HomeMy WebLinkAbout3557; Calavera Hills Phase II Bridge Retaining Walls; Calavera Hills Phase II Bridge Retaining Walls; 2008-02-06GROUP.
DELTA
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February 6, 2008
Simon Wong Engineering
9968 Hibert Street, Suite 202
San Diego, CA 92131
Attention: Mr. James Frost
Subject: Geotechnical Parameters
For Evaluation of Type 5 Retaining Walls
Carlsbad Boulevard Overhead (Bridge No. 57C-134)
Carlsbad, California
Dear James:
In response to your request, we are pleased to provide you geotechnical parameters
for evaluation of the Caltrans Standard Type 5 retaining walls constructed during
seismic retrofit ofthe Carisbad Boulevard Overhead Bridge. The walls experienced
excessive lateral movements following a water main break in the wall backfill. The
bridge is located along Carlsbad Boulevard approximately 0.6 mile northwesteriy of
Carisbad Village Drive (see Figure 1).
1.0 BACKGROOND
1.1 Existing Type 5 Walls
Type 5 retaining walls were constructed to retain the abutment slope east and west
of the Bent 2 footing retrofit. The Type 5 wall west of Bent 2 has a total length of
32 feet and bottom of footing elevation of El. -F24.75 feet, and the Type 5 wall east
of Bent 2 has a total length of 12 feet and bottoha of footing elevation of
El. -1-25.75 feet. Maximum wall design height is 8 feet, and the slope behind the
walls is reportedly about 2h: Iv, although the plans indicated a slope of 1.5h: Iv.
Footings are founded below the adjacent level ground. The foundation plan is
shown in Figure 2.
1.2 Wall Movement
We understand that a water line break occurred in the slope above the Type 5
standard retaining walls constructed below Bent 2, and that excessive wall
movements occurred subsequent to, and as a result of, the water line break. We
further understand that the contractor did not install the drainage system behind the
wall as required by the Standard Plans. Based on review of photographs provided
by Simon Wong, erosion and gullying occurred within the wall backfill, and lateral
movements of the wall of up to about 3 inches occurred. No information is available
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Recomnnended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineering
February 6, 2008
Page 2
on the seepage pattern, subsurface soil erosion, or how much of the wall backfill or
subgrade may have become saturated or eroded.
GROUP
DELTA, B.-j'i.m.iiaaifei
1.3 Previous Geotechnical Studies by Group Delta
Group Delta Consultants (GDC) performed 2 borings at the site, and provided
preliminary geotechnical recommendations for seismic retrofit and liquefaction
evaluation of the bridge structure in the following reports:
• Group Delta Consultants, March 5, 2001, "Recommended Geotechnical
Parameters, CaHsbad Boulevard Overhead (Bridge No. 57C-134), Seismic
Retrofit Project, San Diego County, California, Group Delta Project No. 1-147,"
prepared for Simon Wong Engineering;
• Group Delta Consultants, August 12, 1997, "Liquefaction Report, Carisbad
Overhead, Bridge No. 57C-134, Local Agency Seismic Retrofit Project, San
Diego County, California, Caltrans Contract No. 59Y025, Group Delta Project
No. 1-111," prepared for Moffat and Nichol Engineers.
• Group Delta Consultants, May 23, 1997, "Preliminary Geotechnical Report, Local
Agency Seismic Retrofit Project, Bridge No. 57C-134, Carisbad Overhead, San
Diego County, California, Caltrans Contract No. 59Y025, Group Delta Project
No. 1-111," prepared for Moffat and Nichol Engineers.
Logs and locations of 2 borings by GDC, along with lab test results, are included in
Appendix A.
GDC was not involved in the final design and did not provide design
recommendations for the subject retaining walls.
1.4 Soils Observation and Testing During Construction
Soils and compaction testing services were provided by Testing Engineers San
Diego (TESD) during construction. They performed lab testing of on-site materials
used as backfill including Maximum Dry Density and Optimum Moisture Content
(ASTM D1557), Grain Size Distribution, Sand Equivalent, Expansion Index, and
Direct Shear. TESD also provided field density testing to verify 90 to 95% relative
compaction for fill soils as required by project specifications. Their reports of soil
material testing and field density testing of on-site fill materials are included in
Appendix B.
GDC did not provide geotechnical observation of grading operations or testing
services during construction, and did not observe the Bent 2 excavation, subgrade,
backfill, or construction of the Type 5 walls.
f^:\Projects\_AV\1100\n47 Simon Wong Eng Carlsbad Overhead (bridge replacement)\Wall FaiiureMMT-Recommended Geoteclmical Parameters for Retaining Wall.doc
Recommended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineering
2.0 SUBSGRFACE CONDITIONS
February 6, 2008
Page 3
GDC Boring BH-2 was done near Abutment 1 (see Appendix A). Native soils below
the bottom of footing elevation at Bent 2 are dense to very dense Terrace Deposits
consisting of Silty to Clayey Sand and Sand with Standard Penetration Test (SPT)
blowcount or N-Value of 45 to 86 blows per foot, underlain by Clayey Sandstone
bedrock with SPT N-Value of 71 to 87 blows per foot. Perched groundwater was
found at El. 14.5 feet Boring logs are presented in Appendix A.
