FSR 45 Geotech Report.pdf

January 21, 2010

Mark Colgan, P.E.

Vanasse Hangen Brustlin, Inc.

7056 US Route 7 North Ferrisburgh, VT 05473

RE: Letter Update to our Geotechnical Recommendations Report

USDA Forest Service GMNF FSR 45 Bridges 45.00.4 and 45.02.4, Chittenden, VT Bridge Replacements

GeoDesign #837-19.4

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 two bridge replacements 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 – Chittenden Bridge 45-00.4

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 Chittenden Bridge 45-00.4 replacement is presented in Figure 1 assuming a full scoured out section in front of the footing (i.e., stream side). We assumed a 135 pound per cubic foot unit weight for the glacial till 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.

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.

GMNF Chittenden Brook Bridge Replacements Evaluation and Recommendations – Letter update GeoDesign Project Number: 837-19.4

1/21/10

Figure 1. Factored Net Bearing Resistance for Chittenden Bridge 45-00.4 assuming full scour (no soil left above bottom of footing).

Figure 2. Factored Net Bearing Resistance for Chittenden Bridge 45-00.4 assuming half scour (3 feet of soil left above bottom of footing).

Figure 3. Factored Net Bearing Resistance for Chittenden Bridge 45-00.4 assuming no scour (6 feet of soil left above bottom of footing).

1.2 Bearing Pressure – Chittenden Bridge 45-02.4

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 Chittenden Bridge 45-02.4 replacement is presented in Figure 4 assuming a full scoured out section in front of the footing (i.e., stream side).

For comparison, we have provided additional bearing pressures assuming two other scoured depth conditions in front of the proposed footing. Figure 1Figure 5 presents the recommended bearing pressure if half of the soil in front of the footing was scoured out. Figure 6 presents the recommended bearing pressure if none of the soil in front of the footing was scoured out.

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 4. Factored Net Bearing Resistance for Chittenden Bridge 45-02.4 assuming full scour (no soil left above bottom of footing).

Figure 5. Factored Net Bearing Resistance for Chittenden Bridge 45-02.4 assuming half scour (3 feet of soil left above bottom of footing).

Figure 6. Factored Net Bearing Resistance for Chittenden Bridge 45-02.4 assuming no scour (6 feet of soil left above bottom of footing).

1.3 Expected Settlement

We expect the settlement of the structures at both Chittenden Bridges will be on the order of a ½ to 1 inch or less provided the subgrade is kept firm upon excavation. We expect that 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):

1/21/10

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\837-19.4-Chittenden Bridge #45.00.4\Report\837-19.4 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 45 Bridges 45.00.4 and 45.02.4, Chittenden, VT Bridge Replacements

GeoDesign #837-19.4

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 two bridge replacements 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 each brook crossing evaluated, and our recommendations for bearing pressure and subgrade preparation at each crossing for the replacement bridges.

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) Chittenden Brook Bridge Replacements

Evaluation and Recommendations

Town of Chittenden, Rutland 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. 837-19.4

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 Chittenden Bridges

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

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-7 APPENDIX 4 – SOIL BORING LOGS ................................................................. A-10 APPENDIX 5 – GRAIN SIZE ANALYSIS RESULTS......................................... A-19 APPENDIX 6 – LIMITATIONS............................................................................ A-21

GMNF Chittenden Brook Bridge Replacements Evaluation and Recommendations

12/16/09

1 EXECUTIVE SUMMARY

Two brook crossings were evaluated for this report (as shown on Figures 1 through 2 in Appendix 1) and all sites have firm bearing soils at expected foundation bearing elevation, approximately six feet below stream bed elevation. The subsurface conditions are conducive to shallow foundations for the replacement bridges with our recommended maximum net allowable bearing pressure for these sites of 7,000 psf.

