GR13-15_Ivars_Bridge_Final_Foundation_Report.pdf

Ivar’s Bridge AK IRR FORT 1002(1)

Geotechnical Report No. 13-15

Ivar’s Bridge

FINAL FOUNDATION REPORT

AK IRR FORT 1002(1)

Fort Yukon, Alaska

October 29, 2015

Intentionally blank.

GR01‐15 Page i

Contents

1. INTRODUCTION

1.1 Project Location

1.2 Scope of Project

1.3 Potential Material Source

2. SITE CONDITIONS

2.1 Topography and Site Description

2.2 Average Temperatures and Precipitation

3. SUBSURFACE CONDITIONS

3.1 Site Geology

3.2 Regional Permafrost

3.3 Subsurface Investigations

3.3.1 2007 Borings

3.3.2 2015 Test Pits

3.3.3 Groundwater

3.3.4 Site Permafrost

4. FOUNDATION ANALYSIS AND RECOMMENDATIONS

4.1 Bridge Layout and Foundation Considerations

4.2 Scour

4.3 Seismic Hazard

4.4 Liquefaction Potential

4.5 Permafrost Considerations

4.6 Design Loads and Bearing Resistance

4.7 Design Soil and Geotextile Parameters

4.8 GRS-IBS Layout

5. MATERIAL SOURCES

5.1 Material Source Location

5.2 Previous Material Source Investigation

5.3 Use of Material Source for GRS-IBS Backfill

5.4 Riprap Sources

6. EARTHWORK RECOMMENDATIONS

7. CONSTRUCTION CONSIDERATIONS

7.1 Earthwork Operations

7.2 Dewatering

7.3 GRS-IBS Construction

8. REFERENCES

GR13‐15 ii

Appendices

APPENDIX A

Boring and Test Pit Location Map

APPENDIX B

Unified Soil Classification System Soil Descriptive Terminology Logs of 2015 Test Pits Laboratory Test Reports for 2015 Test Pit Samples

APPENDIX C

Laboratory Test Reports for 2015 Grab Samples from Potential Material Source

APPENDIX D

2007 Geotechnical Study Report

GR13‐15 iii

Figure 1. Project Location Maps.

1 MILE

PROJECT LOCATION

100 MILES

Material Source

Geotechnical Report GR01‐15 page 1

1. INTRODUCTION

This report provides a summary of the site and subsurface conditions, analysis, and final foundation recommendations for the proposed bridge across the Sucker River.

Ivar’s Bridge is also known as the Sucker River Bridge. The proposed bridge replaces a functionally obsolete crossing composed of a series of 24 metal culverts capped with aggregate bedding and a series of concrete slabs. The crossing provides access primarily to subsistence activities for the residents of the village of Fort Yukon as well as connectivity to other communities.

Stationing and elevations referenced in this report are based on the project 95% plan set dated October 15, 2015.

1.1 Project Location

The project site is about 3 miles outside Fort Yukon, Alaska. The coordinates of the bridge site are 66.585138° north, 145.116164° west (WGS 84). The proposed bridge is located about 25 feet north of the existing crossing. The project location is shown in Figure 1 on page iii. The location of the proposed bridge relative to the existing crossing is shown on the Boring & Test Pit Location Map in Appendix A.

1.2 Scope of Project

The existing crossing structure consists of a series of 24 corrugated metal culverts with a concrete surfacing. The existing crossing frequently overtops during spring break up.

The fill around the pipes and below the slab has eroded to the point that large sections of the concrete slab are unsupported. The new bridge will provide unhindered access to an area used by the village for subsistence activities.

The new bridge will be a 150 foot long steel superstructure with a timber deck. The superstructure will be supported on a geosynthetic reinforced soil – integrated bridge system (GRS-IBS) at the abutments. The waterway opening will be 134 feet wide below the bridge.

The GRS-IBS abutments will be protected with riprap.

GRS-IBS abutments consist of well-compacted granular backfill, closely spaced layers of geotextile reinforcement, and concrete masonry unit (CMU) facing. Further explanation of GRS-IBS is provided in Section 4.

Geotechnical Report GR13‐15 page 2

1.3 Potential Material Source

The material source discussed in this report is a source that has been identified as a potential borrow site near Fort Yukon that and been historically used for borrow by the village of Fort Yukon and the ADOT&PF. Discussion of this source and the testing results contained in this report does not relieve the contractor of any requirements contained in the project specifications and does not waive any permitting requirements for this source. Materials from this source may not meet the project specifications.

Surface and subsurface rights of all material sources planned for use on this project must be verified by the contractor prior to use.

Walter Peter, Transportation Coordinator for the Native Village of Fort Yukon, showed Brian Collins, Western Federal Lands Highway Division (WFLHD) Sr. Geotechnical Engineer, a potential material source outside Fort Yukon in June 2015. Mr. Peter indicated that this source has been used in the past by Fort Yukon for roads and the Alaska Department of Transportation and Public Facilities for the airport runway. The material source location is shown on the Fort Yukon map in Figure 1 at the beginning of this report. The site is located about 1 mile east of Fort Yukon. The coordinates of the material source are 66.566942° north, 145.197793° west (WGS 84).

No sources of rock for riprap production are known to exist in the vicinity of Fort Yukon.

We anticipate that riprap material will need to be barged in.

The potential material source is discussed in more detail in Section 5.

Geotechnical Report GR13‐15 page 3

2. SITE CONDITIONS

2.1 Topography and Site Description

The project lies within an area known as Yukon Flats, an alluvial lowland that lies along the Yukon River and its tributaries between Circle and Stevens Village. The project site has generally level terrain. The topography within the Yukon Flats has relief of less than 150 feet (Williams, 1962) and the landforms include river flood plains, alluvial terraces, alluvial fans, and sand dunes.

The bridge site is relatively level terrain. The limits of the proposed bridge alignment on the north side of the existing crossing were cleared of vegetation by the tribe in 2015.

Figures 2, 3 and 4 are site photographs taken when the test pits were completed on June 20, 2015.

The vegetation in the area consists mostly of willow, alder, aspen and birch. At the time of the site visit vegetation had been removed from the proposed bridge alignment.

Walter Peter, Transportation Coordinator for the Native Village of Fort Yukon, indicated that the vegetation was removed earlier in the spring of 2015.

