HomeMy WebLinkAboutCT 02-28; La Costa Condominiums; Third Party Review of Geotechnical Reports; 2008-01-11THIRD PARTY REVIEW OF
GEOTECHNICAL REPORTS
Proposed La Costa Condominiums
Carlsbad, California
Prepared for:
Mr. Frank Jimeno, P.E.
City of Carlsbad
Public Works - Engineering Division
1635 Faraday Avenue
Carlsbad, CA 92008
Prepared by:
Testing Engineers - San Diego, Inc.
7895 Convoy Court, Suite 18
San Diego, California 92111
CONTRACT NO. : 148513
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8002 VI NVF
January 11, 2008 Contract No. 148513.30
Mr. Frank Jimeno, P.E.
City of Carlsbad
Public Works - Engineering Division
1635 Faraday Avenue
Carlsbad, CA 92008
Subject: Third-Party Review of Geotechnical Reports, Proposed La Costa
Condominiums, Carlsbad, California
Dear Mr, Jimeno:
Introduction
As requested Bureau Veritas, Testing Engineers-San Diego has performed a
third-party geotechnical review of the subject project. The purpose of our
review was to provide an opinion on whether the geotechnical aspects of the
project have 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. The following project documents were
provided for our review:
1. "Update Geotechnical report, Proposed La Costa Condominiums",
prepared by American Geotechnical, Inc., File No. 23280.01, dated May
31,2007.
2. "Recommended Remedial Grading Procedures, La Costa
Condominiums", prepared by American Geotechnical, Inc., File No.
23280.01, dated May 31, 2007.
3. "Grading Plans for La Costa Condominiums", Sheets 1 through 9,
prepared by O'Day Consultants, Job No. 98-1027, dated November
2007.
Testing Engineers San Diego
// Bureau Verhas Company Main: 858.715.5800
7895 Convoy Court. Suite 18 lax: 858.715.5810
San Dieso. CA 92111 \v\\\\.us.hureauveritas.com
City of Carlsbad - La Costa Condominiums Contract No. 148513.30
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In addition to the above project documents, the following related documents
were provided by you for our review:
4. "Update Preliminary Geotechnical Report, Lot 185, La Costa Avenue
South Unit 1, Carlsbad, California", prepared by Leighton and
Associates, Inc., Project No. 980161-002, dated October 30, 2002.
5. "Landslide Stabilization Recommendations, Banich Powers, Calso
Landslides, 2416 Sacada Circle, Carlsbad, California 92009", prepared
by American Geotechnical, Inc., File No. 23080.02, dated March 27,
2007.
6. "Landslide Treatment Recommendations, La Costa De Marbella,
Carlsbad, California", prepared by American Geotechnical, Inc., File No.
21381.16, dated May 18, 2007.
Results of Review
Based on the results of our third-party review, it is our opinion that there are
geotechnically-related issues that should be addressed by the project
geotechnical consultant. Our enumerated comments are presented as follows:
1. The Geotechnical Site Map (Plate 1) does not indicate the contact between
many of the geologic units (for example the contact between Tsa and Af
located between the southern portions of Cross Sections A-A' and D-D', or
the contact between Tsa and Qcol near Cross Section L-L' and the contact
between Tsa and Af in the northeast corner of the site. It is our opinion that
the geotechnical map should be revised to show geologic contacts, and the
legend should be revised to indicate the map symbol used to depict
geologic contacts. In addition, the symbols used on the map to depict
geologic contacts should be in general accordance with standard geologic
mapping conventions.
2. The first line in the geotechnical map legend includes a symbol Af/Tsa
which depicts existing fill, buried Santiago Formation. That symbol appears
to be used only once in the southwest corner of the map. In other areas of
the map, the symbol Af/Tsa is circled. That circled symbol is not indicated
on the map legend. By geologic mapping convention, when a symbol is
circled it means the unit is buried. However, the map legend is not clear on
this particular symbol. In the case of this project we would expect that the
"Af in the circled Af/Tsa would not be buried. In any event, the map
symbols and map legend do not match, and the map symbols are not
consistent for this unit. It is our opinion that the geotechnical map and
legend should be revised to correct any inconsistencies. In addition, the
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symbols used on the map to depict the various units, should be in general
accordance with standard geologic mapping conventions.
