J-6_Appendix_A_-_Geotechnical_Evaluation_2014_0828.pdf

August 28, 2014 Project 14-3236G

Mr. Bill Lloyd, PE Great West Engineering, Inc.

Via Email: blloyd@greatwesteng.com

Dear Mr. Lloyd:

Re: Engineering Reconnaissance and Geotechnical Recommendations, Proposed Boat Docks at

Meriwether and Coulter Campgrounds, Helena National Forest, Lewis and Clark County, Montana

The engineering reconnaissance to provide geotechnical recommendations related to the proposed boat docks referenced above has been completed. These services were provided in general accordance with our Subconsultant Agreement for Professional Services dated July 9, 2014.

Project Overview

The Helena National Forest is planning on replacing the existing boat docks at the Meriwether Day Use and Coulter Campground recreational sites on Holter Reservoir. It is our understanding the existing boat docks are hauled by barge from each campground to the Gates of the Mountains facility each fall to prevent damage from ice and winter weather. New boat docks are desired that can either remain in place or have a local area for shore storage. The general layout for the proposed Meriwether and Coulter docks are shown on the attached sketches. Ramps will be provided from the shore to a lateral dock, then multiple boat slips will extend out into the lake. At the Meriwether dock, eight boat slips along with a larger boat slip for the Gates of the Mountains tour boat are anticipated. At the Coulter dock, six boat slips are anticipated. The Meriwether dock will be constructed south of the existing dock while the Coulter dock will be constructed at relatively the same location as the existing dock. If possible, at the Meriwether dock, it is also desired to have a nearby shore-line ramp/dock for the Helena National Forest barge to load and unload equipment.

Field Procedures

The geotechnical reconnaissance was performed by Mr. Gregory T. Staffileno, PE, senior geotechnical engineer with our firm, and Mr. Chad C. Binstock, engineering assistant with our firm, on July 14 and 15, 2014. Mr. Reid Stovall and Mr. Gabe Witham of the Helena National Forest along with Mr. Bill Lloyd of Great West Engineering initially accompanied us during the reconnaissance. The existing boat docks as well as nearby features at each facility were toured.

After the initial meetings, SK Geotechnical personnel performed the reconnaissance of each area.

Observations were made of the lake bed and shore line geologic features. Numerous photographs were taken of each area, some of which are included with this report. To evaluate lake bed conditions, dynamic cone penetrometer (DCP) tests were performed at select locations. The DCP was developed by our firm and the results provide us an indication of lake bed soil conditions. The DCP consists of a 1.2-inch diameter cone mounted on 3/4-inch diameter rod. A 10-pound hammer (weight) falling 24 inches is used to drive the cone into the lake bed. The blows required to drive the cone each 6-inch increment of

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Great West Engineering, Inc. August 28, 2014 Project 14-3236G Page 2 advance or the depth driven in 50 blows was recorded. The results of our reconnaissance are discussed in more detail later in this report.

Geology

The Geologic Map of the Canyon Ferry Dam 30' by 60' Quadrangle, West-Central Montana, by Reynolds and Brandt, 2005, was reviewed prior to the reconnaissance. A portion of this map is attached to this letter. Both docks are situated along valleys emanating from the mountains to the east. Native streams emanate from both of these valleys. As can be seen on the attached Meriwether Dock aerial map, a significant gravel debris flow occurred south of the docks along the stream. It is our understanding that this gravel debris flow occurred in about 2012 after numerous fires in the mountains reduced natural vegetation. During periods of prolonged heavy rain, flooding occurred into the valley which created the debris flow, extending out into the lake. Therefore, we anticipate that the surficial geology in both areas is primarily alluvial sands and gravels associated with the valleys. According to the Geologic Map, the native bedrock along the valley walls is Lodgepole limestone (Ml) in light blue or Mission Canyon limestone (Mm) in darker blue. A bedrock outcrop was observed just east of the proposed Meriwether dock as shown on the attached sketch. This outcrop is also shown in attached photograph 5. The outcrop consisted of limestone, yellowish brown and gray, slightly to moderately fractured, with moderately wide joints. The limestone was judged to be very hard by bedrock standards and fractures were located approximately every 3 inches to 5 feet. The primary bedding surface was 5 degrees/185 degrees dipping at 15 degrees west (towards the lake).

Results

While standing on the docks, we could observe the lake bed materials. In general, the lake bed appeared to consist primarily of gravels, sands, cobbles, and boulders. At the Meriwether dock, we judged the presence of a cobble (4 inches to 12 inches in size) every 1 to 2 feet and a boulder (larger than 12 inches) every 3 to 6 feet. At the Coulter dock, we observed a cobble every 1/2 to 1 foot and a boulder every 2 to 3 feet.

