Analysis of a subsidence phenomenon via DInSAR data and geotechnical criteria

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/265027439

Analysis of a subsidence phenomenon via DInSAR data and geotechnical criteria ARTICLE

CITATIONS

READS

8

14

5 AUTHORS, INCLUDING: Settimio Ferlisi

Dario Peduto

Università degli Studi di Salerno

Università degli Studi di Salerno

30 PUBLICATIONS 175 CITATIONS

25 PUBLICATIONS 219 CITATIONS

SEE PROFILE

SEE PROFILE

Gianfranco Fornaro

Michele Manunta

Italian National Research Council

Italian National Research Council

213 PUBLICATIONS 4,008 CITATIONS

89 PUBLICATIONS 902 CITATIONS

SEE PROFILE

SEE PROFILE

Available from: Michele Manunta Retrieved on: 04 February 2016

Analysis of a subsidence phenomenon via DInSAR data and geotechnical criteria Leonardo Cascini,* Settimio Ferlisi,** Dario Peduto,*** Gianfranco Fornaro,**** Michele Manunta*****

Summary

The paper discusses, using geotechnical criteria, the potentialities offered by the Small BAseline Subset (SBAS) Differential Synthetic Aperture Radar Interferometry (DInSAR) technique to investigate a subsidence phenomenon affecting the urban area of Sarno town (Campania Region, Italy). To this aim, low- and full-resolution data, acquired by ERS-1/2 and Envisat satellites in the period from 1992 to 2004, have been used. In particular, low-resolution DInSAR data, once validated via a comparison with ground levelling data, are analyzed in order to detect the most subsidence-affected areas where damages to buildings were recorded during past surveys. Interesting correlation between the ground deformation gradient vectors and some damage characteristics, such as the direction of building rotation axes, are shown. Then, a preliminary analysis at “building scale” by means of full-resolution DInSAR data is presented in order to study the response of some buildings to ground movements, also deriving the maximum values attained by some relevant parameters to be used in damage criteria. Finally, the usefulness of low- and full-resolution analyses for monitoring of urbanized areas is highlighted by stressing the role that the geotechnical approach can play in DInSAR data validation process as well as in pointing out all the issues to improve the reliability of the obtained results in at “building scale” studies.

1. Introduction Subsidence due to groundwater withdrawals, either for industrial or domestic use, is known to be a major problem in many mega cities around the world, with varieties in spatial extent and severity; in this regard, some of the best known examples are those of Bangkok, Las Vegas, Mexico City, Paris, Shanghai and Tokyo. In particular, B ELL [1994] showed that ground surface settlements reached rates as high as 1 mm/day in some areas of Mexico City; in the case of the centre of Shanghai a total amount of settlements equal to 2 ÷ 3 m was measured from 1921 up to now [CHAI et al., 2004]. As far as the Italian territory is concerned, the subsidence occurring in Emilia-Romagna Region can be mentioned; in particular, the highest settlements were recorded in the territory of Bologna – with a maximum rate of 6 cm/year in the period 1983-1992 – and in Modena area, where an average settlement *

Full Professor, Department of Civil Engineering, University of Salerno, Italy ** Researcher, Department of Civil Engineering, University of Salerno, Italy *** Ph.D. Student, Department of Civil Engineering, University of Salerno, Italy **** Senior Researcher, Institute for Electromagnetic Sensing of the Environment (IREA), CNR, Naples, Italy ***** Ph.D. Student, Department of Electrical and Electronic Engineering, University of Cagliari, Italy- Institute for Electromagnetic Sensing of the Environment (IREA), CNR, Naples, Italy

rate of 1.5 cm/year was attained in the period 19851999 [BENEDETTI et al., 2000]. The widespread distribution of this kind of phenomena and their related effects, in terms of damages to structures and infrastructures, call for a thorough detection of the role played by the factors influencing the magnitude of ground surface settlements in order to properly plan remedial actions. To this purpose, several aspects need to be investigated such as subsoil stratigraphy, physical-mechanical soil properties, groundwater regimen, etc. These investigations, to be useful, need a huge number of settlement measurements in time, that can turn out to be an expensive and time-consuming task if carried out, over large areas, via conventional monitoring techniques (i.e. optical levelling or Global Positioning System (GPS)). In this regard, remote sensing techniques, such as space-born Synthetic Aperture Radar (SAR) operating in the microwave frequency, must be considered an alternative and complementary method for its large area coverage (around 80 x 80 km) and the availability of image archives covering more than a decade. The idea to map ground surface displacements involving large areas via analyses of DInSAR data was firstly proposed by GABRIEL et al. [1989], basing on single interferograms (i.e. using an image pair). The first applications were proposed by GOLDSTEIN et al. [1993] – with measures of ice-stream velocity in Antarctica – and by MASSONNET et al. [1993] – with the co-seismic deformation field generated by the Lander earthquake. Since then, many experiments

RIVISTA ITALIANA DI GEOTECNICA 4/2007

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA showed the potentiality of the technique in detecting deformation phenomena [PELTZER and ROSEN, 1995; RIGNOT, 1998; VADON and SIGMUNDSSON, 1997; LANARI et al., 1998; STRAMONDO et al., 1999; LUNDGREN et al., 2001]. More recently, a significant progress has been introduced via different DInSAR techniques based on stacks of SAR images [FERRETTI et al., 2000; STROZZI et al., 2001; BERARDINO et al., 2002; MORA et al., 2003], following two different types of strategy: the coherence-based methods [BERARDINO et al., 2002; MORA et al., 2003] and the methods based on the so-called permanent scatterers [FERRETTI et al., 2000]. Both types of methods involve large image stacks, which represent a key factor to achieve a fully quantitative DInSAR monitoring [CROSETTO et al., 2005]. The use of these techniques is encouraged by some validations of the DInSAR data in urban areas affected by subsidence/uplift phenomena caused by either mining [CROSETTO et al., 2005] or water extraction [CASCINI et al., 2006; WORAWATTANAMATEEKUL et al., 2003], frequently related to underground construction works [JURINA et al., 2003; FRUNEAU et al., 2003; LE MOUÉLIC et al., 2002]. However, a thorough examination of the obtained results and further validations are absolutely necessary in order to allow the confident use of the technique in engineering practice. To this aim, the paper focuses the attention on the capability of the multipass Small Baseline Subset (SBAS) DInSAR algorithm proposed by BERARDINO et al. [2002] as a powerful tool for a comprehensive study of a subsidence phenomenon at different scales and using geotechnical criteria. Particularly, the ground surface settlements of the Sarno town (Italy), ascribed to groundwater withdrawals, are evaluated at low- and full-resolution, by using data acquired by ERS-1/2 and Envisat satellites in the period from June 1992 to October 2004. Then, the obtained results are interpreted in the framework of two welladopted approaches [S KEMPTON and M C D ONALD ,

1956; BURLAND, 1995] usually aimed to prevent the buildings from settlement-induced damages.

