HomeMy WebLinkAbout; Beech Avenue Sewer Geotechnical; Beech Avenue Sewer Geotechnical; 2008-10-21Oclober 21, 2008
ProjectNo. 106471001
Mr. Mark Biskup
City of Carlsbad
Public Works-Engineering
1635 Faraday Avenue
Caiisbad, California 92008
Subject:
Reference:
Limited Geotechnical Consulting Services
Beech Avenue Sewer
Carlsbad, California
Inland Foundation Engineering, 2008, Preliminary Geotechnical Report, Beech
Avenue Sewer Project, Carlsbad, Califomia: dated February 26.
Dear Mr. Biskup:
In accordance with your request and the authorization ofthe City of Carlsbad, we have pertormed a
review of pertinent background information including the above-refei^ced geotechnical report and
performed a brief site visit in order to conduct a limited geotechnical evaluation regarding the natural
foimational materials that are expected lo be encountered during the construction ofthe Beech Avenue
Sewer Project. We understand that the infonnation we are pixjviding may be used by the project design
team that includes Boyle Engineering and Jacobs Associates.
The project alignment is located along Beech Avenue between Ocean Street and the Washington Street
easement in Carlsbad, California. The project consists of replacing approximately 1,000 feet of existing
sewer with an 8-inch or 10-inch ID. sewer by trenchless methods (i.e. mictx^tunneling or auger boring),
and open trench construction. The sewer will be installed at a depth of up lo approximately 30 feel.
Based on geologic maps pubhshed by the Califomia Geological Survey, our field reconnaissance, and
the referenced geotechnical report, the project alignment is underlain by shallow fill and Pleistocene-
age marine terrace deposits, also referred to as "old paralic deposits." Marine terrace deposit materials
are anticipated to be encountered at the proposed depths ofthe new sewer. Relatively shallow surficial
soils, such as fill can also occur at places along the alignment. However, the primary earth unit to be
encountered in the microtunneling operations is anticipated to be the marine teirace deposits.
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Beech Avenue Sewer
Carlsbad, California October 21,2008
ProjectNo. 106471001
The marine terrace deposits were laid down by shore and near-shore processes during the Pleistocene
epoch at sea levels higher than where they stand today. The predominant constituent is sand, but
boulders, gravel, cobbles, and minor amounts of silt and clay also occur in the unit. An interval de-
scribed as boulders and cobbles in a sand matrix was encountered in boring B-01 that was drilled for
the preparation ofthe referenced geotechnical report. Cohesionless (or caving) sands may also be
present. Based on our experience, overall the Pleistocene marine terrace deposits can be character-
ized as varying from slightly cemented sands to sandstones.
Based on the above, the natural fomiational materials that will be encountered during the constmction of
the Beech Avenue Sewer are expected to have unconfined compressive strengths varying fix)m ten
pounds per square inch (psi) for siighUy cemented sands to approximately 500 psi for sandstones. These
estimates are for the sand/sandstone layers that make up the body ofthe unit However, resistant boulders,
cobbles, and gravel were encountered and may also occur in other areas within the terrace deposits which
may result in difficult drilling conditions for the proposed trenchless constnjction altemadves.
The conclusions provided herein are based on our review of readily available background data and field
observations of exposed surface conditions. Ninyo & Moore has nol perfomied any subsurface explo-
ration of its own for this project. Additional services including subsurface explorafion by Ninyo &
Moore could be performed to fiirther refine our conclusions regarding the subsurface condilions.
We appreciate the opportunity to be of service on this project.
Sincerely,
NINYO & MOORE
Andres Bemal, GE.
Senior Project Engineer
Rl/AB/GTF/ek
Distribution: (1) Addressee
Gregory T. Farrand, C.E.G
Principal Geologist
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