GM 8-20 OR FS ERFO 2017(1)-18(6) Lateral Earth Pressures Memo REV1a.pdf

158377846

M E M O R A N D U M Western Federal Lands Highway Division

610 E. Fifth Street Vancouver, WA 98661

DATE: March 13, 2020

TO: Helen Oppenheimer Project Manager

FROM: Eric Lim Sr. Geotechnical Engineer

SUBJECT: Geotechnical Memo 8-20 Lateral Earth Pressures on Soldier Pile Walls – Revision 1a Rogue River Repairs 2017

OR FS ERFO 2017(1)-18(6)

Rogue-Siskiyou National Forest, Oregon

INTRODUCTION

This memorandum provides lateral earth pressures for structural design of soldier pile walls with and without tieback anchors. The information presented in this memo will be supplemental to our geotechnical recommendations memorandum being prepared under separate cover. This memorandum has been revised based on Cross-Functional Team (CFT) discussions on March 12, 2020.

PROJECT DESCRIPTION

Two soldier pile retaining walls are planned at Mile Posts MP13 and MP20 on the Forest Road 33 (Agness Road) to mitigate landslide movements. Both areas have deep patch backfills within the roadway, but are experiencing loss of ground under the deep patches. The loss of ground is due to landslide movements and is resulting in settlement and damage to the roadway. Due to the numerous previous repair attempts, including multiple rounds of deep patch mitigation, we have recommended a structural repair solution consisting of soldier pile walls with tiebacks where needed.

SOLDIER PILE WALL DESIGN RECOMMENDATIONS

Active Earth Pressures

Design of these soldier pile walls should be based on “active” lateral earth pressures, which assume that the walls are free to deform on the order of 0.001 to 0.002 times the exposed height. This lateral deformation is likely to be accomplished by vertical settlement of up to roughly 0.005 times the height of the wall, which may extend back from the side of the cut a distance equal to roughly the height of the cut. Lesser degrees of settlement may also occur within a setback extending twice as far back. A greater amount of lateral deformation could allow greater vertical settlements. If no structural elements are located within this zone, or if structural elements within the zone can tolerate this degree of settlement, then it would be appropriate to design with active earth pressures. Active earth pressure coefficient equations from the AASHTO Bridge Design Specifications are shown in Figure A-1 to be used for Strength I and Service I limit states.

Figure 1 – Static Active Pressure Coefficient Equations from AASHTO BDS, 2017

For the case of a cantilevered wall system, the applied lateral pressure would be represented by a triangular pressure distribution termed an equivalent fluid density (EFD). Figure A-1 of this memo illustrates the recommended pressure distributions for inside the slide plane and the taper zone (i.e., construction ramps) outside the slide plane at the ends of the walls. We recommend a nominal active pressure equivalent fluid densities as shown in Table 1 for relatively level roadway conditions. Active pressures should be assumed to act on the pile spacing above the excavation base and the grouted pile diameter below the base of the excavation.

Table 1 – Static Lateral Earth Pressure Parameters for Cantilever Walls

Zone 1

Total Unit

Weight, γ (pcf)

Effective Friction Angle, φ’

(deg) Cohesion, c (psf)

Wall or Pile friction, δ (deg)

Coefficient ka

Equivalent Fluid

Density (pcf)

Ground Surface Angle from

Horizontal, β (deg)

Active 1 2,4 135 32 0 16 0.28 38 0 Active 2 3 130 15 0 11 0.53 69 0 Active 3 3 140 28 0 21 0.32 45 0

Notes: 1. See Figure A-1 for zone depths. Zone depths vary and may consist of several soil layers.

2. Using a wall/lagging to soil friction coefficient of 0.50.

3. Using a grouted pile to soil friction coefficient of 0.75.

4. Composite value of deep patch and underlying soils and areas beyond deep patch.

Table 2 – Static Lateral Earth Pressure Parameters for Tieback Walls

Zone 1

Total Unit

Weight, γ (pcf)

Effective Friction Angle, φ’

(deg) Cohesion, c (psf)

Wall friction, δ

(deg) Coefficient ka

Apparent Earth

Pressure (psf)

Ground Surface Angle from

Horizontal, β (deg)

Active 2,3 135 22 0 11 0.41 55 H 0 Notes: 1. See Figure A-1 for zone depths. Zone depths may consist of several soil layers.

2. Using a wall/lagging to soil friction coefficient of 0.50.

3. Composite value of deep patch and underlying slide materials.

For the case of walls with one or more levels of anchors, the applied lateral pressure would be represented by a trapezoidal pressure distribution termed an apparent earth pressure (AEP) distribution. Figure A-1 illustrates the recommended trapezoidal pressure distributions. Soil parameters used for AEPs are shown in Table 2. These pressure diagrams assume the walls will be constructed from the top down.

An additional uniform lateral pressure 75 psf should be added to the above values to model traffic surcharges. This surcharge is approximately equivalent to a uniform vertical traffic live loads of 250 psf on the roadway and wall backfill, and should be adequate to model typical wheeled and track-mounted construction vehicle loading, except for crane outrigger point loads and stockpile loads. These loads should heavier loads should be evaluated during construction if placed a horizontal distance within the height of the wall.

