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Foundation Design Recommendation Technical Memorandum

For

Frick Springs Bridge Lompoc, California

Prepared For The City of Lompoc

BY

August 15, 2006

250 Big Sur Dr., Goleta, CA 93117

Phone: (805) 685-6511 E-Mail: [email protected]

TECHNICAL MEMORANDUM To: MR. MICHAEL W. LUTHER, P.E. Date: August 15, 2006 Senior Civil Engineer Engineering Division City of Lompoc, CA From: Bengal Engineering, Inc.

250 Big Sur Drive Goleta, CA 93117

Subject: Foundation design recommendations for the Frick Springs Bridge. This technical memorandum presents the results of the subsurface exploration and foundation design

recommendations for the proposed Frick Spring Bridge construction project. The project involves

construction of a single span bridge, consisting of a pre-fabricated superstructure supported by two seat-

type abutments. The proposed bridge will span Miguelto Creek at a location approximately four (4) miles

south of the City of Lompoc, California. The location of the project is shown in Figure 1.

Subsurface Conditions Our evaluation of the subsurface conditions at the project site is based on drilling two exploratory borings

at the locations shown in the Plate 1. The exploratory borings are located near and east of the proposed

Abutment 2. No exploratory borings could be drilled near or west of the proposed Abutment 1, because

this area was not accessible to the drill rig. The exploratory borings were drilled on March 3, 2006 using a

CME 75 drill rig. These borings were drilled to a depth of about 35.5 feet below existing ground surface.

Subsurface materials encountered during the drilling were examined and classified in the field in

accordance with the Unified Soil Classification System. Standard Penetration Tests (SPT) blow counts

were measured at selected depths as shown in the Plates A-1 and A-2 included in Appendix A of this

technical memorandum.

Based on the results of our subsurface exploration, the soil profiles at the locations of the exploratory

borings consist of 5 to 10 feet of artificial fill over colluvium and alluvium or natural soils. The thickness of

colluvium and alluvial soils at the boring locations ranged from about 5 to 10 feet. These soil units consist

of mainly Sandy Silt, Silty Clay and Gravel with scattered rock fragment. The above soil layers are

underlain by bedrock or formation material to the maximum explored depth of 35.5 feet. The bedrock

ranged from highly weathered, highly fractured, slightly hard to slightly weathered, slightly fractured,

moderately hard shale, siltstone and sandstone. Features generally associated with tectonic movement

and/or slide debris (e.g. sheared or polished surfaces) were observed within the highly weathered bedrock

encountered between the depths of 10 feet and 19 feet at the location of the Boring B-2. No surface

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Bengal Engineering, Inc.

Frick Springs Bridge Foundation Design Recommendation Technical Memorandum

surface features that would indicate the presence of an active landslide in the area were observed.

Groundwater, except a small amount of perched water at a depth of 10 feet at the location of the

Boring B-1, was not encountered in the exploratory borings at the time of drilling. Groundwater

conditions can vary depending on the time of the year or season, amount of rainfall and other

hydrologic conditions, however.

Seismic Design Recommendations

The project site is located within the westernmost portion of the seismically active Western Traverse

Ranges physiographic province of Southern California. The region has experienced numerous major

earthquakes in the past (e.g., 1927 Mw7.1 Lompoc Earthquake) and is likely to experience significant

earthquakes in the future. Based on the CALTRANS California Seismic Hazard Map (CSHM, 1996), the

Santa Ynez River (SYR) fault is the nearest known active seismic source. This unknown style fault is

located about 6.0 km north of the project site. This fault is capable of generating a Maximum Credible

Earthquake of moment magnitude Mw7.5. Based on CSHM (1996), the project site is located within a

Peak Bedrock Acceleration (PBA) zone of 0.5g.

