THE SURVEY OF SOIL VERTICAL MOVEMENTS IN THE REGION OF BOLOGNA

Lamberto Pieri and Paolo Russo Istituto di Topografia.Geodesia e Geofisica Mineraria Université di Bologna,Italy

Abstract The soil subsidence in Bologna area has been known for at least 30 years.In this paper after a short review of the known phenomenon we comment the planned project of high precision levelling network and some results of the first measurements just carried out with the purpose of checking subsidence. General

It has been known for at least 30 years that the soil in the Bologna area is subsiding.The comparison between the height of some bench marks of Italian levelling network levelled in the period 1897-1902 and relevelled in the period 1947-1957 reveals this phenomenon.On the ground of these data Salvioni (1953,1957) was able to draw some contour lines of equal subsidence in the Po valley.The Bologna area showed a slight vertical sinking of the soil ranging from 10.7 cm to 20.9 cm in that period (fig. 1 ).

APPENNINES

Fig.1 Contour lines in cm of equal subsidence in the period 1897-1957 (Salvioni,1957).

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After 1950 there is a lack of information about the subsiden­ce, however the phenomenon has become increasingly pronounced.

This fact has been brought out by the comparison of the geometric levelling carried out around 1943-50 and some repetitions performed by I.G.M.I, and other public organizations in the period ranging from 1970 to 1973.

The resulting soil movements and their distribution are set out in fig.2

Fig.2 Contour lines in cm of equal susbidence in the period 1943/50 - 1970/73.

One can see between two characheristic bands with a sinking of the soil greater than 1.2 m a steady strip of land in relation to the river Reno. Contour lines converge in the centre of Bologna city at the foot of the hills.With the purpose of checking the phenomenon of vertical sinking of the soil the "Istituto di Topografia" of Bologna University working together with "Ufficio Tecnico del Comune di Bologna" m 1978 carried out two high precision levelling lines crossing the historical centre of the city and repeated the measurements

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10 months later. The comparison of levellings, showed a sinkig of the soil greater than 6 cm/year.

In the same period people began to notice damage in some old buildings in the north Bologna area and an interesting phenomenon concerning the Asinelli tower was observed.Since 1970 we have been performing measurements to check the stabi­lity of the tower which is leaning and during the repetition of the measurements it v/as noticed that a new component in a northerly direction was added to the normal inclination.

In the period from 1974 to 1981 some public organizations have executed a great number of levelling lines. These data providing disconnected relevelled segments do not allow us to draw contour lines. Nevertheless these data brought up to date our knowledge of soil movements.

These measurements were carried out starting from different bench marks and at different time so that it was necessary to uniform the data. For this purpose we assumed as junction points bench mark settings in the more stable areas.

Following the advice of geologists we choose the segment between bench mark 186 and 163 of the line of I.G.M.I, n. 5 (Firenze-Bologna). In particular the bench mark 163 was used for connecting the heights of points belonging to different levelling lines. Fig.3 shows the vertical movements of bench marks along the main levelling lines of I.G.M.I, in the period 1973-1981 .

. 1 9 7 6 ( A G I R ) - 1376 (CADASTRE _ 1978 (JAN.) .. 1978 (DEC) . 1979 (AGI R)

- 1980CI.GMO - 1981(M0DENA LI

f~ REFERENCE: 1973

107 1DB 100 110

- 10

. 2 0

L I N E S N . 5 , 6

1974

5 6

Fig.3 Vertical displacement profiles along levelling lines n.5,6 and n. 15, 17.

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We analysed only the more urbanized area of Bologna. The graph 3.a shows that the area at the foot of the hills is little affected by the soil sinking; going to the north there is a sharp increase in the soil movement in the civic centre (bench mark 5/188 - Nodal 31) with a maximum sinking of 45-9 cm in the period 1973-1978 which, in the linear hypothesis, means a sinking velocity greater than 9 cm/year. The graph 3.b shows a gradual sinking of the soil, increasing from the east boundary to the centre of the town with a maximum of 47.5 cm in the period 1974-1980. In a westerly direction one meets an area almost stable to the river Reno and then one finds a gradual increas in the soil sinking with a maximum of 81.9 cm in the period 1974-1981 which, in the linear hypothesis, corresponds to a sinking soil velocity of 11.7 cm/year.

