GeoStructures: Horw

Case study: SOFO System

Static loading test
Axial displacements
Lateral friction
Concrete and joint quality
Soil parameters evaluation

Project Description

Summary: Prefabricated piles, static load testing, lateral friction evaluation, automatic measurements

Clients: Luzern Canton

Place: Luzern Canton (Switzerland)

Period: 1997 - 1998

Installation: IMM SA / Stump Bohr AG

Structure's short description: Three centrifuged concrete piles were measured in a static-loading test. These 45 cm piles will be used to support a cut-and-cover tunnel. They are of floating type and are installed in a clay-silt-sand soil with layers of gravel. The 3 and 4 m long SOFO sensors were arranged in chains of 7-8 thus forming a 22-26m long multi-point extensometer that was installed in the central hole of the piles. The hole was subsequently grouted.

Aim of monitoring: To measure the lateral friction and the load bearing capacity, the piles were subjected to charges up to 2'000 kN.

Number of sensor: 23

Main Results: The SOFO sensors were selected because of their reliability, precision, ease of installation and the possibility of carrying out automatic measurements. Furthermore the measurement did not require an access to the pile head, greatly simplifying the loading setup. Before, during and a few days after the static loading test, the SOFO sensors were automatically measured using an optical switching unit. The measurement allowed the determination of the pile settlement as a function of the load level and the time, the calculation of the lateral friction, the ultimate load bearing capacity and the evaluation of the soil parameters. The measurements showed that shorter piles could be used to carry the forecasted loads. Figure 1 shows the measured deformation as a function of the applied loads. Each curve represents a sensor. Figure 2 shows the axial deformations for different depths. The reduction in the deformation of the lower sensors indicates that part of the load is dissipated through lateral friction.

Recorded deformations as a function of the applied load and for different depths.

Deformaton distribution as a function of depth

Bibliography:

SOFO?: Monitoraggio di opere del genio civile tramite sensori a fibre ottiche, G. Ballerini, D. Inaudi, S. Vurpillot, ARCHI, architettura e ingegneria, ed. Casagrande, Nr. 1, p 74 -80, 1998

Looking below the surface , D. Inaudi, L. Laloui, G. Steinmann, Concrete Engineering International, Vol 4, No 3, p 39-41, 2000

Verformungsmessungen mit optischen Glasfasersensoren , N. Casanova, Messen in der Geotechnik 98, IGB-TUBS Braunschweig - Institut f? Grundbau und Bodenmechanik Fachseminar, Germany, p 1-14, 1998

Globale Bauwerks?erwachung 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

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