Red_Dirt_Bridge_Geotechnical_Report_FINAL.pdf

Revised Geotechnical Design Report Red Dirt Bridge Replacement

Eagle County, Colorado

July 31, 2015

Submitted To:

AECOM

717 17th Street, Suite 2600 Denver, Colorado 80202

By:

Shannon & Wilson, Inc.

1321 Bannock Street, Suite 200 Denver, CO 80204

23-1-01372-201

ALASKA

CALIFORNIA

COLORADO

FLORIDA

MISSOURI

OREGON

WASHINGTON

WISCONSIN

1321 BANNOCK STREET, SUITE 200

DENVER, COLORADO 80204

PHONE 303•825•3800

FAX 303•825•3801

www.shannonwilson.com 23-1-01372-201

July 31, 2015

AECOM

717 17th Street, Suite 2600 Denver, Colorado 80202

Attn: Mr. Corey Lang

RE: REVISED GEOTECHNICAL DESIGN REPORT, RED DIRT BRIDGE

REPLACEMENT, EAGLE COUNTY, COLORADO

We are pleased to submit our revised geotechnical design report for the above-referenced project.

The enclosed revised report discusses subsurface conditions encountered during the exploration program, summarizes laboratory tests, and provides geotechnical engineering recommendations for the proposed project. The recommendations provided in this revised report supersede those provided in the previous version of this report dated July 14, 2015.

We appreciate the opportunity to be of service to you on this project. If you have questions or require further information, please contact me at (303) 825-3800.

Sincerely, SHANNON & WILSON, INC.

Mark J. Vessely, P.E.

Vice President

ALL:JSC:MJV:GRF

Enc: Revised Geotechnical Design Report

23-1-01372-201-L1/

23-1-01372-201-R1_Final_Rev1.docx 23-1-01372-201 i

TABLE OF CONTENTS

Page

1.0 INTRODUCTION

2.0 SCOPE OF WORK

3.0 SITE AND PROJECT DESCRIPTION

4.0 SUBSURFACE EXPLORATIONS AND LABORATORY TESTING

5.0 SUBSURFACE CONDITIONS

5.1 Regional Geology

5.2 Subsurface Conditions

6.0 GEOLOGIC HAZARDS

6.1 Seismic Hazards and Ground Motion Design Parameters

6.2 Swelling Soil and Bedrock

6.3 Evaporite Dissolution and Subsidence

6.4 Corrosive Soil and Bedrock

6.5 Other Geologic Hazards for Consideration

7.0 GEOTECHNICAL RECOMMENDATIONS

7.1 Bridge Foundations

7.1.1 Drilled Shafts – Axial Resistance

7.1.2 Lateral Resistance

7.2 Retaining Walls

7.2.1 Lateral Earth Pressures

7.2.2 Sliding and Bearing Resistance

7.2.3 MSE Reinforcement Length

7.2.4 Embedment Depth

7.2.5 Global Stability

7.2.6 Wall Drainage

8.0 CONSTRUCTION CONSIDERATIONS

8.1 Drilled Shaft Installation

8.1.1 Drilled Shaft Installation Methods and Equipment

8.1.2 Drilled Shaft Inspection and Observation

8.1.3 Concrete Placement

8.1.4 Cross-Hole Sonic Logging

8.2 Site Preparation

8.3 Temporary Excavations and Support

8.4 Earthwork and Grading

TABLE OF CONTENTS (cont.)

ii

8.4.1 Excavation

8.4.2 Subgrade Preparation and Proof Rolling

8.4.3 Fill Placement and Compaction

9.0 PLAN REVIEW AND CONSTRUCTION OBSERVATION

10.0 LIMITATIONS

11.0 REFERENCES

TABLES

1 Seismic Parameters for Design 2 Corrosivity Ratings Based on Soil Resistivity, Roberge (1999) 3 Sulfate Exposure, ACI (2011) 4 Geotechnical Parameters for Design of Deep Foundations 5 Recommended Design Parameters for Lateral Earth Pressures

FIGURES

1 Vicinity Map 2 Site and Exploration Plan 3 Red Dirt Bridge Generalized Subsurface Profile 4 Map Showing Extent of Evaporite Bedrock 5 Recommended P-Multipliers for Groups Effects 6 Recommended Surcharge Loading for Temporary and Permanent Walls 7 Nominal Bearing Resistance Versus Effective Footing Width, Embedment = 4

Feet 8 Nominal Bearing Resistance Versus Effective Footing Width, Embedment = 1.5

Feet

APPENDICES

A Subsurface Explorations B Laboratory Test Results C Geophysical Investigation D Important Information About Your Geotechnical Report

REVISED GEOTECHNICAL DESIGN REPORT

RED DIRT BRIDGE REPLACEMENT

EAGLE COUNTY, COLORADO

1.0 INTRODUCTION

This revised report presents the results of our subsurface exploration program and provides geotechnical engineering recommendations for the proposed Red Dirt Bridge Replacement Project in Eagle County, Colorado. The revised report summarizes our subsurface explorations, laboratory testing, and geotechnical engineering studies, and presents conclusions and recommendations for design and construction of the bridge.

The recommendations provided in this revised report supersede those provided in the previous version of this report dated July 14, 2015.

