HomeMy WebLinkAboutCT 72-21; TANGLEWOOD; SUPPLEMENTAL REPORT OF SOIL AND GEOLOGIC INVESTIGATION; 1973-04-16•
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SOUTHERN CALIFORNIA TESTING LABORATORY INCORPORATEO
6280 RIVERDALE ST. SAN O-l£GO. CALIF". 92120 • TEL€: 290"4321 • P.O. sax 20627 SAN DIEGO. CA~JF". 92120
7 3 -a 3 B H I G H WAY 1 1 1 PAL MOE S E 'R T, 0. A L IF. 92260. TELE 346-1079 1.
678 ENTERPRISE ST. ESCONDIDO, CALIF.
92025 TEL E 4,8 ~v ~ ~ J,
April 16, 1973 \00 A )
,;lA' -0t' Larwin-San Diego
6150 Mission Gorge Road
San Diego, California 92120
Attention: Mr. ~qalt Wood
SUBJEC'l': Su ort of Soil
Gentlemen:
s· T 312022
Report No. 7
ation
In accordance with your request { Tile herewith submit 'the subj ect.
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The intent of this report is to describe the scope of the engineer-
ing and geologic analysis performed by Southern California Testing
Laboratory, Inc. prior to submission of prior reports pertaining to
the subject project. The scope of this report is in accordance
with a recent conversation between Mr. Levish of Burkland and Asso-
ciates, Mr. Dennis Hannan, Project Geologist,,:. and 'the undersigned.
Please do not hesitate to contact the undersigned if you have any
questions regarding the contents contained herein.'
". We appreciate the opportunity to be of service.
Respectfully submitted j
DES: jle
cc: (4) Submitted
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'Dennis L. Hannan, Project Geologist •
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• 6280 Riverdale St. ~ San Diego, California 92120." Phone 280-4321
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AL.L. I'IEI'OIlT!1 ARE SUBMITTED AI> TH!>: CONFIDENTIAL I"ROI'i!:I1TY OF CLIi!:HTIl. AUTHOIUZATION Fon PUBLICATION OF ourl REP01'lTS. CCNCLU$IONS. OR EX1:RACTIl
FROM OF! r~EGA1'lOING THEM It! 1'l!!IIE:RV!>:D I"ttNOING OUR WIUTTltN APFrlOV"L Atl ... MUTUAL PROTECTION' TO CLIEIUS. THE PUDLIC AND OUNSI£L:vEC.
SUPPLEHENTAL' REPORT OF
SOIL AND GEOLOGIC INVESTIGATION
TANGLEWOOD SUBDIVISION
CARLSBAD, CALIFORNIA .-
SCOPE
The scope of this report is intended to provide a discussion of the soil
and geologic conditions prevalent within the subject site, the analysis
of same by Southern California Testing Labora'tory, In,? 0 and the effect
recommendations and site grading have produced on any adverse conditions.
The previous reports contained the basic field and laboratorytes't datai
therefore, this phase of the previous reports will not be duplicated.
FIELD I~~ESTIGATION
A total of 20 test borings have been extended wi,thin the .subject site
at ·the locations shown on attached Plate No. lAo The test borings were
-. conducted on three separate occasions. Reports presenting our findings i
laboratory tests, analysis and recommendations were submitted upon com-
pletion of each series of test borings.
• The soil conditions revealed by the test borings were confirmed by down
hole logging of numerous test borings. The lack of consistency in strati-
fication observed in the site grading, completed to date, WaS noted in • our field investigation and subsequent reports.
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Down hole logging of Boring Nos. 8 and 9 revealed stratas of intensely
fractured silts and clays. Slickensided surfaces were observed in the
strata. The characteristics and orientation of the slickensided surfaces
were more indicative of those found in areas of differential rebound
of over-consolidated silts and clays than that which would be indicative
'of existing landslides. Although the conditions encountered in Boring No.
8 did not present conclusive evidence of an existing slide ,contact,
recommendations were made to the design engineer to reorient the proposed
cut in this area. The final grading plan indicates a cut slope which
was reduced to a maximum height in the order of 27 feet and reoriented
to generally a north-south cut slope. Based on the infonnation available
at this time, it is our opinion that this 'vas a prudent revision to
the preliminary grading plan. This revision elimina'ted the need for • ") additional detailed investigation and stability analysis in the area
of Boring No.8.
