Automated Geomatic System for Monitoring Historical Buildings During Tunneling in Roma, Italy

Automated geomatic system for monitoring historical buildings during tunneling in Roma, Italy

M. Crespi, F. Giannone & M. Marsella D.I.C.E.A. Geodesy and Geomatic Area of the Faculty of Engineering – Università di Roma La Sapienza, Roma, Italy A. Sonnessa Survey Lab srl – Università di Roma La Sapienza, Roma, Italy

ABSTRACT: The present work is focused on the preliminary results obtained through the geomatic integrated monitoring system currently running at the test site of the Basilica of Maxentius in the Roman Forum. The system is aimed at controlling a number of archaeological sites which can be potentially affected by the tunneling works for a new metro line which is presently under construction. It includes different high precision geomatic sensors controlled by a centralized control station which continuously acquire data at high frequency. In order to identify a reliable processing procedure and assess the quality of the collected data, we started to develop and experiment a preliminary analysis of the data collected in the first period of the system opera- tion (before the start of the excavation works). This activity allowed us to assess the performance of each sensor, focusing on the capability of the system to control also the stability of the monitoring stations. The obtained results will be adopted to better define an automated procedure for future massive data processing.

1 INTRODUCTION

The effects of the underground construction works in urban areas represent a threat for the stability of the overlying buildings and structures. The problem is particularly relevant when the damages can affect archeological sites and historical buildings, such as those located in the historical centre of Rome which, for cultural and architectonic heritage is listed by UNESCO as a World Heritage Site. In view of the works which are planned to start for the construction of the third underground line in Rome (Metro C line), a monitoring system, integrating ground and satellite based geomatic sensors, was designed and Figure 1. Plan view of the Metro C Line between San Giovanni partially installed in a number of test site within the in Laterano and Piazza Venezia. archeological area. The final configuration of the monitoring systems is designed to control the interaction of the tunneling works with a number of 2 SYSTEM CONFIGURATION monuments between San Giovanni in Laterano and Piazza Venezia (Fig. 1). The present configuration includes base station sites The monitoring system is presently operating in a equipped with three geomatic sensors characterized beta version in three test sites: the Basilica of by high precision and by the capability of automati- Maxentius and Constantine in the Roman Forum, cally collecting high frequency data (Dominici et al. Aurelian Walls near Porta Asinaria and the Monu- 2008) (Mat et al. 2010): automatic total stations, ment to Vittorio Emanuele (Vittoriano). GNSS geodetic receivers, bi-axial inclinometers In this work we focus on the procedure adopted to (Tab. 1). Each station sites is also fitted with an unit assess the stability condition of the base station sites for measuring main meteo parameters (tempera- and on the preliminary results obtained from the ture/pressure) useful to extrapolate a corrective analysis of the data collected in the test sites of the model for detrending the displacement time series. Basilica of Maxentius and Costantine in the Roman The total station is a classical topographic instru- Forum. ment used for the measurement of slope distance,

1110 horizontal direction and vertical angles to compute sensors and are used to check the stability of the sta- the coordinates of any unknown point, in our case tion site. They are aimed at complementing the mon- monitoring points, relatively to the total station posi- itoring of short-term (daily) and long-term (seasonal tion (coordinates in the instrument reference sys- and more) stability of the station sites. In addition, tem). In order to compute the coordinates of sur- the site stability check is routinely carried out by the veyed points in an external reference system, it is automatic total stations themselves through a num- necessary to link the measurements to a reference ber of external reference points. Finally, the meteor- point or, better, to a network of points outside the ological sensors are devoted to continuously meas- monitored area. This is carried out by fixing the co- ure pressure and temperature providing daily and ordinates of the total station and estimating an angle seasonal trends that can be compared and jointly an- (the orientation angle) adjusting redundant observa- alyzed with the displacement time series in order to tions with the least square method. In our case the highlight the presence of correlated signals that orientation of the total station, is carried out using a should be not confused with the real structural set of point with known coordinates. movements.

