2. THRUST FORCE Theoretically, jacking thrust is the total needed forces to overcome on frictional resistance around pipe string and tunnel face. Typically jacks are oversized so they can be operated at a lower pressure and maintain a reserve jacking capacity. Effective factors in determining of thrust force in pipe jacking and microtunnelling are as follows:
1- Overburden loading on pipes
2- Ground material during length of drive
3- Water table condition
4- Stress distribution
5- Over cutting area
- Lubricant using
7- Intermediate jacking station
8- Misalignment of the pipes
9- Curve in jacking way
10- Work stoppage
Parameters numbers 1 to 4 are known as site related and numbers 5 to 10 construction related parameters. Also method of excavation is an important parameter in prediction of thrust force. Method that uses lubricant or mud slurry permanently gives lower thrust force than other methods give. It is hard to find a function that involves all above parameters theoretically. Thus it has been tried to predict most of them.