3. The manner in which the geotechnical site map and legend depict
exploratory borings performed by the various geotechnical consultants is
generally clear. The symbols for the various numbered borings are
preceded by an acronym or abbreviation of the company name followed by
the boring number. However, there are a number of borings in the northern
portion of the site whose symbol is a capital "B" followed by the boring
number. This symbol is not indicated on the geotechnical map legend. The
origin of these borings is not indicated on the map or described in the
project geotechnical report. It is our opinion that the geotechnical map
legend should be revised to include a symbol and description of these
borings.
4. There are two exploratory borings shown on the geotechnical map that are
annotated with the symbol LB-1 (one near the northern end of Cross
Section K-K' and another near the northern end of Cross Section l-l'). It is
our opinion that the geotechnical map should be revised to indicate the
correct location and identification of the borings shown. LB-1 is shown on
Cross Section l-l" and again on Cross Section K-K'. This appears to be
related to the above mentioned error since the cross sections are not
located near each other. It is our opinion that the consultant should check
the boring locations and numbers and revise the cross sections to reflect the
correct boring numbers.
5. The boring logs in the project geotechnical report identify the borings
performed by American Geotechnical, Inc. as LGB-1 and LGB-2. The
Geotechnical Site Map (Plate 1) identifies these borings as AGLB-1 and
AGLB-2. It is our opinion that the map and report should be consistent in
their identification of these borings. The report does not indicate the type of
rig used to drill LGB-1 and LGB-2, and it is also not indicated on the boring
logs. It is our opinion that the consultant should provide a description of the
drilling and sampling methods used.
6. On page 6 under the section entitled, "Artificial Fill (Qaf)", the project
geotechnical report indicates that previous site grading was observed and
tested by Benton Engineering, Inc. in about 1970. The report then states
that, "According to the referenced report (Benton, 1979), the fill was
observed and tested during grading and the results were presented in a
report issued in 1970. This report was not available for review." There
are several comments relating to the above statement.
a. The statement, "...the referenced report (Benton, 1979)...", would
seem to indicate that the 1979 report was reviewed and included in
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the references presented in Appendix A. Our review of the
references presented in Appendix A indicates that a 1979 report by
Benton Engineering is not listed. It is our opinion that the
geotechnical consultant should clarify whether the referenced report
by Benton Engineering, Inc. was actually reviewed, and if so, it
should be included along with the other references in Appendix A.
b. The statement, "...the fill was observed and tested during grading and
the results were presented in a report issued in 1970. This report
was not available for review.", makes it clear that although the 1970
report was not available for review, the project geotechnical
consultant has some knowledge of the observation and testing of the
previous fill that was placed on the site, and that it was documented
in a report in 1970. If the report was not available for review, it is not
clear how the consultant knows that the fill was observed and tested
during grading, or even how the consultant even knows of the
existence of such a report. It is our opinion that the project
geotechnical consultant should provide clarification and reference for
the statements regarding the previous grading and existing fill soils at
the site.
7. The legend for the geotechnical map includes a symbol AGTP-2 and
indicates that it shows the approximate location of test pit by American
Geotechnical. There are no symbols on the geotechnical map annotated as
AGTP. There are however, symbols on the map annotated TP-1 and TP-2.
If these are the symbols that are meant to indicate the test pits by American
Geotechnical then it is our opinion that either the geotechnical map or the
map legend should be revised so that the annotation is consistent.
8. There are two test pits identified as TP-2 shown on the geotechnical map. It
is our opinion that the geotechnical map should be revised to reflect the
correct annotation for each test pit.
9. The project geotechnical report identifies that a large portion of the site is
underlain by existing fill soils. Although there are no test results or
evaluation of the existing fill presented, on page 6, the report indicates that
the existing fill is moderately well compacted. On page 13 under section
6.2.1 entitled, "Site Preparation", the report indicates that, "Prior to grading,
the site should be cleared of all surface and subsurface obstructions
including things such as existing structures, fill...." From that section and
statement, it appears that the consultant's opinion is that the fill is unsuitable
to remain in place and should be removed. Recommendations for treatment
of the existing fill is not mentioned in any of the other sections of the report.
It is our opinion that the consultant should provide clarification regarding the
treatment of the existing fill soils.
