This particular condition is related to the geological multistratum composed by lake deposit (essentially based on silt with high contents of consolidated organics) and fluvial deposits based on sand and gravel. At the beginning of the century the lake border was extended into the lake with deposits of incoherent materials of different origins, with an extreme slope· The project foresees of a diaphragm of piles anchored at the head. To optimize the anchorage technique in the various soils of the lake borders (lake deposits and sand/gravel) a monitoring campaign has been launched. The limited financial budget and confined place allowed only the execution of 4 anchorages, all with an anchored part of 7 m installed as follows: Anchored part realized with multiple injections in the lake deposits. Anchored part realized with a Jetting bulb in the lake deposits. Anchored part realized with multiple injections in the sand and gravel soil. Anchored part realized with a Jetting bulb in the sand and gravel soil.·

Aim of monitoring:

The optical fiber sensors SOFO, installed in the anchored part have been placed in this position to verify the optimal length of the anchorage and to determine the force transmission from the bulb to the soil. This allows to study its behavior either at short or long term .· Instrumentation: Three chains of 3 SOFO sensors each have been attached to the anchorage part during its assembly in the factory (the anchorage injected in a traditional way in sand and gravel has not been monitored). The measuring points for all sensors start from the head of the bulb. The length of each sensor is 1.5m 5.5m and 7.0m . The measurements have been carried out during the load test and after some weeks to determine the correlation with the load loss on the head and the behavior of the anchored part. 

Main results:

The figure shows an example of the results obtained during the cable tension. In particular it has to be noted that the head of the bulb gets in compression while the parts of 5.5m and 7m are not transferring any load as no difference between the two sensors is detected. There is a good correlation between the behavior of the bulb and the load measurement on the head of the anchorage.

Related Papers:

1. Monitoraggio mediante sensori a fibre ottiche, N. Casanova, D. Inaudi, G. Colombo, XX Convegno di Geotecnica, Parma, Italy, Patron Editor – (p 395-403) – 1999
2. Globale Bauwerksüberwachung mit Lichtwellenleitersensoren – Funktionsweise des Systems SOFO® und Anwendungsbeispiele, N. Casanova, G. Colombo, D. Inaudi, G. Ballerini, H. Podhradszky, W. Zemann, J. Kling , 13. Sitzung der AG6 – Bautechnik – (p 1-12) – 1999
3. Geo-structural monitoring with long-gage interferometric sensors, D. Inaudi, N. Casanova, SPIE, 7th International Symposium on Smart Structures and Materials, Newport Beach, USA – (Vol 3995, p 164-174) – 2000