavated ring. Both the slurry and grout pressures increase from crown to invert and the gradient was modelled assuming a unit weight of 12 kn/m³ and 21 kn/m³ for the slurry and grout, respectively (see D, E for slurry pressure and F for grout pressure in Figure 1). This tunnel excavation model sequence was continued until the tunnel face reached 122 m (80 rings) into the model, whilst a monitoring plane was assumed 61 m (40 rings) away from the start of the excavation. Figure 3: Measured TBM slurry and grout pressures (at tunnel YL springline) The reinforced concrete tunnel lining was modelled using 8-node, double curved thick shell elements. To account for relatively flexible segmental joints, a bending stiffness reduction factor equal to 0.64 was applied to the continuous ring based upon Muir Wood's formula (Wood, 1975). The annulus grout was modelled as annulus pressure at ring i - 6 (see Figure 2d) along the tunnel circumference and later converted to solid elements after ring i - 7. The details of tunnel structure geometries and material properties are provided in Table 2. At the tunnel boundary, the lining and soil / grout elements shared the same nodes with no-slip contact. In summary, the 3D soil-tunnel-fluid coupled model consists of 25572 elements and 139485 nodes. Table 2: Summary of lining and grout properties Specification Reinforced concrete lining Backfill grout Young's modulus (MPa) 20,000 1000 Poisson's ratio 0.2 0.2 Tensile strength (MPa) 3 2 Compressive strength (MPa) 30 20 Thickness (m) 0.229 0.15

FE RESULTS AND FIELD MEASUREMENTS Figure 4 shows the surface settlement directly above the tunnel profile (y=0) recorded by AMTS at one monitoring point during tunnel excavation. The total station used for the AMTS survey had an angle measurement accuracy of 1º which corresponds to 1.5 mm for this monitoring point. In the figure, Δx = 0 represents the arrival of TBM face at the monitoring plane, while the distance of the TBM behind and ahead the monitoring plane is indicated as - Δx and + Δx, respectively. All of the measurements in Figure 4 were recorded at one surface location. The array of data from -50 m < Δx < 50 m is created from repeated measurements as the TBM approached and passed. The field data are within 3.0 mm indicating wellcontrolled ground deformation during construction. The observed scatter in the data is consistent with the precision uncertainty of the AMTS. The surface settlements are very small and at levels that were inconsequential to the project. We select data from this monitoring point because there is a distinct albeit small change in deformation as the TBM passes. Using a slurry pressure gradient equal to 220 kpa at the springline and a grout pressure gradient of 328 kpa at the springline, the FE model predicts the ground settlement as plotted within the measurement band in the figure. The ground starts to deform when Δx = -30 m, and the deformation gradually increases until TBM moves about 20 m ahead (Δx = 20 m). The majority of the settlement occurs within -10 m < Δx < 10 m, suggesting that the slurry face and annulus pressures are the cause for the slight settlement as the TBM passes the monitoring plane. After the TBM is 10 m past the monitoring plane, the rate of settlement is drastically reduced. These observations are generally consistent with those in Grasmick et al. One significant difference between modelled and observed deformations is that settlements in the FE model appear to occur sooner (Δx = -30 m) than the typical measured settlement (Δx ~= -15 m) (Grasmick et al. 2015). This can be contributed to the fact that soil plasticity was not accounted for in the FE model, resulting in a wider settlement trough. The results verify that the FE model can reasonable match the observed deformation and confirms that the ground deformation can be reasonably limited within several millimetres provided the TBM parameters (e.g. slurry pressure and grouting pressure) are wellcontrolled. Figure 4 Comparison between FE results and experimental data

