Town Center L1B2-CS 990506t5~06/99
THU 13:14 FAX 972 8~-9888
NOKTH AMEKlCAN
PROPERTIES SOUTHWEST
May 6, 1999
Via Fax: (972) 304-3570
6 Pages
Re: Paving Subgrade for the
Tom Thumb Retail ~ Town Center ~ ~
Dear Ken: ~ .... 7~
We are ~ose to completing our rough cut and fill operations for the new Tom Thumb and
retail shops. With regards to the paving notes on the 'Early Grading Plan" (furnished by
D~dy, Anderson & Associates), there is a notation that re~mmends three (3) different
methods of subgrade preparation for sandy soil, day soil or as recommended by the
geote~ and/or the Ci~.
Attached please find the sections of the Alpha Testing repo~ No.~412 (page 28,
secaon 6.5.2) that states:
'If concrete pavement is utilized, the above re~mmend~ pro~dure for mechani~l
lime stabilization would not be required in either clay or sandy clay subgrade areas."
With this letter we are requesting to follow this recommendation for both the private and~
public paving for this site. Fo~unately, ali su~rade materials on the site display a ~
relatively Iow plasti~ index.
Please ~ntact me with any questions at (972) 866-9555.
Mr. Kenneth M. Griffin, P.E. I CRy Engineer
City of Coppell
255 Parkway Boulevard
Coppell, TX 75019-4409
Sincerely,
John Maggiore
Vice President
Copy: Bill Anderson / Dowdy, Anderson and Associates / Fax: (972) 931-9538
14990 Landmark Blvd. · Suite 300 · Dallas, TX 75240-6729
972/866-9555 Telephone
972/866-9888 Fax
%',~[:~9 THU 13:14 FAX 972 866 9888
ALPHA Report No. 96412
NORTH AMERICCN
6.5 P~v~m~nt
Sandy clay or clay encountered near the existing ground surface will probably
constitute the subgrade for most parking and drive areas. Therefore, it is
recommended these materials be improved prior to construction of pave['nent.
Due to the wide spacing of the borings, division of the site into areas with similar
subgrade conditions was not possible. Delineation of areas with similar subgrade
conditions, if required, should be performed during construction after the subgrade
material has been exposed. The specific type of improvement procedures
required in given drive and parking areas will be dependent upon the type of
'Subgrade material present after final subgrade elevation has been achieved. In
the following paragraphs, recommendations are provided for either clay or sandy
clay sUbgrades.
Calculations used to determine the required pavement thickness are based only
on the physical and engineering properties of the materials and conventional
thickness determination procedures. Pavement joining buildings should be
constructed with a curb and the joint between the building and curb should be
sealed. Related civil design factors such as subgrade drainage, shoulder support,
cross-sectional configurations, surface elevations, reinforcing steel, joint design
and environmental factors will significantly affect the service life and must be
included in preparation of the construction drawings and specifications, but were
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~/06/99 THU 13:15 FAX 972 866 9888
NORTH AMERICCN
PHA Report No. 96412
:' not included in the scope of this study. Normal periodic maintenance will be
requir~l for all pavement to achieve the design life of the pavement system.
6.5.1 Asphaltic Concrete Pavement
After final subgrade elevation is achieved, the exposed surface of the pavement
Subgrade soil should be scarified to a depth of 6 inches and mixed with hydrated
lime (by dry soil weight) in conformance with Texas Highway Department Item
260. In areas where clay is exposed, 6 percent hydrated lime would be required
whereas in areas where sandy clay is exposed, 3 percent would be required.
Assuming an in-place unit weight of 100 pcf for the pavement subgrade soils,
these percentages of lime equate to about 27 lbs of lime per sq yard for a clay
subgrade and about 13 ibs'of.lime per. sq yard for a sandyClay subgrade. The
actual amount of lime required should be confirmed by additional laboratory tests
(ASTM C 977 Appendix XI).
1. It is recommended lime stabilization procedures extend at least 1 ft
beyond the edge of the pavement to reduce effects of seasonal shrinking
and swelling, upon the extreme edges of pavement. The soil-lime mixture
should be compacted to at least 95 percent of standard Proctor maximum
dry density (ASTM D 698) and within the range of 0 to 4 percentage points
above the mixture's optimum moisture content.
2. In all areas where hydrated lime is used to stabilize subgrade soil,
routine Atterberg-limit tests should be performed to assure the resulting
plasticity index of the soil-lime mixture is at/or below 15. It should be noted
in some areas, sandy clays with plasticity index values of 15 or less before
lime stabilization may be encountered. Therefore, lime stabilization of
these soils would not be required.
