HomeMy WebLinkAboutEngineer's Letter - Deep Foundation Support Recommendations 11.06.2019K±A
Kumar & Awn Maas, Illi,.
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Company
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office Locations: Denver (AQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
November 6, 2019
Springborn Greenhouses
Attn: Charles Barr
2000 Broadway, Apt. 203
San Francisco, California 94115
chart es (rucharlesbarr.n et
RECEIVED
MAY li u "ITN
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
>ts 1 Ass arli ras
Project No. 19-7-561
Subject: Deep Foundation Support Recommendations, Proposed Greenhouse and Tech
Building, Dry Hollow and Colorado River Roads, Garfield County, Colorado
Dear Mr. Barr:
As requested by SGM, we are providing additional recommendations for the building foundation
support at the subject site. We previously conducted a subsoil study for design of foundations at
the site and presented our findings in a report dated October 14, 2019, Project No. 19-7-561.
The building perimeter will be supported by continuous walls and the interior by isolated
columns. Column loadings range between about 5 to 10 kips for the greenhouse and 20 to 55
kips for the tech building. Considerable grading is proposed to flatten the building area with a
cut depth: up to about 5 feet at the southeast corner of the tech building and about 8 feet of fill
depth at the northwest corner of the greenhouse. At the design building pad grade, soil depths
above dense gravel are estimated at about 12 to 22 feet.
DRILLED PIERS — TECH BUILDING
We recommend straight -shaft piers drilled into the underlying dense gravel soil be used for
foundation support of the tech building. The design and construction criteria presented below
should be observed for a straight -shaft, drilled pier foundation system.
1) The piers should have a minimum diameter of 12 inches and be designed for an allowable
end bearing pressure of 12,000 psf and a skin friction of 1,000 psf for that portion of the
pier embedded in the sand and gravel. Pier penetration through the upper clay
soils/structural fill should be neglected in the skin friction calculations. Settlement of
piers bearing in the dense gravel soils under sustained loading is expected to be minor,
less than r/2 inch and essentially occur during construction. Down -drag from negative
skin friction in the silty clay soils can be neglected provided structural fill is placed well
in advance of the drilled pier installation.
Springborn Greenhouses
November 6, 2019
Page 2
2) All piers should have a total embedment length of 10 feet and a minimum penetration
into the gravel soil of 2 feet.
3) Lateral load analysis of piers can be performed with the software analysis package
LPILE. We believe this method of analysis is appropriate for piers with a pier
embedment length to diameter ratio of 5 or greater. Suggested criteria for LPILE
analysis are presented in the following table:
Soil Type
Silty Clay
Sandy Gravel
Recommended p -y
Curve Model
Stiff Clay w/o Free Water
(Rees)
Sand
Density (pci)
0.065
0.075
Friction Angle (degrees)
-
35
k — Above Water (pci)
30
225
Undrained Cohesion, c (psf)
500
-
Strain Factor, E50
0.010
-
4) No reduction in axial load resistance is needed for piers spaced a minimum of 3 pier
diameters (center -to -center). At one diameter (piers touching), the skin friction reduction
factor for both piles should be 0.5. Interpolation can be used between one and three
diameters.
5) For lateral loading, no reduction is needed of piers in-line with the direction of lateral
loads with a minimum spacing of 6 pier diameters (center to center). If a closer spacing
is required, the modulus of subgrade reaction for the initial and trailing piles should be
reduced according to the following table. Reductions to the modulus of subgrade
reaction can be accomplished in LPILE by inputting the appropriate modification factors
for p -y curves.
Pile Spacing
(Center to Center)
3 Pile
Diameters
4 Pile
Diameters
5 Pile
Diameters
6 Pile
Diameters or
More
Lead Row
0.7
0.85
1.0
1.0
2nd Row
0.5
0.65
0.85
1.0
3rd and Higher
Rows
0.35
0.5
0.7
1.0
6) The pier holes should be properly cleaned prior to placement of concrete. The natural
silty clay soils are generally medium stiff which indicates that casing of the holes should
not be required. Some caving and difficult drilling may be experienced in the bearing
soils due to gravels and cobbles. Placing concrete in the pier hole immediately after
drilling is recommended.
Kumar & Associates, Inc. 0
Project No. 19.7-561
Springborn Greenhouses
November 6, 2019
Page 3
7) The pier drilling contractor should mobilize equipment of sufficient size to achieve the
design pier sizes and depths.
8) A representative of the geotechnical engineer should observe pier drilling operations on a
full-time basis.
HELICAL PIERS - GREENHOUSE
We recommend piers that extend down into the underlying dense gravel soil be used for
foundation support of the greenhouse. The piers should derive their downward load capacity
from the dense. gravel soil and the upper clay soils/structural fill should be neglected in the load
capacity. Down -drag from negative skin friction in the silty clay soils due to structural fill
placement can be neglected for a smooth shaft such as helical piers. Rough shafts such as drilled
and grouted micro -piles may have reduced load capacity assuming a friction angle acting on the
shaft of about 18 degrees for the depth of compressible natural clay soils below the structural fill.
The downward load capacity of the helical piers will depend on the pier shaft and helix sections
and could vary between about 20 to 40 kips. Settlement of piers bearing in the dense gravel soils
under sustained loading is expected to be minor, less than V2 inch and essentially occur during
construction. Battered piers are normally used for lateral load capacity of the foundation. Piers
should be spaced at least 3 feet from center to center to avoid reduction from group action.
Chemical testing was not performed on the onsite soils. Although it has been our experience that
the pier capacity and steel section will greatly exceed the design value and the required design
life of the pier system will not be compromised, the pier contractor should verify that the piers
have adequate corrosion protection for the site. Exterior grade beams and pier caps should have
a minimum depth of 3 feet for frost protection and void form below them is not needed. We
should observe installation of the helical piers on a full time basis.
If you have any questions or need further assistance, please call our office.
Sincerely,
Kumar & Associates, Inc.
Steven L. Pawla
Rev. by: DEH
SLP/kac
cc: SGM, Inc
SGM, Inc
— Jerry Burgess (ierryb(cpsgm-inc.com)
— Mindy Nastal (mindyn(ilsgm-inc.com)
Kumar & Associates, Inc. e
Project No. 19.7-561