HomeMy WebLinkAboutCDP 04-11; CASA DI MARE; INTERIM REPORT OF SITE CONDITIONS AND PRELIMINARY RECOMMENDATIONS; 2003-03-17I
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GEOTECHNICAL EXPLORATION, INC.
SOIL & FOUNDATION ENGINEERING • GROUNDWATER
HAZARDOUS MATERIALS MANAGEMENT • ENGINEERING GEOLOGY
17 March 2003
Mr. Ted Viola
6817 Adolphia Drive
Carlsbad, CA 92009
Job No. 04-8622
Subject: Interim Report of Site Conditions and Preliminary
Recommendations
Casa Di Mare Remodel and Additions
5019 Tierra Del Oro Street
Carlsbad, California
Dear Mr. Viola:
In accordance with your request, Geotechnical Exploration, Inc. is in the process
of investigating soil conditions at the residential lot on Tierra Del Oro Street in
Carlsbad, California. Our work to date has included review of preliminary building
plans, observation of the site and surrounding vicinity, subsurface soil testing (5
hand-auger borings) in the proposed new addition areas and improvement
locations, and laboratory soil testing.
We found the proposed new addition building site to be underlain at depth by
medium dense to dense formational materials and slopewashjcolluvium with
approximately 2 to 3 feet of loose to medium dense fill soils that will not provide
adequate bearing strength for the proposed new additions and improvements. In
addition, up to 6 feet of backfill soil exists adjacent to the lower-level basement
walls. As suchr we recommend that the fill soils either be removed and
recompacted as part of site preparation for the main level improvements or that the
additions and any new structural loads be supported on a system of caissons
founded in the underlying dense formational (terrace deposits) soils. It is our
understanding that a new two-story addition will be added on the east side of the
existing residence, including a new garage and upper level bedrooms. In addition,
7420 TRADE STREET • SAN DIEGO, CA 92121 • (858) 549-7222 • FAX: (858) 549-1604 • E-MAIL: geotech@ixpres.com
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Casa Di Mare Remodel and Additions
Carlsbad, California
Job No. 04-8622
Page 2
a second-story addition will be built over the existing building footprint that will be
supported by new caissons
CONCLUSIONS AND PRELIMINARY RECOMMENDATIONS
The following preliminary conclusions and preliminary recommendations are based
upon the practical field investigation conducted by our firm, and resulting
laboratory tests, in conjunction with our knowledge and experience with the soils in
this area of the City of Carlsbad. Additional or modified recommendations will be
included in our final geotechnical report.
A. Preparation of Soils for Site Development
1. The existing concrete patio, foundations, debris and vegetation on the
building site must be removed prior to the preparation of the areas to receive
new structural improvements and be properly disposed of.
2. If the site is to be re-graded to provide a more uniform, firm soil base for the
proposed new additions and improvements on the main level, the existing fill
soils on the building pad shall be completely removed to a depth of at least 2
to 3 feet, and as per the indications of our field technician based on field
observations. The excavated soils shall be cleaned of any debris and
deleterious materials and watered to approximately optimum moisture
content. Deeper removal and recompaction (up to 6 feet) may be required
adjacent to the basement wall in the front courtyard area.
3. All fill shall be compacted to at least 90 percent of the maximum dry density
determined per ASTM D1557-98, and the soil moisture content shall be at
least 3 percent over the optimum. Should any expansive soils be
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Casa Di Mare Remodel and Additions
Carlsbad, California
Job No. 04-8622
Page 3
encountered they shall be compacted to a moisture content equal to at least
5 percent over the optimum.
4. The allowable soil bearing capacity of the proposed recompacted fill in the
north addition is 2,500 psf for foundations embedded at least 18 inches into
the ground, measured from the lowest adjacent grade. The passive
resistance is 300 pcf. The friction coefficient is 004. The anticipated
maximum settlement of properly compacted fill up to 10 feet in thickness is
on the order of 1 inch. Differential angular rotation should be less than
1/240.
5. All foundation excavations and grading excavations shall be observed and
evaluated by a representative of our firm before they are backfilled with soil
or concrete.
B. Design Parameters for Caissons
All new loads for the proposed second-story addition are to be supported by a
caisson foundation system and the following recommendations should be followed.
We encountered between 2 to 6 feet of loose to medium dense fill soil over medium
dense to dense terrace deposits in the area of the proposed second-story addition.
6. Where caissons are utilized, they shall be designed by the project
Civil/Structural Engineer to support all vertical and lateral loads of the
proposed addition.
