BTS | HARDING PRIZE COMPETITION 2023


Power ON, operating cycle, and rates


The Pressure Ring system works in a similar way to how the TBM advances during the normal operation, where the cycle is limited by the elongation of the thrust cylinders. In the launch case of the system, the maximum


stroke of the external cylinders was 200mm thus, once reached, a new cycle would commence. The cycles of the Pressure Ring System are explained


in the sequence below: 1. Rear lock nut tightens against the cylinder rod 2. Start-up of the hydraulic power pack. Actuate the cylinders so the force is transmitted to the shield. TBM ready to bore. Commence excavation


3. Extension of the hydraulic cylinder as the TBM excavates until the end of the stroke of the cylinder


4. Tighten the inner lock nut inside of the stool to maintain the tension. Once all the nuts are locked the cylinder rod is retracted to initiate a new advance 5. Repeat from point 1


During the operation, the total advance, movement, and the forces applied were monitored on the four different sectors to get a better control of the machine and the excavation. An elongation sensor was installed on each sector


so the squareness of the steel ring could be controlled and to correct any discrepancy created across them. Also, another elongation sensor was installed on one of the cylinders at each sector to read the extension of the cylinder on each cycle. A pressure transmitter indicated the force created by


the cylinders showing both the TBM operator and the jacking system operators the total force applied by the cylinders on each of the sectors so it could be adjusted trying to avoid eccentricity between the left and right sectors. As stated, due to safety and design reasons, the load


applied by the cylinders was limited by a pressure reducing valve avoiding over-tensioning the bars and the structure, which was designed to work up to a maximum total force of 37,500kN. Originally, it was estimated that the operation would


last approximately nine days. This allowed time for the extension of the gantry rails and the structure of the conveyor as the machine advanced from the Silvertown launch shaft.. The extension of those happened at every 5 cycles of the cylinder strokes. The team achieved the goal in seven days.


The table shows two cases with the estimated


duration of a cycle when the cylinders are operating at the maximum and minimum speed. Of course, the rates shown could not always be


achievable for various reasons. For instance, not all the cylinders extended at the same speed so to keep the squareness of the ring, the cycle had to be stopped when the first set of cylinders was about to reach the maximum elongation. On the other hand, there were planned downtimes related to assembly of the gantry rails and the conveyor structure that also affected the advance rate during the operation.


Parameters for the excavation The team decided to split the advance of the TBM into three different scenarios based on the different geological conditions the machine was going to encounter during the launch:


Stage 1 The TBM had to excavate through a full face 50MPa concrete headwall of 3000mm thickness, then GFRP secant piles of 2000mm-diameter. An earth pressure of 0bar in front of the working chamber was maintained keeping the chamber completely empty so the concrete could not harden inside. The maximum thrust force expected was between


2000kN and 9000kN. At this phase, the force varied depending upon the machine location, whether just upon the cradle or mining through the concrete headwall. Thrust force was calculated by assessing friction among other factors, such as cutting force or the towing of the back-up gantries. The least favourable combination of individual resistance was decisive thus a safety factor was included within the assessment.


Stage 2 The TBM had to excavate to 2000mm into soft concrete/ mixed ground at 0.5bar earth pressure in front of the cutterhead. The maximum thrust force expected was between 8500kN and 18,800kN.


Stage 3 The TBM had to excavate to 4000mm into mixed face London Clay and 10MPa soil mixed piles at 1bar earth pressure in front of the cutterhead. The maximum thrust force expected was between 14,000kN and 24,300kN.


Table 1: Estimated duration of the operation based on maximum and minimum speed of the hydraulic cylinder


Total cycle - SF 1.5 (min)


Max. Advance (15mm/min)


Min. Advance (5mm/min)


54.5 94.5


Total duration TBM launch (days)


2.35 12,414.00 4.07 62.07


Total advance (mm)


Strokes No.


Incidents during the launching As the earth pressure increased, the force required to overcome the resistance climbed to a peak of about 24,000kN. At this peak force, a loud noise was heard in the vicinity of the bottom of the steel ring that put on hold the operation. Investigation followed for possible damages to the


structure. It was found that the bottom joint of the steel ring had buckled, several bolts snapped and the inner stiffeners near the joint had slightly deformed.


24 | June 2023


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39