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Glade Dam and Reservoir - PFMA

USSDMay 3, 2018

Jennifer Williams, PE – AECOMDarrin Harris, PE – Black & VeatchJeremy Franz, PE – CO Dam Safety

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1. PURPOSE, CONTEXT AND SCOPE

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1.1 Purpose of the Risk Assessment (PFMA)

• Perform a Potential Failure Mode Analysis on the Preliminary Design of a new embankment dam

• Evaluate design in a failure-mode framework to support, document and track design decisions

• Identify critical design issues that may pose a dam safety concern

• Focus the scope for the Phase 2 field investigations

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1.1 Purpose of the Risk Assessment (PFMA)

• PFMA served as a format to conduct a design review by the Design Team, Regulator and Technical Review Board

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• New 280-ft-tall zoned earthfill embankment dam

• 170,000 acre-ft off-channel reservoir• Hydraulic Structures

– Spillway– Dual Inlet/Low Level Outlet– Multi-level Intake

• Forebay and Pump Station• Conveyance Canals

Glade Dam – Proposed Facilities

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9-10 May 2017

Glade Reservoir Project – PFMA Workshop

Main Dam

Saddle Dam

High Level Outlet Low Level Outlet

Spillway

Outlet Channel

Munroe Canal Bypass Pipeline (followings high

level outlet alignment and continues upstream)

Refined Feasibility Layout

“Rock Knob”

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Refined Feasibility Layout

Rock Knob

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Zoned Earthfill Embankment Dam

30+00

Riprap

CoreEarthfill Shell

FilterDrain

Earthfill Shell

Riprap

Earthfill Shell

Core

Earthfill Shell

Chimney Filter/Drain (core protection)

Blanket Filter/Drain (foundation protection

and seepage conveyance)

9-10 May 2017

Glade Reservoir Project – PFMA Workshop

MAIN DAM, SECTION 30+00

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Overburden Foundation Profile

Interbedded sands and clays with weak soil randomly throughout full depth.

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6 June 2016

Glade Reservoir Project – Preliminary Design and Phase I Investigation

A

A’

Cutoff wall

Grout Curtain

Cutoff wall

Bedrock Geology ProfileRight Valley

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6 June 2016

Glade Reservoir Project – Preliminary Design and Phase I Investigation

E

E’

Grout Curtain

Bedrock Geology ProfileLeft Valley

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1.2 Context of PFMA

• Design Criteria– Colorado Dam Safety Criteria– Owner Operational Requirements

• Meet Current State-of-Practice– Technical Advisory Panel– Potential Failure Mode Analysis

Preliminary Design Requirements

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1.2 Context of PFMA

• Dam Alignment• Foundation Treatment

– Overburden– Bedrock

• Embankment Type– Rockfill vs Zoned Earthfill

• Internal Zoning Geometry• Location of Hydraulic Structures

Design Decisions not Driven by Criteria

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1.2 Context of PFMA• Why qualitative PFMA and not quantitative “Risk”?

• Recognizing this is in early phases of design– Insufficient analyses/design for quantitative estimates

• Process to be repeated in final design advancement stages

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1.2 Context of PFMA

• Colorado Dam Safety Program moving towards “Comprehensive Dam Safety Evaluations” which follow a PFMA approach

• It will dovetail into state inspection program once the dam enters operations

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1.3 Scope of the PFMA• Desired Outcome: Identify PFMs that most

influence non-criteria design components • At this phase of design only focused on PFMs

that resulted in catastrophic release– All PFMs have “high” consequences

• Identified PFMs that could Lead to an “Uncontrolled Release of the Reservoir”– Future phases may consider both failure and

operational PFMs

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1.3 Scope of the PFMA

• PFMs Organized by Loading Type– Normal, – Flood (PMF) – Seismic

• PFMs also Organized by Project Component– Embankment, Spillway, Outlet, etc

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1.3 Scope of PFMA Addressing Uncertainty

• Degree of uncertainty inherent at this phase of design influenced the level of detail and scope of the PFMA

• Rock-knob stability analyses performed using a range of parameters

• Several PFMs that would be better informed/addressed with future quantitative analyses and less so with subjective estimates were not evaluated (i.e. seismic stability, flood overtopping, etc).

