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Buttressing an Aging Gravity Dam with RCC to Remedy Flood Risk
by Michel H. Jubran, Gannett Fleming
 |
| General view of Loch Raven Dam prior to rehabilitation. |
Roller-compacted concrete (RCC) was the material of choice to buttress
Loch Raven Dam, almost a century old cyclopean concrete structure.
The RCC rehabilitation, project initiated in 2002, brought the dam
into compliance with current safety requirements and significantly
increased its spillway capacity.
Loch Raven Dam is a 110-ft (33.5-m) high, 700-ft (213-m) long, concrete
gravity dam located on Gunpowder Falls about 9.5 miles (1 5.3 km)
upstream of its confluence with the tidal flats of the Gunpowder
River in Baltimore County, Maryland. The City of Baltimore built
the dam in two stages from 1912 to 1922 to provide a raw water supply.
Additionally, the dam impounds a reservoir of approximately 72,000
acre-feet or 23 billion gallons (87 billion liters).
The dam has undergone maintenance and cosmetic modifications throughout
the years; however, the height and configuration of its basic operational
and structural features has remained unchanged from 1922 until the
present rehabilitation project. The Corps of Engineers, Baltimore
District, issued a Phase I Dam Inspection Report in 1978. The report
raised concerns about the safety of the dam and during the 1980s
and 1990s, the City of Baltimore authorized additional inspections
and investigations to evaluate the extent of safety deficiencies.
These studies ultimately led to plans for improving the structural
stability of the dam and increasing its spillway capacity so it
would safely pass the current Probable Maximum Flood (PMF) mandated
by the State of Maryland. The City of Baltimore initiated the resulting
three-year construction project to rehabilitate the dam in 2002
at a construction cost of $29 million.
 |
| Cross-section of spillway showing proposed
modifications. |
Approximately 55,000 yd3 (42,000 m3) of RCC
was placed to complete the stilling basin and buttress sections,
and to raise the crest of the dam. This raised the dam height 21.2
ft (6.5 m) while maintaining the existing spillway crest elevation.
Other parts of the Loch Raven Dam rehabilitation included:
- removal and reconstruction of 180 ft (55 m) of a 10-ft (3-m)
diameter water supply conduit situated within the footprint of
the dam rehabilitation
- modifications to existing inspection galleries
- extension of the gallery entrance adit and installation
of a steel door closure
- rehabilitation of existing raw water intake screens, and
replacement of water supply sluice gate operators
- installation of toe drains and internal drains between the
existing dam and RCC buttress and between the RCC buttress and spillway
facing concrete.
Dam operational requirements and constricted site conditions influenced
the design solution and dictated aspects of construction sequencing.
Because the dam reservoir impounds water supply for more than one
million residents in the Baltimore area, maintaining full reservoir
head and continuous water service from the dam during construction
was a design requirement. However, since structural stability was
one of the dam's critical deficiencies, implementation of long-term
design solutions had to be evaluated in context with appropriate construction
condition design case loadings to minimize the potential for destabilizing
the structure during construction. Of particular relevance in this
process was the potential for, and consequences of, major flooding
occurring during the construction of dam improvements.
Another issue, less critical to the long-term design objectives
but more immediate in relation to onsite construction activities,
was the need to provide flow diversion from a relatively large,
300 square mile (777 square km), contributing watershed. To address
the stability of the dam during construction, post-tensioned rock
anchors were installed. Resolution of the overall design objectives
of increasing spillway capacity and providing adequate structural
factors of safety for the PMF event was achieved using RCC to raise
the dam and install buttress sections to provide increased structural
stability.
RCC Mix Design
Gannett Fleming performed trial RCC batching and testing program
by preparing approximately 220 trial cylinders using 18 RCC mix
proportions with various water and cement contents. Testing of the
the cylinders took place at the ages of 7, 28, 56, 90, 180, and
365 days. Based on laboratory testing and subsequent evaluation
of results from onsite test section placement trials, Gannett Fleming
selected mix proportions for RCC production as presented in Table
1.
| Table 1 - RCC Mix Design |
| Mix. No. |
Placement
Location |
Design Mixtures in lb/yd3
(kg/m3) |
Cement |
Fly Ash
(Class F) |
Water |
Aggregate |
Fine |
Coarse |
| 1 |
Buttress and stilling basin |
121
(72) |
121
(72) |
215
(128) |
2270
(1347) |
1512
(897) |
| 2 |
Top 3 feet (0.9 m) of stilling basin |
192
(114) |
286
(170) |
215
(128) |
2103
(1247) |
1400
(830) |
The buttress mix and lower portion of stilling basin (Mix 1) was
designed for an average compressive strength of 2,500 psi (17.2
MPa) at one year, while the upper 3 ft (0.9 m) of stilling basin
(Mix 2) was selected for an average compressive strength of 4,000
psi (27.6 MPa) at 90 days. The gradations of fine and coarse aggregates
specified for RCC production are presented in Table 2.