Considering the importance of soil movements and the pro­bable consequences to the surrounding territory the technical office of Bologna municipality in the year 1981 commissioned some experts of Bologna University (Selli et al.,1981) to plan a study of this phenomenon. The planned program includes seve­ral subjects whose developement was expected within four years in order to know all the aspects of the soil sinking phenomenon. The execution of the programme began in 1982 with the institution of a levelling network,. The measurements of the network have just been concluded and we are now going to elaborate the measurement data. The area under control

The area to be checked is about 460 km and includes the localities where the greatest soil sinking has taken place. First of all we planned to project a high precision levelling network utilizing only new bench marks, but in order to have information on the soil movements from the first measurements, we included in the network as many old bench marks as possible provided they were found to be stable. It has been possible to include 170 old bench marks in the network. Reference bench mark

The problems concerning the vertical datum and the reference bench mark are closely connected with the purpose of the levelling network. All authors agree in considering the repeated levelling as the- most reliable method for measuring vertical soil movements but,when a levelling network is not available it is possible to use scattered relevelled segments (Vanicek and Krakiwski,1982). In fact for a preliminary examination of the subsidence phenomenon we have also used this method even though in a rough way using the available scattered levelling data. Nov/ we have projected and measured for the first time a levelling network the purpose of which is to detect vertical soil movements.

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Concerning the reference bench mark it is pratically impossible to connect the Bologna control network with the Genova tide gauge which gives the mean sea level assumed as vertical datum for the Italian levelling network. In fact the levelling line Bologna-Genova is about 340 km long.

It is therefore clear that the best way is to locate some reference bench marks around the area to be checked .

Unfortunately almost .all those areas are subsiding.

.APPEWNINES

SASSO MARCONI*

Fig.4 Levelling network with distances between consecutive and between nodal bench marks.Such distances are respectively: 1 km and 6.5 km in area n.1; 1 km and 3.5 km in area n.2; 0.5 km and 2 km in area n.3; 0.25 km and 0.5 km in area 4.

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Nevertheless after a careful examination of levelling lines repeated at intervals of several years and after investigations on the spots carried out with the geologist prof. Mario Ciabat-ti, we have been able to find two bench marks that are almost stable: the one is located in "Sasso Marconi" area and the other in "Castel dei Britti" area. As a precaution againts damage, near each one we set up other bench marks on rock or on buildings with foundation resting on rock. Layout of the levelling network The levelling network has a lenght of about 375 km . It has been divided into four parts as regards the density of bench marks. Fig.4 shows the distances between consecutive and nodal bench marks in each area. In the center of Bologna we have the highest density of bench marks for detecting local soil movements in connection with the stability of buildings and with hydraulic problems.

The orientation of the levelling lines is as nearly parallel as possible to the direction of the greatest subsi­dence. For this reason for example the lines near the south side of the river Reno are almost perpendicular to the river­bed. Moreover the levelling lines avoid as far as possible the roads that can be changed in a short time. The bench marks, 475 in number, are located almost always on outer walls of old buildings. We rejected the new buildings or those v/hich might be changed.

In fig. 5 the scheme of the levelling net covering the centre of Bologna is shown. It is easy to see the higher density of bench marks to the north where there are greater soil movements. In the same figure the areas are indicated in which we set up our local levelling networks with the purpose of studyng the movements of buildings or monuments. The survey The levelling measurements have been already carried out using instruments and methods of high precision levelling. Only a few technical lines have been measured with the purpose of connecting old scattered bench marks to the new levelling network. We established the values of discrepancies between forv/ard and backward levelling of a section to be in modulus less than T=3 mm Jî> (D is the length of the section in km). When that discrepancy exceedes T the rejection test must be satisfied.According to this test a levelling x is rejected when

|x-x|>3.7mmv /D (where x is the mean of the separate means of the forward and backward levelling for the section).

In order to remove gross errors and to reduce the accumulation of systematic errors we stated rules concerning levelling.

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procedures,instrumentation testing and rod calibration. We also laid down limit for the loops misclosures.As regard air refraction corrections we did not impose the collection of the atmospheric data because all the area of the levelling network is flat and the atmospheric corrections would be small and uncertain.

