Rock slope rating (RSR)

A rock slope rating (RSR) system has been developed for evaluation of rock slope stability under a variety of geological conditions and engineering requirements. RSR system evaluates the probability of failures for plane and wedge sliding and toppling and circular failures. Probability of each mode of failure is determined individually. The main categories for input parameters are summarized as follows.

1.      Geological features: Various types of slope mass to which the RSR can be applied: massive rock, blocky rock, bedded rock, heavily-jointed rock, soft rock, and hard-soft inter-bedded rock.

2.      Safety requirements: The system classifies the engineering applications of rock slope into four levels of safety, based on the type of engineering structures (e.g., railroad, housing, major highway, spillway, dam abutment, mined road, etc.).

3.      Groundwater conditions: The groundwater condition is classified in terms of its level as compared to the slope height. The options are from completely dry to water level up to 25%, 50%, 75%, or 100% of the slope height. If the condition is unknown, the system makes further inquiry about the climate where the slope is situated. Two options are available: tropical and arid.

4.      Slope geometry: Slope geometry includes orientation, height, angle, and curvature. Three slope shapes can be selected: convex, concave and straight faces.

5.      Joint characteristics: The system requires detailed joint characteristics, including orientation, average spacing, continuity, aperture, filling, and roughness of all joint sets.

6.      Geo-mechanics parameters: Rock density, uni-axial compressive strength, and shear strength of all joint sets are considered in the stability evaluation.

 

The system classifies each factor considered in the stability evaluation into small ranges or sub-divisions, mainly to convert the input slope characteristics into quantitative form. A set of rating is then assigned to these parameters for each failure mode considered. Recognizing that these parameters can have different impacts under different conditions of the rock mass, a set of influencing factors is also defined as multiplying factors for the corresponding parameter. The probability of failure P{f} in percent for each mode can then be calculated by:

P{f} = Σ{Rn In}

where Rn is the rating for each parameter, In is the influencing factor for the corresponding parameter, and n represents type or number of the parameters considered for each slope (varying from 1, 2, 3, 4..n). Probability of failure to classify the quality of slope mass is expressed as follows:

 

Table 2.4. Probability of failure to slope mass quality

P{f}, %

Slope Mass Quality

<20

Highly stable

20-40

Stable

40-60

Fair

60-80

Unstable

80-100

Highly Unstable

 

 

Table 2.5: Rating factors for evaluation of circular failure.

      

Slope height

Slope face angle

Ground water

Degree of weathering

(m)

Rate

Degree

Rate

(%)

Rate

Condition

Rate

5-7

1

20-25

0

0

0

Fresh

2

7-10

5

25-30

1

25

5

Slightly

4

10-15

8

30-35

2

50

10

Moderately

6

15-20

10

35-40

3

75

10

Highly

8

>20

10

40-45

5

100

10

Completely

10

 

45-50

6

Unknown

*5 or 10

Unknown

5

50-55

8

 

 

55-60

9

60-65

9

65-70

10

>70

10

Vegetation

Number of discontinuity

Vibration

Average discontinuity spacing

Condition

Rate

(Sets)

Rate

Conditions

Rate

(mm)

rate

No vegetation

10

≤ 2

1

Near blasting sites

10

<20

10

Grass

7

3

8

Near Main highway

5

20-60

7

Grass n small tree

5

≥ 4

10

No vibration

0

60-200

5

Full grown tree

0

Unknown

5

Unknown

5

>200

0

Unknown

5

 

 

 

 

Unknown

5

*5 for arid climate, 10 for tropical climate

                 

 

The probability of failure is determined by summing the multiplied products between the rating and the corresponding influencing factor. The proneness of failure predicted by the RSR system are finer than those from the SMR system because RSR actually calculates the P{f} values while SMR uses inference method to estimate.  RSR takes into consideration the slope shape, i.e., convex, concave or straight whereas RSR explicitly considers the strength of intact rock, which has different degrees of impact on the stability under different modes of failure and joint characteristics. Further, RSR can handle the slope mass containing inter-bedding of soft and hard rock.