Structural monitoring of a GRP jacking project with CoJack

CoJack-variant for GRP jacking pipes 

The calculation program CoJack, which was developed by S & P Consult GmbH (Bochum, Germany) for the structural monitoring of pipe jacking projects, has meanwhile proven itself in numerous jacking projects with reinforced concrete pipes. For around half a year now, for the first time also a program version has been available that was developed especially for jacking projects with GRP-pipes. It takes into account the material-specific properties of plastic pipes in particular.

Currently, it is being applied in practice for the first time in an ambitious jacking project in the nature protection area Hexbachtal in Essen, Germany. By order of Stadtwerke Essen GmbH (municipal utility), the contractor Epping Vortriebs GmbH from Bocholt, Germany is three jacking distances of a total length of more than two kilometres; this also includes several tight curves that partially run into opposite directions with radii of approx. 600 metres and jacking distances of more than 800 m. GRP-pipes manufactured by Hobas Rohre GmbH (Neubrandenburg, Germany) are used with lengths of 3 m and 6 m, an inner diameter of 1885 mm  and a wall thickness of 85 mm (see Figure 1).

 Structural problem

The central problem when it comes to transferring jacking forces from one pipe to the next – especially with rigid pipes made of reinforced concrete and vitrified clay – is that when driving curves and in steering movements the pipe string does not bend continuously; instead the rather rigid pipe remains straight to a large extent and angular deflections occur at the pipe joints. Here a so-called gaping joint can develop, which again reduces the surface used for pressure transfer and the contact compressive stress inevitably increases. With reinforced concrete and vitrified clay pipes, this undesired effect is reduced by using pressure transfer rings (PTR) made of wood or wood products, which, when loaded, deform due to their low rigidity and connect to the pipe face on both sides, increase the contact area and thus have a stress-reducing effect (see Figure 2).

Here, as a consequence of the high forces the material wood is strained far beyond its elasticity. Plastic compression occurs, which is irreversible and thus remains after unloading. In the course of a jacking project, each pressure transfer ring is loaded and unloaded numerous times, during which the angular deflection changes constantly. It increasingly looses its load-distributing effect and "hardens".

Jacking pipes made of GRP do not require a pressure transfer ring as they themselves deform under increasing jacking force. This is an effect of their higher flexibility and therefore they increase the contact area to the neighbouring pipe. This "intelligent" material behaviour allows the force to be transferred directly from one pipe face to the next. In unloaded condition the pipes only touch in one spot. Increasing the jacking force causes the contact area to grow, whereas the gaping joint decreases (see Figure 3). This effect can continue even up to a complete closure of the joint also in curves.

Another advantage compared to the development of joint gaps with a wooden pressure transfer ring is that the pipe joint does re-deform again into its original condition when unloaded and no plastic deformations remain. During the entire jacking project the pipe material GPR remains in the linear-elastic range, so the deformation behaviour of the gap cannot be calculated as non-linear-elastic by taking to account the geometry of the pipe. Reducing the desired flexibility of the pipe (hardening) does not occur in this case.

This special material behaviour was now implemented in a variant of the structural simulation program and is now for the first time used in the jacking project in the Hexbachtal.

In this construction project already before the start of the jacking CoJack was an important instrument for the structural verification in connection with the structural design. At present, CoJack is serving the purpose of monitoring the construction works and will be used as a basis for the structural evaluation in the approval of the construction after completing the works.

In connection with the planning, CoJack was used for checking the compulsory structural calculation of the jacking pipes with the available planning data. Not only were the scheduled curves taken into account in the pre-calculation of the pipe stresses, but also the permissible fabrication tolerances for the pipes were considered and approaches for steering movements were made. In addition, CoJack did not start from the assumption of a jacking force that is constant during the entire jacking process, but realistically included the successively growing jacking force.

