An extensive investigation of land subsidence in the Euganean geothermal basin, Italy

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Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, 1995.

1 Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). IAHS Publ. no. 234, An extensive investigation of land subsidence in the Euganean geothermal basin, Italy G. GOTTARDI, P. PREVIATELLO & P. SIMONINI Istituto di Costruzioni Marittime e di Geotecnica, Université di Padova, Via Ognissanti 39, Padova, Italy Abstract In order to characterize the phenomenon of man-induced land subsidence taking place in the Euganean geothermal basin, an extensive investigation has been carried out at the University of Padova. This phenomenon was observed in the recent past and related to the withdrawal of hot waters (85 C) from the subsoil. The currently exploited aquifers are located in the deep calcareous bedrock, hydraulically interconnected to the alluvial Quaternary deposits. In the 1960s the production of hot waters for mud bath treatments caused the progressive piezometric lowering (2.5 m year" 1 ) and the shallow layers cooling. A marked phenomenon of land subsidence thus developed, notwithstanding the deepening of draining wells. A 300 m-deep borehole was drilled and continuously sampled in order to investigate the geotechnical properties of the alluvial soils; at the same time an accurate measurement of water withdrawal and piezometric levels was also planned. The effects of high temperatures on soil parameters and on land subsidence occurrence are also taken into account in the paper. INTRODUCTION Phenomena of soil lowering in the Euganean geothermal basin - located at the foot of the Euganean Hills, southwest of Padova (Fig. 1) had been observed several times in the past and were soon related to the withdrawal of hot waters from the subsoil. The area has been well known since Roman times, as reported by many archaeological finds and witnessed by Latin authors like Lucretius, Titus Livius, native of this country, Plinius Maior and others. The present hills are mainly composed of volcanic rocks of Upper Eocenic and Oligocenic age, whereas the present sedimentary rocks are from Upper Jurassic to Lower Oligocène. The whole area is characterized by a close system of intersecting block faults, which caused step-like displacements of the rock formations situated under the alluvial Quaternary sediments. Due to the morphology of the bedrock, the Quaternary deposits - mainly sandy and clayey loose materials have different thicknesses: in Abano Terme they range from about 100 to a maximum of 230 m, whereas in Montegrotto Terme from 0, where bedrock locally crops out, to a maximum of 180 m. Discovering the source origin of thermal waters has always fascinated the researchers: earliest assumptions connected the water heat to the natural burning of imagined underground deposits of sulphur and coal. Until the beginning of this century

2 22 G. Gottardi et al. Fig. 1 Location of the Euganean geothermal basin (hatched area). the heating source was ascribed to the possible presence of magmatic waters; subsequently the idea of the geothermal gradient as main heating cause of the infiltrating rain waters began to develop. In 1973, Piccoli et al. carried out an exhaustive research which led to a novel interpretation - now widely accepted - of the Euganean hydrothermal phenomenon. According to the authors the thermalism source is related to the rain waters that, in infiltrating into the subsoil in the "Piccole Dolomiti" area and nearby zones (about 100 km northwest), gain temperature and salinity. In particular the high temperature of the water (varying from 80 C to 87 C) would be produced by the terrestrial heat at depths around 5000 m - reached by their circulation. Deep waters, moving towards the southeast, reach the area around the Euganean Hills and suddenly go back up through the described system of faults and fractures existing in the local subsoil. LAND SUBSIDENCE IN THE EUGANEAN BASIN Until the end of the last century the waters used for thermal baths spontaneously emerged from springs and lakes, thus confirming that the average piezometric head was originally above the ground level. The increasing demand of water for thermal treatments made it subsequently necessary to drill numerous wells into the shallow Quaternary alluvial sediments. This produced the progressive lowering of the piezometric level at an escalating rate, which reached a maximum in the 1970s. In order to withdraw the waters with higher temperature and salinity levels, many wells were finally deepened into the fractured carbonatic bedrock (mainly the "Biancone" formation, Upper Cretaceous-Upper Jurassic, and in part the "Scaglia Rossa", Lower Eocene p.p.-upper Cretaceous) which now represents the only exploited aquifer with a rather high "equivalent" permeability eq = 10" 3 m s" 1. However small cracks in some well casings could cause a slight withdrawal of groundwater from the alluvial sediments too.

