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, over excavation, 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:1.Pilot tube guide rails2.Pilot tubes to guide the microtunnel boring machine (MTBM)3.External or internal pull rods to improve grade control4.Pipe buoyancy compensation5.Lighter-weight machines6.Controlled excavation rates7.Lightweight and cone-shaped cutter wheels8.Articulated machines9.Extendable overcutters10.Well thought out designsThis paper discusses the various methods and evaluates their effectiveness in controlling grade in very loose to loose and very soft to soft soils.
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