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?