Geotech_Report_-_Pocahontas_Drive_Lift_Station_(24-292)_10.28.24_(1).pdf

This document has been electronically signed and sealed by Keith V. Jacobs (license # 66577) on October 28, 2024. Printed copies of this document are not considered signed and sealed, and the signature must be verified on any electronic copies.

Pocahontas Drive Lift Station Okaloosa County, Florida LMJ File #: 24-292 October 28, 2024

Prepared for

Mr. Blaine Beck, PE blaine.beck@jacobs.com

Prepared by

Larry M. Jacobs & Associates, Inc.

328 East Gadsden Street, Pensacola, Florida 32501 Florida Certificate of Authorization #2184

Keith V. Jacobs, PE Principal Engineer FL Reg. #66577 mailto:blaine.beck@jacobs.com

Pocahontas Drive Lift Station LMJ Project 24-292 October 28, 2024

Subsurface Conditions

▼ The boring encountered slightly silty sand and sand in the upper16 feet where very soft very sandy clay was encountered to 28 feet underlain by sand and slightly silty sand to the bottom of the boring at 41 feet.

▼ The boring was loose and medium dense in the top 13 feet, very loose/very soft to 28 feet, and medium dense to the bottom of the boring at 41 feet.

▼ Groundwater was encountered at 3½ feet below grade at the time of drilling.

▼ Groundwater levels will vary with the tide, the amount of local rainfall, and changes in site drainage characteristics and may be different at other times.

General Comments and Recommendations

▼ Dewatering will be needed to install the lift station, and well points are typically used for lift station construction in similar soils.

▼ We anticipate at least two perimeter rings of wells with one set above the clay layer and one below the clay layer to effectively dewater the sandy soils above and below the clay.

▼ We recommend that well points below the clay layer maintain groundwater a minimum depth of 3 feet below the bottom of the planned excavation, which could be up to 30+/-feet below existing grade (as needed to remove poor soils).

▼ The lift station boring encountered very soft clay at the anticipated lift station bottom depth that is not suitable for support of the lift station.

▼ We recommend over-excavating the soft clay soils beneath the lift station concrete collar down to the underlying medium dense sandy soils per this report.

▼ The clay soils are not suitable for lift station backfill and will need to be hauled offsite.

Note: The above summary is an overview of the report and should not be used by itself for planning, design, and/or construction. See the relevant sections for further details.

Summary

Project Information

The site lift station site is located at the northwest corner of the intersection of Bradford Drive and Pocahontas Drive in Fort Walton Beach, Florida. Based on Google Earth®, existing grade near the existing lift station is roughly 8 feet. At the time of drilling, the existing lift station area was fenced.

Proposed Construction

The project consists of the replacement of the existing lift station. The new lift station is planned to be 6 feet in diameter with a bottom of well depth around 25 feet below existing grade. The lift station is planned to have a concrete collar at the bottom with crushed stone below the concrete. We understand the existing well is planned to be buried in place.

Subsurface Exploration

Our exploration included one Standard Penetration Test (SPT) boring drilled for the proposed lift station to a depth of 41 feet. The boring was drilled using a truck mounted drill rig and was advanced between sampling using a “mud” jetting technique with bentonite drilling mud. The subsurface conditions encountered in the boring can be found on the boring log here.

The above project information is the basis of our recommendations. If this information changes or is incorrect, our office should be notified, and changes to our report may be needed.

Project Information

Dewatering ▼ Dewatering will be needed to install the lift station, and well points are typically used for lift station construction in similar soils.

▼ We anticipate at least two perimeter rings of wells with one set above the clay layer and one below the clay layer to effectively dewater the sandy soils above and below the clay.

▼ We recommend that well points below the clay layer maintain groundwater a minimum depth of 3 feet below the bottom of the planned excavation, which could be up to 30+/- feet below existing grade (as needed to remove poor soils).

▼ Dewatering should occur prior to excavating and should be maintained until backfill operations are complete.

▼ Dewatering pump failure could cause significant disturbance or “boiling” of the bottom of the excavation which could require extensive restoration measures like undercutting and/or replacing of disturbed/loosened soils. LMJ should be notified if any such events occur, and additional recommendations or testing may be needed.

