Cottonwood Geotechnical Report_Amend0001.pdf

1971 West 12th Avenue | Denver, Colorado 80204 | Phone: 303-825-0777 | Fax: 303-825-4252

GEOTECHNICAL INVESTIGATION

SOUTH COTTONWOOD CREEK BRIDGE

FOREST SERVICE TASK ORDER 3-2014

NFSR 344 OVER SOUTH COTTONWOOD CREEK

SECTION 36, TOWNSHIP 14 SOUTH

RANGE 80 WEST

SAN ISABEL NATIONAL FOREST

CHAFFEE COUNTY, COLORADO

Prepared For:

HDR ENGINEERING, INC.

9781 South Meridian Boulevard

Suite 400 Englewood, Colorado 80112

Attention: Rob Thompson

Project No. DN47,498-125

October 29, 2014

Revised December 29, 2015

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TABLE OF CONTENTS

SCOPE

SUMMARY OF CONCLUSIONS

SITE CONDITIONS

PROPOSED CONSTRUCTION

SUBSURFACE CONDITIONS

SITE DEVELOPMENT

FOUNDATIONS

LATERAL LOADS ON ABUTMENT AND WING WALLS

CONCRETE

LIMITATIONS

FIG. 1 – LOCATIONS OF EXPLORATORY BORINGS

FIG. 2 – SUMMARY LOGS OF EXPLORATORY BORINGS

FIG. 3 – GRADATION TEST RESULTS

TABLE I – SUMMARY OF LABORATORY TEST RESULTS

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SCOPE

This report presents the results of our Geotechnical Investigation for the pro-posed South Cottonwood Creek Bridge replacement on National Forest System

Road 344 just east of the Cottonwood Lake spillway about 8.9 miles southwest of

Buena Vista, Colorado. The purpose of this investigation was to evaluate subsurface conditions at the bridge site and to provide geotechnical design and construction criteria. Our report includes descriptions of the proposed construction, subsoil conditions found in our borings, engineering properties of the soils, and recom-mended design criteria. The report was prepared from data developed during our field and laboratory investigations, engineering analysis of the field and laboratory data and our experience with similar projects. The scope was described in our

Master Service Agreement (HDR Master Agreement No. MSA2011-022, Task Order

005, Agreement 0000013063) dated August 29, 2014.

Our recommendations are based on the proposed construction as discussed in this report. Revision of the proposed construction could affect our recommenda-tions. We should be contacted to review our recommendations if plans change. A summary of our findings and conclusions is presented below. Detailed geotechnical recommendations for design and construction are presented in the report.

SUMMARY OF CONCLUSIONS

1. Our borings encountered dense, bouldery sand, gravel and cobble to depths of 7 and 11 feet below the existing ground surface at which point drill refusal was experienced after multiple attempts at each loca-tion. Groundwater was not observed at the time of drilling in our bor-ings.

2. The bridge abutments and wing walls can be founded on footing foun-dations constructed on the sand, gravel and cobble soil. Design criteria are presented in the report.

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SITE CONDITIONS

National Forest Service Road 344 crosses South Cottonwood Creek at a location about 8.9 miles southwest of Buena Vista, Colorado in Section 36, Town-ship 14 South, Range 80 West, in the San Isabel National Forest just east of Cot-tonwood Lake (Fig. 1). The existing bridge is timber on timber abutments with a

W33x130 steel beam supported by two piers at mid-span. The span is about 30 feet and is on a 19 degree skew. The existing bridge is functionally obsolete due to the amount of traffic and bridge travel width. The site is about 75 feet east of the Cot-tonwood Lake spillway where South Cottonwood Creek resumes. The bridge ap-proaches are about 5 feet above the creek. The abutment areas and creek bottom expose silty sand, gravel and cobble with boulders up to about 3 feet long. The area is vegetated with trees, brush and grasses.

Photo 1 – West side of bridge looking east

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Slopes are moderate to steep down to the river from the wing walls and ap-proaches. The presence of frequent large boulders in the vicinity of the bridge implies the alluvium in this valley resists scour reasonably well.

Photo 2 – East side of bridge looking west (midspan beam)

PROPOSED CONSTRUCTION

We understand the new bridge will be about 50 to 60 feet long and 14 feet

(single lane clear width) to 24 feet (double lane clear width) wide. We understand design alternatives that will be considered are composite steel/pre-stress girders with cast-in-place concrete deck or pre-stressed concrete girders with the top flange used as the deck (slab or box girders). Bridge foundations are expected to be

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SUBSURFACE CONDITIONS

The subsoil conditions were explored by drilling two exploratory borings, one near each of the proposed abutments (Fig. 1). The borings were advanced using 4-inch diameter solid stem continuous-flight auger and a truck-mounted CME 45N drill rig. TH-1 was located about 15 feet northwest of the northeast corner of the bridge and TH-2 was located about 20 feet southeast of the southeast abutment corner.