2. The case study Sarno is located in the coastal graben of the Campanian Plain, extending for almost 170 km2, bordered to the south by the carbonatic ridge of Lattari Mountains, to the east by the Sarno Mountains and to the north by the volcanic complex of the Somma-Vesuvius (Fig. 1). The rich water resource of the Campanian Plain and, particularly, of the Sarno area, had been intensely exploited during the centuries. Since the 1940s numerous deep wells were built for agricultural and industrial use; then, by the end of the Second World War, some more huge water-catchments were built, essentially in the Lufrano zone (Fig. 1). Numerous water level measures, carried out inside the working wells [CELICO, 1983; CELICO and DE PAOLA, 1992], allowed to follow the changes of the Campanian Plain groundwater regimen (Fig. 2), caused by the intensive water exploitation. It is worth underlining that this trend altered the rela-

Fig. 1 – Study area. Fig. 1 – Area di studio.

Fig. 2 – Groundwater regimen variation in the Campanian Plain over the period 1978-1989 [CELICO and DE PAOLA, 1992]. Fig. 2 – Modifiche del regime idrico sotterraneo nella Piana Campana tra gli anni 1978 e 1989 [CELICO and DE PAOLA, 1992].

OTTOBRE - DICEMBRE 2007

51

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

52 tionship among groundwater flows and watercourses, causing dangerous cases of groundwater pollution [CELICO and DE PAOLA, 1992]. A similar huge water exploitation has been carried out in Sarno area since 1960 by means of both private and public works (Fig. 3). The private withdrawal, ranging up to 30106 m3/year, has basically a seasonal use. As far as public works are concerned, the first drainage gallery was realized in 1962 near S. Maria la Foce source. It withdrew water volumes larger than 3000 l/sec, all through the 1960s and the 1970s; since the beginning of the 1990s, it did not intercept the stratum anymore and the spring discharges were nearly dried up [CASCINI E. and CASCINI L., 1993a]. Another drainage gallery was built in the first half of the 1970s in proximity of the S. Marina di Lavorate source; the available data underline a progressive reduction of the intercepted discharge from 1500 l/sec (registered in December 1988) down to 150 l/sec in the first 1990s. As a consequence of the progressive decrease of both the gallery withdrawing efficiency and the piezometric levels, in 1987 a first forced withdrawal was built near Cerola spring, then closed in August 1989 for pollution problems involving the groundwater and later reactivated just for seasonal withdrawals [CASCINI and DI MAIO, 1994]. Finally, as a consequence of Mercato-Palazzo deep wells, working since June 1989, the capacity of the spring with the same name decreased from 1800 l/s down to 1000 l/s during the following three years. The correlation between the withdrawals and the impoverishment of the groundwater resources all over the Sarno area is supported by two models, respectively based on statistical and mathematical formulations, capable of relating rainfall data to water discharges [CASCINI E. and CASCINI L., 1993b].

The ongoing captation works, either within or outside Sarno area, resulted in a marked lowering of the groundwater table under the whole urban area; consequently, subsidence phenomena occurred, causing serious damages to numerous buildings. In order to deepen the available knowledge on these phenomena, in 1992 detailed investigations were undertaken. The results achieved by field investigations [CASCINI and DI MAIO, 1994] and the available scientific literature [NICOTERA, 1969] allowed to draw the subsoil stratigraphy within the town centre. Particularly, the subsoil is constituted by a fractured calcareous bedrock covered by detrital and pyroclastic deposits where lenses or layers of very deformable peat, characterized by thickness ranging up to a maximum of 6 metres, are present with an irregular arrangement [C ASCINI and D I MAIO, 1994]. A careful analysis of field investigation results showed that, although the withdrawal effects involved a very large area, the spatial distribution of settlements is not homogeneous, owing to the presence of peat in the subsoil; as a matter of fact, the thicker the peat layers the larger the settlements. In addition, the collected measures pointed out that the settlements were essentially of two kinds: instantaneous, in correspondence of a rapid drawdown of the water-table, and viscous when the pore water pressures assumed constant values. In the following, after a brief description of the SBAS algorithm used for the image processing, the subsidence phenomenon is analized by using the low- and full-resolution DInSAR technique. This is firstly applied and then validated on the basis of topographic measurements and well known criteria in the field of geotechnical engineering.

Fig. 3 – Localization of the main captation works in Sarno town [modified from CASCINI and DI MAIO, 1994]. Fig. 3 – Localizzazione delle principali opere di captazione nel comune di Sarno (SA) [modificata da CASCINI e DI MAIO, 1994].

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

3. Multipass DInSAR basics and the used SBAS algorithm The DInSAR principle is simple: the interference (phase difference) given by the beating (conjugate product) of the response of targets in two SAR images, acquired at different times (whose separation is referred to as temporal baseline) and from different orbits (whose separation is referred to as spatial baseline), is the composition of several terms that can be classified in two main categories: – geometric (deterministic) terms associated to the target-to-radar path difference; – stochastic phase shift (noise) terms. Mathematically, the phase difference Δφ, related to a generic pixel can be expressed as follows: (1) wherein, Δφgeo is the geometric term and Δn the stochastic one. Geometric terms are induced by the target topography in association with the presence of an angular view difference due to the spatial baseline, and a possible target displacement occurring between the passes: (2) wherein λ is the wavelength, Δφtop the topographic phase component and Δs the displacement occurring between the two acquisitions projected in the line of sight. Stochastic contributions are due to: 1) changes of the scattering properties due to temporal modification (temporal decorrelation) and/or to the different interaction mechanisms between the electromagnetic wave and the ground surface associated to the angular view difference (spatial decorrelation); 2) propagation delay variation due to the presence of the atmosphere (Atmospheric Phase Screen, or briefly APS); 3) receiver noise contributions (thermal noise decorrelation); 4) processing artefacts and orbital information inaccuracies. The stochastic terms can be expressed as follows: (3) wherein Δn_low is mainly associated with the APS and to the orbital errors and Δn_high to the decorrelation effects, the thermal noise and processing artefacts. The former is a spatially correlated and temporally uncorrelated contribution and, as consequence, can be reliably estimated only by using a set

