Richard Hanke Making It Sound for Seattle's U-Link
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GROUTING Making it sound for Seattle’s U-Link Richard Hanke, PE, of Malcolm Drilling Company, Figure 1: Map showing the U-Link tunnels and contracts Phaidra Campbell and Colin Lavassar, PE, of Jacobs Associates, and John Sleavin PE, of Sound Transit, discuss the ground improvement program required for the construction of Seattle’s University Link (U-Link) U230 contract, to assist with tunneling operations at multiple locations THE COMPLEX GEOLOGY and congested tunnels from the Capitol Hill Station (CHS) underground utilities of Seattle’s Capitol Hill south to the existing rail within the have posed some of the greatest challenges downtown transit tunnel on Pine Street on Sound Transit’s University Link (U-Link) (Figure 1); a 550ft (168m) long station on light rail U230 tunneling contract, part of the Capitol Hill; and a TBM retrieval shaft at the city’s developing Link Light Rail system. The existing Pine Street Station and Stub Tunnel U-Link project will extend Seattle’s existing (PSST). The Capitol Hill Station interfaces with light rail service from the downtown core of another component of the U-Link project, the the city to the University of Washington via U220 Contract, at the north end, along East Capitol Hill. The extension consists of a series John Street, between Broadway and 10th of tunnels that are being built under multiple Avenue. The U220 Contract includes contracts. This article focuses on one of the construction of twin tunnels between Capitol project’s major components, the U230 Hill and the University of Washington to the Contract, which includes construction of twin north. NORTH AMERICAN TUNNELING JOURNAL 19 GROUTING Tunnels enter and exit at both ends While EPBMs can provide continuous of the Capitol Hill Station (CHS), support to the native soils along the requiring construction of two large majority of the tunnel alignments, it is ground improvement zones to assist difficult to maintain face pressures at with TBM launch and retrieval. Two the break-in and break-out locations smaller ground improvement zones at the CHS and PSST sites, hence the were also needed to receive the TBM need for localized ground at the Pine Street Stub Tunnel (PSST). improvements. The ground improvements were The primary geotechnical concern achieved by constructing overlapping at the break-in and break-out large-diameter jet grout columns to locations was that material may flow provide a solid mass of strengthened around the TBMs and into the impermeable ground. The following excavated box as mining progresses. article explains the need for ground This uncontrolled loss of material can improvement and examines several result in subsurface voids and large major components of the U230 surface settlements. These concerns Contract ground improvement were partially addressed by program: surface and site conditions, dewatering the soils within the tunnel initial design considerations, zones, but layers of saturated soil or construction challenges, quality perched water could still exist. control measures, verification testing, Therefore, additional safety was and in situ conditions post ensured by providing ground construction. improvement at each break-in/break- out location with the primary goal of Subsurface and site conditions reducing the permeability of the The geology at the north end of the native soils. CHS consists of glacial till and diamict The design considerations of the (Qvd) from ground surface elevation two jet grout blocks at the CHS were 327ft (100m) to elevation 300ft relatively similar. The dimensions of (91m); nonglacial fluvial deposits the blocks were calculated based on (Qpnf) between elevation 300ft (91m) the anticipated strength of the jet- to 260ft (80m); and underlying layers grout improved soils, the size and of nonglacial and glacial lacustrine deposits fill (Af) from ground surface elevation 176ft length of the TBMs, and the pillar width (Qpnl and Qpgl). The fluvial deposits are (53.5m) to elevation 140ft (42.5m); landslide between the twin tunnels. The blocks were generally described as slightly silty, gravelly (Qls) and wetland deposits (Qw) between 50ft high, 80ft long and 40ft wide (15m sands to sandy gravels and will flow when elevations 140ft (42.5m) and 115ft (35m); high, 24m long and 12m wide), normal to wet, even under low hydrostatic heads. The and underlying layers of nonglacial lacustrine the tunnel alignment. The dimensions of the fluvial deposits are below the static deposits (Qpnl) and Pre-Vashon diamict blocks provided approximately 20ft (6m) of groundwater table and a dewatering system (Qpgd). The U230 tunnels will break into the improved soils above the tunnels and 10ft is necessary to allow the excavation to PSST excavation between elevations 124ft (3m) below and to the sides. The width was progress. This dewatering system consists of (37.5m) and 104ft (31.5m), and the tunnel based on assumed TBM lengths