HomeMy WebLinkAboutHEARTHFIRE (HOFFMAN) - OVERALL DEVELOPMENT PLAN - 31-95 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTBasements, Drainage Systems and Slabs on Grade
It is our opinion that basement construction is feasible at the site.
The finished lower basement slabs should be placed a minimum of three
(3) feet above existing ground water or adjusted ground water levels
after site grading has been completed. In addition, all lower, level
structures placed in or within three (3) feet of the bedrock stratum
should be provided with a drainage system designed to intercept and
collect potential perched ground water.
Subgrade below slabs on grade should be prepared in accordance
with the recommendations discussed in the "Site Grading and Utilities"
section of this report. Slabs on grade should be designed as, floating
slabs.
GENERAL COMMENTS
It should be noted that this was a preliminary investigation and
that the bearing capacities recommended in this report are based on
preliminary tests. Due to variations in soil conditions and swelling
pressures encountered at the site, it is recommended that additional
test borings be made prior to construction. Samples obtained from the
borings should be tested in the laboratory to provide a basis for
evaluating subsurface conditions.
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the pier. The piers are supported by the bedrock stratum partially
through end bearing and partially through skin friction. It is
recommended that all piers have minimum ten (10) foot lengths and that
they be drilled a minimum of three (3) feet into the firm bedrock
stratum. Based on preliminary test results, piers founded at the above
levels may be designed for a maximum allowable end bearing pressure
between fifteen thousand (15,000) to thirty thousand (30,000) pounds
per square foot. It is estimated that a skin friction between one
thousand five hundred (1500) to three thousand (3000) pounds per
square foot will be developed for that portion of the pier embedded in
the firm bedrock stratum. To counteract swelling pressures which will
develop if the subsoils become wetted, all piers should be designed for a
minimum dead load three thousand (3000) to seven thousand five hundred
(7500) pounds per square foot. Where this minimum dead load
requirement cannot be satisfied, it is recommended that skin friction
from additional embedment into the firm bedrock be used to resist uplift.
To help provide the required skin friction, the sides of the pier drilled
into the bedrock stratum should be roughened. All piers should be
reinforced their full length to resist tensile stresses created by swelling
pressures acting on the pier. It is recommended that all grade beams
have a minimum four (4) inch void between the bottom of the beam and
the soil below. Where the bedrock stratum is encountered below the
ground water, temporary casing of the drill holes may be required. Use
of a rock bit and adequately sized drill rig may be required to penetrate
the lower, denser portion of the bedrock stratum and the lenses of
highly cemented sandstone.
It is strongly recommended that the geotechnical engineer be
present during the drilling operations to (1) identify the firm bedrock
stratum, (2) assure that proper penetration is obtained into the sound
bedrock stratum, (3) ascertain that all drill holes are thoroughly
roughened, cleaned and dewatered prior to placement of any foundation
concrete, (4) check all drill holes to assure that they are plumb and of
the proper diameter, and (5) ensure proper placement of concrete and
reinforcement.
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Stripping, grubbing, subgrade preparation, and fill and backfill
placement should be accomplished under continuous observation of the
geotechnical engineer. Field density tests should be taken daily in the
compacted subgrade, fill, and backfill under the direction of the
geotechnical engineer.
Foundations
In view of the loads transmitted by the proposed residential and
commercial development and the soil conditions encountered at the site, it
is recommended that structures founded in the upper clays a minimum of
two (2) feet above the bedrock stratum be supported by
conventional -type spread footings and/or grade beams. All exterior
footings should be placed a minimum of thirty (30) inches below finished
grade for frost protection. The structural fill should be constructed in
accordance with the recommendations discussed in the "Site Grading and
Utilities" section of this report. The structural integrity of the fill as
well as the identification and undisturbed nature of the soil should be
verified by the geotechnical engineer prior to placement of any
foundation concrete. Based on preliminary test results, footings and/or
grade beams founded at the above levels a minimum of two (2) feet above
the bedrock stratum may be designed for a maximum allowable bearing
capacity between one thousand five hundred (1500) to three thousand
five hundred (3500) pounds per square foot (dead load plus maximum
live load). To counteract swelling pressures which will develop if the
subsoils become wetted, all footings and/or grade beams should be
designed for a minimum dead load between five hundred (500) to one
thousand two hundred fifty (1250) pounds per square foot.
Commercial structures exhibiting heavy concentrated loads and other
structures founded in or within two (2) feet of the bedrock stratum
should be supported by a drilled pier foundation system. Using this
type of foundation system, the structure is supported by piers drilled
into the bedrock stratum and structural grade beams spanning the piers.
Piers should be straight -shaft and should be drilled within plumb
tolerances of one and one-half percent (1-1 /2%) relative to the length of
IM
backhoe having a minimum one and one-half cubic yard bucket may be
needed to excavate the firm bedrock. The extremely dense bedrock may
require blasting as discussed in the "Geology" section of this report.
