This Compilation of TAC Papers was prepared courtesy of

SEM in Seattle - Design and Construction of the C710 Beacon Hill Station Tunnels

Michael Murray

Hatch Mott MacDonald

Stephen Redmond

Obayashi Corporation

Richard Sage

Sound Transit

Franz Langer

Dr. Sauer Corporation

Don Phelps

Hatch Mott MacDonald

Abstract:

Contract C710 is currently under construction as part of Sound Transit's Link Light Rail

connecting downtown Seattle with Sea-Tac Airport. The paper describes the design and

construction of the deep mined station under Beacon Hill using the Sequential Excavation

Method (SEM), also known as the New Austrian Tunneling Method (NATM).

The 55 m (180 ft) deep binocular station includes platform, concourse, cross-passage and

emergency ventilation tunnels together with station egress and ventilation shafts.

The paper describes the geotechnical conditions anticipated and encountered, and the

development of the design from the preliminary design stage through the construction stage.

Following the construction of a Test Shaft during the final design stage, it was realized that the

ground conditions would be difficult, so provision was made for further geotechnical

investigations and ground improvement from the surface during the construction stage.

The construction methods and design details are strongly influenced by the need to ensure safety

during construction. Excavation sequences include twin-sidewall and single-sidewall drifts. A

range of pre-support measures and 'tool-box' items are made available and adopted as necessary.

Details are included on the rates of progress achieved in the safe and successful tunnel

construction to date.

Keywords: Puget Lowlands; Cascade Mountains; Olympic Mountains; test shaft; collector

tunnels (VECP); platform shift; exploratory drilling; jet grouting; dewatering wells; slurry wall

shaft and headhouse; head house and shaft excavation; SEM Construction; barrel vault

installation; platform tunnels; soft ground SEM tunnelling; water-charged glacial deposits.

Murray, M., Redmond, S., Sage, R., Langer, F., Phelps, D. SEM in Seattle - Desgin and Construction of the C710 Beacon Hill Station Tunnels. 2006 Tunnelling Association of Canada Proceedings.

SEM in Seattle - Design and Construction of the C710 Beacon Hill Station Tunnels

Michael Murray Hatch Mott MacDonald

Stephen Redmond Obayashi Corporation

Richard Sage Sound Transit

Franz Langer Dr. Sauer Corporation

Don Phelps Hatch Mott MacDonald

ABSTRACT: Contract C710 is currently tmder construction as part of Sotmd Transit's Link Light Rail cotmecting downtown Seattle with Sea-Tac Airport. The paper describes the design and construction of the deep mined station tmder Beacon Hill using the Sequential Excavation Method (SEM), also known as the New Austrian TUtmeling Method (NATM).

The 55 m (180 ft) deep binocular station includes platform, concourse, cross-passage and emergency ventilation tuunels together with station egress and ventilation shafts.

The paper describes the geotechnical conditions anticipated and encotmtered, and the development of the design from the preliminary design stage through the construction stage. Following the construction of a Test Shaft during the fmal design stage, it was realized that the grotmd conditions would be difficult, so provision was made for further geotechnical investigations and ground improvement from the surface during the construction stage.

The construction methods and design details are strongly influenced by the need to ensure safety during construction. Excavation sequences include twin-sidewall and single-sidewall drifts. A range of pre-support measures and 'tool-box' items are made available and adopted as necessary.

Details are included on the rates of progress achieved in the safe and successful tuunel construction to date.

I INTRODUCTION

Sound Transit (ST) is constructing the 26 km (16 mile) long light rail line from downtown Seattle southwards to Sea-Tac Airport. By 2009 modern low­floor light rail cars will run along the light rail route with a maximum speed of 88 km/h (55 mph). The light rail line will run at street level, on elevated trackways as well as underground. The travel time from Westlake, downtown Seattle, to the Airport will be 36 minutes.

Contract C7l 0 is the only mined tunuel section and is located just south of the downtown area. In addition to the construction of approximately 1.6 km ( one mile) long twin-bored running tunuels and a deep mined station under Beacon Hill, the contract also includes 800 m (one half mile) of aerial structure and an elevated station at the eastern end. Obayashi

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Corporation was awarded the construction contract in June 2004 at a contract price ofUS$280M.

The 1300 m (4,300 ft) long twin running tunnels under Beacon Hill are being mined by an Earth Pressure Balance Tunnel Boring Machiine (EPB­TBM) supplied by Mitsubishi Heavy Industries in Kobe, Japan.

The deep mined Beacon Hill station is being built from a one-square-block site located at the intersection of Beacon Avenue South and McClellan Street South. Future passengers will access the Beacon Hill station by high-speed elevators that transport them 49 m (160 ft) down to the underground platforms.

2 DESIGN

In October 2000, a Jomt venture of Hatch Mott MacDonald and Jacobs Civil Inc. (HMMJ) was awarded a contract for the final design of the Beacon Hill Turmels (0710) segment of the project. Dr. G. Sauer Corporation (DSC) was awarded a sub-contract by HMMJ for the design of the concourse cross adits and platfoml turmels.

