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Reservoir Condition AssessmentHonolulu Board of Water Supply February 5, 2015
Lloyd Soohoo, P.E.
Jon Toyoda, P.E.
CDM Smith
Discussion topics
1. Board of Water Supply (BWS)
2. Why BWS performed reservoir inspections and assessments
3. How the work was conducted
4. Overview of preliminary findings
5. How the work will influence the 30-year CIP
6. Implications of this work for other utilities
Board of Water Supply
Water Master Plan
• Condition Assessment
– Wells and Pump Stations
– GAC Filter Plants
– SCADA
– Corporation Yards
– Pipelines
– Reservoirs � TODAY’s TOPIC
• Water Master Plan
• 30-year Capital Improvement Program (CIP)
• Financial Plan and Rate Study
Why is BWS performing reservoir condition
assessments?
• Reservoirs are critical facilities
• Major repairs or replacement can in some cases take 2+ years
to implement
• BWS routinely performs these comprehensive assessments on
a 10-year cycle
• Results will help identify and prioritize projects for the 30-year
CIP
What types of reservoirs are in the BWS system?
• Configuration
– Buried
– Partially buried
– At-grade
– Elevated
• Size range
– 0.1MG to 6.0MG
• Material
– Concrete (169) – varying
types of designs
– Steel (2)
• Age
– <2 to 90+ years
Reservoir types
Conventionally reinforced
• Passive standard reinforcement
resists hoop tension
• Long track record of success
with minimal maintenance
• Less efficient for larger
capacities
Internal post-tensioned
• Some in service 50+ years
• Tendons in internal wall
ducts actively compress
wall
Reservoir types (continued)
Wire-wound
• 1950s – 1960s tanks have
required varying levels of
maintenance
• Prestressed wires applied to
core wall exterior and covered
with pneumatic mortar
External post-tensioned
• Wire-wound tank repair
• Post-tensioned strands
applied to exterior
• Maintenance, corrosion,
security concerns
Reservoir types (continued)
Strand-wound
• Wire-wound tank repair or
new construction
• Galvanized prestressing
strands applied to core wall to
place wall in compression
• Larger capacities possible
Three evaluation techniques were used
Approach # of
Reservoirs
How selected Purpose
1. Exterior
inspection
171 All active reservoirs Visual inspection of
all reservoirs in
system
2. Interior
ROV
inspection
30 Reservoirs >40 yrs
old not previously
interior inspected
Assess interior
condition of older
reservoirs
3. Analytic
evaluation
(desktop)
17 Representative
samples of different
design/materials
Identify seismic and
wind upgrade
opportunities
1. Exterior inspections
Objectives:
• Document condition of
concrete, rebar, paint,
appurtenances
• Identify components
requiring repair
• Document performance of
prior repair methods over
time
• Identify and prioritize
repairs for inclusion in 30-
year CIP
Exterior inspection items
Upper and lower seals
• Leaks, gaps in seal, and vegetation
in seal
Walls
• Leaks, spalled concrete, and large
cracks
• Sound entire wall surface
• Condition of coating system
Foundation slab
• Leaks, cracks, concrete
deterioration
Exterior inspection items (continued)
Roof
• Ponding, topping material,
cracks, hatches, etc.
Appurtenances
• Vents, guardrail, ladders, security
cameras
Reservoir site
• Vegetation/Landscaping, on site
roadway, access roadway,
perimeter fencing
2. Interior inspections
• Inspect interior of 30
reservoirs using Remote
Operated Vehicle (ROV)
• ROV was disinfected per
BWS-modified Association of
Diving Contractors
International standards
• 1-2 ROV inspections were
performed per day
• Notable observations
documented in narrated
video files
Interior inspections
Efflorescence
Depth below
water surface
Depth below
water surface Bearing
Benefits of periodic inspections
• A regular inspection program can document the reservoir or
component conditions over time
• Allows for rapid evaluation of reservoir/component condition
May 2005 May 2014
1% hollow
area
20% hollow
areas
40% hollow
areas
Exterior and interior inspections
Reservoir components are scored on 1 to 5 scale
– 5 no issues, 1 poor condition/failure imminent.
• Scoring combined with criticality rating will help prioritize
component repairs
• Cost estimates developed for issues needing repair or
replacement
Reservoir inspection – Preliminary findings
1. Vast majority of BWS reservoirs in good to excellent condition
2. Less than 5% of reservoirs were noted to need some form of
near-term repair/rehabilitation
3. The most significant structural issues were observed in wire
wound tanks from the 1950’s and 1960’s
3. Numerical structural analysis (objectives)
• Identify potential opportunities to improve seismic
performance
• Perform seismic analyses of 17 representative reservoir
types/age/configurations using these criteria:
• IBC 2012
• ASCE 7-10
• ACI 350-06 and 350.3-06
• AWWA D100, D110 and D115
• Develop conceptual seismic retrofit details and costs
Seismic hazards can be significant
source: http://earthquake.usgs.gov/earthquakes/states/hawaii/hazards.php
Oahu
Conventionally reinforced is predominant
Connections were generally unanchored and
contained
Pre-1961 Wall-floor connection
Pre-1961Wall-floor connection
Preformed slot in
slab
Preformed slot in
slab
Copper
plate
Copper
plate
Unanchored and partially contained wall-floor
connections
Post-1961 Wall-floor connection
Post-1961 Wall-floor connection
Stepped footing
Wall
outside face
Wall
uncontained on
outside face
Graphite between
wall and mortar
topping
Wall-floor connection effect
Pre-1961
unanchored,
contained
Leading Trailing
Post-1961
unanchored, partially
contained
Leading Trailing
Earthquake load distribution
• Some hydrodynamic pressures are distributed between the
leading and trailing halves
• Pressures vary with angle and depthsource: American Concrete Institute, ACI 350.3-06
Leading half
(pressure)
Leading half
(pressure)
Trailing half
(suction)
Trailing half
(suction)
Finite Element modeling loads
MATLAB was used to generate plate geometry and loads for
modeling in RISA 3D
Numerical structural analysis
Failure modes evaluated:
- Hoop tension
- Vertical flexure
- Horizontal flexure
- Shear
- Wall-roof connections
Vertical
flexure
Hoop
tension
Horizontal
tension
Horizontal
reinforcement
resists hoop
tension
Vertical
flexure
Vertical
reinforcement
resists vertical
flexure
Hoop tension
Partially-contained
Post-1961
Contained
Pre-1961
Contained wall-floor connection performed better than
partially-contained
Contained wall-floor connection performed better than
partially-contained
Vertical flexure
Partially-contained
Post-1961
Contained
Pre-1961
Conceptual retrofit alternative
Add curb to create a contained condition
Curb retrofit Restraint cable retrofit
Conceptual retrofit alternative
Add restraint cables around tank perimeter
Curb retrofit
Application of numerical structural analyses
• Developed cost-effective details to improve seismic performance
that require minimal downtime to implement
• Encourage additional analysis
Implications for other utilities
1. Significant value in regularly scheduled evaluations
– Concrete reservoirs: (10 years)
– Steel reservoirs: (5 years)
– Interior inspections: (dependent on WQ and accessibility)
– Consistent form of documentation
2. Selected designs may warrant more frequent inspections
– Wire wound tanks (1950’s and 1960’s)
– External post-tensioned tanks
3. Does not appear that there is a fixed “expiration date” for
concrete reservoirs
4. Seismic retrofits, if desired, can be implemented at relatively
modest cost.
QUESTIONS?