Conveyor Belt Weigh Scale Measurements, Face Pressures, and Related Ground Losses in EPBM Tunneling

Robert Robinson Shannon & Wilson, Inc., Seattle, Washington Richard Sage Sound Transit, Seattle, Washington Rob Clark Shannon & Wilson, Inc., Seattle, Washington Edward Cording University of Illinois, Urbana-Champaign, Illinois Peter Raleigh Jacobs Associates, Seattle, Washington Clement Wiggins Parsons Brinckerhoff, Inc., Miami, Florida

ABSTRACT: For 95% of the 1,310 m (4,300 ft) long Beacon Hill twin tunnel alignment, muck volumes were within acceptable limits. However in 5%, along a reach of mixed granular and cohesive soils, just east of the mined station, excessive excavation volumes resulted in the formation of nine large cavities and disturbed soil zones. These were discovered 18 months later beneath thick till layers using borings guided by muck conveyor belt weigh scale data. Excess muck volumes from the scale data correlated within 2% of the total grout volumes plus delineated disturbed soil volumes. Large ground losses correlated well with EPBM plenum pressures that were less than ambient groundwater and earth pressures.

INTRODUCTION

The introduction of closed-face tunneling machines in the 1970s has greatly enhanced the ability to construct ever larger tunnels, in more difficult soil and groundwater conditions, at increasing depths. Earth pressure balance machines (EPBMs) and slurry pressure balance machines (SPBMs) have generally been successful in significantly reducing ground losses around the shield and the resulting surface settlement volumes, often to less than 0.5% of the theoretical excavated volume. However, the use of these innovative TBMs has not completely eliminated the potential for damaging ground losses and surface settlements. The designer and contractor must still assess the potential for excessive ground losses and the appropriate means for monitoring and adjusting TBM operating parameters to minimize ground losses. Ground losses around the shield are typically measured using widely-spaced deep settlement points, extensometers, and inclinometers. Surface settlement volumes, generally shaped like a Gaussian probability curve, are surveyed with settlement points.

Runs or inflows of soils at the tunnel face may cause localized ground losses that progress upward as chimney-shaped voids and loosened soil zones. With ground loss and settlement monitoring points spaced 10s to 100s of meters apart, it is unlikely that they will be positioned to effectively measure

localized large losses. Continuous measurement of the weight or volume of the excavated muck provides the best opportunity for assessing ground loss. For most past projects, muck volumes were visually estimated from fill levels and the number of muck cars, to an accuracy of +10 to 25 percent. Variations in volume were often attributed to changes in soil properties, bulking, and groundwater volumes, rather than excessive ground losses. For EPBMs, the use of conveyor belts to transport muck from the screw conveyor provides the opportunity to use conveyor belt weigh scales (scales) to provide considerably more accurate measurements of muck weights versus EPBM advance. Conveyor scales for the bulk material handling industry are typically specified to measurement accuracies of +0.1 to 1%. However, limited tunneling experience with scales, dirty wet working conditions, variable soil unit weight and water content, and frequent recalibration requirements has generally resulted in skeptical assessment of the data, thus relegating weigh scales to useful indicators of excess muck volumes.

Over 70 incidents of localized settlements exceeding 15 cm and isolated sinkholes have been reported on many otherwise successful EPBM and SPBM projects. These settlements have been attributed to localized construction challenges, changes in ground conditions, and operational deficiencies, as listed on Table 1. In most instances, operational deficiencies have included inadequate face pressures, suggesting that more emphasis is needed on assessing EPBM operational parameters that influence ground behavior critical to satisfactory tunnel construction.

