HomeMy WebLinkAboutLot 2-3 Soils Report
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Lawson & Associates
Geotechnical Consulting, Inc.
ROUGH GRADING PLAN REVIEW
FORFAULTSETBACKS
"TRADITIONS" DEVELOPMENT
TRACT29133, CITYOFTEMECULA
RIVERSIDE COUNTY, CALIFORNIA
Project No.: 042364-30
Dated: June 8, 2004
Prepared for:
Mr. Rick Hauser
GALLERY HOMES
31610-2 Railroad Canyon Road
Canyon Lake, California 92587
RECEIVED
JUN 0 B 2004
CITY OF TEMECULA
ENGINEEAIN80EPARTMENT
\
40935 County Center Drive 0 Suite A 0 Temecula, CA 92591 0(909) 719-1076 0 Fax (909) 719-1077
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Lawson & Associates
Geotechnical Consulting, Inc.
June 8, 2004
Project No. 042364-30
Mr. Rick Hauser
GALLERY HOMES
31610-2 Railroad Canyon Road
Canyon Lake, California 92587
Subject:
Rough Grading Plan Review for Fault Setbacks, "Traditions" Development, Tract 29133, City
of Temecula, Riverside County, California
During alluvial removal operations, faulting was observed on the subject site. In order to determine the age of
the faulting, a fault dating specialist, Dr. Thomas Rockwell, with Earth Consultants International (ECI), was
consulted. ECI excavated two (2) fault trenches on the site and determined that the faults observed were part of
the active Wildomar segment of the Elsinore Fault zone (see report by ECI, attached). After delineating the fault
zone, a meeting was held with representatives from the City of Temecula and the County of Riverside
attending. A 20-foot wide setback to be established on both sides of the active fault zone was discussed. This
required that the lot lines be adjusted and the locations of the proposed residential structures on Lot Numbers 2
and 3 be moved out of the fault setback zone (see Figure 1 attached). It is also recommended that the
foundations for Lot Numbers 2 and 3 be post tensioned slabs.
Fault Setback Recommendations
We have reviewed the fault investigation report for Tract 29133 prepared by Earth Consultants International
(ECI). Additionally LGC personnel performed geologic field mapping during rough grading and have assisted
in logging, fault exposures within trenches T-1 and T-2 addressed in the ECI report and are well acquainted
with the onsite geotechnical conditions. Based on our review of the ECI report, active faulting was observed in
a 3-foot wide zone in T-1 and a 13-foot wide zone in T-2 with three (3) potentially active minor faults or
fractures within approximately 65 feet to the east of the main fault (as measured along the trench). ECI
conservatively included these active and potentially active faults (or fractures) in a zone of active faulting
extending a distance of approximately 90 feet as measured along T-2.
Based on our review of the ECI report, it appears that significant active and potentially active faulting (or
fracturing) has been identified and included within their zone encompassing active faulting. The faulting
appears to have been accurately identified and constrained by closely spaced trenches located along the northern
and south eastern margins of the site.
Due to the close trench spacing (400:1: feet) which accurately constrains the limits of active and potentially
active faulting and the conservative zone identified by ECI that encompasses the active and potentially active
fault features, it was agreed that a 20-foot wide fault setback zone be established on both sides of the active
faulting zone.
z..
40935 County Center Drive. Suite A. Temecula, CA 92591 .(909) 719-1076 . Fax (909) 719-1077
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Figure 1 shows the zone of active faulting identified by ECI, the recommended 20-foot wide fault setback zone
and the redesigned lot layout and proposed residential structure location based on the 20- foot wide setback.
Based on our review of the revised design with respect to the recommended setback zone it appears that this
setback will adequately mitigate the potential for damage to the structures from ground surface displacement
due to an earthquake on the Wildomar Fault. No structures intended for human occupancy should be
constructed within this setback zone. The new design as indicated on Figure 1 will satisfy this requirement.
Post Tensioned Slab/Foundation Desi1!n Recommendations
Post tensioned slabs should be utilized for the support of the residential structures on Lot Numbers 2 and
3. We recommend that the foundation engineer design the foundation system using the geotechnical
parameters provided in Table A. These parameters have been determined in general accordance with
Chapter 18 Section 1816 of the UBC, 1997 edition. Alternate designs are allowed per 1997 UBC
Section 1806.2. In utilizing these parameters, the foundation engineer should design the foundation
system in accordance with the allowable deflection criteria of applicable codes and the requirements of
the structural engineer/architect.
