1988-8805 E
Street Address
~/(o1
Category
I
3?5V I
Serial #
93 - () f} 'ì 0 R
I
Description
Name
/l1q3
Plan ck. #
Year
recdescv
(J[b[bO[5[D
ßß(] trßßíÐflJUfB (D[b
ßWruUllJßߌ6
OflJŒ~
8624 CUYAMACA STREET
SUITE F
SANTEE, CALIFORNIA 92071
TEL: (619) 449-5900
September 1, 1987
Mr. Barry Simons
Barob Group
796 Neptune Avenue
Leucadia, CA 92024
Subject:
Geologic Reconnaissance Study
Proposed single family residential
building site
West side of Neptune Avenue, between
Jason Street and Jupiter Street
Leucadia area, San Diego County, California
Our Project No. 24C2
Dear Mr. Simons:
In accordance with your request and authorization, our firm ha!';
performed a geologic reconnaissance study at the above-mentioneò
subject property, which is more particularly referred to as
Assessor's Parcel No. 254-210-13, in the County of San Diego,
. State of California.
The purpose of the reconnaissance stuòy was to evaluate the
prevailing geologic conòitions at the site based on a visual
inspection and reconnaiss'ance mapping of the formational units
exposed in the existing bluff, and to evaluate the overall long-
term stability of the bluff with respect to the currently
proposed site development.
SCOPE OF WORK
The scope of our reconnaissance study included the following:
a site visit and reconnaissance mapping of the bluff at the
project site and its immediate vicinity which was performed
on August 27, 1987;
a review of pertinent geologic maps And lit.eral:nre:
a r~view of topographic maps and stereographic
interpretation of aeria] photographs pertaining to the
general site area: and
the prepAration of this written report
observations, findings, and opinions.
to present
our
Project. No. 24C2
September 1, 1987
Page 2
A listing of the literature, topographic maps, and aerial
photographs that were reviewed and used in the preparation of
this report are presented in Appendix A under "References".
SITE LOCATION AND DESCRIPTION
The project site is located between l320 and l340 Neptune Avenue
in the Leucadia area of San Diego County, California. The
property is located on the west side of Neptune Avenue with a
50-foot frontage along the street, and measures nearly 210 feet
deep towards the west. The front, easterly half of the site
encompasses a relatively flat to very gently, westward sloping
pad which extends ap~roximately 125 feet to the top of the
existing bluff. This pad is separated from Neptune Avenue by a
fill slope that is on the order of 3 feet in maximum height. The
site is presently vacant and existing site improvement!'; include a
small, wooden structure in its southwestern corner ònd a 7-foot
high \vooden fence along the rear of the property. rrhe fence is
set back a distance varying between 5 to 7 feet from the top of
the bluff. On-site vegetation consists of sparse growths of
native grasses and weeds, whereas the bluff is covered with a
"mixture of native grasses and shrubs.
Based on the information provided to us, it is our understanding
that the proposed project consists of the construction of a two-
story residential building in the front-central portion of the
existing pad area. It is furt.her understood that the proposed
building will be of conventional wood-frame construction and be
supported on a system of continuous peripheral and isolated
interior spread footings. Grading fnr the project will be
minimal and involves the preparation of the building pad and the
driveway.
GEOLOGIC FORMATIONS AND SETTING
Figure No. 1 represents the central portion of thE" T1.S.G.S. 7.5
minute Encinitas Quadrangle, showing the location of the subject
property at the west~rly edge of an approximatply 6S~foot high
bluff. The sea bluffs in the vicinity of the project site are
eroded into the seaward edge of a Pleistocene marine cut terrace
that is surmounted by a rounded ridge of "old beach-ridge" or
off-shore bar sand deposit. A visual inspection of the bluff
exposures indicated that the project site is unòerlñin by two
geologic formations: t_he Sñnt.iago Formñtion of Eocene ñge anc'i
Terrace Deposits which are equivalent to the Linc'iavista Formation
^IIIEIJ GEOIICltrW"^' rrH;ltlrrlì:; ItW
11(,;J!\('IIV:lII"""c-,tr""',';'II"'1 ":"!lIf"" ('"!d"",,,,!):"!)"1
, ,
Project No. 24C2
Rpptember 1, 1987
Page 3
of Pleistocene age. In the Leucadia area, the cliffs are
underlain by the Member B of the Santiago Formation as mapped by
Wilson (1972).
