1992-2955 TE
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ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
Project Number 91-27a
August 21, 1991
Mr. John Robinson
507 West A Street
Encinitas, California 92024
Subject: Geotechnical Engineering Report
Sea Bluff Erosion Mitigation
507 West A Street
Encinitas, California
Dear Mr. Robinson:
In accordance with your request, we are pleased to submit
. the following report describing the site geotechnical conditions,
and providing our opinions and recommendations to mitigate the
erosion of the sea bluff on the subject site.
This report contains a summary of our field findings, our
recommendations for mitigation of the existing erosive condition,
a site plan depicting mitigative locations, detail drawings, and
specifications, as appropriate.
Should you have any questions, please do not hesitate to
contact us at your convenience.
Very truly yours,
~~m
CEG 1084; expo 6-30-92
,/~
~
CharlesJ. Randle
RCE 22096; expo 9-30-93
Distribution:
(5) addressee
(1) Schmidt Design Group Attn: Glenn Schmidt
2655 Fourth Avenue
San Diego, California 92103
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
GEOTECHNICAL ENGINEERING REPORT
SEA BLUFF EROSION MITIGATION
507 WEST A STREET
ENCINITAS, CALIFORNIA
INTRODUCTION
In accordance with your authorization, we have completed our
study at the subject site. The purpose of our work was to assess
site soil conditions regarding sea bluff erosion, prepare reports
outlining our findings, and provide recommendations to mitigate
~ea bluff erosion.
A list of reference documents is included in Appendix A. Our
geotechnical evaluation (dated April 26, 1991/Revised May 20,
1991) of the site is attached as Appendix B. Our report dated
August 15, 1991 is attached as Appendix C. Drawings and diagrams
of mitigative methods are shown in Appendix D.
A general site plan is presented as Plate 1 at the end of
this report. The site plan depicts as accurately as possible the
existing topographic and geological conditions, as well as
locations for mitigative methods.
PROJECT LOCATION
According to Reference 1 in Appendix A, the property at
507 West A Street is assessor parcel number 258-042-20. The legal
description includes Lot 1, Block 47 Encinitas, in the County of
San Diego, State of California, according to Map No. 148, filed
in San Diego County June 12, 1883.
The property is located south of the western extension of
A Street, west of 5th Street, and north of the Moonlight State
Beach area in the City of Encinitas, California. The western
portion of the property is a sea bluff down to the Pacific Ocean.
A site location map is presented as Figure 1 in Appendix B.
SITE GEOLOGICAL CONDITIONS
The geological conditions reported and observed at the site
are included as pages 2 through 6 in Appendix B, and in Appendix
C. The general geologic conditions are shown in Appendix B as
Figure 2. Figure 3 in Appendix B depicts our interpretation of
the subsurface geological conditions present at the site.
Page 3
Project Number 91-27a
August 21, 1991
CONCLUSION OF STUDIES
The results of our studies indicate on-going soil erosion of
the sea bluff at the subject site. The cause of the erosion
includes poor surface drainage from this and an adjacent site to
the north, the steep configuration of the sea bluff, the lack of
vegatative growth cover on the slope, and other factors including
human foot traffic and digging on the slope.
If some types of mitigation are not implemented, erosion
will likely continue unchecked at a relatively rapid rate and
affect existing site improvements.
RECOMMENDATIONS
The following recommendations are provided as methods to
mitigate the rate of erosion at the site. These methods are
intended to provide temporary, short term control of the erosion
at the site, while the other longer term solutions are being
considered and planned. All such protections should be considered
temporary since, in our opinion, no protective erosion control
device along eroding coastal bluffs can be considered permanent.
LONGER TERM SOLUTIONS
1. Base Protection
The base of the bluff at the site is composed of Torrey
Sandstone that will erode and retreat at relatively slow
rates as compared to the overlying friable sands. The slow
rate of erosion of the Torrey Sandstone will eventually
cause a response erosion of the overlying friable sands.
Some type of base protection might be considered to be
installed to lessen and/or slow the rate of retreat of the
Torrey Sandstone. Such measures to consider might include
rip-rap armour to blend into an existing rip-rap armour .
along the base of the bluff just south of the site, sea wall,
and/or other types of devices.
2. Upper Property Protection
A longer term solution for protection of the property
at the top of the bluff might include relocation of the
existing residence such that the present east-west alignment
is north-south and the structure set back from the existing
westerly foundation location. This consideration would
Page 4
Project Number 91-27a
August 21, 1991
involve encroachment onto the property north of the subject
lot. Relocation of the residence in such a manner would
result in a longer time use of the property as residential.
Another method to consider for the longer use of the upper
property might include the closure of A Street, installation
of a east-west aligned retaining wall north of the lot, and
lowering the upper portion of the lot by 10 to 15 feet with
access from 5th Street.
SHORTER TERM SOLUTIONS
The following recommendations are to be implemented at
your request. The locations for mitigative devices or site
improvement is shown on the site plan, Plate 1. Drawings
depicting detail and specifications, as appropriate are
included in Appendix D.
1. Site Drainage
Positive surface drainage shall be established for the
site. Positive surface drainage for this report is defined
as drainage away from the residential foundation and sea
bluff top at a gradient of at least 2 percent for at least
5 feet and directed to a suitable outlet. The establishment
of positive surface drainage will include the area to the
immediate north of the lot, since runoff from the adjacent
site directly influences erosion of the sea bluff on the
site. The lot near the bluff top shall be resloped toward
the east to concrete lined drainage ditches. A concrete lined
drainage ditch shall also be installed near the top of the
bluff north of the lot. The locations for the ditches are
shown on Plate 1. A detail drawing for the ditches are
shown as Figure D-l. The ditches shall be sloped to drains
at the locations shown on Plate 1. A detail drawing for the
drains is shown as Figure D-2.
All drainage devices shall have a minimum gradient of
at least 2 percent toward the east and 5th Street. Downspputs
for the residence shall be directed to discharge away from
the foundation, and into either the concrete lined drainage
ditches, or the drains depicted on Plate 1 and Figure D-2.
2. Slope Mitigation
Mitigation for the sea bluff erosion will consist of
the following basic procedure. Removal of the debris cone
at the base of the bluff. Hydroseeding of the sea bluff
face above elevations of about 30 feet. Covering the exposed
Page 5
Project Number 91-27a
August 21, 1991
soil between elevations of about 30 and 7C ~et with a
degradable cover. Selected hand plantings ~lthin erosion
scar areas. Slope watering will be avoided.
The general locations for the procedures outlined
above are shown on Plate 1 for information only. The general
concept plans and specifications being prepared by the
Landscape Architect shall be utilized for the actual field
construction.
The procedures outlined are intended to avoid, as much
as possible, slope disruption during installation operations
and immediately thereafter.
1. The removal of the cone of slope debris must be
performed to remove the reason for persons to jump onto
and slide down the slope. The material can be removed
to another area or spread as a thin cover over the
beach gravels. As a safety measure, it may be prudent
to request closure of A Street west of 5th Street,
except for residents, until the mitigative work has
been completed and the results established. This may
involve a few to several months of time.
2. The hydroseeding operation is expected will cover
most of the exposed slope areas between elevations of
about 30 to 70 feet. Seed, mulch, and nutrient mixture
and actual coverage will be established by the Landscape
Architect.
3. The placement of a cover onto the slope shall be
performed in a manner to avoid, as much as possible,
disruption of the slope soils. We recommend a product
such as Erosion Control Blanket C-125 by North American
Green, or an approved equivalent. The covers shall also
be placed to avoid disruption of significant areas of
established vegetation.
The blanket recommended is intended to reduce
rainfall impact, reduce the velocity of runoff water,
shield the surface soils from wind, and provide a
temporary shield to allow for the growth of hydroseeded
vegetation. The product recommended, when properly
installed will blend well with the existing and a~jacent
slope areas. The cover should consist of a lower layer
applied in a horizontal direction with the slope across
the erosion scar area. A second layer should cover the
lower layer as well as other areas bare of vegatation.
Page 6
Project Number 91-27a
August 21, 1991
The covers should be overlapped as recommended by
the manufacturer with at least a 4 to 6 inch overlap. The
fabric should be stapled as recommended by the
manufacturer with at least 3.5 staples per square yard.
