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Paper TA-T1-104

King County Uses New Shaft Technology on the Ballard Siphon Project

Ade Franklin, Project Manager, King County Wastewater Treatment Division, Seattle, WA Marty Noble, Project Representative, King County Wastewater Treatment Division, Seattle, WA Jeremy Johnson, PE, Resident Engineer, Jacobs Associates, Seattle, WA Doug Genzlinger, PE, Project Manager, Tetra Tech, Seattle, WA Kim Staheli, PhD, PE, President, Staheli Trenchless Consultants, Inc. John Fowler, Project Principal, James W. Fowler Company. As part of a trenchless project in Seattle, Washington, the King County Department of Natural Resources and Parks – Wastewater Treatment Division has achieved a North American first by utilizing the Herrenknecht vertical shaft machine (VSM) to construct a 29.5-foot-diameter (9 m), 156-foot-deep (47.5 m) shaft, which will act as the tunnel boring machine launch shaft. The general contractor, James W. Fowler Company, elected to utilize the VSM to successfully complete the excavation and segmental lining of the shaft as an alternative to its originally planned slurry wall method. Excavation was completed in-the-wet through fill and overconsolidated glacial till, with groundwater at approximately 15 feet (4.6 m) below the ground surface. This paper provides a brief overview of the geotechnical conditions and design considerations for the shaft and structure, outlines the use and capabilities of the VSM, describes the construction technique and installation progress achieved. Also provided is a summary of lessons learned on the project with regard to the use of the Herrenknecht VSM. 1. INTRODUCTION

The Ballard Siphon Replacement Project (BSRP) is located in Seattle, Washington, USA; and is being undertaken by the King County Department of Natural Resources and Parks – Wastewater Treatment Division. The north project site is located in Seattle’s Ballard neighborhood, while the south project site is located in the Interbay neighborhood. The tunnel connecting the two project sites crosses beneath the Lake Washington Ship Canal (Canal), which is a US Army Corp of Engineers maintained navigable channel connecting Lake Washington with the Puget Sound. The BSRP is being constructed to rehabilitate two existing 70-year-old wooden sewer pipes, which cross beneath the Canal at Salmon Bay. In addition, the BSRP will provide additional capacity to the sewer system in order to protect the water quality of the Canal, and to allow for future growth in the North Seattle area. The major project construction activities include:

An 89-foot-deep (27 m) shaft, Forebay Structure, and Regulator Structure addition at the north project site A 134-foot-deep (41 m) shaft, Afterbay Structure, and Junction Structure at the south project site Slip-line rehabilitation of the two existing 36-inch (915 mm) internal diameter (ID) wood-stave pipelines

that currently carry wastewater across the Canal A new 1,977-foot-long (603 m) tunnel connecting the north and south sites, which will be finished with an

88.5-inch (2,250 mm) ID final lining

North American Society for Trenchless Technology (NASTT) NASTT’s 2013 No-Dig Show

Sacramento, California

March 3-7, 2013

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Figure 1 provides a project overview.

Figure 1, Project Overview The shaft located on the south project site (South Shaft) is the focus of this paper. During construction, this shaft was used to launch and stage the tunneling activities. In its final configuration, the shaft will house piping to convey wastewater from the tunnel to the new Afterbay and Junction structures. Project team members are as follows: General Contractor James W. Fowler Company (JWF) South Shaft Design Brierley and Associates Construction Management Jacobs Associates Project Design Tetra Tech Landau Associates Staheli Trenchless Consultants 2. SOUTH SHAFT GEOTECHNICAL CONDITIONS

The Geotechnical Baseline Report (GBR) for the project was completed by Landau Associates, in cooperation with design team members Tetra Tech and Staheli Trenchless Consultants. It was anticipated that four different geologic units would be encountered during construction of the South Shaft: Holocene Fill, Vashon Advance Outwash, Pre-Fraser Interglacial Deposits, and Pre-Fraser Slickensided Deposits. The Holocene Fill unit was encountered to a depth of about 8 feet (2.4 m), and consisted of loose sand and silty sand, and medium stiff sandy clay. The Vashon Advance Outwash Deposits were present from about 8 feet to 18 feet (2.4 to 5.5 m) below the ground surface, and consisted of medium dense, silty, fine to medium sands with gravel. Pre-Fraser Interglacial Deposits were encountered for the next 70 feet (21 m) of excavation, from about 18 feet to 88 feet (5.5 to 27 m) below the ground surface. This glacially overconsolidated unit consisted of interbedded layers of very dense granular (sand and silty sand) and hard cohesive (silt and clay) materials. The final 70 feet of excavation were completed through the Pre-Fraser Slickensided Deposit. This glacially overridden deposit was similar to the finer-grained portions of the Pre-Fraser Interglacial unit above, consisting of very stiff to hard clay containing very small sand-filled slickensides. Groundwater was anticipated to be about 16 feet (4.9 m) below the ground surface. Vibrating wire piezometers around the shaft indicated the groundwater levels were stable throughout shaft construction at about 20 feet (6.1 m) below the ground surface.

