HomeMy WebLinkAboutPD 2020-0036; PLAZA BONITA SLOPE REMEDIATION; GEOTECHNICAL INVESTIGATION OF SLOPE FAILURE 2210 PLAZA BONITA CARLSBAD CA 92009; 2020-06-01g
Geotchnical Engineer
4596 Mouflt Hubbard Avenue, San Diego, CA.92117
Phone/Text: 619.813-8462
Email: martinowen@geotechengineer.com
Job No. 200506
June 1, 2020 To: Ms Marylou Powers .Rij, (i±FJff f) La costa Village HOA
OCT 22 2020 Subject: eotechnical Inspection of Slope Fai1lfr G AND DEVELOPMT 2210 Plaza Bonita ENGINEENG Carlsbad, CA 92009
bear Ms. Powers:
In accordance with your request, I have conducted a geotechnical inspection of the su
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slope failure. The purpose of the inspection was to evaluate the slope failure and provide
recommendations for slope repair and restoration. My inspection was performed on May 6,
2020.
SITE DESCRIPTION
The subject slope is a steep (approximately 1-1/2:1 H:V) west-facing, graded fill slope, about 15
feet high, that descends from the west side of the subject residence down to a lower level and
swimming pool.
The residence is the westerly unit of a two-story, multi-unit townhome building construct
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i
n
1971. The building is (mostly) supported on conventional shallow footings and has lower l
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slab-on-grade floors. However, you indicated that the subject residence previously experienced
foundation settlement and was stabilized by underpinning with pipe piles or other deep
foundations. The date and extent of the foundation repairs are not known.
You also indicated you recently inspected the interior of the residence and no building da
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e
was observed. In addition, your insurance company sent an engineer to inspect the building
and no damage was reported.
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Geotechnical Inspection of Slope Failure Page 2 2210 Plaza Bonita Job No. 200506 Carlsbad, CA 92009 June 1,2020
Slope vegetation is moderate to good and consists of several shrubs and ivy and other ground
cover.
Reportedly there was a recent leak in a sprinkler line near the top of the slope. It is not known
how long the pipe was leaking. The leak is believed to be factor In the slope failure..
DESCRIPTION OF SLOPE FAILURE
The slope failure occurred In the upper portion of the slope below the west side of the
subject residence, following recent, heavy rains.
The failure is a shallow, slump-type landslide, about 3 feet deep maximum, approximately
50 feet long (parallel to slope), and extending about 2/3 of the way down the slope (see
attached Figure 1).
The landslide occurred along a curved failure surface within the slope fill. The soils at the
top of the slope slid downwards resulting in a 3-feet high, vertical scarp and rotated
outwards accumulating as a soil pile or bulge near the bottom of the slope.
The failure is less noticeable at the north end of the slope where there has been less slope
displacement at the bottom.
The failure appears to have occurred due to an increase in weight and loss of shear strength
of the outer Slope fill soils resulting from saturation of the slope soils. The source of water
appears to be a combination of recent, heavy rainfall and water from a leaking sprinkler
pipe.
The residence does not appear to been damaged by the failure. However, the scarp has
exposed some PVC electrical conduits running parallel to the building. The horizontal
distance from the edge. of the residence to the edge of the scarp and conduits is about 5
feet.
I probed the slope soils within and adjacent to the landslide area and the soils were soft and
saturated to a depth of at least 3 feet. Poor compaction of the fill soils appears to be a
factor in the slope failure.
Martin R. Owen PE, GE
Qeotechnical Engineer
Geotechnkal Inspection of Slope Failure
Page 3 2210 PlazaBoñlta
Job No. 200506 Carlsbad, CA 92009
June 1, 2020
The slope is overly steep by today's giading standard
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failure.
SOILS AND GEOLOGY
The slope soils are fill soils consisting of gray brow
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F
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published geologic maps the site is underlain at depth
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Formation.
CONCLUSIONS
The primary cause of the slope failure appears to be satu
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shear strength resulting from a combination of recent hea
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pipe.
The steep slope and poor slope fill compaction appear t
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The slope failure is shallow and relatively minor and t
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to have been damaged or threatened by the failure.
RECOMMENDATIONS
The' slope failure should be repaired and the s
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configuration, as follows:
The slope can be repaired using small grading equipment
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.
The existing slope vegetation within the slope restor
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.
Some of the shrubs may be salvageable and can be reu
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.
The accumulated soils at the bottom of the slope s
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b
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m
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stockpiled on the nearby sidewalk or other area.
In addition, prior to placing fiIl,4eetwidëñiifii m(measured into slope) fill key, at l&ii2-feet -dee, should be excavated at the bottom of th
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(
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)
.
Martin R. Owen PE, GE
Geotechnical Engineer w
- Geotechnical Inspection of Slope Failure Page 4 2210 Plaza Bonita Job Noi 200506 Carlsbad, CA 92009
June 1, 2020
As grading proceeds to the north, where there has been less slope displacem
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,
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y
b
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possible to excavate the fill key further up the slope (instead of at the bottom).
The onsite soils may be reused as fill, provided they are free of organics a
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rocks and cobbles over 6 inches in dimension. Any import fill soils should b
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o
v
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d
b
y
the undersigned or other soil testing company.
