FSR 25 Geotech Report.pdf

January 21, 2010

Mark Colgan, P.E.

Vanasse Hangen Brustlin, Inc.

7056 US Route 7 North Ferrisburgh, VT 05473

RE: Letter updated our Geotechnical Recommendations Report

USDA Forest Service GMNF FSR 25 over Mad River, Warren, VT Bridge Replacement

GeoDesign #837-19.3

Cc: Tristan Cooke, P.E., VHB Aaron Guyette, P.E., VHB

Dear Mark:

GeoDesign is pleased to submit this supplemental letter on the foundation recommendations for the Warren bridge replacement for Green Mountain National Forest (GMNF). This letter updates the bearing pressures and lateral load recommendations given in our original report dated December 16, 2009 to provide LRFD factored resistances for bearing and lateral loading.

1 RECOMMENDATIONS

1.1 Bearing Pressure – East Abutment

The recommended bearing pressure at the proposed bottom of footing elevation (assumed to be designed at six feet below brook bottom elevation) for the proposed bridge over the Mad River on FSR 25 replacement is presented in Figure 1 assuming a full scoured out section in front of the footing (i.e., stream side). We assumed a 120 pound per cubic foot unit weight for the silty sand and gravel and the groundwater table at the ground surface at the location of the footings (stream side).

For comparison, we have provided additional bearing pressures assuming two other scoured depth conditions in front of the proposed footing. Figure 2 presents the recommended bearing pressure if half of the soil in front of the footing was scoured out. Figure 3 presents the recommended bearing pressure if none of the soil in front of the footing was scoured out.

GMNF Bridge over Mad River Replacement Evaluation and Recommendations – Letter update GeoDesign Project Number: 837-19.3

1/21/10

For all tables, the resistance factor used is 0.45 per Table 10.5.5.2.2-1 in the AASHTO LRFD Bridge Design Specification, 4th edition 2007 with 2009 Interim Revisions.

Figure 1. Factored Net Bearing Resistance for GMNF FSR 25 over Mad River assuming full scour (no soil left above bottom of footing).

Figure 2. Factored Net Bearing Resistance for GMNF FSR 25 over Mad River assuming half scour (3 feet of soil left above bottom of footing).

Figure 3. Factored Net Bearing Resistance for GMNF FSR 25 over Mad River assuming no scour (6 feet of soil left above bottom of footing).

1.2 Bearing Pressure – West Abutment

The recommended factored bearing resistance at the proposed bottom of footing elevation (assumed to be designed at above river bottom elevation on bedrock) for the proposed bridge over the Mad River on FSR 25 replacement is 11.7 ksf. This value was determined using published unconfined compressive strength values for Phyllite rock which may be below the actual in situ strength of this rock. If necessary, we can modify this recommendation using actual unconfined compressive strength data obtained from laboratory testing. We can perform this testing on the rock core samples that we obtained during our site evaluation if you request.

The recommended bearing pressure on rock assumes the bedrock is sound and not fractured. We recommend using 8 ksf (as presented in our original report) if the rock is blasted and/or disturbed. We also recommend using a concrete mud mat as a leveling pad if needed once bedrock is exposed (as mentioned in our original report).

1.3 Expected Settlement

We expect the settlement of the east abutment structure will be on the order of a ½ to 1 inch or less provided the subgrade is kept firm upon excavation. We expect the settlement of the west abutment structure to be neglible provided it is founded on firm native bedrock (i.e., not blasted).

If blasting is necessary, settlements will be on the order of a ½ to 1 inch or less provided the placed subgrade material is firmly compacted per our recommendations in our December 16, 2009 report. We expect that these settlements will be largely complete during construction.

1.4 Lateral Earth Pressure against backfilled walls and culverts and Sliding Resistance

Recommended earth pressure design criteria are presented below for free draining compacted Granular Backfill (i.e., VAOT 704.08A):

Horizontal Earth Pressures and Sliding Resistance

Abutments and wing walls should be designed for the lateral earth pressures outlined below.