Based on the subsurface conditions encountered in the exploratory boring, the soil profile at the site can

be classified as Type C as defined in the Caltrans Seismic Design Criteria (SDC, 2004). The

recommended design Acceleration Response Spectrum (ARS) for structure design is shown on the

attached Figure 2. The recommended ARS curve was obtained by applying modifications to the standard

SDC ARS curve for M=7.25±0.25, PBA=0.5g, and Soil Profile Type C due to near fault effects since the

site is located less than 15 km from the controlling seismic sources. The modifications for the near fault

effects were introduced as recommended in the SDC (2004).

The project site is not located within any State of California designated Alquist-Priolo Earthquake Fault

Zone (CGS, 1999). The site is not considered susceptible to surface rupture hazard due to fault

movements. The subsurface soils at the site are not considered prone to liquefaction during seismic

shaking. The potential hazard associated with other secondary seismic hazards including lateral

spreading, slope failure or landslide hazards and tsunami is considered low.

Foundation Design Recommendations Shallow foundations (spread footings) are not recommended for this bridge structure due to scour

concerns and soil conditions at shallow depths. Driven piles are not recommended due to potential driving

difficulties. Based on the subsurface soil conditions including the soil type, dense/hard nature of the

Formation material and the absence of groundwater, deep foundations consisting of Cast-In-Drilled-Hole

(CIDH) piles are considered most appropriate for this project site. Based on the load demand and the

results of our pile capacity analyses, as presented below, 16-inch diameter CIDH piles are recommended

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recommended for the support of the proposed structure.

An analysis was performed to evaluate the geotechnical nominal resistance in axial compression for

different diameter CIDH piles as a function of the pile length. This analysis was performed in accordance

with the method recommended in the Federal Highway Administration (FHWA, 1999) manual entitled

“Drilled Shafts: Construction Procedures and Design Methods”. Soil shear strength parameters including

friction angles for the cohesionless soils and undrained shear strength for the cohesive soils were

estimated based on the measured SPT blow counts. Results of this analysis for the 16-inch diameter

CIDH pile are presented in Figure B-1 in Appendix B. The design pile tip elevation based on the results of

this analysis and the required design nominal resistance in compression is presented in Table 1.

Table 1. Pile Data Table

Nominal Resistance (kips)

Location

Pile Type

Design Loading(kips) Compre-

ssion Tension

Cut-Off Elev. (ft)

Design Tip Elev.

(ft)

Specified Tip Elev.

(ft)

Abut 1 16-inch CIDH 45 90 0 834 814 (1), 812(2) 812

Abut 2 16-inch CIDH 45 90 0 834 814(1), 812(2) 812

Notes: Design tip elevation is controlled by the following demand: (1) Compression, (2) Lateral

An analysis was also performed to evaluate the response of the proposed 16-inch diameter CIDH piles

under the design lateral load from the retained soil. This analysis was performed in accordance with the p-

y method of analysis and using the computer program LPILE Plus 5 (Ensoft, Inc., 2005). The required soil

design parameters determined based on the measured SPT blow counts are presented in Tables 2 and 3

below.

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Table 2. Recommended Design Soil Parameters at Abutment 1 for LPILE Analysis

Layer Approx. Elevation

Range (ft) Generalized Soil

Description Unit Wt.

(pcf)

Average Strength

Parameters

K -value (pci)

ε50

1 842 to 834 Sandy Silt (ML), medium dense

120 c=500 psf, φ=30o

100 0.005

2 834 to 822 Silty Clay (CL)/Weathered

bedrock, Shale, stiff

120 c=2000 psf, φ=0o

667 0.005

3 822 to 817 Shale, moderately weathered, slightly to

moderately hard

125 c= 3000 psf φ=0o

1000 0.005

4 817 to 807 Siltstone/Sandstone, moderately hard

125 c=500 psf, φ=34o

100 0.005

Table 3. Recommended Design Soil Parameters at Abutment 2 for LPILE Analysis

Layer Approx. Elevation Range (ft)

Generalized Soil Description

Unit Wt.