Please note that the post-tensioned design methodology reflected in UBC Chapter 18 is in part based on
the assumption that soil moisture changes around and beneath the post tensioned slabs are influenced
only by climatological conditions. Soil moisture change below slabs is the major factor in foundation
damages relating to expansive soil. The UBC design methodology has no consideration for
presaturation, homeowner irrigation, or other nonclimate related influences on the moisture content of
subgrade soils. In recognition of these factors, we have modified the geotechnical parameters obtained
from this methodology to account for reasonable irrigation practices and 1?roper homeowner
maintenance. In addition, we recommend that prior to foundation construction, slab sub grades be
presoaked to 12 inches prior to trenching. Moisture should be maintained at above optimum levels up to
concrete construction. We further recommend that the moisture content of the soil around the immediate
perimeter of the slab be maintained at near optimum moisture content during construction and up to
occupancy of the homes.
The geotechnical parameters provided in Table A assume that if the areas adjacent to the foundation are
planted and irrigated, these areas will be designed with proper drainage so ponding, which causes
significant moisture change below the foundation, does not occur. Our recommendations do not account
for excessive irrigation and/or incorrect landscape design. Sunken planters placed adjacent to the
foundation, should either be designed with an efficient drainage system or liners to prevent moisture
infiltration below the foundation. Some lifting of the perimeter foundation beam should be expected
even with properly constructed planters. Based on the design parameters we have provided, and our
experience with monitoring similar sites on these types of soils, we anticipate that if the soils become
saturated below the perimeter of the foundations due to incorrect landscaping irrigation or maintenance,
then up to approximately Y2- to I-inch of uplift could occur at the perimeter of the foundation relative to
the central portion of the slab.
Future homeowners should be informed and educated regarding the importance of maintaining a
constant level of soil moisture. The owners should be made aware of the potential negative
consequences of both excessive watering, as well as allowing expansive soils to become too dry. The
soil will undergo shrinkage as it dries up, followed by swelling during the rainy winter season, or when
irrigation is resumed. This will result in distress to the improvements and structures.
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Project No. 042364-30
Page 2
June 8, 2004
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TABLEA-
Preliminarv Geotechnical Parameters for Post Tensioned Foundation Slab Desilln
PARAMETER VALUE
Exoansion Index Medium
Percent that is Finer than 0.002 mm in the Fraction Passing the No. 200 Sieve. < 20 percent (assumed)
Clay Mineral Type Montmorillonite (assumed)
Thornthwaite Moisture Index -20
Depth to Constant Soil Suction (estimated as the depth to constant moisture 7 feet
content over time, but within UBC limits)
Constant Soil Suction P.P. 3.6
Moisture Velocity 0.7 inches/month
Center Lift Edge moisture variation distance, em 5.5 feet
Center lift, y m 2.5 inches
Edge Lift Edge moisture variation distance, e", 3.5 feet
Edge lift, Vm 1.0 inches
Soluble Sulfate Content for Design of Concrete Mixtures in Contact with Site Negligible
Soils in Accordance with 1997 UBC Table 19-A-4
Modulus of Subgrade Reaction, k (assuming presaturation as indicated below) 120 lbs/in'
Minimum Perimeter Foundation Embedment 24
Sand and Visqueen Type 2
Additional Recommendations:
Presoak to 12 inches prior to trenching, maintain at above optimum up to concrete construction
Sand & Visqueen
Type 1
Install a lO-mil Visqueen (or equivalent) moisture barrier covered by a minimum of I-inch layer of sand. Note: The
builder must ensure that the Visqueen has been lapped and sealed and not punctured as a result of being placed in direct
contact with the native soils or by other construction methods.
Type 2
Install a 6-milVisqueen (or equivalent) moisture barrier covered by a minimum of I-inch layer of sand and 2 inches
below. Or, install a lO-mil Visqueen (or equivalent) moisture barrier in contact with the native soils and covered by a
minimum of at least 2 inches of sand. Note: For both options, the builder must ensure that the Visqueen has been lapped
and sealed and not punctured as a result of being placed in direct contact with the native soils or by other construction
methods.
* The above sand and Visqueen recommendations are traditionally included with geotechnical foundation
recommendations although they are generally not a major factor influencing the geotechnical performance of the foundation.
The sand and Visqueen requirements are the purview of the foundation engineer/corrosion engineer and the homebuilder to
ensure that the concrete cures correctly, is protected from corrosive environments, and moisture penetration of the floor is
acceptable to the future homeowners. Therefore, the above recommendations may be superseded by the requirements of the
previously mentioned parties.
Lt
Project No. 042364-30
Page 3
June 8, 2004
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The opportunity to be of service is appreciated. Should you. have any questions regarding the content of this
letter, or should you require additional information, please do not hesitate to contact this office at your earliest
convenience.
Respectfully submitted,
LA WSON & ASSOCIATES GEOTECHNICAL CONSULTING, INC.
