HomeMy WebLinkAbout; Agua Hedionda Lagoon Slope Failure; Agua Hedionda Lagoon Slope Failure; 1999-03-04REPORT
GEOTECHNICAL EVALUATION
OF THE SLOPE FAILURE ALONG
RIGHT-OF-WAY ADJACENT TO
AGUA HEDIONDA LAGOON
CARLSBAD, CALIFORNIA
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Preparedfor
Engineering Department
City of Carlsbad
2075 Las Palmas Avenue
Carlsbad, CA 92009-1576
URSGWC Project No. 58-9911017F.OO-OEC01
March 4, 1999
URS Breinet Wo on warn Ely He
1615 Murray Canyon Road, Suite 1000
San Diego, CA 92108-4314
619-294-9400 Fax: 619-293-7920
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URS Greiner Woodward Clyde
A Division of URS Corporation
March 4, 1999
1615 Murray Canyon Road, Suite 1000
San Diego, CA 92108
Tel: 619.294.9400
Fax: 619.293.7920
Offices Worldwide
Engineering Department
City of Carlsbad
2075 Las Palmas Avenue
Carlsbad, CA 92009-1576
Attention: Mr. Doug Helming
Subject: Geotechnical Evaluation of the Slope Failure
Along Railroad Right-of-Way Adjacent to Agua Hedionda Lagoon
Carlsbad, California
URSGWC Reference No. 589911017FOO-EC01
Dear Mr. Helming:
URS Greiner Woodward Clyde (URSGWC) is pleased to provide the accompanying report,
which presents the results of our geotechnical evaluation for the subject project. Our work was
performed in general accordance with our proposal dated January 11, 1999 and with your verbal
authorization to proceed of January 19, 1999.
This report presents our conclusions and recommendations pertaining to the geotechnical aspects
of the slope repair, as well as the results of our field investigation and laboratory testing. Further
details are provided in the report.
If you have any questions, please give us a call.
Very truly yours,
URS GREINER WOODWARD CLYDE
eo D. Handfelt, R.G.E.
Senior Associate
SMF:
Attachment
teven M. Fitzwilham
Project Manager
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TABLE OF CONTENTS
Section 1 Introduction 1-1
1.1 Scope of Work 1-1
Section 2 Field Investigation and Laboratory Testing 2-1
Sections Site Conditions 3-1
3.1 Geologic and Subsurface Conditions 3-1
3.1.1 Seismicity 3-1
3.1.2 Fill Soils 3-1
3.2 Surface Conditions 3-1
3.2.1 Slope Slough 3-1
3.2.2 Adjacent Slopes 3-2
3.3 Groundwater and Surface Water 3-2
Section 4 Geotechnical Recommendations 4-1
4.1 Geologic and Seismic Hazard 4-1
4.2 Slope Repair 4-1
4.2.1 Site Preparation 4-1
4.2.2 Buttress Fill 4-2
4.3 Erosion Control 4-2
4.4 Surface Drainage 4-3
Section 5 Uncertainties and Limitations 5-1
Figures
Figure 1
Figure 2
Figure 3
Figure 4
Vicinity Map
Photographs of Slope Failure
Schematic Cross Section Through Slope Failure
Compaction and Index Property Data
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SECTIONONF Introduction
This report presents the results of geotechnical services that URS Greiner Woodward Clyde
(URSGWC) has provided with respect to the slope failure along the S.D.N.R. railroad alignment
in Carlsbad, California. The site location is shown on the Vicinity Map (Figure 1). For our
study, we have discussed the project with Mr. Doug Helming representing the City of Carlsbad.
An approximately 60-foot long section of the east-facing slope of an access road along the east
side of the railroad tracks has failed. Photographs of the failure are presented in Figure 2. A
schematic cross-section through the failure is presented in Figure 3. An existing 72-inch wide
trapezoidal storm drain empties into the Aqua Hedionda lagoon at the north edge of the slope
failure. This report presents our conclusions and recommendations regarding the repair of this
slope failure.
1.1 SCOPE OF WORK
The purpose of our geotechnical study was to characterize the site conditions and provide
recommendations for the repair of the slope. The scope of our services included a review of our
files for other projects in the immediate vicinity, field explorations, laboratory testing,
engineering evaluations and analyses, and preparation of this report. This study was designed to
develop conclusions and recommendations regarding the following:
• The geologic setting of the site
• General surface and subsurface conditions
• General extent of the slope failure
• Repair recommendations including slope reconstruction and erosion protection.
