Download Book AutoBlock

AutoBlock Manual
Book Computation
When investigating the safety with respect to sliding, the direction of motion of the
assumed rigid unstable block has to be known. This depends on the orientation of its
bounding discontinuities and on the direction of the resultant force acting on it. Four
cases can be distinguished:
1) Movement along one discontinuity in the direction of the component of the
resultant force parallel to the discontinuity.
2) Movement along two discontinuities in the direction of the component of the
resultant force parallel to the line of intersection.
3) Lifting off (includes toppling, since only equilibrium with respect to force
components, but not to moments is formulated).
4).No movement, e.g. blocks are jammed by existing discontinuities.
In the first two cases other discontinuities may be involved, from which the potentially
unstable blocks move away (so-called tension cracks or TCs). In the third case the
unstable blocks move away from all discontinuity surfaces.
Two different types of computation are available:
 Computation of the Factor of Safety for the conditions defined.
Knowing the type and direction of movement, in the bounding discontinuities the
normal reactions and the existing resultant shear force can be determined (see
literature, e.g. Kovári and Fritz, 1993). After calculating the maximum shear
resistance using the Mohr-Coulomb failure criterion one obtains the factor of safety
as the ratio of the maximum shear resistance to the existing shear force.
 Computation of the required Additional Force (e.g. an anchor force) to reach a
specific factor of safety.
If the factor of safety is below a critical value, often it is aimed to find an additional
force (e.g. a prestressed anchor) which increases the factor of safety to an
acceptable value. By specifying this value and the orientation of the additional
force its required magnitude is determined, whereby the mode and orientation of
movement may be changed dynamically.
Limitation: in the current version of AutoBlock the computation of the required
Additional Force is not yet implemented.
69