rock engineering for a megaton detector

Rock Engineeringfor a

Megaton Detector

Charles NelsonCNA 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)

Vol

ume

(cub

ic m

eter

s)

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 100Cavern Span (m)

Roc

kbol

t Len

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

Roc

kbol

t Spa

cing

(m)

Empirical Values Examples


Shotcrete Thickness vs Rock Quality

0

100

200

300

400

0.01 0.1 1 10 100NGI "Q" Rating

Sho

tcre

te T

hick

ness

(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

Documents

rock stresses

rock material strong

m3 cavern feasible

jointed rockrock masses

massive rockrock masses

blocky rockrock masses

large cavern construction

situ stressesselect