Thomas Dewey
Associate Geologist, CEG 1975
Stephen M. Poole
Principal Engineer, GE 692
TD/SMP/jn
Distribution: (4) Addressee
(2) Riverside County - Attn: Mr. Wayne Harrison
(2) City of Temecula - Attn: Mr. Ron Parks
Attachments: Figure 1 - Recommended Fault Setback Zone ( Rear of Text)
Appendix A - Fault Investigation for Tract 29133, in the City of Temecula, Riverside County,
California, By: Earth Consultants International, ECI Project No. 2410.01, Report dated
May 29,2004. (Rear of Text)
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I Project No. 042364-30
5'
Page 4
June 8, 2004
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APPENDIX A
FAULT INVESTIGATION FOR TRACT 29133
By: EARTH CONSULTANTS INTERNATIONAL
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Eel Project No. 2410.01
May 29, 2004
To:
GALLERY HOMES
31610-2 Railroad Canyon Road
Canyon Lake, California, 92587
Attention:
Mr. Rick Hauser
Subject:
Fault Investigation for Tract 29133, in the City of Temecula, Riverside County,
California
1.0 Introduction and Background
In accordance with your authorization, Earth Consultants International (ECI) has completed an
evaluation of the faulting present at Tract 29133, located on Ynez Road, approximately 950
feet southeast of Pauba Road (Figure 1). Our work consisted of the following:
. Review of a previous geotechnical fault investigation report for the site (Leighton &
Associates, 1988). This investigation did not find evidence of active faulting.
. Directing the excavation of two long trenches across a suspected fault zone discovered
during grading.
. Scraping and logging of the trenches.
. Meeting at the site with representatives for the City of Temecula, and the Riverside
County geologist, Mr. Wayne Harrison.
. Preparation of this report.
We used the Precise Grading Plan for Tract 29133 in the field and as a base for the attached
map (Figure 2). The trench locations shown on Figure 2 were surveyed by the civil engineer
for the project. The location of the temporary grading cut shown on Figure 2 was plotted in
the field using a tape and Brunton compass, measuring from existing survey stakes.
EO was contacted by Lawson & Associates (Lawson), the geotechnical consultant for Tract
29133, to assist with the age determination of a fault encountered during rough grading. The
fault was initially interpreted by Lawson's field personnel from the presence of a sheared clay
unit that was thought to be fault gouge. Lawson opened two short trenches on strike with this
clay near the southeastern corner of the project area. No fault was exposed in the trenches,
although there appeared to be a very minor shear along bedding. A very well-developed
Pleistocene soil capped the Pauba formational strata in the trench closest to the property
boundary, and this soil was not faulted. The soil had already been removed by grading in the
other short trench. To make certain that the interpreted fault did not miss the trenches, it was
decided that the trench closest to the property boundary would be extended in both directions,
herein termed Trench T-1. If the capping Pleistocene soil was found to be unfaulted in T-1,
150 EI Camino Real, Suite 212 ill Tustin, California 92780 ill
Telephones: (714) 412-2666 & (714) 544-5321 ~ Fax (714) 544-5553
~ www.earthconsultants.com
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Project No. 2410.01
May 29, 2004
then the fault exposed during grading would be interpreted as an inactive Pleistocene
structure. It was also determined that a second trench, Trench T-2, would be excavated along
the northern property boundary where the clay bed had first been observed to resolve the
nature and significance of the fault.
The initial Trench T-1 was excavated to about five feet deep, and extended about 130 feet west
and another 110 feet northeast from the projected surface trace of the clay (Figure 2). The five-
foot depth was chosen because the two small trenches had exposed the strong soil horizons
developed into Pauba Formation and this depth was deemed sufficient to locate any significant
fault that cuts the Pauba strata. However, a swale filled with young sediments was
encountered in a portion of T-1, with the Pauba Formation strata expressing opposing bed dips
across this swale. Consequently, this section was deepened and shored, exposing a significant
fault that juxtaposes very different strata of the Pauba Formation and involves the late
Quaternary swale deposits and soil horizons.
Trench T-2 was initially excavated about 110 feet long and to a depth of about five feet along
the northern property line where the fault was expected to be found. The uppermost native
soils had already been largely removed by grading operations, and several minor faults were
exposed in the trench that displaced Pauba Formation strata.
At this point, ECI acquired and analyzed pre-development aerial photography of the area and
determined that the fault exposed in Trench T-1 was likely the primary strand of the Elsinore
fault, as discussed below in Section 2.0 of this report. Tract 29133 lies entirely within the
State's Alquist-Priolo Fault Special Studies zone for this fault. Consequently, Trench T-2 was
extended another 120 feet to the east in order to cross the projection of the main strand, and to
provide a second point on the fault for correlation across the site.
