Surface and Deep Displacements Evaluated by GPS and Inclinometers in a Clayey Slope

Abstract

The continuous deformations of two landslides of a clayey slope of the Italian Southern Apennines cause damage to buildings and infrastructures. In 2004, 11 inclinometer casings were installed to monitor them. In 2006, fixed-in-place probes with continuous data acquisition were installed in two of those casings, in correspondence to the detected slip surfaces. In 2010, such probes went out of use, thus were removed and periodical measurements restarted. The validity of the interpretation of previous localized measurements could thus be ascertained.

In 2006, GPS permanent and non-permanent stations had also been installed. By taking into account a possible mechanism of interaction soil-GPS station, displacements very close to those obtained by nearby inclinometers with fixed casing tip are obtained.

Depending on its particular properties, one of the landslides undergoes uniform displacements in each transversal section of the channel. So, GPS data can also give useful information on the movement of the whole section.

Introduction

The landslides under examination develop in a slope facing the valley of the Basento river, in a site called Costa della Gaveta, east of Potenza, Southern Italy (Fig. 1). They are ancient and complex landslides and develop in the geological formation of the Varicoloured Clays (extensively described by Pescatore et al. 1999), constituted by tectonized, fissured and heterogeneous clay shales. The main characteristics of landslide B are described by Di Maio et al. (2010) who show that displacements can be considered uniform in each transversal section of the earth slide channel. The displacement rate decreases from upslope to downslope because of the increase in the areas of transversal sections, the “soil discharge” being practically constant. Landslide A (Fig. 1) is much more complex. It has been analyzed by various research groups who traced different boundaries. Our current studies seem to show that the zone delimited in Fig. 1 is only a small portion of a large and complex landslide system much more extended towards the eastern side (boundary close to the line with question marks in Fig. 1) than previously evaluated.

Fig. 1

Boundaries of the landslides and location of inclinometer boreholes and GPS stations

For both landslides, the average rate of displacement is in the order of about 1 cm/year, however, some sites are characterized by much higher rates, about 10 cm/year. In these sites, the highway and the railway, whose tunnel crosses one of the accumulation zones, need frequent maintenance and strengthening, besides continuous monitoring by extensometers, inclinometers and topographic measurements. Actually, the displacements have caused some buildings evacuation and the removal of an overpass at risk of collapse (Fig. 2).

Fig. 2

Building severely damaged (a); removal of a pedestrian overpass of the highway (b)

In 2004, the local public administration (Basilicata Region) provided funds for a geotechnical investigation. Eleven continuous coring boreholes were equipped with piezometers, and eleven core-destruction boreholes were equipped with inclinometer casings (Fig. 1). Standard inclinometer measurements started at the beginning of 2005. In 2006, fixed-in-place inclinometer probes, with continuous data acquisition, were installed in two of those casings (in one of which they went soon out of use), in correspondence to the slip surfaces detected by previous periodical inclinometer measurements. In the same period, six GPS permanent stations and ten non-permanent ones were installed, in collaboration with the Geological Survey of Italy – Geophysical Service (Calcaterra et al. 2008).

Calcaterra et al. (2012) compared the displacements obtained by using different instruments. Firstly, they showed the advantage of the integrated system constituted by mobile and fixed-in-place inclinometer probes. In the case under examination, the first periodical measurements showed that most deformations were concentrated in a thin zone around the slip surface. So, a few fixed-in-place probes installed around it were considered sufficient for a satisfactory monitoring of the landslide. Experimental data collected in the meanwhile confirm the validity of the hypothesis. In fact, in July 2010, the fixed-in place-probes were removed from I9 and standard inclinometer measurements by mobile probe along the whole casing height were carried out again. This paper shows that the results are consistent with previous predictions based on the few fixed probes, and allows to ascertain the strong correlation, hypothesized by Di Maio et al. (2010), with the displacements of the other sections of the landslide channel.

Calcaterra et al. (2012) also showed that the surface displacements evaluated by the GPS stations are consistent with the surface displacements evaluated by the inclinometer profiles. This paper reports further data and shows that by taking into account the mechanical interaction soil-GPS station, GPS displacements are very close to those obtained by nearby inclinometers with fixed casing tip.

