Geotechnical_Report.pdf

Subject Final Geotechnical Design Memorandum

Project Name Mallard Avenue Bridge #1339, ACHD Project No. 221019

Attention Chris Dayley/PM Ada County Highway District

From Josh Butler, P.E./Jacobs Engineering Group Inc. (Jacobs)

Date October 18, 2021

Copies to George Van Horn/Jacobs

This technical memorandum has been prepared to provide geotechnical recommendations for use in designing the foundations for the replacement bridge structure as part of the Mallard Avenue Bridge project.

1. Background

This project includes the design and preparation of plans, specifications, and estimates for replacing the existing culverts that convey Loggers Creek under Mallard Drive (ACHD Bridge Number 1339) with a new structure in southeast Boise. The existing crossing includes twin 84-inch diameter corrugated metal pipe

(CMP) culverts that are approximately 82 feet long. The need for the replacement is driven by ACHD’s plan to replace the aging CMP culverts, and also to improve safety and stream passage at this location.

The existing project site is a roadway connecting commercial and residential property on either side of

Loggers Creek, which is a side channel to the Boise River. Loggers Creek flows north through the project site and roughly intersects Mallard Drive at a perpendicular angle, approximately 650 ft west of East

Parkcenter Boulevard and approximately 400 ft south of Parkcenter Park and pond. Figure 1 shows the project location including the two borings drilled as part of the geotechnical exploration.

1.1 Scope of Work

Jacobs has performed geotechnical engineering to provide foundation recommendations for the replacement bridge. The scope of work included the following tasks:

Perform a geotechnical exploration to evaluate subsurface conditions

Provide recommendations for foundation design of the replacement structure, including allowable bearing pressures, estimated lateral earth pressures, design recommendations, and earthwork requirements for construction

Summarize results and recommendations in this Technical Memorandum.

Pavement design for the crossing is not part of the scope of work for this project, and the roadway section will be provided by ACHD. The proposed structure will include a cast-in-place concrete slab over the creek and concrete pavement to carry traffic, to limit the vertical section thickness over the creek channel.

Draft Geotechnical Design Memorandum, 95 Percent Submittal

Figure 1. Project vicinity and boring location plan.

1.2 Limitations

This memorandum was prepared for the exclusive use of ACHD and Jacobs Engineering Group Inc.

(Jacobs) for specific application to the Mallard Drive Bridge Project. This memorandum has been prepared in accordance with geotechnical engineering practice. No other warranty, expressed or implied, is made.

The analysis and design values contained in this memorandum were based on data obtained from a field exploration performed in 2021. The borings advanced for this project indicate subsurface conditions only at specific locations and times, and only to the depths associated with each exploration. They do not necessarily reflect strata or water level variations that may exist between locations. If variations in subsurface conditions from those noted herein are discovered during the course of excavation and construction, reevaluation of the exploration and design analyses may be necessary.

If changes in the nature, design, or location of the facilities are made, the recommendations contained in this report should not be considered valid unless the changes are reviewed and the recommendations of this report modified or verified in writing. Jacobs is not responsible for any claims, damages, or liability associated with interpretation of subsurface data, reuse of the data, or reuse of engineering recommendations without the express written authorization of Jacobs.

2. Technical Information

Jacobs conducted a geotechnical exploration on February 2, 2021, that included advancing two soil borings, one on each side of the existing structure. Locations of the borings (including approximate horizontal coordinates and elevation) are shown in Figure 1. The borings were drilled by Haz Tech Drilling, Inc., based in Nampa, Idaho, under subcontract to Jacobs. A Jacobs geotechnical engineer observed the drilling program and maintained a written log of drilling conditions during the investigation. The boring logs are provided in Attachment A.

For each of these borings, standard penetration test (SPT) samples were collected for laboratory testing and the blow counts were recorded by a geotechnical engineer. The SPT was conducted using a CME automatic hammer. Complete laboratory test results are included in Attachment B.

2.1 Subsurface Profile

Subsurface conditions consist of fill derived from Boise River alluvium, and in-place alluvium, for the full depth of exploration (31.5 ft maximum). This corresponds to an approximate elevation for the limit of exploration of 2683 ft. The proposed bottom of the new stream channel is approximately el. 2706 ft, and the top of footings for the stiff leg culvert structure are proposed to be approximately 2704 ft. This includes a minimum of two feet of streambed material placed in the channel above the top of footings

(refer to Figure 3).

