The key to curved microtunnelling

Jackcontrol AG is an expert in trenchless construction methods having successfully constructed more than 100 km of tunnel worldwide.

The company’s Hydraulic Joint is a pressure transmission ring and force sensor that allows makes curved drives easier than ever before.

It is undeniable that microtunnelling has proven its efficiency worldwide, indirectly recognisable by the fact microtunnelling projects are getting more complex in usually busy urban areas. While straight drives are generally preferred, curved drives are no longer the exception and a pressure transfer ring that optimises the permissible jacking forces between the prefabricated reinforced concrete pipes promotes performing these curved drives even more.

The Hydraulic joint.

Advantages of the Hydraulic Joint

Despite the range of 400 mm as one of the smaller inside diameters (ID) in microtunnelling, the curved drives are first starting at 1,000–1,200 mm ID. The maximum diameter of the reinforced concrete pipes (RCP) for pushing forward the excavating machine is still limited to a range of 4,000 mm ID due to logistical reasons.

Because there are always irregularities at the pipe-ends, concrete on concrete directly would cause point loads and the RCPs would quickly crack. A softer pressure transfer medium between the RCPs is needed, which in most cases is a wooden packer.

The problem that arises with curved drives is the average contact surface between the RCP’s reduces in proportion to the radius of the curve. The permissible jacking force on the RCPs is directly linked to the contact surface.

Pressure transferring of the Hydraulic Joint versus a wooden packer.

To tackle this problem, the Hydraulic Joint was developed at the ETH Zürich University in the early 2000s. The Hydraulic Joint is based on the law of communicating vessels and constructed out of a hydraulic line filled with liquid.

These hydraulic lines can be used as a pressure transfer medium between the RCPs and dimensioned for jacking forces up to thousands of Kilonewtons in accordance with each project.

Because the liquid is trapped in the hose, the liquid will distribute itself among the concrete surfaces under equal pressure. This results in a maximum pressure transfer area even in curves where a wooden packer is limited.

This consequence, combined with the other advantages of the Jackcontrol packages, leads to a new generation of curved drives. This next generation allows operators to design projects that were almost unthinkable until a few years ago.

As an example, in 2019 contractor Bessac could eliminate a difficult planned shaft in Auckland, New Zealand by combining two drives with a tight S-Curve; the total length of the combined drives was 423 m. A 450 m and 150 m radius curve, with pipe dimensions 2,000 mm ID and 2,380 mm OD and 2 m pipe lengths.

The situation plan for contractor Bessac in Auckland, New Zealand.

The Jackcontrol System has proven itself to be superior in more than 260 projects over the last 15 years and is being further developed. Additional strengths of the Hydraulic Joint are the optimum quality assurance because of its limitation of the lateral forces, which is also integrated in the permanent monitoring system.

If the economic benefits of the use of longer jacking pipes in tighter curves is also brought into account, the monitored Hydraulic Joint is the key to successful curved microtunnelling. Not only for an optimised project, but especially for a more manageable project.

For more information visit the Jack Control website.

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