At the time of the site visit there was a pile of cobble material stockpiled on the west side of the crossing, between project Station 104 and 105 on the left side.

Figure 2. Photograph of Proposed Bridge Location, Looking East (ahead) from about Station 105+40.

Geotechnical Report GR13‐15 page 4

Figure 3. Photograph of Proposed Abutment 1 Location, Looking West (Back) from about Station 106+50.

Figure 4. Photograph of Proposed Abutment 2 Location, Looking East (Ahead) from about Station 107+00.

Geotechnical Report GR13‐15 page 5

2.2 Average Temperatures and Precipitation

Table 1 shows the average maximum and minimum temperatures, average monthly precipitation, and average monthly snowfall data for the period from 1899 to 1990 (Western Region Climate Center, 2015).

Table 1: Average Temperature, Precipitation, and Snowfall for Fort Yukon, AK.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual

Avg. Max.

Temp. (F)

-10.9 -3.6 14.7 34.8 56.1 70.9 73.2 66.3 50.6 27.2 1.3 -8.7 31.0

Avg. Min.

Temp. (F)

-27.8 -23.4 -11.5 9.0 32.1 47.9 51.2 44.7 32.1 12.4 -14.2 -24.6 10.7

Avg.

Total

Precip.

(in.)

0.49 0.36 0.28 0.21 0.31 0.73 0.81 1.06 0.79 0.59 0.42 0.52 6.57

Avg.

Total

Snowfall (in.)

6.7 5.1 4.1 2.4 0.6 1.2 0.0 0.0 1.7 6.8 6.5 6.7 41.9

Avg.

Snow

Depth (in.)

19 22 24 15 2 0 0 0 0 2 9 14 9

Note:

1) Period of Record : 01/01/1899 to 03/31/1990.

2) Percent of possible observations for period of record: Max. Temp.: 62.6% Min. Temp.: 62.9%

Precipitation: 64.2% Snowfall: 63.9% Snow Depth: 62.4%.

Geotechnical Report GR13‐15 page 6

3. SUBSURFACE CONDITIONS

3.1 Site Geology

The area at the bridge site is mapped as flood plain and low terrace alluvium (Williams, 1962) that is described as well-stratified layers and lenses of gray to brown coarse to fine well-sorted rounded to subangular gravel and minor amounts of sand and silt, mantled by as much as 25 feet of well –stratified layers and lenses of silt, sand, and locally, organic matter. The thickness of the alluvium is about 100 feet at Fort Yukon (Williams, 1962).

The potential borrow source is located in an area mapped as alluvial fan and related terrace deposits (Williams, 1962) that are described as well-stratified layers and lenses of well-sorted gray to brown coarse to fine gravel containing minor amounts of sand and silt and a few layers and lenses of peat and woody material.

3.2 Regional Permafrost

Fort Yukon is located just north of the Arctic Circle and the mean annual air temperature is below freezing. The Permafrost Map of Alaska (Ferrians, 1965) shows the Yukon Flats area as underlain by discontinuous permafrost and labels two points at Fort Yukon with the depth to the base of permafrost as 18 feet and 390 feet. Temperature observations within permafrost at Fort Yukon indicate that the permafrost near Fort Yukon is warm permafrost with ground temperature measurements just below the active layer averaging about 31 degrees Fahrenheit (CRREL, 1962).

Ball et al (2011) performed 1800 line kilometers of airborne electromagnetic (AEM) and magnetic geophysical survey of Fort Yukon and the Yukon Flats in order to create a three dimensional map displaying the distribution of the underlying permafrost. The depth of permafrost determined through the survey correlated well to a deep borehole drilling in the 1990s (Abraham, 2011). The results of the study indicate most of Fort Yukon and the surrounding area is underlain by permafrost to a depth greater than 300 feet; unfrozen ground (taliks) is generally found below lakes and rivers. Through-going taliks (indicating the entire depth of permafrost is thawed) only appear to occur below larger lakes and rivers. In the vicinity of the proposed bridge across the Sucker River the maps are interpreted to indicate that permafrost extends to a depth of about 100 feet (30 meters) with a thaw bulb below the Sucker River.

3.3 Subsurface Investigations

The subsurface information gathered at the bridge site includes geotechnical borings performed in 2007 by others and test pits completed in 2015 by WFLHD. The locations

Geotechnical Report GR13‐15 page 7 of the borings and test pits are shown on the Boring and Test Pit Location Map in Appendix A.

The soil profile across the site is generally consistent with 2.5 to 8 feet of sandy silt and silty sand overlying relatively clean sand and gravel. Groundwater was encountered near the elevation of the river. Permafrost was encountered in some of the borings and test pits. Further details are provided below.

3.3.1 2007 Borings

In 2007 three geotechnical borings were drilled by Shannon & Wilson as part of a Geotechnical Study for replacement of Ivar’s Bridge (Carroll, 2007). The Geotechnical Study Report is attached in Appendix D. The 2007 borings are in relatively close proximity to the current proposed bridge location as shown on the Boring and Test Pit location Map in Appendix A. Boring 07-1was drilled on the west side of the river near proposed Abutment 1 and Boring 07-3 was drilled on the east side of the river near proposed Abutment 2. Boring 07-2 was also drilled on the east side of the river but was located about 100 feet from the river’s edge.

The borings encountered between 2.5 to 8.0 feet of sandy silt and silty sand overlying layers of sandy gravel, gravelly sand, sand, and sandy silt. Permafrost was encountered in Boring 07-2 and Boring 07-1 at depths below 6.5 and 25 feet, respectively, extending to the termination depth of the borings (40 feet). Permafrost was not encountered in Boring 07-3. Groundwater was encountered in Boring 07-1 and Boring 07-3 at depths of 7 feet and 0.2 feet, respectively. The borings penetrated to depths from 40 to 42 feet below the ground surface.

A 340-lb hammer with a 30-inch drop and a 3-inch outside diameter and 2.5-inch inside diameter split barrel sampler were used in the borings. The number of blows required to drive the sampler 18 inches was recorded and the number of blows required to drive the sampler the final 12 inches of the 18-inch increment was reported as the penetration resistance.