The paper reports some evidence of the usefulness of the integrated instrument system and of its limits. In the case of landslide A, GPS data give a warning on the possibility of a deeper slip surface not clearly detected by inclinometer measurements. In the case of landslide B, because of the displacement uniformity in each transversal section, GPS data can also give qualitative information on deep displacements in the time period between two consecutive periodical inclinometer measurements. However, acceleration or deceleration registered by GPS was found to be due to the superficial soil layer and not relevant for civil protection purposes.

Inclinometer Measurements

Eleven core-destruction boreholes, up to 50 m deep, were equipped with inclinometer casings in the two landslides under examination (Fig. 1). First profiles, as in the case of I9 in the landslide B (Fig. 3c), were almost uniform from the ground to the slip surface, with the localization of shear displacements in a thin zone around the slip surface. So, to carry out a continuous and not expensive monitoring, in July 2006, only three fixed-in-place probes (85 cm long) were installed in the casing I9, one across the slip surface and the other two above and below it, in the landslide body and in the stable soil respectively. In particular, the central probe was installed with its barycentre at a depth of 25 m from the ground level and the other two probes at 20 and 30 m respectively (Fig. 3a). As expected, the upper and lower probes did not register any inclination increment in about 3 years, whereas the central probe registered increasing inclination (Fig. 3b). The shear displacement on the slip surface was obtained by attributing such inclination to a 1 m thick soil layer. This interpretation made the results of the continuous measurements consistent with those of periodic ones. Under the hypothesis that above and below the slip surface no deformations have occurred, similarly to the zones around the upper and lower probes, displacement profiles were obtained like that reported in Fig. 3c by a dotted line. It is worth noting that the corresponding azimuth profile (Fig. 3d) is consistent with those obtained by standard periodical measurements and constant with time.

Fig. 3

Installation scheme of the fixed-in-place probes in I9 (a); angle increment with respect to the vertical for the three fixed-in-place probes (b); displacement profiles obtained by standard mobile probe, and by the three fixed-in-place probes (c); azimuth profiles and azimuth against time of the central probe (d)

In January 2009, the fixed-in-place probes went out of use. Di Maio et al. (2010) observed that, in the contemporaneous period of monitoring, the displacements of I9 at 25 m depth were about twofold those evaluated in I8 at about 37 m depth (Fig. 4) even during displacement accelerations. On the basis of the evaluation of the areas of transversal sections, they hypothesized that one of the possible causes of this strong correlation is the constancy of soil discharge in the landslide channel. So, after January 2009, in the lack of other data, the Authors extrapolated the displacements in I9 from those in I8. In July 2010, the fixed-in-place probes were removed and inclinometer measurements were carried out by mobile probe again. Figure 4 shows that the displacements in I9 at 25 m depth thus evaluated on the slip surface are equal to those inferred from I8 and consistent with the displacements previously determined by the fixed-in-place probes. A strong correlation has been found also with the displacement in I10. In the case of the channel of landslide B, a little information on a part can thus give information on the other parts.

Fig. 4

Longitudinal section B′B″ of landslide B with inclinometer profiles (a); displacements in I9 at 25 m depth obtained by different instruments and displacements in I8 at 37 m depth multiplied by 2 (b)

GPS Measurements

The GPS network consists of six permanent stations and ten non-permanent ones (Fig. 1). Measurements started in July 2007 for the former and in July 2006 for the latter. The master permanent station was installed in a stable area, upslope from the landslides. The other five permanent stations (F1–F5), all located in the two landslides, were equipped with a dual frequency geodetic antenna (Calcaterra et al. 2012). Three permanent stations (F1, F3 and F5) were installed on concrete columns, with a foundation 1.5 m deep (Fig. 7b). The other two were placed on a low concrete wall (F2) and on a building (F4) with shallow foundations (about 1.5 m depth). The results are reported in Figs. 5 and 6. The permanent station F3 registered the highest horizontal displacement and a noticeable vertical one. The other stations underwent lower horizontal displacements and consequently their vertical displacements are very low or still in the error order of magnitude.

Fig. 5

Horizontal displacements of permanent GPS stations

Fig. 6

Horizontal and vertical displacements of permanent F3 station

Five GPS non-permanent stations are situated on the top of inclinometer casings by means of apposite mobile adapters. The other five are positioned mainly on walls of various types. The few available results (Figs. 8, 9, and 10) are significant if compared to those of the other instruments.