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3. SOUTH SHAFT DESIGN

The South Shaft provides temporary ground support to allow the tunnel and Afterbay Structure to be built; however, it is not part of the permanent facility. Because of this, King County specified that the shaft be contractor-designed. The Contract Drawings specified the shaft center point and the tunnel invert elevation, and the Contract Specifications listed several acceptable construction methods suitable to the ground conditions, including caisson, slurry diaphragm wall, and ground freezing. The contractor was free to choose the other variables, including the shaft diameter. 3.1. Contractor Design

JWF originally planned to construct the 136-foot-deep (41 m) (minimum finished depth) South Shaft using slurry wall techniques, assuming a shaft outside diameter (OD) of 58 feet (17.7 m). However, cost disputes with its slurry wall subcontractor, blamed on the delay between the contract bid and award dates, forced JWF to choose a different construction technique. Several options were investigated to construct the shaft, including ground freezing. Eventually, a 29.5-foot (9 m) ID caisson was chosen, to be installed using Herrenknecht’s vertical shaft machine. 3.2. Vertical Shaft Machine

The vertical shaft machine (VSM) is a mechanized piece of equipment developed by Herrenknecht to excavate shafts in a variety of soil or rock conditions. The type used on the Ballard Siphon project is designed for soil applications, and is capable of excavating a shaft with a 29.5 foot ID (Figure 2).

Figure 2, Herrenknecht VSM Machine at the BSRP South Shaft

Current VSM capabilities include shafts with internal diameters ranging from 18 feet (5.5 m) to 39.4 feet (12 m), and depths of up to 280 feet (85 m). This was the first time a VSM had been used in North America, so to ensure the shaft would be installed by qualified and experienced personnel, JWF partnered with Herrenknecht to complete the shaft. The soil version of the VSM is essentially a modified caisson sinking operation, with the modifications consisting of how the shaft is excavated, and how the shaft lining, or caisson, sinks. 3.2.1. Ring Beam

The first step in building a shaft using the VSM is construction of the ring beam, which is essentially a foundation to support the VSM. Because of the VSM machinery supports the entire weight of the caisson during shaft installation, the ring beam is much more robust than the typical guide walls used for slurry panel or secant pile operations. The ring beam for the South Shaft had a rectangular cross section, with a depth of 6 feet (1.8 m), and a width of 5.75 feet (1.75 m). Eight #8 steel bars ran circumferentially around the outside face of the ring beam; nine or eleven #8

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bars were present around the inside face, depending on location; six or eight # 8 bars ran around the top of the beam, depending on location; and ten or thirteen #11 bars were present around the bottom of the beam, depending on location. These were all tied together with #6 ties at 5.5-inch (140 mm) spacing around the entire circumference. The design required concrete with a 5,000 psi (34.5 MPa) 28-day strength. The internal diameter of the ring beam was 32.47 feet (9.9 m), allowing for a 1.95-inch (49.5 mm) gap between the caisson outside diameter and the ring beam internal diameter.

3.2.2. Shaft Excavation

Shaft excavation is accomplished with a mechanized excavation arm attached to the bottom of the shaft lining system (Figure 3). The excavation arm is centrally located in the shaft, and attached to the shaft walls at six locations via three gripper arms where embedded steel plates are fitted with shoes to allow the excavator to be raised and lowered as needed. The excavation arm is capable of rotating 180 degrees around the shaft, and pivots from the center of the shaft towards the edge. A horizontal rotating cutterhead (Figure 4), very similar to what is found on a typical roadheader, is positioned at the end of the excavation arm. As the soil is excavated, it is hydraulically lifted as slurry to a soil separation plant located at the ground surface. Because of the hydraulic lifting of the shaft muck, excavation is completed in the wet. Following a trip through a typical soil separation plant, relatively soil-free water is recirculated back to the shaft to maintain the desired water level. A major advantage of the VSM excavation arm is its capability to excavate beyond the outside diameter of the shaft lining. This is useful during shaft sinking to significantly reduce the friction levels along the outside of the shaft lining, allowing caisson installations at greater depths, and in harder soils, compared to traditional caisson installation methods. This overexcavation capability also has significant advantages related to the design and installation of structural invert slabs, which is discussed below. The VSM used on the Ballard Siphon project was capable of excavating about 19.5 inches (500 mm) beyond the shaft lining extrados.