2.0 SCOPE OF WORK

Our services were conducted in general accordance with our Task Order No. 6 with AECOM, effective January 8, 2015 under Master Services Agreement No. 12S-15391-CO05. We completed the following tasks for the project:

Coordinated subcontractors for drilling borings and completing the geophysical surveys

Observed, logged, and collected soil and rock samples from two auger borings and two continuous core borings

Completed laboratory testing on selected soil and rock samples from the borings

Evaluated geotechnical data and completed geotechnical engineering analyses to develop conclusions and recommendations for design of the project

Evaluated subsurface conditions relative to construction of the project

Prepared this report

3.0 SITE AND PROJECT DESCRIPTION

The existing Red Dirt Bridge is located approximately 15 miles northeast of Dotsero, Colorado along Colorado River Road near the intersection with Red Dirt Creek Road (Figures 1 and 2).

Existing topography along Colorado River Road is mountainous with numerous steep to vertical rock slopes and steep embankments. The bridge and road alignment traverses approximately southwest to northeast in the project area. The Union Pacific Railroad runs parallel to Colorado

River Road and Red Dirt Bridge, and there is an existing railroad bridge approximately 40 feet east of the current bridge. On the north side of the project area, the existing road alignment is constrained to the west by a near-vertical rock slope about 80 feet high and a debris flow channel between the rock slope and roadway. On the south side of the bridge, the road is constrained to the west by the banks of the Colorado River and to the east by a gravel residential roadway.

The proposed project consists of reconstruction of Red Dirt Bridge over the Colorado River.

Based on preliminary plans provided by AECOM, the new two-lane bridge is proposed to be three spans, 230 feet long, and 34.5 feet wide (including curbs). The spans will be 75 or 80 feet long and supported by deep foundations. To accommodate the proposed grades and widened approach, retaining walls will also be required at each abutment.

4.0 SUBSURFACE EXPLORATIONS AND LABORATORY TESTING

A field exploration and geotechnical laboratory testing program was implemented to explore and evaluate subsurface conditions at the proposed bridge location. The subsurface exploration program consisted of drilling and sampling two auger borings and two continuous core borings, designated SW-01 through SW-04, from February 16 through 24, 2015. The approximate locations of the borings are shown on Figure 2. Appendix A presents a discussion of the drilling and sampling procedures used to complete the borings. Appendix A also presents the individual exploration logs and an explanation of the symbols and terminology used on the logs.

Geotechnical laboratory tests were completed on selected samples retrieved from the borings to estimate soil and rock index and engineering properties. Tests included natural water content, grain size distribution, Atterberg limits, unconfined compressive strength, and corrosion.

Laboratory test methods and results are provided in Appendix B. The natural water content, fines content, and Atterberg limits are also shown on the individual boring logs included in Appendix A.

5.0 SUBSURFACE CONDITIONS

5.1 Regional Geology

At a regional scale, the project area is located on the western slope (west of the continental divide) of Colorado. The site is about 15 miles northeast of Dotsero, Colorado and is within a canyon cut by the Colorado River. To the south near Gypsum, high deformation created by upwelling of less dense evaporate deposits of the Eagle Valley Formation can be observed and is expressed by dozens of faults and folds. In general, these deposits and high deformation are exposed approximately one mile south of the project area, but are known to interfinger with strata nearby (Kirkham and Scott, 2002).

About seven miles north of the project site, broader, more continuous structure exists. At this location, the axis of a large doubly plunging syncline that begins near Vail dissipates just as it crosses the canyon cut by the Colorado River. From this point south along the Colorado River, the exposed rocks become older.

Shannon & Wilson reviewed a regional geologic map (Tweto and others, 1978) to form an understanding of subsurface conditions at the project site. Based on our review, the surface geologic mapping performed by Tweto and others (1978) is consistent with our subsurface observations at the site. Based on our exploration program and geologic literature review, we grouped the subsurface materials into three general geologic units: fill, alluvium, and Minturn Formation bedrock. The upper and lower portions of the exposed cliffs at Red Dirt Bridge are Weber-Maroon Formation and Minturn Formation, respectively. Data from our geotechnical borings also indicates that the bedrock underlying the bridge is Minturn Formation. Minturn Formation varies from gray, pale-yellow, and red sandstone, conglomerate, and shale. Weber Sandstone is described as yellow-gray sandstone and Maroon Formation is grayish-red sandstone, conglomerate, and mudstone (Tweto and others, 1978).

5.2 Subsurface Conditions

A generalized subsurface profile is presented as Figure 3. At the bridge approaches, loose to very dense fill materials were encountered beneath the pavements in borings SW-01 and SW-04 to depths of about 12 and 7 feet, respectively. The fill generally consisted of poorly graded gravel with silt and sand. We anticipate the fill is associated with construction of the existing bridge.

Underlying the fill and in the river channel, our borings encountered alluvium consisting of medium dense to very dense, sand and gravel with variable amounts of silt and occasional cobbles and boulders to about 1 to 3 feet in size.

The bedrock encountered in the borings primarily consisted of very fine-grained, calcareous sandstone interbedded with siltstone and shale. The bedrock was encountered from elevations of about 6343 to 6346. The bedrock was generally fresh with moderate weathering observed on joint surfaces. Observed discontinuities included several low angle joints along bedding planes and several high angle joints with iron staining that were present in the rock core.

During drilling, borings SW-01 and SW-04 encountered groundwater at depths of 19.2 and 16.9 feet (approximate elevations of 6,355 and 6,356 feet), respectively. Groundwater fluctuations are likely and will be heavily influenced by water levels in the Colorado River.

