HomeMy WebLinkAboutRecommendations for Drilled Pier Design 12.11.17H-PVKUMAR 5020 Coung Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7989
Fax (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado
December ll,2Ol7
Giard Homes
Attn: Roger Giard
1431 AþortRoad
Rifle, Colorado 81650
(Ro ger. giard @ giardhomes.com)
REGEIVED
Subject:Recommendations for Drilled Pier Design, Proposed Main Residence, Big
Mountain Ranch, County F.load252, North of Rifle, Garfield County, Colorado
Dear Roger:
As requested, we are providing design recommendations for a drilled pier foundation to support
the main residence at the subject site. Our recommendations for the pier design are presented in
this report. The services are part of our agreement for construction observation and testing for
the project to Giard Homes dated December 5,2017 .
Background Information: We previously conducted a preliminary subsoil study at the site and
presented our findings in a report dated November 7, 2016, Project No. 16-7-485. We also
performed a subsurface study with drilled pier recommendations for the nearby Ranch House at
the site, submitting our findings in a report dated May 10,2017, Project No. 17-7-263.
Proposed Construction: The main residence will be a relatively large one and two story wood
frame structure over a partial walkout basement level. Ground floors will be structurally
supported over crawlspace in the basement and non-basement areas, and slab-on-grade in the
main level attached gra3e. Based on information provided in our preliminary subsoil study, it is
desired to found the building on drilled piers due to the movement sensitive type structure
planned. Cut depths will range from about 5 to 15 feet for the building excavation.
If building plans, location or grading are significantly different from those described above, we
should be notified to re-evaluate the recommendations contained in this report.
Subsurface Conditions: Boring 2 from the preliminary study was located at the current
proposed main residence site. The subsurface conditions encountered in the boring, below about
Geotechnical Engineering I Engineering Geology
Matedals Testing I Environmental
JAN 2 2 2017
GARFIELD COUNTY
COMMUNITY DEVELOPMElI¡Ï
Project No. 16-7-485.01
Giard Homes
December Il,20I7
Page2
IVz feet of topsoil, consisted of about SYz feet of stiff to very stiff, sandy clay with scattered
gravel underlain by medium hard to hard, weathered claystone that became less weathered and
very hard claystone/siltstone bedrock below a depth of about LLVzfeet down to the boring depth
of 20 feet. prilling in the bedrock with depth was difficult due to its hardness but auger drilling
refusal was not encountered to the drilled depth of about 8/z feet into the very hard bedrock. The
subsurface conditions are generally similar to the other three borings drilled nearty for the
preliminary study. No free water was encountered in Boring 2 or in the other three borings when1 r'
drilled in October 2016.
Discussion: The soils and weathered bedrock at the site typically possess a swell potential. A
drilled pier foundation can be used to penetrate the expansive materials to place the bottom of the
piers in a zone of relatively stable moisture conditions and make it possible to load the piers
sufficiently to resist uplift movements. In addition to their ability to reduce differential
movements caused by expansive materials, straight-shaft piers have the advantage of providing
relatively high supporting capacity and should experience a relatively small amount of
movement.
Drilled Pier Recommendations: The design and construction criteria presented below should
be observed for a straight-shaft drilled 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.
Piers should also be designed for a minimum dead load pressure of 12,000 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.
Uplift on the piers from structural loading can be resisted by utilizing 757o of the
allowable skin friction value plus an allowance for the weight of the pier.
The piers should be at least 12 inches in diameter and should penetrate at least
three pier diameters into the bedrock. A minimum penetration of 10 feet into the
bedrock and a minimum pier length of 20 feet are also recommended. The 20 feet
4)
2)
3)
H.P*KUÍVIAR
Proiect No. 1 6.7-485.01
Giard Homes
December lI,2Ol7
Page4
nearby preliminary study borings. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation and/or drilling is performed. If conditions
encountered during construction appear to be different from those described in this report, we
should be notified at once so re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications of the recommendations presented herein. We recommend on-site observation
of excavations, the pier drilling, and testing of structural fill by a representative of the
geotechnical engineer.
If you have any questions or need further assistance, please call our office.
Sincerely
I.I-P* KU
David A. Young, P
Rev. By: SLP
DAYlkac
cc Eggers Architecture - Don Eggers (don.eggers@esgersarchitecture.com)
KRM Consultants - Tim Hennum (tim@krmconsultants.com)
H-PVKUMAR
Proiect No. 1 6-7-485.0'l
Giard Homes
December ll,20l7
Page 3
minimum depth is measured from the ground surface near the top of pier or
adjacent excavation depth, whichever is greater.
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.
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 bedrock 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 utilizerl 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. We
recommend a slump in the range of 6 to g inches.
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 cobbles and possible cemented bedrock zones. Concrete should be
placed the same day the pier hole drilling is completed.
l0) Although free water was not encountered in the borings at the site, some seepage
in the pier holes may be encountered during drilling. If water cannot be removed
prior to placement of concrete, the tremie 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.
l1) 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.
l2) A representative ofthe geotechnical engineer should observe pier drilling
operations on a full-time basis.
Limitations: This report has been prepared in accordance with generally accepted geotechnical
engineering principles and practices in this area at this time. We make no warranty either
express or implied. The conclusions and recommendations submitted in this report are based
upon the data obtained from the exploratory boring drilled at the location and to the depth
indicated, the proposed type of construction and our experience in the area including the other
H-P*t4¡¡y¡4¡¡
Proiect No 16-7-485 O1