• The strata of intensely fractured silts and clays encountered in Boring
No. 9 did not reveal sufficient ponsistency to indicate the presence
of an existing slide contact in this area.
• Samples vJ"ere obtained from the fractured and slickensided stratas for
determination of residual shear strength as described in this report.
• LABORATORY TESTING
Laboratory tests were performed on samples obtained from the test borings
in accordance with A.S.T.M. standards to determine the general physical
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. 'characteristics of the prevailing soils. Residual shear tests were
• performed on samples obtained from Boring Nos. 8 and 9. The results
of these tests indicate a residual soil strength parameter of ¢ = 22.0°,
c = 110 psf and'¢ = 18.5°, c = 220 psf, respectively.
• Detailed re.sul ts of our laboratory tests are presented in Report No.
2.
• During site grading, a sample was hand cut from an existing slide contact.
located approximately 200 feet east of Boring No. 16. The existing slide
debris, which represents less than 3,000 cubic yards of material,. will be
• removed or buttressed during site grading. Additional investigaotion will
determine the corrective measures best suited for this condition. The
sample would be classified as a CH and possess a substantially higher
.... e degJ~ee of sJ_ickensided surfaces than the. snnlplevs encount-er-ed iIi Bori11g~
'0' Nos. 8 and 9. This sample, when tested for residual shear, produced
residual strength parameters of ¢ = 10° f C = 100 psf. A reviev" of the
• site grading completed to date indicates that the highly frac·tured silts
and clays encountered in the area of Boring Nos. 8 and 9 are not similar
to the fractured and slickensided soils encountered in the area of the
• above described existing slide.
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ENGINEERING ANALYSIS
The fact that the general grading concept was established prior to camp le-
tion of our Report No. 2 presented a condition whereby only three major
'slopes to be constructed during site grading required slope stability
analysis. This does not include the proposed cut slope in the area of
. Boring No. 8 which was revised as described above. These. three major
slopes consisted of the following:
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SCT 312022 April 16, 1973 P?lge'Four
1) The fill slope located along the northeast boundary of 'the site,
northerly of Boring No. 4.
2) A cut slope on the north side of Elm Street located easterly of
Boring No. 14.
3) A cut slope on the south side of Elm Street immediately east of
El Camino Real.
The above slopes and prevailing soil conditions were analyzed through the
use of a Burrows 5700 computer. The computer program does analyze a
minimum of 27 potential failure planes for any given slope and soil condi-
tion. The computer analysis is continued until the center of radius
of the failure circle producing the least factor of safety falls within
a prede·termined grid. The computer printout presents the factor of
safety with6ut a seismic accelleration factor of O.lg. The above de-
scribed analysis produced the following conclusions on the three major
exposed slopes:
1) Fill Slope Along the Northeast Boundary
2)
The computer analysis produced a factor of safety of 1.70
without seismic forces and 1.42 with seismic forces, based
on the slope as designed, utilizing the remolded soil strength
parameters of ¢ = 33°, c = 250 psf, ~ = 138 pcf.
Cut Slope North of Elm Street
A detailed analysis of this slope showed that an increased
factor of safety could be obtained by increasing the proposed
bench width in lieu of the standard reduction in ~lope ratio.
Therefore, recommendations were presented, in our Report No. 2A
dated September 15, 1972 for the design of this slope. The
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seT 312022 April 16 r 1973 Page Five
computer analysis produced a factor of safety of 1.41 without
seismic forces. Utilizing direct shear results and a seismic
accelleration of O.lg, the factor of safety will be in excess
of 1.5. The residual soil strength parameters of ¢ = 22°,
c = 110 psf, = 137 pdf were utilized in these calculations.
3) Cut Slope South of Elm Street
Based on the absence of continuous bedding or jointing planes
and slickensided surfaces encountered in this cut, the stability
analysis was based on direct shear results with the recommendation
tha't detailed geologic inspection be performed during site grading.
Based on the inspection completed to date, no evidence of exist-
ing slide contact or continuous bedding and/or jointing planes
requiring the utilization of residual shear parameters, have
been observed in the slope. In the event a major bedding or
jointing plane system is observed with, an out-of-slope dip, re-
co~nendations will be submitted for any corrected measures required.
