Unseen Utility Alternatives
Small Diameter Trenchless Tunneling Equipment Offers Contractors Evasive Applications
By Laura Anderson
Trenchless technologies can be effective methods of eliminating construction’s social and surface restoration costs. The trenchless approach often presents cost savings given that pipe installation can take place below existing utility lines to avoid disruption or
relocation. In many crowded and urban settings, contractors often find that trenchless applications like pipe jacking and microtunneling are the most economic choices when pipe is installed at least 25 ft below the ground.
The method that the contractor chooses is dependent upon many factors. The proverbial “it depends” can become frustrating and understandably so. When it comes to new trenchless installation, there is no clear-cut set of factors that determine the correct small diameter tunneling method, however, the following
information may help narrow down options.
Small diameter pipe jacking is generally used to install pipe on line and grade for drives of 1,000 ft or less, using pipes ranging in size from 4.25- to 52.5-in. outside diameter (OD). The two methods of small diameter pipe jacking are slurry microtunneling and pilot tube microtunneling (PTMT).
The pipe is “jacked” forward by a power source on a frame that is against a trust block.
Microtunneling is generally referred to as non-manned, remote-controlled, continuously supported pipe jacking. The application most suitable for this method is gravity flow pipeline installations. This option is recommended for contractors whose job specifications demand a small launch shaft in a high water table area that requires earth pressure balance. This method can be used for ground conditions that are significantly below the water table and typical drive lengths from 300 to 1,000 ft or more. Microtunneling jobs can take varying amounts of time and depend on the distance of pipe to be installed, crew experience level, pipe length and soil conditions. Production rates can vary from 20 to 160 ft in a 10-hour shift.
 The microtunneling boring machine (MTBM) is operated by a crew member from a control console located above ground outside of the shaft. The ground for the tunnel is excavated by the cutter head on the MTBM. As it is thrust forward by the jacking frame, a piece of final product pipe is placed behind it. The forward advancement on the pipe string, combined with the rotating, steerable action of the cutter head removes the spoils and forces them to the crusher cone. The slurry mixed with small spoils travels through lines to a slurry separation system and eventually returns to the machine face. The pipe installation and the spoil removal in this method occur simultaneously.
PTMT is generally used for new installation of pipe for gravity flow pipelines from 4.25 to 48 in. The machine can operate out of a minimal diameter shaft (Akkerman’s 308A jacking frame requires a mere 8-ft shaft) and provides pinpoint accuracy through the use of a LED target viewed through theodolite guidance. The LED target is sighted down the center of the pilot tubes. PTMT is commonly referred to as “auger boring meets microtunneling.” PTMT systems will provide pinpoint accuracy up to around 350 ft, although distances that exceed this length have been reported. PTMT performs well in most soil conditions with the exception of large cobble or solid rock.
Pilot tubes, with an appropriate steering head mounted on the front, plot the course and establish line and grade. Next, augers and casings perform the excavation of the spoils then the pipe string follows directly behind it. As pilot tubes, the augers and casings provide final product pipe installation, while a section is removed from the reception shaft. This three-pass method continues until the final product pipe is in place. Operators monitor the progress of the target through the theodolite on a monitor in the launch shaft, alongside the jacking frame.
Learning how to best operate a microtunneling system takes expertise. Operators must learn how to maneuver the console and observe the monitor’s inputs and outputs. Prospective operators will not find operation to be an insurmountable task however. Manufacturers recommend that crew members undergo intensive training on how to most effectively operate and care for their microtunneling system.
 PTMT jobs can take varying amounts of time and depend on the distance of pipe to be installed, crew experience, pipe length and ground conditions. On average, PTMT systems can jack pilot tubes a complete drive length of 200 to 300 ft in a 10-hour shift. Some single drive jobs can be erected and dismantled in a week and some take longer. The actual production rate of the machine hinges upon crew experience and ground conditions.
Training to operate a PTMT generally takes about two days since operation is not as complex as the microtunneling system. The most difficult and most critical part of operating the PTMT is getting the guidance system on line and grade.
 The PTMT has been used for many applications in the sewer and water industries, including gravity flow installation, pull back of utilities and providing guidance for auger boring rigs, as well as installing pilot tubes
for probing.
“Around half of Akkerman guided boring machine (GBM) system applications provide accuracy for auger boring rigs,” states Rob Tumbleson, vice president of sales and marketing at Akkerman Inc. “Akkerman continually learns of new and innovative ways that customers utilize their GBMs.”
Recently, a user pulled back heating coils under the foundation of a frozen vegetable packing plant floor to thaw frozen areas. Another contractor used the GBM pilot tube system for installing new pipe under the concrete floor inside of a major bottling plant for a water filtration system. Overall, there are a few new technologies in the PTMT and microtunneling markets.
“Akkerman’s new 4812 GBM system can jack 2-meter long pipes with an OD up to 48 in. from a 12-ft shaft with 200 tons of jacking force, 100 tons of pullback force and 20,000 ft-lbs of torque” states Tumbleson.
“This machine presents contractors with a variety of pipe diameter and length options. Additionally, it features longer dual-wall pilot tubes and augers and casings to increase productivity. The jacking thrust is advanced through a single and continuous hydraulic action and the guide rails are low for ease of functionality. Together, this requires fewer joints to go the same distance, thus reducing project costs.”
 Keyhole microtunneling frames are also gaining popularity, as they allow the customer to operate a high-capacity jacking frame out of a minimal diameter launch shaft. Akkerman’s MT860K, MT875K and MT890K keyhole jacking frames are designed to operate out of a minimal diameter shaft (16 ft for the MT860K and 20 ft for the MT890K). They feature up to 800 tons of thrust capacity while advancing a pipe with a maximum OD of 75 in. (up to 60-in. OD pipe for the MT860K and up to 90-in. OD pipe for the MT890K). They are able to push 10-ft laid length pipe sections and up to 20-ft lengths with add-on components and a larger shaft size. The keyhole frame uses keyed locks on the thrust block in order to continuously advance the machine.
Generally, contractors interested in purchasing a small diameter pipe jacking system should ask the manufacturer for a list of current customers and recently completed projects. They should also call project owners to ascertain their opinion on the quality of the completed job. Most contact and job information can be secured from the Internet. It is important to note that each job will dictate the most appropriate small diameter tunneling equipment choice and there is not a one-size-fits-all platform. Contractors will find that their small diameter pipe jacking investment will quickly pay off and their experience in the trenchless market will enhance their profitability.
Laura Anderson is a marketing specialist with Akkerman Inc., Brownsdale, Minn. |
