HomeMy WebLinkAboutObservation of Excavation 06.27.2012Gtech
HEPWORTH - PAWLAK GEOTECHNICAL
June 27, 2012
Greg Boyd
1253 Kings Row
Carbondale, Colorado 81623
greggohrc@gmaiLeo in
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5020 (minty Road 15=1
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Phone: 970-'-145.7c)Ati
Fax: 970-9d 5-,ti=154
email: hpgeu'Nhvgeurech.com
Job No. 112 209A
Subject: Observation of Excavation, Proposed Addition, Lot 1 Kings Row
Subdivision. 1253 Kings Row, Missouri Heights, Garfield County,
Colorado
Dear Mr Boyd:
As requested, the undersigned representative of Hepworth-Pawlak Geotechnical, Inc.
observed the excavation at the subject site on June 22, 2012 to evaluate the soils exposed
for foundation support. The findings of our observations and recommendations for the
foundation design are presented in this report. The services were performed in
accordance with our agreement for professional engineering services to you, dated June
22, 2012.
The proposed 950 square foot addition will be a one-story wood frame structure over
partial crawlspace, partial slab -on -grade. Foundations were designed based on an
assumed allowable soil bearing pressure of 2,000 psf. Foundation walls acting as
retaining walls were designed to resist lateral earth loading by assuming an equivalent
fluid unit weight of 50 pcf. The existing house was built in 1978 and is a one-story wood
frame structure over a basement level.
At the time of our visit to the site, the foundation excavation had been cut in two levels
from 1 to 31/2 feet below the adjacent ground surface. The soils exposed in the bottom of
the excavation consisted of medium stiff, sandy clay. Results of a swell -consolidation test
performed on a sample taken from the site, shown on Figure 1, indicate the soils have low
to moderate compressibility under light to moderate loading and have a minor swell
potential when wetted under a constant light load. No free water was encountered in the
excavation and the soils were slightly moist. The existing basement wall backfill appears
to extend out about 3 feet from the basement wall. The footings had been formed and
rebar had been doweled into the basement wall. In the northeast portion of the addition,
we observed what appeared to be an existing concrete wall below the footing forms
Parker 303-841-7119 • Colorado Springs 719-633-7562 • Silverthorne 970-468-1989
Greg Boyd
June 27, 2012
Page 2
extending south off the southeast corner of the house. We understand that this is actually
a narrow footing for a previous greenhouse located on the south side of the house. We
asked the structural engineer to look at this old foundation condition and we understand
that the footing can be placed over the old concrete.
Considering the conditions exposed in the excavation and the nature of the proposed
construction, spread footings placed on the undisturbed natural soil designed for an
allowable soil bearing pressure of 2,000 psf can be used for support of the proposed
addition. The exposed soils tend to compress or expand when wetted and there could be
some post -construction movement of the foundation if the bearing soils become wet.
This movement would likely be differential to the existing house. Footings should be a
minimum width of 18 inches for continuous walls and 2 feet for columns. Loose and
disturbed soils in footing areas should be moistened and compacted or removed and the
bearing level extended down to the undisturbed natural soils. Exterior footings should be
provided with adequate soil cover above their bearing elevations for frost protection.
Continuous foundation walls should be reinforced top and bottom to span local anomalies
such as by assuming an unsupported length of at least 10 feet. Foundation walls acting as
retaining structures should also be designed to resist a lateral earth pressure based on an
equivalent fluid unit weight of at least 50 pcf for on-site soil as backfill. A perimeter
foundation drain should be provided to prevent temporary buildup of hydrostatic pressure
behind retaining walls taller than about 2 to 3 feet. Structural fill placed within floor slab
areas can consist of the on-site soils or a suitable imported material compacted to at least
95% of standard Proctor density at a moisture content near optimum. Backfill placed
around the structure should be compacted and the surface graded to prevent ponding
within at least 10 feet of the building. Landscape that requires regular heavy irrigation,
such as sod, and sprinkler heads should not be located within 6 feet of the foundation.
The recommendations submitted in this letter are based on our observation of the soils
exposed within the foundation excavation and do not include subsurface exploration to
evaluate the subsurface conditions within the loaded depth of foundation influence. This
study is based on the assumption that soils beneath the footings have equal or better
support than those exposed. The risk of foundation movement may be greater than
indicated in this report because of possible variations in the subsurface conditions. In
order to reveal the nature and extent of variations in the subsurface conditions below the
excavation, drilling would be required. It is possible the data obtained by subsurface
exploration could change the recommendations contained in this letter. Our services do
not include determining the presence, prevention or possibility of mold or other biological
Job No. 112 209A
GGE?UPtech
Greg Boyd
June 27, 2012
Page 3
contaminants (MOBC) developing in the future. If the client is concerned about MOBC,
then a professional in this special field of practice should be consulted.
If you have any questions or need further assistance, please call our office.
Sincerely,
HEPWORTH — PAWLAK GEOTCHNICAL, INC.
Etlllyy,��i
f
Daniel E. Hardin, P.> 24443
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Attachment Figure 1 — Swell -Consolidation Test Results
DEH/ksw
cc: SGM, Inc — Attn: Nathan Torrez (NathanT[c�sgm-ine.com)
Job No. 112 209A
GecPtech
COMPRESSION - EXPANSION (% )
1
1
2
3
4
5
Moisture Content = 7.9
Dry Density = 103 pcf
Sample of: Sandy Clay
From: Footing Grade - South Side of Addition
percent
Expansion
upon
wetting
01
1 0 10
APPLIED PRESSURE (ksf )
100
112 209A
Gtech
HEPWORTH-PAWLAK GEOTECHNIrCAL
SWELL -CONSOLIDATION TEST RESULTS
FIGURE 1