# Presentation: 007189

## Stability & ConstructabilityOptimization Opportunities in theDesign & Construction of Underground Space

- Chris Laughton PhD, PE, C.Eng.
- Project Manager for Underground Design & Construction Fermi National Accelerator Laboratory.

## Optimization Potential

- Some project are **rigid** -> core functions override engineering preferences for most stable & most practical
- Point-Connecting or Corridors - utility, transit, accelerators, beamline detectors (Long Baselines?)..
- Mining – “ore-centric” layouts, short-term access, low FOS
- Some projects are more **flexible**....
- Hydropower, storage (dry good and fluids), public spaces - engineers can pick host rock, orientations, shapes, dimensions..
**DUSEL openings may have some flexibility** - potential to optimize key engineering aspects of the design to enhance self-supporting ability of rock and improve practicality and safety of construction while respecting core functions

## End-User Requirements

- Space
- Alignment, cross-section, volume (detectors), connections..
- Structures (end-user driven)
- Soffit: Anchors, partitions, rails, cranes, trays, racks, shields..
- Invert: stability against vibrations, destress, overstress, swell..
- Services (ideally some reuse of construction utilities)
- HVAC, Water, Power, Communication, Data Acquisition..
- ES&H (on-site and off-site)
- Egress, access, air quality, noise, groundwater, lighting etc..
- Document Needs -> before developing solutions (data first)
**Integrate design and construction engineers’ preferences in to the Baseline**.
- Early Integration - fewer changes, time/cost savings.

## Geology, Geology, Geology

- Explore before you draw..pick the best host rock mass..
- Modicum of data/rational analyses needed at start - simple is OK
- RMC’s guidance only ~ questionable application in high stress?
- Modeling is a powerful, but good input is critical..garbage in..
- Likely Stability Issues at DUSEL:
- Stress-Driven Yield and/or Burst (overstress)
- Gravity-Driven Fall-Out (blocks, wedges, soil-like fill)
- Water pressure and inflow (erosion, shear strength reduction)
- Combinations of the above
- Early Site Investigation Objectives (reduce uncertainties):
- Rock - Intact rock strengths
- Stress - In Situ Stress levels/orientations
- Fracture - Discontinuities
- Water - head, permeability, estimates flow locations and rates)

## DUSEL Rock Mass Assumptions..

- Basis of Conceptual Design ~ data + assumptions
- Representative Behaviors (routine variability)
- Local Adversities ~ frequency/severity
- Pre-SI Baseline Documentation of both _Knowns & Unknowns_ -> no more sophisticated than the data can support!! (KIS, S)
- More assumptions = more contingency
- Rule #1 - avoidance preferred to mitigation (e.g. SI first)
- Pending SI - assume a hard & blocky rock mass
- Relatively strong and abrasive intact rocks 100MPa+
- Containing fractures and fracture zones, some with water
- Subject to significant stress at depth

## Stability of Underground Openings 

- Underground, two forms of instability often observed:
- Geo-structurally-controlled, gravity-driven processes leading to block/wedge fall-out
- Stress driven failure or yield, leading to rockburst or convergence
- (after Martin et al. IJRM&MS, 2003)
- Note: structure and stress can act in combination to produce failure and adding water can exacerbate failure or reduce the FOS against failure through the action of flow and/or pressure

## Orientation of Major Excavations

- Consider Orientation with respect to Stress Field and Geo-Structure (discontinuity-bound blocks/wedges)
- 1) If there is a major fault or fracture zone in the volume of a major excavation find a new site! (e.g data before design!)
- 2) If a single dominant discontinuity set is present
-  Minimize gravity-driven fall-out by placing the long axis of the excavation sub-perpendicular to the strike of the discontinuity set.
- 3) If multiple sets are present avoid placing the long axis parallel to any - give more weight to sets most likely to cause instability.
- 4) If high stresses are unavoidable at a site
- Destabilizing forces..gravity always..rock stress/water pressure sometimes
- A little stress and fracture can aid stability
- Minimize yield, slabbing, rockburst activity avoid placing the long axis of the perpendicular to the principal stress (~15-30 degrees from parallel, after Broch, E. 1979).

## Rock Fracture - Orientation

- Single Set of planes of weakness. Stability is a function of Excavation Axis:
- Maximize - Strike Perpendicular
- Minimize - Strike Parallel
- More typically multiple sets of planes of weaknesses..
- Maximize by avoiding having any strike close to parallel to axis.