The Santiago Formation (Member B) is composed of an interbedded
sequence of sandstone, mudstone, anrJ claystone that were
deposited in a marine and nearshore marine environment. The
lower l5 to 20 feet of the cliff at the subject site reveal
moderately to well indurated mudston~ strata of the Santiago
Formation. These strata are approximately horizontal or dip very
gently (less than 3 degrees) in a northwesterly direction.
The upper 45 to 50 feet of the bluff is composed of poorly to
moderately indurateB Plei!';tocene sAndy deposits which lie
unconformably on the Santiago Formation. 'T'he friñhle sanrJstone
is massive and only faintly stratifierJ horizontally. In
addi tion, a surf ic ial topsoil layer which is es t irna terJ to be 18
to 24 inches in thickne!';s, ônd consists of a brown, si]ty fine to
medium sand, was observed ñt the top of the bluff.
A cross-sectional profile of the bluff was constructerJ by using a
tape and hand level and is illustrated in Section A-A' on Figure
No.2. A narrow, sandy beach lies directly in front ana along
the toe of the sea cliff. The lower (basal) portion of the sea
cliff is eroded directly by storm waves into the Santiago
Formation and exhibits a variable slope gradient of ñpproximately
0.5:1 to 2:1 (horizontal:vèrtical) with a 2- to 4-foot wiãe ledge
at approximate mid-height. Further to the north and south of the
subj ect property, this basal portion varies both in height as
well as in steepness, and in some places, has a near-vertical
slope gradient. The upper + 7- to 8-foot portion of the Santiago
Formation is deeply weathered and highly fractureò, rmd has an
undulatory unconformable contact with the overlying Pleistocene
sandy deposits. Active seepage was noted at anrJ along this
contact ñt the subject property as well as in the cliffs further
to the north and south of the si te. l\bove the contact, the
friable S'andstones extend upwards at an approximate slope
gradient of l:l and slightly steeper all the way to the top of
the bluff. Where concentrateò surface runoff has come over the
top (rim) of the bluff, the otherwise smooth, mass wastage slopes
have been severely gullied.
GEOLOGIC ßAZARDS
Several published and unpublished reports by Wilson, Hannan,
Kennedy, and others, have called attention to minor, northeas t-
^lllrrJ r,rPTFClfrjW/l1 rr~r;lflf f n~ II ('
Rr,;", rllv;,rl1"r;, <;1""", ,;""" , ,""""", ('"",.."" , q:'f)n
Project No. 24C2
. September l, 1987
Page 4
trending faults exposed at various places along the sea cliffs of
the San Diego County coast. The faults that are located closest
to the subject site, based on a map compiled by Hannan (l975),
are shown on Figure No.1. None of these faults are known to be
active or to be associated with earthquake hazard. No faults or
indications of faulting were observed in th~ bluff exposures at
the subject site. Only a few northeast-trending fractures/joints
were noted in the Santiago Formation outcrops, dipping between 55
to ñ5 degrees to the north.
The site is, however, located near the offshore continuation of
the northwest-trending Rose Canyon Fault, which is regarded by
most local geologist as an active fault with considerable seismic
risk. An earthquake o'riginating on this fault, at a probable
epicentral distance of only a few miles, would pose a major
seismic hazard to the site and may result in bedrock
accelerations in the Santiago Formation on the order of 0.5g,
with somewhat higher accelerations in the overlying Pleistocene
sediments, especially if it is saturated near the unconformity.
Liquefaction of the saturated Pleistocene spdiments as a result
of seismic shaking during a major earthquake is another potential
"hazard. An evaluation of the potential occurence of ñ major,
techtonic event on the Rose Canyon Fault is beyond the scope of a
reconnaissance study. Furthermore, it must be noted that such an
event would result in extensive regional damage rather than being
confined to the subject site.
Our reconnaissance of the cliffs in the vicinity of the subject
site and review of pertinent aerial photographs revealed the
presence of several landslides further to the south of the
project site as illustrated on Figure No.1. The landslides
involve only the Pleistocene terrace deposits and appear to be
the result of spring sapping. No landslides or evidence of past
sliding in the bluffs were observed at the subject site and
adjacent properties to the north.
In appears that the slides were initiated by sapping, ñS springs
emerged where the unconformity between the Eocene mudstones and
the overlying Pleistocene sandstones was truncated by earlier
stages of the sea cliffs. Spring sapping would become important
again in the retreat of the upper portions of the bluff only if a
greatly increased f low of groundwater (seepage) occu r red or if
the wave-cu t basal portion of th~ c 1 if f sat. bei'lch leve 1 were to
retreat significantly. Unrler present circ\1msti'lIlCPS, we do not
foresef' a substantji'll increase in gro\11ldwë1ter (lischarge in the
local cliffs.