These recommendations were developed from soil borings that were drilled at least 10 feet below proposed bottom of footing elevation at each of the bridge crossings (see Figure 3 and 4, Appendix 2). The allowable bearing pressures were estimated using the observed soil type from the soil borings and based on SPT correlations.

Sieve analyses were performed on soil samples within the expected bearing soil strata for the proposed bridge crossings to support scour analysis by VHB at each brook crossing. 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 two brook crossings in the GMNF that are due for replacement per the June 19, 2009 GMNF 2009-2010 Roads and Facilities Maintenance Activities letter authored by Meg Mitchell, Forest Supervisor. The bridges on FSR 45 are Aquatic Passage Bridge Reconstruction projects.

These project locations are presented in Figures 1 through 2. We have provided recommended bearing pressures for each of the proposed bridge foundations. Our evaluation included soil borings and grain size analysis and through interpretation of this data, we are providing foundation recommendations for the brook 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)

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 locations and provide geotechnical design criteria for the foundation design for each brook crossing.

12/16/09

2.2 Available Information

The specific type of planned replacement bridge structures was not available to us while preparing this report. We assumed conventional single span bridges 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 two brook crossings. 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 bridges 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 two bridge locations are in the Green Mountain National Forest, in the Town of Chittenden, in Rutland County as presented in Figure 1 in Appendix 1. For clarity, the two brook crossing sites will be referred to in this report by the abbreviations presented in Table 1.

Table 1 Site Locations and Site Abbreviations.

Site Location Site Abbreviation Forest Service Road 45 over Chittenden Brook Br. No. 45.00.4 Forest Service Road 45 over Chittenden Brook Br. No. 45.02.4

3.1 Location

The Bridge Nos. 45.00.4 and 45.02.4 are located in Chittenden, VT in National Forest Land next to the Joseph Battell Wilderness in the GMNF as presented in Figure 2 (see Appendix 1).

3.2 Geology

Soils and bedrock encountered in 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 mapped as, Schist from the Hoosac Formation at Bridges 45.00.4 and 45.02.4. Table 2 presents this information for each site.

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Table 2 Mapped Soil and Bedrock Information.

Site Mapped Elevation (feet)

Mapped Surficial(1) Mapped Bedrock(2)

Br No.

45.00.4

~1305 Glacial Till

HOOSAC FORMATION – Quartz-sericite-albite-biotite-chlorite SCHIST characterized by albite porphyroblasts – biotite and garnet porphyroblasts common southward – locally carbonaceous.

Br No.

45.00.4

~1765 Glacial Till

HOOSAC FORMATION – Quartz-sericite-albite-biotite-chlorite SCHIST characterized by albite porphyroblasts – biotite and garnet porphyroblasts common southward – locally carbonaceous.

(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 brook at the crossing as shown in Figures 3 and 4 in Attachment 2. Soil borings were drilled by Transtech Drilling Services on November 3 and 4, 2009. The soil borings were sampled to approximately 10 feet below the proposed bottom of footing elevation for each bridge crossing (i.e., 6 feet below brook elevation), except where there was 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 figures in Appendix 2 are approximate based on visual observations with reference to the bridge location and edge of brook 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 3 Drawing names and date received.

Drawing Name Site Name Date Received from VHB Hand drawn sketch BR. NO. 45-00.4 n/a Hand drawn sketch BR. NO. 45-02.4 n/a

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3.4 Subsurface Description

3.4.1 Overview

In general, all soil borings terminated in firm material. The soil borings typically terminated 10 feet below the proposed bottom of footing for the brook crossings in glacial till soil. The soil type encountered at the proposed bottom of footing elevation (i.e., bearing elevation six feet below stream elevation) for the bridges was also firm glacial till. The following sections describe the encountered soil types. All 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 replacements. 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.

