Rippability Analysis

Rippability is the ease with which soil or rock can be mechanically excavated.  The rippability of rock is controlled by numerous parameters including uniaxial strength, degree of weathering, abrasiveness, and spacing of discontinuities.  Typically, weathered rocks, highly stratified or laminated rocks, and rocks with extensive fracturing are rippable.  Conversely, massive or crystalline rocks, and rocks with no planes of weakness are typically non-rippable.

Rippability versus seismic velocity for a range of rock types (Caterpillar, Handbook of Ripping).

Seismic refraction has historically been the geophysical method utilized to indirectly predetermine the degree of rippability.  This has been the favored method based on extensive field tests conducted by the Caterpillar Company, relating seismic velocity to rippability.  The seismic velocity as measured in seismic refraction surveys provides a qualitative measure of the rock strength and the presence of major fractures.  In a typical seismic refraction survey the data are processed and the results are presented as a layered model, with each layer consisting of a constant seismic velocity.  Modern tomographic surveys can provide higher resolution models of variations in seismic velocity with depth, allowing for a more detailed analysis.  The electrical resistivity method can be used as a complementary method with seismic refraction surveys, since resistivity is useful in distinguishing competent from weathered rock in most cases.

The example below displays a seismic refraction tomography survey conducted over slag heaps at an old smelter site to aid in remediation planning for the area.  The upper panel displays the results of the tomographic modeling of seismic velocity; the interface between layer 1 and layer 2 of the more traditional layered earth model result is also shown for comparison.  Analysis of the seismic velocity combined with borehole data across the site produced the layered model of rock strength in the middle panel.  This has been constrained by the raypath coverage (sensitivity of the model) in the bottom panel.