^lllrn mJHrClftW;^1 fll(~trJlrHS Ifl("
R(,?I1 ("11\':11""";1 Sir"'" (""1ft" r <;",,11"'. ('"Id"""" q;'()7!
Project No. 24C2
September 1, 1987
Page 5
Another mode of bluff failure in the area of the project site
involves the "excavation" of small caves in the basal portions of
the cliffs due to storm wave action and subsequent collapse of
the roofs of the caves. Our observations indicate that the
location and formation of the caves appear to be controlled by
fracturing/jointing in the basal mudstone units. No caves exist
at the subject site, but several were observed at the adjoining
properties to the south.
Published literature pertaining to cliff retreat and erosion
along the San Diego County coastline indicate a wide range in
rates of erosion, depending largely on specific location and type
of lithologic unit(s) at the base of the cliffs. Fairly recent
research data made available by G. Kuhn suggests that cliff
erosion, particularly in some areas along the North County
coastline, may be a more serious problem than previously
believed. The date presented seems to indicate that there is no
uniform rate of erosion and that, in fact, cliff erosion or
retreat appears to occur as a somewhat instantaneous response to
a particular set of circumstances at any given site. His
findings are based on historic evidence which indicates that
major storm waves, particularly when combined with excessive
'precipitation over extended periods of time, are responsible for
"instant" massive block failures (landslides) which account for
the major erosional damage along the coastline. In areas where
the basal portions of th~ cliffs are made up of geologically
younger (Pliocene or younger) formational units or unconsolidated
alluvial deposits, the potential for major cliff retreat/erosion
is much greater.
The published rates of cliff erosion/retreat for the general
Encinitas-Leucadia area indicate an average rate of bluff top
(rim) retreat of several inches to a few feet per year, until a
"stable" mass wastage slope of about 35 degrees is obtained.
Accordingly, if a 35 degree slope is projected from the contact
between the Santiqgo Formation and Pleistocene sandy deposits,
the rim of the assumed "stable" cliffs would lie along a line
approximately 20 feet back from the present (existing) bluff top.
However, the ultimately stable mass-wastage slope would depend on
the degree of cementation and weathering of the friable sandstone
strata, and would be highly vulnerable to gullying by
concentrated runoff, uncontrolled pedestrian traffic seeking
òccess to the beach, and landsliding as ñ result of spring
sapping. It must be noted th~t the ahov~-mentinned rates of
erosion repr~sent avprage long-term rates of cliff
retreat/erosion but do not take into accol1nt perjoòic events
where substantial losses of bluff tops ònd cJiffs occur as a
^IIlffJ(',Fnrrr:I!r ('^, rr¡(;IrHIH';lrw
Rr,;"1 (II\,"',,-H':! SI",.., '~"""', ~;,ll""" (.11""""" O""ìI
Project No. 24C2
September l, 1987
Page fi
result of block failures/landslides which have been attributed to
major storms and periods of high rainfall in the past.
SUMMARY
The major, potential geologic hazards which could affect the
proposed site development are a seismic hazard posed by the
nearby located Rose Canyon Fault and bluff stabi 1 i ty. We feel
that it would not be practically nor economically feasible to
design an "earthquake-resistant" project, particularly for a
single lot development. With respect to cliff erosion or
retreat, the subject property is not much different from all
other bluff top properties along the Encinitas-Leucadia
coastline.
The bluff top residential properties in the Encinitas-Leucadia
area have been developed intermittently during the past 50 to 60
years, with only a few vacant lots remaining at this time. The
older properties were developed with no or few restrictions and
have been subjected to various amounts of damage by cliff erosion
and landsliding. The regulatory agencies having jurisdiction
'over the coastal areas have been especially interested in the
problem of severe erosion and landslide hazards along the sea
cliffs at Encinitas and Leucadia, and their policies over the
years have become increasingly stringent in that area, partly
reflecting environmental considerations and partly due to
legislation requiring engineering and geologic studies in the
interest of public safety and welfare.
In order to apply the regulations fairly and consistently, it is
necessary to realize that erosion and recession of the cliffs in
this area has already progressed generally well beyond the
influence of the steep wave-cut cliffs at the head of the beach.
