HIGH PRECISION GPS MONITORING FOR LANDSLIDE HAZARD AND LAND STABILITY EVALUATION IN THE CERCA DEL CIELO URBANIZATION, PONCE, PUERTO RICO

The Cerca del Cielo urbanization suffered a major landslide that initiated in the summer of 2007, which was followed by major surges and displacements in the late fall. The landslide site is located in the Ponce municipality, within three distinctive Oligocene–Miocene age sedimentary units in and around the area: Juana Diaz Formation, Mudstone and Chalk unit. The study area lies entirely within the Juana Diaz Formation that is typically a troublesome geologic unit due to its plastic clay size particles that leads to expansive soil wide spread throughout the city of Ponce and causing large slope stability problems. The slip surface inclined 50º in the head and 15º near the center and horizontal in the foot, lies underneath the chalk colluvium within a highly plastic clayey residual soil that developed over the underlying weathered mudstone unit. Global Positioning System (GPS) has been widely used in landslide monitoring to detect superficial displacements up to the millimeter level. Our continuous GPS measurements back in 2008 suggest that the kinematics of the landslide displacements was dominated by local precipitation. On September 2008 a tropical disturbance dumped 50 cm of rain on the landslide area temporarily accelerated the sliding, which generated a rapid displacement of 1m horizontally and 0.5m vertically within 4 days and then slowed after this major surge and displacement. During 2009 the average creep of was 1cm/month. A similar pattern was seen for the year 2010 where preliminary result show ~35mm total displacement. We installed 18 GPS campaign stations in 2011 to delineate stable and unstable areas inside and outside the sliding mass. Our results indicate that the landslide is still moving but with a much smaller rate of 2 mm/month. The area of sliding mass has not increased during the past 2 years. The change in creep rate may be related to the lack of heavy rainfalls over the period or the displacement of the center of mass and resultant reduction of the driving force.