Excavations All excavations should be made at a safe slope (1.5H:1V minimum) or sheeting, shoring, or trench boxes should be used. We expect that sheeting will be used to install the lift station.

Note that significant rainfall events and flowing groundwater (seepage forces) through excavation walls can be destabilizing, and stormwater runoff, seepage, and erosion should be controlled for all excavations. The contractor is solely responsible for designing and constructing safe excavations that maintain stability during construction. All excavations should be in accordance with the latest local, state, and federal safety regulations. We recommend that the sheet piles be designed by a registered professional engineer licensed in the state of Florida, which is also an OSHA requirement for excavations deeper than 20 feet.

▼ Estimated unit weights (γ), friction angles (φ), and cohesion (c) values are noted on the boring log if needed for sheet pile design/analysis. These estimated soil parameters are based on the SPT values, published correlations, and our experience with similar soils.

▼ Note that the unit weights provided are moist/buoyant.

▼ We recommend installing sheets that penetrate a sufficient depth into the medium dense soils encountered beneath the very soft clay layer, and a second waler (or struts) may be needed in addition to a top waler.

Vibration Discussion ▼ Driving sheet piles creates ground vibrations, and these vibrations can cause disturbance or damage to nearby existing structures.

▼ We recommend performing pre-existing conditions surveys of the nearby structures prior to any pile driving, which includes taking photos and documenting the existing conditions including obvious existing cracks or damage.

▼ We also recommend vibration monitoring during sheet pile installation and removal, and LMJ can provide the above services upon request.

Earthwork

Backfill Recommendations ▼ Backfill material should be the soil types listed in the following table.

▼ Excavated native soils can be used as backfill around the lift station except for the clay soils, which are unsuitable for use as backfill. Clay soils excavated for foundation construction should be hauled offsite.

▼ All backfill should be free of organic or deleterious materials and non-plastic.

▼ Samples of any imported fill material, if needed, should be submitted to the geotechnical engineer for testing and evaluation prior to shipment to the site.

Backfill Recommendations

Material Type

Lift Thickness (in) Hand Operated

Equipment Large Equipment

Hand Operated

Equipment Sand or Slightly Silty Sand 10-12 6 Plate Tamper

Silty/Clayey Sand 6-8 4 Jumping Jack

Compaction ▼ Backfill material should be moisture conditioned to within 2% of its optimum moisture content prior to compaction.

▼ The sand soils should be relatively easy to compact and will require ample moisture to achieve compaction. It is best to thoroughly wet dry sandy soils prior to compaction.

▼ We do not recommend using a large vibratory roller for this project because they can draw up groundwater and cause damage or disturbance to the nearby residences. A smaller, utility trench roller may be used.

▼ Compaction of backfill should be per the below table and should be verified by LMJ at the frequency in the below table.

Compaction and Testing Recommendations

Site Element Minimum

Compaction

(ASTM D1557)

Minimum Compaction Testing Frequency Per Lift

Backfill for Over-Excavation 95% 3 Minimum

Crushed Stone Under Concrete Collar 98% 3 Minimum

Backfill Around Lift Station 95% 2 per 12-inch increment of backfill

Earthwork

Lift Station Recommendations

▼ Dewatering will be needed to install the lift station, and recommendations for dewatering are provided in the earthwork section here.

▼ The lift station boring encountered very soft clay at the anticipated lift station bottom depth that is not suitable for support of the lift station.

▼ We recommend over-excavating the soft clay soils beneath the lift station concrete collar down to the underlying medium dense sandy soils.

▼ The 25-foot sample had very soft clay, and the 30-foot sample had medium dense slightly silty sand indicating that the conditions changed between samples. Based on the boring, undercut depths of roughly 2-4 feet could be needed to remove the very soft clay.

▼ The over-excavation should be widened a minimum of half the depth of the over-excavation on all sides of the concrete collar.

▼ We normally recommend placing a minimum of 12 inches of well-graded crushed stone beneath the concrete collar (such as FDOT Graded Aggregate Base or GAB).

