HomeMy WebLinkAboutPrelim Geotech Engineering Investigation 10.07.24ffi GTL ITHOMPSON
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October 7,2024
KBBS Holdings, LLC
248 Skipper Drive
Carbondale, CO 81623
Attention: Kara Byrne
Subject:Prelim inary Geotechnical Engineering I nvestigation
Lookout Mountain Wedding & Events Center
Parcel No. 21 851 2402034
Garfield County, Colorado
CTLIT Project No. GS06903.000-115
CTLIThompson, lnc. (CTLIT) completed a preliminary geotechnicalengineering
investigation for Lookout Mountain Wedding & Events Center on Parcel 218512402034 in
Garfield County, Colorado. The geotechnical engineering opinions and preliminary
recommendations in this letter are to provide the client with information to assist with planning
for the proposed buildings. A design-level geotechnical engineering investigation should be
performed to finalize recommendations for the buildings after civil engineering and architectural
plans are available. The investigation would include excavating several additional exploratory
pits at specific building locations.
Site Gonditions
The Lookout Mountain Wedding & Events Center is proposed on Parcel No.
218512402034, which is located on the east end of Lookout Mountain in Garfield County,
Colorado. The property is a 4O-acre parcel that is accessed from Lookout Mountain Road
(County Road 120) on the south flank of the mountain. A vicinity map with the location of the
site is included as Figure 1. The propefty is on a localized, topographic ridge that trends down to
the southeast. The county road passes through the property with a switch-back turn that
contours around the ridge. An aerial photograph of the property is shown on Figure 2.
An old gravel pit is in the southeast paft of the property. The pit is accessed by an
existing, unimproved driveway from the county road. An abandoned wellhead is west of the
gravel pit near the end of the driveway. Vegetation on the property is predominantly thick
Gambel Oak with undergrowth of Big Sagebrush and native grasses. Ground surface elevation
within the proposed development areas range between 8440 feet and 8400 feet. Naturalground
along the ridge within the area of the proposed buildings generally slopes down to the southeast
at grades between 5 and 15 percent.
Proposed Construction
CTLIT was provided with a conceptual master plan for the Lookout Mountain Wedding &
Events Center that was prepared by The Land Studio (dated August 7,2A24). A cluster of ten,
tiny homes is proposed on the ridge in the northwest part of the property. We understand the
tiny homes will be one-story, pre-manufactured buildings placed on cast-in-place foundations. A
preliminary plan provided to us (dated September 17,2024) indicates that a concrete patio and
CTllThompson, lnc.
Denver, Fort Collins, Colorado Sprinqs, Glenwood Sprinqs, Pueblo, Summit Countv - Colorado
Chevenne, Vllyoming and Bozeman, Montana
stainuay will be constructed at the entrance of each tiny home. An events building is proposed
near the abandoned wellhead. Plans have not been developed for the building. Development
will include construction of a new access road and parking areas. The proposed development is
shown on Figure 3.
Site Geoloqv
As part of our preliminary geotechnical engineering investigation, we reviewed geologic
mapping by the Colorado Geological Survey (CGS) titled, "Geologic Map of the Glenwood
Springs Quadrangle, Garfield County, Colorado", by Kirkham, Streufert, Cappa, Shaw, Allen
and Schroeder (dated 2009). The map indicates the localized, topographic ridge in the area of
the proposed development is underlain by bedrock of the Miocene Epoch (Neogene Period) that
is derived from flows of basalt, basaltic andesite, and trachybasalt.
The geologic mapping indicates the basaltic bedrock is underlain by bedrock of the
Maroon Formation (Permian and Pennsylvanian Periods). The Maroon Formation is mainly
reddish-brown sandstone, conglomerate, mudstone, siltstone, and claystone. Subsurface
information from our exploratory pits indicates the basaltic bedrock is not present above the
Maroon Formation bedrock in the proposed development areas at the site. The map indicates
the bedding in the Maroon Formation dips down to the southwest at an angle of about 23
degrees. This is generally unfavorable from a large-scale, slope stability standpoint.