Table 1: technical features for the total station and the bi-axial 2.1 The Basilica of Maxentius Test site ______inclinometer ______Total Station The Basilica of Maxentius constitutes a monumental ______Observable Accuracy complex among the more imposing in the archeolog- Horizontal direction and vertical angles 0,5” ical zone of Rome. The construction, begun by ______Distance 1mm+1ppm Maxentius in 303 A. D., was interrupted by his death in 312, during the Battle "ad Saxa Rubra". In 313, ______Bi-axial inclinometer Measuring range Accuracy Constantine completed the Basilica of which only ______one entire side and the main apse of the shorter side From To ______presently remains. Today, with part of the bold [mrad] [mrad] [mrad] vaults rising more than 35 metres from the ground. -1,5100 1,5100 +/-0,0047 -2,5100 2,5100 +/-0,0141 The monitoring system installed in the site of the Basilica of Maxentius is designed in order to ob- -3,0000______3,0000 +/-0,0471 serve the remains of the monument by means of Automatic total station measures the monitoring three “monitoring towers/stations: ”Moncone”, posi- points every four hours. The GNSS receivers pro- tioned inside the Basilica and monitoring the south- vide 3D coordinates in a global reference frame west side, “Villino Rivaldi”, positioned outside the (Fastellini et al. 2011), while inclinometer measures Basilica and monitoring the north-east side, and two inclination angles along two perpendicular di- “Corrado Ricci”, positioned in Via dei Fori Imperiali rections (X and Y direction) respect to the vertical and monitoring the north-west part of the building position. Every measurement is acquired in the sen- (Fig. 2). sor own reference frame and subsequently trans- formed in a common reference frame named MetroC reference system. Presently, a daily solution is calculated for the GNSS position, while inclinometer pro- vides its measurements every 20 minutes. Since at the present stage of construction, the tunneling has not yet reached the monitored monuments, the main objective of the monitoring system is to observe the behavior of the building in unper- turbed conditions. Thus, the data analysis can permit to evaluate the standard daily/seasonal displacement patterns which the structures undergo. This aim is obviously of crucial importance since it will allow to detect and quantify the movements which are not in- duced by the tunneling itself. Such movements can be filtered out enabling to highlight only critical Figure 2. Monitoring stations controlling the Basilica of structural deformations which can be used for early Maxentius warnings when given thresholds will be exceeded. The geomatic sensors are installed in suitable Figure 3 shows a tower instrumented with an au- sites and utilized for different aims: the main sensors tomatic total station, a GNSS geodetic receivers and are the automatic total stations devoted to the direct a bi-axial inclinometers. control of the structures, while the bi-axial incli- The whole monitoring system is totally automat- nometers and GNSS receivers behave as auxiliary ed following the user setting for acquisition rate,

1111 number of points to measure, measuring method, has been configured to acquire the coordinates of the etc.. Warning messages can be also set by the user if complete set of prisms every four hours. measurements exceed a given threshold, highlight- For each monitoring station, a set of reference points ing risks for the stability of the Basilica. (hereafter PR) of known coordinates, positioned on surrounding buildings which are not overlying the excavation area are provided in order to calculate the orientation of the total station.

Figure 5 Position of the reference points

2.2 Analysis of the stability of the Monitoring Figure 3. Geomatic sensor installed on the monitoring towers towers The GNSS receivers and the bi-axial inclinometers The total stations are devoted to measure the co- are adopted to collect data useful to monitor the sta- ordinates of over 300 high precision micro-prisms bility of the towers. In case of manifest variations positioned on the Basilica of Maxentius (Fig 4). due to local instability of the station sites and not linked to the tunneling works, the collected data permit to estimate corrections for adjusting the measurements acquired on the MP by the automatic total stations. Due to the different accuracy of the two sensors, inclinometer can be used for detection short-term motions, while GNSS receiver monitors the position of the monitoring tower in the long period. Furthermore, if the GNSS solutions (baselines) are referred to an external and stable GNSS station, it is possible to obtain displacements framed into a reference system which is not influenced by local in- stabilities. Nowadays, the baselines are calculated relative to Moncone station which up to now has proved to be the most stable site. Future improve- ments of the system are scheduled in order to in- clude the GNSS receiver into a network controlled

Figure 4. Micro-prisms positioned on the Basilica of Maxentius by the Sapienza M0SE permanent station. (clockwise from the top: prisms measured by Moncone, Cor- As an additional method for controlling the stabil- rado Ricci and Villino Rivaldi stations) ity of the station, MPs measured by two or three stations have been selected on the Basilica. They can Prisms are distributed over the whole structure to be used in order to compare measurements provided allow the observation of the global behavior of the by different stations and acquire further information building and also along directions which are more on the stability of the monitoring towers. likely to evidence critical displacements. In this first stage of the monitoring activity, with the aim of val- idation the system and understanding the behavior of the Basilica in undisturbed conditions, the system