As recommended by the manufacturer, because of local
loose soil conditions, the use of 18 inch or longer
staples may be required. The tops of the covers shall
be stapled with at least two levels of three foot long
pins spaced every 3 feet horizontally, and offset one
foot for the second level three feet below the first
level.
An outline of the material specifications is
contained as Figure 0-3.
4. Selected handplantings shall be performed within the
erosion scar areas as shown on the plans being prepared
by the Landscape Architect. The handplantings will be
performed in such a manner as to avoid impact to the
work previously performed. Slope watering will be
avoided.
The work proposed is expected will occur in the
late summer to early fall of 1991.
3. Safety
The safety of personnel on-site, and adjacent to the
work area, during installation activities is the responsibility
of the contractors performing the installation of devices
as described on pages 4, 5, and 6. We cannot be responsible
for the safety of personnel, including our own personnel,
during the installation of devices described on pages 4, 5,
and 6.
4. Earthwork
Except for the work as described herein, no significpnt
earthwork is planned for the installation of erosion control
devices described herein on pages 4, 5, and 6.
5. Performance of Work
All installation work shall be performed in accordance
with this report, and the plans and specifications being
prepared by the Landscape Architect. All installation work
shall be performed utilizing sound construction practices,
in accordance with the manufacturer minimum requirements for
Page 7
Project Number 91-27a
August 21, 1991
the product.
6. Observation Services
All work described herein on pages 4, 5, and 6 shall be
performed under the observation services of the engineer.
All slope mitigation work performed with the plans and
specifications prepared by the Landscape Architect shall be
performed under the observation service of the Landscape
Architect.
7. Limits of Work
No other work shall be performed at the project site
except as described on pabges 4, 5, and 6 herein. Any
changes or deviations from the work as described herein
shall be approved by the engineer, or for aspects regarding
the plans and specifications of the Landscape Architect, by
the Landscpae Architect.
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Project Number 91-27a
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Legend for Site Erosion Mitigation
This plan has been modified from the Topographic Plan prepared
by La Costa Engineering. A geological map and cross section are
presented as Figure 2 and 3 in Appendix B.
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Description
Location for the installation of drainage
ditches - see detail Figure D-1.
Location for the installation of 24 inch area
drain - see detail Figure D-2.
Location for the installation of 6 inch area drain
for downspout discharge.
Location of 6 inch diameter pipe drains to street,
Schedule 40 PVC or approved equivalent. See detail
Figure D-4 for subsurface placement. Arrows
indicate planned flow direction.
Location of pipe outlet to street. See detail
Figure D-5.
Area to reslope to surface drain to the east.
Area to be hydroseeded, selected areas to be
covered, and handp1anted. Refer to the plans and
specifications being prepared by the Landscape
Architect.
Cone of Slope Debris to be removed or leveled.
Plate 1: Site Plan - 507 West A Street, Encinitas, California
APPENDIX A
REFERENCES
1. Artim and Associates, 1991a; Geotechnical Evaluation,
507 West A Street, Encinitas, California; Project number
91-27, dated April 26, 1991/Revised May 20, 1991.
2. Artim and Associates, 1991b; Site Observations - Sea
Bluff, August 13 and 14, 1991, 507 West A Street,
Encinitas, California; Project number 91-27, dated
August 15, 1991.
3. Eisenberg, L.I., 1983; Pleistocene Marine Terrace and
Eocene Geology, Encinitas and Rancho Santa Fe Quadrangles,
San Diego County, California; San Diego State University,
Masters Thesis, 386 pages.
4. Geotechnical Exploration, Inc. 1991; Geotechnical
Investigation of Coastal-Erosion and Bluff-Stability,
Stroscher Residence, 1118 Neptune Avenue, Encinitas,
California; Job number 90-5877, dated March 15, 1991.
5. Kuhn, G.G. and Shepard, F.P., 1984; Sea Cliffs, Beaches,
and Coastal Valleys of San Diego County; University of
California Press.
6. La Costa Engineering, 1991; Topographic Map, 507 West
A Street, Encinitas, California; scale 1"=20'.
7. Lough, C.F., 1979; Assessment of Degree of Risk Along
Coastal Bluffs in Unincorporated Portion of San Diego
County; San Diego County Department of Planning and Land
Use.
8. San Diego Association of Geologists, 1989; The Seismic
Risk in the San Diego Region; Proceedings of Workshop,
June 29-30, 1989.
9. Wilson, K.L., 1972; Eocene and Related Geology of a
Portion of the San Luis Rey and Encinitas Quadangles,
San Diego County, California; University of Californiad
Riverside; Masters Thesis, 135 pages.
APPENDIX B
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Mr. John Robinson
507 West "A" Street
Encinitas, California 92024
Subject: Geotechnical Evaluation
507 West "A" Street
Encinitas, California
References:
1. Eisenberg, L.I., 1983, Pleistocene Marine Terrace and
Eocene Geology, Encinitas and Rancho Santa Fe Quadrangles,
San Diego County, California; San Diego State University,
unpublished Masters Thesis, 386 pages.
2. San Diego Association of Geologists, 1989, The Seismic
Risk in the San Diego Region; Proceedings of Workshop,
June 29-30, 1989.
3. Wilson, K.L., 1972, Eocene and Related Geology of a
Portion of the San Luis Rey and Encinitas Quadrangles,
San Diego County, California; University of California,
Riverside; unpublished Masters Thesis, 135 pages.
Introduction
In accordance with your authorization of April 5, 1991, we
have performed a geotechnical evaluation of the property located
at 507 West "A" Street, Encinitas, California. The purpose of our
work was to assess the general site and soil conditions and to
prepare a report outlining our findings as well as recommendations
for mitigation and/or repair of potential problem features observed.
The property is assessor parcel number 258-042-20. The legal
description includes Lot 1, Block 47 Encinitas, in the County of
San Diego, State of California, according to Map No. 148, filed
in San Diego County June 12, 1883.
The property is located south of the western extension of
"A" Street,-west of 5th Street, and north of the Moonlight State
Beach area. The western portion of the property is a sea bluff
down to the Pacific Ocean. A site location map is presented as
Figure 1. The base map utilized was page 24 of The Thomas Guide
San Diego County, 1991 edition.
A general site plan was prepared for reference of location
of, and presentation of observations, test excavations, and
geological units. The site plan is presented as Figure 2 and the
base map was a topographic map prepared by La Costa Engineering,
Page 2
Project Number 91-27
April 26, 1991/Revised May 20, 1991
undated. A cross section of the site depicting our interpretation
of the subsurface conditions is presented as Figure 3.
Project construction plans and documents were not readily
available for review. Soils reports including any compaction
results for the site were not available for review.
Scope of Services
The scope of our services has included the following tasks:
* Review available pertinent reports relative to geotechnical
cond~tions at the subject site.
* Perform a visual site assessment of the site setting, and
general conditions.
* Perform a field analysis of the on-site subsurface soils
including geological logging and description of soils in
the sea bluff, and two shallow test excavations to examine,
sample, and test the foundation soils of the site building
pad. All logging and descriptions were performed by a
certified engineering geologist.
* Laboratory testing of representative soils including
expansion index, grain size (seive), and direct shear.
* Analysis and interpretation of our field findings and
laboratory test results. Evaluation of data and preparation
of this report with recommendations for mitigation and/or
repair of certain observed features.
* Processing and preparation of this report.
Site Geologic Setting
The site is located in an area identified by Eisenberg
(reference 1) to be underlain by a Pleistocene Beach Ridge and
deposits. Eisenberg also identifies that the Beach Ridge deposit
is about 40 to 50 feet thick in the site area and is underlain
by the Eocene Torrey Sandstone.
Our review of reference 2 indicate that there are no known
active faults in the vicinity of the site. Wilson (reference 3)
reports a northeast trending fault near the site. at Moonlight
State Beach; however, Eisenberg notes (page 38) that the fault
"does not cut the Pleistocene terrace". Eisenberg also notes
Page 3
Project Number 91-27
April 26, 1991/Revised May 20, 1991
that the bedrock along the base of the sea bluff contains faults,
"none of which can be seen to cut the Pleistocene terrace."
