NNN99 Rock Engineering for the Next Very Large Underground Detector D. Lee Petersen
Overview Rock engineering 101 Cavern size & shape Construction methods Feasibility Historical projects Numerical modeling Empirical design Cost estimate Assumptions Variations Breakdown by parts Challenges
Rock Engineering 101 Rock material strong, stiff, brittle Weak rock > Strong concrete Strong in compression, weak in tension Postpeak stength 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
Rock Engineering 101 Massive rock Rock masses with few discontinuities, or Excavation dimension < discontinuity spacing
Rock Engineering 101 Jointed or blocky rock Rock masses with moderate number of discontinuities Excavation dimension > discontinuity spacing
Rock Engineering 101 Heavily jointed rock Rock masses with a large number of discontinuities Excavation dimension >> discontinuity spacing
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, σ v σ h unless there are active tectonic forces
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
Cavern size & shape Cylinder Torus Straight SuperK Soudan 2
Cavern Size & Shape
Torus Options 60m x 60m x 47.7m radius 50m x 50m x 63.7m radius 40m x 40m x 99.5m radius
Torus Options
Construction methods Drill & blast Small top headings Install rock support Large benches
Is a 10 6 m 3 Cavern Feasible? Previous cavern projects Numerical modeling Empirical design methods
Is a 10 6 m 3 Cavern Feasible? 1,000,000 Volume (cubic meters) 800,000 600,000 400,000 200,000 Existing NG Caverns 0 0 20 40 60 80 100 120 Span (m)
Numerical Modeling
Rock Strength Mohr-Coulomb strength criterion Strong c = 18e6 MPa φ = 40 deg Intermediate 1 c = 12e6 MPa φ = 35 deg Intermediate 2 c = 6e6 MPa φ = 30 deg Weak c = 3e6 MPa φ = 28 deg
Failure Zones, Cylindrical Cavern Strong Intermediate Weak
Failure Zones, 60m x 60m Torus Strong Intermediate Weak
Failure Zones, 50m x 50m Torus 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 Quality
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 20 Rockbolt Length (m) 15 10 5 0 0 20 40 60 80 100 Cavern Span (m) Empirical Data Cavern Spans
Rockbolt Spacing vs Rock Quality 3 Rockbolt Spacing (m) 2 1 0 0.01 0.1 1 10 100 NGI "Q" Rating Empirical Values Examples
Shotcrete Thickness vs Rock Quality 400 Shotcrete Thickness (mm) 300 200 100 0 0.01 0.1 1 10 100 NGI "Q" Rating Empirical Values Examples
Cost Estimate Assumptions Undeveloped site Civil construction environment Highway access Massive scale is cost neutral Advantages of scale offset disadvantages Shafts are 600 meters deep Access tunnels are 2,400 meters long Ancillary space: 2.5 % Mobilization, Bond, etc.: 15% Permits, Fees, Engin., Environ.: 20%
Cost Estimate Assumptions Access tunnel cost includes portal & limited site work Shaft cost includes shaft collar, equipment, & limited site work Components not included: Linings (floor & walls) Waterproofing Outfitting Experiment equip. Ventilation Water system Estimate is in 1999 U.S. $ No explicit contingency costs! What is the contingency for an unknown site, unknown country, unknown access, no design work, etc.??
Cost Summary $450,000,000 $400,000,000 $350,000,000 $300,000,000 Cost (1999 $) $250,000,000 $200,000,000 $150,000,000 $100,000,000 $50,000,000 $0 Cylinder Q=100 Cylinder Q=3 Cylinder Q=0.1 Straight Q=100 Straight Q=3 Straight Q=0.1 Torus Q=100 Torus Q=3 Torus Q=0.1 Cavern Type & Rock Quality Horizontal Access Vertical Access
Cost Summary 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 Involve geotechnical engineers throughout the process