The underground station layout at contract award is shown in Fig.1 and consists of twin shafts and a complex configuration of vehicle, pedestrian and ventilation tunnels. The inverts of the platform tunnels are 49 m (160 ft) below ground surface. The platform turmels are 116 m (380 ft) long and spaced 45 m (146 ft) apart, center to center.

Fig. I. Station Layout at Contract Award.

The SEM was selected as a means of progressively excavating and supporting the ground. The specified sequences are designed to expose and stabilize the ground in limited incremental widths and heights. Standard support measures are used throughout and these include specified round lengths, fiber-reinforced flashcrete applied to newly exposed surfaces, lattice girders andlor steel arches installed at predetennined intervals, and reinforced shot crete support ranging from 20 cm to 30 cm (8 inches to 12 inches) in thickness, depending on the final opening dimensions.

The fmal lining consists of fiber reinforced in-situ concrete varying from 30 cm to 35 COl (12 to 14 inches) for the nonnal pedestrian access areas of the station turmels, platform tunnels and concourse cross ad its. For the remaining tunnels, fiber reinforced shotcrete varying from 20 cm to 40 cm (8 to 15 inches) is specified. Junctions are reinforced with steel reinforcement. A waterproofing system is installed between the initial and final lining consisting

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of a geotextile fleece and PVC membrane with an injection system.

2. 1. Geological Conditions Seattle is located within the central portion of the Puget Lowland, an elongated topographic and structural depression bordered by the Cascade Mownains on the east and the Olympic Mountains on the west. The lowland is characterized by a series of north-south trending ridges separated by deeply cut ravines and broad valleys, the result of glacial scouring and sub-glacial erosion. The area may have been subjected to six or more major glaciations with an ice thickness up to 915 m (3000 ft). The glacial and interglacial soil units are typically of limited lateral extent. A high degree of variation is evident locally to the extent that some lmits cannot be reliably correlated between adjacent borings.

Subsurface conditions were investigated in three different phases associated with the Conceptual Engineering (CE), Preliminary Engineering (PE), and Final Engineering (FE) stages of design. A total of 73 investigation borings were drilled specifically for the project during the period 1998 and 2003.

A simplified geologic model was developed for the Beacon Hill Twmel and included in a Geotechnical Baseline Report (GBR). The model groups the geologic units into six engineering classes having sinlilar physical and engineering properties as follows:

• Class I - Loose to Dense Granular Deposits • Class 2 - Soft to Very Stiff Clay and Silt • Class 3 - Till and Till-Like Deposits • Class 4 - Very Dense Sand and Gravel • Class 5 - Very Dense Silt and Fine Sand • Class 6 - Very Stiff to Hard Clay

The GBR was included as a contract docwnent, and was intended to assist bidders in evaluating the requirements for excavating and supporting the ground, and in preparing their bids. The baseline conditions presented in the report are used by Sowld Transit to evaluate any differing site condition claims.

2.2. Test Shajl In 2003, a 46 m (150 ft) deep Test Shaft was constructed within the design footprint of the Beacon Hill Station Main Shaft. The shaft was 5.5 III (18 ft) in diameter from the growld surface to 32 m (105 ft) below ground surface, and 1.8 m (6 ft) in diameter from 32 m to 47 m (105 to 153 ft) . The primary objectives of the Test Shaft were to confirm the nature of the ground and groundwater conditions during construction, and provide an opportunity for bidders to view the subsurface materials prior to bidding the project. The Test Shaft indicated the extreme

variability of the soils and groWldwater conditions which had a profoWld impact on the shaft and tWlnel design . A report on the Test Shaft was included in the contract documents in the Geotechnical Data Report.

2.3. Connector Tunnels - VECP The four 24 m (80 ft) long connector tlUmels positioned between the platform ttumels and the mOIling tWlnels were sized to accommodate the large transverse ventilation ad its (TV A) framing into them in addition to allowing the passage of the TBM through them. Obayashi proposed a Value Engineering Change Proposal (VECP) to drive three of the four SEM connector ttumels by the TBM, leaving behind as the final lining the pre-cast bolted one pass lining associated with such a TBM. It was proposed in the VECP to mine the two eastern connector ttumels and one of the western connector tunnels with the TBM thereby reducing a portion of SEM excavation of the station and also reducing the jet grout zones which would perhaps have required inclined drilling beneath a secondary arterial street Right-Of-Way.

This resulted in a redesign jointly developed with HMMJ for the jWlctions between the TV As and the mrming tWlnels by allowing the systematic removal of the segments and providing a SEM jWlction chamber. Also as part of the VECP, the west damper chamber (DC) required reconfiguring to accommodate its construction from the southwest TVA. Also the cast­in-place final lining for the remaining southwest connector tlUmel would be replaced by a steel fiber reinforced shotcrete (SFRS) final lining in the arch.