of 30ft (9m) deep wells around the perimeter of the box eyes should be within the landslide and and the desire to have at least 10ft (3m) of and vacuum well points in the northwest glacial lacustrine deposits. These deposits the final tunnel lining sealed and securely corner of the excavation, where the largest have relatively high fines content and are grouted within improved soils before the ground water inflows are expected. The TBM anticipated to be firm to slow raveling when cutterhead of the TBM breached the safety of drives break into the CHS between elevation unsupported. While the static groundwater the improved zone. A secondary goal of the 263ft (80m) and 283ft (86m). table was estimated to be at elevation 104ft ground improvement at the CHS was to The subsurface conditions at the south end (31.5m), ground improvement was still improve the performance of the excavation of the CHS box varied considerably compared required at both break-in locations. support systems to limit shoring to those at the north, in that the layer of displacements (see Figure 2 for the locations fluvial deposits was much thinner. At the Design considerations & rationale of the jet grout zones at the CHS site). south end of the CHS box the soils consisted The U-Link tunnels are being mined with While the design considerations for the of glacial till and diamict from ground surface earth-pressure balance (EPB) TBMs (see p7). two CHS jet grout blocks were similar, the elevation 325ft (99m) to elevation 270ft (82m); fluvial deposits from elevation 270ft Figure 2: Locations of Jet Grout Zones for the Capitol Hill Station (CHS) (82m) to 265ft (80.5m); and underlying layers of glacial lacustrine deposits. Despite the relatively thin layer of fluvial deposits, ground improvement was still required, and the dimensions of the improved block were identical to those used at the north. The primary reason for ground improvement in this zone was uncertainty about ground conditions because of the highly variable nature of the given geology. The subsurface conditions at the Pine Street Stub Tunnel (PSST) consist of artificial 20 NORTH AMERICAN TUNNELING JOURNAL GROUTING two zones posed significantly different levels mode. The southbound tunnel jet grout without the need to consider reducing of risk. The south zone is within the confines geometry was also controlled by the existing injection rates and energy levels. The six of the U230 CHS construction site and is not deep sewer line that ran perpendicular to the interconnected test columns were installed adjacent to any major buildings or streets. alignment. During the design phase this with specific attention given to observation However, the north block is constructed utility was to remain operational during the of grout return specific gravity and beneath the public right-of-way on East John production of the ground treatment; communication between adjacent fresh Street and is within 25ft (7.5m) of a large however, this restriction was lifted during the columns. Intercommunication between apartment building. The north zone thus construction phase. adjacent columns installed fresh-on-fresh is posed a greater geotechnical challenge, it is During the design and tendering phases, the first form of feedback in terms of also the interface between the U220 and jet grouting operations were not envisioned erosional performance or geometry achieved. U230 contracts. As such, the challenges to be permitted to encroach onto Pine Street Continual monitoring of grout return specific associated with the tunnel break-in at the or on the city sidewalk. Using 3D modeling, gravity eventually yields a site-specific north end of the station were considered to typical locations and angles for the jet grout database with respect to soil type and depth be much greater than those at the south. columns were determined, as shown in that can later be correlated to column The construction of the north jet grout Figure 4. However, these restrictions were diameter. block was also complicated by the presence of multiple buried and overhead utilities and Quality control measures Figure 4: Model of possible locations and high traffic volumes on East John Street. angles of jet grout installation at the In situ wet samples were taken from various Although the jet grouting operation could PSST depths of the fresh jet grout columns on a encroach within the sidewalk of East John daily basis. A soilcrete retrieval tool was Street traffic lanes had to remain open. A 3D fashioned onto the drill string and lowered model was subsequently created to into a fresh column to obtain a discrete determine the potential locations and angles sample at a given depth. Samples were cast of the jet grout columns that would be into 2in (50mm) by 4in (100mm) cylinders for necessary to avoid the utilities, stay within the testing of unconfined compressive strength project site, and achieve the desired level of (UCS) per ASTM D4832 and determination of improvement, as shown in Figure 3.