OTTOBRE - DICEMBRE 2007

53

of interferograms, rather than a single acquisition pair, as better explained in the following. Accordingly, if stochastic contributions are “low”, or “kept low” via averages or ad-hoc filtering procedures, by knowing the target height from an external Digital Elevation Model (DEM), the topography contribution can be subtracted from the interferogram (differential interferogram generation), thus accessing to the deformation measure. Indeed, after topography removal, the differential phase Δφdiff can be written as follows: (4) wherein Δz is the possible topography error associated with the external DEM, ctop is the phase conversion coefficient and depends on the geometric parameters of the two acquisitions; in particular, it is proportional to the spatial baseline. Thus, if Δz and Δn are low or properly mitigated, the target displacement, or better, its component along the radar line of sight (l.o.s.) can be reliably measured with an accuracy of the order of the radar wavelength (5.6 cm for the C-Band radar systems mounted onboard the ERS and Envisat satellites of the European Space Agency). Processing artefacts can be limited by using accurate algorithms for focusing and, particularly for the alignment (registration) of the two images before the image beating. Orbital information inaccuracies can be controlled because they appear as low order spatial contributions on images covering 80x80 km, such as those associated to a frame of ERS and Envisat-ASAR (ERS-like mode) data. Thermal noise decorrelation is generally negligible, unless target backscattering is somewhat low (weak targets or targets in shadow). On the other hand, spatial decorrelation is relevant only at large spatial baseline (say 150m for ERS and Envisat case) over homogeneous scenes [FORNARO and MONTI GUARNIERI, 2002]. This noise contribution can be tackled, by either limiting the baseline, by restricting the analysis to a network of bright targets (permanent scatterers), or reduced by ad-hoc filtering techniques. Temporal decorrelation is a much more critical issue that strongly impact the final monitoring in terms of spatial coverage. Temporal decorrelation [ZEBKER et al., 1992] has been observed, at C-Band, on time scales as short as a few hours in vegetated areas experiencing windy conditions. On the other hand, a phase correlation over several years in nonvegetated areas was demonstrated by U SAI and KLEES [1999]. Qualitatively arid is much better than forest, dry conditions are better than wet, and long radar wavelengths are better than short ones. With respect to this aspect, application of the DInSAR technique to urban areas is particularly favourable.

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

54 In such a case, due to the presence of stable and bright (manmade) targets, high correlation may be observed even at temporal separation of several years. Atmospheric contribution shows correlation lengths on the order of 0.5-1 km spatial separation. Together with DEM inaccuracy, which introduces errors in the topographic contribution cancellation for the generation of the differential interferogram especially at large baseline, this factor represents a major limitation for the accuracy of standard twopass DInSAR. APS mitigation, the possibility to track the deformation s(tn), and not only to determine its variation Δs between two time instants, and the retrieval of the residual topography Δz in (4) are the fundamental advances offered by recent multipass DInSAR techniques (such as Permanent Scatterers and SBAS), over standard two-pass differential interferometry, that allow accurate monitoring and precise localization. To achieve such goal, multipass techniques exploit the benefits of jointly processing a large number of observations, typically more than 30. The peculiarity of SBAS to work with small baselines offers the advantage of limiting the influence of topography errors and the effects of spatial decorrelation when mapping deformations at large scale. Moreover, SBAS allows naturally the implementation of DInSAR analysis at both low-resolution (medium scale) and full-resolution (large scale), thus providing a flexible tool for investigating wide as well as localized ground deformations. The low resolution processing [BERARDINO et al., 2002] is in any case carried out prior to the small scale analysis to identify and filter out the APS from the full resolution data. The first step of the SBAS algorithm is the unwrapping [COSTANTINI and ROSEN, 1999] on a sparse grid of the wrapped interferograms. Starting from the unwrapped phase interference, the absolute phase at the different passes is evaluated via an inversion of a linear equation system that describes all the interferogram beating. The topographic and the linear deformation components are evaluated from knowledge of the spatial and temporal baselines distribution. Such components are subtracted from the data and, finally, the non linear deformation component and the APS signal are separated via a spatio-temporal filter that accounts for the different spatial and time-frequency characteristics of the two contributions. The results are deformation sequence (deformation time series) for all the pixels (80m x 80m ground size, on average) that retain sufficient stable scattering characteristics over the monitoring interval. APS and low resolution deformations are used to phase calibrate the data stack at the full resolution for the subsequent small scale analysis [LANARI et al., 2004]. In particular, residual topography and

linear deformation components are estimated from the phase calibrated data stack via a maximization of a proper merit figure (the temporal coherence) that is sensitive to the scattering stability. The resulting small scale residual topography allows achieving accurate target localization in the subsequent geocoding process that brings the target from the radar reference system to a chosen output cartographic system (typically UTM WGS-84). Then phase residuals are used, together with the large scale velocity estimate, to track the deformation at full resolution and, thus, to attach deformation time series information to each target in the output target location map. Buildings (thanks to the presence of dihedral or trihedral structures or to tiled/corrugated roofs) or in general manmade structures (especially if metallic) are well mapped and monitored by the full resolution DInSAR analysis.

4.The Small Baseline Subset (S.B.A.S.) algorithm implementation In the case study, low- and full-resolution analyses, almost 80×80 m and 20×5 m (range-×-azimuth) pixel spacing respectively, were performed. Azimuth is approximately parallel to the NorthSouth direction, whereas range is approximately parallel to the East-West direction. The dataset consists of 83 images acquired by the ERS-1/2 and ENVISAT systems, spanning the time interval from June 1992 to October 2004 (descending orbits; track 36, frame 2781). The adopted procedure furnished low-resolution SAR pixels exhibiting a regular spatial distribution (Fig. 4a), while the full-resolution SAR pixels (Fig. 4b) had an improved localization of the measures on elements where the signal attains the highest coherence, mainly buildings in urban areas (Fig. 5). The validation process of the DInSAR data is discussed in the following sections. It started with a point-wise comparison with traditional ground levelling (mean square deviation of 0.3 mm/km) data referring to a net of 18 benchmarks (Figs. 4a and 4b). Particularly, six sets of topographic measurements were available: four of them were the result of a first levelling survey, carried out from July 1992 to October 1993 [CASCINI and DI MAIO, 1994]; two additional surveys have been accomplished in March 2004 and in January 2006. 4.1. Low-resolution processing results The ground settlements, corresponding to the measured l.o.s. displacements for each available

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

Fig. 4 – Ortophoto of the study area with localization of ground levelling benchmarks and coherent SAR pixels at both low (a) and full (b) resolution [CASCINI et al., 2007]. Fig. 4 – Ortofoto dell’area di studio con localizzazione dei capisaldi topografici e dei pixels SAR coerenti sia a bassa (a) che ad alta (b) risoluzione [CASCINI et al., 2007].

Fig. 5 – Low- and high-resolution SAR pixels distribution. Fig. 5 – Distribuzione spaziale dei pixels SAR a bassa ed alta risoluzione.

SAR pixel, have been computed assuming that ground displacements mainly occur along the vertical direction [CASCINI et al., 2006]. Referring to four sample pairs of ground levelling benchmarks and low-resolution SAR pixels (the nearest to each single benchmark), Figure 6 shows a good agreement between settlements values ob-

OTTOBRE - DICEMBRE 2007

tained, for the time period 1992-1993 when the highest settlement rates occurred, via the use of the DInSAR technique and the analysis of ground levelling data, so confirming the abovementioned assumption. Furthermore it also highlights that remotely sensed settlements increase almost linearly in time from the beginning of 1995, reaching cumulative values that again fit the most recent available levelling measurements (March 2004, January 2006). Moreover, the spatial distribution of cumulative ground settlements confirms that the highest values are measured in correspondence of the thickest peat layer, as in the case of the 526 benchmark [CASCINI and DI MAIO, 1994]. With the aim of passing from a point-wise to a spatial analysis, both levelling and DInSAR results were interpolated and two (Figs. 7a, b) dimensional maps of cumulative settlements were generated, referring to the geodetic measurements acquired over the period July 1992-October 1993 and to the DInSAR data processed for the period June 1992-November 1993 [CASCINI et al., 2006]. Obviously, due to the denser spatial distribution of SAR pixels, the map obtained by adopting the DInSAR technique shows higher spatial detail than those derived from the analyses on levelling data, although similar patterns are globally obtained. Moreover, DInSAR subsidence patterns (Fig. 7b) seem to confirm the

55

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

56

Fig. 6 – Settlement point-wise comparisons (from low resolution DInSAR and ground levelling data). Fig. 6 – Confronti puntuali tra i cedimenti misurati con livellazioni topografiche e con la tecnica DInSAR a bassa risoluzione.