Load factors for earth pressures shall be taken from Table 3.4.1-2 in AASHTO LRFD Bridge Design Specifications. Resistance factors for bearing and sliding resistance shall be taken from Table 10.5.5.2.2-1 in AASHTO LRFD Bridge Design Specifications.

Granular Backfill

Coefficient of Active Earth Pressure, Ka = (1-sinf)/(1+sinf) = 0.28 Coefficient of At-Rest Earth Pressure, Ko = (1-sinf) = 0.44 Coefficient of Passive Earth Pressure, Kp = tan2(45+f/2) = 3.5 (Rankine) Backfill Density, = 135 pounds per cubic foot (pcf) Friction angle, f = 34 degrees

Coefficient of friction, (ultimate) f, (from DM-7.2 Table 7.2-63)

Mass footing concrete on Granular Backfill or crushed stone = 0.55 Mass footing concrete on Natural Glacial Till Soil = 0.55

Reliance on passive pressure in front of foundation/abutments is not recommended due to scour potential.

1.5 Closure

If you have any questions regarding these updated recommendations, please call our office.

Sincerely, GeoDesign, Inc.

Shawn P Kelley, Ph.D. John E. Lens, P.E.

Associate Senior Principal

O:\Client related\CL-800 to 899\837-Vanasse Hangen Brustlin\837-19.3-Warren Bridge\Report\837-19.3 Letter update to Rpt 100121.doc

December 16, 2009

Mark Colgan, P.E.

Vanasse Hangen Brustlin, Inc.

7056 US Route 7 North Ferrisburgh, VT 05473

RE: Geotechnical Recommendations Report USDA Forest Service

GMNF FSR 25 over Mad River, Warren, VT Bridge Replacement

GeoDesign #837-19.3

Cc: Tristan Cooke, P.E., VHB Aaron Guyette, P.E., VHB

Dear Mark:

GeoDesign is pleased to submit this report on the foundation recommendations for the bridge replacement in Warren, VT for Green Mountain National Forest (GMNF). This report includes the site evaluation data that we collected (i.e., soil boring logs and grain size analysis), our summary of the subsurface conditions at the river crossing and our recommendations for bearing pressure and subgrade preparation for the replacement bridge.

We appreciate the opportunity to be of service to you on this project. If you have any questions, please contact me at 802-674-2033 (Ext. 209).

Sincerely, GeoDesign, Inc.

Shawn P Kelley, Ph.D. John E. Lens, P.E.

Associate Senior Principal

Enclosed: GMNF Bridge evaluation report

Submitted: One electronic copy via email (mcolgan@vhb.com, tcooke@vhb.com, aguyette@vhb.com) and one hard copy mailed U.S. Post.

U n i t e d S t a t e s D e p a r t m e n t o f A g r i c u l t u r e ( U S D A )

F o r e s t S e r v i c e , E a s t e r n R e g i o n

Green Mountain National Forest (GMNF) Bridge over Mad River Replacement

Evaluation and Recommendations

Town of Warren, Washington County, VT

Vanasse Hangen Brustlin, Inc.

Mr. Mark Colgan, P.E.

7056 US Route 7 North Ferrisburgh, VT 05473

802-425-7788

VHB Project No. 57263.11 GeoDesign Project No. 837-19.3

December 16, 2009

54 Main Street, P.O. Box 699, Windsor, Vermont 05089

(T) 802.674.2033, (F) 802.674.5943 i

TABLE OF CONTENTS

TABLE OF CONTENTS........................................................................................................... I