While, in some other areas along the San Diego County coastline,
excessive undercutting of the wave-cut cliffs exerts an influence
all the way to the top of the sea cliffs, in the I..eucadia-
Encinitas area the upper part of the sea cliffs have almost
approached a stable mass-wastage slope of approximately 35
degrees in the Pleistocene sandy deposits. It is noted that the
amount of retreat or the rate of retreat of the wave-cut portion
of the cliffs will be relatively insignificant because the
shoreline j,n the Encinitas-Leucadia area has already reached a
^I' IFIJ GFOrfTI¡r~1('^1 rrmlTlrrn<; IrIC
Rf>?-1 (:11\,;,\111:1("1 SII,,"I, ~lIlt" r, <:;'111""'. (';old"'11"1 0;>[\11
Project No. 24C2
September 1, 1987
Page 7
relatively straight and essentially stable configuration
associated with the classical stage of "maturity" in the
geomorphic classification of erosional shorelines. Thus, the
question of retreat of the rim of the bluffs and the appropriate
setback for structures must be regarded primarily in light of
mass wastage processes as well as the engineering characteristics
of the Pleistocene sandy deposits and foundation design
considerations for each particular project. It is our
understanding that you have retained another consultant to
prepare a soils report which will include foundation design
recommendations for the project.
Uncohtrolled surface drainage that is permitted to flow directly
over the top of the bluffs, indiscriminate construction of
staircases and/or unrestricted foot traffic on the bluffs, anò
lack of proper maintenance of bluff planting have been, the major
man-induced contributing factors influencing bluff stability
particularly in regards to the older developments. The property
owner can and should make the necessary efforts to reduce the
rate of erosion by providing and maintaining proper drainage
control, planting of deep-rooted and durable native groundcover
and shrubs, and preventing pedestrian traffic on the bluff.
In our opinion, the proposed building setback of 40 feet from the
top of the bluff i~ reasonable and adequate. This setback is
considered adequate to provide protection from intermittent
crumbling of the rim of th~ cliffs and natural grading of the
bluff by mas~ wasting. However, adherence to this setback cannot
guarantee safety against massive block failure or landsliding of
the cliffs due to aggravated spring sapping or liquefaction of
the saturated sandy deposits during a major earthquake. Such
protection is far beyond the scope of the present regulations and
powers of the regulatory agencies, especially as applied to a
single lot rather than a lñrge scale subdivision or land
development project.
The observations, findings, and opinions presented in this report
are based partly on our visual observations of the present site
conditions, partly on an evaluation df the information gathered
from our review of pertinent 1 iterature and maps, and partly on
our general experience with similar site and geologic conditions.
This reconnaissance ~tudy has been conducted in accordance with
the standard of care currently used in the preparñtion of
geologic reconnaissance studies by other geologic and engineering
geologic consultants practicing in the San niRgo area. No other
warranty is expressed or implied.
^Iurn (;[(11 reIHII!!'\ rrH;lfJf r w; IIW
gr;;-><'1 (:">"1111'1'",' Sfl""', (~I"t" f, "-co,,"," r'~lof"",t:' ():,r\71
Project No. 24C2
September 1, 1987
Page 8
Should you have any questions regarding the contents of this
report, please feel free to contact our office. This opportunity
to be of service is sincerely appreciateò.
Figure Nos. 1 and 2 are parts of this report.
Respectfully submitted,
ALLIED GEOTECHNICAL
~
Tiong J. I.lem
CEG lOl2
ENGINEERS, INC.
TJI./cb
Distr.
( 3 )
(1)
Mr. Sjirk W. Zijlstra
Addressee
^IIIFfl(~F()rfC"rW'^1 rr C;lflffF';lrw
Rr,7"""\,lfll;""';t,,'rl,r",,'~r ';""100",1"",",,0:'1171
,JL "',
','\ V "', j
<lIlt', i', , -
"'n'A 'J~"rrT"~
, . . '
, "
, '
LA
-- ...:
.-' ,
LEGEND
",- fauff /oeaf/on (Hannan, 1975)
([~ lands/ide location ba!Jed on aerial pholo~
0
~ooo
2,000
-1
~
FIGURE No:1
SITE LOCA TION MA P - fìErRESENTS TilE CENTRAL
PORTION OF THE U 5, G, 5 7.5 MINUTE ENCINITAS TOPOGRAPIIIC
QUADRANGLE, 1968 EDIT/ON, PI10TOREVISED 1975
PRO J EC T No 2 4 C 2
ALL/ED GEOTECHNICAL ENGINEERS, INC
ltJ
~I
~
ltJ
~
~
ltJ
~
:)
h
Q..
ltJ
~
(t
"
j
SCALE" 1"= 20~
REFERENCE
,
I
()
It)
'~.....
,.,"...;,,"......_1Ji8j o ¡.!iIi ¡~.,
"..,,'._,",-...., '...