Fig. 7 – Settlements a) measured with ground levelling (July 1992 – October 1993) and b) obtained via the DInSAR technique (June 1992 – November 1993), [modified from CASCINI et al., 2006]. Fig. 7 - Mappe di deformazione relative a: a) misure di livellazione (luglio 1992-ottobre 1993); b) dati SAR (giugno1992- novembre 1993), [modificata da CASCINI et al., 2006].

knowledge on the spatial distribution of the geological strata and their thickness. As a matter of fact, settlements increase moving from North to South and from West to East, reaching their maximum values in the easternmost map corner corresponding to the Cerola zone (Fig. 3), where the thickest peat layers were recorded during previous investigations [CASCINI and DI MAIO, 1994]. As far as the relationship between the magnitude of settlements (both absolute and differen-

tials) and the building damage occurrence is concerned, a first attempt was made by CASCINI et al. [2006] using low-resolution DInSAR results. In particular, the deformation gradient magnitude, computed on the ground pixel grid starting from DInSAR measured settlements, was compared with the localization of damages (Fig. 8). These latter were recorded to some buildings, during a field survey at the beginning of 1992, that is before either DInSAR and ground levelling meas-

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

Fig. 8 – Deformation gradient map from DInSAR data (June 1992 – November 1993) and spatial distribution of damaged buildings [modified from CASCINI et al., 2006]. Fig. 8 – Mappa dei gradienti di deformazione ottenuta dall’analisi di dati DInSAR (giugno 1992 - novembre 1993) con indicazione degli edifici danneggiati [modificata da CASCINI et al., 2006].

urements started. However, the open cracks, monitored in some damaged buildings with the aid of displacement transducers from October 1992 to September 1993, highlighted a progressive increase of the damage severity [C ASCINI and D I MAIO, 1994]; this confirms the ground settlement

irreversibility during the period (June 1992 - November 1993) which the map of Figure 8 refers to. The recorded damages essentially consist of i) noticeable rigid tilts between adjacent buildings (Fig. 9) or ii) sub-vertical cracks – whose width increases from the bottom to the top – as in the case of the ancient low-rise masonry building with shallow foundations (Fig. 10) labelled with the number 3 in Figure 8. It is interesting to point out that both

Fig. 9 - Damages to the reinforced concrete building (labelled with n. 1 in Fig. 8) located in the proximity of the Cerola zone (photo dated July 2004). Fig. 9 – Danni all’edificio in cemento armato (indicato con il n. 1 in Fig. 8) situato in prossimità della sorgente Cerola (foto del luglio 2004).

Fig. 10 – Damages to the masonry building (labelled with n. 3 in Fig. 8) located in the proximity of the Cerola zone (photo dated July 2004). Fig. 10 – Danni all’edificio in muratura portante(indicato con il n. 3 in Fig. 8) situato in prossimità della sorgente Cerola (foto del luglio 2004).

OTTOBRE - DICEMBRE 2007

57

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

58

Fig. 11 – Settlement point-wise comparisons (from ground levellings and full resolution DInSAR data). Fig. 11 – Confronto puntuale tra cedimenti (da misure di livellazione e dati DinSAR ad alta risoluzione).

cracks and axes of building rotation are almost always normal to gradient deformation directions, shown in Figure 8 as arrows. The absence of damages to some buildings, where significant gradient values are present, is not surprising for several reasons. First of all, inside this critical zone, buildings are essentially of reinforced concrete and more recent than the aforementioned masonry buildings. Moreover a large part of the zone is occupied by a public garden. Outside the critical zone, only one reinforced concrete building, signed by the number 2 in Figure 8, has experienced a sub-vertical crack in correspondence of a “weak” section. 4.2. Full-resolution processing results As far as the validation of full-resolution DInSAR data is concerned, Figure 11 shows the comparison between DInSAR and geodetic data, with reference to the same sample of benchmarks chosen for low-resolution data validation (Fig. 6). The diagrams confirm that a good fitting of geodetic data can be achieved, independently from the different size of the coherent SAR pixel area. A key point of the full-resolution DInSAR data is the precision in estimating the residual topography that allows accurate geo-localization of the pixels monitored by the DInSAR technique. Moreover, the presence of more than a single phase centre, located on building roofs (Fig. 12) allows the detection of local effects induced by the subsidence phenomenon, as described in the following section.

Fig. 12 – Close-up view of the spatial distribution of coherent SAR pixels detected at full resolution on damaged buildings: a) building on reinforced concrete; b) Church and Town Hall. Fig. 12 – Distribuzione spaziale dei pixel a maggiore coerenza derivanti dall’impiego della tecnica DInSAR ad alta risoluzione: a) edificio in cemento armato; b) Chiesa e Municipio.

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

Fig. 13 – Longitudinal and transverse profiles on the Church (left) and Town Hall building (right). Fig. 13 – Sezioni longitudinali e trasversali sulla Chiesa (sinistra) ed il Municipio (destra).

5. Analysis of full-resolution processing results The building under examination (Fig. 12b) was constructed before 1900. It is a masonry structure composed by two attached blocks corresponding to a Church and the Town Hall (Fig. 13); this latter has 4 floors and it is 20 m high. The vertical bearing walls have an average thickness, locally exceeding 1 m; the foundations are of shallow type. Their bases are not horizontal in the case of the Town Hall because the back part is founded on the calcareous bedrock whereas the front one rests on pyroclastic soils, with a total height difference of 4 metres. Since the Town Hall building and the Church form a unique brickwork block (Fig. 14a), the analysis of their crack patterns must be jointly carried out, taking into account that both structures were damaged by the earthquake occurred in November 1980 [NIGRO, 1992]. Focusing the attention on the damages due to ground settlements, it can be noticed that the cracks (Fig. 14b), wide up to few millimetres, are aligned along a single vertical section of the Town Hall, following the east-west building orientation and 25 m far from the northernmost bound of the building (Fig. 13). These cracks, firstly recorded during a field survey in 1992 [N IGRO , 1992], were considered the consequence of the southward rotatory motion of the building, on an axis localized in correspondence of its foundations and whose direction is almost parallel to the principal façade; this latter, in turn, seemed virtually crack free. More recently, a field survey carried out by the Authors in November 2006 highlighted that

OTTOBRE - DICEMBRE 2007

Fig. 14 – a) A view of the building; b) projections (A-A and B-B) of a sample cross sections, reported in Figure 13, highlighting the damages recorded during the field survey carried out on 1992 (sketch by hand [NIGRO, 1992]). Fig. 14 – a) Vista dell’edificio; b) proiezioni (A-A e B-B) di una sezione, riportata in Figura 13, con indicazione dei danni registrati durante i sopralluoghi del 1992 (schizzo a mano [NIGRO, 1992]).