The parameters for subgrade and backfill materials herein are based on soils
information obtained during construction of the walls and provided by others, and
we did not perform an investigation of the wall failure. We understand that
temporary excavations were made to El. 17 feet to expose the Bent 2 footing to
allow for footing retrofit. This resulted in excavations of up to about 8 feet below
the Type 5 retaining wall footings founded at El. 24.75 to 25.75 feet. The
excavation below the Type 5 wall footings were reportedly backfilled with
non-expansive on-site granular materials (Sand with Silt and Silty Sand) compacted
to 95% relative compaction. Walls were reportedly backfilled with compacted
non-expansive on-site granular soils with minimum Sand Equivalent of 20 as
required by Caltrans Standard Specifications. Tests on backfill performed by TESD
showed that final fills were non-expansive, low fines content, granular materials with
high friction angle and achieved the specified relative compaction. Test results of
the reported fill materials also showed the following data:
• Abutment 5 Spoils of Footing Overexcavation - Yellowish Brown Poorly
Graded Sand with Silt (SP-SM): % passing No. 200 sieve = 7%, Expansion
Index = 1 (very low), Maximum Dry Density^ 113.5 pcf and Optimum Moisture
= 12% (ASTM D1557), Sand Equivalent = 23, cohesion of 450 psf and friction
angle of 37 degrees;
• Abutment 1 Spoils of Footing Overexcavation - Orange-Brown Silty Sand
(SM): % passing No. 200 sieve = 15%, Expansion Index = 4 (very low),
Maximum Dry Density= 123.5 pcf and Optimum Moisture = 11% (ASTM
D1557), Sand Equivalent = 20, cohesion of 1450 psf and friction angle of
.35 degrees.
These are the materials reportedly placed as compacted fill below the Type 5
footings and behind the walls. GDC has not observed or tested the original or
existing condition of the backfill below the wall footings or behind the wall.
ri:\Projects\_AV\l]00\ll47 Simon Wong Eng Carlsbad Overhead {bridge replacement)\Wal! FailureMl47-Recommended Geotechnical Parameters for Retaining Wall.doc
Recommended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineering
3.0 GEOTECHNICAL PARAMETERS FOR WALL EVALUATION
February 6, 2008
Page 4
Based on the reported subgrade and backfill properties (Appendix B), the following
parameters may be used to evaluate the subject Type 5 retaining walls for both
as-designed conditions and in the event of buildup of hydrostatic pressures.
3.1 Assumptions
We have made the following assumptions in our analysis:
• Earth pressures and bearing capacity analyses assume backfill soils below the
footings and behind the walls are properly compacted to project density
specifications, and that the engineering and grain size properties are as
presented in TESD's reports in Appendix B;
• Unsaturated conditions are applicable for the case where an adequate drainage
system is present behind the wall to prevent buildup of hydrostatic pressures;
• Saturated conditions, hydrostatic lateral pressures, and uplift on the footing are
applicable in the event of a serious water main break and absence of any
drainage system behind the wall.
3.1 Soil Parameters
Based on data provided by TESD, we recommend the following soil parameters for
the compacted granular backfill:
• Total Gnit Weight (not saturated) = 125 pcf;
. Total Gnit Weight (Saturated) = 132.5 pcf;
• Cohesion = 0
• Friction Angle = 35 degrees; and
• Wall Friction Angle = 25 degrees.
3.2 Lateral Earth Pressures
Active earth pressures were computed using Coulomb's method using the
parameters listed above. Assuming no hydrostatic pressures behind the walls, the
recommended earth pressures are summarized as:
• Active Earth Fluid Pressure (Level) = 36 pcf (minimum per Caltrans);
• Active Earth Fluid Pressure (2:1 slope) = 46 pcf;
• Active Earth Equivalent Fluid Pressure (1.5:1 slope) = 64 pcf; and
• Active pressures for sloping backfill may be inclined at 25 degrees to the
horizontal.
NAProjects\_AV\n00\n47 Simon Wong Eng Carlsbad Overhead (bridge replacement)\Wall FailureMl 47-Recommended Geotechnical Parameters for Retaining Wall.doc
Recommended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineenng
February 6, 2008
Page 5
For evaluation of the wall with hydrostatic pressures, the active earth pressures
above assumed water level are as listed above, and below the assumed water level
active earth pressures are reduced but full hydrostatic pressure is added to the active
pressures below the water level:
• Saturated Active Pressure (Level) =18 pcf;
• Saturated Active Earth Fluid Pressure (2:1 slope) = 23 pcf;
• Saturated Active Earth Equivalent Fluid Pressure (1.5:1 slope) = 32 pcf;
• Hydrostatic Pressure = 62 pcf (add to soil active pressure below assumed water
level);
• Active pressures for sloping backfill may be inclined at 25 degrees to the
horizontal; and
• Hydrostatic pressure is assumed to act horizontally.
