Key to that progress is the development of a new pressure transmission ring, the hydraulic joint and a new understanding of the structural behaviour of reinforced concrete pipes found in the research project Response and dimensioning of reinforced concrete pipejacking tubes by the Swiss Federal Institute of Technology in Zurich.

Together with corresponding hardware and software tools, these two items lead to an efficient and secure online monitoring of the installation process that respects the geological boundary conditions and adapts to both the installation method and the real steering movements induced by the site personnel/site equipment.

Actual remote real-time navigation systems guide precisely even through complex alignments, guarantee quick recognition of misalignment and enable site personnel to keep steering/correction movements in adequate limits without harming the pipes. The monitoring hardware and software warns the site personnel before damage occurs, resulting in much less damage and essential cost savings.

Structural pipe issues

The understanding of the structural behaviour of pipes during the jacking process is often based on empirical estimation rather than on systematic structural analysis of the pipes and joints.

In almost every case the jacking process is the deciding load combination for concrete pipes and it can be said that if the pipes can get jacked on their end position without any damage then they will stay free of damage for the rest of their lifetime.

In addition, wood materials used as pressure transmission rings between the pipe faces under curved alignment induce considerable forces normal to the pipe axis, often causing damage to the pipes. Damage can range from cracks and concrete spalling – reducing the durability of the pipes up to the complete failure of the pipes – resulting in high costs and delays for replacement and/or rehabilitation.

The hydraulic joint enables higher security levels with up to a six times smaller radii of curvature in comparison to wooden pressure transmission rings. In combination with existing up-to-date automatic navigation systems, the hydraulic joint enables long and complex alignments using existing pipe jacking or microtunnelling equipment. In addition designers and contractors get a much wider range of application.

Load and response of jacked reinforced concrete pipes

Besides permanent load pressures such as earth, water and traffic load pressures pipes are exposed to significant additional loads while completing the jacking process. These loads can be explained by the three standard cases shown symbolically in Figure 1.

Case 1 shows a pipe in a straight section of the pipeline, case 2 depicts a pipe in a curved section with a constant radius and case 3 shows a pipe standing in the turning point between two inversely orientated curvatures. A good approximation of the pipes can be assumed as stiff in their axial direction, while the global curvature is located in the soft joints. The distribution of axial deformation and pressure is shown in Figure 2 and leads to significant pressures acting on the pipes while being jacked.

The following explanations have been simplified to assume that the mechanical characteristics of wooden materials are constant around the pipes circumference. Furthermore, as a first step only the initial deformation of the joint is discussed without considering the influence of plastic deformation of repeatedly loaded wood material.

Under these assumptions, the distributions of axial deformation and pressure in case 1 keeps uniform with the thrust Q lying exactly in the pipe axis. In case 2, the axial deformation is triangular with an open gap on the outer side of the curve. According to the mechanical law of the joint material Q is moving off from the pipe axis inducing an eccentricity called e. Ignoring the influence of friction pipe surface and soil Q on the two pipe’s faces stays constant and in line. Due to geometrical reasons lateral deflection forces U are induced that usually mean not a significant load for the pipes.

The sense of deflection/curvature inverts between the two pipe’s faces in case 3 – Q on each face of the pipe is not in line anymore. To fulfill the moment equilibrium the pipe bears itself against the surrounding soil with lateral bedding forces B as result. Depending on the size of e and the pipe’s geometry, B can reach a similar size as the resulting jacking force Q itself. Of course, a cylinder shell made of a reinforced concrete can only sustain a lesser load to the cylinder axis than parallel to the axis, and consequently these bedding forces – B – mean a significant danger of damage to the pipes.

In a large scale test series held at the Institute of Structural Engineering of the ETH Zurich/Switzerland, deformation and failure mechanisms of eccentrically loaded pipes have been investigated. Seven reinforced concrete pipes each with an inner diameter of 1 m, wall thickness of 13 cm and length of 1.5 m were tested.

The pipes were axially loaded by a compact jack (3) posed eccentrically on the pressure ring. The opposite horizontal bearing (7) was placed again with maximum eccentricity but below the pipe axis. The lateral reaction, B, was supported by a group of laterally placed, radial oriented compact jacks (9); the number of these lateral jacks could have been varied between 1 and 4 for each group, so different soil and bedding conditions could have been investigated.

Figure 4 summarises the failure mechanisms observed in the test series. (a) shows a 4-inch bending mechanism, (b) a transversal mechanism, (c) a punching of the pipe wall and (d) the spall-off of the inner/outer concrete cover. While mechanism (b) appears only under special conditions, the other mechanisms can be observed again and again, starting usually with the longitudinal cracks of mechanism (a) followed by the spalling of the concrete cover (d). Having hard soil conditions or hard obstacles in the soil such as boulders, unknown foundations etc, a punching of the pipe wall (c) can occur without warning.

With no more discussion about the consequences of these damages it can be said that they decrease the pipe’s durability or even sustainability and have to be avoided. Out of theoretical considerations and the observations made at the described test series it becomes quite clear that the most efficient way to avoid these damages is to minimise the lateral bedding forces B. Since B is proportional to e and Q – and high Q-values are of interest for practical reasons – e needs to be minimised and one way to do so is to find a pressure transmissions ring/joint with improved mechanical characteristics – for example, the hydraulic joint.

