HomeMy WebLinkAboutDriller Pier Foundation Recommendations 02.24.2017HPaKUMAR
Geotechnical Engineering 1 Engineering Geology
Materials Testing 1 Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkgienwood@kumarusa,com
February 24, 2017
Heath Cotter
752 Cactus Court
Rifle, Colorado 81650
heathcotter@gmail.com
Office Locations: Parker, Glenwood Springs, and Summit County, Colorado
RECEIVE[
DEC 1 9 2011
GARFIELD COUNTY
JMMUNITY DEVELOPMENT
Project No. 17-7-111
Subject: Driller Pier Foundation Recommendations, Proposed Residence, 899 Gage Road,
Garfield County, Colorado
Gentlemen:
As requested by Stephen Kesler, we are providing drilled pier recommendations as an addendum
to our previous subsoil study report, dated January 26, 2017, Project No. 17-7-111. We
understand that the previously recommended spread footing with minimum dead load pressure to
help mitigate the expansive soil condition at the property is not practical for the residence design.
PIER FOUNDATIONS
Straight -shaft piers should be drilled into the claystone/sandstone bedrock to support the
proposed structure.
The design and construction criteria presented below should be observed for a straight -shaft pier
foundation system:
1) The piers should be designed for an allowable end bearing pressure of 30,000 psf
and an allowable skin friction value of 3,000 psf for that portion of the pier in bedrock.
2) Piers should also be designed for a minimum dead load pressure of 10,000 psf
based on pier end area only. If the minimum, dead load requirement cannot be achieved, the pier
length should be extended beyond the minimum penetration to make up the dead load deficit.
This can be accomplished by assuming one-half the allowable skin friction value given above
acts in the direction to resist uplift. A minimum pier diameter of 12 inches is recommended.
3) Uplift on the piers from structural loading can be resisted by utilizing 75% of the
allowable skin friction value plus an allowance for the weight of the pier.
4) Piers should penetrate at least three pier diameters into the bedrock. A minimum
penetration of 5 feet into the bedrock and a minimum pier length of 20 feet are recommended.
5) Piers should be designed to resist lateral loads assuming a modulus of horizontal
subgrade reaction of 50 tcf in the clay soils and a modulus of horizontal subgrade reaction of 200
tcf in the bedrock. The modulus values given are for a long, 1 foot wide pier and must be
corrected for pier size.
Heath Cotter
February 24, 2017
Page 2
6) Piers should be reinforced their full length with at least one #5 reinforcing rod for
each 14 inches of pier perimeter to resist tension created by the swelling materials.
7) A 4 -inch void form should be provided beneath grade beams to prevent the
swelling soil and rock from exerting uplift forces on the grade beams and to concentrate pier
loadings. A void form should also be provided beneath pier caps.
8) Concrete utilized in the piers should be a fluid mix with sufficient slump so that
concrete will fill the void between the reinforcing steel and the pier hole.
9) Pier holes should be properly cleaned prior to the placement of concrete. The
drilling contractor should mobilize equipment of sufficient size to effectively drill through
possible cemented bedrock zones.
10) Although free water was not encountered in the borings drilled at the site, some
seepage in the pier holes may be encountered during drilling. Dewatering equipment may be
required to reduce water infiltration into the pier holes. If water cannot be removed prior to
placement of concrete, the tretnie method should be used after the hole has been cleaned of spoil.
In no case should concrete free fall into more than 3 inches of water.
11) Care should be taken to prevent the forming of mushroom -shaped tops of the
piers which can increase uplift force on the piers from swelling soils.
12) A representative of the geotechnical engineer should observe pier drilling
operations on a full-time basis.
FLOOR SLAB CONSIDERATIONS
Expansive soils could potentially heave the floor slab if subjected to increased moisture. We
should evaluate the subgrade for expansion potential at time of excavation and before pier
drilling. Sub -excavating and placement oft feet of road base could be used to help mitigate the
heave potential. Slip joints should also be used at the bottom of non -load bearing partition walls
to prevent upward loading of the structure in the event of floor slab heave.
If you have any questions or need further assistance, please call our office.
Sincerely,
H -P4- KUMAR
16222
Steven L. Pawlak, P. ', '7 /49.
SLP/kac �,sr): Pa'MIA. T r'
11. 'o1.
• y
cc Westar, Inc — Stephen Kesler (westararofnet)
H-PikKUMAR
Project No. 17-7-111