HomeMy WebLinkAboutGeotechnical Investigation.pdfGeotechnical Investigation
Lots 20, 27, 28, 42 and 48
Pinyon Mesa
Garfield County, Colorado
Project No. 27-362
January 15, 2008
Prepared for:
Skyview Developments
Attn: Angela Tourney
1024 Centre Avenue
Suite 100, C & D
Fort Collins, Colorado 80526
Prepared by:
Yeh and Associates, Inc.
170 Mel Ray Road
Glenwood Springs, Colorado 81601
Phone: 970-384-1500
Fax: 970-384-1501
Lots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
Table of Contents
Page
PURPOSE AND SCOPE OF STUDY 1
PROPOSED CONSTRUCTION 1
SITE CONDITIONS 1
SUBSURFACE CONDITIONS 1
SITE DEVELOPMENT 2
FOUNDATIONS RECOMMENDATIONS 3
BELOW -GRADE WALLS 4
SLABS -ON -GRADE 4
SUBSURFACE DRAINAGE 4
SURFACE DRAINAGE 5
LIMITATIONS 5
Figure No.
Site Location 1
Test Hole Locations 2
Appendices No.
Test Hole Logs A
Laboratory Test Results B
tots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
PURPOSE AND SCOPE OF STUDY
This report presents the results of our geotechnical investigation for five proposed
single-family residences on Lots 20, 27, 28, 42 and 48 at Pinyon Mesa Subdivision in Garfield
County, Colorado. The location of the subject sites are presented on Figures 1 and 2. The
purpose of the study was to develop recommendations for design and construction of single-
family residences on the subject lots.
The field investigation consisted of drilling one exploratory test hole within the building
envelope on each lot. Samples of the subsurface materials were collected and returned to our
laboratory for testing. The results of our field and laboratory programs were evaluated to
develop recommendations for building foundations and surface and subsurface drainage. This
report summarizes the information obtained during our investigation and presents our
recommendations based on the proposed construction and subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residences will be one to two-story wood framed structures with or without
basements. For non -basement structures, foundation excavations will likely be on the order of 3
to 4 feet deep. For basement structures, foundation excavations will likely be on the order of 8
to 10 feet deep. We anticipate the main level floor will be structurally supported. Foundation
loads will likely be on the order of 1,000 to 3,000 pounds per linear foot. The lots included in
this investigation are presented on Figure 2.
SITE CONDITIONS
Pinyon Mesa is located south of Glenwood Springs, Colorado and more particular, about
one mile up and on the south side of County Road 114 from the intersection of Highway 82.
The subject lots were vacant within a developed subdivision. Lot 27 had an existing stock pile
covering the majority of the building envelope at the time of investigation. Vacant lots are
located around the subject lots. Lots 20, 27, 28, 42 and 48 were situated on the eastern half of
the subdivision. Streets were paved and utilities have been installed. The site sloped down to
the west at grades of about 4 to 13 percent, becoming steeper around Lot 42. The building
envelopes on Lots 20, 42 and 48 were partially cleared of scrub and the entire site was covered
with snow at the time of this investigation.
SUBSURFACE CONDITIONS
An exploratory test hole was drilled within the building envelope on each lot. The test
holes were drilled to depths between 25 and 35 feet. Modified California and split spoon
1
Lots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
samples were collected at specified depths. California samples were collected using a 2 -inch
I.D. sampler driven into the subsoils with a 140 -pound hammer falling 30 inches. The number of
blows needed to drive the sampler constitutes the blow count. For example, a value on the log
(Appendix A) of 24/12 indicates the sampler was driven 12 inches with 24 blows of the hammer.
The blow count can be used as a relatively measure of material stiffness or density. Split spoon
samples were obtained in the same manner, but with a 1.5 -inch I.D. The collected samples
were transported to our laboratory where they were examined and classified. Laboratory tests
included moisture content, dry density, swell/consolidation, grain size analysis and Atterberg
limit testing.
