Rock Engineering for a Megaton Detector Charles Nelson CNA Consulting Engineers

Rock Engineeringfor a

Megaton Detector

Charles Nelson

CNA Consulting Engineers

January 2002 CNA Consulting Engineers

Overview

• Rock engineering 101• Cavern size & shape• Construction methods• Feasibility

– Historical projects– Numerical modeling– Empirical design

• Other considerations

January 2002 CNA Consulting Engineers

Rock Engineering 101

• Rock “material” — strong, stiff, brittle– Weak rock > Strong concrete– Strong in compression, weak in tension– Postpeak strength is low unless confined

• Rock “mass” — behavior controlled by discontinuities– Rock mass strength is 1/2 to 1/10 of rock

material strength

• Discontinuities give rock masses scale effects

January 2002 CNA Consulting Engineers

Rock Engineering 101

• Massive rock– Rock masses with few

discontinuities, or– Excavation dimension

< discontinuity spacing

January 2002 CNA Consulting Engineers

Rock Engineering 101

• Jointed or “blocky” rock– Rock masses with

moderate number of discontinuities

– Excavation dimension > discontinuity spacing

January 2002 CNA Consulting Engineers

Rock Engineering 101

• Heavily jointed rock– Rock masses with a

large number of discontinuities

– Excavation dimension >> discontinuity spacing

January 2002 CNA Consulting Engineers

Rock Engineering 101

• Rock stresses in situ– Vertical stress weight of overlying rock

– ~27 Kpa / m 16.5 MPa at 610 m

– ~1.2 psi / ft 2,400 psi at 2000 ft

– Horizontal stress controlled by tectonic forces (builds stresses) & creep (relaxes stresses)

– At depth, v h unless there are active tectonic forces

January 2002 CNA Consulting Engineers

Rock Engineering 101

• What are the implications for large cavern construction?– Find a site with good rock

– Characterizing the rock mass is JOB ONE

– Avoid tectonic zones & characterize in situ stresses

– Select size, shape & orientation to minimize zones of compressive failure or tensile stress

January 2002 CNA Consulting Engineers

Cavern size & shape

January 2002 CNA Consulting Engineers

Cavern Size & Shape

January 2002 CNA Consulting Engineers

Construction methods

• Drill & blast

• Small top headings

• Install rock support

• Large benches

January 2002 CNA Consulting Engineers

Is a 106 m3 Cavern Feasible?

• Previous cavern projects

• Numerical modeling

• Empirical design methods

January 2002 CNA Consulting Engineers

Is a 106 m3 Cavern Feasible?

0

200,000

400,000

600,000

800,000

1,000,000

0 20 40 60 80 100 120Span (m)

Vo

lum

e (

cu

bic

me

ters

)

Existing NG Caverns

January 2002 CNA Consulting Engineers

Numerical Modeling

January 2002 CNA Consulting Engineers

Failure Zones, Cylindrical Cavern

Strong Intermediate Weak

January 2002 CNA Consulting Engineers

Failure Zones, Straight Cavern

Strong Intermediate Weak

January 2002 CNA Consulting Engineers

Empirical design methods

• Appropriate during feasibility assessments

• Require classification of the rock mass

• Most commonly used today:

– Bieniawski RMR rating

– NGI Q rating

• NGI Q rating used in the following

January 2002 CNA Consulting Engineers

Rock Quality Assumptions

• Q=100– One joint set; rough, irregular, undulating joints with tightly

healed, hard, non-softening, impermeable filling; dry or minor water inflow; high stress, very tight structure

• Q=3– Two joint sets plus misc.; smooth to slickensided,

undulating joints; slightly altered joint walls, some silty or sandy clay coatings; medium water inflows, single weakness zones

• Q=0.1– Three joint sets; slickensided, planar joints with softening or

clay coatings; large water inflows; single weakness zones

January 2002 CNA Consulting Engineers

Rock Quality

Q=100 Q=3 Q=0.1

January 2002 CNA Consulting Engineers

Rock Quality

January 2002 CNA Consulting Engineers

Rock Quality

January 2002 CNA Consulting Engineers

Rock Quality

January 2002 CNA Consulting Engineers

Rock support methods

• Rockbolts or cable bolts– Provides tensile strength & confinement

• Shotcrete– Sprayed on concrete

– Provides arch action, prevents loosening, seals

• Concrete lining– Used when:

• Required thickness exceeds practical shotcrete thickness• Better finish is needed

January 2002 CNA Consulting Engineers

Rockbolt Length vs Cavern Span

0

5

10

15

20

0 20 40 60 80 100

Cavern Span (m)

Ro

ck

bo

lt L

en

gth

(m

)

Empirical Data Cavern Spans

January 2002 CNA Consulting Engineers

Rockbolt Spacing vs Rock Quality

0

1

2

3

0.01 0.1 1 10 100

NGI "Q" Rating

Ro

ckb

olt

Sp

acin

g (

m)

Empirical Values Examples

January 2002 CNA Consulting Engineers

Shotcrete Thickness vs Rock Quality

0

100

200

300

400

0.01 0.1 1 10 100

NGI "Q" Rating

Sh

otc

rete

Th

ickn

ess

(mm

)

Empirical Values Examples

January 2002 CNA Consulting Engineers

Cost Categories

Excavation

Haulage

Support

Access Tunnel

Ancillary Space

Mobilization,Bond, etc.

