Technical Insights By Glenn Boyce, PhD, PE, and Norman Joyal, PE, GE
In microtunneling, a microtunnel boring machine (MTBM) is pushed through the ground by advancing pipe from a jacking shaft. Ground behavior at the face of excavation depends on the composition of the ground. It is broadly characterized as clay (more cohesion, less friction), and silt and sand (less cohesion, more friction). These materials are often mixed, interlayered, and changing along the tunnel. Cohesion creates a soil mass that remains standing when excavated. Friction provides short-term support to a cohesionless soil mass before it begins to ravel, run, or flow when excavated.
A slurry MTBM provides constant face pressure to counterbalance earth and groundwater pressures by pumping engineered drilling fluid (EDF) into the MTBM face. The EDF pressure should be kept greater than the earth and groundwater pressures. In silty and sandy materials, the EDF is forced into the interstitial spaces between particles, forming a filter cake, which acts as a membrane, allowing the ground to be stabilized via friction and cohesion. As the MTBM advances, the jacking pipe advances through a stable bored hole supported with pressurized lubrication, resulting in less friction and lower jacking forces.
The drilling fluid should consist of water and bentonite, a clay material that absorbs free water into its structure while maintaining its consistency. When the ground is already stiff, firm, and impermeable such that drilling fluid cannot penetrate the interstitial spaces, the percentage of bentonite can be reduced. However, introducing conditioners and additives to the slurry is beneficial in controlling clay behavior such as swelling and stickiness, or minimizing the breakdown of its structure into clay platelets.
Unfortunately, bentonite is often omitted entirely from the drilling fluid because of the mistaken assumption that clay in the ground is as good as bentonite. Most existing natural clays do not have the water-absorbing qualities of bentonite. Using only water with existing natural clay often results in no formation of a filter cake, and a fluid with a “chocolate milk shake” consistency that flows into the MTBM in an uncontrolled manner. This results in overexcavation, leading to surface settlements and sinkholes.
The U.S. industry recognizes (ASCE Microtunneling Standard No. 36-15) that ground with more than 20% soil particles passing a No. 200 sieve (<75 μ) is cohesive (and known as “fines”), and ground with less than 20% soil particles passing the sieve is cohesionless. Unfortunately, for slurry microtunneling, the definition for cohesive ground is incomplete. Ground with <20% clay behaves much differently than ground with <20% silt. However, both are categorized as fines. One solution is to revise the definition of cohesive soils to be consistent with the Unified Soil Classification System (ASTM D2487), and to include hydrometer testing to differentiate the constituent percentages of silt and clay to better determine if the soil mass will behave as a firm or flowing mass.
Using bentonite in slurry microtunneling is not complicated, but it requires constant vigilance, monitoring, and testing to maximize the slurry’s beneficial attributes in ground support. When ground conditions and behavior are understood, overexcavation, settlement, and sinkholes can be prevented with the proper use of bentonite in the drilling fluid.