Temp Shoring Geotech LetterCOBALT
GEOSCIENCES
November 13, 2019
Troy Grasseth
G2 Contracting
G2contractingolive.com
RE: Temporary Shoring
Proposed Single -Family Residence
15712 — 72nd Avenue West
Edmonds, Washington
Cobalt Geosciences, LLC
P.O. Box 82243
Kenmore, Washington 98028
In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to
discuss temporary shoring as part of basement retaining wall construction at the referenced site.
Area Description L Proposed Construction
The site is located at 15712 — 72IId Avenue West in Edmonds, Washington. The proposed
construction includes a new residence with basement level located about 18 feet west of 72IId
Avenue West. We understand that excavation work was recently performed to create a temporary
excavation for basement retaining wall construction.
The excavation was near vertical and about 12 to 14 feet in height, and located several feet west of
the roadway and near the east property line. Since that time, the area was backfilled (buttressed)
to prevent sloughing and erosion until a temporary shoring plan could be developed.
This report provides recommendations for temporary Ultra block wall temporary shoring located
between the basement and previous cut excavation. We understand that the basement retaining
wall has been redesigned to limit footing size. Based on our review, there is 15 to 17 feet of
horizontal space between the back of the new footing and previous temporary excavation and an
additional 3 to 4 feet to 72IId Avenue West.
Temporary shoring would need to be at least 10 feet in height with variable backslope conditions
depending on the walls proximity to the previous cut.
Shallow Soil Conditions
We observed exposed soils in the area of the wall cuts and building. The shallow soils are
consistent with weathered glacial till. It appears that dense to very dense unweathered glacial till
is present below the site at 4 to 6 feet below existing elevations and extending below proposed
excavation depths.
It is typically feasible to create near vertical temporary excavations in glacial till provided they are
protected from erosion and for relatively short durations.
www.cobaltgeo.com (2o6) 331-1097
November 13, 2019
Page 2 of 3
Temporary Shoring
Recommendations
The existing near vertical cuts must be protected from erosion and sloughing following fill
buttress removal and until the retaining wall system is backfilled. We recommend utilizing an
Ultra block wall located between the new footing and previous near vertical cut. Ultra blocks are
interlocking concrete blocks with dimensions of 2.5 feet tall, 2.5 feet deep, and 5 feet long. There
are options for the location of the wall based on the geometry; however, all buttress fill must be
removed as part of temporary shoring placement. Ultra blocks should be on site prior to buttress
removal.
Based on the anticipated soil and slope conditions, a 10 to 12 feet tall Ultra block wall will be
required. The wall height necessary will depend on the location of the block wall with regard to
the road and basement wall. The backslope should have a maximum magnitude of 12 degrees
(4H:1V).
We recommend a minimum 6 inch embedment below the base of the building foundation
elevation and a wall batter of 4 to 6 degrees from vertical. Fill could be placed and compacted in
front of the wall to create the embedment.
The wall should have two double rows of Ultra blocks at the base (5 foot depth) placed in an
alternating pattern to increase rigidity. The upper 2 to 3 blocks may be single -block depth (2.5
feet) in an alternating brick pattern. Figure 1 shows the block layout, drainage rock location and
width, and options for backfill zones depending on the location of the wall.
As discussed, there is adequate space to locate the temporary block wall closer to the cut or the
basement. If the wall is located closer to the cut, we recommend removal of all buttress fill,
followed by wall construction and backfill placement. Backfill should consist of 2 to 4 inch clean
angular rock with a minimum width of 6 inches between the base blocks and cut. As the wall
transitions to single -block depth, the rock zone will widen to at least 3 feet.
If the temporary wall is located closer to the new residence, it may be possible to utilize
compacted structural fill between the drain rock zone and the previous cut. If this is performed,
the geotechnical engineer should be on site to verify buttress fill removal and replacement with
compactable structural fill up to subgrade. Two to four inch quarry rock should still be used as a
drainage layer between the wall and fill soils and all fill should be compacted to at least go
percent of the modified proctor. It is unlikely that the in -place glacial till fill will be suitable for
use as structural fill in this area.
The geotechnical engineer must be on site for all excavation and wall construction work. Once the
basement wall is in place, the blocks may be removed and replaced with wall backfill material. All
fill should be compacted as structural fill.
