HomeMy WebLinkAboutPreliminary Geotech Engineering Investigation 10.07.24ffi GTLITHOMPSON
:,N ,,i',r;:! .: , ?.
October 7,2024
KBBS Holdings, LLC
248 Skipper Drive
Carbondale, CO 81623
Attention: Kara Byrne
Subject:Preliminary Geotechnical Engineering lnvestigation
Lookout Mountain Wedding & Events Center
Parcel No.218512402034.
Garfield County, Colorado
CTLIT Project No. GS06903.000-115
CTLlThompson, lnc. (CTLIT) completed a preliminary geotechnic_af _elgjneering
investigation for Lookout Mountain Wedding & Events Center on Parcel 218512402034 in
Garfield County, Colorado. The geotechnicalengineering 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 civilengineering and architectural
plans are available. The investigation woutd include excavating several additionalexploratory
pits at specific building locations.
Site Conditions
The Lookout Mountain Wedding & Events Center is proposed on Parcel No.
218512402034, which is located on the east end of Lookout Mountain in Garfield Gounty,
Colorado. The property is a 40-acre parcelthat is accessed from Lookout Mountain Road
(County Road i2O) ontne south flank of the mountain. A vicinity map with. the.location of the
iit" ir included as Figure 1. The property is on a localized, topographic ridge that trends down to
the southeast. The county road passes through the property with a switch-back tum that
contours around the ridge. An aerial photograph of the property is shown on Figure 2.
An old gravel pit is in the southeast part of the propefi. The pit is accessed by an
existing, unimpioved driveway from the county road. An abandoned wellhead is west of the
gravel-pit neai the end of the driveway. Vegetation on the property is predominantly thick
6am6el 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;idg'e within the aiea of the proposed buildings generally slopes down to the southeast
at grades between 5 and 15 Percent.
Prooosed Gonstruction
CTLIT was provided with a conceptual master plan for the Lookout Mountain Wedding &
Events Center that was prepared by The Land Studio (dated August 7,2O24r. A cluster of ten,
tiny homes is proposeO bn ine ridge in the northwest part of the property. We understand the
tiny nomes wiil O6 one-story, pre-manufactured buildings placed on cast-in-place foundations' A
pr"liminary plan provided to us (dated September 17,2A241indicates that a concrete patio and
CTllThompsglt. lnc.
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Site Geolosy
As part of our preliminary geotechnicalengineering investigation, we reviewed geologic
mapping Uy tne Colorado Geologicat Survey (CGS) titled, "Geologic Map of the Glenwood
Springs Ouadrangle, Garfield Gounty, Colorado", by Kirkham, Streufert, Cappa, Shaw, Allen
"nO
d"nroeder (dlted 20Og). The mip indicates the localized, topographic ridge in the area ol
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 basaftic bedrock is underlain by bedrock of the
Maroon Formation (Permian and Pennsylvanian Periods). The Maroon Formation is mainly
reddish-brown sandstone, congtomerate, mudstone, siltstone, and claystone. Subsurface
information from our exptoratory pits indicates the basaltic bedrock is not present above the
Maroon Formation bediock in tire 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. tnis is generally unfavorable from a large-scale, slope stability standpoint.
Several faults are below and adjacent to the subject parcel. The fautts are generally
parallelwith the localized, topographic iidge. 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 lnvestisation
For our preliminary geotechnical engineering investigation, subslrface conditions at the
site were investigated ny bnserving excavaiion of four exploratory pits. Two exploratory ptq !lP-
1 and TP-z')were excavated in the proposed area of the tiny homes and two pits (TP-3 and.TP-
4) were exiavated nearthe proposid events building. The pits were excavated with a trackhoe
on September 11 ,2024 at the approximate locations shown on the attached Figure 3.
Exploiatory excavation operations were directed by our engineer who logg^ed 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 inches of topsoil, underlain by 4.5 to Tfeet clayey sand and sandy clay that is interbedded
with clayey gravet. Sandstone bedrockwas encountered at depths of 5 t9I feet The hardness
of the uiaiok in Tp-1 and Tp-2 made excavation deeper than 6 inches into the sandstone not
practical.
Subsoils found in our exploratory pits, TP-3 and TP4, generally consisted of about 6
inches of topsoil or 2.Sfeet of fiil and 6 inches of clayey sand, underlain by 6 to 8 feet of silty
gravel. Sandstone bedrock was encountered at a deplh of I feet in TP-3. The hardness of the
bedrock made excavation deeperthan 6 inches into the sandstone not practical-
stainray will be constructed at the entrance of each tiny home. An events building is proposed
nearthe 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.