Figure 3, VSM Excavation Assembly Figure 4, VSM Excavation Cutterhead

3.2.3. Shaft Sinking

Caisson sinking is another VSM modification. Traditional caissons have a cutting shoe at the bottom of the caisson, and the shaft sinks via controlled bearing failures as material is excavated from within the shaft adjacent to the cutting shoe. Bentonite is often injected along the outside of the shaft to keep friction forces low enough to allow the shaft to continue sinking. Shaft depth is a limitation for traditional caisson installations because of increasing friction along the outside of the caisson as it sinks deeper, and the difficulty of maintaining shaft verticality at large depths. Soil hardness is also a limitation for traditional caissons because of the need for controlled bearing failures at the cutting shoe in order to keep the shaft sinking. The caisson used with the VSM still has a cutting shoe at the bottom; however, the overexcavation capability of the VSM means the shaft sinking does not entirely rely upon it. Overexcavation below the cutting shoe negates the need for the localized bearing failures at the shoe to keep the shaft sinking, allowing caisson installation in harder soils.

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The overexcavation beyond the shaft outside diameter reduces the friction forces that develop along the outside of the shaft, allowing for deeper caisson installations. The VSM excavation arm cuts beneath the caisson cutting shoe; therefore, the caisson itself must be independently supported during the entire installation process. This is achieved by four large hydraulic jack assemblies (Figure 5), which sit at the ground surface and contain cable bundles that attach to steel shoes embedded within the bottom of the caisson lining system. The cable bundles travel from the bottom of the caisson to the surface hydraulic jacks along the outside of the lining system, support the caisson during the excavation cycle, and then lower the caisson incrementally as needed. The four jacks operate independently, and are used in conjunction with inclinometers mounted inside the caisson lining to control the verticality of the shaft during installation. This allows for much greater control of shaft verticality than with traditional sinking methods. Throughout excavation, three surface winches (Figure 6) hold cables that attach to the VSM excavation assembly. These winches are capable of lifting the excavation assembly if maintenance or repairs are required, and they retrieve the excavation assembly following completion of shaft excavation. Raising and lowering the excavation assembly is relatively quick, as remotely operated pins release the gripper arms from the steel support shoes, and the entire assembly travels along a simple guide system installed inside the caisson as the shaft is sunk.

Figure 5, VSM Hydraulic Caisson Jack (1 of 4) Figure 6, VSM Excavation Assembly Winch (1 of 3) 3.3. Shaft Lining

The caissons walls installed with VSM can be constructed with precast concrete segments, or in lifts with cast-in-place concrete as is common for typical caisson construction. JWF chose to use the precast segmental lining, constructed using forms supplied by Herrenknecht. The segments were cast by Bethlehem Pre-Cast at its Cashmere, WA facility. Segment dimensions were predetermined, based on the geometry of the existing Herrenknecht forms. Each of the 45 segment rings had an ID of 29.5 feet (9 m), a height of 3.28 feet (1 m), and contained four individual segments. The segments were 15.8 inches (400 mm) thick, and contained a continuous EPDM rubber gasket around the perimeter to prevent groundwater intrusion at the segment boundaries. Segment rings were attached across the horizontal joints by eight (two per segment) equally spaced and continuous 0.7-inch (18 mm) steel rods, joined by couplers at the joints. To aid with segment installation, the circumferential face of each segment also contained three centering dowels, similar to the type used with tunnel segmental lining systems. Individual segments were connected to each other across the vertical joints by two angled 0.9-inch-diameter (24 mm) bolts. Finally, each segment contained a 1-inch (25 mm) grout port near its center point to facilitate bentonite injection during shaft sinking, and grout injection during shaft completion. Almost all of the segment rings were constructed with steel fibers, which comprised the majority of the concrete reinforcement. Steel bar reinforcement was only used around the lifting lugs to allow the segments to be removed