3 Investigation of land subsidence in the Euganean geothermal basin, Italy 23 Due to its economical and environmental importance, in 1966 the Italian Department of Industry decided to impose a united management of the local geothermal resources. Only after that date piezometric levels over the entire region together with the total production of hot waters began to be systematically monitored and recorded. The volume of water pumped in the two urban centres of Abano and Montegrotto in the period satisfying the demands from more than 2 million daily visitors per year - was about 20 millions m 3 year 1 with a maximum of over 25 millions in 1985 (Fig. 2). Since the exploitation was mainly concentrated in such area of about 20 km 2, the upper solid line in Fig. 2 represents the 90% of the total amount withdrawn from the entire basin (about 30 km 2 ). The average piezometric level trend showed in the same period not only a progressive reduction - especially in the 1970s (up to 2.5 m year" 1 ) - but also clear seasonal cycles as a consequence of the corresponding demand and touristic flow (Ballestrazzi et al, 1991). It was apparent how quick the system could reply to substantial variations of the volume of water withdrawn. This would suggest that aquifers are in general rapidly and continuously supplied with the previously described deep circulation system and therefore that their exploitation is still possible, even if only partially and under controlled conditions: it is now estimated that, if all the wells were shut down, there would be a sudden recovery of the piezometric head up to above the ground level again. The elevation measures in the thermal area started not organically in 1959, but high precision levelling began only in The last survey was carried out in Main results are summarized in Fig. 3 (Antonelli et al, 1995), where the isokinetic lines representing the total settlement in the period are drawn. Two marked subsidence cones are clearly identifiable, located in the centres of Abano and m xio 20 TOTAL WITHDRAWAL ABANO TERME MONTEGROTTO TERME \ A 8 T i "i r ~i i \ i i i i i 1~ t i Fig. 2 Withdrawal in the period \ < o - YEARS

4 24 G. Gottardi et al. ^ ia MONTEGROTTO Fig. 3 Isokinetic lines during (Antonelli et al., 1995). Montegrotto. Data from repeated levellings during along the north-south profile indicated with the A-A' line in Fig. 3 are plotted in Fig. 4. The subsidence rate, which has been estimated to have exceeded 20 mm year" 1 in the recent past, appears to have turned into a decreasing trend in the late 1980s. mm H Fig. 4 Settlement profile in along the A-A' line (Antonelli et al., 1995).

Investigation of land subsidence in the Euganean geothermal basin, Italy 25 SITE INVESTIGATION OF ALLUVIAL SEDIMENTS With the aim of keeping under control the land subsidence occurrence in the

5 Investigation of land subsidence in the Euganean geothermal basin, Italy 25 SITE INVESTIGATION OF ALLUVIAL SEDIMENTS With the aim of keeping under control the land subsidence occurrence in the Euganean basin and, more generally, of optimizing the management of geothermal resources, a detailed and multidisciplinary investigation was therefore promoted by the local authority (the "Regione Veneto") and commissioned to the University of Padova. One of the main objectives was the investigation of the geotechnical properties of soils forming the alluvial Quaternary deposit, in order to provide as accurate as possible parameters for the calibration of the finite element model which represents the final and most attractive step of the research. A borehole named " Aponus 2" and 300 m deep a depth never achieved previously in the area was therefore drilled and continuously sampled. The engineering simplification of the soil profile, stressing the permeability and compressibility properties, is proposed in Fig. 5 (Gottardi et al., 1994); the layer subdivision is not always unambiguous because of the silty nature of most sediments. The Quaternary deposit extends locally as far as 196 m and is followed by the carbonatic fractured bedrock indicated as "Biancone". Three main aquifers, alternating with more or less impervious aquitards composed of silty clay and clayey silt of low plasticity, can be singled out within the alluvial sediments: the first one (10-13 m), composed of silty sand, is too shallow to be of practical interest for thermal purposes, while the other two (48-57 m and m), made by medium and fine sand, provide rather satisfying characteristics of permeability (k = 10" 5-10" 4 ms" 1 ). It was already noticed that they are no longer exploited. In correspondence to each aquifer a standpipe piezometer was then installed (labelled PI, P2 and P3 in Fig. 5), both to check the possible hydraulic interconnections and to measure water level variations during the year. Figure 6 shows these records starting from July 1990: measures labelled P4 refer to the borehole itself which was subsequently deepened as far as 465 m into the bedrock. All water levels lie well below the ground surface and show a clear seasonal trend according to the corresponding volume withdrawn; furthermore depressions generated in the deep aquifer appear to propagate towards the other shallower aquifers. A general recovery in reply to recent withdrawal limitations is also evident. Finally, on the right-hand side of Fig. 5, the temperature log represents the result of one of the many geophysical tests performed in the deep borehole: starting from about 45 C at the top, the water temperature reaches (and in many cases exceeds) 80 C in correspondence of the bedrock. Unfortunately no data measured directly in the Quaternary deposits are available, but the actual temperature gradient should not be dissimilar from the borehole trend of Fig. 6. GEOTECHNICAL CHARACTERIZATION A total of 113 samples (33 undisturbed and 80 remoulded) were obtained from the deep borehole. Figure 7 summarizes the main geotechnical properties of the alluvial sediments. Atterberg limits show that most samples lie within the category of very silty clays (10 < plasticity index (/ ) < 22, 35 < liquid limit (vv L ) < 50) and very few samples are high plastic clays. The water content profile indicates that the soil below