The standard penetration test (ASTM D 1586) specifies the use of a 140-lb hammer with a 30-inch drop and a 2-inch outside diameter split barrel sampler. Corrections were applied to the reported field penetration resistance values in the 2007 borings to account for the larger hammer weight and larger sampler size in order to convert them to equivalent standard penetration test (SPT) values. In addition, corrections were applied to account for the sample depth (rod length) and overburden pressure. The corrected SPT resistance value is referred to as (N1)60.

3.3.2 2015 Test Pits

Four test pits were conducted by WFLHD at the locations of the proposed GRS-IBS abutments on June 20, 2015. Test pits TP-1 and TP-2 were near proposed Abutment 1

Geotechnical Report GR13‐15 page 8 and TP-3 and TP-4 were near proposed Abutment 2. A locally available Case 580 loader/excavator was used to dig the test pits. Soils encountered in the test pits were described in accordance with Federal Lands Highway Soil Description and Identification Guidelines. Frozen soils were described in following ASTM D 4083 Standard Practice for Description of Frozen Soils (Visual-Manual Procedure).

The excavator encountered refusal in frozen ground at depths of 4 and 5 feet in TP-2 and TP-1, respectively. TP-3 and TP-4 were terminated at depths of 6 and 7 feet, respectively, because the water table was encountered close to the ground surface and the unfrozen sand and gravel caved continuously during attempts to extend the excavation.

The test pits encountered organic silt, silt, and sandy silt layers to depths of 3 to 6.5 feet overlying gravel, sand with gravel, and gravel with sand that extended to the termination depths of the test pits of 4 to 7 feet. The elevation of the contact between the silty layer and the underlying sand and gravel layers was approximately 986.5 near proposed Abutment 1 and about 983.5 near proposed Abutment 2.

Frozen ground was encountered in TP-1 and TP-2 below a depth of 2 feet. The frozen ground extended to the termination depth of the test pits which was determined by excavator refusal at depths of 4 to 5 feet. In TP-3 and TP-4 frozen ground was encountered below a depth of 1 foot but the excavator penetrated through the frozen silt and sandy silt layer at depths of 4.5 to 6.5 feet and penetrated into an unfrozen saturated gravel layer. The frozen layers encountered in the test pits are within the estimated depth of the active layer so they are interpreted to be seasonally frozen.

Laboratory test reports for samples taken from the test pits are contained in Appendix B.

3.3.3 Groundwater

Groundwater was observed in the borings between depths of 0.2 to 7 feet below the ground surface on June 4 to 6, 2007.

During the test pit excavations on June 20, 2015, the water level encountered in test pits ranged from 1 to 3 feet below the ground surface. Groundwater was encountered in the gravel layer below the frozen layers in TP-3 and TP-4. The groundwater filled the test pit holes to approximately the elevation of the Sucker River which was stagnant and estimated to be between elevation 987 and 988 feet on the project datum at the time of the test pit investigations.

The groundwater table is expected to be coincident with the Sucker River surface elevation. Perched water is likely to be present in permeable soil layers overlying permafrost.

Subsurface water is likely to be encountered during foundation excavation at or near the elevation of the river. Perched water is also likely to be encountered in soil layers above

Geotechnical Report GR13‐15 page 9 permafrost, possibly at higher elevations than the river surface. Water elevations are likely to fluctuate with fluctuations of the Sucker River, particularly during spring break up and following periods of heavy precipitation.

3.3.4 Site Permafrost

The results of the 2007 borings are consistent with the AEM results discussed in Subsection 3.2. Permafrost was encountered in Boring 07-2 and Boring 07-1 at depths below 6.5 and 25.0 feet, respectively, and extending to the termination depth of the borings (about 40 feet). Permafrost was not encountered in Boring 07-3, which is the closest boring to the Sucker River channel, but may be present below the bottom of the boring. Layers of ice were not encountered in any of the borings.

Frozen ground that was encountered in the Test Pits is interpreted to be within the zone of the active layer and therefore not interpreted to be permafrost.

Geotechnical Report GR13‐15 page 10

4. FOUNDATION ANALYSIS AND RECOMMENDATIONS

4.1 Bridge Layout and Foundation Considerations

The proposed bridge will be a 150-foot long steel plate girder superstructure. Spread footings, Geosynthetic Reinforced Soil – Integrated Bridge System (GRS-IBS) and driven piles are the foundation types that were considered for this site.

Spread footings and GRS-IBS are both shallow foundation alternatives. A conventional concrete spread footing foundation was considered but not chosen because a GRS-IBS will be more a more flexible system, have faster construction, and be more cost effective.

A typical cross-section through a GRS-IBS abutment is shown in Figure 5. For this project the bridge superstructure will consist of steel girders so a precast concrete footing (not shown in the figure) will be used at the beam seat location to transfer the superstructure load to the top of the abutment. GRS-IBS technology consists of closely spaced layers of geosynthetic reinforcement and compacted granular backfill and concrete masonry unit (CMU) facing to create a reinforced soil foundation, abutment, and integrated approach for the bridge. The application of GRS-IBS has several advantages.

The system is easy to design and cost effective to construct. It can be quickly built in variable weather conditions with labor and equipment and can easily be modified in the field. GRS-IBS is more flexible compared to other foundation alternatives during seismic events. The GRS-IBS system integrates the superstructure into the abutment and approach embankment and thereby eliminates the bump at the end of the bridge that typically occurs with deep foundations.

Figure 5. Typical GRS-IBS Cross-Section (from Adams et al, 2011).

Geotechnical Report GR13‐15 page 11

Driven piles were also considered. The cost of driven piles would be high compared to the GRS-IBS option. The permafrost encountered in the borings would create difficult driving conditions and require predrilling of the piling. These site conditions increase the risk of costly contract modifications for driven pile foundations on this project.

Based on these considerations GRS-IBS abutments are the recommended foundation type for this project.

4.2 Scour

Carolyn Sourek, WFLHD Hydraulic Engineer, provided scour design elevations and scour countermeasure design recommendation in the Final Bridge Scour Analysis Summary report dated July 20, 2015. The channel thalweg is at elevation 985.0 on the project datum. The abutment scour is at elevation 981.0 on the project datum. The recommended scour countermeasure is a 4-foot thick layer of class 5 riprap sloped at 1V:2H in front of the abutments.

