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Why does settlement occur in microtunneling?

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Settlements occur in microtunneling, or other tunnel construction methods in two forms: large settlements and systematic settlements. The cause of large settlements is the over-excavation by microtunneling machine leading to the loss of stability at the tunnel face and the formation of empty space above the tunnel. The occurrence of large settlements is attributed to the improper operation of the tunneling machine or rapid unexpected changes in ground conditions.

Systematic settlements are mainly caused by the collapse of the radial overcut between the jacking pipe and the excavation. The annular space between the jacking pipe and the excavation is essential in microtunneling and pipe jacking for the following purposes:

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(i) Reduction of jacking forces
(ii) Injection of the lubrication
(iii)Steering of the microtunneling boring machine

During tunneling, the soils may collapse onto the pipe, resulting in subsidence at the ground surface. Systematic settlements can be controlled by limiting the radial overcut and by filling the annulus with bentonite lubricant during tunneling, and with cement grout after tunneling is completed.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.

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Comments
  • Tarig Elwasila September 28, 2013 at 11:30 am

    Please let me know how to calculate in figure the quantity of bentonite to fill the over cut portion during jacking operation and cement grout to fill the anulaus space between soil and jacked pipes to minimize surface(Road) settlement.
    for example: excepted settlement after calculation is =4.7mm

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