Safety First: Entrance Ring and Rubber Seal in Microtunnelling
If you want to go home safely,you must be able to walk
In microtunnelling, there is only one golden rule — Safety First.
In my previous post, we discussed Setting the Jacking Frame, which is the foundation of any successful drive.
Today, we will focus on another extremely critical component:
Entrance Ring and Rubber Seal
and why they are so important in microtunnelling operations.
🔹 Why Entrance Ring & Rubber Seal Are Critical
The Entrance Ring and Rubber Seal work as a protective barrier that prevents water, sand, and soil from flowing into the shaft.
If this system is missing or poorly installed:
Water flow control will fail
Slurry water will escape through the shaft
Jacking operation may become impossible
More importantly, if water comes out together with sand or soil, serious problems can occur:
Sudden increase in jacking pressure
Formation of voids around the shaft
Under pressure, soil from above may collapse
The shaft can get filled with water, sand, and soil
High risk of machinery damage, especially electrical equipment
I will add a photo below showing such a situation.(Photo No :05)
From this, you can clearly understand how important the Entrance Ring really is.
🔹 Installation of the Entrance Ring
1️⃣ Center Point Alignment
The Entrance Ring must be installed exactly on the center point.
Measure carefully from the alignment center
Ensure equal clearance on all sides
No side should have less space than the others
The first photo clearly shows proper centering.
2️⃣ Proper Fixing and Support
Once positioned correctly:
•Secure the ring uniformly on all sides
•Use welding or wall plugs as required
•The ring must be stable and immovable
Any movement at this stage can create major problems later.
🔹 Breaking / Chipping the Shaft Wall (Very Important)
Before starting breaking or chipping, never rush.
⚠️ Mandatory Check Before Breaking
Before breaking the wall, you must:
Drill small holes to check:
•Water pressure
•Amount of sand coming with water
If:
Water pressure is high
Sand flow is excessive
👉 Grouting must be done first to stabilize the ground.
Dry or controlled conditions are always safer.
❗ If you break the wall without checking, water, sand, and soil can rush in suddenly, causing immediate failure.
🔹 Correct Method of Breaking / Chipping
As shown in the photo: (no:2)
Break the wall along the inside edge of the Entrance Ring
•Ensure:
•No concrete projections remain
•No steel bars or reinforcement are left exposed
The surface must be clean and smooth.
🔹 How Much Space Should Be Cleared?
The required clearance depends on:
•Cutter disc diameter
•Cutter length
•Rubber seal thickness
✅ Example:
If the cutter disc requires 100 mm clearance,
you should clear at least 120 mm to ensure:
•Smooth cutter rotation
•No obstruction
•Safe and comfortable entry
🔹 Sealing All Gaps Properly
As shown in the image:
Seal the entire circumference using cement or grout
Ensure no empty space remains
Even small gaps can cause:
•Water leakage
•Soil movement
•Long-term operational failure
This step must not be ignored.
If the Entrance Ring and Rubber Seal are not installed properly, water can suddenly rush into the shaft along with soil and sand.
This may completely flood the pit and make further operation impossible.
Before attempting a second break or restart, never try to clean the soil or sand directly.
Grouting is mandatory first to stabilize the ground and control water pressure.
If grouting is not done, soil, sand, and water will continue to flow uncontrollably, causing repeated collapse and serious risk to machinery, especially electrical components.
🔹 Mandatory Safety Precautions
⚠️ Always wear full personal protective equipment, especially:
•Safety goggles (very important during breaking/chipping)
•Helmet, gloves, and protective clothing
🚨 Final Safety Warning
During the final stage of breaking, extreme caution is required.
When the concrete layer is fully removed and you reach the soil layer:
•Soil may suddenly collapse
•Water and sand may rush in without warning
•Always stay alert and prepared.
✅ Final Words
A correctly installed Entrance Ring and Rubber Seal may not look impressive,
but it is one of the most important elements of a successful microtunnelling drive.
Even a small mistake here can lead to major failures later.
If you have any questions about Entrance Ring installation or face problems on site, feel free to comment below. I will share solutions based on real field experience.
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