DR9306-SY 960228 Engineering Consultants
CHANNEL BANK STABILITY
STREAM G-6/BETHEL SCHOOL ROAD
COPPELL, TEXAS
February 28, 1996
10555 Newkirk Slreel
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: ADDENDUM NO. 1 - Geotechnical Consulting Services
Channel Bank Stability
- Stream G6/Bethel School Road
Coppell, Texas
PBT Project No. 101-002
Dear Mr. Griffin:
This letter is issued as Addendum No. 1 to Patton, Burke & Thompson's (PBT's) earlier report for the
purpose of providing additional analyses and recommendations regarding alternatives for the geotechnical
engineering design and construction of slope enhancements. The following discussion is to be included as a part
of PBT's original report dated January 11, 1996.
During the previous study, it was found that the calculated factor of safety for the west bank of the G-6
Stream, particularly at Stations 1+00, 1+50 and 2+00 in the subject study area (see Figure 1 for the locations of
_ these three stations), was less than 1.5 and considered inadequate for a long-term condition. Furthermore, it is
understood that the city desires for the original upper railroad-tie wall to be installed back to it's original plan
location on the slope, particularly around the bank/pool area. In this letter report, two selected cross sections, bank
- area (Station 1+00) and bank/pool area (Station 2+00), were considered for further analysis. Also, the reinforced
concrete channel is included in all analyses, and it is recommended that the following additional conclusions be
incorporated into our original report:
· Bank/pool area (Station 2+00): Three additional basic cases were investigated in this area. Case (1): A
railroad-tie wall was installed at the original location on the slope. The factor of safety was calculated to
- be around 1.1 which is lower than 1.5 and similar to that obtained in the previous report for the current or
alter construction condition. Further analysis with the original upper raikoad-tied wall in place and
Patton, Burke & Thompson
Engineering Consultants
The City of Coppell
Stream G6/Bethel School Road, Addendum No. 1
Page 2
_ reconstructing the remaining slope to stay eastward of the existing pool concrete decking, resulted in a
minimum factor of safety in the order of 1.2. From these basic analyses, it appears that an improvement
in overall slope stability in this area cannot significantly be affected by only the installation of the original
_ railroad-tied wall and/or modification of the remaining slope. Case (2): An additional height of 3-feet by
3-feet gabions wall was added to the currently existing channel to increase the height of the original gabions
wall, and to reconstruct by flattening of the remaining slope. Results of this basic analysis indicated that
_ the calculated factor of safety was generally in the area of 1.1. Case (3): In this case consideration was
given to some form of structural wall, generally along the plan alignment of the original railroad-tie wall.
One alternative would be to install straight sided drilled shafts which penetrated significantly, for slope
_ stability considerations, below the level of the current existing concrete channel bottom and were spaced
close enough to prevent soil fi-om sliding between shat~s. The case of utilizing 12-inch diameter and 15-feet
long shafts are schematically presented on attached Figure 2. The factor of safety was significantly
- increased for the improved slope, increasing fi-om around 1.1 to a range varying from 1.9 for shaf~ spacings
of 10-feet on centers to 2.2 for 5-foot shaf~ spacings. We would not recommend pier spacings any larger
than 10-feet center to center, and if piers extended above the final soil slope they would need to be
- connected with some form of wooden cross members.
~ Bank area (Station 1+00): There were three (3) additional basic cases analyzed for this general area. Case
-- (1): The anticipated zone of soil that is currently in tension (i.e., has cracked to the surface) would be
excavated, recompacted and reconditioned to the original long-term properties of the on-site fill materials.
The factor of safety for this case was found to be around 1.2 which was generally the same as the calculated
- factor of safety for the current after-construction case. Case (2): This approach inVolved complete removal
of the tension zone materials and reconstructing a final slope which was flattened to approximately 6 to 1
_ (H:V:), as shown on attached Figure 3. Analysis results indicated that the factor of safety for this procedure
was considerably improved and should be slightly higher than 1.5. Case (3): Again, a 12-inch diameter and
15-feet long straight sided drilled shaft wall would be constructed generally along the western edge of the
_ backfill for the recently constructed gabions wall, as shown on attached Figure 4. The factor of safety, for
either a shaf~ spacing of 5-feet on centers or shafts on a 10-foot spacing, was greater than 1.5 and
significantly increased over the current calculated condition.
In conclusion, it appears that the results of these additional analyses indicate the following:
-- 1. Reconstruction of the original upper railroad-tie wall and/or modification of the existing slope (i.e,,
staying east of the existing concrete pool slab) and/or addition of another height ofgabions wall do
not result in satisfactory long-term stability for the bank/pool area (Station 2+00).
2. A straight-sided drilled shaf~ retaining structure with shafts spaced no farther than 10-foot center
to center would create a satisfactory long-term factor of safety for the bank/pool area.
3. Recompaction and reconstruction of the soil tension area in the bank area around Station 1+00 will
not result in a long-term satisfactory factor of safety. However, excavation of the soil tension area
-- and flattening the back slope to around 6 to 1, for a minimum distance of 30-feet west of the
channel face of the existing gabion wall, will result in a long-term factor of safety slightly greater
than 1.5.