Boring MDB-1 was advanced east of the creek and in the westbound lane of Mallard Drive, through the asphalt roadway. Asphalt thickness was typically 4 inches, overlying 4 inches of base aggregate at this boring and also at MDB-2. Beneath a thin roadway section, mixed gravel, cobbles, and sand were encountered to 31.5 ft bgs. This material consisted of rounded alluvium, with a density of medium dense to very dense, typically with more than 50 percent gravel and cobbles, approximately 30 percent sand, and less than 10 percent fines. As noted by drilling resistance, some zones are dominated by more sand while other zones are dominated by more cobbles. This material extends to the maximum explored depth of 31.5 feet bgs.

Boring MDB-2 was advanced west of the creek, in the eastbound lane of Mallard Drive. Beneath the asphalt roadway section, mixed gravel, cobbles, and sand were encountered in this boring, very similar in density and structure to MDB-1.

Groundwater was encountered in both borings, at a depth between 7.8 and 8.4 ft bgs at the time of drilling. This depth is roughly equivalent to the water surface elevation within the creek at the time of drilling. Subsurface properties are summarized in Table 1, and more detailed descriptions of the subsurface conditions encountered can be found in Attachment A.

Table 1. Subsurface Conditions — Mallard Drive Bridge #1339

Soil Type

Depth to

Top of

Layer

(feet)

Layer

Thickness

(feet)

Consistency/

Density/Hardness

Field SPT

N-Value γmoist

(pcf)

Estimated φ’ (deg)

Estimated su

(psf)

Poorly Graded

Gravel, Cobbles, and

Silty Sand

<1 >30 Medium Dense to Very

Dense

48 (avg.;

range is 15 to Refusal)

125 34 0

Abbreviations:

SPT = standard penetration test γmoist = in situ moist unit weight of soil pcf = pounds per cubic foot psf = pounds per square foot

Refusal is >50 blows per 6 inches φ’ = effective stress friction angle su = undrained shear strength

N/A = not applicable

3. Evaluation and Recommendations

The proposed structure is a concrete stiff leg box culvert with an opening that is roughly 22 ft wide by 48 ft long, with a precast concrete slab to support traffic. A preliminary plan view of the structure is shown in

Figure 2; a cross section of the box culvert is shown in Figure 3.

Figure 2. Proposed box culvert structure plan view.

Figure 3. Proposed box culvert structure section.

3.1 Bearing Resistance

The box culvert will be supported on compacted structural fill overlying native soils. Recommendations pertaining to the load and resistance factor design (LRFD) foundation design for the proposed bridge are presented in the next sections. Table 2 shows the resistance factors recommended for use in the design of spread footings.

Table 2. Resistance Factors for Shallow Foundation Design

Limit State Bearing Resistance

Bottom Sliding

(cast-in-place on sand)

Passive Earth Pressure

Resistance on the Side of the Footing

Strength I 0.45 0.80 0.50

Extreme I/Service I 1.00 1.00 1.00

Note: Source: AASHTO, 2014; Table 10.5.5.2.2-1.

The ultimate (nominal) bearing resistance was evaluated in accordance with the American Association of

State Highway and Transportation Officials’ (AASHTOs’) (2020) guidance. The ultimate bearing resistance is used as the Extreme I Limit State Resistance (resistance factor is equal to 1). For the box culvert foundation, the recommended ultimate bearing resistance is 11,100 to pounds per square feet (psf).

For the Strength I Limit State, a resistance factor is applied to the net bearing resistance (nominal resistance minus vertical earth pressure). As listed in Table 2, a resistance factor of 0.45 applied to the net bearing resistance defined the Strength I Limit State bearing resistance is 5,000 psf. It is anticipated that settlement of the structure will be less than 1 inch and will occur immediately following construction.

3.2 Earth Pressures and Lateral Resistance

Well-graded sand and gravel backfill is recommended for backfill behind the abutments and wingwalls.

The wingwalls and abutment walls must be designed to resist static lateral earth pressures. These lateral earth pressures were estimated following methods summarized in AASHTO (2020) for retaining structures.

Three states of static soil pressure are identified in the AASHTO document, depending on the amount and direction of wall movement:

• At-rest state. These walls are restrained from yielding.

• Active state. These walls are free to move away from the backfill. The amount of movement to mobilize active earth pressures is very small for granular materials—typically 0.001 to 0.002 times the wall height for granular soils.

