Slurry Wall - Saskatchewan Mining Association

Rocanville Slurry Wall
Presented by: D. Chad LePoudre – Vice President, SNC-Lavalin
SMA Conference – Saskatoon, SK
October 29, 2014
Speaker Introduction
D. Chad LePoudre, P.Eng.
Vice President, Geoenvironmental LOB
SNC-Lavalin
Education
- B.Sc. Civil Engineering
Employment History
- Came to SNC-Lavalin through acquisition of MDH
in 2011
- Joined MDH in 2003
Acknowledgment
Dr. Moir D Haug, P.Eng.
Senior Technical Advisor
Previous: President, MDH Engineered
Solutions
Previous: Department Head, Civil and
Geological Engineering, U of S
Moir has always been readily available to
provide guidance and lend his
significant experience to this project.
Acknowledgement
Thank you to PotashCorp to allowing us to
present today
Outline
1.
2.
3.
4.
5.
6.
7.
Speaker Introduction
Outline
What is a slurry wall?
Site orientation / layout
Timeline
Technical Summary
Challenges
a) Technical
b) Construction
c) Management
8. Lessons learned
What is a slurry wall?
A bit of a misnomer. Really what we’re constructing at Rocanville is
1. Soil-bentonite cut off wall; and,
2. Cement-bentonite cut off wall (at remote locations).
The expression ‘slurry’ wall is used since we have a slurry-supported
excavation
Note: Other types of cutoff walls
exist. This is referred to as a
‘conventionally excavated soilbentonite cut-off wall’.
Slurry wall
Consider a typical stratigraphic cross-section
Slurry Wall
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Clay Shale
Slurry Wall
Kmax = 1x10-9 m/s
3 to 6 orders of magnitude difference
or
1,000 to 1,000,000 times difference
K =~ 1x10-3 m/s to 1x10-6 m/s
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Glacial Till
Sand & Gravel
Clay Shale
Excavation
Chiseling on Rocks
Backfill Mixing
Location
K2
Timeline
•
•
•
2008 EIS submission – Rocanville West Expansion Project
• TMA Expansion and Slurry Wall included as part of the expansion
• Drilling Investigation to support slurry wall design
• Landowner consultations – Alignment redesign
2008 Fall – Construction Tendering
2009
• Start of construction
• Supplementary drilling for new alignment
2012 – Anticipated completion of construction
2014 – revised completion of construction schedule
•
2015 – Actual completion of construction…
•
•
Brief Technical Summary
•
•
•
•
•
•
•
10.7km in length
~24m to 51m depth
361,000 vertical square meters
• One of largest in the world
6-7 years construction (2009 – 2015)
Max K = 1x10-9 m/s
Approximately $90+ million
Site investigation, Stratigraphic
interpretation, Permitting, Design, EPC for
slurry wall, working platform and related
activities
Backfill gradation
•
•
Approximately 4-8 engineering staff
Approximately 8-40 contractor staff
•
Much more boring stuff (maximum sand
content, backfill slump, backfill survey
frequency….. QA/QC)
Challenges (highlights)
Technical
Alignment selection
Slurry wall depth
Potential for Arching
Construction
Very large project
Site Constraints
Management
Multiple construction seasons
Alignment Selection
QU’APPELLE RIVER VALLEY
Groundwater Discharge
to Alluvial Sediments
G
Welby
Channel
Aquifer
West
Channel
Aquifer
G’
Private Landowners
1
2
1
Unconfined aquifer at depth due to
discharge to Qu’Appelle River
Valley
2
Stacked unconfined
aquifers due to
discharge
Alignment Selection
Stage 1 plus
-Image presented in
EIS
-Existing TMA =
~233ha
- Stage 1 + Stage 2 =
~330ha
-2 stages due to south
landowner
negotiations
Slurry Wall Design Depth
Stn. 5+906.62
Stn. 7+912.53
Slurry Wall Design Depth
Potential for Arching?
