Geological strength index_-slope: an adaptation of the geological strength index system for use in the rock slope stability assessment

Abstract

The Geological Strength Index (GSI) system is the basis of parameters used in the Hoek-Brown failure criterion for rock mass strength estimation. The author tested this system and here suggests a modified GSI called Geological Strength Index-_slope (GSI_slope). The modified system combines two different existing approaches: the GSI system and Slope Mass Rating (SMR). The purpose of GSI_slope is to allow engineering geologists to quickly evaluate the stability of natural and excavated slopes or open-pit mining in the field. GSI_slope is computed by subtracting a constant value of 10 and the multiplication of adjustment factors for discontinuity orientation and slope (F1, F2, and F3, based on the parallelism of discontinuity and slope, discontinuity dip angle, and the difference between the inclination angle of discontinuity and slope) from GSI, and adding field groundwater rating to it. Modified curves are also proposed in this work to determine the accurate ratings of the adjustment factors. The results of this work are compared to the values obtained from equations of continuous-SMR and SMR-value itself for both the adjustment factors and GSI_slope values. The comparison showed that the proposed curves and GSI_slope equation are valid and easy to use for estimating the adjustment factors’ ratings and GSI_slope value.

KEYWORDS:
slope stability; geomechanical system; geological strength index; rock mass rating; slope mass rating; geological strength index-_slope

INTRODUCTION

This study introduces a new system, the Geological Strength Index-_slope (GSI_slope), which can be used for rapidly evaluating rock slope stability in the field. It offers simple ideas about stability conditions and instability modes.

GSI_slope system combines two different existing approaches: GSI and Slope Mass Rating (SMR) systems. The GSI system is applied as a tool to determine the rock mass strength (Hoek and Brown 1997, 2019, Hoek and Diederichs 2006, Marinos and Carter 2018), and the SMR is applied to determine the stability condition (Romana 1985).

The GSI application for slopes has not been currently probable. The GSI_slope system uses adjustment factors of Romana's (1985, 1993) SMR. Field guidelines permit the rapid use of this system for the rock slopes.

The GSI was innovated by Hoek (1994), Hoek et al. (1995), and Hoek and Brown (1997) to gain victory over the defects in Bieniawski's (1976, 1989) rock mass rating (RMR) for very poor-quality rock masses. This system supplies an estimate for the reduction of the rock mass strength for various geological conditions, which can be done in the field (Hamasur 2009).

The GSI is one of the excellent rock mass classification systems that is employed to assess very weak and heavily jointed rock mass. In addition to generalized Hoek-Brown constants, modulus of deformation, and properties of strength for an approximate style of tunnels and caverns, it can be applied by other ways of dealing and associated with rock mass properties. The difference of this system from other geotechnical systems is as follows: it uses field observation, represented by the structure of rock mass and discontinuity surface conditions, throughout the assessing method of rock mass and is strongly considered a practical tool for estimating the rock mass strength properties needed for the pre-stability task of engineering projects (Hussian et al. 2020).

After the growth of the GSI system, various researchers throughout the world have achieved research on multiple sides of the GSI system to adapt the weakest, jointed, and heterogeneous rock mass for the design of engineering projects (Marinos et al. 2005, Hoek et al. 2013, Vásárhelyi and Kovács 2017, Hoek and Brown 2019). The basic GSI chart, for use with jointed and very weak rocks, is shown in Fig. 1.

Figure 1.
Basic GSI chart.

GSI_SLOPE IDEA, TABLES, AND CURVES

The proposed GSI_slope is computed by subtracting a constant value of 10 and the product calculation of adjustment factors for orientation of discontinuities and slope of the SMR (F1, F2, and F3, based on the parallelism of discontinuity and slope, discontinuity dip angle, and the difference between the inclination angle of discontinuity and slope) from GSI, and adding an actual field groundwater rating (WR) to it, as follows (Eq. 1):

(1)$$GS{I}_{slope}=GSI-10+WR+(F1.F2.F3)$$

The GSI = RMR₁₉₇₆ (Hoek et al. 1995, Hoek and Brown 1997), and the GSI value is equal to the rating for the first four parameters of RMR₁₉₇₆ (unconfined compressive strength of intact rock, rock quality designation [RQD], discontinuity spacing, and discontinuity condition); also, the rock mass should be assumed to be completely dry, and a rating of 10 is assigned to the groundwater value (Hoek et al. 1995, Hoek and Brown 1997); so, to determine the actual field WR, the value of 10 can be subtracted from the GSI value, and determine the groundwater condition in the field, the rating can be estimated from Bieniawski's RMR table (Table 1) (Bienawski, 1976).