A summary of the borings and approximate elevations are as follows:

SW-01 (near Southwest Abutment) o Top Elevation = 6373.6 feet o Bedrock Depth/Elevation = 31 feet/6342.6 feet o Groundwater Depth/Elevation = 19.2 feet/6354.4 feet

SW-02 (near Pier 2) o Top Elevation = 6373.8 feet o Bedrock Depth/Elevation = 30.5 feet/6343.3 feet o River Depth/Elevation = 18.7 feet/6355.1 feet

SW-03 (near Pier 3) o Top Elevation = 6373.9 feet o Bedrock Depth/Elevation = 31 feet/6342.9 feet o River Depth/Elevation = not measured

SW-04 (near Northeast Abutment) o Top Elevation = 6373.4 feet o Bedrock Depth/Elevation = 27 feet/6346.4 feet o Groundwater Depth/Elevation = 16.9 feet/6356.5 feet

6.0 GEOLOGIC HAZARDS

6.1 Seismic Hazards and Ground Motion Design Parameters

Colorado is comprised of areas of low to moderate potential for damaging earthquakes. It is not possible to accurately estimate the timing or location of future earthquakes, because the occurrence of major earthquakes is relatively infrequent and the historical earthquake record in Colorado is short (about 140 years). The closest known active faults near the project area are the Frontal fault and Killarney faults. The Frontal fault is north-northwest striking and primarily a high angle normal fault located approximately 30 miles east of the project area. It forms the eastern margin of the Gore and Tenmile Ranges. Based on geomorphic features along the fault trace, this fault is suspected to have been active less than 130,000 years ago based on geomorphic features along the fault trace (Widmann, 1997). The Killarney faults are northwest striking and inferred to be normal faults. These faults are located about 23 miles northwest of the project area and are thought to have last moved in the late Cenozoic (Widmann, 1999).

Because of the distance from the site to these relatively inactive faults, the potential for ground surface fault rupture is low.

Liquefaction may occur in loose, saturated, cohesionless soils when subjected to earthquake ground shaking. Based on the medium dense to dense sand and gravel encountered at the project site, and the relatively low peak ground acceleration (PGA) for this area, the liquefaction potential is low.

The American Association of State Highway and Transportation Officials (AASHTO) site class is determined using SPT N-values and/or shear wave velocities measured in the upper 100 feet of the soil profile. Based on geophysical surveys completed by Olson Engineering (Appendix C), the shear wave velocities in the upper 100 feet (Vs100) range from 1,669 to 2,053 feet per second (ft/sec). Using criteria for Vs100 in the AASHTO Load and Resistance Factor Design (LRFD) Bridge Design Specifications (2014), Site Class C is recommended for the proposed Red Dirt Bridge Replacement.

Ground motion parameters were determined for the project site using the USGS U.S. Seismic Design Maps Web Application (USGS, 2014) and the 2009 AASHTO LRFD Bridge Design Criteria (the design parameters have not changed in the current 2014 manual). A summary of the seismic design ground motion parameters are provided in Table 1.

6.2 Swelling Soil and Bedrock

Many of the soil/rock formations in Colorado are susceptible to volume change by swelling/shrinking. This geologic phenomenon has the potential to cause substantial damage to lightly-loaded structures (especially pavements and retaining walls) when exposed to water. Due to a lack of argillaceous overburden and bedrock encountered during our subsurface explorations, the site has low swell potential.

6.3 Evaporite Dissolution and Subsidence

Large areas of Colorado are underlain by Mesozoic and/or Paleozoic evaporate deposits (White, 2012). As stated by White (2012), the evaporate bedrock, which contains evaporative minerals, can dissolve in the presence of fresh water. The dissolution of such rocks can alter the ground and surface water flows, and create subsurface voids such as caverns, open fissures, and solution pipes. Subsidence of the ground surface and sinkhole openings are a geologic hazard. Based on our review of the Colorado Map of Potential Evaporite Dissolution and Evaporite Karst

Subsidence Hazards (White, 2012), the project site is within the Eagle Collapse Center and is near the north end of the evaporate bedrock zone along the Colorado River (Figure 4).

While the rock core from borings SW-01 and SW-04 indicated evaporite layers and calcite filled vugs, subsurface voids were not observed at the exploration locations and depths. Additionally, Olson Engineering reviewed the refraction tomography for potential karst features. Olson observed a low velocity zone within the bedrock along Line 4. However, they indicated that true voids are generally characterized by low ray density zones in the ray coverage plots. At the low velocity location within the bedrock along Line 4, Olson reported there was no such feature in the ray density plot. Further discussion is provided in Appendix C.

While the geophysical data did not identify potential voids, there is uncertainty in the level of the evaporite subsidence hazard. To reduce the potential risk of this hazard, we understand the drilled shaft design will ignore the base resistance and only use side resistance. Based on information provided by AECOM, the increase in project cost is negligible because the depth of embedment is largely controlled by lateral requirements.

6.4 Corrosive Soil and Bedrock

The clay soil and bedrock materials in Colorado can be corrosive to substructure elements. To assist in estimating the corrosion potential at the site, samples of the overburden and bedrock were tested for pH, resistivity, water soluble sulfates, and chlorides. The results are presented in Table B-1 in Appendix B.

The resistivity measured in the two samples was 1,600 and 3,300 ohm-cm. Based on the FHWA 2010 Drilled Shaft Manual (Brown and others, 2010), the resistivity values suggest an aggressive subsurface environment for metal in contact with these materials. Roberge (1999) also provides corrosivity ratings based on soil resistivity (Table 2). The resistivity values measured in the samples indicate corrosive to highly corrosive subsurface conditions.

