Rock Engineering for a Megaton Detector

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


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



• Massive rock– Rock masses with few

discontinuities, or– Excavation dimension

< discontinuity spacing



• Jointed or “blocky” rock– Rock masses with

moderate number of discontinuities

– Excavation dimension > discontinuity spacing



• Heavily jointed rock– Rock masses with a

large number of discontinuities

– Excavation dimension >> discontinuity spacing



• 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



• 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


Cavern size & shape


Cavern Size & Shape


Construction methods

• Drill & blast

• Small top headings

• Install rock support

• Large benches


Is a 106 m3 Cavern Feasible?

• Previous cavern projects

• Numerical modeling

• Empirical design methods


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


Numerical Modeling


Failure Zones, Cylindrical Cavern

Strong Intermediate Weak


Failure Zones, Straight Cavern

Strong Intermediate Weak


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


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


Rock Quality

Q=100 Q=3 Q=0.1


Rock Quality


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


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


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


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


Cost Categories

Excavation

Haulage

Support

Access Tunnel

Ancillary Space

Mobilization,Bond, etc.

Permits, Fees,Eng, etc.


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)


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


U.G. Space Considerations

• Common facilities (infrastructure & usable space)

• Cavern shapes & sizes

• Laboratory-experiment relationship

• Special needs


Common Facilities


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


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?


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


Compact Layout


Compact vs. Open Layout?

• Open layout– Better isolation

– Reduced impact during expansion

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


Cavern Shapes

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


Cavern Shapes


Cavern Shapes

• Avoid complex intersections

• Avoid closely spaced, parallel excavations

• Overexcavation & underexcavation are common


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?


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


Other Experience

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


Underground Owners:

• Interact with building code officials

• Prepare & enforce design / construction standards

• Control tenant improvements

• Control occupancy

• Restrict structural modifications


Underground Owners:

• Restrict chemicals & hazardous materials

• Require regular maintenance

• Provide labor or preferred contractors for improvements

• Typically make all improvements


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


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


Special Needs

• Shape

• Shielding

• Clean rooms, clean lab?

• Radon control

• Magnetic field cancellation

• Power use or reliability

• Heat generation


Special Needs (cont.)

• Water supply

• Flammable detector materials/gasses

• Suffocating gasses

• Occupancy

• Hours of access


Salt Cavern


Hard Rock Cavern

Rock Engineering for a Megaton Detector

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