Some concern regarding potential
for Arching of Backfill due to:
Depth
Strength of backfill
Inconsistent consolidation
Potential for Arching?
Masters Study Research Project and Laboratory Program
“Time Dependant Differential Consolidation of Slurry Trench Backfill” Rahul Vishwanath Mukherjee, Dr. Moir D. Haug.
- research sponsored by the Saskatchewan Potash Producers Association
That the potential for
arching can be minimized
by adjusting the gradation
of the backfill material.
That
this
change
in
gradation should not result
in an increase in hydraulic
conductivity.
Potential for Arching
a)
Arching theory
b)
Lateral squeezing theory
Forces due to Arching

Effect of fine content on consolidation and hydraulic conductivity
Depth (m)
Ground surface / top of the slurry wall
Due to arching???
Geostatic = ρgh
Due to accumulated
side wall friction
Stress (kPa)
Vertical stress distribution with depth of the wall
σv = B*ϒ/ (µ*k) *(1- exp-(µ*k*z/B))
K = 0.35
ϒ = 18 kN/m3
0
Calculated Vertical Stress KPa
100
200
300
400
500
600
0
5
Geostatic stress
B = 0.6 m
10
Depth (m)
B=1m
15
B= 1.5 m
B=2m
20
25
30
PCS
35
Field Vertical Stress vs Depth for Different Trench Widths
Findings
1. The angle of internal friction for backfill mixes decreased with the
increase in fine content (f = 32o to 23o for 10% to 50% fines, respectively).
2. The hydraulic conductivity for backfill mix decreased with the increase
in fine content and vertical stress (i.e. from 1E-05 m/s to 1E-10m/s).
3. The co-efficient of lateral earth pressure decreased from 0.38 to 0.35 with
depth.
4. Arching was found to limit vertical stress in the backfill to less than 5m
geostatic equivalent for a 1m wide wall.
Potential for Arching
Instrumentation Installation
Potential for Arching
Instrumentation Installation
Potential for Arching
Instrumentation Installation
Potential for Arching
Instrumentation Installation
850.0
510.00
Pore pressure
800.0
500.00
Horizontal stress
Total vertical stress
490.00
750.0
Pressure (kPa)
480.00
700.0
470.00
650.0
460.00
600.0
450.00
550.0
440.00
500.0
430.00
05/08/2011 0:00 25/08/2011 0:00 14/09/2011 0:00 04/10/2011 0:00 24/10/2011 0:00 13/11/2011 0:00 03/12/2011 0:00 23/12/2011 0:00
Days
Back fill elevation (m)
Blackfill elevation
Construction Challenges
1. Very Large Project (world record)
a) Only large, qualified contractors were invited to bid
b) Several risks for remote contractors
•
•
Water supply
Local contractors (working platform, surcharge berm)
2. Site Constraints
a)
b)
c)
d)
Geological
Other Landowners
Environmental
Ancillary (roads, rail, power, water, gas, infrastructure)
3. Floods!
Ancillary Infrastructure
Low strength cement-bentonite panels for roads and rail
Low strength cement-bentonite panels for roads
Water supply at Qu’Appelle
Natural Drainage
Water video
Record runoff / precipitation events in
2012
And then again in 2014
Management Challenges
• Almost complete turnover of staff:
• PotashCorp Site Management
• Contractor Management and Laborers
• MDH/SNC staff (actually not too bad)
• Consultant was acquired by large Engineering Company
• Contractor was acquired by Equity Firm
• Management of change difficult when staff turnover
Summary and Lesson
Learned
• The ‘Stanley Cup’ of geotechnical projects
• Excellent communication between Owner and Consultant,
due largely to committed site and project staff
Lessons Learned:
a) Perhaps a bonus/penalty clause in the contract
b) Record keeping is essential
c) Staff bonus for project completion
d) Opportunity for Design/Build approach
Thank you
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