3.4.2 Chittenden Bridges

Six soil borings were performed at the two bridge sites in Chittenden. Each boring was drilled at the bridge deck ground elevation. BR. NO. 45-00.4 had four soil borings drilled. Borings B-3 and B-4 were drilled on the south side of brook. Borings B-5a and B-5b were drilled on the north side of the brook. B-3 was terminated 15 feet below the bridge deck because the sampler refused (we believe it was on a boulder/cobble in Glacial Till based on the findings in boring B- 4). A second boring on the south side (B-4) was drilled nine feet away from B-3 and terminated 22 feet below the bridge deck in weathered rock. B-5a was drilled to 5 feet below grade into the abutment fill soils. This boring was terminated at 5 feet below grade because of encountering a suspected dead-man from the bridge abutment. A second boring on the north side (Boring B-5b) was drilled 2.5 feet from B-5a and terminated at 26 feet below grade. Figures 5 and 6 present photographs of BR. NO. 45-00.4. The generalized subsurface profile beginning at the ground surface at the approach roadway at BR. NO. 45-00.4 is as follows:

Sand & Gravel –FILL(S&G) – A layer of medium dense Sand and Gravel consisting of fine to coarse Sand, 10 to 20% fine to coarse Gravel with 0 to 20% Silt was observed to about 7 to 10 feet thick. Assumed old ground consisting of dark silt with organics was observed below the S&G layer and was about 2 foot thick.

Silty Sand (SS) – A loose layer of Silty Sand consisting of loose fine to coarse Sand, 10 to 20% fine to coarse Gravel, and 20 to 35% silt was encountered below the Sand & Gravel fill layer.

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Glacial Till – Very dense glacial till was observed underlying the SS layer.

The sieve analysis sample for B-4 was in the Glacial Till layer (at the bearing elevation) and contained 20% silt. The sieve analysis sample for B-5b was in the Glacial Till layer (at the bearing elevation) and contained 40% silt.

BR. NO. 45-02.4 had two soil borings drilled. Borings B-1 was drilled on the south side of brook. Borings B-2 was drilled on the north side of the brook. B-1 terminated 28 feet below the bridge deck in decomposed bedrock and B-2 terminated in Glacial Till at 22 feet below grade.

Figures 7 and 8 present photographs of BR. NO. 45-02.4. The generalized subsurface profile beginning at the ground surface at the approach roadway at BR. NO. 45-00.4 is as follows:

Sand & Gravel –FILL(S&G) – A layer of medium dense Sand and Gravel consisting of fine to coarse Sand, 10 to 20% fine to coarse Gravel with 0 to 20% Silt was observed to about 5 feet thick.

Old ground (OG) – Very loose assumed old ground (Organic Silt) was observed below the S&G layer and was about 5 feet thick.

Gravelly Sand and Silt (GSS) – A medium dense to very dense layer of fine to coarse Sand, 10 to 20% fine to coarse Gravel, 20 to 35% silt was encountered below the OG layer.

Glacial Till – Very dense glacial till was observed underlying the GSS layer.

The sieve analysis samples for B-1 and B-2 were in the Glacial Till layer (at the bearing elevation) and contained 50% to 80% silt.

3.4.3 Groundwater

Groundwater was inferred to be at or just below the brook elevation at the Chittenden Brook crossings in Chittenden, VT.

4 RECOMMENDATIONS

4.1 Bearing Strata/elevation

The bearing strata for BR NO. 45.00.4 will be in the dense glacial till stratum. This stratum begins about 12 to 14 feet below the ground surface at the soil boring locations (i.e., top of bridge deck elevation). We expect that the foundation bearing elevation will be six feet below current bottom of brook elevation (as indicated on the soil boring logs).

The bearing strata for BR NO. 45.02.4 will be in the dense glacial till stratum. This stratum begins about 18 to 19 feet below the ground surface at the soil boring locations (i.e., top of bridge deck elevation). As with BR NO. 45.00.4, we expect the bearing elevation will be six feet below current bottom of brook elevation (as indicated on the soil boring logs). Per boring B-2, a

12/16/09 thin layer of Sandy Silt may be above the glacial till on the north side of the brook. This should be excavated down to the glacial till below for bearing of the bridge abutment footings.