1 EXECUTIVE SUMMARY

2 INTRODUCTION

2.1 Objective

2.2 Available Information

2.3 Organization of Report

3 SITE LOCATION AND SUBSURFACE DESCRIPTION

3.1 Location

3.2 Geology

3.3 Subsurface Explorations

3.4 Subsurface Description

3.4.1 Overview

3.4.2 Warren Bridge site

3.4.3 Groundwater

4 RECOMMENDATIONS

4.1 Bearing Strata/Elevation

4.2 Bearing Pressure

4.3 Expected Settlement

4.4 Lateral Earth Pressure against backfilled abutments

4.5 Material and Compaction Requirements

4.5.1 Working mat/Leveling Pad

4.5.2 Soil Material

4.5.3 Compaction Requirements

5 CONSTRUCTION CONSIDERATIONS

6 CONSTRUCTION DOCUMENT REVIEW AND FIELD OBSERVATION

7 LIMITATIONS

8 APPENDICES

ii

APPENDIX 1 – SITE LOCATION.......................................................................... A-1 APPENDIX 2 – SOIL BORING LOCATIONS....................................................... A-4 APPENDIX 3 – SOIL BORING PHOTOGRAPHS ................................................ A-6 APPENDIX 4 – SOIL BORING LOGS ................................................................... A-8 APPENDIX 5 – GRAIN SIZE ANALYSIS RESULTS......................................... A-11 APPENDIX 6 – LIMITATIONS............................................................................ A-13

GMNF Bridge over Mad River Replacement Evaluation and Recommendations

12/16/09

1 EXECUTIVE SUMMARY

The Mad River crossing in Warren, VT was evaluated in this report (as shown on Figures 1 and 2 in Appendix 1) and firm bearing soil and bedrock were encountered at or above expected foundation bearing elevation, approximately six feet below stream bed elevation. The subsurface condition is conducive to shallow foundations for the replacement bridge with our recommended maximum net allowable bearing pressures are between 5,000 and 8,000 psf (8,000 psf is recommended for design on rock although 16,000 psf may be used if the bedrock is sound and not fractured).

These recommendations were developed from two soil borings that were drilled 10 feet below proposed bottom of footing elevation on the east side and 3 feet below bottom of footing elevation on the west side(see Figure 3, Appendix 2). The allowable bearing pressures were estimated using the observed soil type from the soil borings and based on SPT correlations and corresponding presumptive allowable bearing pressures as given by the Department of the Navy Facilities Engineering Command, Design Manual 7.2 (1986) and AASHTO 2LRFD Bridge Design Specifications 4th edition 2007 with 2009 Interim Revisions.

Sieve analyses were performed on soil samples within the expected bearing soil strata for the proposed bridge crossing to support scour analysis by VHB. These data are presented in Appendix 5.

We recommend that a field engineer from our office observe the excavations for the proposed bridge footings. The purpose is to evaluate the in situ soil conditions for consistency between our interpretation, our recommendations and actual field conditions.

2 INTRODUCTION

This report presents our subsurface evaluation and the geotechnical engineering we performed for the Mad River crossing in the GMNF that is due for replacement per the June 19, 2009 GMNF 2009-2010 Roads and Facilities Maintenance Activities letter authored by Meg Mitchell, Forest Supervisor. The bridge on FSR 25 is an Aquatic Passage Bridge Reconstruction project.

The project location is presented in Figures 1 and 2. We have provided recommended bearing pressures for the proposed bridge foundation. Our evaluation included soil borings and grain size analyses and through interpretation of these data, we are providing foundation recommendations for the river crossings.

A copy of this referred letter by GMNF may be found here:

(http://www.fs.fed.us/r9/forests/greenmountain/htm/greenmountain/links/projects/docs/other/cale ndar_year_09/gm_roadsmaint_notice_19jun09.pdf)

12/16/09

2.1 Objective

The planned objective of this report is to provide VHB with a characterization of the subsurface conditions at the expected replacement bridge location and provide geotechnical design criteria for the foundation design for river crossing.