Penetrometer tests DCP-1 through DCP-3 were performed at the Meriwether dock, and the results are indicated on the attached sheet. The lake bed was encountered at a depth of 18 inches, 60 inches, and 102 inches below the water level, respectively. DCP results typically ranged from 1 to 54 blows for 6 inches of penetration to depths of about 2 to 6 feet below the lake bed, indicating the alluvial gravels were primarily medium dense to dense, although loose right at the surface of the lake bed. Below these depths, DCP results were 50 for 3 inches and 50 for 4 inches, which is practical refusal for our equipment.

Therefore, below depths of 2 1/2 to 6 feet, the DCP results indicate the gravels are either dense, or more cobbles and boulders are present.

DCP-4 through DCP-7 were performed at the Coulter boat dock at the locations indicated. At DCP-4, 50 blows for only 1 inch of penetration occurred right at the lake bed. At the other locations, DCP results generally ranged from 12 to 50 blows for 6 inches of penetration to depths ranging from 1 1/2 to 3 1/2 feet below the lake bed, indicating medium dense to dense conditions. Below these depths, DCP results ranged from 50 for 2 inches to 50 for 5 inches. Therefore, the DCP results indicate dense or more cobbles and boulders are present right at the lake bed right at or up to 3 1/2 feet below the lake bed.

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Existing retaining walls are currently present at both boat docks. At the Meriwether dock, a two-tiered wood retaining wall has been constructed along the shore line to the existing boat ramp, which will also be present for the proposed boat ramp further south. The retaining wall is shown in attached photographs 2 and 4. Photograph 4 also shows our personnel observing the 40-degree slope above the retaining wall to a height of about 10 feet, which decreases to a 20-degree slope. The slope consisted of silty gravel with sand, American Society for Testing and Materials (ASTM) soil GM. Photographs 6 and 7 show the gravel debris flow, which occurred south of the Meriwether dock. The debris flow extended 10 to 30 yards out into the lake and generally becomes finer-grained closer to the lake. The picnic shelter located about 100 yards from the lake shore was buried by the gravel debris flow, as shown in photograph 7.

Photographs 8 through 11 show the Coulter boat dock. A gabion basket retaining wall has been constructed along this shore line. The slope of the ground (most likely fill) behind the gabion wall was approximately 20 degrees decreasing to about 5 degrees.

Analysis and Recommendations

Limitations

As indicated in our proposal dated January 21, 2014, the analysis and recommendations contained in this report are only based on our observations of exposed surfaces, DCP tests, and engineering experience.

Specific borings along the dock shore line and in the lake have not been performed, primarily due to the areas only being accessible by boat and excessive costs for barge drilling. Supporting the docks will either require installing deep foundations or earth-supported concrete structures. Obtaining subsurface soil, lake bed, and groundwater information for these structures is important to avoid the risk of encountering unforeseen conditions and subsequent change orders. In this case, we were unable to perform soil borings to evaluate subsurface soil, groundwater, and lake bed conditions, and therefore the information should be considered circumspect. We recommend providing contingencies for unforeseen conditions during construction.

Discussion

Because dock systems are subjected to uplift and lateral loads, one of the best types of foundation systems to use is often helical piers. Helical piers are a deep foundation installed a minimum of 12 diameters below the ground surface or lake bed to provide resistance to these forces. Based on our observations, however, significant cobbles and boulders were observed along the lake bed at both locations. We also anticipate cobbles and boulders are buried beneath and behind the existing retaining walls. Helical piers are not capable of being installed through cobbles greater than about 4 to 6 inches in size. Basically, the helical piers torque out or shaft wrap prior to meeting the required embedment depth, resulting in inadequate uplift resistance. Therefore, it is our opinion helical piers are not suitable for supporting either boat dock.

Driven piling are another method of supporting boat docks along shores. Driven piling are larger and stronger than helical piers, however, they require larger equipment and cranes for installation. It is our opinion driven piling are also not feasible for installing deep foundation systems at the docks due to the significant costs needed to transport cranes and piling to each of the sites. Furthermore, subsurface soil information is critical in evaluating driven pile depths and capacities. In this case, subsurface soil

Project 14-3236G Page 4 information is not available, and bedrock could be present at relatively shallow depths. This bedrock could stop the piling at depths shallower than necessary. In other words, we do not know whether or not the piling will work even if costs were not an issue.

