rock mechanics and its effects on …...rock mechanics and its effects on spillway modification...

ROCK MECHANICS AND ITS EFFECTS ON SPILLWAY MODIFICATION DESIGN

AEG Annual MeetingAshville, NC19 August 2019

Coralie Wilhite, PG, SPRAT IUS Army Corps of EngineersSacramento, CA

Co-authors USACE: Ken Pattermann, PE, GE Jerilynn HilmarVanessa Bateman, PG, PE

Presenter

Presentation Notes

Based on a paper we prepared for the 2019 Highway Geology Symposium.

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• Success (now Schafer) Dam and Spillway were completed in 1961 on the Tule River in central California.

• The project includes a zoned earthen embankment dam, Frazier Dike, and the emergency spillway.

PROJECT SCOPE

• It provides flood risk reduction, ag water supply, and recreation.

• A feasibility study proposed to raise the SW 10 ft and widen it ~165 feet.

• Modification was approved in Fall 2018 and design is currently underway.

Presenter

Presentation Notes

Completed in 1961 – 6 miles upstream of Porterville. provides flood damage risk reduction, agricultural water supply, and recreation to the local community and central valley. Draining an area of 630 square miles the Tule originates as three forks, that join to form Lake Success Raise - to provide additional flood risk reduction and irrigation water supply. A subsequent baseline risk assessment found the main risk drivers to be extreme loading events and confirmed that the proposed raise and widening would further reduce the flood related risks. Spillway is about 700 feet north of the main dam right abutment.

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• Project Purpose: To widen the spillway and move the road above PMF

SPILLWAY MODIFICATION – PHASE I

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65% Phase I Grading Plan

Presenter

Presentation Notes

Today I will focus on Phase I and right abutment analyses. Left abutment is still being investigated. This grading plan shows the initial Design slopes based on the initial stability analysis that this presentation summarizes – these are being re-assessed with recent data. Yes, the goal comes at the beginning and end, but it is the whole reason behind this work. To have discussions early and often regarding project needs, purpose, and end product. Then plan, investigate, analyze, design accordingly to decrease risk of unknowns, increase confidence, and decrease change of differing site conditions or issues during construction.

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• Input data: Site geologic and structural mapping, sub surface investigations, downhole televiewer, geophysical surveys, lab testing, erodibility study (pending), historical investigations and analyses.

• Assumptions: Safety. Some failure is OK. Scour doesn’t damage structure. Road closed during flow events. etc.

PROJECT OVERVIEW

Right Spillway Abutment

Left Spillway Abutment

Main Dam

Existing Sill

Presenter

Presentation Notes

Existing sill (at 652.5 feet) is 3 feet wide at the top, 8 feet wind at the bottom and is 6 feet deep. Reinforced key block were also embedded into the abutments at the sill location and measure 4.5 feet thick and 8-12 feet wide and anchored to the wall with 8 foot #8 bars. Safety to personnel during construction and along road for life. Failure OK if not causing harm to structure, road, or people. Can’t block spillway flow. Road will be closed during high water and all flow events.

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Depth of erosion from 1966 flow vs. as-build conditions

PREVIOUS FLOW EVENT

• Historical Flow in 1966 with a max flow of 8,300cfs.

• An Erodibility Study is in process for the site.

Max 8,300cfs

Right Spillway Abutment

1966 erosion and headcutting

Main Dam

Presenter

Presentation Notes

Bottom is a 2006 survey that shows depth of scour (mostly) from the 1966 flow event vs. as-build elevations.