4.3 Seismic Hazard

The American Association of State Highway and Transportation Officials (AASHTO) criteria for seismic design of ordinary bridges, allows for damage to occur when the structure is subjected to 1000-year strong ground motion, but does not allow for collapse of one or more spans and loss of life. Bridges may suffer damage that requires repair or partial replacement. AASHTO, however, requires bridges to withstand small, more frequent earthquakes without damage.

The procedures outlined in Section 3.10 of the AASHTO LRFD Bridge Design Manual 7th Edition (2014) were followed to characterize the seismic hazard. These procedures use a design earthquake with a return period of 1000 years, or a 7 percent probability of exceedance (PE) in 75 years.

According to the site class definitions specified in Table 3.10.3.1-1 of ASSHTO-LRFD Bridge Design Specifications 6th Edition, 2012 (AASHTO-LRFD). Site Class D is defined as stiff soil with average Standard Penetration Test (SPT) blow count for the upper 100 feet of the soil profile between 15 and 50 blows per foot, moisture content less than 40%, and a PI value less than 20.

The AASHTO Seismic Design Parameters Software (version 2.10) was used to determine the following ground motion parameters for the site. Horizontal peak ground acceleration (PGA), short-period spectral acceleration (Ss), and long-period spectral acceleration (S1), for the referenced AASHTO weak rock site class (Site Class B) and a 1000-year return period (7 percent probability of exceedance in 75 years).

Geotechnical Report GR13‐15 page 12

Based on the subsurface profile at the bridge site, and assuming the subsurface conditions encountered in the borings continue to a depth of at least 100 feet deep, the site would meet the Site Class D conditions. The spectral accelerations were scaled to the referenced AASHTO Site Class D using the site factors recommended in Tables 3.10.3.2- 1, 2, and 3 in AASHTO manual (2014).

Table 2. Summary of Seismic Parameters (7% PE in 75 years).

Site Class (by Soil Type and Profile) D

Horizontal Peak Ground Acceleration (PGA) 0.10g

Horizontal Response Spectral Acceleration at Period of 0.2 sec (Ss) 0.23g

Horizontal Response Spectral Acceleration at Period of 1.0 sec (S1) 0.09g

Site Factor for Zero-Period on Acceleration Spectrum (Fpga) 1.60

Site Factor for Short-Period Spectral Acceleration (Fa) 1.60

Site Factor for Long-Period Spectral Acceleration (Fv) 2.40

Modified Horizontal Peak Ground Acceleration (As) 0.16g

Modified Horizontal Response Spectral Acceleration at Period of 0.2 sec (SDS) 0.36g

Modified Horizontal Response Spectral Acceleration at Period of 1.0 sec (SD1) 0.20g

Seismic Zone 2

The modified peak ground acceleration and spectral acceleration values in Table 2 were used to develop the 5-percent-damped-design response spectrum shown in Figure 6 below.

Figure 6. 5-Percent-Damped-Design Response Spectrum.

Geotechnical Report GR13‐15 page 13

4.4 Liquefaction Potential

The general soil profile at the site consists of a 2.5 to 8 foot layer of sandy silt overlying saturated sandy gravel and gravelly sand. The average corrected SPT (N1)60 value at the site, based on the penetration resistances reported in the unfrozen layers from the 2007 borings, was determined to be 35 blows per foot. Since the corrected penetration resistance was greater than 25 blows per foot and the seismic hazard at the site is just above the limits of seismic zone 1 it is interpreted that the liquefaction hazard at the site is very low. Seasonally frozen ground and permafrost is not susceptible to liquefaction.

Further liquefaction analysis was not required.

4.5 Permafrost Considerations

In permafrost areas structural loads are normally transferred through the active layer to the thermally stable layer. Based on our analysis of weather data from the Fort Yukon weather station (Western Regional Climate Center, 2015) we estimated that the depth of the active layer varies between 7 to 15 feet in Fort Yukon, depending on the surface conditions. Where the ground cover consists of vegetation the estimated active layer depth varies from 7 to 9 feet and where the surface cover consists of gravel we estimate that the active layer depth is between 14 to 15 feet. Thus we anticipate that the depth of the active layer will increase due to the removal of the existing vegetation at the proposed bridge site. However, the new bridge will include construction of approach embankments and GRS-IBS abutments at elevations 10 to 15 feet above the existing surface and the new fill will provide insulation of the underlying material that will offset the deepening of the active layer due to vegetation removal.

The soils encountered in the borings and test pits near the proposed abutment locations below the proposed bottom of the reinforced soil foundation (RSF) elevation consist of relatively clean sand and gravel layers that generally have between 1 and 6 percent fines (percent of material by weight passing the No. 200 sieve). Layers of ice would results in 100 percent settlement of those layers when they thaw; however, the borings did not encounter any layers of ice. Therefore, these soils are interpreted to be thaw stable and there is a low risk of large settlements occurring due to decay of existing permafrost at the site. Sand and gravel with less than 6 percent fines has a very low tendency to heave when it freezes.

Predicting future settlement caused by degradation of the permafrost due to climate change would be a complex analysis that is outside the scope of this report. However, a sensitivity analysis was performed in order to evaluate potential effects on the bridge.

AASHTO (2014) indicates that bridges can tolerate angular distortions up to 0.008 radians (0.458 degrees) between adjacent foundations. For this bridge with 146 feet from bearing to bearing that equates to 14 inches of differential settlement. We estimated thaw depth using a procedure developed for design of the trans-Alaska pipeline (Nelson et al, 1983). In the analysis we made the assumption that the clean sands and gravels extend below the depths investigated in the borings. The results of our analysis indicate that the

Geotechnical Report GR13‐15 page 14 thaw extending to a depth of 100 feet would result in thaw settlement of about 14 inches.

This settlement would likely occur in similar magnitudes at both bridge abutments so differential settlement would likely be considerably less than the limit of 0.008 radians recommended in AASHTO (2014). We interpret that it is highly unlikely that one abutment would settle the estimated maximum of 14 inches while the other abutment would undergo no settlement.

Minor amounts of thaw settlement will likely go unnoticed because the GRS-IBS abutment and bridge superstructure will settle together. The system is inherently flexible and minor differential settlements will go unnoticed. Since limited subsurface information was available for this site, as a precaution, it is recommended to detail the abutments so that the superstructure can be jacked back into position to correct differential settlements.