2.2 Available Information

The specific type of planned replacement bridge structure was not available to us while preparing this report. We assumed a conventional single span bridge with cast-in-place concrete abutments. If other systems are considered, our office should be informed so that the design/construction recommendations provided herein may be evaluated for applicability.

2.3 Organization of Report

This report is divided into eight chapters including an appendix chapter. Chapter 1 has an executive summary of the report, and Chapter 2 is an introductory chapter. Chapter 3 presents the site location and subsurface description for the river crossing. Chapter 4 presents the foundation recommendations and recommended subgrade preparation. Chapter 5 presents construction considerations for the installation of the foundation elements for the proposed bridge and Chapter 6 presents the construction document review recommendations and field observation recommendations. Chapter 7 presents the report conclusions and Chapter 8 has the appendices for the report.

3 SITE LOCATION AND SUBSURFACE DESCRIPTION

The river crossing location is in the Green Mountain National Forest, in the Town of Warren in Washington County, as presented in Figure 1 in Appendix 1. For clarity, the river crossing will be referred to as Warren Bridge through out this report

3.1 Location

The Warren Bridge is located in Warren, VT in the Breadloaf Wilderness in the GMNF as presented in Figure 2 (see Appendix 1).

3.2 Geology

Soils and bedrock encountered in the boring logs generally match published mapping data provided on the Surficial Geologic Map of Vermont (Doll, 1970) and the Centennial Geological Map of Vermont (Doll, 1961). The Surficial Map indicates predominantly glacial till. Bedrock is

12/16/09 mapped as Phyllite and Schist from the Pinney Hollow Formation at the Warren Bridge site, Table 1 presents this information.

Table 1 Mapped Soil and Bedrock Information.

Site Mapped Elevation (feet)

Mapped Surficial(1) Mapped Bedrock(2)

Warren Bridge

~1240 Glacial Till

PINNEY HOLLOW FORMATION – Pale Green quartz-sericite-chlorite PHYLLITE and SCHIST with abundant magnetite, chloritoid, phyllite and schist, quartz-sericite-albite-chlorite schist and rare beds of carbonaceous and schistose quartzite.

(1) Doll, C.G., Stewart, D.P., and MacClintock, P. (1970). “Surficial Geologic Map of Vermont.”

Vermont Geologic Survey, Map, scale 1:250000.

(2) Doll, C.G., Cady, W.M., Thompson Jr, J.B., and Billings, M.P. (1961). “Centennial Geologic Map of Vermont.” Vermont Geologic Survey, Map, scale 1:250000.

3.3 Subsurface Explorations

GeoDesign subcontracted a soil drilling company to drill soil borings on each side of the river at the crossing as shown in Figure 3 in Attachment 2. Soil borings were drilled by Transtech Drilling Services on November 5, 2009. The soil borings were sampled to approximately 10 feet below the proposed bottom of footing elevation (i.e., 6 feet below river elevation), except where there was shallower bedrock or Glacial Till refusal encountered.

GeoDesign personnel observed and logged the soil borings. Photographs from the site are presented in Attachment 3. The soil boring logs are provided in Attachment 4.

The locations of the soil borings shown on the attached figure in Appendix 2 are approximate based on visual observations with reference to the bridge location and edge of river made in the field by GeoDesign personnel. The soil borings locations should be considered accurate only to the degree implied by the method used. Ground surface elevations were estimated from a topographic program by DeLorme (Topo USA 6.0).

Table 2 Drawing names and date received.

Drawing Name Site Name Date Received from VHB

Hand drawn sketch Warren Bridge n/a

12/16/09

3.4 Subsurface Description

3.4.1 Overview

In general, all soil borings terminated in firm material or bedrock. One soil boring terminated 10 feet below the proposed bottom of footing for the bridge crossings and the other soil boring terminated three feet below the proposed bottom of footing elevation. The soil type encountered at the proposed bottom of footing elevation (i.e., bearing elevation six feet below stream elevation) for the bridge was silty sand and gravel soil in one boring and bedrock in the other boring. The following section describes the encountered soil types in more detail. The soil boring logs may be found in Appendix 4. The soil boring logs infer boulders/cobbles with depth when drill rig chatter was observed while augering. An estimated quantity cobble/boulder count is not given as none were physically observed.