The closest active faults to the site include the Elsinore
Fault about 35 to 40 miles northeast of the site; the Coronado
Bank Fault about 20 to 30 miles southwest of the site; and the
offshore projected extension of the Rose Canyon Fault about 3
to 5 miles west of the site. There are many other active faults
in the southern California area that could generate ground
shaking at the site; however, for purposes of this report reference
2 contains information to suggest that ground shaking at this
site would be greatest from a maximum probable earthquake
generated on one of the branches of the Rose Canyon fault within
10 miles of the site. Information in reference 2 suggests that
the maximum probable earthquake for the Rose Canyon Fault is
about a 6.5 Richter Magnitude event. The occurrence of such an
earthquake within 10 miles of the site could generate ground
acceleration at the site of greater than about 0.5g.
The conditions to consider at the site from the aspects of
earthquakes and faulting include ground fault rupture,
liquefaction, ground shaking, and ground failure. Since there are
no known or mapped active faults on the site the probability of
ground fault rupture appears to be very low. Since the soils that
underlie the site are medium dense to dense sand the probability
for liquefaction appears to be low. From the aspects of ground
shaking this site is no different than numerous other sites
around the Encinitas area and the site should perform as well as
other sites in Encinitas with similar geological conditions. From
the aspects of ground failure the site condition to consider will
include the sea bluff along the west part of the property and the
water seepage near the contact of Pleistocene deposits and the
Torrey Sandstone.
The regional ground
be at and just above sea
water are known to occur
above the contact of the
Sandstone.
water table in this area is expected to
level. Local perched and seasonal perched
within the sea bluff area, and especially
Pleistocene deposits and the Torrey.
Site Field Conditions
Site field work was performed on April 8 and 10, 1991.
During our field work we observed the following:
* The site is located south of the west extension of "A"
Street. The extension consists of asphalt driveway access
to the subject property and a property located north of
Page 4
Project Number 91-27
April 26, 1991/Revised May 20, 1991
the "A" Street extension.
* Surface drainage from the extension area appears will flow
east, west toward the sea bluff, and south toward the site.
No provisions were observed in the "A" Street extension area
to divert runoff away from the sea bluff or property to the
south and toward 5th Street.
* The lot was noted to be about 9 to 12 feet above 5th
Street to the east.
* The site is developed with a single story residential
structure. The structure includes a basement area with
access from the east.
* A retaining wall about 6 to 8 feet high is located just
south of the south property boundary. The adjacent pad
to the south of subject site is at an elevation below the
subject site.
* Site development includes a concrete drive/parking area;
wood fence to northwest corner of residence; deck area on
south side of residence; and two low retaining walls near
southeast and northeast corners of residence.
* Elevations for the subject site pad are from about 82 feet
near northwest corner of residence to about 70 feet near
northeast access drive. The pad has two general levels:
the west level varies from about 79 to 81 feet; and the
east level varies from about 70 to 75 feet.
* The top of bluff is about 15 to 18 feet from the main
floor of the residence, but the foundation of the basement
area is likely about 20 to 25 feet from the face of the
bluff.
* We observed that at least two slope failures had occurred
on the bluff face during and immediately after the heavy
rains- of March 1991. The failure areas were about 3 to 15
feet wide, about 1 to 3 feet deep, and several feet long.
Secondary accelerated erosion has also occurred during
the rainstorms.
* The materials in the bluff are composed of the approximate
sequence of description from top to the bottom. The upper
8 to 10 feet of soils are composed of light reddish-brown,
silty sand. These soils are lightly cemented with iron-
oxide. The materials tend to erode and stand temporarily
Page 5
Project Number 91-27
April 26, 1991/Revised May 20, 1991
for longer periods of time than the underlying material.
* Underlying the reddish-brown silty sand is about 35 to
40 feet of pale brown, very fine sand. These soils are
clean and friable with virtually no cohesive material. A
grain size test of this material is included in the back
of this report.
* Underlying the pale brown sand is about 20 feet of very
dense sandstone of the Torrey Sandstone. The contact of
the upper Torrey Sandstone is delineated by a stained
red-brown layer about 2 feet thick likely a result of
water seepage. This bedrock unit stands in relativily
more stable steep inclinations. We did not observe any
fracture zones or faults in this unit.
* The base of the bluff is marked by piles of eroded sand
and debris from the bluff. Below the base to the water is
a beach composed primarily of cobbles. Figure 4 presents
a photograph presentation of the bluff face and beach area.
* The bluff face is nearly void of vegetation. Some growths
of ice plant grow from the top of the slope, but otherwise
the bluff face is bare of vegetation.
* Test excavations T-l and T-2 were made at the locations
shown on Figure 2. Each excavation was made to a depth of
4 feet, soils examined, described, and sampled.
* Our knowledge of the site area indicates that sea bluffs
along this area suffer ~from irregular and episodic
failures, especially along the tops of the bluff. The
average rate of retreat of the base appears to vary from
about 1/2 to 1 inch per year up to about 6 inches per
year. The upper bluff however, as the lower bluff
retreats, often remains in an unstable condition for some
time until an erosive force from the top of the bluff
causes a large section to fail and "run" down the slope.
* We observed that erosion appears to be more rapid at this
site than at the adjacent nearby sites.
* This section of sea bluff will continue to erode at a
rate more rapid than adjacent properties unless some
type of mitigation is implemented.
* Except as described herein there are no other geological
hazards known to be present on the site.
Page 6
Project Number 91-27
April 26, 1991/Revised May 20, 1991
* The slope at the subject site is as steep as 65 feet
vertically along a horizontal depth of about 60 feet.
This inclination is slightly steeper than l:l( horizontal
to vertical). The lower Torrey Sandstone is capable of
standing in the near vertical for long periods of time:
however, the overlying friable sands will tend to erode
to an angle of repose of about 26 degrees. In addition,
the upper friable sands are extremely prone to rapid
erosion.
* During a walking review of the approximate 2 miles of
bluff north of Moonlight State Beach in March, 1991, we
observed several sites at which vegetative growth was
well established in the friable sands. Certain species of
small trees and shrubs appeared to be concentrated at the
contact between bedrock and friable sand. Water seepage
at these areas appeared to be not present, or less than
adjacent sites. Certain species of shrubs appeared to be
well established within the friable sand zone.
* The test excavations encountered medium dense to dense,
reddish-brown, slightly silty sand. The upper several
inches in each excavation appeared to be loose and
disturbed, possibly as a result of landscaping and/or
site construction activities. An expansion test on the
soil in T-l indicated they have a very low potential for
expansion. A direct shear test of soil at the base of T-2
indicated an angle of 31 degrees and 100 pounds cohesion.
Summary of Key Site Conditions
The site slope is about 65 feet high at an approximate 1:1
inclination. The upper 40 to 50 feet of the slope is underlain by
friable sands. The angle of repose for these sands is likely
about 26 degrees. These soils are actively eroding.
A set back line of 2:1 horizontal to vertical (26 degrees)
from the approximate contact of the Torrey Sandstone and friable
sands would extend to the base of the existing structures
basement foundation.
Mitigation of the present erosion conditions should be
implemented to protect the property at the upper portion of the
lot. Such protections should be considered temporary since, in
our opinion, no protective devices along eroding coastal bluffs
can be permanent.
Page 7
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Conclusions and Recommendations
The results of our study indicate on-going soil erosion of
the bluff at the subject site. The cause of the erosion includes
poor surface drainage from this and an adjacent site to the
north, the steep sea bluff on the west part of the site, the
friable nature of the upper sands in the bluff, and the lack of
vegetative growth on the slope. If some types of mitigation are
not implemented, erosion will continue and affect site improvements.
We recommend that the following be utilized for mitigation
of the site erosion conditions.
1. Positive surface drainage should be established for the
site. Positive surface drainage for this report is
defined as drainage away from foundations and sea bluff
top at a gradient of at least 2 percent for at least 5
feet. Establishment of positive surface drainage will
also need to include the site area immediately adjacent
to the north of the lot, since runoff from the adjacent site
directly influences erosion of the site bluff. All areas
near the bluff top should be regraded to slope toward the
east or into concrete lined drainage swales away from the
sea bluff top. All drainage should have a minimum
gradient of at least 2 percent toward the east and 5th
Street. Eave gutters and downspouts should be installed
on the residence with downspouts discharging away from
the foundation and sea bluff top and utilizing non-
erosive devices. Installation of positive drainage will
likely include several area drains and/or collection
points discharged to 5th Street.