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10. The project geotechnical report indicates the presence of ancient landslides
and recent landslide movement along the southern portion of the site. The
report does not indicate which landslides are active and which are not.
Recent landslide movement indicates that calculated safety factors for the
existing conditions should be less than 1.0. Appendix D (Slope Stability
Calculations) of the project geotechnical report indicates that the existing
conditions have calculated safety factors well in excess of 1.0. This would
seem to indicate that assumptions or factors used in the analyses may not
be representative of the actual site conditions. As an example, the stability
analysis of the existing landslide condition along Cross Section D-D' was
calculated with a circular failure surface and a block failure surface. The
calculated safety factors for the two failure surfaces were reported as 1.34
and 2.36, respectively. The calculated safety factors appear to be higher
than would be expected for an unbuttressed landslide condition. It is our
opinion that the consultant should identify which landslides are active and
comment on why the existing calculated safety factors of those landslides
are greater than 1.0.
11. As a follow up to the previous comment number 10, we will use Cross
Section D-D1 from the project geotechnical report as one example to
illustrate why it is our opinion that the assumptions or factors used in the
stability analyses may not be representative of the site conditions. If an
analysis of the pre-landslide or no-landslide condition was performed with
the soil strength parameters used in the analysis of Cross Section D-D' the
calculated safety factor would be similar to, but likely greater than, the
calculated safety factor of 1.94 for the no-landslide condition reported for the
proposed back cut along D-D'. If the pre-landslide safety factor of the slope
was 1.94 (or greater), it would not seem plausible for a landslide to have
occurred in that slope, and as represented in the project geotechnical
report, there do not appear to be any geotechnical factors or conditions
present that would cause the occurrence of a landslide in the subject slope.
However, the landslide is present, which would indicate that the
assumptions or factors used in the analysis are not representative of the
existing site conditions. Some of the factors which may not be
representative are discussed in the following sections.
a. The assumed soil strength parameters used in the stability analyses may
be too high. The project geotechnical report appears to use assumed
shear strength and unit weight parameters for the various units in the
stability calculations presented in Appendix D of the report. Laboratory
testing of these units does not appear to have been performed by the
project geotechnical consultant. We note that the assumed soil strength
parameters are significantly higher than the parameters used in the
referenced geotechnical report by American Geotechnical, Inc. dated
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May 18, 2007 (reference 4) for an adjacent project to the west (La Costa
De Marbella). It is our opinion that the project geotechnical report should
identify the origin or basis of the assumed soil parameters used in the
analysis.
b. The project geotechnical report indicates that the onsite landslides
occurred along shear surfaces developed within the claystone beds and
laminations of the Santiago Formation. We generally agree that bedding
plane shears form the basal slip surfaces of the landslides. It is also our
experience that the back of the landslides are also sheared zones that
can generally develop along high-angle joint or fracture systems. The
failure surface for such landslides would not be a circular failure surface
as analyzed in the slope stability calculations (Appendix D of the project
geotechnical report). The failure surface would have a relatively flat
basal failure surface (parallel to bedding), and have a relatively steep
back scarp that was planar to sub-circular. The stability analysis of such
a failure surface would include modeling of the relatively weak sheared
basal rupture and sheared back scarp. The soil strength parameters
along the sheared bedding and fracture surfaces would typically be
lower than those assumed in the project geotechnical report. Our review
of the boring logs that were downhole-logged by Leighton and
Associates, Inc., and presented in the project geotechnical report,
indicates that factures and sheared fracture zones are common in the
"undisturbed" formational materials beyond the limits of the landslides.
The presence of relatively weak zones developed along joint and/or
fracture systems could help explain the presence of landslides in slopes
that are generally comprised of relatively high strength geologic
materials. It is our opinion that the project geotechnical consultant
should comment on the presence of sheared bedding and fracture zones
identified in the exploratory borings within the "undisturbed" formational
materials. The comments should address why the presence of these
relatively weak zones is not reflected in the slope stability analyses
performed for the subject project.
12. The report does not specify the seismic factor used to calculate seismic
stability. It is our opinion that the consultant should indicate the seismic
factor used in the calculations.