Sieve analyses were performed on soil samples from the anticipated strata at the proposed foundation bearing surface (i.e. strata within six feet below stream bottom) for the proposed bridge replacement. These tests were performed on soil samples from SPT sampling in the soil borings. The soil boring samples did not contain particles larger than coarse gravel and therefore those size particles were excluded from the sieve analysis (i.e., cobbles and boulders were not included in the grab samples due to the size of these particles). The sieve results are presented in Appendix 5.

Bedrock was encountered on the west side of the Mad River above the proposed bottom of footing elevation. No soil samples were analyzed on this side of the river as the footing will bear on the in situ bedrock.

3.4.2 Warren Bridge site

Two soil borings were performed at Warren Bridge. B-1 was performed on the east river bank at the bridge deck elevation. B-2 was performed on the west river bank at the bridge deck elevation.

Figure 4 and 5 present photographs of the site. The generalized subsurface profile beginning at the ground surface at the approach roadway at the Warren Bridge is as follows:

Silty Sand & Gravel – FILL and NATURAL (SSG) – A layer of loose to medium dense Silty Sand and Gravel consisting of fine to coarse Sand and fine to coarse Gravel with 10 to 20% Silt was observed at each side of the river. On the west side, this material was medium dense behind the abutment. On the east side, this material was loose to medium dense behind the abutment and then became dense at and below the proposed bearing elevation.

Silt/Fine Sand (SFS) – Below the Silty Sand and Gravel layer on the east side of the river was a dense layer of Silt and Fine Sand. This layer consisted of fine to coarse Sand, 20 to 35% Silt with 10 to 20% fine Gravel.

12/16/09

Bedrock –Bedrock was encountered in B-2 at 9 feet below the bridge deck and was underlying the SSG. The RQD of the cored rock sample was 83%. Bedrock was also observed along the shore line of the west bank and was observed to be phyllite (see Figure 3 in Appendix 2). Bedrock was inferred to be 25 feet below the bridge deck on the east side of the bridge underlying the SFS at boring B-1.

The sieve analysis samples from B1 were in the SSG layer (at the bearing elevation) and these samples contained 17% silt. There was no sample taken from the west side of the river as bedrock was encountered at the bearing elevation.

3.4.3 Groundwater

Groundwater was inferred to be at the river elevation at the Mad River crossing in Warren, VT.

recommendations.

4 RECOMMENDATIONS

4.1 Bearing Strata/Elevation

On the west side of the river, shallow rock is expected (i.e., 0 to 3 feet below the river bottom interpolated between boring B-2 and exposed bedrock to the North of the bridge). The bridge abutment footing can bear on this bedrock after any very soft weathered rock is removed and exposed rock slope is satisfactory (i.e., flat enough to prevent sliding). The bedrock surface may be chipped or blasted to create steps in rock or the footing can be pinned if the exposed rock slope is beyond tolerable values.

On the east side, bedrock was encountered 10 feet below the river bed. The soil encountered at six feet below the river bottom (approximately 14 feet from the ground surface at the boring location), is a medium dense to dense silty sand and gravel and the proposed footings may bear on this stratum (as indicated on the soil boring logs) as long as scour risk is acceptable.

4.2 Bearing Pressure

The recommended bearing pressures at the proposed bottom of footing elevation (assumed to be designed at six feet below river bottom elevation or top of bedrock, whichever is higher) for the proposed bridge site are presented in Table 3. We have recommended a slightly lower bearing pressure for the east side abutment than our engineering analysis indicated due to the expected conditions during construction. If a higher value is needed we can provide further recommendations for construction to limit soil disturbance and allow for a higher value. We have also recommended a lower bearing pressure on the bedrock due to the unknown rock condition and limited subsurface data collected on the west side. It is recommended that the

12/16/09 lower value be used in design. If a higher value is needed, we can provide construction recommendations for rock preparation to allow for a higher value.

