HomeMy WebLinkAbout3303; Agua Hedionda Sewage Pump Station; Cannon Rd, Proposed Undercrossing Carlsbad Ranch; 1995-06-13LEIGHTON AND ASSOCIATES, INC
Geotechnicol and Environmental Engineering Consultants
June 13,1995 •ay
Project No. 4930489-09
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To: Carltas Company
5600 Avenida Encinas, Suite 100
Carlsbad, California 92008-4452
Attention: Mr. Chris Calkins
Subject: Proposed Undercrossing, Cannon Road and Armada Drive, Carlsbad Ranch Phase I,
Carlsbad, California
References: Leighton and Associates, Inc., 1993, Preliminary Geotechnical Investigation, Units
I and n (Lots 1 through 7), Carlsbad Ranch Phase I, Carlsbad, California, Project
No. 4930489-01, dated July 22, 1993
, 1995, Geotechnical Investigation, Proposed Golf Cart Undercrossing,
Cannon Road, Carlsbad, California, Project No. 4930489-09, dated April 26, 1995
Introduction
In accordance with the request of the City of Carlsbad (City), we have prepared this letter regarding
the proposed undercrossings at the site. Since the preparation of the referenced reports, an
additional Undercrossing has been proposed at the site crossing Armada Drive. The purpose of this
letter is to provide a review of the geotechnical conditions in that area and to provide supplemental
recommendations for both undercrossings.
Armada Drive Undercrossing
The Armada Drive Undercrossing will be located at approximately Station 3+60 and will be similar
in design and construction to the Undercrossing proposed at Cannon Road. The geotechnical
conditions in this area consist of a thin veneer of topsoil overlying dense terrace deposits. Trench
T-5 excavated near the street centerline in this area encountered 2 feet of topsoil in this location.
Proposed grading in this vicinity will involve removal of this potentially compressible topsoil, minor
amounts of weathered terrace deposits and only minor cuts and fill. Excavation for the Undercrossing
will require a cut into the terrace deposits. The results of chemical testing performed on soils within
Phase I indicate the soils are suitable for the installation of the proposed Undercrossing.
3934 MURPHY CANYON ROAD, SUITE B205, SAN DIEGO, CALIFORNIA 92123 (619) 292-8030 • (800) 447-2626
FAX (619) 292-0771
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4930489-09
Conclusions and Recommendations
The Armada Drive undercrossing has essentially the same geotechnical conditions as at the Cannon
Road undercrossing, hence the geotechnical recommendations previously presented are also
applicable at this location with the following exceptions.
• Slurry backfill was previously recommended for trench backfills that extended within 5 feet of the
top of an arch. The purpose of the slurry backfill was to eliminate the need for compaction
equipment within this zone which has a potential to damage the arch. We understand that others
have a concern over the use of this slurry. As an alternative, the trenches in this area can be
backfill with soils compacted to a minimum of 95 percent of the soils' maximum dry density. To
avoid damage to the arch and utilities, only light weight compaction equipment should be utilized.
• The City has also expressed concern over the deflection of the arch that may occur as part of site
construction. Therefore, we recommend the implementation of a monitoring program during site
construction. This should include monitoring during construction to verify that the observed
deflections are within 2 percent of the deflections directions in accordance with ASTM D789-88
(shape control). During installation, the contractor shall implement a deflection monitoring
program that can be monitored continuously throughout the construction process by the
contractor, civil and geotechnical engineer and representatives of the City of Carlsbad. In
addition, the top of the arch should be surveyed at various locations by the project civil engineer
prior to and during backfilling operations.
• After backfill operations are complete and prior to installation of utilities above the arch, we
recommend that load testing of the arch be performed. The arch should be loaded to design
loading (H-20) and monitored for deflection by the city inspector, civil and geotechnical
engineers. The civil engineer will provide tolerable limits for deflection of the proposed utilities
on the project plans. Should these deflections be exceeded, additional recommendations to
protect the proposed utilities will be warranted. If measured deflections are within acceptable
tolerances, installation of the proposed utilities may proceed after written approval is received.
• We also recommend that all construction joints be designed as watertight joints to minimize the
potential for future seepage-related problems.
• The Cannon Road undercrossing is to be constructed and the ends buried temporarily. The
undercrossing will eventually be uncovered and incorporated into future development plans. In
order to bury the outlets, we understand that the ends will be sealed with a plywood bulkhead
covered with compacted fill. In order to be stable at this vertical gradient the fill soils should be
reinforced with a geogrid fabric placed horizontally at 1 foot vertical intervals in the fill. The
fabric should wrap and overlie the top of fill lift adjacent to the bulkhead The basal layer of the
fabric should extend 15 feet horizontally away from the bulkhead (5 foot on sides). The upper
layer of geofabric should overlap the fill lift a minimum of 5 feet The geogrid layer should
extend a minimum of 12 inches above the top of the arch. The geogrid fabric should consist of
Amoco 2002 or approved equivalent The attached Figure 1 illustrates the installation of the
geogrid reinforcement
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LflGHTONAHDASSOCIATB, IMC
4930489-09
• During construction of the undercrossings, continuous observation and testing of the backfill
process should be performed. In addition, prior to backfilling, the chemical characteristics of the
proposed backfill soils should be tested prior to use to determine conformance with the project
specifications.
If you have any questions regarding our report, please contact this office. We appreciate this
opportunity to be of service.
Respectfully submitted,
LEIGHTON AND ASSOCIATES, INC.
Michael R. Stewart, CEG1349 (Exp. 12/31/95)
Director of Geology
G. Franzone, RCE 39552
tor of Engineering
MRS/kar
Attachments: Figure 1 - Backfill Detail
Distribution: (2) Addressee
(2) City of Carlsbad
Attention: Ms. Sherri Howard
(2) O'Day Consultants
Attention: Mr. Jerry O'Connell
(2) Harrison Company
Attention: Mr. Steve Harrison
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PLAN VIEW
NO SCALE
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Temporary
bulkhead'
Proposed undercrossing
(15' wide X 10' high)
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v
5'mm
15'
SIDE VIEW
NO SCALE
Proposed roadway
\
\-Future slope
Soil backfill
(95% minimum
relative compaction)
\
\
^Proposed undercrossing
5'
15'
Geogrid reinforced
"backfill
Soil backfill
-(90% minimum
relative compaction)
\
± 1
\\
\
Temporary ^^*
bulkhead — '
(designed by others)
0'
(r(((r((( - ^«
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Geogrid reinforcement
"Amoco 2002 or approved
equivalent, 12" vertical
spacing
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BACKFILL DETAIL
ROADWAY UNDERCROSSINGS
Project No.:
Scale:
Engr./Geol.:
Drafted By:
Date:
4930489-09
NO SCALE
JGF/MRS
KRD
JUNE 13. 1995
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FIGURE NO. 1