Centrifuge tests to predict loading conditions on a bored tunnel due to consolidation settlements
Randstadrail is a future light-rail link between Rotterdam, The Hague and Zoetermeer in the Netherlands. The project involves 2 single track shield tunnels in Rotterdam, each with a diameter of 6.5 m and a length of 2.4 km. In some sections, the top part of the tunnels will be placed in a soft holocene layers. Settlements of these layers in the livetime of the tunnel of 100 years would induce external forces to exert strongly on the tunnel lining. To predict the extra stresses on the tunnel known as the negative skin friction, the use of an elastic formulation was at hand. Because uncertainties are not negligible, the calculation method needs to be verify. Therefore GeoDelft was commissioned by Gemeentewerken Rotterdam / Ingenieursbureau to perform centrifuge tests.
In the Rotterdam city area, the geotechnical profile consists of 17.5 m of soft Holocene layers (peat and organic clay), lying on a the Pleistocene sand layer of 30 m thick. The freactic water level is about 2 m below NAP. These geotechnical profile was scale down 1:65 in the centrifuge tests. The tunnel was embeded in the sand layer at 135 deg. Two tests were perfomed with without and with a load consiting of a sand layer of 1.3 m in prototype scale.
The increase of the soil stress on top of the tunnel compared to the stress in the clay during the consolidation in the centrifuge conditions shows the generation of the negative skin friction. The tests results were compared to the results of the FEM calculations, using the Hardening Soil Model of PLAXIS 7.2. Parameters of the clay needed to carry out the calculations like the strength and the stiffness were determined from triaxial and Ko-CRS tests. It was found that the stress distribution around the tunnel coincided rather wel with the measured distribution from the tests. The horizontal and vertical soil pressures are reached the maximum values on the crest of the tunnel while they decrease to zero at the level of the tunnel axis.
Conclusion
It was concluded that the negative skin friction exerting on a tunnel as a consequence of consolidation of soft soil layers can be determined in centrifuge tests. From the tests results and back analyses using FEM calculations, it was found that there are 30 to 50 % less skin friction than compared to the elastic formulation.
