Golder_Associates_2015_Geophysical_Investigation_Memorandum_-_April_2015.pdf

0424151dpf1_RPA-Middle Fork_Geophysical Investigation.Docx

Golder Associates Inc.

18300 NE Union Hill Road, Suite 200

Redmond, WA 98052 USA Tel: (425) 883-0777 Fax: (425) 882-5498 www.golder.com

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April 24, 2014 Project No. 1403984

Mr. Brian Collins Federal Highway Administration Western Federal Lands Highway Division 610 E. Fifth Street Vancouver, WA 98661

Through:

Brian Wacker Robert Peccia & Associates PO Box 5653 825 Custer Avenue Helena, MT 59604

RE: MIDDLE FORK SNOQUALMIE RIVER ROAD, 2015 GEOPHYSCIAL INVESTIGATION

MEMORANDUM, WA PFH 29-1(1) TASK ORDER NO. T-11-002, MULTI-DISCIPLINE IDIQ NO.

DTFH70-10-D-00016

Dear: Mr. Collins

This memorandum presents the results of our 2015 geophysical investigation of designated portions of the Middle Fork Snoqualmie River Road (also known as Forest Highway (FH) 29, Forest Service Road 56, and King County Road 9899 (Lake Dorothy Road). Golder Associates Inc. (Golder) provided services as a subconsultant to Robert Peccia & Associates located in Helena, Montana. Golder’s scope of work was identified in the Statement of Work (Task Order Number T-11-002) issued by the Western Federal Lands Highway Division (WFLHD) of the Federal Highway Administration (FHWA), dated April 28, 2011, and Modification No. 001, dated May 9, 2011.

1.0 PROJECT UNDERSTANDING AND BACKGROUND

The project sites are located between MP 2.6 and 12.6 on the Middle Fork Snoqualmie River Road, as shown in Figure 1. WFLHD is currently reconstructing Middle Fork Snoqualmie River Road. The existing road is a gravel surfaced road varying between 16 to 22-feet wide. The new road will generally be constructed to a 20-foot wide surface (two 10-foot lanes) except areas constrained to 18-feet wide to minimize environmental impacts. We understand the new upgraded road surface will be paved.

The services requested by the WFLHD to support the design efforts documented in this memorandum include seismic refraction surveys to evaluate the nature and thickness of overburden and to collect seismic velocity information for ripability assessment at specific locations along the project corridor.

2.0 GEOPHYSICAL SURVEY

The seismic refraction method was employed to evaluate the nature and thickness of overburden and to collect seismic velocity information and assess rippability of rock outcrops adjacent to the current road alignment within potential cut slopes in the preliminary design.

Mr. Brian Collins April 24, 2015 Federal Highway Administration 2 1403984

2.1 Methodology and Field Procedure

Seismic refraction is a method commonly used to determine the compressional velocity and for mapping the thickness of alluvium and depth to bedrock. The method requires a seismic energy source to introduce seismic waves into the subsurface. The seismic waves penetrate the overburden and travel along interfaces where there is an increase in compressional velocity. Typically this increase in the compressional velocity is the water table or the top of the bedrock. While the seismic waves are traveling along this surface, they continually refract seismic waves back to the ground surface where they are detected by geophones. The geophones convert the acoustic energy in the ground to an electric signal that is transmitted by the geophone cable to the seismograph. The seismograph detects the arriving electric signals with respect to time and stores the records digitally for future data processing. The data is processed to determine the compressional velocity of the earth material through which the energy has traveled and to model the subsurface geology based on the velocity data. This geophysical model depicts the earth in cross-section showing the velocity and thickness of the subsurface layers below the seismic line.

Table 2-1 summarizes the seismic refraction data collection effort.

Table 2-1: Seismic Refraction Line Locations

Seismic Refraction Line

Location (Stationing in Preliminary Plan set dated 05-Apr-2011) Line

Length (ft) Offset

2015-SL1 (Figure 1) 279+30 to 281+60 230 ~15-25’ below RT cut line;

~20-50’ RT of CL

2015-SL2 (Figure 1) 280+50 149.5 ~20-169.5’ RT of CL

2015-SL3 (Figure 1) 281+50 138 ~15-153’ RT of CL

2015-SL4 (Figure 2) 273+15 to 274+53 138 ~20-30’ RT of CL

Seismic refraction data for lines SL-1, SL-2, SL-3, and SL-4A were collected using a 24-channel Geometrics GEODE seismograph coupled with 24 Mark products 30-Hz geophones. The 24 geophones were spaced at 6, 6.5 or 10 foot intervals to make a 138 to 230-foot “spread”.

A 16-pound sledge hammer stuck against a steel plate was used as the seismic source. The seismic energy from multiple hits to the plate were “stacked” to improve the signal-to-noise ratio. A minimum of 9 shot points were recorded for each spread, with shot locations at 5 and 20 to 35 feet off both ends of the spread and at 5 locations within the spread. Project stationing and horizontal positon was estimated using a fiberglass measuring tape from staked locations. Elevation data was collected relative to the current road center line elevation reported in the preliminary plan set using a hand level and stadia road to sight points along each seismic survey line.