Table 1.
Groundwater rating.

Groundwater	General conditions	Completely dry	Moist only	Water under moderate pressure	Severe water problems
Rating		10	7	4	0

• Source: Bienawski (1976).

The GSI system assumes a very favorable discontinuity orientation, its rating is set to zero, and the RMR system of Bienawski (1976; 1989) does not give a precise rating of the discontinuity orientation condition because it depends on the personal diligence and judgment. The most important geotechnical system that offers a precise rating for the discontinuity orientation condition in the rock slopes is the SMR of Romana (1985, 1993) and Tomás et al. (2007); so, in this study, the last one is preferred.

The adjustment rating of the discontinuity orientation is the product of the same three factors proposed by Romana (1985) for the SMR system, and is given as follows:

• (i)

  F1 is the rating of the difference in dip direction between discontinuity and slope face or between the plunge direction of two discontinuities and slope face;

• (ii)

  F2 is the rating of the dip angle of discontinuity or plunge angle of the intersection line of two discontinuities;

• (iii)

  F3 is the rating of the difference in dip angle between discontinuity and slope dip angles or between the plunge angle of the intersection line of two discontinuities and slope angle (Hamasur et al. 2020).

Instead of the tables of Romana (1985) and Anbalagan et al. (1992) and the equations of Tomás et al. (2007), the rating of these three adjustment factors (F1, F2, and F3) can be obtained from the curves shown in Figs. 2–5. The researcher of this article benefited from the equations of Tomás et al. (2007) in drawing these curves.

Figure 2.
Variation of rating for the adjustment factor no. 1 (F1).

Figure 3.
Variation of rating for the adjustment factor no. 2 (F2).

Figure 4.
Variation of rating for the adjustment factor no. 3 (F3) (Planar & Wedge failure).

Figure 5.
Variation of rating for the adjustment factor no. 3 (F3) (Toppling failure).

COMPARISON BETWEEN CONTINUOUS-SMR AND GSI_SLOPE

To prove the validity of GSI_slope system, the data of rock mass structure and surface conditions of discontinuities were used from Verma et al. (2011) and Hamasur and Qadir (2020) in order to determine the rock mass GSI value for all the 4 slope locations and all the 10 slope stations, as shown in Fig. 6.

Figure 6.
Value of the GSI in the 10 rock slope stations (circles with number) of Hamasur and Qadir (2020), and in the 4 slope locations (circles with letter and number) of Verma et al. (2011).

Verma et al. (2011) assigned the groundwater condition as completely dry for slope locations no. 1, 2, and 3, with the rating value of 10, and moist (damp) for slope location no. 4, with the rating value of 7. Also, Hamasur and Qadir (2020) determined the rating of the actual field groundwater to be equal to 7; this value is the average of dry condition (May-October) that has a value of 10 in RMR₁₉₇₆ and water under moderate pressure (November-April) that has a value of 4 in RMR₁₉₇₆.

Adjustment factors (F1, F2, and F3) were determined from the attitude of discontinuities and slope by modified curves of this study; then, the product calculation of these three factors was determined. The GSI_slope value for 19 data sets was calculated in the mentioned 10 slope stations, using Eq. 1, and the results are shown in Table 2.

Table 2.
Comparison between continuous-SMR and GSI_slope (data of continuous-SMR are from Hamasur and Qadir 2020).

Slope	Failure	Continuous-SMR				GSIslope (from this study)
Station	Type	RMRb (1989)	F1.F2.F3	SMR	GSI	GSI-10	F1.F2.F3	WR	GSIslope
1	FT	63	-21.28	41	65	55	-20.00	7	42
	DT	63	-9.65	53	65	55	-9.09	7	53
2	WS	65	-23.62	41	66	56	-24.51	7	38
	FT	65	-20.27	44	66	56	-19.20	7	44
3	WS	64	-45.94	18	67	57	-44.23	7	19
	FT	64	-21.11	42	67	57	-19.84	7	44
4	WS	62	-29.44	32	65	55	-30.26	7	32
	FT	62	-23.56	38	65	55	-22.50	7	39
5	PS	63	-44.17	19	65	55	-42.35	7	20
	WS	63	-52.68	10	65	55	-50.63	7	11
DT	63	-0.54	62	65	55	-0.50	7	61
6	WS	70	-23.46	46	71	61	-24.60	7	43
	FT	70	-17.72	52	71	61	-17.25	7	51
7	PS	67	-32.64	34	67	57	-32.49	7	32
	FT	67	-22.04	44	67	57	-22.00	7	42
8	WS	69	-45.12	23	72	62	-43.89	7	25
	FT	69	-19.85	49	72	62	-18.84	7	50
9	PS	73	-40.50	32	73	63	-40.19	7	30
10	FT	71	-17.65	53	75	65	-16.10	7	56