Since an influence has been shown of the earth's magnetism on automatic levelling instruments we recommended the use of automatic nonmagnetic levels.

m LOCAL CONTROL • J O F ~ ^ = f ^ â = â ê i j ^ M NETWORKS i r n j ? ^ / ^ ^ ^ t r

® BENCH MARKS '&.' ^ OJH\??

500 m •ï-ï>a».i'ï> 4.'"

Fig. 5 Scheme of the levelling network of the centre of Bologna with the sites in which local control networks have been set up.

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In the case of the Bologna levelling network the cause of the more dangerous systematic error is the considerable speed of subsidence. In fact during the measurement of the levelling network large soil movements can occur so that we need to reduce the levelling data to a common period. However we laid it down that the measurement period had to be at most 45 days. Moreover we indicated some levelling lines we called "criticals" to be measured first of all and in any case within 30 days (fig.6). Employing at the same time 7 levelling crews we completed this programme. First results Levelling network measurements have just been carried out; after the corrections due to the calibration rods and after the technical and administrative controls we will perform the adjustment of the network.

Fig. 6 Levelling loops misclosures W in mm and critical lines.

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LINES N 5,6

0

10

20

30

40

50

60

70

80

90

100

110

120 cm

- REFERENCE 1973

107 108 109 110 111 CENTRE

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1974

4 5

LINES N. 15,17

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10_

20_

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Fig. 7 Vertical displacements profiles along levelling lines n.5,6 and n. 15, 17-

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The Bologna municipality autiorized us to present some provisional results of the levelling concerning only the urban centre.

Fig.6 shows the loops in which misclosures exceed ~{L, L being the length in km of the loops. These errors are small and this means that also the adjustment corrections will be small. Therefore the provisional data of height differences we present here are not very different from those obtained after adjustement is carried out. In Fig. 7 we give the soil movements along the same levelling lines shown in fig.3. The grey area shows the soil movements which appeared from previous measurements.

The trend of the subsidence is confirmed, moreover we have new data relating to the northern area of the town in which the maximum subsidence (bench mark N.6/10) is found to be 116.3 cm in the period 1973-1983 which, in the linear hypothe­sis, corresponds to a sinking velocity of 11.6 cm/year. In Fig.8 we set out the sinking of two bench marks of great interest in the period ranging from 1950 till now.

100

120-

140

160

220 -

240

1974 76 79 81 83

\ 140

160

180 „

200

220

\

..17/11

0

ml ^\^DOLOGNA J

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Fig.8 Diagrams of sinking of bench marks n.6/5 and n. 17/11 with respect to time.

Conclusions The first results of measurements besides confirming the satisfactory layout of the survey, show that the phenomenon of subsidence in Bologna is proceeding with a disturbing velocity which has obtained 11.6 cm/year.

We hope to have, as soon as possible, the data resulting from the adjustment of the network in order to have an overall description of subsidence in the Bologna area.

We also urge the Bologna municipality to consider the absolute necessity of carrying out the programme elaborated in 1981 by the experts (Selli et al.,1981 ) in order to acquire knowledge of all the aspects of the subsidence phenomenon. Acknowledgements We have to thank Prof. M. Ciabatti who gave us geological support for the choice of stable bench marks and the land-surveyor G. Scuderi who gave us a great deal of help and has directed the levelling measurements.

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References Energy, Mines and Resources Canada, 1978: Specifications and

Recommendations for Control Surveys and Survey Markers, Survey and Mapping Branch, Ottawa, Canada, 45 p.

Salvioni, G. , 1953: Primo contributo sulla comparazione dei ri— sultati fra la nuova rete altimetrica fondamentale e la vec-chia livellazione di precisione; Bollettino di Geodesia e Scienze affini, vol.XII, n.I, p.87-99.

Salvioni , G., 1957: I movimenti del suolo nell'Italia centro-settentrionale. Dati preliminari dedotti dalla comparazione di livellazioni; Bollettino di Geodesia e Scienze Affini, vol.XVI, n.3, p.325-366.

Selli, R., Ciabatti, M., Pieri, L., Russo, P., 1981: Progetto per lo studio dell'abbassamento del suolo sul territorio bo-lognese, Comune di Bologna, 20 p.

Vanicek, P., Krakiwsky, E., 1982: Geodesy: The Concepts, North Holland Publishing Company, Amsterdam, Netherlands, 691 p.

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