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DG

DGL

HW with DG, MW, and quartzite - IF

MW/HF

Qtzit

East

West

DGHW

HW

HW

IJ

MW/IF with areas of HW

MW/IF with areas of HW

DG

MW/HF with areas of SW

North-East

South

DG

DGDG

IF

HWMW/HF with areas of HW/IF

ABCDEFGH K

HW with areas of DG and MW. IF

MW/HF with areas of HW/IF



North-East

South-West

HW - Highly Weathered Rock

SW - Slightly Weathered Rock

HF - Highly Fractured RockIF - Intensely Fractured Rock

MF - Moderately Fractured Rock

MW - Moderately Weathered Rock

UW - UnWeathered Rock

A - Structural Data Groupings

- Approximate Slide Boundary- Shear

- Approximate Weathering Boundary

DG - Decomposed RockKEY

GEOLOGIC MAP – RIGHT ABUTMENT

HW with DG, MW, and quartzite - IF

Presenter

Presentation Notes

Long history of geology – earliest are sedimentary and volcanic deposits, followed by folding and metamorphism. Then massive igneous intrusions during Orogenic events that caused more metamorphism and folding and faulting. Then more intrusions of batholiths followed by intrusions of dikes, sills, etc. and erosion throughout the whole process to finally expose the Sierra Nevada granitic core. Pleistocene uplift and erosion gave us the approximately current geomorphology. Right abutment is mostly composed of granitic rocks with pockets and veins of quartzite and meta-basalt. Generally weathering grade increases with depth, but two HW zones extend to the spillway floor along what appear to be sheared meta-basalt units. Quartzite tends to be of high grade no mater it’s depth. Approximately the top 10 to 60 feet of the upper slope is decomposed to HW with higher quality rock below. Highest quality rock is at and near the existing sill (10+00) new OGEE planned at ~ 8+50.

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INVESTIGATIONS

• 2 more geophysical lines were just completed high on the right abutment.• 5 more downhole explorations (with televiewer and testing) are in process.

Map of borings and seismic lines for the right spillway abutment

N

Presenter

Presentation Notes

Many more borings and Investigations in the 50s for dam construction and in 2001, 2003, 2007 for borrow studies, lab testing, spillway mod investigations. geophysical lines across the site.

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RIGHT ABUTMENT SEISMIC PROFILES

Correlation with borings: ~3,500 fps is approx. equal to the bottom of HW/rippable rock

Presenter

Presentation Notes

Example profiles from the top of the right abutment. These lines have been correlated with onsite borings to determine what velocities are approximately equal to what weathering. 3,500 fps is approximately equal to the bottom of HW rock and the bottom of ripability.

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Direct shear lab specification test sheet

LABORATORY TESTING

Direct shear test results Phase 1 (initial explorations)

Clay seams in 1F-19-28

NEW

find

ing…

Presenter

Presentation Notes

Direct shear samples tested on natural fractures at 1 to 2 times the mag. of the insitu stress at the fracture depth. Discuss low SW results and sensitivity analyses. Also note clay found in upstream hill – high and low angle – ¼ + inch thick. Currently being worked into design.

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LABORATORY TESTING

UCS Testing results from 2019 and earlier

Weathering Grade Average Unconfined

Compressive Strengths (psi)

Unit Weight (lbs/ft3)

Angle of Internal Friction (Phase 1 Lab Results)

(φ) Unweathered 29,000 165 76

Slightly Weathered 20,500 155 33* Moderately Weathered 8,500 150 58

Highly Weathered 1,500 145 49

Material properties used as baseline for analyses

Presenter

Presentation Notes

Odd values due to what appeared to be incorrect weathering labeling in the earlier testing program and due to unknown fractures in the samples. We have analyzed the data and come up with average UCS for analyses. Use 10% of UCS for anchor design or max 600psi. ling from earlier testing.

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A

B

D

C

EF

G

H

N

SLOPE ANALYSIS

Slopes Analyzed - Right Spillway Wall

Slope Stike Dip Dire Max HeightApprox. Location (Centerline

Stationing)A 080 170 115 5+25 to 8+00B 065 155 135 8+00 to 10+50C 050 140 105 10+50 to 13+50D 035 125 60 13+50 to 14+25

Slope is ~850 ftlong and changes orientation

Presenter

Presentation Notes

Original slope is 1:1 – constructed without benching, slope support, or drainage. Performed slope stability analyses like an open pit model. Ever 15 degrees along the existing spillway slope Slopes listed in table.

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CURRENT SLOPE

• 1:1 – No benches, slope support, or drainage.

• Evidence of small scale slide, wedge, and topple failures.

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STRUCTURAL ANALYSIS

Resultant stereonet used for failure analyses

Right abutment data set -

surface and subsurface

data

Selected joint sets

Set Name Strike (right) Dip Dip Direction

Set K 091 11 181 Set L 187 17 277

Set M 083 85 173 Set N 237 58 327 Set O 010 70 100 Set P 021 30 111 Set Q 353 46 083

Presenter

Presentation Notes

Initially all data (right and left) were combined, but the scatter was so great that the slopes were looked at individually. Still a lot of scatter, but some joint sets start to pop. 7 Joints were selected for the right abutment.

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KINEMATIC ANALYSIS

Example: Wedge failure potential at 25 degrees on slope A (DD170) along P and Q.

• Ran kinematic analysis for planar, wedge, and topple failures for slopes A, B, C, and D.

