HomeMy WebLinkAbout; Carlsbad Blvd Las Encinas Creek & Bridge; Carlsbad Blvd Las Encinas Creek & Bridge; 2009-02-05February 5, 2009 BEYOND ENGINEERING
SDB044900
Mr. John J. Cahill
City of Carlsbad
Department of Public Works - Engineering
1635 Faraday Avenue
Carlsbad, CA. 92008
SUBJECT: LAS ENCINAS CREEK BRIDGE COMPARATIVE STUDY
Dear Mr. Cahill:
Nolte Associates Inc. (Nolte) has completed a comparative study of three alternatives for
the Las Encinas Creek Bridge replacement. The three potential bridge alternative designs
considered are a cast in place box culvert, a prefabricated arch system, and a precast box
culvert.
BACKGROUND
The existing bridge over Las Encinas Creek currently carries southbound traffic along
Carlsbad Boulevard. The structure was originally built in 1913, widened in 1924, and
widened again on both sides in 1928. The current structure consists of 12 cast-in-place
reinforced concrete T-beam girders. The bridge is approximately 24' long by 68' wide.
Due to the severe deterioration of the structure on the west side, traffic has been shifted to
the east side of the bridge where the girders are in much better condition.
BRIDGE STRUCTURE ALTERNATIVES
Nolte developed three preliminary alternatives for the replacement of Las Encinas Creek
Bridge. The new bridge will have a proposed width of 45'-2" from edge of deck to edge
of deck, consisting of two 12' lanes, a 5* shoulder with a concrete barrier Type 732 on the
east side, an 8' shoulder/bike lane with a sidewalk and concrete barrier Type 80SW on the
west side. (See attached drawing). We are proposing to maintain the current centerline
roadway,' which would avoid any additional impacts beyond the current limits of the
existing structure. The new bridge width will be approximately 24' less than the existing,
however, its increased length will allow for a greater flow width to convey the 100-year
storm event. Alternative 1 is a cast in place, 12' x 8' triple cell reinforced concrete box
culvert. Alternative 2 is a precast reinforced concrete arch measuring 36' x 8'.
Alternative 3 is a precast, 12' x 8' triple cell reinforced box culvert.
HYDRAULIC CHARACTERISTICS
Nolte produced models subjecting all of the proposed bridge alternatives to the 100-year
storm peak discharge flow. This flow (2,000 cfs) was obtained from the Master Drainage
Study performed by Brown and Caldwell. The model assumed all of the altematives to
have wingwalls/headwalls flared at 45 degrees on the east (intake) side only of the
bridge. The proposed alternatives have been sized to allow conveyance of this flow
without causing objectionable backwater effects. The resulting water surface elevation
for Alternatives 1 and 3 is 12.77 feet and 13.23 feet for Alternative 2. The flow velocity
in Alternatives 1 and 3 is approximately 2 feet per second faster than Alternative 2.
POTENTIAL SCOUR
Although the predicted velocities are less than 5 feet per second, which is considered to
be non-erosive velocities, consideration should also be given to the continuous tidal
action which might undermine the structure's foundation. Therefore, scour measures will
need to be addressed further during the design phase. Some common anti-scour measures
that could possibly be utilized are cut-off walls with the placement of rock rip rap, or
geomat. Currently, a combination of cut-off walls and rock rip rap are in place, and we
anticipate the new structure to have the same protection.
DURATION OF CONSTRUCTION
Construction times between the proposed bridge altematives are noticeably different.
The approximate duration of construction for Alternative 1 can be estimated at 4 months.
This duration represents the time when the road is closed for bridge removal through
completion of the replacement bridge. For Alternative 2, the total duration of
construction would be approximately nine weeks from the time of system purchase. This
time frame would include; shop drawing completion, steel order, fabrication of precast
units, delivery and installation. During the time prior to installation, the contractor could
demolish the existing stmcture, prep the site and complete necessary foundation work.
This difference in construction time is not only reflected in the overall project cost, but
also in the impact to local southbound traffic which will be detoured through Avenida
Encinas. The approximate duration of construction for Alternative 3 is similar to
Alternative 2.
CONSTRUCTION COST DIFFERENCE
Please note that since all three alternatives will require bridge removal, similar amounts
of excavation and backfill, AC surfacing, architectural treatments, and other common
items such as approach roadway improvements, the cost of these items is not being
reflected in the following totals. The estimated cost of the cast in place box culvert is
$250,000. Purchase, delivery, and installation of the Precast CON/SPAN Arch is
estimated to be $255,000. The estimated cost of purchase, delivery, and installation of
the precast box culvert is $218,000.
CONCLUSION & RECOMMENDATIONS
Alternative 1
CIP Box Culvert
Alternative 2
Precast Arch
Alternative 3
Precast Box
Culvert
Qioo Water Surface @
Upstream Face , 12.77 ft 13.23 ft 12.77 ft
Qioo Water Velocity @
Upstream Face 4.59 ft/sec 2.56 ft/sec 4.59 ft/sec
Number of Cells needed to
Convey Flow 3- 12'x 8' 1-36'x 10' 3-12'x8'
Creek Bottom Concrete Concrete Concrete
Duration of Construction 4 months 9 weeks 9 weeks
Approximate Cost* $250,000 $255,000 $218,000
*Does not include excavation/backfill/bridge removal/AC surfacing/architectural treatments, etc.
Bridge hydraulic performance, community impact (duration of construction), structural
stability, and construction costs were all considered in the evaluation of the three bridge
alternatives.
Although it appears to be the most cost-effective option, the precast box culvert
alternative presents two factors that require careful consideration:
1. Structural stability - The three cells of the precast box. culvert are installed as
separate units next to each other. According to the manufacturer, a steel plate will
tie them together with grout placed between them. It is our opinion that this type
of connection could lead to corrosion, cracking, and differential settlement
between the individual cells. Future maintenance costs could become
problematic.
2. Potential debris accumulation at the inlet of the box culvert alternatives should
also be considered. This situation could also prove to be a recurring maintenance
cost for the City. From a design standpoint, a triple cell box culvert will need to
be oversized to account for the potential debris accumulation.
In addition to the above, the visual impact of each alternative from the beach goers'
standpoint may become an issue. A single arch would definitely give more of an open
feeling as compared to a 3-cell box, and therefore gain easier approval from the local
community.
Vy,
In summary, all three altematives for the. Las Encinas Creek Bridge replacement have
their own merits and disadvantages. Our findings indicate that the hydraulic performance
of the precast arch system will be superior to the box culvert alternatives, especially with
respect to flow velocities.
Considering that Alternative 2 has a relatively short construction time, is more
aesthetically pleasing, and has a better hydraulic performance, Nolte recommends
replacing the existing structure with a precast CON/SPAN Arch for the following
reasons:
1. Short constmction time significantly reduces impacts to southbound traffic.
2. If and when the southbound lanes are re-aligned in the future, it may be possible
to easily remove the units and use them at another location with similar
conditions.
3. Cost difference is less than 17% when compared to the precast box, and less than
5% when compared to the cast-in-place alternative.
Please consider the information provided in this letter to formulate your opinion as to
which bridge alternative you feel is best for the City.
If you have any questions concerning the contents of this letter, please feel free to contact
me at 858.385.2128.
Sincerely,
Nolte Associates, Inc.
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