2.2 Results

Seismic data processing was completed using PickWin and PlotRefA programs, both part of the SeisImager 2D software package commercially available from Geometrics. For each seismic shot point, the time of the first seismic arrival at each geophone was carefully measured in PickWin. Geophone and shot geometry were then entered into PlotRefA, along with the measured first arrival times and surface topography. This information was used to generate a best-fit tomographic model of compressional seismic velocity.

Federal Highway Administration 3 1403984

Percent root mean square (RMS) error of the velocity models ranged from 5 to 15 percent; which is higher than ideal, and likely the result of:

Extremely steep slopes Irregular surface conditions Heterogeneous subsurface conditions

A panel in Figures 1 and 2 depicts the approximate locations of refraction lines in relation to the stationing in Preliminary Plan Set dated April 5th, 2011. The seismic refraction interpretations are presented as a cross-sections of modeled compressional (a.k.a. p-wave) seismic velocity. These velocity models are shown in the separate panels of Figures 1 and 2.

As described in Golder’s August 23, 2011 memorandum (Middle Fork Snoqualmie River Road, Rock Cut Investigation and Design Recommendations Memorandum, WA PFH 29-1(1) Task Order No. T-11-002, multi-discipline IDIQ No. DTFH70-10-D-00016), modeled compressional seismic velocity was used to categorize the expected subsurface conditions below the seismic lines. Areas of the subsurface with modeled seismic velocities between 0 and 6,000 feet per second (fps) are interpreted to be unsaturated sediments and organic matter. Subsurface materials with modeled velocities between 6,000 and 10,000 fps are interpreted to be saturated sediments and rock; with 8,000 to 10,000 fps interpreted to be somewhat weathered or fractured rock and materials above 10,000 fps interpreted to be competent rock.

The thick black line between “hot” colors indicating high seismic velocities and “cool” colors indicating lower seismic velocities marks the approximate velocity where bedrock is interpreted to exist based solely on seismic data.

As shown in Figures 1 and 2, the interpreted bedrock topography is irregular and bedrock velocity is highly variable. These features of the seismic models are consistent with observations at the various sites of investigation where rock is near the surface in some places and soil cover is apparent in others.

Rock rippability can be empirically correlated with seismic velocity and is based, in part, on the equipment being used for the earthwork. Ripping is typically performed by tractor-mounted equipment. The size of the tractor (dozer) required for ripping can be determined by a rippability assessment for the rock. The strength and competency of the rock material as well as rock structure (joints, bedding planes, or foliation planes) will determine the ease of rippability. Rock that is too strong to be ripped may be fragmented with explosives.

Table 2-2 depicts an example rippability chart for a Caterpillar D9L ripper for various materials. As shown in the chart, the velocity correlated to rippability of materials is variable based on the type of rock. For this report <8,000 fps (Black line on the profiles in Figures 1 and 2) was chosen to represent rippable material and >9,000 fps was chosen to represent non-rippable material based on observations that the rock material was predominantly metasedimentary and that a Caterpillar D9L would be used for construction. If other equipment is proposed, this assessment should be revisited.

The west side of SL1 and a 20-foot section in the middle of SL-4 appear to have non-rippable rock very near the surface. Difficult ripping conditions appear likely in the proposed cuts in the vicinity of station 274+00 (30 or more feet RT) and at 279+40 to 280+00 as indicated by the high seismic velocity in these areas. However, the blocky nature of the rock created by systemic joint and foliation patterns may allow more ripping than the seismic velocity measurements suggest if the fractures and joints in the rock can be exploited by the ripper tooth.

Federal Highway Administration 4 1403984

Table 2-2: Example Rippability Chart

3.0 LIMITATIONS OF GEOPHYSICAL INVESTIGATION

Golder services are conducted in a manner consistent with the level of care and skill ordinarily exercised by other members of the geophysical community currently practicing under similar conditions subject to the time limits and financial and physical constraints applicable to the services. Seismic refraction is remote sensing geophysical method that may not detect subsurface features, faults, or differentiate lithology because of site conditions. Modeled depths and velocities are generally assumed to be accurate to +/- 15% of reported values; however, survey conditions may produce results outside of these bounds.

Furthermore, it is possible the seismic data may, upon intrusive sampling, prove to be misinterpreted.

We trust the foregoing is satisfactory for your current needs. We look forward to your review comments and will be glad to respond to any questions you may have.

GOLDER ASSOCIATES INC.

David P. Findley, LG, LEG Peter E. Fahringer, LG Associate-Engineering Geologist Associate-Senior Geophysicist

DPF/PF/km

List of Tables (In text)

Table 2-1 Seismic Refraction Line Locations Table 2-2 Example Rippability Chart

List of Figures

Figure 1 Seismic Lines 2015-SL-1, 2015-SL-2, and 2015-SL-3 Interpreted Refraction Results Figure 2 Seismic Line 2015-SL-4 Interpreted Refraction Results

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2015-04-23

01403984

SEISMIC LINES 2015-SL1, 2015-SL2, & 2015-SL3

INTERPRETED REFRACTION RESULTS

MIDDLE FORK SNOQUALMIE

NEAR TANNER, WA

Robert Peccia & Associates AND Federal Highway Administration Western Federal Lands Highway Division

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2015-04-23

01403984

SEISMIC LINE 2015-SL4

INTERPRETED REFRACTION RESULTS

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NEAR TANNER, WA

Robert Peccia & Associates AND Federal Highway Administration Western Federal Lands Highway Division

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