KBBS HOLDINGS, LLC Page 2 ot 6
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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 difiicult and could require a hoe ram aftachment 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
classiff 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 forflatter
slopes. Contractors are responsible for site safety and providing and maintaining safe and
stable excavations. The contracto/s gualified person should identifu 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.
Subexcavation and Struetunl Fill
Footing foundations will be appropriate for buildings construcled 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 2feet 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 largerthan 4 incfies, organics, and debris. The potentialfor 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
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Page 3 of 6
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from the site can likely be used as backfitl, provided they are free of rocks larger than about 4-
inches in diameter, organics, and debris.
Anticipated Buildinq Foundations
The naturalsoils atthe site are relatively heterogeneous. We judgethese soils are
generally suitable for support of footing foundations, but variable support condilions exist. To
ireate more uniforyn support conditions and reduce the potential for differential building
movement, we anticipate that subexcavation of the soils to a depth of about 2teet below
footings will be needed. The subexcavated soils will need to be replaced with densely
compicted, structuralfillas discussed in the Subexcavation and StructuralFillsection.
We expec-t an allowable bearing pressure of less than 3,000 psf for footings on the
structuralfill. A design-level geotechnicalengineering investigation should be performed to
develop recommendations for the buildings after civil engineering and archite,ctural plans are
developed. This would include excavating additionalexploratory pits at specific building
locations.
Slab-On-Grade Construction
Plans indicate that a concrete patio and stairway 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 struc'tures can be
utilized, provided'the soils below these elements are subexcavated to a depth of about Zfeet.
The subexcavated soits will need to be replaced with densely compacted, structural fill.
ln general, we expect slab-on-grade construction will perform well at this site, provided
proper design and construction methods are utilized. Recommendations for slab-ongrade
construction wiil neeA 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 Drainage
KBBS HOLDINGS, LLC
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 thioughout the year, and local perched groundwater can developduring
times 6f neavy precipitationbr seasonal snowmelt and runoff. Frozen ground during spring
runoff can alsb creaie a perched condition. These conditions can cause wetting of foundation
soils, hydrostatic pressuies on below-grade walls, and wet or moist conditions in belowgrade
areas, such as crawl spaces.
To mitigate problems associated with groundwater seepage and infiltration of surface
water into foundation backfill soils, foundation drains should be anticipated around the
perimeters of belowgrade areas of the buildings. The drains should discharge via positive
gravity oulets or ;gad to sump pits where water can be removed by pumping. Additionaldetails
ior tn6 drain systems will need to be developed for the buildings during the design{evel
geotechnical engineering investigation-
t A atAa t| tt^t arat i ala
Page 4 ot 6
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Surface Drainaqe
Surface drainage is criticalto 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 potentialfor 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 allfoundation backfill soils. Landscaping
should be carefully designed and maintained to minimize irrigation near foundation walls.
Goncrete
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 318-19 states the sulfate exposure class is
Not Applicable or 50.
SULFATE EXPOSURE CTASSES PER ACI 318.19
A) Percent by mass in soildetermined by ASTM C1580
For this level of sulfate concentration, ACI 318-19 Code Requiremenfs 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 geotechnicalengineering opinions and preliminary recommendations in this letter
are to provid--e the client with information to assist with planning for the proposed buildings' A
design-level geotechnical engineering investigation should be performed to fi nalize
recoirmendaiions for the buildings after civil engineering and architectural plans are available.
The investigation would include excavating severaladditionalexploratory pits at specific building
locations.
We believe this preliminary geotechnicalengineering 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 waranty, express or implied, is made. We are
available to discuss the contents ol this letter. Please contact us if you have questions or need
additional information.
KBBS HOLDINGS, LLC
' Water:Soluble Sulfate, (SOr) in
SoilA
(o/ol
Exposure Classes
< 0.10s0NotApplicable
0.101o 0.20slModerate
lo 2.00s2Severe
s3 > 2-OOVerv Severe
a A61td, al lan I aaa? a rra trrFhhlal^ o Ft 'F't-^
Page 5 of 6
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38298
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GTLITHOMPSON, I
D
Reviewed by:
Division MaEng
Attachments: Figure 1
Figure2
Figure 3
- Aerial Photograph
- Proposed Development
Figure 4 - Summary Logs of Exploratory Pits
Figures 5 and 6 - Gradation Test Results
Table l- Summary of Laboratory Testing
Map
KBBS HOLDINGS, LLC Page 6 of 6
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o 10(t0 20(x'
SCATE l- - 2OlJf)'
KIBS tpl.Dll€s, LLtC
LCTI(CT'T M'IINTAIN Ti'HIilrl ATTrl FN'FNIB (IFNTFP
SATELLITE IMAGE FROM MAXAR
(coPYRGHr 2024)
NOTE:
Viclnlty
It---
ffiLEGEND:
TP-l APPROXIMATE LOCATION OF
I EXPLORATORY PIT
NOTE:o r50 5oo
SCALB 1'- 5@'
PARCEL BOUNDARY
SATELUTE IMAGE FROM GOOGLE EARTH
(DATED JULY 19, 2023)
Aerial
TP_1
TP-2
TP_3
TP-4
(cR 120)
KBBS Hcx.oIt\lGIS, LI.c
Loor(our uourintru WEDDING AilD arENTs cENTER Dlralaararlh
LEGEND:
TP-l APPROXIMATE LOCATION OF
I DPLORATORY PIr
NOTE:
tr
o
SALg l- - JOd
BASE DRAWING BY fiE IAND STUDIO, INC.