2. Soil samples of the friable slope sands should be
collected and an analysis performed to assess the type of
vegetation that may survive, and thrive, at the site
slope conditions. The species of vegetation that appear
to be performing well in similar slope and soil condition
along the bluff north of the site should be analysed to
evaluate their suitability to adapt to this site. Based
on this study a landscape plan should be developed and
implemented at the subject sea bluff slope. Such a plan
should avoid, as much as possible, slope disruption during
planting operations. Slope watering should be avoided if
possible, and if deemed necessary might consist of a
degradable drip system. .
3. The base of the bluff area is composed of Torrey
Sandstone that will erode and retreat at relatively slow
Page 8
Project Number 91-27
April 26, 1991/Revised May 20, 1991
rates. The slow retreat will continue to cause a response
erosion of the upper bluff friable soils. However, when
the recommendations contained in numbers 1 and 2
above are implemented, the resultant erosion will be
better controlled. Some type of base protection might
be installed to lessen and/or slow the rate of retreat of
the Torrey Sandstone base. Such measures could include
rip-rap armor to blend into an existing armor just south
of this site.
4. For safety of the property at the top section of the
bluff and to persons at the base of the slope, we
strongly recommend that planning and implementation of at
least the first two recommendations begin immediately.
5. For longer term protection of the property, other
considerations include relocation of the existing
residence such that the present east-west alignment is
north-south and set back from the present foundation. This
consideration would involve encroachment onto the
property north of the subject lot. Relocation of the
residence would result in a longer time use of the
residential property.
6. The final mitigation plans and specifications should be
prepared by this firm and approvals obtained prior to the
order, or purchase, of any materials or construction.
7. We assume no responsibility or liability for work
performed by others at the subject site.
8. Safety of personnel on-site during mitigation construction
is the responsibility of the contractor performing the
work.
9. Should you desire, we can obtain cost estimates for
mitigation recommendation number 1, and cost estimate to
perform the additional study for recommendation 2. Otþer
course of actions will be dependent on the directions
and requirements of the City of Encinitas and the Coastal
Commission.
10. Additional services may be required dependent upon the
directions and requirements provided from the City of
Encincitas and the Coastal Commission.
Page 9
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Should you have any questions or require additional service,
please do not hesitate to contact us.
Very
/
~~--,
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Charles J. Randle
RCE 22096; expo 9-30-93
~~~
Ernest R. Artim
CEG 1084; expo 6-30-92
Distribution:
(3) addressee
Page 10
Project Number 91-27
April 26, 1991/Revised
May
20,
1991
~
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.....
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'-- ...
SITE
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Base
Map:
The Thomas Guide
1991 edition
San Diego County,
North
page
24
Scale:
1
inch
2900
feet
Figure
1 :
Site Location Map
507 West "A" Street,
Encinitas,
California
Page 11
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Legend for Figure 2 and 3
Qb
Beach cobble and sand deposit
Qt
Quaternary Beach Ridge deposits
Tt
Torrey Sandstone
~'
/
geologic contact
~
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2:1 erosion line
no fractures were observed at site exposures
no faults were observed at site exposures
no joints were observed at site exposures
bedding is generally cross-bedded or massive
contact between Torrey Sandstone and friable sand is
relatively horizontal
T-2
x
approximate location of test excavation
A AI
Igeological cross-section
scale of original drawings is 1 inch = 20 feet
reduced scale of report drawing is the same horizontally
and vertically. Refer to bar scale on Figure 2.
Figure 2a: Legend for Figures 2 and 3
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Generalized Geologic Cross-
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Page 14
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Figure 4:
Photograph view of slope and base area. The subject
site is the central section of the slope. Note the
cobble beach area; the eroded sand at the base of the
slope as a cone shape; and the reddish-brown stained contact
of the Torrey Sandstone below contact and the friable
sand above. Also note the lack of vegetation along this
segment of bluff. The palm trees and brush at the top of
slope are an adjacent property, and the debris sand cone
to the right of photograph is an adjacent property to the
south. The view also includes the rip-rap armor near the
adjacent southern property.
Page 15
Project Number 91-27
April 26, 1991/Revised May 20, 1991
LABORATORY TEST RESULTS
Expansion Testin9
Sample Location
Description
Expansion Index
Classification
T-2
0-4 feet
Fine sand
-0-
Very Low
Direct Shear Test
Sample Location
Friction Angle
Cohesion
T-2
4 ft depth
31 degrees
100
Grain Size
(as a percent passing the listed screen size)
Sample Location
#4
#8
#16
#30
#50
#100
#200
10 below top of
slope
100
98
92
80
30
16
7
APPENDIX C
Project Number 91-27
August 15, 1991
Mr. John Robinson
507 West A Street
Encinitas, California 92024
Subject: Site Observations - Sea Bluff
August 13 and 14, 1991
507 West A Street
Encinitas, California
Reference Report:
Geotechnical Evaluation, 507 West A Street, Encinitas,
California; prepared by Artim and Associates, project
number 91-27, dated April 26, 1991/Revised May 20, 1991.
This letter will serve to document observations made on
August 13 and 14, 1991 at the subject site area. On August
13, 1991, we had made a visit to obtain measurements of the
residence from the top of bluff for use in preparation of plans
and specifications for mitigation of sea bluff erosion. At the
time of our visit, we observed what appeared to be a new slope
failure. We observed that the bluff in an area of friable sand
appeared to have failed and undermined the top of the slope.
However, upon closer examination we observed footprints, both
bare and with shoes, from a rough path area into the undermine
area. The footprints appeared relatively small and we assume the
prints were made by children. We also noted evidence for digging
in the undermine area, which we assume had been done by the
children who had left the footprints. Because of our concern for
our safety, we did not make any ventures into the back portion of
the undermine area; however, we could clearly observe footprints
near the rear of the undermine area. The footprints appeared to
originate from a brush covered top of slope at the extreme west
end of the A Street extension.
On August 14, 1991, we returned to obtain measurements of
the undermine area. Utilizing a light pole with a tape measure
attached, we were able to obtain rough measurements of the
amount of soil erosion involved in the apparent vandal damage.
We obtained measurements of an undermined slope area of about ,3
to 5 feet deep, 5 to 9 feet high, and about 9 to 12 feet wide. We
estimate that about 25 cubic yards of soil erosion by vandals
occured recently at the subject site. The undermine area poses an
immediate threat to persons at the top of the bluff, as well as
to persons at the base of the bluff. The undermine area also
might impact the methods being considered for mitigation of slope
erosion. .
Based upon our conversations with persons at the base of the
slope, young people have been observed to slide down the slope
Page 2
Project Number 91-27
August 15, 1991
onto the debris cone at the base of the slope. Alledgedly, this
type of activity has been common ever since the cone formed in
February to March, 1991. Therefore, a significant portion of the
debris created since the initial failure of February to March,
1991, may in fact be the result of human vandal related activities,
and not "natural" erosion forces.
We also noted on August 14, 1991 after a section of the
undermine area failed, that the soils appeared to be very moist.
This seemed to be unusual for the type of material and the weather
at this time of year. The very moist area appeared to be about
13 feet below the pad of the lot. This elevation for moisture, the
type of material, and the time of year suggest another source for
the moisture in the slope. It may be prudent to check water lines
in the adjacent A Street and 5th Street, including service lines,
for possible leakage. If necessary, repairs should be performed as
quickly as possible.
We will incorporate the information observed into our plans
and specification for site erosion mitigation.
Should you have any questions, please do not hesitate to
contact us.
Very truly yours,
U\~
Ernest R. Artim
CEG 1084; expo 6-30-92
Distribution:
(2) Addressee
ProJect Number 91-27
August 15, 1991
Photograph depicting the undermined area apparently created as
a result of digging on the slope by persons. Assess appears to
have been from the upper left of the photo along a foot trail
identified at the base of the vertical exposure at the top of the
bluff. The excavated and subsequently collapse soils have collected
at the base of the slope onto the previous debris cone created
by the slope failure of February to March, 1991. Photographs of
the slope taken in July, 1991 indicate their was no digging or
undermining activity prior to those photos.
