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Landslides

Seismic refraction surveys were conducted on two recent landslides (one translational slide and one rotational slump) that damaged a highway in the Caribbean National Forest, Puerto Rico. These surveys on recent landslides provided insight into the dimensions and depths of their failure planes. Field observations and drill-hole data were used to verify the accuracy of refraction data.
The seismic velocity measured in the overlying colluvium and saprolitic soils at a translational slide was 350 m/s.
The seismic velocity of the saprolite below the failure plain ranged from 1,000 to 1,500 m/s. The velocity in the deeper weathered bedrock measured about 7,300 m/s.
The thickness of colluvium ranged from 3 to 5 m on the toe of the slump, and the thickness of saprolitic soil above the head scarp ranged from 4 to 8 m. Estimated depth to the top of the quartz-diorite bedrock ranged from 12 to 24 m.
The partially exposed failure plane was at the top of the saprolitic layer with velocities in the 1,000 to 1,500 m/s range.

A rotational slump at a second site had a measured seismic velocity of less then 400 m/s in a 6-m thick layer of colluvium and saprolite, which, based on drill-hole data, graded to saprolite at a depth of 1 to 2 m. A seismic velocity contact in the saprolite was found at this 6-m depth; the seismic velocity in the saprolite below this contact was 870 m/s.
The seismic velocity of weathered quartz-diorite bedrock underlying the saprlite was 2,040 m/s. Depth to bedrock from seismic data was calculated to be 20 meters, which compared reasonably well with depth of exposed bedrock (15 m) in an adjacent slump. Drill-hole data obtained from other surveys confirmed the depth to bedrock at 20 m.
The possible failure plane of this rotational slump was revealed in a velocity contact within the saprolite. The surface, as noted above, was 6 m deep and was confirmed with drill-hole data, which indicated a two- to three-fold increase in blow counts; this represents an increase in saprolite density below the contact.
The presence of a perched water table was observed overlying this surface.

Larsen, M.C., 1995, Use of seismic refraction techniques for investigating recent landslides in a tropical rain forest in Puerto Rico in Miller, R.L., Escalante, J.A., Reinemund, J.A., and Bergin, M.J., eds., Energy and Mineral Potential of the Central American-Caribbean Regions, Springer-Verlag, Berlin, p. 411-414.

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