HomeMy WebLinkAboutCT 02-28; LA COSTA CONDOMINIUMS; LANDSIDE STABILIZATION; 2008-02-19la American GeotechnicaUnc.
SOIL, FOUNDATION AND GEOLOGIC STUDIES
February 19, 2008 File No. 23080-02
O'Day Consultant
2710 Loker Avenue West
Carlsbad, CA 92010
Attention: Mr. John P. Strohminger
Subject: INTERiUM REPAIR RECOMMENDATIONS
Landslide Stabilization
Banich, Powers, Calso Landslides
2416 Sacada Circle
Carlsbad, California
Reference: LANDSLIDE STABILIZATION RECOMMENDATIONS
Banich, Powers, Calso Landslides
2416 Sacada Circle
Carlsbad, California
Report Prepared by American Geotechnical, Inc.
Dated March 27, 2007 (FN: 23080-02)
Dear Mr. Strohminger:
This report presents a description of our progress concerning the landslide stabilization for the
Banich, Powers, and Calso landslides. To date, three tieback retaining walls have been
installed. The grading work to remove the main landslide has not yet been started. Details
concerning the repair work are as follows:
1. Tieback Walls: In our opinion, the three tieback walls have been completed substantially
in conformance with the plans and specifications. We have provided the City Inspector
with a copy of our field dailies and the tieback testing results. Because of the observed
soil conditions and safety concerns, it was recommended during construction that the
tieback walls be moved downward and outward relative to their original proposed
locations. The as-constructed locations of the three tieback walls are shown in red on
the enclosed plans.
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File No. 23080-02
February 19, 2008
Page 2
2. Slope Inclination: The upper part of the slope was originally recommended to be built at
a 1.5:1 (horizontahvertical) slope ratio. However, in order to keep the rear yard top of
slope concrete patio and in order to cover the tieback walls, the slope inclination for the
repair area will need to be steepened between 1.2:1 and 1.4:1 (horizontahvertical)
(average 1.3:1) depending on the actual location. Please note that this steepened slope
ratio of 1.3:1 in average is close to the pre-existing slope inclination of the upper part of
the slope prior to failure. Hence, we propose to re-build the slope to the same inclination
as existed prior to failure with adequate soil reinforcement.
On the enclosed plans, we have shown in red the proposed construction change from a
1.5:1 slope inclination to about 1.3:1 slope inclination in average for the upper part of the
slope. The proposed slope inclination for the upper part of the slope will vary from 36 to
40 degrees maximum depending on the actual location.
In terms of gross slope stability, the factor of safety is unchanged for the upper part of
the slope due to the support provided by the three tieback walls and additional soil
reinforcement. For surficial slope stability, our calculation indicated that the slight
steepening ofthe slope ratio from 1.5:1 to 1.2:1 will not dictate a decrease in vertical
spacing of the geogrid as illustrated in the attached calculation in Appendix A. We have
utilized a slope angle of 40 degree (about 1.2:1 slope ratio) in our surficial slope stability
calculations. As can be seen in the calculation, with a slope angle of 40 degree (about
1.2:1 slope ratio), a geogrid spacing of 24 inches will still provide a factor of safety of 1.5
for surficial stability at the site. Nonetheless, we recommend that the vertical spacing for
the geogrid reinforcement be reduced from 24 inches to 18 inches. With the vertical
spacing of the geogrids reduced to 18-inch, lighter strength of geogrid materials may be
used at the closer spacing.
Originally, Tensar BX1200 or similar, Synteen SF35 and Mirafi 3XT were specified.
BX1200 is the lightest grid in the Tensar product line that meets the project requirement
Synteen and Mirafi make a lighter grid suitable for the recommended 18-inch revised
spacing. Both Synteen SF20 and Mirafi 2XT are acceptable. All grids should be placed
from the face of the slope to the heel of the back-cut. Where that distance is less than
liAmerican Geotechnical, Inc.
File No. 23080-02
February 19, 2008
Page 3
10-feet, the grid should be laid out front to back, then continued onto the next 18-inch
level. In this case, the grid may be cut-off at the face of the succeeding layer or cut at a
total length of 10-feet, whichever is less.
