Att. 8 Geotechnical Report 240906.pdf

Nicole Kesselring, P.E.

Enman Kesselring Consulting Engineers 61 Prospect Street Rutland, VT 05701

September 6, 2024 File No. 4880.00

Re: Geotechnical Engineering Report Lincoln Woods Trail Reconstruction Design Lincoln, New Hampshire

Dear Ms. Kesselring:

Enclosed is our Geotechnical Engineering Report for the Lincoln Woods Trail Reconstruction Design project in Lincoln, NH. The report presents results of the subsurface exploration program, geotechnical evaluations, and provides geotechnical engineering recommendations and construction considerations for the project.

We trust this report meets the needs of the project. If you have any questions, please call Shawn Kelley at (802) 391-8514.

Very truly yours, SANBORN, HEAD & ASSOCIATES, INC.

Shawn P. Kelley, Ph.D.

Project Director

Rebecca E. Bentley, P.E.

Vice President

SPK/REB:spk

Encl. Geotechnical Engineering Report

P:\4800s\4880.00\Source Files\Geotech Report\20240906 Lincoln Woods Cover Ltr.docx

TABLE OF CONTENTS

1.0 INTRODUCTION

2.0 SITE AND PROJECT DESCRIPTION

3.0 PURPOSE AND SCOPE

4.0 SUBSURFACE EXPLORATION PROGRAM

4.1 Test Borings

4.2 Geophysical Surveying

5.0 GEOTECHNICAL LABORATORY TESTING

6.0 SUBSURFACE CONDITIONS

6.1 Topsoil

6.2 Sand and Gravel

6.3 Granite Bedrock

6.4 Groundwater Conditions

6.5 Estimated Depth to Bedrock

7.0 GEOTECHNICAL ENGINEERING RECOMMENDATIONS

7.1 Slope Stability Evaluations

7.1.1 Modeling Approach

7.1.2 Proposed Slope Repair Details

8.0 CONSTRUCTION CONSIDERATIONS

8.1 Slope and Retaining Wall Subgrade Preparation Recommendations

8.2 Anchored Mesh Placement Recommendations

8.3 Fill Placement and Compaction Requirements

8.4 Material Re-use

9.0 CONSTRUCTION MONITORING

FIGURES

Figure 1 Locus Plan Figure 2 Subsurface Exploration Location Plan

APPENDICES

Appendix A Limitations Appendix B Test Boring Logs Appendix C Geophysical Survey Report Appendix D Geotechnical Laboratory Test Results Appendix E Slope Stability Evaluation Results Appendix F Material Specifications

1.0 INTRODUCTION

Sanborn, Head & Associates, Inc. (Sanborn Head) has prepared this geotechnical engineering report for Enman Kesselring Consulting Engineers (EKCE) in support of the proposed Lincoln Woods Trail Reconstruction Design (Project) located along the Lincoln Woods Trail in Lincoln, NH (Site). These services have been completed in accordance with our proposal dated October 27, 2020, and is subject to the limitations included in Appendix A.

Elevations in this report are in feet (ft) and referenced to the North American Vertical Datum of

1988 (NAVD88).

2.0 SITE AND PROJECT DESCRIPTION

The Site is an approximately 500-foot long section of the Lincoln Woods Trail beginning approximately ¾ mile north of the trailhead on NH Route 112 (Figure 1). The Lincoln Woods Trail is a heavily trafficked hiking trail that generally follows the west bank of the East Branch Pemigewasset River. The trail is within the Pemigewasset Ranger District of the White Mountain National Forest. We understand that the trail generally follows the alignment of a historical railroad line.

The existing trail generally sits on a bench on the west side of the generally north-south trending river valley. The elevation of the trail at the Site ranges from approximately elevation (El.) 1228 to 1234. The East Branch Pemigewasset River flows to the south at the base of the valley and just to the east of the existing trail. The elevation at the base of the riverbank ranges from approximately El. 1204 to El. 1210 at the Site.

A DigSafe utility clearance performed by Sanborn Head did not identify any subsurface utilities in the vicinity of the Site at the time of this report.

We understand that the existing trail was damaged following flooding caused by tropical storm Irene in 2011 and additional flooding in 2017. Flooding of the East Branch Pemigewasset River resulted in a failure of the west bank of the river and damage to the hiking trail. The existing trail has been temporarily rerouted around the failed area. It is assumed that subsequent flood events may lead to a closure of the trail if additional damage is sustained.

The Project consists of the design and construction of a permanent repair for the damaged section of the hiking trail.

3.0 PURPOSE AND SCOPE

The purpose of this report is to describe the subsurface conditions at the Site, evaluate the stability of the existing slope, and provide geotechnical engineering recommendations for stabilization of the slope and pavement rehabilitation. Specifically, our scope of services included the following:

A subsurface exploration program consisting of test borings and geophysical surveying to evaluate the subsurface conditions at the Site and obtain soil samples for laboratory testing;

September 6, 2024

Geotechnical Report

4880.00

Geotechnical laboratory testing on select soil samples to assist with classification of soils encountered and evaluation of their engineering properties;

Stability modeling of the slope to evaluate the existing conditions at the Site and proposed repair alternatives; and

A report presenting the results of the subsurface exploration program, Sanborn Head’s recommended slope repair system and construction considerations, test boring logs, geotechnical laboratory testing results, and an exploration location plan.

4.0 SUBSURFACE EXPLORATION PROGRAM

Sanborn Head performed a subsurface exploration program between November 2 and November 9, 2020, consisting of four (4) test borings (SH-1 through SH-3 and SH-5) and a seismic refraction survey along two lines as shown on Figure 2.

4.1 Test Borings

The test borings were drilled by New England Boring Contractors of Derry, New Hampshire using a Mobile Drill B-29 track-mounted drill rig. The test borings were advanced using drive and wash drilling techniques with 5-inch, 4-inch and 3-inch inside diameter (I.D.) flush-jointed casing to depths between 10.7 ft and 29.2 ft below ground surface (bgs).