This VECP option seemed appealing initially. However, upon further review of the schedule, the advantages were overcome by events and the subject was dropped. At the same time that the VECP was being developed, additional information gradualIy became available from the post-award borings and indicated that the geological conditions at the east end of the station were more difficult than originally expected.

While the VECP solution would have reduced, but not completely eliminated large portions of jet grouting within the confmes of a city street in a quiet neighborhood, anything that could be done to reduce the quantity of jet grout would have substantial financial, political, and environmental benefits.

2.4. Platform Shift Out of the connector ttumel VECP was conceived the first Platfonu Shift design. The groWld to the west of the main shaft was fOWld to be much better (stiff clays and tills), so much so that the platfomls could be shifted 27 m (88 ft) to the west without drastic changes in the interior details of the station itself

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while at the same time utilizing the shafts (main and ancilIary) in their original locations (the slurry walIs had already been constructed). As part of the scheme, the west TV A was relocated further west and three additional angle drilIed probes were perfonned to confinu tbe suitability of the relocated position.

Further borehole information confinued that the shifted east DC and part of the east TV A were still partly located in sands. Again to avoid the necessity for further jet grouting from the surface or groWld improvement from within the shaft, a second scheme to shift the east TVA to al ign directly with the ancillary shaft was developed. This time the redesign would have the benefit of deleting the east DC by relocating the ventilation dampers into extended platform tWlnels. Further ventilation analysis confinned that this arrangement would be acceptable. As a result the west TV A was reconfigured similar to the east TV A with the dampers relocated to the extended platform tlUmels and a smaller jWlction chamber replaced tbe west DC which was no longer sized to house the dampers.

Mal"l Shall.

RunlWlO Tunnel

Fig. 2. Station Layout with Shifted Platroml Twmels

This design scheme displaced the VECP and was developed in stages by HMMJ with input from Obayashi and ST. This scheme (see Fig. 2) is being implemented by ST.

3 PRETREA TME T AND SHAFT CONSTRUCTION

3.1. Exploratory Drilling A subsurface drilIing program was specified for the construction phase by the designers, and the infonnation ga ined enabled the geotechnical interpretations to be rermed. Obayash; carried out more than 50 borings using mud rotary and son ic core

recovery methods between August 2004 and November 2005. These were used to adjust the extent of jet grouting and dewatering. For example, the geological profiles at the east sections of the northbound and southbound platform tunnels were revised to indicate a long thick section of sand present at the tunnel crown level. The borings were also used for the installation of the surface instrumentation including inclinometers, extensometers and piezometers. The piezometers indicated that there was approximately 15 m (50 ft) of water head in these sands which, if oot removed or the ground modified, would have resulted in a " flowing ground" condition upon excavation.

During SEM mining, systematic probing ahead of the face is carried out. Also for each twmel section, a horizontal probe hole is cored to augment the geotechnical interpretations. This information is reviewed during the daily SEM meetings.

Fig. 3. Aerial View of Beacon Hill Construction Site

3.2. Jet Grouting Jet grouting was carried out from the surface and targeted zones of sand within the tunnel profile as pretreatment for the SEM tunnels. Jet grouting was defined as the furnishing and installation of overlapping jet grouted coluoms to allow tunnel excavation with minimal water ingress and to provide a stable crown and face for excavation. The volwnes and locations of ground treatment were specified on the contract drawings and Obayashi was responsible for the design of jet grouting to achieve the required vol woes and perfomlances. The original contract indicated known areas requiring jet grout pretreatment as approximately 15 m (50 ft) along the west Longitudinal Ventilation Adit (L VA) and a 15 m (50 ft) zone of the east Damper Chamber (DC). Condon Johnson/Soletanche JV was awarded the subcontract in 2004. After some initial test colwllns late in 2004, the production work continued between January 2005 and December 2005. Inclined holes were originally

62

specified especially to alleviate restncttons of groutillg under Beacon Avenue at the west LV A and near private property to the east of the station. With the platfoml shift described earlier, this was no longer a concern and the columns were drilled vertically from within the staging area (see Fig. 3). The targeted column spacing was generally on a triangular grid of 1.5 m (4 .75 ft) centers. Predrilling was carried out using a Klemm 806 rig drilling to the top of the planned jetted zones. A Klemm KR 3012 drill rig with a 24 m (80 ft) mast and high pressure pwnps were used for the jetting (see Fig. 4). Each hole was surveyed with an inclinometer and results plotted to confinll there was no divergence. Construction quality testing included in-situ permeability testing and core recovery with associated compressive strength testing. Generally the strength results achieved were between 3 MPa (400 psi), the specified minimum, and 20 MPa (3000 psi). A total volwne of approximately 4200 m' (5500 yds') was injected using over 500 deep colunms. The majority of this treatment was to target the sands in the eastern sections of the northbound and southbound platform tunnels over a 46 III (150 ft) length and 18 m (60 ft) length respectively, with additional jet grouting perfonlled in the twmel breakout zones of the main and ancillary shafts. In cross section, the target area includes the sands within the tunnels and a zone at least 1.2 m (4 ft) in thickness outside of the tunnel initial lining in the sands.

