Geotechnical Report with addenda.pdf

SEA

SAILORS ENGINEERING ASSOCIATES, INC.

1675 SPECTRUM DRIVE • LAWRENCEVILLE, GEORGIA 30043 • TEL (770) 962-5922 • FAX 962-7964

May 12, 2023

Mr. Ken Peters, P. E.

Project Manager

Engineering Management, Inc.

303 Swanson Drive

Lawrenceville, Georgia 30043

RE: Geotechnical Investigation for

Two Proposed Retaining Walls

Future Helen Walkway Project

Project No. GPAS: A-311-007921-1

South Main Street

Helen, White County, Georgia

SEA Job #231-048

Ken:

In accordance with your written authorization, Sailors Engineering Associates, Inc. has completed the geotechnical investigation for the subject project and is pleased to submit this report with our conclusions and recommendations.

GENERAL

We understand that EMI, Inc., working for the city, is providing civil design services for a Main

Street sidewalk extension project in downtown Helen, White County, Georgia. The project is along and to the south of State Route 75 (Main Street) from just east of the Helen City Hall, continuing west, to a point just east of a private, one-way, roadway named Ber Weg. EMI, Inc.

has also been tasked with facilitating the design through the design review of the Georgia

Department of Transportation. We understand that in that process, based on some unique drainage issues, the need for two retaining walls to be built adjacent to (south of) and along the sidewalk alignment has arisen.

Initially, the plans were modified to simply include GDOT standard walls for various, typical wall heights (a 6 foot and less wall standard option, and a 6 to 8 foot tall wall standard option).

However, upon deeper review and given Helen’s steep topography, GDOT has become concerned about the steep slopes behind the subject walls and has called for this foundation/geotechnical investigation for the walls.

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From our review of provided plans for the project and/or discussions with Mr. Ken Peters of

EMI, Inc., Retaining Wall #1 is currently to be an 81 foot long structure that is nearest the roadway named Ber Weg (near and just east of Paul’s Steakhouse in Helen, Ga.). The wall’s maximum height here will approach, but be just less than, 7 feet and its planned alignment calls for the wall face to be between about 9.2 horizontal feet (@ wall station 0+00, the wall’s easternmost end) to about 9.5 horizontal feet (@ wall station 0+81, the wall’s westernmost end), south of the State Route 75’s southernmost edge of pavement. [We note that Mr. Ken Peters of

EMI, Inc. has indicated that he can reduce these horizontal distances from the southern edge of pavement to the front face of the wall between about 1.7 feet (@ wall station 0+00) to about 2.0 feet (@ wall station 0+81), for wall constructability, if necessary.]

Also from plan review and/or discussions with Ken, Retaining Wall #2 is to be a 79 foot long structure that is nearest the Helen City Hall. This wall’s maximum height will be slightly shorter than Retaining Wall #1, but still will exceed 6 feet and its planned alignment also calls for the wall face to be between about 9.7 horizontal feet (@ wall station 0+00, the wall’s easternmost end) to about 10.5 horizontal feet (@ wall station 0+79, the wall’s westernmost end), south of the State Route 75’s southernmost edge of pavement. [Here, Ken Peters has indicated that he can reduce the horizontal distances from the southern edge of pavement to the front face of the wall between about 2.2 feet (@ wall station 0+00) to about 3.0 feet (@ wall station 0+79), for wall constructability, if necessary.]

The original purpose of our investigation was to drill in the proposed footprints of the subject walls, determine the presence of any unsuitable soil conditions, near surface ground water or rock that would adversely affect construction costs and to provide recommendations for wall soil subgrade preparation for standard, cantilevered Portland cement cast-in-place retaining walls.

AREA GEOLOGY

White County, Georgia is located in the physiographic province known as the Piedmont which extends from the Hudson River at the north to Alabama at the south. The Piedmont is the least mountainous part of the Appalachian Highlands. The surface of the Piedmont can be described as a broadly undulating or rolling topography with low knobs or ridges, and valleys 30 to 300 feet thick. The underlying crystalline rocks of the Piedmont are metamorphic schists, gneisses, quartzites and slates, and igneous granites and gabbros. According to published geologic literature and mapping, the base rock in the area surrounding the subject site consists of

Metagraywacke/mica schist of the Precambrian-Paleozoic period.

