2026 Industry Outreach Presentation.pdf

Patillas Dam Seismic Safety Modification Industry Outreach, San Juan, Puerto Rico 6/11/2026

Presentation Outline

• Project Status

• Facility Background:

• Location

• Site Overview

• Existing Outlet Works

• Powerplant

• Spillway and Operations

• Justification for Modification

• Design Objective

• 60% Design:

• Design Objective

• Construction Sequencing

• Site Access and Use

• Outlet Works Isolation

• 60% Design Overview

• Construction Considerations

• Site Visit Logistics

• Q&A

Project Status

• 60% Design transmitted December, 2025

• Experiencing design delays due to federal staffing challenges

• Expected out for solicitation in late 2027

• NEPA/SHPO expected by early 2027

• FEMA/HUD Funding requires completion and closeout by May 2033

Project Location – Patillas, Puerto Rico

Location

• Confluence of the Río Patillas and Río Marín

• Río Marín referred to as Maton River in historical documents

Presenter Notes Presentation Notes Point out spillway

Site Access

• Primary access through route 1

• Gravel over culverts

• Crest access through route 2 (blue)

• Paved, narrow

• Canal access to route 3

(green dash)

• Unimproved two-track

• All routes unaffected by spillway flows

Spillway K-8 School

Site Overview

CONTROLLED

Patillas Dam

Powerhouse (abandoned)

Tunnel uptake shaft Overflow weir

Outlet Works Control Tower

Residences (encroachment) Access from offsite

Abandoned canal discharge structures

Spillway opposite side of abutment, out of scope

Patillas Canal (buried) and canal gate structure

Bathrooms (abandoned)

Forebay uptake shafts

Crest access (gated)

Toe access (gated)

Description of Dam

• Semi-hydraulic fill dam, built in 1914

• Concrete Crest Wall Elevation (El.) 238.8

• Embankment shown as 232 on many drawings before camber

• Crest length = 1,067 ft, width = 20 ft

• Top of Active Conservation (TAC) El. 222.0; Storage = ~11,000 acre-ft

• Reservoir restriction since 2015 at El. 212.0. The 1972 spillway ogee El. 191.0 (formerly 212)

• Top of Dead Storage El. 133.6 (Outlet-Works Intake-Tower Trashrack Sill)

• Min. Freeboard = 16.8 ft

• Structural height = 147 ft; Hydraulic Height = ~130 ft (El. ~92 to 222)

• Slopes = 2H:1V to 3H:1V u/s; and 1.5H:1V to 2H:1V d/s (from crest to toe)

• No toe drain system or filter, very limited drawings

Construction Methods – Semi-Hydraulic Fill

• Dumped fill from 40-foot-tall railroad trestles

• Sluiced “clay-rich” core fill (specified as 33% min.) fill to settling pond via flume

• No compaction of any materials, grouting, etc.

• Note variation in downstream shell particle sizes

Patillas Dam (1914) – Idealized sections

Upstream Reservoir Downstream

Starter Dikes Starter Dikes

Concrete Cutoff Wall

Core Pool

View of Powerhouse, Right Abutment, Emb.

Embankment and Left Abutment

Presenter Notes Presentation Notes No signs of seepage noted

Patillas Dam Site Geology

• Quaternary Alluvium/Colluvium (Qac)

• Composed of a heterogeneous mixture of soil materials ranging from clay to boulder size.

• Undifferentiated Tertiary and Cretaceous Plutonic Rocks (TKp)

• Composed primarily of granodiorite and diorite with various stages of weathering and hydrothermal alteration.

• Localized zones of extrusive flow igneous rock such as andesite and volcanic sandstone.

Patillas Dam Exploration Maps

CPT-1CPT-2CPT-3

Patillas Dam Exploration Results

Outlet Works Tunnel

• 20 ft wide, 20 ft tall horseshoe shaped, drill and blast for ~600 feet under right abutment

• Lined with ~12” of unreinforced concrete

• Inspected and tested in 2011, ~3000 psi. Good condition other than crown voids. Limited seepage through liner.