6 26 G. Gottardi et al. SOIL PROFILE PIEZOMETERS PI P2 Pi PI. TEMPERATURE C 20 io "= SILTY CLAY SILTY SANO LAYERS OF CLAYEY ANO SANDY SILT 50 LAYERS OF SILTY CLAY ANO CLAYEY SILT MEDIUM- FINE SANO A SILTY CLAY 100 LAYERS OF CLAYEY ANO SANOY SILT SILTY CLAY FINE SANO SILTY CLAY 200_ (m) GRAVEL ANO C0B8LES MINIMUM PIEZOMETR. LEVEL V ( ) CARBONATIC BEDROCK Fig. 5 Simplified soil profile from the deep borehole. 90 m has a natural water content either at or under its plastic limit and that the liquidity index (I L ) generally tends to decrease with depth. This is due to the large vertical effective stress at those depths and does not necessarily imply extremely over consolidated deposits. Standard one-dimensional oedometer compression tests were performed on twentysix 70 mm diameter specimens. In Fig. 7 the vertical preconsolidation stresses determined from e-log a' plots using the Casagrande method is reported. On the same figure the trend of vertical effective stress is drawn, considering both a linear (u = yz,

7 Investigation of land subsidence in the Euganean geothermal basin, Italy 27 _ o.oo I Fig. 6 Water level measurements in where: y = 9.81 kn m" 3 and z = depth) and in situ measured piezometric pore pressure distributions. Total vertical stresses were calculated using an average soil density equal to 19 kn m" 3. Note that only few samples are slightly overconsolidated, especially in the upper layers of the Quaternary deposit, whereas the others appear to be normally consolidated or slightly under consolidated with respect to the present effective stress situation. The oedometric modulus M was calculated in correspondence of the in situ vertical effective stress and its values are also reported in Fig. 7. Even if experimental data are rather scattered, a general increase of M with depth can be observed. The power equation M = 490 z 0,84 (where if z is in m, M is in kpa), fitting the experimental data, was assumed as a reasonable trend of soil stiffness with depth. Fig. 7 Main geotechnical parameters of alluvial sediments from laboratory tests.

8 28 G. Gottardi et al. Finally values of the coefficient of consolidation C v have been determined from oedometric settlement-time plots - in the normally consolidated range using the Terzaghi one-dimensional consolidation theory and the Casagrande method. The values of C v vs. depth range from about " 7 to " 6 m 2 s" 1 (Fig. 7). A large scatter of experimental data is again observed and no particular trend of C with depth appreciated. THERMAL EFFECTS ON CONSOLIDATION PROPERTIES It should be kept in mind that the Quaternary deposits are permeated by hot water, characterized by a temperature level up to 85 C, which is substantially different from standard long-term geotechnical laboratory tests. Many studies have established that soil behaviour is significantly influenced by temperature and particularly the consolidation rate. Two types of conditions can be considered in relation to the temperature influence on soil behaviour: isothermal and non-isothermal. In the case of the Euganean basin no changes of the temperature profile have been developed for the last 20 years and, consequently, isothermal conditions in the alluvial Quaternary deposits seem to take place. In such conditions the coefficient of compressibility C c can be assumed constant for a given soil, independently of the temperature level (Campanella & Mitchell, 1968). Therefore, also the oedometric modulusm = 2.3(1 + e)a'/c c is practically not affected by temperature even if a slight decrease of the void index is observed when temperature is increased. DEPTH 38.0 i 45.3 T( C) 50 0 D a'(kpa) Fig. 8 Influence of temperature on consolidation rate.

Investigation of land subsidence in the Euganean geothermal basin, Italy 29 In order to characterize the variation of consolidation rate with temperature, some special oedometer tests were also