Several faults are below and adjacent to the subject parcel. The faults are generally
parallelwith the localized, topographic ridge. We believe these faults are generally inactive. The
gravel pit on the property is indicated on the geologic map. The map does not indicate
significant hazards on the property.
Preliminarv Subsurface lnvestiqation
For our preliminary geotechnical engineering investigation, subsurface conditions at the
site were investigated by observing excavation of four exploratory pits. Two exploratory pits (TP-
1 and TP-2) were excavated in the proposed area of the tiny homes and two pits (TP-3 and TP-
4) were excavated near the proposed events building. The pits were excavated with a trackhoe
on September 11 ,2024 at the approximate locations shown on the attached Figure 3.
Exploratory excavation operations were directed by our engineer who logged conditions
encountered in the pits and obtained representative samples of the soils. Summary logs of
subsurface conditions found in our exploratory pits are included as Figure 4.
Subsoils found in our exploratory pits, TP-1 and TP-2, generally consisted of about 6 to
12 lnches of topsoil, underlain by 4.5 to 7 feet clayey sand and sandy clay that is interbedded
with clayey gravel. Sandstone bedrock was encountered at depths of 5 to I feet. The hardness
of the bedrock in TP-1 and TP-2 made excavation deeper than 6 inches into the sandstone not
praeiicai.
Subsoils found in our exploratory pits, TP-3 and TP-4, generally consisted of about 6
inches of topsoil or 2.5 feet of fill and 6 inches of clayey sand, underlain by 6 to 8 feet of silty
gravel. Sandstone bedrock was encountered at a depth of 8 feet in TP-3. The hardness of the
bedrock made excavation deeper than 6 inches into the sandstone not practical.
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING & EVENTS CENTER
CTLIT PROJECT NO. GS06903.000.115
Page 2 of 6
Observations during excavation indicated the clayey sand is medium dense, the sandy
clay is stiff, the clayey gravel and silty gravel are dense, and the sandstone is very hard.
Groundwater was not encountered in our exploratory pits at the time of excavation. The pits
were backfilled immediately after exploratory excavation operations were completed.
Soil samples were returned to our laboratory where typical samples were selected for
pertinent testing. Laboratory testing included gradation analysis and Atterberg limits. Gradation
test results are shown on Figures 5 and 6. Results of laboratory testing are summarized in
Table l.
Earthwork
Excavations
Our subsurface information indicates that excavations in the overburden soils at the site
can be made with conventional, heavy-duty excavation equipment. Excavations into the bedrock
will be ditficult and could require a hoe ram attachment on a trackhoe.
From a "trench safety" standpoint, sides of excavations must be sloped or retained to
meet local, state, and federal safety regulations. The overburden soils at this site will likely
classify as Type B and Type C soils, based on OSHA standards governing excavations. The
sides of temporary excavations deeper than 5 feet, and above groundwater, should be no
steeper than 1 to 1 (horizontal to vertical) in Type B soils and 1.5 to 1 in Type C soils.
Groundwater seepage into excavations can cause slumps and sloughing and the need for flatter
slopes. Contractors are responsible for site safety and providing and maintaining safe and
stable excavations. The contractor's qualified person should identify the soils encountered in
excavations and ensure that OSHA standards are met.
Our experience in similar geology and topography in the area indicates the upper soils
may become saturated during snowmelt in spring and early summer months. Groundwater
seepage could be encountered in foundation excavations during these months.
Suhexcavation and Structural Fill
Footing foundations will be appropriate for buildings constructed at the site. A slab-on-
grade floor is a reasonable alternative for the events building. However, the natural soils at this
site are relatively heterogeneous. To create more uniform support conditions and reduce the
potential for differential building movement, the soils below footings and floor slabs will need to
be subexcavated to a depth of about 2 feet and replaced with densely compacted, structural fill
We judge that the structural fill can consist of the excavated soils, provided they are free
of rocks larger than 4 inches, organics, and debris. The potential for differential movement, and
recommendations for mitigation, will need to be addressed for each building during a design-
level geotechnical investigation.