• Originally controlled with two circular slide gates in outlet works tower

• Downstream gate was removed in the 1960s, added a 54-inch steel pipe to convey flows to the now abandoned 1980s hydroelectric power plant

• Outlet works feed Patillas Canal

Outlet works plan – Modified 1961

Outlet works section - 1961

• ~393 feet of existing 54-inch pipe

• ~325 feet of tunnel through bedrock

• Remaining ~68 feet of concrete forebay

Outlet Works Control Tower Design Drawings

CONTROLLED

Downstream gate removed, upstream leaksup to ~28 cfs Still confirming this leakage rate

Outlet Works Tunnel Interior

• 20 ft high, 20 ft wide horseshoe shaped tunnel

• 12-inch-thick concrete tunnel lining

• Limited access through an 18-inch-thick bulkhead wall

Outlet Works Tunnel Interior Tunnel uptake shaft

Outlet Works Forebay

Outlet works discharge detail

• Tunnel uptake shaft was original discharge point, bypassed by 1960s construction of forebay and forebay uptake shaft

• Construction of 54” conduit and connection to powerhouse

• Powerhouse constructed ~1980s

• Canal contains an overflow weir

• Patillas canal buried in 72” conduit in 1990s

Outlet Works Forebay 1961 Construction Views

• No reinforcement or other engineering drawings available for demolition reference, additional photos show some rebar installation

• USBR including concrete outline drawings based on LiDAR

Canal Headworks

Powerhouse

• To be demolished

• No RCRA controlled metals/coatings identified

• Transformers emptied

• Design drawings available

Site Overview Refresh

Patillas Dam

Powerhouse (abandoned)

Outlet Works/Forebay Structures, tunnel entrancePatillas Canal (buried) and canal gate structure

Control Tower

Residences

Abandoned secondary canal discharge structure

Spillway opposite side of abutment, out of scope

Spillway – Out of Scope

• Storm flows do not impact site access

• Flows around site

• Can draw reservoir to El. 191 ft

(~47 feet below crest)

Operations

• Patillas Canal supplies water to:

• PRASA coal powerplant

• Municipal drinking water use

• Irrigation users

• Typical use of 17-21 cfs

• Patillas Reservoir is used by another municipal water filtration plant

• Minimum RWS 185

• Ideally no interruptions in canal water service

• PRASA powerplant has limited on site water storage

Justification for Modification

• Uncompacted hydraulic fill materials are vulnerable to liquefaction at ~>500-year EQ

• Upstream crest loss and overtopping are likely

• Many residences within the dam inundation area <3 miles downstream

• Facility Background:

• Location

• Site Overview

• Existing outlet works

• Powerplant

• Spillway and Operations

• Justification for Modification

• 60% Design:

• Design Objective

• Construction sequencing

• Site access and use

• Outlet Works Isolation

• 60% Design Overview

• Construction Considerations

• Construction of a secondary zoned embankment dam overtop the existing dam

• Uses portion of the existing dam expected to remain in place for partial support

• Requires an outlet works replacement and extension

60% Design Objective

60% Construction Sequencing

1. Installation of a bypass pumping system for canal deliveries

2. Primary and secondary isolation of the outlet works

1. Must occur in the tower, dewatering of the tunnel upstream of the tower may cause collapse of the tunnel

3. Demolition of forebay, powerplant, existing outlet works, misc features

4. Stripping of the embankment, abutments, dewatering and excavation

5. Construction of new outlet works and valve houses

6. Placement of tunnel CLSM backfill

7. Placement of key trench filter and embankment fill

8. Construction of three toe drains, filter material

9. Placement of remaining zoned embankment fill

Site Use Map – Existing Conditions

• Leased area for Contractor use

• Access constraints from PR-799 (next slides)

• Primary access through route 1

• Gravel over culverts

• Crest access through route 2 (blue)

• Paved, narrow

• Canal access to route 3

(green dash)

• Unimproved two-track

• All routes unaffected by spillway flows

Spillway K-8 School

Site Access Considerations

• Adjacent elementary school will be impacted by construction

• PREPA is coordinating alternative access for student pickup/dropoff

• Can also widen corner with PR-799

• Tight corners from PR-799

• Abandoned building at intersection is encroachment on PR DOT property

• Requires improvement for semi-trailer access (next slide)

Constraints

• Demolition of existing structure and widening of access road to be included in contract

• Minor grading also required

• Single barrel culvert

Double Culvert Restriction

Site Access Considerations

• Access Road Culvert over outlet works discharge channel

• Tight turning radius, either requires improvement or use of private culvert to the South (next slide)