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from the forms prior to the concrete reaching its full design strength. The concrete 28-day compressive strength was 6,000 pounds per square inch (psi) (41.4 MPa), and a dosage rate of 65 pounds per cubic yard (38.6 kg/m3) of concrete was required for the steel fibers. Segment rings #1, #3, and #5 (numbered from the bottom of the caisson up) contained traditional reinforcement necessitated by the additional loads imposed upon them. Ring #1 served as the caisson cutting shoe, while rings #3 and #5 contained the steel embeds that ultimately supported the VSM excavator assembly. 3.4. Shaft Invert

Hydrostatic uplift is very often a concern for deep shaft invert slabs. The capability of the VSM to excavate below the caisson and its cutting shoe, and beyond the outside diameter of the caisson walls, allows unique design options for an invert slab that can resist significant hydrostatic uplift forces. Long-term depressurization of the groundwater surrounding the South Shaft was not an option, so a structural invert slab capable of resisting about 140 feet (42.7 m) of hydrostatic pressure was necessary. In order to accomplish this, the excavation for the invert slab was extended below the caisson bottom and past the caisson outside diameter, aided by relatively stable soils at the shaft bottom and the wet excavation method. The radial motion of the VSM excavation arm meant the invert slab excavation was shaped like an inverted dome, with the excavation bottom at the center of the shaft extending approximately 10 feet (3 m) below the caisson cutting shoe, and the excavation bottom at the caisson wall extending about 5 feet (1.5 m) below the cutting shoe. The maximum thickness of the completed concrete invert slab was 13 feet (3.9 m) at the shaft center point because of the invert concrete extending several feet into the shaft lining. The invert slab excavation also extended approximately 19 inches (500 mm) beyond the outside diameter of the caisson walls for the full shaft perimeter. This plug geometry allowed the invert concrete to encapsulate the bottom of the caisson lining, forming an inverted “champagne cork” shaped plug capable of resisting the hydrostatic uplift forces. The invert slab overpour also aided the completed shaft structure in resisting the hydrostatic uplift forces, and the concrete encapsulation of the caisson bottom proved very effective in limiting groundwater intrusion into the shaft at the base. The monolithic shaft invert was unreinforced, constructed using 5,000 psi strength (34.5 MPa) concrete, and was placed in the wet to maintain stability of the shaft bottom. 4. CONSTRUCTION

Construction of the South Shaft began on January 31, 2012, and the shaft was fully completed by around the end of June 2012. The following sections chronicle the major activities involved in the shaft construction. Figure 7 shows the duration of shaft construction in calendar days.

Figure 7, South Shaft Construction Duration

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4.1. Ring Beam

As detailed in Section 3, the first step in building a shaft using the VSM is construction of the ring beam, which is a foundation to support the VSM. The ring beam for the South Shaft had a rectangular cross section, with a depth of 6 feet (1.8 m), and a width of 5.75 feet (1.75 m). Ring beam construction took 22 calendar days, beginning on January 31, 2012, and ending on February 22, 2012. 4.2. Setup And Calibration

Setup for the VSM operation began by installing five segmental lining rings, necessary because the VSM excavation machinery is supported by rings #3 and #5. The bottom two rings sit within the ring beam, while the top three extend above it (see Figure 2 above). Following assembly, the excavation unit was lowered into the completed segmental rings, the necessary services were attached, and the excavation equipment was calibrated and tested. Figure 8 shows the VSM excavation arm during calibration and testing.

Figure 8, VSM Excavation Arm during Calibration

The complete setup and calibration process lasted 56 calendar days, beginning February 23, 2012, and ending April 18, 2012. Within that period, ring installation and VSM assembly took about 35 calendar days; attachment of the excavation machinery to the segments and connection of the required services (cables, hoses, power, etc.), required about 7 calendar days; and testing and calibration took 14 calendar days. 4.3. Shaft Excavation