## Rock Fracture - Size/Scale Effects

- Larger Excavation -> increased potential for blocky fall-out

## High & Low Stress 

- Excavation results in stress redistribution at perimeter:
- Low Stress or Tension: mobilized shear strength will be low - Failure!
- High Stress: locally, tangential stresses may exceed rock strength - Failure!
- Above conditions can result in fall-out (walls, crown)
- Geometry of fall-out material a key consideration
- Ideally eliminate or limit the zones of both high and low stress around the perimeter

## Mitigating Stress -Section Shape

- Minimum Boundary stresses occur when the axis ratios of elliptical or ovaloid openings are matched to the in situ stress ratio after Hoek+Brown
- Nice to keep the bottom flat. However, some designers go the whole hog (counter arch..), Sauer..

## High-Stress Failure Zones

- Not always practical to have circular/elliptical sections..
- Stress concentration will occur as a function of stress field/orientation and excavation shape
- Shaded areas show where rockburst or yield is most likely to occur around a horseshoe opening under three types of principal stress orientation..
- Vertical
- Horizontal
- Inclined

- After Selmer-Olsen+Broch

## Stress-Driven Instability can be Severe

- Severity Prediction?
-  relative to Virgin Stress vs. Intact Strength Ratio
- Overstress Failures
- Under moderate stress regime aim to even-out the distribution of stresses to avoid local stability problems, as discussed
- Under higher stress localize stress concentrations to reduce unstable area and costs of support...

- After Hoek+Brown

## Section & Support Mitigation

- Strategy for Minimizing Impact of Overstress
- Vertical Principal Stress
- Reduce potential for buckling/slabbing by avoiding long perimeters sub-parallel to principal stress - “low” excavations
- Horizontal and Inclined Principal Stresses
- Focus and support highly stressed volume at discrete locations around the section by increasing radii of curvature of section to concentrate loading
- bolt support can be used to stabilize areas of concentrated loading

- after Selmer-Olsen+Broch

## Mitigation Step: Opening Separation

- Virgin stress conditions are modified when openings are made, at the perimeter (hydrostatic stress)
- Radial stress zero
- Tangential stress 2x virgin
- 2 circular openings
- Shared diameter, a
- In hydrostatic stress field
- Minimal Interaction if distance between openings centers is greater than 6a
- In high stress situations, ensure openings do not overly encroach on zones of influence

- After Brady & Brown

## Methods & Means Assumptions

- Drill and Blast preferred
- Flexible Heading Operations can Accommodate
- Alignment and Section Changes
- Support and Treatment Changes
- Pre-Conditioning/Cautious Blasting Options
- TBMs - capable of higher productivity, but
- Rigid Heading Operations
- Changes  -> Major Utilization  drops (~50-90%)
- Potential R&D tool - exploratory long, straight tunnels + uniform, good rock
- Roadheaders - “Hard-Rock” Challenged
- Potential R&D toll - ref. ICUTROC initiative
- Raise/Blind Bore Equipment
- Inclined/Vertical Shaft Drilling
- Stabilization Measures
- Bolts and Cables (pre-’ post reinforcement..)
- Super Skins/Liners (spray-on,  c-i-p..)
- Final Liners (Paint, shotcrete, Gunite, .waterproofing..)

## Designing Practical Solutions

- Underground Construction Engineers often complain that “the design of a structure is not always made with due respect to modern construction.” (Brannsfor &Nord, Skanska)
- To improve the constructability of underground structures it is worthwhile including active construction engineers in the development of the design concepts.. (Laughton, ‘01)
-  Some examples on improving constructability..

## Layout for Optimized Construction

- In general capital costs underground are productivity-driven
- In Tunnels..Minimize “Layout Gymnastics”...Avoid
- _Steep ramps_ (>8-10%)  = significant productivity reductions (haulage etc.)
- _Long curves_ - long straight sections/short switch-backs preferred
- _Mining in close proximity to existing structures_ - cautious blasting is slower
- _Multi-pass sections_ -> use largest mechanized equipment that can get down!
- _Routine Changes_ -> standardize excavation/support procedures when possible
- _Incompatibilities between equipment/materials systems_ -> match capacities/sizes
- _Impractical section transitions_ -> design/draw as it will be built
- Additionally...in Multi-Pass Operations/Caverns...Avoid
- _Bottoms-up Mining_ -> prefer top-down work under a supported crown
- _Wide, short excavations_ with high span:depth ratios -> benched volumes give higher productivity/require less reinforcement compared to headings
- In Wet Ground...Avoid
- _Downhill mining_ - achieve gravity drainage

## Practicalities..Sections Transitions

- Right angled intersections can be problematic
- Drill/blast will typically produce bell-shaped transitions - why not draw it like that (end-user might be able to better adapt installations to reality!)?
- Difficult to mine to line and grade
- Liable to be under low stress/tension

- Selmer-Olsen & Broch

- Long-Section

## Practicalities..Access Tunnels

- Excavation methods of today make it possible to use long inclined drifts.. provided that the drifts are correctly shaped, so that maximum transport capacity is obtained. This cannot be achieved by constructing the drifts as spirals: curves should be kept to a minimum and be as short as possible. Straight reaches promote high speed and consequently greater capacity (also yields improved visibility/safety, ideal passing places etc..).