The concentration of water soluble sulfates measured in the two samples from the site during this study was 0.02 and 0.12 percent by weight. Based on classifications as defined by ACI-318-11 (ACI, 2011) (Table 3), these test results suggest a negligible to moderate degree of sulfate attack on concrete exposed to site soils (exposure class S0 or S1).

We recommend that a specialist in corrosion-resistance design review the results included in Table B-1 to determine actual construction materials and methods based on the test results.

6.5 Other Geologic Hazards for Consideration

As discussed in Section 3.0, the existing road alignment on the north side of the bridge is constrained to the west by a debris flow channel which is adjacent to a near-vertical rock slope.

The debris flow channel is at the base of the rock slope and contained by a berm adjacent to the roadway that appears to be constructed from granular soils derived from the project vicinity and possibly debris flow deposits. The debris flow berm ranges from a height of approximately 2 feet at the southwest end to 10 feet at the north end where the berm intersects the native ground and channel. The berm appears to be relatively old and is well vegetated with grasses and bushes. No vegetation was observed within the channel, which suggests there is a recurrence of flow in the channel.

We recommend that precautions be taken during construction to avoid disturbance to the existing conditions of the channel and berm. Additionally, the debris flow discharge north of the bridge location should not be obstructed and future debris flows should be anticipated.

7.0 GEOTECHNICAL RECOMMENDATIONS

7.1 Bridge Foundations

We understand that conventional drilled shafts will be used for foundation support at the two piers and drilled shafts with an H-pile core will be used for foundation support at the abutments.

Regardless of the selected foundation type(s), we anticipate difficult installation conditions due to the presence of boulders. Design recommendations for drilled shafts are presented in the following sections.

7.1.1 Drilled Shafts – Axial Resistance

The design criteria presented herein uses LRFD criteria that were developed based on recommendations presented in the AASHTO LRFD Bridge Design Specifications (AASHTO, 2014). Axial resistance parameters, which are summarized in Table 4, were developed based on methods described in FHWA publication NHI-10-016, Drilled Shafts: Construction Procedures and LRFD Design Methods (Brown and others, 2010).

The nominal unit side resistance values were estimated from Equation 13-19 in the 2010 FHWA manual (Brown and others, 2010). The resistance calculated by this equation utilizes a reduction factor based on rock quality designation (RQD) and the condition of the rock joints.

The nominal unit base resistance values were estimated from Equation 13-21 in the 2010 FHWA manual (Brown and others, 2010). This equation provides a method of calculation for shafts bearing on sedimentary rock with primarily horizontal discontinuities, where discontinuity spacing is at least 1 foot, and discontinuity aperture does not exceed 0.25 inch (Brown and others, 2010).

Drilled shafts can be designed for tip and side resistance in the bedrock. Consistent with local practice, we recommend ignoring side resistance in the overburden. The side resistance should also be ignored above the potential scour depth. Based on our analyses, we recommend a minimum drilled shaft penetration of 5 feet into the bedrock (see Table 4).

The drilled shaft axial resistance parameters presented in Table 4 are nominal values. For service and extreme event limit state conditions, a resistance factor of 1.0 should be applied to the nominal resistance values. The recommended resistance factors for strength limit state conditions are provided in Table 4. If a non-redundant single shaft is used to support a bridge pier, then the strength limit state resistance factor (see Table 4) should be reduced by 20 percent in accordance with the AASHTO LRFD Bridge Design Specifications (AASHTO, 2014). The nominal axial resistance parameters presented herein do not require reduction due to shaft group action, provided the shafts are spaced a horizontal distance of at least two shaft diameters, center-to-center.

Assuming that drilled shafts are constructed using good installation techniques and equipment based on criteria described in the 2010 FHWA manual (Brown and others, 2010), we anticipate that total settlement for drilled shafts at the service limit state will be approximately ½ inch. We anticipate that differential settlement between piers will be about 50 to 75 percent of this value. About 75 percent of the settlement is anticipated to occur as the load is applied with the remaining settlement occurring within the first year following construction. Drilled shaft construction considerations are discussed further in Section 8.1.

7.1.2 Lateral Resistance

Lateral loads acting on the structure from wind, seismic, and other loadings are typically resisted by the passive earth pressure against the caps, the frictional resistance developed between the sides of the cap and surrounding soils, and the lateral resistance provided by the deep foundation members. The lateral behavior of the shafts is highly dependent on the degree of fixity of the top of the shaft.

Frictional sliding resistance at the base of the cap should be ignored because a deep foundation-supported structure may not transmit load directly to the soil beneath the cap.

Similarly, passive soil resistance against the cap and frictional resistance along the sides of the cap should be ignored because of the relatively small allowable design deflections and the potential loss of support against the cap during a scour event. As a result, the lateral resistance will come from the deep foundation elements.

We assume that the computer program LPILEPLUS (Ensoft, 2012) will be used to evaluate the lateral behavior of the drilled shafts, including estimating lateral deflections and determining required embedment depths. LRFD design methods for lateral loading of drilled shafts utilize a resistance factor of 1.0 and are controlled by allowable movement criteria. Geotechnical parameters required for input into LPILEPLUS are presented in Table 4. These input parameters are for an individual drilled shaft. If groups of drilled shafts are used, the effects of group interaction should be considered when evaluating the horizontal load-deflection behavior. When using the P-Y method of analysis, the values of P (load) should be factored appropriately to account for the group effects. Recommended values of P-multipliers for loading perpendicular and parallel to a line of shafts are provided on Figure 5.