1112 3 DATA ANALYSIS

The results presented in the following are obtained from the analysis of the data collected by the “Cor- rado Ricci” monitoring station. The dataset is the most relevant among the three monitoring stations because of its completeness. However, the station was initially affected by insta- bilities due to the foundation setting. Therefore the stability analysis carried out on this station was useful to implement the workflow that will be followed to adjust the position of all the other geomatic sensors, when the tunneling will approach the Basilica. The analysis of the data was carried out following these steps:  least square adjustment of the Total station observations to estimate the orientation angle  detection of the outliers for each time series provided by the different sensors  stability analysis of the monitoring station  detrending of the monitored points

3.1 Total station adjustment

The orientation angle of the total station was esti- Figure 6. Time series of the N4M03MP01/02E points calculat- mated with the least squares method starting from ed using PR to estimate the orientation angle the coordinates of the reference points; the position of the total station was considered fixed. If all the reference points, assumed to be stable because positioned outside from the area affected by excavation works, show similar trends, instability of the monitoring station can be supposed. The average displacements (computed on each planimetric com- ponent) of the reference points is assumed to be the displacement of the monitoring station. Furthermore it is possible to compare the displacement vector with the data provided by the GNSS sensors and bi- axial sensors.

In Figure 6, we show the results obtained using Figure 7. Time series of the N4M03MP01 points calculated us- the reference points to estimate the orientation angle ing MP to estimate the orientation angle of the total station which evidence the presence of a similar trend in two different time series (MP- N4M03MP01E and MP- N4M03MP02E) and a quite noisy measurement patterns on the horizontal components. Deeper investigation highlighted that the orientation angle, calculated using the point installed on the structure (Fig. 7) provided better results. Considering that the distance from the reference points to the total station range from 20 to 60 m, the improvement can be attributed to the decreasing of the atmospheric effects with the distance between total station and measured point. Therefore, the results presented in the following paragraphs are computed using the point of the structure as PR.

In Figure 8 are indicated points used as PR and Figure 8. Point belonging to the Basilica and used as PR to cal- the MP analyzed culate the orientation angle of the total station are marked with the yellow dot. Analyzed MP are marked with the sky-blue tri- angle

1113 3.2 Detection of outlier The outlier detection analysis for the data acquired by the total station, carried out on the time series coordinates, has been divided in two steps: in the first step a threshold value has been fixed (5 cm) in order to remove blunders. In the second step time series have been split into six sub-series, grouping measurements by time acquisition, with the aim of com- paring data acquired in the same atmospheric condition (i.e. temperature, pressure etc). Finally, a moving average has been applied on the sub-series coordinates E,N,U. Additionally, due to the different acquisition rate of the GNSS sensor (1 Hz with a daily solution) and of the bi-axial inclinometer (20 min), outlier analysis was carried out applying a classical moving average method on the complete time series. Table 2 shows the results of the outlier detection procedure performed on six monitoring points and six reference points. The time series were de-trended using the orientation approach based on MP positioned on the Basilica. The PR are characterized by an higher number of outliers, probably due to the longer distance from the base station in comparison with the MP. Figure 9. The time series for the point N4M03MP01E before (up) and after (down) the outlier detection ______Table 2 Statistics of the outlier detection procedure ______TYPE ID N° OBS. Outlier Distance 3.3 Stability analysis of the monitoring station ______% [m] In the following graphs (Fig. 10, 11 and 12) the PR N4M03MP01E 2395 25 75,60 trends observed on the Corrado Ricci monitoring PR N4M03MP02E 2407 10 69,80 PR N4M03MP03E 2395 10 73,08 station using GNSS receiver, inclinometer and PR PR N4M03MP04F 2394 8 54,12 are shown. The three different sensors evidence sim- PR N4M03MP05E 2411 9 67,40 ilar trends that are in accordance with the trend ob- ______PR N4M03MP08E 2395 8 61,70 served on the monitoring points. This demonstrate ______Avg 2400 12 the motion of the monitoring station. MP N4M03MP23D 2776 12 71,10 MP N4M03MP22B 2792 8 58,00 MP N4M03MP15C 2788 9 71,20 MP N4M03MP11C 2791 8 56,30 MP N4M03MP07F 2769 11 70,60 ______MP N4M03MP06A 2793 8 60,90 ______Avg 2785 9

In Figure 9, an example of the results obtained after the outlier detection on N4M03MP01E time series is shown. It is worth to mention that this pre- processing analysis is very relevant for improving the capability of the system not only to detect but also to validate in short time real displacements in the active phase of the works. Figure 10. Graph shows the displacement of the Ricci monitoring station observed on the PR.