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2. POTENTIAL FAILURE MODE ANALYSIS

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PFMA Approach• Facilitated Team

– Design Team (Black & Veatch-AECOM: 11 participants)• Geology, geotech, hydraulic structures, constructability leads

– Owner (5 participants)– Technical Review Board (4 participants)– Regulator (Colorado Dam Safety: 3 participants)

• Review Materials and Project Overview Presentation

• Initial Brainstorm List Developed by Design Team Provided for Comment

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PFMA - ApproachPFMA Workshop• Initial brainstorm list reviewed (60 PFMs)• PFM Screened by Team based on:

– Those judged non-credible– Those better informed by future analyses– Prioritized those with key influence on design details– Noted reason for no further development

• Developed selected PFMs• TAP Responses to Questions Posed• Summarize Take-Aways

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PFMA - Format• Qualitative • Tabular format• Similar to FERC or Reclamation/USACE Best

Practices, but…• Did not Categorize based on Likelihood

– Early design of a new dam, therefore ranking PFMs or eliciting probabilities not warranted

– Identified PFMs will be addressed through design advancement.

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PFM Identification and “Screening”

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PFM Identification and “Screening”

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Insights and Recommendations• Process was an Effective Design Review Tool• Identified Key Considerations to be Investigated

by Design Team early in design• Process helped identify the most critical data

gaps needed for Phase II investigation• Resulted in justification to evaluate an alternate

dam alignment – a significant design change• Similar process will be beneficial at future final

design phase and construction phase

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Insights and Recommendations

• Excellent example of how simplified Risk/PFMA analysis can be used for design

• Earlier is generally better• Living document which will be revisited during

future design milestones & construction– Value engineering proposals– Verification of field conditions– Response to RFIs and design change orders

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Insights and Recommendations

• Operation Phase– Monitoring thresholds – Detection– Focused inspections – Detection – Emergency actions – Intervention

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LIMITATIONS, DECISIONS, RISK COMMUNICATION AND

LESSONS LEARNED

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Limitations of Approach Used• Qualitative

– Did not provide numeric annualized risk estimate to compare to tolerable guidelines.

– All PFMs will need to be addressed and designed to be “remote”

• Based on early design details– Limited design analyses had been completed– Focused on geologic characterization and “typical”

design details• Evaluated only select number of PFMs that

were most influential to design advancement

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Decision and Recommendations

• Embankment Dam– Evaluate new dam alignment to reduce seepage

and stability concerns of the weaker rock abutment

– Evaluate stability of rock knob to identify satisfactory offset

– Foundation treatment to be required under entire core and downstream filter area. Upstream shell area should be spot treated with filter material

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Decision and Recommendations

• Hydraulic Structures– Move spillway off weathered rock knob– Evaluate required drawdown criteria using both

Bureau of Reclamation and SEO criteria

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Resulting Design Modifications

Feasibility Alignment

Preliminary Alignment

Feasibility Spillway

Preliminary Spillway

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Glade Dam – Proposed Facilities

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Value Added by Using PFMA-Approach

• Built consensus of stakeholders in transparent manner and allowed owner to understand risk drivers

• In this case, a significant alignment modification was made with less impact to the design schedule/budget

• Provides a valuable design-review tool for the duration of design as well as a performance-review tool for the life of the structure.

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Risk Communication• PFMA provided justification for design

modifications• Will be used to document design reasoning for

future periodic comprehensive dam safety evaluations for the life of the structure

• Used by owner to justify to project sponsors/participants the increase in project cost due to the revised embankment alignment

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Lessons Learned

• Review materials, and pre-prepared workshop materials and PFM list were important for efficient execution during the workshop

• Allow sufficient time to work through important matters thoroughly, but keep team on task.

• Don’t schedule a 12-hour day (lose focus)• Technical diversity of the team is beneficial

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Lessons Learned

• Having Regulator participate in the PFMA as part of the Review Board had advantages:– Streamline our review timeline– Develop understanding of the project early– Deeper understanding of TAP/BOC/PRB comments– Provide perspective on Regulator’s Rules and

Process– Reduce Owner Risk

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Lessons Learned

• PFMA at this early stage will allow critical design features to be tracked through future design phases and construction

• Owner perspective: Valuable tool to identify major design changes early in the process