13. The northern and eastern limits of the landslide in the southwest portion of
the site (near Cross Section D-D') and the northern limits of landslides in the
eastern portion of the site (near Cross Sections B-B' and K-K') are not
clearly delineated on the geotechnical map. It is our opinion that the
geotechnical map should be revised to show the limits of the landslides
(similar to the limits of the landslide near Cross Section C-C'. Using
standard geologic mapping symbolization, if the limits are buried, they
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should be dotted, if the limits are uncertain and buried, they should be
dotted and queried.
14. As shown on the Geotechnical Site Map, in the northeast portion of the site,
an irregular area roughly 20 feet by 40 feet is indicated by light green
shading (the same shading used for the landslides shown on the map).
This feature spans the geologic contact between Qcol and Tsa. The feature
is not annotated with a map symbol and it is not identified by annotation on
the map or in the map legend. It is our opinion that the consultant should
identify this feature and revise the geotechnical map and legend so that it is
appropriately depicted.
15. On Plate 3, Cross Section l-l' identifies a "proposed buttress". This buttress
is not indicated on the geotechnical map and the buttress is also not
indicated in the stability calculations presented in Appendix D. It is our
opinion that if the consultant is recommending a buttress in this area, the
location and recommended limits of the buttress should be presented
appropriately on the geotechnical map. Stability calculations for the buttress
should also be included.
16. The cross sections presented on Plates 2 and 3 generally include depictions
of existing and proposed grades. Some of the cross sections also depict
proposed remedial landslide grading. We assume that the benching
depicted at the back and toe of the landslides on the cross sections
indicates that the landslides will be completely removed during remedial
grading. Cross Section D-D' depicts proposed remedial landslide grading
similar to the other cross sections. However, since Cross Section D-D'
lacks the depiction of benching at the back and toe of the landslide, it is not
readily apparent if the landslide will be completely removed. Our review of
the stability calculations presented in Appendix C would seem to indicate
that the landslide may be removed during remedial grading. It is our opinion
that the consultant should clarify the recommended remedial grading, and if
the landslide depicted on Cross Section D-D' is to be removed during
grading, the cross section should be revised to show the benching at the
back and toe so that the cross section is consistent with the other cross
sections presented.
17. As a follow up to our previous comment 16, Cross Section D-D' apparently
depicts queried failure surfaces that extend southerly to the property
boundary and beyond the approximate limit of buttressing (as shown on the
Geotechnical Site Map). It would appear that these failure surfaces will be
unsupported during excavation of the recommended buttress backcut, and
that failure of these surfaces could potentially affect off-site properties. It
does not appear that these failure surfaces will be removed during remedial
grading and it does not appear that these failure surfaces were modeled in
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the slope stability calculations presented in the project geotechnical report.
It is our opinion that the project geotechnical consultant should address
potential failure of the recommended backcut along these surfaces, and if
necessary provide appropriate recommendations to mitigate the potential to
damage to adjacent properties if a failure were to occur.
18. Cross Section B-B' depicts proposed remedial landslide grading. We
assume that the benching depicted at the back and toe of the landslide
indicates that the landslide will be completely removed both on site and on
the adjacent property to the south. Cross Section B-B' dos not depict the
proposed finished grade of the slope along the section. It is our opinion that
Cross Section B-B' should be revised to depict the proposed grades.
19. As a follow-up to the previous comment number 18, Cross Section B-B'
depicts a relatively steep backcut proposed to remove the existing landslide.
According to the cross section, this backcut could undermine support at the
toe of an existing fill slope and residential structure on the adjacent property
to the south. The existing subsurface conditions indicated the cross section
for this existing fill slope do not match the conditions modeled in the
computer slope stability analysis for this section presented in Appendix D. It
does not appear that the stability analysis for Cross Section B-B' included
analysis for potential failure of the existing fill slope either as it exists or in its
undermined condition. It is our opinion that the project geotechnical
consultant should review the cross section and stability analysis for this area
and comment on the slope stability. The consultant's review and comment
should also address temporary stability of the existing fill during backcut
excavation and provide recommendations for temporary support of the
slope and existing structure, if needed.