59

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

60

Fig. 15 – Settlements computed along the longitudinal profile (1) nearest to the principal façade of the building (observation period starting from 8th June 1992 and referred to three significant time intervals). Fig. 15 – Andamento dei cedimenti cumulati, a partire dall’8 giugno 1992 e per tre intervalli temporali significativi, di punti disposti lungo la sezione longitudinale (1) più vicina alla facciata principale dell’edificio.

damages observed in 1992 haven’t worsen during the last 14 years. As a matter of fact, the still visible cracks recorded in the past didn’t change with time, in terms of both their length and width; the remaining cracks, on the other hand, have been filled. Basing on these collected information and starting from June 8th, 1992, the available DInSAR data over the building were analyzed and interpolated on a grid of 5 x 5 m in order to obtain settlements along sample cross-sections (e.g. longitudinal and transverse profiles), respectively parallel and perpendicular to the principal building façade (Fig. 13). The computed settlements were obtained assuming a pure shear mode of deformation of the building, without any strain (either of compressive or tensile type) in the vertical direction; as a conse-

quence, the horizontal displacements were disregarded and the building settlements were computed referring to the SAR pixels located on its roof. Settlements show a well-defined trend along the longitudinal profiles. In particular, referring to profile 1, the computed settlements for a given time of observation are practically independent of the distance computed along the building façade, for both the Church and the Town Hall (Fig. 15). On the contrary, sagging and hogging zones can be observed (Fig. 16) as the distance from the principal façade of the monitored building increases (profile 2). Referring to this latter profile - around 14 meters far (along the north-south building direction) from Town Hall section where the damages concentrate - the sagging zone, where the maximum settle-

Fig. 16 – Settlements computed along the longitudinal profile (2) farthest from the principal façade of the building (observation period starting from 8th June 1992 and referred to three significant time intervals). Fig. 16 – Andamento dei cedimenti cumulati, a partire dall’8 giugno 1992 e per tre intervalli temporali significativi, di punti disposti lungo il profilo longitudinale (2), più distante dalla facciata principale dell’edificio.

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

Fig. 17 – Settlements computed along the transverse profile (1) on almost half part of the extension of the Church. Fig. 17 – Cedimenti calcolati lungo il profilo trasversale (1) su circa metà della profondità della Chiesa.

Fig. 18 – Settlements computed along the transverse profile (2) on the Town Hall building. Fig. 18 – Cedimenti calcolati lungo il profilo trasversale (2) riguardante il Municipio.

ments occur, is located just in correspondence of the above mentioned weak section, along the east-west building direction. As far as the transverse profiles are concerned (Fig. 13), the Church (Fig. 17) seems to exhibit less differential settlements than the Town Hall building (Fig. 18), that is undergoing a rotation southward, as it was already supposed by NIGRO [1992] at the end of the damage survey carried out in 1992. In conclusion, the processed full-resolution DInSAR data outline the building response to the subsidence phenomena; moreover they allow the use of the classical settlement damage criteria, usually adopted in engineering practice in order to avoid building damages due to ground movements. Among these damage criteria, the empirical approach given by SKEMPTON and MCDONALD [1956] and the structural engineering approach proposed by BURLAND [1995], were used. As far as the criterion of SKEMPTON and MCDONALD [1956] is concerned, βmax limiting values of the so-called angular distortion – or relative rotation – β are correlated to the attainment of peculiar level of damage severity in a building. In particular, referring to structures settling under their own weight,

OTTOBRE - DICEMBRE 2007

the value of βmax = 1/300 is assumed as the angular distortion threshold value for first cracking in panel wall and load-bearing walls, while the value of βmax = 1/150 is considered as a threshold for severe cracking and structural damage; finally, an allowable value of βmax = 1/500 is recommended in order to avoid cracking. The criterion proposed by BURLAND [1995] is based on the use of interaction diagrams relating limiting values of relevant parameters, i.e. the deflection ratio Δ/L and the horizontal strain εh (i.e. the average strain due to the horizontal movement of the building portion bounded by two reference points). The interaction diagrams model a loadbearing wall as a weightless, linear-elastic, isotropic beam of length L, height H and unit thickness. According to BURLAND et al. [2004], H is taken as the height from foundation level to the eaves (the roof is usually ignored), while L corresponds to the length of the building in the hogging or sagging zones (limited by the points of settlement profile inflexion). The deflection ratio and the horizontal strain limiting values are related to the attainment of a “normal degree of damage severity” in the

61

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

62 building. In particular, six categories of damages are contemplated, where categories between 0 to 2 are related to aesthetic damage while effects on serviceability and structural stability are connected to categories 3 to 5. When dealing with problems devoted to the prediction of the likely level of settlement-induced damage in a given building, the BURLAND’s method [1995] is applied assuming that the foundations can conform to the deformed shape of the ground surface in greenfield conditions. It follows that the obtained results can be conservative, as discussed by FRANZIUS et al. [2004]. On the other hand, the interaction problem is, in general, not negligible and the role played by the building stiffness on the prediction of damage severity cannot be disregarded [VIGGIANI and STANDING, 2001]. In order to apply the abovementioned approaches to a case study in which damages have already been recorded, the profile of the relative rotation β and the deflection ratio Δ/L (with reference to settlement occurred from June 8th, 1992 to October 7th, 2004) were plotted versus the distance along the building façade. Referring to the ground movement definitions reported in Figure 19, β represents the rotation angle with respect to the horizontal plane of the line joining two reference points, rela-

tive to the rigid body rotation ω of the superstructure or a well defined part of it; Δ is the distance from a given point to the line connecting two reference points on either side; L is the distance between the two reference points defining Δ and Δ/L and is considered positive for upward concavity (sagging) and negative for downward concavity (hogging) [BURLAND and WROTH, 1974]. Figure 20 shows the profiles of relative rotation β calculated by taking values of the curve slope at continuous points along its length and not taking into account the rigid-body rotation. As it can be observed, referring to longitudinal profile 1, the computed relative rotation attains values low enough to justify the absence of cracks in correspondence of the principal building façade. As for the longitudin-al profile 2, the value of βmax is just over 0.0017 (1/588) and coincides with the point of maximum inflection, as it would be expected given the method of determining β. Taking into account that the building under examination was already damaged when DInSAR measurement started, the criterion of SKEMPTON and MCDONALD [1956] substantiates the verified absence of damage severity increase during the period of Town Hall building monitoring.