Foundation Wall Backfill
Proper placement and compaction of foundation wall backfill is important to reduce
infiltration of surface water and settlement from consolidation of backfill soils. This is especially
important for backfill areas that will support concrete patio slabs and stairs. The soils excavated
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING & EVENTS CENTER
crLlr pRoJEcr No. Gs06903.000-115
Page 3 of 6
from the site can likely be used as backfill, provided they are free of rocks larger than about 4-
inches in diameter, organics, and debris.
Anticipated Buildinq Foundations
The natural soils at the site are relatively heterogeneous. We judge these soils are
generally suitable for support of footing foundations, but variable support conditions exist. To
create more uniform support conditions and reduce the potential for differential building
movement, we anticipate that subexcavation of the soils to a depth of about 2 feet below
footlngs wlll be needed, The subexcavated soils will need to be replaced with densely
compacted, structuralfill as discussed in the Subexcavation and Structural Fill section.
We expect an allowable bearing pressure of less than 3,000 psf for footings on the
structuralfill. A design-level geotechnical engineering investigation should be performed to
develop recommendations for the buildings after civil engineering and architectural plans are
developed. This would include excavating additional exploratory pits at specific building
locations.
Slab-On-G rade Gonstruction
Plans indicate that a concrete patio and stainnray are planned for each tiny home. Plans
for the events building have not been developed. The soils at the site are heterogenous with
variable slab support characteristics. We anticipate that concrete slabs and structures can be
utilized, provided the soils below these elements are subexcavated to a depth of about 2 feet.
The subexcavated soils will need to be replaced with densely compacted, structural fill.
l^ ^^^^.^l ..,^ ^-^^^l ^l^L ^^ ^-^)^ ^^-^4-..^ri^- ..,:ll -^J^'- ,.,^ll ^r tL:^ ^ir^ --^..:l^lilr vgrrsrari vvs E^yrtut Drdu-L,lt-Ytdutt rrvltDt!urrurJl! wt!! Ptttt\,ilt vvcll dt ltilD uil.gr Ptuvtuttu
proper design and construction methods are utilized. Recommendations for slab-on-grade
construction will need to be developed for the buildings after civil engineering and architectural
plans are developed. Additional subsurface investigation will be required as part of the design-
level geotechnical engineering investigation.
Subsurface Drainaqe
Our experience in similar geology and topography in the area indicates the upper soils
become saturated during snowmelt in spring and early summer months. The extent and rate of
seepage will fluctuate throughout the year, and local perched groundwater can develop during
times of heavy precipitation or seasonal snowmelt and runoff. Frozen ground during spring
runoff can also create a perched condition. These conditions can cause wetting of foundation
soils, hytJrostatic pressures on below-grade walls, and wet or moist conditions in below-grade
areas, such as crawl spaces.
To mitigate pi'obiems associaied wiih groundwaier seepage arrd irrfiitration of sut'raee
water into foundation backfill soils, foundation drains should be anticipated around the
perimeters of below-grade areas of the buildings. The drains should discharge via positive
gravity outlets or lead to sump pits where water can be removed by pumping. Additionaldetails
for the drain systems will need to be developed for the buildings during the design-level
geotechnical engineering investigation.
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING & EVENTS CENTER
CTLIT PROJECT NO. GS05903.000-115
Page 4 of 6
Surface Drainaqe
Surface drainage is critical to the performance of building foundations, floor slabs, and
structural components. lnfiltration of surface water into the backfill soils adjacent to a building
can increase the potential for movement of the structure. Site grading adjacent to the buildings
should be designed and constructed to rapidly convey surface water away from the buildings in
alldirections.
We recommend the buildings be provided with gutters and downspouts. Roof
downspouts should discharge well beyond the limits of all foundation backfill soils. Landscaping
should be carefully designed and maintained to minimize irrigation near foundation walls.
Concrete
Concrete in contact with soil can be subject to sulfate attack. Our subsurface information
in the area of the site indicates the soils have water-soluble sulfate concentrations of less than
0.10 percent. As indicated on the table below, ACI 318-'l 9 states the sulfate exposure class is
Not Applicable or S0.
