LAND SUBSIDENCE IN THE AREA OF MODENA, PO VALLEY, NORTHERN ITALY

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1 LAND SUBSIDENCE IN THE AREA OF MODENA, PO VALLEY, NORTHERN ITALY A. Colombetti, R. Gelmini - Institute of Geology, Univ. of Modena, Italy M. Pellegrini - Dept. Earth Sciences, Univ. of Bologna, Italy N. Paltrinieri - Municipality of Modena, Italy A. Zavatti - U.S.L. (= Public Hygene Board) no. 16, Modena, Italy Abstract The artificial land subsidence phenomena which occurred in the town of Modena (Italy) during the past decades are described, emphasizing the geological and hydrogeological conditions of the subsoil. The subsidence is clearly provoked by lowering of the piezometric surface, estimated in about 10 m. This lowering causes variations of the pore and effective pressure values in the silty and clayey levels present in the first 120 m of depth, corresponding, from a hydrogeological point of view, to a multicompartimental monostratum aquifer. Land subsidence reached maximum absolute lowering values little inferior to 1 m, fully justified by the geotechnical characteristics of the subsoil. The differential lowerings of the ground have induced remarkable lesions in the various buildings, due to the underground anisotropy. Moreover, some anomalies of the groundwater chemistry (F and Zn ions) have been recorded and are perhaps connected with the clays'interstitial water squeezing. Introduction The township of Modena is placed in the Apenninic margin of the Po Valley (northern Italy), at a level of about 35 m above sea level and at a distance of 15 Km from the first elevations of the Apennines range and of 140 Km from the Adriatic sea coast-line. During the past decades, the urban area and its outskirts have been interested by remarkable artificial subsidence phenomenon, with absolute ground lowering which reaches maximum values of about 1 m, causing severe damage to many monuments and buildings of the historical town centre. This situation was put into evidence when some geodetic levelling surveys were carried out between 1974 and 1977, and when, in the following years, some papers were published describing the static conditions and restoration methodology of the Dukes'Palace (XVIIth Century). This is one of the most important monumental buildings interested by lesions caused by differential lowerings of the foundation ground (Cestelli Guidi, 1978; Croci, 1980; Righi, 1980; Martinotti et Alii, 1981) In the 1980,the Municipality of Modena started an organic study of the phenomenon, planning and setting out a new geodetic levelling network, connected with a bench mark situated in a geologically stable area of the Apennines (Russo, 1984). At the present, with the sponsorship of the Municipality of Modena, specific geotechnical surveys of the town centre underground are being executed. These surveys consists of Dutch 805

2 r.. o.- l ALLUVIAL DEPOSITS -----~4 CLAY fryv-x-vj SAND l^±-^^\ MARL Fig. 1 - Deep geological conditions beneath the town of Modena (Pieri and Groppi, 1981) u, m A: upper, middle Miocene; 1, urn, PI: lower upper-middle pliocene; Q: marine Quaternary. friction-cone penetrometer tests, geotechnical boring and laying out of Casagrande piezometers and strain meters, aimed at the individuation of the ground thickness subject to subsidence induced strains, and to a better definition of the town subsoil geotechnical characteristics. About these paramètres only a few papers have been published up to now (Cancelli, 1984; Cancelli and Pellegrini, 1984). At the present, instead, there is a good knowledge of the hydrogeological characteristics of the town, both for the geometrical and physical structure of the aquifer (Colombetti et Alii, 1980; Pellegrini and Zavatti, 1980) and for the nappe flow field and its evolution in space and time (Various Authors, 1981-a). Geological and hydrogeological setting Information regarding the geological characteristics of the underground of Modena derives mainly from geophisical logs and from boring lithostratigraphies performed for the searches of hydrocarbons and ground waters. Beneath the town of Modena, the substratum formations at the base of the alluvial cover are met at an average depth of 250 m, 2-3 Km South of the town centre and 350 m from the ground-level, northwards. The bedrock is made up of a clay sequences with rare conglomerate levels in a transition environment, and of marine clays. The 'marine formation top corresponds to upper-middle Pleistocene, while the total thickness of the Plio-Quaternary marine formations (overconsolidated clays and weakly cemented sands) is found between 2,000 and 3,000 m of depth, respectively South and North of the town. Fig. 1 shows the deep geological conditions beneath the town of Modena (Pieri and Groppi, 1981). The alluvial cover (upper-middle Pleistocene-Holocene) is constitued by alternations of gravels with a sandy or silty-sandy matrix and by silty-clayey levels. Fig. 2 shows the structural conditions of the alluvial cover of the high Modena plain (Colombetti at Alii, 1980); in the first 180 m of underground 806