Closure
The information presented herein is based upon professional interpretation utilizing standard
practices and a degree of conservatism deemed proper for this project. We emphasize that this
report is valid for this project as outlined above and for the current site conditions and should not
be used for any other site. The contractor and owner are responsible for project safety, risk to
adjacent properties, and excavation stability.
www.cobaltgeo.com (206) 331-1097
November 13, 2019
Page 3 of 3
Temporary Shoring
Sincerely,
Cobalt Geosciences, LLC
a'(NONYI�
WA
4896
L
Exp. 6/26/2020
Phil Haberman, PE, LG, LEG
Principal
PH/sc
www.cobaltgeo.com (206) 331-1097
Max. 4H:1V Roadway
Slope at 3 Feet Height
Ft. Min; --I
�"2&m III
For total wall heights j
Of Up to 12 feet l
Area of Compacted Backfill, Angular Rock,
or Cut Native Soils (See Report)
4-6'
Batter
2 to 4 Inch Angular Quarry Rock )
III
io Min.
i Max. F—
o.5Ft. Min.
I -III
I — s Feet (Zypicaj) I--� Min. 4 Inch Diameter Perforated PVC Pipe
6 Inches (Schedule 40)
Minimum
NOTES:
For use with 2.5'x2.5'x5' interlocking concrete blocks Not to Scale
Backfill zone may consist of compacted structural fill, clean angular
rock, or cut native soils depending on wall location. If compacted
fill is used, it must be compacted to at least 90 percent of the modified proctor HO N r
ASTM D1557 Test Method in lifts up to subgrade and verified through /y
testing and monitoring during placement. v OF W AI
Plastic sheeting should be draped over the top of the wall, extending
over the backfill zone and secured on the edge of roadway pavement
Cobalt to verify keyway, drainage, backfill, soil conditions, 9G
and block installation during construction ��'�C• '� 548548 E
SS�ONAL
Exp.6-26-2020
Temporary Shoring
GravityWall
Layout
Cobalt Geosciences, LLC
P.O. Box, WA
Kenmore, WA 98028
COBALT15712
- end Avenue West
Edmonds, Washington
Figure i
wwW.cobahgeo.com
cob'dtgeo(ogmail.com
Project:
Location:
Designer:
Date:
Section:
Design Method
Design Unit:
New Project
Site Location
xxx
11 /12/2019
Section 1
NCMA_09_3rd_Ed
UltraBlock
SOIL PARAMETERS
Retained Soil:
Foundation Soil:
Leveling Pad:
GEOMETRY
UltraWall
cP
coh
y
38 deg
50 psf
120 pcf
38 deg
50 psf
120 pcf
40 deg
0 psf
130 pcf
Crushed Stone
Design Height:
12.00 ft
Wall Batter/Tilt:
0.0/ 6.00 deg
Embedment:
0.50 ft
Leveling Pad Depth:
0.50 ft
Slope Angle:
12.0 deg
Slope Length:
3.0 ft
Slope Toe Offset:
0.0 ft
Vertical b on Single Depth
FACTORS OF SAFETY
Sliding:
1.50
Bearing:
2.00
Live Load:
0 psf
Live Load Offset:
0.00 ft
Live Load Width:
0 ft
Dead Load:
0 psf
Dead Load Offset:
0.0 ft
Dead Load Width:
0 ft
Leveling Pad Width:
5.92 ft
Overturning: 1.50
13
C
C
c
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 1
RESULTS
FoS Sliding: 3.02 (Ivlpd)
Bearing: 1,908.89
Name
Elev.[dpth] ka
9.78[2.22] 0.201
1X
1X
7.33[4.67]
0.201
1X
4.89[7.11]
0.198
2X-2X
2.44[9.56]
0.314
FoS Overturning:
FoS Bearing:
Ma Paq Paqd (PaC)
74
292
0 0
— 0 0
56
110
645
0
0
164
1910
0
0
282
2.83
15.09
4:16 fix
1X
V.. -L
X4.89
-
18
100.00
182
100.00
481
43.13
1628
16.90
2X-2X 0.00[12.00] 0.282 L 2654 j 0 0 1 333 1 2321 1 3.02 (3.-1
Column Descriptions:
ka: active earth pressure coefficient
Pa: active earth pressure
Paq: live surcharge earth pressure
Paqd: dead surcharge earth pressure
(PaC): reduction in load due to cohesion
PaT: sum of all earth pressures
FSsl(lvl Pad): factor of safety for sliding at each layer. (FS sliding below the leveling pad)
FSot: factor of safety of overturning about the toe.
