Safe Utility Tunnelling Through Tailor-Made Hard-Software

The planning and execution of pipe jacking projects is based on the regulations in the currently valid DWA worksheets A 125 "Pipe Jacking and Related Methods" of December 2008 and A 161 "Static Calculation of Jacking Pipes" of March 2014. In particular, the formulations in A 161 completely dispense with unnecessary, absolutely formulated restrictions (for example, the curve radius or the jacking force) and even provide information on the necessary static proof when using alternative pressure transfer concepts other than the conventional wooden pressure transfer ring.

Static jacking support (Monitoring of joint gap differences)

On this basis, so-called pipe joint monitoring systems have been established (e.g. CoJack from STEIN Ingenieure), which now do much more than just monitor the pipe joints. Rather, these systems serve as a continuous static control instrument during jacking to determine the current stress on the jacking pipes and even enable a constantly updated forecast of the steering movements and jacking forces that will just be permissible in the future.

CoJack's hardware includes a sensor system installed at the construction site for continuous and complete recording of the most important jacking data. In a protected area of the internet, the course of the jacking force and the course of the actually jacked route curvature (scheduled curves including steering movements) are graphically displayed. The corresponding diagrams also contain the corresponding current limit values, which are continuously confirmed or updated in the background by means of static calculations accompanying the jacking process. 

Thus, the observer can always judge at first glance whether the current jacking parameters are within the permissible range. Dangerous tendencies can be detected at an early stage so that countermeasures can be taken in time. This illustration of the otherwise mostly completely invisible static stresses on the jacking pipes is not only used by the Employer and construction supervisors as a control instrument. The jacking companies carrying out the work have also come to appreciate and even learn to use the transparency of the jacking data.

They can use CoJack to prove that the permissible pipe stresses are being adhered to and, if necessary, have a statically secure basis for optimising their jacking progress. CoJack provides information as to whether higher jacking forces are permitted if necessary or with which force the jacking may be continued immediately after an excessive steering movement. The safety level prescribed in the regulations is always maintained.

The optional measurement and display of the jacking force not only at the main jacking station, but also at all installed intermediate jacking stations, contributes to this. This even works when the intermediate jacking stations are not actively used at all and are only passively pushed along. For this purpose, the jacking cylinders have to be extended just a few centimeters.  Thus, the jacking force applied by the main jacking station is transmitted within the passive jacking cylinders of the intermediate jacking station via the oil cushion (see Figure 1).

By observing the jacking force progression, it is possible to localise jacking sections with increased jacking resistance if necessary. An additional help for the interpretation of the jacking force diagrams is the representation of the jacking cylinders expansion of the intermediate jacking stations, which allows a local and temporal allocation of the intermediate jacking stations activities.

Lubrication and measuring systems

If the positions or the sections with increased resistances are located in this way, modern lubrication systems can condition the grouting of the annular space in a very specific way. Not only with a sense of proportion and gut feeling, but also volume-controlled and/or pressure-controlled, a defined quantity of the lubricant and supporting agent (bentonite) can be introduced into the annular gap between soil and jacking pipe at each lubrication station (position in the pipe string) or even at each lubrication outlet.

If the grouting is carried out sufficiently slowly, i.e. with sufficiently low flow rates and speeds, the injection pressure at the lubrication outlet can even be estimated via the pressure transducer located above ground.

In recent years, quality-enhancing further developments have been made not only in lubrication technology but also in measurement technology. Although the basic techniques of measurement for machine control by means of laser or gyro compass have remained the same, the associated software has developed in great strides. For example, modern surveying systems can now be set up by remote control, and remote maintenance from the manufacturer's office avoids unnecessary journeys.

The greatest advantage is the reduction of downtimes, which on the one hand represent a large cost factor and on the other hand can lead to technical problems in the form of increased jacking resistance when restarting.

In addition, software products are available on the market which not only document the data mentioned in Worksheet DWA-A 125, Section 7.2.6, in tabular form in compliance with the regulations in printable form, but also prepare them and display them in clearly understandable form online in a graphic format. CoJack covers the statically relevant part of this data and additionally compares the measurement data with the permissible data in a graphical form. This also allows the observer who does not necessarily want to delve into the finer details of pipe statics to draw the right conclusions from the measurement data.

The same algorithm allows CoJack to bypass the standard approaches of the DWA-A 161 for jacking with several curves, which are far on the safe side, without violating the regulations of the A 161. If no more precise proof is provided, the rules and regulations state that a heavily preloaded pressure transfer ring must be assumed from the second curve onwards, so that the calculated permissible jacking force often falls to a painfully low value. Exceptions are permitted when using more precise methods.