Fig. 4. Vertical Jet Grouting Rigs

3.3. Dewaterillg Wells A system of vacuWJl-enhanced deep dewatering wells was specified to reduce the hydrostatic pressure where the twmel excavations were expected to encowHer penlleable soils below the water table. Sound Transit was responsible for the well system design. Obayashi are responsible for the proper installation, operation,

maintenance of the pumping wells and operation system. Wells are generally spaced 15 m (50 ft) apart along both sides of the twmels, and at depths between 33 m and 55 m ( 11 0 and 180 ft). A total of 39 wells and 10 observation wells were installed. As each group of wells was installed, pump tested and brought online, the drawdown effects were noticeable. Tbe pumps are two horsepower and have a capacity of 110 Vmin (30 gpm) PWllping against the heads described above. Total pumping volwne (steady state condition) generally ranges between 110 and 190 Vmin (30 and 50 gpm). The pumps are checked daily and maintenance is performed when necessary.

3.4. Slurry Wall Shaft and Headhouse Observations of the ground bebavior from U,e Test Shaft during the fmal design stage resulted in redesigning both the main shaft and ancillary shaft lining from SEM to using slurry walls.

The main shaft diaphragm is approximately 16 m (52 ft) in diameter and is 55 m (182 ft) deep. This work was perfomled by Soletancbe using a hydro­fraise machine mounted on a Liebherr crawler crane cutting a I m (3 ft-4 inch) thick panel. The ancillary shaft diaphragm is approximately 9 m (30 ft) in diameter and has a depth of 51 m (167 ft). This work too was done with the same hydro fraise used for the main shaft except the cutting wbeels were changed to cut a thirmer wall at 0.9 m (3 ft-2 incb).

The bentonite slurry transported the cutt ings to a separation plant complete with screens, cyclones, and centrifuge for return to the excavation. A Cat 320 was used to muck a pit constructed from tbe basement of one of the houses demolished to clear tbe site. Rebar cages were tied on site with block-outs for invert slab niches and with pipe sleeves for instrwnentation. The cages with their attacbments were lowered into U,e bentonite and suspended from a structure on U,e guide wa lls. Concrete trucks backed up to boppers setting on tremie pipes to deliver approximately 3500 m3 (4,600 yds3) of cOllcrete to the main shaft and 1800 m3 (2,300 yds3) to U,e ancillary shaft. Since the upper 18 m (60 ft) of circular main shaft slurry wa ll and of the circular anci llary sbaft slurry wall was to be demolished while the interior excavation of the beadhouse basements was being done, a lean mix was used in the upper reaches of the slurry wall panels.

The headbouse basement diaphragm wall was 0.8 m (2 ft-8 incb) thick and 19 m (62 ft) deep. This work was perforoled by Soletanche using a conventional cable grab mounted on a Leibherr crawler crane. The grab deposited the material directly into trucks queued on site. The work was orcbestrated so U,at some of the ancillary shaft headbouse wall panels were constructed while some of the main shaft panels were constructed.

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3.5. Head House and Shaft Excavalioll The main shaft was excavated using a Hitachi 330

Excavator witb breaker and Cat 320 for excavating and loading muck skips (see Fig. 5) . ine cubic meter (twelve cubic yard) muck skips were lifted to the surface and tipped into a muck bin at the collar using a Kobelco 2000 (200 MT) lattice boom crawler crane. The main sbaft diaphragm walls which extended through the beadbouse were demolished as the excavation advanced. This was all carefully choreographed with tbe installati on of several rows of multi-strand tie-backs which extended approximately 2 1 m (70 ft) into the surrounding growld.

After the headbouse excavation was complete, a cap beam approximately 1.8 m (6 ft) tall was foroled and poured, tying in dowels protruding from U,e top of the slurry wall panels. Soon after, the interior excavation of the circular shaft continued down to the bottom using the Cat 320 excavator and the same muck skips described above.

Upon reaching tbe bottom, the subgrade was excavated to a "dished" shape, rebar dowels were installed, and "submarine" style concrete invert was poured as a provisional shaft bottom. Upon reaching the design strength, the invert was backfilled with spoi ls to develop a working platform for commencement of the break-in to the SEM twmels.

Fig. 5. West Headhouse Excavation

4 SEM CONSTRUCTJO

4.1. SEM Organi=alion Under the oversight of the Tunnel Manager, Obayasbi employed an experienced SEM Manager to control tbe day-to-day SEM activities along with a Site Manager responsible for Beacon Hill Station. Obayashi entered into an agreement with Beton and Monierbau USA, Inc. (Evansvill e, [ndiana) to provide

key SEM staff. Toonel excavation and suppon act ivities continue on a six day 24 hour working schedule. Generally two crews are working three shifts of 8 hours duration. Experienced SEM Superintendents and SEM Project Engineers are on si te continuously to facilitate immediate decision making at the face. These individuals are supponed by Walkers and Shift Engineers respectively.