%D/H: ratio of based depth to height (warning for narrow walls, < 35%)
FoS O'
79.34
8.89
3.66
3.71
2.83
%DOH
100%
53%
35%
51%
41%
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 2
DESIGN DATA
TARGET DESIGN VALUES (Factors of Safety)
Minimum Factor of Safety for the sliding along the base FSsl =1.50
Minimum Factor of Safety for overturning about the toe FSot =1.50
Minimum Factor of Safety for bearing (foundation shear failure) FSbr =2.00
MINIMUM DESIGN REQUIREMENTS
Minimum embedment depth Min_emb =0.50 ft
INPUT DATA
Geometry
Wall Geometry
Design Height, top of leveling pad to finished grade at top of wall H =12.00 ft
Embedment, measured from top of leveling pad to finished grade emb =0.50 ft
Leveling Pad Depth LP Thickeness =0.50 ft
Face Batter, measured from vertical i =0.00 deg
Slope Geometry
Slope Angle, measured from horizontal a =12.00 deg
Slope toe offset, measured from back of the face unit STL_offset =0.00 ft
Slope Length, measured from back of wall facing SL_Length =3.00 ft
NOTE: If the slope toe is offset or the slope breaks within three times the
wall height, a Coulomb Trial Wedge method of analysis is used.
Surcharge Loading
Live Load, assumed transient loading (e.g. traffic)
LL = 0.00 psf
Live Load Offset, measured from back face of wall
LL_offset =0.00 ft
Live Load Width, assumed strip loading
LL_width = 0.00 ft
Dead Load, assumed permanent loading (e.g. buildings)
DL = 0.00 psf
Dead Load Offset, measured from back face of wall
DL_offset =0.00 ft
Dead Load Width, assumed strip loading
DL_width = 0.00 ft
Soil Parameters
Retained Zone
Angle of Internal Friction
cp = 38.00 deg
Cohesion
coh =50.00 psf
Moist Unit Weight
gamma=120.00 pcf
Foundation
Angle of Internal Friction
cp = 38.00 deg
Cohesion
coh =50.00 psf
Moist Unit Weight
gamma=120.00 pcf
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 3
NOTES ON DESIGN UNITS
The wall section is designed on a 'per unit width bases' (lb/ft/ft of wall or kN/m/meter of wall). In the calculations the
software shows lb/ft or kN/m, neglecting the unit width factor for simplicity.
The weights for the wall unit are shown as Ibs / ft3 (kN / m3). For SRW design a 1 sf unit is typically 1 ft deep, 1.5 ft
wide and 8 inches tall (or 1 ft3). therefore a typical value of 120 pcf is shown. With larger units the unit weight will
vary with the size of the unit. Say we have 4 ft wide unit, 1.5 ft tall and 24 inches deep with a tapered shape (sides
narrow), built with 150 pcf concrete. We add up the concrete, the gravel fill and divide by the volume and the results
may come out to 140 pcf, as shown in the table. The units with more gravel may have lower effective unit weights
based on the calculations.
Hollow Units
Hollow units with gravel fill are treated differently in AASHTO. If the fill can fall out as the unit is lifted, then AASHTO
only allows 80% of the weight of the fill to be used for eccentricity (overturning calculations). In the properties page
for the units the weight of the concrete may be as low as 75 pcf. This is the effective unit weight of the concrete only
(e.g. the weight of the concrete divided by the volume of the unit). The density of the concrete maybe 150 pcf, but not
the effective weight including the volume of the void spaces used for gravel fill.
Rounding Errors
When doing hand calculations the values may vary from the values shown in the software. The program is designed
using double precision values (64 bit precision: 14 decimal places). Over several calculations the results may differ
from the single calculation the user is making, probably inputting one or two already rounded values.
