TRENCHLESS | PROJECT REPORT
Cutterhead after completion.
4. MICROTUNNELLING The slurry-type MTBM provided continuous face support and groundwater control while advancing through Eugene Formation sandstone. Line and grade were maintained using a laser guidance system, allowing for continuous monitoring and correction throughout the drive. Upon completion of the casing installation, the carrier pipes were installed, and the works completed with cellular concrete placed inside the remaining annular space, as required.
4.1 Installation metrics - anticipated Design-phase assumptions included estimated jacking forces based on casing diameter, alignment length, rock strength, and frictional resistance. Estimates of anticipated slurry volumes, advance rates, and cutter wear were developed to support equipment selection and schedule planning. Maximum jacking forces were evaluated by Delve Underground, estimating the face pressure required to advance the machine while considering skin friction between the jacked steel casing and the ground. Since this was a maximum estimation for evaluating structural and hydraulic components required for the drive, the main bearing limit of 1500kN (150 tons) was used as face pressure. The skin friction was calculated to be 2.68kN/m2 (0.025 tons/ft2) [1]. These anticipated forces combined over the length of the drive were roughly 2640kN (265 tons) maximum, which was used to check the thrust block, jacking frame, and pipe for structural and hydraulic capacity. An intermediate jacking station (IJS) was not
determined to be required based on jacking loads, but a telescopic can with grippers was utilised on the project as a contingency if excessive roll was encountered, or required jacking forces exceeded expectations. Cutter wear was not anticipated to be an issue
based on the abrasivity indicated in the geotechnical reports, but there were provisions in place for cutterhead intervention during the drive if required. These provisions and planning included a cutterhead camera, face access, spare ground engagement tooling, and a groundwater pump test protocol.
REFERENCES ● [1] Thomson, J.C. (1993) ‘Pipe Jacking & Microtunneling’, Springer, 1993. 22 | July 2026
4.2 Installation metrics - encountered Actual installation data demonstrated jacking forces, advance rates, and slurry pressures consistent with or better than anticipated design values. The advance rates varied based on the hardness of the rock as encountered by the MTBM. Softer or weaker material broke up into finer material when engaged by the cutting tools, which resulted in slightly hindered production. The cutterhead was designed to excavate in rock excess of 103 MPa (15,000 psi) UCS, which resulted in higher production rates. These material changes were not only witnessed by the jacking parameters, but also the size of the material coming off the separation plant; weaker sandstone became saturated into the slurry, removed in small particles, while the harder material produced more efficient rock chips. The encountered jacking forces were much lower than
anticipated as a result of proper lubrication and excellent integrity of the annular space in the Eugene Formation. The average required jacking force was less than 500kN (50 tons) combining face pressure and skin friction, with roughly 180kN (18 tons) required to advance the MTBM into the Reception Shaft. These lesser jacking forces did not require any use of the telescopic can. While there was a plan in place for cutterhead tool
change during the drive, it was ultimately not required. The operator witnessed steady cutterhead advancement and no spikes to the jacking forces that would indicate excessive wear. Upon reception of the machine, the tooling showed minimal wear, with some of the paint still on the machine.
4.3 Encountered conditions During the microtunnel installation, two significant conditions were encountered: drainage of excessive surface site water; and, clay content of material. The Launch Shaft was located at the end of the large
open excavation where the tanks would be constructed. The bottom of this excavation was consistent bedrock, with the lowest section being the microtunnel Launch Pit. Any existing groundwater was handled via sump pumps within the shaft as anticipated. However, during large concrete pours unrelated to the microtunnelling, excessive site water use resulted in flooding of the Launch Shaft. This was overcome by several dewatering wells installed near the Launch Shaft as well as sump pumps, installed prior to launch of the MTBM. The stronger rock in the Eugene Formation resulted
in excellent production rates and separation plant performance, but some of the weaker rock encountered dissolved into slurry as expected. However, the Formation contained some clay content that quickly raised the mud weight in the system. Flocculants and polymers were added to the slurry to remove the clay content, but several tank drains were required, utilising vac trucks.
This paper has been used with permission of the North American Society for Trenchless Technology (NASTT) and edited for space. The full paper can be found at https://
knowledgehub.nastt.org/. Copyright 2026 by NASTT. The original paper was distributed at NASTT 2026 No-Dig Show.
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 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