Table 1 Interpreted causes of 47 sinkholes or settlement incidents deeper than 15 cm for EBPM tunnels (after BTS 2005, Shirlaw et. al. 2003)

Cause of Incidents Total Number

Comments

Problem during TBM repairs 7 3 during replacement of cutterhead tools 2 during maintenance of plenum 2 during maintenance of tail seals

Obstructions 4 4 unforeseen obstructions Ground 16 8 unforeseen ground conditions

7 mixed face conditions 1 poorly compacted fill

Over-excavation 16 11 after decision to lower plenum pressure to improve conditions 5 plenum pressure lower than had been instructed 15 of 16 instances of over-excavation attributed to operator error

Unknown causes 6 Mixed-face conditions 13 Various glacial deposits, soil over rock, decomposed

granite, alluvium Inappropriate technical decisions 13 Contractor decision to reduce face pressure to

enhance soil conditioning, reduce cutterhead wear Exit/entry to launch/target portals 11 11 at launch or reception portal due to poor eye seals,

alignment problems, and TBM pressure reduced because of insufficient reaction

THE BEACON HILL PROJECT

The Beacon Hill Project is a 1.6 km (1 mi) long segment near the center of the 50 km (31 mi) long Sound Transit Link alignment currently in various stages of design and construction from SeaTac Airport on the south, Everett on the north, and Bellevue to the east of Lake Washington. The Beacon Hill Project consists of 1,310 m (4,300 ft) of twin tunnels that pass east-west through the 90 m (300 ft) high glacially sculpted Beacon Hill (Figure 1). The tunnels are primarily in glacially over-consolidated hard clayey till and hard fractured and slickensided lacustrine clay with interbeds of very dense silt, sand, and gravel. The project includes a 121 m (400 ft) long mined station with twin platform tunnels constructed by the sequential excavation method. Borings for instrumentation, dewatering wells, and geologic probe holes drilled prior to construction defined the extent of layers and lenses of dense sand and gravel within the hard till and clay to the east end of the station, prompting a 26m (88ft) westward shift of the station. The GBR indicated groundwater levels about 25 m above the station, but construction borings showed perched water levels about 11m above tunnel crown at the east end of the station.

Figure 1. Plan and Geologic Profile of the Tunnels

Sequence of Tunnel Construction

The twin tunnels were excavated with an earth pressure balance machine (EPBM) (Figure 2) and supported with 1.5m (5 ft) long rings of bolted, gasketed, concrete segment lining (Redmond, 2007). The specifications required that the EPBM be continuously operated in closed-face mode at a minimum 0.34 MPa (50 psi) face pressure to counteract groundwater and soil pressures. The EPBM was launched January 2006 from the west portal for the southbound (SB) tunnel, reconditioned at the station, continued

to the east portal, partially disassembled, transported to the west portal, and relaunched July 2007 for the northbound (NB) tunnel. The EPBM was equipped with a data acquisition system to monitor, collect, and transmit data from over 200 sensors used to measure operating parameters including: soil conditioner volumes, face pressures at 5 sensors (Figure 3), and other EPBM sensors. The contractor also installed two dynamic conveyor belt weigh scales (scales) to monitor muck weight on the trailing gear conveyor versus TBM advance. Scale data was collected from a display screen in the operators cabin.

Figure 2. Mitsubishi Earth Pressure Balance Machine 6.5 m (21.1 ft) Diameter

The western 545 m (1,800 ft) of both tunnels were primarily in hard till-like soils with fairly uniform scale data indicating negligible volume losses, to an apparent accuracy of +/- 5% per day, as shown on the left half of Figure 4. Surface settlements were consistently less than 6 mm (0.25 in.) at centerline survey points and borehole extensometers, reflecting surface volume losses of less than 0.25%. With EPBM re-launch from the mined station, the scales indicated highly fluctuating values, reflecting up to three times the normal muck volumes over the initial 90 to 110m (300 to360ft) of both tunnel alignments (middle of Figure 4). The contractor interpreted the scales to be erroneous, possibly related to the re-launching process, and initiated recalibration, maintenance, and eventual replacement of both scales, with no visible effect on the erratic data. At about 110 m (360 ft) east of the station the scales again reflected normal muck flows of about 99 tonnes (110 tons) per ring for the remainder of both tunnels.

Additional surface surveys, borehole instrumentation measurements, and contact liner grouting were initiated at the request of Sound Transit due to the fluctuating scale data. Follow-up surveys of houses, streets, and utilities indicated less than 10 mm (0.4 in.) of total surface settlement with no related damage. One of the four borehole extensometers in this area indicated higher than normal deep settlements of up to 150 mm (6 in.) at 10 m (30 ft) above the tunnel, prompting injection to refusal of an additional 5 m3 (6.5 cy) of grout through the tunnel liner.