Bedrock used as fill should be broken into pieces less than six (6)
inches in diameter. Proper placement of the bedrock as fill may be
difficult, and a disc or other mixing equipment may be needed to obtain
uniform moisture and proper compaction. The bedrock should be used in
open and planted areas or in the lower portion of fill below paved and
building areas.
In computing earthwork quantities, an estimated shrinkage factor
of eighteen percent (18%) to twenty-three percent (23%) may be used for
the on -site soils compacted to the above -recommended density. A
shrinkage factor of fifteen percent (15%) to twenty percent (20%) may be
used for the bedrock used as compacted fill.
Utility trenches dug four (4) feet or more into the upper soils and
weathered bedrock should be excavated on stable and safe slopes, or the
excavations should be properly shored. The firm bedrock may be
excavated on near -vertical slopes. Excavation of the firm bedrock may
require the use of heavy-duty construction equipment equivalent to a
backhoe having a minimum one and one-half cubic yard bucket and/or
possibly blasting. Where utilities are excavated below ground water,
dewatering will be needed during placement of pipe and backfilling for
proper construction. All piping should be adequately bedded for proper
load distribution.
Backfill placed in utility trenches in open and planted areas
should be compacted in uniform lifts at optimum moisture to at least
ninety percent (90%) of Standard Proctor Density ASTM D 698-78 the full
depth of the trench. The upper four (4) feet of backfill placed in
utility trenches under roadways and paved areas should be compacted at
or near optimum moisture to at least ninety-five percent (95%) of
Standard Proctor Density ASTM D 693-78, and the lower portion of these
trenches should be compacted to at least ninety percent (90%) of
Standard Proctor Density ASTM D 698-78. Addition of moisture to
and/or drying of the subsoils may be needed for proper compaction.
Proper placement of the bedrock as backfill may be difficult.
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u
The site is underlain by the Cretaceous Pierre Formation. The
Pierre Shale is not known to contain economic deposits of coal, limestone
or quarry rock. Sand and gravel was not encountered at the site, and
economic deposits of these materials are not anticipated. Extensive
uranium minerals are not common in the Pierre Formation, and
background levels of radioactivity are anticipated to be low .in this area.
However, the possibility does exist that abnormally high radiation rates
may exist on the property. A detailed study of radon gas or radiation
hazards at the site is beyond the scope of this report. It is suggested
that a more detailed study be made prior to construction to more
accurately determine if radiation hazards exist at the site.
Site Grading and Utilities
Specifications pertaining to site grading are included below and in
Appendix C of this report. It is recommended that the upper six (6)
inches of topsoil below building, filled and paved areas be stripped and
stockpiled for reuse in planted areas. The upper sir. (6) inches of the
subgrade below building, paved and filled areas should be scarified and
recompacted two percent (2%). wet of optimum moisture to at least ninety
percent (90%) of Standard Proctor Density ASTM D 698-78. (See
Appendix C.) Where this subgrade compaction cannot be obtained due
to saturated or unstable subsoil conditions, the subgrade should be
stabilized by use of granular pit -run material or by the use of
geotextiles. It is anticipated that stabilization of the wet marshy area
adjacent to the lake will be required if this area is filled. Finished
subgrade below building, filled and paved areas should be placed a
minimum of three (3) feet above existing ground water. Fill should
consist of the on -site soils or imported material approved by the
geotechnical engineer. Fill should be placed in uniform six (6) to eight
(8) inch lifts and mechanically compacted two percent (2%) wet of
optimum moisture to at least ninety-five percent (95%) of Standard
Proctor Density ASTM D 698-78.
Bedrock encountered at the site may be used as fill material.
Heavy-duty. construction equipment equivalent to a D-8 tractor or a
associated downwarping of the Denver Basin to the east. Relatively flat
uplands and broad valleys characterize the present-day topography of
the Colorado Piedmont in this region. The site is underlain by alluvial
and residual soils of Pleistocene and/or Recent Age. These soils are
underlain by bedrock of the Cretaceous Pierre Formation.
Sandstone bedrock outcrops were located along the southeast side of
Richard's Lake at the southwest corner of the property. Bedrock
underlies the central and high point of the property at depths three and
one-half (3-1/2) to ten and one-half (10-1/2) feet below the surface, and
in the northwest portion of the property at depths thirteen (13) to
areater than fifteen (15) feet below the surface. It is estimated that the
bedrock underlies the remainder of the site at depths of approximately
fifteen (15) to twenty-five (25) feet below the surface. A depth to
bedrock contour map is included in Appendix A. The regional dip of
the Pierre Shale in this area is slight and in an easterly direction.