20. The cross sections presented on Plates 2 and 3 do not present any
groundwater data. Typically groundwater and seepage conditions with
depths encountered are annotated adjacent to the boring symbols on cross
sections. Considering the importance of groundwater to the stability of the
project area especially during grading when relatively steep temporary
slopes will be excavated into the hillside, it is our opinion that the cross
sections should be modified to depict the groundwater and seepage
conditions as encountered in the borings. Plotting the groundwater
conditions on the cross sections will also help the project grading contractor
identify potential troublesome areas and assist in planning his work.
21. Surface water seepage was indicated on the Site Plan (Figure 1) contained
in the referenced Landslide Stabilization Recommendations report by
American Geotechnical, Inc., dated March 27, 2007 (reference 5). Since
the Landslide Stabilization Recommendations report is technically not a
project document, and for reasons presented in the previous review
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comment number 19, it is our opinion that the seepage condition should be
reported in the project geotechnical report and identified on the geotechnical
map.
22. Appendix C of the project geotechnical report presents the results of
laboratory tests. Review of the test results indicate that laboratory tests
were performed on three samples. The report does not identify the origin of
the samples (i.e. boring or test pit number and depth). It is our opinion that
the origin of the samples should be identified. It is our opinion that the
geotechnical consultant should comment whether the laboratory tests on
these three samples provide the basis for their investigation and evaluation
of the engineering properties of the onsite soils.
23. The project geotechnical report appears to use assumed shear strength
and unit weight parameters for the various units in the stability calculations
presented in Appendix D of the geotechnical report. Laboratory testing of
these units does not appear to have been performed by the project
geotechnical consultant. It is our opinion that the report should identify the
origin or basis of the assumed soil factors used in the analysis.
24. There appear to be no borings in the hillside above the flat pad area in the
easternmost portion of the site in the vicinity of Cross Sections l-l' and
Cross Section L-L'. The map depicts an area mapped as colluvium.
Cross Section L-L' depicts a slope with a ratio of approximately 1.5 to 1
(horizontal to vertical) in a unit mapped as colluvium. The colluvium is
perched on the existing slope. Cross section l-l' also depicts colluvium
perched on the existing slope. The depth of the colluvium is queried on
Cross Section l-l', but is not queried on Cross Section L-L'. The basis for
the geology depicted on the Geotechnical Site Map (Plate 1), and the
cross sections is not clear. The geotechnical conditions differ from those
presented in previous geotechnical reports. Since these cross sections
are used to analyze stability in this area, it is our opinion that the project
geotechnical consultant should provide clarification for the basis of the
geotechnical conditions depicted for this area.
25. As a follow-up to the previous comment number 24, the stability
calculations for Cross Sections l-l' and L-L' do not appear to include
analysis of the stability of the colluvium perched on the relatively steep
hillside above the project site. Colluvium is generally known to be
relatively weak and prone to failures in sloping terrain. Cross Section L-L'
depicts a slope with a ratio of approximately 1.5 to 1 (horizontal to vertical)
within the unit mapped as colluvium. It is our opinion that the stability
analysis for this area should include analysis of the stability of the
colluvium. If the colluvium is found to have a less than acceptable factor
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of safety against failure, the project geotechnical consultant should
provide appropriate recommendations for mitigation of potential instability.
26. The Geotechnical Site Map (Plate 1) of the project geotechnical report
depicts an approximate limit of buttressing and possible off-site grading
along Cross Section L-L'. Review of Cross Section L-L' (Plate 3) indicates
that the limit of buttressing is not reflected on the cross section. It is our
opinion that the project geotechnical consultant should revise the cross
section so that the recommended buttress in this area is correctly
depicted.
27. On the Geotechnical Site Map (Plate 1) of the project geotechnical report,
a line (shown as long dashes with dots) identifies, "possible limits of
grading". Based on the map and cross sections, the nature of the grading
within this limit is relatively clear in the western approximately two-thirds of
the site. However, the nature of this grading is not clear in the eastern
approximately one-third of the site. The possible grading is not shown on
the map or cross sections, and it is not discussed in the project
geotechnical report. It is our opinion that the project geotechnical
consultant should discuss the nature of possible grading in this area. In
addition, the "possible limits of grading" line is not closed at the northern
property line (indicating that the possible limit extends north beyond La
Costa Avenue. If this line is, as it is annotated, the "possible limits of
grading", it should be closed along the northern property line along La
Costa Avenue.
