stability analysis for the right abutment … · typical fracture: planar shape; smooth surface;...

BUILDING STRONG®and Taking Care of People!

“The views, opinions and findings contained in this report are those of the authors(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation.”“The views, opinions and findings contained in this report are those of the authors(s) and should not be construed as an official Department of the Army position, policy or decision, unless so designated by other official documentation.”

1File Name

Michael Nield, Senior Engineering GeologistU.S. Army Corps of Engineers, Huntington WVLRD Dam Safety Production CenterDam Safety Modification Mandatory Center of Expertise

STABILITY ANALYSIS FOR THE RIGHT ABUTMENT SPILLWAY ISABELLA DAM CA

AEG Annual MeetingSeptember 2017

Main Dam

BUILDING STRONG®and Taking Care of People!2

Stability Analysis for the Right Abutment Spillway – Isabella Dam, CAOUTLINE

A. PROJECT INFORMATION

B. SPILLWAY STABILITY

C. CUT-SLOPE STABILITY

1. Project Location2. Project Description3. Site Geology

1. Founding Elevations2. Plane Analysis3. Wedge Analysis

1. Stereonet Analysis2. Construction Concerns


Isabella Dam – Project LocationVicinity Map

ISABELLA DAM

Kern RiverWatershed

ISABELLA DAM

Bakersfield


Isabella Dam – Site Plan

4

MAIN DAM• 185 ft high• Zoned earth fill (almost homogeneous)• Foundation primarily granitic bedrock

AUXILIARY DAM• 100 ft high• Homogeneous silty sand • Foundation = alluvial soils

and bedrock

SPILLWAY• Ungated spillway• Ogee Weir

Constructed 1948-1953Primary Purposes: Flood control (~ 74%); Irrigation (~21%); Non-Federal Hydropower (~5%)

OUTLET WORKS• Hydropower


Erosion Along the Conduit

Erosion Through a Crack Near the Main Dam Crest

Undersized spillway

Hydrologic Overtopping

Seismic Stability of Borel Conduit

and Tower

Kern Canyon Fault (Previous Interp: Inactive)

Rupture Auxiliary Dam

Internal Erosion and Liquefaction Potential of the

Foundation Alluvium

Cross Section – Kern Caynon Fault – Auxiliary Dam

Primary Issues – Potential Failure Modes


Vicinity Map

ISABELLA DAM

Kern RiverWatershed

ImmediateInundation

Area

SubsequentInundation

Area

Consequences of Dam Failure - Inundation


Isabella Dam – Proposed Modifications

Auxiliary Dam

Kern Canyon Fault

Main Dam

ExistingService Spillway


Main Dam

Auxiliary Dam


Kern Canyon FaultProposed Emergency Spillway

UpstreamLabyrinth Weir

Emergency Spillway Physical Model

Proposed Modifications – Emergency Spillway


Main Dam

Auxiliary Dam



Proposed Modifications – Emergency Spillway


Auxiliary Dam

Kern Canyon Fault

Raise Top of Main Dam by 16’ Downstream Filter & Drain

Proposed Emergency Spillway

Main DamCross Section - Main Dam

Proposed Modifications – Main Dam


Auxiliary Dam


Main Dam


Proposed Modifications – Main Dam


Auxiliary Dam

Kern Canyon Fault

Raise Top of Auxiliary Dam by 16’ Downstream Buttress with Filter & Drain



Main Dam

Section - Auxiliary Dam Inside Fault Zone

Section - Auxiliary Dam Outside Fault Zone

Proposed Modifications – Auxiliary Dam


Auxiliary Dam

Kern Canyon Fault

Raise Top of Auxiliary Dam by 16’ Downstream Buttress with Filter & Drain



Main Dam

Proposed Modifications


Main Dam

Existing Service Spillway

State Route 155

Right Abutment Spillway

3D Image of Proposed Right Abutment Spillway

Proposed Modifications – Right Abutment Spillway


Site Geology Cretaceous-aged Alta Sierra Granite

Varying degrees of weatheringAlta Sierra (late Cretaceous)

Kern River Granite

(late Cretaceous)

Mount Adelaide (early Cretaceous)

Mount Adelaide (early Cretaceous)

