This list of microtunneling terms and definitions is courtesy of the ASCE Microtunneling Standards Committee and forms part of the CI 36 Standard Design and Construction Guidelines for Microtunneling.
adapter ring: In microtunneling, a fabricated ring, usually made from steel, that is mounted on the first pipe of the pipe string. This ring is intended to transfer the jacking force from the microtunnel boring machine’s (MTBM’s) bearing area to the jacking pipe and to create a waterproof seal between the machine and the spigot of the first joint.
advance rate: Forward progress over a given period of time; includes penetration rate, makeup time, downtime, and idle time; typically measured in feet or meters per shift.
annular space: The theoretical volume between the gauge cut and the outside radius of the jacking pipe times the length of the installation, equal to annulus x length of tunnel.
annular thickness: The radial distance between the excavated radius created by the gauge cutter and the outside radius of the jacking pipe. The radial distance is equal to the overcut (gauge cut) plus the shield cut.
annulus: The theoretical area between the gauge cut and the pipe outside diameter (OD) equal to (π x (gauge cut OD2 – pipe OD2)/4). It is the combined area created by the overcut and the shield cut.
auger boring: A technique for forming a bore from a jacking or drive shaft to a receiving shaft by means of a rotating auger with cutting tools. The casings are jacked forward sequentially in a cyclic process while the auger is turned. Spoils are removed back to the drive shaft by helically wound auger flights rotating in the steel casing. The equipment may have limited guidance and steering capability.
auger MTBM: A type of microtunnel boring machine that uses auger flights to remove the spoils through a separate, smaller casing placed through the jacking pipe.
backfill grouting: Grout injected into the void space between the carrier pipe and casing after the drive is completed.
belly pans: Shims or other steel plates added to the bottom of the MTBM to adjust the machine’s location within the excavation. Also known as belly plates.
Berlin construction method: A sewer collection system developed in Berlin, Germany, whereby sewer laterals are brought to a manhole and then dropped into the sewer interceptor. This method of connecting laterals to sewer is more compatible with trenchless installations.
boulder: Per ASTM D653, a rock fragment, usually rounded by weathering or abrasion, with a dimension of 12 in. (305 mm) or more.
buried objects: Items in the ground that can affect MTBM advancement and line and grade control. Buried objects that prevent continued advancement along the design path, which includes tolerances, are called obstructions.
caisson: A watertight chamber used in shaft construction under water. The ground within the chamber is excavated allowing the chamber to sink under its own weight into the ground.
carbon footprint: The amount of carbon dioxide generated and caused by a construction project, including any traffic effects.
carrier pipe: Pipe for conveyance of water, gas, sewage, or other products and services. Some carrier pipe can be installed by direct jacking (i.e., one-pass microtunneling).
cased bore: A bore in which a pipe, usually a steel casing, is inserted simultaneously with the boring operation.
casing: A jacked pipe in a two pass system that supports a bore. The casing is not a carrier pipe.
cemented soils: Soils in which the particles are held together by a naturally occurring or man-made chemical agent that changes the soils’ physical and mechanical properties.
charge line: See feed line.
closed-face: Term referring to one mode of operation of a traditional TBM.
cobble: Per ASTM D653, a rock fragment, usually rounded or semirounded, with dimensions between 3 and 12 in. (75 and 305 mm).
cohesionless soils (noncohesive soils): Earth materials containing less than 20 percent soil particles passing the No. 200 sieve. Any free-running soil, such as sand or gravel, whose strength depends on friction between particles.
cohesive soils: Earth materials containing 20 percent or more soil particles passing the No. 200 sieve.
compression ring: A ring fitted between the end-bearing area of the leading pipe bell and the trailing pipe spigot to help distribute jacking forces more uniformly. A compression ring is attached to the trailing end of each pipe and is compressed between the pipe sections during jacking. The compression ring compensates for steering corrections, pipe misalignment, and pipe dimensional tolerances. Also referred to as a packer.
contact grouting: Grout injected into the theoretical space between the jacking pipe and the ground after the drive is completed.
contaminated plume: The underground trace of an environmental pollutant.
contract documents: Documents prepared by the owner and project engineer for bidding and for awarding a project; they can include bid forms, general conditions, special conditions, technical specifications, drawings, geotechnical data reports, and geotechnical baseline reports.
control console: An electronic unit typically inside a container located on the ground surface that controls the operation of the microtunneling machine. As the machine operator drives the tunnel from the control console, electronic information is transmitted to the control console from the MTBM. This information includes head position, steering angle, jacking force, penetration rates, machine-face torque, slurry feed and return flow rates and pressures, and laser position. Some control consoles are equipped with a computer that tracks and stores the data and allows real‑time analysis of the tunnel drive.