APPENDIX D
Project Number 91-27a
August 21, 1991
..
Co
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3' min
Ditch bottom may
be rounded at the option
of the contractor.
3" 47D.C.2DDD concrete or
3" 2500 psi, air placed concrete
with 1\t,"xl~" 17 gage stucco
nettingo
TYPE A
5' min
.u ,20m;," '
,~,~'~; ~.\/:% .'.
"¡;"}~."; 'l '~.
" "" .,,¿
slope
3" 470.C.2000 concrete or /
3" 2500 psi, air placed concrete
with IYJ"xIYl" 17 gage stucco
netting.
c
'e
3' min
bottom may be rounded at
the option of the contractor.
TYPE B
TYPE C
NOTES
1. Longitudinal slope of lined ditch shall be 2% minimum.
LEGEND ON PLANS
~ ===::> . ~
Figure D-1: Description and Specifications of Drainage
Ditches
Project Number 91-27a
August 21, 1991
LEGEND ON PLANS
:-.~
SECTION A-A
-
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-- B
-
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PLAN
--- Finish Grad,
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17 bend down
8" inlO will
l
1 YJ" IJp, [ T
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Figure D-2: Description and Specifications of Drains
1 Rounded Pipe Ends.
"1
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2'
SECTION B-B
Project Number 91-27a
August 21, 1991
Purpose
The purpose of the material is to reduce rainfall impact to
the surface soil, reduce the velocity of runoff water, shield the
surface soils from wind, and provide a temporary shield to allow
for the growth of hydroseeded vegetation.
Composition
The product erosion control blanket C125 by North American
Green is recommended by the manufacturer for areas of "high
velocity runoff and extreme slopes". The product is made of
"tough, long-lasting coconut fiber, non photodegradable netting
and is sewn together. Although coconut fiber is degradable, it
will not disintegrate for several years".
Specifications
Material Content
Coconut Fiber
Weight
Netting
(heavy wt)
Thread
100%
0.50 Ib/sq. yard
both sides
UV stabilized
100% polyester
Physical Specs
Width
Length
Weight
Area
6.5 feet
83.5 feet
30 Ibs
60 sq. yards
Figure D-3: Description and Specifications of Slope
Cover Material
Project Number 91-27a
August 21, 1991
~
C'I
invert elevation
~ ~ 4" clearance {minI
SECTION
NOTES
{O¡ indicates minimum relative compaction.
Figure D-4: Description and Specifications of Drain Outlet
Pipe Placement
Project Number 91-27a
August 21, 1991
A
Sidewal k cuts per
Std. Dwg. G.l1
"..- ---------.2 --
L--------- ----
B
B
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....
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Co
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ci:
A
PLAN
...
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....
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:;
c..:>
Gutter
SECTION
. ".,
.. . '
¡/
Pip I to bl finishld
flush with curb.
c:
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~
Gutter
'.
L.. 3 112" ...
~ ~.'" ...?~~min .,:l5r.'.~.
~i.,D '"
. ...,
6" ."" .
Typ.
SECTION A-A
. ""'.' .'.'
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OpllOnll bruk linl for cuts
in ,.iSling curb Ind gutter.
SECTION B-B
APPROVED DRAIN
PIPE SIZES
J" 6" to 8" CURB FACE
4" 8" CURB FACE
6" 10" CURB FACE
NOTES
1. Pipe shall be one continuous length from property line to curb line.
2- Multiple pipes to be set a minimum distance of 0/2 apart.
J. Concrete shall be 520-C-2500
4. Pipe shall be circular asbestos cement. cast iron or rigid plastic.
Figure D-5: Description and Specifications of Drain Pipe
Outlet to Street
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ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
Project Number 91-27b
November 15, 1991/Revised December 19, 1991
Mr. Paul B. Webb
Coastal Planner
Califor~ia Coastal Commission
San Diego Coast Area
3111 Camino Del Rio North, Suite 200
San Diego, California 92108-1725
Subject: Emergency Permit Request - On Site Drainage Improvements
507 West A Street
Encinitas, California
Reference Report:
"Geotechnical Engineering Report, Sea Bluff Erosion
Mitigation, 507 West A Street, Encinitas, California",
Artim and Associates, Project number 91-27a, dated
August 21, 1991.
Dear Mr. Webb:
In conformance with the request of our client, Mr. John D.
Robinson, and your letter dated October 23, 1991, we are issuing
this request for an emergency permit to mitigate bluff erosion
at 507 West A Street. This action is necessary because of vandal
activity on the slope. This activity constitues a sudden and
unexpected occurrence that presents and/or poses a hazard to
the structure and improvements at 507 West A Street, and to
persons at the top of the bluff as well as on and/or below the
bluff, and demands immediate action to prevent or mitigate loss
or damage to life, health, property.
This report lists those actions proposed to be taken under
the emergency permit.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
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Page 2
Project Number 91-27b
November 15, 1991jRevised December 19, 1991
I
Should you have any questions, please do not hesitate to
contact us.
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Very truly yours,
~~
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Ernest R. Artim
CEG 1084: expo 6-30-92
&
Charles J. Randle
RCE 22096: expo 9-30-93
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Distribution:
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(2) addressee
(1) Mr. John D. Robinson
507 West A Street
Encinitas, California 92024
(2) Mr. Bill Weedman
City of Encinitas
527 Encinitas Boulevard
Encinitas, California
Major Use Permit Application
Number 91-156 MUP
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EMERGENCY PERMIT WORK
SEA BLUFF EROSION MITIGATION
ON SITE DRAINAGE IMPROVEMENTS
507 WEST A STREET
ENCINITAS, CALIFORNIA
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INTRODUCTION
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The geotechnical conditions at the subject site including
the project location, site background, conclusions, and the
geotechnical conditions are documented and described in the
reference report. The reference report includes a summary of the
field findings, recommendations for mitigation of the existing
erosive condition, plan depicting mitigation locations, detail
drawings and specifications, as appropriate.
This report lists the nature of the emergency at the site,
the potential hazard to existing structures and improvements, and
the potential hazard to persons at the top of the bluff as well
as on and/or below the bluff. This report also outlines only
those actions to be taken under the emergency permit.
NATURE OF EMERGENCY
On or before August 13, 1991 vandals had excavated into the
bluff at the subject site. The vandalism was reported in our
letter dated August 15, 1991. The excavation was at least 3 to 5
feet deep, 5 to 9 feet high, and about 9 to 12 feet wide made
into the side of the slope creating an undermine area. The
undermine area was reported to pose "an immediate threat to
persons at the top of the bluff, as well as to persons at the
base of the bluff." The potential for, and the subsequent,
accelerated erosion also poses an immediate threat to the
property improvements and residential structure at 507 West A
Street, Encinitas, California.
At the time of the vandalism, recommendations were already
being prepared to mitigate and control erosion of the sea bluff
at the subject site. Those recommendations are contained in the
reference report.
EMERGENCY PERMIT WORK
The following conditions will be addressed by the emergency
permit work. Poor surface drainage from this and an adjacent site
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Page 4
Project Number 91-27b
December 19, 1991
to the north will be controlled. All improvements will be on
site at 507 West A Street, Encinitas, California.
Positive surface drainage shall be established for the site.
Positive surface drainage for this report is defined as drainage
away from the residential foundation and sea bluff top at a
gradient of at least 2 percent for at least 5 feet and directed
to suitable outlets.
The lot near the bluff top shall be sloped slightly toward
the east to a concrete lined drainage ditch and covered with a
blanket of 10 to 12 mil visqueen. The visqueen shall be
overlapped by at least 6 inches and shall be pinned to the
ground surface and covered with pea gravel or wood bark. A
concrete lined drainage ditch shall be installed along the north
property line near the bluff top to divert off site runoff from
the slope face to area drains. All drainage devices shall have a
minimum gradient of at least two percent toward the east and 5th
Street. Downspouts for the residence shall be directed to
discharge away from the foundation, and into either the concrete
lined drainage ditches, one of the two area drains depicted on
Figure 1, or tightlined to the drain outlet pipes. A legend for
Figure 1 is shown as Figure la. Detail for the concrete lined
ditches, area drains, drain outlet pipes and other detail are
show~ as Figures 2 through 5.