28. The proposed remedial landslide grading shown by the project
geotechnical consultant on Cross Section K-K' indicates a finished slope
on the order of 90 feet high with a slope ratio of 1.5 to 1 (horizontal to
vertical). The report does not address the stability of this slope nor does it
provide recommendations that should be incorporated into design and
construction so that this slope will have an acceptable factor of safety
against deep-seated and surficial instability. It is our opinion that the
project geotechnical consultant should address the stability of this
relatively steep and high slope.
29. On page 19, in the section entitled, "Off-Site Fill Slopes", the geotechnical
report states that slope stability analysis indicates that the 1.5 to
1 (horizontal to vertical) off-site fill slopes above the southern property
boundary are subject to surficial failure. The report also indicates that if
one of these slopes were to experience a failure, there could be some
impact on the proposed condominium development. The report mentions
some methods that could be considered to mitigate the potential for
damage to the project due to a failure of one of the off-site slopes. It is our
opinion that the project geotechnical consultant should provide an
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analysis/description of the types of failure and impacts that might be
experienced by the proposed development due to a failure of one of the
off-site slopes. The project geotechnical consultant should also review the
project design and comment whether the design has incorporated
appropriate features to mitigate the potential for damage to the proposed
project due to failure of the off-site slopes.
30. As a follow-up to the previous comment number 29, the project
geotechnical report does not include an analysis or statement regarding
the gross or mass stability of the off-site 1.5 to 1 (horizontal to vertical)
slopes and the potential impact of these slopes on the proposed project. It
is our opinion that the project geotechnical consultant should comment on
the gross stability of existing slopes and impacts to the project site due to
potential slope instability. The project geotechnical consultant should
provide appropriate recommendations to mitigate potential adverse
impacts, if necessary.
31. On Page 12, the project geotechnical report recommends that off-site
drainage that is currently being directed over the southern slope be
collected and directed to a suitable disposal area. It is our opinion that the
project geotechnical consultant should review the project plans and
comment whether the design has incorporated appropriate features to
control surface drainage currently directed to the site from off-site sources.
32. The project geotechnical report indicates that nearly the entire slope along
La Costa Avenue (northern project boundary) will be stabilized with a
buttress. However the buttress is not continued easterly along La Costa
Avenue beyond the proposed main entrance to the project. The majority
of the existing slope along La Costa Avenue both east and west of the
proposed main entrance is comprised of existing fill. If a buttress is
recommended to stabilize the existing fill slope and proposed
improvements along La Costa Avenue west of the main entrance, it is not
clear why it would not be continued to stabilize the existing fill slope and
proposed improvements east of the main entrance. It is our opinion that
the geotechnical consultant should comment on the reason that the
recommended buttress is not continued east of the proposed main
entrance.
33. Review of the project geotechnical report indicates that nearly all of the
onsite slopes will require a stabilization buttress. The standard detail for
buttress subdrains provided in the project geotechnical report indicates
that the recommended vertical spacing of buttress subdrains (backdrains)
is about 12.5 feet, and the horizontal spacing of buttress subdrain outlets
should be limited to about 100 feet. It is our opinion that the locations of
the recommended subdrains and the subdrain detail should be shown on
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the project plans. Based on the recommendations for subdrains
presented in the project geotechnical report, it appears that on the order of
50 to 100 separate subdrain outlets may result from the remedial grading
(not counting the outlets for the recommended retaining wall subdrains).
The subdrain detail shows outlets discharging at the slope face. On page
26 under section 6.8 entitled, "Site Drainage", the project geotechnical
report states that, "No drains should be allowed to empty adjacent
foundations or over slopes." It is our opinion that the project geotechnical
consultant should provide recommendations regarding appropriate
discharge for the recommended subdrains that will be constructed as a
part of this project. The recommendations should consider protection of
the outlets, periodic clearing of vegetation or other blockage, and
designation of, or recommendations for who should be the responsible
party for maintaining the outlets. In view of the number of discharge
outlets that may result from the proposed grading, consideration should be
given to discharging all subdrain outlets into the project storm drain
system. In addition, the locations of the drains and outlets should be
surveyed and the locations shown on the as-graded map for future
reference.
