geotechnical engineering centre cricos provider no 00025b the mystery of unsaturated soil mechanics...

Geotechnical Engineering CentreCRICOS Provider No 00025B

THE MYSTERY OF UNSATURATED SOIL MECHANICS – SOME MINING

APPLICATIONS

Australian Geomechanics Society, SA & NT Chapter

Monday 15 July 2013, Adeliade

Professor David J WilliamsEmail: D.Williams@uq.edu.au

Geotechnical Engineering CentreCRICOS Provider No 00025B 2

Overview

Unsaturated soil mechanics continues to play poor relation to saturated soil mechanics

An unsaturated soil at a given density performs better than same soil in a saturated state

• Some mining applications:– Wetting-up and drain down of waste rock dumps– Drain down, desiccation and rewetting of mine tailings– Drain down, desiccation and re-wetting of product coal– Bearing capacity and deformation of mine wastes– Shear strength and compressibility of clay-rich spoil– Performance of geo-covers placed on mine wastes

INTRODUCTION

Key Unsaturated Soil Mechanics Parameters

1. Shear strength:– Or capacity of a soil to support load– Simplistically, shear strength can be tested under

unsaturated conditions (adding “cohesion”)

2. Compressibility:– Or deformation of a soil under an applied load– Simplistically, compressibility can be tested under

unsaturated conditions

3. Permeability (hydraulic conductivity):– Or rate of drainage under an applied load– Assessed by SWCC and ksat testing

Mining and Mineral Processing Wastes• Coarse-grained wastes (surface dumps):

– Overburden (typically up to 200 mm) or waste rock (up to 1 m), particularly from surface mining (limited from underground mining)

– Coarse reject (typically 50 mm) from coal processing– Slag or scats (typically 15 mm) from smelting– Spent heap leach material (typically 15 mm)

• Fine-grained wastes (tailings storage facilities):– Crushed and ground tailings (typically silt-size)– Erosion sediments (fines)

Waste Rock and Coarse Processing Waste Dumps

Waste rock dump

Pumped co-disposal of coal washery wastes

Coarse reject dump

Tailings Storage Facilities

Metalliferous tailings Iron ore tailings Coal tailings

Red mud Sand mining tailings In-pit coal tailings

WETTING UP AND DRAIN DOWN OF WASTE ROCK DUMPS

Impact of Waste Rock Wetting-Up

0 200 400 600 800 1000 1200 1400 1600 1800 2000

RAINFALL (mm/year)

Kalgoorlie, WA, average annual rainfall = 250 mm

Net evapotranspirative

Net infiltrative Moistureexcess

Potential impact of waste rock dump wetting-up

Average annual rainfall ~600 mm

Wetting-Up and Drain Down of Coarse-Grained Wastes

50 min ~60 mm

25 min ~30 mm

80 min ~100 mm

Start-up

Moisture Profile after 100 mm

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4

LENGTH (m)

Surface Infiltration into Cadia’s Trial Waste Rock Dump

Changing Infiltration into Cadia’s Trial Waste Rock Dump with Time

November 2007

May 2006

Note differential settlement-induced ponding

Average Base Seepage Beneath Top and Side Slopes of Cadia’s TWRD

Trigger Rainfall and Delay for Base Seepage Following Rainfall

0 100 200 300 400 500 600 700 800 900 1000

CUMULATIVE RAINFALL (mm)

Flat top lysimeter rainfall trigger

Angle of repose lysimeter rainfall trigger

Flat top lysimeter delay

Angle of repose lysimeter delay

Trend linefor trigger rainfall

Trend linefor delay

Estimated Wetting-Up and Continuum Breakthrough

0 1 2 3 4 5 6 7 8 9 10

Assumed rainfall infiltration of 450 mm(equivalent to 1.5 x 10-8 m/s)

January 2010

Possible actualbase seepage

Continuum breakthrough at:~25% saturated for fresh WR

~60% saturated for weathered WR

Estimated Time to “Continuum Breakthrough” of Waste Rock Dumps

0 20 40 60 80 100 120 140

Time to continuum breakthrough (years)

Start for 15 m height Full for 15 m heightStart for 30 m height Full for 30 m heightStart for 60 m height Full for 60 m heightStart for 120 m height Full for 120 m height

