HomeMy WebLinkAboutFile Documents.520 N 8th St.0032.2017 (27).ARBKH - P ti KU MAR 5020 County Road 154
rle.rwnrw Srximc rn aiam
Geolechnir l Engineer -,ring Geology Phone: (970) 945-7908
Materials Testing I Envirc-s e
Fax: (970)945-8454
Email: hpkglenwood@kumamsa.mm
Olfim Locations: Parker, Glenwood Springs, and Silverihome, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED BASEMENT ADDITION
520 NORTH 8'h STREET
ASPEN, COLORADO
PROJECT NO. 17-7-157
FEBRUARY 13, 2017
PREPARED FOR:
JAMIE TISCH
520 NORTH 8'h STREET
ASPEN, CO 81611
(jamictisch(a'�aol.com]
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY....................................................................................... I -
PROPOSEDCONSTRUCTION.................................................-------........................................ 1 -
SITE CONDITIONS................................................................................................................... 1 -
FIELDEXPLORATION ........... -................................................................................................. 2 -
SUBSURFACE
-
SUBSURFACE CONDITIONS.................................................................................................. 2 -
FOUNDATION
-FOUNDATION BEARING CONDITIONS............................................................................_. 3 -
DESIGN
-
DESIGN RECOMMENDATIONS........................................................... 3 -
-FOUNFOUNDATIONS
DATIONS..................................................................................................................... 3 -
FOUNDATION AND RETAINING WALLS .................. .............................................. .... .: 4 -
FLOORSLABS .................................. .................................................................................. .: 5 -
UNDERDRAIN SYSTEM.............................-------"'......."'-"'---'---------............................... 5 -
SLOPESTABLIZATION.....................................................................................................: 6 -
DRYWELL.................. '...................................................... ........ ............................................ 6 -
SURFACE DRAINAGE................................................................-------....................._....-...- 6 -
7 -
FIGURE I - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - GRADATION TEST RESULTS
TABLE 1 - PERCOLATION TEST RESULTS
H -P a KUMAR Project No. 17-7-157
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed basement addition to the
existing residence located at 520 North 8's Street, Aspen, Colorado. The project site is shown on
Figure 1. The purpose of the study was to develop recommendations for the foundation design.
The study was conducted in accordance with our proposal for geotechnical engineering services
to Jamie Tisch dated Febmary 1, 2017.
A field exploration program consisting of an exploratory boring was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification and other engineering
characteristics. The results of the field exploration and laboratory testing were analyzed to
develop recommendations for foundation types, depths and allowable pressures for the proposed
building addition foundation. This report summarizes the data obtained during this study and
presents our conclusions, design recommendations and other geotechnical engineering
considerations based on the proposed concoction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed basement addition will include removing an existing swimming pool and enlarging
the area for habitable space. The footprint of the existing building will essentially not change.
The existing building is a one story split level structure. Ground floor will be slabon-grade in
the addition. Grading for the structure is assumed to be relatively minor with cut depth of about
12 feet. We assume relatively light foundation loadings, typical of the proposed type of
construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The existing residence is a one-story, split level building. The lot was covered with 1 to 2 feet of
snow and heavily vegetated with deciduous and coniferous trees at the time of our field
H -P —,KUMAR Pmjecl No. 17-7-157
-2 -
exploration. The terrain was nearly flat with a gentle slope down to the northeast. On the south
side of the driveway, an 18 -inch high retaining wall separated vegetated areas from the driveway.
A dry irrigation ditch ran along the western portion of the lot.
FIELD EXPLORATION
The field exploration for the project was conducted on Febmary 10, 2017. One exploratory
boring was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The
boring was advanced with 4 -inch diameter continuous flight augers powered by a truck -mounted
CME45B drill rig. The boring was logged by a representative of H-P/Kumar. Drilling access
closer to the proposed addition area was not possible due to existing trees.
Samples of the subsoils were taken with a 1'% inch I.D. spoon sampler. The sampler was driven
into the subsoils at various depths with blows from a 140 pound hammer falling 30 inches. This
lest is similar to the standard penetration test described by ASTM Method D-1586. The
penetration resistance values are an indication of the relative density or consistency of the
subsoils. Depths at which the samples were taken and the penetration resistance values are
shown on the Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface conditions encountered at the site is shown on Figure 2. The
subsoils consist of about 3 feet of organic sandy clay and silt with gravel fill overlying dense,
silty gravel and sand with cobbles and possible boulders. Drilling in the coarse granular soils
with auger equipment was difficult due to the cobbles and boulders.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and gradation analyses. Results of gradation analyses performed on small diameter drive
samples (minus 1 V: inch fraction) of the coarse granular subsoils are shown on Figure 3.