FIELD INVESTIGATION

On March 14, 2023, we mobilized our drill rig to the site to perform a geotechnical investigation of the subsurface soils in the area of each wall. Our Soil Test Boring numbers 1 and 2 (see

Boring Logs and a Boring Plan, attached) represent soils for Retaining Wall #1 and Soil Test

Boring numbers 3 and 4 (appended) represent soils for Retaining Wall #2. The test borings were extended through soils by mechanical drilling procedures using continuous spiral hollow auger

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flights with a steel fingered Hawthorne bit as the cutting device. The consistencies of the underlying soils were determined by Standard Penetration Testing in accordance with ASTM

Specification D1586. Samples were obtained with a standard 1.4 inch I.D., 2.0 inch O.D., split tube sampler as illustrated in the Appendix. The sampler was first seated 6.0 inches to penetrate any loose cuttings; then it was driven an additional foot with blows of a 140 pound hammer falling 30.0 inches. The number of hammer blows required to drive the sampler each 6.0 inch increment is recorded in the Boring Logs. The number of blows required to drive the sampler the final foot is the standard penetration resistance, an indicator of soil strength.

No ground water was encountered during the drilling operations and for safety purposes we backfilled the boring holes immediately upon drilling. The elevation of the water table in Helen fluctuates during the year and is directly related to the amount of rainfall in the months prior to observation.

It is noted that the elevations on the Boring Logs for the four soil test borings were interpolated from a two foot contour interval, provided set of construction plans (with some topographic information), but it is not known if these plan elevations have been field verified. Nonetheless, the boring surface elevations should be considered approximate.

SUBSURFACE CONDITIONS

Retaining Wall #1

At soil test boring B-1, below about 3 feet of soft upper fill soils described as brown silty sand with a little rock fragments, we penetrated firm sandy residual or saprolitic soil underlain by very firm partially weathered rock. At 16 feet below the B-1 ground surface (at an elevation of about

1409.5 feet) we encountered mass rock, inferred by auger refusal.

Soil test boring B-2 initially penetrated about 5 feet of fill soils of which the upper approximate 3 feet are relatively soft. Very firm rocky fill exists at the soil fill base and under the rocky fill we encountered about 2 feet of very firm saprolitic, partially weathered rock. At 7 feet below the B-

2 boring surface (at an elevation of about 1421.0 feet) we encountered mass rock, inferred by auger refusal.

Retaining Wall #2

Below about an inch of topsoil, soil test boring B-3 penetrated about 7 vertical feet of medium firm residual soil followed by about 12 feet of firmer residual sandy materials. Under the sand we penetrated about 0.4 feet of partially weathered rock before terminating the boring at an elevation of about 1404.6 feet.

Below about an inch of topsoil, soil test boring B-4 penetrated about 5 feet of fill soils of which the upper approximate 3 feet were relatively soft. Very firm rocky fill was at the fill base and under the fill (at an elevation of about 1418.0 feet) we encountered mass rock, inferred by auger refusal.

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Mass rock (inferred by auger refusal) and hard drilling materials were encountered as noted on the individual Logs of Boring and in the Conclusions and Recommendations section of this report. No ground water was encountered in our soil test borings, but we note that they were not left open for any stabilized readings and, instead, were backfilled immediately upon drilling for safety purposes.

CONCLUSIONS AND RECOMMENDATIONS

Retaining Wall #1 and the Easternmost portion of Retaining Wall #2:

Based on a March 29, 2023 and a May 1, 2023 reconnaissance of the site performed by our Mr.

Jim Sailors, P. E., Ken Peters, P. E. (and Mr. Norm Amend, P. E. of Timberidge Construction on

May 1, 2023 only), it is our opinion that about ¾ of the western portion of Retaining Wall # 2 can utilize the wall designs generally as detailed on sheets S1, S2 and S3 in the provided construction plans (for the Main Street Sidewalk Extension for the city of Helen, dated May of

2022). However, for the remaining approximate ¼ of the Retaining Wall #2’s eastern end and all of Retaining Wall #1, it is our opinion that wall design, taking into account some amount of excavation back into the existing steep slopes at the site, will be necessary and top down, soil nail wall design and construction for wall constructability should be performed. These walls will need to be designed by a licensed professional engineer knowledgeable in wall design with steep existing slope retention and the higher pressures these conditions bear on the wall taken into account for, at a minimum, wall sliding resistance, local and global wall and slope stability. In our experience these types of walls have most often been designed and built by a design-build wall contractor such as Timberidge Construction.

For the portion(s) of wall that will be design-build, top-down, soil nail walls, the soft soils and utilities that currently exist between the planned wall(s) and the S. R. 75 edge of pavement can be left in place because the design will not count on wall bearing support in these areas.