DATED AUGUST 7, 2024.
Proposed
t0
TP-w-?,
TP-3
w*4
KBSHoBrst U,.C
lml(grT uclrirfrN wEF[tE AlD hrENrg cEttlfEB Fla.rrrla*narJ'
tl--l I Y-Z
15
TOPSOIL, SAND, CLAYEY, MOIST, DARK
BROWN.
FILL, GRAVEL, SAND, COBBLES. CI3YEY'
MEDIUM DENSE, MOIST, DARKBROWN.
CLAYEY SANDAND SANDY CTAY, MEDIUM
DENSE OR STIFF, MOIST, RUST, BROWN.
(sc, cL)
GRAVEL. CLAYEY, SANDSTONE COEBLES
AND BOULDERS, ANGUI.AR. DENSE, MOIST
(GC).
GRAVEL, SILTY, WEATHERED
CONGLOMERATE, DENSE, MOIST. TAN,
YELLOW, BROWN. (GM)
SANDSTONE BEDROCK, VERY HARD,
MOIST. RUST.
r r-o lrl
INDICATES BULK SAMPLE OBTAINED FROM
EXCAVATED SOILS.
:ffir
00
55
FullrlIL
IFclljo
Ful
UJt!
IFo-
EIo
1o
15
LEGEND:
F
KBES HOLTXNGS, LtC
t tlrlrnr rllnt rlrilN ufftln t FElrc ntlffi
INDICATES DEEPER EXCAVATION NOT
PRACTICAL DUE TO HARD BEDROCK.
NOTES:
1. THE PITS WERE EXCAVATED WIT}I A TRACKHOE ON
SEPTEMBER 11,2A24. THE PITS WERE BACKFILLED
IMMEDIATELY AFIER EXPLORATORY EXCAVATION
OPERATIONS WERE COMPLETED.
2. GROUNDWATER WAS NOT ENCOUNTERED IN THE
EXPLORATORY PITS DURING THIS INVESTIGATION'
3. THESE LOGS ARE SUBJECT TO TTIE EXPIAMTIONS,
LIMITATIONS, AND CONCLUSIONS CONTAINED
IN THIS LETTER.
Summary Logs
of Exploratory Pits
T
E
w
E
aaYA
w
n
ffi
GRAVELSANDS
COARSE cosst-EsCOARSFINEFINEMEDIUMcr-AY (Pr-ASnC) TO SILT (MIN-PIASTI0)
A}IALYSISAT{ALYSlS;
cllrl
?Flrlc.
F2ulotu,o.
970o
$eoFz.
850E,l,lc40
76.2 127 200
152
I
0
80
90
100
30
20
t0
9.52 19.1 36.1.001 o.qrz .qr5 .mo .019 .037
100
CLEAR SOTJARE OPE {II{GS
3rr
U.S. STANDARO SERIES
.'ro0 '5ll'40'3{l '16 '10'8
TIMEREATINAS
60lllN. 19il11,1. 4MlN. 'l tllN. ?ln 0
10
20
30
40
50
e0
70
.o71 .149 ."*o.or.* 1.19 2'O Lgg 4-76
DruETEROF PARTICIE IN MITI.IMEIERS
25HR.7HR
a5 li[N. t5 MlN.