Dam

Right Abutment Spillway Moderate to severely fractured

Light gray, medium grained, hard

Existing Service Spillway

Geologic Map – US Geologic Survey


Exploratory Drilling

Main DamRaised 16’

1F-13-13 1F-13-14 1F-13-15 1F-13-16 1F-13-17

1F-16-03

1F-16-04

1F-16-02

Proposed Right

Abutment Spillway

Boring Location Plan

Inclined BoringVertical Boring

Core Samples

Down-Hole Camera Images

Exploratory Drilling

Down-Hole Camera


Granite - Weathering Decomposed Granite: reddish brown,

soft, soil-like w/ relic bedrock structure, under 2,000 psi UCS

Moderately Weathered Granite: light reddish gray, moderately hard to hard, difficult to pluck individual grains with knife, 8,500 to 10,000 psi UCS.

Slightly or Unweathered Granite: Light gray, hard, 10,000 to 25,000 psi UCS.

Highly Weathered Granite: light reddish brown, soft to mod. hard, individual grains can be easily plucked with a knife, 2,000 – 8,500 psi UCS. Predominant rock type at spillway site.

Boring F1-13-15 Showing Varying Degrees of Weathering


Discontinuities

Moderate to intensely fractured

Typical Fracture: planar shape; smooth surface; occasionally open, iron stained or clay coated; and continuous (10’ to +100’ length)

Distinct Joint Sets (A - K)

Shear Zones

Main Dam

SR 155

Exposure at Site - Showing Low-Angled Joints

Aerial View - Showing High-Angled Joint Sets

BUILDING STRONG®and Taking Care of People! 20

Potential Spillway Erosion - Consequences

Oroville Dam - Spillway ErosionIsabella Dam – Right Abutment Spillway


Right Abutment Spillway - Design

MainDam

Plan View

Profile View

Upstream Dam Crest Downstream


Founding Elevation – Typical Upstream of Dam Monolith2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

HW

HW

MW

DG

SOIL (SM)

Boring F1-16-02

0100RQD WEATHERING

Temporary Detour Road

Foundation Concrete

DG Decomposed Granite

HW Highly Weathered Granite

MW Moderately Weathered Granite

SW Slightly Weathered Granite

UW Unweathered Granite

Soil

21

Cross Section – Station 101+70

Approximate Base of Near-Surface

Decomposed Granite

Foundation surface will be free of all soil, Decomposed Granite and loose rock.

Foundation geometry will consist of stair-stepped series of horizontalsurfaces.

DRAIN

Spillway

Top of Ground

Preliminary Foundation Elevations


Founding Elevation – Typical Dam Crest Monolith2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

SP

Boring F1-13-14

Foundation Concrete






Soil

21


Approximate Base of Near-Surface Decomposed Granite



DRAIN

Top of Ground

Dam Embankment

Approximate Top of Rock

Spillway

HW

HW

MW&

HW

DG

GP&

GM

Raised Dam Embankment



Founding Elevation – Typical Downstream Monolith2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

Boring F1-16-04

Foundation Concrete






Soil

21


Approximate Base ofNear-Surface Decomposed Granite



DRAIN

Top of Ground

Foundation Drain

Spillway

HW

DG

SW

Temporary Detour Road Preliminary Foundation Elevations


Preliminary Foundation ElevationsUpstream

Foundation Concrete Profile View

DownstreamDam Crest

Foundation DrainsJoint Drains

Core Samples from Boring 1F-16-02

Final foundation elevations will be established in the field.

Experienced on-site geologist is required.

Localized problematic areas are expected.

Dental concrete may be required.


Spillway Structure - Design Details

Expansion Joint DetailSection - Downstream - Foundation Drain

Joint Drain Detail Profile View

Waterstop

Dowel Bars

Joint Drain

Foundation Drain

Foundation Drains

Joint Drains


Planar Sliding Stability – Input Parameters

Sliding Resistance for Highly Weathered Granite:phi = 33 degrees, cohesion = 0 psi

5 degree waviness or i angle (as measured in field)

Maximum design earthquake: horizontal PGA 0.3g, vertical PGA 0.21g. Seismic coefficient 0.2 (2/3 of 0.3g)

Bedrock Exposures – Joint Orientations

Dam Construction Profile


Planar Sliding – Downstream Monolith – Load Cases2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