Comparison Between Different Measurements

It is possible to compare the displacements of the GPS antennas to the cumulative displacement obtained by inclinometers at 0.25 m depth from the ground surface. Figure 7 schematizes the cases of non-permanent station on the top of the inclinometer casing (Fig. 7a) and that of permanent antenna on the top of concrete column (Fig. 7b). Most of inclinometer casings underwent increasing inclination with time. So, the displacement of point P in Fig. 7a can be obtained from that of P′ by subtracting the component Δs. The same procedure was used in the case of Fig. 7b under the hypothesis that the column with its foundation tilted rigidly with an inclination equal to the average inclination of the upper 1.5 m of the nearby casing. Figure 8 shows that the displacements of point P thus evaluated are very close to those evaluated by means of inclinometers.

Fig. 7

Schemes used to compare GPS displacement (point P′) and cumulative inclinometer superficial displacement (point P)

Fig. 8

Comparison between displacements evaluated by inclinometers and GPS in points P and P′ defined in Fig. 7

Also the other GPS antennas located on rigid structures provided data practically coincident with those of inclinometers. Figure 9 reports for the landslides A and B other superficial displacements evaluated with the different instruments. The figure shows that, almost in any position, measurements relative to different instruments are close to each other. One exception is constituted by GPS CS06, positioned on a wall close to inclinometer I6. As shown by Fig. 10, in this case the GPS station registers displacements higher than those, negligible, of the inclinometer.

Fig. 9

Displacements at 0.25 m depth evaluated by the different instruments: landslide A (a), landslide B (b)

Fig. 10

Comparison between non-permanent GPS station CS06 and surface I6 inclinometer displacements cumulated from 40 m depth. At the end of 2008 the casing was no more accessible

The displacements of CS06 result almost parallel to the wall, so the influence of possible wall tilting can be excluded. As a matter of fact, superficial displacement of inclinometer I6 reported in the figure is cumulated from a depth of 40 m up to the ground surface, whereas the casing is 50 m long. This because a few months after the installation, the casing became not accessible beneath 40 m depth. At that time, it was not clear why. On the basis of current results, it seems reasonable to hypothesize the cause was the existence of a slip surface at that depth.

The difference between the superficial and the deep displacements is small (Figs. 9 and 11). So, GPS data can also give information on deep displacements in the time period between two consecutive periodical inclinometer measurements, or when the inclinometer casings go out of use.

Fig. 11

Displacements on the slip surfaces against time for both landslides

However, GPS data cannot be used to evaluate the landslide acceleration on the slip surface. In fact, while the superficial displacement rates seem to be interested by seasonal variation, above all in the accumulation zone, the rate of deep displacement is almost constant (Fig. 11). In particular, the comparison between the deep displacements evaluated by fixed-in-place probes in I9 (Fig. 4) and superficial displacements in the nearby GPS station F5 (Fig. 5) shows that to the acceleration of this latter does not correspond acceleration on the slip surface. The mechanics of deep displacements is currently under examination and is analyzed in a companion paper presented at this same conference (Vassallo et al. 2013).

Conclusions

This paper reports the results of inclinometer and of GPS measurements in two slow, deep and ancient clayey landslides. Mobile and fixed-in-place inclinometer probes, permanent and non-permanent GPS stations on various supports were used. Installation was performed, and data interpretation was carried out, so as to make results of the different instruments consistent with each other. This allows to take advantage from the integrated monitoring system. In fact, a satisfactory monitoring can be obtained by a few fixed-in-place probes with continuous data acquisition, installed around the deep slip surfaces previously detected by standard measurements along the whole casing height. Furthermore, if the kinematics of the landslide is sufficiently studied, and if the difference between superficial and deep displacement is clearly understood, GPS continuous data can give significant information in the time period between periodical inclinometer measurements or when the inclinometer casings go out of use. GPS data can also be used to understand the landslide mechanisms but, alone, in this type of landslide, cannot be used to give alarm to population. On the other hand, GPS data can indicate the possibility of the existence of slip surfaces deeper than the inclinometer.