Generally, the subsoils encountered in our test holes consisted of silt and/or clay. Lot 20
terminated on very dense gravel, cobble and boulders. Lot 27 had an existing stock pile
covering the majority of the building envelope. The fill (stock pile) consisted of clay materials
and was approximately 8 feet in height. Lot 42 had a silty sand layer between the clay and clay -
silt layers. Bedrock was not encountered to the maximum depths explored. The silt and clay
samples tested had 54 to 78 percent fines (passing NO. 200 sieve) with liquid limits on the silt
being non -liquid and non- plastic. The silty sand had 38 percent fines. Five silt and clay
samples generally exhibited low consolidation (-0.2 to -1.8 percent) with one sample exhibiting
moderate consolidation of -3.5 percent when wetted under an applied pressure of 1,000 psf.
The laboratory test results are presented in Appendix B and are summarized in the Summary of
Laboratory Test Results table.
Groundwater was not encountered during drilling, and the subsoils were slightly moist to
moist. We believe variations in ground water conditions can occur. The magnitude of the
variation will be largely dependent upon local irrigation practices, the duration and intensity of
precipitation, site grading changes, and the surface and subsurface drainage characteristics of
the surrounding area.
SITE DEVELOPMENT
We understand cuts of about 3 to 4 feet from existing grade may be necessary to reach
foundation level for crawlspace construction and 8 to 10 feet from existing grade for basement
construction, with the exception of Lot 27, where we assumed the 8 feet of stock piled material
would be removed prior to construction. Fill placement should be minor. Areas to receive fill
should be stripped of vegetation, organic soils and debris. The on-site soils free of organic
matter, debris and rocks larger than 6 inches can be used in fills. Fill should be placed in thin,
2
tots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
loose lifts of 8 inches thick or less, moisture conditioned to 0 to 3 percent above optimum
moisture content and compacted to at least 95 percent of maximum standard Proctor dry
density (ASTM D 698). Placement and compaction of fill should be observed and tested by a
geotechnical engineer.
FOUNDATIONS RECOMMENDATIONS
Based on the results of our subsurface investigation, we judge these lots as low
collapsible risk overall and therefore believe the residences can be supported on footing
foundations placed on the natural soils or properly compacted fill. Based on our experience and
laboratory test results, we believe there is a low risk of consolidation and therefore a low risk of
foundation movement provided the following recommendations are followed. The following
design and construction details should be observed for spread footings placed on the natural
soils or properly compacted fill.
1. Foundations should be constructed on undisturbed, natural soils. Loose, disturbed
soils encountered at foundation level should be removed and replaced with
compacted fill or the foundation should be extended to undisturbed soils.
2. Footing foundations can be designed for a maximum allowable soil pressure of 2,000
psf. The design pressure may be increased by 1/3 or as allowed by local code,
when considering total loads that include wind or seismic conditions.
3. Continuous wall footings should have a minimum width of at least 18 inches.
Foundation pads for isolated columns should have a minimum dimension of 24
inches.
4. Resistance to sliding at the bottom of the footing can be calculated based on a
coefficient of friction of 0.30. Passive pressure against the side of the footing can
also be considered for the sliding resistance if it is properly compacted. Passive
pressure can be estimated based on an equivalent fluid density of 300 pcf for a level
backfill.
5. Grade beams and foundation walls should be reinforced to span undisclosed loose
or soft soil areas. We recommend reinforcement sufficient to span an unsupported
distance of at least 10 feet.
6. The soils below exterior footings or exterior edges of slabs should be protected from
freezing. We recommend the bottom of footings be constructed at least 3 feet below
finished exterior grade or as required by local municipal code.
7. All foundation excavations should be observed by a representative of the
geotechnical engineer prior to placement of concrete.
3
tots 20, 27, 28, 42 and 48
Pinyon Mesa
Project No. 27-362
BELOW -GRADE WALLS
Crawlspaces or basements are planned below the main level for these residences.
Foundation walls that extend below grade should be designed for lateral earth pressure where
backfill is not present to the same level on both sides of the wall. For walls that can deflect or
rotate about 0.5 to 1 percent of the wall height, the wall can be designed for "active" earth
pressure conditions. For a very rigid wall where negligible deflection can occur, an "at -rest"
lateral earth pressure condition can be used. Typically, below -grade walls for residences can
rotate under normal design loads, and this deflection results in acceptable performance.
If on-site soils are used as backfill, we recommend using an equivalent fluid density of
45 pcf for design of below grade walls. This value assumes that some minor cracking is
acceptable. If negligible deflections are desired, a higher equivalent fluid density of 60 pcf
should be used for design. These equivalent densities do not account for sloping backfill,
surcharges or hydrostatic pressure.
