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Engineering • Consulting • Testing
August 11, 1999
Ranch Builders, Inc.
1011 Arbor Avenue
Fort Collins, Colorado 80525
Attention: Mr. Dennis Lane
Job No. 534-95007
Subject: Preliminary Recommendations to Mitigate Swell Potential for a Portion of
Wabash Street, Four Seasons Subdivision, Fort Collins, Colorado
Specifications: "Design and Construction Criteria, Standards and Specifications for
Streets, Sidewalks, Alleys and Other Public Ways, City of Fort Collins,
Colorado, City Engineering Office, July 1996.
Dear Mr. Lane:
In accordance with your request we are performing a geotechnical evaluation to
determine pavement thickness for the subject project. At the client's request and to
expedite the project, PSI is providing the following two recommendations in an effort to
mitigate the swell potential of subgrade soils. The soils in question are location on a
short portion of proposed Wabash Street and classify as A-7-6 and are located between
South Shield Street (station 10+00) and Station 12+70. The remaining portion of the
street in our opinion does not require any additional treatment before placement of
pavement section.
Option 1.
Overexcavation to a depth of two feet and scarify the third foot below the proposed
asphalt section. The removed soils should be replaced with a type A-6 clayey soil and
moisture condition and compacted to 95% maximum dry density and within 0 to 4
percent of optimum moisture content using a standard Proctor. The proposed treatment
area would be between South Shields Street (station 10+00) and approximately station
12+70. A field engineer from PSI should be on -site to verify the overexcavation location
during construction.
Option 2.
In our opinion another suitable method of mitigating the swell potential of the clay soils
on this site is to use Fly -ash stabilization. In our experience a 10% class C fly -ash has
been used effectively on similar clay soil types. A full depth treatment of 18" should be
used between South Shields Street (station 10+00) and approximately station 12+70. To
provide a reasonable transition between subgrade characteristics, the fly ash stabilized
section should taper to zero thickness at station 14+25. A field engineer from PSI should
be on -site to verify the stabilization location during construction.
Professional Service Industries, Inc. • 4765 Independence Street • Wheatridge.. CO 80033 • Phone 303/424-5578 • Fax 303/423-5625
The final geotechnical evaluation for pavement thickness design should be completed
within the next five business days. If we can be of further service please contact us at
(303)424-5578.
Sincerely,
PROFESSIONAL SERVICES INDUSTRIES
Derek Ulchla, P.E.
Department Manager
Reviewed y
K. Craig aughn, E
Vice President