Somple of clay SANDY
From TP-1AT7FEET
Somple of SAND,CLAYEY (SC)
From TP-2AT4FEET
KBBS HOLDINGS, LLC
LOOKOUT iIOUNTAIN WEDDING AND EVENTS CENTER
PROJECT NO. GS06903.000-ll5
GRAVEL ls olo
sllracGV &o/o
PTAST|CITY lnOex -
GRAVEL 31 olo
SILT & CLAY 30 o/o
PTASTICITYINDEX
SAND 19 o/o
LIQUID LIMIT 28 o/o
5 o/o
SAND 39 o/o
LnurdarMlT
%
Gradation
Test Results
GRAVELSAilDS
COARSE coEErEsCOARSFlNErlED|UMFINEcl.AY (PTASTIC) TO SILT (NOI$PrASnc)
-//-
----1-
1(xt
s0
80
9toz.6
F",
850e.ulqro
3{'
m
10
0
50
60 MtN. 19lltN. 4 MlN. t illN. '200
10
20
30
70
80
90
100
s.52 19.1 3O.1 n., '1u"*.001 0.0(n .(n5 .m9 .019 .Gl?
CIEARSOT'ARE OPENIN(iS
3rr gt4. 1W 3" 5'A
U.S. STANDARD SERIES
'1(x)-50.,{}'30.16.1o.8
.o74 .149 .*oor.* 1.19 2.O 2.* 1.76
ONilEIER OF PARTICLE IN TIILLIiffiTERS
ZSHR. TilR
45utN. r5MlN.
FIG,5
ffi
GRA\,EI-SANDS
CdRSE cosStEsCOARSFI}IEMEDIT.IIIFINECLAY (PIAST|C) TO SILT(M)ttlflAsTlC)
ANALYSIS
clul
=fut,
2706
$oo
1n mo
152
90
10
m
1(n
70
80
m
100
30
m
10
0 0.52 r9.1 36.1 762.mr 0.qI2 .(xE .ms .otg .037
5|ar$TAI'IDARD SERIES CTEAR SQUARE OPENI'.GSIT'E READINGS
60 MrN. 19 MrN. a MlN. 1 MlN. 'm0
.o71 ,tils .*o.o'* 1-19 LO L* 1-78
O'AMEIER OF PARTICIE lN MITIIMETERS
'r0o 50'40'30 '16 '10'825HR. 7HR.
45MlN. 15MlN.
Somple of _eB _vE-L, t!!T.Y_(qM)FTom TP-3ATsFEET
Somple of eRRvel,SILTY (GM)
From TP-4AT6FEET
KBBS HOLDINGS, LLC
LOOKOUT IIOUNTAIN WEDDING AND EVENTS CEI'ITER
PROJECT NO. GS06903.000-115
GRAVEL 69 %
slLr a cLAV io "t"
PIASTICITY IHOCX
SAND 21 o/o
o/o
oh
LIQUID LIMIT
SAND 22 olo
Lnu|DaaMli ofi'
-- o/o
GRAVEL
SILT & CLAV
PLASTICITY INDEX
71 o/o
7 o/o
Gradation
Test Results
GRAVELSANDS
coARs€COBEIESCOARSFll,lEIGDIUMFINECLAY (PLATSTE) TO SILT (ltlOl$PUSnC)
1(x)
90
80
(go
z.6h
F2p
G.ul04o
30
20
10
-----t-
_4
-t-
-
-
J--
t0
3{'
1fi!
0 1n 2@
152
5{'
60
70
80
s-az 19.1 36.1 76.2.00r o.qt2 .005 .fltg .010 .037
Th,E CLEAR SOUARE OPENINGS
3rr 3l1P 1t/i f 5'eU.S. STANDARD SERIES
-r00 '50'40'3{l '16 '10'8
.o71 .1.t9 .*o.or.* 1-ts 2'0 2'3'i 4'76
DNMETER OF PARTICLE IN MILLIMEIERS
qtMfi{. tgUlN. ,lMlN. l ll,ltN' 2625HR. 7HR.
/t5 Mlll. t5 Mlll.
FIG.6
TABLE I
SUMMARY OF LABORATORY TEITING
crLlT PROJECT NO. GS06903.000-115
ffi
DESCRIPT]ON
CLAY. SANDY (CL)
SAND. CLAYEY (SC)
GRAVEL. SILTY (GIVI)
GMVEL. SILW(GM)
PASSING
NO.200
SIEVE
(Vsl
66
30
10
7
PERCENT
SAND
(%)
1S
39
21
22
PERCENT
GRAVEL
(oh,
15
31
69
71
sotrsf,.E
SULFATES
(oh]
.SWELL
(Yo)
PLASTICITY
INDEX
(o/ol
5
LIQUID
LIMIT
(%)
28
DRY
DENSIW
(PCF)
MOISTURE
CONTENT
(%)
12.4
6.3
23.4
DEPTH
(FEET)
7-8
4-5
5.6
6-7
EXPLORATORY
PIT
TP-1
TP-2
TP.3
TP.4
Page 1 of 1