For obtaining a high quality, molithically pressed lining for tunnels in sands and clayey sands at nautral moisture content, the following basic conditions should be fulfilled:
The shove of the shield should be so chosen that, consistent with the construction of the shield and the force of the jacks, a uniform, continuous movement is ensured throughout the shove.
The pressing unit should have facilities for rotation, essential for carrying out work along the chosen track without eccentricity, while the pressure, built up by the unit, should be adequate for repressing of concrete and compaction of the soil all around the lining perimeter.
The formwork should not cause harmful mechanical effects on the lining during the setting and hardening of concrete.
The composition of the concrete mix should be so chosen that its repressing ability is preserved during the entire process of advance of the shield.