For retaining wall #1, our two soil test borings (B-1 and B-2) initially penetrated about 3 vertical feet of soft soils before reaching materials acceptable to support a standard cantilevered wall in bearing capacity (a bearing capacity of a recommended 2,500 psf). Some upper undercutting and compacted stone or firmer compacted soils replacement to remediate/build a subgrade satisfactory for a standard, cantilevered wall support would be necessary at the Retaining Wall

#1 location, if a cast-in-place wall is built here (not recommended).

Currently a buried gas line and a water line extend both longitudinally along the Retaining Wall

#1 alignment and, in places, transverse to Retaining Wall #1. It is likely that the soft soils encountered in B-1 and B-2 are due to soft utility backfill soil presence. In addition to some undercut and replacement of soft upper soils in the wall #1 bearing footprint, these utility lines, if determined to be in the footprint of the wall footing, would need to be relocated and removed;

provided a standard cantilevered wall is the wall of choice for this location.

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Our two soil test borings (B-3 and B-4) initially penetrated between about 0 feet (@ wall station

0+79, western end of the wall) and 3 feet (@ wall station 0+00, eastern end of the wall) of soft soils before vertically reaching materials acceptable to support the wall in bearing capacity (a bearing capacity of a recommended 2,500 psf). Therefore, some undercutting and compacted stone or firmer compacted soils replacement to remediate/build a subgrade satisfactory for a standard, cantilevered wall support will be necessary for Retaining Wall #2, if any portion of it is built as a cast-in-place wall.

Currently a buried sanitary sewer line and a water line extend both longitudinally along (or in the vicinity of) the Retaining Wall #2 alignment and, possibly in places (water), transverse to

Retaining Wall #2. It is possible that the soft soils encountered in B-4 are due to some soft utility backfill soil presence, and this soft soil could extend beyond B-4. Again, as in the case of

Retaining Wall #1, in addition to some undercut and replacement of soft upper soils in the wall

#2 bearing footprint, these utility lines, if it is determined that they are in the footprint of the wall footing, would need to be relocated and then removed for the portion of the wall that will be a standard, cantilevered cast-in-place wall. These utilities should not underlie the wall or exist within or under a cast-in-place wall footing.

For any portion of standard, cantilevered cast-in-place wall, upon excavation to expose the proposed base of wall subgrades, our representative should visit the project and, taking the information obtained from the borings, evaluate the soil subgrades to determine the soil’s suitability to either be built upon as-is, or to recommend what depth undercutting and replacement with firmer materials capable of supporting the design bearing capacity will be needed. In this case, the contractor should be prepared to undercut soft soils as deep as 3.0 feet and replace them with firm compacted materials.

Gradually sloping portion of Retaining Wall #2:

The following recommendations/conclusions/wall parameters pertain to the portion of Retaining

Wall No. 2 where a standard, cantilevered cast-in-place wall can be constructed:

1. All areas to receive cast-in-place wall or its backfill should be stripped of organic material and topsoil prior to the commencement of construction. The topsoil should be stockpiled on-site for future use in landscaped areas (if approved by the owner), disposed of in a designated area on-site, or wasted off-site. Topsoil should not be used as structural fill.

2. All areas to remain at grade or to receive fill should be proof-rolled with a loaded tandem-axle dump truck or, if not possible, visually evaluated in the presence of a representative of the Geotechnical Engineer. Cut areas should be proof-rolled/evaluated once rough subgrade has been reached. Any soft soils encountered during proof-rolling/evaluation should be undercut and replaced with suitable compacted materials.

Because soft soils were encountered in our soil test borings, they may be located at other nearby locations in the wall supporting footprint. It is possible that some of these soft soils will be removed during site stripping and grading operations, however, any remaining soft soils should be undercut and replaced with suitable compacted fill or

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compacted crushed stone. The full extent of the area requiring remediation and the thickness of stabilization stone or re-compacted structural fill will have to be determined during construction.

4. All fill material to be utilized on the project should be free of organic or otherwise deleterious materials and compacted to minimum dry densities corresponding to 98% of maximum dry density, and at moisture contents within +/- 3% of optimum moisture content, as obtained by Standard Proctor, ASTM D698.

Fill should be placed in lifts not to exceed 6.0 inches in compacted fill thickness in mass fill areas, and as needed to obtain the required compaction in ditch lines and foundation wall backfill.

