HomeMy WebLinkAbout19_SUITABILITY_OF_SOILS19.1Suitability of Soils
suitability of soils
section
19
19.2 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission
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19.3Suitability of Soils
SOILS INVESTIGATION : SUBDIVISION ROADS & POND SITES
BAR SLASH X RANCH, OLD STAGE ROAD, ASPEN, COLORADO
On April 10, 2003, Buckhorn Geotech, Inc. conducted an investigation of soil conditions
at proposed roadway routes and two pond sites on the Bar Slash X Ranch near Aspen,
Colorado. This work was performed at the request John Lifton. The purpose of this
investigation was to evaluate the soils for the construction of roads leading to house
sites and for the development of ponds. The investigation consisted of a site inspection,
excavation of 11 test pits, logging and testing of materials encountered, and analysis of
available data. The following report presents the findings of our investigation and our
geotechnical engineering recommendations for road design and pond development.
Test Pit Locations
A mini-excavator was used to excavate the eleven test pits on the property. As shown
on the attached Site Map, four test pits (TP#1, TP#2, TP#3, TP#4) were excavated in
the section of proposed roadway south of Old Stage Road, two test pits (TP#5, TP#6)
were located in an existing pond site north of the farm house, three test pits (TP#7,
TP#8, TP#9) were located in the section of proposed roadway north of Old Stage Road,
and two test pits (TP#10 and TP#11) were excavated in a second proposed pond site.
Mapped Soils Information
According to the Soil Survey of Aspen-Gypsum Area, Colorado (Soil Conservation
Service: 1992), soils on the property are mapped as Morval Loam (6 to 25% slopes)
and Uracca, moist-Mergel complex (6 to 12%, 12 to 25% and 25 to 65% slopes).
Morval Loam soils are deep, well drained soils formed on alluvial fans derived
dominantly from basalt. The main concerns regarding development on this soil are
shrink-swell potential, steep slopes, excessive fines, and it can contain large stones.
Uracca soils are deep, well drained soils formed in alluvium derived from mixed igneous
and metamorphic rocks, while Mergel soils are deep, well drained soils on terraces and
concave valley side slopes formed in alluvium and mixed glacial outwash. All strata of
Uracca-Mergel complex soils, including the ground surface, contain abundant gravel,
cobbles and boulders. Major limitations for development on these soils include the
abundance of large stones and boulders, tendency to erode, and steep slopes.
Our observations of the subsurface conditions were generally similar to those mapped
by the SCS. In the more low-lying areas tested, including the irrigated fields, we
generally found 2 to 3 feet of fine-grained (silt and clay) soils and organics overlying a
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19.4 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission
more granular, gravelly to cobbly substratum. In the more upland areas on low ridges
and terraces, we found gravelly to cobbly soils at the surface.
Soils on Roadway Corridors
As mentioned, four test pits (TP#1, TP#2, TP#3, TP#4) were excavated in the section of
proposed roadway south of Old Stage Road and three test pits (TP#7, TP#8, TP#9)
were located in the section of proposed roadway north of Old Stage Road, as shown on
the attached Site Map. For ease of discussion, we will refer to the southern section,
which is adjacent to the golf course and south of Old Stage Road, as the southern
roadway and we will refer to the section north of the farmhouse and Old Stage Road as
the northern roadway. The results of individual laboratory tests are attached and there
is also a summary table of these results presented for ease of reviewing the results.
The following photograph was taken looking south from near TP#4 at the road
alignment as it passes through the southern roadway area: The stakes mark the center
of the proposed roadway. This area consists of mostly irrigated pasture/hayfield (TP#1,
TP#2, TP#3), while some consists of modified upland areas (TP#4).
