ASSESSMENT AND MONITORING OF A SFRC RETAINING STRUCTURE: MEASUREMENT ISSUES (PART 2)

Transcription

1 ASSESSMENT AND MONITORING OF A SFRC RETAINING STRUCTURE: MEASUREMENT ISSUES (PART ) Marco Scaioni (1), Mario Alba (1), Alberto Giussani (1) and Fabio Roncoroni (1) (1) Politecnico di Milano, Polo Regionale di Lecco, via M. d Oggiono 18/a, 3900 Lecco, Italy Abstract This paper deals with a solution for the problem of monitoring 3-D displacements of Control Points on a SFRC retaining structure designed for stabilizing unstable slopes (see Part 1). A robotic total station was used to perform measurements during both placement stage and first months of standard life. To compare all observations taken at different epochs, the instrument was georeferenced into a Local Reference System which is permanently materialized through monuments. Even though displacements interesting the shelters were always under the statistical significance level of measurements, some trends were easily outlined. On the other hand, empirical tests showed that the achievable accuracy of this monitoring technique is better than how much is theoretically expected. Indeed, the repositioning of the total station on the same relative position with respect to the observed panels, it allows to reduce many systematic effects intrinsic to observations. A twofold achievement was reached. Firstly, measurements made possible to compare foreseen and real static behaviour of the retaining structures. Secondly, a monitoring system to be applied in such cases was setup and tested. Its capabilities and limitations were then outlined. Moreover, the total station based monitoring system is integrated by other sensors able to measure local displacements inside and on the surface of panels (FOS and Vibrating Wire Sensors). Keywords Monitoring, engineering geodesy, robotic total station, retaining structures, risk mitigation 1. INTRODUCTION In the Part 1 [1] of the paper an innovative retaining structure for the stabilization of unstable slopes has been presented. In particular, a real application to the reinforcement of a ground slope face in Caslino d Erba (Como, Italy) has been described. The part will focus

2 on the monitoring systems adopted to assess the placement phase and the first 4 months of standard life of the installed shelters. Indeed, results obtained from monitoring are really important to investigate the structural behaviour of these panels, considering that this is the first application of them. Despite of the relatively small size of the interested slope (see Fig. 1a), the local geological conditions are quite complex (unhomogeneous material, uneven surface, internal water circulation), resulting in a difficult forecast of the effective geotechnical response of the panels to the external solicitations. The role of monitoring system is then relevant in both placement phase and during the standard life. Measurements are planned to be taken for a period of 5 years from installation. In addition, current technical specifications (e.g. EuroCode7) do not give specific information on monitoring techniques to be adopted in such applications. A second important aim of this research is then to test different instruments and methods, in order to assess the achievable accuracy and the feasilibility of their application for control in a long period (durability, stability of the reference system, maintenance). An expected finding from this research is to give a contribute in the definition of a protocol for monitoring shelters applied to unstable slopes. Different monitoring sensors have been installed from the placement phase so far. To measure the 3-D displacements of a set of control points positioned on the 4 panel surface, a robotic total station is applied. This instrument is repositioned at each observation epoch and georeferenced in a geodetic network in order to enable comparisons between measurement taken in a long period. In Section instruments, characteristics of the application and some methodological aspects to be used in similar cases are introduced. In addition, results achieved in both placement stage and in the first 4 months after installation are described and analysed in Sections 3 and 4, respectively. Eventually, one of the shelters was equipped by a 6 Fibre Optic Sensors (FOS see []) Figure 1: On the left (a), an outlook of the slope in Caslino d Erba just after the end of panel installation. In the middle (b), two configurations of control points (CPs) for monitoring displacements by total station; in the upper part, the solution adopted for panel 100, in the lower that for 00, 400, 500; red rings on the panel 500 highlight CPs used for the monitoring during placement stage. On the right (c), project drawings (front and retro views) and an image of a CP to be fastened into the panel structure

