Geotechnical_Memo_GM39-16.PDF

GEOTECHNICAL MEMORANDUM GM39-16

To: Kevin Bracy, PMP / David Evans and Associates Date: August 12, 2016

(ISSUED 10-05-16)

GRI Project No.: 5726-A

From: George A. Freitag, CEG; Michael W. Reed, PE; and Jason D. Bock, PE

Re: Geotechnical Consultation Mirror Lake Trail Relocation Task Order No. DTFH7015F19005 Government Camp, Oregon

At your request, GRI has completed additional consultation for the Mirror Lake Trail Relocation project in Government Camp, Oregon. The purpose of the consultation was to provide foundation recommendations for several proposed signs and evaluate global stability of the project retaining walls.

The global stability assessment included slope stability modeling using the proposed new site grades and structures. This memorandum describes the work accomplished and provides our conclusions regarding foundation recommendations for the proposed signs and the global stability of the two proposed retaining walls at the project site.

GRI recently completed a geotechnical investigation for the Mirror Lake Trail Relocation project, which is summarized in our August 11, 2016, geotechnical report, titled “Geotechnical Report, Mirror Lake Trailhead Relocation, OR DOT 26(2), Clackamas County, Oregon, Task Order No. DTFH7015F19005.

PROJECT DESCRIPTION

Construction of the proposed Mirror Lake Trailhead relocation will include the construction of two walls; a welded wire mechanical stabilized earth (MSE) wall (known as the MLT wall) along the south and west sides of the proposed mirror lake trailhead parking lot and a cast in place concrete retaining wall along the north side of the North Access Road (known as the NAR Wall).

SITE DESCRIPTION

Topography and Subsurface Conditions The project site is currently occupied by a combination of paved highway, paved roadways, and forested areas. The ground surface slopes gently from the northeast to the southwest at an average slope of about 8H:1V (Horizontal to Vertical) with localized areas as steep as 3H:1V. Subsurface conditions encountered in our explorations generally consist of alluvial sand with variable amounts of silt, gravel, cobbles, and boulders to the maximum depths explored. In general, the silt content decreases with depth, and the gravel, cobble, and boulder contents increase with depth. Groundwater was not encountered in our explorations, and we anticipate the regional groundwater level is located deep with the coarse grained soils.

SIGN FOUNDATIONS

We understand two monument signs and an interpretive sign will be constructed as part of the project. We anticipate sign foundations will be designed in accordance with the American Association of State Highway Transportation Officials (AASHTO) LRFD (Load Resistance Factor Design) Specifications for Structural Supports for Highway Signs, Luminaires, and Traffic Signals, 1st Edition. In our opinion, spread footings established in the near-surface alluvial soils or structural fill can be designed for a nominal bearing resistance of 8 ksf for the strength limit state. For the service limit state, we recommend a nominal bearing resistance of 3.0 ksf to limit footing settlement to less than 1 in. Spread footings should be at least 2 ft wide and embedded at least 2 ft below the lowest adjacent grade. We recommend the sign footings be underlain by a minimum 6-in. thickness of compacted crushed rock meeting requirements for crushed rock base as outlined in FP-14 (FHWA, 2014), Table 703-2, Class C. The crushed rock should have a maximum size of about 11/2 in. and should extend at least 12 in. beyond the edges of the footing. It may be preferable to cap the open-graded crushed rock with 2 in. thickness of crushed rock base outlined above to facilitate grading of the surface. Local areas of softer or organic-rich subgrade may require deeper overexcavation and should be evaluated by a qualified geotechnical engineer.

In accordance with the AASHTO LRFD design manual, resistance factors must be applied to the nominal bearing resistance, nominal sliding resistance, and nominal passive resistance for spread footing design.

The recommended resistance factors for footing design are summarized in the following table.