ECI logged both trenches in detail, and from these exposures, we determined the activity of
each exposed fault strand. In this report, we first discuss the aerial photograph analysis and it's
bearing on the location of the Elsinore fault through the project site. We then discuss the
stratigraphy and faults exposed in each of the two trenches, and based on clear relationships
between the faults and the modern soil horizons, determine the width of the active fault zone
through the site.
Both trenches were placed at property boundaries, as the area in-between was already graded
(removal of compressible soils and fill placed).
2.0 Aerial Photograph Analysis
We analyzed stereo-paired aerial photographs of the project area taken in 1983 before much
of the region had been developed (Figure 3). The Wildomar strand of the Elsinore fault is
clearly visible on these photographs as an alignment of deflected and offset drainages, aligned
notches in ridge-lines, probable scarps, and a linear hillslope to the southeast of the project
site. We determined the scale of the photographs for the project area (the scale varies from the
center of the photos outward towards the edges) by measuring the distance between two
intersections (Pauba/Ynez Roads to Santiago/Ynez Roads). We then determined the distance
between the same two intersections from the USGS 7.5' topographic maps of the area
(Temecula and Pechanga Quadrangles). We then scaled the photographs in Adobe Photoshop
and superposed the interpreted photograph with the topographic map.
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The project site boundary was taken from the Precise Grading Plan and also superposed on the
combined topographic map and aerial photograph (Figure 4). Finally, we took the Alquist-
Priolo (A-P) Earthquake Fault Zone Map, with the State's interpreted location of the Wildomar
strand of the Elsinore fault, and superposed the location of the fault and A-P Zone onto Figure
4. ECI's interpretation coincides precisely with that of the A-P map, and places the main fault
through the location of Trench T-1 in the area of the swale. Based on this, we interpret the
fault exposed in T-1 as the Wildomar strand of the Elsinore fault.
3.0 Significance of the Elsinore Fault
The Elsinore fault is one of the primary strands of the San Andreas fault zone in southern
California (Kennedy, 1977; Rockwell and Lamar, 1986; Magistrale and Rockwell, 1996) and
transfers about 10 percent of the current plate motion. The fault extends 260 km from near the
Mexican border northwestward through Temecula to the north end of the Santa Ana
Mountains, where it branches northward into the Whittier and Chino faults. The fault, with
about 10 km of total strike-slip during the past two million years (Kennedy, 1977; Hull, 1990)
has been repeatedly active during the Holocene, resulting in its designation by the State as an
Alquist-Priolo Earthquake Fault zone. In Temecula, the primary strand of the Elsinore fault is
the Wildomar fault.
The slip rate on the Elsinore fault has been determined from offset Pleistocene alluvial fan and
fluvial deposits at about 5:t2 mm/yr both north of the site in the Temescal Valley (Millman and
Rockwell, 1986), and to the south near Pala (Vaughan and Rockwell, 1986; Vaughan, Thorup
and Rockwell, 1999). In Murrieta, Rockwell et al. (2000) show that 10.5 m of right-lateral slip
has occurred in the past 1,900 years on the Wildomar fault, yielding a late Holocene rate of
about 5 mm/yr, similar to the long-term rate for the entire zone. Farther southward, near
Palomar Mountain, the fault splays into the Elsinore and Earthquake Valley faults, with about 3
mm/yr taken by the main Elsinore fault to the south (Magistrale and Rockwell, 1996).
The Holocene earthquake history of the fault is less well-determined, but paleoseismic studies
have documented six surface rupturing earthquakes in the past 1,000 years along the Glen Ivy
North fault (Temescal Valley segment of the fault). At Agua Tibia Mountain, the Wildomar
strand ruptures in large earthquakes about every 450-750 years, and last ruptured between
A.D. 1655 and 1810 (Vaughan, Thorup and Rockwell, 1999). Considering the 5 mm/yr strain
accumulation rate and the occurrence of surface ruptures every 450-750 years for the
Wildomar strand, these values argue for 2.5-3.75 m (8-12 feet) of slip per event, equating to
earthquakes in the Magnitude 7 range.
4.0 Trenching Investigation
As described above, the trenches were excavated in phases to cross the entire fault zone to
sufficient depth to resolve location and activity. The results from each trench are discussed
separately, as the exposed stratigraphy and of the fault zone are quite different between the
two trenches.
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4,1 Trench T-1
Trench T-1 extended along the southern and eastern property lines to shadow any possible
projection of the clay zone/suspected fault encountered during grading (Figure 2 and Plate 1).
The trench exposed units of the Pauba Formation, as well as younger colluvial and alluvial
deposits in the area of the swale between trench stations 40 and 100 (see plate 1). For most of
the length of the trench, the Pauba Formation and its associated surface soi I make up the
entirety of the exposed units.