• Passive state. These walls move toward the backfill. The movement to mobilize passive earth pressures is much larger than for active earth pressures—typically 0.01 to 0.02 times the wall height.

For compacted backfill, the lateral earth pressures given as equivalent fluid unit weights in Table 3 should be used as a basis of design. The resulting active and passive forces act at a distance H/3 above the base of the abutment wall or base of the spread footing, where H is equal to the height above the lowest adjacent grade.

Table 3. Static Lateral Earth Pressure Summary

Earth Pressure Condition Coefficient of Lateral Earth

Pressure (K) Equivalent Fluid Weight (pcf)

At-rest, Ko 0.44 57

Active, Ka 0.25 33

Passive, Kp 6.90 896

Notes:

1. Coefficient of lateral earth pressure reported for horizontal backfill for at-rest and active conditions, and horizontal foreslope within a distance equal to or greater than three times the wall embedment for the passive condition.

2. Passive pressure assumes interface friction is 50 percent of the backfill friction angle. No reductions made for deformation compatibility.

The lateral earth pressure coefficients and equivalent fluid unit weights were estimated for imported clean, free-draining granular soil. This material was assumed to have a compacted total unit weight of 130 pounds per cubic feet (pcf) and a friction angle of 34 degrees. The wingwalls and abutment walls should also be designed to resist surcharge loads of nearby at-grade structures, material stockpiles, or vehicles.

Effects of surcharge loads can be accounted for by multiplying the surcharge load by the coefficient of earth pressure given in Table 3.

3.3 Construction Considerations

Considerations for performing earthwork including excavations, subgrade preparation, and fill and backfill are discussed next.

3.3.1 Excavations

Excavations should be sloped and shored as necessary to protect workers and existing facilities, and to comply with federal, state, and local regulations. Excavation slopes shall be stepped as necessary, so that maximum vertical dimensions for any bench is 4 feet or less. Material encountered in the borings suggest free draining sand, gravel, and cobble will be present—perched or saturated zones are unlikely in this material, but may be encountered if fine-grained zones of fill are present for the existing embankment crossing. Saturated zones in this material or beneath the groundwater table are anticipated to present significant challenges with temporary stability and the extent of excavation. Excavations beneath the groundwater table are anticipated to require shoring to prevent excessive sloughing.

Diversion of surface water (Loggers Creek streamflow) will be required during construction of the stiff leg culvert foundations. Cofferdams and active dewatering may also be necessary for inspecting and preparing subgrade, and for forming and pouring footings.

3.3.2 Subgrade Preparation

Loose material encountered in excavations should be overexcavated and replaced with structural fill. The subgrade should be inspected for loose zones or zones of excessive moisture. Loose or wet soil encountered at the subgrade should be excavated and replaced with structural fill and compacted to

95 percent relative compaction in accordance with ASTM D698. A Type I Subgrade Separation Geotextile should be placed over the prepared subgrade surface, as specified in the Idaho Transportation

Department (ITD) Standard Specifications for Highway Construction (2017 edition).

3.3.3 Fill and Backfill

A minimum of 6 inches structural fill is recommended below the structure foundation and should be placed over the separation fabric following subgrade preparation. Structural fill or backfill adjacent to the abutments and leveling material beneath foundations should consist of well-graded granular material, with a maximum of 10 percent fines (passing U.S. No. 200 Sieve). Structural fill should be placed with a maximum loose lift thickness of 8 inches and compacted to a minimum of 95 percent relative compaction

(in accordance with ASTM D698).

4. References

American Association of State Highway and Transportation Officials (AASHTO). 2020. LRFD Bridge Design

Specifications. September. 9th Edition.

Bowles, J.E. 1977. Foundation Analysis and Design. 5th Edition. McGraw Hill.

Idaho Transportation Department (ITD). 2017. Standard Specifications for Highway Construction.

Naval Facilities Engineering Command. 1986. Design Manual 7.01 Soil Mechanics.

Vesic, A.S. 1973. “Analysis of Ultimate Loads of Shallow Foundations.” Journal of the Soil Mechanics and

Foundations Division, American Society of Civil Engineers, Vol. 99, No. SM1.

