HomeMy WebLinkAboutARROWHEAD COTTAGES - MJA/FDP - FDP160004 - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORT (3)Everitt Companies
3003 E. Harmony, Suite 400
Fort Collins, Colorado 80528
Attn: Mr. Stan Everitt
Re: Subsurface Exploration Addendum Report
Residential High Rise Condominium Project
Centre for Advanced Technology P.U.D. - 71h Filing
Fort Collins, Colorado
EEC Project No. 1082101
Mr. Everitt:
Earth Engineering Consultants, Inc. (EEC) personnel conducted a subsurface exploration study for
the proposed residential high rise condominium project within the Centre for Advanced
Technology P.U.D. — 7`h Filing development in Fort Collins, Colorado in October of 2008, and
prepared a report detailing our findings and geotechnical engineering design recommendations
based on the subsurface conditions encountered. For further information, please refer to our
"Subsurface Exploration Report" dated October 13, 2008, EEC Project No. 1082101. We have
recently been requested by the project design team, to provide supplemental foundation design
criteria based on the project consisting of an "at -grade parking structure" along with 3-residential
condominium levels constructed above; as opposed to the 3-story residential condominium
structures as previously presented in our October 2008 report.
We understand this project involves the construction of three (3) residential "high-rise"
condominium structures, and an associated pavement area, planned for construction at the
northeast corner of Centre Avenue and Worthington Circle on Tracts A and C of the Centre for
Advanced Technologies - 7rh Filing in Fort Collins, Colorado. The residential buildings will be
4-story structures, (i.e., an at -grade parking structure with 3 levels of residential type units
above). Foundation loads for the structures are expected to be moderate with continuous wall
loads less than 5 kips per lineal foot and individual column loads up to about 250 kips. Floor
loads are expected to be light to moderate.
4396 GREENFIELD DRIVE
WINDSOR, COLORADO 80550
(970) 5 4 5 - 3 9 0 8 FAX (970) 663-0282
Earth Engineering Consultants, Inc.
EEC Project No. 1082101
April 21, 2008
Page 2
SUBSURFACE SOIL AND GROUNDWATER CONDITIONS
The subsurface conditions generally consisted of the following. Native, essentially cohesive
subsoils, classified as sandy lean clay, lean clay, and clayey sand were encountered beneath the
surficial topsoil/vegetative layer, and extended to the depths explored or to the course granular
stratum below. In the deeper borings, (i.e., soil borings B-2 and B-4) a course clayey sand and/or
silty sand with gravel was encountered beneath the upper cohesive materials and extended to the
depths explored, approximately 30-feet below site grades.
At the time of drilling, free water was observed in soil borings B-1 through B-3 at approximate
depths of 13-1/2 to 18-feet below site grades. Groundwater was not encountered in boring B-4
to maximum depths of exploration, approximately 15-feet. When checked several days after
drilling, groundwater was measured in borings B-1 through B-3 at approximate depths of 14-1/2
to 16-1/2-feet below site grades, while free water was still not observed in boring B-4 to
maximum depths of exploration.
GEOTEHNICAL ENGINEERING ANALYSIS AND RECOMMENDATIONS
General
Based on the subsurface conditions encountered in the test borings as well as the anticipated
maximum loading conditions, we recommend the proposed 4-story, "high-rise" condominium
structures be supported on conventional spread footings bearing on a zone of approved
engineered fill material. Care will be needed to evaluate the anticipated bearing materials for
each building footprint, to verify that spread footing foundations are bearing on suitable
materials. Footings should be placed on similar like material to reduce the potential for
differential movement. We anticipate floor stabs, and exterior flatwork could be supported on
newly placed and compacted fill soils or on ground modified native soils. Low to moderate
swell was observed in the near surface cohesive subsoils. Care will be needed to evaluate the
swell of near surface soils at the time of construction.
Earth Engineering Consultants, Inc.
EEC Project No. 1082101
April 21, 2008
Page 3
Site Preparation
Although final site grades were not available at the time of this report, based on our
understanding of the proposed development, we anticipate about 1 to 3-feet of
approved/imported fill material may be necessary to achieve design grades. After stripping and
completing all cuts and prior to placement of any fill or site improvements, we recommend the
exposed soils be scarified to a minimum depth of 12-inches, adjusted in moisture content to
within ±2% of standard Proctor optimum moisture content and compacted to at least 95% of the
material's standard Proctor maximum dry density as determined in accordance with ASTM
Specification D-698.
Fill soils required for developing the building, pavement and site subgrades, after the initial zone
has been stabilized, should consist of approved, low -volume -change materials, which are free
from organic matter and debris. It is our opinion the on -site cohesive clay soils could be used as
general site fill, provided adequate moisture treatment and compaction procedures are followed.
We recommend fill materials be placed in loose lifts not to exceed 9 inches thick and adjusted in
moisture content, generally ±2% of optimum moisture content, and compacted to at least 95%, of
the materials maximum standard dry density as determined in general accordance with ASTM
Specification D-698. If the site cohesive lean clay soils are used as general fill material, care
will be needed to maintain the recommended moisture content prior to and during construction of
overlying improvements.