Seismic activity in the area is anticipated to be low; therefore, from a
structural standpoint, the property should be relatively stable. It is
anticipated that the weathered rock and the majority of the firm bedrock
may be excavated by conventional heavy-duty construction equipment
equivalent to a D-8 tractor and ripper or a large backhoe having a
minimum yard and one-half bucket. However, the sandstone becomes
dense at the lower depths, and a layer of densely cemented sandstone
was encountered at the surface of the bedrock stratum in Boring 9.
This densely cemented sandstone and dense sandstone at depth may
require the use of special heavy-duty excavation equipment or possibly
blasting for excavation. Due to the relatively flat to gently -rolling
nature of the site, geologic hazards due to mass movement caused by
gravity, such as landslides, mudflows, rock falls, etc., are not
anticipated. With proper site grading around structures and proper
drainage for streets and drive areas, erosional problems at the site
should be minimal. Portions of the wet area located adjacent to the
property in the northwest part of the site may lie within the development
area. It is recommended that construction not be placed within these
wet areas without proper draining and filling.
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(4) Ground Water: At the time of the investigation, free ground
water was encountered in Borings 14, 15, 16, 18 and 19 at
depths eight and one-half (8-1 /2) to thirteen and one-half
(13-1 /2) feet below the surface. No free ground water was
encountered in the remaining borings drilled at the site to the
depths explored. Water levels in this area are subject to
change due to seasonal variations, fluctuations in the level of
Richard's Lake Reservoir No. 6 and the small pond adjacent to
the site and irrigation demands on and/or adjacent to the
property. In addition, where ground water is not already
encountered on top of the bedrock stratum, surface water may
percolate through the upper subsoils and become trapped on
the relatively impervious bedrock stratum, forming a perched
ground water condition.
RECOMMENDATIONS AND DISCUSSION
It is our understanding that the site is to be developed for
residential and commercial use. The majority of the area will consist of
single-family homes and multi -family apartments. Commercial development
is planned in the southeast corner of the site. A school site is planned
for the property as well. The site will be served by water and
sanitation districts.
Geology
The proposed site is located within the Colorado Piedmont section of
the Great Plains physiographic province. The Colorado Piedmont, formed
during Late Tertiary and Early Quaternary time (approximately sixty-five
` million (65,000,000) years ago), is a broad, erosional trench which
separates the Southern Rocky Mountains from the High Plains.
Structurally, the property lies along the western flank of the Denver
Basin. During the Late Mesozoic and Early Cenozoic Periods
(approximately seventy million (70,000,000) years ago), intense tectonic
activity occurred, causing the uplifting of the Front Range and the
t
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properties of the soils encountered. Moisture contents, dry unit
weights, unconfined compressive strengths, water soluble sulfates, swell
potentials, and the Atterberg limits were determined. A summary of the
test results is included in Appendix B.
SOIL AND GROUND WATER CONDITIONS
The soil profile at the site consists of strata of materials arranged
in different combinations. In order of increasing depths, they are as
follows:
(1) Silty Topsoil: The majority of the area is overlain by a six
(6) inch layer of silty topsoil. The topsoil has been
penetrated by root growth and organic matter and should not
be used as a bearing soil or as a fill and/or backfill material.
(2) Sandy Silty Clay: This stratum underlies the topsoil and
extends to the depths explored and/or the bedrock below.
The silty clay is plastic, contains varying amounts of sand and
exhibits generally moderate bearing characteristics in its dry
to near -saturated in situ condition. When wetted, the clay
stratum exhibits slight to moderate swell potential.
(3) Sandstone-Siltstone-Claystone Bedrock: The bedrock was
encountered in Borings 2, 8, 9, 10, 12 through 16 and 18 at
depths three and one-half (3-1/2) to thirteen (13) feet below
the surface. The upper one-half (1/2) to four (4) feet of the
bedrock is highly weathered; however, the underlying
interbedded sandstone, siltstone and claystone is firm to dense
and exhibits high to very high bearing characteristics. A thin
layer of dense, highly cemented sandstone was encountered in
the upper portion of the bedrock stratum in Boring 9. When
wetted, the claystone-siltstone portion of the bedrock exhibits
slight to high swell potential.
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SITE LOCATION AND DESCRIPTION
The site is located north of Richard's Lake Road west of County
Road 11 adjacent to Richard's Lake Reservoir No. 6, northeast of Fort
Collins, Colorado. More particularly, the site is described as a tract of
land situate in Section 30, Township 8 North, Range 68 West of the
Sixth P.M., Larimer County, Colorado.
The site is located north of the Fort Collins Country Club along the
north side of Richard's Lake Road on the west side of County Road 11.
The property is bordered on the northeast by a residential development
and on the northwest by Douglas Road. The majority of the site
consists of fallow farm land vegetated with grain stubble. The property
is bordered on the south and west by Richard's Lake Reservoir No. 6.
