RocFall™

As discussed in Section 1.4, RocFall™ is a commercially available, 2D, hybrid, probabilistic rockfall trajectory model. RocFall™ was selected for use in modelling rockfall at Redcliffs for this thesis on the basis of simplicity, speed of running simulations, and availability of the programme. It was also considered appropriate for the geology and geometry of the site, but a primary aim of this study has been to evaluate its applicability as a runout modelling technique.

1.5.1 Model Assumptions

In order to make the model simple, the following assumptions were made (Stevens, 1998): • Each rock is modelled as an infinitely small particle, so there is no interaction between

particles.

• Rocks are not considered to have size, however their mass is considered in calculating their kinetic energy. Mass is not used in any of the equations for calculating the motion of the rocks.

• The mass of rocks is constant throughout each simulation, and rocks cannot break or split into multiple pieces during the simulation.

• Frictional resistance of the air is not considered.

These assumptions are required to limit the number of variables in the model, however some of them contribute to shortcomings in the model, particularly the lack of consideration of a rock’s mass, the inability to consider interaction between particles, and the lack of fragmentation consideration. Pfeiffer & Bowen (1989) suggest that the assumption that fragmentation will not occur represents the worst-case scenario presented by the largest rock that remains intact while travelling down a slope. Conversely, by excluding fragmentation,

the model does not include any consideration of flyrock resulting from fragmentation, particularly at the base of the slope, where fragments can be propelled beyond the main toe of the talus slope. This was an issue at Redcliffs where indentations in buildings suggest flyrock passing 2-4m above the toe of the slope. In this case, consideration of fragmentation is necessary in interpreting results, as it is not included in modelled results.

1.5.2 Input Parameters used in RocFall™

Coefficients of Restitution

Pfeiffer & Bowen (1989) state that “slope material properties influence the behavior of a rock rebounding from the slope”. These rebounding rocks are represented by normal (Rn) and tangential (Rt) coefficients of restitution, where the “normal direction is perpendicular to the impacted surface, and the tangential direction is parallel to the impacted surface, at the point of contact” (Piteau and Associates Limited, 1980; Wu 1984; in Pfeiffer & Bowen 1989). These authors further define these parameters, saying “Rn is a measure of the degree of elasticity in a collision normal to the slope, while Rt is a measure of the resistance to movement parallel to the slope”. Richards et al. (2001) provide a further review using RocFall™ as the analysis method, from which values for Rn and Rt were considered in selecting initial values for these parameters in modelling.

Friction Angle (Phi)

The friction angle as used in RocFall ™ is a specific input parameter that controls the mode of movement downslope, rather than an internal friction angle of a rock material. The following description explains the use of the parameter as defined by the RocFall™ software:

“The friction angle is chosen based on the particle shape and the mode of movement. [It] is the critical angle of the slope segments for the purpose of rocks moving downslope. If the slope segment is inclined more than this angle, the rocks will move downslope, if it inclined less than this, they will come to rest on the segment” (RocScience 2003).

Because the friction angle controls the mode of movement, this parameter represents a significant approximation of block shape, in the crude form of ranging from long flat slabs to spherical rocks (Figure 1.7). Long flat slabs are most likely going to topple and slide, so a higher friction angle is used. In contrast, a spherical rock will tend to roll, so a low friction angle (close to zero) is used (Figure 1.7).

Figure 1.7 (Left) Illustration of the friction angle as used in RocFall (TM) (after RocScience 2003). (Right) Interpretation of representative block shapes as represented by friction angles.

Slope Roughness

Within RocFall™ slope geometries are input as vertices, with a line segment joining adjacent vertex points. Slope roughness is used to model local variations in geometry, on a scale that is measured between the vertices entered as the slope geometry (RocScience, 2003). Figure 1.8 shows this variability graphically, where the dashed line represents the line segment joining two adjacent vertices, and the solid line represents the adjusted profile after roughness has been calculated.

Slope roughness is represented by a normal distribution, where the mean value is calculated directly from the slope geometry, and the user controls the roughness by adjusting the standard deviation. As the standard deviation is increased, variability of the generated slope compared to the measured slope will get more pronounced, the rocks are more likely to bounce in directions increasingly different from the angle of the slope segment, and the rock paths will look more “unpredictable” or “unusual” (RocScience 2003).

Angular Velocity

Angular velocity refers to the “time rate at which an object rotates, or revolves, about an axis” (“Angular Velocity” 2012). In RocFall™, consideration of this parameter in calculations can affect the downslope transportation of rocks as discussed in Section 5.5.3. According to material published by RocScience (2003), the initial value of angular velocity is not as influential in the calculations that produce runout paths as the consideration of the concept itself. The article also states that as a general rule, the initial angular velocity value is very small and often zero, due to most rocks starting with very little movement but rocks begin to rotate quite quickly as they travel downslope. Because of this, consideration of angular velocity is the only aspect of this parameter that has been varied during modelling.

Rockfall Source Areas

RocFall™ defines the rockfall source areas on a slope as point- or line- seeders. A point seeder releases simulated rocks from a single point on the slope profile, whereas a line seeder allows the user to select a continuous area from which simulated rocks can originate.