7.2 Retaining Walls

Four retaining walls, labeled A through D on the preliminary plans provided to us, are currently proposed to be constructed at the bridge abutments. The preliminary plans indicate a maximum wall height of about 10 feet and we understand the walls may consist of mechanically stabilized earth (MSE) walls or cast-in-place concrete (CIPC), cantilevered walls. Recommendations for both wall types are provided below.

During construction, the gravel residential access road to the east of the southwest abutment may be affected by retaining wall B. However, assuming the east edge of the proposed roadway is similar to the existing, we do not anticipate permanent impacts to the access road.

7.2.1 Lateral Earth Pressures

The lateral earth pressures against retaining walls depend on many factors, including surcharge loads, the type of adjacent soil, drainage provisions, and whether or not the top of the wall can yield or deflect laterally/rotate during and after excavation. Recommended lateral earth pressure parameters for the walls were developed based on recommendations by AASHTO (2014). Our recommended earth pressure parameters are provided in Table 5. Additional recommendations and assumptions are as follows:

We recommend Structural Backfill conforming to the requirements provided in the latest edition of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 (2014).

Active and at-rest earth pressures provided in Table 5 assume the walls are backfilled with properly compacted (see Section 8.5.3), Structural Backfill in a 1H:1V (horizontal to vertical) zone extending upward from the base of the wall from a point

1.5 feet behind the wall.

Where walls are free to deflect, they can be designed for the active condition. Where a deflection greater than 0.001 times the height of the wall (0.001H) cannot occur, they should be designed for at-rest conditions.

The earth pressures in Table 5 assume the walls have a vertical wall face and horizontal back slope, and subsurface drainage is provided such that hydrostatic pressures do not develop.

If the conditions and assumptions above are not met, we should be notified so that we may revise our recommendations.

Surcharge loads from motor vehicles/traffic and construction equipment will induce additional lateral loads on retaining walls. The pressures provided in Table 5 do not account for surcharge loads. Lateral loads due to various types of surcharges may be calculated by multiplying the surcharge by the at-rest earth pressure coefficient (K0) or active earth pressure coefficient (KA), depending on if the wall is free to yield as discussed above. The earth pressure coefficients are also shown in Table 5. Surcharge loads should be applied as recommended on Figure 6.

Seismic earth pressures should be added to the static earth pressures acting on retaining walls and structures. Seismic earth pressures should be calculated in accordance with AASHTO (2014). The seismic parameters for calculation of seismic stability are provided in Table 1.

7.2.2 Sliding and Bearing Resistance

The coefficient of friction for sliding resistance is provided in Table 5 and our recommended nominal bearing resistance can be obtained from Figures 7 and 8 for effective footing widths between 4 and 10 feet. The bearing resistance has been provided for the strength limit state and a service limit state for 1 inch of settlement. We recommend the service limit state bearing resistance be used for design. Appropriate resistance factors are also provided on Figures 7 and 8.

The bearing resistance values provided on Figures 7 and 8 assume horizontal ground in front of the wall footing and that scour protection will be provided such that horizontal ground and 4 feet of embedment will exist throughout the life of the structure. Additional assumptions are shown in the notes on Figures 7 and 8. If 4 feet of embedment is not possible, we have provided bearing resistance values for 1.5 feet of embedment.

At the time this report was prepared, the retaining wall details were not available. Once retaining wall details and cross sections are finalized, we should be given the opportunity to review the plans and determine if our recommendations need to be revised.

7.2.3 MSE Reinforcement Length

To reduce potential for compound stability to control the design of MSE walls, we recommend a minimum L/H ratio (reinforcement length/wall height) of 0.7, or a reinforcement length of 8 feet, whichever is greater. The reinforcement lengths may need to be increased to meet internal, external (sliding and overturning), or compound stability requirements. These failure modes should be evaluated by the MSE wall vendor/designer as these failure modes depend on the particular reinforcement type and spacing selected by the wall vendor/designer.

The reinforcement lengths may also need to be increased to meet global stability. As discussed below, global stability analyses should be completed once the details of the walls are finalized.

7.2.4 Embedment Depth

Where the ground surface in front of the wall face is near horizontal, the base of the reinforced zone (MSE walls) or the base of the wall (CIPC, cantilevered walls) should be a minimum of 18 inches below the ground surface. If the ground surface in front of the wall is sloped, the bottom of the reinforced zone (MSE walls) or bottom of the wall (CIPC, cantilevered walls) should be located a minimum of 18 inches below the elevation at which there is a 4-foot horizontal distance from the wall face to the slope face in front of the wall. A 4-foot wide horizontal bench should also be provided in front of walls bearing on sloping ground.

The MSE walls should be embedded a minimum of 4 feet below the ground surface for frost mitigation. CIPC, cantilevered walls should also be embedded at least 4 feet for frost mitigation.

7.2.5 Global Stability

Once final grading and wall type and geometries are determined, the global stability for the wall should be checked.

7.2.6 Wall Drainage

Based on conditions encountered in the subsurface explorations (see Section 5.2), we do not anticipate the presence of groundwater near the base of the walls, with the exception of high flow periods in the Colorado River. Nevertheless, surface water can infiltrate wall backfill, regardless of the permeability of the backfill. Therefore, we recommend providing drainage measures that reduce the potential for water to accumulate in the backfill and for hydrostatic pressures to act on the wall face. Typical drainage measures include free-draining backfill and a drainage system at the base of the reinforced and retained fill zones. The specific drainage measures should be provided by the MSE wall vendor/designer. We also recommend free-draining backfill be placed in front of the wall.