In the southern roadway area (TP#1, TP#2, TP#3 and TP#4), we generally found 1.7 to
3 feet of dark brown, moist, organic-enriched sandy to fine sandy silt with some clay and
10 to 20% subrounded to rounded gravel (Unified Classification: SM-ML). This soil is
relative soft and easy to excavate with a mini-excavator and has low to moderately low
density. The fine-grained soil was found to be non-plastic and has a moisture content of
17.0 to 18.5% (see Atterberg Limits results for sample “GS1”). A sample, collected at a
depth of 2 to 3 feet in TP#1, was found to be composed of 40.2% fines (mostly silt),
35.6% sand (mostly fine sand), and 24.2% gravel (see Sieve Analysis results for sample
“GS1”). A swell/consolidation test was performed on a sample taken at a depth of 1.75
feet in TP#1 and represents the dark brown sandy silt with some gravel (see
Swell/Consolidation test results for sample “FG”). Under a seating pressure of 100
pounds per square foot (psf) and left at its in-situ moisture content of 18.5%, the sample
compressed 0.3%. When inundated with water, the sample swelled 0.02%. Upon the
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19.5Suitability of Soils
addition of progressively increasing pressures to 2,000 psf, the sample consolidated a
total of 9.9%. The initial dry density of this sample was 90.6 pounds per cubic foot (pcf).
The amount of swelling pressure generated within the sample is negligible. The fine-
grained soil is underlain by brown to reddish-brown, silty sand with 40 to 80%
subrounded to subangular gravels, cobbles and occasional small boulders. This rocky
substratum is moderately dense to dense. No groundwater or bedrock was
encountered to a depth of 2.25 to 4 feet.
TP#4 was anomalous to the other soils in the southern roadway area because it is
located in a disturbed upland area. The profile for TP#4 indicates up to 0.5 feet of
topsoil underlain by reddish-brown, moist, silty sand and gravel with subrounded to
subangular cobbles and some small boulders (Unified Classification: GP-GM). This soil
appears to be local fill material and is moderately dense with 50 to 70% rock. A sample,
collected at a depth of 1 to 1.5 feet in TP#4, was found to be composed of 13.4% fines
(clay and silt), 36.2% sand, and 50.4% gravel (see Sieve Analysis results for sample
“GS2”). This coarse-grained soil was found to be non-plastic and has a moisture
content of 6.2% (see Atterberg Limits results for sample “GS2”).
The following photograph was taken looking north from near TP#8 in the northern
roadway area. This area consists of a terrace containing irrigated pasture (seen on the
left side of the photograph) and a lower terrace containing non-irrigated sagebrush
(seen on the right side of the photograph).
In the northern roadway area (TP#7, TP#8 and TP#9), we found 0.7 to 2 feet of organic-
enriched silty loam soil with roots and 10 to 20% gravel. This was underlain by brown to
red-brown, gravelly silt to silty fine sand that is variably clayey with 40 to 70% gravels
and cobbles. The soils in the upper, silty soil (0.7 to 2 feet) are loose with relatively low
density (Unified Classification: ML-SM). The deeper, coarser soils are rocky and
moderately dense to dense (Unified Classification: GM). Laboratory tests of the
shallower, silty soil, collected at a depth of 0.5 to 1.5 feet in TP#9, indicate that it has a
moisture content of 14.2 to 15.4%, is non-plastic, and is composed of 27.5% fines
(mostly silt), 44.3% sand, and 28.2% gravel (see Atterberg Limits and Sieve Analysis
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19.6 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission
results for sample “GS4”). A swell/consolidation test was performed on a sample taken
at a depth of 0.5 feet in TP#9 and represents the dark brown sandy silt with some gravel
(see Swell/Consolidation test results for sample “NO”). Under a seating pressure of 100
pounds per square foot (psf) and left at its in-situ moisture content of 15.4%, the sample
compressed 0.6%. When inundated with water, the sample swelled 0.01%. Upon the
addition of progressively increasing pressures to 2,000 psf, the sample consolidated a
total of 10.0%. The initial dry density of this sample was 88.2 pounds per cubic foot
(pcf). The amount of swelling pressure generated within the sample is negligible. The
deeper, coarser soil, sampled at a depth of 1 to 2 feet in TP#8 was found to be non-
plastic, has a moisture content of 20.7%, and is composed of 23.5% fines (silt and clay),
35.3% sand, and 41.2% gravel (see Atterberg Limits and Sieve Analysis results for
sample “GS3”).