Production shaft excavation began on April 19, 2012, and finished on May 17, 2012, requiring 29 calendar days (25 working days at 6 days per week) to excavate and install 40 rings and excavate for the invert slab, totaling about 140 feet (42.7 m) of excavation. This is an average of 5.6 feet (1.7 m) per working day; however, excavation for the invert slab took longer than expected because of the time necessary to separate the clay spoils from the shaft water as discussed below. Installation of three segmental rings (9.8 feet [3 m]) was achieved on most of the production excavation days. The primary issue encountered during shaft excavation was the ability of the soil separation plant to effectively remove the clay from the water prior to it being returned back to the shaft, due to its fine-grained nature. JWF’s soil separation system consisted of two 10,000 gallon (38,854 L) slurry tanks, one vertical clarifier, one Derrick flo-line primer, three Brandt LCD2 shakers, one Cobra shaker with 15-inch (380 mm) cones, one Derrick DE 7200 centrifuge, and two Brandt CF2 centrifuges. It was discovered early that excavation had to be stopped if the specific gravity of the slurry leaving the soil separation plant exceeded about 1.2, because at that value just as much material was being returned to the shaft as

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was being excavated. There were multiple days when the separation plant was left running after hours to remove material from the shaft water, and when excavation had to be stopped because of high slurry specific gravity readings. This issue became more pronounced as the shaft got deeper and the volume of water in the shaft increased, explaining why the excavation for the invert slab took longer than expected. Despite this issue, 140 feet (42.7 m) of shaft were excavated and lining installed in less than a month. One other minor issue came up during shaft excavation. At a depth of approximately 70 feet (21.3 m), very dense granular soils, or possibly a large boulder or zone of nested cobbles, were encountered. It was difficult to determine exactly what was being encountered by inspecting the cuttings. This zone was present for several feet, and caused the slurry intake pipe at the bottom of the shaft to become clogged. The clog was quickly cleared in less than 4 hours by winching the excavation machinery to the surface. Neither the VSM machinery nor the shaft lining system suffered any damage as a result of encountering this zone, and production was not significantly impacted other than by the short delay to clean the clogged intake pipe. 4.4. Invert Placement And Shaft Completion

Preparation for placement of the structural invert slab included removal of the VSM equipment and cleaning of the water within the shaft, both of which happened simultaneously. Removal of the VSM equipment, including all equipment except for the hydraulic jacks and cables still supporting the caisson, was completed in approximately four days. Following VSM removal, an access platform was placed across the shaft and two tremie pipes were installed on either side of the platform, both of which were used during placement of the invert slab concrete. One day prior to concrete placement, sonar measurements were taken of the invert slab excavation to confirm that it had not collapsed. The entire preparation process required 14 calendar days, and the invert slab was placed on June 1, 2012. A total of 459 cubic yards (350 m3) of concrete were placed over approximately 5 hours and 20 minutes, using two tremie pipes and two pump trucks. The final step in construction of the South Shaft involved grouting around the caisson lining, unwatering the shaft, and cleaning the muck from the shaft bottom. Because of difficulties in effectively removing the fine-grained soil from the shaft water, approximately 8 to 10 feet (2.4 to 3.0 m) of very loose material had settled out on top of the invert slab and had to be removed. Grout was pumped through several grout ports embedded into the segmental lining near the bottom of the shaft prior to the shaft unwatering. This entire process was completed in approximately 28 calendar days; however, it could have been completed in a much shorter time if it would have been critical to the schedule. Figure 9 shows a view of the completed shaft.

Figure 9, Looking Down the Completed South Shaft

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5. SUMMARY AND LESSONS LEARNED

The Herreknecht VSM proved to be an effective and efficient method for excavating the South Shaft on the Ballard Siphon Project. No major problems were encountered during shaft excavation, and the 29.5-foot diameter (9 m), 156-foot deep shaft (47.3 m) (134 feet [41 m] deep in final configuration) was installed in 150 calendar days. The shaft could have been completed even faster if the schedule had required it, and if this had not been the first time the VSM was used locally. In addition, the completed shaft verticality was very accurate, and groundwater infiltration into the completed shaft was minor, at 2 to 3 gallons per minute (7.6 to 11.4 liters per minute), despite the shaft being subjected to about 115 feet (35 m) of groundwater head. The following is a summary of observations and lessons learned from the Ballard Siphon South Shaft installation:

The Herrenknecht VSM is an efficient method for installing deep caissons in difficult soil conditions.

The capability of the excavation assembly to excavate below and beyond the outside diameter of the caisson lining allows for unique solutions to designing a structural invert slab to resist hydrostatic uplift forces.

Effective soil separation is critical to maintaining shaft excavation rates.

The 150 calendar day installation time could have been reduced by about 30-plus days, had the schedule required.

6. REFERENCES

King County Department of Natural Resources and Parks – Wastewater Treatment Division. 2009. Ballard Siphon Replacement Project Rebid Contract Documents, Contract C00507C10, Seattle, WA, US.