HomeMy WebLinkAboutCT 02-28; La Costa Condominiums; Slope Compaction Requirements; 2010-12-09VeyifBl Testing Engineers - Vertical Five
Geotschnicat, Consf/udion Inspection & Mafeifate Tesffng Consufiante
December 9. 2010 Contract No.: 148513.32
Mr. Frank Jimeno, P.E.
City of Carisbad
Public Worl<s - Engineering Division
1635 Faraday Avenue
Carisbad, CA 92008
Subject: Third-Party Review of Slope Compaction Requirements, Proposed La
Costa Condominiums, Carlsbad, Caiifornia
References: 1. "Compaction Requirements for Slope Reconstruction, Proposed La Costa
Condominiums. Lot 185, La Costa Avenue South - Unit 1, Carlsbad Tract
No. 02-28, Carisbad, California", prepared by American Geotechnica! inc., File
No. 23280-12. dated October 21, 2010.
1. "Alternate Landslide Remedial Recommendations, Proposed La Costa
Condominiums, Lot 185, La Costa Avenue Soutii - Unit 1, Carlsbad Tract No. 02-28,
Carisbad, California", prepared by American Geotechnical Inc., File No. 23280.01,
dated March 11, 2009.
Dear IVIr. Jimeno:
As requested Testing Engineers - Vertical Five (TE), has performed a geotechnical review of
the subject slope compaction requirements as presented in reference 1. The purpose of the
review was to provide an opinion on whether the geotechnical aspects of the project been
identified and appropriately addressed in the project geotechnical documents. This letter
presents the results of our third-party review. Our review is based on geotechnical data
presented in the project geotechnical documents and our professional judgment. We have
not performed subsurface investigation, laboratory testing or independent geotechnical
analysis for the project.
Results of Review
In the above referenced letter (reference 1). the project geotechnical consultant has
recommended that the earthwork compaction requirements for graded slopes be reduced
from a minimum of 90% relative compaction (based on ASTM D-1557) to a minimum of 85%.
The referenced letter does not provide revised stability calculations or the results of laboratory
tests of samples of the onsite soils with which to base the revised compaction
recommendations. Based on the results of our third-party review, it is our opinion that there
are geotechnicaliy-related issues that should be addressed by the project geotechnical
consultant Our enumerated comments relating to the reduced slope compaction
recommendations are presented as follows:
1. The original slope stability analyses for the slope construction assumed that fill would
be compacted to a minimum of 90% relative compaction (ASTM D-1557). Those
analyses used assumed soil strength and unit weight factors associated with onsite fill
soils compacted to 90%. Fill soils compacted to 85% relative compaction would have
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Proposed La Costa Condominium Project
Cartlsbad, California
Proposal No.: 3802.10.235
lower shear strength parameters and by definition lower unit weight than the same
soils compacted to 90%.
It is our opinion that the geotechnical consultant should comment on the reduction in
assumed soil strength and unit weight associated with the revised compaction
recommendations. The consultant's comments should be based on results of
laboratory shear tests on samples of the onsite soils remolded to 85% relative
compaction. The consultant should also provide revised slope stability analyses for
the project that use the reduced soil strength and unit weight values. The analyses
should also reflect any revisions in the buttress or finished slope geometries.
It is our opinion that the manufacturer of the geogrids used in the slope reconstruction should
comment on the applicability/suitability of the use of their product in slopes where soil
compaction is less than 90%.
Testing Engineers - Vertical Five appreciates being of service to you in this matter. If you have
any questions or require additional information, please do not hesitate to contact either of the
undersigned at (858) 715-5800.
Respectfully submitted,
Testing Enginbers - Vertical
Cailos E. Acero, RCE
Project Engineer
Gene Custenborder, CEG
Principal Engineering Geologist
CA/GC:ws
Distribution: (1) Addressee (via e-mail)
V^cst f-ggj/^g Engineers - Veriical Five