Fill soils should be placed and compacted. in 6 to 8 inches thick lifts/layers
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bottom of the fill key and working upwards.
•
The fill should be moistened or dried as
necessary to slightly above optimum moisture conditions and compacted by
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means to at least 90 percent relative compaction based on ASTM D 1557.
The 5-feet wide minimum fill width should be maintained as the fill is brought u
p
.
T
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should be benched into the original slope soils in 2 feet minimum, vertical bench
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(
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attached Figure 1).
The upper portion of the slope should be overbuilt and cut back.
Horizontal layers of geogrid fabric should be installed at 4 feet maximu
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increments to provide extra strength.
The slope grading and compaction and placement of geogrid should be observed
a
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s
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d
as necessary by the undersigned or other soil testing company.
OTHER OPTIONS
You asked for other repair options to be considered such as constructing a retainin
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bottom of the slope.
Constructing a retaining wall at the bottom of the slope instead of regrading t
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o
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i
s
n
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t
a
preferred option. The retaining wall would have to be strong enough and high en
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(
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5 feet) to withstand the soil pressures exerted by the landslide and would probably
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supported on drilled, concrete caissons extending several feet into the grou
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.
I
t
w
o
u
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d
b
e
necessary to perform a subsurface soil investigatiàn and structurally design th
e
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a
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l
,
a
n
d
t
o
obtain permits from the City of Carlsbad. A retaining wall solution would also leave
t
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e
s
l
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i
n
its present, failed condition.
Martin R. Owen PE, GE Geotechnical Engineer
Geotethnical InspectiOn of Slope Failure Page 5 2210 Plaza Bonita Job No. .260506 Carlsbad, CA 92009
June Ia 2020
Constructing a lower (3 to 44eet high) Wall as part of a grading solution is a possibl
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t
i
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b
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t
the only perceived benefit is that It would allow you to reconstruct the slope at a flatter angle.
The wall would still have to be designed and permitted.
Construction of a retaining wall at the top of the slope would provide support to the existing
building foundations and conduits. However, the wall would do nothing to stab
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i
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landslide and further slope movement .ould.stilI occur.
SLOPE LANDSCAPING
The slope should be re-landscaped with shrubs and deep-rooted ground cover s
u
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h
a
s
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.
Moderate irrigation will be required to establish the landscaping.
FRENCH DRAIN AT BOTTOM OF SLOPE
You also indicated you would like to construct a French drain at the bottom of the sl
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t
o
intercept possible below-ground seepage.
A French drain should be at least 3 feet deep and 12 inches wide. A 4-inch di
a
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t
e
r
,
perforated SDR 35 or better PVC pipe wrapped in a filter sock should be placed at the bo
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the trench and the trench backfilled with Caltrans Class 2 Permeable Material dra
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k
(
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and sand mix). The pipe should flow at a 1 percent minimum gradient to an approve
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.
LIMITATIONS
Please note I have reviewed no construction plans or reports as part of this Inspectio
n
,
a
n
d
have performed no subsurface exploration or soil testing. The above conclusio
n
s
a
n
d
recommendations are based on my inspection and experience with other, similar
p
r
o
p
e
r
t
i
e
s
i
n
San Diego County.
This report provides no warranty, either expressed or implied, concerning future b
u
i
l
d
i
n
g
o
r
slope performance. Future damage from geotechnical or other causes Is a possibility.
Martin R. Owen PE, GE
Geotechnical Engineer
Geotechnical inspection of Slope Failure Page 6 2210 Plaza Bonita
Job No. 200506 CaliSbad, CA 92009
June 1, 2020
This opportunity to be of service is appreciated. If you have any questions, ple
a
s
e
d
o
n
o
t
hesitate to call or contact me.
Very truly yours,
Martin R. OWen PE, GE
Geotechnical Engineer
Plo. 23155
Ezp. 12131121
Attachments: Figure 1 and 2
Martin R. Owen PE, GE
Geotechnical Engineer
Geoiechnlcal Inspection of Slope Failure
Page 7 2210 Plaa Bonita
Job No. 200506 Ca1lsbad, CA 92009
June 1, 2020
FIGURE 1
'SLOPE FAILURE
ScHEMATIc ONLY
EAST-WEST SECTION
2210 TffROUGH••'SLOPE Plaza
Bonit
E— 5'
Scarp Original Slope
l.5:l(H:V) 39 Approx.
•1
___
•
Soil Displacement/ 15'
Failure Bulge fr6m Slope Approx.
Surface Failure
Lower
Level
Martin R. Owen PE, GE
Geotechnical Engineer
Geotechnical Inspection of Slope Failure Page 8 2210 PL= Bonita
Job No. 200506 Carlsbad, CA 92009 June 1,2020
FIGURE 2
SLOPE REPAIR
SCHEMATIC ONLY
EAST-WEST SECTION
2210 THROUGH SLOPE
Plaza
Bonita
<-5' -)I
.Scarp Restored Slope - N 1.5:1(H:V) N
/
Approx.
N /•
Geofabric Layers
at 4' Max.
Height Intervals
15'
Approx. 2'HighMin.
Fill Benches
Lower
Level
2' Deep
Fill yin
~51 *I
M.
Martin R. Owen PE, GE
Geotechnicul Engineer