Figure 3. Five Pressure Sensors in Cutterhead Muck Chamber of EPBM

Down the rabbit hole?

Eighteen months after relaunching the EPBM from the station on the NB alignment, a homeowner one block east of the station noticed a rabbit-sized hole in the flower bed near her front porch. Inspection showed this hole to be the top of a 7 m (23 ft) deep, 7.6 m (25 ft) diameter dome-shaped cavity in sand and till located within a meter of centerline of the 35 m (110 ft) deep tunnel. The cavity was immediately filled with 178 m3 (231 cy) of controlled density fill (CDF) as directed by Sound Transit. A rapid-response investigation into possible causes of the large cavity was initiated by Shannon & Wilson, Inc. and Dr. Ed Cording. A review of operational data from the EPBM indicated an 8 m (25 ft) long erratic scale zone beneath the location of the cavity, with a calculated excess muck volume of 399 tonnes (443 tons) or 200 m3 (260 cy), shown on the Figure 4 plot of excess ground losses. Review of scale data revealed 9 possible over-excavation zones totaling 4,893 tonnes (5,394 tons), or 2520 m3 (3,296 cy) of ground loss above the two tunnels, in excess of the normal 110 tons of muck excavated per 1.5 m (5 ft) of advance, as shown on Figure 5.

Phased Ground Loss Exploration and Grouting

A phased exploration and grouting program was initiated to locate other possible cavities and loosened soil zones and remediate the ground losses with various forms of grouting. Table 2 presents the quantities

Figure 4. Muck Conveyor Belt Weigh Scale Output for Both Alignments of placed grout, delineated disturbed zones, and a resulting estimate of the bulking attributed to ground loss. Phase 1 - The initial investigation consisted of 6 borings located at zones of excessive scale values. The first boring, 15 m (50 ft) east of the station on NB centerline, drilled thru 12.7 m (42 ft) of hard till and into an 11 m (37 ft) deep dry void underlain by disturbed soil . The void was backfilled with 458 m3 (595 cy) of CDF, indicating an average 7.3m (24ft) diameter cavity, with a volume equal to 101 percent of the excess scale data of 773 tonnes (1,005 tons) or 454 m3 (590 cy). With this initial verification of the weigh scale data, Sound Transit authorized 26 additional borings, and a trial micro-gravity geophysical survey to rapidly assess the extent of the ground losses. Borings were drilled primarily along tunnel centerlines where ground losses were most likely to occur. The micro-gravity survey guided several borings to the north of the SB alignment to locate void SB-1 that had drifted northward. Voids were generally capped by till layers 5 to 24 m (17 to 80 ft) below ground surface. Most voids were dry, as indicated by drilling fluid losses and downhole camera observations. The 9 predicted ground loss areas were located and backfilled with CDF totaling 1957 m3 (2,544 cy) or 77% of the theoretical volume. Angle holes drilled beneath houses were much less successful in locating ground losses that had apparently progressed upward to indeterminate elevations. Five check borings drilled adjacent to the tunnels in high ground loss areas encountered very dense to hard soils within 2 to 3 m (5 to 10 ft) of the tunnel sidewalls, where soils over the tunnel crown were disturbed (loose to medium dense), showing that large ground losses likely originated in the tunnel face and progressed upward over time.

Phase 2 Compaction grouting was implemented to locate and grout the remaining 558 m3 (725 cy) of possible ground loss. The grout holes were spaced 4.5 m (15 ft) along tunnel centerlines, where there was the best chance for encountering voids and disturbed soil zones. Holes were drilled vertically down to 4.5 m (15 ft) above the tunnel, and angled holes were drilled beneath 3 houses above tunnel centerlines. No additional voids were encountered along the NB alignment. A 3 m (10 ft) high void was encountered at a depth of 21 m (70 ft) along the SB alignment, and was backfilled with 45 m3 (59 cy) of CDF in ground loss zone SB-2. Grout equivalent to 9% of the anticipated total ground loss volume was injected

Figure 5, Excess Muck Volumes Along Both Alignments

at pressures of 2 to 4 MPa (300 to 600 psi) as shown on Table 2. Based on scale data, 14% of the ground losses remained unaccounted for.