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SECTIONTWO Held Investigation and laboratory Testing
Our field investigation consisted of a site reconnaissance and geologic mapping of the slope
failure and sampling of near surface soils. The field investigation was performed by a geologist
from our firm on January 21, 1999.
Surface soil samples were collected and visually classified in accordance with the ASTM D2488
classification system. The samples were then returned to our laboratory for further examination
and testing. The classifications were then substantiated by performing grain size analyses.
Results of the grain size analyses are presented in Figure 4. The suitability of on-site soils for fill
was evaluated by performing a laboratory compaction test (ASTM D1557). Results of the
compaction test are also presented in Figure 4. The testing was performed in general accordance
with ASTM standards.
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SECTIOHTHREE Srte Conditions
OUT knowledge of the site conditions has been developed from a review of area geology and field
and laboratory programs undertaken for this project.
3.1 GEOLOGIC AND SUBSURFACE CONDITIONS
The site is located along a man-made crossing of the Aqua Hedionda Lagoon. The site is
approximately 1800 feet east of the Pacific Ocean shoreline. The site is underlain by artificial fill
over Pleistocene-age terrace deposits over the Tertiary-age Santiago Formation.
3.1.1 Seismicity
The Rose Canyon-Offshore Zone of Deformation-Newport Inglewood fault zone contains the
nearest active faults to the site. This zone of faults is located offshore of the site at a distance of
approximately 3 to 4 miles based on marine geophysical surveys. Other active faults in the
vicinity include the Coronado Bank and San Clemente fault zones located further offshore at
distances of approximately 20 miles and 30 miles, respectively, and the Elsinore fault zone
located approximately 25 miles to the northeast.
3.1.2 Fill Soils
The failed slope is within fill soils composed of silty fine sand. The near surface fill soils along
the backscarp of the slide are loose. The fill soils at the toe of the slide were submerged by
approximately 1 to 2 feet of water at the time of our site reconnaissance. These soils are very
loose and could be easily penetrated with a footing probe. Buried rip rap was encountered at a
depth of approximately 2 feet below the surface soils at the toe of the failed area.
3.2 SURFACE CONDITIONS
3.2.1 Slope Slough
The slope failure is approximately 60 feet in length and 30 feet in height. Based on visual
observations at the site, the toe of the slope has retreated by approximately 15 feet. The scarp of
the slide is located approximately 3 to 5 feet east of the east edge of the dirt access road. No
distress has been observed in the access road. Slope inclinations within the slope failure area
vary from nearly vertical to approximately 5:1 (horizontal:vertical). A typical cross section of
the slope failure is presented as Figure 3.
A 12-inch diameter high pressure gas pipeline that traverses the failed area is currently
unsupported in that area. This pipeline is located approximately 6 feet down (vertically) from the
top of the slope. Gunite has been applied to two portions of the slope directly below the gas
pipeline. Sand bags have been placed along the boundaries of the sloughing area. Concrete
barriers have also been placed between the slough area and the access road.
The outfall of a 72-inch wide trapezoidal storm drain is located just north of the slope failure. At
present, there is no erosion control/slope protection existing below this outfall. It is our opinion
that water from the storm drain had eroded away the toe of the slope, undermining the slope, and
inducing the failure. Small trees, grassy vegetation and debris are located within the failure area.
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SECTIONTHREH Site Conditions
3.2.2 Adjacent Slopes
The adjacent undisturbed fill slopes have inclination of approximately 1.5:1. An approximately
three foot high nearly vertical slope is located at the toe of the nearby undisturbed slopes. Rip
rap has been placed in front of the nearly vertical portions of the adjacent slopes. A 5 to 10 foot
wide beach of sand is located between the toe of the rip rap at the toe of the slope and the lagoon
water. These adjacent slopes are covered with grassy vegetation. Small to moderate sized
erosional channels were observed on these surface slopes.
3.3 GROUNDWATER AND SURFACE WATER
No groundwater or seeps out of slope were encountered during our site reconnaissance. Water
was flowing from the storm drain into the lagoon at the time of our site visit. The water from
Agua Hediondo Lagoon laps up against the toe of the slope failure. Groundwater at the site is
tidally influenced.
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SECTIONFQUR Geotechnlcal Becommendadons
The discussions, conclusions, and recommendations presented herein are based on information
provided to us, review of available information, results of field explorations, laboratory testing,
empirical correlations, engineering analyses, and professions judgment.