3 The concept was to test this kind of sensor, which were inserted in the inner of the concrete structure in order to measure local linear displacements. Furthermore, 3 panels were also equipped by vibrating wire sensors, which were placed on the external downstream face. These sensors allow to measure local linear deformation, but in different position with respect to FOS. Eventually, load cells were installed on 3 out of the 4 ground anchor cables of each shelter. All sets of sensors are connected to their respective data acquisition units, which can be remotely controlled via GPRS connection. Considering the assessment of panel behaviour is still on-going and will concern the future 5 year period, results achieved with these sensor will be presented in next publications.. TOTAL STATION BASED MONITORING SYSTEM The placement stage of 4 SFRC framed precast panels was monitored by using total station measurements and a set of retro-reflecting targets. These will be then used for the monitoring during the standard life of shelters. Different Control Point (CP) configurations were adopted, as can be seen in Figure 1b. Panel 100 was equipped with 9 CPs, which were previously evaluated as being the minimal configuration to assess the static condition of such a structure, considering its deformation and not only rigid displacements. On the other hand, due to the experimental character of this application, other panels were equipped with a larger number of CPs (#13). Furthermore, some CPs were directly fixed into the slope to check relative displacements of the panel with respect to this. Specific targets were designed for this application. These are made up of two elements, one to be fastened in the concrete structure, and the other to be fixed with screws to the previous one (Fig. 1c). The removable element reports a retro-reflecting target for electronic range-finder measurement..1 Monitoring equipment The instrument adopted for all measurement stages is a total station Leica TCA003 (see [3] and Fig. b). It is capable of measuring azimuthal readings (α) and zenithal angles (Ζ) with an nominal standard deviation of σ Να,Ζ = ±0.15 mgon (ISO ). The phase-shift rangefinder enables it to measure distances with a nominal precision which can be evaluated by the formula σ Νd = ±(k 0 + k 1 d), according to ISO The coefficient k 1 might be negletted in this application, because the distance d (in km) from total station to target keeps limited to a few decades of metres. The value of k 0 depends on the used reflector. In case of high-precision prism (such those adopted for the materialization of the reference system see sub-sec.., k 0 = 1 mm; in case of target tapes used as CPs on the panels, k 0 = 3 mm). As described in the following, the TCA003 is placed at every time on a topographic tripod an setup. Among the other facilities, this total station is equipped by motorized axes which allow it to autonomously aim on targets, if a list of approximate coordinates is already available. This option, integrated by the possibility of Automatic Target Recognition (ATR), allows one to speed up the monitoring stage. The total station can be georeferenced into an existing reference system by centering it on a point with known coordinates, and by azimuth orientation towards another known point. In addition, TCA003 is capable to get itself georeferenced by 3-D inverse intersection. In this case, if a set of points (at least ) with known coordinates is available, the total station can measure their position in its intrinsic reference system, and then computing a 3-D roto-translation to get the best fit into the ground

4 reference system. Information about the achieved accuracy of georeferencing is also provived in real-time.. Setup of reference system A Local Reference System (LRS) was established in the area of the slope to be reinforced. Materialization of LRS was made by two 3-D monuments. When the system to measure displacements during both placement and standard life was designed, a solution based on a single total station stand-point was selected. Despite the lack of redundancy on each CP measurement, this solution allowed a fast data acquisition, which was a mandatory requirement during the monitoring of the placement stage. In addition, the presence of several obstacles in front of the slopes (permanent crane, yard vehicles) limited the position from which the whole surface of panels could be seen. Different is the case of monitoring of standard life after the end of construction works, when all existing obstacles would be removed. In this case, total station measurements could be also performed through intersection from two stand-points. A concrete platform at the base of the slope (point 100 in Figure a) was selected as total station stand-point. In order to reduce the setup error of the total station in the whole error budget (see Sub-section.3), the positioning of a stable (or removable) pillar would have been the best solution. Unfortunately, on the platform where point 100 lies no permanent structures were allowed to be built up. Then the adopted solution was to materialize the LRS through a set of 5 Ground Control Points (GCP) widespread around stand-point 100. On each of them, a high-precision retro-reflecting prism can be accurately replaced at every measurement epoch. The total station is approximately setup in correspondence of a nail on the platform, in order to take measurements from the same position. However, precise 3-D coordinates of instrumental centre and azimuth orientation are determined at each epoch in indirect way by means of inverse intersection on GCP. The measurement of GCP coordinates was achieved at epoch 0, before starting the placement of panels. As can be seen in Figure a, a redundant geodetic network was setup, based on two instrumental stand-points (100 and 00). The mean accuracy of GCP measurement resulted ±0.6 mm in x direction, ±0.9 mm in y and ±0.3 mm in z. The lower accuracy along y is motivated by the higher contribute of the distance to the measurement of this coordinate. In Figure a, -D error ellipses (1σ) of GCP coordinates estimated from the Least Squares adjustment of the network are shown [4]..3 Theoretical precision One of the goals of this research is to evalute the best procedure for monitoring retaining structure in different stages of their life. Even though the theoretical precision of the total station Leica TCA003 in standard applications is well know from manufacturer specifications, for specific situations like that is considered here, empirical on-the-field evalutation is needed. The error budget affecting the CP measurement is made up of two main contributes. The first one concerns the georeferencing of the total station into the LRS at each measurement epoch. This accounts for the precision of the instrumental centre coordinates (x 0 =[x 0 y 0 z 0 ] T ), expressed through the corresponding covariance matrix C 0, and the precision of orientation of the y axis in the plane xy (σ α0 ). This plane is horizontal because the main total station axis is accurately setup to be aligned along the local plumb line (z axis).