Result Rounding
As noted above the software is based on double precision values. For example, using an NCMA design method an
allowable factor of safety of 1.5 the software may calculate a value of 1.49999999999999, since this is less than 1.5,
it would be false (NG), even though the results shown is 1.50 (results are rounded to 2 places on the screen). In the
design check we round to 2 decimal places to check against the suggested value (1.49999999999 rounds to 1.50).
Given the precision of the calculation, this will provide a safe design even though the 'absolute' value is less than the
minimum suggested.
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 4
RETAINING WALL UNITS
STRUCTURAL PROPERTIES:
N is the normal force [or factored normal load] on unit to unit interface
The unit to unit shear is N x Tan(0.0) + 17796.0
N is the normal force [or factored normal load] on the base unit
The default base unit to leveling pad shear is 0.8 N tan(40 deg.) or
may be the manufacturer supplied data.
Table of Values:
Unit
Ht (in)
Width (in)
Depth (in)
Equiv_Density (pcQ
Equiv_CG (in)
Cap
14.75
59.00
29.50
140.00
14.75
Full
29.50
59.00
29.50
140.00
14.75
Double
29.50
59.00
59.00
140.00
29.50
Triple
29.50
59.00
88.50
140.00
44.25
15 in Tall Unit
14.75
59.00
29.50
140.00
14.75
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 5
FORCE DETAILS
The details below shown how the forces and moments are calculated for each force component. The values shown
are not factored. All loads are based on a unit width (ppf / kNpm).
Layer
Block Wt
X-Arm Moment
Soil Wt X-Arm
Moment
846.08
2.26
1909.62
60.28
3.66
220.87
�1
2
3
846.08
846.08
2.00
1692.21
206.53
3.65
753.41
1.74
1474.79
353.47
3.64
1286.28
4
1692.15
2.72
4594.70
0.00
5.17
0.00
5
1692.15
2.46
4159.88
Block Weight (Force v) = block: 5,923 X-Arm = 2.45 ft
Soils Block Weight (Force v) = 620 ppf X-Arm = 3.77 ft
Active Earth Pressure Pa = 2,654 ppf
Pa_h (Force H) = Pa cos(b batter) = 2,654 x cos(28.55.5) = 1,924 ppf
Y-Arm = 4.17 ft
Pa_v (Force V) = Pa sin(b batter) = 2,654 x sin( 28.55.5) = 1,298 ppf
X-Arm = 4.50 ft
Passive Earth Pressures
Passive earth pressures are used for resistance of the Leveling Pad, but may be extended upward to assist
with the resistance of the wall facing for walls that have deep embedments.
Passive Earth Pressure:
kp = 4.20
Pp = 517.20 ppf
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 6
CALCULATION RESULTS
OVERVIEW
UltraWall calculates stability assuming the wall is a rigid body. Forces and moments are calculated about
the base and the front toe of the wall. The base block width is used in the calculations. The concrete units and
granular fill over the blocks are used as resisting forces.
EARTH PRESSURES
The method of analysis uses the Coulomb Earth Pressure equation (below) to calculate active earth
pressures. Wall friction is assumed to act at the back of the wall face. The component of earth pressure is assumed to
act perpendicular to the boundary surface. The effective 6 angle is 6 minus the wall batter at the back face. If the
slope breaks within the failure zone, a trial wedge method of analysis is used.
EXTERNAL EARTH PRESSURES
Effective 6 angle (3/4 retained phi) 6 =28.5 deg
Coefficient of active earth pressure ka =0.282
External failure plane p = 62 deg
Effective Angle from horizontal Eff. Angle =84.48 deg
Coefficient of passive earth pressure: kp = (1 + sin(T)) / (1 - sin(g)) kp =4.20
(os(�i + i)2
Ka:-
a r sh�'Pi +
cas(i) •c05(5i— i�l 1 +
cos(6' —
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 7
W0: stone within units
W1: facing units
W2: stone over the tails
W9: Driving force Pa
W10: Driving Surcharge load Paq
W11: Driving Dead Load Surchage Paqd
FORCES AND MOMENTS
The program resolves all the geometry into simple geometric shapes to make checking easier. All x and y
coordinates are referenced to a zero point at the front toe of the base block.