▼ The over-excavation to remove unsuitable clay could be backfilled using compacted sandy soils or crushed stone, and backfill should be placed and compacted and compaction verified per the earthwork section of this report.

▼ If a well-graded crushed stone is used to backfill the undercut, it should be placed in 12-inch lifts compacted to a non-yielding surface up to 1 foot below the bottom of the collar.

▼ The crushed stone immediately under the concrete collar should be compacted for a minimum depth of 12 inches to a minimum of 98% of the material’s Modified Proctor density

(ASTM D1557).

▼ The bottom of the lift station over-excavation should be evaluated by LMJ to verify and document the removal of the unsuitable clay, and we recommend putting a note on the plans requiring this.

▼ To help reduce differential settlement between the well and connections such as piping, the lift station could be filled with water and allowed to settle prior to making piping connections to the well.

Lift Station

STANDARD PENETRATION TEST BORING

ALL BORING LOCATIONS ARE APPROXIMATE

N

B-1

09-04-24

DARK BROWN SILTY SAND (6")(TOPSOIL)(SM)

A

DEPTH

(FT)

PROBE

(IN)

SURFACE

0.5

1.5

2.5

HA

TAN/ORANGE LOOSE SLIGHTLY SAND (SP/SM)

TAN WITH ORANGE SEAMS LOOSE TO MEDIUM DENSE SAND (SP)

WHITE MEDIUM DENSE SAND (SP)

DARK BROWN/BLACK SLIGHTLY SILTY SAND (SP/SM)

GRAY VERY LOOSE SLIGHTLY SILTY SAND (SP/SM)

GRAY VERY SOFT VERY SANDY CLAY WITH CLAYEY

SAND POCKETS (CH)

GRAY/GREEN VERY SOFT VERY SANDY CLAY (CH)

GRAY MEDIUM DENSE SLIGHTLY SILTY SAND (SP/SM)

WHITE/LIGHT GRAY MEDIUM DENSE SAND WITH SOME

CLAYEY SAND POCKETS (SP)

w=81.5

LL=100

PL=35

PI=65

LI=0.71

w=98.3 -200=52.7 w=13.3 -200=6.2 φ

100/50 γ γ

31105/55

2485/35

90/40

31105/55 c 250 c 250 c

Boring Location Map

Boring

L

Test Results

Laboratory testing for this project included wash #200 sieve tests, natural moisture content tests, and one Atterberg Limits tests run on selected spilt spoon samples to assist in soil classification and to evaluate and document soil properties. A pocket penetrometer was used to estimate the approximate cohesion of the clay samples. The laboratory test results are shown on the boring log adjacent to the samples tested.

Lab

Basis of Recommendations

Recommendations rendered herein are based on assumed and/or design information available at the time of this report, the subsurface conditions encountered in the test boring, generally accepted geotechnical engineering principles and practices, and our experience with similar soil and groundwater conditions. Should final project information or existing conditions differ from the information used in this report, or should any soil conditions not discussed in this report be encountered during construction, our office should be notified and retained so that this report can be modified as needed. LMJ should be provided the final plans and specifications for review to determine if any changes to our report are needed based on the final design and that our recommendations have been properly interpreted.

This report and any correspondence are intended for the exclusive use of our client for the specific application to the project discussed. LMJ is not responsible for the interpretations, conclusions, or recommendations made by others based on the information in this report.

Regardless of the care exercised in performing a Geotechnical Exploration, the possibility always exists that soil and/or groundwater conditions will differ from those encountered at the specific boring locations. In addition, construction operations may alter the soil conditions.

Therefore, it is recommended that a representative from LMJ be involved during the construction phases discussed in this report.

Test Methods

Standard Penetration Test

The Standard Penetration Test (SPT) consists of driving a 2-inch diameter split spoon sampler into the ground using a 140-pound hammer dropped 30 inches. The number of blows required to drive the sampler one foot (after seating it 6 inches) is referred to as the blow count or “N” value and represents the relative density of subsurface soils. “N” values can be found on the boring logs. The SPT borings were drilled in general accordance with ASTM D1586 using a truck mounted drill rig. Each sample was removed from the sampler, classified in the field by the driller, and packaged for visual classification by our engineering staff and laboratory testing. The borings were sampled using an autohammer. FDOT converts autohammer to safety hammer using a factor of 1.24.