Increasingrainfall

Increasing time

Increasing dump height

17Typical mine life ~20 years, duringwhich time WRD is likely to be uncovered

DRAIN DOWN, DESICCATION AND REWETTING OF MINE TAILINGS

Impact of Tailings Slurry Deposition

0 200 400 600 800 1000 1200 1400 1600 1800 2000

RAINFALL (mm/year)

Kalgoorlie, WA, average annual rainfall = 250 mm

Net evapotranspirative

Net infiltrative Moistureexcess

Average annual rainfall ~600 mm

Assuming 0.5 m/year rate of rise at 25% solidsor 1 m/year at 50% solids

Mt Keith Tailings Laboratory Column Test

Settled tailingsDesiccated tailings

Re-flooded tailings

Comparison of Field and Laboratory SWCC Data for Mt Keith Tailings

0.1 1 10 100 1000 10000 100000 1000000

MATRIC SUCTION (kPa)

Average lab. re-drying SWCC Average lab. re-wetting SWCC

Field data to 0.2 m depth Field data below 0.2 m depth

Lab. column drying data

Tempe re-drying

Tempe re-wetting

Column dryingand re-drying

Laboratory Regime Field Regime

Comparison Between Field and Laboratory Hydraulic Conductivities

1.0E-22

1.0E-20

1.0E-18

1.0E-16

1.0E-14

1.0E-12

1.0E-10

1.0E-08

1.0E-06

0.1 1 10 100 1000 10000 100000 1000000

Estimated field function

Average laboratory function

Sensors on Towers in Trial TSF Cell

Piezometer

TDR moisture sensor

Matric suction sensor

Deposition in Trial TSF Cell

Desiccated tailingsFinal deposition

Early deposition Further deposition

Suction Data Collected from Northern TowerDeposition over 18 Months

0123456789

101112131415

0 50 100 150 200 250 300 350 400 450 500 550 600

1 @ 2 2 @ 2 3 @ 2 4 @ 2 5 @ 26 @ 2 7 @ 2 8 @ 2 9 @ 2 10 @ 211 @ 2 12 @ 2 13 @ 2 14 @ 2

Deposition Deposition

Matric Suction vs. Time

Suction Data Collected from Northern TowerDeposition from 17 May to 19 July & 26 Oct to 16 Nov 2007

0 10 20 30 40 50 60 70 80 90 100

) 01-Jun-07 15-Jun-07 01-Jul-0715-Jul-07 01-Aug-07 15-Aug-0701-Sep-07 15-Sep-07 01-Oct-0715-Oct-07 01-Nov-07 15-Nov-0701-Dec-07 15-Dec-07 31-Dec-07

4.8 mm on 28 July 0.2 mm on 28 Sept19.6 mm on 12 Dec

Matric Suction vs. Depth

DRAIN-DOWN, DESICCATION AND RE-WETTING OF PRODUCT COAL

Dewatering and Atmospheric Effects on Product Coal

Coarse Fine Ultra-Fine

Field stockpile

11.5 m

SWCCs of Product Coal Size Fractions

0.1 1 10 100 1000 10000

Composite - Drying data Composite - Wetting data Composite - Drying curveCoarse - Drying data Coarse - Wetting data Coarse - Drying curveFine - Drying data Fine - Wetting data Fine - Drying curveUltra-fine - Drying data Ultra-fine - Wetting data Ultra-fine - Drying curve

Duration of Applied Pressure – Centrifuging of Fine Product Coal

0.001 0.01 0.1 1 10 100

TIME (hours)

~ 22.5%

50% dewatering

90% dewatering

3.8 min 48 min5-15 s

41 to 42%

Duration of Applied Pressure – Vacuum Filtration of Ultra-Fine Coal

0.001 0.01 0.1 1 10 100

TIME (hours)

1-2 min

35 to 38%50% dewatering

90% dewatering

2.3 min 15 min

Vacuum filtration vs. Briquetting of Ultra-Fine Product Coal

0.01 0.1 1 10 100 1000 10000 100000 1000000

Filtered ultra-fine - Drying curve

Briquetted ultra-fine - Drying data

Briquetted ultra-fine - Wetting data

Sampling Field Coal Stockpile for PSD and Moisture Content

Wetting-Up of Model Coal Stockpile

Stockpile Total Moisture Content Distribution

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

DISTANCE FROM CENTRELINE (m)

