In the jacking process, the jacking forces applied by the jacking station must be transmitted from pipe to pipe via non-restrained joints. In order to also ensure a sufficiently large force transmission surface for angulations, pressure transfer rings are inserted. However, their load-distribution effects decline continuously during jacking as a result of the large plastic deformation thus continuously increasing the load on the pipe.

Although this phenomenon has been known for some years, this is only accounted by an empirical increase in the modulus of elasticity, which does not provide sufficient safety for long jacking distances. With the pipe jacking simulation program ‘CoJack’ it is now possible to calculate the actual stress of jacking pipes during the entire jacking process.

Statics problem

The central problem in the transfer of jacking forces from pipe to pipe is that, especially during steering movements and curved paths, angular deflections occur in the joints and thus a so-called gaping joint develops. The pressure transfer area becomes smaller with the effect that the contact compressive stress increases. This undesired effect is usually moderated by utilising pressure transfer rings (PTR) of wood or wooden materials, which are deformed under the stress due to the low stiffness and which abut on both pipe faces. They enlarge the contact surface and thus act in a stress reducing manner (see Figure 1).

However, due to the large forces, the wooden material is loaded far beyond its limit of elasticity. Thus, not only elastic, but also plastic compressions that are not reversible occur and also remain after the removal of the load. In the course of a jacking process, each pressure transfer ring is subjected to numerous applications and releases of force, during which the angulations change constantly. The ring increasingly loses its load-distributing effect and ‘hardens’.

Previous calculation methods

So far in the common calculation methods this non-linear hardening behaviour has only been taken into account in a very simplified manner by a constant modulus of elasticity for the pressure transfer ring, which as a consequence has to be chosen far on the safe side due to its high inaccuracy.

Thus, the current structural calculation of the jacking pipes only represents a ‘snapshot’ of the jacking, in which exclusively one particular angular deflection of the pipes, one jacking force and especially only one stiffness of the pressure transfer ring is taken into account in the form of the modulus of elasticity. The time- and load-dependent, highly non-linear material behaviour of the pressure transfer ring remains completely ignored. As a result, the actual stress of the pipe concrete and thus the actual safety level remains fully unknown during the entire jacking procedure. The results are either very short measured jacking sections, which unnecessarily add to the costs of the construction measure, or an increased risk of overstressing the pipes.

Despite the prescribed acquisition and recording of the most important jacking parameters in the standards and regulations, a constant updating of the structural calculations with correction of the permissible jacking force (increase or decrease) that takes into account the specific characteristics of the pressure transfer ring cannot be carried out.

The described problem is often the main cause of damage or destruction during jacking.

Spalling at the external pipe face occurs particularly often, since the stress is usually highest at this point, but the concrete cover in particular is unable to take up overstresses and splinters. This typical damage is often invisible from the inside of the pipes, but is a decisive influence on the durability of the entire construction. Therefore, within a short period or a few years, even severe damages are likely to occur as a consequence.

The new program – CoJack

With the new calculation program CoJack developed by Prof. Dr. -Ing. Stein & Partner GmbH a practical and powerful instrument is now available that increases the safety and economic efficiency of pipe jackings. Among other things, for the first time it takes into account the decisive input parameters which have a great influence on pipe stress, but which have been ignored in the previous methods of calculation:

• Non-linear stiffness behaviour of the pressure transfer ring, which can be determined individually for any jacking project by means of a laboratory experiment;

• The loading history of the pressure transfer ring for each point of time in the jacking;

• Changes in the stiffness and the geometry of the pressure transfer ring while jacking;

• Development of the longitudinal force in the course of the jacking; and,

• Sequence of the traversed path geometry (path and gradients) with steering movements.

Due to its modular structure, the simulation program is not only an instrument for structural planning, but also for the construction process and the final acceptance. Within the scope of planning it serves for testing the structural calculation with planning data. Instructions are given for the construction process concerning the development of the jacking force and the steering movements.

Within the scope of construction process the jacking data used in planning are constantly replaced by the measurement values (jacking forces, geometric measurements) determined at the construction site (see Figure 3). Thus, on the one hand, the level of safety achieved for the already traversed jacking path can be determined and, on the other hand, the prediction for the remaining distance that is still to be traversed can be improved and thus the planning can be updated with reference to the current state of information. Only CoJack offers this permanently updated prediction concerning the further jacking, which means a major increase of safety. Particularly with unexpected jacking situations (for example incorrect steering), new instructions for the ongoing construction process are developed, which usually allow the jacking to be continued without overstress.

Within the scope of the final acceptance, the entire jacking procedure can be retraversed by way of calculation on the basis of the measurement data and the corresponding level of safety can be determined for any point of time. If necessary, overstressed pipes can be named in order to carry out further investigation for possible damages.