Other Test Methods

Wash #200 Sieve (ASTM D1140), Moisture Content (ASTM D2216), and Atterberg Limits

(ASTM D4318).

Appendix

LEGEND

SAFETY HAMMER AUTOMATIC HAMMER

GRANULAR SOILS

SPT

BLOWS/FOOT (N) RELATIVE DENSITY

0-2 VERY LOOSE

3-8 LOOSE

9-24 MEDIUM DENSE

25-40 DENSE

> 40 VERY DENSE

COHESIVE SOILS

BLOWS/FOOT (N) RELATIVE DENSITY

<1 VERY SOFT

1-3 SOFT

4-6 MEDIUM STIFF

7-12 STIFF

13-24 VERY STIFF

> 24 HARD

GRANULAR SOILS

SPT

BLOWS/FOOT (N) RELATIVE DENSITY

0-3 VERY LOOSE

4-10 LOOSE

11-30 MEDIUM DENSE

31-50 DENSE

> 50 VERY DENSE

COHESIVE SOILS

BLOWS/FOOT (N) RELATIVE DENSITY

0-1 VERY SOFT

2-4 SOFT

5-8 MEDIUM STIFF

9-15 STIFF

16-30 VERY STIFF

> 30 HARD

SAND

SILTY SAND

CLAY

PEAT

SLIGHTLY

SILTY SAND

SILT

CLAYEY

SAND

GRAVEL

GROUNDWATER NOT ENCOUNTERED

AT TIME OF DRILLING

STANDARD PENETRATION

RESISTANCE IN BLOWS PER FOOT

STANDARD PENETRATION

RESISTANCE USING AUTOHAMMER

ENCOUNTERED GROUNDWATER

LEVEL

ENCOUNTERED PERCHED WATER

LEVEL

NUMBER OF BLOWS REQUIRED (50)

TO ADVANCE SPLIT SPOON SAMPLER

A SPECIFIC DISTANCE (2) INCHES

SPLIT SPOON SAMPLE ADVANCED

UNDER WEIGHT OF ROD AND

HAMMER

HAND AUGER

SHELBY TUBE SAMPLER

NATURAL MOISTURE CONTENT (%)

FINES PASSING #200 SIEVE (%)

ORGANIC CONTENT (%)

LIQUID LIMIT

PLASTIC LIMIT

LIQUIDITY INDEX

APPROXIMATE COHESION VALUE

(PSF) BASED ON POCKET

PENETROMETER READINGS

SATURATED VERTICAL HYDRAULIC

CONDUCTIVITY (FT/DAY)

DRY UNIT WEIGHT (PCF)

ESTIMATED MOIST UNIT WEIGHT

(PCF)

ESTIMATED BUOYANT UNIT WEIGHT

(PCF)

ESTIMATED ANGLE OF INTERNAL

FRICTION (DEGREES)

w

Kv γ d

Appendix

NOTES

1) SPT BORINGS PERFORMED

IN GENERAL ACCORDANCE

WITH ASTM D1586

2) SUBSURFACE CONDITIONS

ARE AT BORING LOCATIONS

AND ACTUAL CONDITIONS

BETWEEN BORINGS MAY

VARY

3) ALL CLASSIFICATIONS ARE

BASED ON VISUAL

EXAMINATION UNLESS

ACCOMPANIED BY

LABORATORY TEST RESULTS

4) BOUNDARIES BETWEEN SOIL

LAYERS SHOULD BE

CONSIDERED APPROXIMATE

AS THE ACTUAL TRANSITION

MAY BE GRADUAL

5) DEPTH OF BORING IS BELOW

EXISTING GRADE AT TIME OF

DRILLING

6) ELEVATIONS, IF SHOWN,

WERE ESTIMATED FROM

PROVIDED TOPOGRAPHIC

SURVEY

7) COLORS USED FOR BORING

HATCHING MAY NOT

REPRESENT THE ACTUAL

SOIL COLORS