Design and Construction of a 200-foot-deep, 24-foot-diameter Conveyance System

Boise, IdahoSeptember 14, 2009

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Conveyance System Access Shaft

Daniel R. Williams, P.E.

Project Overview

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Project Location Map BrightwaterWWTP

BellevueSeattle

EffluentConveyanceSystem

Treatment PlantTreatment Plant

•• 36 MGD in 2010; Peak Flow 130 36 MGD in 2010; Peak Flow 130 MGDMGD•• 54 MGD in 2040; Peak Flow 170 54 MGD in 2040; Peak Flow 170 MGDMGD•• Membrane bioMembrane bio--reactor WWTPreactor WWTP•• Reclaimed water facilitiesReclaimed water facilities

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•• Reclaimed water facilitiesReclaimed water facilities

Conveyance TunnelsConveyance TunnelsLength (feet)

Outside Diameter

East 13,890 18’

Central 19,020 11,590

17’17’

West 21,033 13’

PREDESIGN REPORT:

q Describes a BWP large enough to serve as a retrieval shaft for two TBMs

q Location based maximum tunnel length and

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q Location based maximum tunnel length and depth based on hydraulics

§ Tunnel BT3 is 3.8 miles in length§ Tunnel BT4 is 3.9 miles in length

q Anticipates an inside diameter of 30 feet

Brightwater Effluent System

PROFILE

PLAN

Ballinger Way Portal – Design Phase

TECHNICAL MEMORANDUMS:

q Tunnel and Portal Access TM

§ Describes inspection intervals and access requirements for Brightwater System

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q Technical Advisory Committee Report

requirements for Brightwater System

§ Inspection and maintenance at BWP by remote operated vehicles (10’ x 10’)

§ Proposes a drilled shaft 10 feet in diameter

TASK: Evaluate consequences of reducing the

Ballinger Way Portal – Design Phase

inside diameter of the BWP from 30 feet to 10 feet.

Why reduce the diameter?

• Large diameter BWP could take 15 months to construct at a cost of $13M.

• Drilled shaft BWP could be constructed in several months

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• Drilled shaft BWP could be constructed in several months for a cost of $1.5M.

Problems?

• Complex tunnel-to-portal and tunnel-to-tunnel connections (Drilled shaft not large to receive TBMs).

Soil Conditions at BWP / Tunnel Interface:

Flowing or unstable soil from scattered sand, silty sand, and silty gravel layers; hydrostatic head of 150 feet at base of BWP

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feet at base of BWP

Inspection / Maintenance Requirements:

• Initial inspection after ten years of operation• After 7.0 or larger quake• 30-year intervals• Submersible and crawler-type ROVs

Ballinger Way Portal – Design Phase

ALTERNATIVES:

#1 Small Diameter Portal (i.e., Drilled Shaft)

1a – Portal above Tunnel Intersection

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#2 Large Diameter Portal

1b – Portal West of Tunnel Intersection

1c – Portal East of Tunnel Intersection

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Ballinger Way Portal – Design Phase

Constructability and Risk (Drilled Shaft Alternative):

• Unstable flowing soil conditions where tunnel-to-tunnel and tunnel-to-portal connections occur.• Groundwater pressure at connections is 60-plus psi.

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• Groundwater pressure at connections is 60-plus psi.• Requires grouting (in absence of dewatering) to safely conduct hand-mined excavations.• Effectiveness of grouting is difficult to verify; incomplete grouting could compromise hand-mined excavations, exposing workers and the tunnels to soil and groundwater.• Increases the likelihood of Contractor claims and schedule delays.

Ballinger Way Portal – Design Phase

Schedule Impacts (Drilled Shaft Alternative):

• Drilled shaft cannot be constructed independently of tunnels (so drilled shaft is on the critical path).

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tunnels (so drilled shaft is on the critical path).

Cost (Drilled Shaft Alternative):

• Estimated cost of drilled shaft alternative is $7.2M. Large diameter portal alternative costs ranged from $6M to $14M.

Ballinger Way Portal – Design Phase

ALTERNATIVE #2Large Diameter Portal

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Ballinger Way Portal – Design Phase

TASK: Evaluate the following four alternative excavation support systems:

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– Sequential Excavation w/ Depressurization– Open Caisson– Ground Freeze Wall– Slurry Diaphragm Wall

Ballinger Way Portal – Design Phase

Alt 2a – Sequential Excavation with Depressurization

• Alternative was ruled out because of concerns about volume of groundwater

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concerns about volume of groundwater removal and ability of Contractor to keep shaft dewatered.

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Alt 2bOpen Caisson

• Alternative was ruled outbecause of concerns about plumbness and keeping

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plumbness and keeping caisson moving downward at the depths required.

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Alt 2cGround Freeze Wall

• Alternative was allowed because of past success on other projects similar in size

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other projects similar in size and scope – represented best chance for success.

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Alt 2dSlurry Diaphragm Wall

• Alternative was allowed because of past success on other projects similar in size and scope – represented best chance

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scope – represented best chance for success.

Ballinger Way Portal – Design Phase

Alternative Costs:

1. Sequential Excavation w/Depressurization -- $8.19M2. Ground Freeze Wall -- $14.56M3. Slurry Diaphragm Wall -- $9.17M4. Caisson -- $6.3M

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4. Caisson -- $6.3M

Ballinger Way Portal – Design Phase

Reclaimed Water Pipeline – Pipe Support Design

• 20-inch diameter pipe, 170-foot-tall column

• Thermal expansion when temp rises from 40-deg to 90-deg = 0.65 inches

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• Thermal expansion when temp rises from 40-deg to 90-deg = 0.65 inches

• Force imparted by “pinned” pipe = 165,000 lbs

Therefore, pipe supports designed to allow for thermal expansion

• Polymer material is placed between the steel “strap” and the pipe itself, which allows the pipe to move vertically within the pipe support.

• Polymer material is used for skids on casing spacers and is used on boat docks.

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