- Plan

## Practicalities..Shaft Access

- Rock falls are often a problem if the shaft opens out directly into the rock cavern where work is in progress.  It is therefore better to position the shaft somewhat to one side and make a horizontal connection.

- Cross-Section

## Practicalities..Cavern Access

- It is not always self evident where an adit should  enter in a rock cavern.
- General agreement that if the rock cavern is short, <150m, the adit should come in at the end.
- Where the cavern is longer, it maybe more cost-effective to start in the middle and work two faces.

- Plan View

## Practicalities - Cavern Access

- The cavern long section shown below is suitable for rock caverns where volume is a functional demand. No extra tunnel tunnel is constructed for excvating the benches: it is sufficient to have an inclined drift in the rock cavern.

- Long-Section

## Cavern Cost Study - Layout

- Economy in rock cavern construction - oil storage..
- Looking for the “cheapest unit volume”
- Norwegian experience in hard rock at relatively shallow depth (stress an occasional a problem)
- after E.D Johansen, ‘79.

- Hard Rock Cavern - Cost Model Geometry

- Long-Section

- Cross-Section

## Cavern Cost Study - Findings

- Excavation Costs
- Unit cost (Nk/m3) reduced as span increased
- Reduction most marked in the 10-20m span range
- Reinforcement Costs
- In good rock - slight drop in unit cost (Nk/m3) calculated with increased span (10-20 m range)
- When rock conditions are less favorable, the costs of reinforcement can  increase rapidly with increasing span.

##   Cavern Cost Study - Conclusions 

- Rock Caverns with Spans > 20m
- Reductions in excavation cost ~ relatively small compared to potential for increase in reinforcement cost
- Many 20m+ caverns have been built, but
- Reinforcement needs can increase rapidly
- Designers and builders perception of risk will be critical to affordability -> how good is the ground?, how well are its characteristics known?
- Reserve detailed design until the ground is adequately characterized - conduct trade-off design/cost studies before committing to a large span design
- Choosing a span greater than the rock mass can reasonably allow is the greatest error a designer can make, after Johansen

## One Possible Generic Lab Layout 

## Contract Optimization

- Clear Definitions
- Scope - including ground behaviors
- Acceptability of Alternates
- Allow bidder to match facility to his/her specific skill-se/tools/materials
- Risk - register/allocate/address
- Risk allocated to party best able to address it
- Pre-qualify
- Streamlined roles and responsibilities
- Authority and responsibilities aligned
- Real-time, on-site decision making
- Variable conditions = variable response (in many contracts some variability may be potentially “unexpected”..DSC)
- Agreement on range of treatment, excavation and support options (Design-as-you-go!)

## Concept Development Steps

- 1) Find a Volume of Rock Mass Suitable to House the Required Underground Opening(s)
- Tie-in to existing excavations etc..
- 2) Orientation of Long Axis
- 3) Cross-sectional Size and Shape
- 4) Inter-Spacing Between Excavations
- Ensure that the costs and contingencies that are developed truly reflect the uncertainties in the rock mass conditions and the construction process
- after Selmer-Olsen & Broch

## Summary - Concept Optimization

-  Not rocket science but a modicum of engineering input during the concept development may reduce cost and risk..
- Not only.. End-User Needs
- But also..(if you need it we can build it, but we’d prefer..)
- Design Engineer Preferred (Stability)
- Characterize potential adverse ground behavior(s) - to include realistic worst-case scenarios (forewarned-forearmed)
- Identify the best “rock-compatible” engineering solution(s)
- Construction Engineer Preferred (Practical, Cost-Effective)
- Meet end used demands more safely and at lower cost and risk
- accommodate designer’s range of adverse ground conditions/behaviors
- Assumes change is acceptable (Constructability, VE  Review framework)
- Early integration of needs and preferences is key
- Explore before you draw -> when possible let geology guide design (easier to change the design than the rock!)

## Other Opportunities..

- Proposal #99: Wine Storage?

- Thanks for Your Attention

- Central California Wine Cave

- Large Electron Positron