Split-spoon sampling was conducted at 5-foot intervals from the ground surface to refusal at all test boring locations in general accordance with ASTM D1586 (2-inch diameter sampler driven 24 inches by blows from a 140-pound hammer falling freely for a 30-inch drop). The number of blows required to drive the sampler each 6-inch increment was recorded, and the Standard Penetration Resistance (N-Value) was calculated as the sum of the blows over the middle 12 inches of penetration. Split-spoon refusal was defined as less than 6 inches of penetration for 100 blows from a 140-pound hammer.

Representative soil samples were collected from the split-spoon samples and stored in jars for later review. A Sanborn Head representative observed the test borings and classified the soil samples based on visual estimates of grain-size distribution and plasticity using a modified Burmister system.

Rock coring was conducted with an NX core barrel in general accordance with ASTM D2113 at test boring locations SH-1, SH-2 and SH-5. Rock core samples were described and logged in the field by a Sanborn Head representative. The core descriptions included percent recovery, Rock Quality Designation (RQD), orientation and frequency of fractures, observed fracture infilling or coatings, the weathering state of the core, and other characteristics of note. The RQD was calculated for each core run in bedrock by dividing the total length of the rock core segments longer than 4 inches over the total length of the core run.

Groundwater levels at each test boring location were estimated from the moisture condition of the samples obtained during drilling and by water level readings measured by Sanborn Head field personnel at the time of drilling. Groundwater measurements during drilling may not represent stabilized groundwater levels. It should be noted that groundwater levels will vary depending on seasonal variations in temperature and precipitation and may also be influenced by nearby utilities and structures, drilling methods, and other factors that differ from conditions at the time the measurements were made.

Test boring logs, prepared by Sanborn Head, are included in Appendix B.

4.2 Geophysical Surveying

Sanborn Head retained Hager-Richter Geoscience, Inc. of Salem, New Hampshire (HRGS) to perform a seismic refraction survey to evaluate the depth to bedrock at the Site. The survey was performed on November 2, 2020, by three (3) HRGS field representatives and was observed by a Sanborn Head engineer.

Seismic refraction data was acquired along two (2) transects (Seismic Lines 1 and 2) totaling 470 linear feet. Seismic Line 1 was located along the original alignment of the Lincoln Woods Trail at the top of the failed slope, and Seismic Line 2 was located on the west bank of the East Branch Pemigewasset River near the toe of the slope.

The approximate locations of the seismic lines are shown on Figure 2. Details of the survey and complete results are included in Appendix C.

5.0 GEOTECHNICAL LABORATORY TESTING

Geotechnical laboratory tests were performed on select samples obtained from the test borings. Geotechnical laboratory tests were performed by GeoTesting Express, Inc. of Acton, Massachusetts. Grain size analyses were performed on four (4) soil samples collected from the test borings in accordance with ASTM D6913.

The complete laboratory test results are included in Appendix D.

6.0 SUBSURFACE CONDITIONS

In general, the subsurface conditions encountered at the Site consisted of topsoil, sand and gravel, boulders, and granite bedrock.

6.1 Topsoil

Topsoil was encountered at test boring locations SH-2, SH-3 and SH-5. Where encountered, this layer is between 0.3 and 2 ft thick, and typically consists of moist, very loose to loose, brown to black, SILT or fine to coarse SAND, with varying amounts of gravel, leaves, roots, and other organic fragments. SPT N-Values in the topsoil ranged from 2 blows per foot (bl/ft) to 7 bl/ft, with an average of 5 bl/ft at the test boring locations.

6.2 Sand and Gravel

Sand and gravel was encountered at all test boring locations. Where encountered, this layer is between 7.6 ft and up to at least 28.9 ft thick, and typically consists of moist to wet, tan or brown, fine to coarse SAND or fine to coarse GRAVEL, trace to some silt. Refusal was encountered in the sand and gravel stratum in all of the test borings. SPT N-values in the sand and gravel ranged from 4 bl/ft to greater than 100 bl/ft, with an average of 76 bl/ft at the test boring locations.

Boulders were encountered and cored at varying depths within the sand and gravel stratum in test borings SH-1, SH-2 and SH-5. Where cored, the boulders were between 1.2 and 5 ft thick and typically consisted of fine to coarse grained GRANITE. Obstructions that were not cored were also encountered at all the test boring locations; the test borings were advanced through obstructions using a rollerbit and casing with a spin shoe.

6.3 Granite Bedrock

Granite bedrock was encountered at test boring location SH-1. Where encountered, bedrock generally consists of hard, fresh to moderately weathered, gray or pink, medium to coarse-grained GRANITE. Observed fractures are moderately steep, very close to closely spaced, have occasionally stained surfaces and no infilling, and are smooth, planar, and open. The RQD of the recovered rock core is 11%.

6.4 Groundwater Conditions

Groundwater level measurements were measured at the conclusion of drilling at each test boring location between November 2 and November 9, 2020, prior to backfilling the test borings. Measurements were taken from the ground surface. Groundwater was encountered in test borings SH-2 and SH-5 at depths of 18 ft bgs and 16 ft bgs, respectively (approximately El.

1209 and El. 1211).

6.5 Estimated Depth to Bedrock

The geophysical survey indicates the top of bedrock elevations along Seismic Line 1 at the top of the slope range from approximately El. 1194 to El. 1213. Bedrock elevations along Seismic Line 2 near the toe of the slope range from approximately El. 1204 to El. 1210.

7.0 GEOTECHNICAL ENGINEERING RECOMMENDATIONS

The following recommendations have been made based on our current understanding of the proposed Lincoln Woods Trail Reconstruction Design project as described in this report. In general, these recommendations are based on the results of the subsurface investigation program performed by Sanborn Head, published correlations with soil properties, and the following documents:

Federal Highway Administration (FHWA) Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects (FP-14);

FHWA Load and Resistance Factor Design for Highway Bridge Superstructures Reference Manual (LRFD Manual);

Scope of Work for Lincoln Woods Trail Reconstruction Design, issued by the Pemigewasset Ranger District of the White Mountain National Forest;

7.1 Slope Stability Evaluations

7.1.1 Modeling Approach

Sanborn Head used Slide 2018 by Rocscience, Inc. of Ontario, Canada, to evaluate the global stability of the existing slope. Slide is a comprehensive 2-dimensional limit-equilibrium program that analyzes potential failure surfaces to calculate the minimum factor of safety based on the input geometry, soil properties, and loading conditions, and is widely accepted in engineering practice. The section analyzed in Slide is located at approximately Sta. 3+25, as shown in Appendix E.