No free water was encountered in the boring at the time of drilling and the subsoils were moist to
slightly moist with depth.
H -P g KUMAR Pmlecl No. 17-7-157
-3 -
FOUNDATION BEARING CONDITIONS
The natural granular soils encountered in the boring are adequate for support of spread footing
foundations. Man -placed fill and debris from previous site development should be completely
removed from beneath the proposed addition area. The addition as planned is slab -on -grade and
structural fill can be used to reestablish design subgrade after the fill and debris have been
removed.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building addition be founded with spread footings
bearing on the natural granular soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils should be designed for
an allowable bearing pressure of 4,000 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 feel for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 42 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be reinforced lop 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
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of [his report.
H-P=KUMAR Project No. 17-7-157
4-
5) All existing fill and debris from previous site development, and any loose or
disturbed soils should be removed and the footing bearing level extended down to
the relatively dense natural granular soils. The exposed soils in footing area
should then be moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 50 pef for backfill consisting
of the on-site granular soils. Cantilevered retaining structures which are separate from the
residence and can be expected to deflect sufficiently to mobilize the full active earth pressure
condition should be designed for a lateral earth pressure computed on the basis of an equivalent
fluid unit weight of at least 40 pef for backfill consisting of the on-site granular soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95% of the maximum standard Proctor density.
Care should be taken not to overcompacl the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
H -P � KUMAR Project Na 17-7-157
-5 -
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.50. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 pef. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be a granular material compacted to at least
95% of the maximum standard Proctor density at a moisture content near optimum.
FLOORSLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade
construction. To reduce the effects of some differential movement, floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. The requirements forjoint spacing and slab reinforcement should be established by the
designer based on experience and the intended slab use. A minimum 4 -inch layer of free -
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
material should consist of minus 2 -inch aggregate with at least 5011. retained on the No. 4 sieve
and less than 2% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area that local perched groundwater can develop during times of heavy precipitation or
seasonal runoff. Frozen ground during spring runoff can create a perched condition. We
recommend below -grade construction, such as retaining walls, crawlspace and basement areas,
be protected from wetting and hydrostatic pressure buildup by an underdmin system.
H -P _ KUMAR Project No. 17-7-157
6 -
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free -draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum I % to
a suitable gravity outlet. Free -draining granular material used in the underdrain system should
contain less than 2% passing the No. 200 sieve, less than 505/o passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least 1'/: feet deep.
SLOPE STABLIZATION
The City of Aspen requires an engineered excavation slope stabilization plan if proposed
foundations are within 15 feel of neighboring structures or public travel ways. The plan is not
required if excavations are leu than 5 feet below the existing grade or further than 15 feet from
travel ways and less than 15 feet deep. The proposed building addition area is near the east and
south property lines and slope bracing could be required depending on the addition location, size
and excavation depth. Slope bracing through use of a variety of systems such as grouting, micro
piles and soil nails should be feasible at the site. A shoring contractor should provide design
drawings to support the proposed excavation slopes where needed. Other City requirements may
also be applicable.
DRYWELL
Drywells and bio-swales are often used in the Aspen area for site runoff detention and disposal.
The natural granular soils encountered are typically free draining and should be suitable for
surface water treatment and disposal as needed. The results of percolation testing performed in
Boring 1, presented in Table 1, indicate an infiltration rate between about 4 to 8 minutes per
inch. The bedrock is generally known to be relatively deep in this area and groundwater level
was not encountered to the boring depth of 12 feet. The drywell should have solid casing down
to at least basement level and perforation below that level.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the addition has been completed:
H -P g KUMAR Project No. 17-7-157
-7-
I) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of 3
inches in the first 10 feet in paved areas. Free -draining wall backfill should be
capped with about 2 feet of the on-site finer graded soils to reduce surface water
infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least 5
feel from foundation walls.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied -
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory boring drilled at the location indicated on Figure I, the proposed type of
construction and our experience in the area- Our services do not include determining the
presence, prevention or possibility of mold or other biological 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. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory boring and variations in the subsurface
conditions may not become evident until excavation is performed- If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may he made.