5. Fill slopes should be constructed to a maximum of 2.0H:1.0V. Fill slopes should be compacted in horizontal lifts not to exceed 6.0 inches in compacted thickness as fill is placed.

6. All undercut and fill operations should be monitored by a representative of the

Geotechnical Engineer. He should perform sufficient density tests to verify that specified compaction is obtained.

7. The following parameters can be used for any cantilevered reinforced concrete retaining wall design:

a. For soils encountered on the subject site, assume they are non-cohesive with an internal friction angle of 30 degrees and a moist unit weight of 120 pounds per cubic foot

b. Equivalent fluid pressure for walls not allowed to move: 65 pounds per cubic foot

c. Equivalent fluid pressure for walls allowed to move sufficiently to develop active conditions: 40 pounds per cubic foot (Ka=0.33)

d. For soils behind and in front of walls where they will not be disturbed by future erosion or construction, use a passive condition equivalent fluid pressure of 335 pounds per cubic foot (Kp=2.8). A safety factor of 2 should be used for wall design to reduce the movement needed to develop passive pressure

e. Drainage features such as weep holes and porous backfill should be utilized

f. Allowable net bearing pressures for footing design: 2,500 psf

g. Coefficient of friction against sliding: 0.4

h. Compaction of backfill within 10.0 feet of walls should be performed with hand operated equipment, such as walk behind compactors and/or mechanical tamping equipment (for example Wacker Packers)

i. If sloping backfill or toe conditions exist, the Geotechnical Engineer should be consulted for additional recommendations

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The soils encountered on site, with the exception of the topsoil, will be suitable materials for backfill against reinforced concrete retaining walls, provided they meet the requirements previously mentioned regarding structural fill.

8. Ground water was not encountered during our drilling operations and is not expected to have a significant effect on any cast-in-place wall construction at the site.

9. Auger refusal, hard drilling materials and weathered rock were encountered in some of the test borings performed on the site. The following table lists locations, depths of hard consistency soils/weathered rock, auger refusal or depths of penetration without auger refusal for each location.

Boring

No.

Hard Consistency

Soil or Weathered

Rock

Auger Refusal

Penetration

Without Refusal

Being Encountered

Depth Elevation Depth Elevation Depth Elevation

(ft) (ft) (ft) (ft) (ft) (ft)

B - 1 8.5 1417.0 16.0 1409.5 ----- -----

B - 2 2.5 1425.5 7.0 1421.0 ----- -----

B-3 19.0 1405.0 ----- ----- 19.4 1404.6

B-4 3.5 1419.5 5.0 1418.0 ----- -----

Based on the above table, significant rock removal is not anticipated for any cast-in-place wall construction.

If we can be of further service to you on this project, please contact us at your convenience.

Respectfully submitted, SAILORS ENGINEERING ASSOCIATES, INC.

Peyton T. Duncan, P.E.

Senior Engineer

Jim D. Sailors, P.E.

Principal Engineer

APPENDIX

SOIL CONSISTENCY DESIGNATIONS

(Based on results of Standard Penetration Tests performed according to ASTM Specification D-1586-84)

NUMBER OF BLOWS (“N”): Shall be defined as the number of blows of a 140 pound hammer falling free a distance of 30 inches required to drive a standard split spoon sampler (2” O.D. and 1.4” I.D.) 1 foot.

When the sample is primarily cohesionless, use the following consistency table:

NUMBER OF BLOWS (N) CONSISTENCY DESIGNATION

0 - 4 Very Loose

5 - 10 Loose

11 - 30 Medium

31 - 50 Dense

51 or more Very Dense

When the sample is primarily cohesive, use the following consistency table:

NUMBER OF BLOWS (N) CONSISTENCY DESIGNATION

0 - 2 Very Soft

3 - 4 Soft

5 - 8 Medium

9 - 15 Stiff

16 - 30 Very Stiff

30 or more Hard

SAMPLING PROCEDURES

Soil Sampling & Penetration Testing is performed in accordance with ASTM D1586-84.

The Standard Penetration Resistance is the number of blows of a 140 pound hammer falling 30 inches to drive a 2.0 inch O.D., 1.375 inch I.D. split barrel sampler one foot.

The Undisturbed Sampling Procedure is performed in accordance with ASTM Specification D1587-83.

B-1 B-2

B-3 B-4

BO

RI

NG

P

LA

N

S M ain

St He len , W hit e C ou nty

, G A

SE

A J ob N o.