The geometry of the existing slopes was based on a plan and cross-sections provided by EKCE dated November 24, 2020. Generalized subsurface conditions were developed based on subsurface conditions observed in adjacent test borings and geophysical data. Groundwater levels were conservatively assumed based on those observed in the test borings adjusted as necessary during calibration of the model.

A pedestrian surcharge load of 90 pounds per square foot (psf) was added across the full width of the existing trail as indicated by the LRFD Manual.

Soil properties used in the model were estimated based on observations made during the subsurface exploration program, the results of the geotechnical laboratory testing, correlations with similar soils, and our engineering judgement. Soil properties used in the models are included in Appendix E.

An existing conditions model was generated and analyzed prior to evaluating proposed stabilization systems. The model was calibrated by adjusting soil properties and groundwater conditions until a failing condition similar in size and shape to that observed in the field was established. These calibrated soil properties and groundwater conditions were used for subsequent analyses of proposed stabilization alternatives.

As part of the detailed analysis of the selected repair alternatives, Sanborn Head analyzed both static and seismic loading conditions. Seismic analysis was conducted using pseudo-static methods with vertical and horizontal site-specific load coefficients of 0.162g and 0.081g, respectively, as indicated by the Structural Engineers Association of California online database and corresponding to Site Class C.

Target minimum factors of safety of 1.3 under static loading conditions and 1.1 under seismic loading conditions were assumed for the proposed repairs. Results of the slope stability modeling, which show a schematic layout of the proposed repair, are included in Appendix E.

7.1.2 Proposed Slope Repair Details

Based on our review of the modeling results and observations made at the Site, we believe that the failure mechanism at the failure section is surficial erosion due to river action during high water events. The stability model was used to evaluate proposed repair alternatives at the analyzed section. An iterative approach allowed for the development of an optimized design.

In general, the proposed repair consists of regrading the existing slope to a maximum grade of 1 vertical to 1.5 horizontal (1V:1.5H) and armoring the regraded slope surface with a stone fill blanket. The stone fill blanket should be a minimum of 4 feet thick and constructed of Class 3 riprap (FP-14 Table 705-1) above the ordinary high-water level. The stone fill blanket should be constructed of Class 7 riprap (FP-14 Table 705-1) below the ordinary high-water level to protect against scour. The stone fill blanket should be placed on a sand cushion a minimum of 9 inches thick as specified in Appendix F.

As an alternative to the stone fill blanket, an anchored mesh system such as ARMORMAX by Propex Global or similar may be used on the slope face above the 100-year flood elevation.

Following installation of the anchored mesh system, the slope should be seeded in accordance with the recommendations provided by the manufacturer of the system. The final design of the anchored mesh system should be performed by the manufacturer of the system, with review by Sanborn Head. The manufacturer should provide construction specifications and installation details that may be used by the Contractor during installation of the system.

8.0 CONSTRUCTION CONSIDERATIONS

8.1 Slope and Retaining Wall Subgrade Preparation Recommendations Prior to placement of the sand cushion and stone fill, all vegetation, topsoil, loose, sloughed, or other deleterious material shall be removed from the slope. The exposed subgrade should be graded such that surface water runoff is promoted prior to placement of the sand cushion. The prepared subgrade should be observed by the geotechnical engineer prior to placement of fill materials.

During placement of stone fill, the Contractor shall place individual stones to maximize the surface area contact with adjacent stones. To the extent practical, stone surfaces as-placed should dip toward the slope face.

8.2 Anchored Mesh Placement Recommendations

If selected, the anchored mesh system should be installed by an approved installer in accordance with final design details and installation guidance provided by the manufacturer of the system. The subgrade below the anchored mesh system should be prepared as described in Section 9.1.

8.3 Fill Placement and Compaction Requirements

Where a raise-in grade is required below the stone fill blanket, Granular Backfill material meeting the requirements of FP-14 Section 704.03(a) should be used. The sand cushion placed directly below the stone fill blanket should consist of Select Borrow meeting the requirements of FP-14 Section 704.07. Alternative soil types used as fill proposed by the Contractor may be considered and should be reviewed by Sanborn Head prior to placement.

Fill material should be placed in lifts no greater than 12 inches in loose thickness and compacted to a minimum of 95 percent of its maximum dry density as measured by ASTM D1557.

8.4 Material Re-use

We anticipate that on-Site soils may be suitable for stockpiling and reuse as Granular Backfill, Select Borrow, or Structural Backfill, provided the soil(s) meets the requirements outlined in Appendix F. Excavated material that is to be reused as fill should be stockpiled in such a manner that promotes water runoff and limits the saturation of the material.

9.0 CONSTRUCTION MONITORING

We recommend that Sanborn Head be provided the opportunity to review the design plans and specifications to confirm that the recommendations of this report have been properly incorporated. We further recommend that our firm be retained during construction to observe excavation of existing soils, subgrade preparation, and placement and compaction of soil and stone fill to confirm that the recommendations in the report are being implemented as intended in the field. The geotechnical engineer in the field should observe the work for compliance with the recommendations in this report and assist in the development of design changes should subsurface conditions differ from those anticipated prior to the start of construction.

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Figures

PEMIGEWASSET RIVER-

EAST BRANCH

SITE

KANCAMANGUS

HIGHWAY

LINCOLN WOODS

TRAILHEAD

LINCOLN, NEW HAMPSHIRE

ENMAN KESSELRING CONSULTING ENGINEERS

SHEET NO. 1

FILE NO. 4880.00SCALE: 1" = 1000'

DATE: JANUARY 2021 CHECKED BY: IPD

DRAWN BY: LET

LINCOLN WOODS TRAIL RECONSTRUCTION DESIGN

LOCUS PLAN

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ORTHOPHOTO CAPTURED FROM MICROSOFT

CORPORATION EARTHSTAR GEOGRAPHICS 2020.

