V> N c o-0 0 OJ 0 Q) ,:. V> ~ "' l> 0 c. "0 c. ~ ::;· v;· a· 0 ::> "' n I ,. Q. V> n or iii" ::> ~ n" ' ::r ,... lD lD ~ CJ) z p ::0 (;I (.o) .... ~ v\ ""~ f\ \'' ~ ~r:\' .... , -' ~'

65/68/1995 14:45 2145314389 STANTECH ENGINEERING ~;: 5'1an~ec.h llllleeri11 ~-8May 1996 Job No. 1596-0026 Mr. John Baumgartner, P.E. Public Works Director Town of Addison 16801 Westgrove
· POBox 144 Addison, TX 75001 Re: Addison Circle Subbase Dear Mr. Baumgartner: 92oo King Anhur Drive Dallas, Texas 75247·3610 Phone 214/631-4372 Fax 2141631-4380 PAGE 62 In accordance:
with your request, we are pleased to provide this opinion letter concerning the proposed alternate subbase at the referenced project. The main question is whether or not the proposed
alternate would be equal in quality to the originally-specified subbase. The proposed alternate is to substitute either in 15itu or processed Austin Chalk for lime stabilized soil as
subbase for concrete pavements. We understand that this proposal was made because street grading exposed the firm, tan weathered Austin Chalk in most areas. This weathered chalk is rock-like
in appearance, much harder than soil, and would be difficult to process for lime stabilization. Since it has the appearance of rock, and crushed rock or gravel can often serve excellently
as subbase, the substitution was suggested. It is our opinion that over time the chalk will probably react more like soil and therefore generally be an unsuitable subbase. This opinion
is discussed in the following paragraphs. The project may be described as a dense residential and/or light commercial development. It is obviously intended to be a high-quality development
compatible with other area construction. We understand that the Town of Addison shares construction costs with the developer, Columbus Realty Trust. Huitt-Zollars is the civil engineer,
Fugro
85/88/1995 14:45 2145314388 Job No. 1596-00211 Mr. JobnBawngartner,l'.E. Town of Addison 8May 1996 Page2 STANTECH ENGINEERING PAGE 83 StanTecb Engineering Co. McClelland (Southwest),
Inc., the engineering laboratory, Gibson the prime contractor, and Drew Excavating a subcontractor. Original plans and specifications were for construction of a six-inch concrete pavement
on a six-inch subbase of lime-stabilized subgrade soil. As described in the preceding paragraph, an alternate was proposed when the weathered Austin Chalk, rather than soil, was exposed
by grading. The contractor excavated down only to rough grade in those areas with the firmest weathered chalk, and down to a depth of eight inches below rough grade in other areas. The
contractor has since segregated excavated soil from chalk, obtaining the chalk primarily from on-site utility trenches and other below-grade excavations, placed it over a portion of
the over-excavated areas, and processed it in-place with a high speed mixer. Our first contact on the project was April 29th in a telephone call from Mr. Baumgartner. One or both of
the undersigned visited the site soon thereafter and again May 6th. Mr. Baumgartner was present during the first site visit. Present during the second . were Mr. Saad Hineidi, P.E.,
and Mr. Pat Nichols ofFugro-McClelland (Southwest), Inc., and Mr. Bryant Nail of Columbus Realty Trust. In between these visits, one of our technicians visited the site and obtained
three samples of the possible or proposed alternate materials. During the second site visit, at which time some of the chalk had been processed, we obtained two more samples of the chalk
material. While at the site, we examined the pulverized material and found it to have the properties of clay. We believe that the question of subbase suitability should be examined first
by considering the purpose of the subbase, how the originally-proposed material would satisfY this purpose, and then compare how the proposed alternate would perform. Since by far the
most common type of pavement failure in this area is pothole formation or deflec-tion caused by loss ofsubgrade support due to pumping, the main purpose of the subbase is to provide
a non-pumping material beneath the pavement. Common pavement design and analysis procedures assume the presence of a non-pumping subbase. The secondmost common type offailure is pothole
formation or deflection caused by loss of support over volumetrically unstable soils such as expansive soils or poorly compacted fill. Therefore, the secondary purpose of the subbase
is to provide uniform support as compared to 
I estes/1995 14:45 21453143Be lob No. 1596-0028 Mr. John Bawngartner, P.E. Town of Addison 8May 1996 Page3 STANTECH ENGINEERI~~ PAGE e4 StanTecb Eoginerring Co. natural soils and/or
subgrade wbich may dissected by utility trenches. Lime stabilized soil, when constructed properly, has proven to be suitable for these purposes. The weathered chalk in its current condition
is a relatively strong material. In fact, good non-pumping subbases generally provide better support for the pavement than the pumping clay subgrade soils common in the area. However,
for concrete pavements, strength variations between support media can provide only minimal improvement to overall pavement life and load-carrying capacity. For this reason, we are of
the opinion that the strength of the finally-selected subbase material is much less important than whether or not it will possess the characteristic of a non-pumping material. One of
our concerns with the proposed alternate Is derived from our experience. The challc appears hard and and rock-like when first exposed, but weathers and degrades into ~~~~~~~~~~=~~~~~~
quality Austin Chalk weather completely into clay during the normal life of a pavement.· Our laboratory tests, attached, illustrate that the chalk possesses clay properties wbich will
become evident during weathering. Additionally, it should be noted that the samples of crushed chalk that were placed in pans of water in our laboratocy last week have already started
to slake into clay. We are of the opinion that this would occur at the site. With res~ to the areas where harder tan weathered chalk is exposed at rough grade, we are of the opinion
that these materials should weather more slowly. Good support may be provided in many areas for the full design life of the pavements. However, we feel that some areas, probably less
than a majority, will weather into clay during the design life of the pavement and that there will be failures due to pumping and loss of support. This would be an obvious disadvantage
to using the exposed chalk as subbase. Another disadvantage would be that utility trenches and natural variations in support will not have the ameliorating benefit of a constructed subbase.