• Has been used for heavy construction in the past but is unrated

• Straighter alignment onto private land available for lease

• Landowner has allowed PREPA to use access for large equipment in the past

Single barrel culvert

Outlet Works – Discharge Channels

Canal Regular Flows – 25 cfs, 100 cfs max

Evacuation Flows – 600 cfs max, “Old Patillas River” Channel

Spillway Discharge Channel into Patillas River

• Downstream slide gate raised and concreted in place to install existing 54” steel conduit, gate stem removed

• Remaining 60-inch slide gate leaks ~28 cfs measured at the canal

• Isolation is required to remove and replace the 54-inch conduit

Isolation of Outlet Works – High Risk

D/S gate raised and concreted in place, stem removed

Gate Chamber Tunnel View

• Design intends to tie in at expansion joint flange with welded connection, weld repair remaining concrete encased stub

Outlet works tower gates – scan results

• USACE conducted a dive inspection earlier in 2026

• Attempted to scan potential bulkhead location upstream of the existing gate

• Unable to cover most of survey area due to camera limitations and dive conditions

Outlet Works Gate Drawdown May 2026

• Gate leakage floods ~1/3 of the

42-inch discharge pipe at the powerplant

• Allows for access down the tower to the gate chamber

• Existing ladder rungs are deteriorated and unsafe

• Facility Background:

• Location

• Site Overview

• Existing outlet works

• Powerplant

• Spillway and Operations

• Justification for Modification

• 60% Design:

• Design Objective

• Construction sequencing

• Site access and use

• Outlet Works Isolation

• 60% Design Drawing Overview

• Construction Considerations

60% Design Overview

• Temporary Canal Outlet Bypass

• Outlet Works Isolation

• Demolition

• Excavation

• Final Grade

• Site Grading

• Excavation and Dewatering

• Outlet Works

• Tunnel encasement

• Piping and Valves

• Gate House and Valve House

Canal Outlet Works Bypass

• Canal deliveries must continue uninterrupted throughout construction

• Deliveries for a coal power plant, irrigation and municipal water

• Brief (<4 hour) downtimes may be coordinated in advance with

PREPA

Canal Deliveries During Construction 0-25 cfs

• Diesel-electric pumping system on barge near outlet works tower

• Three 12-inch HDPE lines running along R abutment to buried canal discharge structure

• Contractor designed and operated

• Example alignment shown in orange

Outlet Works Isolation

• Can start after temporary bypass installation

• Could involve a bulkhead at the

24-inch square manhole and at the existing blind flange

• Weld repair of the existing 54-inch stub requires dry conditions

• Likely be Contractor designed to enable flexibility

Diver installed bulkhead, high risk – do not pursue 24-inch manhole bulkhead with sump through penetration for vent pipe Demolish existing gate and concrete to access slot Partial bulkhead to allow sump to dry out e. stub Secondary (tertiary?) sacrificial valve, abandoned in place

Demolition

Demolition – Powerplant and Forebay

• Powerplant drawings are available in 60% package as FIO

• Very limited forebay drawings

Demolition – Tunnel Interior/Forebay

Demolition Tunnel Interior

Demolition – Limited Construction Photos

• No reinforcement drawings available

Excavation Plan

• Rolling excavation for key trench to limited area open

• Stripping to fractured bedrock along abutments

• Shaping on right abutment to eliminate overhangs and vertical

Excavation – Typical Section

Dewatering and Excavation Instrumentation

Key Trench Profile

• Key trench excavated to elevation 90 or fractured bedrock

• Allows for adequate capture of foundation seepage

• Limits downstream deformations

• Abutment shaping not shown yet

• Competent bedrock much deeper

Final Grade – Draft 90%Plan

Final Grade – Typical Section

Material Zone Requirements Commercially Sourced – Limited Borrow

• Zone 1 – Semi pervious, granular with some non-plastic fines

• Max Particle Size: 0.75”, 90-100% passing No. 4 Sieve, 40-75% passing the No. 200 sieve

• Minimum Plasticity Index: 0, Maximum Liquid Limit: 30

• Zone 1A – Transition zone to existing dam shell material

• Considering a geotextile instead

• Zone 2 – Filter Sand – ASTM C33 Fine Concrete Sand

• Zone 3 – Filter Gravel – No. 67 Gravel

• Zone 3A – Key Trench Rockfill – Screened quarry pit run material

(<10% fines)

• Zone 4 – Miscellaneous Fill – Similar to Zone 3A with more allowable fines, potential re-use of Key Trench excavation

Toe Drains

• Designed for up to 15,000 gpm

• 24-inch HDPE pipe at 1% slope

• Separated by concrete encased 54-inch outlet works pipe.