Seismic Considerations

The project is located within a seismic zone 4 with a modified peak seismic ground acceleration coefficient, As, of 0.48 g; therefore, AASHTO guidance would typically be to design for earthquake performance. However, it is often difficult and cost prohibitive to design landslide repairs to also resist design level earthquakes. In our opinion, the Extreme Event Limit State should be waived in the scope of the repair due to the ERFO nature of the repairs, low traffic volume, and likelihood of other slope failures from a large earthquake. Based on our meeting on March 12, 2020, we understand that the CFT concurs with this assessment and that seismic loading will not be added to this ERFO repair project.

Passive Earth Pressures

Lateral resistance for the walls could be provided by passive pressure on the buried portion of the solider piles and a bracing system of tieback anchors, where needed. Figure A-1 illustrates the recommended truncated triangular pressure distribution for passive resistance.

The recommended nominal passive pressures are shown in Table 5 below and on Figure A-1.

The passive pressure may be applied over 2.5 times the diameter of the concreted soldier pile section or the pile spacing, whichever is less, neglecting the upper 2 feet below the excavation base. Per AASHTO Bridge Design Specifications (BDS) (AASHTO, 2017) Table 11.5.7-1 Resistance Factors for Permanent Walls, a reduction factor of 0.75 should be applied for passive resistance of vertical elements.

Table 5 – Passive Lateral Earth Pressure Parameters

Zone

Total Unit

Weight, γ (pcf)

Effective Friction Angle, φ’

(deg) Cohesion, c (psf)

Wall or Pile friction, δ (deg)

Coefficient ka or kp

Equivalent Fluid

Density (pcf)

Ground Surface Angle from

Horizontal, β or i (deg)

Passive 1 1,2,3 130 15 0 11 - 0 -27 Passive 2 1,2 140 28 0 21 1.16 160 -27 Passive Lock-off 1

125 29 0 22 6.2 770 0

Notes: 1. Using a grouted pile to soil friction coefficient of 0.75.

2. Assuming a 1V:2H effective slope in front of the wall.

3. Represents the shear zone falling away from the wall.

TIEBACK DESIGN RECOMMENDATIONS

The anchor portion of the tieback must be fully located a sufficient distance behind the retained excavation face to develop resistance within the stable soil mass. In addition, the anchors must be located below the sliding mass and embed into stable schist bedrock. As shown on Figure A-1, we recommend the anchorage be attained behind the assumed slide planes that are approximately 15 feet below the road at MP 13 and 16 feet at MP 20.1. The zone in front of the above described plane is called the “no-load zone”.

The no-load portion of the tieback should also be a minimum length of 15 feet and protected with bond-breakers, such as plastic sheathing. All tieback holes within the no-load zone should be immediately backfilled with sand/cement grout to prevent loss of ground. The tendons should be specified for Class I Corrosion Protection for tieback anchors as recommended by the Post-Tensioning Institute (PTI) for permanent anchors (PTI, 2014).

Anchor holes should be drilled in a manner that will minimize loss of ground and not disturb previously installed anchors. During the drilling, caving could occur if loose, wet or saturated zones are encountered. Drilling with a continuous-flight auger or casing would reduce the potential for loss of ground. The designers and walls subcontractor should particularly note the presence of existing facilities adjacent to the project site, including buried utilities, as they may affect the location or extent of the anchor holes. In addition, the selected tieback anchor installation should be subject to performance testing, including verification testing and proof-loading. The deep patch geosynthetic layers may be drilled through for the tiebacks.

Use of the design values presented herein is dependent on a well-constructed anchor. We recommend that concrete be placed in the drilled tieback anchor hole by tremie methods such as pumping through a hose placed in the bottom of the hole or pumping through the center of a continuous-flight auger. In this way, the grout is forced up through the anchor zone under pressure, with the resulting anchor more likely to be continuous. The grout should not be placed into the anchor zone by simple gravity methods such as flowing down a chute. We recommend that a qualified geotechnical engineer or their representative monitor all tieback installations.

With a tieback wall system, we estimate a nominal cement grout-soil adhesion of 75 pounds per square inch (psi) could be used for the based on PTI published values for soft shales.

However, the actual adhesion value will depend on the contractor’s mean and methods, such as using pressure grouting, secondary grouting, etc. Therefore, the design loads are typically provided on the plans, and it is up to the contractor to construct on anchor bond zone to resist the specified load. A minimum bond zone length of 10 feet is recommended. Per AASHTO BDS Table 11.5.7-1 Resistance Factors for Permanent Walls, a reduction factor of 1.0 may be applied for design resistance where the pullout resistance is proof-tested during construction.

In addition, all tiebacks should be proof-loaded to at least 130 to 150 percent of design resistance. The tieback testing program should be approved by WFLHD and observed by a qualified geotechnical engineer.