controls: Part of the microtunneling system that allows synchronized excavation, removal of spoils, and jacking of pipe to balance forward movement with excavation so that ground settlement and heave are managed.
crossing: Pipeline installation in which the primary purpose is to provide one or more passages beneath a waterway, road, or other surface obstruction.
crown: The highest point of the pipe or tunnel.
cut and cover: See open-cut.
cutability: The capacity of rock to be spalled, ripped, chipped, and broken into smaller pieces to allow the advance of the MTBM. Key properties affecting cutability include the frequency of discontinuities, unconfined compressive strength, tensile strength, hardness, and abrasiveness of the rock.
cutter chamber access: See face access.
cutter wheel: Any rotating tool or system of tools on a common support that excavates at the face of a bore.
delay time: Lost time associated with the owner’s issues.
design specification: A document that establishes specific requirements the contractor must use, including means and methods. A design specification creates an implied warranty that if the contractor uses the specified means and methods along with industry-accepted good practices, the constructed product will meet the specification requirements.
desktop study: A compilation and review of available site, geotechnical, and other information pertinent to a project. Typically performed at the outset of a project, before field explorations and detailed design.
dewatering: The act of removing groundwater or lowering the groundwater elevation, using a system of wells and pumps.
differential cut: The spiral, screwlike pattern created on the excavated surface as the gauge cutter is advanced while being rotated around the bored periphery of the cutter wheel.
direct-jacked pipe: Pipe installed in the ground and serves as the casing and carrier pipe.
discharge line: See return line.
downtime: Time lost when the MTBM is unable to operate; generally associated with equipment failure.
drilling fluid: Water that may contain additives, including bentonite, polymers, soda ash, surfactants, or other materials, to enhance stability and excavatability. If the drilling fluid contains additives and is design to have specific properties, it is considered to be an engineered drilling fluid, whether specified by the engineer or selected by the contractor. Some additives may increase the effectiveness of the drilling fluid and reduce adhesion of the spoils (cuttings). The drilling fluid is used in a closed-loop system for transporting spoils and for counterbalancing earth and groundwater pressures during microtunneling.
drive: Designation of the pipe installed from a jacking shaft to a reception shaft.
drive shaft: See jacking shaft.
earth piercing: The use of a tool that comprises a percussive hammer within a suitable, generally torpedo-shaped casing. The hammer may be pneumatic or hydraulic. The term is usually associated with nonsteered devices.
effective stresses: In a saturated mass of soil or rock, the total stress P minus pore water pressure, Uw.
emergency recovery shaft: See rescue shaft.
engineered drilling fluid: See drilling fluid.
entrance seal: See launch seal.
entry ring: See launch seal.
EPB machine: Earth pressure balance tunneling machine, by which mechanical pressure is applied to the material at the face and controlled to provide the correct counterbalance to earth and groundwater pressures in order to prevent heave or subsidence. The term is not applicable to microtunneling systems for which the primary counterbalance of earth and groundwater pressures is supplied by pressurized slurry.
exit seal: Same as a launch seal except for the retrieval of the machine at the reception shaft. Used in high groundwater and unstable soils to prevent loss of ground.
exit shaft: See reception shaft.
eye: The opening in the shaft wall through which the MTBM is launched.
face: The location where excavation is taking place.
face access: Access to the location where excavation is taking place, typically through the cutting chamber. MTBM configuration may only allow limited personnel access to the back of the cutter wheel from within the pipe string. An air lock may also be required in the rear of the MTBM to maintain air pressure for counterbalancing earth and groundwater pressures.
face pressure: Earth and groundwater pressures applied against the cross-sectional area of the microtunnel face.
factor of safety: The ratio of the structural capacity of a system to expected or actual loads on the system.
feed line: Pipeline that transports drilling fluid from the slurry separation plant to the face of the MTBM. Also known as a charge line.
filter cake: A thin layer of clay or polymer from the slurry at the face and perimeter of the formation being excavated. The filter cake is formed through filtrate loss.
filtrate: A liquid that has passed through a filter.
fines: Portion of a soil finer than a No. 200
(75 mm) US standard sieve.
frac-out: See inadvertent return and hydrofracture.
gauge cut: The outermost projection of the excavation, measured as a diameter. The gauge cut is subject to cutter wear and to the differential cut.
gauge cutter: The cutter that creates the gauge cut and is subject to wear.
gel: Per ASTM D653, the condition of a liquid that has begun to exhibit measurable shear strength.
