{"id":6680,"date":"2026-10-06T05:59:16","date_gmt":"2026-10-06T05:59:16","guid":{"rendered":"https:\/\/www.pipetechservice.com\/?p=6680"},"modified":"2026-10-06T05:59:19","modified_gmt":"2026-10-06T05:59:19","slug":"bentonite-and-lubrication-systems-in-microtunneling","status":"publish","type":"post","link":"https:\/\/www.pipetechservice.com\/es\/bentonite-and-lubrication-systems-in-microtunneling\/","title":{"rendered":"Bentonite and Lubrication Systems in Microtunneling: A Practical Guide"},"content":{"rendered":"

Bentonite lubrication (also called annular or pipe-jacking lubrication) is the practice of filling the annular gap between the pipe string and the ground with a bentonite-based suspension so that jacking friction stays low enough to reach the reception shaft. It does two jobs at once: it lubricates the pipe-ground interface, and it supports the surrounding soil so the annulus does not close onto the pipe. Here the suspension injected into the annulus is the lubricant; the circulating fluid that supports and clears the face is the slurry, and the two are specified separately.<\/p>\n\n\n\n

This guide is for owners specifying the works, contractors preparing a bid, and the engineers and supervisors who run the drive. It covers the practical decisions in order: how the annular gap and overcut are dimensioned, why the suspension behaves differently in silt and in gravel, what the friction numbers are worth and where they come from, how the suspension is mixed and swollen, where it is injected along the drive, why the entry seal decides whether any of it works, how lubrication interacts with face support, and what to record on every push so a failing film is detected before the jacking force rises.<\/p>\n\n\n\n

\"Bentonite
Figure 1. A lubrication spread on a microtunneling site: mixing and holding tanks, injection pump and the lines that feed lubricant into the annular gap at intervals along the pipe string.<\/figcaption><\/figure>\n\n\n\n

What Bentonite Lubrication Actually Does<\/h2>\n\n\n\n

The annulus created by the cutterhead or shield has to be filled with bentonite immediately, because bentonite lubrication reduces skin friction both as a lubricating film and by supporting the surrounding soil.<\/p>\n\n\n\n

Bentonite lubrication is specified as a fluid problem rather than a pumping problem. The pump, the injection points and the control regime only deliver the fluid; whether the drive benefits depends on whether the suspension’s properties match the ground it is injected into.<\/p>\n\n\n\n

\"Cross-section
Figure 2. The annular gap and its injection points. The gap has to be created by the cutterhead or shield, filled immediately, and then maintained for the length of the drive.<\/figcaption><\/figure>\n\n\n\n

Dimensioning the Annular Gap: Overcut Is a Design Decision<\/h2>\n\n\n\n

The annular gap, also called the annulus, is created by the overcut, and overcut is a design decision. The larger the outside diameter, the larger the overcut.<\/p>\n\n\n\n

Overcut is set by three inputs in order: the design rule for the pipe diameter (about 5 mm to 50 mm by radius, small end for roughly 300 mm OD and large end for about 3,000 mm OD), the specification cap where one applies (30 mm or 2% of pipe outside diameter, whichever is smaller), and the steering room the alignment actually needs.<\/p>\n\n\n\n

Project-specific boundary conditions always enter the decision: cover depth, permissible settlement, pipe length, bends, and the deviations the control system will inevitably produce. Some specifications impose a firm cap \u2013 a published water-industry specification requires the overcut of the tunneling shield not to exceed 30 mm or 2% of the pipe outside diameter, whichever is smaller.<\/p>\n\n\n\n

Applies to rigid jacking pipe driven in soil on a controlled line and grade. Not applicable where the drive is in rock, or where the alignment needs a curvature the pipe string cannot follow: there the annulus is set from the drive design and confirmed against the actual ground and pipe.<\/p>\n\n\n\n

Whether the annulus is cut by the machine itself \u2013 usual for slurry machines \u2013 or by a shield with an internal cutterhead is a project decision, and the internal cutterhead alternative is worth discussing where material is strongly rolling. An overcut that is too small can stop the jacking altogether or damage individual pipes: because every controlled drive deviates from the nominal axis, a generous annulus gives the string room to follow the machine’s steering without being forced through the cavity. In rock drives the damage pattern is characteristic, with radial cracks forming approximately in the middle of a pipe.<\/p>\n\n\n\n

Conclusi\u00f3n:<\/strong> the gap has to be wide enough for the deviations the control system will actually produce, not just for the nominal pipe outside diameter. A bentonite lubrication system can only work with a gap to fill, which is why the overcut is settled before the fluid specification.<\/p>\n\n\n\n

Why Lubrication Works: Ground Behavior and the Friction Numbers<\/h2>\n\n\n\n

The same bentonite suspension supports the annulus by two different mechanisms depending on the pore size of the ground, and knowing which mechanism is doing the work determines how the fluid should be specified.<\/p>\n\n\n\n