Troutbrooke Slope Stabilization Project Class Environmental Assessment

Transcription

1 Troutbrooke Slope Stabilization Project Class Environmental Assessment Community Liaison Committee Meeting #1 November 24 th, 2010

2 Agenda Project Purpose Class Environmental Assessment Process Results of Monitoring and Risk Assessment Overview of Conceptual Options Discussion Period Next Steps

3 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.

4 Project Location

5 Project Objectives Provide long-term, low maintenance protection against erosion and slope instability Prevent future property damage and reduce risk to public safety Include enhancements to terrestrial habitat wherever possible Ensure compatibility with the surrounding physical, biological, social and cultural environment

6 Class EA Planning and Design Process Initiate Class EA Nov 10 Prepare Baseline Environmental Inventory Nov 10 Evaluate Alternatives Nov/Dec 10 Select Preferred Alternative Jan 11 Analyze Environmental Impacts Feb 11 File Report for Review Mar 11

7 Role of CLC To assist TRCA in obtaining public input on the project To identify items of concern related to the design of the project To assist in resolving issues of concern

8 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 Significant risk of additional movement within slope fill near slope crest Installed inclinometer casings at #51 and 49 no significant movement found

9 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 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

10 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)

11 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 About 8,000 m 3 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

12 Soils Strength Moisture Fill Geotechnical and Slope Stability Assessment Typical Borehole Log Glacial Till Clay and Silt Approx. Water Table

13 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) 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

14 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

15 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

16 Option 1: Do Nothing

17 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

18 Option 2: Remove Fill and Replace with an Engineered Slope

19 Option 2a: Remove Fill and Replace with an Engineered Slope Sort existing fill and re-compact at 2.5 H : 1 V

20 Option 2b: Remove Fill and Replace with an Engineered Slope Remove existing fill and import granular fill at 2.0 H : 1 V

21 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

22 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

23 Option 3: Remove Existing Fill & Replace with an Engineered Mechanically Stabilized Earth Wall

24 Option 3: Remove Existing Fill & Replace with an Engineered Mechanically Stabilized Earth Wall

25 Option 3: Remove Existing Fill & Replace with an Engineered Mechanically Stabilized Earth Wall

26 Option 3: Remove Existing Fill & Replace with an Engineered Mechanically Stabilized Earth Wall

27 Option 4: Greenspace Acquisition

28 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 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