34. Sheet 3 of the referenced project grading plans prepared by O'Day
Consultants (reference 3) shows a proposed detention basin in the western
portion of the site at the toe of the slope that descends from the southern
site boundary. It appears that this is an unlined basin and that it is located
up-gradient from proposed residential structures and improvements. The
proposed detention basin is not addressed in the project geotechnical report
It should be noted that the referenced geotechnical report by Leighton and
Associates, Inc. dated October 30, 2002 (reference 4), indicates that
ponding of water should be avoided. It is our opinion that the project
geotechnical consultant should review the proposed detention basin and
comment whether water infiltration from the unlined basin could adversely
affect the proposed improvements, and if necessary, provide appropriate
recommendations to mitigate any adverse impacts.
35. The geotechnical map (Plate 1) presented in the referenced report by
Leighton and Associates, Inc. dated October 30, 2002 (reference 4)
indicates the presence of a colluvium-filled drainage underlying a portion of
the site adjacent to La Costa Avenue westerly of the proposed main
entrance to the project. The Leighton map indicates that the colluvium is
now buried by existing fill soils. It is not clear why the project geotechnical
consultant has chosen not to include that feature on the geotechnical map
provided in the project geotechnical report. It is our opinion that the project
geotechnical map should depict the known geotechnical features underlying
the site, and that the project geotechnical map should be revised to indicate
this feature.
City of Carlsbad - La Costa Condominiums Contract No. 148513.30
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36. We understand that repair for a slope failure of the north-facing slope along
the southern site boundary is currently being performed. Recommendations
for the slope repair were presented in the referenced American
Geotechnical, Inc. report dated March 27, 2007 (reference 5). Repair of the
slope includes reconstruction with geogrid reinforcement, subsurface and
surface drainage features with a surface drainage energy dissipater. It is
not clear how the repaired slope and its features will interface with
stabilization measures that are recommended for the adjacent slopes on the
subject La Costa Condominiums project. The project geotechnical
documents do not address the interfacing of the slope stabilization that is
ongoing and the proposed stabilization that is planned for the subject
project. It is our opinion that the project geotechnical consultant should
comment on the proposed remedial slope grading at the site and its impacts
on the slope stabilization reconstruction that is currently being done on the
adjacent property to the south. The comments should include discussion on
the interfacing of the current slope stabilization and the proposed remedial
grading stabilization that is recommended for the subject project (i.e.
grading interface, geogrid interface, surface and subsurface drainage
features interface).
37. As a follow-up to the previous comment number 36, it is our opinion that the
project geotechnical consultant should comment on whether the conditions
exposed or encountered during the current slope stabilization will require
modifications to the recommendations presented in the project geotechnical
documents for the subject La Costa Condominiums project.
38. Our review of the project geotechnical report indicates that the project
geotechnical consultant has drilled only one exploratory boring within the
boundary of the subject project. The geotechnical report includes logs of
exploratory borings drilled during previous investigations by other
consultants. The consultant's interpretation of subsurface conditions
(identification and characterization of subsurface geologic units and existing
fill, geometries of landslides, characterization of groundwater underlying the
site, etc.) appears to be based on data developed during previous
investigations by other consultants. If the consultant is relying principally on
the geotechnical data developed by others, it should be so stated. The
project geotechnical consultant should comment on the applicability and
suitability of the work performed by other geotechnical consultants as it
relates to the proposed development. Unless the project geotechnical
consultant provides alternate geotechnical recommendations which are
based on sound geotechnical analysis, the consultant should state that he
agrees with the interpretations and recommendations presented in the
reports of the other consultants. If the project geotechnical consultant does
not agree with the findings presented in the previous geotechnical reports,
City of Carlsbad - La Costa Condominiums Contract No. 148513.30
Page 14 of 16
he should so state and provide the reason(s) for disagreement along with
appropriate geotechnical data/analysis supporting his position. Previous
reports that are relied on for characterization of the site (not just the logs of
the borings) should become permanent additions to the project geotechnical
documents. The previous geotechnical reports should also be submitted for
third-party review.
