Engineering geology

SSR Classification System

The SSR (Slope Stability Rating) classification system (Taheri et al., 2006a) is a new rock mass classification system which has been proposed in Iran to study the stability of fractured rock slopes with non-structural control failures (Table 1). This system is proposed based on modified GSI which was described before, and Non-linear Hoek-Brown failure criterion.

In SSR, beside the Geological Strength Index (GSI), five more parameters have been taken into account. The SSR values of the rock mass can be determined from Table 1, by summing the rating value of all the parameters. It should be mentioned that the rating values of the second parameter (UCS) in table 1 is the difference from an old version (Taheri et al., 2006b) and have been modified in this study.

Table 1. Rock mass classification with SSR (Slope Stability Rating) system for preliminary evaluation of slope stability
Table 2. Rock groups defined for SSR rating system

SSR Slope Design Charts


To provide a useful tool for the preliminary design of rock slopes, several stability analysis was performed using the limit equilibrium code, CLARA.
Based on the studied slopes, several rock slopes were assumed with heights ranging from 25 to 400 meters and slope face angles from 30 to 70 degrees.

Different discontinuity and intact rock properties, slope excavation methods and environmental characteristics such as groundwater and earthquake force were considered. The SSR value was determined for each of the studied slopes, with various geometry and safety factors, as well. Four design charts were prepared for factors of safety 1.0, 1.2, 1.3 & 1.5. Figure 1. shows the chart associated with F.S = 1.

The design charts for different factors of safety have similar shapes but different positions in which as the factor of safety increases, curves shift to the right side to provide more conservative designs. These charts provide the relationships between the safe slope angles versus slope height, at the different factor of safety as a function of the calculated SSR values for the rock masses.

Figure 1. Rock slope design chart, based on SSR values of the rock masses (F.S=1.0)

Field Verifications

Considering fields of application of the SSR system, it is important that the system is validated for the intended use. For this purpose, the system was reviewed using several slope sites. Table 3 represents the geometrical and stability conditions of 6 slope sites from Iran which had been studied previously to propose the SSR system.

Base on the information obtained in the site investigations, the SSR values of the slopes were determined. Amongst the 9 cases studied cases there were two unstable slopes and others are in the stable condition.

Table 3. Summary of slope data from case studies in Iran

Another database of rock slopes from Australia was also studied in this work. Table 4 summarized these case records (Douglas, 2002). 37 cases were selected from 13 open-pit mines. Each mine may have several stable/unstable slopes denoted as a, b, c, etc.

The rating of rock mass structures which is represented by GSI was determined by Douglas (2002). In this database, there are two groundwater conditions, dry and moderate pressure, which the corresponding rating was determined. Furthermore, since the database is based on slopes in mining operations, the rating for normal blasting (production blasting) is allocated to all.

UCS and rock type rating was obtained based on the information which made available by Douglas (2002) and finally the SSR values were calculated for each case. It should be mentioned that in both databases the slopes were studied in a static condition which implies the zero-rating for earthquake force parameter.

Table 4. Part of the Summary of Slope Data from Case Studies in Australia

The two databases from Iran and Australia combined into one database and presented in Figure 2. The database consists of 46 slopes, of which 28 were stable and 18 had failed. All failures were non-structural i.e. thoroughly fractured rock mass.

To study the consistency of original design charts with the database, a comparison of the failed case records and SSR design chart on the verge of failure (FS=1) is presented in Figure 3. General observation shows that several of the cases do not confirm the design curves and original design curves would have indicated the use of steeper slopes.

Figure 2. Relationship between SSR and height in the combined database
Figure 3. Comparison of failed cases in database with original
design charts (FS=1)

Conclusions

  1. The rock mass classification systems, developed in recent years for slope stability problems have some limitations, especially in analyzing the stability of slopes in closely jointed or crushed rock masses.
  2. In order to establish a modified rock classification system for assessing the stability of rock slopes with non-structurally controlled anticipated failures, the Geological Strength Index (GSI) has been found to be a useful and reliable tool.
  3. SSR (Slope Stability Rating) is a new rating system which has been proposed to study the stability of fractured rock slopes with non-structurally controlled failures.
  4. By calculation the SSR value of a rock mass, the safe slope face angle can be determined by using the proposed slope design charts. In these charts, the safe slope angles have been plotted against slope height and SSR value of rock masses.
  5. The system was reviewed on the basis of a database consists of 46 rock slopes from Iran and Australia. The modified rock slope design charts are proposed for maximum excavation angle (FS=1) and also for some conservative excavation angles (FS= 1.2, 1.3, 1.5).

Adapted by A. Taheri; University of Adelaide

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