Care should be exercised after preparation of the subgrades to avoid disturbing the subgrade
materials. Positive drainage should be developed away from the structures to avoid wetting of
subgrade materials. Subgrade materials becoming wet subsequent to construction of the site
improvements can result in unacceptable performance.
Foundation Systems — Conventional Type Spread Footings
The cohesive to slightly cohesive upper native subsoils appear to exhibit a slight potential for
settlement wider the anticipated heavy wall and column loads for the 4-story structures.
Significant post -construction settlement of the structure(s) would be expected for footing
Earth Engineering Consultants, Inc.
EEC Project No. 1082 101
April 21, 2008
Page 4
foundations designed at the maximum design loads supported directly on these native subsoils at
relatively shallow depths.
To reduce the potential for post -construction settlement caused by consolidation of the native in -
situ cohesive soils under heavily loaded conditions, we suggest placing the foundation system on a
zone of imported/structural fill material. The foundation bearing zone should be over -excavated
and/or "cut to grade" to accommodate placement of a minimum of 3-feet of structural fill below
the bottom of all footings. The over -excavated native materials should be stockpiled for reuse in
other areas of the site. The over -excavation in the building footprint areas/footing areas should
extend laterally in all directions at least 8-inches from the edges of the footings for each 12-inches
of over -excavation. The on -site upper cohesive subsoils should not be used as fill in these
foundation related areas.
Foundation fill materials used to develop foundation bearing should consist of approved imported
structural fill materials which are free from organic matter and debris. Structural fill consisting of
CDOT Class 5, 6 or 7 aggregate base course materials could be considered. The foundation
backfill/bearing stratum fill materials should be placed in loose lifts not to exceed 9 inches thick
and adjusted to a moisture content of ±3% of optimum moisture content, and compacted to at least
98% of standard Proctor maximum dry density per ASTM Specification D-698 or, as appropriate,
70% of relative density.
After placement of the fill materials, care should be taken to avoid excessive wetting or drying of
those materials. Bearing materials which are loosened or disturbed by the construction activities
or materials which become dry and desiccated or wet and softened should be removed and
replaced or reworked in place prior to construction of the overlying improvements.
The outlined steps for preparing bearing materials will significantly reduce but not eliminate the
potential for settlement of the building with consolidation of the underlying materials.
Overexcavation of a greater depth of material could be considered to further reduce the potential
for post -construction settlement. Preloading or surcharge loading could also be used to reduce
future settlement potential.
Earth Engineering Consultants, Inc.
EEC Project No. 1082101
April 21, 2008
Page 5
Conventional footing foundations could be supported directly on the structural fill backfill soils
as outlined above. For design of footing foundations bearing on a zone f imported structural fill
compacted to at least 98% of standard Proctor maximum dry density, we recommend using a net
allowable total load soil bearing pressure not to exceed 2,500 psf. The net bearing pressure
refers to the pressure at foundation bearing level in excess of the minimum surrounding
overburden pressure. Total load should include full dead and live loads.
For foundation construction, full -tithe quality control should be performed by EEC to verify
compliance to the project specifications. All footings should be placed on similar soils to reduce
the potential for differential movement between soil types. Quality control/field monitoring
should include but not limited to moisture content, dry density, percent compaction, and lift
thicknesses for the fill material being placed.
Exterior footings and foundations in unheated areas must be protected from frost action. The
normal depth of frost protection in this area is estimated to be around 30-inches. We recommend
continuous wall footings have a width of at least 16-inches. Isolated column pads should have
dimensions of at least 24-inches by 24-inches. Based upon the structural loading conditions
provided, larger footing sizes will be needed to accommodate actual foundation load and design
requirements.
Footings should be proportioned to reduce differential foundation movement. Total movement
resulting from the assumed structural loads is estimated to be on the order of about 1-inch over the
entire length of the building. Differential movement for use of conventional type spread footings
throughout the site should be on the order of 1/2 to 3/4 of the estimated total movement. Additional
foundation movements could occur if water from any source infiltrates the foundation soils;
therefore, it is imperative proper drainage be provided in the final design and during construction
for the structure.
Foundation excavations should be observed by the geotechnical engineer. If the soil conditions
encountered differ significantly from those presented in this report, supplemental recommendations
will be required.
Earth Engineering Consultants, tnc.
EEC Project No. 1082101
April 21, 2008
Page 6
The remaining geotechnical engineering recommendations concerning design and construction
of the proposed site improvements as presented in our October 2008 are valid and should be
implemented. We appreciate the opportunity to be of service to you on this project. If you have
any questions concerning the enclosed report, or if we can be of further service to you in any
other way, please do not hesitate to contact us.
Very truly yours,
Earth Engineering Consultants, Inc.
David A. Richer, P.E.
Senior Geotechnical Engineer
DAR/LLL/dla
Reviewed by:
Lester L. Litton, P.E.
Principal Engineer
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