SS9301-SY 940921SS ?3 -°/
reed engineering
GE3MQ)Mp
GEOTECHNICAL CONSULTANTS
September 21, 1994
Project No. 1998
City of Coppell
Engineering Department
P.O. Box 478
Coppell, Texas 75019
ATTN: Mr. Ken Griffin, P.E.
Re: Supplemental Recommendations
Slope Stability Analysis
Grapevine Creek - Beltline Road at Mockingbird Lane
Coppell, Texas
Gentlemen:
For reasons of constructibility and economy, consideration is being
given to substituting an additional row of piers for the proposed
shear key at the toe of the slope. Supplementary recommendations
associated with this alternative are discussed herein. These
recommendations are applicable considering that all other remedial
measures proposed in the geotechnical report are carried out.
The area between the proposed 33 -inch diameter piers and the toe of
the slope should be over - excavated and recompacted as outlined in
the Earthwork section of the geotechnical report. The excavation
should extend at least one foot into unweathered shale below the
bentonite seam Considering that this is accomplished, the lower
portion of the slope may be stabilized by a single row of 18 -inch
diameter concrete piers. A minimum of 12 piers on maximum 8 -foot
horizontal centers is recommended. The center line of the piers
should approximately parallel the slope break between the lower and
upper slope. The piers should be founded a minimum of six feet
into unweathered shale below the bentonite seam The top of the
piers should extend to Elev. 446 or higher. The piers should be
reinforced with 1 -1/2 percent steel throughout their full depth.
Plate 17 of the geotechnical report has been modified to reflect
the alternative discussed above and is attached hereto.
The remedial measures proposed herein, as well as within the
geotechnical report, are applicable for stabilization of the
existing landslide at the referenced site. It has been observed
that erosion is occurring at the toe of the slope upstream of the
existing landslide. The presence of several trees in the lower
2424 STUTZ DRIVE ■ SUITE 400 ■ DALLAS, TEXAS 75235 ■ 214/350 -5600 ■ (FAX) 214/350 -0019
City of Coppell
Project No. 1998
September 21, 1994
Page 2
portion of the upstream slope has prevented a slide from occurring
in this area to this date. However, some of these trees are in the
process of being undercut by stream erosion at the toe. If erosion
of the toe continues unchecked, it is possible that new slides
could develop upstream of the existing landslide in the future.
This is of particular concern if the trees along the edge of the
bank are undercut by erosion. Consideration should, therefore, be
given to providing some means of erosion protection along the toe
of the bank upstream of the existing landslide.
We trust this information will be sufficient for your use. Should
you have any questions concerning our findings or if you desire
additional information, please do not hesitate to call.
Sincerely,
REED YyNGINEERING GROUP, INC.
F. i y Smith, P.G.
z resident
Ronald F. Reed, P.E.
Principal Engineer
FWS /RFR /aap
copies submitted: X2) City of Coppell /Mr. Ken Griffin, P.E.
(1) Craig Olden, Inc. /Mr. Craig Olden
NOTES:
1. Excavate min. 1' into undisturbed soil at back of slide, and
min. 1' into unweathered shale below bentonite seam along base
of slide. BacXfill in accordance with Earthwork section of
report.
ELEV.
(feet)
470-
LIMITS OF EXCAVATION
460
1' MAX.
1
450
APPROX: TOP
UNWEATHERED SHALE
3.5 .
1 1
C
O
MIN. 5% SLOPE
3. Construct min. 2' -thick drainage blanket with durable crushed
stone (ASTM C -33, Size 67 or coarser) wrapped in filter fabric
(ADS 600, or equivalent) .
4. Install min. 6" dia. perforated drainage pipe (ADS N -12, or
equivalent) at base of drainage blanket.
5. Tee min. 6" dia. solid drainage pipes (ADS N -12, or equivalent)
into perforated drainage pipe on max. 12' horizontal centers.
Slope solid drainage pipes min. 5% to discharge at toe of slope
approx. 1' above "normal" creek flow level.
6. Install three rows of 33" dia. concrete piers, seven piers per
row. Space piers 12' on center, offsetting adjacent rows by
6'. Space each row 8' on center (See Plate 18 for plan layout
of piers) . Minimum penetration - 6' into unweathered shale.
Top of piers to be at El. 450 or higher. Reinforce piers with
1 -1/2% steel.
7. Regrade slope to approx. pre -slide topography. Cover regraded
slope with erosion control fabric (BonTerra S2, or equivalent)
and revegetate.
8. Install 9" gabion mattress on lower slope up to El. 450 or
EL. 450 higher. Extend mattress min. 6' beyond toe of slope, and min.
1.75 ; 10' beyond ends of slide. Anchor mattress as necessary.
L
440 \— APPROX. ELEVATION �1 (I I - ( ( = 1 ( i ( = i i I (f = I (� I ���� N��A\(
OF BENTONITE SEAM
6' MIN.
430
0 5' 10' 20'
(Idealized Section - Not for Construction)
x H �I ecn o � , 3 6&f VVJ I AS4aV o AC
0 1
H J s . G ul r► he b
f' �_ J0 cfe ,--- 5 t0fJ:_> .
6` I Li 6 ,ray s W -
�(��z►ioY,
-fhz Z