In summary, the soil profile found in the upper 1.7 to 3 feet in the southern roadway
area and in the upper 0.7 to 2 feet in the northern roadway area is dominated by low
density, compressible, organic-enriched soils. Below this soil is a silty to slightly clayey
sand with gravel and cobbles in a dense matrix. No bedrock or groundwater was
encountered in any of these test pits. We also observed no signs of a fluctuating water
table in these two areas.
Roadway Design
Further laboratory evaluation of the native soils was performed for determining roadway
suitability. Standard Proctor and California Bearing Ratio (CBR) tests were performed
on 4 soil samples collected from TP#1, TP#4, TP#8 and TP#9. The results of these
tests are presented in the table below.
sample
#
location of
sample
Unified
Classifi-
cation
maximum
dry
density
(pcf)
optimum
moisture
content
(%)
CBR @0.1”
penetration
CBR @0.2”
penetration
BS1 TP#1@1-3’SM-ML 109.4 15.4 1.7 1.7
BS2 TP#4@1-
1.5’GP-GM 124.5 11.3 1.0 0.8
BS3 TP#8@1-2’GM 104.4 18.2 1.1 1.2
BS4 TP#9@0.5-
1.5’SM-ML 110.0 15.7 0.7 0.6
These results indicate relatively low maximum dry densities and extremely low CBR
values. This means that the native soils in the upper 2 to 3 feet are generally unsuitable
for construction of a roadway surface directly upon it. Consequently, these soils would
need to be stabilized or removed and replaced with more suitable and denser material.
The following roadway sections are recommended for this property:
�Remove all surface vegetation, debris and large rocks. Remove additional material
as needed to reach a uniform grade below finished roadway for selected option.
�Moisten and proof-compact subgrade.
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19.7Suitability of Soils
�Construct roadway section as either of the following:
Option 1:6” of ¾” roadbase (CDOT Class 6)
28” of 3”minus aggregate (CDOT Class 1)
Option 2:4” of ¾” roadbase (CDOT Class 6)
16” of 3”minus aggregate (CDOT Class 1)
Mirafi 600X Fabric
�All fill is to be placed in horizontal lifts not more than 6 inches deep with each lift
being compacted to at least 95% of Modified Proctor Density.
Soils in Pond Sites
As mentioned, two test pits (TP#5, TP#6) were located in an existing pond site north of
the farm house and two test pits (TP#10 and TP#11) were excavated in a second
proposed pond site, as shown on the attached Site Map. The first pond site appeared
to have been used as a small catchment area and was essentially bare and non-
irrigated. The following photograph, taken looking roughly southeast, shows the ground
cover and site conditions of the first pond site.
TP#6
TP#5
At TP#5 and TP#6, we found 0.3 to 1.3 feet of gray-brown to black, silty clay to clayey
silt with highly weathered shale fragments.This appeared to be used a pond liner. This
clayey soil was underlain by soft, low density, red-brown sandy to clayey silt with up to
10% gravels. In TP#5, the soils became rockier below a depth of 3 feet, but the matrix
was still dominated by the fine-grained silty interstitial soil. In TP#6, a sandy gravel with
silt stratum was encountered at a depth of 4.3 feet. No groundwater or bedrock was
encountered in either test pit to a depth of 4.5 feet.
Laboratory tests of the silty clay to clayey silt with shale fragments found in the upper
0.3 to 1.3 feet in TP#5 and TP#6 indicate a natural moisture content of 28.6% and its
Unified Classification is CL-ML. A swell/consolidation test was performed on a sample
of the shaly clay surface soil, taken at a depth of 0.5 feet in TP#6 (see
Swell/Consolidation test results for sample “DI”). Under a seating pressure of 100
pounds per square foot (psf) and left at its in-situ moisture content of 28.6%, the sample
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19.8 Bar Slash X Ranch LLC - Annexation and Stage Road PUD/Subdivision: Final Submission
compressed 1.0%. When inundated with water, the sample swelled 0.8%. Upon the
addition of progressively increasing pressures to 2,000 psf, the sample consolidated a
total of 3.8%. The initial dry density of this sample was 82.7 pounds per cubic foot (pcf).