When the trench was deepened in the area of the swale, it was found to coincide with the
presence of a major fault. The fault juxtaposed very different strata of the Pauba Formation,
with bedding dipping into the fault on each side. On the northeast, stratified conglomerate
and sandstone was seen to dip to the southwest and steepen towards the fault. To the
southwest, the poorly stratified Pauba was seen to gently dip to the southwest from the
westernmost end of the trench to station 145, but then steepen dramatically in a tight fold at
station 145, to a dip of about 50 degrees to the north.
In Trench T-1 the fault is a three foot-wide zone of intense shearing that clearly displaced
stratified alluvium near the bottom of the trench. Above the stratified alluvium, very massive
colluvial deposits are present that obscure the fault. Opening of the trench caused large blocks
of soil in the colluvium to break out along fracture surfaces, but when these blocks were
scraped clean and the wall smoothed out, we could not see actual shears extending up
through the colluvium to the surface. We attribute this to the massive nature of the colluvial
soil and to the likelihood that when surface ruptures occur, the soil will simply melt back into
place, leaving very little trace in the colluvium. This is very different than the expression of the
fault in Trench T-2, as discussed below in Section 4.2.
4.1.1 Age of the Pauba Formation
The Pauba Formation is a middle Quaternary sequence of stratified fluvial and alluvial fan
deposits that fill much of the Elsinore Trough from Temecula northwestward to lake Elsinore.
The age of the Pauba Formation is poorly determined but is considered middle Pleistocene
because it contains the Bishop Tuff (-710,000 years in age) (Kennedy, 1977). The actual age
of individual members within the Pauba could vary substantially and may be as old as early
Quaternary (1.5-2 million years).
At the project site, the minimum age of the Pauba is indicated by the strength of the capping
argillic B soil horizon. In Trench T-1, the argillic horizon is a reddened (SYR 4/Sd) sandy clay
with prismatic structure, extremely hard consistence, and continuous thick clay films.
Comparison to dated soils in the Elsinore Trough (Millman, 1988; Millman and Rockwell,
1986) demonstrate that soils with these qualities are a minimum of 120,000 years, dating to at
least the last interglacial period. Further, the soil horizon has developed across the folded
underlying Pauba stratigraphy, indicating that the formation is considerably older than the age
of the soil. Considering that the Pauba is seen to be strongly folded in Trench T-1 (fold hinge
around station 145), and considering that the folding must predate the modern topography into
which the soil is developed, these relationships argue that the Pauba on site is considerably
older than 120,000 years. We take the age for this unit to be middle Quaternary, as previously
described in the literature (Kennedy, 1977).
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4,1.2 Age of Colluvial Units
The colluvium exposed in Trench T-1 was composed of two sub-units, with the lower unit
delineated by a buried argillic soil. The upper colluvial unit expresses an A horizon (A, and
locally an A2) over massive silty pebbly sand that is dark and generally enriched with organic
humus. A very weak subsoil could be locally observed as clay staining on grains in a B
horizon position (indicated as a Btj on Plate 1). Late Holocene soils developed in the offset
channel deposits at Murrieta (Rockwell et aI., 2000) expressed similar staining and were
radiocarbon dated at less than 1,900 years. We take the upper colluvium to be late Holocene
in age.
The buried colluvial unit is capped by a dark brown (10YR 3/2m) argillic horizon with
common thin and few moderately thick clay films on ped faces and in pores. The clay films
were thickest immediately overlying the fault, and we interpret this to represent the influence
of faulting on the localized accumulation of secondary clay. The argillic horizon is fairly
weakly formed and is similar to late Pleistocene to early Holocene soils in Temescal Valley
(Millman and Rockwell, 1986) so we infer a similar age for these deposits. The colluvium is
underlain by faulted alluvium and Pauba Formation.
4.1.3 Age and Width of Faulting in Trench T-1
Considering the massive character of the colluvial soils exposed in Trench T-1 between
stations 40 and 100 above the Pauba Formation, and considering the geomorphic expression
of the fault through the project site in precisely this location as seen in the 1983 aerial
photographs, we consider it likely that all units are faulted but that the fault is only visibly
expressed in the lower colluvium, alluvium and bedrock. As mentioned above, the lower
colluvial argillic horizon fell out in blocks during opening of the trench, a clear indicator of
recent fracturing, even though the fault was not visible after the trench wall was scrapped flat.
The width of the fault zone was about three feet as viewed in the plane of the trench face,
which is not perpendicular to the fault. Thus, the actual width of the fault is less than 3 feet.