4.2 Bearing Pressure

The recommended bearing pressure at the proposed bottom of footing elevation (assumed to be designed at 6 feet below brook bottom elevation) for the proposed bridge site is presented in Table 4.

Table 4 Recommended Net Allowable Bearing Pressures for each Site.

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

BR NO. 45-00.4 Glacial Till 20% to 40% 7,000 Softening of subgrade soils underwater – subgrades need protection

BR NO. 45-02.4 Glacial Till 50% to 80% 7,000 Softening of subgrade soils underwater – subgrades need protection

4.3 Expected Settlement

We expect the settlement of the structures at the BR. NO. 45-00.4 and 45-02.4, will be on the order of a ½ to 1 inch provided the subgrade is kept firm upon excavation. We expect that settlements will be largely complete during construction.

4.4 Lateral Earth Pressure against backfilled abutments

Recommended earth pressure design criteria are presented below:

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Table 5 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**

* 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 Soil.

4.5 Material and Compaction Requirements

4.5.1 Working mat

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 footings.

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.

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.

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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 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 at exposed subgrade level and being able to respond promptly. Having on-site geotechnical representation will be critical for the construction of these brook crossings.

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. This requires protecting the shallow footing subgrades from becoming loosened. Having a relatively dry excavation will assist in this requirement. The expected quantity of inferred cobbles and boulders from the boring logs (B-3, B-4, and B-1) and what was observed at the ground surface (as seen in Figures 3 and 4 in Appendix 2) may make installation of a sheet piling system difficult and the contractor should be made aware of such conditions. 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.

While bedrock is unlikely, 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 concrete.

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 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.

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In confined areas, smaller vibratory compactors may be used, provided maximum stone size matches the compaction equipment per Table 6.

Table 6 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 6 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 will also review geotechnical aspects of contractor/design submittals 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. As such, GeoDesign should observe that the earthwork and footing bearing surface preparation are being performed per the construction documents.

7 LIMITATIONS

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

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8 APPENDICES

APPENDIX 1 – SITE LOCATION

tnewman Polygon tnewman Text Box Location of Bridge Replacements tnewman Line tnewman Typewritten Text tnewman Typewritten Text tnewman Text Box FIGURE 1. Site Location dhumphrey Text Box

APPENDIX 1- SITE LOCATIONS

GMNF Bridges Replacement and Foundation Recommendations Town of Chittenden, VT GEODESIGN File # 837-19.4 Date 12/15/2009 dhumphrey Text Box

APPENDIX 1- SITE LOCATIONS

GMNF Bridges Replacement and Foundation Recommendations Towns of Chittenden, VT GEODESIGN File # 837-19.4 Date 12/15/2009 Approximate Scale 1" = 1 mile dhumphrey Text Box

BR. NO. 45.02.4

dhumphrey Text Box Figure 2 - Chittenden Bridges Location Plan dhumphrey Text Box

BR.NO. 45.00.4

spk Typewritten Text

HANCOCK

spk Typewritten Text

ROCHESTER

spk Typewritten Text

GOSHEN

spk Typewritten Text spk Typewritten Text spk Typewritten Text

CHITTENDEN

spk Typewritten Text

PITTSFIELD

spk Typewritten Text

JOSEPH BATTELL

WILDERNESS

APPENDIX 2 – SOIL BORING LOCATIONS

APPENDIX 3 – SOIL BORING PHOTOGRAPHS

Figure 5 Photograph of Chittenden BR. NO. 45.00.4

Figure 6 Photograph of BR. NO. 45.00.4 abutment.

Figure 7 Photograph of Chittenden BR. NO. 45.02.4.

Figure 8 Photograph of BR. NO. 45.02.4 Abutments.

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 borings 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 Replacements for the Green Mountain National Forest in Chittenden, 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.