The amount of swelling pressure generated within the sample was calculated to be 270
psf. A swell/consolidation test was performed on a sample of the deeper, clayey silt
with fine sand soil, taken at a depth of 3.3 feet in TP#6 (see Swell/Consolidation test
results for sample “DM”). Under a seating pressure of 100 pounds per square foot (psf)
and left at its in-situ moisture content of 18.4%, the sample compressed 0.3%. When
inundated with water, the sample swelled 0.2%. Upon the addition of progressively
increasing pressures to 2,000 psf, the sample consolidated a total of 1.9%. The initial
dry density of this sample was 103.1 pounds per cubic foot (pcf). The amount of
swelling pressure generated within the sample was calculated to be 210 psf.
The following photograph, taken looking northeast, shows the second pond site as
TP#10 was being excavated. The approximate position of TP#11 is also indicated on
the photograph.
TP#11
TP#10
At the second pond site (TP#10 and TP#11), which was in a grassy irrigated field, we
found 2.2 to 2.75 feet of dark brown, moist to wet, organically-enriched, silt with trace
clay and variable amounts of sand with 10 to 20% gravels and occasional cobbles. This
soil has low to moderate density, has a moisture content of 20.5 to 22.3%, and is
classified as SM-SC (see Atterberg Limits results for samples “GS5” and “GS6”). The
sample collected from a depth of 1 to 2 feet in TP#10 was found to be non-plastic, while
the sample collected from a depth of 1.7 to 2.2 feet in TP#11 was taken of a clayier
pocket and was found to have a Plasticity Index (PI) of 13. A soil with a PI of less than
15 is considered to have low potential for swelling when exposed to water and shrinking
when dried. Gradation Analysis of these two samples from TP#10 and TP#11 indicate
that they are composed of 34.2 to 36.0% fines (silty and clay), 39.1 to 47.4% sand, and
16.6 to 26.7% gravel (see Sieve Analysis results for samples “GS5” and “GS6”). The
fine-grained, organic-enriched silty soil is underlain at a depth of 2.2 to 2.75 feet by silty
sand with 50 to 70% gravels and cobbles. No groundwater or bedrock was
encountered to a depth of 3 to 3.5 feet.
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19.9Suitability of Soils
In summary, the soils at both pond sites below a shallow surface layer have low
plasticity and are dominated by more silt than clay. These soils are therefore not
impervious as a natural pond liner. A gravelly substratum was found at the pond sites.
At TP#5, the gravel was encountered at a depth of 3 feet, at TP#6 the gravel stratum
was found at a depth of 4.3 feet, and at the second pond site the gravel layer was found
at a depth of 2.2 to 2.75 feet. This gravelly substratum is a stable base but would serve
to transmit groundwater and percolating surface water.
Pond Development Recommendations
The upper soil zone (0.3 to 1.3 feet at Pond 1 and 2.2 to 2.75 feet at Pond 2) should be
removed from both the embankment and reservoir areas of both ponds. This material
can be used for landscape purposes and topsoil, but cannot be used for pond
construction. The gravelly substratum can be used for general embankment
construction when placed and compacted as directed by the design engineer. Because
of the soil permeability and potential for erosion, the entire reservoir of both ponds and
their spillways should be lined with impermeable material. This material (plastic or
rubberized sheet) must be installed per manufacturer’s requirements so that all joints
and penetrations are completely sealed and the liner is protected from environmental
degradation and damage from wildlife and livestock.
Closure
Thank you for the opportunity to perform this soil investigation for you. When you are
ready to construct the roadways and ponds, we can assist with design and materials
testing. If you require any of these services or have any questions regarding this report,
please do not hesitate to give us a call.
Respectfully Submitted,
August 14, 2003
Thomas E. Griepentrog, P.E.
TEG/lbh
Enclosures: Site Map, Soil Logs, Summary Table of Laboratory Test Results, Swell/
Consolidation graphs, Sieve Analysis results, Atterberg Limits results, Proctor graphs,
CBR results
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