9 Investigation of land subsidence in the Euganean geothermal basin, Italy 29 In order to characterize the variation of consolidation rate with temperature, some special oedometer tests were also carried out. In this case the oedometer apparatus was installed in a thermostatic cell, capable of maintaining the soil sample at constant temperature (25 C and 50 C in this case): C v -values at 50 C in the normally consolidated range (Fig. 8) are higher than those obtained from oedometric tests performed at 25 C, for soils sampled at 38.0 and 45.3 m. Similar results have been found by Burghignoli & Desideri (1988), and can be partially explained taking into account the variation of water viscosity and specific weight, which considerably affects the hydraulic conductivity of soils. The secondary compression coefficient C a was finally measured in all the oedometric tests and for the Quaternary cohesive deposits lies between and No appreciable differences of C a -values were detected in samples tested at 25 C and 50 C, thus confirming the experimental work of Miliziano (1992). CONCLUDING REMARKS The heavy exploitation of hot water and the consequent piezometric lowering are responsible for the widespread phenomenon of land subsidence in the Euganean geothermal basin, which has been detected and monitored over about the last 20 years. On the basis of the detailed mechanical characterization of cohesive soil layers, a rough estimate of the final consolidation settlement produced by the maximum piezometric lowering has been produced simply by assuming one-dimensional vertical deformations and by using the proposed trend of the modulus M with depth. The following hypotheses were also introduced in calculating consolidation settlements: homogeneous and uniform soil within each layer, incompressible granular materials and linear increasing with depth of the vertical effective stress increment induced by the piezometric lowering (Gottardi et al., 1994). The total final settlement s tot thus emerges from the following simple expression: where H h M, are respectively, the thickness and oedometric modulus of the n compressible layers and Aa' t is the vertical effective stress increase. The so computed settlement is equal to about 50 cm, but it should be observed that all the above approximations lead to an upper limit of the total settlement (Ricceri & Butterfield, 1974). In fact the hypothesis of homogeneity and uniformity within each cohesive layer is unfortunately far away from the actual in situ soil profile, where thin sub-layers of silt and silty sand may easily modify the behaviour of the whole layer. A better estimate of subsidence settlement can be obtained by the three-dimensional finite element numerical model (Antonelli et al., 1995), which has been calibrated on the basis of the above mentioned geotechnical parameters. Unfortunately accurate measurements of ground level variations have not been available since the beginning of the phenomenon of land subsidence, which had already been recognized in the early 1960s. However the subsidence rate, estimated to have exceeded 20 mm year" 1 in the recent past, has been found to show a decreasing trend in the late 1980s.

10 30 G. Gottardi et al. A comparison of the two orders of magnitude - measured and computed - would suggest that most of the expected settlements have already taken place and that the phenomenon is now being exhausted, as confirmed also by the most recent le veilings. Therefore, land subsidence in the Euganean region appears to be in its final stage, provided that no further piezometric level lowering in the aquifers of the alluvial deposits occurs. Finally the land-subsidence phenomenon, which normally takes long time to fully develop, in the Euganean basin is speeded up by the presence of hot groundwaters: lower water viscosity and specific weight at high temperature imply higher permeability and thus higher rate of consolidation. Acknowledgement The work has been kindly supported by the "Regione Veneto Segreteria Régionale per il Territorio, Dipartimento per la Geologia e le Attività Estrattive". REFERENCES Antonelli, R., Fabbri, P., Iliceto, V., Majorana, C, Previatello, P., Schrefler, B. & Sedea, R. (1995) The geothermal Euganean field. A subsidence modelling approach. In: Proc. World Geothermal Congress (Florence) (in press). Ballestrazzi, P., Brighenti, G., Ciancabilla, F., Dainese, A. & Schiesaro, G. (1991) Ricerca sulla subsidenza nell'area termale di Abano Terme (PD). InvestigationPei Ed., Parma. Burghignoli, A. &Desideri, A. (1988) Influenzadellatemperatura sulla compressibilitàdelleargille. In: Proc. oftheconf. "Deformazionl del terreni ed interazione terreno-strutturain condizionidi esercizio" (Monselice, Padova), Campanella.R. G. &Mitchell,J. K. (1968) Influenceoftemperaturevariationson soil behaviour./. Geotech. EngngASCE 94(3), Gottardi, G., Previatello, P. & Simonini, P. (1994) Geotechnicalcharacterizationof land subsidence in the Euganean thermal basin. In: Proc. 7th Int. Congr. IAEG (Lisbon), Miliziano, S. (1992) Effettidellatemperaturasul comportamento meccanico délie terre coesive. PhD Thesis, Universitàdi Roma "La Sapienza". Piccoli, G., Dal Prà, A., Sedea, R., Bellati, R., Di Lallo, E., Cataldi, R., Baldi, P. & Ferrara, C. (1973) Contributoalia conoscenza del sistema idrotermale Euganeo-Berico. Proc. Ace. Naz. Lincei, VIII, Roma 11, Ricceri, G. & Butterfield, R. (1974) An analysis of compressibility data from a deep borehole in Venice. Géotechnique 24(2),

Land subsidence due to groundwater withdrawal in Hanoi, Vietnam

Land Subsidence (Proceedings of the Fifth International Symposium on Land Subsidence, The Hague, October 1995). 1AHS Publ. no. 234, 1995. 55 Land subsidence due to groundwater withdrawal in Hanoi, Vietnam