Fig. 19 – Definition of the parameters related to settlement occurrence [BURLAND and WROTH, 1974]. Fig. 19 – Definizione delle grandezze legate all’accadere di cedimenti [BURLAND and WROTH, 1974].

Fig. 20 – Processed relative rotations from two longitudinal profiles parallels to the building façade. Processed data refer to settlements occurred from June 8, 1992 to October 7, 2004. Fig. 20 – Rotazioni relative calcolate per due profili longitudinali paralleli alla facciata dell’edificio. I dati sono relativi ai cedimenti calcolati dall’ 8 giugno 1992 al 7 ottobre 2004.

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

Fig. 21 – Processed deflection ratios from two longitudinal profiles parallels to the building façade. Processed data refer to settlements occurred from June 8, 1992 to October 7, 2004. Fig. 21 – Rapporti di inflessione calcolati per due profili longitudinali paralleli alla facciata dell’edificio. I dati sono relativi ai cedimenti calcolati dall’ 8 giugno 1992 al 7 ottobre 2004.

The resulting Δ/L curves are plotted in Figure 21, showing maximum values of deflection ratio of about 0.067 % in the sagging zone where the settlement diagram shows the maximum curvature, i.e. the damage is most likely to occur. In order to improve the building damage analysis, the maximum values of the processed deflection ratio were introduced in the damage criterion given by BURLAND [1995]. In such a case, an important issue is the assessment of the elastic moduli (E and G), or of the E/G ratio, which can be assigned to the equivalent beam adopted to model the masonry building response. However, the assessment of the E/G ratio is a very difficult task, as it depends on several building details such as the structural typology (also considering the mechanical properties of the construction materials), mode of deformation, geometry, different amount and position of cracking in which deformations tend to concentrate. These information should be collected through accurate investigations; if this approach cannot be followed, a parametric study may be pursued [VIGGIANI and SOCCODATO, 2004]. BOSCARDING and CORDING [1989] also observe that “typically, the E/G ratio for a solid masonry beam would be expected to be somewhat greater than the E/G ratio for an isotropic material. In addition, the presence of openings in the wall would further increase the E/G ratio”. On the other hand, BURLAND et al., [2004] further state that “load bearing wall undergoing hogging were well represented by E/G = 0.5, with the neutral axis in the bottom of the beam due to the restraint offered by the foundations”. Due to a lack of information about the above mentioned details for both the Church and the Town Hall and considering that the limiting relationship between Δ/L and L/H for an isotropic beam

OTTOBRE - DICEMBRE 2007

is in agreement with several case of damaged masonry bearing walls undergoing sagging [BURLAND and WROTH, 1974], the ratio E/G was assumed equal to 2.6 (with Poisson coefficient ν’ = 0.3) in the present analysis. In such a way, the global response of the building, in both sagging and hogging mode of deformation, was modelled referring to an isotropic linear-elastic equivalent beam. Moreover, the neutral axis of the model beam was positioned in the middle, for sagging, and at the bottom edge, for hogging. Finally, horizontal strains was neglected (εh = 0), having assumed, in the DInSAR data analysis, that the horizontal displacements of the building are equal to zero. Referring to the maximum values of the deflection ratios processed for the Town Hall building (Fig. 21), the BURLAND’s criterion allows to demonstrate that, for the considered model beam, the cracking is controlled by shearing, in both mode of deformation (Figg. 22 and 23). Moreover, for the adopted E/G ratio, the magnitude of maximum processed deflection ratios are below the boundary of “moderate” (category 2) or “very slight” (category 0) normal degree of the damage expected severity [BURLAND and WROTH, 1977], respectively for sagging and hogging, assuming the building initially undamaged. The obtained results confirm that, for sagging mode of deformation, the damage criterion of SKEMPTON and MCDONALD [1956] is less conservative than that proposed by BURLAND [1995]. Furthermore, since the damages were already recorded before the analysis started, the previous results should be confirmed by an increase of the existing damage severity, from June 1992 up to October 2004; this, is not proved by the field surveys. However, it is worth stressing that the results derived by the use of

63

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

64

Fig. 22 – Interaction diagram relating Δ/L to εh for the case of an isotropic linear-elastic beam (E/G = 2.6), with L/H = 0.71 and neutral axis at the middle, undergoing sagging . Fig. 22 – Diagramma di interazione tra Δ/L e εh per una trave con comportamento elastico-lineare isotropa (E/G = 2.6), con L/H = 0.71 ed asse neutro in mezzeria, che si deforma in modo sinforme.

Fig. 23 – Interaction diagram relating Δ/L to εh for the case of an isotropic linear-elastic beam (E/G = 2.6), with L/H = 0.71 and neutral axis at the bottom edge, undergoing hogging. Fig. 23 – Diagramma di interazione tra Δ/L e εh per una trave con comportamento elastico-lineare isotropa (E/G = 2.6), con L/H = 0.71 ed asse neutro sul bordo inferiore, che si deforma in modo antiforme .

BURLAND’s criterion are influenced by the value to be assigned to the equivalent beam stiffness in order to adequately model the building response to ground movements; this issue, moreover, is further complicated in the case of historical and monumental structures [VIGGIANI and SOCCODATO, 2004], i.e. the Church and the Town Hall building. Finally, an ad-

vanced damage analysis calls for the availability of DInSAR data of both ascending and descending orbits, in order to derive the true direction of displacement vectors and, therefore, to estimate, in such a way, the horizontal displacements that could play a relevant role in building damaging [BOSCARDIN and CORDING, 1989; BURLAND et al., 2004].

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA

6. Conclusion

References

The present work is focused on the analysis and interpretation, via geotechnical criteria, of low- and full-resolution DInSAR data processed for the urban area of Sarno (Southern Italy), which is affected by a subsidence phenomenon. With reference to ground surface settlements, the results obtained via low-resolution analysis highlight that the adopted DInSAR technique is able to furnish settlement data in agreement with the measures achieved via traditional optical levelling. As for the damages recorded to the buildings during past surveys, the DInSAR maps allow an analysis of the deformation spatial gradient, both in magnitude and direction, showing a strong correlation between the gradient vectors, i.e. the steepest variation direction, and the cracks or building rotation axes, which are almost always orthogonal to the gradient vectors, in agreement to basic physical considerations. The analysis also highlights the main factors concurring to damage generations that can be considered: structural typologies (i.e, masonry, reinforced concrete, etc.), rises, year of construction, position and orientation of the building, thickness of the underlying deformable soils, etc. Such encouraging results suggest that low-resolution DInSAR data could be used in an early detection of ground surface settlements over large areas. This is, for instance, the case of the Campania Region, which is widely affected by subsidence phenomena due to huge groundwater withdrawals. As far as full-resolution analysis is concerned, SAR coherent pixels of about 10x10 m allow to significantly improve the spatial settlement distribution and their pattern at building scale. In this regard, the analysis of a sample building performed by DInSAR data processing substantiates the peculiar crack pattern observed during field surveys. Moreover, the analysis suggests an appropriate improvement of DInSAR data set that, also with reference to essentially vertical displacements, should be collected referring to both the ascending and the descending orbits. In such a way, the values of relevant parameters (deflection ratio, relative rotation,…), widely adopted in engineering practice, can be obtained thus allowing an appropriate analysis of building damages at detailed scale. Of course, this and other issues need to be properly deepened before the DInSAR technique is confidentially used to model typical engineering problems at building scale. However, since now, the performed analyses emphasize the role played by the geotechnical studies on the processing and validation of DInSAR data and on the acceptance of their reliability as well.