• 2 discharge points – Parshall Flume and V-Notch weir

• ~18% slope along left abutment to above crest

• Follows property boundaries along left abutment

• Allows for roadway improvements, Contractor Use

Limits of Disturbance

Site Plan

• Contractor use area leased from adjacent land owner

• Surveyors identified potential encroachment on PREPA property based on historic records

• Designers have generally avoided the encroachment areas

• Drainage swales at toe and abutment groins to convey surface water

• Designed for 100-year storm

• “Gate House” to house 54-inch pipes and valves

• “Canal Valve House” to house 36-inch canal supply line

• Access both from Patillas Canal access road and existing embankment road

• Crest access from existing paved road

• 30% showed additional access from dam toe but not achievable with land boundaries

New access under design for 90%

Outlet Works

• 600 cfs capacity enables drawdown outside high inflow periods

• 3D lidar scan in June 2025 used to model new pipe alignment

• Milling of rock in the tunnel required to add clearance for encasement rebar

• Tunnel abandoned and backfilled with CLSM

• Considering cellular concrete

• Contact grouting along crown (not shown at 60%)

71Outlet Works – Conduit Encasement

Outlet Works Iso View

• Concrete encased 54” steel conduit from existing outlet

• 36” steel conduit bifurcation to supply Patillas Canal with 0-100 cfs

Outlet Works – Downstream Features

Canal Stilling Basin

Canal Stilling Basin

Evacuation Stilling Basin

Outlet works Piping isometric view

• Outlet works guard and regulating gates

• Blind flange for future powerplant

• Butterfly valve and plunger valve on canal supply line to regulate flows

• Split sleeve couplers for potential differential settlement

• Facility Background:

• Location

• Site Overview

• Existing outlet works

• Powerplant

• Spillway and Operations

• Justification for Modification

• 60% Design:

• Design Objective

• Construction sequencing

• Site access and use

• Outlet Works Isolation

• 60% Design Drawing Overview

• Construction Considerations

Site Use

Stockpile Area (Ac) Stage Zone 1 Zone 1A Zone 2 Zone 3 Zone 3A Zone 4 TOTAL

1 - - 1.40 1.40 1.50 1.50 5.80

2 1.50 0.75 0.75 0.75 - 4.60 8.35

3 3.70 1.50 1.00 1.00 - 3.70 10.90

4 3.00 1.40 0.50 0.50 - 1.20 6.60

• Staging Area (Leased): 16 Acres

• Borrow Stockpiles:

Embankment Stages

Proportional Volume (CY) Stage Zone 1 Zone 1A Zone 2 Zone 3 Zone 3A Zone 4 TOTAL

1 - 817.00 20,683.0

22,030.0

27,807.0 0 28,208.00 99,545.00

2 28,636.00 9,587.00 9,718.00 9,693.00 - 146,864.0

204,498.0

3 104,833.0

29,386.0

12,244.0

12,238.0 0 - 95,568.00 254,269.0

4 68,613.00 21,576.0 0 2,463.00 2,478.00 - 15,964.00 111,094.0

Total 202,082.0

61,367.0

45,108.0

46,439.0

27,807.0

286,604.0

669,407.0

Outlet Works – Existing Features

Weather Impacts

• Rain Days:

• Throughout the year, but heaviest in the Fall

• 5 to 10 rain days per month

• Fall Shutdown

• No Embankment work assumed August 1 to October 31

Construction Schedule

• Award: 9/2027 (assumed)

• On-site: 12/2027

• Bypass Operation: 3/2028 – 3/2030

• Demo and Tunnel Extension: 3/2028 – 3/2030

• Embankment: 6/2028 – 4/2031

• Contract Complete: 7/2031 (estimated)

• Duration: 1460 Calendar Days (4 Years)

Site Visit Logistics

• 9 am departure from Estadio Municipal

Gaspar Cochran

• Bring steel toes and hard hats

• PR-184 (red line) is not advised – narrow, windy and slow

• Eastadio

Municipal Gaspar Cochran, PR-3 Patillas 00723, Puerto Rico

Mike Henrick mhenrick@usbr.gov

Stefano Truschke struschke@usbr.gov

Questions?

mailto:mhenrick@usbr.gov mailto:struschke@usbr.gov