Soldier Piles and Lagging

Soldier piles for walls are typically set in pre-augered holes and backfilled with lean or structural concrete. Vertical loads on such piles could be resisted by a combination of friction and end-bearing below the base of the excavation. We recommend a nominal side friction value of 1,000 psf and a nominal end-bearing value of 15 ksf (kips per square foot) for design.

Side friction should be neglected within the upper 10 feet due to the presence of slide materials. The end-bearing value is predicted on embedment of at least 10 feet below the design grade (i.e., base of slide materials). An appropriate reduction factor should be applied for design resistance. Per AASHTO BDS Table 11.5.7-1 Resistance Factors for Permanent Walls, Article 10.5 applies for axial compressive resistance of vertical elements. Per AASHTO BDS Table 10.5.5.2.4-1 Resistance Factors for Geotechnical Resistance of Drilled Shafts, a reduction factor of 0.60 for side resistance in IGMs and 0.55 for tip resistance in IGMs.

We recommend lagging, or some other form of shoring, be installed in all areas. Due to soil arching effects, lagging may be designed for 50 percent of the lateral earth pressure used for walls design. Prompt and careful installation of lagging will reduce potential loss of ground.

The requirements for lagging should be made the responsibility of the walls subcontractor to prevent soil failure, sloughing and loss of ground and to provide safe working conditions. We recommend the voids between the lagging and soil be backfilled with lean-concrete backfill.

However, the backfill should not allow potential hydrostatic pressure to build-up behind the wall. Drainage mats should be placed on the soil behind the wall. Timber lagging should be pressure treated with an appropriate wood preservative, and all cuts ends treated on-site with the same preservative.

REFERENCES

AASHTO (2017), AASHTO LRFD Bridge Design Specifications, 8th Edition, 2017, American Association of State Highway and Transportation Officials, Washington, D.C.

Post Tensioning Institute (PTI) (2014), PTI Recommendations for Prestressed Rock and Soil Anchors.

CLOSING

Please contact Eric Lim at (360) 619-7826 or Eric.Lim@dot.gov with any questions regarding this memorandum.

CC: Sara Loftus, Structural Engineer Benjamin Oltmann, Bridge/Structures Functional Manager Nick Farny, Engineering Geologist Geotechnical File

Attachment: Figure A-1: Lateral Earth Pressures for Strength I & Service I Limit States mailto:Eric.Lim@dot.gov

Excavation Base

X to Design Grade

Assume no resistance

D e p th

D

B

Finish Grade

(Slide materials)

0 D psf

Passive 1

38 H psf

Active 1

MP 20 from STA 201+76 to 202+80 MP13 from STA 17+98 to 20+46

Cantilever Section Inside Slide Plane

D e s ig n H e ig h t, H friction nominal

1.0 ksf fs = end bearing nominal qa = 15 ksf above design grade.

to act of the pile spacing

4. Traffic surcharge assumed include a reduction factor.

are nominal and do not smaller. Passive pressures spacing, whichever is pile diameter or the pile times the grouted soldier assumed to act over 2.5

3. Passive pressures are below design grade.

over grouted pile diameter above design grade, and to act over pile spacing

2. Active pressure assumed explanation of parameters.

1. See report text for

Notes:

& SERVICE I LIMIT STATES

PRESSURES FOR STRENGTH I

FIG. A-1: LATERAL EARTH

STATE PROJECT

NUMBER

SHEET

U

S

S u r_ ft

D c m y f il e s \p w p r o d u c ti o n

\d

\C a p e M e a r e s L a te r a l

E a r th

P r e s s u r e s .d g n

OR FS ERFO 2017(1)-18(6)

:3

P

M

A u g u s t

C h e c k e d b y

D e s ig n e d b y

69 D psf

Active 2 = 75 psf

Traffic Surcharge

Roadway elevation = 0

Elev. -10 ft

Elev. -20 ft

45 D psf

Active 3 160 D psf

Passive 2

Design Grade

Elev. -12 ft

N o L o a d Z o n e

Min. Bond Length = 10' Tieback Anchor Typ

D e s ig n H e ig h t, H

Single or Multiple Row Tiebacks and 16 ft for MP20.1 of 15 ft for MP 13 AND below a depth behind this line Locate all anchors

Finish Grade Elev. -10 ft

= 75 psf

Traffic Surcharge

Design Grade

(Hn)3

H13

D e p th

D

(Slide materials)

0 D psf

Passive 1

160 D psf

Passive 2

Roadway elevation = 0

Elev. -20 ft

H

Hn

Pa

B

H

Pa per Equation 3.11.5.7.1-2 Active AEP

' m in

' m in

D e p th

D m in

B

Finish Grade

38 H psf

Active 1

MP 20 from STA 201+52 to 201+68, STA 202+88 to 203+04 MP 13 from STA 17+74 to 17+90, STA 20+54 to 20+70

Cantilever Section Outside Slide Plane

D e s ig n H e ig h t, H

= 75 psf

Traffic Surcharge

Roadway elevation = 0

Elev. -10 ft

45 D psf

Active 3 160 D psf

Passive 2

Design GradeElev. -12 ft m in m in