Both borings were within about a few inches of the elevation of the bridge deck.

Borings were surveyed to the top of the bridge’s southeast abutment which was assigned as a temporary benchmark (assumed elevation = 100 feet). Practical auger drilling refusal occurred at a depth of 11 feet in TH-1 and 7 feet in TH-2. A number of attempts were made to advance past these refusal depths with no success. Drilling was supervised by our field representative who logged the soils found in the borings and obtained samples. Graphical logs of the borings, including results of field pene-tration resistance tests and some laboratory test data are shown on Fig. 2.

Soil samples obtained during drilling were returned to our laboratory and vis-ually examined by the geotechnical engineer for this project. Laboratory testing was then assigned, and included natural moisture content and dry density, and sieve analysis. Results of laboratory tests are presented on Fig. 3 and summarized in

Table I.

Subsoils encountered in boring TH-1 consisted of about 7 feet of silty to sandy gravel with cobbles and boulders. TH-2 encountered about 11 feet of silty to sandy gravel with cobbles and boulders. Groundwater was not encountered during drilling in our borings. The gravel was medium dense to very dense based on pene-tration resistance and our judgment. Bedrock was not encountered. Samples of the

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SITE DEVELOPMENT

The soils penetrated by our borings can likely be excavated with heavy-duty excavation equipment. Practical drill refusal was recorded at 7 and 11 feet in our borings. Cobbles and boulders will be encountered and removal may require extra effort. We recommend the owner and the contractor become familiar with applicable local, state and federal safety regulations, including the current Occupational Safety and Health Administration (OSHA) Excavation and Trench Safety Standards. We believe the sandy gravel will classify as Type C. Type C soils require a maximum slope of 1.5:1 for temporary excavations. Excavations below groundwater will likely slough and have to be laid back, possibly to 3:1 or shallower.

Diversion of South Cottonwood Creek or possibly installation of cofferdams may be required for foundation construction. Dewatering sumps and pumps may be required during construction to remove water and construct foundations on a dry surface. If pumps are required, they will need to be set at least 1.5 feet below the excavation floor. Water should be pumped down through the foundation soils. Water flowing up through the soils will weaken the soils. Multiple sumps may be necessary.

Fills may be required on both sides of the bridge. All vegetation and organic topsoil should be removed prior to fill placement. All backfill should meet “Standard

Specifications for Construction of Roads and Bridges on Federal Highway

Projects USDOT-FHA FP-14”.Fill can consist of on-site sand and gravel soils that meet the requirements of Section 704.04 Structural Backfill. Native cobbly sand and gravel soils will probably have to be screened to remove oversized particles. Clay or organic soils should not be used as fill. Imported fill (if required) should contain 100 percent passing the 3-inch sieve, less than 15 percent passing the No. 200 sieve, and a plasticity index less than 6.

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Backfill should be placed in loose lifts not exceeding 6 inches thick, moisture conditioned to within 3 percent of optimum and compacted to at least 95 percent of

AASHTO T99 (Standard Proctor). Fill will settle; any improvements placed over fill should be designed for movement. Timing of this settlement is very difficult to antici-pate because of the large number of variables. Properly compacted fill may settle about ½ to 1 percent of the fill height.

FOUNDATIONS

The soils encountered at anticipated foundation levels consist of silty sand and gravel with cobbles and boulders. In our opinion, the best foundation alternative is to construct culverts, bridge abutments or wing walls on spread footing founda-tions. We believe potential settlement should be within allowable tolerance for the type of construction planned. Deep foundations such as drilled piers or driven piles were considered but eliminated because the cobbles and boulders would make them difficult to install.

It is likely foundation excavations will expose cobble and boulders larger than

6 inches. Soft or organic clayey layers may also be present but are not expected. In order to mitigate concern for point loading and uneven foundation support, we recommend the foundation excavations be made to grade to establish whether soft or oversized materials are present and whether stabilization will be necessary

Foundation excavation and preparation should be made per FP-14. Design criteria for footings are presented below:

1. Based on an estimated angle of internal friction for cobbly sand and gravel of 35 degrees, we calculated an ultimate bearing capacity of 17,000 psf for footings buried 3 feet. Using LRFD criteria, footings can be designed using an ultimate bearing capacity of 17,000 psf and a service load resistance factor of 0.45. Compacted sand, gravel and cobble soil has a bulk density of about 130 pcf.