7. For vertical loading, all end bearing caissons should be embedded at least 5
feet into dense natural materials, to be approved upon observation by a
representative of this firm. Dense natural soils are typically encountered
between 2 and 6 feet from the surface. For lateral loading deSign, the
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Casa Oi Mare Remodel and Additions
Carlsbad, California
Job No. 04-8622
Page 4
caisson length may require additional embedment. It is important, when
drilling excavations for caissons that utilize end-bearing strength, to limit the
amount of loose material on the bottom of the excavation. Therefore, we
recommend that caissons be designed with a minimum diameter of 24 inches
in order to facilitate observation of the excavations and allow for easy hand-
tool removal of material on the bottom. For end-bearing capacity caissons,
no slough over 1 inch in thickness shall remain at the bottom of the
excavation before concrete placement. The drilling contractor shall provide
an appropriate cleaning tool to satiSfy this requirement. Otherwise, shoring
installation and hand-tool cleaning (or another acceptable option) will be
required.
Frictional caissons may start getting positive vertical frictional resistance
starting at an approximate depth equal to -5 feet from the surface. The
average allowable frictional resistance per shaft square foot is 300 psf from -
5 ft. to -15 feet in depth; and 500 psf from -15 to -25 feet in depth. Wet
caisson weight to be deducted is 1/3 of their weight in the soil embedded
part. The part of caisson above ground needs to be considered in its full
weight.
8. The minimum center-to-center spacing of caissons, in a direction
perpendicular to the temporary seismic or with a lateral load, shall be 3
caisson diameters. For caissons located in the same line of the applied
lateral load, the shadow effect produces a reducing effect in their combined
individual lateral load capacity. For 24-inch diameter caissons spaced 10 feet
center-to-center, the reduction factor shall be equal to 2.2, for the sum of
the individual capacities. If the spacing is 12 feet center-to-center, the
reduction factor is 1.8. The reduction factor for caissons spaced at least 16
feet apart is 1.
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Cas a Di Mare Remodel and Additions
Carlsbad, California
Job No. 04-8622
Page 5
9. The allowable end-bearing capacity is 6,000 psf for caissons penetrating at
least 5 feet into dense formational soils (terrace deposits). This end-bearing
capacity has already deducted the down drag force produced by existing fills.
The caisson weight to be considered is only one-third of the actual weight of
buried caisson. The actual needed caisson length and embedment into
formational soils shall be established by the structural engineer based on the
length needed to adequately support the total vertical and lateral loads
included in the design.
10. For lateral earthquake or wind load resistance, the structural engineer may
use any method that conSiders the equilibrium of forces and moments.
Some structural engineers like to use the fixity concept. Based on a free-
head caisson with diameter equal to 24 inches, the concrete modulus of
elasticity, and a horizontal sub-grade reaction of the loose fills, we
recommend that fixity depth to be considered in the calculation be not less
than 8 feet from the soil surface. The maximum moment produced by the
lateral load may be calculated by multiplying the lateral load times one-half
the total distance between the point of application of lateral load at top of
caisson load application above the ground plus the buried depth to fixity
depth.
If a balance of forces is calculated based on the applied lateral forces and
reaction soil forces, the following allowable passive (equivalent fluid) forces
are recommended: for existing loose fill soils 130 pcf and for formational
soils 300 pcf. The passive resistance shall be measured from a depth of
caissons at least 3 feet from the surface. The passive resistance of the
caissons may be considered applicable on a projected surface equal to 2.5
times the diameter of the caisson multiplied by the vertical length being
considered.
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Cas a Di Mare Remodel and Additions
Carlsbad, California
Job No. 04-8622
Page 6
11. Another option chosen by some engineers is the use of the pole equation
presented in USC Section 1806.8.2. to calculate minimum depth of
embedment of the caissons due to lateral loads. The maximum lateral
bearing of fill soil is 1,300 psf; 3,600 psf for formational soils.
12. The recommended allowable end bearing vertical capacity for the caissons
already includes the effect of negative friction produced by the existing fills
as well as the buried caisson weight. Any caisson weight above the soil
surface shall be considered as dead load and shall be deducted from the net
end bearing capacity. The depth to medium dense to dense formational soils
in the explored area was encountered to range from approximately 2 to 6
feet from the existing split-level pad surface.
13. Caisson drilling operations shall be performed under the continued
observations of a representative of our firm to confirm the penetration into
formational soils.
14. The design and construction of the caissons shall be in accordance with the
recommendations presented above, the current USC reqUirements accepted
by the City of Carlsbad, and also in accordance with ACI 336, 3R-93 Design
and Construction of Drilled Piers, of the American Concrete Institute. The
contractor shall follow all the safety procedures required by Cal OSHA.
15. It is also our recommendation that the caissons excavations be filled with
concrete within 2 days after the excavations are completed, to help reduce
the risk of soil caving, mud or slough intrusion, etc.
We will keep you advised as to the completion of our final report. We appreciate
this opportunity to be of service. Should you have any questions, please feel free
to contact our office. Reference to our Job No. 04-8622 will help to expedite a
response to your inquiries.
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Casa Di Mare Remodel and Additions
Carlsbad, California
Respectfully submitted,
GEOTECHNICAL EXPLORATION, INC.
J iser, Project Coordinator
Senior Project Geologist
Jaime A. Cerrosl P.E.
Job No. 04-8622
Page 7
R.C.E. 34422/G.E. 2007
Senior Geotechnical Engineer
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