PERFORMANCE OF WORK
All installation work shall be performed in accordance with
this report. All installation work shall be performed utilizing
sound construction practice, and in accordance with the minimum
requirements of the manufacturer of the products. All work
described herein shall be performed under the observation of the
geotechnical consultant. A report of observation of installation
and as-built configuration shall be submitted upon completion and
approval of the work outlined herein.
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Figure 1: Site Plan for On Site Drainage Improvements
507 West A Street, Encinitas, California
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Symbols
Description
......
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Location for the installation of drainage
ditches - see detail
@]
Location for the installation of 24 inch area
drain - see detail
c
Location for the installation of 6 inch area drain
for downspout discharge.
>
Location of 6 inch diameter pipe drains to street,
Schedule 40 PVC or approved equivalent. See detail
Figure D-4 for subsurface placement. Arrows
indicate planned flow direction.
0
Location of pipe outlet to street. See detail
~
Area to res10pe to surface drain to the east.
Figure la: Legend for Figure 1- On Site Drainage Improvements
507 West A Street, Encinitas, California
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0
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~ 18 " min
I
Ditch bottom may
be rounded at the option
of the conlractor.
3" 470.C.2000 concrete or
3" 2500 psi, air placed concrete
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TYPE A
I
18" min
TJ.
.;:~. 1:1
. . ..:.i
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. bottom may be rounded at
the option 01 the contractor.
I
TYPE B
TYPE C
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NOTES
1. Longitudinal slope of lined ditch shall be 2% minimum.
LEGEND ON PLANS
I
====- ==:!:> ===-
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Figure
2
: Description and Specifications of Drainage
Ditches
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LEGEND ON' PLANS
:..~
SECTION A.A
Figure
2 : Description and Specifications of Drains
-
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--- Finish Grid.
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L 4" clearance (mln)
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SECTION
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NOTES
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(O¡ indicates minimum relative compaction.
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Figure
4: Description and Specifications of Drain Outlet
Pipe Placement
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SECTION a-a
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APPROVED DRAIN
PIPE SIZES
J" 6" to 8" CURB FACE
4" 8" CURB FACE
6" 10" CURB FACE
NOTES
1. Pille shall be one continuous length from property line to curb line.
2. Multigle pipes to be set a minimum dinance of 0/2 apart.
J. Concrete shall be 520.C.2500
4. Pipe shall be circular :ement. cast iron or rigid plastic.
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Figure
5: Description and Specifications of Drain Pipe
Outlet to Street
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ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
Ý ! - I 'It; ^1 ¡J r~
Project Number 91-27b
November 15, 1991/Revised December 19, 1991
Mr. Paul B. Webb
Coastal Planner
Califor~ia Coastal Commission
San Diego Coast Area
3111 Camino Del Rio North, Suite 200
San Diego, California 92108-1725
Subject: Emergency Permit Request - Off Site Rip Rap Wall
507 West A Street
Encinitas, California
Reference Report:
"Geotechnical Engineering Report, Sea Bluff Erosion
Mitigation, 507 West A Street, Encinitas, California",
Artim and Associates, Project number 91-27a, dated
August 21, 1991.
Dear Mr. Webb:
In conformance with the request of our client, Mr. John D.
Robinson, and your letter dated October 23, 1991, we are issuing
this request for an emergency permit to mitigate bluff erosion
at 507 West A Street. This action is necessary because of vandal
activity on the slope. This activity constitues a sudden and
unexpected occurrence that presents and/or poses a hazard to
the structure and improvements at 507 West A Street, and to
persons at the top of the bluff as well as on and/or below the
bluff, and demands immediate action to prevent or mitigate loss
or damage to life, health, property.
This report lists those actions proposed to be taken under
the emergency permit.
635 "C" Street. Suite 405 . San Diego. C aliromia 92101. (619) 544-9155
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Page 2
Project Number 91-27b
November 15, 1991/Revised December 19, 1991
Should you have any questions, please do not hesitate to
contact us.
Very truly yours,
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Y4~
Ernest R. Artim
CEG 1084~ expo 6-30-92
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Charles J. Randle . "1;.(' õtci.l\~<~'
RCE 22096~ expo 9-30-93
Distribution:
(2) addressee
(1) Mr. John D. Robinson
507 West A Street
Encinitas, California 92024
(2) Mr. Bill Weedman
City of Encinitas
527 Encinitas Boulevard
Encinitas, California
Major Use Permit Application
Number 91-156 MOP
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EMERGENCY PERMIT WORK
SEA BLUFF EROSION MITIGATION
OFF SITE RIP RAP WALL
507 WEST A STREET
ENCINITAS, CALIFORNIA
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INTRODUCTION
The geotechnical conditions at the subject site including
the project location, site background, conclusions, and the
geotechnical conditions are documented and described in the
reference report. The reference report includes a summary of the
field findings, recommendations for mitigation of the existing
erosive condition, plan depicting mitigation locations, detail
drawings and specifications, as appropriate.
This report lists the nature of the emergency at the site,
the potential hazard to existing structures and improvements, and
the potential hazard to persons at the top of the bluff as well
as on and/or below the bluff. This report outlines only those
actions to be taken under the emergency permit.
NATURE OF EMERGENCY
On or before August 13, 1991 vandals had excavated into the
bluff at the subject site. The vandalism was reported in our
letter dated August IS, 1991. The excavation was at least 3 to 5
feet deep, 5 to 9 feet high, and about 9 to 12 feet wide made
into the side of the slope creating an undermine area. The
undermine area was reported to pose "an immediate threat to
persons at the top of the bluff, as well as to persons at the
base of the bluff." The potential for, and the subsequent,
accelerated erosion also poses an immediate threat to the
property improvements and residential structure at 507 West A
Street, Encinitas, California.
Erosion of the base of the bluff has progressed to a level
that requires protection of the base of the bluff in order to
protect property and persons at the top of the bluff as well as
persons on and/or below the bluff. Recommendations have been
prepared to mitigate and control erosion of the sea bluff at the
subject site. Those recommendations are contained in the
reference report.
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Page 4
Project Number 91-27b
December 19, 1991
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EMERGENCY PERMIT WORK
The following conditions will be addressed by the emergency
permit work. The base of the bluff will be protected by a rip rap
wall. All work performed at the base of the bluff will be off
site of 507 West A Street, Encinitas, California.
Mitigation of sea base retreat and protection of the base of
the sea bluff will consist of placement of a rock revetment. The
area of placement will require excavation of the loose beach sand
and cobbles to form a key into formational bedrock. In addition,
the sand debris cone at the base of the slope will be removed by
spreading it around the local beach area outside the limits of
the proposed rip rap revetment. All excavated loose sand and
cobbles will be spread out around the local beach area outside
the limits of the proposed rip rap revetment. The top of the rock
revetment will extend to an elevation of approximately 25 feet
(MSL). The location of, and plans and specifications for the rip
rap wall are attached.
PERFORMANCE OF WORK
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All installation work shall be performed in accordance with
this report. All installation work shall be performed utilizing
sound construction practice, and in accordance with the attached
plans and specifications. All work described herein shall be
performed under the observation of the geotechnical consultant.
A report of observation of installation and as-built configuration
shall be submitted upon completion and approval by the consultant
of the work outlined herein.
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GEOTECHNICAL AND ENVIRONMENTAL CONSULTANTS,
ARTIJvI & ASSOCIATESi
635 "I::' STREET, SUITE 405, SAN DIEGO, CA 92101 II
TELEPHONE (619) 544-9155
CIVIL ENGINEERING CONSULTANT
CHARLES J. RANDLE. PE
325E> WIN'" STRf.ET, SUITE f, (), ;,AN DlmO, CII !1~lln
TELEPHONE (619) 221-0878
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.
ARTIM & ASSOCIATES
Project Number 91-27
Geotechnical and Environmental Consultants
.
April 26, 1991/Revised May 20, 1991
Mr. John Robinson
507 West "A" Street
Encinitas, California 92024
.
Subject: Geotechnical Evaluation
507 West "A" Street
Encinitas, California
References:
.
