HomeMy WebLinkAboutCT 13-03; ROBERTSON RANCH-RANCHO COSTERA; UPDATE TO PRELIMINARY SOILS INVESTIGATION AND REPORT OF COMPACTED FILLED GROUND - ADDENDUM 1; 2017-07-31Lake
Im
P.O. Box 1195
92040
TVS
side, Calffomia
(619) 36O
July 31, 2017
K.F. Stephens, Inc.
1500 West 11th Avenue
Suite 24
Escondido, CA 92029
SUBJECT: File No. 1147H1-16
UPDATE TO Preliminary Soils Investigation and
Report of Compacted Filled Ground - Addendum I
Proposed Residential Building Site
La Costa Vale
7601,7603 and 7605 Romeria Street
City of Carlsbad
Dear Mr. Stephens:
In response to the Geotechnical Report Review dated June 6, 2017, Permit Numbers
GR2016-0055, 0056, 00579, here is the additional information requested.
Please see Plate I for the location map.
The subject properties are located at the southwest corner of La Costa Avenue
and Romeria Street in Carlsbad, California (see Location Map, Figure 1). All
three parcels are identified with Subdivision Lots numbers and street address
as, (southerly) Lot 20, 7605 Romeria Street, (middle) Lot 21, 7603 Romeria
Street, (northerly) Lot 22, 7601 Romeria Street, from La Costa Vale -Unit No.
1, Lots 20 through 22, per Subdivision Map number 7457, Recorded on
October 18, 1972. The three sites are contiguous rectangular shaped residential
lots. The lots are currently vacant of any residential structural developments
other than a low growth of scattered weeds, grasses, and several shrubs and
two trees.
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11421 Woodside Ave., Suite C
Santee, California 92071
7 -31 -17 (619) 562-0500
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K. F. Stephens File No 1147H1-16 July 31, 2017
The Topography of the lots consist of three level building pads, that are stair
stepped from 6 to 9 feet high with 2:1 documented fill slopes between each lot.
The lots drain towards Romeria Street. The proposed construction is a single
family residence on each of the graded lots and retaining walls on the slopes
between the lots. The structural design engineer should be referred to for the
foundation design and the structural loads.
There were no additional subsurface explorations done at this time. The
recommendations were based on the information included in the Preliminary
Soils Investigation dated October 22, 1971, Project No. 71-10-18F, Interim
Reports on Compacted Filled Ground dated March 1, 1973 & April 2, 1973 and
the Final Report of Compacted Filled Ground dated February 28, 1974 prepared
by Benton Engineering, Inc., Project No. 72-8-18D and the Geotechnical
Feasibility Study dated October 26, 2015 prepare-d by Hetherington
Engineering, Project No. 7802.1.
The laboratory testing performed by Soil Testers consisted of a maximum dry
densities and optimum moisture contents of the fill materials as determined by
the A.S.T.M., D1557-78, Method A, which uses 25 blows of a 10 pound
rammer falling from a height of 18 inches on each of 5 layers in a 4 inch
diameter 1/30 cubic foot compaction cylinder; an expansion index, A.S.T.M.
D4829-1 1 A specimen is prepared by compacting a test soil into a metal ring at
a degree of saturation of 50+1-2%. The specimen and the ring are then placed
in a consolidometer. A vertical confining pressure of 6.9 kPa (llbf/in2) is
applied to the specimen and the specimen is then inundated with distilled
water. The deformation of the specimen is recorded for 24 h or until the rate of
deformation becomes less than 0.005 mm/h (0.0002 in/h) whichever occurs
first. A minimum recording time of 3 h is required; a direct shear test,
A.S.T.M. D3080-11, the test method consists of a lacing the test specimen in
the direct shear device, applying a predetermined normal stress, providing for
drainage or wetting of the test specimen, consolidating the specimen under the
normal stress, unlocking the frames that hold the test specimen and applying a
shearing force to shear the specimen. Generally three or more specimens are
tested, each under a different normal stress to determine the effects upon shear
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K. F. Stephens File No 1147H1-16 July 31, 2017
resistance and displacement The range in normal stresses should be
appropriate for the soil conditions being investigated.
6. A review of the Geotechnical Feasibility Study prepared by Hetherington
Engineering, Inc. dated October 26, 2015 found no evidence of faulting or
landsliding within the subject site. The site is likely underlain by Santiago
Formation bedrock.
7. The site location S = 0.75 1 is considered seismic design category D.
TABLE 1:1.6.1 SEISMIC DESIGN CATEGORY BASED ON SHORT
PERIOD RESPONSE ACCELERATION PARAMETER
Value of s
Occupancy Category
I or U In IV
SDS<0,I67 A A A
0.167 < SDS <0.33 B B C
033<SDS<0.50 C C D
0.50 S05 D D D
8. Liquefaction is a phenomenon in which loose, saturated soils lose shear strength
under short-term (dynamic) loading condition. Ground shaking of sufficient
duration can result in the loss of grain-to-grain contact in potential liquefiable
soils due to a rapid increase in pore water pressure, causing the soil to behave as
a fluid for short period of time.