4.1 GEOLOGIC AND SEISMIC HAZARD
The primary geologic and seismic hazard potentially affecting the site is moderate to severe
ground shaking in response to either a local moderate or more distance large magnitude
earthquake. No known active faults cross the site and the area is not within an Alquist-Priolo
special Studies Zone for fault rupture hazard.
The Rose Canyon Fault Zone dominates the seismic exposure of coastal San Diego, including the
site area. The maximum credible earthquake for the Rose Canyon fault zone consists of a three
segment rupture and an estimated IV* magnitude event. In our opinion, the maximum credible
earthquake for the project is a magnitude (Mw) 6.5 event, which corresponds to a single segment
rupture along the nearest fault segment. Although not specifically studied for this project, soil
liquefaction induced settlement may occur in the event of seismic shaking.
It is our opinion that the slope failure was caused by erosional undermining of the slope by
waters discharged from an adjacent storm drain. We do not believe that the failure was a deep
seated slope instability. We understand that riprap and an energy dissipator is planned for the
adjacent storm drain outfall. That, along with additional protection on the surface and toe of the
repaired slope should mitigate the potential of a future failure.
4.2 SLOPE REPAIR
We recommend that the failed area be repaired by rebuilding to match the 1.5:1 inclinations of
the adjacent unfailed slopes. It is our opinion that the global stability of a 1.5:1 slope is adequate,
however, measures will need to be undertaken to control erosion on the slope. The repair should
be completed by removing the existing loose soils and constructing a soil buttress to rebuild the
slope. The general extent of the slope repair is presented in Figure 3. We recommend that a
pregrading conference be held at the site with the owner, contractor, civil engineer, and
geotechnical engineer in attendance.
4.2.1 Site Preparation
We recommend that existing stockpiles of soil, vegetation, and other debris and rubble be
removed and disposed of off-site. We also recommend that soils containing organic matter be
removed from the slope area.
We recommend that material be removed from the backscarp of the failed area to create a %: 1
backslope for the soil buttress. Loose materials at the toe of the failed area should be
overexcavated down to firm material, or to a maximum depth of 3 feet below the existing ground
surface, to provide a foundation for the buttress. This excavation will need to be done below the
lagoon water level. This overexcavation should extend from the toe of the backcut to the
proposed new toe of slope; this horizontal distance is up to about 15 feet. The overexcavation
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SiCTIOHFOUR _ Geotechnlcal Recommendations
and the backcut should extend laterally at least 5 feet into either side of the slope failure. We
recommend that these excavations be observed by a qualified geologist or engineer to verify
subsurface conditions.
4.2.2 Buttress Fill
We recommend that backfills for the buttress placed below the lagoon level and for a vertical
distance of 1 foot above the lagoon level (at the time of filling) be a 1 inch nominal size crushed
rock. This 1-inch gravel should meet the specifications of Section 200-1.2 of the Standard
Specifications for Public Works Construction (Green Book). The gravel can be placed in a
single lift and should be compacted with a minimum of 4 passes with a heavy vibratory roller. A
filter fabric should be placed on top of the gravel backfill prior to placement of the upper buttress
fill soils. The filter fabric should have a minimum grab strength of 90 pounds and placed in
accordance with Section 300-10. 1 . 1 of the Green Book.
Select fill materials should be used for the upper buttress fills. Select fill is defined as
nonexpansive (expansion index less than 50) granular (Unified Soil Classification of SM or
coarser) materials. The select fills should contain no rocks or hard lumps greater than 6 inches in
maximum dimension. Materials that contain perishable, spongy or otherwise compressible
nature materials should not be used for the buttress fill. Materials that are removed from the face
of the backscarp and the buttress foundation overexcavation should generally be suitable for use
as select fill.
Backfill materials should be placed in layers that, when compacted, should not exceed 8 inches.
Each layer should be spread evenly and should be thoroughly mixed during the spreading to
obtain uniformity of moisture and material in each layer. After each layer has been placed,
mixed, and spread evenly, it should be uniformly compacted to relative compaction that is
indicated by test to be not less than 90 percent. Relative compaction is defined as the ratio
(expressed in percent) of the in-place dry density of the compacted fill divided by the maximum
laboratory dry density evaluated in accordance with the ASTM Test Method D1557. A copy of
results from an ASTM 1557 test on existing site soils is presented in Figure 4.
The surface of the repaired slope should also be compacted to a minimum of 90 percent relative
compaction. This could be accomplished by trackwalking of the slope, or overbuilding the slope
and cutting back to the finished profile.