(pcf)

Average Strength

Parameters

K-value (pci)

ε50

1 842 to 834 Sandy Silt (ML), medium dense (fill)

120 c=500.0 psf, φ=30o

100 0.005

2 834 to 823 Siltstone,, highly fractured and weathered, slightly hard.

125 c= 500psf, φ=34o

90 0.005

4 817 to 807 Siltstone/Sandstone, moderately hard

125 c=500 psf, φ=34o

100 0.005

Results of the laterally loaded pile analysis are presented in Appendix B of this report. These results are

also summarized in Table 4 below.

Table 4. Summary of Laterally Loaded Pile Analyses

Applied Load Location

Lateral Load (kip)

Moment (kip-ft)

Pile Top Deflection

(inch)

Maximum Moment, Mm

(kip-ft)

Depth of Mm (ft)1

Maximum Shear, Vmax

(kip)

Depth of Vmax (kip)1

B-1 13.6 42.5 0.15 58.8 2.4 14.6 5.8 B-2 13.6 42.5 0.15 55.0 2.0 13.6 0.0

Note: Depth from the pile cut-off elevation.

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The design pile tip elevations based on the results of the laterally loaded pile analyses is also presented in

Table 1. Based on these results and the proposed pile cut-off elevation of 834 feet above Mean Sea

Level (MSL), the specified tip elevation for the proposed 16-inch diameter CIDH piles is 812 feet above

MSL for both abutments.

Foundation Settlement The total settlement of the top of the 16-inch diameter CIDH piles founded at the specified tip elevation

and subjected to an axial compression load equal to the design nominal axial resistance is estimated to be

less than 0.5 inches. Differential settlement between the two abutments may be taken as equal to half of

the total settlement. The majority of the estimated foundation settlement should occur immediately after

construction. Abutment Backfill In order to reduce static lateral earth pressure, the structure backfill placed immediately behind the

abutment walls can be mechanically stabilized using reinforcing tension elements such as geo-synthetics.

It is estimated that a lateral movement on the order of about 0.1 inches of the 8 feet high abutment wall

will be required to mobilize full active pressure. A gap of this amount between the back of the abutment

wall and the face of reinforced-soil should reduce lateral earth pressure on the abutment wall.

It is recommended that a 3-inch thick compressible low-density polystyrene (geofoam) sheet be placed

between the abutment wall and the reinforced abutment backfill. Care should be exercised during the

placement and compaction of the abutment backfill to avoid applying significant pressure on the geofoam

sheet. One method of achieving this is to use a thin (2- to 3-inch thick) wooden slip form between the

abutment backfill and the geofoam during placement and compaction of the last layer of the reinforced

backfill. This slip form is removed after the required soil compaction is achieved and used with the

construction of the next layer. This process should leave a small gap between the abutment backfill and

the geofoam that can be backfilled with self-compacting clean coarse sand or pea gravel after the

construction of the reinforced abutment backfill is completed. Only small compaction equipment such as

hand-held vibrating plate should be used near the abutment wall. Subsequent lateral movement, if any, of

the reinforced abutment backfill should results in only a small compression of the geofoam without

transferring any significant lateral load to the abutment wall.

For design purpose, we conservatively recommend that the lateral earth pressure be taken as about 3/4 of

that estimated for the un-reinforced abutment structure backfill assuming a friction angle of 34 degree.

For seismic design, the passive resistance of the abutment backfill may be estimated as recommended in

the SDC (2004).

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Pile Construction Considerations/Recommendations

Somewhat difficult drilling conditions should be anticipated due to dense/hard conditions of the

formation material.

Depending on the time of the year and rainfall conditions, some seasonal or temporary perched

groundwater may be present near the formation and the overburden contact.

Concrete should be poured in the dry hole. The bottom of the drilled hole should be cleaned of any

loose material prior to pouring concrete.

Temporary steel casing, if used, should be installed with an impact hammer. Jetting and/or

vibration should not be allowed. Temporary casing, if used, should be withdrawn as the concrete is

poured.

