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March 2018

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Microtunneling is a technique used to make small diameter tunnels to install pipes by jacking method. Due to the small tunnel diameters (150 mm to 3600 mm), the tunnel machine is controlled remotely from the surface. Keep the tunnel diameter ½ inch to 4 inches bigger than the required diameter to decrease friction force while pushing the pipes. Bentonite slurry is filled in the annular space between the tunnel and the liner to act as a lubricant and to prevent the collapse of tunnel inner edge.

Applications of Microtunneling

  • Utility pipes in congested areas like water, sewer, oil & gas, electrical etc.
  • Highway, river and railway crossings
  • Deeper installations

Benefits of Microtunneling

  • No disturbance to traffic flow.
  • Construction possible in all weather conditions.
  • Laying pipes with an alignment that is curvature
  • Savings in construction costs.
  • Low environment impact

Microtunneling Procedure

Excavate adequate size pits to the required level for drive and reception shafts. The pit size should accommodate the tunneling equipment and provide for safe operation.
Make a concrete base slab foundation to support the tunnel equipment.
Provide a thrust wall as per the tunneling machine requirement. The base slab and thrust walls should able to withstand the expected forces of tunnel operation.
Install entrance ring at the entry wall through which microtunnel machine can pass
Install hydraulic jacking frame at a required level
Install equipment like the mobile crane, pumps, bentonite mixing arrangements, pipes etc.
Lower the tunnel machine into the drive shaft and set-up guidance system
Make all connections like electrical, electronics, communication, slurry, hydraulic, pumps, pipes etc.
Conduct pre-operation checks and do the test run
Fix the laser in the pit bottom aligned with horizontal and vertical grade.
Start microtunnel machine operation. The hydraulic jacks thrust forward the tunnel machine through the strata.
Stop the excavation once the machine reaches the required position to place a pipe. Disconnect all connections and lower the pipe to be installed on to the jacking frame. Reconnect the cables and pipes again. Jack the pipe and machine forward.
Examine the alignment regularly
Repeat the steps of pipe jacking, disconnection of cables & hoses, lowering the pipe, then reconnect cables & hoses and jacking the pipe and machine forward until the tunnel machine exit the ground into the reception shaft.
Monitor groundwater and soil pressures, cutting head torque, slurry pressure continuously and control the slurry and feed flow. Always maintain the head pressure to withstand soil pressure to prevent abrupt settlement or uplift.
On completion of the jacking operation, inject cementitious grout to substitute the bentonite in the annular space between tunnel inner face and liner outer face. The grout is injected through the injection ports installed on the pipes.
Remove tunnel machine, jacking frame, and all other equipment from driving and reception shafts.

Bentonite Slurry:

The bentonite suspension serves as a lubrication agent and as a way of support during pipe jacking process. The slurry is pressed outwards through injection nozzles in the backup pipe and fills the space between overcut and pipe. The liquid film around the pipe reduces the friction between the pipe casings and as a result also the tunneling force required. For tunneling in loose soil, the bentonite suspension acts as a support to avoid subsidence.

Quality Assurance and Quality control

Pipe inspection:

Inspect all pipes visually for any cracks and deformities.

Bentonite slurry:

Monitor and check continuously the density and viscosity of the bentonite mix. Ensure the parameters are within specified range.

Grout compressive strength:

Make a set of three (3) compressive strength test specimen for every line and check for the compressive strength.

Settlement monitoring:

Mark the grid monitoring points at 2.0 meters center-to-center. Fix pins/mark with paint spots and take the reference levels. Check levels at the monitoring points after the construction of the pit, prior to the commencement of tunneling works, after completion of the line. Evaluate the readings of the surface levels at the grid points and observe for any settlement.

  • All equipment shall be inspected, calibrated & certified
  • Follow pre-operation checklist before commencing the jacking activities and QC inspection checks during operation.

Health, Safety, and Environment

  • Carry all activities with due regard to the health and safety of all workers and staff including third parties. Maintain all equipment in a safe condition and operate in a safe manner.
  • Don’t allow any person in the close vicinity of the job site without the specific permission and knowledge of authorized persons. Inform other parties employed on the site of the dangers relating to the microtunneling operations.
  • Provide suitable access and egress as required to allow safe movement of the equipment without injury to persons or damage to property.
  • Store all oxygen and acetylene cylinders in the open air and secure suitably to prevent unauthorized usage. Place all cylinders upright. Fit all cylinders with suitable regulators, pressure gauges, and flashback arrestors when in use.
  • Deliver all substances to site with a relevant Material Safety Data Sheet.

Existing utilities:

Detect and mark out on site any high voltage cables, signal and communication cables, drainage, etc.

Emergency procedure:

Display emergency contact numbers clearly at the site. In case of an accident or emergency, the qualified first aider will administer first aid. If a more serious accident occurs, contact emergency contact numbers.

Personnel Protective Equipment

  • Brief all site personnel on Personnel Protective Equipment requirements.
  • All site personnel shall wear safety equipment such as coveralls, hard hats, safety footwear, etc. All personnel is to be aware of the site safety plan and are to comply with its content.
Personal Protective Equipment.When to wear?
Safety helmet.Mandatory at all times.
Safety shoes.Mandatory at all times.
High visibility vest.Mandatory at all times.
Safety hand gloves.Mandatory at all times.
Ear protection.During tunneling operation.
Eye protection.During tunneling operation.
Welding helmet and mask.When welding/hot cutting.
Full body harness.When working at height.
Other PPE.As required.

Featured image credits: By Sabino Freitas Correa [CC BY-SA 4.0 ], from Wikimedia Commons