3. Landscaping: After completion of slope repairs at the site, it is recommended that
landscaping for the slope be established as soon as possible.
We appreciate the. opportunity to be service. If you should have any questions or concerns,
please do not hesitate to contact our office.
Respectfully submitted,
AMERICAN
President/CEO
G.E. 103
li-chu (Jerry) Huan
Chief Engineer
G.E. 2601
GWA/JH:dd/dmc
Enclosures: Appendix A - Surficial Slope Stability Calculations
Distribution: 3 - Addressee (Overnight)
wpdata/20000/23080-02.gwa.clmc.feb.2008.lnteriumRepairReport
laiAmerican Geotechnical, Inc.
File No. 23080-02
February 19, 2008
APPENDIX A
Surficial Siope Stability Calculations
Slope and Soil Strength Data:
Slope angle, a =
Cohesion, c =
Friction angle, (j) =
Total saturated unit weight, YT =
Depth, d =
Soil Type (USCS Symbol) =
40 deg.
100 psf
33 deg.
125 pcf
4 ft
SC
1. Check existing unreinforced slope (FG = 0)
c + (YJ - Yw)dcos^atan(j)
F.S. =
Yxdsinacosa
2. Design slope reinforced with geogrids
FS required =
0.79 NG
1.5
FQ = Geogrid force
e = Force required by geogrids to achieve designed F.S.
N = (YT • Yw)dLcosa + Fosina
S = cL/cosa + [(YT-Yw)dLcosa + Fosinajtancj)
= L/cosa[c + (YT- Yw)dcos^atan(j)] + Fcsinatancj)
FS = S/P =
L/cosa(c + (YT-Y„)dcos^atan(j)] + Fcsinatancj)
(dLYxsina - Fccosa)
FG[FScosa + sinatancj)] =FSdLYTsina - L/cosa[c + (YT - Yw)dcos^atan(t)]
L/cosalFSyxdsinacosa - c - (YT - Y„)dcos^atan(|)]
FScosa + sinatan(t)
Let e = force required per unit vertical height of slope = Fo/Ltana
FSYxdcosa =
c/sina =
(YT - Yw)d(cos^a/sina)tan(|) =
FScosa + sinatantj) =
574.53
155.57
148.45
1.57
Hence, e=
[FSyT-dsinacosa - c- (yT-Yw)dcos^atan(j)]
(1/sina)
FScosa + sinatan(|)
172.68
GEOGRID DESIGN
AMERICAN GEOTECHNICAL F.N. 23080.02 FEB. 2008
TABLE
A
Consider: Tensar BX1200 Geogrid;
Tj = Long term allowable design strength for geogrids =
Use TH =
530
350
lb/ft
lb/ft
Vertical spacing of geogrids required, s = Tj/e =
Use s =
13.12 ft Geogrid
2.03 ft
24.32 inch
inch spacing 18
Use LG = m =
3. Check Anchorage Length
LB = d/tana = 4.77
Anchorage length, L^ = LQ - LB =
L = LQtana =
Normal stress, w^ = YT((d + L)/2) =
Cj = 0.8
TA = 2CjWAtan(t) = 974.88
'rA = TALA= 8146.42
FSA = TA/TJ = 23.28
8.36 ft
11.01 ft
938.24
OK
176.76
4. Calculate Bond Length and Factor of Safety
w' = ((0 + d)/2)YT- - ((0 + dcos^a)/2)Y„ =
TB = 2cjW'tan(t) = 183.67
TB = TBLB= 875.54505
FSB = TB/TJ = 2.50 OK
END OF DESIGN
NOTE:
1) Both Synteen SF 20 and Mirafi 2XT are acceptable
2) All grids should be placed from the face of the slope to the heel of the back-cut. Where that
distance is less than 10-feet, the grid should be laid out front to back, then continued onto
the next 18-inch level. In this case, the grid may be cut-off at the face of the succeeding layer or
cut at a total length of 10-feet, whichever is less.