6.3 12.3 12.9 8.5 11.1 11.9 10.0 12.1 9.5 7.4 10.5 12.6 8.2

7.7 8.2 7.7 7.7 7.8 7.8 6.3 7.5 6.0 5.5 3.7

4.7 7.7 7.7 7.5 6.9 7.6 7.2 7.8 3.5

7.0 6.5 6.8 5.3 5.6 3.2

6.7 6.2 5.7 6.1 4.6

12.1 12.1 12.3 11.2 10.5 9.2 5.0

5% total moisture content contour7.5% total moisture content contour10% total moisture content contour

BEARING CAPACITY AND DEFORMATION OF MINE WASTES

Tailings Bearing Capacity Issues

“Bow-wave”

Too thin a cover over soft tailings

“Bow-waving” of crusted tailings

Covering Wet Tailings –Victorian Gold Mine

Surcharging edge of tailings

Spreading thin cover

D6 dozer

Ponded water due to drainageof excess pore water pressure

2-3 m surcharge

1 m cover, avoiding“bow wave” failure

Shear Strength Profiles of Coal Tailings

Safe Height of Fill on Coal Tailings

H = Nc.sv/F.~5.14.sv/(3 x 18)

~0.095 sv

SHEAR STRENGTH AND COMPRESSIBILITY OF CLAY-RICH

Jeebropilly Spoil – Relative Unscalped PSDs

CLAYWeathered

Rock Unweatheredcoarse-grained

Rock Unweatheredfine-grained

Mt Arthur Spoil – Relative Unscalped PSDs

Gravimetric Moisture Content vs. Matric Suction

Jeebropilly rocky spoil is less dense and wetterthan Hunter Valley spoil, although similar degree

of saturation, and hence matric suction

Jeebropilly Weathered Rock PSDs

Comparison of Direct Shear Friction Angles (-2.36 mm)

0 200 400 600 800 1000

NORMAL STRESS (kPa)

Jeebropilly Weathered Rock tested dryJeebropilly Weathered Rock tested wetMt Arthur Sandstone tested dryMt Arthur Sandstone tested wet

c' = 0 kPa, ' ~ 26.6o

c' = 0 kPa, ' ~ 35.6o

c' = 0 kPa, ' ~ 41.5o

c' = 0 kPa, ' ~ 40.7o

Note substantial softening on wetting upof clay-rich Jeebropilly Spoil

UPC 150 mm, 10 MPa Consolidometer

76 & 150 mm Consolidometer Testing of Jeebropilly Weathered Rock

Note low loose density and substantial consolidationon loading and wetting of clay-rich Jeebropilly Spoil

76 & 150 mm Consolidometer Testing of 3-Month Old Mt Arthur Sandstone

Note high loose density and limited consolidationon loading and wetting of Mt Arthur Sandstone

Large Texture Changes on Weathering of Jeebropilly Weathered Rock (-19 mm)

0 days 8 days (32.6 mm rain)

21 days (+6.8 mm rain)15 days (+23.2 mm rain)

Large PSD Changes on Weathering of Jeebropilly Weathered Rock (-19 mm)

Settlement of Jeebropilly Weathered Rock with Time on Weathering (-19 mm)

Minimal Texture Changes on Weathering of Mt Arthur Sandstone (-19 mm)

0 days 7 days (4.0 mm rain) 14 days (+39.2 mm rain)

28 days (+6.8 mm rain)21 days (+47.8 mm rain)

PERFORMANCE OF STORE AND RELEASE COVERS PLACED ON

MINE WASTES

Influence of Climate – Natural Recharge (Beekmann et al., 1996)

ARIDSEMIARID Av. rainfall ~600 mmpa

~70 mm~2.2 x 10-9 m/s

Store and Release Cover on Mine Wastes

Evaporationfrom ponded water

Evapotranspirationfrom vegetated surface

Rainfall

Infiltration and storage

Nominal 0.5 m compacted seal

Minor infiltration

Capillary break over tailingsor trafficked waste rock surface

Limited oxygendiffusion

and infiltration

Angle of repose waste rockor tailings

Seepagealong ~1% slope

Nominal 1 to 2 mloose, rocky soil mulch

Minor infiltration

Limited oxygendiffusion

and infiltration

Seepagealong ~1% slope

Evapotranspirationfrom vegetated surface

Rainfall

Infiltration and storage

Nominal 0.5 m compacted seal

Seepagealong slope

Nominal 1 to 2 mloose, rocky soil

Store/releasecover

Construction of Store and Release Cover on Kidston’s Waste Rock Dumps

Rocky soil mulch, capable of storingup to 300 mm rainfall/m thickness

Compacted sealing layer

Vegetation of Kidston’s Store and Release Cover12-month old trees (no

grasses) – December 200218-month old trees & 6-month

old grasses – July 2003

2-year old trees and 1-year old grasses

0.35 0.75 1.37 1.75

Sensor Depths (m)