1. Eisenberg, L.I., 1983, Pleistocene Marine Terrace and
Eocene Geology, Encinitas and Rancho Santa Fe Quadrangles,
San Diego County, California; San Diego State University,
unpublished Masters Thesis, 386 pages.
.
2. San Diego Association of Geologists, 1989, The Seismic
Risk in the San Diego Region; Proceedings of Workshop,
June 29-30, 1989.
.
3. Wilson, K.L., 1972, Eocene and Related Geology of a
Portion of the San Luis Rey and Encinitas Quadrangles,
San Diego County, California; University of California,
Riverside; unpublished Masters Thesis, 135 pages.
Introduction
.
In accordance with your authorization of AprilS, 1991, we
have performed a geotechnical evaluation of the property located
at 507 West "A" Street, Encinitas, California. The purpose of our
work was to assess the general site and soil conditions and to
prepare a report outlining our findings as well as recommendations
for mitigation and/or repair of potential problem features observed.
.
The property is assessor parcel number 258-042-20. The legal
description includes Lot 1, Block 47 Encinitas, in the County of
San Diego, State of California, according to Map No. 148, filed
in San Diego County June 12, 1883.
.
The property is located south of the western extension of
"A" Street, west of 5th Street, and north of the Moonlight State
Beach area. The western portion of the property is a sea bluff
down to the Pacific Ocean. A site location map is presented as
Figure 1. The base map utilized was page 24 of The Thomas Guide
San Diego County, 1991 edition.
.
A general site plan was prepared for reference of location
of, and presentation of observations, test excavations, and
geological units. The site plan is presented as Figure 2 and the
base map was a topographic map prepared by La Costa Engineering,
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
ARTIM & ASSOCIATES
.
Page 2
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
undated. A cross section of the site depicting our interpretation
of the subsurface conditions is presented as Figure 3.
.
Project construction plans and documents were not readily
available for review. Soils reports including any compaction
results for the site were not available for review.
Scope of Services
.
The scope of our services has included the following tasks:
* Review available pertinent reports relative to geotechnical
conditions at the subject site.
.
* Perform a visual site assessment of the site setting, and
general conditions.
.
* Perform a field analysis of the on-site subsurface soils
including geological logging and description of soils in
the sea bluff, and two shallow test excavations to examine,
sample, and test the foundation soils of the site building
pad. All logging and descriptions were performed by a
certified engineering geologist.
* Laboratory testing of representative soils including
expansion index, grain size (seive), and direct shear.
.
* Analysis and interpretation of our field findings and
laboratory test results. Evaluation of data and preparation
of this report with recommendations for mitigation and/or
repair of certain observed features.
* Processing and preparation of this report.
.
Site Geologic Setting
.
The site is located in an area identified by Eisenberg
(reference 1) to be underlain by a Pleistocene Beach Ridge and
deposits. Eisenberg also identifies that the Beach Ridge deposit
is about 40 to 50 feet thick in the site area and is underlain
by the Eocene Torrey Sandstone.
.
Our review of reference 2 indicate that there are no known
active faults in the vicinity of the site. Wilson (reference 3)
reports a northeast trending fault near the site at Moonlight
State Beach; however, Eisenberg notes (page 38) that the fault
"does not cut the Pleistocene terrace". Eisenberg also notes
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
ARTIM & ASSOCIATES
.
Page 3
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
that the bedrock along the base of the sea bluff contains faults,
"none of which can be seen to cut the Pleistocene terrace."
.
The closest active faults to the site include the Elsinore
Fault about 35 to 40 miles northeast of the site; the Coronado
Bank Fault about 20 to 30 miles southwest of the site; and the
offshore projected extension of the Rose Canyon Fault about 3
to 5 miles west of the site. There are many other active faults
in the southern California area that could generate ground
shaking at the site; however, for purposes of this report reference
2 contains information to suggest that ground shaking at this
site would be greatest from a maximum probable earthquake
generated on one of the branches of the Rose Canyon fault within
10 miles of the site. Information in reference 2 suggests that
the maximum probable earthquake for the Rose Canyon Fault is
about a 6.5 Richter Magnitude event. The occurrence of such an
earthquake within 10 miles of the site could generate ground
acceleration at the site of greater than about 0.5g.
.
.
.
The conditions to consider at the site from the aspects of
earthquakes and faulting include ground fault rupture,
liquefaction, ground shaking, and ground failure. Since there are
no known or mapped active faults on the site the probability of
ground fault rupture appears to be very low. Since the soils that
underlie the site are medium dense to dense sand the probability
for liquefaction appears to be low. From the aspects of ground
shaking this site is no different than numerous other sites
around the Encinitas area and the site should perform as well as
other sites in Encinitas with similar geological conditions. From
the aspects of ground failure the site condition to consider will
include the sea bluff along the west part of the property and the
water seepage near the contact of Pleistocene deposits and the
Torrey Sandstone.
.
.
The regional ground
be at and just above sea
water are known to occur
above the contact of the
Sandstone.
water table in this area is expected to
level. Local perched and seasonal perched
within the sea bluff area, and especially
Pleistocene deposits and the Torrey
.
Site Field Conditions
Site field work was performed on April 8 and 10, 1991.
During our field work we observed the following:
.
* The site is located south of the west extension of "A"
Street. The extension consists of asphalt driveway access
to the subject property and a property located north of
I
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
. ARTIM & ASSOCIATES
.
Page 4
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
the "A" Street extension.
.
* Surface drainage from the extension area appears will flow
east, west toward the sea bluff, and south toward the site.
No provisions were observed in the "A" Street extension area
to divert runoff away from the sea bluff or property to the
south and toward 5th Street.
.
* The lot was noted to be about 9 to 12 feet above 5th
Street to the east.
* The site is developed with a single story residential
structure. The structure includes a basement area with
access from the east.
.
* A retaining wall about 6 to 8 feet high is located just
south of the south property boundary. The adjacent pad
to the south of subject site is at an elevation below the
subject site.
.
* Site development includes a concrete drive/parking area;
wood fence to northwest corner of residence; deck area on
south side of residence; and two low retaining walls near
southeast and northeast corners of residence.
.
* Elevations for the subject site pad are from about 82 feet
near northwest corner of residence to about 70 feet near
northeast access drive. The pad has two general levels:
the west level varies from about 79 to 81 feet; and the
east level varies from about 70 to 75 feet.
.
* The top of bluff is about 15 to 18 feet from the main
floor of the residence, but the foundation of the basement
area is likely about 20 to 25 feet from the face of the
bluff.
.
* We observed that at least two slope failures had occurred
on the bluff face during and immediately after the heavy
rains of March 1991. The failure areas were about 3 to 15
feet wide, about 1 to 3 feet deep, and several feet long.
Secondary accelerated erosion has also occurred during
the rainstorms.
.
* The materials in the bluff are composed of the approximate
sequence of description from top to the bottom. The upper
8 to 10 feet of soils are composed of light reddish-brown,
silty sand. These soils are lightly cemented with iron-
oxide. The materials tend to erode and stand temporarily
.
635 II C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
ARTIM & ASSOCIATES
.
Page 5
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
for longer periods of time than the underlying material.
.
* Underlying the reddish-brown silty sand is about 35 to
40 feet of pale brown, very fine sand. These soils are
clean and friable with virtually no cohesive material. A
grain size test of this material is included in the back
of this report.
.
* Underlying the pale brown sand is about 20 feet of very
dense sandstone of the Torrey Sandstone. The contact of
the upper Torrey Sandstone is delineated by a stained
red-brown layer about 2 feet thick likely a result of
water seepage. This bedrock unit stands in relativily
more stable steep inclinations. We did not observe any
fracture zones or faults in this unit.
.
* The base of the bluff is marked by piles of eroded sand
and debris from the bluff. Below the base to the water is
a beach composed primarily of cobbles. Figure 4 presents
a photograph presentation of the bluff face and beach area.
.
* The bluff face is nearly void of vegetation. Some growths
of ice plant grow from the top of the slope, but otherwise
the bluff face is bare of vegetation.
.
* Test excavations T-l and T-2 were made at the locations
shown on Figure 2. Each excavation was made to a depth of
4 feet, soils examined, described, and sampled.
.