Northeast and west-southwest of this zone, Pauba Formation was continuously exposed to the
ends of the trench and was only broken on the west by very minor shears with less than two
inches of throw. There also appeared to be minor shearing along a bedding surface in the area
of the fold hinge. None of these minor fractures affected the strongly developed surface soil
and are considered inactive. Thus, along the southern margin of the project site, the width of
active faulting is taken to be less than three feet.
4.2 Trench T-2
Trench T-2 was initially excavated to cross the suspected fault trend, and then extended once it
was determined that the main Elsinore fault was crossed in Trench T-1 and projected east of
the end of T-2. Consequently, the southern wall was logged for the western part of the trench
whereas the northern wall was logged from the main zone to the east (Plate 1). The area of the
trench had already been graded, and the western portion of the trench had sustained deeper
cuts than the eastern part.
The fault zone is broader in T-2 than in T-1, with secondary strands west and east of the main
zone. As discussed below, there is direct evidence that the faults to the west are older,
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inactive strands whereas the strands to the east are minor but probably have sustained some
Holocene rupture during large events.
The main fault is a complex of subparallel shears that affect all units to the surface. The
surface is the current grade line and the native soil has been locally removed by grading.
Nevertheless, topsoil was found in wedges penetrating downward into the main fault zone,
indicating recently active strands.
4.2.1 Age of Units in Trench T-2
In Trench T-2, two major units are delineated that contain age information. Northeast of the
main fault, a colluvial unit is exposed that is capped by a soil with a weakly formed argillic
horizon (trench stations -15 to -55, Plate 1). Lying stratigraphically beneath the colluvium, the
Pauba Formation is exposed for most of the length of the trench.
The Pauba Formation is well-bedded sandstone interbedded with siltstone, and has a
completely different character than the Pauba strata exposed in Trench T-1, presumably
because Trench T-2 exposed a deeper and older part of the section. This inference is based on
the observation that away from the fault to the southwest, the bedding in Trench T-2 dips
southwest and projects beneath the stratigraphic level of the section exposed in Trench T-1. In
addition, bedding mapped in the temporary cut slope east of T-2 is dipping to the southwest
(Figure 2). Furthermore, the Pauba Formation exposed near the eastern end of Trench T-1
projects above the ground surface (also dips southwest) above the sandstone exposed in T-2
and in the cut slope in the northeastern corner of the project area. These observations indicate
that the Pauba Formation in Trench T-2 predates (is lower in the Pauba section) than that
exposed in T-l. Consequently, it also must be significantly older than 120,000 years, the
estimated age of the soil capping the folded section in T-1.
The "late Pleistocene" colluvium expresses a soil that is identical to the buried argillic horizon
described in the colluvial swale in Trench T-1, although it has been largely removed by
grading. Between trench stations -70 and -85 (Plate 1), most of the profile appears preserved,
with the exception of the topsoil (A) horizon. The argillic horizon is slightly redder (7.5-10 YR
3/4d) than that exposed in T-1, and has moderately developed subangular blocky structure, a
hard consistence, and common clay films in pores and on ped faces. We compared this soil to
those described and dated by Millman and Rockwell (1986) and assign a latest Pleistocene age
for this unit, with the possibility that it is early Holocene in age.
4.2.2 Faulting in Trench T-2
The faults exposed in Trench T-2 were each evaluated for significance and age of activity. For
ease of discussion, each fault presented in Plate 1 is numbered from 1 through 13 from west to
east. The main zone is collectively grouped as fault 9, as there were so many strands that
involved the truncated modern soil and we consider all of them potentially active. The rest are
treated individually, although faults 1 through 8 form a group with similar characteristics and
are discussed together.
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4.2.2.1 Faults 1 through 8
Faults 1 through 8 comprise an older group of faults between Stations 5 and 60 that were
active at the time of Pauba deposition. Of these, faults 4 and 8 are the most significant
structures and probably have substantial strike-slip as they sustain significant mismatches in
stratigraphy across them. However, these two faults, along with fault strands 5 and 6, are all
clearly overlain by unfaulted Pauba strata, hence, they have not been active since the middle
Quaternary.
Fault 7 is a minor fault considered part of this group of older faults, although there was several
inches of dip separation expressed on the uppermost Pauba strata exposed below the grading
cut. The primary reason that this fault is grouped with the inactive strands, in addition to its
location within them, is that all of the faults are structurally similar and form a group of shears
that currently dip northeast but are perpendicular to the Pauba strata. That is, prior to folding
of the Pauba, these faults are interpreted to have been vertical, forming a flowering upward
(palmtree) structure. Fault 7 is one of the least expressed of this group with only minor
mismatches in stratigraphic units. This observation suggests that total slip on fault 7 is very
minor. The grading at the site eliminated the option of seeing whether this fault is overlain by
unbroken Pauba, and the modern soil was removed. Nevertheless, there is no indication of
any late Quaternary activity such as fissuring filled with modern soil, as can be seen with
several other minor fault strands northeast of the main zone.