4.6 Design Loads and Bearing Resistance

WFLHD Structural Engineer, Adam Fisher, provided the bridge foundation loads per abutment via email on July 27, 2015, summarized in Table 3.

Table 3. Summary of Bridge Loads per Abutment.

Type of Load Unfactored Load & Service Limit State

Strength Limit State Factored Load

Beam Dead Load 94.5 kip 118 kip

Deck Dead Load 43 kip 54 kip

Railing Dead Load 15 kip 19 kip

Footing Dead Load 118 kip 148 kip

Live Load 307.5 kip 538 kip

The superstructure loads will be transferred to the top of the GRS-IBS abutment through a precast concrete footing. The precast concrete footing dimensions are 8 feet-2 inches wide by 21 feet-2 inches long by 2 feet-6 inches thick with a backwall that will be approximately 5 feet-8 inches high. Based on these dimensions and the loads in Table 3 the bearing stress at the base of the precast footing are shown in Table 4.

Geotechnical Report GR13‐15 page 15

Table 4. Summary of Bearing Stress at Base of Precast Footing.

Type of Load

Bearing Stress – Unfactored &

Service Limit State Strength Limit State

Bearing Stress

Bridge Dead Loads, qb 1.6 ksf 2.0 ksf

Bridge Live Loads, qLL 1.8 ksf 3.1 ksf

Total 3.4 ksf 5.1 ksf

The calculated strength limit state factored bearing resistance at the top of the GRS-IBS abutment was calculated to be 9.9 ksf, which exceeds the factored bearing stress of 5.1 ksf.

4.7 Design Soil and Geotextile Parameters

Table 5 shows the soil parameters used in the GRS-IBS analysis. The bottom of the reinforced soil foundation (RSF) for the GRS-IBS will bear at elevation 978.5 on the project datum. This is below the upper layer of sandy silt and silty sand encountered in the subsurface investigation test pits and borings. The RSF will bear on the underlying relatively clean sand and gravel layer.

Table 5. Soil Parameters used for GRS-IBS Design.

Property Units

Foundation Soil

Retained Soil

Reinforced Soil

RSF Soil

Unit Weight pcf 120 120 135 135

Friction Angle deg 36 36 40 40

Cohesion pcf 0 0 0 0

Maximum Particle Size in. - - 1.0 -

The design was completed assuming the geotextile will be a woven geotextile with an ultimate tensile strength of at least 4800 lb/ft and tensile strength at 2 percent strain of at least 1000 lb/ft.

4.8 GRS-IBS Layout

The bridge layout was dictated by the hydraulic conditions in order to provide adequate clearance below the bottom of the superstructure and in order to provide adequate embedment of the GRS-IBS for scour protection.

The precast concrete footings will be approximately 8.2 feet high including the backwalls (measured from base of footing to finished grade). The height of the GRS-IBS with

Geotechnical Report GR13‐15 page 16

CMU facing (measured from the bottom of the precast footing to the scour elevation) will be approximately 15.4 feet at Abutment 1 and 14.2 feet at Abutment 2. These heights are approximate because the final elevations may vary slightly based on the bridge manufacturer’s approved shop drawings. The reinforced soil foundation (RSF) will extend 2.5 feet below the base of the GRS-IBS abutment.

The GRS-IBS abutment will be 26 feet-2 inches wide and the wing wall will extend 15 feet back at a 90 degree angle from the abutment face. The RSF limits will extend 2.5 feet horizontally outside the limits of the CMU facing.

The main geotextile reinforcement lengths in the GRS-IBS will match the wing wall lengths of 15 feet. The main reinforcement layers are installed between subsequent layers of CMU blocks with a vertical spacing not exceeding 8 inches. The bearing bed zone, directly below the precast concrete footing, will include 5 additional geotextile layers at mid-CMU block height, each 10 feet in length, resulting in vertical spacing no greater than 4 inches between subsequent layers.

The integration zone, behind the precast footing, will include 8 layers of geotextile reinforcement evenly spaced between the bottom of footing elevation and bottom of the roadway aggregate section. Each layer wraps back behind the precast concrete footing with a 4 foot long tail.

Solid CMU blocks will be used for the GRS-IBS facing in order to provide additional strength against ice impact during spring break-up, except that the top 3 rows of blocks will be hollow and then filled with concrete to tie them together.

Geotechnical Report GR13‐15 page 17

5. MATERIAL SOURCES

The material source discussed in this report is a source that has been identified as a potential borrow site near Fort Yukon that and been historically used for borrow by the village of Fort Yukon and the ADOT&PF. Discussion of this source and the testing results contained in this report does not relieve the contractor of any requirements contained in the project specifications and does not waive any permitting requirements for this source. Materials from this source may not meet the project specifications. Surface and subsurface rights of all material sources planned for use on this project must be verified by the contractor prior to use.

5.1 Material Source Location

Walter Peter, Transportation Coordinator for the Native Village of Fort Yukon, brought Brian Collins, WFLHD Sr. Geotechnical Engineer, to a potential material source outside Fort Yukon in June 2015. The material source location is shown on the Fort Yukon map in Figure 1 at the beginning of this report. The site is located about 1 mile east of Fort Yukon. The coordinates of the material source are 66.566942° north, 145.197793° west

(WGS 84).

5.2 Previous Material Source Investigation

In 2007-2008 HDL Engineering Consultants completed soil borings and test pit investigations on behalf of the Alaska Department of Transportation and Public Facilities in order to evaluate this source for use in support of proposed improvements at the Fort Yukon Airport (Dilley, 2008). The investigation included 20 solid stem auger borings and 20 test pits. Test results on samples from the test borings indicate the fines content (percent by weight passing the No. 200 sieve) in the gravelly sand ranges from 5.9 to

19.2 percent, with the exception of one boring (M-13) which had a fines content of 36.5 percent (Dilley, 2008). The percentage of sand in samples from the borings ranged from

32.6 to 72.4 percent and the percentage of gravel ranged from 12.2 to 58.3 percent (Dilley, 2008).

Test results from Dilley (2008) for composite samples combined from multiple borings and test pits are summarized in Table 6.

Geotechnical Report GR13‐15 page 18

Table 6. Laboratory Test Results for Combined Material Site Samples (reproduced from Dilley, 2008).