SH-2

SH-3

SH-5

SH-1

SH-4

4880.00

PROJECT NUMBER:LINCOLN WOODS TRAIL RECONSTRUCTION DESIGN

LINCOLN, NH

ENMAN KESSELRING CONSULTING ENGINEERS

DATE:

PROJECT MGR:

REVIEWED BY:

DESIGNED BY:

DRAWN BY:

L. TRACY

L. TRACY

I. DONOVAN

S. KELLEY

JANUARY 2021

I. DONOVAN

PIC:

NOTES:

1. BASEMAP WAS PROVIDED BY ENMAN KESSELRING CONSULTING ENGINEERS OF

RUTLAND, VERMONT ON NOVEMBER 3, 2020.

2. THE TEST BORING WAS ADVANCED BY NEW ENGLAND BORING CONTRACTORS OF DERRY,

NEW HAMPSHIRE USING A MOBILE DRILL RIG BETWEEN NOVEMBER 2 AND NOVEMBER 9, 2020.

3. TEST BORINGS LOCATIONS WERE SURVEYED IN THE FIELD BY OTHERS.

LEGEND:

LOCATION AND DESIGNATION OF TEST BORING

DESCRIPTIONDATE BYNO.

EXPLORATION LOCATION PLAN

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SH-1

Appendix A

Limitations

LIMITATIONS

Explorations

1. The analyses, recommendations, and designs submitted in this report are based in part on the data obtained from subsurface explorations. The nature and extent of variations between these explorations may not become evident until construction. If variations then appear evident, it will be necessary to re-evaluate the findings of this report.

2. The generalized soil profile described in the text is intended to convey trends in subsurface conditions. The boundaries between strata are approximate and idealized, and have been developed by interpretation of widely spaced explorations and samples; actual soil transitions may be more or less gradual than indicated. For specific information, refer to the soil boring logs.

3. Water level readings have been made in the soil borings at the times and under the conditions stated on the soil boring logs. These data have been reviewed and interpretations have been made in the text of this report. However, it must be noted that fluctuations in the level of the groundwater may occur due to variations in rainfall, temperature, and other factors differing from those occurring at the time measurements were made.

Review

4. In the event that any changes in the nature, design, or location of the proposed project are planned, the conclusions and recommendations contained in this report shall not be considered valid unless the changes are reviewed and conclusions of the report modified or verified in writing by Sanborn Head.

Construction

5. It is recommended that Sanborn Head be retained to provide soil engineering services during the earthwork and specialty geotechnical construction phases of the work. This is to observe compliance with the design concepts, specifications, or recommendations and to allow design changes in the event that subsurface conditions differ from those anticipated prior to the start of construction.

Use of Report

6. This report has been prepared for the exclusive use of Enman Kesselring Consulting

Engineers for the Lincoln Woods Trail Reconstruction Design project located in Lincoln, NH in accordance with generally accepted soil and foundation engineering practices. No other warranty, expressed or implied, is made.

7. This report has been prepared for this project by Sanborn Head and Associates, Inc. for design purposes only. Contractors using this report to prepare a bid for site work acknowledge that its scope is limited to design considerations only.

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Appendix B

Test Boring Logs

2.5

6.5

8.5

S-1

S-2 C-1

C-2

0 - 2

4 - 4.1

4.1 - 5.5

7.6 - 10.7

-----4'-----

-----7'-----

-----7.6'-----

-----10.7'-----

24/2

0/0 17/17

37/35

-----0'-----

SAND &

GRAVEL

BOULDERS

SAND &

GRAVEL

GRANITE

30/0"

S-1 (0 to 2'): Loose, brown, fine GRAVEL and Sand, little Silt, very few Root fragments. Wet.

S-2 (4 to 4.1'): No Recovery.

C-1 (4.1 to 5.5'): Boulder. REC=100%.

C-2 (7.6 to 10.7'): Hard, fresh to moderately weathered, gray/pink, medium to coarse-grained, GRANITE, fractures shallow to moderately steep, very close to closely spaced, FeO-stained surfaces, no infilling, smooth, planar, open. Moderately fractured. REC=95%. RQD=11%.

Boring terminated at 10.7 feet bgs.

Rollerbit through obstructions approximately 6 feet to 6.5 feet and 7 feet to 7.6 feet.

Depth of Hole

Depth of Casing

Drill Rate

(min/ft)

Depth (ft)

Date Finished: 11/03/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time Ref. Pt.

11/02/20Drilling Company: New England Boring Contractors

Foreman: W. Hoekele

Sheet: 1 of 1

--- No Groundwater Encountered

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 4" Drive and Wash

Sampling Method: 2" Split Spoon, Auto Hammer / NX Core Barrel

Date Started: 11/02/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-1 Ground Elevation: 1229 ± feet Datum: NAVD 1988

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IN

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2/

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Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

Data

Remarks

4.75

4.5

8.75

4.75

S-1

S-2

S-3

S-4

C-1

0 - 2

4 - 6

9 - 11

14 - 14.8

16 - 21

-----0.3'-----

-----16'-----

24/8

24/12

24/10

10/5

60/32

-----0'-----

TOPSOIL

SAND &

GRAVEL

BOULDERS

100/4"

S-1A (0 to 0.3'): Leaves/Roots.

S-1B (0.3 to 2'): Loose, tan, fine to coarse GRAVEL, some Sand, trace Silt. Moist.

S-2 (4 to 6'): Medium dense, tan/brown, fine to coarse SAND, some Gravel, little Silt. Moist.

S-3 (9 to 11'): Very dense, tan/gray, fine to coarse GRAVEL and Sand, trace Silt.

Moist.

S-4 (14 to 14.8'): Very dense, tan/brown, fine to coarse GRAVEL and Sand, trace Silt. Moist.

C-1 (16 to 21'): Boulders. REC=53%.