Such a method is CoJack, which is in full compliance with the rules and regulations, but usually - due to the internal upstream determination of a jacking specific load history - allows significantly higher jacking forces. This sometimes makes the second curve in the route possible in the first place. It is pleasing that the static safety is not reduced by this, but even increased by the more precise calculation.

The hydraulic joint

In contrast, the latest developments in pressure transfer means for the transfer of jacking force from pipe to pipe can be classified as pure hardware products. The aim is to replace the transfer of dynamic forces by means of a "crushed" chipboard ring, which seems rather anachronistic to today's engineers, with a modern method. Most pressure transfer rings made of different materials failed due to their suitability or cost and have been used in practice only sporadically so far.

Only the hydraulic joint, on the other hand, is becoming increasingly important and, after overcoming teething troubles, is a technical and often also economic alternative, if one compares the higher direct costs for the joint itself compared to the wooden pressure transfer ring with the indirect savings (longer jacking pipes, higher jacking forces, faster installation, more diverse possibilities for line management).

The jacking force is transferred very elegantly via a water cushion enclosed in the hydraulic hose. Since liquids have the unique property of exerting the same pressure at every point and in every direction, the pipe end faces are subjected to the same pressure all around, even in tight bends, which can also be calculated quite easily depending on the jacking force. The stress on the pipes is therefore

• uniform without local stress peaks,
• easy and precise to calculate
• largely indepedent of joint bending (cornering).

In addition, the hydraulic joint has no "memory" and, unlike the wooden pressure transfer ring, is not adversely affected by an unfavorable load history (e.g. high tensions in previous curves). Since the static calculation of the pipe stresses contains fewer imponderables, the security with regard to overstressing is greater.

The latest developments in hydraulic joint manufacturing even allow a technically and economically interesting dual function of the hydraulic joint (TuSo HD-Sealing system).  In the jacking phase, the hose has the described function as a pressure transfer medium. After completion of the jacking in the operating phase, the hose also takes over the function of the internal joint seal, thus saving the installation of a complex caulking seal, which would have to be tailor made and correctly installed for each joint.

The hydraulic hose alone does not constitute a seal due to its surface and hardness, but a coating with an additional sealing hose made of softer elastomer prevents the penetration of the waste water. Its hardness is optimised in such a way that on the one hand it transmits the jacking forces undamaged via its wall, but on the other hand it clings to the concrete surface with such a positive fit that it fulfils the function of an internal joint seal (also known as an internal seal). The necessary contact pressure is applied solely via the jacking force.

Optionally, once the pipeline has been immovably integrated into the shaft structure, a defined pressure can be set and documented in the hydraulic hoses. A pressure control and readjustment is possible at defined intervals in order to adjust the jacking pipe position to any changes in the situation.

In this way, the appropriately conditioned hydraulic joint not only increases the static safety during jacking, but also the safety with regard to corrosion of the pressure transfer ring.

In addition, the hose can of course also serve as an internal abutment for grouting processes. However, it must be remembered that the grouting material introduced will more or less impair the articulation of the pipe connection, depending on its rigidity. The introduction of a cement-bound grout leads to a complete stiffening and thus to an undesirable rigid pipe string.

Any small subsequent ground displacement, for example as a result of adjacent construction measures, changes in the groundwater level or mining-related subsidence, leads to uncontrolled damage in the form of cracks or spalling on the pipes, so that the already self-sealing hose without grouting represents the safer solution.

Summary

In recent years, numerous new and further developments have contributed to a significant increase in static safety in the area of pipe jacking, although the number of pipe jacking sections that can be classified as difficult in the tenders of network operators is increasing. The willingness to carry out measures in difficult ground, with long lengths and daring routes with S and space curves has increased. Using new methods of monitoring, online statics, pressure transfer methods and improved methods of lubrication and surveying, these jacking projects are often mastered without major problems in combination with the extensive experience of the jacking companies.

The most far-reaching innovations that both the clients and the construction companies first had to familiarise themselves with were the joint monitoring programs (static monitoring systems) as software and the hydraulic joint as hardware. In the meantime, these applications, which were initially viewed with suspicion, have become widely established. Table 1 shows under which conditions the hydraulic joint should be used for a safe jacking project and when a static jacking support (for example with CoJack) is sufficient. Furthermore, it remains to be seen to what extent the increase in transparency regarding the jacking data, which is highly esteemed by all sides, will continue.

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