During the design stage, agreement was reached with Soood Transit that the Designer should be represented on site during the implementation of the SEM design. As mentioned earlier, Hatch Mon MacDonalcllJacobs Joint Venture (HMMJ) was responsible for the detailed design of all turmels and ponals, shafts and mined station ttmnels, including the fmal lining and waterproofing system. Dr Sauer Corporation (DSC) assisted with the SEM design and waterproofing design for the Station as a sub­consultant to HMMJ. During construction, HMMJ and DSC provide a team of experienced SEM engineers and SEM inspectors to assist the Construction Management team (parsons Brinckerhofl) in providing engineering oversight of the SEM excavation and suppon activities. ST's geotechnical consultant, Shannon & Wilson (S& W), is represented on site providing oversight on geotechnical activities.

As pan of the regular commwlications required for the control of the SEM work, daily on-site meetings are beld following a joint inspection of all the SEM faces. Topics discussed include current activit ies, planned activities for the next 24 hours and instrOOlentation results. The meetings are always attended by representatives of Obayashi, ST and HMMJIDSC and a pannering approach adopted by the parties helps to ensure open commwlication. Current progress, instrumentation results and agreements reached during these daily meetings on field decisions to better adjust the SEM to actual groood conditions are entered into a Journal Book and signed by the Obayashl SEM Manager and the HMMJIDSC SEM Engineer. SEM activities are also included in more fonnal Weekly Progress Meetings used to discuss all C71 0 activities in a larger fOCWll.

On a weekly basis, sbotcrete strength results are summarized and discussed at the SEM daily meetings. Tills allows close control of any potential problems and the tinlely agreement on any necessary mitigation measures.

Construction Work Plans are developed by Obayasill for each of the toonels for review and approval by ST. Any changes to these plans are discussed in the SEM daily meetings. In addition, contingency plans were developed and these include

64

procedures to implement additional toonel suppon measures.

A 'Required Excavation and Suppon Sheet' (RESS) is produced for all toonel sections to assist commooications. The RESS confimls the excavation sequence, required suppon, tool box items etc. and is coootersigned by the relevant panies.

Geologic mapping is performed during each excavation cycle. The face maps are jointly agreed between ST and Obayashi and coootersigned. Photographs are taken to complete the records. Along with the borehole data, the geotechnical model is constantly updated and presented to the interested panies as interpreted geological sections.

4.2. Wesl Longiludillal Venl Adil The first SEM excavation was for a 3 III (lOft) long section of the 7 m (23 ft) wide west LV A using a top beading, bench and inven sequence. This was turned ooder as the shaft went by with the intention of completing the remainder of this long decline later "from the bonom up". This section was completed over a two week period in JWle 2005. A Cat 320B excavator with a milling head attachment, suitable for mining tllrough the jet grouted columns and the dense clay material was used (see Fig. 6). After completion of the west LV A, shaft excavation continued to gain access to the Concourse Cross Adit (CCA) top beadings.

.. ,) ;oY , .. .r.,.. . .. "Lr J.

6. West LV A Excavation

4.3. COllcourse Cross Adils The CCAs, with an excavated width of approximately 14 m (45 ft) and heigbt of 12.5 m (4 1 1\) are the largest turmel openings on the project (see Fig. 7). The north and south adi ts, each 20 m (67 ft) long, connect the main shaft to the platfonn turmels. A grouted barrel vault pipe arch was installed in the crown over the full length of each CCA prior to turmel excavation.

(0 ...."

1-------1381m ' .. 5· .... 1-------'

Fig. 7. Concourse Cross Adit Cross Section

4.3.1 Barrel Vault Illstallation Specialty subcontractor orthwest Cascade Inc. and Obayashi jointly installed multiple rows of perforated steel pipes above the crown of both CCAs from the main shaft. On the basis of more favorable conditions in the south CCA, two rows of pipes were reduced to one. Pipes were drilled using a Klemm KR 806-3 hydraulic rig at 45 cm (18 inch) centers with 10 cm (4-inch) diameter used for the shortest pipes temlinating at the platform tunnel junction and a larger 15 cm (6-inch) diameter for the longest pipes extending beyond the headwall up to 23 m (75 ft) in length. The pipes were drilled as lost casing using "J" teeth welding into the lead casing pipe. After cleaning, each pipe was swveyed and then weak cementlhentonite grout dams were placed to ensure micro-fine cement would not run along the annulus. After the grout dams set up, stage grouting in 1.5 m (5 ft) sections using a double packer system and microfine MC-500 portland cement grout was perfonned. TIle refusal criteria was 85 I (3 cubic ft) grout per 30 COl (lineal ft) pipe or holding 14 bar (200 psi) for 10 minutes in sand zones and 5 minutes in clay/silt zones. The target strength was originally 14 MPa (2000 psi) after 48 hours but was later changed to 3.5 MPa (500 psi) in 24 hours. The required positional tolerance of I % was confirmed using a down-the-hole Maxibore horizontal incl inometer. A total pipe length of approximately 1980 m (6500 ft)

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was drilled and grouted in 7 weeks with the crews working two 10-hour shifts/day.