H -P ; KUMAR
Project No. 17-7-157
-S -
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
'/' 7r
'Or
Shane M. Mello, Staff Engineer
Reviewed by:
Steven L. Paw
SMM/kac
Cc: Thunderbowl Architects — Matthew Smith(mattheMQthunderbowlarchitects.coml
H -P le KUMAR Project No. 17-7-157
_
\e
fL(�
'BENCHMARK
EL 7889.85
AS GIVEN,.
/
PROPOSED
BASEMENT
1
IT
15 0 IS 30
APPROXIMATE SCALE—FEET
Ill 17-7-157 1 H -P (UMAR I LOCATION OF EXPLORATORY BORING I Fig. 1
BORING I
LEGEND
EL 7891.5'
0
®FILL: ORGANIC SANDY CLAY AND SILT WITH GRAVEL, VERY
STIFF, MOIST, DARK BROWN.
30/12
GRAVEL AND SAND (GM); SILTY, WITH COBBLES, POSSIBLE
BOULDERS, DENSE, SLIGHTLY MOIST, MIXED BROWN. ROUNDED
ROCK.
5
L DRIVE SAMPLE, 1 3/8 -INCH I.D. SPUN SPOON STANOARD
I■ PENETRATION TEST.
16/12
30/12 DRIVE SAMPLE BLOW COUNT. IG THAT 30 BLOWS OF
A 110 -POUND ER 3DIS
3D INCHES WERE REQUIRED
WC=3.6
i0 DRIVE THE SAMPLER 12 INCHES.
SAP 12 INC
H=3]
10 -200=21
NOTES
50/4
I. THE EXPLORATORY BORING WAS DRILLED ON FEBRUARY 10, 2017
WIN A 4 -INCH DMMETEN CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATION OF THE EXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
15
SITE PLAN PROVIDED.
3. THE ELEVATION OF THE EXPLORATORY BORING WAS MEASURED
BY HAND LEVEL AND REFERS TO THE BENCHMARK ON FIG. I.
4. THE EXPLORATORY BORING LOCATION AND ELEVATION SHOULD
BE CONSIDERED ACCURATE ONLY TO THE DEGREE IMPLIED BY
THE METHOD USED.
5. THE ONES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN
MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORING AT
THE TIME OF DRIWNC.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
t4 = PERCENTAGE RETAINED ON NO. 4 SIEVE ASTM 0 422);
-200 = PERCENTAGE PASSING N0. 200 SIEVE ASTM D 1140).
E -1I 17-7-157 1 H-RWtJ AR I LOG OF EXPLORATORY BORING I Fig. 2 1
yR
I'l—,
01
AN ma ma xu r
r1AILtINETERS•.n •.• a w� pp.a
—Iss
[IIAMETER
r
OF FANiICLES IN
SENO
giAKL
CLAY TO 51LT
COBBLES
PINE MEDIUM COARSE
PINE COARSE
CRAWL S) % SAND
Q X SKY .0 CLAY 21 %
LOU10 UNIT
PLASTICITY INDEX
SAMPLE OF; SPRY Spnd And Crawl
FROM: V rk, 1 O 6- and IV
4
Inm 4.1 .uulb so, �ry �e
vmq.. nJ�n •
4 ��VV
np
1F
F �.1X�n
j�
SI.W..Ivurvry4vin Np lso Amina—
e¢vbz. �IlnYSIu'111. 61u CV6
ee
onJ/o. 4!u B�p.
17-7-157
H—PiKUMAR
GRADATION TEST RESULTS Fig. 3
H-PIKUMAR
TABLE 1
PERCOLATION TEST RESULTS
PROJECT NO. 17-7-157
HOLE NO.
HOLE
DEPTH
(INCHES)
LENGTH OF
INTERVAL
(MIN)
WATER
DEPTH AT
START OF
INTERVAL
(INCHES)
WATER
DEPTH AT
END OF
INTERVAL
(INCHES)
DROP IN
WATER
LEVEL
(INCHES)
AVERAGE
PERCOLATION
RATE
(MINJINCH)
B-1
116
2
26
25%
%
4
4
6
4
6
6
251/2
25
%
25
24%
%
24%
1/2
24Y.
24
/
24
23'/.
%.
Note: The percolation test was conducted in the completed 4 -inch diameter borehole
on February 10, 2017.