| Table 2 - RCC Aggregate Gradation |
| Sieve Size |
Percent Finer by Weight |
Fine Aggregate |
Coarse Aggregate |
| 1-1/2 in. (37.5 mm) |
100 |
100 |
| 1 in. (25 mm) |
100 |
95 - 100 |
| 1/2 in. (12.5 mm) |
100 |
25 - 60 |
| 3/8 in. (9.5 mm) |
100 |
— |
| No. 4 (4.75 mm) |
85 - 100 |
0 - 10 |
| No. 8 (2.36 mm) |
65 - 85 |
0 - 5 |
| No. 16 (1.18 mm) |
45 - 65 |
— |
| No. 50 (300 µm) |
15 - 30 |
— |
| No. 100 (150 µm) |
12 - 27 |
— |
| No. 200 (75 µm) |
5 - 15 |
0 - 2 |
Construction
 |
| RCC placement on the east section of spillway
while the west section is operating. |
The joint venture of ASI-Cianbro received the rehabilitation construction
contract on August 2002. Construction began in September that year
and is scheduled for completion in September 2005. Construction
alternated between one half of the dam and then the other to accommodate
diversion of flow during construction. Work on each side was phased
to sequentially install the rock anchors, followed by stilling basin/buttress
construction, and finally the raising of the non-overflow portion
of the dam.
To improve bonding between the RCC buttress and existing dam, a
layer of previously installed shotcrete surface treatment was removed
from the dam's face prior to the RCC placement. This was accomplished
by a combination of mechanical demolition and hydrojetting to remove
debris and treat concrete fractures that would otherwise diminish
the quality of bond between the RCC
and the existing dam. In addition, before RCC placement, drilled
and grouted tie anchors were installed in the downstream face of
the dam. During RCC placement, bedding mix was placed on each lift
at the contact zone between the existing dam and the RCC buttress
to also improve the bond along the contact surfaces. The bedding
mixture consisted of cement, sand and water.
 |
RCC nearing completion on the west section
of spillway while
the east section is operating. Note the RCC mixing plant
and conveyor system in the background. |
RCC was batched onsite using a Johnson-Ross batch plant with a
capacity of 4 yd3 (3 m3). Conveyors delivered
batched RCC to the placement area, transporting the material approximately
700 ft (213 m) horizontally and 100 ft (30.5 m) vertically. From
the conveyor, a CC 200-24 Rotec Crane and telescopic conveyor with
360 degrees of swing and a horizontal reach of up to 200 ft (61
m) delivered the RCC to the lift surface. A dozer spread the RCC
in a 12-inch (300-mm) lift thickness and placed it against the previously
placed conventional concrete mix.
All exposed surfaces of the new construction were faced with air-entrained
conventional concrete. Within the spillway the facing consisted
of 18-inches (460-mm) of conventionally reinforced concrete constructed
after the completion of RCC placement. Facing for the non-overflow
sections of the dam consisted of a 15-inch (380 mm) nominal width
of conventional concrete placed concurrent with RCC. A Rustler III
mobile concrete batch plant batched the concrete facing onsite and
delivered it to the placement area using crane and bucket. RCC was
then delivered to the lift surface, spread and compacted adjacent
to the freshly placed facing concrete. Prior to initial set of the
concrete, internal vibrators were used at the zone of contact between
the facing concrete and RCC to aid in consolidation.
 |
| A view of the dam after completion of RCC
placement on the west end prior to placing reinforced concrete
facing and weir. |
Spillway RCC was over-placed approximately 6 inches (150 mm) beyond
the design contact plane of RCC and facing concrete. A combination
of mechanical, manual, and high-pressure water jetting trimmed excess
RCC prior to placing facing concrete. No. 10 anchor bars were embedded
8 ft (2.4 m) into the RCC lifts at approximately 2-ft (0.6-m) vertical
and 4-ft (1.2-m) horizontal spacing. The contractor utilized slip-forming
techniques to place facing concrete in continuous vertical sections.
The spillway crest was reshaped to accommodate the design head for
the raised dam. This work involved demolishing the top of the existing
spillway and constructing a new spillway ogee crest. Conventional
concrete rather than RCC was used in the upper portion of the replacement
section because of space limitations and forming requirements to
achieve the design shape. A roller bucket and stilling basin were
constructed within the lower portion of the spillway. The roller
bucket was made of conventionally formed reinforced concrete and
the stilling basin slab entirely of RCC.
Vital Statistics
Remediation Features
Height of dam above foundation
prior to remediation................................. 110 ft (33.5
m)
New height of dam ...................................131.2 ft (40
m)
Overall length of dam................................700 ft (213
m)
Height of spillway crest above foundation... 90 ft (27.4 m)
Spillway width.......................................... 288 ft
(87.8 m)
Original downstream slope at spillway........ 1V:0.62H
New downstream slope at spillway............. 1V:1H
RCC lift thickness..................................... 12 in. (300
mm)
Original spillway capacity.......................... 27% PMF
New spillway capacity............................... 100% PMF
 |
| View of completed Loch Raven Dam. |
Credits
Owner: City of Baltimore, Baltimore, Md.
Engineer: Gannett Fleming, Inc., Camp Hill, Pa.
Contractor: ASI - Cianbro Joint Venture
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a printed version of this case study.
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