troutbrooke slope stabilization project class …review of rock fill dam • rock fill dam located...
TRANSCRIPT
Troutbrooke Slope Stabilization Project
Class Environmental Assessment
Community Liaison Committee
Meeting #1Meeting #1
November 24th, 2010
Agenda
• Project Purpose
• Class Environmental Assessment Process
• Results of Monitoring and Risk Assessment• Results of Monitoring and Risk Assessment
• Overview of Conceptual Options
• Discussion Period
• Next Steps
TRCA Erosion Control Monitoring
and Maintenance Program
• Program purpose is to protect public infrastructure, parklands,
recreational trails, and residential dwellings threatened by
erosion and slope instability issues arising typically from
historic planning and development decisions. historic planning and development decisions.
Project Location
� Provide long-term, low maintenance protection against erosion
and slope instability
� Prevent future property damage and reduce risk to public safety
Project Objectives
� Include enhancements to terrestrial habitat wherever possible
� Ensure compatibility with the surrounding physical, biological,
social and cultural environment
Class EA Planning and Design Process
Initiate Class EA – Nov ’10
Prepare Baseline Environmental Inventory – Nov ’10
Evaluate Alternatives – Nov/Dec ’10Evaluate Alternatives – Nov/Dec ’10
Select Preferred Alternative – Jan ’11
Analyze Environmental Impacts – Feb ’11
File Report for Review – Mar ’11
Role of CLC
• To assist TRCA in obtaining public input on
the project
• To identify items of concern related to the • To identify items of concern related to the
design of the project
• To assist in resolving issues of concern
Geotechnical and Slope Stability Assessment
• Geotechnical Investigation in 1991 following spring slope movement
• Movement had taken place through previously placed earth fill & retaining walls
• Dwellings did not appear to be affected• Dwellings did not appear to be affected
• Significant risk of additional movement within slope fill near slope crest
• Installed inclinometer casings at #51 and 49 – no significant movement found
Geotechnical and Slope Stability Assessment
• Geotechnical Investigation in 2009 following spring slope movement
• Movement through earth fill from #51 to 43, exposed foundation wall at #45
• Boreholes drilled, hand auger samples, inclinometers installed behind #45 & 41• Boreholes drilled, hand auger samples, inclinometers installed behind #45 & 41
• Study concluded:
• Slope conditions adequately safe and stable against deep seated slides
• Significant risk of additional slides in upper fills & retaining structures near
slope crest & dwellings
• Ongoing monitoring recommended
• Further investigation recommended to allow for final design
• Preliminary recommendations for remediation
Geotechnical and Slope Stability Assessment
• Current Study: initiated late September 2010
• Three additional boreholes (I1 to I3) and inclinometers on table land
between #49/47, behind 43 and 39. Now inclinometers at #51 to #39.
• Thirteen additional fill thickness boreholes (H1 to H13)
Geotechnical and Slope Stability Assessment
• Site Stratigraphy:
• Earth fill and rubble extends to depths of 1.1 to 7.6 m near the dwellings and
reduces to no fill part way down the slope
• Underlying native soils consist of competent very stiff or dense glacial till
deposits, overlying hard clay and silt
• All investigations (1991, 2009, 2010) found that the native soils were consistent• All investigations (1991, 2009, 2010) found that the native soils were consistent
• About 8,000 m3 of fill across site
• Ground water
• Consistently found in piezometers at ~ Elev. 155 m (about 4 to 5 m below
grade), within the glacial till deposit
Geotechnical and
Slope Stability
Assessment
• Typical Borehole Log
Soils Strength Moisture
Fill
Glacial
TillTill
Clay and
Silt Approx. Water
Table
Review of Rock Fill Dam
• Rock fill dam located 130 m west of #51, within Black Creek flood plain
• During flood events, water contacts slope toe
• 100 year storm water elevation ~ 145 m, about 2 m above toe (11 m below
slope crest)slope crest)
• Concern about “tea bag effect” or capillarity due to suction
• Capillarity / suction can only occur in unsaturated soils
• Clay and silt layer have moisture contents of 20 to 26%, and is therefore
already in saturated condition
• Even if not saturated, overlying glacial till is too coarse to be subject to
capillarity
• If capillarity had led to slope instability, it would have been near toe of
slope, not in the upper, oversteepened earth fill well above the native soils
Review of Options
• “Do Nothing”
• Remove existing fill and replace with an engineered slope
• Remove existing fill and replace with an engineered
mechanically stabilized earth wall
• Greenspace acquisition
Option 1: “Do Nothing”
• Upper, over-steepened earth fill slope will eventually self-flatten to a stable
inclination of about 2 H : 1 V
• Provide fencing, frost protection to foundations, ongoing monitoring
Advantages
• Low cost
• Low construction disturbance
• Low valley land impact
Disadvantages
• Very low level of stabilization to
slopes
• High tableland loss
• High potential impact to dwellings
• Unknown timeframe
Option 1: “Do Nothing”
Option 2: Remove Fill and Replace with an
Engineered Slope
• Remove existing fill and replace with one of three options:
• Sort existing fill and re-compact at 2.5 H : 1 V (~ 22 deg.)
• Replace with imported granular fill at 2.0 H : 1 V (~ 27 deg.)
• Replace with geogrid reinforced granular fill at 1.5 H : 1 V (~ 34 deg.)
• Re-vegetate final slope configuration
Advantages
• Medium level of slope stabilization
• Low impact to dwellings
• Known time frame
Disadvantages
• High construction disturbance
• Some tableland loss
• Little to no tableland in back yards
Option 2: Remove Fill and Replace with an
Engineered Slope
Option 2a: Remove Fill and Replace with an
Engineered SlopeSort existing fill and re-compact at 2.5 H : 1 V
Option 2b: Remove Fill and Replace with an
Engineered SlopeRemove existing fill and import granular fill at 2.0 H : 1 V
Option 2c: Remove Fill and Replace with an
Engineered Slope
Remove existing fill and replace with geogrid reinforced granular fill at 1.5 H : 1 V
Option 3: Remove Existing Fill & Replace with an
Engineered Mechanically Stabilized Earth Wall
• Remove existing fill and replace with a mechanically stabilized earth wall
with a face angle of 1 H : 1 V (~ 45 deg.)
• Geogrid reinforcement in structure
• ‘Soft’ vegetated face
• Backfilled with imported granular fill
Advantages
• Highest level of stabilization
• Low impact to dwellings
• Creation of tableland at each dwelling
• Known time frame
Disadvantages
• High construction disturbance
• Highest cost
Option 3: Remove Existing Fill & Replace with an
Engineered Mechanically Stabilized Earth Wall
Option 3: Remove Existing Fill & Replace with an
Engineered Mechanically Stabilized Earth Wall
Option 3: Remove Existing Fill & Replace with an
Engineered Mechanically Stabilized Earth Wall
Option 3: Remove Existing Fill & Replace with an
Engineered Mechanically Stabilized Earth Wall
Option 4: Greenspace Acquisition
Next Steps
• CLC members to complete feedback forms and return to TRCA by Friday
December 3, 2010
• TRCA to work with Terraprobe to modify/add/omit alternative options based
on input receivedon input received
• Next CLC meeting tentatively scheduled for mid-January 2010 to discuss
evaluation of alternatives and to select the preferred option
• Final CLC meeting will be held in March to discuss the project plan