Attachment A

Boring Logs

SS-1

SS-2

SS-3

SS-4

13-17-20 (37)

7-8-7 (15)

20-26-32 (58)

8-15-13 (28)

1.0

5.0

10.0

15.0

2.5

6.5

11.5

16.5

HOT MIX ASPHALT AND BASE MATERIAL

POORLY GRADED SAND WITH GRAVEL [FILL]

(SP)

brown, dry to moist, dense, approx. 25% fine to coarse gravel

SILTY SAND WITH GRAVEL [FILL] (SM)

brown becoming dark gray below 5.8' bgs, moist, medium dense, approx. 20% fines, approx. 15-20% fine gravel

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

gray, wet, very dense, subrounded to subangular gravel, fine to medium grained sand, approx. 10% fines

POORLY GRADED GRAVEL WITH SAND (GP)

gray to light gray, wet, medium dense, subrounded fine to coarse gravel, approx. 30-40% coarse grained sand, clean, granitic

1.1 1.5

1.5 1.5

1.5 1.5

1.3 1.5

4" HMA, 4" gravel base

SPT conducted with CME Autohammer

Driller notes gravel and cobbles below 2.5 ft., relatively easy/loose drilling

Groundwater observed at 7.8' bgs 15 minutes after break

Driller notes some cobbles

6"-6"-6" (N)

PROJECT NUMBER:

DRILLING CONTRACTOR : Haz Tech (M. Corn)

LOCATION : NE Corner of Mallard Dr. and Loggers Crk Crossing in WB roadway

STANDARD

PENETRATION

TEST RESULTS

#TYPE

D3463900 MDB-1

BORING NUMBER:

START : 2/2/2021

DRILLING METHOD AND EQUIPMENT : HSA, HQ Triple Tube (CME 85)

PROJECT : Mallard Ave Bridge #1339

SOIL DESCRIPTION

SHEET 1 OF 2

INTERVAL (ft)

SOIL BORING LOG

LOGGER : J. Butler, PE

D E

P T

H B

E

LO

W S

U R

F A

C E

A N

D E

LE

V

A T

IO

N ft)

G R

A P

H

IC

L O

G

DEPTH INTERVAL, SOIL NAME, USCS GROUP

SYMBOL, COLOR, MOISTURE CONTENT,

RELATIVE DENSITY OR CONSISTENCY, SOIL

STRUCTURE, MINERALOGY

RECOVERY (ft)

END : 2/2/2021WATER LEVELS : 7.8 ft bgs

ELEVATION :

COMMENTS

SS-5

SS-6

SS-7

15-49-34 (83)

14-27-40 (67)

18-32-30 (62)

20.0

25.0

30.0

21.5

26.5

31.5

POORLY GRADED GRAVEL WITH SAND (GP)

gray to light gray, wet, medium dense, subrounded fine to coarse gravel, approx. 30-40% coarse grained sand, clean, granitic, single large piece of gravel wedged in shoe

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

gray, wet, very dense, mostly fine gravel with few >2", coarse grained sand, approx. 10% fines

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

gray to tan, wet, very dense, more coarse gravel than SS-6, medium to coarse grained sand, approx. 15% fines Bottom of Boring at 31.5 ft below ground surface.

1.4 1.5

1.4 1.5

1.4 1.5

Driller notes some cobbles

Boring backfilled with bentonite and compacted cold patch at surface

6"-6"-6" (N)

PROJECT NUMBER:

DRILLING CONTRACTOR : Haz Tech (M. Corn)

LOCATION : NE Corner of Mallard Dr. and Loggers Crk Crossing in WB roadway

STANDARD

PENETRATION

TEST RESULTS

#TYPE

D3463900 MDB-1

BORING NUMBER:

START : 2/2/2021

DRILLING METHOD AND EQUIPMENT : HSA, HQ Triple Tube (CME 85)

PROJECT : Mallard Ave Bridge #1339

SOIL DESCRIPTION

SHEET 2 OF 2

INTERVAL (ft)

SOIL BORING LOG

LOGGER : J. Butler, PE

D E

P T

H B

E

LO

W S

U R

F A

C E

A N

D E

LE

V

A T

IO

N ft)

G R

A P

H

IC

L O

G

DEPTH INTERVAL, SOIL NAME, USCS GROUP

SYMBOL, COLOR, MOISTURE CONTENT,

RELATIVE DENSITY OR CONSISTENCY, SOIL

STRUCTURE, MINERALOGY

RECOVERY (ft)

END : 2/2/2021WATER LEVELS : 7.8 ft bgs

21-26-45 (71)

30-38-34 (72)

4-12-20 (32)

50/6" (R)

1.0

5.0

10.0

15.0

2.5

6.5

11.5

15.5

HOT MIX ASPHALT AND BASE MATERIAL

POORLY GRADED GRAVEL WITH SILT AND SAND

[FILL] (GP-GM)

brown/gray, dry, very dense, mixed lithology, coarse gravel, medium grained sand, approx. 5% fines, rock flour from SPT

POORLY GRADED GRAVEL WITH SILT AND SAND

[FILL] (GP-GM)

brown/gray, dry, very dense, mixed lithology, coarse gravel, medium grained sand, approx. 5% fines, rock flour from SPT

SILTY SAND WITH GRAVEL (SM)

gray, wet below 11 ft., medium dense to dense, approx.

