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Proposal Stability Requirements Flipbook PDF
Proposal Stability Requirements
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2.2
GUIDELINES FOR SLOPE STABILITY ANALYSIS
The design criteria for the slope stability based on the “Geotechnical Manual for Slopes” of the Hong Kong Geotechnical Engineering Office (GEO) is adopted in the stability analysis for existing and proposed mitigated slopes. GEO recommends that the appropriate factor of safety (FoS) against failure shall be based on the risk of loss of life and economic loss.
Table 1 shows the recommended factor of safety against loss of life and against economic loss by GEO. These factors of safety are based on groundwater conditions resulting from a tenyear return rainfall. As such, the geotechnical stability of the proposed cut slope is to be designed to achieve a minimum factor of safety against failure of 1.4 as recommended by GEO.
Table 1: Recommended FoS for new slopes of ten-year return of rainfall. Recommended FoS against loss of Economic Risk Risk of life
life for a ten-year return of rainfall Negligible
Low
High
Recommended FoS against
Negligible
> 1.0
1.2
1.4
economic loss for a ten-year
Low
1.2
1.2
1.4
return of rainfall
High
1.4
1.4
1.4
However, Public Woks Department (PWD) suggests a minimum FoS of 1.5 considering 75 years design period for reinforced or treated slope. For back analysis of existing or natural slopes, a minimum FoS of 1.25 considering 75 years design period is adopted. Table 3 provides typical design criteria for geotechnical works adopted by PWD and Road Engineering Association of Malaysia (REAM).
2.3 CLASSIFICATION OF SLOPE AND HILLY TERRAIN MITIGATION
The slopes are categorized based on classification in Dewan Bandaraya Kuala Lumpur (DBKL) slope guidelines given in Table 2 below.
1
Table 2: Slope classification based on DBKL slope guidelines Class
Description
I
In situ slope gradient less than 15o
II
In situ slope gradient between 15o to 25o.
III
In situ slope gradient between 25o to 35o.
IV
In situ slope gradient more than 35o
2
Table 3: Typical Design Criteria CRITERIA for Geotechnical Proposed by PWD and REAM SOME TYPICAL DESIGN FORWorks GEOTECHNICAL WORKS DESIGN COMPONENT
1.
Slope / Embankment (not on soft ground)
Unreinforced
MODE OF FAILURE
1.1
Local & Global Stability
MINIMUM FACTOR OF SAFETY
DESIGN LIFE (durability of materials)
1.25
75 yrs Analysis should be according to GEOTECHNICAL MANUAL FOR SLOPES (1984), GEO Hong Kong
Reinforced or Treated
1.2
Local & Global Stability
1.5
75 yrs
75 yrs
5 years post construction settlement : (i) within 50m from structures approach < 100mm (ii) within 100m remote from structures < 100mm Total Settlement