Rollerbit through obstruction from 7 feet to 8 feet

5" casing to 9 feet

4" casing to 15 feet

Rollerbit to through obstruction from 14.8 feet to 16 feet

Depth of Hole

Depth of Casing

Drill Rate

(min/ft)

Depth (ft)

Date Finished: 11/09/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time 29.2' 10 Minutes

Ref. Pt.

11/09/20Drilling Company: New England Boring Contractors

Foreman: B. Raiche

Sheet: 1 of 2

27'15:40 Ground Surface18'

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 5"/4"/3" Drive and Wash

Sampling Method: 2" Split Spoon, Auto Hammer / NX Core Barrel

Date Started: 11/09/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-2 Ground Elevation: 1227 ± feet Datum: NAVD 1988

B O

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IN

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Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

S-5

C-2

S-6

S-7

S-8

21 - 21.6

22 - 23.1

23.1 - 24

24.5 - 25.8

29 - 29.2

-----21'-----

-----21.9'-----

-----23.1'-----

-----29.2'-----

7/0

13/3

11/8

15/11

2/2

BOULDERS

SAND &

GRAVEL

BOULDERS

SAND &

GRAVEL

100/1"

100/5"

100/3"

100/2"

S-5 (21 to 21.6'): No Recovery.

C-2 (22 to 23.1'): REC=23%.

S-6 (23.1 to 24'): Very dense, tan/brown, fine to coarse GRAVEL and Sand, trace Silt. Wet.

S-7 (24.5 to 25.8'): Very dense, tan, fine to coarse SAND, some Silt, little Gravel. Wet.

S-8 (29 to 29.2'): Very dense, tan, fine to coarse SAND, some Gravel, little Silt. Wet.

Boring terminated at 29.2 feet bgs due to split spoon refusal.

3" casing to 22 feet

Depth of Hole

Depth of Casing

Drill Rate

(min/ft)

Depth (ft)

Date Finished: 11/09/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time 29.2' 10 Minutes

Ref. Pt.

11/09/20Drilling Company: New England Boring Contractors

Foreman: B. Raiche

Sheet: 2 of 2

27'15:40 Ground Surface18'

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 5"/4"/3" Drive and Wash

Sampling Method: 2" Split Spoon, Auto Hammer / NX Core Barrel

Date Started: 11/09/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-2 Ground Elevation: 1227 ± feet Datum: NAVD 1988

B O

R

IN

G L

O G

C :\U

S E

R S

\I D

O N

O V

A N

\D E

S K

T O

P \V

P N

L O

C A

L F

O

LD

E R

\4

.0

\4

.0 0.

G P

J

S A

N B

O R

N H

E A

D V

.G

LB

S A

N B

O R

N H

E A

D V

.G

D T

2/

/2

Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

S-1

S-2

S-3

S-4

0 - 2

4 - 4.8

9 - 11

14 - 16

-----0.5'-----

-----18'-----

24/11

10/4

24/10

24/12

-----0'-----

TOPSOIL

SAND &

GRAVEL

100/4"

S-1A (0 to 0.5'): Loose, black, SILT, little Sand, common Organic fragments, few Root fragments.

Moist.

S-1B (0.5 to 2'): Loose, tan, fine to coarse SAND and Gravel, trace Silt. Moist.

S-2 (4 to 4.8'): Very dense, brown, fine to coarse SAND, some Gravel, little Silt, very few Root fragments. Moist.

S-3 (9 to 11'): Very dense, tan, fine to coarse GRAVEL and Sand, trace Silt. Wet.

S-4 (14 to 16'): Very dense, tan, fine to coarse GRAVEL and Sand, trace Silt. Wet.

Boring terminated at 18 feet bgs. No refusal encountered.

Roller bit through obstructions from 4.5 feet to 8.5 feet 4" Casing to approximately 4.5 feet

Rollerbit through obstruction at approximately 9.5 feet to 13.5 feet.

Rollerbit through obstruction at approximately 16.5 feet to 18 feet.

Test boring terminated due to damage to spin shoe on 3" casing.

Depth of Hole

Depth of Casing

Depth (ft)

Date Finished: 11/06/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time Ref. Pt.

11/05/20Drilling Company: New England Boring Contractors

Foreman: B. Raiche

Sheet: 1 of 1

--- No Groundwater Encountered

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 4"/3" Drive and Wash

Sampling Method: 2" Split Spoon, Automatic Hammer

Date Started: 11/05/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-3 Ground Elevation: 1228 ± feet Datum: NAVD 1988

B O

R

IN

G L

O G

C :\U

S E

R S

\I D

O N

O V

A N

\D E

S K

T O

P \V

P N

L O

C A

L F

O

LD

E R

\4

.0

\4

.0 0.

G P

J

S A

N B

O R

N H

E A

D V

.G

LB

S A

N B

O R

N H

E A

D V

.G

D T

2/

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Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

S-2

S-3

S-4

S-5

0 - 2

4 - 4.4

9 - 9.1

15 - 17

19 - 19.8

-----2'-----

24/10

5/4

1/0

24/14

10/5

-----0'-----

TOPSOIL

SAND &

GRAVEL

WOH

100/5"

50/1"

100/4"

S-1A (0 to 0.5'): Leaves/Roots.

S-1B (0.5 to 2'): Very loose, orange/brown, fine to coarse SAND, little Gravel, trace Silt, very few Root fragments. Moist.

S-2 (4 to 4.4'): Very dense, tan/gray/pink, fine to coarse GRAVEL, little Sand, trace Silt. Wet.

S-3 (9 to 9.1'): No Recovery.

S-4 (15 to 17'): Very dense, tan/gray, fine to coarse SAND and Gravel, little Silt. Wet.

S-5 (19 to 19.8'): Very dense, tan/gray, fine to coarse SAND and Gravel, little Silt. Wet.

Rollerbit through obstructions from

4.4 feet to 8.5 feet.

4" casing to 9 feet.

Rollerbit through obstruction from 9 feet to 10 feet and 10.5 feet to 13 feet.

Depth of Hole

Depth of Casing

Drill Rate

(min/ft)

Depth (ft)

Date Finished: 11/05/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time 5 Minutes

Ref. Pt.