-- Patton, Burke & Thompson I
Engb~
The City of Coppell
Stream G6/Bethel School Road, Addendum No. 1
Page 3
4. If it is not feasible to flatten the slope in the general bank area around Station 1+00, then a straight-
- sided drilled shaft wall, similar to the one discussed for Station 2+00, would provide a very
satisfactory long-term factor of safety.
We appreciate the opportunity to provide these additional analyses and conclusions to the City. We would
be pleased to discuss, as in the past, any questions which may arise concerning this study. If we can be of'further
_ assistance, please contact us at your convenience.
Respectfully submitted,
PATTON, BURKE & THOMPSON
Che-Hung Tsai, Ph.D.
Staff'Engineer
· ·
CHT/JWB/jp
101-002.adl
Dist: Th~ City of Coppeli (3)
Patton, Burke d~ Thompson
Engineering Consultants
FIGURES
II
Jones Residence
NORTHLAKE WOODLANDS EAST PH. VI ........ ~
LOT 9-R, BLOCK d
475.18
475.25 STA. 2+00
SPA
STA. 1+50.~ ~.. ....
DECK
4.74.67
75
STA. 1+00 .).72
Station 1+00 from this PO~
survey is approximately
Station '9+40 for the 472.5~
Stream G-6 construction 472,72 472.33
plans dated September 1994. .
472.61 '"'
i 472,63 ~ 472.56
472.62 D~C~
472.56 ' '*
~ 472i60~72~49" 472.56 472.51
× 472,52
~- f-f- Scale: 1" = 12.5'
/----------- Note: Information on this figure taken
"' from City of Coppell's Sheet 1
~c-- of 2 for Project No. DR 93-06.
.... ;3.69 Survey performed week of December"
18, 1995.
459.11
458.75
10' COt4~lE FLU~I --
- ~--46-4.~ ~o~ c^e~o, w^L~ 463.27
i × 464.80
464!77 SURVEY & X-SECTION LOCATIONS
St(]. 1+00, 1+50 and 2+00
Channel Bank Stability
Stream O-6/Jones Residence
Job No. 101-002 Coppell, Texas
A: FILL B: CLAY, sandy C: FILL (wall backfill)
7= 120 pcf 7= 125 pcf 7= 120 pcf
C= 60 psf C= 200 psf C= 60 psf
~-- 16° ~= 18' ~= 15'
D: Gabions Wall (2'x2') E: Concrete Shaft & Soil F: Concrete Channel
7= 135 pcf 7= 150 pcf 7= 150 per
C= 0 psf C= 7200 psf for a spacing of 5-feet C= 36000 psf
~= 50° C= 3600 psf for a spacing of lO-feet e= 0°
~_- 0·
Failure Plane
Pool A
LDNG TERM (S±o, 2+00)
(1). Critical F.S.= 2.2 (for a shaft spacing of 5-feet)
(2). Critical F.S.= 1.9 (for a shaft spacing of Ia-feet) Bank / Pool Area
MOD{F{£D CROSS SECT{ON
AT STATION 2+00
Scale: Channel Bank Sfabilily
Stream G-6/Jones Residence
Job No. 101-002 0 15-feet Coppell, Texas
Paffon, Burke & Thompson Figure 2
A: CLAY, sandy B: FILL (wall backfill)
7---- 126 pcf 7= 120 pcf
C= 200 psf C= 50 pst
¢= 18' ¢= 15° I Critical F.S.= 1.5
C: Gabions Wall (3'X3') D: Concrete Channel
7= 135 pcf y= 150 pcf
C-- 0 psf C= 36000 psf
0= 50' ~= 0'
Failure Plane
D ~A~~.'l (H:V) Slope
LONG TERM (S'to, 1+00)
Bank Area
MODIFIED CROSS SECTION
AT STATION 1+00
Scale: ~ Channel Bank Stability
Stream G-6/Jones Residence
Job No. 101-002 o IS-feet
Coppell, Te×os
Parian, Burke & Thompson Figure ,5
A: Tension Area B: CLAY, sandy C: FILL (wall backfill)
?= 120 pet ?= 126 pet 7= 120 pet
C= 0 pst C= 200 pst C= 60 pst
e= 0° e= 18' ~-- 16°
D: Gabions Wall (2'x2') E: Concrete Shaft & Soil F: Concrete Channel
7= 136 pet 7= 150 pet 7= 150 pcf
C= 0 pst C= 7200 pst for a spacing of 5-feet C= 36000 pst
~= 60° C= 3600 psf for a spacing of lO-feet e= O'
~_- 0·
Failure Plane
LDNG TERM (St:o, 1+00)
(1). Critical F.S.= 2.9 (for a shaft spacing of 5-feet)
(2). Critical F.S.= 2.0 (for a shaft spacing of lO-feet) Bank Area
MODIFIED CROSS SECTION
AT STATION 1+00
Scale: ~ Channel Bank Sfabilily
Stream G-6/Jones Residence
Job No. 101-002 0 15-feet
Coppell, Texas
Patron, Burke & Thompson Figure 4