Investigation into factors affecting jacking force: a case study

This study investigates the influencing factors affecting jacking loads during slurry pipe-jacking works at four drives in the Shulin district sewer network in Taipei County, Taiwan, with lengths between 73 and 126 m. The main factors affecting jacking loads during tunnelling may include: (a) overcut annulus and volume of injected lubricant; (b) work stoppages; (c) geology; and (d) misalignment. In the four pipe-jacking drives, the jacking forces are represented using the baseline technique. Results show that local variations (increasing or decreasing) of jacking force are ascribed to the varying face resistance due to driving between coarse soil and fine soil governed sand or gravel deposit or driving into and away from a buried wooden log. The increase in jacking loads could also be due to increasing friction resistance resulting from the pipe deviation being greater than a threshold value of 60 mm. Excessive injected volumes of lubricant result in very low pipe frictions incurred during pipe-jacking of the four drives and are reflected through the back-analysed mu(avg) values, which vary from 0.02 to 0.09. Jacking load increases due to either overnight stoppages or short breaks are more pronounced in poorly graded gravel or sand deposit than in clayey gravel or clayey sand deposit.