Faults 1,2 and 3 are the least significant faults in Trench T-2, based on the observation that all
stratigraphic units could be matched across the shears and reconstructed with little or no
strike-slip. Fault 1 cuts up into a massive sand unit in the Pauba Formation and could not be
traced up to the grading cut. Similarly, fault 3 could not be traced to the cut surface and
expresses only very minor slip that appeared to die out both upward and downward. Fault 2,
in contrast, was graded off and does continue up to the graded surface. However, none of
these faults has any indication of recent activity in the form of fissuring or inclusion of modern
soil, and all are clearly very minor. Consequently, they were grouped with the inactive
strands.
4.2.2.2 Faults 9 and 10 - The Main Zone
A major zone of faulting was exposed between stations 0 and -20 that we interpret as the main
zone, and are shown as fault zones 9 and 10 on plate 1. Within this zone, we identify at least
seven individual major shears and numerous secondary shears. The actual width of the zone
is less than that visualized in Plate 1 because the faults strike at about 450 to the trend of the
trench. Nevertheless, this -13 foot-wide zone is a complex of major shears that juxtapose the
Pauba Formation on the southwest with late Pleistocene colluvium to the northeast. Pauba
strata northeast of the fault are presumably present beneath the colluvium, as exposed farther
east in the trench. Some fissures within this zone are filled with soil from the A horizon (now
removed), and the remnants of the argillic horizon below the artificial fill are in fault contact.
Further, this fault zone aligns with the major geomorphic expression of the fault in the 1983
aerial photographs and is the most significant group of structures exposed in Trench T-2. All of
these observations support the contention that this is the Wildomar fault and that it is present
on site, as suggested in the A-P map.
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4.2.2.3 Faults 11, 12 and 13
Northeast of the main zone, three separate shear zones are expressed that break the Pauba
Formation; two of these (faults 11 and 12) cut the late Pleistocene colluvium whereas the other
(fault 13) appears to affect the soil developed across the Pauba.
Fault 11 appears to be a minor fault that drops the contact between a sandstone and siltstone
within the Pauba Formation by up to 2 feet. Shears could be traced into the sandstone and
flattened upward, suggesting that these were active during deposition of the Pauba Formation.
One shear appeared to crack into the "late Pleistocene" colluvium, although no displacement
could be resolved in the contact between the colluvium and the Pauba. The contact is
significantly bioturbated, so minor slip could be missed. Nevertheless, major slip is precluded
and this fault may not be active.
In contrast, Fault 12 juxtaposes the late Pleistocene colluvium against the Pauba Formation
sandstone across an intensely bioturbated, subvertical zone of soil filling above the bedrock
fault. The fault itself shows only minor vertical separation on a contact within the Pauba, but
considering the involvement of the late Pleistocene colluvium and the infilling of soil, we
consider this strand likely to be active.
Fault 13 is another minor fault within the Pauba Formation that appears to affect the soil. A
zone of bioturbation was observed to extend from the argillic horizon downward to the
bedrock fault, although no shears or displacement could be seen in the soil itself. The vertical
separation in the Pauba Formation is minor, about 3-6 inches, but mismatches across the
trench suggest some strike slip. We interpret this as a very minor secondary fault that probably
just cracks with large earthquakes, and the cracking has facilitated and encouraged
bioturbation downward into the fault zone. There is no direct evidence that the soil is faulted,
but the presence of this zone of soil mixing, extending from the bedrock fault upward into the
argillic horizon, is sufficiently suspicious, such that we conclude this fault could experience
some minor movement in the future, if the main fault should rupture again. Consequently, we
include fault 13 as part of the active zone.
East from fault 13, continuous unbroken Pauba bedding was exposed in the trench wall and in
the temporary cut slope above. This stratagraphic sequence, which consisted of well-bedded
to laminated fine-grained to coarse-grained sand with minor thin clayey silt beds, was exposed
in the temporary grading cut for 85 feet east of the end of Trench T-2 (Figure 2). In the
northeast corner of the project area, several minor shears are present that were evaluated
during the grading. None of these produced sufficient displacement to produce recognizable
vertical separation in the Pauba Formation strata. Consequently, these shears are not
considered part of the active zone although cracking has occurred along them at some point in
the Pleistocene.
As discussed earlier in this report, a sheared clay projecting across the site was suspected as a
fault zone. This clay was exposed in the south wall of Trench T-2 and was determined to be a
sedimentary unit within the Pauba Formation that had been faulted and folded such that during
grading, the clay day-lighted at the surface and appeared as a narrow zone of dark gray clay
on the graded surface.