gel time: The measured time interval between the point when the slurry mix is in a fluid state and the point when it begins to form a gel.
gravel: Per ASTM D653, rounded or semirounded particles of rock that will pass a 3-in. (76.2-mm) sieve and be retained on a No. 4 (4.75-mm) US standard sieve.
ground cover: Distance between the ground surface and the crown of the pipe.
groundwater: All subsurface water, as distinct from water on the surface.
grouting: The process of filling voids or of modifying or improving ground conditions. Grouting materials may be cementitious, chemical, or composed of other mixtures.
guidance system: System that locates the actual position of the MTBM relative to the laser or other device. The laser or other device should be referenced to the design line and grade.
guided boring method: Another term for the pilot tube method, in which an auger boring power unit is used with a pilot tube’s guidance and steering system.
heave: Measurable upward movement of the ground or structure as the result of the excavation process.
horizontal directional drilling (HDD): A surface-launched trenchless technology for the installation of pipes, conduits, and cables. HDD creates a pilot bore along the design pathway and reams the pilot bore in one or more passes to a diameter suitable for the product, which is pulled into the prepared bore in the final step of the process.
HQ: A diamond core wire line tube drill bit size where the hole diameter is 96 mm and the core diameter is 63.5 mm.
hydrofracture: A special form of inadvertent return in which drilling fluid reaches the surface or waterway. Also called a frac-out.
idle time: Lost time when the MTBM is not in operation; not including makeup time or downtime.
in-line microtunneling: A method of replacing an existing pipe by filling the pipe with flowable fill to prevent fluid loss and then excavating the entire pipe and surrounding ground with an MTBM for a new installation.
inadvertent return: The loss of drilling fluid, including slurry and lubrication, from the slurry or lubrication system. A special form of inadvertent return, where the fluid exceeds the strength and confining pressure of the ground and reaches the surface or waterway, is called a hydrofracture or frac-out.
inclinometer: (1) A geotechnical instrument used to measure horizontal deflection transverse to an installed vertical pipe casing by passing a probe through a casing (via two sets of grooves in the casing). (2) An instrument used within the MTBM to measure pitch and roll.
inlet line: See feed line.
interjack pipes: Pipes specially designed for use with an intermediate jacking station.
intermediate jacking station (IJS): A fabricated steel cylinder fitted with hydraulic jacks, which is incorporated into a pipeline between two specially fabricated pipe segments. Its function is to provide additional thrust in order to overcome skin friction and distribute the jacking forces over the pipe string on long drives.
invert: The lowest point of the pipe, tunnel, or shaft.
jacking force: The total force required to overcome the face pressure component and the frictional resistance component along the pipe to allow forward movement of the MTBM and the trailing pipe string.
jacking frame: A structural component that houses the hydraulic cylinders used to propel the microtunneling machine and pipeline. The jacking frame cradles the MTBM and jacking pipes, and serves to distribute the thrust load to the pipeline and the reaction load to the shaft wall or thrust block.
jacking loads: See jacking force.
jacking pipes: Pipes designed to be installed using pipe jacking techniques.
jacking shaft: Excavation from which trenchless technology equipment is launched for the installation of a pipeline. The jacking shaft may incorporate a thrust wall to spread reaction loads to the ground and an entry ring to control inflows of groundwater and soil at the portal.
jacking shield: A fabricated steel cylinder from within which the excavation is carried out either by hand or machine. An articulated steering section and steering jacks are incorporated into the shield to allow it to be adjusted to control line and grade. The term is associated with traditional tunneling.
Jersey barrier: See K-rail.
K-rail: A modular concrete or plastic barrier used to separate lanes of traffic. Also known as Jersey barrier.
laser: A device commonly incorporated into the guidance system and used to track alignment and grade during the tunneling or jacking operation.
launch seal: A mechanical seal, usually consisting of a rubber flange that is mounted to the wall of the jacking shaft. The flange seal is distended by the MTBM as the machine passes through the shaft. The seal is intended to prevent the flow of groundwater, soils, slurry, and lubrication into the shaft during tunneling operations.
lubrication: The act of injecting a fluid, normally bentonite and/or polymers, to reduce the skin friction and jacking forces on the jacking pipe during installation. The fluid fills the annular space.
makeup time: The time spent to add a pipe segment and make connections.