3.3 Lateral Resistance
Resistance to sliding is provided by a combination of friction along the base of
footing and passive resistance on the front face of footing. Since the Type 5 wall
has a key, the sliding in front of the key occurs through the soil, and behind the key
sliding occurs along the concrete soil interface. The following friction coefficients
and passive resistance may be used for checking the design.
• Friction Coefficient, soiVsoil = 0.70
• Friction Coefficient, soil / concrete = 0.45
• GItimate Passive Fluid Pressure = 440 pcf (unsaturated);
• GItimate Passive Fluid Pressure = 210 pcf (saturated).
Sliding and passive resistance may be combined. A factor of safety of 1.5 is
required for sliding. For the case of hydrostatic pressures, uplift on the bottom of
footing due to water should be included in the sliding analysis (sliding friction
resistance is friction coefficient multiplied by the difference between the resultant
vertical force and the hydrostatic uplift force). Overturning is considered adequate if
the resultant on the base of footing falls within the middle 1/3 of the footing.
3.4 Bearing Capacity
We used Terzaghi's bearing capacity equation to compute the ultimate bearing
capacity of the wall footing. The Standard Plan for a Type 5 Wall is shown in
Figure 3. For a Type 5 wall with design height of 8 feet, footing width (B) is 6.5 feet
and bottom of footing is embedded a minimum of 2 feet below finished grade.
Maximum toe pressure for the worst case loading condition (Case IV, infinite 1.5:1
slope) is 4.2 ksf. Due to loading eccentricity, the pressure may be assumed
triangular on the footing base, with maximum toe pressure of 4.2 ksf and heel
N:\Projects\_AV\llOO\n47 Simon Wong Eng Carlsbad Overhead (bridge replacement)\Wall FailureMl 47-Recommended Geotechnical Parameters for Retaining Wall.doc
Recommended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineering
February 6, 2008
Page 6
pressure of 0 ksf. This places the resultant force at 1/3 the footing width behind the
toe. Following Meyerhofs procedure, the equivalent footing width (B') assuming the
resultant centered on the equivalent footing is 2/3 of the total width, or 4.33 feet,
and the equivalent uniform bearing pressure acting on the equivalent footing width
is 0.5*(4.2 ksO*B / (2/3B) = 0.75*4.2 ksf = 3.15 ksf Gsing Terzaghi's formula, a
friction angle of 35 degrees, saturated subgrade soil conditions with water at the
ground surface, and an equivalent footing width of 4.33 feet, the ultimate bearing
capacity is computed as 12.4 ksf Factor of safety is computed as the ultimate
bearing capacity over the applied bearing pressure (using the equivalent footing
width). Factor of safety is then FS = 12.4 / 3.15 = 3.9. This is greater than the
normally allowed FS=3.0, and therefore even with saturated subgrade and 1.5:1
backfill slope the bearing capacity is adequate. For flatter backfill slope and/or
unsaturated subgrade, factors of safety will be higher.
4.0 CONCLUSIONS
GiUJLJP
The eariih materials reported by TESD and used as backfill behind the wall are
suitable as wall backfill and if properly compacted meet or exceed the Caltrans
requirements for structure backfill of the Type 5 wall;
The materials reported by TESD and used as backfill below the wall footing are
suitable as footing subgrade if properiy compacted and exceed the bearing
capacity requirements for the Caltrans Type 5 wall up to 8 feet, even if the
subgrade materials were to become saturated;
Based on the data provided on wall subgrade and backfill materials and reported
degree of compaction, the wall would be expected to perform adequately under
normal circumstances without saturation of the backfill;
Likely and possible causes of the wall distress include:
o The water line break saturating the backfill;
o Buildup of hydrostatic pressures on the back of wall and below the
footing due to lack of proper drainage system;
o Water from the pipe failure eroding and washing out the backfill
material resulting in loss of material and loosening of the backfill;
o Water from the pipe failure built up in the backfill seeping below and
around the footing ("under-seepage") resulting in removal of subgrade
soil (piping) and loosening of the subgrade;
o Actual pressures exceeding the design pressures due to the reasons
listed above, and uplift pressures below the footing, both resulting in
reduced sliding and overturning factors of safety and wall movements.
n:\Projects\_AV\llOO\l 147 Simon Wong Eng Carlsbad Overhead (bridge replacement)\Wall FailureMl 47-Recommended Geotechnical Parameters for Retaining WalLdoc
Recommended Geotechnical Parameters for Retaining Wall Evaluation
Carlsbad Blvd. Overhead
Simon Wong Engineering
5.0 ATTACHMENTS
February 6, 2008
Page 7
The following Figures and Appendices are attached and complete this letter report:
Figure 1
Figure 2
Figure 3
Vicinity Map
Foundation Plan
Type 5 Wall Standard Plans
Appendix A
Appendix B
Existing Geotechnical Data from GDC
Field and Laboratory Test Results from TESD
We hope this report meets your immediate needs. We appreciate the opportunity to
assist you in this important project. If you have any questions, please call us at (949)
609-1020.
Sincerely,
GROUP DELTA CONSULTANTS, INC.
Curt Scheyhing, P.E., G.E.
Senior Engineer
Kul Bhushan, Ph.D., G.E. (Exp. 12/31/09)
President
GROUP
J
\
DELTA
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