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5,0 Summary and Conclusions
A major fault bisects Tract 29133 and was exposed in Trenches T-1 and T-2. Analysis of 1983
aerial photography indicates that this fault is the Wildomar strand of the Elsinore fault, the
primary fault strand in the Temecula Valley area. The exposed fault is also coincident with the
Elsinore fault, as mapped by the California Geological Survey for the Alquist-Priolo Earthquake
Fault Zone (A-P zone).
The active strands of the fault comprise a zone approximately three feet wide in Trench T-1
along the southern property boundary, widening to about 13 feet on the northern property
boundary, with three potentially active minor faults or fractures within about 65 feet to the east
of the main fault. The narrow fault zone in Trench T-1 had a strike of about N40W, as
measured with a Brunton compass in the trench. In contrast, the main zone faults in Trench T-
2 have a slightly more westerly strike: we excavated the bounding fault on the east edge of the
main fault zone and resolve a strike of N53W for this fault. Most of the main zone faults were
too complex to be correlated across the trench. The average strike of the main fault across the
site is about N50W.
We map the zone of active faulting across the project site by taking the outermost active faults
in Trench T-2 and connecting them to the main fault exposed in Trench T-1 (Figure 2). This
results in a wedge-shaped zone across the site that opens northward towards the broader zone
of deformation expressed in Trench T-2. Within the area of the trenches and grading cuts we
observed, the active zone is well-defined, with no observed Holocene deformation outside of
the active zone.
Please note that the active zone shown on Figure 2 is not a setback zone. The homes should
be setback from the outer limits of the active zone, to a distance recommended by Lawson and
Associates.
Should you have any questions regarding this report, or require additional information, please
do not hesitate to contact us. We thank you for the opportunity to be of service.
Respectfully Submitted,
EARTH CONSULTANTS INTERNATIONAL, INC.
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Kay St. Peters, CEG 1477
Project Consultant
Dr. Thomas Rockwell
Senior Consultant
Distribution: (5) Addressee
(6) Lawson and Associates, fne.
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REFERENCES CITED
California Division of Mines and Geology, 1990, Alquist-Priolo Special Studies Zone,
pechanga Quadrangle, Revised Official Map dated January 1, 1990.
California Division of Mines and Geology, 1990, Alquist-Priolo Special Studies Zone,
Temecula Quadrangle, Revised Official Map dated January 1, 1990.
Hull, B.G., 1990, Seismotectonics of the Elsinore-Temecula Trough, Elsinore fault zone,
southern California: Ph.D dissertation, University of California, Santa Barbara, 233p.
Kennedy, M.P., 1977, Recency and character of faulting along the Elsinore fault zone in
Riverside County, California: California Division of Mines and Geology Special Report 131,
12p.
Lamar, D.L., and Rockwell, T.K., 1986, An overview of the tectonics of the Elsinore fault zone:
in guidebook and Volume on Neotectonics and Faulting in Southern California (P. Ehlig, ed.),
cordilleran section, Geological Society of America, pp. 149-158.
Magistrale, H., and Rockwell, T., 1996, The central and southern Elsinore fault zone, southern
California: Bulletin of the Seismological Society of America, v.86, no. 6, pp.1793-1803.
Millman, D.E., 1988, Neotectonics of the northern Elsinore fault, Temescal Valley, southern
California: unpublished MS thesis, San Diego State University, 248p.
Millman, D.E., and Rockwell, T.K., 1986, Neotectonics of the Elsinore fault in Temescal
Valley, California: in Guidebook and Volume on Neotectonics and Faulting in Southern
California (P. Ehlig, ed.), Cordilleran Section, Geological Society of America, p. 159-166.
Rockwell, T.K., Bergmann, M., and Kennedy, M., 2000, Holocene slip rate of the Elsinore fault
in Temecula Valley, riverside County, California: in Geology and enology of the Temecula
Valley, Riverside County, California (B.B. Birnbaum and K.D. Cato, eds.), pp 105-118.
Vaughan, R., and Rockwell, R., 1986, Alluvial stratigraphy and neotectonics of the Elsinore
fault zone at Agua Tibia Mountain, southern California: in Guidebook and Volume on
Neotectonics and Faulting in Southern California (P. Ehlig, ed.), cordilleran Section, Geological
Society of America, pp. 177-192.
Vaughan, P.R., Thorup, K.M., and Rockwell, T.K., 1999, Paleoseismology of the Elsinore fault
at Agua Tibia Mountain, southern California: Bulletin of the Seismological Society of America,
v. 89, no. 6, pp. 1447-1457.
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