DR9306-CS 960111 ' .l'anuaa'~ 11, 1996
10555 Newkirk Slreet
Suite .530
Dallas, Texas 75220
214.831.1111
FAX 214.831.0800
The City of Coppell
255 Parkway Blvd.
P.O. Box 478
Coppell, Texas
Attn.: Kenneth M. Griffin, P.E.
Assistant City Manager
City Engineer
Re: Geotechnical Investigation Services
Channel Bank Stability
Stream G-6/Bethel School Road
Coppell, Texas
PBT Project No. 101-002
Dear Mr. Griffin:
Patton, Burke & Thompson (PBT) has completed a geoteclmical investigation for a portion of' the above
referenced channel bank and hereby submit our findings and conclusions. This assignment was carried out in
general accordance with our discussions and PBT's proposal dated October 12, 1995.
Our firm appreciates the opportunity to be of professional geotechnical engineering service to the City, a.nd
particularly the assistance provided by you and your staff. We would be pleased to discuss any questions which
may arise concerning this report. If we can be of further assistance, please contact us.
Respectfully submitted,
PATTON, BURKE & THOMPSON
Che-Hung Tsai, Ph.D. Jack W. Burke, P.E.
Staff Engineer Principal
CI IT/JW13/jp
lOl-O02.rpt
Patton, Barke & Thompson
Engineerbg Consultants
I I It
Stream G-6/13ethel School Road
Page 10
movement, less than 1-inch. A suction approach calculation method, developed by Dr. Gordon McKeen, University
of New Mexico, indicated a potential movement of between 2 to 2-1/2-inches under current subsoil conditions.
Also, the VOLFLO computer program, developed primarily by Texas A&M University, was used to estimate
potential vertical movements. Depend/ng on the parameters selected to model the' seasonal moisture variation, total
swell potential for the existing site clay soils ranges from l-l/4 to 1-3/4-inches, and the range of total shrinkage
(i.e., settlement) potential due to the existing large trees (see Photos 6 and 7 in Appendix C) ranges from 1-1/2 to
2-inches.
6.0 CONCLUSIONS
The findings and analysis of the observed distress damage at the subject Jones study site and residence, as
specifically relates to the geotechnical engineering aspects of this study, are based primarily on the provided
available information, site observations, geotechnical investigations and our past experience with construction
activities and slope stability problems. The findings and analysh, in our opinion, are adequate to form a professional
basis for a conclusion on the basic question of''Did the construction activities for the new stream G-6 channel
improvements cause the observed study site movements (i.e., the pool area) and/or the observed signs of distress
and movement in the main house?".
In conclusion, the observed movements and distress, in our opinion, can be attributed to the following main factors
or conditions:
1. Existing subsurface conditions, at both the bank/pool study area and the main residence, consist of a
significant depth of old, man-made fills which due to their variability are susceptible to movements (both
I vertical and horizontal) over time.
I 2. Extensive landscaping several large mature trees and irrigation watering systems throughout the property
creates major oppommities for large seasonal variations in water contents of subsurface man-made fill
materials.
I
Puiton, Burke & Thompson
Engineering Consultants
Lt
.~ ~" ~ Stream O-6/Bethel School Road
Page 11
3. Construction activities for the G-6 Stream drainage improvement project did lower the critical factor of
~ safety for bank stability at the study site, both for the during construction and the alter (current)
construction conditions.
4. Although the factor of safety for gross stability at the study site was reduced during construction, the
created critical factor of safety is still considered adequate for normal, relatively short-term construction
periods. Furthermore, these reduced factors of safety (generally in the range of 1.2 to 1.4) did not within
themself create an unstable bank and the resulting observed movements and distress.
5. Although the reduced long-term after consm~ction stability of the existing bank has also been reduced (i.e.,
as compared to the original before construction condition), the current critical factor of safety is greater
_,1 than unity (i.e., 1.0) and is not the cause of the observed distress and movements to date. However, due
to the normal practice that the critical long-term factor of safety for bank stability should be at least 1.5,
the current factor of safety for the west bank of the G-6 Stream (particularly in the study site area) is
considered inadequate.
6. Observed movement and cracking distress to-date for the main residence was not caused by the
construction activities for G-6 Stream drainage improvements. Furthermore, the subject cracking distress
and movements in all probability are the result of past shrinkage settlement of the structure foundation due
to the large, mature trees in the near vicinity.
7.0 LIMITATIONS
This study is limited in that the findings and analysis rely upon information provided by City as well as others and
the geotechnical investigations performed by PBT several months after the movements actually occurred.
Therefore, should any conditions other than those described in this report be encountered or be known by others
to exist, it is recommended that Patton, Burke & Thompson be notified so that supplemental interpretations, if
required, can be provided.
Patton, Burke · Thompson
Engineering Consultants
"- -- Stream G-6/Bethel School Road Page 12
The findings, analysis and conclusions presented in this report are based on the assumption that subsurface
conditions do not vary appreciably from those encountered at the borings. Furthermore, the subsurface information
presented in this report does not constitute a direct or implied warranty that the subsurface conditions at the boring
locations can be dkectly interpolated or extrapolated.
Patton, Burke & Thompson has prepared this report to assist the City of Coppell in their eValuation of the study
site. We have developed our findings avxi conclusions in accordance with locally accepted professional geotechnical
engineering principles and practices at the time of the investigatiOn. We make no other warranty either express or
implied. Our analyses are based on the results of the field exploration, laboratory tests, analysis, and our
interpretations of subsurface conditions.
Pu~ton, Burb & Thompson