SULFATE EXPOSURE CLASSES PER ACI 318-19
Exposure Glasses
Water.Sciluble :Srilfate
SoilA
(o/;\
(SO+) in
Not Applicable SO < 0.10
Moderate S1 0.10 to 0.20
Severe s2 0.20 to 2.00
Very Severe S3 > 2.00
A) Percent sulfate by mass in soil determined by ASTM C1580
For this level of sulfate concentration, ACI 318-19 Code Requirements indicate there are
no cement type requirements for sulfate resistance.
Limitations
Our exploratory pits provide a preliminary characterization of subsurface conditions in
the areas of the proposed tiny homes and events building. Variations in the subsurface
conditions not indicated by the pits will occur.
The geotechnical engineering opinions and preliminary recommendations in this letter
are to provide the client with information to assist with planning for the proposed buildings. A
design-level geotechnical engineering investigation should be performed to finalize
recommendations for the buildings after civil engineering and architectural plans are available.
The investigation would include excavating several additional exploratory pits at specific building
locations.
We believe this preliminary geotechnical engineering investigation was conducted with
that level of skill and care ordinarily used by geotechnical engineers practicing under similar
conditions in the locality of this project. No warranty, express or implied, is made, We are
available to discuss the contents of this letter. Please contact us if you have questions or need
additional information.
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING & EVENTS CENTER
GTLIT PROJECT NO. GS06903.000-115
Page 5 of 6
D
o38298
ffi
CTLITHOMPSON,
mes D. Ke
ncipal
Attachments: Figure 1 Map
Figure 2 - Aerial Photograph
Figure 3 - Proposed Development
Figure 4 - Summary Logs of Exploratory Pits
Figures 5 and 6 - Gradation Test Results
Table I - Summary of Laboratory Testing
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING & EVENTS CENTER
crLlT PROJECT NO. cS06903.000-115
Reviewed by:
Division Ma
Page 6 of 6
ffi
0 1000 2000
SCALE: 1'- 2000'
KBBS HOLDINGS, LLO
LOOKOUT MOUNTAIN WEDDINO ATTJD EVENTE OENTEN
cTL/f PROJECT NO. GSO6903.OOO-1 15
SATELLITE IMAGE FROM MAXAR
(CoPYRTGHT 2024)
NOTE:
Vicinity
Map Flg. 1
LEGEN D:
TP-1 APPROXIMATE LOCATION OFI EXPLORATORY PIT
NOTE:
tr
o 150 J00
SCALE: 1'- 300'
KBBS HOLDINGS, LLC
L@KOUT MOUNTAIN WEDDINGAND EVENTS CENTER
oTL/T PROJECT NO. GSO5903.OOO-1 15
PARCEL BOUNDARY
SATELLITE IMAGE FROM GOOGLE EARTH
(DATED JULY 19, 2023)
Aerial
Photograph
TP_1
TP-2
TP_5
TP-4
(cR 120)
Flg. 2
LEGEND:
TP-1 APPROXIMATE LOCATION OF
I EXPLOMTORY PIT
ffi
NOTE:
0 150 300
SCALE: 1'- 300'
KtsBS Holdlnga,lJ,.C
LOCIKOUT MOI'NTAN WEDDINO AhID EVENTS CENTEN
oTUT PROJECT NO. GSO6903.OO0-115
BASE DRAWING BY THE I.AND STUDIO, INC.
DATED AUGUST 7, 2024.
Proposed
Development Fb.
TP-1
TP*2
TP-3
TP.4
g
TP-1 TP.2
10
15
TOPSOIL, SAND, CLAYEY, MOIST, DARK
BROWN.
FILL, GRAVEL, SAND, COBBLES, CI.AYEY,
MEDIUM DENSE, MOIST, DARK BROWN,
CI.AYEY SAND AND SANDY CTAY. MEDIUM
DENSE OR STIFF, MOIST, RUST, BROWN.
(sc, cL)
GMVEL, CLAYEY. SANDSTONE COBBLES
AND BOULDERS, ANGULAR, DENSE, MOIST
(GC).