1-0

RE

TA

IN

IN

G

WA

LL

S -

H

EL

EN

, G

A

N

0 100 20050 Feet

1 inch = 100 feet

Legend !< Boring Locations

Retaining Wall

Co ord ina te

Sy ste m:

N

AD

S tat eP lan e G eo rgi a W es t F

IP

S 1

2 F ee t

G

IS:

E.

H.

La st

Ed ite d:

3/1

7/2

3 4 :40

:34 P

M

SA

ILO

RS

EN

GI

NE

ER

IN

G

AS

SO

CI

AT

ES

, IN

C.

EN

VI

RO

NM

EN

TA

L/G

EO

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CH

NI

CA

L

SP

EC

TR

UM

D

RI

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LA

WR

EN

CE

VI

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E, G

EO

RG

IA

(77 0)

2-5

F

AX

-79

SE

A

Im ag ery S ou rce s:G oo gle /M ax ar

(20

M

AR

B-2 B-1

B-4 B-3

1" : 50' 1" : 50'

SHEET 1 OF 1

BORING NO.

CONTRACTED WITH: B-1

JOB NO. DATE:

PROJECT NAME: 231-048 03/14/23

No. TYPE BLOWS/6"

1425.5

Drilling very firm - drilling through thin rock lens

Drilling hard

21-50/2.0"

1409.5

5 SS

Auger refusal at 16.0 feet

50/6.0"

9 4 SS

3 SS 10-19-20 6

Brown silty sand with a little rock fragments

(fill) 1 SS 3-1-2 Drilling soft

Brown sand with a trace of silt (saprolite) 2 SS 12-20-21 Drilling firm

L O G O F B O R I N G

Engineering Management, Inc.

Helen Retaining Walls

LOCATION: South Main Street - Helen, Georgia

ELEV.

(ft)

DESCRIPTION DEPTH

IN

FEET

SAMPLES

Permaco Mudbug with

Rope / Cathead Hammer

Topsoil: 1.0" brown sand with a little silt and organics

NOTES

CONTRACTED WITH: B-2

JOB NO. DATE:

PROJECT NAME: 231-048 03/14/23

1428.0

Drilling hard

3 SS 40-50/4.5"

1421.0 7

Brown sand with a trace of silt (saprolite)

Auger refusal at 7.0 feet

Brown silty sand with a little rock fragments and a trace of organics (fill) 1 SS 4-4-4 Drilling medium

3 2 SS 50/6.0" Drilling firm

L O G O F B O R I N G

Engineering Management, Inc.

Helen Retaining Walls

LOCATION: South Main Street - Helen, Georgia

ELEV.

(ft)

DESCRIPTION DEPTH

IN

FEET

SAMPLES

Permaco Mudbug with

Rope / Cathead Hammer

Topsoil: 1.0" brown sand with a little silt and

CONTRACTED WITH: B-3

JOB NO. DATE:

PROJECT NAME: 231-048 03/14/23

1424.0

Grayish brown sand with a trace of silt

(saprolite)

Boring terminated at 19.4 feet

1404.6

6 SS 37-50/5.0"

14 5 SS 5-7-11

SS 8-17-17 Drilling firm

Light brown sand with a trace of silt

(saprolite)

9 4

Brown sand with a trace of silt and rock fragments(saprolite)

3 SS 3-3-6

Brown silty sand (residuum) 1 SS 4-4-6 Drilling medium

3 2 SS 3-3-5

L O G O F B O R I N G

Engineering Management, Inc.

Helen Retaining Walls

LOCATION: South Main Street - Helen, Georgia

ELEV.

(ft)

DESCRIPTION DEPTH

IN

FEET

SAMPLES

Permaco Mudbug with

Rope / Cathead Hammer

Topsoil: 1.0" brown sand with a little silt and

CONTRACTED WITH: B-4

JOB NO. DATE:

PROJECT NAME: 231-048 03/14/23

1423.0

Drilling hard

1418.0 5

Auger refusal at 5.0 feet

2 SS 4-11-50/3.0"

Brown sand with a little silt and rock fragments (fill) 1 SS 2-3-5 Drilling medium

Drilling firm

L O G O F B O R I N G

Engineering Management, Inc.

Helen Retaining Walls

LOCATION: South Main Street - Helen, Georgia

ELEV.

(ft)

DESCRIPTION DEPTH

IN

FEET

SAMPLES

Permaco Mudbug with

Rope / Cathead Hammer

Topsoil: 1.0" brown sand with a little silt and