Another deep foundation system which may be suitable for the proposed docks is micropiles. Micropiles are basically very small diameter (3 to 10 inches) drilled concrete piers. The advantage of micropiles is the equipment is relatively small, and therefore easier to transport to the site. The micropiles are also installed using drilling methods and advancing casing. Drilling and casing can be advanced through alluvial gravels and sands as well as bedrock to the desired embedment depth. We wish to point out, however, that micropiles require specialized contractors and equipment. When considering the docks are only barge accessible, significant costs to mobilize the equipment to each dock site should be anticipated.

Another approach to consider for the boat docks is concrete structures. Concrete bulk heads can be installed along the shore line to anchor the ramps. The bulk heads can either be precast concrete or cast-in-place. In the lake, concrete deadman anchors can also be placed to support the boat ramps. Cobbles or chains are attached to the deadman anchors to hold the boat ramps relatively in place.

It is therefore our opinion that either micropiling or concrete structures can be used to support the proposed boat ramps. Our recommendations regarding these foundation systems are discussed in more detail below. The actual selection will be made as design progresses, and additional geotechnical analysis and recommendations may be necessary.

Micropiles

Methodology. The micropile alternative was evaluated in general accordance with the Federal Highway Administration, Micropile Design and Construction Guidelines Publication No. FHWA-SA-97-070. We assumed the micropile would utilize Type A grout placement which is gravity placement by tremmie pipe, and the grout would be neat cement grout. We also assumed gravel and sand would be present the full micropile depth and an ultimate grout to ground bond strength of 2.0 kips per square foot was used for our design. For our analysis, we neglected the micropile capacity in the upper 3 feet assuming it would be cased and/or subject to disturbance. We have also assumed the micropile will not be subjected to scour effects.

Type. We recommend installing 6- to 8-inch diameter micropiles. We recommend advancing casing for these micropiles due to the presence of gravels, sands, cobbles, and boulders in the lake bed. The casing will help prevent sloughing during advancement and contain drilling fluids or grout required to stabilize the borehole. It is our opinion the upper 3 feet of casing should be left in place below the lake bed as additional protection of the micropiles upon completion. Less expensive drilled and grouted hollow bars were also considered, but with this method, the grout cannot be contained and would cause contamination of the lake bed.

Depth. Table 1 below summarizes anticipated embedment depths and uplift capacity for 6- and 8-inch diameter micropiles.

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Table 1. Micropile Depths and Capacities

Diameter Minimum Depth below Lake Bed

(feet)

Minimum Grouted Bond

Length (feet)

Allowable Uplift Capacitya

Allowable Bearing

Capacitya

6 inches 11 8 10 kips 10 kips 8 inches 9 6 10 kips 10 kips aIncludes a minimum factor of safety of 2.5 against tensile or compressive failure.

It is our opinion the micropiles can either be installed along the shore line or in the lake bed to support the boat ramps. We wish to point out, however, that site specific subsurface soil borings were not performed and unforeseen conditions or obstructions could be encountered, resulting in delays and/or change orders.

Although sands and gravels were assumed for our soil strength parameters, the contractor should be prepared to drill through boulders and/or bedrock also. The micropile installed below the lake bed will need to be completed below the water surface so careful attention to final grout placement depth will need to be followed. Divers for cutting of casing, pile finishing, and anchor attachments will also be required.

We recommend a minimum of one static tensile load test be performed at the beginning of construction to better evaluate the pile uplift capacity. We recommend providing a contingency for this approach, if selected.

Concrete Structures

Type. As previously indicated, it is our opinion the concrete structures can be utilized to support the boat ramps. A variety of concrete structures could be considered, including rectangular or square masses of concrete, shallow-spread footings with wingwalls or columns, cast-in-place structures, or precast structures.

Depth. Concrete structures along shore lines will be subjected to frost. We recommend providing 5 feet of earth cover over the bottoms of these concrete structures for frost protection.

Anticipated Subgrade. Although no site specific borings were performed, our observations indicate the concrete structures will primarily be supported on gravels, sands, cobbles, and boulders. Directly behind the retaining walls, we anticipate these materials will most likely consist of existing fill associated with the construction of the retaining wall structures. In the lake bottom, we anticipate these materials will be alluvial deposits, although some fill (and possible obstructions) associated with the previous developments could also be encountered.

Bearing Capacity and Settlement. We anticipate that shore line concrete structures will most likely encounter existing fill associated with the wood and gabion retaining walls. We anticipate this existing fill was not well compacted during placement, and some consolidation will likely occur in the future. We recommend designing for 2 to 3 inches of fill consolidation, and consequent concrete structure settlement in the future. Therefore, somewhat flexible connections need to be provided allowing them to settle without adverse effects. To reduce the amount of settlement, we recommend assuming an allowable bearing capacity of 1,500 pounds per square foot (psf) for the existing fill and/or native gravels, sands, cobbles, and boulders.