39. The project geotechnical report indicates the presence of expansive soil
within the planned development, and on Page 20 states that, "Because of
the large quantities of moderately to highly expansive soils present within
the building pad areas, it may not be possible to selectively grade the
building pad areas without importing material such that the overall
characterization of the soils within five (5) feet of grade are either non-
expansive or have low-expansivity." The expansion index testing (2 tests)
presented in the project geotechnical report resulted in medium and high
expansion potential. In consideration of the expansive soil conditions
expected at the site, it is not clear why the foundation recommendations
provided in the project geotechnical report address foundation design and
construction on soils that are non-expansive or have low-expansivity. Note
that the referenced geotechnical report by Leighton and Associates, Inc.
dated October 30, 2002 (reference 4), presents foundation
recommendations for soils with expansion potential ranging up to very high
(post-tensioned foundation and slab system). Unless the project
geotechnical consultant is certain that soils with very low to low expansion
potential will be imported to the site to construct the building pads, it is our
opinion that the recommendations for foundation design and construction
provided in the project geotechnical report should reflect the expansive soil
conditions that are expected at the site. If the project geotechnical
consultant is certain that soils with very low to low expansion potential will
be imported to the site to construct the building pads, it should be stated in
the project geotechnical report.
40. The project geotechnical documents recommend that a representative of
the project geotechnical consultant be on site during construction to provide
observation and testing services. It is our opinion that an engineering
geologist should also be present on site during construction to observe and
map the geologic conditions exposed/encountered. The mapped geologic
conditions should be presented on an as-graded geologic map and included
with the as-graded geotechnical report for the project.
41. The project geotechnical documents recommend installation of slope
inclinometers and surface survey monuments above the proposed buttress
backcut along the southern property boundary. The project documents
indicate that the inclinometers and survey monuments will be surveyed
regularly to monitor potential slope instability. It is our opinion that the
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Page 15 of 16
results of the monitoring should be provided to the City on a weekly or at
least bi-weekly basis, and if significant movement is detected the results
should be provided on a daily basis.
General geologic/geotechnical standards of practice comments:
42. When borings are not located along a cross section, but are projected to the
cross section, the distance and direction of projection is typically annotated
on the cross section. Our review indicates that many of the borings
depicted on the cross sections were "projected" to the cross section lines.
Of all the borings that were projected only one boring was annotated to
indicate that it was projected (B-4 on Cross Section J-J'). The distance of
projection was not provided. It is not clear why the consultant chose to
indicate that one of the borings was projected, but failed to do so for all of
the other borings that were projected.
43. The total depth of the borings was not shown on the cross sections.
Typically when borings are depicted on cross sections, the total depth of the
boring is annotated, usually above or below the boring symbol.
44. LB-5 is located approximately on the Cross Section C-C', but is not shown
on the cross section. The consultant has shown AGLB-2 which is located
further from the cross section than LB-5. It is not clear why the LB-5 was
not included on the section.
45. The geologic cross sections (Plates 2 and 3), depict buildings which,
although not identified as such, appear to be existing structures outside of
the project boundary. Cross Section J-J" (Plate 3) is the only section that
depicts a building within the project boundary. Although this building is
also not identified, we assume that this building does not exist since it is
not identified on the project plans or addressed in the site description. To
eliminate confusion, if buildings are shown on cross sections, they are
typically identified as either existing or proposed as appropriate.
46. On page 5 paragraph 2, the first two lines of the project geotechnical report
make reference to a report by Leighton and Associates in 1999, and
indicate that the report is referenced in Appendix A. The 1999 Leighton
report is not listed in Appendix A, as stated.
47. On page 15 line 1, it appears that some unit of measure was apparently
omitted after the statement that, "A minimum of five (5)..."
48. On page 15 line 2, page 16 line 18 and page 20 line 8, it seems that the
consultant has misused the term "aerial" as it relates to the extent of the
proposed improvements. Typically, the term "aerial" refers to airborne or
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Page 16 of 16
Contract No. 148513.30
in-flight, or something similar. Perhaps the more correct term would be
area).
The opportunity to be of service to the City of Carlsbad on this project is
appreciated. If you have any questions regarding our review, please contact
our office.
Respectfully submitted,
Testing Engineers - SAN DIEGO, Inc.
A Bureau Veritas Company
Gene Custenborder, CEG
Senior Engineering Geologist
GC/VO:cs
Van Olin, PE, GE
Principal Geotechnical Engineer
Distribution: (5) Addressee