GEOGRID DESIGN
AMERICAN GEOTECHNICAL F.N. 23080.02 FEB. 2008
TABLE
A
GEOGRID CALCULATIONS
BANICH, POWERS, CALSO
CARLSBAD, CALIFORNIA
STABILITY ANALYSIS INPUT & OUTPUT
a d c '1' F Yt Xof F e x
(slope) (depth) (C) (<!>•) existing (sat soil) XH:1V design e/dY„
degrees feet psf degrees pcf slope ratio psf psf
output input input input output input input input output output
45.0 4.0 100 33 0,73 125.0 1.00 1,5 180,2 0,72
38.7 4.0 100 33 0.82 125,0 1,25 1,5 169.2 0,68
33.7 4,0 100 33 0.92 125,0 1.50 1,5 149.7 0,60
29.7 4.0 100 33 1.03 125,0 1,75 1.5 124.7 0,50
26.6 4.0 100 33 1.15 125.0 2,00 1,5 95.8 0.38
24.0 4.0 100 33 1.27 125.0 2,25 1.5 64.1 0.26
21.8 4.0 100 33 1.39 125.0 2.50 1.5 30.4 0,00
BASIC PARAMETERS VERTICAL GEOGRID SPACING
a d c * F Soil USCS Td Ci S=Tj/e S=Ta/e
(slope) (depth) (C) (<t>') existing (Coarse to (BX1200) (soil) (BX1200) (BX1200)
degrees feet psf degrees ...Fine) lb/ft feet inches
input output output output output
45.0 4.0 100 33 0.73 sc 350.00 0.80 1,94 23,3
38.7 4.0 100 33 0.82 sc 350.00 0.80 2,07 24,8
33.7 4.0 100 33 0.92 sc 350.00 0,80 2.34 28,1
29.7 4.0 100 33 1.03 sc 350.00 0,80 2.81 33,7
26.6 4.0 100 33 1,15 sc 350.00 0,80 3,65 43,8
24.0 4.0 100 33 1.27 sc 350.00 0,80 5,46 65,5
21.8 4.0 100 33 1.39 sc 350.00 #N/A 11.52 138,2
BASIC PARAMETERS ANCHORAGE LENGTH & FS
a d c it> F L. L w. T F
(slope) (depth) (C) (<!>') existing (grid) (anchor) F=T.A^d
degrees feet psf degrees meters feet feet psf lb/ft lb/ft
input output output output output output
45.0 4.0 100 33 0.73 4,00 9.12 13.12 1070 10139 28,97
38.7 4.0 100 33 0,82 4,00 8.12 10,50 906 7644 21,84
33.7 4.0 100 33 0,92 4,00 7.12 8.75 797 5894 16.84
29.7 4.0 100 33 1,03 4,00 6.12 7.50 719 4569 13.06
26.6 4.0 100 33 1,15 4.00 5,12 6,56 660 3511 10.03
24.0 4.0 100 33 1,27 4,00 4,12 5,83 614 2630 7.52
21.8 4.0 100 33 1,39 4,00 #N/A #N/A #N/A #N/A #N/A
BASIC PARAMETERS BOND LENGTH & FS
a d c 'I' F Lb U W Tgub Tb F
(slope) (depth) (C) existing (bond) (avg.) (avg.)
degrees feet psf degrees feet psf psf psf lb/ft
output output output output output output
45.0 4.0 100 33 0,73 4,00 62 188 195 780 2,23
38.7 4.0 100 33 0.82 5,00 76 174 181 903 2,58
33.7 4.0 100 33 0,92 6,00 86 164 170 1020 2,91
29.7 4.0 100 33 1.03 7.00 94 156 162 1134 3.24
26.6 4.0 100 33 1,15 8,00 100 150 156 1248 3.57
24.0 4.0 100 33 1,27 9,00 104 146 151 1363 3,90
21.8 4,0 100 33 1.39 #N/A m/A #N/A #N/A #N/A #N/A
SURFICIAL STABILITY ANALYSIS & GEOGRID DESIGN by William S, McCann; SEP 25, '91
GEOGRID SPACING CHART FOR FS = 1.50
(d=4')
70
65
60
55
50
BX1200
GEOGRID
VERTICAL 45
SPACING
(inches)
40
35
30
25
20
10 20 25 30
SLOPE (degrees)
50