Analysis of friction development at microtunneling case study

Abstract

In 2005, a pipeline construction was undertaken under the river IJ in Amsterdam. The microtunneling method was used and it consisted of a closed front TBM of 1800mm diameter over a length of 785m in Pleistocene Sand and extremely soft Holocene sediments and anthropogenic sediments. This construction was accompanied by instrumentation that registered the drilling process with force measurements in the main jacks and intermediate jacking stations, strain in the concrete, joint width, tilt of the element and displacement measurements.
All this data was analysed with the focus on the friction development over the entire boring length. The first part of data analysis was to plot and describe the findings of the available parameters, along the total route, that could have an influence on the friction. Next, the microtunnel route was divided into six sections based on changes in soil conditions and in alignment so the analysed parameters (horizontal and vertical deviations, tilt, main jacking force , front force and friction) could be correlated. In order to better understand the results, a Pearson’s correlation analysis was created to identify any statistically relevant correlation between the available parameters. The final analysis was performed to estimate the impact that subsequent pipe segment installations have on the friction over time at a specific location.
The friction development over the entire length at the boring under the river IJ (less than 2kPa, with the exception of the start) was compared with the friction coefficient value described in the NEN 3650 when overcut and lubrication are used for concrete pipes (f =7.5 kPa). The friction coefficient is overestimated during design phase and can be optimized. Also for all six sections, after a standstill, an increase in friction is observed. At locations where correlation between alignment and forces are apparently present, the horizontal deviation is observed as the influencing parameter. This is also confirmed by the Pearson’s correlation analysis results. Regarding the impact that subsequent pipe segment installations have on the friction, the results of this analysis clearly shows a tendency for a decrease in friction when considering soil type and changes in alignment.
Overall, this work indicates the need of a more thorough friction prediction calculation to be included in the design standards. One that includes more influencing parameters other than overcut and lubricant. Such understanding would enable more accurate predictions in future projects, reducing both risks and costs.