     

The result are requirements for the construction in the form of (temporary) limiting values for the development of the jacking force and for the steering movements. The maximum values for the jacking forces given here and for the maximum angular deflections in the joints (measurable as joint gap differences) are to be looked at as reference values, whose (moderate) exceeding does not lead to a stand-still of the jacking, but which – with CoJack – simply leads to a shift of limiting values for the further jacking in the overall context.

 Starting with the installation of the first pipes, now – in connection with the construction – the jacking data applied during planning are step-by-step replaced by values measured on the construction site (jacking forces, geometrical measuring) (Figure 4). In this way, on the one hand the safety level that was achieved for the jacking distance that has already been covered can be determined and on the other hand, the prediction on the remaining distance that is still to be covered can be improved. Thus, the structural planning is continuously updated with reference to the respective latest level of information.

This continuously renewed structural foresight onto the further jacking process is based on an algorithm that was especially developed for CoJack. This represents a clear gain in safety and provides a reliable foundation for decisions concerning the process technology. Especially in unexpected jacking situations (such as obstacles, steering errors) variants in the further procedure can be run through and momentous decisions can be made on a reliable structural basis and on short notice. The most competitive option is set within the program – always provided the agreement by the client – and is taken into consideration during the further construction. Under the monitoring of CoJack often even after a strong steering movement (bend in the pipe string) or a temporal exceeding of the permissible jacking force, the jacking can be continued without delay without overstressing the pipes and possibly with additional requirements or restrictions.

If required, under the "supervision" of CoJack even increased jacking forces can be applied in order to a free a stuck jacking, for instance. Here, the safety that the pipes are not damaged is always ensured. In connection with the approval of the construction, in a calculative way later the entire jacking distance can be covered again by using the measuring data acquired during jacking and, if necessary, additionally by comparison to a final measuring of the pipe string. The corresponding safety level can be determined for any point in time of the jacking project and for each pipe. Where necessary, single overstrained pipes can be named in order to introduce further investigation of possible damage.

Gap measurement system GMS

When monitoring a running jacking project, CoJack depends on precise and up-to-date measuring values from the construction site. For this purpose, the partner company VMT GmbH (Bruchsal, Germany) has developed the measurement system GMS, which is especially designed to meet the requirements made by CoJack and whose sensor technology continuously and automatically records, archives and visualises the jacking parameters and provides them for the online-request through CoJack (see Figure 5).

For jacking projects with GRP-pipes, special sensors have been developed, which measure the distortion of the pipes during jacking. They were used in the jacking project Hexbachtal. For the jacking works in Essen, the sensor system for measuring of the distortion covers two three metre long pipes (pipe 2 and pipe 3) and the three neighbouring joints. To record the special deflection in the joints and the special distortion of the pipes, on each measuring level three sensors are arranged along the circumference. So here a total of 15 distance meters are used, which are combined to two measuring cross sections from a calculative point of view. They provide a redundant and rather clear picture of the distortion of the path covered (Figures 6 and 7).

Pressure sensors are integrated into the hydraulic lines at the main jacking stations and the intermediate jacking stations. They record the actually applied pressures, from which the gap measurement system calculates the respectively resulting forces of the corresponding jacking stations. Distance meters at the starting shaft and at the intermediate jacking stations provide the necessary information on the activities of the intermediate jacking stations and the progress of the jacking (Figures 8 and 9). With information in the system editor of the computer at the construction site each sensor is given an individual CanBus address, via which it is identified and read out.

Visualisation

On the monitor of the GMS system computer on the construction site, the status of the installed sensors is displayed constantly (see Figure 10). Also the dimensions of the joint gap of the single cross sections at the springings, crown and invert are displayed.

The data recorded can be printed and displayed in graphics and in tables. Important information such as the behaviour of the measuring cross sections or the temporal progression of the forces applied onto the main and intermediate jacking stations are presented in diagrams, which are updated constantly and in real time. Not only on the computer at the construction site, but also irrespective of the location on any computer with internet access with the corresponding access authorisation the present data can be retrieved and visualised. So besides the construction site personnel, for instance, also the site manager and the client in the office is informed about the present status of the jacking including the most important jacking parameters.

