La subsidenza nell'area urbana di Parma

Abstract

At the request of the City Administration and the Town Water Authority (AMPS), the Institute of Geology (Parma University) started a study programme on the subsidence of the town of Parma, a small city in the Po valley (N Italy) in 1990. Subsidence indicates ground settlement. Vertical movements may be either negative or positive and can be due to either sediments compaction, or tectonic or isostatic movements, in some areas, vertical movements may also be caused by anthropic subsidence induced by land reclamation works, by the load of buildings and especially by fluid pumping from the underground. Natural subsidence cannot be controlled. Subsidence induced by anthropic activities can and should be kept within safety limits to reduce damage to the environment and buildings. Signs of ground movement have long been known in Parma, as instability phenomena in both historic and recent buildings indicate. Subsidence data is unavailable up to the end of 1980's. Research to document the occurrence of ground settling in the town and to identify causes started in 1990 as part of a hydrogeologic study aimed at a qualitative and quantitative evaluation of groundwater resources in the Parma plain. High precision levelling was designed on the basis of the town's radial development, using 14 IGM (Military. Geographic institute) bench marks and 24 reference datum points located on AMPS wells in the town area (Figure 1). Parma is crossed by the IGM levelling lines No. 40 (Parma-Verona), No. 20 (Parma-Piacenza), No. 17 (Parma-Bologna) and No. 33 (Parma- Sarzana). Only this latter one crosses the Apennines and has been used both as the origin reference datum, because it falls in an area not subjected to subsidence, and as the point connecting the origin reference datum to the town. The 1990 and 1991 geometric levellings started from bench mark No. 102/33 located on the walls of the Piantonia parish church (el. m 277.8737 in 1952), the IGM trigonometric station chosen by the firm undertaking topographic measurements. In 1992, on the basis of geologic considerations, the origin reference datum point was moved uphill and bench mark No. 200 (el. m 448.6750 in 1991) was located on the Belvedere Hotel, adjacent to the State Route 62 and built on the Mt Prinzera's ophiolite. This paper presents the results of four cycles of annual measurements from 1990 to 1993 showing that ground settlement in various zones of the town are noticeable and different in time. The present situation is synthesized in Tables 1-3, in the diagrams and in the map completing the text. In Table 1 the 1990 elevations of the town's IGM bench mark are compared to previous elevation data for each bench mark, obtained with measurements carried out from 1982 to 1985. Data of single leveilings and total settlements in the period 1990-1993 of all bench marks in the town are given in Table 2. Figures 2 and 3 show data extrapolated from bench marks along lines which are considered significant owing to the town's structure. The large quantities of water pumped from the undergound in the last 20 years have caused the progressive lowering of the water level in most AMPS wells (see Table 3) and may be considered as being the principal cause of subsidence. Figure 4 indicates the variations in elevation of bench marks located on the AMPS wells in the period 1990-1993. Local subsidence rate varies in time, probably because of variations in the lithological characteristics and grain size distribution of the quaternary sediments subjected to yearly variations of water extraction and recharge. As a precaution, AMPS has put out of operation wells in the town centre. Further gathering of precision leveilings will allow a better definition of ground settlement patterns and may allow identification of anthropic factors.