ST9801-CS000405 MEMORANDUM
FROM THE
DEPARTMENT OF ENGINEERING
To:
From:
Date:
RE:
Ken Griff'm, P.E., Director of Engineering & Public Works
~4~
Kent Co!!in~, P.E., Civil Engineer
April 5, 2000
Recommendation for Coppell Road pavement and subgrade design change
(ST98-01)
During construction of the Coppell Road water line, observations were made that the
trench material in the east-west section of the road (SH121 to the box culvert) was a high quality
sandy material. Preliminary discussions with Larry Davis led to questioning the specification for
lime-treated subgrade. My understanding of lime was that it should be used to lower the plasticity
of a material and establish a working base. It is questionable whether either of these could be
achieved by treating this material with lime.
Subsequent to our discussions, I contacted Kevin Young with Freese and Nichols to
inquire about the soil investigation that took place during the design process. I learned that Rone
Engineers had completed a geotechnical study, dated October 1998, that included a soil analysis
for four boring locations. The results of the study indicated that the subgrade material in the east-
west portion of Coppell Road were non-plastic. No specific plasticity index values were
provided. This report continues to state that lime stabilization will not be required. Finally, the
report states that if stabilization is desired, cement should be used (p.4, See. 5.2). I am not sure
why the project was specified to use lime stabilization in light of this study.
The issue of pavement design has been examined more closely in the recent weeks. I
contacted Kevin Young again on March 6, 2000 requesting that he contact Rone Engineers on
this issue. In response, Rone Engineers issued a letter to Freeese and Nichols dated March 8,
stating that lime or cement is not of value in regards to the plasticity of the soil. The letter. further
states that, if properly compacted, the subgrade "will provide adequate long term strength for the
pavement section..
On April 4, Mike Martin and I met with Mike Sharp with Dallas County at the job site to
get an opinion on the subgrade/pavement issue. His opinion was that the existing subgrade
material would be adequate as a subgrade material as long as it is confined laterally, which it will
be. He mentioned that it is not uncommon to use compacted subgrade and increase the strength
and/or thickness of the concrete pavement where the existing material is non-plastic.
The next issue is whether or not the pavement thickness needs to be increased. The
specification for the Coppell Road project was 8" of reinforced concrete paving. An additional
inch of pavement would result in a 9" reinforced concrete. roadway. The Subdivision Ordinance
"CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN"
specifies 7" thick pavement for a C2U. Hence, the pavement thickness will already be increased
by an inch when compared to the typical specification.
At my request, Ed Bell Construction provided costing on April 3, 2000 for several
options that might be pursued in lieu of the lime stabilization. Their estimates include Coppell
Road from SH121 to Parkway Blvd. Two of the options include increasing the cement content in
the mix by a half-sack or increasing the thickness of the paving by 1 inch. Based on the estimate,
the cost would be $49,609.13 for the thickness increase and $7,991.00 for the half-sack of
concrete. However, based on the yards of subgrade stabilization shown for the east-west portion
of Coppell Road, the City would save $48,354 by deleting lime stabilization. Note that these
costs will be reduced by approximately 11% if the limits of the pavement modification are
shortened to end at the box culvert.
Considering all of the information gathered, my recommendations for Coppell Road from
SH121 to the box culvert are as follows. Regarding subgrade modification, I recommend
compacting the existing material to 95% of theoretical density with a moisture content at
optimum to plus 4%, per the Subdivision Ordinance (p.80, item E). Further, given the project
specification of 8" of concrete paving and the fact that this exceeds the City requirements for a
C2U, I recommend maintaining this concrete thickness. Finally, considering the minimal cost of
adding a half-sack of cement to the concrete mix and the potential benefit of the increased
strength, I recommend addition of this requirement to the pavement design. The resulting
pavement design would be 8" of compacted subgrade, 8" of reinforced concrete paving, with a
modification to the mix design from a 5-sack mix to a 5 ~h-sack mix. The estimated cost savings
to the project if these recommendations are implemented would be $35,923. I would welcome the
opportunity to discuss this matter with you at your convenience.
"CITY OF COPPELL ENGINEERING - EXCELLENCE BY DESIGN"