TP-3 TP-4
INDICATES BULK SAMPLE OBTAINED FROM
EXCAVATED SOILS.
=ffi
00
5
Fullrl
TL
IF
o-l!o
Llllull!
IFI
uJ(]
10
15
LEGEND:
F
T INDICATES DEEPER EXCAVATION NOT
PRACTICAL DUE TO HARD BEDROCK.
NOTES:
1. THE PITS WERE EXCAVATED WITH A TMCKHOE ON
SEPTEMBER 11,2024. THE PITS WERE BACKFILLED
IMMEDIATELY AFTER EXPLOMTORY EXCAVATION
OPEMTIONS WERE COMPLETED.
2. GROUNDWATER WAS NOT ENCOUNTERED IN THE
EXPLOMTORY PITS DURING THIS INVESTIGATION
3. THESE LOGS ARE SUBJECT TO THE EXPLANATIONS,
LIMITATIONS. AND CONCLUSIONS CONTAINED
IN THIS LETTER.
Summary Logs
of Exploratory Pits
E
w
GRAVEL, SILTY, WEATHERED
CONGLOMEMTE, DENSE, MOIST. TAN.
YELLOW, BROWN. (GM)
SANDSTONE BEDROCK, VERY HARD,
MOIST. RUST.
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN IA,EDOING & EVENTS CENTER
CTLIT PROJECT NO. GS06903.003-1.15
7aEl1t
.[
FIG.4
ffi
SANDS GRAVEL
MEDIUM COARS FINE COARSE COBBLEScLAY (PIASTTC) TO SILT (NON-PIASTTC)
FINE
ANALYSIS SIEVE ANALYSIS
2ro(t,o
s60
F2
850u
UJo40
30
20
10
0
0
10
20
30
40
50
60
70
80
90
100
--
-**
oulz
FI
FzL,otulI
90
80
'16 '10'8
100
e.52 re.r 36.1 76.2 12151!00.00'l 0.002 .005 .009 .019 .037
5'6" I'
U,S. STANDARD
'100 '50'40'30
CLEAR SOUARE OPENINGS
3/8' 3t4" 1vl', 3'
TIME READINGS
60 MlN. 19 MrN. 4 MlN. 1 MlN. ',200
.o74 .149 ,297 .590 1.19 2.0 2.X8 4.76
0.42
OIAMETER OF PARTICLE IN MILLIMETERS
25 HR. 7 HR.
45 MlN. 15 MlN.
Somple
From
of cr_Ry,SANDY (CL)GRAVEL
stLT & CLAY 66 Yo
PlAsrcrry truoei
*
GRAVEL
15% SAND 19%
o/o
Vo
TP-1AT7FEET
SILT & CLAY 30
PI.ASTICITY INDEX -
LTQUTD LrMrT 28
31 SAND
LIQUID LIMIT
Gradation
Test Results
5
Somple of sRtrto, cLAyEy (sc)
FTom TP.2 AT 4 FEET
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING AND EVENTS CENTER
PROJECT NO. GS06903.000-115
o/o
o/o
39 oA
o/o
o/o
SANDS GRAVEL
COARSE COBBLESFINEMEDIUMCOARSFINEcrAY (PLASTTC) TO SrLT (NON-P|-ASTTC)
SIEVE ANALYSISANALYSIS
10
20
30
40
50
60
70
80
90
100
.07.1 ,149 .297 .590 1.19 2.0 2.38 4.76 9.52 19.1 36.1 76.2 127,-200---'o.42---- 152
DIAMETER OF PARTICLE IN MILLIMETERS
=
=
-----7 a--
gtoz6U'
f60
Fz
850e.
UJ
%o
oulzaFul&Fz
UJotui
o.
8'
0
90
80
60 MtN. 19 MtN. 4 MlN. 1 MlN. '200
100
.001 0.002 .005 .009 .019 .037
CLEAR SOUARE OPENINGS
3/8' 3t4" 1w', 3" 5'6'
U,S. STANDARO SERIES
'100 '50 '40'30 '16 '10 '8
30
20
10
0
25 HR. 7 HR.
4s MrN. 15 MtN.
