Rock Engineering for a Megaton Detector

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Rock Engineeringfor aMegaton Detector

• Charles Nelson
• CNA Consulting Engineers

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Overview

• Rock engineering 101
• Cavern size shape
• Construction methods
• Feasibility
• Historical projects
• Numerical modeling
• Empirical design
• Other considerations

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Rock Engineering 101

• Rock material strong, stiff, brittle
• Weak rock gt 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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Rock Engineering 101

• Massive rock
• Rock masses with few discontinuities, or
• Excavation dimension lt discontinuity spacing

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Rock Engineering 101

• Jointed or blocky rock
• Rock masses with moderate number of
  discontinuities
• Excavation dimension gt discontinuity spacing

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Rock Engineering 101

• Heavily jointed rock
• Rock masses with a large number of
  discontinuities
• Excavation dimension gtgt discontinuity spacing

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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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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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Cavern size shape
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Cavern Size Shape
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Construction methods

• Drill blast
• Small top headings
• Install rock support
• Large benches

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Is a 106 m3 Cavern Feasible?

• Previous cavern projects
• Numerical modeling
• Empirical design methods

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Is a 106 m3 Cavern Feasible?
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Numerical Modeling
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Failure Zones, Cylindrical Cavern
Strong
Intermediate
Weak
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Failure Zones, Straight Cavern
Strong
Intermediate
Weak
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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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Rock Quality Assumptions

• Q100
• 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
• Q3
• 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
• Q0.1
• Three joint sets slickensided, planar joints
  with softening or clay coatings large water
  inflows single weakness zones

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Rock Quality
Q100
Q3
Q0.1
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Rock Quality
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Rock Quality
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Rock Quality
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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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Rockbolt Length vs Cavern Span
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Rockbolt Spacing vs Rock Quality
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Shotcrete Thickness vs Rock Quality
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Cost Categories
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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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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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U.G. Space Considerations

• Common facilities (infrastructure usable space)
• Cavern shapes sizes
• Laboratory-experiment relationship
• Special needs

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Common Facilities
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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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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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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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Compact Layout
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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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Cavern Shapes

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

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Cavern Shapes
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Cavern Shapes

• Avoid complex intersections
• Avoid closely spaced, parallel excavations
• Overexcavation underexcavation are common

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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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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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Other Experience

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

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Underground Owners

• Interact with building code officials
• Prepare enforce design / construction standards
• Control tenant improvements
• Control occupancy
• Restrict structural modifications

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Underground Owners

• Restrict chemicals hazardous materials
• Require regular maintenance
• Provide labor or preferred contractors for
  improvements
• Typically make all improvements

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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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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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Special Needs

• Shape
• Shielding
• Clean rooms, clean lab?
• Radon control
• Magnetic field cancellation
• Power use or reliability
• Heat generation

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Special Needs (cont.)

• Water supply
• Flammable detector materials/gasses
• Suffocating gasses
• Occupancy
• Hours of access

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Salt Cavern
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Hard Rock Cavern
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