Subsurface_Soil_Conditions_Report_-_Neary_and_Blackburn_Borings.pdf

February 2, 2024 Project No. SCR-1854

CITY OF SANTA CRUZ PUBLIC WORKS

Attention: Cina Seyedkazemi 809 Center Street, Room 201 Santa Cruz, CA 95060

Subject: Subsurface Soil Conditions

Reference: Sewer Rehabilitation Project Blackburn and Neary Streets

Santa Cruz, California

Dear Mr. Seyedkazemi:

As requested, we drilled two (2) exploratory borings for the sewer rehabilitation project proposed along Blackburn and Neary Streets in Santa Cruz, California, Figure 1. The purpose of our borings was to provide logs of the subsurface soil conditions along the proposed sewer alignment.

Field Exploration Subsurface conditions at the site were explored on 1 February 2024 with two (2) exploratory borings drilled with 6-inch diameter solid stem augers advanced with truck mounted drilling equipment. Our borings were drilled 10 and 15 feet deep. The approximate locations of the exploratory borings are indicated on Figure 1.

The soils observed in the test borings were logged in the field and described in accordance with the Unified Soil Classification System (D2487 and D2488), Figure 2. The Test Boring Logs denote subsurface conditions at the locations and times observed, and it is not warranted they are representative of subsurface conditions at other locations or times.

Representative soil samples were obtained from the exploratory borings at selected depths, or at major strata changes. These samples were recovered using the Standard Terzaghi Sampler (T). The penetration resistance blow counts for the (T) noted on the boring logs were obtained as the sampler was dynamically driven into the in-situ soil. The process was performed by dropping a 140-pound hammer a 30-inch free fall distance and driving the sampler 6 to 18 inches and recording the number of blows for each 6-inch penetration interval. The blows recorded on the boring logs present the accumulated number of blows that were required to drive the last 12 inches.

Subsurface Soil Conditions

The Santa Cruz County Geologic Map indicates the site is underlain by Alluvial deposits, undifferentiated (Holocene), which are described as, “Unconsolidated, heterogeneous, moderately sorted silt and sand containing discontinuous lenses of clay and silty clay. Thickness highly variable; may be more than 100 ft thick near coast”.

Our borings encountered clay the entire depth. The clay is soft becoming very soft around 7 feet.

Dees & Associates, Inc.

SCR-1854 | 2/2/24

Groundwater Groundwater was encountered 5 feet below grade at Blackburn Street and 4 feet below grade at Neary Street. Groundwater levels denote groundwater conditions at the locations and times observed, and it is not warranted it is representative of groundwater conditions at other locations or times. Groundwater levels can vary due to seasonal variations and other factors not evident at the time of our investigation.

Discussion In general, there is a high caving risk for trenches excavated below the groundwater table and a moderate risk for trenches excavated above the groundwater table. Trenches will likely have to be shored and possibly dewatered to facilitate open cut construction. If groundwater levels are high at the time of excavation, the trench walls could start to cave in before the shoring can be set. Buoyancy and high groundwater should be considered in design of the underground utilities.

The subgrade soils below the pavement are soft so we recommend compacting the subgrade soil to at least 95 percent relative compaction where pavements will be constructed. The road section consisted of 3.5 inches of asphalt over 6 inches of baserock at Blackburn Street, and 3 inches of asphalt over 4 inches of baserock at Neary Street. The road sections do not appear appropriate for the soil conditions, a thicker baserock section should be considered.

Very truly yours, DEES & ASSOCIATES, INC.

Rebecca L. Dees Geotechnical Engineer G.E. 2623

Copies: 1 to Addressee

Engineer-1 K Stamp 2025

SCR-1854 | 2/2/24

APPENDIX A

Site Vicinity Map/Boring Site Plan

Unified Soil Classification System

Logs of Test Borings

Figure 1

B-2

B-1

SCR-1854 | 2/2/24

THE UNIFIED SOIL CLASSIFICATION SYSTEM

MAJOR DIVISIONS

GROUP

SYMBOLS

TYPICAL NAMES CLASSIFICATION CRITERIA

C O

A R

S E

-G R

A

IN

E D

S O

IL

S

M O

R E

T H

A N

H A

L F

O F

M A

T E

R

IA

L I

S L

A R

G E

R T

H A

N N

O

S

IE

V

E S

IZ

E

(T H

E N

O

S

IE

V E

S

IZ

E I

S A

B O

U T

T H

E S

M A

L L

E S

T P

A R

T

IC

L E

V

IS

IB

L

T O

T H

E N

A K

E D

E Y

E

G R

A V

E

S

M

R E

T H

A N

H A

L F

O F

C O

A R

F R

A C

T

IO

N I

S L

A R

G E

R T

H A

N

S

IE

V E

S

C

E

G

A V

E

F

IN

E

S

GW

Well-graded gravels, gravel-sand mixtures, little or no fines

Wide range in grain sizes and substantial amounts of all intermediate particle sizes

GP

Poorly graded gravels, gravel-sand mixtures, little or no fines

Predominantly one size or a range of sizes with some intermediate sizes missing

Not meeting all gradation requirements for GW

R

A V

E

W

IT

H F

IN

F

IN

E S

GM

Silty gravels, gravel-sand-silt mixtures

Non plastic fines or fines with low plasticity Atterberg limits below “A” line or

