HomeMy WebLinkAboutB18-0275 soils.pdf H-P ti KU mAR5020 County Road 154
Glenwood Springs,CO 81601
Geotechnical Engineering I Engineering Geology Phone:(970)945-7988
Materials Testing I Environmental Fax:(970)945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver(HQ),Parker, Colorado Springs, Fort Collins, Glenwood Springs,Summit County,Colorado
September 10, 2018
Paragon Homes Denver
Attn: Ed Venerable
6460 S. Quebec Street#400 O z-7
Englewood, Colorado 80111 U1111- �OQ
ed@paragonhomesdenver.com
Project No. 18-7-555
Subject: Observation of Excavation,Proposed New Eastern Half Duplex Addition,Lot 27,
Block 7,Vail Village 1st Filing, 175 Forest Road, Vail, Colorado
Dear Ed:
As requested, a representative of H-P/Kumar observed the excavation at the subject site on
September 5,2018 to evaluate the soils exposed for foundation support. The findings of our
observations and recommendations for the foundation design are presented in this report. The
services were performed in accordance with our agreement for professional engineering services
to Paragon Homes Denver dated September 5,2018.
The eastern half of the duplex residence has been removed and is being re-built. The new
structure will be one and two story wood frame construction over a walkout basement level
daylighting to the north. The ground floors will be slab-on-grade. The building has been
designed to be supported on spread footings assuming an allowable soil bearing pressure of
2,000 psf. The foundation design was based on an excavation observation report for the rebuild
of the western half of the duplex by H-P Geotech dated July 22, 2008,Job No. 108 400A. We
have been provided a copy of that report.
At the time of our site visit, the foundation excavation which was essentially complete had been
cut in multiple levels from about 21/2 to 13 feet below the adjacent ground surface. The
excavation had partially undermined the adjacent building foundation and site retaining walls.
The soils exposed in the bottom of the excavation consisted primarily of medium dense to dense,
silty sand and gravel with cobbles. In the deeper cut area in the southeast corner, fractured
sandstone bedrock was exposed at subgrade. At the shallower cuts for the middle portion step
down and in the northeast corner down and of the excavation was a shallow depth of fill over the
natural granular soils that we recommended be removed. The soils were too rocky and the
bedrock to hard and fractured to obtain undisturbed samples for swell-consolidation testing. The
results of a gradation analysis performed on a disturbed bulk sample of the natural granular soils
(minus 3 inch fraction) obtained from the excavation are presented on Figure 1. There was some
minor groundwater seepage at the bottom of the middle bench area step down in the excavation
grade. The soils were generally moist to very moist in the seepage area and the bedrock was
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Paragon Homes Denver
September 10, 2018
Page 2
slightly moist. The structural engineer has made recommendations addressing the existing
building foundation undermining in a letter dated September 5, 2018.
Considering the conditions exposed in the excavation and the nature of the proposed
construction, spread footings placed on the undisturbed natural granular soil and bedrock
designed for an allowable bearing pressure of 2,000 psf can be used for support of the proposed
residence. There could be some differential foundation settlement due to the variable bearing
conditions and assumed variable loadings. Footings should be a minimum width of 16 inches for
continuous walls and 2 feet for columns. All existing fill and any loose disturbed materials in
footing areas should be removed and the bearing level extended down to the undisturbed natural
granular soils or bedrock, and the subgrade compacted. Exterior footings should be provided
with adequate soil cover above their bearing elevations for frost protection. Forty-eight inches of
frost cover is typically used in the Vail area. Continuous foundation walls should be reinforced
top and bottom to span local anomalies such as by assuming an unsupported length of at least
12 feet. Foundation walls acting as retaining structures should also be designed to resist a lateral
earth pressure based on an equivalent fluid unit weight of at least 50 pcf for on-site granular soil,
excluding topsoil and oversized (minus 6 inch)rocks, as backfill. A perimeter foundation and
lateral floor slab underdrains as needed should be provided to prevent temporary buildup of
hydrostatic pressure behind the basement walls and prevent wetting of the lower level. The
underdrain system should include at least 6 inches of free draining gravel below basement level
floor slab. Structural fill placed within floor slab areas can consist of the on-site sand and gravel
soils compacted to at least 95% of standard Proctor density(SPD) at a moisture content near
optimum. Backfill placed around the structure should be compacted to at least 90% SPD(95%
in pavement areas) and the surface graded to prevent ponding within at least 10 feet of the
building. Some settlement of deeper foundation wall backfill should be expected even though
the fill is compacted to project specifications which could result in distress to facilities
constructed on the backfill. Landscape that requires regular heavy irrigation, such as sod, and
sprinkler heads should not be located within 5 feet of the foundation.
The recommendations submitted in this letter are based on our observation of the materials
exposed within the foundation excavation and do not include subsurface exploration to evaluate
the subsurface conditions within the loaded depth of foundation influence. This study is based
on the assumption that materials beneath the footings have equal or better support than those
exposed. The risk of foundation movement may be greater than indicated in this report because
of possible variations in the subsurface conditions. In order to reveal the nature and extent of
variations in the subsurface conditions below the excavation, drilling would be required. It is
possible the data obtained by subsurface exploration could change the recommendations
contained in this letter. Our services do not include determining the presence,prevention or
possibility of mold or other biological contaminants (MOBC) developing in the future. If the
H-Pk
Project No. 18-7-555
Paragon Homes Denver
September 10, 2018
Page 3
client is concerned about MOBC, then a professional in this special field of practice should be
consulted.
If you have any questions or need further assistance,please call our office.
Sincerely,
H-P KI M ' t `` 00888so®®sri
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David A. Young,P.1 ��O Cr`o ig0.
DAY/kac `10 , 0JVAIOA
attachment Figure 1, Gradation Test Results
cc: Jay Nobrega(jay@vailconstructiongroup.com)
Mountain Home Construction—Jim Meehan(jmeehan-mhc@comcast.net)
H-PtKUMAR
Project No. 18-7-555