Jean-Mathieu Nocquet

Monitoring the La Clapiere Landslide (French Alps) by continuous GPS

C. Pambrun, J.-M. Nocquet, J. Virieux, Y. Gugliemi, F. Cappa


Introduction

The Clapiere landslide is located in the Tinée valley (southern French Alps). It is a large landslide (with a volume exceeding 5.10 m ) near the town of Saint-Etienne de Tinée and results from a destabilization of a part of the left bank of the NW-SE oriented Tinée river. It is bordered on its southeastern side (at the right on figure 1) by the Rabuons brook. The lanslide is divided into 3 main parts limited by two pre-existent faults moving at different velocities. The northeastern area moves at ~380 cm/yr, the main sliding area at 40-90 cm/yr and the northwestern part at 20-70 cm/yr. Azimuth of displacement are consistent over the whole sliding area with a general deviation of 10-20° to the south relative to the main slope direction, that suggests a complex mechanism of sliding. Recent dating suggests that the slope failure initiated 11 kyr ago, with a multistage evolution including stages of acceleration and decceleration. Rupture has been continuous since the beginning of the century.

GPS monitoring system

In order to monitor the landslide activity, a Near Real- Time GPS survey system has been developped. Two bifrequency GPS (Ashtech Mira-Z) have been installed in the stables areas surrounding the landslide and are used as reference sites. Two single-frequency GPS receiver and antenna were installed within the sliding area. CLP1 is located in a fast sliding area representative of the dynamics of the main part of the landslide. The second site CPL2 has been installed at the edge of the main landslide area on a block intermediate between the main sliding area and the stable area. Data are transmitted in real time through a radio link system described below. Data are stored in a PC that uses a telephone line to send the data to Geosciences Azur (Sophia-Antipolis, near Nice) where the analysis is performed.

GPS station setup

 

The GPS station with the landslide consist in a single-frequency Ashtech G12 receiver with a Ashtech Geodetic 4 antenna, a radio modem, a 60W solar panel and a 30 Ah battery. The reference sites are equiped with a bifrequency Ashtech Mira-Ze (equivalent to ÁZ) and standard Choke-Ring antennas. The monumentation for the sites located within the landslide is a tripod anchored to the soil. The monumentation for reference sites is a 1.5 m high concrete pillar anchored to the bedrock.

 

 

Real-time transmission & rinex translation

 

 

We use radio modems (SATEL 3AS/869) that have buffer capabilities to send data to a master PC using a multiplex signal. Every 30s each modem attached to the receiver sends a identifier message to the master PC through the radio link. The time of sending is shifted by 5 secondes between each receiver. Once received, the message is converted into the RINEX format in real-time. The master PC is connected to a telephone line and sends the data once a day to the laboratory located at Sophia-Antipolis. We are planning to upgrade the system with an ISDN connection that will enable us to get the data every hour and perform near real-time processing.

Preliminary results

The experiment has now been running for about 2 years. The data are processed on a routine basis at Geosciences Azur. We show here after some preliminary results obtained using the GPS commercial processing package Ashtech-Solution that has been done as a quality check. The time series show the daily position of site CLP1 relatively to the reference site CLAP (see figure 2). The displacement cumulated over the 500 days of measurement is ~30 cm on each component. However, the displacement time series clearly show that the motion is not linear with time and that the landslide (at least a part of it) is subject to phase of acceleration and deceleration that last several weeks. After an acceleration stage observed during the winter 2003-2004, the velocities show a steady decreasing trend since March 2004 corresponding to a slow decceleration of the sliding. The time series both in displacement and velocities also show higher frequency signal, that might be correlated with the rainfalls occurences.

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