1114 Table 3 Correlation coefficient for the time series INB-Total ______Station and GNSS- Total Station ______Correlation coefficient INB Total Station 0.98 ______GNSS Total Station 0.99

3.4 Analysis of the observed MP Figure 14 shows the MP –N4M03MP22B time series before and after the removal of the trend observed on the monitoring station. The corrected series do not show any more the significant Figure 11. Ricci GNSS receiver time series. displacement that was clearly due to the station motion. On the contrary a trend of smaller amplitude evidence the presence of seasonal signature.

Figure 12. Ricci bi-axial inclinometer time series.

Figure 13. Planimetric displacement for the total station (m), Figure 14. time series for the point N4M03MP22B with (up) the bi-axial inclinometere (gon) and the GNSS (m). and without (down) the trend of the monitoring station

It should be mentioned that the bi-axial inclinom- In this preliminary stage, in absence of tunneling eter furnishes as output data angular values; so the works, the analysis has been focused on the behavior computation of a lever (distance between the rota- of a few selected MPs. The building has been divid- tion point and the centre of the inclinometer) is ed into three zones “on the strength of” the height needed in order to convert angle measurements in from the ground plane: zone A on the top, zone B in length measurements. This procedure will be im- the middle and zone C near the base of the edifice plemented in the analysis tool in order to automati- (Fig. 15). cally transform the observations into horizontal and vertical displacements. Figure 13 show a comparison between the horizontal displacements obtained from the three sensors while Table 3 contains the correlation coefficient between the inclinometer, the GNSS and the total station time series, both highlighting the good agreement between all the observations.

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Figure 15. Zones selected for the analyses.

The following graphs (Fig. 16-18) evidence that the same trend is visible for all sectors, apparently showing a magnitude decreasing with the inclination angle and distance from the ground. This can be ex- plained both as an influence of the atmospheric effects on the measurements but also, in consideration of the peculiarity of the Basilica, as structural “weakness” on the upper portion of the structures.

Figure 17. Time series for zone B.

Figure 16. Time series for zone A.

1116 The analysis was primarily aimed at accuracy as- sessment of each sensor and in their capability of de- tecting short period stability of the monitoring station. The analysis carried out on the data provided by different sensors installed on the monitoring stations highlighted the same trend which is clearly the consequence of an instability probably due to the characteristics of the foundation ground. The total station trend was obtained using points belonging to the structure as reference points. The same trend was confirmed by the inclinometer and by the GNSS receiver. During the tunneling phase, due to the difficulty to separate the displacement of monitored points from the displacement of the stations, GNSS receivers and bi-axial inclinometer will provide a more useful tool to correct/adjust the observed data. An automatic procedure was implemented in order to remove outliers from the time series coordinates. Results obtained from monitoring points time- series coordinates analyses indicates that no movements are underway. Observed displacements are di- rectly linked to the seasonal variations of atmospheric parameters. As previously underlined, the experience con- ducted in the ante operam phase was useful to set up the workflow that will be followed to adjust the position of the geomatic sensors to correct the observed data, when the tunneling will approach the Basilica.

REFERENCES

Dominici D., Fastellini G., Radicioni F., Stoppini A. 2008 An integrated monitoring system for the monumental walls of Amelia, 13th FIG International Symposium on Deformation Figure 18. Time series for zone C. Measurements and Analysis – 4th IAG symposium on Geod- esy for Geotechnical and Structural Engineering, Lisbon 4 CONCLUSIONS Fatellini G., Radicioni F., Stoppini A. 2011 The Assisi land- slide monitoring: a multi-year activity based on geomatic techniques, Applied Geomatics DOI 10.1007/s12518-010- The integrated geomatic monitoring system designed 0042-9 in order to highlight the interaction of the tunneling Mat R.I., Jasmee J., Zahrullaili Y., Abd. Manan S. 2010, A for the new Metro C line with the overlying struc- Feasibility Study of Building Structural Deformation Moni- tures, is an example of monitoring system which toring Using Global Positioning System (GPS), Terrestrial takes advantage of many redundant data provided Surveying Technique (TST) and Crack Gauge Measure- ment (CGM). 6th International Colloquium on Signal Pro- by different sensors. cessing & Its Applications (CSPA) In this work we present and discuss preliminary results obtained from the analysis of the monitoring towers devoted to monitor the Basilica of Maxentius in the Roman Forum. The final configuration will allow to control the monument by means of three monitoring towers, each one equipped with a total station, a GNSS geodetic class receivers and a bi- axial inclinometers. We started to develop a joint analysis of all the available data, focusing on the monitoring station called Corrado Ricci. The exper- imented analysis methodology will be extended to future massive data processing.

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