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THU 13:16 FAX 972 866 9888 NORTH AMERICCN
Report No. 96412
Mechanical lime stabilization of the pavement subgrade soil will not prevent
normal seasonal movement of the underlying untreated materials. Pavement and
other fiat work will have the same potential for movement as slabs constructed
directly on the existing soils.' Therefore, good perimeter surface drainage with a
minimum slope of 2 percent away from the pavement is recommended. The use
of sand as a leveling course below pavement supported on expansive clays
should be avoided, .Normal maintenance, of pavement should be expected over
· the life of the structures.
~004
Texas Triaxial Classification tests have not been performed for this specific
project, but our previous experience with similar soils indicates the Texas Triaxial
valUe for lime stabilized clay and 'sandy clay will be about four (4) while the Texas
Triaxial value for natural untreated clay or sandy clay will be about six (6). Using
the above values and assuming normal traffic for a 15-year project life, the
following pavement sections are recommended:
1. The pavement section in parking areas can consist of at least 5 inches
of asphaltic concrete composed of 3.5 inches of binder under 1.5 inches
of surface course overlying a lime stabilized clay and/or sandy clay
subgrade.
2. In drive areas, 6 inches of asphaltic concrete (4.5 inches of binder
under 1.5 inches of surface course) overlying a lime stabilized clay and/or
sandy clay subgrade should be adequate.
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THU 13:16 FAX 972 866 9888 NORTH AMERICCN ~005
Report No. 96412
3. The coarse aggregate in the surface course should be composed of
angular crushed limestone rather than smooth gravel.
6.5.2 Portland Cement Pavement
If concrete pavement is utilized, the above recommended procedure for
mechanical lime stabilization would not be required in either clay or sandy clay
Subgrade areas. However, prior to construction of pavement on untreated clay
or sandy clay subgrade soil, the exposed subgrade should be scarified to a depth
of at least 6 inches and compacted to at least 95 % of standard Proctor maximum
dry density (ASTM D 698) and within the range of I percent below to 3
percentage points above the material's optimum moisture content.
Pavement can then consist of 5 inches of adequately reinforced concrete in both
parking and drive traffic areas..Theoretically, a thinner pavement section is
possible in parking areas; however, to provide adequate concrete cover for bar
reinforcing steel, a 5-inch thick pavement section is required. Joints in concrete
paving should not exceed 15 ft.
6.6 Pavement Specifications
Pavement should be specified, constructed
requirements:
and tested to meet the following
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~ THU 13:16 FAX 972 86~.6 9888
NORTH AMERICCN
Report No.
1.
96412
Hot Mix Asphaltic Concrete: Texas SDHPT Item 340, Type B Base
Course (binder), Type D Surface Course.
Portia'nd Cement Concrete: Texas SDHPT Item 360. Specify a
minimum compressive strength of 3,000 lbs per sq inch at 28 days.
Concrete should be designed with 5 _.+ 1 percent entrained air.
'Lime Stabilized Subgrade: Texas SDHPT Item 260. An estimated
3 to 6 percent of hydrated lime (by dry soil weight) should be
applied to existing soil which has been scarified to a depth of 6
inches. The actual amount of lime required should be confirmed by
additional laboratory tests prior to construction.
a. The soil-lime mixture should be compacted to at least 95
percent of standard Proctor maximum dry density (ASTM D
698) and within the range of 0 to 4 percentage points above
optimum moisture. The moisture content of the subgrade
should be maintained until the pavement surface is placed.
b. In all areas where hydrated lir~e is utilized to stabilize ,the
subgrade soil, reurine Atterberg-limit tests should be
performed prior to completion of construction to assure, the
· resulting. Plasticity index of'the soil-lime mixture 'Will'be at/or
below 15. Gradation, Atterberg-timits and density tests
should be performed at a frequency of I test per 5000 sq ff
of pavement.
Re-compacted Subgrade: On-site materials should be scarified to a
depth of at least 6 inches and re-compacted to at least 95 percent
of standard Proctor maximum dry density (ASTM D 698) and within
the range of 1 percent below to 3 percentage points above the
material's optimum moisture content. The moisture content of the
subgrade should be maintained until the pavement surface is
placed. Density tests should be performed at a frequency of 1 test
per 5000 sq fl of pavement.
006
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