Borings TP-North TP-West TP-East TP-South

Grain Size Distribution

Cobbles (>3”) 0% 0% 0% 0% 0%

Gravel (#4 – 3”) 24.7% 63.1% 55.0% 50.4% 49.4%

Sand (#200 - #4) 62.6% 32.6% 40.1% 39.5% 44.4%

Fines (<#200) 12.8% 4.3% 5.0% 10.1% 6.2%

Apparent Specific Gravity NA 2.65 2.65 2.65 NA

LA Abrasion 16% 14% 15% 15% 15%

Nordic Abrasion NA* 6.6% 6.8% 6.6% NA*

Degradation 90 87 90 87 82

Sodium Sulfate 0.9% 0.7% 0.6% 0.7% 0.8%

*Insufficient material for testing.

5.3 Use of Material Source for GRS-IBS Backfill

Normally, GRS-IBS abutments are constructed with manufactured crushed open-graded backfill with a high angle of internal friction. Well compacted, high quality granular backfill is a critical component of the GRS-IBS technology. At the request of the Fort Yukon Native Tribe, we evaluated use of this local material source, assuming that minimal material processing will occur. The material source consists of subrounded to rounded sand and gravel. Samples from the proposed material source were sent to WFLHD by representatives from the tribe. Two of the samples were submitted to the WFLHD Materials Laboratory for particle size analysis and Atterberg limit tests. Three other samples were sent to Turner Fairbank Highway Research Center (TFHRC).

TRHRC has a large 12 inch shear box that was used to determine the internal angle of friction for the proposed material source. The test reports for the 2015 grab samples from the potential material source are contained in Appendix C.

Based on the results of our testing and other historical data from this source (Dilley, 2008), material from this source is suitable for use as GRS-IBS backfill. However, testing indicates that some of the material within the site is outside the specified gradation limits so the proposed material should be tested prior to incorporation into the GRS-IBS abutments. Dilley (2008) reports that samples from the test pits and borings in the eastern portion of the material site had higher percentages of fine grained material (percent passing the No. 200 sieve).

5.4 Riprap Sources

No sources of rock for riprap production are known to exist in the vicinity of Fort Yukon.

We anticipate that riprap material will need to be barged in and hauled to the site. The nearest suitable riprap source is unknown.

Geotechnical Report GR13‐15 page 19

6. EARTHWORK RECOMMENDATIONS

The soil profile across the site is generally consistent with 2.5 to 8 feet of sandy silt and silty sand overlying relatively clean sand and gravel. Groundwater was encountered near the elevation of the river, but may also be encountered perched on top of permafrost.

Permafrost was encountered in some of the borings and test pits.

The RSF will bear at elevation 978.5 on the project datum on the relatively clean sand and gravel deposits. The approach embankments will bear on the surficial layer of sandy silt and silty sand material. We recommend placement of reinforcement geotextile below the limits of all fill embankments. The reinforcement geotextile below embankments should be the same product used as reinforcement in the GRS-IBS abutments. We anticipate that the approach embankments will settle 6 to 12 inches due to consolidation of the sandy silt and silty sand and due to thaw settlement of frozen layers near the ground surface. This magnitude of settlement should be acceptable below a gravel surfaced roadway. The integration zone of the GRS-IBS will provide a relatively smooth transition and mask differential settlement between the GRS-IBS abutment and the approach embankments.

We anticipate that the approach embankments will be constructed with relatively clean sand and gravel imported from the local material source discussed in Section 5.

Construction considerations for earthwork operations are contained in Section 7.

Geotechnical Report GR13‐15 page 20

7. CONSTRUCTION CONSIDERATIONS

7.1 Earthwork Operations

The soil profile across the site is generally consistent with 2.5 to 8 feet of sandy silt and silty sand overlying relatively clean sand and gravel. Groundwater was encountered near the elevation of the river, but may also be encountered perched on top of Permafrost.

Permafrost was encountered in some of the borings and test pits.

The upper layer of sandy silty and silty sand will not be suitable for embankment material. The underlying relatively clean sand and gravel is good embankment material;

however, it is likely to be saturated and will therefore not be suitable to embank.

We anticipate that the approach embankments will be constructed with relatively clean (<10% passing the No. 200 sieve) sand and gravel imported from the local material source discussed in Section 5. The locally available gravel consists of subrounded to round particles which may make achieving the specified level of compaction more difficult, especially adjacent to the GRS-IBS CMU wall facing.

The upper layer of sandy silt and silty sand is loose/soft and will not likely support construction equipment. The Case 580 excavator/loader became stuck during the test pit excavations in 2015. A layer of geotextile reinforcement will be placed below the entire approach embankments. Two feet of compacted sand and gravel should be placed on top of the geotextile before any other construction traffic uses it.

Frozen soil should be anticipated in excavations. Frozen soil layers encountered at the site may be either seasonally frozen ground or permafrost. The active layer depth at the site is estimated to range from 7 to 15 feet; closer to 7 feet where the ground surface is covered with vegetation and closer to 15 feet where the ground surface is gravel roadway.

The relatively small Case 580 loader/excavator used to dig the test pits reached practical refusal in some of the frozen layers.

Frozen soil cannot be used as backfill for the GRS-IBS abutment nor for the approach embankments.

7.2 Dewatering

The elevation of the bottom of the RSF is 978.5 feet on the project datum. This elevation is well below the river surface elevation. The relatively clean sand and gravel at the RSF bearing elevation has a relatively high permeability and it is likely that the RSF excavations will require dewatering. If the excavation is inundated with water, dewatering is required so the backfill can be placed and compacted in the dry. The sand and gravel encountered in the borings will cave if left unsupported when excavations extend below the groundwater table.

Geotechnical Report GR13‐15 page 21

7.3 GRS-IBS Construction

The fundamental component of the GRS-IBS system is closely spaced layers of geotextile reinforcement with well-compacted granular backfill.

Careful attention should be given to the installation of the first row of blocks. Since all other courses of block are built off the first row, it is essential to ensure that the bottom row is level and even for construction.

It is highly recommended that the contractor reviews the following resources that provide further details on successful construction of GRS-IBS abutments.