8.0 CONSTRUCTION CONSIDERATIONS

8.1 Drilled Shaft Installation

8.1.1 Drilled Shaft Installation Methods and Equipment

Specifications and installation methods should be in general accordance with our recommendations and Section 565 of the Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 (FHWA, 2014).

Drilled shafts will obtain their capacity from within the bedrock. Our experience indicates heavy duty drill rigs using rock auger drill methods can penetrate sedimentary bedrock.

Based on the laboratory test results completed on rock core samples, the unconfined compressive strength (UCS) of the rock varied from about 1,800 to 19,000 pounds per square inch (psi). The UCS of samples from the upper 10 feet of rock was generally less than about 6,000 psi. High strength, strongly cemented layers of siltstone/sandstone should be expected and may result in more difficult and slower drilling or the need to switch to rock coring methods. These layers are variable in location and thickness. The means and methods of drilled shaft installation should be determined by the drilled shaft contractor. Further, as required by Section 565.04 of FP-14 (FHWA, 2014), the drilled shaft contractor should provide a construction plan that describes the proposed drilled shaft installation and construction methods.

Based on our subsurface explorations, approximately 27 to 31 feet and 9.5 to 13 feet of sand and gravel with boulders exists at the abutments and river bottom, respectively.

Groundwater was also encountered within the alluvium. We anticipate that temporary casing sealed into the bedrock will be required to prevent raveling, caving, and flowing conditions in the overburden. Where casing is used, it should be pushed, rotated, vibrated, or driven into the bedrock. An oscillator system can also be considered to advance the casing through the river alluvium and into the bedrock. Compared to the more traditional method of lowering the casing after advancing the drilled shaft excavation into bedrock, an oscillator system will likely assist in maintaining the location/alignment of the excavation. The drilled shaft contractor will ultimately be responsible for selecting the means and methods of casing installation. The inside diameter of the casing should be equal to or larger than the specified drilled shaft dimensions. Where the casing is sealed into the bedrock bearing zone, bedrock penetration to achieve design capacity should begin at the bottom of the casing. If penetration of more than 5 feet of casing into the bedrock occurs, we should be notified so that we may provide appropriate reduced side resistance parameters. Even with casing, groundwater can infiltrate into drilled shafts from perched water or within fractured or more permeable zones within the bedrock. Hence, the Contractor should be prepared for underwater concrete placement using a tremie pipe.

Cobbles and boulders will likely be encountered during drilled shaft installation and result in difficult and slow drilling. Cobbles and boulders can sometimes be excavated by conventional augers, but modified single-helix augers, designed with a taper and sometimes with a calyx bucket mounted on the top of the auger, a.k.a. boulder rooters, are generally more successful at extracting smaller boulders (Brown and others, 2010). However, the extraction of large boulders and rock fragments can cause considerable difficulty and significantly reduce drilling production. Boulders that are solidly embedded can likely be cored, while coring through boulders loosely embedded in soil may be ineffective. The removal of loosely embedded boulders may require breaking the boulder in the hole with percussion methods or a rock breaker tool (or other appropriate methods).

Based on the maximum boulder size of about 3 feet encountered in our borings, we recommend using at least a 42-inch diameter auger to facilitate the removal of large boulders and boulder fragments. It is possible that boulders larger than 3 feet in size may be encountered during construction. The drilled shaft contractor should be prepared to excavate and remove boulders using appropriate methods. As previously stated, the construction plan submitted by the drilled shaft contractor should detail the proposed methods to extract boulders and rock fragments.

Installing temporary casing may be difficult if relatively clean, saturated sand and gravel are encountered during construction. If slurry methods are required to stabilize the excavation, we recommend the use of polymer or mineral slurry in accordance with Section 565.06 (b) of FP-14 (FHWA, 2014). The drilled shaft contractor should not be permitted to use natural or uncontrolled slurry methods during construction. Construction of drilled shafts using wet methods, i.e., slurry, is typically more difficult than constructing shafts using dry methods.

Because a wet excavation cannot be easily visually observed, good construction practices, particularly the recommendations discussed in Section 8.1.2, are critical to constructing shafts that perform adequately. Wet installation methods and specifications should be in accordance with Section 565 of FP-14 (FHWA, 2014).

8.1.2 Drilled Shaft Inspection and Observation

In accordance with Section 565.06 (a)(1) of FP-14 (FHWA, 2014), a geologist or engineer familiar with the subsurface conditions at the project site should observe drilled shaft installation to maintain a boring log of material excavated from the drilled shaft and to document other pertinent information during construction The hole should be cleaned in accordance with Section 565.06 (a)(2) of FP-14 (FHWA, 2014) and observed by the geologist or engineer prior to pouring concrete. The drilling and concreting process should be relatively continuous with minimal stoppage of work between the completion of drilling, cleaning the hole, and the placement of concrete after setting the rebar cage. In addition, because concrete may need to be placed under water using a tremie pipe, quality construction techniques and observations by the geologist or engineer will be essential to drilled shaft installation and to confirm the shafts are installed in accordance with Section 565 of FP-14 (FHWA, 2014).

8.1.3 Concrete Placement

Groundwater inflow into drilled shafts from fractured or more permeable zones within the bedrock is possible. Pumping and/or tremie concrete placement may be required if significant water inflow develops in the bedrock or below the temporary casing, or shafts are constructed using wet methods. Tremie placement should be used if wet methods are used to construct the shafts, the excavation does not meet the criteria established in Section 565.06 (a)(2) of FP-14 (FHWA, 2014) for dry construction methods, or free-fall of the concrete (if the shaft meets the requirements for the dry construction method) measured from the bottom of the chute is more than 25 feet.. The Contractor should be prepared to address these issues.