Limitations Due to the topography and hydrologic conditions, inherent uncertainty in the geologic processes,

and the absence of an exploratory boring near Abutment 1, some variations in the subsurface

conditions at the abutment locations than those encountered at the location of the exploratory

borings should be expected. If subsurface conditions encountered during construction are

significantly different than those presented in this report, this office should be contacted immediately

so that modifications to the foundation recommendations, if needed, could be provided. This report

has been prepared in accordance with the state of practice at this time in Southern California. No

other warranty as to the content of this report is either intended or implied.

If you have any questions or comments on the content of this memorandum, please contact the

undersigned at 805-685-6511.

Very truly yours,

Md. Wahiduzzaman, P.E.

Principal

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-References- California Department of Transportation (2004), Seismic Design Criteria, Version 1.3, Sacramento, California. California Geological Survey (CGS, 1999) Earthquake Fault Zones in California, Special Publication 42. Ensoft (2005), LPILE Plus 5 , A Program for the Analysis of Piles and Drilled Shafts Under Lateral Loads, Austin, Texas. Federal Highway Administration (1999), Drilled Shafts: Construction Procedure and Design Methods, FHWA Report No. FHWA-IF-99-025, Washington, D. C. California Seismic Hazard Map (CSHM, 1996), Based on Maximum Credible Earthquakes (MCE), Prepared by L Mualchin, California Department of Transportation, Revision 1, July, 1996.

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FIGURES AND PLATES

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Bengal Engineering, Inc.10 of 24


0

0.4

0.8

1.2

0 0.5 1 1.5 2 2.5 3 3.5 4

Period (sec)

Spec

tral

Acc

eler

atio

n (g

) Standard SDC ARS

Recommeded Design ARS

Earthquake Magnitude =7.5, PBA=0.5g, Soil Profile Type C Damping = 5 %

Figure 2. Recommended ARS Curve for the Frick Spring Bridge

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APPENDIX A

EXPLORATORY BORING LOGS

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APPENDIX B

RESULTS OF AXIAL AND LATERALLY LOADED PILE ANALYSIS

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0

5

10

15

20

25

0 50 100 150 200Ultimate Skin Friction (kips)

Pile

Len

gth

(ft)

Soil Profile at Boring B-1

Soil Profile at Boring B-2

Figure B-1. Pile Lenth vs. Ultimate Skin Friction for 16-inch Diameter CIDH Piles

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Lateral Deflection (in)

Figure B-2. Depth vs. Lateral Deflection for the Soil Profile at Boring B-1

Dep

th (f

t)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

24

68

1012

1416

1820

22

H=13.6 kips, M=42.5 kip-ft

19 of 24


Unfactored Bending Moment (in-kips)

Figure B-3. Depth vs. Moment Curves for the Soil Profile at Boring B-1

Dep

th (f

t)

-100 0 100 200 300 400 500 600 700 8000

24

68

1012

1416

1820

22

H=23.6 kips, M=42.5 kips

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Shear Force (kips)

Figure B-4. Depth vs. Shear for the Soil Profile at Boring B-1

Dep

th (f

t)

-14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 120

24

68

1012

1416

1820

22


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Lateral Deflection (in)

Figure B-5. Depth vs. Lateral Deflection for the Soil Profile at Boring B-2

Dep

th (f

t)

0 0.02 0.04 0.06 0.08 0.1 0.12 0.140

24

68

1012

1416

1820

22


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Unfactored Bending Moment (in-kips)

Figure B-6. Depth vs. Moment for the Soil Profile at Boring B-2

Dep

th (f

t)

-100 0 100 200 300 400 500 600 7000

24

68

1012

1416

1820

22


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Shear Force (kips)

Figure B-7. Depth vs. Shear for the Soil Profile at B-2

Dep

th (f

t)

-8 -6 -4 -2 0 2 4 6 8 10 120

24

68

1012

1416

1820

22


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