3 SASSUOLO FORMIGINE MODENA km 200 Fig 2 - Cross section of the alluvial deposits of R. Secchia fan. Alluvial deposits (middleupper Pleistocene-Holocene): 1) silts and clays; 2) sandy; 3) gravels with sandy matrix; 4) gravels and conglomerates. 5) Marine formations of lower Pleistocene age. Alluvial fan boundary: 6) present, 7) recent, 8 ancient. 9) Boundary between continental and marine deposits 10) Limit of investigated strata. m fill the gravelly levels have a percent thickness ranging from 20 to 40% (Various Authors, 1981-b). This thickness usually decreases from S to N, turning away from the Apenninic margin. The lithostratigraphic logs of some deep wells, drilled S and SW of Modena, have showed that beyond m and up to the top of marine formations (300 m circa), the gravelly levels have reduced frequence and thickness, less than 5% of the considered interval. The top of the first gravelly levels, always around Modena, is found at a depth between 17 m in the southern part of the town, and 35 m beyond in the northern one. The alluvial deposits, on which the town of Modena was built, belong to the alluvial fan of the River Secchia and to those of other minor water courses. The alluvial fan of the R. Secchia extends for about 70 Km, corresponding, from a hydrogeological point of view, to a sector of the widespread Po Valley aquifer system. This sector, although affected by the recharge of the R. Secchia, is not defined by boundary conditions with no-flows or with space and time univocal flows. The boundary conditions of the R. Secchia alluvial fan are as follows: the hill margin corresponds to a no-flows boundary (impermeable); the fan apex to a fixed flow, with recharge from the river to the aquifer. Up to a depth of 120 m, measured near Modena, but varying and generally increasing from S to N according to 307

4 35 < \ GROUND SURFACE H»N \ \ > o n I i o SINGLE LEVEL METER FITTING CURVE l i i i i i i 1 i i 1 i l i i \ \A ' i i i i 1 i i i i i i t i i 1 i i i E ra 500 Fig. 3 - Evolution of groundwater piezometric level (h) in Modena (top part) from 1940 to 1983, compared with rainfalls (ha): 1) piezometric level; 2) rainfall average value from 1850; 3) idem, for ; 4) idem, for decades; 5) idem, annual values for the depositional model of the fan itself, the alluvial deposits from a multicompartimental monostratum aquifer with unconfined waters in the high sector of the plain, with confined waters near and N of Modena due to the presence of thick impermeable covering layers (Pellegrini and Zavatti, 1980). At the present, the groundwater of the R. Secchia fan tends to a deficit. The inflows are represented by underdrainage from the water-course (0.5 m /s) in the upper plain, by infiltration of rainfalls and irriguous waters (from canals), and they have been estimated at about 3m 3 /s. The inventory deficit is caused by many factors: first, the groundwater overdraft, at least equal to inflows, is estimated just for the Municipality of Modena at about 42.8*10 rrp/year, half of which is for domestic consumption (Various Authors, 1981-a). Secondly, the inflows have decreased during the last 30 years, because of the urbanization of widespread permeable areas in the high plain, which reduced by 20% the rainfall infiltration areas available. Moreover the excavation of gravels from the river bed has canalized its shape, decreasing the underdrainage toward the aquifer. The variation of the grounwaters inventary and of its factors is shown by the development trend of the potentiometric surface in the Modena area. 808

5 GROUND SURFACE _ \ \ u"lii s -..h-,\. / t; N I s' v, ' ' \ ii-4 ] / * < \\ n 1-'' i-* 1 V h " S J 1 / Hi", 1 z "~" I n in M n 111 n III i ii ii i in li 11 il m i [in n in 11 Mm n in 111 [ii i ii in ii i hi ii in i M 111 m lit ' llj I.I ' 1.1 > NI < >. '... ' 111 ' I.I 1975 h JJIL LLLU lili LiU Fig. 4 - Evolution of groundwater piezometric level h (top part) from 1975 to 1983 in Modena, compared with rainfalls ha (bottom part): detailed analysis. The water level decreased from 1.80 m above ground-level, during the pre-war years, to the ground-level between , to m in 1960, and up to -9 m average value of 1983 (Fig. 3).However during , cyclic intervals were recorded with development trend of opposite sign; the extreme values ranged between -12 m in July 1976 and - 3 m in June During the years , characterized like 1979, by a negative trend, the low total flow rates of the R. Secchia and the precipitations, definitely below average (about 50% of previous values: Fig. 4), played a major role. In the years such a correlation between rainfall and piezometric levels does not take place, because of the pumpages which had the most important part in the groundwater level lowering. In the past two years the marked groundwater lowering, caused by a reduction of the rainfall inflows, was relatively limited due to a decrease of the overdraft, since the introduction of water recycling plants in some of the main water-consuming industries of the town. Fig. 5 shows in detail the groundwater potentials in the area of Modena during 1983, considering the water level in 55 wells. The potentiometric surface appears conditioned by 809