Portions of the high pressure gas main will be unsupported during construction activities. The
contractor should be responsible for providing adequate support of this pipeline during
construction activities. Manually operated compaction equipment will be required in the vicinity
of the gas main.
4.3 EROSION CONTROL
The toe of the reconstructed slope should be protected with rip rap placed up to elevation of the
existing slope rip rap and the rip rap proposed for the energy dissipator at the end of the new
storm drain. The rip rap should consist of '/z ton rock and meet the requirements of Section
200-1.6 of the Green Book. The rip rap should be placed to interlock. A filter fabric should be
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SEGTIONFOUR Beotechnlcal Recommendations
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placed below the rip rap to prevent migration of the fill soils through the rip rap. This filter
fabric should have a minimum grab strength of 200 pounds and be placed in accordance with
Section 300-9.1.1 of the Green Book.
The newly graded slope surface should be protected from surface erosion with permanent
revegetation. To provide temporary erosion control until the vegetation is established, a
biodegradable erosion control blanket (rolled or hydraulically applied) should be installed with or
over the selected seed mix. The rolled erosion control blanket should be comprised of a straw-
coconut blend with netting; or the hydraulically applied blanket should be a blended fiber matrix
with long fibers and a high strength adhesive that does not lose its strength upon rewetting.
4.4 SURFACE DRAINAGE
We recommend that surface drainage at the top of slope be controlled to prevent ponding and
infiltration of surface water into the slope. We recommend that a soil berm be constructed to
direct water away from the slope face.
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SECTIOMFJVF DnceitaMlisiHUnludMS
We have observed only a very small portion of the pertinent subsurface conditions. The
recommendations made herein are based on the assumption that soil conditions do not deviate
appreciably from those found during our field investigation. We recommend that URSGWC
review the grading plans to verify that the intent of the recommendations presented herein has
been properly interpreted and incorporated into the contract documents. We further recommend
that the site earthwork be observed by a qualified engineer or geologist to verify that site
conditions are as anticipated, or to provide revised recommendations, if necessary.
This report is intended for design purposes only and may not be sufficient to prepare an accurate
bid.
Geotechnical engineering and the geologic sciences are characterized by uncertainty.
Professional judgments presented herein are based partly on our understanding of the current
conditions and partly on experience. Our engineering work and judgments rendered meet current
professional standards; we do not guarantee the performance of the project in any respect.
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CARLSBAD %\v<&
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"Reproduced with permission granted by THOMAS BROS MAPS. This map is
copyrighted by THOMAS BROS. MAPS. It is unlawful to copy to reproduce all
or any part thereof, whether for personal use or resale, without permission."
SITE VICINITY MAP
CARLSBAD SLOPE FAILURE
DRAWN BY: CM CHECKED BY:PROJECT NO: 589911017F.OO-OEC01 DATE: 2-3-99 FIGURE NO: 1
URS Greiner Woodward Clyde
SLOPE FAILURE, LOOKING SOUTHWEST, 12-INCH
GAS MAIN WITHIN FAILED AREA.
SLOPE FAILURE, LOOKING NORTH. TRAPEZOIDAL OPEN CHANNEL
STORM DRAIN AT TOP OF SLOPE.
PHOTOS OF SLOPE FAILURE
CARLSBAD SLOPE FAILURE
FN: SECTION DRAWN BY: CM CHECKED BY:DATE: 2-10-99 PROJECT NO: 991107F-EC01 FIGURE NO: 2
I/as Greiner Woodward Clyde
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COMPACTION CURVE
Test Method: •ASTMD1557 BASTMD698 +CA-DWR:S-10 O Other Effort
Compaction Procedure: B Specimen Preparation Method: Moist
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Molding Water Content (%)
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PARTICLE-SIZE DISTRIBUTION CURVE
GRAVEL SAND
COBBLES COARSE | FINE COARSE) MEDIUM
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GRAIN SIZE IN MILLIMETERS
NOTATION: • Representative of entire sample • Representative of compacted specimen • Representative of compacted specimen
and entire sample
Exploration Sample Depth OPT.WC MAX. DUW LL P
No. No. (ft) (%) (pel)
B-1 1 9.0 128.5 NT N
PROJECT NAME: Carlsbad Slope Failure
PROJECT NUMBER: 589911 01 7F
I Description and/or Classification
Brown silty Sand (SIM)
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COMPACTION AND INDEX FIGURE No 4
PROPERTY DATA
SR-400 (11/98) (SNA)cpcl_b11 URS Greiner Woodward Clyde