Troutbrooke Slope Stabilization Project Class …Review of Rock Fill Dam • Rock fill dam located 130 m west of #51, within Black Creek flood plain • During flood events, water

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 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


• 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)


• 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 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

Documents

Troutbrooke Slope Stabilization Project Class …Review of Rock Fill Dam • Rock fill dam located 130 m west of #51, within Black Creek flood plain • During flood events, water