RESISTANCE FACTORS FOR SPREAD FOOTING DESIGN

Strength Limit State

Service Limit State

Extreme Limit State

Bearing Resistance: 0.45 1.00 1.00 Sliding Resistance (Cast-in-Place Concrete): 0.80 1.00 1.00

Passive Component of Sliding Resistance: 0.50 1.00 1.00

Foundations established a minimum of 2 ft below grade can use passive resistance in all directions. We recommend computing this resistance on the basis of an equivalent fluid having a unit weight of 250 and 500 pcf (nominal passive resistance) for the service and service limit states. This design passive earth pressure would be applicable only if the sign footing is cast neat against undisturbed soil, and/or if backfill for the footings and the abutment is placed as granular structural fill.

All spread footing excavations should be made with excavators equipped with a smooth-edged bucket and observed by a geotechnical engineer or engineering geologist. Areas of soft or otherwise unsuitable material encountered at subgrade elevation should be removed and replaced with compacted crushed rock in accordance with the Structural Fill section of this report. If groundwater is encountered in the footing excavation, it should be removed prior to placing concrete.

SLOPE STABILITY

The global stability of the walls was evaluated using subsurface information developed by GRI, as described in our August 11, 2016, geotechnical report. The slope stability models used for our analysis were based on two cross sections created using the subsurface characterization from our geotechnical report in combination with topographic information from the regional aerial lidar survey published by DOGAMI (2007). Each cross section was evaluated using two models representing static and seismic loading conditions. The cross sections used for our analyses are shown on the attached slope stability models, Figures 1 through 4.

The slope stability analyses were conducted with the assistance of SLIDE 7.0, a limit-equilibrium slope stability analysis software published by Rocscience Inc. of Toronto, Ontario, Canada. The factor of safety for each model was calculated using the Spencer method of slices, which satisfies both force and moment equilibrium. Seismic loading cases considered a peak ground acceleration (PGA) of 0.27 g using a horizontal acceleration coefficient (Kh) of 0.135 applied as a pseudo static load. The factor of safety against sliding is defined as the ratio of the forces resisting movement to the forces driving movement of the soil mass. As the factor of safety decreases and approaches 1.0, the relative stability of the slope is considered to decrease. Factors of safety less than 1.0 infer the soil mass is not in equilibrium, and movement is likely to occur.

MLT Wall

The cross section for the MLT wall was taken at station 1+84.93, which corresponds to the most critical section (tallest portion of the wall) of the proposed retaining wall. Soil properties used for this evaluation are based on observations in our explorations, laboratory testing, and our experience with similar soils in the area.

Results of our slope stability evaluation indicate the proposed topography, with a 200 psf surcharge load to account for vehicles, has a global factor of safety of about 1.7 and 1.4 for static and seismic loading conditions, respectively. The factor of safety against sliding is defined as the ratio of the forces resisting movement to the forces driving movement of the soil mass. In our opinion, the results of our slope stability evaluation indicate the risk of deep-seated or large-scale slope instability surrounding the MLT wall is relatively low.

NAR Wall

The cross section for the NAR wall was taken at station 1+93.3, which corresponds to the most critical section (tallest portion of the wall) of the proposed retaining wall. Soil properties used for this evaluation are based on observations in our explorations, laboratory testing, and our experience with similar soils in the area.

Results of our slope stability evaluation indicate the proposed topography has a global factor of safety of about 1.7 and 1.3 for static and seismic loading conditions, respectively. In our opinion, the results of our slope stability evaluation indicate the risk of deep-seated or large-scale slope instability surrounding the MLT wall is relatively low.

LIMITATIONS

This memorandum summarizes the results of our geotechnical consultation for the proposed signs and two site retaining walls. The scope is limited to the specific project and location described herein, and our description of the project represents our understanding of the significant aspects of the project. In the event that any changes in the location of the project elements as outlined in this memorandum are planned, we should review the changes and modify or reaffirm the conclusions of this report in writing.

Renews 1/2017

Renews 12/2016

The opinions, findings, and conclusions submitted in this memorandum are based on our site reconnaissance, slope stability modeling, subsurface explorations completed as part of our previous geotechnical investigation, and other sources of information discussed in this memorandum. In the performance of subsurface investigations, specific information is obtained at specific locations at specific times. However, it is acknowledged that variations in soil conditions may exist between exploration locations. This memorandum does not reflect any variations that may occur between these locations. No warranty, express or implied, is provided.

