11_-_geotech_report.pdf

August 14, 2012

HDR Engineering, Inc.

303 East 17th Avenue, Suite 700 Denver, Colorado 80203-1256

Attention: Rob Thompson, P.E.

Subject: Geotechnical Investigation USDA Forest Service

Contract No. AG-82X9-C-11-0004, Task Order 06-2011 Aquatic Organism Passage (AOP) Culvert Replacement Red Sandstone-Muddy Pass Forest Service Road 700.1 White River National Forest Eagle County, Colorado Project No. DN45,814-125

This report presents the results of our Geotechnical Investigation for the proposed Aquatic Organism Passage (AOP) Culvert Replacement on National Forest Service Road (NFSR) 700.1 about 5 miles northeast of Wolcott, Colorado (Fig. 1). The purpose of this investigation was to evaluate the surficial geology and likely ground conditions at the site and provide geotechnical design and construction criteria. Our report includes descriptions of the proposed construction, estimates of subsoil and ground water conditions, engineering properties of the soils, and recommended design criteria. This report is based on conditions exposed during a reconnaissance visit by the author, and our experience with similar conditions and projects. The scope was described in HDR’s Task Order 2, Task Order Agreement No. 0000107800 dated October 13, 2011.

SITE DESCRIPTION AND GEOLOGY

National Forest Service Road 700.1 crosses Muddy Creek about 1.64 miles east of Eagle County Road 131 in the White River National Forest (Fig. 1), at

39.745863 north and -106.660245 degrees west, at an elevation of about 7320 feet.

The existing culvert consists of two corrugated metal pipes. The roadway is about 14-20 feet wide at the crossing. The existing culverts have about 2 feet of soil cover. Our site visit was conducted by David A. Glater, P.E., C.P.G. on October 22, 2011.

The project area is a mountain stream surrounded by scattered to thick brush. The stream gradient is estimated at about 2 to 3 percent. The existing culvert embankments are moderately sloping, generally in the range of about 2H:1V to 3:1. The creek bed exposes sandy clay with scattered to rare cobbles.

Rock outcrops were not observed in the culvert area. Nearby hillslopes expose weathered Pierre Shale bedrock.

HDR ENGINEERING, INC.

AOP CULVERT REPLACEMENT –RED SANDSTONE – MUDDY PASS

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Geologic mapping by Lidke (Geologic Map of the Wolcott Quadrangle, USGS Map I-2656, dated 1998) shows Cretaceous-age sedimentary shale from the Pierre Shale formation is beneath the site. We believe the bedrock is at least 5 feet below the stream bed, based on the valley geometry. The bedrock is covered by sandy clay alluvium. Ground water levels close to the culvert are likely to be about coincident with the stream bed grade.

We performed a sieve analysis of material from the culvert embankment.

The sample had 16 percent gravel retained on the No. 4 Sieve and 60 percent silt and clay fines passing the No. 200 Sieve. Atterberg limits testing indicated a liquid limit of 28 percent and a plasticity index of 10 for the material. Water-soluble sulfates were non-detect at a level of 0.01 percent. The native material is low plasticity silty and sandy clay. This material should be considered to have moderate to high erosion potential.

PROPOSED CONSTRUCTION

We understand a replacement culvert is planned at or near the existing location, to enhance ability for aquatic organisms to traverse the creek below the road. The new culvert or bridge may consist of a counter-sunk culvert, an open bottom arch, 3-side precast concrete bridge, or precast concrete abutments and wing walls with prestressed concrete or treated timber slab superstructure. If selected, the open bottom or 3-sided bridge will be constructed on a stem wall and spread footing foundation. The structures will be placed consistent with a reference reach longitudinal profile to prevent excessive aggradation or headcutting. The use of riprap will be minimized and generally used only to protect the slopes of the fill around the structure ends and footings. Only minor alignment or elevation changes of the channel are expected. The finish grade of the road over the structure may be raised or lowered depending upon the selected structure type. Foundation loads are anticipated to be light to moderate.

DESIGN AND CONSTRUCTION CONSIDERATIONS

The site soils are expected to be low-plasticity sandy clay with scattered cobbles. We believe this material can generally be excavated with heavy-duty excavation equipment. We recommend the owner and 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 soils will classify as Type B soil. Based on OSHA regulations, maximum soil slope inclinations of 1:1 (horizontal to vertical) are required for temporary excavations in dry conditions. Excavations in soil below ground water will likely slough and have to be laid back, possibly to 6:1 or shallower.

Diversion of Muddy Creek or possibly installation of cofferdams may be required for bridge/culvert construction. Dewatering sumps and pumps will likely be required during construction to remove water and construct foundations on a reasonably dry, stable 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

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Multiple sumps may be necessary.

Fills may be required on both sides of the culvert or bridge. Any areas that will support new construction or improvements should have all vegetation and organic topsoil removed prior to fill placement. Fill can consist of on-site silty sand and gravel soils. Clay or organic soils should not be used as fill. Imported fill (if required) should contain between less than 35 percent passing the No. 200 sieve, a plasticity index less than 15, and a liquid limit less than 30 percent. Soil lumps, rocks or boulders larger than 6 inches in diameter should be broken up or discarded prior to placement. Larger materials may be suitable for rip rap armor.