5 Figure : On the left (a), layout of the geodetic network adopted to establish the Local Reference System (LRS) at every monitoring epoch. Planimetric standard error ellipses (confidence level 39%) are drawn, while bars represent the standard error on z coordinates. On the right (b), picture of the total station Leica TCRA003 during works in Caslino d Erba Estimations of both C 0 and σ α0 can be obtained by Least Squares simulation of the inverse intersection needed to get the total station georeferencing, in the hypothesis of normal errors only. In this problem, all GCPs are considered fixed weighted points, with associated standard deviations derived from the adjustment of geodetic network at epoch 0. This computation resulted in a std.dev. for the total station instrumental centre of σ x0 = ±0.4 mm, σ y0 = ±0.7 mm and σ z0 = ±0.5 mm. These a priori estimated values were confirmed when real measurements were used. In this case, even better estimates were achieved: σ x0 = ±0.3 mm, σ y0 = ±0.5 mm, σ z0 = ±0.3 mm. The second contribute to the error budget is given by the precision of angle and range measurements performed by the total station. These can be expressed through the covariance matrix C α =diag( σ α,σz, σd ). The diagonal form of C α is due to the stochastic independence among observations. While the std.dev. of range can be directly evaluated as described in Sub-sec..1, in case of angles some further contributes have to be considered. For azimuthal readings α, the total uncertainty is given as: σ = σ σ (1) α N α + σ α 0 + σ T + vα where σ Να is the nomimal std.dev. of azimuthal readings, which has been previously introduced in Sub-sec..1; σ Τ is the std.dev. of target pointing, which is ±1/d mrad in case ATR is used, and ±60 /M in case of manual collimation (M is the telescope magnification, which is 30x in case of TCA003 see [5]); σ vα accounts for the contribute (ν α ) of the

6 residual verticality error (ν) on azimuthal readings. According to [6], this error can be expressed as : v α = vsin( α α1)ctan( Z) () being α 1 usually unknown. Then, if the std.dev. of ν is σ ν, the corresponding std.dev. of ν α can be computed through propagation, after substitution of sin(α-α 1 ) = 1: σ = ctan( Z) (3) να σ ν In case of TCA003, the std.dev. of vertical axis compensation is σ v = ±0.1 mgon. In a similar way, the following expression can be written in case of zenith angles: σ = σ + σ + σ (4) Z NZ T v In order to propagates errors on observations to CP coordinates, the relation between these is needed: [ d sin Z sin d sin Z cos d cos Z ] T + x0 x = α α (5) In the hypothesis that systematic errors have been completely removed, the accuracy of a single CP can be estimated as: C x = C 0 + J C J (6) x T α x where J x is the matrix of derivatives. From the second term in Eq. (6), it results that the accuracy of CPs depend also on their relative position with respect to the total station standpoint. Thus, the theoretical accuracy achievable on all the 4 panels is in the order of σ x σ z = ± 1.0 mm and σ y = ±.5 mm. To discriminate between real displacements and measurement uncertainty a statistical test is performed [7]. Considering the displacement of a generic coordinate j={x,y,z} between epochs i and i+1, this is distributed as a standard normal random variable: Δ + j N 0, σ j, i σ j, i+ 1 By establishing a significance level α=5%, displacements can be assumed as real if: Δ j 1.96 σ =. 78σ i i (7) (8) In the case of CP displacements, thresholds for real displacements can be derived from (8) to be as ±.8 mm in x and z coordinates, and ±7 mm in y. In reality, this evaluation disregards the fact that measurements are taken at different epochs with the same geometric layout. Similar systematic errors are then repeated and can be removed by computing differences. In conclusion, the real significance of displacements is inferior to the theoretical one, even though no way to analytically express this is known.