Several gravel and dirt roads traverse the site. These roads are access
roads for existing oil and gas well pumps. The site is dominated by a
small knoll or hill located in the south-central portion of the site, and
the property, in general, exhibits good positive drainage from the crest
of this hill to the southwest, southeast, northeast and northwest. The
area adjacent to Richard's Lake is vegetated with native grasses, yucca
and cactus. Low bedrock outcrops are located along the shore of
Richard's Lake. A small pump house is located along the east side of
Richard's Lake, and an above -ground water line extends northward from
the lake through the property. A pump station is located adjacent to
the extreme northeast portion of the site. A small low area is located
in the southwest . portion of the site and is vegetated with trees. A
small lake and adjacent wet or marshy area are located in the northwest
portion of the area, but the lake is not included within the property.
The remains of a farm shed and an old homestead are located on the top
of the ridge, near the south end of the property. Several large dead
cottonwood trees which hav+ut down are located in this area.
LABORATORY TESTS AND EVALUATION
Samples obtained from the test borings were subjected to testing in
the laboratory to provide a sound basis for evaluating the physical
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REPORT
OF A PRELIMINARY
GEOTECHNICAL INVESTIGATION
SCOPE
This report presents the results of a preliminary geotechnical
evaluation prepared for the proposed residential and commercial
development located adjacent to Richard's Lake west of County Road 11,
northeast of Fort Collins, Colorado. The investigation included test
borings and laboratory testing of samples obtained from these borings.
The objectives of this study were to determine the geologic
characteristics at the site and to evaluate the subsurface conditions at
the site relative to the proposed construction.
SITE EXPLORATION
The field exploration, carried out on November 7 and 12, 1986,
consisted of drilling, logging, and sampling nineteen (19) test borings.
The locations of the test borings are shown on the Test Boring Location.
Plan and Geologic Map included in Appendix A of this report. Boring
logs prepared from the field logs are included in Appendix A. These
logs show soils encountered, location of sampling, and ground water at
the time of the investigation. A summary of. the test results is included
in Appendix B.
The borings were advanced with a four -inch diameter, continuous -
type, power -flight auger drill. During the drilling operations, a
geotechnical engineer from Empire Laboratories, Inc. was present and
made continuous observations of the soils encountered. A visual
evaluation of the site was made by an engineering geologist of Empire
Laboratories, Inc. on November 13, 1986.
no
0
Empire Laboratories, Inc.
GEOTECHNICAL ENGINEERING 8 MATERIALS TESTING
P.O. Box 503 • (303)484-0359
301 No. Howes • Fort Collins. Colorado 80522
November 20, 1986
Hoffman Development
P. 0. Box "Gil
Fort Collins, Colorado 80522
Attention: Mr. Max Hoffman
Gentlemen:
We are pleased to submit our Report of a Preliminary Geotechnical
Investigation prepared for the proposed commercial and residential
development located on Richard's Lake Road, northeast of Fort Collins,
Colorado. The accompanying report presents our findings in the
subsurface and our recommendations based upon these findings.
Very truly yours,
EMPIRE LABOR TO IES, INC
it R. She od
E G I ; t
F Gf PROfcss
2575
Senor ng(neering eo ogls
Reviewed by:
Chester C. Smith, P.E.
President
cic
cc: Mr. Rodger Prinsloe
P.O. Box 1135
Longmont, Colorado 80502
(303)776-3921
i
Branch Offices
P.O. Box 1744
Greeley. Colorado 60632
(303) 351.0460
Member of Consulting Engineers Council
P.O. Box 10076
Cheyenne, Wyoming 82003
(307) 632.9224
TABLE OF CONTENTS
Table of Contents ..............................................
Letter of Transmittal ..........................................
Report.........................................................
Appendix A ....................................................
A-1
Test Boring Location Plan
and Geologic Map ..................
A-2
Key to Borings ...............................................
A-3
Log of Borings ...............................................
A-4
Depth to Bedrock Contour
Map ...............................
A-9
Appendix B....................................................
B-1
Summary of Test Results .....................................
B-2
Appendix C....................................................
C-1
Appendix D....................................................
D-1
SoilsMap ....................................................
D-2
Soils Descriptions ...........................................
D-3
REPORT
OF A PRELIMINARY
GEOTECHNICAL INVESTIGATION
FOR
RICHARD'S LAKE P.U.D.
LARIMER COUNTY, COLORADO
HOFFMAN DEVELOPMENT
FORT COLLINS, COLORADO
PROJECT NO. 6762-86
m
EMPIRE LABORATORIES, INC.
301 NORTH HOWES STREET
FORT COLLINS, COLORADO 80521
000 MERRICK
Engineers & Architects
p