11/05/20Drilling Company: New England Boring Contractors

Foreman: B. Raiche

Sheet: 1 of 2

11:00 Ground Surface16'

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 4"/3" Drive and Wash

Sampling Method: 2" Split Spoon, Auto Hammer / NX Core Barrel

Date Started: 11/04/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-5 Ground Elevation: 1227 ± feet Datum: NAVD 1988

B O

R

IN

G L

O G

C :\U

S E

R S

\I D

O N

O V

A N

\D E

S K

T O

P \V

P N

L O

C A

L F

O

LD

E R

\4

.0

\4

.0 0.

G P

J

S A

N B

O R

N H

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2/

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Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

4.5

C-1

S-6

22.9 - 26.9

27 - 27.8

-----22.9'-----

-----26.9'-----

-----27.8'-----

48/16

9/6

SAND &

GRAVEL

BOULDERS

SAND &

GRAVEL

100/3"

C-1 (22.9 to 26.9'): Boulders. REC=33%.

S-6 (27 to 27.8'): Very dense, gray/tan, fine to coarse GRAVEL, some Sand, trace Silt.

Wet.

Boring terminated at 27.8 feet bgs due to split spoon refusal.

3" casing to 27 feet.

Depth of Hole

Depth of Casing

Drill Rate

(min/ft)

Depth (ft)

Date Finished: 11/05/20

Checked By: S. Kelley

Depth to Water

Stab.

Time

Groundwater Readings

Date Time 5 Minutes

Ref. Pt.

11/05/20Drilling Company: New England Boring Contractors

Foreman: B. Raiche

Sheet: 2 of 2

11:00 Ground Surface16'

Project: Lincoln Woods Trail

Location: Lincoln, NH

Project No.: 4880.00

Drilling Method: Mobile Drill B-29 with 4"/3" Drive and Wash

Sampling Method: 2" Split Spoon, Auto Hammer / NX Core Barrel

Date Started: 11/04/20

Logged By: I. Donovan

Sanborn, Head & Associates, Inc.

Log of Boring SH-5 Ground Elevation: 1227 ± feet Datum: NAVD 1988

B O

R

IN

G L

O G

C :\U

S E

R S

\I D

O N

O V

A N

\D E

S K

T O

P \V

P N

L O

C A

L F

O

LD

E R

\4

.0

\4

.0 0.

G P

J

S A

N B

O R

N H

E A

D V

.G

LB

S A

N B

O R

N H

E A

D V

.G

D T

2/

/2

Sample No.

Depth (ft)

Pen/ Rec (in)

Log

StratumSample Information

Description Spoon Blows per 6 in

Geologic DescriptionField Testing

Appendix C

Geophysical Survey Report

HAGER-RICHTER

GEOSCIENCE, INC.

©2020 Hager-Richter Geoscience, Inc.

GEOPHYSICAL SURVEY

LINCOLN WOODS TRAIL

LINCOLN, NEW HAMPSHIRE

Prepared for:

Sanborn | Head & Associates, Inc.

187 Saint Paul Street, Suite 201 Burlington, Vermont 05401

Prepared by:

Hager-Richter Geoscience, Inc.

8 Industrial Way - D10 Salem, New Hampshire 03079

File 20MH27 November 2020

GEOPHYSICS FOR THE ENGINEERING COMMUNITY

SALEM, NEW HAMPSHIRE

Tel: 603.893.9944

FORDS, NEW JERSEY

Tel: 732.661.0555

SALEM, NEW HAMPSHIRE • FORDS, NEW JERSEY

www.hager-richter.com

November 30, 2020 File 20MH27

Shawn P. Kelley, PhD, PE Project Director Sanborn | Head & Associates, Inc.

187 Saint Paul Street, Suite 201 Burlington, Vermont 05401

Phone: 603.232.8758 Cell: 207.232.5832 Email: jennifer.baron@gza.com

RE: Geophysical Survey Lincoln Woods Trail Lincoln, New Hampshire

Dear Dr. Kelley:

In this report, we summarize the results of a geophysical survey conducted by Hager-Richter Geoscience, Inc. (HRGS) at the above referenced site in Lincoln, New Hampshire for Sanborn | Head & Associates, Inc. (Sanborn | Head). The scope of the survey and the area of interest was specified by Sanborn | Head.

INTRODUCTION

The site is located on the Lincoln Woods Trail in the White Mountain National Forrest in Lincoln, New Hampshire. The general location of the site is shown in Figure 1. According to information provided by Sanborn | Head, a slope failure has occurred on the west bank of the East Branch Pemigewasset River located approximately 1 mile north of the Kancamagus Highway. In response, the Lincoln Woods Trail has been relocated west away from the top of the slope failure. As part of a geotechnical investigation of the slope failure and trail stabilization, Sanborn | Head requested a geophysical survey be conducted in the vicinity of the slope failure to determine the depth and configuration of the bedrock surface.

The area of interest (AOI) for the geophysical survey was specified to be the accessible portions of the former Lincoln Woods Trail above the failed slope and on the west bank of the East Branch Pemigewasset River below the Lincoln Woods Trail. Figure 2 shows the location of the seismic refraction survey lines relative to the trail and proposed borings staked by Sanborn | Head prior to the geophysical survey.

Geophysical Survey

Lincoln, New Hampshire File 20MH27 Page 2

OBJECTIVE

The objective of the geophysical survey was to determine the depth and configuration of the bedrock surface in an area of interest specified by Sanborn | Head.

THE SURVEY

Michael Howley, P.G., Bryan Carnahan, and Sean Reid of HRGS conducted the geophysical survey on November 2, 2020. The project was coordinated with Shawn Kelley, PhD, P.E. of Sanborn | Head. Ian Donovan, also of Sanborn | Head, was on site for the duration of the field work and specified the area of interest.

The geophysical survey was conducted using the seismic refraction method. Seismic refraction is the industry standard for the determination of the depth and configuration of bedrock. Data analysis and interpretation were completed at the HRGS offices. Original data and field notes will be retained in the HRGS files for a minimum of three years.

