The viscoelastic closure of the underground cavities and the dissolution of the mine structures through fresh water flooding caused subsidence above the deep kainite San Cataldo mine, Sicily (Italy), opened in the early 60s. The present paper describes the instruments installed and measurements taken to observe the phenomena. The systems used were: automatic deep wire extensometers in the rock mass above the mine; measurements by surveying methods on the area above the cavities and in the neighbourhood; inclinometers in the slope above the mine; piezometer sensors in the mine; microseismic monitoring; remote sensing. A simple model has been taken to interpret all the various data collected, and to compute the time for the fresh water to fill all the underground cavities.
The San Cataldo mine is in central Sicily, near Cal anissetta. The mine was opened in 1962, and production stopped in 1979. The original owner of the mine, Montecatini, built the industrial treatment plant on a gentle slope 150 m above the salt deposit. Kainite from neighbouring mines was also treated at this plant, and so the plant went on working until 1986, when it had to be closed and fenced for safety reasons. In the late 60s a significant amount of water had started leaking into the mine from the rock mass above the salt, and the flooding had to be contained by continuous pumping. At the same time the tanks of the floating plant on the surface began tilting towards the foot of the slope. In 1983 a geotechnical, hydrogeological and rock-mechanics related study was begun in order to assess the origin of the tilting, quantify the subsidence in the area above the mine, and propose possible remedial measures. The study revealed that the phenomena were related to the behaviour of the underground cavities. Initially the elasto - plastic behaviour of the rock mass (in particular: a) the weakening process due to salt dissolution; b) the approach of failure conditions of the rock mass above the cavities) was considered to be the cause. However, after several years of observation and monitoring, it became apparent that the viscoelastic behaviour of the orebody was the main factor. The present paper describes the investigations carried out so far, the monitoring methods used and the most important results obtained.
The surface layout of the San Cataldo mine is shown on Figure 1. The slope where the plant and the tanks are located is at 320 * 350 m above sea level. Mining was carried out on 5 main levels, from +185 to -142 m asl (Figure 2). Both the room and pillar method and the sub-level sloping method were used to mine 3 kainite layers. Mining started at the top and worked downwards with occasional filling. Between +185 and 0 m asl the 3 layers are approximately vertical. Below this level, the layers dip about 45°. The total volume of cavities created is about 2.5 Mm³, due to the high extraction ratio.