FHWA provides a GRS-IBS construction video located at the following website:

http://www.fhwa.dot.gov/everydaycounts/technology/grs_ibs/multimedia.cfm

This construction video also can be viewed on the FHWA's YouTube channel at:

http://www.youtube.com/user/USDOTFHWA#p/a/u/0/w_5WFoAdoUw

The construction video follows the guidance outlined in the GRS-IBS Implementation Guide, FHWA-HRT-11-026, dated January 2011, which is available at the following website:

http://www.fhwa.dot.gov/publications/research/infrastructure/structures/11026/index.cfm

Geotechnical Report GR13‐15 page 22

8. REFERENCES

AASHTO, 2014, AASHTO LRFD Bridge Design Specifications, Seventh Edition, American Association of State Highway and Transportation Officials, Washington, D.C.

Abraham, Jared (2011) A Promising Tool for Subsurface Permafrost Mapping: An Application of Airborne Geophysics from the Yukon River Basin, Alaska, U.S.

Department of the Interior, USGS Fact Sheet 2011-3133, 4p.

Adams, M., Nicks, J., Stabile, T., Wu, J., Schlatter, W., and Hartmann, J. (2011) Geosynthetic Reinforced Soil Integrated Bridge System, Interim Implementation Guide, Report No. FHWA-HRT-11-026, Federal Highway Administration, 169p.

Ball, L.B., Smith, B.D., Minsley, B.J., Abraham, J.D., Voss, C.I., Astley, B.N., Deszcz- Pan, M., and Cannia, J. (2011) Airborne Electromagnetic and Magnetic Geophysical Survey Data of the Yukon Flats and Fort Wainwright Areas, Central Alaska, June 2010, USGS Open-File Report 2011-1304.

Carroll, N.M. (2007) Final Geotechnical Study, Proposed Sucker River Bridge, Fort Yukon Alaska, submitted to PDC Inc. by Shannon & Wilson, Inc., Fairbanks, Alaska.

Dilley, L. (2008) Geotechnical Report for Fort Yukon Airport Improvements and Material Site Study, Fort Yukon, Alaska, AKSAS Project No. 62650, Hattenburg, Dilley & Linnell Engineering Consultants, State of Alaska Department of Transportation and Public Facilities.

Ferrians, O.J. (1965) Permafrost Map of Alaska, Department of the Interior, USGS Miscellaneous Investigation Series, Map I-445.

Nelson, R.A., Luscher, U., Rooney, J.W., and Stramler, A.A. (1983) Thaw Strain Data and Thaw Settlement Predictions for Alaskan Soils, Proceedings of the Fourth International Conference on Permafrost, University of Alaska, Fairbanks, Alaska, p 912- 917.

U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) (1962) Ground Temperature Observations, Fort Yukon, Alaska, U.S. Army Cold Regions Research and Engineering Laboratory, Hanover, New Hampshire, Technical Report 100.

Western Regional Climate Center, Western U.S. Climate Historical Summaries, Ft.

Yukon, Alaska (503175), http://www.wrcc.dri.edu/cgi-bin/cliMAIN.pl?ak3175, accessed October 6, 2015.

Williams, J.R. (1962) Geologic Reconnaissance of the Yukon Flats District Alaska, U.S.

Geologic Survey Bulletin 1111-H, U.S. Department of the Interior.

Brian.Collins Rectangle

Geotechnical Report GR13‐15 October 2015

Boring and Test Pit Location Map

N

T o F o rt Y u ko n

NOTE:

2.

1.

of FHWA survey data.

August 2007. Boring elevations estimated from contour plot the Site Plan in Shannon and Wilson's Geotechnical Study dated

Boring locations were placed based on the locations shown on survey data.

points. Test pit elevations estimated from contror plot of FHWA

Test pit locations determined by measuring offset from control

LOCATION MAP

BORING & TEST PIT

IVARS BRIDGE

Existing crossing structure

Proposed bridge

TP15-01

TP15-02

TP15-03

TP15-04

STATE PROJECT

NUMBER

SHEET

U

S S u r ft

D c m y fi le s p w p r o d u c ti o n d a k t-fo r t1 ta .d g n

AK IRR FORT 1002(1)

A

M

O c to b e r

C h e c k e d b y

D e s ig n e d b y

BH07-01

BH07-02

BH07-03

Geotechnical Report GR01‐15 October 2015

Unified Soil Classification System Soil Descriptive Terminology

Logs of 2015 Test Pits Laboratory Test Reports for 2015 Test Pit Samples

SOIL DESCRIPTIVE TERMINOLOGY

CONSISTENCY OF COHESIVE SOILS

SPT N-Value (blows/ft) Consistency Field Identification

0 - 1 Very Soft Easily penetrated several inches by first

2 - 4 Soft Easily penetrated several inches by thumb

5 - 8 Firm Moderate effort required to penetrate several inches by thumb

9 - 15 Stiff Readily indented by thumb but penetrated only with great effort

16 - 30 Very Stiff Readily indented by thumbnail

> 30 Hard Indented with difficulty by thumbnail

RELATIVE DENSITY OF

COHESIONLESS SOILS

SPT N-Value (blows/ft) Relative Density

0 - 4 Very Loose

5 - 10 Loose

11 - 30 Medium Dense

31 - 50 Dense

> 50 Very Dense

PARTICLE SIZE

Term Sieve Size

Boulder > 12” (>300mm)

Cobble 3” - 12” (75mm - 300mm)

Gravel – coarse ¾” - 3” (19mm - 75mm)

Gravel – fine #4 - ¾” (4.75mm - 19mm)

Sand – coarse #10 - #4 (2.00mm - 4.75mm)

Sand – medium #40 - #10 (0.425mm - 2.00mm)

Sand – fine #200 - #40 (0.075mm - 0.425mm)

Silt & Clay < #200 (<0.075mm)

MOISTURE CONTENT

Dry Dusty, dry to the touch

Moist Damp, but no visible free water

Wet Visible free water, saturated, over optimum

S-1

S-2 sandy ORGANIC SOIL, black, moist to wet, low plasticity, homogeneous seepage observed between 1.5-2' (on top of frozen soil) frozen below 2' sandy SILT (ML), dark gray, frozen Nbe, low plasticity, homogeneous poorly graded GRAVEL with sand (GP), gray-brown, frozen Vx, fine to coarse sand, fine subrounded to rounded gravel, homogeneous

Bottom of test pit at 5.0 feet due to practical refusal of excavator in frozen layer

Ground temperature measurements:

T = 49.9 deg F @ 0.3 ft T = 43.2 deg F @ 1.0 ft T = 37.0 deg F @ 1.5 ft T = 31.7 deg F @ 2.0 ft T = 31.5 deg F @ 2.7 ft

2.0

2.8

5.0

FIELD

BLOW

COUNT

(Recovery)

LATITUDE (DEGREES): N 66° 35' 7.12" (66.58531°)

LONGITUDE (DEGREES): W -145° 6' 59.36" (-145.11649°)

ELEVATION: 989' [El. referenced to project datum.]

