Grade Control for Microtunnels in Very Loose and Very Soft Soils
Abstract
Microtunneling has made many advances over the years and is now used in a variety of difficult ground conditions. One of these is low (zero) blow count soils. These very loose (non-cohesive) and very soft (cohesive) soils are prone to increased settlement, overexcavation, and diving of the microtunneling machine with loss of grade control. The first reaction of most owners and engineers is “let’s do ground improvement,” with jet grouting often being their solution. The result is a substantial increase in construction costs. The introduction of grout also changes the chemistry of the ground and can “kill” the slurry used by the machine in no time. There are many other solutions with far less cost impact. These methods include the use of:
- Pilot tube guide rails
- Pilot tubes to guide the microtunneling boring machine (MTBM)
- External or internal pull rods to improve grade control
- Pipe buoyancy compensation
- Lighter-weight machines
- Controlled excavation rates
- Lightweight and cone-shaped cutter wheels
- Articulated machines
- Extendable overcutters
- Well thought out designs
This paper discusses the various methods and evaluates their effectiveness in controlling grade in very loose to loose and very soft to soft soils.
2010
International No-Dig Show Proceedings
Glenn Boyce
Jacobs Associates
Mark Havekost
Jacobs Associates
Craig Camp
Jacobs Associates
Norm Joyal
Jacobs Associates