4.3.2 Excavation and Support Due to the large size of the openings and the expected difficult ground conditions especially in the crown, the excavation sequence was prescribed as a twin­sidewall drift each with top heading, bench and invert followed by the center drift top heading, bench and invert. Finally the temporary sidewalls were removed in stages to form the completed ring shape. At Obayashi's request, the center drift sequence was changed and the center drift top heading size was increased to pennit tbe use of the Liebherr 900 excavator (see Fig. 8). This top heading was driven all the way to the headwall before removal of tbe bench and completion of the invert closure (see Fig. 9).

The excavation for the south CCA commenced with the breakout of the shaft slurry wall concrete in August 2005. The Liebherr 900 excavator with a purpose-made rotating boom was used to excavate the side-wall drift top headings. Various tool box items were used including face bolts, pocket excavation and welded wire fabric. TIle overlapping II m (35 ft) long probe holes drilled ahead of the face were generally dry with the exception of the probes in the southeast ad it which provided small flows of less than 4 Vmul (one gpm). A sand dyke was first encountered in the soutbeast side drift with localized flowing sand in the crown requiring the use of tool box items such as grouted pipe spiles and well points. Pocket excavation was necessary in this area with the immediate application of flashcrete. The sand dyke was again enCOlUltered in the center drift top heading, and following the previous experience was more effectively handled prunarily with the use of pocket excavation, grouted pipe spiles and additional shot crete. The south CCA was completed following

the construction of the main shalt base slab and temporary backfilling in February 2006.

The north CCA commenced in October 2005 and at the time of writing was just completed.

CCA

4.4. Platform Tllnnels The southbound and northbound platform tunnels as originally designed are approximately 103 m (338 It) long each, with excavated dimensions of approximately I I m (37 It) wide and 10 m (32 It) high (see Fig. 10). The length of each platform was increased to 132 m (434 It) for the platfoml shilt redesign. The excavation and support of the southbound platform tunnel commenced with the breakouts from the south CCA lining for both the east and west drives in March 2006. Two rows of grouted pipe spiles were used as pre-support in the breakout zones. Initially the advance length was 1.2 m (4 It) but this was later increased to 1.4 III (4 ft 6 inch). The shotcrete thickness is 35 cm (14 inch) including 5 cm (2 inch) flashcrete. Reinforcement is 2 layers of 6x6 WI2xWI 2 mesh. Lauice girders are installed at 1.2 m (4 ft) centers close to the face of each excavated round. Steel TH girders are installed in the temporary sidewalls for ease of later removal. The invert of the first side drift leads the top heading of the second by a minimum of 7 m (24 ft). Systematic probe drilling is carried out in each top beading generally over an I I III (35 ft) length with a minimum 5 III (16 ft) overlap. To date the west drive has encountered prinlarily dry si lty clay conditions with some localized sand pockets. Varying quantities of rebar spiles of length 4 m ( 12 ft) and spacing 30 Col ( I ft ) are driven in advance of the crown at each round of the top heading. The excavation sequence requires the completed invert to be a maximunl distance from the top heading face of

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10m (34 It). Following invert construction of 2.5 m (8 ft) in one cycle, the sequence follows with the excavation and support of a 1.2 III (4 It) long section of bench then a 1.2 m (4 ft) top heading followed by another bench and top heading again before the next invert cycle (see Fig. I I). A larger Liebherr 932 tunnel excavator is used to excavate the platform twmels. The temporary side-wall is removed generally in 2.5 m (8 ft) stages ensuring that the joints at the crown and invert are carefully inspected and constructed and maintaining a minimum of 2.5 m (8 ft) of intact side-wall from the completed invert. The invert shotcrete is protected with a minimum I III (3 It) layer of temporary backfill. Any local seepage water encowllered at the face is collected in pipes held in position by shotcrete and channeled away.

1------ 11 '3m (",.~)------I

Fig. 10. Platform Tunnel Cross Section

At the time of writing the east drive of the southbound platform tunnel had been completed approximately 8 m (25 ft) in the first side drift of the top heading. A sand lense ahead of the face, which was originally detected from Ule subsurface drilling program, has resulted in additional probing from the ttmnel face. Along with the additional probes, drainage lances were installed to better understand this complex piece of ground and to take advantage of whatever drainage effects could be realized from underground.

iT"l ~ , ,

Fig. II . Platfonn Tunnel Excavation Sequence

Fig. 12. View of the Southbound Platform Tunnel from the South CCA

The northbound platform twmels are expected to start shortly with the barrel vault installation from within the north CCA. A double row of 18m (60 ft) long pipes was designed for the start of the northbowld platform twmels in both directions. However, because of more favourable ground conditions identified during the supplemental geologic exploration and as a result of the north CCA excavation, the double row has been reduced to a single row. Also the pipes in the west direction have been reduced to 12 m (40 ft) lengths. Excavation of this tunnel will commence upon completion of the barrel vault and the excavation of the southboWld platform west drive (see Fig. 12).

