SLOPE STABILITY DEFINITION & DETERMINATION GUIDELINE

Transcription

1 LONG TERM STABLE LINE: consists of the Stability Component and the Erosion Component STABILITY Definition: The setback gradient line measured from the toe of the slope, or channel assuming the location of the toe remains fixed with time. (See Figures, 3 and 4b and 4c). Factors for Consideration: soil strength changing load conditions groundwater conditions weathering of slope face slope geometry increases in surface runoff over slope condition of vegetation Method of Calculation: There are three methods of establishing this component. Each method is progressively more involved as indicated in Figure 2. Factor of Safety: Minimum Factor of safety of.5 is required. EROSION Definition: The regression of the slope toe/channel bank due to erosion over the design life of the structure at the crest of the slope and is measured as a horizontal distance. (See Figures, 4b, and 4c). Factors for Consideration: proximity of the slope toe to the sediment load carried by the watercourse watercourse average and peak flow rates and velocities of the watercourse susceptibility of the soils to erosion increases in surface runoff over slope type and extent of vegetation weathering of slope face Method of Calculation: fluvial geomorphological processes affecting the reach within which the site is located. As outlined in Figure 4a, the distance from the toe of the valley wall to the watercourse channel bank as well as the design erosion allowance must be determined. The erosion is measured horizontally from the top of the channel bank or the location of the bankfull flow, whichever is lower in elevation (Figure 4c). DEVELOPMENT Definition: A minimum allowance from the identified slope hazard area to take into account external conditions which could have an adverse effect on the existing natural conditions of the slope. This setback distance maybe superseded by more stringent municipal or provincial requirements. For minimum allowance refer to CVC s Watershed Planning and Regulation Policies (200). Factors for Consideration: provide an access point along the crest of the slope allow for the placement of sediment control measures and limit of working easement if necessary. keep heavy equipment away from the provide tableland area for potential future revegetation slope allow for the redirection of surface flows away form the slope hazard area and/or reforestation (e.g. Credit Valley Conservation Authority planting programme) Method of Calculation: Measured as the horizontal distance from the approved top of bank or from the combined distance derived from the Stability and Erosion Components whichever is the greater. (see Figure ). February 204 /8

2 Figure. Typical Valley Wall Slopes FIGURE a: STABLE WELL VEGETATED VALLEY WALL WITH WIDE FLOODPLAIN OR EROSION PROTECTION NORMAL RIVER WATER LEVEL VALLEY WALL DEVELOPMENT FUTURE DEVELOPMENT WALL SHALLOWER THAN GRADIENT FIGURE b: OVERSTEEPENED VALLEY WALL WITH WIDE OR EROSION PROTECTION PROPERTY LINE STABILITY DEVELOPMENT NORMAL RIVER WATER LEVEL GRADIENT LINE FIGURE c: OVERSTEEPENED VALLEY WALL SUBJECT TO TOE EROSION PROPERTY LINE STABILITY EROSION DEVELOPMENT NORMAL RIVER WATER LEVEL GRADIENT LINE February 204 2/8

3 Figure 2. Stability Component Determination PROCEDURE FOR ESTABLISHING STABILITY OF TOTAL STABILITY (S.C.) IS A FUNCTION OF THE GEOMETRY AND THE SOIL, BEDROCK AND GROUNDWATER CONDITIONS. IS HEIGHT GREATER THAN 2m? NO (S.C. = 0) YES REQUEST PROFILE IS FLATTER THAN 3 H:V? YES (S.C. = 0) NO DETERMINE BASED ON ASSUMED GRADIENT LINE EQUAL TO 3H:V DOES CONFLICT WITH PROPONENTS WORK? NO (S.C. DETERMINED) YES STUDY REQUIRED BY GEOTECHNICAL ENGINEER TO REFINE BASED ON SITE SPECIFIC CONDITIONS: PROCEDURE A) OR B) OR BOTH DEPENDING ON CONDITIONS AND PROPOSED WORKS. A) ROUTINE INVESTIGATION B) DETAILED INVESTIGATION CLASSIFY SOIL AND GROUNDWATER CONDITIONS AT SITE BY TEST PITS AND/OR HAND AUGER HOLES USE GENERALIZED GUIDELINES (FIGURE 3) GEOTECHNICAL BOREHOLES WITH PIEZOMETER INSTALLATIONS FOR DETAILED SOIL CLASSIFICATION AND GROUNDWATER LEVELS USE SITE SPECIFIC STABILITY ANALYSES TO DETERMINE SAFE DISTANCE (MIN. FACTOR OF SAFETY =.5) S.C. DETERMINED S.C. DETERMINED; MIN F.S. =.5 February 204 3/8