6 Fig. 5 - Detailed survey of groundwater potentiometric May 1982 (m above sea level). surface in Modena, presence of important pumping stations, giving rise to drawdown in the piezometric levels. The geotechnical characteristics of the subsoil have been illustated by Cancelli (1984) and by Cancelli and Pellegrini (1984) and can be summarized as follows. In the first part of the subsoil, usually investigated during foundation ground tests (up to m), the prevailing lithotype is made up of more or less silty clays, in great part inorganic. The degree of consistency of these materials is widely varying: the undrained Cu cohesion ranges between 20 and 100 KPa, with the highest values concentred mainly in the first m from the ground level (this being an index of overconsolidation due to nappe fluctuations and surface drying). As for the shear resistence in drained conditions, the ( ) angle generally varies between 20 and 24 ; c' cohesion often differs from zero, conforming to the degree of overconsolidation of part of the deposit 810

7 (Cancelli, 1984). Some geotechnical investigations carried out in the North outskirt of the town, have shown that the first 10 m of clays are over-consolidated, with an overconsolidation ratio OCR generally ranging from 2 to 4. Below a depth of 10 m the soil seem to be normally consolidated, but also slightly overconsolidated lenses are present at random (0CR==2) Considering the alluvial deposition environment,the most likely cause of overconsolidation might be found in repeated periods of exposition to the air, afterwards covered by new alluvial deposits (Cancelli and Pellegrini, 1984); at deeper levels the natural aging of the deposits should also be considered. In the urban area, in particular between 10 and 15 m in depth, some samples are definitely underconsolidated; this may be related to the marked lowering of the water table in recent decades, thus explaining the present phenomenon of increased land subsidence. Values of maximum consolidation pressure inferior to the present geostatic pressure were obtained also for the deep clay layer underlying the Dukes'Palaces, so that the subsidence phenomenon involves at least in part also deeper layers, and isn't limited only to the first clayey level. The presence of underconsolidated layers is not found in the North sector of the town, where the lowering of the piezometric level has been up to now much more contained (within a few meters). Land subsidence The town of Modena is located in the southern sector of the Po Valley, which is a large sedimentary basin, where during the Pliocene and even more in the Quaternary a differentiated subsidence began, which was very marked in nearly all the plain, reaching one of the highest worldwide values: a few kilometers North of Modena the Pliocene base is at a depth of over 6,000 m. Referring to the average formations thickness values recorded near Modena, the following values indicating the subsidence rate may be obtained: 0.4 mm/year in the Pliocene (5.4 M M years b.p.); 1.2 mm/year in the lower-middle Pleistocene (1.8 M M years b.p.); 0.37 mm/year in the middle-upper Pleistocene (0.8 M years b.p.); 3 mm/year in the last 2,000 years, up to 1945; mm/year in the last 20 years. The value for the last 2,000 years was obtained considering that the Roman archaeological level is buried under 6 m of alluvial deposits and presuming that absolute topographic altitudes are substantially unvaried due to compensation by alluvial deposits. The geodetic levelling surveys carried out between 1972 and 1983 and the relative data elaborations (Various Authors, 1981-a; Russo, 1984) showed that near the town of Modena the land subsidence rate starting from the pre-1970 years, has increased at lest tenfold compared with the last 2,000 years circa. The most pronounced lowering between 1950 and 1979 took place in the town centre with maximum values of 83.7 cm. Although the evolution of this phenomenon 811