30% subrounded gravel, approx. 20% fines

POORLY GRADED GRAVEL WITH SAND (GP)

gray, wet, very dense, large granitic gravel in shoe

1.5 1.5

1.5 1.5

1.5 1.5

0.5 0.5

3.5" HMA, 4.5" base

SPT conducted with CME Autohammer

Groundwater measured at 8.4 ft. bgs after 15 minute break

Very "boney" slow drilling on large cobbles below 12 ft.

6"-6"-6" (N)

PROJECT NUMBER:

DRILLING CONTRACTOR : Haz Tech (M. Corn)

LOCATION : SW Corner of Mallard Dr. and Loggers Crk Crossing in EB roadway

STANDARD

PENETRATION

TEST RESULTS

#TYPE

D3463900 MDB-2

BORING NUMBER:

START : 2/2/2021

DRILLING METHOD AND EQUIPMENT : HSA, HQ Triple Tube (CME 85)

PROJECT : Mallard Ave Bridge #1339

SOIL DESCRIPTION

SHEET 1 OF 2

INTERVAL (ft)

SOIL BORING LOG

LOGGER : J. Butler, PE

D E

P T

H B

E

LO

W S

U R

F A

C E

A N

D E

LE

V

A T

IO

N ft)

G R

A P

H

IC

L O

G

DEPTH INTERVAL, SOIL NAME, USCS GROUP

SYMBOL, COLOR, MOISTURE CONTENT,

RELATIVE DENSITY OR CONSISTENCY, SOIL

STRUCTURE, MINERALOGY

RECOVERY (ft)

END : 2/2/2021WATER LEVELS : 8.4 ft bgs

19-27-50/5.5" (R)

13-24-30 (54)

26-49-50/3" (R)

20.0

25.0

30.0

21.5

26.5

31.2

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

gray, wet, very dense, fine to coarse gravel, coarse grained sand, approx. 5% fines

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

brown to tan, wet, very dense, fine to coarse gravel, coarse grained sand, approx. 10-15% fines

POORLY GRADED GRAVEL WITH SILT AND SAND

(GP-GM)

gray, wet, very dense, fine to coarse subrounded granitic gravel, coarse grained sand, approx. 5-10% fines Bottom of Boring at 31.2 ft below ground surface.

1.5 1.5

1.4 1.5

1.2 1.5

Driller notes less heavy cobbles below 24 ft.

Short zones of fast drilling (predominantly sand?)

Boring backfilled with bentonite and compacted cold patch at surface

6"-6"-6" (N)

PROJECT NUMBER:

DRILLING CONTRACTOR : Haz Tech (M. Corn)

LOCATION : SW Corner of Mallard Dr. and Loggers Crk Crossing in EB roadway

STANDARD

PENETRATION

TEST RESULTS

#TYPE

D3463900 MDB-2

BORING NUMBER:

START : 2/2/2021

DRILLING METHOD AND EQUIPMENT : HSA, HQ Triple Tube (CME 85)

PROJECT : Mallard Ave Bridge #1339

SOIL DESCRIPTION

SHEET 2 OF 2

INTERVAL (ft)

SOIL BORING LOG

LOGGER : J. Butler, PE

D E

P T

H B

E

LO

W S

U R

F A

C E

A N

D E

LE

V

A T

IO

N ft)

G R

A P

H

IC

L O

G

DEPTH INTERVAL, SOIL NAME, USCS GROUP

SYMBOL, COLOR, MOISTURE CONTENT,

RELATIVE DENSITY OR CONSISTENCY, SOIL

STRUCTURE, MINERALOGY

RECOVERY (ft)

END : 2/2/2021WATER LEVELS : 8.4 ft bgs

Attachment B

Laboratory Test Results

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GRAVEL: 54%

SAND: 40%

SILT/CLAY: 6%