We recommend concrete be designed and placed in accordance with Section 565.08 of FP-14 (FHWA, 2014). Drilled shaft defects in cased shafts are frequently the result of inadequate head of concrete, particularly when combined with marginal or low slump concrete.

If a truck-mounted pump is used to tremie concrete, pull-out of the pipe can occur if a pressure surge causes upward boom movement. Adequate methods should be established to measure and confirm that minimum head requirements are met throughout the concrete placement process.

8.1.4 Cross-Hole Sonic Logging

As recommended in Sections 565.07 and 565.09 of FP-14 (FHWA, 2014), Crosshole Sonic Logging (CSL) should be used to evaluate the integrity of all production drilled shafts.

CSL is a non-destructive testing method that requires steel tubes to be installed in the drilled shaft and tied to the rebar cage. The tubes are attached to the interior of the rebar cage and then the cage is lowered into the hole and the concrete is placed. After the concrete has cured, a sound source and receiver are lowered, maintaining a consistent elevation between source and sensor. A signal generator generates a sonic pulse from the emitter which is recorded by the sensor. Relative energy, waveform, and differential time are recorded and logged. This procedure is repeated at regular intervals throughout the shaft. By comparing the graphs from the various combinations of access tubes, a qualitative idea of the soundness of the concrete throughout the drilled shaft can be interpreted.

8.2 Site Preparation

We recommend that brush and other vegetation be cleared, and roots and stumps be removed from all areas to be graded. All surface and subsurface structures associated with current development of the site, including foundations, pavements, utility poles, fence poles, underground utilities and other deleterious material, should also be removed. Any existing surficial topsoil and soil containing organics should be stripped and removed from all areas. The depth of this removal is anticipated to vary, but generally be relatively thin. Topsoil and organic-rich soils are not considered suitable for reuse as fill and should be removed from the site or stockpiled for reuse in re-vegetation.

All clearing and grubbing, and removal of structures and obstructions, should be completed in accordance with Sections 201, 202, and 203 of FP-14 (FHWA, 2014).

8.3 Temporary Excavations and Support

The safe slope for the excavation of subsurface materials depends on many factors, including:

(1) the presence of groundwater; (2) the type, density, and shear strength of the subsurface materials; (3) the depth of excavation; (4) the presence of adjacent facilities; (5) surcharge loading adjacent to the excavation (including excavated material, existing dead or live loads, and construction equipment); and (6) time of construction. Considering these factors, unshored, temporary excavation slopes may be possible in areas around the site. However, we recommend that unshored, temporary excavation slopes only be utilized above the groundwater level (either current level or lowered by dewatering). Constructing excavation slopes below the groundwater level may cause slope instability due to the seepage of groundwater into the excavation.

To the extent that temporary excavation slopes are possible, such slopes are anticipated to be relatively flat because of the presence of the granular fill and alluvium material. For cost estimating and planning purposes only, we recommend assuming temporary excavations above groundwater level are sloped at 1.5H:1V, consistent with OSHA Type C soils. Temporary excavations should be in accordance with Section 208.03 of FP-14 (FHWA, 2014).

8.4 Earthwork and Grading

Earthwork, including embankment construction, placement of fill, and subgrade preparation, should conform to the requirements provided in Division 200 of FP-14 (FHWA, 2014).

8.4.1 Excavation

The fill and native soil can likely be excavated using conventional excavating equipment, such as a rubber-tired backhoe or tracked hydraulic excavator. To reduce disturbance of subgrade soils, we recommend that at least the last foot of excavation be completed with an excavating bucket that has a flat plate over the teeth.

8.4.2 Subgrade Preparation and Proof Rolling

All wall and embankment subgrades (including areas requiring excavation) should be stripped, scarified to a depth of at least 6 inches, moisture conditioned, and compacted to a dense and unyielding condition and to the requirements described in Section 204 and 208 of FP-14 (FHWA, 2014). Additionally, subgrades should be protected from drying or wetting in excess of what is required to achieve the specified compaction requirements.

The compacted surface should be proof-rolled with pneumatic tire equipment with a minimum axle load of 18 kips per axle, or equivalent. Any areas that are identified as being loose, soft, or yielding during proof-rolling should be compacted in place, removed and reconditioned, or replaced with Structural Backfill. Care should be taken during proof-rolling and subgrade preparation to avoid disturbing subgrade soils and supporting soils that will remain in place, as they can rut and pump under repeated construction traffic. The final subgrade surface should be sloped to promote positive drainage and kept free of water at all times.

Leaving the subgrade elevation high until final grading begins is a means to reduce the potential for disturbance to the final subgrade materials.

8.4.3 Fill Placement and Compaction

Structural Backfill conforming to FP-14 (FHWA, 2014) should be used to backfill zones behind retaining walls. Fill materials should be placed in horizontal lifts, with the loose lift thickness not to exceed 8 inches for heavy equipment compactors and 4 inches for hand-operated compactors. Thinner lifts may be required, depending on the contractor’s equipment.

Compaction of backfill adjacent to walls can result in higher lateral earth pressures against the wall. Therefore, we recommend small equipment that is hand-operated be used within a line extending upward from the base of the structure at 0.5H:1V, to a maximum horizontal distance of 5 feet from the face of the wall.