D E

P T

H ft

E

LE

V ft)

Sheet 1 of 1

BORING LOG (US Customary Units)

S A

M P

LE

DRILLING METHODS:

DATE FINISHED:

TOTAL DEPTH: 5' (BOTTOM ELEV: 984')

LIQUID LIMIT

AFTER DRILLING

Grab Sample

AT COMPLETION

G R

A P

H

IC

L O

G

DESCRIPTION

Case 580 Backhoe

106+01, 0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

989.0

988.0

987.0

986.0

985.0

984.0

983.0

982.0

981.0

980.0

979.0

Station and offset determined by measuring distances from project control points. Elevation estimated from project contour map. GPS coordinates determined using recreational GPS (WGS 84).

NOTES:

BORING Test Pit TP15-01

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DATE STARTED:

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FEDERAL HIGHWAY ADMINISTRATION

VANCOUVER, WASHINGTON

GEOTECHNICAL SECTION

Walter Peter

WEATHER:

PROJECT: Ivar's Bridge AK IRR FORT 1002(1)

N/A

WATER CONTENT (%)

2'

WATER LEVELS

HAMMER:

WHILE DRILLING

DRILLER:

DRILL: Backhoe

D E

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H ft

6/20/2015

S A

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6/20/2015

STATION, OFFSET:

PLASTIC LIMIT

Typewritten Text

TP15-01

TP15-01

S-3 sandy ORGANIC SOIL, black, moist to wet, nonplastic, homogeneous seepage from 1 in thick gravelly pocket at 1 ft sandy SILT, dark gray, frozen Nbe, nonplastic, homogeneous poorly graded SAND with silt and gravel (SP-SM), gray-brown, frozen Vx, fine to coarse sand, fine subrounded to rounded gravel, homogeneous

Bottom of test pit at 4.0 feet due to practical refusal of excavator in frozen layer

Ground temperature measurements:

T = 48.6 deg F @ 0.5 ft T = 44.7 deg F @ 1.0 ft T = 32.1 deg F @ 2.0 ft T = 31.7 deg F @ 2.5 ft T = 31.5 deg F @ 3.0 ft

2.0

3.0

4.0

FIELD

BLOW

COUNT

(Recovery)

LATITUDE (DEGREES): N 66° 35' 7.15" (66.58532°)

LONGITUDE (DEGREES): W -145° 6' 59.80" (-145.11661°)

ELEVATION: 990' [El. referenced to project datum.]

D E

P T

H ft

E

LE

V ft)

Sheet 1 of 1

BORING LOG (US Customary Units)

S A

M P

LE

DRILLING METHODS:

DATE FINISHED:

TOTAL DEPTH: 4' (BOTTOM ELEV: 986')

AFTER DRILLING

Grab Sample

AT COMPLETION

G R

A P

H

IC

L O

G

DESCRIPTION

Case 580 Backhoe

105+92, 0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

990.0

989.0

988.0

987.0

986.0

985.0

984.0

983.0

982.0

981.0

980.0

Station and offset determined by measuring distances from project control points. Elevation estimated from project contour map. GPS coordinates determined using recreational GPS (WGS 84).

NOTES:

BORING Test Pit TP15-02

COMPANY:

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N/A LOGGER: B. Collins

Sunny, 80s F

DATE STARTED:

0.0

FEDERAL HIGHWAY ADMINISTRATION

VANCOUVER, WASHINGTON

GEOTECHNICAL SECTION

Walter Peter

WEATHER:

PROJECT: Ivar's Bridge AK IRR FORT 1002(1)

N/A

WATER CONTENT (%)

1'

WATER LEVELS

HAMMER:

WHILE DRILLING

DRILLER:

DRILL: Backhoe

D E

P T

H ft

6/20/2015

S A

M P

LE

R

6/20/2015

STATION, OFFSET:

TP15-02

S-4

S-5

ORGANIC SOIL, black, moist, homogeneous sandy SILT (ML), dark gray, moist to frozen Vx below 1 ft, nonplastic, homogeneous poorly graded GRAVEL with sand (GP), gray-brown, wet, fine to coarse sand, fine subrounded to rounded gravel, homogeneous test pit filled with water immediately after penetrating gravel layer at 4.5 ft, filled to 1 ft depth below ground surface

Bottom of test pit at 6.0 feet due to caving in gravel below water table

Ground temperature measurements:

T = 49.0 deg F @ 0.1 ft T = 31.9 deg F @ 1.0 ft T = 31.5 deg F @ 2.5 ft

0.5

4.5

6.0

FIELD

BLOW

COUNT

(Recovery)

LATITUDE (DEGREES): N 66° 35' 6.36" (66.5851°)

LONGITUDE (DEGREES): W -145° 6' 56.63" (-145.11573°)

ELEVATION: 988' [El. referenced to project datum.]

D E

P T

H ft

E

LE

V ft)

Sheet 1 of 1

BORING LOG (US Customary Units)

S A

M P

LE

DRILLING METHODS:

DATE FINISHED:

TOTAL DEPTH: 6' (BOTTOM ELEV: 982')

AFTER DRILLING

Grab Sample

AT COMPLETION

G R

A P

H

IC

L O

G

DESCRIPTION

Case 580 Backhoe

107+37, 0

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

988.0

987.0

986.0

985.0

984.0

983.0

982.0

981.0

980.0

979.0

978.0

Station and offset determined by measuring distances from project control points. Elevation estimated from project contour map. GPS coordinates determined using recreational GPS (WGS 84).

NOTES:

BORING Test Pit TP15-03

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