Phase 3 Verification borings were drilled to assess the effectiveness of prior grouting, locate and treat any remaining ground losses, and provide a 3-D definition of disturbed soils over the tunnels. A 12 m (40 ft) wide pattern with exploration borings spaced 3 m (10 ft) apart was laid out along both alignments. Phase 3 progressed in 5 steps of increasing complexity, public inconvenience, and cost. The results of each step were assessed to determine the need to progress to the next step. Step 1 involved vertical mud rotary borings in truck-accessible areas. Step 2 involved inclined sonic-core holes beneath residences. Step 3 involved vertical holes drilled with hand portable rotary drill rigs, between houses, in back yards, off public right-of-way and on steep streets. Step 4 involved portable rotary drilling inside basements of three residences. Step 5 involved compaction grouting, which was eventually considered unnecessary.

Phase 3 placed 149 m3 (115 cy) of grout and more importantly defined the distribution of disturbed zones of loose to medium dense soils totaling over 7,770 m3 (10,100 cy) above both tunnels, capped with hard glacial till, and grout. The 3 phases of grouting and CDF placement totaled 2,257 m3 (2,952 cy) of the estimated ground loss. Phase 3 identified zones of disturbed soils, shown on Figure 6. Bulking factors for very dense granular soils that have been disturbed to a medium dense to loose condition have been found to range from 2 to 8%, averaging about 4%. An average 4% bulking accounts for about 308 m3 (400 cy) of lost ground resulting in 7,770 m3 (10,100 cy) of disturbed ground. Figure 7 also shows that the disturbed zones have been capped with zones of CDF or grout that inhibits upward migration of disturbance into overlying

Table 2 Cumulative Grouting and Bulking Volumes

NB Tunnel Alignment

SB Tunnel Alignment Total

Weigh Scale Loss 1,913 cy 1,383 cy 3,296 cy

No. of

Holes Volume,

cy. No. of Holes

Volume, cy.

Volume, cy.

Cumulative Volume,

cy. % Fill Factor

Phase 1, Placed 21 1,632 17 912 2,544 2,544 77.2%Phase 2, Placed 26 87 34 206 293 2,837 86.1%Phase 3, Placed 47 71 61 44 115 2,952 89.6%

% Fill 93.6% 84.0% 89.6%

Approximate Delineated Disturbed Ground 5,015 5,120 10,100 Estimated Originating Ground Loss (4%) 130 161 291 Total Volume Accounted For 94 1,920 112 1,323 3,243

% Volume Accounted For 100.4% 95.7% 98.4%

soils. The soils overlying the grout-filled voids generally consist of glacial till that prevented the upward migration of the voids. The 3 phases of grouting accounted for 89.6% of the ground loss, and the identified disturbed zones account for an additional 8.8 % of ground loss, assuming 4% bulking, and thus accounting for 98.4% of the total ground loss.

EPBM Operation Parameters

EPBM operational data for the SB tunnel indicates that for 90 to 105 m (300 to 350 ft) east of the station, face pressures of 0 to 0.12 MPa (0 to 17.4 psi) were typically used, substantially less than the specified minimum 0.34 MPa (50 psi). In this tunneling interval, at the end of each mining cycle, the measured earth pressures in the plenum frequently climbed, and in some cases more than doubled, during the ring-build stage, indicating that the external earth and water pressure exceeded the plenum pressure. Piezometers east of the station measured groundwater pressures of 0.12 MPa (17.4 psi) above tunnel crown, while EPBM plenum pressures, measured by the upper pressure sensor during mining for the 16 rings east of the station, were less than 0.06 MPa (7.8 psi) or only 50% of what was necessary to adequately balance only the groundwater pressure. As noted earlier, the BTS (2005) and Shirlaw (2003) have noted that ground losses may occur when TBM face pressures are not sufficient to counterbalance earth and groundwater pressures. Guglielmetti (et. al. 2007) recommends that TBM face pressure should exceed groundwater head plus active to at-rest earth pressure with some margin of safety. Review of ground conditions encountered east of the station indicates that face pressures of at least 0.3 MPa