Seismic refraction data were acquired along two transects, designated as Survey Lines 1 and 2 totaling 470 linear feet. Photo 1 below shows a portion of the Seismic Line 2 setup acquired on the west bank of the East Branch Pemigewasset River. The positions of the survey lines were recorded using a Trimble GeoX7 CM GPS receiver coupled with an external Zephyr-2 antenna.

The approximate elevations along the geophysical survey lines was also determined from plans provided by Sanborn | Head.

Photo 1. View south along Seismic Line 2 acquired on the west embankment of the East Branch Pemigewasset River in the area of the slope failure.

Lincoln, New Hampshire File 20MH27 Page 3

EQUIPMENT AND PROCEDURES

Seismic Refraction. Seismic refraction data were acquired along two (2) transects located on the former Lincoln Woods Trail above the failed slope and on the west bank of the East Branch Pemigewasset River below the Lincoln Woods Trail. Figure 2 shows the locations of the seismic refraction transects.

We used our 48-channel seismograph (two 24-channel Geometrics Geodes) connected to, and controlled by, a notebook PC computer. The software provides for the acquisition, display, plotting, filtering and storage of seismic data. The seismogram image presented in real time on the notebook screen allows the operator to verify the quality of the data. The stored digital data are later transferred at the end of the field day for storage, backup, and future data processing.

The Geodes were coupled to two 24-element seismic spread cables for a total of 48 geophones.

The geophones measure only the vertical component of the compressional wave energy, and their resonant frequency is 14 Hz. The geophones are equipped with a vertical 3-inch spike that is pressed into the soil so that the geophone case is contacting the ground surface. A geophone spacing of 4 and 6 feet was used.

A piezoelectric crystal is used as a trigger device. The trigger is attached to the hammer, and upon impact emits a small electrical impulse that travels via a cable to the seismograph to start the seismograph recording. The timing mechanism in the seismograph is factory calibrated and does not require additional calibration according to manufacturing specifications.

Seismic energy was provided by a 12-lb sledgehammer striking a metal base plate. We record seven "shots" per cable spread - one shot off each end of the cable, one shot at each end of the cable, and three shots interior to the cable, as access allowed. The number of stacks per shot location is variable, and the quality of the stacked seismic signal for each shot location was verified in the field with the visual display. The seismic refraction data were acquired using a 100-millisecond recording length and a sample interval of 0.02833 milliseconds.

The seismic data were analyzed using the Generalized Reciprocal Method (GRM) of seismic refraction interpretation. The method is described in detail in Palmer (1980)1. GRM allows for some variation in the surface topography as well as lateral variation in the seismic velocity of the upper layers. The method uses the principle of migration whereby the refractor need only be planar over a short distance, thus allowing the calculation of depth to an undulating interface. In addition, GRM is relatively insensitive to dip angles as high as 20o, unlike most other methods that can be sensitive to dips as low as 5o. GRM also allows for the calculation of depth below each geophone instead of below only the shot points as in the Time-Intercept and Crossover Distance methods. The GRM software that we use for data analysis (IXRefraX by Interpex) contains several internal tests for data consistency.

1 Palmer, Derecke (1980) The Generalized Reciprocal Method of Seismic Refraction Interpretation, Society of Exploration Geophysicists, 104 p.

Lincoln, New Hampshire File 20MH27 Page 4

The results are used to construct an interpreted velocity profile of the subsurface for each seismic line. The velocities of seismic waves are functions of the types of geologic material through which they pass. One can thus infer the general subsurface stratigraphy from the velocities determined. Seismic velocities are expressed in feet per second (fps).

LIMITATIONS OF THE METHOD

HRGS MAKES NO GUARANTEE THAT ALL BEDROCK WAS DETECTED

IN THIS SURVEY. HRGS IS NOT RESPONSIBLE FOR DETECTING

BEDROCK THAT NORMALLY CANNOT BE DETECTED BY THE

METHODS EMPLOYED OR THAT CANNOT BE DETECTED BECAUSE OF

SURFACE OR SUBSURFACE CONDITIONS. THE ACCURACY

(STANDARD DEVIATION) OF THE APPARENT DEPTHS OF RELATIVELY

COMPETENT BEDROCK DETERMINED BY THE SEISMIC REFRACTION

SURVEY IS ABOUT ± 10% OF THE APPARENT DEPTH OF BEDROCK, OR

± 2 FEET, WHICHEVER IS GREATER. THE BEDROCK MODELS SHOWN

AS PROFILES OR LISTED AS TABULAR DATA SHOULD NOT BE USED

FOR CONTRACT BEDROCK REMOVAL QUANTITIES.

Seismic Refraction. Like all geophysical methods, the seismic refraction method is based on the assumption that the local geology is uncomplicated. In particular, the seismic refraction method assumes that interfaces between geologic materials correlate with sharp increases in seismic velocity and that the interfaces between geologic units are relatively flat lying. The method is not very sensitive to lateral variations within layers, and relatively subtle features such as fracture zones within bedrock are generally difficult to detect unless there is a topographic expression of the feature. The accuracy of the method is degraded in areas with strong topographic relief and/or where the interfaces have apparent dips greater than about 20̊. In general, the standard error of depths determined is about 10% or 2 feet, whichever is greater.

Where two materials do not exhibit contrasting velocities, or where velocities gradually increase with depth, a clear refracted signal is not generated, and the GRM method cannot be used to distinguish the two materials. In some cases, the "geophysical contact" between materials with contrasting velocities does not correlate exactly with the "geologic contact." For example, where a highly weathered bedrock is overlain by a dense material such as till, the velocity range of the weathered bedrock might overlap or approach the velocity range of the till, and the two materials cannot be distinguished seismically. In such cases, the depth determined by GRM is the depth of competent bedrock, which might be located at some depth below the geologic contact.