INFLUENCE OF TBM PARAMETERS ON GROUND DEFORMATION Face Pressure and Annulus Pressure In practice, there are a number of empirical approaches used to estimate the required slurry pressure. A widely used rule of thumb approach is where the required face support equals the active lateral earth pressure, pore water pressure, or a combination of the two, plus a nominal safety margin as summarised by Kanayasu et al. (1995) and Grasmick et al. (2015). According to the rule of thumb summarized above, face pressure adopted in tunnel YL (220 kpa) approximately equals to the geostatic total lateral earth pressure of 228kPa (lateral effective pressure 63 kpa + water pressure 165 kpa) at the springline, and thus successfully limited ground deformation. The slight deformation observed is possibly a result of the slightly lower slurry pressure at the crown of the face and shield annulus compared to the total vertical stress as discussed in detail in Grasmick et al. As a parametric study, two extreme slurry pressure conditions are examined to further evaluate the effect of slurry pressure on ground deformation, assuming grouting pressure remains constant as 328 kpa: 1) face pressure = water pressure + 20 kpa (i.e. 185 kpa = 165 kpa + 20 kpa) as the lower pressure bound; 2) face pressure = geostatic total earth pressure (water pressure + lateral geostatic effective earth pressure assuming K 0 conditions and no arching) (i.e. 308 kpa = 165 kpa + 143 kpa) as the upper bound. Figure 5 compares the ground settlement under different slurry pressure cases. Note that the applied slurry annulus and slurry face pressures are tied together given the assumed continuity between the excavation chamber and annulus. The longitudinal settlement profiles remain similar, whereas the higher slurry pressure reduces the settlement. The decrease in settlement observed for Δx > 20 m is a numerical artefact, i.e., a result of the transition from grout pressure to grout properties. This net heave is not a realistic phenomenon, albeit the major findings from the FE results still hold. Results suggest that at this site, the lower bound pressure may be adopted to ensure tunnel stability as long as the induced settlement is within allowance. On the other hand, the upper bound pressure limits the ground deformation within 1 mm for sensitive structures nearby, but it in turn requires greater machine thrust for excavation. Grouting Pressure Figure 5: The effect of slurry pressure on ground deformation At the shield s tail, grout is injected into the tail void to control the ground deformation around the liner gap. In the FE model, the tail gap is assumed to be fully filled with grout, whereas the grouting pressure is varied from 278 kpa to 398 kpa, a reasonable fluctuation. Figure 6

compares the ground deformation under different annulus pressures assuming slurry face and annulus pressures remain equal to 220 kpa. The higher grouting pressure reduces the ground deformation behind the tunnel face (Δx > 20 m), although the effect is negligible. This further supports the observation that slurry pressures at the face and shield annulus are the primary culprit for the slight ground deformation observed. Similar to the effect of annulus slurry pressure described before, the unloading of grouting pressure behind the shield tail (Δx > 20) allows the soil under the tunnel to heave and thus results in ground surface net heaving. However, this is a numerical induced result and not realistic. The higher grouting pressure induces more net heaving, since the soil heaves more after unloading of the high pressure. Figure 6 The effect of grouting pressure on ground deformation CONCLUSIONS A 3D finite element model was developed to investigate the tight control of ground deformation observed during slurry shield tunnelling in Queens Bored Tunnel project. The FE model considers TBM parameters such as face pressure, annulus pressure and grouting pressure during pressurized-face tunnelling, and the computed results are in line with the field data. Furthermore, the observed ground settlement trends suggest that the face and annulus slurry pressure is the likely cause for the slight deformation observed as further supported in Grasmick et al. Further analysis is needed to isolate the effect of the slurry pressure at the face and annulus relative to the vertical and lateral earth pressures to determine the exact causes for the slight settlement. In addition, the FE model was used to explore the influence of TBM parameters on the ground deformation. Results from the parametric study underline the relative importance of the effect of slurry face pressure and slurry annulus pressure. The FE analysis results suggest that moderate changes in grouting pressure (± 50 kpa), assuming the tail void is entirely filled with grout and that the grout cures to full strength by the time excavation of the current ring is completed, does not significantly influence the ground deformation. ACKNOWLEDGEMENTS The authors would like to thank the Metro Transit Authority (MTA) for providing us with data for the analysis.

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