* Our knowledge of the site area indicates that sea bluffs
along this area suffer from irregular and episodic
failures, especially along the tops of the bluff. The
average rate of retreat of the base appears to vary from
about 1/2 to 1 inch per year up to about 6 inches per
year. The upper bluff however, as the lower bluff
retreats, often remains in an unstable condition for some
time until an erosive force from the top of the bluff
causes a large section to fail and "run" down the slope.
.
* We observed that erosion appears to be more rapid at this
site than at the adjacent nearby sites.
* This section of sea bluff will continue to erode at a
rate more rapid than adjacent properties unless some
type of mitigation is implemented.
.
* Except as described herein there are no other geological
hazards known to be present on the site.
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
ARTIM & ASSOCIATES
.
Page 6
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
.
* The slope at the subject site is as steep as 65 feet
vertically along a horizontal depth of about 60 feet.
This inclination is slightly steeper than l:l( horizontal
to vertical). The lower Torrey Sandstone is capable of
standing in the near vertical for long periods of time;
however, the overlying friable sands will tend to erode
to an angle of repose of about 26 degrees. In addition,
the upper friable sands are extremely prone to rapid
erosion.
.
.
* During a walking review of the approximate 2 miles of
bluff north of Moonlight State Beach in March, 1991, we
observed several sites at which vegetative growth was
well established in the friable sands. Certain species of
small trees and shrubs appeared to be concentrated at the
contact between bedrock and friable sand. Water seepage
at these areas appeared to be not present, or less than
adjacent sites. Certain species of shrubs appeared to be
well established within the friable sand zone.
.
* The test excavations encountered medium dense to dense,
reddish-brown, slightly silty sand. The upper several
inches in each excavation appeared to be loose and
disturbed, possibly as a result of landscaping and/or
site construction activities. An expansion test on the
soil in T-l indicated they have a very low potential for
expansion. A direct shear test of soil at the base of T-2
indicated an angle of 31 degrees and 100 pounds cohesion.
.
Summary of Key Site Conditions
.
The site slope is about 65 feet high at an approximate 1:1
inclination. The upper 40 to 50 feet of the slope is underlain by
friable sands. The angle of repose for these sands is likely
about 26 degrees. These soils are actively eroding.
.
A set back line of 2:1 horizontal to vertical (26 degrees)
from the approximate contact of the Torrey Sandstone and friable
sands would extend to the base of the existing structures
basement foundation.
.
Mitigation of the present erosion conditions should be
implemented to protect the property at the upper portion of the
lot. Such protections should be considered temporary since, in
our opinion, no protective devices along eroding coastal bluffs
can be permanent.
.
635 II C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
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ARTIM & ASSOCIATES
.
Page 7
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Geotechnical and Environmental Consultants
Conclusions and Recommendations
.
The results of our study indicate on-going soil erosion of
the bluff at the subject site. The cause of the erosion includes
poor surface drainage from this and an adjacent site to the
north, the steep sea bluff on the west part of the site, the
friable nature of the upper sands in the bluff, and the lack of
vegetative growth on the slope. If some types of mitigation are
not implemented, erosion will continue and affect site improvements.
.
We recommend that the following be utilized for mitigation
of the site erosion conditions.
.
1. Positive surface drainage should be established for the
site. Positive surface drainage for this report is
defined as drainage away from foundations and sea bluff
top at a gradient of at least 2 percent for at least 5
feet. Establishment of positive surface drainage will
also need to include the site area immediately adjacent
to the north of the lot, since runoff from the adjacent site
directly influences erosion of the site bluff. All areas
near the bluff top should be regraded to slope toward the
east or into concrete lined drainage swales away from the
sea bluff top. All drainage should have a minimum
gradient of at least 2 percent toward the east and 5th
Street. Eave gutters and downspouts should be installed
on the residence with downspouts discharging away from
the foundation and sea bluff top and utilizing non-
erosive devices. Installation of positive drainage will
likely include several area drains and/or collection
points discharged to 5th Street.
.
.
.
2. Soil samples of the friable slope sands should be
collected and an analysis performed to assess the type of
vegetation that may survive, and thrive, at the site
slope conditions. The species of vegetation that appear
to be performing well in similar slope and soil condition
along the bluff north of the site should be analysed to
evaluate their suitability to adapt to this site. Based
on this study a landscape plan should be developed and
implemented at the subject sea bluff slope. Such a plan
should avoid, as much as possible, slope disruption during
planting operations. Slope watering should be avoided if
possible, and if deemed necessary might consist of a
degradable drip system.
.
.
3. The base of the bluff area is composed of Torrey
Sandstone that will erode and retreat at relatively slow
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
.
Page 8
Project Number 91-27
April 26, 1991/Revised May 20, 1991
.
rates. The slow retreat will continue to cause a response
erosion of the upper bluff friable soils. However, when
the recommendations contained in numbers 1 and 2
above are implemented, the resultant erosion will be
better controlled. Some type of base protection might
be installed to lessen and/or slow the rate of retreat of
the Torrey Sandstone base. Such measures could include
rip-rap armor to blend into an existing armor just south
of this site.
.
4. For safety of the property at the top section of the
bluff and to persons at the base of the slope, we
strongly recommend that planning and implementation of at
least the first two recommendations begin immediately.
.
5. For longer term protection of the property, other
considerations include relocation of the existing
residence such that the present east-west alignment is
north-south and set back from the present foundation. This
consideratiön would involve encroachment onto the
property north of the subject lot. Relocation of the
residence would result in a longer time use of the
residential property.
.
6. The final mitigation plans and specifications should be
prepared by this firm and approvals obtained prior to the
order, or purchase, of any materials or construction.
.
7. We assume no responsibility or liability for work
performed by others at the subject site.
.
8. Safety of personnel on-site during mitigation construction
is the responsibility of the contractor performing the
work.
.
9. Should you desire, we can obtain cost estimates for
mitigation recommendation number 1, and cost estimate to
perform the additional study for recommendation 2. Other
course of actions will be dependent on the directions
and requirements of the City of Encinitas and the Coastal
Commission.
10. Additional services may be required dependent upon the
directions and requirements provided from the City of
Encincitas and the Coastal Commission.
.
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
. ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
.
Page 9
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Should you have any questions or require additional service,
please do not hesitate to contact us.
.
Very truly yours,
.
/...> //// //
. // --_/
¡:~L._L ýjr"--:'__Ji,¿ /'--
Charles J. Randle
RCE 22096; expo 9-30-93
.
~~
.
Ernest R. Artim
CEG 1084; expo 6-30-92
Distribution: (3) addressee
.
.
.
.
.
/-,
635 II C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
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.
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.
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ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
.
Page 11
Project Number 91-27
April 26, 1991/Revised May 20, 1991
Legend for Figure 2 and 3
.
Qb
Beach cobble and sand deposit
Qt
Quaternary Beach Ridge deposits
Tt
Torrey Sandstone
.
"
/
geologic contact
".
,.
2:1 erosion line
no fractures were observed at site exposures
.
no faults were observed at site exposures
no joints were observed at site exposures
bedding is generally cross-bedded or massive
.
contact between Torrey Sandstone and friable sand is
relatively horizontal
.
T-2
x
approximate location of test excavation
A A'
Igeological cross-section
scale of original drawings is 1 inch = 20 feet
reduced scale of report drawing is the same horizontally
and vertically. Refer to bar scale on Figure 2.
.
.
.
Figure 2a: Legend for Figures 2 and 3
.
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155
.
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Generalized Geologic Cross-
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.
ARTIM & ASSOCIATES
Geotechnical and Environmental Consultants
.
Page 15
Project Number 91-27
April 26, 1991/Revised May 20, 1991
LABORATORY TEST RESULTS
.
Expansion Testing
Sample Location
Description
Expansion Index
Classification
T-2
0-4 feet
Fine sand
-0-
Very Low
.
Direct Shear Test
Sample Location
Friction Angle
Cohesion
T-2
4 ft depth
31 degrees
100
.
Grain Size
(as a percent passing the listed screen size)
Sample Location
#4
#8
#16
#30
#50
#100
#200
.
10 below top of
slope
100
98
92
80
30
16
7
.
.
.
.
t
635 "C" Street. Suite 405 . San Diego. California 92101. (619) 544-9155