HomeMy WebLinkAboutObservation of Excavation 3.10.21il*fr E{ur¡¡*¡' ¿ir J.ir:;figr¡ii¡[t::,;r tï;;i,, "
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March I0,2AZI
Roger Esheiman
7378 County Road 100
Carbondale, Coiorado B 1 ó23.'rai¡l¡ç'!it¡tt.{(¡4gtl¡-¿Ìtl'r:oqi
project No. 2r:/ -zs'/
Subject: Observation of Pit Excavation, Proposed ADU, l3'/B County Road 100, Garfield
County, Colorado
Dear Roger:
As requested, the undersigned representative of Kurnar &, Associates observed thc pit excavation
at the subject site on Maroh I0,2A21b evaluate the soiis exposed for founciation support. The
fîndings of our obselations and recorumendations fol the foundation support are presented in
this report. The services were perfonned in accordance with our agr:eoll'rent for professional
engineering serviees to you datcd March 10,2021
The proposed ADU will be a one story, wood flame stÍuciLlre over a wallcout basement, ai:out
30 fbet by 40 fcet in pian size. The building col'nels were stalcocl in the fielcl.
At the time of our visit to the site, we observed a pit excavated with a Case 5B0SK backhoe in
the northeast part of the building area. The pit was about 6 fbet cleep and showed the natural
soils to oonsist of about 8 to 10 inches of topsoil overlying mediutn clense, silty sandy giavel
with cobbles down to the'bottom of the pit at 6 feet. The trackhoe bucket teeth were scraping on
rooks in thc bottom of the pit, so there may be srnall bouiders encountered in the building
exeavation. 'We undetstand that the bottorn of the backhoe pit is roughly proposed slab grade.
No frce water was encountered in the pit excavation and the soils were slightly moist.
Considering the conditions exposed in the pit excavation and the nature of the proposed
oonstruction, spread footings placed on the undistulbeci natural soil designed for an allowable
soii trearing pressure of 2,000 psf can be used for support of the proposed ADU. 'lire exposed
matrix soiis tend to eompress when wetted and there could be some post-eonstruetion settiement
of the foundation if the bearing soils become wet. Footings should be a minimurn width of
1ó inches for continuous walls and 2 feet for columns. I-oose and clisturbed soils in footing areas
should be compacted or removcd and the bearing levei exfended clowrr to the undistul'bed natural
soiis. The bearing soils s.hould be protected against r?ost. Exterior footings should be provided
rvith adequate soil cover (36 inches) above their bearing elevations lol lrost protection.
Continuous foundation wails should be rcinfbrced top and bottom to span local anoinalics such
as by assuming an unsupportecl lengfh of at least 10 feet. Foundation waiis aeting as retaining
structures should also be designcd to resist a lateral earth pressure based or1 an equivalent fluid
Roger Eshclman
Mareh IA,2A21
PageZ
unit weight of at least 45 pef for on*site granular soil as backfill excluding roclc larger than 6
inches, A perimeter foundation drain shoulcl be provided to prevent temporat'y buildup of
hydrostatie pressure behind the basement walls aud prevent wetting of the lower level.
Structural frll plaeed witirin fïoor slab areas oarl consist of ths on-site soils compac;tedto at least
95% ofstandarci Proctor clensity at a moisture cotttent near optimum. We recûmfllend a 4 to
6-ineh-thick layer of %-inoh scroened rook be piaced ciirectly below the basement slab for
dlainage and to act as a break for eapillary moisture rise. Backfill placed around the sJrureture
should be eompaeteci and the surfaee graded to prevent ponding within at ieast 10 feet of the
building.
The recommendations submitted in this letter are based ori our observation of the soils exposed
within the pit excavation down to proposed slab grade and do not inelude subsurface exploration
to evaluate the subsurface eonditions withiu the ioaded clepth of foundation influence. This
study is baseci on the assurnption that soils at greater depth have equal or better support than
those exposed in the pit. The risic of foundation movement may be greater than indieated in this
report because of possible variations in the subsurface r:onditions. TVe should observe the
cornpletecl foundation exeavation prior to piacement of footings to further verifu the soil
conelitions. Our seruices do not inciude determining tho presence, prevention or possibility of
mold or other biological contarninants (MOBC) doveloping in the fi-rture. If the eiient is
consemed about MOBC, then a professional in this special field of practice should be consulted.
if you have any questions or need further assistanee, please call our office.
Sincereiy,
E. Hardin,
Rev, by: SLP
DEH/kae
cc: Colorado Shuetural - Mike Albraney !!ì¡]1ç.û{¿çgl-ar.iyltEil_tlll!!1i}1.-!i{¡1
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