ABSTRACT: A large-scale landslide in an open pit mine located in weathered gneiss unit in Menderes Massif occurred on

19.12.2018. The open pit mine failure occurred on a fairly shallow failure surface (maximum depth: 17 m), running

almost parallel to the slope face. At that time, the overall slope height was 80 meters (elevations: 490 m – 410 m) and

overall slope angle 25°. The failure involved around 700 × 103 m3 of rock. Significant displacements in rock slopes

in the pit occurred before the failure surfaces had fully developed. This study aims to investigate the causes of the

landslide and to monitor slope movements in the landslide area before the failure. While monitoring the movement

(between 2017 and 2018, 750 days), amount of rainfall per day and per hour was also measured and recorded.

The amount of rainfall has been evaluated in conjunction with the time-dependent slope movement data (timedependent

surface displacement measurements) by using the software “GeoMoS” to determine the relationship

between the geological structure and the landslide mechanism. Some small scale instabilities have been observed in

the hanging wall beforehand. The large scale failure of the hanging wall, however, has progressed during the periods

of heavy rainfall, suggesting that the variations in groundwater pressures triggered larger displacements along the

developed failure surface. During dry weather, the slope moved at a rate of about 3.5 mm/day. Slope movement

slightly accelerated in open pit due to the work of third overburden removal at the date of 04.12.2018. Until the

uncontrolled overburden removal at the slope toe (14.12.2018), the amount of the total resultant displacement of

the slope increased up to 100 cm and the rate of slope movement was calculated as 10 cm/day. Finally, from the

uncontrolled excavation of the slope toe (14.12.2018) to 19.12.2018 (when the landslide occurred) the movement

accompanied by the intense rainfall, occurred as 160 cm/day. Total resulting displacement of the slope at the end

of this measurement campaign was at the amount of 10.5 m. In addition, some indicators before the failure were

determined to define the different stages of the landslide. It was also determined that the most important indicators

are the displacements, heaves, and shear-strain accumulation at an elevation near the slope toe.