8 in time is not yet fully understood, the average annual subsidence may be calculated at 4 to 8 cm/year in the areas of greatest intensity. The subsidence, highly differentiated in the town centre in relation to the ground anisotropy, is in general more marked North of the centre, where the water pumpages for industrial consumption are concentrated and where the silty-clay sequences prevail (over 80% in the first 120 m in depth) over the coarser ones. The differential lowerings have naturally caused lesions in numerous public and private buildings in the town centre, some of which are important monuments. Also a tract of the R. Secchia dyke on the NW outskirt of the town had to be raised by 70 cm, a height corresponding more or less to the soil lowering. As for the causes of differential subsidence, registered around Modena, in recent years, like other towns in the Po Valley, there is no doubt about their being artificial, considering the anomalous lowering rates compared with historical or geological ones. There is in fact a clear connection with the lowering of the water-table recorded on both a regional (Various Authors, 1981-a) and a local level (Martinotti et Alii, 1981). This connection is particularly evident comparing the soil lowering-isolines (Russo, 1984) and the potenziometric map surveyed in Modena in May 1982 (Fig. 5). Groundwater chemical alterations The chemistry of the groundwaters of the R. Secchia alluvial fan is influenced by the feeding conditions of the river. The waters have a high saline content (Chlorides, Sulphates and Bicarbonates) and a marked hardness (Calcium), up to over 50 French hydrothimetric degrees. The groundwater temperature varies between 10 and 13 C, increasing slightly up to a depth of 200 m altogether, in conformity with all the Po Plain basin, except a few areas, which is characterized by extremely low geothermal gradients. In the last few decades an increase in the temporary hardness of the water-supply wells of the Municipality of Modena, due to an increase of Bicarbonates. This increase is difficult to explain for now: it may be due to the increase of bacterial activity in the farm land of the permeable feeding areas with free CO2 following organic pollution; or to acid rains caused by air pollution; or lastly to variations of the geochemical conditions underground, resulting from a high rate of exploitation of the aquifer. This last hypothesis could be linked to the anomalous presence of Fluorine and Zinc ions in the waters; however there are no available reference data for the past. In groundwaters the Fluorine isocone lines reproduce the geometry of the gravelly alluvial fans, with values up to 0.1 mg/1, signifying that these ion concentrations are inversely proportional to be thickness of the gravelly levels of the aquifer. North of Modena values up to 1 mg/1 are reached, corresponding to thin sandy-gravelly aquifer levels, interbedded between silty-clayey layers. If on the one hand polluting sources can be excluded, on the other there is a clear connection with the underground lithostratimetry and with the exploitation rate of the aquifer. 812

9 Analogous conclusions may be drawned examining the area distribution of the Zinc concentrations in the groundwater, and also on the base of a detailed study carried out on the hydrochemistry of the waters from a well South of Modena always referred to the Zinc ion. The well, which drains different levels of the aquifer system, contains waters with larger concentration of Zinc ion, corresponding to the aquifer levels subject to a most intense pumping in the surronding area. The anomalous concentration of these ions, usually present in limited quantities and of a little significance for hydrogeological characterisation, could be determined by the lowering of the piezometric surface and by a subsequent variation of tension in the clays. This phenomenon could be considered concurrent with artificial subsidence: consolidation of the clays and sqeezing of the interstitial waters (Various Authors, 1981-a). Conclusions In the township of Modena natural land subsidence has gradually increased from the Pliocene up to the present day, with rate values that from the initial 0.3 mm/year have reached 3 mm/year in the last 2,000 years (Fig. 6). 10U ( BI-LOG SCALE) 10 I I Fig. 6 - Rate values of subsidence from Pliocene to the present in the area of Modena. I I ^ CO CO iri ^ o CM t (year) Subsidence in the post-war period, with a rate of a fiew cm/year, may obviously be attribuited to artificial causes. Moreover a clear connection has been shown between land subsidence and the evolution of the piezometric surface which in Modena has undergone a lowering of about 10 m. As for geotechnical interpretation of this phenomenon, the data available at present are too few, especially because of the complete lack of data relating to deep layers; therefore the following considerations necessarily rely on a large-scale extrapolation. The first 120 m of soil subject to an increase in tensional state can be referred to; this thickness corresponds to the aquiferous layer ("multicompartimental monostratum") undergoing pumpage. Only 80 of these 120 m will be considered, excluding the first 10 m already consolidated and 30 which may be attributed to gravelly layers. According to Cancelli and Pellegrini 813