Figure 6. Profile of CDF-Filled Voids and Disturbed Soils Along Southbound Tunnel

(43.5psi) would have been needed to preserve groundwater and soil stability. The launch/reaction frame anchorage within the station failed during launching of the EPBM on

the SB alignment, thus limiting the thrust that could be applied by the TBM until sufficient grouted liner was placed. The contractor subsequently minimized face pressures and associated thrusts during launching of the EPBM on the NB alignment from the station. However, the upper plenum pressure sensor indicates pressures under 0.13 MPa (18.9 psi) during tunneling for 82 m (270 ft) east of the station. In a full face of hard clay or till, such low pressures may not result in ground loss, however much of this length of tunnel contained sand layers and lenses, which would likely flow if not adequately supported. Figure 7 illustrates that ground losses increased significantly when face pressures were less than about 80 % of the ambient face pressure during ring building and for more prolonged shutdowns.

CONCLUSIONS

Well installed and maintained conveyor belt weigh scales are likely accurate to better than + 5% and may be more accurate than +2%, as demonstrated at Beacon Hill, when averaged over several rings. However, in situ soil unit weights, quantities of conditioners, changes in moisture content and variations in geology must be known to accurately interpret the scale data.

Operating the EPBM at or above ambient pressure will minimize settlement and decrease the risk of forming subsurface voids.

Forensic analysis of TBM data can be an effective means of determining the sources of suspected over excavation and necessary remedial methods.

Figure 7. EPBM Plenum Pressure and Conveyor Belt Scale Summaries

Ground improvement methods may be needed at launching and receiving portals to compensate for limited thrust reaction and face pressures insufficient to balance external earth and soil pressures.

Voids caused by over-excavation resulted from operation of the EPBM in a partially closed mode with insufficient face pressure.

Lack of surface settlement does not mean over-excavation is not occurring. Very dense materials when disturbed will bulk and compensate for some ground loss.

RECOMMENDATIONS

EPBMs should be equipped with at least two belt weigh scales fully integrated into the TBMs data acquisition system with scale data displayed at the operators console and logged continuously.

Belt scale measurements should be frequently compared with field classified and weighed muck samples of known volume.

Plots of at least the last 8 hours of face pressure sensor data should be continuously updated on the operators console to determine that the TBM is consistently operated above the ambient pressures.

Halt tunneling operations, investigate, and take corrective actions if any critical parameters are not as required or expected. Effective use of an EPBM requires continuous assessment of TBM operating conditions and measured ground behavior.

REFERENCES

British Tunneling Society (BTS), 2005, Closed-Face Tunneling Machines and Ground Stability A Guideline For Best Practice, Thomas Telford Books, London

Guglielmetti, V., Grasso, P., Mahtab, A, and Xu, S. 2007, Mechanized Tunneling in Urban Areas Design Methodology and Construction Control, Taylor & Francis Group, London, 507 pgs.

Redmond, S., Tattersall, S., Garavelli, N., Kudo, T., Raleigh, P., and Lehnen, M., 2007, Driving the Twin Running Tunnels by Earth Pressure Balance Machine on the Sound Transit C-710 Beacon Hill Station and Tunnels Project, Proceedings of the Rapid Excavation and Tunneling Conference, Toronto, Canada, June 2007, p. 793-807.

Robinson, R., Clark, R., Sage, R., Cording, E., 2011, Searching for Cavities on Beacon Hill, North American Tunneling Journal, June/July, p. 14-18

Shirlaw, J.N., Ong, J.C.W., Rosser, H.B., Tan, C.G., Osborne, N.H., and Heslop, P.E., 2003, Local settlements and sinkholes due to EPB tunneling, Proceedings of the Institution of Civil Engineering, Geotechnical Engineering 156, October2003, Issue GE 4, p 193-211