The Design of Deep Rock

Shafts on the Rondout-West

Branch Bypass TunnelChristopher E. Dianora, EIT, NYCDEP

Matt Sorrell, PE, NYCDEP

Eileen Test, PE, McMillen-Jacobs Associates, Inc.

2

NYC Water Supply

Location of

Bypass

Tunnel

Project

3

Rondout-West Branch Bypass Tunnel

4

Rondout-West Branch Bypass Tunnel

5

Design of Shaft 5B

BT

-1 C

on

trac

t

~ 8

20ft

BT

-2

~7

0ft

5ft

Thick

Wall

Tunnel Portal

Domed Invert

30ft ID

BT-2 Contract

6

Design Of Shaft 6B

BT

-1 C

on

tract

~ 6

55ft

BT

-2

~ 1

11

ft

33ft ID

Tunnel Portal

Drainage Tunnel

6ft Thick Walls

Domed Invert

BT-2 Contract

7

Geologic and Hydrologic Conditions

Shaft 5B – 875ft GW Head

Normanskill Formation

Shaft 6B – 700ft GW Head

Mount Merino Formation

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Loading

1 Mile

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Loading

10

Closed Formed Solutions

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Comparison to Existing Shaft 6

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Finite Element Model

Shaft 5B Shaft 6B

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Shaft 6B Results

Redistribution of Hoop Stress around Openings

DIncrease in

Stress

~ D/2 above

and below

opening

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Shaft 6B Results

Hoop Flexure at Openings

Inward

Bending

Outward

Bending at

Centerline

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Shaft 6B Results – Drainage Tunnel Portal

Hoop Axial Stress

Critical Band

Hoop Flexure Stress

Critical Band

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Shaft 6B Results – Drainage Tunnel Portal

Longitudinal Flexure StressLongitudinal Axial Stress

Axial Tension +

Inward Bending

at Springline

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Shaft 6B Results

Redistribution of Hoop Stresses Causes

In-Plane Shears near Openings

Locations of

Max In-Plane

Shears

Locations of

Max In-Plane

Shears

18

Tunnel Stubs

Shaft 6B Connection Tunnel Stub – Hoop Flexure

Inward

Deflection at

Springline

Outward

Deflection

at Crown

and Invert

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Shaft 6B Moment-Thrust Diagram

Shaft 6B Intersection

Controlling Load Case: 1.2(D+EH) + 1.6(L+G), EH= External GW, L= Live, G = Ground

Elements in Critical

Band, DT Crown, and

DT Invert

Elements above

and below tunnel

stubs

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Liner Reinforcement

o Shaft 5B: 5ft thick wall, doubled-up #10 bars @ 8” E.W., E.F.

o Shaft 6B: 6ft thick wall, doubled-up #11 bars @ 8” E.W., E.F.

o Tunnel Stubs: 4 ft thick walls,

Hoop: Doubled-up #10 bars @ 6”/8” (Shaft 5B/6B) E.F

Longitudinal: #11 bars @ 6” E.F.

o f’c = 4,500 psi; fy = 60 ksi

o Radial shear reinforcement near portals and tunnel stubs

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Domed Invert - Free Body Diagrams

R = 0

Pros:

Minimum Excavation

Cons:

Very Large Bending

Moment

R = Shaft Radius

Pros:

No Bending; Pure

Axial Compression

Cons:

Increased Excavation

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Domed Invert Design

Transferred

to Rock

Transferred

to Rock

Transferred

to Shaft Wall

Transferred

to Shaft Wall

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Domed Inverts – Not a Bad Idea!

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Conclusions

• Applicability of Finite Element Modeling to Shaft Design

• Geometric Considerations – Center, Size, Separation

• Sufficient Width of Liner Above and Below Portal is

Crucial

• Model Demonstrated Peak Bending and Shears Not in

Same Location

• Use of Vertical Steel as Membrane Shear Reinforcement

• Tunnel Stubs Can Be Used to Brace Portals

• Dished Slabs Are More Effective than Flat Bottoms for

High Head Applications

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Questions?