Table 3 Recommended Net Allowable Bearing Pressures.

Site Expected Soil

Type at Bearing Elevation

Silt Content of soil matrix at Bearing Elevation

Recommended Maximum net

Allowable Bearing

Pressure (psf)

Special Subgrade Construction

Issues

Warren Bridge – West side

Bedrock n/a 8,000 (disturbed/blasted rock )

16,000 (sound rock)

Rock Removal (e.g., Bedrock hoe ramming/blasting) may be necessary for placement of bridge footing against sloping rock at edge of river

Warren Bridge – East side

Silty Sand and Gravel

17% 5,000 Softening of subgrade soils underwater – subgrades need protection

4.3 Expected Settlement

We expect the settlement of the east abutment at the Warren Bridge will be on the order of a ½ to 1 inch provided the subgrade is kept firm upon excavation. We expect that settlement will be largely complete during construction. Settlement at the west abutment on bedrock will be negligible.

4.4 Lateral Earth Pressure against backfilled abutments

Recommended earth pressure design criteria are presented below:

Table 4 Recommended Earth Pressures.

Static Condition During Wall Construction Recommended Equivalent Fluid Weight of

Soil Backfill (pounds per cubic foot) At-Rest Pressure Behind Rigid Foundation Walls (Fixed at top before backfilling)

60*

Active Pressure Behind Walls Free to Rotate at Top During Backfilling

40*

Passive Resistance in Front of Walls 0**

12/16/09

* These unit weights assume use of free draining compacted Granular Fill backfill against the walls, as described above. Also, where the equivalent wall pressure is less than 240 pounds per square foot (i.e., near the top of wall), use a uniform pressure of 240 pounds per square foot until wall pressure reaches this value, to account for locked in compaction stresses.

** reliance on passive pressure in front of foundation/abutments is not recommended due to scour potential.

We recommend a design friction factor of 0.6 (ultimate) at the base of the wall footings bearing on Granular Material and on leveling pad founded on bedrock.

4.5 Material and Compaction Requirements

4.5.1 Working mat/Leveling Pad

We recommend that a 6 inch working mat consisting of ¾ inch crushed stone (VAOT material 704-02B) enveloped in non-woven geotextile fabric (VAOT material as defined in Table 720- 04A listed as 649.41) be placed beneath the proposed east abutment footing.

Fabric should be needle-punched non-woven material. Seams should be overlapped a minimum of 12 inches. During backfill placement, the drop height should not exceed 3 feet and equipment traffic should be kept off the fabric.

Footings proposed to bear on bedrock should be founded on a 6 inch concrete mud mat leveling pad. For footings bearing partly on bedrock and partly on soil, GeoDesign should evaluate those conditions during construction and provide surface preparations recommendations at that time.

This is to limit the risk of an abrupt hard spot beneath the footing.

4.5.2 Soil Material

Granular Fill for Structures (as shown in Table 704.08A – Granular Fill for Structures, VAOT 2006 Standard Specifications for Construction) is recommended as structural backfill below bridge footings and behind bridge abutment walls. Section 15.1.2 – Spread Footings on Soil from the VAOT Structures Manual provides additional information.

Material beyond the structural backfill behind the abutment and below the roadway should meet VAOT 703.02 Earth Borrow requirements (i.e., reuse existing borrow). Material excavated from the abutment and approach roadway area may be reused as Earth Borrow provided debris and wood/organics are removed and materials can meet compaction requirements.

12/16/09

4.5.3 Compaction Requirements

Compaction should comply with VAOT Specifications where appropriate (i.e., VAOT 204.08).