10 Fig. 7 - Alluvial fan of the R. Secchia: 1) alluvial deposits (with contour lines); 2) terraced alluvial deposits; 3).- impermeable marine formations of the Apenninic margin. Fan boundaries: 4) ancient, 5) recent, 6) present. (1984), assuming that the average total pressure variations is constant in time and in depth with Ap = lookpa, and attributing to compressible soil a primary compressibility of Re = 0.2 (without considering aging), a general oedometric packing would be obtained equal to 1.2 m. The absolute maximum lowering values, reported by Russo (1984) which are less than 1 m, give plausibility to this hypothesis. The present tendency of land subsidence in Modena should be towards the attenuations of the phenomenon, provided that the water table is kept at constant values in the future. The result of the geodetic levelling carried out in 1983 on the control network, which recorded maximum differential lowerings of 25 mm/year, increasable up to 30 considering also geological subsidence (compared with mm of the previous years), could confirm this evolutive trend. In Modena, the land subsidence seems to be accompanied also by anomalies in the groundwater composition, at least for what concerns the presence of some ions usually present in small quantities in the alluvial deposits groundwares, such as Zinc and Fluorine, among the considered 814

11 parameters, that are influenced by the squeezing effect of the clay interstitial waters. Were this hypothesis verified, a long-term substantial alteration of the waters chemistry could be reached, with possible consequences for the use of groundwaters hydric resources (drinkable waters). The decision adopted by Municipal Authorities of Modena tends to privilege the use of surface waters, at least for industrial use; this is consistent with a policy for the rational use of water resources aiming to oppose land subsidence, since adequate norms are still lacking in Italy. References Cancelli, A., 1984, Geotechnical properties of Modena subsoil: Proc. 3rd Interat. Symp. on Land Subsidence, Venice, 10 p. Cancelli, A. and Pellegrini, M., Problemi geologici e geotecnici connessi al territorio della citta' di Modena: Proc. 2nd National Congr. ASS.I.R.CO on "La citta' difficile", Ferrara (Italy), 10 p. ' Cestelli Guidi, C, 1978, Riflessi della subsidenza dei suoli sulla stabilita' délie opère e interventi di salvaguardia: Proc. National Conf. on "I problemi della subsidenza nella politica del territorio e della difesa del suolo", v. 8, Pisa (Italy), p Colombetti, A.; Gelmini, R. and Zavatti, A., 1980, La conoide del fiume Secchia: modalita 1 di alimentazione e rapporti col fiume (Province di Modena e Reggio nell'emilia): Quad. I.R.S.A. - C.N.R., no. 51-1, Rome, p Croci, G., 1980, Gli interventi nel Palazzo Ducale di Modena, interessato da fenomeni di subsidenza: Proc. 14th National Congr. of Geotechnics, v. 1, Florence (Italy), p Martinotti, G.; Barbolini, F.; Cerone, F.; Pozzese, P. and Ricelio, L., 1981, Consolidamento e restauro dell'accademia Militare di Modena: Edilizia Militare, Rome, 61 p. Pellegrini, M. and Zavatti, A., 1980, II sistema acquifero sotterraneo fra i fiumi Enza, Panaro e Po: alimentazione delle falde e scambi fra falde, correlazioni idrochimiche: Quad. I.R.S.A. - C.N.R., no. 51-1, Rome, p Pieri, M. and Groppi, G., 1981, Subsurface geological structure of the Po Plaine, Italy: Nat. Counc. Research (C.N.R.), publ. no. 414 of "P.F. Geodinamica", Rome, 13 p. Righi, P.V., 1980, Rilevamenti geotecnici nel sottosuolo del Palazzo Nazionale di Modena: Proc. 14th National Congr. of Geotechnics, v. 1, Florence, p Russo, P., 1984, Results of recent levelling in region of Modena: 3rd Internat. Symp. on Land Subsidence, Venice, 10 p. Various Authors, 1981-a, Piano per la tutela e l'uso delle risorse idriche:docum. Comprensorio Modena, 18-1, Municipality of Modena (Italy), 662 p. Various Authors, 1981-b, Contributi tematici per la conoscenza dell'idrogeologia Padana: Quad. I.R.S.A. - C.N.R., 51 (2), Rome, 70 p. 815

GEOTECHNICAL PROPERTIES OF MODENA SUBSOIL: A PRELIMINARY REPORT

GEOTECHNICAL PROPERTIES OF MODENA SUBSOIL: A PRELIMINARY REPORT A. Cancelli Geotechnical Section, Dept. of Structural Engineering, Technical University Milano, Italy Abstract Results of field and laboratory