PI < 4

Above “A” line with

4 < PI < 7

are borderline cases requiring use of dual symbols

GC

Clayey gravels, gravel-sand-clay mixtures

Plastic fines Atterberg limits above “A” line with PI > 7

S A

N D

R E

T H

A N

H A

L F

O F

C O

A R

F

A C

T

IO

N I

S S

M A

L L

E R

T H

S

IE

V E

S

IZ

E C L

E

N

F

IN

E

S

SW

Well-graded sands, gravelly sands, little or no fines Wide range in grain sizes and substantial amounts of all intermediate sizes missing

SP

Poorly graded sands, gravelly sands, little or no fines

Predominantly one size or a range of sizes with some intermediate sizes missing Not meeting all gradation requirements for SW

S A

N D

S W

IT

H

F

IN

F

IN

E S

) SM Silty sands, sand-silt mixtures

Non plastic fines or fines with low plasticity

Atterberg limits below “A” line or

PI < 4

Limits plotting in hatched zone with

4 < PI < 7

are borderline cases requiring use of dual symbols SC

Clayey sands, sand-clay mixtures

Plastic fines

Atterberg limits above “A” line with PI > 7

F

IN

E -G

R A

IN

E

D S

O

IL

R E

T H

A N

H A

L F

O F

M A

T E

R

IA

L I

S S

M A

L L

E R

T H

A N

N O

S

IE

V

E

T H

E N

O

S

IE

V

E S

IZ

E

I S

A B

O U

T T

H E

S M

A L

L E

S T

P A

R T

IC

IS

IB

L

E T

O T

H E

N A

K E

D E

Y

IL

T S

A N

D C

L A

Y

(L

IQ

U

ID

L

IM

IT

ML

Inorganic silts and very fine sands, rock flour, silty or clayey fine sands, or clayey silts with slight plasticity

CL

Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays

OL

Organic silts and organic silty clays of low plasticity

S

IL

T S

A N

D C

L A

(L

IQ

U

ID

L

IM

IT

MH

Inorganic silts, micaceous or diatomaceous fine sandy or silty soils, elastic silts

CH

Inorganic clays of medium to high plasticity, organic silts

OH

Organic clays of medium to high plasticity, organic silts

*Gravels and sands with 5% to 12 % fines are borderline cases requiring use of dual symbols.

RELATIVE DENSITY OF SANDS

AND GRAVELS

DESCRIPTION BLOW / FT**

VERY LOOSE

LOOSE

MEDIUM DENSE

DENSE

VERY DENSE

0 – 4 4 – 10

10 – 30 30 – 50

OVER 50

CONSISTENCY OF SILTS AND CLAYS

DESCRIPTION BLOWS / FT**

VERY SOFT

SOFT

FIRM

STIFF

VERY STIFF

HARD

0 – 2 2 – 4 4 – 8

8 – 16 16 – 32

OVER 32

**Number of blows of 140 pound hammer falling 30 inches to drive a 2 inch O.D. 12 vertical inches.

TEST BORING LOG SCR-1854

Blackburn Street

LOGGED BY: CL DATE DRILLED: 2/1/24 BORING TYPE: 6 Inch Solid Stem BORING NO: 1

D

EP

TH

fe e t)

SA

P

LE

N

SOIL DESCRIPTION

U

SC

S

SO

IL

T

Y P

FI

EL

D B

LO

U

B

U

T*

ET

D

EN

SI

TY

(P

F)

IS

TU

R E

IN

TU

IS

TU

R E

SA

TU

R A

ED

H

ES

IO

N

P

SF

P H

I A N

LE

A

SS

IN

S

IE

LA

ST

IC

IT

Y

I N

EX

1-1

1-2-1

1-3

1-4-1

3.5 inches Asphalt over 6 inches Baserock

Dark grayish brown CLAY, moist, soft (Expansive)

Groundwater at 5 feet

Dark grayish brown mottled orange CLAY, wet, soft

Dark grayish brown CLAY, wet, soft

Grayish brown CLAY, wet, soft

Boring Terminated at 15 Feet Groundwater Encountered at 5 Feet

CH

110.5

108.2

501 MISSION ST. STE. 8A | SANTA CRUZ, CA 95060

www.deesgeo.com | (831) 427-1770

FIGURE 3

* Blow count converted:

L = Field Blow Count / 2

TEST BORING LOG SCR-1854

Neary Street

LOGGED BY: CL DATE DRILLED: 2/1/24 BORING TYPE: 6 Inch Solid Stem BORING NO: 2

D

EP

TH

fe e t)

SA

P

LE

N

SOIL DESCRIPTION

U

SC

S

SO

IL

T

Y

FI

EL

D B

U

B

U

T*

Y D

EN

SI

(P

F)

IS

TU

R E

IN

TU

IS

TU

R E

SA

TU

R A

ED

H

ES

IO

N

P

SF

P H

I A N

LE

A

SS

IN

S

IE

LA

ST

IC

IT

Y

I N

EX

2-1

2-2

2-3

3 inches Asphalt over 4 inches Baserock

FILL?, Mottled black and dark gray brown CLAY with a few red friable gravels, moist, soft to firm

Groundwater at 4 feet

Very dark gray with strong brown mottling CLAY, wet, soft to firm

Dark gray CLAY, wet, very soft

Boring Terminated at 11.5 Feet Groundwater Encountered at 4 Feet

CH

501 MISSION ST. STE. 8A | SANTA CRUZ, CA 95060

www.deesgeo.com | (831) 427-1770

FIGURE 4

* Blow count converted:

L = Field Blow Count / 2