The computer analysis produced a factor of safety of 1.53
based on soil strength parame-ters of ¢ = 29°, c = 225 psf, =
137 pcf., In consideration of the distance from the exposed slope
to proposed s·tructures, a seismic accelleration factor'was not
utilized in these calculations.
The trace of an inactive fault was observed southwest of the subject
site during the initial stages of our investigatipn as shown on our
previous reports. During site grading, the evidence of this fault was
Le 'observed within the subject site. The geology section of this report
contains a detailed discussion of seismic activity in the area of the
site.
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An inspection of this discontinuity from the soil en9ineering standpoint
showed that the prevailing soils were very dense with no indication of poten .....
tially differential in situ soil conditions. Within this zone, the follow-
ing corrective measures have been taken.
The natural soils exposed within 3 feet of finished grade have
been removed and recompacted to 90 percent of maximum dry density.
0rther, all foundations "hich "ill encroach on an area 15 feet
wide along the fault trace will be reinforced With~f~o~u;r;;;N~o~.iiie5i;~~;;;,I
It is our opinion that the above recommendations will prevent differential
• movement within the proposed structure, due to dissimilar in situ soil
conditions .
• A review of the final grading plan will show that the adverse soil and • " geologic conditions encountered wi thin the subj ect site will be eliminated
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by the proposed site grading, with the exception of those for which correc-
tive measures have been taken.
SITE GEOLOGY
Mapping of the site has utilized five mappable sub~ivisions of the mat-
erials with regard ,to their engineering significance, basic soil varia-
tions, and sedimentary bedrock conditions. These five engineering mat-
erial types have been delineated on Plate No. lA and are as follows:
I} Qal -Alluvium consisting of brown to dark brown, medium dense
to denser silty-clayey sands, silts and clays which cover the
canyon bottoms. These soils exist in a loose,compressible con-
dition for a depth of' 1.5 feet or more and have required addi-
tional compactive effort prior to the placement of structural
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fills. In some areas of particularly loose, saturated alluvium,
it has been necessary to remove and recompact as much as 4 to
5 feet of material.
Qt -Terrace deposits consisting of interbedded silty-clayey
sands, silts and clays are located approximately below elevation
150 feet only in the extreme northwesterly corner of the site.
These deposits are undoubtedly a combination of materials which
reflect ancient variations in sea level and stream conditions
at the head of Buena Vista Lagoon. Construction of building
pads in this area will encounter moderate to highly' expansive --
soils.
Qls -Landslide involved materials 'may exist in these areas.
Boring No. 8 encountered several closely spaced slickensided
rupture surfaces within fractured olive mudstone material. The
dip of these surfaces was noted to be in the order 'of 20 to 24
degrees in a northerly direction suggesting sliding may have
occurred along these planes. It must be noted, however! that
the olive silt-clay in which these rupture surfaces were observed
characteristically contains an abundance of randomly oriented
clay filled joint planes which are not a result of sliding.
Similar conditions of jointing have been noted within many of
the clay-silt beds during excavation and this condition exists
in areas tha't are not slide involved and, which are not slide
prone (in excavation beneath the area betv.reen Boring Nos. 2 and
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4) Tss' -Eocene Santiago Formation consisting primarily of
white-pale yellow, dense, coarse to medium grained, massively
to cross bedded silty and clayey sands was mapped above approxi-
mately elevation 200-210 feet. Included in this un~t are widely
scattered lenticular olive clay and silt beds whose random dis-
tribution does not allow accurate geologic mapping. Further,
accurate mapping is hampered without easily recognizable and
traceable marker beds. Tss has been delineated as a separate
unit from the underlying materials (Tsm) because of a greater
percentage of slightly expansive to nonexpansive sands. It is
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this unit which produces materials with an overall low expan-
sive potential that may be utilized to cap finished pads expos-
ing moderate to highly expansive soils. Materials from this
area would also be suitable for use in buttresses requiring
soils with higher strength parameters than those common to on-
site clays and silts.