SLABS -ON -GRADE
Based on our investigation, garage and driveway slabs will likely be underlain by silt and
clay soils. We believe there is a low risk of poor slab -on -grade performance due to collapsible
soils. The on-site soils, free of organics and debris, are suitable to support lightly loaded slabs -
on -grade. Slabs should be separated from all load bearing walls and columns with expansion
joints that allow vertical movement. Control joints should be used to reduce damage from
shrinkage cracking. All fill below slabs should be compacted to at least 95 percent of maximum
standard Proctor dry density within 2 percent of optimum moisture content.
SUBSURFACE DRAINAGE
Groundwater was not encountered during our investigation. Surface water typically
flows through permeable wall backfill and collects at the backfill and natural soil interface
resulting in saturated foundation soils and/or wet crawlspace conditions. To reduce water
accumulation outside foundation walls and reduce moist crawlspace and basement conditions,
a foundation drain could be installed around the exterior of the foundation walls. Drains could
be installed in crawlspace areas after completion of construction, if groundwater develops. If
groundwater or highly saturated soils are encountered during foundation excavation on these
lots, we should be contacted for additional recommendations.
4
'Lots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
A typical drain should consist of a 4 -inch diameter, perforated pipe encased in free
draining gravel. The gravel should be % to 1.5—inch washed rock with less than 5 percent fines.
The drain should be provided with a gravity discharge such as a sump pit where water can be
removed by pumping or be daylighted. The pipe should be sloped at a minimum of 1 percent
and should be installed 12 to 18 inches away from and parallel to the footing foundation. The
bottom of the pipe should be at least 2 inches below the bottom of footing level at the high point.
Crawlspace areas should also be provided with adequate ventilation.
SURFACE DRAINAGE
Surface drainage is crucial to the performance of foundations and flatwork. We
recommend the ground surface surrounding the building be sloped to drain away from the
structure. We recommend a slope of at least 6 inches in the first 10 feet for landscape areas
and a minimum slope of 1 percent for paved areas. Backfill around foundations should be
moisture conditioned and compacted as recommended in the SITE DEVELOPMENT section.
Roof downspouts and drains should discharge beyond the backfill area.
LIMITATIONS
The analyses and recommendations presented in this report are based upon our data
obtained from the borings at the indicated locations, field observations, laboratory testing, our
understanding of the proposed construction and other information discussed in this report. It is
possible that subsurface conditions may vary between or beyond the points explored. The
nature and extent of such variations may not become evident until construction. If variations
appear, we should be contacted immediately so we can review our report in light of the
variations and provide supplemental recommendations as necessary. We should also review
the report if the scope of the proposed construction, including the proposed loads, finished
elevations or structure locations, change from those described in this report. The conclusions
and recommendations contained in this report shall not be considered valid unless Yeh and
Associates reviews the changes and either verifies or modifies the conclusions of this report in
writing.
The scope of services for this project did not include, specifically or by implication, any
environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or identification or
prevention of pollutants, hazardous materials or conditions or biological conditions. If the owner
is concerned about the potential for such contamination, conditions or pollution, other studies
should be undertaken.
5
Lots 20, 27, 28, 42 and 48
Pinyon Mesa Project No. 27-362
The report was prepared in substantial accordance with the generally accepted
standards of practice for geotechnical engineering as exist in the site area at the time of our
investigation. No warranties, express or implied, are intended or made. The recommendations
in this report are based on the assumption that Yeh and Associates will conduct an adequate
program of construction testing and observation to evaluate compliance with our
recommendations.
YEH AND ASSOCIATES, INC. Reviewed by:
Keith E. Asay
Staff Engineer
6
Richard D. Johnson, P.E.
Senior Geotechnical Engineer
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Project 27-362
Site Location
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APPENDIX A
TEST HOLE LOGS
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GEOTECHNICAL ENGINEERING CONSULTANTS
Project: Pinyon Mesa
Project Number: 27-362
Legend for Symbols Used on Test Hole Logs
Sample Types
Modified California Sampler. The symbol 24/12 indicates that 24 blows from a 140 pound
hammer falling 30 inches was used to drive 2 -inch I.D. sampler 12 inches.
