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Rock Engineeringfor a
Megaton Detector
Charles Nelson
CNA Consulting Engineers
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January 2002 CNA Consulting Engineers
Overview
• Rock engineering 101• Cavern size & shape• Construction methods• Feasibility
– Historical projects– Numerical modeling– Empirical design
• Other considerations
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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
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January 2002 CNA Consulting Engineers
Rock Engineering 101
• Massive rock– Rock masses with few
discontinuities, or– Excavation dimension
< discontinuity spacing
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January 2002 CNA Consulting Engineers
Rock Engineering 101
• Jointed or “blocky” rock– Rock masses with
moderate number of discontinuities
– Excavation dimension > discontinuity spacing
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January 2002 CNA Consulting Engineers
Rock Engineering 101
• Heavily jointed rock– Rock masses with a
large number of discontinuities
– Excavation dimension >> discontinuity spacing
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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
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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
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January 2002 CNA Consulting Engineers
Cavern size & shape
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January 2002 CNA Consulting Engineers
Cavern Size & Shape
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January 2002 CNA Consulting Engineers
Construction methods
• Drill & blast
• Small top headings
• Install rock support
• Large benches
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January 2002 CNA Consulting Engineers
Is a 106 m3 Cavern Feasible?
• Previous cavern projects
• Numerical modeling
• Empirical design methods
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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
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January 2002 CNA Consulting Engineers
Numerical Modeling
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January 2002 CNA Consulting Engineers
Failure Zones, Cylindrical Cavern
Strong Intermediate Weak
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January 2002 CNA Consulting Engineers
Failure Zones, Straight Cavern
Strong Intermediate Weak
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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
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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
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January 2002 CNA Consulting Engineers
Rock Quality
Q=100 Q=3 Q=0.1
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January 2002 CNA Consulting Engineers
Rock Quality
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January 2002 CNA Consulting Engineers
Rock Quality
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January 2002 CNA Consulting Engineers
Rock Quality
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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
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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
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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
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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
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January 2002 CNA Consulting Engineers
Cost Categories
Excavation
Haulage
Support
Access Tunnel
Ancillary Space
Mobilization,Bond, etc.
Permits, Fees,Eng, etc.
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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)
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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
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January 2002 CNA Consulting Engineers
U.G. Space Considerations
• Common facilities (infrastructure & usable space)
• Cavern shapes & sizes
• Laboratory-experiment relationship
• Special needs
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January 2002 CNA Consulting Engineers
Common Facilities
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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
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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?
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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
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January 2002 CNA Consulting Engineers
Compact Layout
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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
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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
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January 2002 CNA Consulting Engineers
Cavern Shapes
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January 2002 CNA Consulting Engineers
Cavern Shapes
• Avoid complex intersections
• Avoid closely spaced, parallel excavations
• Overexcavation & underexcavation are common
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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?
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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
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January 2002 CNA Consulting Engineers
Other Experience
• Kansas City, MO, converted limestone mines widely used for warehouse & manufacturing
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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
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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
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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
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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
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January 2002 CNA Consulting Engineers
Special Needs
• Shape
• Shielding
• Clean rooms, clean lab?
• Radon control
• Magnetic field cancellation
• Power use or reliability
• Heat generation
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January 2002 CNA Consulting Engineers
Special Needs (cont.)
• Water supply
• Flammable detector materials/gasses
• Suffocating gasses
• Occupancy
• Hours of access
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January 2002 CNA Consulting Engineers
Salt Cavern
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January 2002 CNA Consulting Engineers
Hard Rock Cavern
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January 2002 CNA Consulting Engineers