Uplift - Groundwater

Cross Section – Downstream of Dam – 14o Plane


Top of Ground

Spillway

Potential Failure Plane(47.8’ length, 14 degree inclination)

Required Factor of

SafetyDry Jointsno uplift Usual

Unusual

Extreme

Extreme

1.5

1.3

1.1

1.1

Rain - Wet Jointsfull upliftFlood PMFfull upliftEarthquake MDEno uplift

Load Case14o Plane

Loading Condition


Planar Sliding – Downstream Monolith – Load Cases2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

Cross Section – Downstream of Dam – 20o Plane


Top of Ground

Spillway

Potential Failure Plane(109’ length, 20 degree inclination)

Load Case20o Plane

Loading Condition

Required Factor of


Unusual

Extreme

Extreme

1.5

1.3

1.1

1.1

Rain - Wet Jointsfull upliftFlood PMFfull upliftEarthquake MDEno uplift


2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

SP

21



DRAIN

Top of Ground

Dam Embankment

Spillway

2680Planar Sliding – Dam Crest – Load Cases - Construction

Load Case20o Plane

Loading Condition

Required Factor of


Unusual

1.5

1.3Rain - Wet Jointsfull uplift

Uplift - Groundwater

Potential Failure Plane(47.8’ length, 14 degree inclination)


2D Planar Sliding Equation


2D Planar Sliding Stability – Summary & Sensitivity


3D Wedge Analysis

Wedge - Two Joints w/ Tension Plane

Hypothetical Wedge

Major Joint Sets


Detrimental Wedges

Major Joint Sets Detrimental Wedges

Utilized Swedge program from Rocscience

Two detrimental wedges:Joint Sets A&CJoint Sets D&E

Sliding resistance of Highly Weathered Granite: phi = 33o, c = 0 psi w/ 5o waviness added


Wedge Configurations

Joint Sets D&EWith Tension Plane

Joint Sets D&EW/O Tension Plane

Joint Sets A&CWith Tension Plane

Joint Sets A&CW/O Tension Plane

Tension Plane Tension Plane


Wedge Analysis – Load Cases

Dry JointsUsual

Rain EventWater Filled Joints

Unusual

PMF FloodWater Filled Joints

Extreme

MDE EarthquakeDry JointsExtreme

Weight of StructureAdded

HorizontalForce

Uplift

Added Weight of PMF Flow

Earthquake Force


3D Wedge Stability – Summary & Sensitivity

Sensitivity – Wedge D&E w/ Tension Plane (wet)


Cut-Slope Stability2680

2660

2640

2620

2600

2580

2560

0-20-40-60-80-100 20 40 60 80 100

SP

Foundation Concrete

21


Approximate Base ofNear-Surface Decomposed Granite

DRAIN

Top of Ground

Dam Embankment

Approximate Top of Rock

Spillway

Raised Dam Embankment


Cut-Slope


Kinematic Analysis – Input Parameters

Sliding Resistance for Highly Weathered Granite:phi = 33 degrees, cohesion = 0 psi

Existing Slope


Stereographic Projection – Planar Failure

West Side Cut Slope ½:1 East Side Cut Slope ½:1



Stereographic Projection – Wedge Failure



Stereographic Projection – Toppling Failure


Future Construction Concerns Overhead boulders or rock fall during

excavation.

Mechanical excavation methods

Joints with detrimental orientations and low sliding resistance (clay filled or fault gouge)

Localized problematic areas of Decomposed Granite.

Experienced on-site geologist is required

Construction Contract Status

Plans and Specifications completed

Advertised and Bids received

Notice to Proceed - soon

Approximately 4 years to construct

Boulders Above Proposed Cut-Slope

Hillside Above Lake Isabella


Stability Analysis for the Right Abutment Spillway – Isabella Dam, CACONCLUSIONS / TAKE AWAYS

1. Sliding stability for structures can be determined by utilizing equations and methods typically used for cut-slope design. (If the proposed structure is on the side of a hill)

2. Bedrock weathering can have significant impact on rock strength and influence structural design.

3. How can anyone can talk so much about such a small project?


Stability Analysis for the Right Abutment SpillwayIsabella Dam, CA

stability analysis for the right abutment … · typical fracture: planar shape; smooth surface;...

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