How to choose the most appropriate method for your project
The debate regarding what method of mining to select for infrastructure pipe installation continues to grow. Debate of construction means and methods is healthy for the industry. However, an accurate and realistic discussion of means and methods concerning when to select the guided boring method or microtunneling based on drive length, soil conditions and other project constraints is key to project success and the real value to the construction community as well as the municipalities and respective tax payers funding the projects.
Definition of Methods
Microtunneling is a “trenchless construction method for installing pipelines” with four key attributes: 1) remote control is used in operating the microtunneling boring machine, 2) a guidance system is used, 3) pipe jacking is used to install pipelines, and 4) continuous pressure is exerted at the excavation face to balance groundwater and earth pressures (ASCE/CI 36-14).
In contrast, guided boring method (GBM), also known as pilot tube method and errantly as pilot tube microtunneling, employs an auger boring power unit with a pilot tube’s guidance and steering system. Originally designed for sewer laterals, GBM has been used for jacking casing sizes up to 60 in., when combined with hydraulic powered reaming heads or conventional auger boring rigs. The guided boring method requires multiple passes that upsize from the pilot tube in order to install the casing or carrier pipe. GBM, however, is not microtunneling or any variation thereof (ASCE/CI 36-14). Guided boring method is a simply auger boring with pipe jacking.
Like almost any construction method there are pros and cons that help guide the owner and engineer in selecting the method best suited for the project. As noted above, GBM is not microtunneling as defined by ASCE (www.asce.org), and is limited in its application.
Negatives of GBM include limited drive length, limited compatible ground conditions, deflection and/or obstruction of the pilot tube. While drives using GBM have can reach 600-700 linear feet (lf), the recommended drive lengths are between 200-400 lf in order to maintain accuracy of line and grade and reduce the risk of possibly failing to complete the drive. This is considerably shorter drive lengths than possible with microtunneling.
GBM is best suited for cohesive soils above the water table. It is incompatible for ground with gravel, cobbles, boulders, or rock, whereas microtunneling is suitable for most all ground conditions. Bony or rocky ground conditions can cause obstructions or deflections of the pilot tube that can limit the accuracy of GBM.
Microtunneling can be used in mixed face and mixed reach soil conditions. Additionally, GBM typically uses extremely short lengths of jacking pipe which requires more pipe joints. This creates a greater potential for leaks and problems along the pipeline.
So now that we have discussed the limitations for GBM, it is time to look at what conditions are conducive for the method. For short drives (less than 300 lf), GBM can work out of jacking and receiving shafts as small as 8 ft ID. Larger GBMs used for attempts at long drives (greater than 300 lf), typically require 12- 14-ft jacking pits, which are small and well suited for urban settings. However, using conventional auger boring equipment to upsize casing from pilot tube requires conventional jack outs 24- to 34-ft long, negating much of the GBM shaft benefit.
Costs are almost always drivers when selecting the means and methods for pipeline construction. For this reason, using GBM can yield lower costs. When vitrified clay pipe (VCP) is the carrier pipe, for example, GBM costs are especially low. Another cost savings is in the equipment itself, which is less costly compared to microtunneling. The lower investment cost allows for more competition since the capital investment cost barrier is lower. The U.S. market is huge and, consequently, there are many contractors from which one can choose.
There are several other factors that lower costs for GBM, including fast and easy setup, bore, and recovery compared to microtunneling projects. Personnel required for GBM projects are relatively unskilled except for the GBM operator, which also keeps costs lower. Finally, boom trucks may be used for support equipment instead of cranes.
When selecting between microtunneling and the guided boring method, the owner is best served to not consider cost alone, but the conditions (ground, groundwater, drive length, pipeline diameter and material) that impact the means and methods and ultimate success of trenchless pipeline installations. As noted, both GBM and microtunneling have pros and cons.
The greatest attributes of GBM are lower costs and excellent accuracy, especially for shorter drives. Alternatively, microtunneling is far better suited for difficult and varied ground conditions, working in groundwater, longer drives, and a broader range of pipeline dimensions and materials while still being able to maintain accuracy in line and grade. In short, guided boring methods are not microtunneling and should not be considered a suitable substitute for the same without a realistic and viable assessment to the existing project conditions and constraints.
Gerald Lowe is a Project Manager with Bradshaw Construction Corp. Before joining Bradshaw, Lowe served honorably as a naval officer working as a Civil Engineer Corps and Foreign Area Officer, & Basic Diving Officer in assignments in the United States, Africa, Europe, Afghanistan and the Middle East. He is pursuing a Doctor of Management (Project Management).