To be potentially liquefiable, a soil is typically cohesionless with a grain-size
distribution generally consisting of sand and silt. It is generally loose to medium
dense, saturated, and subjected to sufficient magnitude and duration of ground
shaking. The soils encountered at the subject site are documented fills soils
consisting of grey, silty, clayey sands compacted to a minimum of 90% relative
compaction. Native soils underlying the fill materials are claystone likely
underlain by Santiago Formation bedrock and the site sits 108 to 122 feet
above sea level. Due to the absence of water and based on the on-site soil type it
is our opinion the liquefaction potential will be low to negligible. It is not
anticipated to be a design issue.
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K. F. Stephens File No 1147H1-16 July 31, 2017
The subject site is approximately 3.6 miles from the Pacific Ocean and
approximately 108 to 122 feet above mean sea level. According to McCulloch
(1985) the potential in the San Diego County coastal area for 100 year and 500
year tsunami waves is approximately five and eight feet or less. This suggests
that there is a low probability of a tsunami reaching the site based on elevation
above sea level.
Please refer to attached grading plans, sheet 2 of 4 for each of the lots.
Please refer the attached grading plans sheet 4 of 4. It is our understanding the
final elevation difference, will be within 12 inches of existing grade on each of
the pads.
We conclude that the proposed development of the site is feasible from a
geotechnical standpoint provided the recommendations in this report and in our
UPDATE TO Preliminary Soils Investigation and Report of Compacted Filled
Ground dated March 1, 2017 are incorporated into the design and construction
of the project.
The proposed development of the 3 parcels would have negligible impact on
adjacent properties.
The existing slopes on the 3 parcels are 2:1 documented fill slopes. It is our
opinion that the existing slopes are stable against any deep seated or surficial
failures. Drainage and stability should be addressed by an engineer if any
additional changes are made on this site that would affect the existing slopes.
The proposed development on each of the lots have the structures with
deepened footings and set back a minimum of 18 feet from the property line
and a minimum of 20 feet to a maximum of 25 feet from the top of slope to
prevent the potential for slope creep.
Since the walls do not exceed six feet in height, there are no recommendations
for the grading operations to perform the back cuts for the proposed retaining
wall.
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K. F. Stephens File No 1147H1-16 July 31, 2017
16. The structural design engineer should design the foundation for the onsite soil
conditions. Based on the test results, a safe allowable bearing value of at
least 1500 pounds per square foot for 12 inch deep footings may be used in
designing the foundations and slab for the proposed structures. This value
may be increased by one third for wind and/or seismic loading. This value
may be increased according to the CBC Table 18A-I-A by 20 percent for
each additional foot of depth and or width to a maximum of 3 times the
designated value.
. The soils shall be compacted to a minimum of 90% relative compaction and
foundation excavations a minimum of 30 inches in depth.
The site shall not permit any storm water to drain towards the structure.
All drainage for a minimum of five feet shall drain away from all structures.
The finish floor elevation shall be a minimum of six inches above the finish
-C.
Total and differential settlement is expected to be within industry adopted
standards as long as the recommendations presented in our report are adhered to.
Differential settlement is expected to be less than 1 inch in 30 feet for
foundations bearing on bedrock or the softer natural soils or fill soils.
The soils of all three lots are considered to be highly to critically expansive as
stated in the Benton Engineering report dated April 2, 1973. From our field
observation and lab testing, we found the sites to be highly expansive with an
expansion index of 104. Due to the possibility of critically expansive soils
referenced in the Benton report, we are amending our recommendations to the
following:
The following foundation depth is a minimum due to the expansive nature of the
soil and must be used in order to prevent possible damage to the proposed
structures from adverse effects of expansive soils. These recommendations for
footings and slabs are based upon the soil type only and do not take into
consideration the structural requirements.
Use continuous exterior perimeter foundations (including door openings). They
should be founded a minimum of 30 inches below lowest adjacent grade with a
minimum width of 12 inches.
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K. F. Stephens File No 1147H1- 16 July 31, 2017
U Interior footing depth should exceed 24 inches below top of slab or 24 inches
below lowest adjacent grade for raised floor construction with a minimum
width of 12 inches.
Reinforce continuous concrete foundations as a beam with at least one #5 bar
positioned 3 inches above the bottom of footing and one #5 bar positioned 1
inch below top of foundation or top of finish floor.