7 3. ANALYSIS OF OBSERVED DISPLACEMENTS DURING PLACEMENT The installation stage of retaining structures was monitored by total station measurements, by using the technique described in the previous Section. Measurements concerned a subset of 6 CPs only, due to the small time available between loading steps. In addition, a CP fixed on the ground was measured to check displacements of the slope with respect to shelter. In Figure 1b the positions of CPs for panel 500 are reported, being this the case reported in this paper as example. The total station Leica TCA003 was placed in correspondence of point 100, setup on a topographic tripod, and georeferenced. Then it wasn t removed as far the placement of a panel was ended. In this case, all measurements taken during the same session are likely to not be affected by systematic georeferencing errors due to instrument repositioning. Figure 3: Displacements of CPs on panel 500 during loading steps of central anchor cables; point 543 is directly fixed on the ground Figure 4: Displacements of CPs on panel 500 during loading steps of lateral anchor cables

8 A preliminary loading of both central anchor cables was carried out in order to put the panel close to the slope. The first reading epoch occurred at this stage. Then four loading steps were contemporarily applied to central cables, up to reach the service ability condition. Secondly, both lateral anchor cables were tensioned through 4 loading steps. Results of measured displacements are shown in Figures 3 and 4 for central and lateral anchor cable loading, respectively. In order to discriminate between the rigid displacement of the panel and its deformation due to loading, a further analysis was carried out. Relative 3-D displacements Δs j of all CPs between consequent epochs were considered. In case of only rigid movement, interpolation of all Δs j with a linear function should well fit data. On the contrary, the presence of higher residuals would mean the presence of a deformation component. To avoid the influence of possible measurement errors, the estimate of the linear function interpolating relative displacements of CPs was performed by using a L1 robust technique. In Figure 5, mean and std.dev. (error bars) of 3-D residuals with respect to the interpolating plane computed at different loading steps are reported. Steps from 1 to 4 refer to the loading of central cables, while steps from 5 to 8 to lateral ones. Results show that relative residuals are always lower than the significance level of measurements. However, the larger deformation occurs after the first loading step (no. 5) of lateral anchor cables. During the following steps, this deformation is partially reabsorbed and residuals are small again. displacements (mm) Analysis of panel 500 relative deformation during loading displacements (mm) Analysis of panel 500 absolute deformation during loading loading steps loading steps Figure 5: Mean of residuals (and std.dev.s) with respect to interpolating plane of CPs relative (on the left) and absolute (on the right) displacements; these have been computed with respect to step 0 4. ANALYSIS OF OBSERVED DISPLACEMENTS AFTER PLACEMENT The displacement monitoring of the 4 panels started just after the end of the placement phase. Up today, 3-4 monthly measurement epochs were acquired. This activity is planned for a period of 5 years. Measurements were worked out by following the procedure proposed in Section. Coordinates of points at the first available epoch were used as reference to compute relative displacements.

