HomeMy WebLinkAboutCT 01-01; CALAVERA HILLS VILLAGE; CLARIFICATION OF TRENCH BACKFILL RECOMMENDATIONS; 2003-05-05c-1OI-OI
Geotechnical *Geologic * Environmental
5741 Palmer Way • Carlsbad, California 92008 • (760) 438-3155 • FAX (760) 931-0915
May 5, 2003
W.O. 3459-B-SC
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Calavera Hills II, LLC
2727 Hoover Avenue
National City, California 91950
Attention: Mr. Mark Carpenter
Subject: Clarification of Trench Backfill Recommendations, Calavera Hills II, Carlsbad
Tract 00-02, Drawing 390-90, City of Carlsbad, San Diego County, California.
References: 1. "Review of Grading and Trench Backfill Recommendations, Calavera Hills
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00-02, Drawing 390-90, City of Carlsbad, San Diego County, California,
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dated August 16, 2002, by GeoSoils, Inc.
"Preliminary Geotechnical Evaluation, Calavera Hills II, College Boulevard a
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Road Bridge and Thoroughfare, District No. 4 (B & ID), City of Ca
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W.O. 2863-A-SC, dated January 24,2001, by GeoSoils, Inc.
"Greenback, Standard Specifications for Public Works Constructi
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Standards, Inc., 2000 edition.
Dear Mr. Carpenter:
In accordance with your request GeoSoils, Inc. (GSl), has reviewed our
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regarding grading and trench backfill recommendations (see Referenc
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prepared this letter for the purpose of providing clarification regarding the tr
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recommendations presented. The scope of our work has included a
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referenced documents and preparation of this letter report.
As a result of conversations with your office, a review of the referenced repor
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this office (Reference No. 1 and No. 2), and a review of the "Greenbo
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(Reference No. 3), the following recommendations may be incorporated into th
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construction of the project. Unless specifically superceded in the tex
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recommendations presented in the referenced reports remain valid an
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Existing Recommendations
As discussed in Reference No. 1, site grading within the public right of
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areas) will generally consist of either filling per plan or overexcavating cut ar
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filling to grade. The following two general conditions were discussed, a
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"Plan Cut Street - Overexcavated within affected bedrock areas to at least 1 foot
below lowest utility invert in the street right-of-way. Filled to subgrade with 6-inch
minus processed material."
2. "Plan Fill Street - Generally filled with a mixture of soil and rock fragments derived
from cut excavation. Fills may consist of 12-inch minus material placed from 1 foot
below lowest utility invert to 1 foot below subgrade elevations. Fills placed below a
depth of 1 foot below invert may be placed as compacted fills consisting of 12-inch
minus material. Rock fragments larger than 12 inches may be placed as rock
blankets, windows, or buried individually in pits, as recommended for oversize
material in the referenced report (at least below the lowest utility). If necessary,
6-inch minus processed material may also be used in plan fill areas."
Based on the type of fill condition presented above, recommendations for trench backfill
were also presented in Reference No. 1, as follows:
"Materials generated during trenching in plan fill areas may be used as trench backfill for
trenches with a width of 3 feet or greater. Trenches less than 3 feet in width should be
backfilled with 8-inch minus material. Trench spoils generated in overexcavated cut areas
(streets and pads) are anticipated to consist 6-inch minus processed material suitable for
use as backfill."
Greenbook Criteria
A review of Section 306-1.3.1 in Reference No.3 indicates that trench backfill may consist
of 6-inch minus material where the trench is less than 3 feet in width. Backfill containing
rock fragments, up to 12-inch minus material, are specified as backfill where the trench is
greater than 3 feet in width, to within 5 feet vertically of the finished surface. Rocks greater
than 21/2 .inches in diameter are not recommended in backfill within 1 foot of finished
subgrade.
Discussion
The current recommendations presented in Reference No. 1, and herein, allow for 8-inch
minus material to be placed in a trench with a width of at least 15 inches. Backfill
containing material up to 12 inches in diameter may be placed in a trench at least 3 feet,
or greater, in width. Based on our review, these recommendations are generally consistent
with those presented in the Greenbook (Reference No. 3). It is our opinion that using 8-inch
minus material should not adversely affectthe performance of the backfill material provided
that a sufficient amount of fines is present in the fill to allow for adequate compaction, and
to minimize void space. Furthermore, bringing 12-inch minus material to within 1 foot of
finish subgrade in the wider trenches (i.e., greater than 3 feet in width) should not adversely
affect the performance of the trench from a geotechnical viewpoint, provided that the trench
backfill is properly compacted to a minimum relative compaction of 90 percent.
Calavera Hills II, LLC • W.O. 3459-B-SC
Carlsbad Tract 00-02 • May 5, 2003
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GeoSouls, Inc.
Specific gradation requirements with respect to the percentage of sand, gravel, cobble, etc.,
for backfill were not noted in the "Greenbook." However, periodic sampling and gradation
analyses of representative samples of fill materials placed is being performed. This testing
is primarily done to access the amount of fines in the fill forthe purpose of evaluating that
there is an adequate amount of fines in the fillto minimize void space and to produce a
compacted fill that can be tested, using conventional methods, such as the sand cone
(ASTM D-1 556) and the Nuclear Method (ASTM D-2922). A general baseline of 40 percent
fines has been established for this project.
It should be noted that backfill generated from areas containing processed (crushed)
material, will generally be on the order of 6-inch minus in diameter. Backfill material
generated from areas containing unprocessed fill, generated from cut excavation, may
contain material up to 12 inches in diameter. The removal of some oversize material (i.e.,
greater than 8 inches) from the backfill may be necessary in this case.
Limitations
The conclusions and recommendations presented herein are professional opinions. These
opinions have been derived in accordance with current standards of practice and no
warranty is expressed or implied. Standards of practice are subject to change with time.
GSI assumes no responsibility or liability for work, testing, or recommendations performed
or provided by others. This report may be subject to review by the controlling authorities
for this project.
The opportunity to be of service is greatly appreciated. If you have any questions
concerning this report, or if we may be of further assistance, please do not hesitate to
contact any of the undersigned.
Respectfully subm
GeoSoils, Inc.
Robert G. Crisman
Engineering Geolc
RGC/DWS/JPF/jk
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Reviewed by:
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David W. Skelly
Civil Engineer, F 47857
Distribution: (1) Addressee
(1) McMillin Companies, Attention: Mr. Don Mitchell
(1) McMillin Companies, Attention: Mr. Ken Screeton
(3) Job Site, Attention: Mr. Tom LaMarca
Calavera Hills II, LLC
Carlsbad Tract 00-02
Fi1e:e:\wp9\3400\3459b.cot
GeoSoils, Inc.
W.O. 3459-B-SC
May 5, 2003
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