The depth relations of the water table and bedrock may constitute a significant problem for processing with GRM. This problem is that of a "blind layer." A blind layer occurs where the thickness of the saturated overburden is less than about half the depth of bedrock. In such cases, the water-saturated material immediately above bedrock is "blind" in the sense that no refracted seismic energy from it will be received as a first arrival of seismic energy, and all methods used to reduce the seismic data to determine the depth of bedrock, the objective of this survey, use

Lincoln, New Hampshire File 20MH27 Page 5 only first arrivals. Thus, the saturated layer will not be detected where it is close to bedrock, and most methods of seismic data reduction will indicate that bedrock is considerably shallower than it actually is. Although GRM, the method used by HRGS to reduce the seismic refraction data, does not use first arrivals through the water saturated zone (because there is none to use) in such cases, GRM determines the depth of bedrock correctly by using the average velocity of the saturated and unsaturated zones.

RESULTS

HRGS conducted a seismic refraction survey to determine the depth and configuration of the bedrock surface in the vicinity of a slope failure along the west bank of the East Branch Pemigewasset River. Seismic refraction data were acquired along two transects, designated as Seismic Lines 1 and 2 totaling 470 linear feet. Seismic Line 1 was located on the former Lincoln Woods Trail above the failed slope, and Seismic Line 2 was located on the west bank of the East Branch Pemigewasset River below the former Lincoln Woods Trail. The locations of the geophysical survey lines are shown in Figure 2.

The results of the seismic survey are shown in profile form in Figures 3 and 4 for Seismic Lines 1 and 2, respectively with 3x vertical exaggeration. The depth and elevation of the bedrock surface interpreted from the seismic refraction survey are also included in Table 1.

Data Quality. The quality of the seismic refraction data ranges from fair to good. A measure of the accuracy of the data can be obtained by comparing the bedrock depths determined seismically with depths reported from nearby borings that encountered refusals on assumed bedrock, or cored bedrock. Borings SH-2 and SH-3 were installed along Seismic Line 1 after the geophysical survey was completed. Draft boring logs provided by Sanborn | Head, attached in Appendix 1, indicate that refusal was encountered in SH-2 at a depth of 29.2 feet bgs. The depth of refusal encountered in boring SH-2 matches very well with the depth of bedrock determined from seismic refraction data of 29.5 feet bgs at the same location. Boring SH-3 was terminated at 18 feet bgs after encountering an obstruction at 16.5 feet bgs. The depth of refusal reported in the draft boring log for SH-3 is much less than the depth of bedrock determined from seismic refraction data of 33.5 feet bgs at the same location. We interpret this discrepancy to be due to boring SH-3 likely encountering a boulder rather than bedrock at a depth of 16.5 feet.

Based on the results of comparing seismically determined depths with reported depth of bedrock in borings in data acquired for similar projects, we estimate the accuracy (standard deviation) of the depths of competent bedrock determined by the seismic refraction survey to be about ± 10% of the depth of bedrock, or ± 2 feet), whichever is greater.

Material Velocities. The results of the seismic refraction survey for the site are shown in profile form in Figure 3 and 4 with 3x vertical exaggeration. Materials with two or three distinct velocity ranges were detected at the Site. The upper material exhibits a velocity range of 1,050 to 1,600 feet per second (fps) and is interpreted to consist of mostly unsaturated soils.

Lincoln, New Hampshire File 20MH27 Page 6

An intermediate material present only along Seismic Line 1 exhibits a velocity range of 2,400 to 6,000 feet per second (fps) and is interpreted to consist of saturated soils. The lower material exhibits a velocity range of 14,100 to 18,900 fps and is interpreted to consist of bedrock. Where the top of bedrock is highly fractured and/or deeply weathered, it might exhibit lower velocities that cannot be detected as a distinct layer on the basis of the seismic refraction data. Thus, the top of rock determined on the basis of seismic refraction data is generally the top of relatively competent bedrock, which might be located somewhat below the geologic contact between the overburden and bedrock.

Depth and Configuration of the Bedrock Surface. Seismic profiles showing bedrock elevation determined by seismic refraction for Seismic Lines 1 and 2 are presented in Figures 3 and 4, respectively. The depth of competent bedrock determined by seismic refraction varies between about 19.2 feet and 35.2 feet below ground surface for Seismic Line 1 and between about 6.9 feet and 12.4 feet for Seismic Line 2. The elevation of competent bedrock determined by seismic refraction along Seismic Lines 1 and 2 varies between about 1,194 to 1,213 feet above sea level, for a total relief of about 19 feet. In general, the bedrock surface undulates and is deepest to the south along Seismic Line 1, with a bedrock low located near the center of Seismic Line 2.

The results of the seismic refraction survey along Seismic Lines 1 and 2 are listed in tabular form in Table 1. Note that the profile plots in Figures 3 and 4 represent non-unique models, and bedrock elevations between actual data points represent interpolated values that may differ significantly from actual bedrock elevations. The bedrock model shown as profiles or listed as tabular data should not be used for contract bedrock removal quantities.

CONCLUSIONS

Based upon the results of the geophysical survey conducted by HRGS along the Lincoln Woods Trail in the White Mountain National Forrest in Lincoln, New Hampshire for Sanborn | Head & Associates, Inc. in November 2020, we conclude that:

• The depth of competent bedrock determined varies between about 19.2 feet and 35.2 feet below ground surface for Seismic Line 1 and between about 6.9 feet and 12.4 feet for Seismic Line 2.

• The elevation of competent bedrock along Seismic Lines 1 and 2 varies between about 1,194 to 1,213 feet above sea level, for a total relief of about 19 feet.

LIMITATIONS ON USE OF THIS REPORT

This letter report was prepared for the exclusive use of Sanborn | Head & Associates, Inc.

(Client). No other party shall be entitled to rely on this Report, or any information, documents, records, data, interpretations, advice or opinions given to Client by Hager-Richter Geoscience, Inc. (HRGS) in the performance of its work. The Report relates solely to the specific project for which HRGS has been retained and shall not be used or relied upon by Client or any third party

Lincoln, New Hampshire File 20MH27 Page 7 for any variation or extension of this project, any other project or any other purpose without the express written permission of HRGS. Any unpermitted use by Client or any third party shall be at Client's or such third party's own risk and without any liability to HRGS.

HRGS has used reasonable care, skill, competence and judgment in…