Permits, Fees,Eng, etc.

January 2002 CNA Consulting Engineers

Cost Conclusions

• Costs are sensitive to:– volume

– rock quality

• Costs are insensitive to:– Cavern shape

• Costs are moderately sensitive to:– Horizontal vs. vertical access (within ranges

considered)

January 2002 CNA Consulting Engineers

Challenges

• Find the best possible rock in an acceptable region

• Find a site with feasible horizontal access

• Explore co-use opportunities

• Develop layouts amenable to low cost excavation methods

• Give Geotechnical considerations as much weight as possible

January 2002 CNA Consulting Engineers

U.G. Space Considerations

• Common facilities (infrastructure & usable space)

• Cavern shapes & sizes

• Laboratory-experiment relationship

• Special needs

January 2002 CNA Consulting Engineers

Common Facilities

January 2002 CNA Consulting Engineers

Common Facilities

• What common facilities are beneficial/desirable?– Power, water, sewer, communications

– Machine shop, assembly areas??

– Storage, clean rooms??

• How should common space be allocated between underground & aboveground?– Administration, storage

January 2002 CNA Consulting Engineers

Common Facilities• Radon control

– Should the whole lab have radon control or just certain areas?

– What is the best means? Sealing? Outside air?

• Lab cleanliness standards– 100? 1,000? 10,000?

– What standards for what spaces?

– What are the requirements for the various experiments?

January 2002 CNA Consulting Engineers

Compact vs. Open Layout?

• Compact layout– Allows more interaction

– Common space is more usable

– Reduced infrastructure costs

– Reduced cost to provide multiple egress ways

– Preserves underground space

January 2002 CNA Consulting Engineers

Compact Layout

January 2002 CNA Consulting Engineers

Compact vs. Open Layout?

• Open layout– Better isolation

– Reduced impact during expansion

• Essential to create a Master Plan that will guide lab development

January 2002 CNA Consulting Engineers

Cavern Shapes

• Use simple shapes, e.g. rural mailbox• Avoid inside corners• Avoid tall, narrow shapes• Roof costs the most

January 2002 CNA Consulting Engineers

Cavern Shapes

January 2002 CNA Consulting Engineers

Cavern Shapes

• Avoid complex intersections

• Avoid closely spaced, parallel excavations

• Overexcavation & underexcavation are common

January 2002 CNA Consulting Engineers

Laboratory-Experiment Issues

• What are the issues?– Different sources of funding

– Shared responsibilities

– Shared liabilities

– Users/tenants rights

– Conflict resolution

– Decommissioning (escrow funds?)

– Private tenants?

January 2002 CNA Consulting Engineers

Specific examples

• How many caverns does the lab provide? 0? 1? 2? More?

• Cavern sharing?– Large caverns are cheaper

– Shared caverns create conflicts

• What is the logical boundary between lab-provided services and experiment-provided services?– Power, heating & cooling, clean rooms

– Storage space, assembly space

January 2002 CNA Consulting Engineers

Other Experience

• Kansas City, MO, converted limestone mines widely used for warehouse & manufacturing

January 2002 CNA Consulting Engineers

Underground Owners:

• Interact with building code officials

• Prepare & enforce design / construction standards

• Control tenant improvements

• Control occupancy

• Restrict structural modifications

January 2002 CNA Consulting Engineers

Underground Owners:

• Restrict chemicals & hazardous materials

• Require regular maintenance

• Provide labor or preferred contractors for improvements

• Typically make all improvements

January 2002 CNA Consulting Engineers

What is not the same?

• Funding– Typical UG space, tenants pay

– For NUSL, lab funding & experiment funding are separate

• Special needs– Typical UG space, special needs limited

– For NUSL, everything is special

January 2002 CNA Consulting Engineers

What is not the same?

• Common space– Typical UG space, limited common space

– For NUSL, extensive common space

• Shared space– Typical UG space, share only infrastructure

– For NUSL, experiments may share caverns

January 2002 CNA Consulting Engineers

Special Needs

• Shape

• Shielding

• Clean rooms, clean lab?

• Radon control

• Magnetic field cancellation

• Power use or reliability

• Heat generation

January 2002 CNA Consulting Engineers

Special Needs (cont.)

• Water supply

• Flammable detector materials/gasses

• Suffocating gasses

• Occupancy

• Hours of access

January 2002 CNA Consulting Engineers

Salt Cavern

January 2002 CNA Consulting Engineers

Hard Rock Cavern

January 2002 CNA Consulting Engineers