# Impact of Rock Mass Strength Anisotropy with Depth on Slope Stability Under Excavation Disturbance
## 3 Approaches to the Use of Disturbance Factor on Rock Slope Excavations  
As mentioned above, technical processes, such as mechanical excavation or blasting, used for the excavation of rock masses cause a decrease in the strength of rock masses compared to in the undisturbed condition due to the increase in discontinuity apertures and the development of new cracks. The latest version of the Hoek and Brown criterion, introduced by [PERSON] et al. [4], was used in this study. A practical guideline for the selection of disturbance factor (D) was also defined by [PERSON] et al. [4]. In the guideline, the lower value of \"D\" for slope case was recommended as 0.7. It means that, except natural slopes, the undisturbed rock mass condition cannot be achieved even if mechanical excavation methods such as digging or ripping are used in slope cases. Probably underlying this suggestion is the foresight that a rock mass can be loosening close to the excavation surface due to the removal of overburden, which may also cause disturbances. Although it is possible that the disturbance factor can reflect the degree of disturbance to the rock mass by using the disturbance factor depending on the excavation method, uncertainty about how the disturbance will change with depth, especially for deep excavation slopes, still remains in the literature. After the introduction of the disturbance factor to the criterion, only a few studies have addressed the use of the disturbance factor depending on degree of disturbance for especially deep slope excavations. Within the scope of this study, the five approaches given below were taken into consideration. In addition to the recent modification to [PERSON] et al. [10] by [PERSON] et al. [14], an \"S\" shaped decay formulation proposed in this study was also applied to the same slope profile, along with a comparison of the five approaches. In the subsequent section, the impact of rock mass strength anisotropy with depth on slope stability under excavation-induced disturbance was assessed through slope stability analysis. The approaches used in this study, together with an \"S\" shaped decay equation proposed in this study, are explained as follows. * The first approach (AP1): a disturbance factor (D) independent from depth was applied to the entire rock mass of the analyzed slope cross section. * The second approach (AP2): Conditions for blasted rock that can be excavated by digging efficiently; however, the blast-damaged rock mass lies between the digging limit and the insitu rock mass. The properties of this blast damaged rock mass will control the stability of the slope that remains after digging of blasted rock has been completed [8]. [PERSON] and [PERSON] [8] defined the following approximate relationships which can be used as a starting point in judging the extent (thickness, T) of the blast damaged zone resulting from open-pit mine-production blasting:
* Large production blast, confined and with little or no control: T = from 2 to 2.5 H.
* Production blast with no control but blasting to a free face: T = from 1 to 1.5 H.
* Production blast, confined but with some control, e.g., one or more buffer rows: T = from 1 to 1.2 H.
* Production blast with some control, e.g., one or more buffer rows, and blasting to a free face: T = from 0.5 to 1 H.
* Carefully controlled production blast with a free face: T = from 0.3 to 0.5 H.
* The third approach (AP3): [PERSON] et al. [10] proposed a quantified approach for determining transitional rock mass disturbance in open pit slopes related to mining excavation. The researchers accepted that the disturbance gradually decreases from the excavation surface to the depth. In this approach, it is assumed that the rock mass is fully disturbed depending on a specific depth from the surface (FDL), and the degree of disturbance gradually decreases to match the in situ rock mass (undisturbed) condition from the FDL to the depth of the undisturbed level (UDL).