S ~ 1.0, w ~ 45%

Wet season avS ~ 0.7, w ~ 20%

Dry season minS ~ 0.2, w ~ 5%

Compacted Clayey layer

Rocky soil mulch

0200400600800

Oct-97

) Total for 97/98 = 623 mm

Oct-97

0.35 0.75 1.37 1.75

Sensor Depths (m)

S ~ 1.0, w ~ 45%

Wet season avS ~ 0.7, w ~ 20%

Dry season minS ~ 0.2, w ~ 5%

Compacted Clayey layer

Rocky soil mulch

Up to 2-week lag

k ~2 x 10-8 m/s

Kidston – Field SWCC Data and Fitted Curves

0.1 1 10 100 1000 10000 100000 1000000

0.35 m depth (field data)0.35 m depth (fitted curve)1.00 m depth (field data)1.00 m depth (fitted curve)1.37 m depth (field data)1.37 m depth (fitted curve)1.77 m depth (field data)1.77 m depth (fitted curve)

Unreliable

Desiccatedsurface

Wetsealinglayer

Store and Release Net Percolation

0 2 4 6 8 10 12 14 16 18 20

AGE (years)

Kidston

Mt Whaleback 2 m ROM-av

Mt Whaleback 2 m ROM-peak

Mt Whaleback 4 m ROM

Goldstrike

Mt Whaleback Store and Release Cover (O’Kane et al, 2012)

CRICOS Provider No 00025B

Note: No base sealing layer and constructed with

coarse-grained ROM waste rock, i.e. poorly-

constructed

Geotechnical Engineering CentreCRICOS Provider No 00025BCRICOS Provider No 00025B

Mt Whaleback Store and Release Cover (O’Kane et al, 2012)

Note: A well-constructed store and release cover

should store ~250 mm/m before percolating

Kidston – Schematic Wetting-Up and Drain Down with Time

0 5 10 15 20 25 30 35 40

TIME (years)

During operation Post covering

Rainfall infiltration,diminishing exponentially

as surface compactsand with cover

Base seepage, increasing asdump wets-up, diminishing

exponentially with coverdue to drain down

Final seepage= net percolation

~7 mm/year,on average

NEW GEOTECHNICAL ENGINEERING CENTRE AT

THE UNIVERSITY OF QUEENSLAND

New Geotechnical Engineering Programs @ UQ• Commenced February 2012• Funded by Golder Associates, Rio Tinto,

AngloGold Ashanti and BHP Billiton, in partnership with UQ– Industry Partners each contributing ~$150,000/year– Matching funding from UQ– Total funding of $6 million over 5 years– 3 new academic appointments, making a total of 7:

• Professor of Rock Mechanics (to be appointed)• Lecturer in Rock Mechanics (Dr Nazife Erarslan)• Lecturer in Hydrogeology (offered)

– Plus supporting PostDocs

The GEC Vision• New Civil & Geotechnical Engineering and Mining

& Geotechnical Engineering Dual Major Programs will produce:– Civil and Mining Engineering graduates with a

specialisation in Geotechnical Engineering, unique in Australia! (Canada has 5 or 6 such programs, e.g., making their graduates much sought after by Australian companies)

– Graduates in high demand from Geotechnical and Mining Engineering Consultants, Mining Companies, and Civil and Mining Contractors

– New Programs will have 70-75 graduates/year– Collaborative research with our industry partners

Civil & Geotechnical Engineering4-Year Program

2 (6%) Soil MechanicsCourses → 10

(31%) GeotechnicalEngineering Courses

(in Green)

STAT2201 Analysis of Eng & Sci Data

CIVL2130 Env Issues, Monitoring & Assessment

First Year Elective (or MATH1050, to make up

for MATHS C)

BE Civil & Geotechnical EngineeringFirst Year Elective

(Recommended: ERTH1501 Earth Processes &

Geological Materials for Engineers)