UNFACTORED LOADS
Name
Factor y
Force (ii) Force (H)
X-len
Y-len Mo
Mr
Face Blocks(W1)
1.00
5923
—
2.45
—
14529
Soil Weage(W2)
1.00
620
--
3.77
--
2337
LVIPad(VV18)
1.00
336
—
--
—
--
—
Pa
1.00
--
1924
—
4.17
8025
--_
Pa_v
1. 00
1298
--
4.50
—
—
5837
Sum V / H
1.00
8177
1924
Sum Mom
8025
22703
i
1
!— L —
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 8
BASE SLIDING
Sliding at the base is checked at the block to leveling pad interface between the base block and the leveling
pad. Sliding is also checked between the leveling pad and the foundation soils.
Forces Resisting sliding = W1 + W2 + Pav
5,923 + 620 + 1,298 N =7,841 ppf
Resisting force at pad = 0.8 N tan(slope) + intercept
7,841 x 0.8 tan(40.0) Rf1 =5,779
where L is the base block width
With tilt, the resisting force is 'SumV * cos(tilt)' + Rf * sin(tilt) + Df * sin(tilt)
because the unit is sliding 'upslope'. The program also checks sliding through the pad,
taking the minimum value for Re. The result is correct, the equation shown is not complete.
Friction angle is the lesser of the leveling pad and Fnd cp =38.00 deg
N1 includes N (the leveling pad) + leveling pad (LP)
7,841 + 336 N1 = 8,177 ppf
Passive resistance is calculated using kp = (1 + sin(38))/(1 - sin(38)) kp = 4.20
Pressure at top of resisting trapezoid, d1 = 0.50 Fp1 = 252.22
Pressure at base of resisting trapezoid, d2 = 1.52 Fp2 = 765.13
Depth of trapezoid depth = 1.02
Pp = (Fp1 + Fp2) / 2 * depth 517.20
Resisting force at fnd = (N1 tan(phi) + c L) + Pp
8,177 x tan(38) + 50 x 5.42 + 517 Rf2 = 7,175
where LP = Ivl pad thickness * 130pcf * (L + Ivl pad thickness/2)
[the value printed is the minimum of sliding through the leveling pad or the foundation soil.
Driving force is the horizontal component of Pah
1,924
FSsl = Rf / Df
Df =1,924
FSsI =3.02 / 3.73
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 9
OVERTURNING ABOUT THE TOE
Overturning at the base is checked by assuming rotation about the front toe by the block mass and the soil
retained on the blocks. Allowable overturning can be defined by eccentricity (e/L). For concrete leveling pads
eccentricity is checked at the base of the pad.
Moments resisting eccentricity = M1 + M2 + MLvIPad + MPav
14,529 + 2,337 + 5,837 Mr=22,703 ft-Ibs
Moments causing eccentricity = MPah + MPq
8,025 Mo =8,025 ft-Ibs
e=L/2- (Mr -Mo)/N1
e=4.92/2 - (22,703 - 8,025) /8,177 a =0.59
e/L = 0.12
FSot = Mr / Mo
FSot=22,703 / 8,025 FSot =2.83
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 10
ECCENTRICITY AND BEARING
Eccentricity is the calculation of the distance of the resultant away from the centroid of mass. In wall design
the eccentricity is used to calculate an effective footing width.
Calculation of Eccentricity
SumV = (W1 + W2 + Pa_v)
e = L/2 - (SumMr - SumMo)/(SumV)
e=4.92/2 - (14,678 /7,841.23)
Calculation of Bearing Pressures
Quit =c*Nc+q*Nq+0.5*y*(B')*Ng
where:
Nc =61.35
Nq =48.93
Ng =78.02
c =50.00 psf
q = 120.00 psf
B'=B-2e+Ivlpad=4.24ft
Gamma(LP) =130 pcf
e =0.586 ft
Calculate Ultimate Bearing, Qult Qult=28,806 psf
Bearing Pressure = (SumVert / B') + ((2B + LP depth)/2 * LP depth * gamma) sigma=1908.89 psf
Calculated Factors of Safety for Bearing Qult/sigma =15.09
Note: Calculations and quantities are for PRELIMINARY ANALYTICAL USE ONLY and MUST NOT be used for final
design or construction without the independent review, verification, and approval by a qualified professional
engineer.
UltraWall 5.0.18138 Page 11