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2. If loose or soft soils are exposed in the excavations, the bottom of the excavation can be stabilized by crowding angular rock particles (1 to 5 inches in size) with heavy equipment, until a firm subgrade is achieved. Organic soils such as peat should be removed, if present, and replaced with crowded angular rock.

3. Footings should have a minimum width of at least 24 inches. Larger sizes may be required, depending upon the loads and structural sys-tem used.

4. Footings must be protected from frost action. Normally 3.5 feet of frost cover is assumed in this area. The footings should also extend below depths of anticipated scour unless otherwise protected from erosion and undermining during peak channel flows.

6. The completed foundation excavation should be observed by a repre-sentative of our firm to verify subsurface conditions are as anticipated from our borings and suitable to support structures.

LATERAL LOADS ON ABUTMENT AND WING WALLS

The abutment and wing walls will be subjected to lateral earth pressure which is dependent on the height of wall and type and configuration of backfill. For walls which are restrained from rotating slightly, we recommend the walls be designed to resist the “at rest” earth pressure plus hydrostatic pressure and structural or traffic surcharges. Walls free to rotate slightly should be designed for the “active” condition.

Site soils and Structural Backfill allow use of an equivalent fluid density of 35 pcf for active conditions and 55 pcf for at-rest conditions. Compacted sand, gravel and cobble soil has a bulk density of about 130 pcf. A coefficient of friction of 0.70 for concrete or timber sliding on site soils with a resistance factor of 0.8 can be used to calculate lateral resistance. A passive resistance calculated using 450 pcf equivalent fluid density (EFD) and a resistance factor of 0.5 can be assumed for native soil or similar fill provided that it will not be removed in the future.

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The surcharges referred to above can be from traffic, sloping backfill, etc.

Hydrostatic pressure could be from water soaking down through backfill after flood stage. The hydrostatic pressure can be reduced by providing a drain and/or weep holes behind the abutment wing walls. Wall backfill should be placed in 6-inch maximum loose lifts, moisture conditioned to within 3 percent of optimum and com-pacted to at least 95 percent of standard Proctor maximum dry density (AASHTO T

99, per the requirements of FP-14.

CONCRETE

Concrete in contact with soil can be subject to sulfate attack. Our experience with projects in this region suggests water-soluble sulfate concentrations are likely less than 0.1 percent. Sulfate concentrations less than 0.1 percent indicate Class 0 exposure to sulfate attack for concrete in contact with the subsoils, according to the

American Concrete Institute (ACI). For this level of sulfate concentration, ACI indi-cates no special provisions need to taken for the cement used for concrete in con-tact with the subsoils. In our experience, superficial damage may occur to the ex-posed surfaces of highly permeable concrete, even though sulfate levels are rela-tively low. To control this risk and to resist freeze-thaw deterioration, the water to cementitious material ratio should not exceed 0.50 for concrete in contact with soils that are likely to stay moist due to surface drainage or high water tables. Concrete should be air entrained.

LIMITATIONS

Our borings were located to obtain information relating to the subsurface conditions at the site. Variations in the subsurface conditions not indicated by the borings will occur. If subsurface conditions vary from those found in our borings, we should be contacted to review our opinions and recommendations given in this report. We should observe foundation excavations to verify soils are as anticipated from our borings. We should observe and test fill placement.

TABLE I

SUMMARY OF LABORATORY TEST RESULTS

PASSING

BORING DEPTH MOISTURE DRY NO. 200 SOIL TYPE

CONTENT DENSITY SIEVE

(ft) (%) (pcf) (%)

TH-1 0-5 3.6 14 GRAVEL,SILTY (GM)

TH-1 4 4.8 123 12 GRAVEL,SILTY (GM)

TH-2 0-5 2.1 11 GRAVEL,SILTY (GM)

TH-2 4 4.0 135 11 GRAVEL,SILTY (GM)

HDR ENGINEERING, INC.

SOUTH COTTONWOOD CREEK BRIDGE

PROJECT NO. DN47,498-125

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Upstream Elevation

Downstream Elevation

Looking Downstream

Looking Upstream

Looking Southeast

Looking Northwest

Northwest Driveways

Sharp Curve at Southeast Approach

Location of Upstream Bypass – Looking Northwest

Location of Upstream Bypass – Looking Southeast

Weight Limit Sign and Associated Bridge on the Access Road Approaching the Site