S,ltt:..1wO PROPERTY BOUNDARY SURVEY HAS BHN PER-
FO. BY ALLIED GEOTECHNICAL ENGINEERS, INC.,
Tm LOCATIONS OF TIlE PROPERTY LINES AS SHOWN Of '
THIS DRAWING ARE BASED ON INFORMATION SUPPLIED
TO US, AND TIIEREFORE MAY OR /My NOT 8E RElIA8LF.
SURVE'y' PLAT PREPARED B'y' P SILV/1 i SCALE I" '" 20' , DATED: AUGUST 3/,1987 AND SITE PLAN PREPARED BY
S. W" Z I J L S T R A , SCA L E.' 1/8" = I' - 0" , D ATE D: J U L Y 18 , I 9 87 .
2 10 ' -..I¡II......
-- ~ .
--,-1 I EXISTlNG -'--, ,
1_~_u:~OtrNGJ \-~ - , ---=
EXIST. TOP
-,.... :i- 40' SE18I1CK-"U4_,\_OF BL"U"FF
A \-=;
L"~r t~
~J
PIlYJPOSED RESIDF.NCE
/'
L
210' ~-.,,-
FIGURE No: 2
/'
....
]
EXISTING FENCE'-
! ' \
SITE PLAN SHOWING 1-1 PPROXIAfA TE LOCArlON OF PROPOSED
RESIDENCE IN RELA TION TO EXISTING TOP OF BLUFF.
NEPTUNE AVENUE
LEUCADIA AREA
SA N DI EGO COUN T Y, CALIFORNIA
L~~ð
LL ~~ ~= ~--- -
70
60
50
40
30
20
10
0
TERRACE DEPOSITS (plø/!rfocent!J
";:'~ð',r,¡JjbrPE"BBLE AND SNF:L.L LA)ER i ~ 6.. 121NCNES THICK
--" -- " --
---...- - ------. ---- ,-- SANTIAGO FORM 11 nON (eocene)
-- "--- . --- --, "..--
---- -- .
------'.. -----
-r----.- I I
I I
,-
10
3D
80
40
20
50
60
ro
CROSS- SECTIONAL PROFILE OF BLUFF (A- A ')
SCALE: I": 20' (HORIZONTAL AND VERTICAL.)
,.".".. "-;-0"'"
AI ,.
- -..
. .
Project No. 24C4
September l, 1987
^PPRNDIX ^
References:
Artin, E., 1981, Sea Cliff Retreat: A Case Sturly at Oceanside,
California, in Abbott, P. and O'Dunn, S., editors, Geologic
Investigatiõ'ñ of the San Diego Coastal Plain: San Diego
Association of Geologists Field Trip Guirlehook, p. A4-89.
California, State of, Department of Navigñtion and Ocean
Development, July, 1977, Shoreline Erosion }\long the Southern
California Coast, 6 pp., Appendix and Maps.
Hannan, D.L., 1975, Faulting in the Oceanside, Carlsbad, and
Vista Areas, Northern San Diego County, California, in Ross, A.
and Dowlen, R., editors, Studies on the Gp,ology of Camp
Pendleton, and Western San Diego County, California: San Diego
Association of Geologists, p. 56-59.
Lee, L., Pinckney, C., and Remis, C., April, 1976, SE'a Cliff Base
Erosion, paper presenterl at the AgCE National Water Resources anrl
Ocean Engineering Convention h~ld in San Diego, California in
April of 1976, 13 pp.
Wilson, R.L., 1972, F.ocen(3 FInd Relûterl Geology of a Portion of
the San Luis Rey anr] F.ncinitëH:; Quadrang les, San Diego County,
California, unpublished Master's Thesis, University of
California, Riverside
MapS/Plans:
San Diego County topographic maps, Coordinate Nos. 330-1671, 1960
and 1975 editions, scale: l" = 200'
U.S.G.S. 7.5 Minute Encinitas TOpographic Quadrangle, dated 1968
(photo revised 1975), scale: 1" = 2000'.
Site Plan prepared by Sjirk W.
scale: 1/8" = 1.0'.
Zijlstra,
di'lted .July 18,
1987,
Survey Plan prepared by Peter Silvn,
scale: 1" :-: 20'.
rlatç:>(1 l\t1gust
31,
1987,
Aerial Photos:
U.S. Soil Conservation Service aerial phot.ographs Nos. l\XN-8M-95
& 96, dated April 11, 1953.
San Diego County aerial photographs Nos.
December 13, 1978.
2l0-1SR-36 & 37,
dated