Concrete Slab-On-Grade should be designed by the project's structural
engineer based on anticipated loading conditions. Soil Testers recommends
that conventional reinforced concrete SOG for this project be founded on 6
inches of Class H Virgin Aggregate Base (With approximately 2% +1- of
optimum moisture content and 90% compaction, relative to the lab maximum
dry density, ASTM D 1557), overlying a 12 inch thick zrãeqiate1y
placed and compacted structural fill. We also recommend that a moisture
barrier be provided by a membrane, visqueen 10 mils in minimum thickness
or equivalent, be placed at top of well compacted Class H Aggregate Base,
and midway between 4 inches of moist clean sand.
Floor slabs, as a minimum, should be 5 inches thick with #4 reinforcing steel
at 16" on-center each way. Reinforcement should be placed at mid-height of
the slab. The final slab thickness and reinforcement should be determined by
the structural design engineer. Control joints should be provided in
accordance with the recommendations of the structural design engineer.
As an alternative, post-tensioned, PIT, slabs designed by a qualified structural
design engineer may be considered. Geotechnical recommendations for the
design of P/T SOG will be provided upon request.
19. The three lots step up as the street goes south from La Costa Avenue. Please
refer to question #14 for the structural set back from the slope along the back
property lines. Per section 1808.7 through 1808.7.2, the grading shows
retaining walls between each lot in a north to south condition. And as for the
west to east there lies a large descending slope that goes into an Environmental
habitat area which starts from within the level portion of each lot and enters the
open space by the City of Carlsbad. The Civil Engineer has designed
subdrains and a berm preventing storm water from going over the top. The
City is the responsible party to maintain the slopes. As for the internal slopes
K. F. Stephens File No 1147H1-16 July 31, 2017
they are designed with a single to double retaining wall to support the grade
differences. And the measurements will come from the top of wall. The current
slopes have been constructed as a 2:1 slope meeting the 45 degree angle being
sufficient to provide protection from drainage, erosion and shallow failures.
In reviewing the Grading plans and structural retaining wall designs, the only
items that are of concerns is the back fill material and drainage behind the
retaining walls. The items can be shown in either plans sets. No back fill
material shall be expansive.
We will review the building plan set and prepared a letter of foundation plan
review upon receipt of the updated plans.
There was an error in our previously referenced report; the following is the
corrected paragraph:
Resistance to horizontal movement may be provided by allowable soil passive
pressure and/or coefficient of friction of concrete to soil. The allowable passive
pressure may be assumed to be 250 psf at the surface and increasing at the rate of
150 psf per foot of width and or depth. These pressures assume a frictionless
vertical element, no surcharge and level adjacent grade. If these assumptions are
incorrect, we should be contacted for values that reflect the true conditions. The
values are for static conditions and may be increased 1/3 for wind and/or seismic
loading. The coefficient of friction of concrete to soil may be safely assumed to be
0.4. For a 2:1 wall backfill, the design wall height "H" = the actual wall height "h"
+ (1/2) h. Everything else remains the same, use the design parameter accordingly,
except the H = h + (1/2)h. For example: if the wall height is 10 feet then the design
height is for a 15 foot high wall not a 10 foot high wall.
The recommendation for the cement type is in accordance with ACI 318,
comes from the structural design engineer for designing any type of structural
concrete which this code provides the minimum requirements for the materials,
design and building detailing of the structural concrete and where applicable,
nonbuilding structures. Please refer to the testing on corrosion
recommendations for the structural engineer to select the correct concrete.
'A
K. F. Stephens File No 1 147H1-16 July 31, 2017
26. It is our understanding the final pad elevations will be approximately 1 foot lower
than the existing elevations. The existing fill materials should be removed to the slab
subbase, scarified 12 inches and recompacted to at least 90 percent of maximum dry
density in accordance with the Grading Specifications in this report in order to provide
adequate support for the proposed new structure. The recompaction should extend at
least five feet outside the proposed building footprint and all foundation
excavations should have a minimum of 12 inches of documented fill soils
uniformly under the bottom of the footings. Organic materials and roots must be
removed from the soils before replacement. A representative of our firm must
approve removal down to competent soils free of vegetation and roots. We will
observe the grading operation and perform a finish grade test in the fill placed. A
representative of this company must inspect the foundation excavations prior to
placement of forms and reinforcing steel to insure that adequate depth has been
achieved. Our recommendations are based upon on the soil type encountered and do
not take into consideration the proposed bearing load.
If we can be of any further assistance, please do not hesitate to contact our office. This
opportunity to be of service is sincerely appreciated.
Plate I, Final Grading Plans sheets 1-4 for Lots 20, 21 and 22 respectively and the
Geotechnical Feasibility Study prepared by Hetherington Engineering are parts of this
report.
Respectfully submitted, FV~OFES
9L NO.0 034442
Chin C. Chen, RPE C 34442
CCC/mlj CIVI
CA if