The main advantage of using the alternate in these areas would be the cost savings from omitting stabilization. In summary, it is our opinion that the proposed alternate will not provide
an equal subbiiSe to the original design. In the over-excavated areas, at least, we believe that the 
\ \ 65/68/1996 14:46 2146314386 STANTECH ENGINEER!~ PAGE 65 1ob No. 1596-0028 StanTech Engineering Co. Mr. 1ohn Baumgartner, P.E. Town of Addison 8 May 1996 Page4 original specifications,
modified as needed to account for the removal and replacement procedure that is being followed in lieu of processing in place, should be follow~. In the other areas, those with the firmest
exposed chalk, an evaluation can be made considering the cost savings. The cost savings due to use of the proposed alternate ClUI be calculated; however, we know of no method to predict
the cost of increased maintenance or reduced service life due to gradual weathering of the chalk into pumping subbase. In view of these uncertainties, we would be inclined to use the
original specifications for the entire project. We trust that these tests and opinions will meet your needs at this time. If there arc any questions, or if we may be of additional assistance,
please contact us at 631-4372. Yours sincerely, StauTech Engineering Co. 1SH:jsh filespec c:\fd\18\bj60508l.doc 
\ 65/68/1996 14:46 2146314380 STANTECH ENGINEERit-1'"-Client: Project: U.S. STD. Sieve Sizes (Total %Passing) ~~ --5tanrsciJ laglaurlna ea. 9200 King Arthur Drive Dallas, Toxa& 75247·3610
Phons 214/631-4372 Fax 214/S31·4380 Mr. John Baumgartner, P.E. Public Works Director Town of Addison P.O. Box144 Addison, Tx. 75001 Addison Circle Date: 8 May 96 ST Job No.: 1596-0028
Report No.: 01 SOIL/LIMESTONE ANALYSES SUMMARY Rec'd Rec'd Rec'd 05-03-96 05-03-96 05-03-96 PAGE 66 ASTMC136 Lab# 9039 Lab#0040 Lab#9041 2" 100 1-00 100 No.4 27 26 32 No.40 6 4 10 No.200
0.7 0.6 1.3 No. 200 Decantation • ASTM C 117 35 47 20 (After 24 hours soaking) Liquid Limit _iLL) -ASTM D 4318 36 44 32 Plastic Limit (PL) • ASTM D 4318 17 18 16 Plasticity Index (PI)-ASTM
0 4318 19 28 16 Sample Descriptions: Lab Number 9039-Tan Weathered Limestone Lab Number 9040 ·Tan Weathered Limestone with Very Dark Brown Clay """"-""' · """u""'"""' --"-·· ~ ~ .....
, •• >". ,. ~ ...,.. ~"""" r '-··71 -.. ,.., 'r~ ..... ~·9([1';,\:.,.., '""·.' -. --.J. . //1' '' ~·o ~ ~ '"\' -.,. "' tl '· '·) ;.,._ * \·.•;•~ cyOn J. Co P.E. ~'f![, .... ~ .. --~~-,·'·~
li!" ~YNUON JAY O)A '~ ~ ... ;;,••v .. ~•~~··•"•"vfi"~~.,.'~..J ·' "'\ ~-~'01 _,.,., t·' ·~ """' \..V£1 ,...~ '-l,r r·f .. ,of.. .. .,_ ~-· .. t _..., t~ Gl ..... ,. -"'"'(\ ~ .....
.._·,,;., 'il' "" ... ,. .. l I fS I ~ .. •.• ~··· .. , ~~ "'i(J~'c.. ••• ~~"•·\> ... , '\\ •1o:1 ~~ ~~"" \.'-'~~ ....