All fill material should be placed in horizontal lifts and be compacted to a dense and unyielding condition. Granular soils (AASHTO classification A-1, A-3, A-2-4, and A-2-5 soils) should be moisture treated to within 2 percent of optimum moisture content and compacted to at least 95 percent of the maximum dry density, as determined by AASHTO T99 (per Section 208 of FP-14).

9.0 PLAN REVIEW AND CONSTRUCTION OBSERVATION

We recommend that we be retained to review the geotechnical aspects of the plans and specifications prior to bidding the work to determine that they are in accordance with our recommendations.

Geotechnical design recommendations are developed from a limited number of explorations and tests. Therefore, recommendations may need to be adjusted in the field. To this end, we recommend that a construction observation and monitoring program be implemented for the project and that Shannon & Wilson be retained to monitor the geotechnical aspects of construction, particularly the installation of drilled shafts for the bridge foundations, wall foundations, excavations, and fill placement. This monitoring would allow us to confirm that conditions encountered are consistent with those indicated by the explorations and provide recommendations should conditions be revealed during construction that are different from those anticipated.

10.0 LIMITATIONS

The analyses, conclusions, and recommendations presented in this revised report are based on our understanding of the project and a limited number of subsurface explorations and laboratory test results. We assume that these explorations are representative of the subsurface conditions beneath the site; that is, the subsurface conditions everywhere are not significantly different from those disclosed by the explorations.

This revised report was prepared for the exclusive use of AECOM and the Central Federal Lands Highway Division for the use in design of the Red Dirt Bridge Replacement. It should be made available to prospective contractors and/or the Contractor for information on factual data only, and not as warranty of subsurface conditions, such as those interpreted from the exploration logs and presented in the discussion of subsurface conditions included in this revised report.

Unanticipated soil conditions are commonly encountered and cannot be fully determined by a limited boring and testing program. Such unexpected conditions frequently require that additional expenditures be made to attain a properly constructed project. Therefore, some contingency fund is recommended to accommodate such potential extra costs.

Within the limitations of scope, schedule and budget, the analyses, conclusions and recommendations presented in this revised report were prepared in accordance with generally accepted professional geotechnical and geological principles and practice in this area at the time this revised report was prepared. We make no other warranty, either express or implied.

The scope of our services did not include an evaluation regarding the presence or absence of hazardous or toxic materials in the soil, surface water, groundwater, or air, on or below or around this site. If such contamination exists, it would not be possible to determine it within this limited scope of work.

11.0 REFERENCES

American Association of State Highway and Transportation Officials, 2009, AASHTO LRFD bridge design specifications, Customary U.S. units, 4th edition: Washington, D.C., American Association of State Highway and Transportation Officials.

American Association of State Highway and Transportation Officials, 2014, AASHTO LRFD bridge design specifications, Customary U.S. units, 7th edition: Washington, D.C., American Association of State Highway and Transportation Officials.

American Concrete Institute (ACI), 2011, Building Code Requirements for Structural Concrete and Commentary, Farmington Hills, Michigan, ACI 318-11.

Brown, D.A, Turner, J.P, Castelli, R.J., 2010, Drilled Shafts: Construction Procedures and LRFD Design Methods: NHI Course No. 132014, Geotechnical Engineering Circular No. 10.

Washington, D.C., Report No. FHWA-NHI-10-016.

Ensoft, Inc., 2012, LPILE Plus 2012, A program for analyzing stress and deformation of individual piles or drilled shafts under lateral load, Austin, Texas.

Kirkham, R.M., and Scott, R.B., 2002, Introduction to late Cenozoic evaporite tectonism and volcanism in west-central Colorado, in Kirkham, R.M., Scott, R.B., and Judkins, T.W., eds., Late Cenozoic Evaporite Tectonism and Volcanism in West-Central Colorado:

Geological Society of America Special Paper 366, p. 1-15.

Roberge, P.R., 1999, Handbook of Corrosion Engineering, McGraw-Hill.

Tweto, Ogden, Moench, R.H., and Reed, J.C., 1978, Geologic map of the Leadville 1 degree x 2 degrees quadrangle, northwestern Colorado: U.S. Geological Survey, Miscellaneous Investigations Series Map I-999, scale 1:250,000

U.S. DOT and FHWA, 2014, Standard Specifications for Construction of Roads and Bridges on Federal Highway Projects, FP-14 English Units: U.S. Department of Transportation and Federal Highway Administration, available from http://flh.fhwa.dot.gov/resources/pse/specs/.

U.S. Geological Survey (USGS), 2014, U.S. Seismic Design Maps Web Application: available:

http://geohazards.usgs.gov/designmaps/us/application.php.

White, J.L.,2012, Colorado Map of Potential Evaporite Dissolution and Evaporite Karst Subsidence Hazards (map-discussion booklet accompanies map): Colorado Geological Survey, Department of Natural Resources, Denver, Colorado, scale 1:500,000.

http://ngmdb.usgs.gov/Prodesc/proddesc_9855.htm http://ngmdb.usgs.gov/Prodesc/proddesc_9855.htm http://flh.fhwa.dot.gov/resources/pse/specs/ http://geohazards.usgs.gov/designmaps/us/application.php

Widmann, B.L., compiler, 1997, Fault number 2302, Frontal fault, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 03/12/2015 02:48 PM.

Widmann, B.L., compiler, 1999, Fault number 2336, Killarney faults, in Quaternary fault and fold database of the United States: U.S. Geological Survey website, http://earthquakes.usgs.gov/hazards/qfaults, accessed 03/12/2015 03:14 PM.

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SEISMIC PARAMETERS…