.Split Spoon Sampler. The symbol 50/3 indicates that 50 blows from a 140 pound hammer
falling 30 inches was used to drive 1.5 -inch I.D. sampler 3 inches.
Other Symbols
1 Indicates practical drill rig refusal.
Soil Lithology
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1
Clay, silty, slightly sandy, soft, moist to very moist, brown (Fill).
Silt, slightly sandy, very stiff, slightly moist to moist, brown (ML).
Clay, silty, slightly sandy, stiff to very stiff, slightly moist to moist, brown (CL).
Gravel, cobble, small boulders, very dense, slightly moist, brown (GP).
Clay -Silt, slightly sandy, silty, very stiff, slightly moist, brown (CL_ML).
Sand, slightly silty, slightly gravelly, medium dense, slightly moist, brown (SM).
NOTES:
1. The test hale was drilled on December 13, 2007 using 4 -inch continuous
flight auger.
2. Groundwater was not encountered during this investigation.
3. Subsoil descriptions are subject to explanations within the report.
Figure A-2
APPENDIX B
LABORATORY TEST RESULTS
Consolidation( -)/Swell(+), %
Consolidation( -)/Swell(+),
2.0
0.0
-2.0
-4.0
-6.0
Graph 1
WATER ADDED
•
0.1
2.0
0.0
-2.0
-4.0
-6.0
10
Applied Normal Pressure, ksf
100
Graph 2
•
WATER ADDED
0.1
Applied Normal Pressure, ksf
10
100
Graph
Number
Boring
Number
Depth
(ft)
Natural Dry
Density
(pcf)
Moisture
Content
(%)
Swell(+) /
Consolidation(-)
(%)
Soil Description
SWELL /
CONSOLIDATION
GRAPH
1
Lot 20
4
93
7.0
-3.5
Silt, sandy (ML)
Drawn By: KEA
2
Lot 28
9
94
7.9
Clay, sandy (CL)
Checked By: RDJ
Job No:
27-362
Project Name:
Pinyon Mesa
YEH & ASSOCIATES, INC.
Figure B-1
Consolidation( -)/Swell(+),
Consolidation( -)/Swell(+),
2.0
0.0
-2.0
-4.0
-6.0
Graph 1r
WATER ADDED
01
2.0
0.0
-2.0
-4.0
-6.0
Applied Normal Pressure, ksf
10 100
Graph
2
WATER ADDED
•
01
Applied Normal Pressure, ksf
10 100
Graph
Number
Boring
Number
Depth
(ft)
Natural Dry
Density
(pct)
Moisture
Content
(%)
Swell(+) /
Consolidation(-)
(%)
Soil Description
SWELL /
CONSOLIDATION
GRAPH
1
Lot 42
9
90
8.1
-0.2
Clay, sandy (CL)
Drawn By: KEA
2
Lot 48
4
91
7.7
-1.8
Clay, sandy (CL)
Checked By: RDJ
Job No:
27-362
Project Name:
Pinyon Mesa
YEH & ASSOCIATES, INC.
Figure B-2
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Consolidation( -)/Swell(+), °A
-6.0
Graph 1 r
WATER ADDED
•
0.1
2.0
0.0
-2.0
-4.0
-6.0
Applied Normal Pressure, ksf
10 100
Graph 2I
01
Applied Normal Pressure, ksf
10 100
Graph
Number
Boring
Number
Depth
(ft)
Natural Dry
Density
(pct)
Moisture
Content
(%)
Swell(+) /
Consolidation(-)
(%)
Soil Description
SWELL /
CONSOLIDATION
GRAPH
1
Lot 48
14
91
8.0
-0.9
Clay, sandy (CL)
Drawn By: KEA
2
Checked By: RDJ
Job No:
27-362
Project Name:
Pinyon Mesa
YEH & ASSOCIATES, INC.
Figure B-3
Summary of Laboratory Test Results
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Soil Description
'Silt, sandy (ML)
'Silt, sandy (ML)
(Clay, sandy (CL)
Clay, sandy (CL)
Clay, sandy (CL)
Clay, sandy (CL)
Sand, silty (SM)
Clay, sandy (CL)
Clay, sandy (CL)
1
Clay, sandy (CL)
Swell (+) /
Consolidation (-)
under 1,000 psf
(%)
-3.5
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