The metamorphic gneiss boulders appear relatively durable from a standpoint of freeze-thaw or wet-dry conditions.

We recommend fill be placed in loose lifts not exceeding 8 inches, moisture conditioned and properly compacted. Fill should be moisture conditioned to within 2 percent of optimum moisture content. Fill placed below structures should be compacted to at least 95 percent standard Proctor maximum dry density (ASTM D 698). Backfill for walls and should be compacted to at least 95 percent standard Proctor maximum dry density (ASTM D 698). Fill will settle; any improvements placed over fill should be designed for movement. Timing of this settlement is very difficult to anticipate because of the large number of variables.

Properly compacted fill may settle about 1 percent of the height.

We believe the site is Class B for seismic design purposes. The soils at anticipated foundation levels may consist of sandy clay with scattered cobbles.

Spread footing foundations are suitable to support culvert or bridge abutments and wing walls. We believe potential settlement should be less than an inch, within allowable tolerance for the type of construction planned.

It is likely foundation excavations will expose cobbles and boulders larger than 6 inches. In order to mitigate concern for point loading and uneven foundation support, where foundation excavations expose oversized materials, we recommend additional cut be made to allow for one or two layers of Geocel mat filled with 2-inch minus sand and gravel. Bedrock may be present in foundation excavations. If so, the foundation may be constructed directly on the rock. The footings can be constructed on undisturbed natural soils if they are reasonably dry, stable, and do not expose oversized materials. If soft soils are encountered or soils are significantly disturbed during excavation or dewatering, they should be removed and replaced or stabilized as discussed below. Over-excavation, if required should extend laterally at least one foot outside footing lines. If desired, crushed rock can be inundated with grout to cement the particles to better control erosion/scour and provide a “mud mat” for construction of footings.

Footings should have adequate depth to protect from scour and freezing.

Foundations should be at least 2 feet below river bed level. A detailed scour analysis may result in a deeper or shallower recommendation. Foundation design criteria are presented below:

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1. Footings can be designed for a maximum allowable soil pressure of 2,000 psf.

2. If loose or soft soils are exposed in the excavations, the bottom of the excavation can be stabilized by using crushed rock replacement to a depth of about 1 foot. The rock should be rolled with heavy equipment until a firm substrate is achieved. Organic soils such as peat should be removed, if present in excavations, 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 system 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.

5. The completed foundation excavation should be observed by a representative of our firm to verify subsurface conditions are as anticipated from our borings.

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. A coefficient of friction of 0.35 for concrete or timber sliding on site soils can be used to calculate lateral resistance. A passive resistance calculated using 250 pcf equivalent fluid density (EFD) can be assumed for native soil or similar fill provided that it will not be removed in the future.

These values have not been factored. The structural engineer should apply appropriate factors of safety in design.

Equivalent fluid densities which could be used to calculate the lateral active earth pressure on abutment and wing walls are as follows:

Type of Backfill Equivalent Fluid

Density for “At-Rest” Condition (pcf)

Equivalent Fluid Density For “Active”

Condition (pcf)

Imported clean medium to coarse sand and gravel containing less than 5 percent fines. 50 32

On-site sandy clay 75 55

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If the lower equivalent fluid densities are used in the design, the specified granular backfill must occupy at least the zone between the wing wall and a plane rising at a slope of 1:1 (horizontal:vertical) from the inboard toe of the footing.

Surcharges or hydrostatic pressure should be considered.

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 8-inch maximum loose lifts, moisture conditioned to within 2 percent of optimum and compacted to at least 95 percent of standard Proctor maximum dry density (ASTM D 698). A dry density of 120 pcf, saturated density of 135 pcf and a friction angle of 24 degrees can be used to calculate the appropriate lateral earth pressures in the design of the abutments.

Dynamic loads from compactive effort should be considered by the structural engineer. Placement of fill should be observed and tested by a representative of our firm during construction.

Slopes for fill embankments around the culvert or bridge should ideally be 3H:1V or shallower to allow for easier re-vegetation and reduce erosion compared to slopes at angle of repose. Finished slopes of 2H:1V are acceptable where sufficient materials or space does not allow use of shallower slopes.

CONCRETE

Concrete in contact with soil can be subject to sulfate attack. Testing implies the site sandy clay contains insignificant water-soluble sulfate levels.

Normal Type I/II Concrete is acceptable to use on this site.

In our experience, superficial damage may occur to the exposed surfaces of highly permeable concrete. To control this risk and to resist freeze-thaw deterioration, the water-to-cementitious materials 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 have a total entrained air content of at least

4.5 percent and a maximum of 7.5 percent for freeze-thaw durability.

LIMITATIONS

Our field reconnaissance was conducted to obtain information relating to the geology and likely subsurface conditions at the site. Variations in the subsurface conditions may occur. If subsurface conditions vary from those assumed, 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. We should also observe and test fill placement.

We believe this investigation was conducted in a manner consistent with that level of care and skill ordinarily used by geologists and geotechnical engineers practicing in this area at this time. No warranty, expressed or implied, is