Marsh funnel viscosity: A measure of the flow rate of a fluid through a calibrated funnel. A Marsh funnel is typically used to measure drilling fluid and slurry viscosity, in seconds per quart (sec/qt) of fluid. The Marsh funnel viscosity of water is 26 sec/qt.
maximum allowable jacking force: The largest jacking force that can be applied to the jacked pipe, allowing for an appropriate factor of safety.
maximum anticipated jacking force: The largest anticipated jacking force required to advance jacking pipe and machine along a drive.
microtunneling: A construction method to install pipe into the ground with no imposed size limitations provided the following are used during construction:
mixed face: An interface within the excavated tunnel zone between two geological units that have a significant contrast in engineering properties(e.g., rock overlain by soft ground, or very soft, low-strength soil overlain or underlain by a very stiff, high-strength soil).
mixed ground: The excavated tunnel zone, without a defined interface, that contains soil and coarse fractions of gravels, cobbles, and boulders (i.e., silt with boulders). This term is not currently defined by ASTM or USCS.
mixed reach: A significant change in ground type, characteristics, or behavior along the excavated tunnel between the jacking and receiving shafts. This term is not currently defined by ASTM or USCS.
muck: Spoils or removal of same.
N-value: The number of blows per foot required to drive a standard penetration test (SPT) soil sampler into the ground during geotechnical exploration. The N-value is often used to estimate in situ consistency of clays and silts and relative density of sands.
noncohesive soils: See cohesionless soils.
NQ: A diamond core wire line tube drill bit size where the hole diameter is 75.7 mm and the core diameter is 47.6 mm.
obstruction: Any buried object that lies completely or partially within the cross section of the microtunnel and that impedes continued forward progress along the design path and within allowable tolerances.
one-pass: A procedure by which the carrier pipe is directly jacked into the ground without a casing.
open-cut: A method by which access is gained to the required underground level for installation, maintenance, or inspection of a pipe, conduit, or cable. The excavation is then backfilled, and the surface is reinstated.
open-face: Term referring to one mode of operation of a traditional TBM.
overcut: The theoretical difference between the radial measurement of the gauge cut and the MTBM shield; equal to (gauge cut OD – MTBM OD)/2. Actual overcut is reduced as the gauge cutter is worn and because of the differential cut.
over excavation: A process of excavating more material than the theoretical volume of the tunnel based on diameter and advanced distance. Over excavation can lead to ground settlement and the formation of voids.
packer: See compression ring.
penetration rate: Instantaneous excavation distance per time while the MTBM is operating, typically measured in inches per minute or millimeters per minute.
perched water: An accumulation of groundwater that is above the regional groundwater table in an unsaturated zone.
performance specification: A document that establishes the performance criteria the constructed product must meet. A performance specification leaves the selection of means and methods to the contractor.
piercing tools: See earth piercing.
piezometer: A specialized monitoring well that provides discrete screens and underground seals to produce a measurement of the groundwater pressure at specific intervals below ground. The groundwater pressure may be measured using open standpipes or vibrating wire transducers.
pilot tube method: A multistage method of accurately installing a product pipe to line and grade by use of a guided pilot tube followed by upsizing to install the product pipe. The pilot tube method does not fit within the definition of microtunneling.
pipe brake: A mechanical device designed to prevent the MTBM and pipe string from moving back into the shaft.
pipe eating: See in-line microtunneling.
pipe jacking: A system of using hydraulic jacking from a drive shaft to directly install pipes behind a shield machine so that they form a continuous string in the ground.
pipe lubricant: See lubrication.
pipe ramming: A trenchless installation method whereby a percussive hammer is attached, via an adapter, to an open-end casing, which is then driven through the ground. To create an open casing, the spoils within the casing are removed after the drive is completed, or periodically during the drive.
pipe string: The succession of joined individual pipes being used to advance and support the excavation.
pitch: The upward or downward angle of the MTBM, measured from a theoretical horizontal plane passing through the longitudinal axis of the MTBM.
plastic viscosity: A measure of the internal resistance to fluid flow, expressed as the tangential shear stree in excess of the yield stress divided by the resulting rate of shear.
plowing: A condition where the MTBM is being steered in one direction, yet continues along a undesired direction.
pore water pressure: The pressure of the groundwater held within a soil or rock: in gaps (pores) between particles.
potholing: Small, carefully dug, nonmechanized excavation, used to locate a utility or other subsurface feature.
product pipe: See carrier pipe.
push ring adapter: Mechanical component mounted on the thrust ring to prevent the thrust ring from coming into contact with and damaging the pipe collar.
receiving shaft: See reception shaft.
reception shaft: Excavation into which the microtunneling equipment is driven and recovered.
rescue shaft: An unplanned additional shaft required to remove obstacles/obstructions and/or retrieve or repair the MTBM. The rescue shaft may need to function as a jacking shaft to complete the drive.