9 A first look at observed displacement show very small displacements. These are lower the significance limit of the adopted measurement technique. On the other hand, some systematic effects appeared, which were probably due to the uncertainty of total station georeferencing or to a global sliding of the whole slope. To assess the origin of this problems, the averaged displacements computed on all CPs of each panel were computed. In particular, relative displacements of different panels along the same direction resulted to be highly correlated among them (ρ= ). In addition, standard deviations of displacements results limited under the significance level, meaning that all CPs of the same shelter moved in homogeneous way. A further analysis concerned the comparison of displacements concerning CPs on the panels and CPs fixed on the ground, which always resulted not to significantly differ from each to other. This findings confirmed the assumption about a systematic effect on all measurements. In order to mitigate this problem, measurements on CP on the ground are used to correct other CP coordinates. For instance, in case of displacements along x between epochs i and i+1, the gravity center of CPs on the ground is computed (x Gi ). Thus corrected relative displacement of CP j is evaluated as follows: ( x x ) ( x x ) c Δ x j = j, i 1 G, i + 1 j, i G, i + (9) In Figure 6 the total displacements averaged on all CPs of panels 400 and 500 are reported. Graphics display values of total displacements of different coordinates before and after the data correction by means of relation (9). As you can see, systematic trends were removed, and moreover the relative variations between consequent epochs are very low, especially for panel 500. This result means that no significant changes involve the position of these structures in the observation period. Confirmation of this comes from records of load cells. Indeed, no load increase was observed during the first 4 months of standard life (see [1]) Findings concerning other panels (100 and 00) are here omitted, but they behaved in a similar way..0 Averaged total displacements - panel Averaged total displacements - panel displacements (mm) epoch 4 x y z x corr y corr z corr displacements (mm) epoch 4 Figure 6: Total displacements of CPs positioned on panels 400 and 500. Continuous and hidden lines refer to measurements taken before and after correction (8), respectively

10 5. CONCLUSIONS In this paper the monitoring activities carried out during the placement stage and the first 4 months of standard life of 4 SFRC retaining structures in Caslino d Erba (Como, Italy) are described. This application features a strong experimental character, being a part of a larger project aimed to investigate the soil-structure interaction and to improve the current techniques to stabilize ground slopes. In addition, a second aim of this research is to setup a reliable and precise monitoring system to be applied to check out the behaviour of such structures in different moments of their life. The application in Caslino d Erba is made up of three sets of sensors for the measurement of local linear displacements: fiber optic sensors, vibrating wire sensors and load cells. Global 3-D displacements of all panels have been monitored by using a total station based system. This is capable to take measurements concerning a set of control points placed on the panels downstream face. Even though the evaluated displacements are lower than the theoretical statistical significance, some trends have been clearly outlined. This achievement is motivated by the repetition of the same observation scheme along different measurement epochs. By this approach, some systematic errors which cannot be a priori modelled are however removed. In addition, georeferencing errors are compensated by using measurements taken on stable points on the ground slope. Thus the system has shown to be suitable to detect relative displacements in the range 1 mm. However, smaller changes are not significant for such a structure. The integration to other sensor has not been analysed in detail so far. First comparisons have outlined a good agreement, but further results will be provided in future works. Moreover, the development of an expert system to make an integrate evaluation of all acquired data is foreseen. ACKNOWLEDGEMENTS Authors would like to acknowledge D. Brambilla and L. Casiraghi for the work carried out on this project during their Degree Thesis at Politecnico di Milano (008). This project has been partially funded under the national research program COFIN 004 (Prot ) leaded by Prof. C. Monti (Politecnico di Milano). REFERENCES [1] di Prisco, M., di Prisco, C., Dozio, D., Galli, A. and Lapolla, S., Assessment and monitoring of a SFRC retaining structure: structural issues (Part 1), Proc. of S.A.CO.MA.TI.S., Varenna (Italy), 1-3 Sept 008. [] Ansari, F., 'Sensing Issues in Civil Structural Health Monitoring', (Springer Verlag, 005). [3] Leica Geosystems, ' lgs_553.htm', last accessed on 19 th May 008. [4] Wolf, P.R. and Ghilani, C.D., 'Adjustment Computations: Statistics and Least Squares in Surveying and GIS', (Wiley-Interscience, 1997). [5] Kuang, S., 'Geodetic Network Analysis and Optimal Design: Concepts and Applications', (Ann Arbor Press, Michigan, USA, 1996). [6] Anderson, J.M. and Mikhail, E.M., 'Surveying: Theory and Practice', (McGraw-Hill Science/Engineering/Math, 1997). [7] Sansò, F., 'Il trattamento statistico dei dati', (CLUP, Milano, Italy, 1989). [8] di Prisco, M., Dozio, D., Galli, A., Lapolla, S. and Alba, M., 'HPFRC plates for ground anchors', Tailor Made Concrete Structures', Proceedings of the Int. FIB Symposium, Amsterdam (The Nederlands), 19-1 May, 008 (CRC Press, Taylor & Francis Group, London)