For this purpose, the authorized user simply needs to log onto the website "cojack.eu" with his user name and password. Afterwards, he gets the opportunity to retrieve the most important present data of the jacking in a graphically processed way. The constantly updated diagrams particularly show the distortion of the pipe string as a consequence of driving curves and steering movements as well as the forces applied by the main and intermediate jacking stations up to the present point of time.

This on-line-connection also supplies CoJack with up-to-date measuring data in order to conduct the jacking-synchronous simulations. This connection is even set-up bidirectional, so – if required – the settings on the computer at the construction site can be changed and adapted to the latest calculation results. In this way necessary tightening of the limiting values or possible liberalisation are implemented directly on the computer on the construction site and become visible for all people involved.

The first jacking section in Essen's Hexbachtal

Meanwhile, the first jacking section has been successfully jacked with GRP jacking pipes in the Hexbachtal. After jacking 350 m, the force sensor, that had been used until then, sustained a damage; so from this position onwards a pressure sensor had to be reverted to, which however was unable to differentiate which one of the three steps of the telescope station were active at that time. For this reason the diagram presents the forces that were calculated from the measured pressures from all three steps. At any rate, the permissible jacking force of 5800 kN was not needed.

The second jacking section in the Hexbachtal is currently being executed.

Summary

With CoJack clients, contractors and engineering consultants are equipped with a strong method for structural simulation and monitoring of pipe jacking projects, whose application range could now be extended to GRP pipes. CoJack reduces the hitherto existing risk factor of overstraining the pipes during jacking and thus decisively increases safety and economic efficiency of jacking projects.

In connection with the planning, CoJack serves for checking the mandatory structural calculation of the jacking pipes with the available planning data and defines limiting values for the construction.

Starting with the installation of the first pipes, within the scope of the construction the jacking data applied during planning are step by step replaced by values measured on the construction site (jacking forces, geometrical measuring). On the one hand this allows the user to determine the achieved safety level for the jacking distance covered and, on the other hand, to improve the prediction for the remaining distance that has to be jacked. In this way the planning and especially the structural calculation are constantly updated with reference to the current information level.

In connection with the approval of the construction the entire jacking work must be re-driven in a calculative way by means of the measuring data collected during jacking and, where necessary, additionally by checking them against a final measurement of the pipe string. For the entire jacking work, the also the safety level must be determined for each pipe.

The measurement system GMS (Gap Measurement System), which was especially adapted to the needs of CoJack by the partner company VMT GmbH and whose sensor system continuously and automatically takes up, archives and visualises jacking parameters and provides them for the online-recall by CoJack, supplies the necessary present measuring values from the construction site.

The integrated recording of the currently applied jacking forces at the main jacking stations and intermediate jacking stations allows a consistent and clear documentation of the forces calculated online with regards to a certain point in time and a special jacking station.

This data recorded by the GMS is nearly able to completely reconstruct the jacking process of the pipe string and they provide a very broad foundation for further calculations.

Thus, CoJack represents an important tool for implementing and enforcing trenchless construction.

Literature:

[1] Vogler, G.: Rohrvortrieb – Beanspruchung von Vortriebsrohren bei Kurvenfahrten. BFT (2002), Issue 7, S.50-61
[2] Stein, D.: Trenchless Technology for Installation of Cables and Pipelines. Stein & Partner, Bochum, 2005.
[3] Beckmann, D.: CoJack - A New Statics Method of Computing and Controlling Pipe Jacking
[4] Beckmann, D. ; Stein, R. ; Uhlenbroch, A. ; Fabri, T.: Tunnelling and Underground Space Technology incorporating Trenchless Technology Research

Internet
www.cojack.eu

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