MINE3121 Mining Geomechanics

(Fundamentals of Rock Mechanics ) or ERTH1501

CIVL2360 Reinforced Concrete

Structures

CIVL4270 Geotechnical Investigation

and Testing (New)

ERTH3250 Hydrogeology (New)

1ENGG1100

Introduction to Engineering Design

MATH1051 Calculus & Linear Algebra 1

First Year Elective

2ENGG1200

Introduction to Engineering Problem Solving

MATH1052 Multivariate Calculus & ODEs

ENGG1400 Statics & Dynamics

3 CIVL2330 Structural Mechanics

MATH2000 Calculus & Linear Algebra II

CIVL2410 Traffic Flow Theory and

Analysis2n

4CIVL2340

Introduction to Structural Design

CIVL2131 Fluid Mechanics

CIVL2210 Fundamentals of Soil

Mechanics

5 CIVL3340 Structural Analysis

CIVL3140 Catchment Hydraulics

CIVL3210 Geotechnical Engineering (Applied Soil Mechanics )

6 CIVL3350 Structural Design

CIVL3141 Catchment Hydrology

CIVL3420 Transportation Systems

Engineering

7CIVL4511 or CIVL4512

Civil Design

MINE4120 Mine Geotechnical

Engineering (Applied Rock Mechanics)

Geo Elective or Geo Project/Thesis

(CIVL4560 or CIVL4580) or MINE3121

8CIVL4520

Civil Engineering Management

ERTH2004 Structural Geology or

Geo Elective or Geo Project/Thesis

(CIVL4560 or CIVL4580)

CIVL4280 Advanced Rock Mechanics

EXISTING

NEWNEW

Mining & Geotechnical Engineering4-Year Program

5 (15%) MiningGeomechanics

Courses → 12 (38%)Geotechnical

Engineering Courses(in Green)

STAT2201 Analysis of Eng & Sci Data

MINE2123 Structural Mechanics in Mining

BE Mining & Geotechnical Engineering

1ENGG1100

Introduction to Engineering Design

MATH1051 Calculus & Linear Algebra 1(or MATH1050, to make up

for MATHS C)

First Year Elective

ERTH1501 Earth Processes &

Geological Materials for Engineers

First Year Elective (or MATH1052 Multivariate

Calculus & ODEs)

3 MINE2105 Introduction to Mining

ERTH2004 Structural Geology2n

4CIVL2210

Fundamentals of Soil Mechanics

MINE2210 Physical & Chemical

Processing of Minerals

MINE2106Resource Geology &

Surveying

MATH2000 Calculus & Linear Algebra II

2ENGG1200

Engineering Modelling and Problem Solving

MATH1052 Multivariate Calculus

& ODEs(or MATH1051 Calculus &

Linear Algebra 1)

ENGG1400 Statics & Dynamics

6 MINE3123 Mine Planning

MINE3123 Mine Ventilation I

MINE3125 Rock Breakage

5 MINE3120 Resource Estimation

MINE3121 Mining Geomechanics

(Fundamentals of Rock Mechanics )

MINE3122 Mining Systems

7MINE4122

Mining Geomechanics Research Project I

MINE4124 Hard Rock Mine Design &

Feasibility

8MINE4123

Mining Geomechanics Research Project II

MINE4125 Coal Mine Design &

Feasibility

CIVL4270 Geotechnical Investigation

and Testing (New)

MINE2101 Fluid Mechanics

MINE4120 Mine Geotechnical

Engineering (Applied Rock Mechanics)

MINE4121 Mine Management

MINE4128Coal Mine Strata Control

CIVL4280 Advanced Rock Mechanics

ERTH3250 Hydrogeology (New)

EXISTING

EXISTINGEXISTING

EXISTING

Expected Geotechnical Engineering Graduations

Postgraduate Research Students• PhD students in Geotechnical Engineering:

– Currently 20– > 5 applications pending

• Research topics include:– Mine tailings water cycle– Effectiveness of liners for water storage ponds– Settlement of high coal mine spoil– Flow in coal seams– Behaviour of screw auger piles in clay– Problematic clay-rich coal mine tailings– Mining below the groundwater table– Erosion of granular structures– Computational geomechanics– Development and application of spatial Time Domain Reflectrometry

geotechnical engineering centre cricos provider no 00025b the mystery of unsaturated soil mechanics...

b waste rock

b mining

b wetting

b introduction slide

waste rock dumps slide

b impact of waste rock

startup slide

b tailings storage facilities

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