5) Tsm -This unit encompasses a wide variety of interbedded and
interlensing Clayey-silty sands, silts, and clays of the.,Eocene
Santiago Forma·tion. The white~light gray-pale yellowr dense,
medium to fine grained, silty and clayey sands of this unit are
massivly bedded. Some faint bedding is noticeable only rarely
and has not proven suitable for obtaining reliable bedding
attitUdes. Lensing with, and in irregular contact with, the
clayey and silty sands are massively bedded pink-olive-yellow,
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hard silts and clays. These silts and clays are the predominant
materials of the Tsm unit and are of moderate. to high expansive
potential. The pinkish-olive mudstone encountered in many of
the borings is characteristically fissured with randomly oriented
slickensides exposed on freshly broken su~faces. The fractured
and fissured nature of this mudstone does not appear to be re-
stricted to areas of sliding or faulting (south cut of Elm Avenue)
and may be due simply to differential rebound of over-consolidated
sediments. The shear strength of this soil type has been care-
fully considered with respect to slope stability. Topsoils over-
lying this unit (Tsm) usually consist of 1.5 to 3 feet of sur-
ficial clays ranging in color from dark brown-gray to dark
yellow brown and possess a moderate to high~'expansive potential.
These topsoils are subject to slip-outs on steeper sJopes (gr~ater
than 30°) and numerous examples of this are evident throughout
the site.
• Landsliding:.. Ancient landsliding referenced above as Qls I or possibly
landslide involved sediments, was suggested only in the area of Boring
l No.8. The previously mentioned planes dipping 20 to 24 degrees in a • northerly direction, whether due to sliding or jointing, were 'felt to
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be a serious enough condition that planning moved a 1.5:1 cut slope 20
feee Im.yer in elevation and oriented the slope north-south to avoid inter-,-. •• secting the unfavorably dipping planes.
A recent slide of very limited extent was mapped in the southern portion
• of the site (between Boring Nos. 8 and 13) and resulted due to ever steep-
~ ening of canyon slopes adjacent to an actively eroding drainage course
and the unfavorable distribution of fissured clays. site grading has
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totally "removed the slide debris in this area. A slightly larger" recent
slide was noted just south of the property and east of Boring No. 14;
also due to over steepening of canyon slopes. Both these recent slides
appear to have failed on slope gradients of approximately 1.5:1 or steeper
and for slopes exceeding 25 to 50 feet in height.
The proposed configuration of cuts and fills appears to be such that
the canyon and valley portion of the site will be filled and those cuts
proposed for areas designated as possibly landslide involved are of limited
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height and extent. Possible exceptions' to this may be in the areas of
Boring Nos. 8, 16 and 17 where careful geologic reconnaissance is being
conducted during excavation. Our inspection of the cuts in these areas
should, in our opinion, provide the necessary information to evaluate
possible slide hazards should they exist,
During the excavation of a 1.5:1 twelve foot high cut slope 200 feet
east of Boring No. 16, a slope failure occurred on what appears to be
a pre-existing-rupture surface or a low angle j oint plane dipping out-·
of-slope. Additional investigation of this area will be undertaken to
fully evaluate the existence of slide hazards or adversely dipping fracture
planes. Recommendations for stabilizing this area will be forth coming
pending the investigation of this failure.
Bedding and Jointing: Bedding attitudes have not been obtainable from
the Tss-Tsm materials due to the massively to cross bedded conditions.
In those areas where outcrops were available for inspection contacts
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'between silt-clay beds and ·the sands indicate horizontal bedding. A
• cut slope excavation at the east end of Cross-Section B-B 1 further indicates
horizontal bedding. One questionable attitude near Boring No. 19 was
obtained during excavation of this area and suggests a northeasterly
• strike with a slight 0-5 degree dip to the northwest (west of the fault) •
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. Inspection of cut slopes in Qt material just north of the site also indicate
horizontal bedding.
Jointing of the sands, silts, and clays appears to be very intensive
to moderately intensive. Generally, the joint planes have vary;i.ng strikes
but usually have high angle dips exceeding 40 degrees. Occasional low
angle dips of several degrees have been noted in the dark olive clays
and. silts. It is difficult to distinguish between' what may appear to
be low angle joint planes or partings along bedding planes. ~hose joint
planes in the silts and clays are rarely without clay coatings~ slicken-
sided and non-slickensided, and the clay coating is often a dark olive.