• PS: Planar sliding; WS: Wedge sliding; FT: Flexural toppling; DT: Direct toppling; SMR: Slope Mass Rating; F1, F2, and F3 are adjustment factors of continuous-SMR; GSI: Geological Strength Index; WR: water rating; GSIslope: GSI-10+(F1.F2.F3)+GW.

The adjustment factors F1, F2, and F3 calculated from the curves of this study represent similar results to those obtained from the continuous-SMR equations, despite the presence of ± 3º changes in the (F1. F2. F3) product calculation by the two methods in some cases. In addition, the calculated GSI_slope value from this study has approximately similar results compared to the continuous-SMR value, despite the presence of ± 3º changes by the two methods in some cases (Tables 2 and 3). However, these small changes do not affect the stability classes and conditions because the GSI_slope value has ranges of 20 scores between classes and conditions, as shown in Table 4.

Table 3.
Comparison between continuous-SMR and GSI_slope (data of continuous-SMR are from Verma et al. 2011).

Slope Station	Failure Type	Continuous-SMR			GSIslope (from this study)
		RMRb (1989)	F1.F2.F3	SMR	GSI	GSI-10	F1.F2.F3	WR	GSIslope
Location 1	PS	65	-19.99	45.01	66	56	-22.03	10	43.97
Location 2	PS	60	-33.15	26.85	60	50	-36.00	10	24.00
Location 3	FT	65	-15.39	49.61	66	56	-18.75	10	47.25
Location 4	FT	50	-24.37	25.63	47	37	-20.50	7	23.50

• PS: Planar sliding; FT: Flexural toppling; SMR: Slope Mass Rating; F1, F2, and F3 are adjustment factors of continuous-SMR; GSI: Geological Strength Index (the GSI value obtained from projection of et al information on GSI-Chart); WR: water rating; GSIslope: GSI-10+(F1.F2.F3)+GW.

Table 4.
Description of GSI_slope stability classes and conditions.

GSIslope value	100←81	80←61	60←41	40←21	< 21
Stability class	I	II	III	IV	V
Stability condition	Completely stable	Stable	Partially stable	Unstable	Completely unstable

• Source: modified from Romana (1985).

STABILITY CLASSES AND CONDITIONS

After calculating the GSI_slope value, the stability classes and conditions of the rock slope can be determined from Table 4, which is modified from Romana's (1985) description table of slope mass rating classes to adapt to GSI_slope.

CONCLUSION

Rock mass engineering classification systems are a global system for those who use them. Engineering classification systems are widely utilized to predict the possible failures in the rock slopes, such as SMR, continuous-SMR, and Q-slope; these systems are among the most important classifications for slope stability assessment. Nevertheless, the GSI system does not use it to assess the rock slope's stability conditions.

In this study, the author tested and suggested a modified GSI called GSI_slope. The purpose of GSI_slope is to allow engineering geologists to quickly assess the stability of natural and excavated rock slopes or open-pit mining in the field. In addition, modified curves for evaluating adjustment factors have been proposed. The results from this work are compared to the values obtained from equations of continuous-SMR and SMR-value itself for both the adjustment factors and GSI_slope values. The comparison showed that the proposed curves and GSI_slope equation are valid and easy to use in the field for estimating the adjustment factors’ ratings and GSI_slope value.

ARTICLE INFORMATION

• Manuscript ID: 20220044.
  How to cite this article: Hamasur G.A. 2023. Geological strength index_-slope: an adaptation of the geological strength index system for use in the rock slope stability assessment. Brazilian Journal of Geology, 53(1): e20220044. https://doi.org/10.1590/2317-4889202320220044

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Publication Dates

• Publication in this collection
  05 May 2023
• Date of issue
  2023

History

• Received
  18 June 2022
• Accepted
  05 Dec 2022