4 Figure 3. Generalized Stability Setback Guidelines GUIDELINES FOR ESTABLISHING STABILITY OF TOTAL FOR TYPICAL BEDROCK / SOIL CLASSIFICATIONS WITHIN THE CREDIT RIVER WATERSHED = H LIMESTONE / DOLOMITE BEDROCK H TOE GRADIENT LINE.4 = H SHALE BEDROCK SHALE BEDROCK.4 H TOE GRADIENT LINE 2 = H SAND AND GRAVELLY SAND DRY H TOE 2 GRADIENT LINE 2.5 = H CLAY TILLS THE GRADIENT LINE IS TO BE DETERMINED BY GEOTECHNICAL ENGINEER FOR ALL OTHER SOILS INCLUDING: CLAY (NON TILL) S SAND S WITH WATER SEEPAGE FILL S 2.5 GRADIENT LINE H TOE February 204 4/8

5 Figure 4a. Erosion Component Determination PROCEDURE FOR ESTABLISHING EROSION OF TOTAL EROSION (E.C.) IS A FUNCTION OF THE RATE OF EROSION OF THE STREAM BANK OR VALLEY WALL. DETERMINE WIDTH (F.P.) : DISTANCE FROM TOE OF VALLEY WALL TO EDGE OF STREAM BANK IS F.P. GREATER THAN 5m? YES (E.C. = 0) DETERMINE DESIGN TOE EROSION ALLOWANCE (D.E.A.) FROM TABLE BELOW OR CALCULATE BASED ON HISTORIC RECORDS FOR SITE CALCULATE E.C. : E.C. = D.E.A. F.P. E.C. DETERMINED Notes: - E.C. cannot be less than zero - if F.P. is greater than D.E.A. then E.C. = 0 SUGGESTED DESIGN TOE EROSION ALLOWANCE MATERIAL AT CHANNEL BANK OR BANK FULL BANK CONDITION ACTIVE EROSION OF BANK EROSION CURRENTLY NOT EVIDENT EXISTING EROSION PROTECTION IN PLACE AND MAINTAINED ALONG BANK LIMESTONE DOLOSTONE 2m m 0 SHALE 5m 2m 0 COHESIVE SOILS; SILTY CLAYS, CLAYEY SILTS COHESIONLESS SOILS; SILTS, SANDS 8m 4m 0 5m 7m 0 February 204 5/8

6 Figure 4b. Calculation of Erosion Component of Total Setback with Defined Valley Slope (A) WIDTH GREATER THAN DESIGN EROSION ALLOWANCE EROSION OF (E.C. = 0) (F.P.) DESIGN EROSION ALLOWANCE (D.E.A.) (B) WIDTH LESS THAN DESIGN EROSION ALLOWANCE EROSION OF (E.C. = D.E.A. F..P.) (F.P.) DESIGN EROSION ALLOWANCE (D.E.A.) (C) NO AT THE TOE OF EROSION OF (E.C. = D.E.A.) NO (F.P. = 0) DESIGN EROSION ALLOWANCE (D.E.A.) February 204 6/8

7 Figure 5. Example of Stability, Erosion and Development Setback Components REAR PROPERTY LINE = 2m (IDENTIFIED HAZARD AREA) FRONT LOT LINE REAR LOT LINE TOP OF BANK Development Setback Component Erosion Component = 5m Stability Component = 6m ELEVATION IN METERS RECOMMENDED ZONING RESIDENTIAL OPEN GREENBELT/HAZARD LAND EXISTING SPACE OVERBURDEN.7 STABLE GRADIENT FOR OVERBURDEN BEDROCK 00 SLIDE DEBRIS 00 TOE OF 95 CREDIT RIVER STABLE GRADIENT FOR SHALE BEDROCK 90 February 204 7/8

8 Figure 6. The Physical Features of a Typical Valley VALLEY PRIMARY TOP OF BANK TABLELAND TOP OF BANK UPPER VALLEY TABLELAND PLATEAU (TERRACE) SECONDARY TOP OF BANK WATERCOURSE & CHANNEL VALLEY LOWER VALLEY TOE OF VALLEY FLOOR/ TOE OF CHANNEL BANK TOP OF CHANNEL BANK (Bank full flow location) BED OF CHANNEL (Lowflow) February 204 8/8