Submitted for GRI, George A. Freitag Michael W. Reed, PE Jason D. Bock, PE Associate Principal Project Engineer

References:

AASHTO, 2015, “LRFD Specifications for Structural Supports for Highway Signs, Luminairies, and Traffic Signals, 1st Edition”

FHWA, 2014, “Standard Specifications For Construction of Roads and Bridges on Federal Highway Projects – FP-14”, Federal Project 2014

Oregon Department of Geology and Mineral Industries (DOGAMI), 2007, Lidar: Northwest Oregon and Portland Metro Area.

5726-A GEOTECHNICAL CONSULTATION MEMO

This document has been submitted electronically.

200.00 lbs/ft2

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Distance, ft

E le v a ti o n ft

Material Name Color Unit Weight

(lbs/�3) Strength Type

Cohesion

(psf)

Phi

(deg)

Select Granular Backfill 130 Mohr-Coulomb 0 36

Granular Structural Fill 130 Mohr-Coulomb 0 35

Silty Sand 120 Mohr-Coulomb 0 34

Sand 125 Mohr-Coulomb 0 35

1.7

Analysis Description Trailhead Parking Lot - Sta. 1+84.93 - Static

File Name Mirror Lake Trailhead Parking Lot Retaining Wall - Static.slim

Scale 1:250

Drawn By Jason Bock, PE

Figure # Figure 1

Date

8/11/2016, 8:38:17 AM

Project

Mirror Lake - Trailhead Relocation

SLIDEINTERPRET 7.017

200.00 lbs/ft2

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Distance, ft

E le v a ti o n ft

Material Name Color Unit Weight

(lbs/�3) Strength Type

Cohesion

(psf)

Phi

(deg)

Select Granular Backfill 130 Mohr-Coulomb 0 36

Granular Structural Fill 130 Mohr-Coulomb 0 35

Silty Sand 120 Mohr-Coulomb 0 34

Sand 125 Mohr-Coulomb 0 35

0.135

1.4

Analysis Description Trailhead Parking Lot - Sta. 1+84.93 - Static

File Name Mirror Lake Trailhead Parking Lot Retaining Wall - Seismic.slim

Scale 1:250

Drawn By Jason Bock, PE

Figure # Figure 2

Date

8/11/2016, 8:38:17 AM

Project

Mirror Lake - Trailhead Relocation

1.71.71.71.7

10 20 30 40 50 60 70 80 90

Distance, ft

E le v a ti o n ft

Material Name Color Unit Weight

(lbs/�3) Strength Type

Cohesion

(psf)

Phi

(deg)

Select Granular Fill 130 Mohr-Coulomb 0 36

Cement Concrete 145 Infinite strength

Sand 125 Mohr-Coulomb 0 35

Barlow Trail

Analysis Description North Access Road Wall - Static

File Name Mirror Lake North Access Road Wall - Static.slim

Scale 1:250

Drawn By Jason Bock, PE

Figure # Figure 3

Date

8/12/2016, 10:36:31 AM

Project

Mirror Lake Trailhead Relocation

1.31.31.31.3

10 20 30 40 50 60 70 80 90

Distance, ft

E le v a ti o n ft

Material Name Color Unit Weight

(lbs/�3) Strength Type

Cohesion

(psf)

Phi

(deg)

Select Granular Fill 130 Mohr-Coulomb 0 36

Cement Concrete 145 Infinite strength

Sand 125 Mohr-Coulomb 0 35

Barlow Trail

0.135

Analysis Description

Mirror Lake Trailhead Relocation

North Access Road Wall - Seismic File NameScale

1:250 Drawn By

Jason Bock, PE

Figure #

Mirror Lake North Access Road Wall - Seismic.slim Figure 4Date

8/12/2016, 10:36:31 AM

Project