BELL F.G. (1994) – Fundamentals of Engineering Geology. Blackie Academic & Professional. BERARDINO P., FORNARO G., LANARI R., SANSOSTI E. (2002) – A new algorithm for surface deformation monitoring based on small baseline differential SAR interferograms. IEEE Transactions on Geoscience and Remote Sensing, 40, 11, pp. 23752383. BENEDETTI G., DRAGHETTI T., BITELLI G., UNGUENDOLI M., BONSIGNORE F., ZAVATTI A. (2000) – Land subsidence in the Emilia Romagna Region, Northern Italy. Proc. of the Sixth International Symposium on Land Subsidence, Carbognin, Gambolati, Johnson (Eds.), La Garangola (Padova, Italy) Edition, 1, pp. 61-76. BOSCARDIN M.D., CORDING E.J. (1989) - Building Response to Excavation-Induced Settlement. Journal of Geotechnical Engineering, ASCE, 115, 1, pp. 1-21. BURLAND J.B., WROTH C.P. (1974) - Allowable and differential settlement of structures, including damage and soil-structure interaction. Proc. Conf. on Settlement of Structures, Cambridge University, pp. 611-654. BURLAND J.B. (1995) – Assessment of risk of damage to buildings due to tunnelling and excavations. Invited special lecture. Proc. 1st International Conference on Earthquake Geotechnical Engineering. IS-Tokyo ’95, pp. 1189-1201. B URLAND J.B., M AIR R.J., S TANDING J.R. (2004) – Ground performance and building response due to tunnelling. Advances in Geotechnical Engineering. Proc. of the Skempton Conference. Thomas Telford, London, pp. 291-342. CASCINI E., CASCINI L. (1993a) – Forecasting spring flow time series. Journal of the Italian Statistical Society, 3, 1, pp. 1-23. CASCINI E., CASCINI L. (1993b) – Spring flow analysis for water resources utilization. SPRUCE II (Statistics in Public Resources, Utilities and in Care Environment), Rothamsted Experimental Station, 1316/9/93, Harpenden, England. CASCINI L., DI MAIO C. (1994) – Emungimento delle acque sotterranee e cedimenti nell’abitato di Sarno: analisi preliminare. Rivista Italiana di Geotecnica, 3, pp. 217-231. CASCINI L., FERLISI S., FORNARO G., LANARI R., PEDUTO D., ZENI G. (2006) – Subsidence monitoring in Sarno urban area via multitemporal DInSAR technique. International Journal of Remote Sensing, 27, 8, pp. 1709-1716. C ASCINI , L., F ERLISI , S., F ORNARO , R., P EDUTO D., MANUNTA M., ZENI, G. (2007) – Low and high resolution differential interferometry monitoring of the Sarno urban area. Rivista Italiana di Telerilevamento, 38, pp. 23-33.

OTTOBRE - DICEMBRE 2007

65

CASCINI - FERLISI - PEDUTO - FORNARO - MANUNTA

66 CELICO P. (1983) – Idrogeologia dei massicci carbonatici, delle piane quaternarie e delle aree vulcaniche dell’Italia centro-meridionale (Marche e Lazio meridionali, Abruzzo, Molise e Campania). “Quaderni” della Cassa per il Mezzogiorno, 4/2, 1-225, 1 carta f.t. Roma. CELICO P., DE PAOLA P. (1992) – La falda dell’area napoletana: ipotesi sui meccanismi naturali di protezione e sulle modalità di inquinamento. Gruppo Scient. It. Studi e Ricerche. Atti Giorn. di studio “Acque per uso potabile – Proposte per la tutela ed il controllo della qualità, Milano, pp. 387C-412C. CHAI J.C., SHEN S.L., ZHU H.H., ZHANG X.L. (2004) – Land subsidence due to groundwater drawdown in Shanghai. Géotechnique, 54, 2, pp. 143-147. C OSTANTINI M., R OSEN P.A. (1999) –A generalized phase unwrapping approach for sparse data. Proc. of Geoscience and Remote Sensing Symposium, IGARSS ‘99, 1, pp. 267-269. CROSETTO M., CRIPPA B., BIESCAS E. (2005) – Early detection and in-depth analysis of deformation phenomena by radar interferometry. Engineering Geology, 79, pp. 81-91. FERRETTI A., PRATI C., ROCCA F. (2000) – Nonlinear subsidence rate estimation using the permanent scatterers in differential SAR interferometry. IEEE Trans. Geoscience Remote Sensing. 38, 5, pp. 22022212. FORNARO G., MONTI GUARNIERI A. (2002) – Minimum Mean Square Error Space-Varying Filtering of Interferometric SAR Data. IEEE Trans. Geoscience Remote Sensing, 40, pp. 11-21. FRANZIUS J.N., POTTS D.M., ADDENBROOKE T.I., BURLAND J.B. (2004) – The influence of building weight on tunnelling-induced ground and building deformation. Soils and Foundations, 44, 1, pp. 25-38. FRUNEAU B., DEFFONTAINES B., RUDANT J.P., LE PALMENTIER A.M., COLESANTI C., LE MOUELIC S., CARNEC C., FERRETTI A. (2003) – Conventional and PS differential SAR interferometry for monitoring vertical deformation due to water pumping: the HaussmannSt-Lazare case example (Paris, France). Proc. of the 3rd Int. Workshop on ERS SAR Interferometry (FRINGE 2003), Frascati (Italy). ESA SP-550, available also online: http://earth.esa.int/ fringe03/proceedings/ GABRIEL A.K., GOLDSTEIN R.M., ZEBKER H.A. (1989) – Mapping small elevation changes over large areas: differential radar interferometry. Journal of Geophysical Research, 94 (B7), pp. 9183-9191. GOLDSTEIN R.M., ENGLEHARDT H., KAMB B., FROLICH R.M. (1993) – Satellite radar interferometry for monitoring ice sheet motion: application to an Antarctic ice stream. Science, 262, pp. 1525-1530. JURINA L., FERRETTI A., FRANCHIONI G. (2003) – Valutazione degli effetti di scavi in falda sui cedimenti strutturali di edifici mediante utilizzo di tecniche satellitari SAR. Convegno Internazionale “Crolli e affida-