In addition, all backfill soils beneath and within ‘zone of influence’ of footings as defined as a plane beneath the edge of the footing extending beneath the footing on a slope of 1 Horizontal to 1 Vertical should be compacted to 95% Modified Proctor as defined in ASTM 1557 (or AASHTO T180) and within 2 percent of optimum water content.

Earth borrow should be compacted per VAOT 203.11 (d), 90% Standard Proctor as defined in ASTM 698 (or AASHTO T99). The material within the top 24 inches of the road surface (or embankment surface), should compacted to not less than 95% of the maximum dry density.

Field moisture content should follow the specification in VAOT 203.11 (d) as well.

Table 5 should be used as a guide for soil compaction requirements.

5 CONSTRUCTION CONSIDERATIONS

A key construction consideration is the potential for encountering local variations in the soils and bedrock at exposed subgrade level and being able to respond promptly. Having on-site geotechnical representation will be critical for the construction of this river crossing.

Inflow of water into the excavation during construction will most likely occur and measures to control water inflow are critical to this spread footing design bearing on soil. A firm and stable subgrade is critical for limiting footing settlement on soil. This requires protecting the shallow footing subgrades from becoming loosened. Having a relatively dry excavation will assist in this requirement. The installation of a sheet piling system may be difficult due to the exposed cobbles and boulders observed on the east bank of the river (as seen in Figure 3 in Appendix 2) and the contractor should be made aware of such conditions. The soil boring B-1 encountered inferred cobbles and boulders at 24 feet below grade and there was no inference of cobbles and boulders between the ground surface and 2 4 feet at the boring location. Dewatering pumps will most likely be required in these six foot deep excavations. A pre-planned cofferdam and dewatering design is needed for this construction.

The site may require bedrock removal to create a level pad for the proposed replacement bridge footings.

If blasting is used, we recommend that the overblast rock material have large rock pieces removed from the subgrade and the remaining rock be densified with at least four passes in each of two orthogonal direction with a vibratory compactor able to apply a minimum dynamic force of 8000 pounds per foot of drum width. If appropriately sized compaction equipment can not be used, overblast material should be removed and replaced with a concrete mud mat.

Any overblast rock material must be limited to 15 inches in average diameter. If larger blast-rock is observed at the subgrade elevation, that material should be removed and replaced with

12/16/09 smaller rock fill to be able to fill the voids and be compacted. Maximum loose lift thickness is 16 inches. Granular fill may also be placed and compacted using the requirements presented in Table 5.

In confined areas, smaller vibratory compactors may be used, provided maximum stone size matches the compaction equipment per Table 5.

Table 5 Recommended loose lift thickness for fill and maximum number of passes of compaction equipment.

Maximum Loose Lift Thickness

Minimum Number of Passes

Compaction Method

Maximum Stone Size

Below Structures and Pavement

Less Critical Areas

Below Structures and Pavement

Less Critical Areas

Hand-operated vibratory plate or light roller in confined areas

4” 6” 8” 6 4

Hand-operated vibratory drum rollers weighing at least 1,000#

6” 8” 10” 6 4

Light vibratory drum roller, minimum dynamic force 3,000# per ft. of drum width

6” 10” 14” 6 4

Medium vibratory drum roller, minimum dynamic force 5,000# per ft. of drum width

8” 12” 18” 6 4

Large vibratory drum roller, minimum dynamic force 8,000# per ft. of drum width

10” 16” 24” 6 4

Rock fill should be a well-graded mixture of rock with maximum size matching the compaction equipment being used (i.e., Table 5 requirements).

A 1 foot thick layer of choker material should be placed over top of the compacted overblast rock material. The chocker material should consist of a well-graded mixture of particles to fill the voids in the rock fill surface and should not contain any clayey or silty soil. We recommend a woven geotextile fabric at the surface of the choker layer (VAOT material as defined in Table 720-04A listed as 649.11 with elongation criteria at failure ≥50%).

