A Closer Look at Prevailing Civil Engineering Practice - European ...
A Closer Look at Prevailing Civil Engineering Practice - European ...
A Closer Look at Prevailing Civil Engineering Practice - European ...
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A <strong>Closer</strong> <strong>Look</strong> <strong>at</strong> <strong>Prevailing</strong> <strong>Civil</strong> <strong>Engineering</strong> <strong>Practice</strong>, Wh<strong>at</strong>, Why and How<br />
Vincent T. H. CHU<br />
enhances the cutterhead to be extended. The gripper in the middle section<br />
of TBM is mobilized so th<strong>at</strong> it pushes against the tunnel walls to react the<br />
boring forces. As these forces are dissip<strong>at</strong>ed, it allows the install<strong>at</strong>ion of<br />
lining segments during tunnel so th<strong>at</strong> it increases the speed of tunneling.<br />
Upon completion of a trust stroke, the grippers are retracted and the end<br />
portion of TMB is pushed against the front shield by thrust cylinders.<br />
Double shielded TBM is normally used in rock str<strong>at</strong>a with geological fault<br />
zones and when a high r<strong>at</strong>e of advancement is required. Single shielded<br />
TBM is more suitable to hard rock str<strong>at</strong>a.<br />
13. In Slurry Shield TBMs, should high or low slurry pressure be<br />
maintained to support excav<strong>at</strong>ed face<br />
In Slurry Shield TBMs, the slurry supports the wall of tunnel face in a<br />
manner similar to diaphragm wall. The bentonite forms a filter cake on the<br />
tunnel face on which the slurry exerts its pressure.<br />
The face of a slurry shield tunnel boring machine is stabilized by bentonite<br />
slurry, which is kept under pressure. If the slurry pressures provided are too<br />
low, instability of the face results with occurrence of large settlements. On<br />
the contrary, in case the slurry pressures are kept too high, it leads to an<br />
excessive loss of bentonite and significant soil disturbance would occur.<br />
14. Is NATM a tunneling method or a tunneling concept<br />
In the original version of the New Austrian Tunneling Method, it is a<br />
tunneling concept with a set of main principles as follows:<br />
(i) Applic<strong>at</strong>ion of thin-spayed concrete lining<br />
(ii) Closure <strong>at</strong> invert to form a complete ring as soon as possible<br />
(iii) Measurement of deform<strong>at</strong>ion until equilibrium is <strong>at</strong>tained<br />
It is a design concept in which the ground (soil or rock) enclosing the<br />
opening becomes a load-bearing element through form<strong>at</strong>ion of ring-like<br />
body. It uses all available means to develop maximum self-supporting<br />
capacity to provide support of the underground opening. In essence, it<br />
makes use of geological stress of surrounding soils to stabilize the opening.<br />
Therefore, NATM is a tunneling philosophy and concept r<strong>at</strong>her than<br />
excav<strong>at</strong>ion techniques.<br />
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