bilità delle struttura”, Università degli Studi di Napoli “Federico II”, Napoli, 15-16 maggio 2003, pp. 325-337. LANARI R., LUNDGREN P., SANSOSTI E. (1998) – Dynamic Deformation of Etna Volcano Observed by Satellite Radar Interferometry. Geophysical Research Letters, 25, 10, pp. 1541-1544. LANARI R., MORA O., MANUNTA M., MALLORQUI J.J., BERARDINO P., SANSOSTI E. (2004) – A Small Baseline Approach for Investigating Deformations on Full Resolution Differential SAR Interferograms. IEEE Transaction on Geoscience and Remote Sensing, 42, 7, pp. 1377-1386. LE MOUÉLIC S., RAUCOULES D., CARNEC C., KING C. (2002) – Ground uplift in the city of Paris (France) revealed by satellite radar interferometry. Geoscience and Remote Sensing Symposium, 2002. IGARSS’2002. IEEE International, 2, pp. 12221224. LUNDGREN P., USAI S., SANSOSTI E., LANARI R., TESAURO M., FORNARO G., BERARDINO P. (2001) – Modeling Surface Deformation observed with SAR Interferometry at Campi Flegrei Caldera. Journal of Geophysical Research, 106, B9, pp. 19355-19367. MASSONNET D., ROSSI M., CARMONA C., ADRAGNA F., PELTZER G., FEIGL K., RABAUTE T. (1993) – The displacement field of the Landers earthquake mapped by radar interferometry. Nature, 364, pp. 138-142. MORA O., MALLORQUI J.J., BROQUETAS A. (2003) – Linear and nonlinear terrain deformation maps from reduced set of interferometric SAR images. IEEE Trans. Geoscience Remote Sensing, 41, 10, pp. 22432253. NIGRO E. (1992) – Field survey report. Unpublished. PELTZER G., ROSEN P.A. (1995) – Surface Displacement of the 17 May 1993 Eureka Valley, California, earthquake observed by SAR interferometry. Science, 268, pp. 1333-1336. RIGNOT E. (1998) – Fast recession of a west Antartic glacier. Science, 281, pp. 549-551. SKEMPTON A.W., MAC DONALD D.H. (1956) - Allowable Settlement of Structures. Proc. Institute of Civil Engineers, Part III, vol. V, pp. 727-768. STRAMONDO S., TESAURO M., BRIOLE P., SANSOSTI E., SALVI S., LANARI R., ANZIDEI M., BALDI P., FORNARO G., A VALLONE A., B UONGIORNO M.F., F RANCES CHETTI G., BOSCHI E. (1999) – The September 26, 1997 Colfiorito, Italy, Earthquakes: Modeled Coseismic Surface Displacement from SAR Interferometry and GPS. Geophysical Research Letters, 26, 7, pp. 883-886. S TROZZI T., W EGMULLER U., T OSI L., B ITELLI G., SPRECKELS V. (2001) – Land subsidence monitoring with differential SAR interferometry. Photogrammetric Engineering Remote Sensing, 67,11, pp. 1261-1270. USAI S., KLEES R. (1999) – SAR interferometry on a very long time scale: a study of the interferometric character-

RIVISTA ITALIANA DI GEOTECNICA

ANALYSIS OF A SUBSIDENCE PHENOMENON VIA DINSAR DATA AND GEOTECHNICAL CRITERIA istics of man-made features. IEEE Trans. Geoscience Remote Sensing, 37, 4, pp. 2118-2123. VADON H., SIGMUNDSSON F. (1997) – Crustal deformation from 1992 to 1995 at the midatlantic ridge, southwest Iceland, mapped by satellite radar interferometry. Science, 275, pp. 193-197. VIGGIANI G.M.B., SOCCODATO F.M. (2004) – Predicting tunnelling-induced displacements and associated damage to structures. Italian Geotechnical Journal, 38, 4, pp. 11-25. VIGGIANI G.M.B., STANDING J.R. (2001) – “The Treasury” Ch26. Building response to tunnelling. Case studies from the Jubilee Line Extension, London. Burland J.B., Standing J.R. and Jardine F.M. eds. (CIRIA Special Publication 200. CIRIA and Thomas Telford), 2, pp. 401-432. WEGMULLER U., STROZZI T., TOSI L. (2000) – Differential SAR interferometry for land subsidence monitoring: methodology and examples. Proc. of the Sixth International Symposium on Land Subsidence, Carbognin, Gambolati, Johnson (Eds.), La Garangola (Padova, Italy) Edition, 2, pp. 93-105. WORAWATTANAMATEEKUL J., HOFFMANN J., ADAM N., KAMPES B. (2003) – Urban deformation monitoring in Bangkok metropolitan (Thailand) using permanent scatterers and differential interferometry technics. Proceedings of the third International Workshop on ERS SAR Interferometry (FRINGE 2003), Frascati (Italy). ESA SP-550. Available also on-line: http://earth.esa.int/ fringe03/proceedings/. ZEBKER H.A., VILLASENOR J. (1992) – Decorrelation in Interferometric Radar Echoes. IEEE Transactions on Geoscience and Remote Sensing, 30, pp. 950959.

OTTOBRE - DICEMBRE 2007

Analisi di un fenomeno di subsidenza con l’ausilio di tecniche interferometriche DInSAR e criteri geotecnici Sommario

Nella presente memoria si discutono le potenzialità d’impiego di una tecnica interferometrica differenziale, che si avvale dell’algoritmo Small BAseline Subset (SBAS) per il processamento di dati acquisiti da Radar ad Apertura Sintetica, nello studio del fenomeno di subsidenza che interessa l’area urbana di Sarno (Regione Campania, Italia). A tal fine si utilizzano e analizzano, attraverso il ricorso a metodi propri dell’Ingegneria Geotecnica, dati DInSAR a bassa ed alta risoluzione, acquisiti dai satelliti ERS-1/2 ed Envisat nel periodo compreso tra gli anni 1992 e 2004. In particolare, i dati DInSAR a bassa risoluzione, una volta validati attraverso il confronto con misure topografiche, sono analizzati al fine di individuare le aree urbanizzate maggiormente affette dal fenomeno, all’interno delle quali si sono registrati danni a numerosi edifici. Si mostrano, altresì, interessanti correlazioni tra i vettori rappresentativi dei gradienti spaziali di deformazione della superficie topografica ed alcune caratteristiche dei danni registrati, come la direzione dell’asse attorno al quale alcuni edifici sono ruotati rigidamente. Si presentano, quindi, i risultati preliminari di uno studio a “scala di edificio” che si avvale dell’impiego di dati DInSAR ad alta risoluzione. Le analisi condotte, finalizzate allo studio del comportamento di edifici a seguito di cedimenti della superficie topografica, consentono di enucleare i massimi valori attinti da alcuni dei parametri significativi che intervengono in alcuni criteri di danneggiabilità. Si evidenzia, infine, l’utilità delle analisi derivanti dall’impiego di dati DInSAR a bassa ed alta risoluzione, rimarcando il ruolo che l’approccio geotecnico può giocare sia nel processo di validazione del dato satellitare che nella individuazione degli aspetti da contemplare per migliorare l’affidabilità dei risultati ottenuti in studi condotti a scala di edificio.

67