Joint planes in the silty-clayey sands are often coated vIi th dark bluish-
green clay. Slopes containing this'condition have also been inspected
during grading. Our inspec·tion during grading has recognized and mapped
many of the joint planes and we find that, in addition to generally steep
attitudes, there are many conchoidal fracture planes within the more
dense, hard silts and clays r whereas -the joint plane.s in the silty and
clayey sands are more consistent. Out-of-slope dips on fracture planes
exceeding 10-20 degrees are being carefully analyzed where hoted in the
·dark olive clays and silts.
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Faulting: Due to the serious lack of previous mapping in the area of
• Carlsbad and Oceanside (also the area south of Carlsbad to Del J~1ar),
the .availability or fault information is poor. Student mapping by California
State University, San Diego geology department has been strictly reconnais-
• sance in nature and has not proven to be reliable. The Santa Ana Sheet,
Geologic Map of California, 1965, snows several northeasterly trending
faults in the Camp Pendleton area some 15 miles northwest of the site.
• Mapping completed in late 1972 by Ken Wilson (personal communication)
shows the presence of northeas-terly trending faults in areas east of
the subject site and south of Agua Hedionda Creek 3 miles southeast of
• the site. These faults do not intersect the subject project.
A fault was noted southwest of the subject site several hundred feet south
of the intersec·tion of Elm Avenue and El Camino Real. An approximate
atLitude at tllis location and subsequent attitudes during grading suggest
an approximate northeast strike of between 10-20 degrees and an easterly-
southeasterly dip of between 43 and 73 degrees. Faulting along this trend
appears ·tq be restricted to a zone approximately 15 feet wide in which
dark olive clay gouge and slickensided stirfaces may be observed. Due
-to the lack of conclusive stratigraphic marker beds, the amount of throw
• has not been calculated and no estimate of offset is feasible at this
time. The direction of slickensides does indicate either normal or reverse
dip slip.
• The fault trace does not exhibit the physiographic features such as sag
ponds, offset drainages, springs, and fault scarps that are common to
• proven active faults. A southwest extension of the fault does not appear
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to demonstrate noticeable offset of the Pleistocene terrace materials
• (reddish sands and gravels) and most importantly, the general surrounding
area does not contain any mapped earthquake epicenters (Natural Resources
Inventory Fault Map, 1972, San Diego County) e It might also be noted
• that the tectonically active faults in southern California are northwesterly
faults, whereas faulting in the Oceanside-Carlsbad area is northeasterly
and may, with regard to the lack of seismic activity, be regarded as
e probably inactive. c. Ro Allen stipulates that current displacements
'on faults with high degrees of activity in the recent geologic past (last
10,000 years) are the most likely candidates for fu"cure activity (]I,llen,
e, C. R., st. Amand, P., Richter, C. F., and Nordquist, J.J., 1965, "Rela-
tionship between seismicity and geologic structure in the southern Cali-
fornia region:, Bull. Seismological Soc. America, vol. 55, p. 761) ~
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, Work conducted for Southern California Edison and San Diego Gas ,and ElecJcric
.. Company! s San' Onofre nuclear generating plant by ~..;restern' Geophysical
Company of America, Houston, Texas, consisted of a geophysical survey
". of the offshore region opposite of San Onofre and south to San Diego.
Special emphasis was placed on the location of faults and the age of
cessation of their movements. Seismic reflection and refraction surveys
'. conducted on three different horizons comprised of the sea floor sediments,
the upper Miocene, and the effective acoustic basement rocks (locally
the San Onofre Breccia or top volcanics) 'resulted in a comparison of
• the faults on these horizons. Western Geophysical's conclusion stated
that activity has significantly diminished through geologic time, and
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region is relatively stable by comparison to adjacent regions of southern
California (Final Report, Southern California Edison and San Diego Gas
and Electric Company, San Onofre Offshore Investigations, Western Geophysical
Company of America, Houston, Texas, March 7, 1972, in San Onofre Nuclear
Geperating Sta'tion Units 2 and 3, Amendment No. 11 to Preliminary 'Safety
Analysis Report, p. 43).
• It is our opinion tha·t wi thin the present geologic framework of the Oceanside-
Carlsbad area and through the use of available information, the discontinuity
noted crossing the subject site shmvs no features which would suggest
'. recent seismic activity.
Geology By: Dennis L. Hannan
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