6 CONSTRUCTION DOCUMENT REVIEW AND FIELD OBSERVATION

To maintain continuity between the design and construction phases, GeoDesign recommends that it be given the opportunity to review the construction documents. GeoDesign also requests the opportunity to review geotechnical aspects of contractor/design submittals (e.g.., cofferdams) for consistency with the specifications and geotechnical design intent.

12/16/09

Our design recommendations are based on GeoDesign providing construction observation of subgrades before backfilling. This will provide us the opportunity to evaluate that the subgrades are as anticipated, and to recommend adjustments as warranted.

7 LIMITATIONS

This report is subject to the limitations presented in Appendix 6.

12/16/09

8 APPENDICES

APPENDIX 1 – SITE LOCATION

tnewman Text Box Location of Bridge Replacement tnewman Typewritten Text tnewman Typewritten Text tnewman Text Box FIGURE 1. Site Location dhumphrey Text Box

APPENDIX 1- SITE LOCATIONS

GMNF Bridge Replacement and Foundation Recommendations Town of Warren, VT GEODESIGN File # 837-19.3 Date 12/11/2009 tnewman Polygon tnewman Line dhumphrey Text Box

APPENDIX 1- SITE LOCATIONS

GMNF Bridges and Culvert Replacement and Foundation Recommendations Towns of Warren, Chittenden, Weston, VT GEODESIGN File # 837-19.3 Date 12/11/2009 Approximate Scale 1" = 1 mile dhumphrey Text Box Figure 2 - Warren Bridge Location Plan dhumphrey Text Box GMNF FSR25 over Mad River spk Line spk Typewritten Text

BREADLOAF

WILDERNESS

spk Typewritten Text spk Typewritten Text

WARREN

spk Typewritten Text

GRANVILLE

spk Typewritten Text

LINCOLN

spk Typewritten Text

RIPTON

spk Typewritten Text

APPENDIX 2 – SOIL BORING LOCATIONS

APPENDIX 3 – SOIL BORING PHOTOGRAPHS

Figure 4 Photograph of Warren Bridge.

Figure 5 Photograph of Existing Abutment at Warren Bridge.

APPENDIX 4 – SOIL BORING LOGS

APPENDIX 5 – GRAIN SIZE ANALYSIS RESULTS

APPENDIX 6 – LIMITATIONS

Explorations

1. The analysis and recommendations submitted in this memorandum are based in part upon the data obtained from widely spaced 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 reevaluate the recommendations of this memorandum.

2. The generalized soil profiles described in the text are intended to convey trends in subsurface conditions. The boundaries between strata are approximate and idealized and have been developed by interpretations of widely spaced explorations and samples; actual non-engineered fill / native soil transitions are probably more erratic. For specific information, refer to the exploration logs.

3. Water level readings have been made in the exploration soil borings at times and under conditions stated on the soil boring logs. These data has been reviewed and interpretations 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 occurring since the time measurements were made.

Review

4. In the event that any changes in the nature, design, or location of the proposed structures are planned, the conclusions and recommendations contained in this report should not be considered valid unless the changes are reviewed and conclusions of this report modified or verified in writing by GeoDesign, Inc. It is recommended that this firm be provided the opportunity for a general review of final design and specifications in order that geotechnical engineering recommendations may be properly interpreted and implemented in the design and specifications.

Uses of Report

5. This geotechnical engineering report has been prepared for this project by GeoDesign, Inc.

This report has been prepared for the exclusive use of VHB, Inc. and their design team for specific application to the proposed Bridge Replacement for the Green Mountain National Forest in Warren, VT in accordance with generally accepted soil and foundation engineering practices. No other warranty, express or implied, is made.

6. This geotechnical engineering report has been prepared for this project by GeoDesign, Inc.

This report is for design purposes only and is not sufficient to prepare an accurate bid.

Contractors wishing a copy of the report may secure it with the understanding that its scope is limited to design considerations only.