Practical experience

The special interest in increasing the controllability and therefore the operational safety of sophisticated jacking measures especially on the part of the constructors but also on the part of the construction companies has led to an increasing demand of this new S&P service in Germany and abroad. This has brought about important practical experiences, which in turn allow for practical enhancements and the optimisation of CoJack.

Cojack provides the following aspects:

• The determination of the load of the pipes due to impacts in the direction of the pipe axis for every pipe and at any point in time during the jacking process;

• The safe use of increased jacking forces;

• The safe continuation of the jacking process after steering failures and the exceedance of the jacking force;

• The observation and evaluation of scenarios with regard to the potential development of the jacking force and the steering on the residual jacking section;

• A complete and comprehensible documentation of the jacking process; and,

• In many accomplished projects, the application of CoJack covered all three important phases of jacking measures: planning, monitoring and acceptance.

However, the main application field of CoJack is the simulation that accompanies the construction process.

The necessary measuring data was transmitted daily from the construction site to the office independent of the location via email in a previously fixed format, which made a prompt office simulation possible. In general, the limits fixed at the planning stage were met and CoJack thus provided additional control and documentation, which especially enhanced static safety.

However, in several cases unexpected incidents occurred during the jacking process. These had not been considered in the planning and would have led to damages of the pipes, a longer standstill or even to a stop of the jacking process if CoJack had not been used as an accompanying measure.

During an approximately 250 metre jacking with several planned alterations of the curvature, angular deflections that exceeded the permissible limit by twice its value were found in the pipe joints at a station after 20 metres. It was obvious that all further pipes had to pass this curvature and that the longitudinal force would naturally increase from pipe to pipe. However, the immediately executed simulation and updated static prognosis showed that the pressure transfer rings were only sparsely grouted due to the small distance between the point of failure and the starting manhole and could very well disperse the jacking force. Figure 4 shows the stresses of the third pipe with an instantaneous jacking level of 170 metres that had already occurred (to the left of 170 metres) and the results of the static prognosis (to the right of 170 metres). The stresses at the curvature remained below the permissible values due to the small jacking force. As the pre-damage caused in the transfer pressure ring was so little, jacking could continue with a moderate adaptation of the permissible
jacking force.

Figure 5 shows the simulation for the pipe that is most stressed (pipe 50) before the end of the jacking process. The pipe was situated exactly in the bend when the maximum jacking force was applied. However, the distinctive grouting of the pressure transfer ring did not have any impact on the further jacking process as the pipe had reached its final position before it could approach the second bend.

Using CoJack as an immediate monitoring device and applying special care in steering, it was possible to complete the measure
successfully with the least delay. The practical application of the CoJack online service has met the high expectations of the constructors. The jacking processes simulated by means of CoJack can basically be classified into four categories (A-D):

• Green area: In many cases it could be proved that all jacking guidelines had been adhered to according to the pre-simulation and that the pipe stress always remained below the permissible level (category A).

• Yellow area: In several cases the permissible steering movements (category B) or the permissible jacking forces (category C) were exceeded to the extent that the jackings should have been interrupted or stopped. However, CoJack has proved that at the emergence time of these conditions no overload had occurred yet. Furthermore, a simulation of the residual section with updated planning data was carried out by means of CoJack, which took into account the ‘pre-damage’ of the pressure transfer ring, and thus a static prognosis was produced. In all cases the jacking process could be continued with additional requirements and successfully completed.

• Red area: Moreover, CoJack was used to carry out subsequent calculations of critical pipe jacking processes that had already been completed. In one case the additional simulation revealed pipe stresses as high as the breaking strength of the concrete. The pipes with the highest stress in the pipe string as well as the locations of potential damages were identified. Although the pipes appeared to be completely undamaged, more specific investigations followed which revealed spallings at the outer concrete layer. Unfortunately, the damage had already occurred here. A timely application of CoJack would have prevented the damage. Apart from constructors it is especially innovative jacking companies that use CoJack to secure their economic success. They have realised that CoJack is an important decision support instrument when it comes to optimising the pipe jacking and both to providing highest quality and to using technical measures like interjacking applications and lubrication only if it is inevitable. This is why CoJack makes pipe jacking not only safer and quicker but also more cost-effective and economical.

Conclusion

Numerous experiences in practice have clearly shown that CoJack serves as a reliable static control device and as a quick decision support for the evaluation of unexpected incidents during the jacking process. CoJack guarantees the prevention of unnecessary standstills as well as maximum safety and economic efficiency by providing more transparency and a scope for action and decisions not available before.