4.5. fniliai Sholcrele Lining The design specifications require shotcrete compressive strengths of 14 MPa (2000 psi) at 24 hours and 34 MPa (5000 psi) at 28 days. Dry-mix fiber reinforced shotcrete is specified for the minimUlll 5 cm (2-inch) thick flashcrete layer (see Fig. 13). The remaining initial lining thickness of generally 30 cm (J 2-inches) is sprayed using wet-mix

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reinforced with two layers of welded wire fabric. Panels for shotcrete testing are sprayed daily in the tunnels during the initial lining shotcrete application. Two panels for dry-Illix and two panels for wet-mix provide the necessary number of cores for off-site testing by an independent laboratory. On-site testing facilities are used by Obayashi's QC department primarily to check early strengths. In general the results are consistently better than those speci tied. However, following some sporadic low 24 hour test results for the dry-mix during the early stages, procedures were improved especially for handling the panels during transportation. In-situ cores are taken occasionally to verify the panel results. The sporadic low 24 hour results, taken from panels, bave been checked and found acceptable when in-situ cores were taken and tested. Penetration nail testing is also used unofficially to verify the early strength gain for shot crete less than 12 hours old.

The shot crete thickness is measured and controlled in the tield primarily us ing the lattice girders once they have been surveyed in position. Overbreakloverexcavation is general ly fi lied with the flashcrete layer.

Pre-bagged shotcrete was initially used for the dry-mix until confidence was gained with the on-site batcher. The on-site batcher is a 46 m' fhr (60 cubic yd/hr) volumetric batcher and it is planned to be augmented with a second 76 n,'fbr (100 cubic yd/hr) weigh batcber dedicated generally for wet-mix shotcrete. Other improvements include reconfiguring the shotcrete punlP system so that the pwups are located underground in the south CCA and supp lied by drop holes from the batcher. This will reduce the lengths over which shotcrete is punlped thereby reducing the risk of blockages and downtime.

Shotcrete nozzlemen are required to be experienced and have ACI certification. In addit ion, panels are sprayed by each nozzleman to allow shotcrete cores to be visually inspected and categorized. A shortage of skilled nozzlemen was experienced in the early stages, and on-site training of nozzlemen is helping to overcome this shortage. All shotcrete is sprayed by hand generally from man­baskets with some limited use of the Oruga shotcrete mobile robot. When space allows, a larger Spraymobile Robot will be introduced to the headings.

Obayashi initiated a redesign to replace the specified welded wire fabric reinforcement willl steel tibers for the platform tlUlnel initial lining. Flexural strength testing is required from beams sawed from test panels. After some initial difficulties primarily with the fiber dosage control and blocked shotcrete lines, this change was temporarily suspended. Steel

fibers wiU be reintroduced once the second batch plant is operational.

Fig. 13. Flashcrete for the Southbound Platform Tunnel Breakout

4.6. Tool Box The excavatiol! sequences and ground support measures specified on tbe drawings are augmented with discretionary additional excavation and ground support measures that are installed based on the encountered ground conditions. These measures (tool box items) provide pre-support, support or ground improvement around or within the tunnel. They include rebar spiles, grouted pipe spiles, metal sheets, face wedge, pocket excavation, face bolts, permeation and fracture grouting, soil nails, additional reinforced shot crete, and vacuum dewatering. The tool box items are installed as approved or directed by Sound Transit. For plalUling and estimating purposes a table of quantities of tool box items was included in the GBR and the Contract Price Schedule. The GBR table is reproduced as Table I.

St:M Toofbos 1If-1M

r ! j H ~f 1 :.

H I 1 > S~I SUllo .. "n!_8fh: ! 1 l 1 h

,~ I!A '" 'A ,~ " l.P

=C;:-n!:i..t C-... T_1o

121Z 110' ll1 ,. " ...,

" "" ","",c_Clt .. ·AI.h!. ~~n-:I_

c-. .... T_~ 1111 '" '" '" U )1-10 " """ :::!7:6""t T~_ v_

"'-• .,.IU" TR"". no ,. '" I~ V_"-- Adott ". '" " TUTAI. Ql',unTTU:S ,." -." I ... .. "" " , .....

Table I. Tool Box Items from Ule GBR

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4.7. Instrumentation An extensive array of approximately 54 instrwnents has been installed by Obayashi from the surface in advance of tunnel excavation. 1b.is includes extensometers and inclinometers for monitoring ground movements. Readings are generally taken 2-3 times weekly depending on tbe proximity of the advancing twmel face. With the exception of the surface settlement readings taken by CH2M Hill for Sound Transit and readings taken in the tWlllel by Obayashi, Shannon & Wilson is responsible for taking the readings, consolidating all data (CH2MHill, Obayashi and S&W) and presenting the data on behalf of Sound Transit. To date the maximunl recorded surface settlement is approximately 6 nlln (0.25 inch).

