ak pfh 58(1)_NeckLakeRoad_Rockery_TM 1.21.2019.pdf

Golder Associates Inc.

2121 Abbott Road, Suite 100, Anchorage, Alaska, USA 99507

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The Alaska Department of Transportation & Public Facilities (ADOT&PF) in cooperation with the US Forest

Service (USFS) is proposing approximately eight miles of road upgrades for the Neck Lake Road on Prince of

Wales Island in southeast Alaska. The project begins at the intersection of National Forest Service Road (NFSR)

25 and Forest Highway (FH) 43 and ends southeast of the Whale Pass float plane dock. Western Federal Lands

Highway Division (WFLHD) of the Federal Highway Administration (FHWA) is delivering the project on behalf of

ADOT&PF and the USFS. WFLHD has selected Robert Peccia and Associates (RPA) to provide engineering services for the project. RPA contracted Golder Associates Inc. (Golder) to provide geotechnical engineering support.

RPA contracted Golder to produce a Geotechnical Design Report, dated October 5, 2018, presenting the geotechnical analysis. This technical memorandum is a supplement to the Geotechnical Design Report and provides recommendations for rockery walls to be used along the alignment. The stations and cross sections used in the development of the recommendations were provided by RPA and dated October 12, 2018.

1.0 ROCKERY WALLS

Construction of rockery walls are planned in select areas along Neck Lake Road. Based on our review of the current cross sections, the rockery walls have exposed wall heights ranging from approximately 7 to 11 feet, which does not include the portion of the wall buried at the toe. Design soil parameters were developed for the wall locations based on the available subsurface information obtained from the nearby boreholes and test pits.

Based on the available subsurface information, the proposed rockery walls are located in areas where bedrock is within 7 to 8 feet of ground surface, as presented in Table 1.

Table 1: Rockery Wall Information

Rockery Wall Station Range Rockery Wall Height (feet)1 Average Depth to Bedrock (feet)

141+15 to 141+65 10 8

146+33 to 146+85 11 7

Notes: 1) Based on total height of the retained soil and rock identified in RPA Mainline Cross Sections

TECHNICAL MEMORANDUM

DATE January 21, 2019 Project No. 1788309

TO Jeff Key, PE Robert Peccia and Associates (RPA)

CC Eric Cannon, CPG

FROM John Thornley, PE EMAIL john_thornley@golder.com

NECK LAKE ROAD, AK PFH 58(1), GEOTECHNICAL RECOMMENDATIONS FOR ROCKERY WALLS,

PRINCE OF WALES, ALASKA

Jeff Key, PE Project No. 1788309

Robert Peccia and Associates (RPA) January 21, 2019

1.1 Analysis

Golder calculated global stability, settlement, and bearing resistance for select but representative cross sections at each wall location. The cross sections were developed based on representative sections chosen from the geometries shown on the RPA Mainline Cross Sections dated October 12, 2018. A subsurface profile was developed for each cross section based on the available borehole and test pit information. The parameters used for the analyses are based in part on the available laboratory data, field descriptions of the soils, and typical published relationships for engineering soil properties.

The rockery walls are designed assuming the following soil properties:

Rock unit weight: 150 pcf

Effective friction angle: 40°

Unit Weight of retained soil: 130 pcf

Effective friction angle of retained soil: 35°

For the stability analyses, the capacity to demand ratio (CDR) was calculated in general accordance with FHWA’s

“Rockery Design and Construction Guidelines” (FHWA-CFL/TD-06-006). The recommendations below provide

CDR values greater than unity for overturning, sliding resistance, and bearing capacity. Global stability was also evaluated and a factor of safety greater than 1.3 and 1.1 were maintained for static and seismic loading conditions, respectively.

1.2 Recommendations

Golder recommends that the rockery walls be constructed in general accordance with FHWA’s “Rockery Design and Construction Guidelines” (FHWA-CFL/TD-06-006). Based on the available information, Golder anticipates that the rockery walls will be located in areas with shallow bedrock and will typically be used in transitions in and out of rock cuts. Golder recommends that soil is removed to the bedrock surface prior to construction of the rockery walls. A two to six-inch leveling course should be placed on the bedrock surface and compacted prior to construction of the wall. The base of the excavation should be level or inclined back slightly (on the order of 5 percent) into the slope. The base of the excavation should never slope towards the face of the rockery wall.

The height of any single tiered rockery wall should not exceed 12 feet without direction from the project engineer.

The wall face should be constructed with a batter of no steeper than 1H:4V. Excavation behind the wall should be a minimum of one foot at the base of the wall and extend up at a slope that is safe and meets OSHA temporary slope stability requirements, as verified by the contractor. Golder recommends that a nonwoven geotextile meeting FHWA Class 1 specifications be placed behind the wall prior to construction if the in-situ backslope soils are fine-grained. A minimum of one foot (horizontal) of crushed rock material should be placed behind the wall to act as a backdrain. The crushed rock should consist of material meeting the gradation specifications presented in

Table 2. The top of the backdrain should be covered with non-woven geotextile and a minimum 6-inch thick layer of impervious soil to reduce the likelihood of infiltration of surface water. Compacted granular fill should be placed behind the backdrain and should be benched into the existing slope. The compacted granular fill should be placed until grade is established behind the rockery wall, starting at the back-side of the backdrain.

Jeff Key, PE Project No. 1788309

Robert Peccia and Associates (RPA) January 21, 2019

Table 2: Crushed Rock Gradation Specifications

Sieve Size Percent by Mass Passing Designated

Sieve (AASHTO T27 & T11)

150 mm (6 in.) 100

100 mm (4 in.) 0.0 - 25

19.0 mm (3/4 in.) 0.0 - 15

4.75 mm (No. 4) 0.0 – 5.0

75 µm (No. 200) 0.0 – 2.0

Rockery wall heights will vary with the topography and slope configuration. Table 3 provides base width dimensions based on exposed height of the rockery wall where soil is retained. A minimum embedment depth of

2 feet is recommended. If a combination of competent rock and soil are to be retained the minimum base width may be reduced, as directed by the project engineer.

Table 3: Minimum Rockery Wall Base Width by Height

Rockery Wall Height, Exposed (feet)

Minimum Base

Width (feet)

12 8

10 7

8 6

6 4

2.0 USE OF TECHNICAL MEMORANDUM

This technical memorandum was prepared for the exclusive use of WFLHD and RPA during design of the Neck

Lake Road reconstruction project. If there are significant changes in the nature, design, or location of the facilities, we should be notified so that we may review our conclusions and recommendations in light of the proposed changes and provide a written modification or verification of the changes.

There are possible variations in subsurface conditions between explorations and also with time. Therefore, inspection and testing by a qualified geotechnical engineer should be included during construction to provide corrective recommendations adapted to the conditions revealed during installation.

Unanticipated soil conditions are commonly encountered that cannot fully be determined by a limited number of explorations or soil samples. Such unexpected conditions frequently result in additional project costs in order to build the project as designed. Therefore, a contingency for unanticipated conditions should be included in the construction budget and schedule.