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Lateral Earth Pressures. Concrete bulk heads can be provided along the shore line to provide lateral support to the boat ramps. A schematic for the Meriwether dock has been prepared and is attached to this letter. As can be seen, the concrete bulk head must be placed behind the existing retaining walls such that the passive pressure failure surface does not impact the existing active pressure failure surface behind the existing retaining wall. Assuming a friction angle of 42 degrees and a moist unit weight of 135 pounds per cubic foot (pcf), the active failure surface is 66 degrees from horizontal and the passive failure surface is 24 degrees, as shown. Therefore, the bulk heads should be placed at least 8 feet behind the existing retaining walls. In addition, they must be placed below the top of the existing wood or gabion retaining walls. This will provide a horizontal surface to at least the top of the retaining walls, allowing full mobilization of the passive earth pressure. We recommend using an equivalent passive earth pressure of 500 pounds per square foot per foot of depth (psf/ft) above the high water level (HWL) and 300 below HWL. The coefficient of friction between the bulk head and the underlying subgrade will also provide some resistance to lateral forces. For cast-in-place concrete, it is our opinion a coefficient of friction of

0.40 can be used for design. For precast concrete, we recommend using a coefficient of friction of 0.30 for design.

The lateral earth pressures and coefficient of friction indicated above do not include factors of safety.

Appropriate factors of safety should be included when designing concrete structures to resist lateral earth forces.

Uplift Resistance. Along and above the shore line, backfill placed above concrete bulk heads can be assumed to resist uplift. Resistance to uplift is provided by the weight of the concrete in the bulk head plus the wedge of soil above the bulk head. For cohesionless soils, this wedge is formed by planes extending upward and outward from the top edges of the bulk head or footing at an angle of 20 degrees from vertical. Above HWL, we recommend assuming a moist unit weight of 135 pcf. Below HWL, we recommend assuming a submerged unit weight of 72 pcf. These values do not include factors of safety, and appropriate factors of safety should be utilized during design.

We recommend backfill placed above concrete bulk heads in the shore line be compacted. Preferably, only 6-inch minus material should be used, and it should be compacted to a minimum of 95 percent of its maximum dry density determined in accordance with ASTM Method of Test D 698 (standard Proctor).

Due to the difficulty of performing the compaction tests, we recommend performance criteria be established, including the following.

• All backfill should be placed in loose lifts no thicker than 8 inches.

• Use 6-inch minus material containing sand and gravel.

• Compact backfill with hand-operated wackers (jumping jacks) a minimum of three passes in two directions.

• During compaction, observe consolidation of the backfill, and additional compaction effort applied if material continues to consolidate during compaction effort.

Lake Deadman Anchors. Concrete structures can also be used as deadman anchors in the lake. We anticipate these structures will be precast and transported to the site by barge. They are then carefully placed at the desired location and attached to the boat ramps using cables or chains. Sizing and spacing of the lake anchors should be determined during design. It is our opinion the lake anchors can be placed directly on the lake bed. Some leveling of the lake floor should be performed by divers prior to placement. The weight of the concrete structure is then considered for uplift and lateral resistance. Any horizontal forces from river flow will also need to be considered in the design.

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Attachments:

Geology Map Meriwether Dock Aerial Sketch Layout Sketch Photographs 1 through 7 DCP-1 through DCP-3 Coulter Dock Aerial Sketch Layout Sketch Photographs 8 through 11 DCP-4 through DCP-7 Meriwether Dock Bulk Head Limitation Sketch

GEOLOGY MAP

Engineering Reconnaissance

Proposed Boat Dock Holter Lake, Helena National Forest

North of Helena, Montana

Drawn by: USGS Date 2005

Project: 14-3236G

Scale: None FIGURE

Sheet 1 of 1 1

North

Meriwether Dock

Coulter Dock

1. Existing Meriwether Dock, Looking North 2. Existing Meriwether Dock and Wood Retaining Walls

3. Meriwether Dock: Lake Bed Cobbles and Boulders in Vicinity of New Dock 4. Meriwether Dock: Hill Side above Existing Wood Retaining Wall

5. Meriwether Dock: Rock Outcrop 6. Meriwether Dock: Gravel Debris Flow South of Dock

7. Meriwether Dock: Gravel Debris Flow Surround Picnic Shelter

8. Existing Coulter Dock and Gabion Retaining Walls 9. Existing Coulter Dock

10. Coulter Dock Gabion Walls South of Existing Dock 11. Coulter Dock Dynamic Cone Penetrometer Test