• ɸ=33 for initial then ran sensitivity analyses.

• Structural orientations become more favorable as we move upstream and as the slope is flattened.

• Results from this analysis were input into SWedge and RocTopple to check for block size and FS.

AB

D

CN

Slopes

Presenter

Presentation Notes

Can use Swedge to model slides by identifying a basal plan with 2 release planes.

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Topple failure potential on joints P&K

ɸ=33ᵒ

Wedge failure potential on joints P&Mɸ=30ᵒ

Planar failure potential on joint P

ɸ=30ᵒ

Wedge failure potential on joints P&Q ɸ=25ᵒ

A B

DC

SENSITIVITY ANALYSIS

• Ex: Slope C Sensitivity Analysis for ɸ:o This slope has the most

significant potential failures.

o Slope dip direction is 140 degrees.

o Slope = 1:1 and results in planar, wedge, and topple failure at varying friction angles.

AB

DC

N

Slopes

Presenter

Presentation Notes

Due to the unusual direct shear lab results (especially in SW samples) and due to the finding of clay seams, a sensitivity analysis was run on all slopes for all types of failures. From this the critical friction angle was found at which failure become a potential.

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SENSITIVITY ANALYSIS

• Example: Slope C Sensitivity Analysis for Grade:o All slopes at 1:1 have topple potential.o Most slopes have wedge or planar potential at ɸ’s close to 33ᵒ. o Considering material variability (geology, clay seams, lab) these

values could be possible onsite.o Varied grade to decrease or remove failure potential.

Potential wedge failure on joint sets P&M at ɸ=30

Shallow the slope to 1.2:1 (40 degrees)

Potential wedge failure goes away.

Example:

Presenter

Presentation Notes

Finally slope grades were run to see which produced stable slopes, then run to see which produced failure, but failures that could be mitigated by scaling, anchoring, or other methods. Went with those slopes

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BLOCK ANALYSIS

Example of existing wedge onsite. FS: 0.9238Vol: 120,033 ft3

NS

Example: Worse Case -Slope D at φ=30 degrees and a 1:1 slope Wedge Failure on P and M

Slope ~1.4:1 (36deg)FS: 0.9238Vol: 2,791 ft3

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FS: 0.9085Vol: 44,968 ft3

B

TULE RIVER – SLOPE ANALYSIS

Example: Slope D at φ=30 degrees and a 1:1 slope – Planar Failure

Slope ~1.3:1 (38deg)FS: 0.9238Vol: 2,791 ft3

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• Slopes stability varied based on orientation, structural data, rock properties, and water table.

• Most of the slopes had failure potential at 1:1 and ɸswithin site variability.

• As the slope was laid back most potential either went away or decreased in size so that they could be scaled or anchored in place.

FINDINGS

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From this analysis and findings the following slope grades are being designed:

• Colluvium, alluvium, soil, decomposed rock, or similar and greater than five (5) feet in thickness shall be designed at 2H:1V

• Slopes above the road bench in MW or worse rock should be designed at 1.5H:1V

• Slopes below the road bench in MW or better rock should be designed at 1.2H:1V

SLOPE GRADES

Generalized x-section

Presenter

Presentation Notes

NOTE: we solved the majority of the slope stability on this site by laying the slope back since it is already a very large excavation project, but another alternative would have been to design a different type of slope stabilization like pattern anchoring.

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• Update all analyses incorporating new site data

• Complete block analysis

• Complete slope support design: anchoring, drainage, spot shotcrete, netting, etc.

PATH FORWARD

Block ID*Block Depth into face (ft)

*Block Width along face (ft)

*Block Height along face (ft)

Approx. Total Volume of block

(cu.ft.)

Block Failiure Type

001 Right 3 6 3 54 Topple002 Right 4 6 4 96 Planar003 Right 4 6 4 96 Topple004R** 10 25 6 1500 Planar

005R 6 10 4 240 Topple006R 3 4 3 36 Planar

007R** 40 70 20 56000 Planar008R 4 10 4 160 Topple

009R* 38 40 15 11400 Wedge

Right Abutment

Onsite Block Cast Findings/Potential

Presenter

Presentation Notes

Analyses have decreased design and construction risk by reducing the number of onsite unknowns and allowing the design team to proceed forward with increased confidence in slope geometries and less risk of significant differing site conditions during construction

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THANK YOU :O)

rock mechanics and its effects on …...rock mechanics and its effects on spillway modification...

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