FIG. 5
ffi
SANDS GRAVELGLAY (PLASTTC) TO SrLT (NON.pl-ASTtC)
FINE MEDIUM COARS FINE COARSE COBBLES
SIEVE ANALYSIS
--{-
-l
.-
-lt--t-
':7'l--:.=,
+F-
--f+--t-t.-
-F-
-t-t-
t_-t-
-l----F----------F ------------..--
-I:*
--F}--'-
-}------+---
---,---i--.1=_
-f---|1=_---t---
-t-
-t-
I .,-
2roo
@
{60Fz
850ttrla40
76.2 127 200
152
'4
90
80
10
0100
30
20
10
0
70
80
90
100.001 0,002 .005 .009 .019 .037 9.52 19.1 36.1
5"6'8"
TIME READINGS
60 MlN. 19 MtN. 4 MtN. 1 MtN. .200
U.S. STANDARD SERIES
'100 .50 '40 .30 .'16 .10 .8
CLEAR SOUARE OPENINGS
3/8' 3r4" 1w', 3',
.074 .149 .?97 .590 1.19 2.0 2se 4.76
o.42
DIAMETER OF PARTICLE IN MILLIMETERS
20
30ffz
340btFso6(.ttU60 0.
25 HR. 7 HR.
45 MlN. 15 MlN.
Somple of GRAVEL, stlTy (cM)From Tp-sAisFEil
Somple of cnnvqr_, sllly__(cl4)From fp-+nfopeEr
GRAVEL 69 SAND 21
srLT & CLAY 10
PLASTICIry INDEX
LIQUID LIMIT
GRAVEL 71 o/o SAND 22
stLT & oLAY 7 o/o LIQU|D LtMtT
PLASTtCtry truOri
Gradation
Test Results
o/o
o/o
o/o
o/o
o/o
%
o/o
o/o
KBBS HOLDINGS, LLC
LOOKOUT MOUNTAIN WEDDING AND EVENTS CENTER
PROJECT NO. GS06903.000-l ts
SANDS GRAVELCLAY (PLASTTC) TO StLT (NON-PLASTIC)
FINE MEOIUM COARS FINE COARSE COEBLES
ANALYSIS SIEVE ANATYSIS
--*-
--t-
-.
-
(970z6bp0
Fz
u50tu,o't0
8'
0
127 200
152
90
r00
10
2A
70
80
90
100
.001 0.002 .005 .009 .019 .037 9.52 19.1 36.1 76.2
TIME READINGS
60 MrN. 19 MrN. 4 MtN. I MtN. .200
U.S. STANDARD SERIES.,100.50.40.30.16'10.8 CLEAR SOUARE OPENINGS
3/8' 3/4' 1n', 3' 5'6'
30
20
10
0 .074 .149 .257 .590 1.19 2.0 2.C8 4.76
0.42
DIAMETER OF PARTICLE IN MILLIMETERS
308z*fi
tF50fr
ot
eoH
25 HR. 7 HR.
45 MtN. 15 MtN.
FIG.6
TABLE I
SUMMARY OF LABORATORY TESTING
GTLIT PROJECT NO. GS06903.000-115
ffi
DESCRIPTION
CLAY. SANDY (CL)
SAND. CLAYEY (SC)
GMVEL. SILTY (GM)
GRAVEL. SILTY (GM}
PASSING
NO.200
SIEVE
(Yo)
66
30
10
7
PERCENT
SAND
(%)
19
39
21
22
PERCENT
GRAVEL
(Yo)
15
31
69
71
SOLUBLE
SULFATES
(Tol
*SWELL
(Yo)
ATTERBERG LIMITS
PLASTICIry
INDEX
(Yol
5
LIOUID
LIMIT
(Y.|
28
DRY
DENSITY
(PCF)
MOISTURE
CONTENT
(o/ol
12,4
6.3
23.4
DEPTH
(FETD
7-8
4-5
5€
6-7
EXPLORATORY
PIT
TP-1
-lP-2
TP-3
TP4
Page 1 of 1