return line: Pipeline that transports slurry from the face of the MTBM to the slurry separation plant. Also known as a discharge line.
rider: A shallow collection line that runs along the microtunneled trunk sewer to a drop manhole.
rock: Per ASTM D653 and ISRM, any naturally formed aggregate of mineral matter occurring in large masses or fragments.
roll: The angle of rotation about the theoretical longitudinal centerline of the MTBM, measured from the 12 o’clock position.
settlement: Measurable downward movement of the ground or of an overlying utility or other structure as the result of excavation or dewatering.
settlement point or marker: An instrument installed at ground surface or driven into the ground to monitor vertical ground deformations before, during, and after construction.
shield cut: The theoretical difference between the radial measurement of the MTBM shield and the jacking pipe equal to (MTBM OD – pipe OD)/2.
skinning: The act of fitting a steel cylinder over the MTBM shield body to increase the diameter of an existing MTBM with the use of an appropriately upsized cutter wheel. Also referred to as upsizing.
slurry: A mixture of drilling fluid and spoils.
slurry chamber: A chamber in which excavated material is mixed with slurry for transport through the return line to the separation plant at the surface. The slurry chamber is located behind the cutter wheel of a slurry microtunneling machine.
slurry lines: Parallel hoses or pipes that transport spoils and slurry from the face of a slurry microtunneling machine through the return line to the ground surface for separation, and then return the slurry to the face for reuse through the feed line (the feed line is also known as a charge line).
slurry separation: A process in which excavated material is separated from the circulation slurry. Mechanical separation is typical, although gravity separation in pits or tanks is possible with coarse-grained soils.
spacers: Mechanical structures used to transfer the jacking force from the jacking thrust ring to the pipe and to accommodate lengths of pipe that are longer than the stroke length of the jacks.
specials: Pipe sections immediately ahead of and behind the IJS. Specials have ends that are specifically manufactured to physically accommodate the IJS.
spoils: Earth, rock, and other materials excavated during the installation process. Also referred to as cuttings.
sump pump: A device placed at the base of a shaft or in an MTBM to collect and remove fluids incidental to the construction process and resulting from shaft leakage, and to prevent the excavation equipment from flooding.
surface water: Any water encountered or collected at ground level.
thrust block: An engineered structure, located between the jacking frame and the shaft wall, that distributes the jacking force developed by the hydraulic jacking frame over a large surface area to the ground behind the back wall of the shaft.
thrust ring: A fabricated ring that is mounted on the face of the jacking frame. It is intended to transfer the jacking force from the jacking frame to the thrust-bearing area of the pipe section being jacked.
tooling: Ground-engaging elements of the cutter wheel designed to excavate and penetrate the ground.
top hat: A steel can with a faceplate bolted to a reception shaft wall to receive the arriving MTBM.
trailing can: A principal module that is part of a shield machine such as an MTBM or tunnel boring machine (TBMs); its use depends on the installation dimensions required and the presence of an articulated joint to facilitate steering.
trenching: See open-cut.
trenchless technology: A family of construction techniques for installing or rehabilitating underground infrastructure with minimal disruption to surface traffic, businesses, and residents. Also includes technologies for inspection, leak location, and leak detection with minimal disruption and minimal excavation from the ground surface.
tunnel horizon: The vertical band within the ground where the tunnel is excavated. Includes one tunnel diameter above the crown and one tunnel diameter below the invert.
two-pass: A procedure by which the carrier pipe is installed within a jacked casing.
unanticipated return: See inadvertent return.
upsizing: Process of creating a larger diameter MTBM than a machine with the original factory configuration. Also referred to as skinning. Also, any method that increases the cross-sectional area of an existing pipeline, replacing it with a larger cross-sectional area pipe.
utility monitoring point (UMP): An instrument designed to measure movement of an individual utility when exposed to construction activities.
utility tunneling: A construction method for excavating an opening beneath the ground without continuous disturbance of the ground surface. The excavation is of sufficient diameter to permit personnel access, and allow excavation, transport of spoils, and erection of a ground support system.
waler: Beam used within a braced excavation as support to control wall deflections.
water jetting: Process of using the internal cleansing mechanism of the cutter head, by which high-pressure water is sprayed from nozzles to help remove cohesive soils.
yaw: The angle of the MTBM to the left or right of a theoretical vertical plane passing through the MTBM’s longitudinal axis.
zone of influence: Volume of ground that could possibly be affected by settlement or heave from tunnel mining, shaft excavation, pile driving, or other construction activities.
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