Obayashi is responsible for the timely installation of the twmel instrunlentation at each monitoring section, generally at 15 m (50 ft) spacings in the platform tWlllels. A typical section consists of optical targets used to monitor the deflection of the lining, earth pressure/shot crete stress cells and strain gauges fixed to the lattice girders. All instrunlents are installed prior to shotcrete application. mitial readings are taken within hours to ensure valuable data on defomlation is not lost.

Obayasbi employs a Professional Land Surveyor to implement the survey program. The optical targets are generally read to the required accuracy of 0.15 mm (0 .006 ft). TIlere was a period during Ule initial excavation of the CCAs when it was necessary to use a tape extensometer to supplement the surveyed data. However, given Ule congestion at the bottom of the shaft, improvements were made to the survey allowing the continuation of Ule optical method. The data is presented using Eupalinos software and discussed at the Daily SEM Meetings. The data is monitored against specified Utreshold and limiting values, and also for wlusual trends. Although in some cases threshold and limiting values are reached, the readings are generally well within expectations. For example, Ule maximum recorded roof settlement of the south CCA was 15 nun (O.60-inch).

4.8. Constrllction Sequence/Schedule The SEM excavation sequences in the various mined station ad its, tunnels and other wlderground openings of the stat ion are prescribed on the drawings. The sequencing of the mined station tunnels excavation relative to each other is flexible. Obayashi has freedom in selecting construction methods, equipment, procedures and sequences, subject to the approval of Sowld Transit. The contract requires Ulat the platfonn tunnels be excavated prior 10 TBM arrival.

Obayashi are employing three crews working from the main shaft. At the time of wriling, the crews

.. .. .. • • • • • • • • • ,. • • • • .. • • • ,. • ,. ,. ,. • • • • • " III

" I)

• • .. • • • • • •

had just completed mining of the north CCA and are progressing on the southwest platform tunnel, the southeast platform tunnel, and the installation of barrel vault pipes over the northbound platform tunnels concurrently. In addition, there is a crew working out of the ancillary shaft to execute the emergency tunnel and the east transverse ventilation adits.

The crews consist generally of 10 individuals on each shift. After a learu1ng curve, the cycle times have been reduced to approximately 6 hours per round to date (Fig. 14) which approximates to 6 m (20 ft) per week in the platform tunnel. Where possible and dependent on the soil conditions and the deformation moultoring, field changes are made to assist production and reduce the cycle times. For example, the specified completion of all support prior to the next excavation cycle has so far been relaxed to allow the second layer of mesh and fmal shotcrete layer to be delayed by up to a maximum of three rounds.

The total duration of all the SEM excavation is scheduled to be 24 months out of the total 48 month contract.

project worthy of presenting to the engineering community, and advancing the technical boundaries of soft ground tunneling in the USA.

REFERENCES

1. Tattersall C., M. Murray, 1. Laubbichler, F. Langer. SEM Tuuueling Underway in Seattle - Construction of the Beacon Hill Station and Tuuuel. NAT 2006.

2. Phelps D., J. Gildner, C. Tattersall, J. Laubbichler, McAllister. Design and Risk Management Strategy for the Sound Transit Beacon Hill Station and Tuuuels -RETC2005.

3. Robinson R., M. Kueker, M. Lehnen, S. Warren, McAllister. Impacts of Geotechnical Issues on Design of the Beacon Hill Tunnel and Station Project - RETC 2005.

4. Hatch Mott MacDonaJd Jacobs. Geotechnical Baseline Report 2004.

5. Tattersall c., T. Gregor, M. Lehnen. Design and impact of the Beacon Hill Station exploratory shaft program - NAT 2004.

6. Laubbichler J., T. Schwind, G. Urschitz. Bencinuark for the future: the largest SEM soft ground tunnels in the United States for the Beacon Hill Station in Seattle -NAT 2004.

Platformtunnel- Weekly Progress: Hours per Sidewall Drift 4 feet Rounds of TH, B or Invert

55.0 50.0 45.0 40.0

I!! 35.0 ::: 30.0

~ ~5:g 15.0 10.0 5.0 -fffi:'".-c,--,-:,~ 0.0 -P""--'-r==

2 3

Fig. 14. Platform Tuuuel Average Cycle Times

5 CONCLUSION

4 5

Soft ground SEM tunneling in such variable ground conditions as the local water-charged glacial deposits in Seattle. presents significant technical challenges. These challenges were recognized with the provision of a robust design with appropriate excavation support, pre-support, and available 'tool box' items. Along with having suitably experienced field staff, an open partnering approach between the parties has been a key to the safe and successful tunnel structures excavated to date.

The authors would like to thank all the staff and workforce involved in making this such an interesting

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