The Hydraulic Joint

The idea of the hydraulic joint is based on two principles:

* The principle of communicating vessels saying that the pressure level inside other communicating vessels is constant; * The perfect reversible mechanical behaviour of non-compressible fluids is independent from the deformation or loading history.

The hydraulic joint is a hoselike vessel filled with a fluid placed between the faces of two following pipes. The fluid pressure inside that vessel is to be considered as the reaction of the jacking force, Q, being transferred from one pipe to the other.

It can be said that if the specific contact area, a – as seen in Figure 5 – stays constant along the pipe’s circumference, the acting point of the resulting jacking force, Q, is coincident to the pipe axis and the eccentricity thereby is equal to zero independent of the size of the deflection angle of the joint. Since the lateral bedding forces, B, are proportional to e in this idealised case, B would vanish.

The described idealised behaviour of the hydraulic joint is realistically not applicable. In contrast, the initial ring-shaped hydraulic joint as shown in Figure 5 proved to be practicable and still reduces the eccentricity of the thrust by dimensions shown later in the text.

Before the hydraulic joint is exposed to the jacking force it receives an initial deformation inducing an initial contact area. This is done by opening the hydraulic joint’s end tap so that the fluid can get out from the hydraulic joint without resistance while the joint is getting compressed. The effect of that initial deformation/contact area first is a reduction of the fluid pressure p inside the hydraulic joint and second is a significant reduction of the dependency of the eccentricity e from the deflection angle of the joint.

Using the formula below, the size of the line load, q, transferred by the hydraulic joint from pipe to pipe can be calculated depending on deformation, δ, and fluid pressure, p, while k stands for the axial stiffness of the Hydraulic Joint.

formula.tifIntegrating the formula over the pipe’s circumference leads to the size of the resulting jacking force Q and on a similar integration the eccentricity of the thrust e can be calculated. Based on that knowledge reliable structural analysis can be done using the easily measurable sizes p and δ.

To show the effect of the hydraulic joint a case study has been completed concerning a typical misalignment and correction that can appear on every pipe jacking job.

The calculation of this study was done in 90 steps for a pipe with an inner diameter of 1.2 m and a misalignment that corresponds to a radius of 560 m. The study involved a hydraulic joint and a conventional joint made of chip board with a thickness of 29 mm.

It can be seen that the eccentricity of the hydraulic joint measured values up to six times smaller than the pressure transmission ring made of chipboard. Since the size of the lateral bedding forces, B, is proportional to e the effect and reduction of risks for pipes being jacked is obvious.

On the other hand, the mechanical characteristic of the hydraulic joint led to a significant reduction of the allowable radii of curvature while the maximum allowable jacking force stays unchanged. Table 1 gives an overview of possible radii of curvature depending on the pipe’s outer diameter, OD, and length, L.

Table 1 confirms that the use of the hydraulic joints opens new dimensions for the definition of the geometry of jacked pipelines and in this way gives owners and designers a new range of applications while using pipe jacking and microtunnelling techniques.

Possibilities and practical experiences

The reversible mechanical characteristics of the hydraulic joint ensure it is possible to develop an efficient algorithm doing online analysis of the pipe structure during the jacking process, based on the knowledge of the hydraulic joint’s deformation and the fluid pressure. Around that algorithm, hardware and software tools have been developed by Jackcontrol AG, a subsidiary of the Swiss Federal Institute of Technology in Switzerland. Figure 6 shows the scheme of that monitoring system.

Sensor units placed by the joint between two leading pipes and every intermediate jacking station collect sensor data that is transferred by a bus system outside the pipeline to the control unit. In the control unit, sensor data is being saved and transferred automatically via the Internet to the control centre.

On site, the control unit calculates the maximum allowable jacking force based on the actual sensor data for each pipe corresponding to the geological boundary conditions while taking permanent loads like earth pressure, traffic loads into consideration. If the actual jacking forces approach the calculated limit values then personnel on site receive an alarm by flash light/horn before damage occurs, allowing the jacking process to be interrupted.

Real-time monitoring means that most of the known damages on jacked concrete pipes can be avoided. The real-time structural analysis of the jacked pipes allows the jacking process to run even when the planned or unplanned alignment is getting complicated.

After trial applications in 2004, hydraulic joints started commercial use in 2005. So far, more than 20 pipelines with a total length of more than 4.5 km have been successfully completed. Pipe sizes ranged from DN 1200 mm up to DN 2500 mm and two thirds of the pipelines have had curved alignments. The smallest radius of curvature was 110 m using pipes DN 1200 mm with a length of 3 m.

The hydraulic joints are either removable or permanent. Removable joints are removed after jacking and can be reused – the annular space between the pipes can be left empty or filled, with mortar, PU-foam or a joint profile, depending on the specific requirements. Permanent joints stay in place after jacking, serving as seals in the radial direction. Sealing in the tangential direction is achieved by grouting the space between the hydraulic joint’s spiral turns with a hydrophilic product that swells in contact with water.

In the majority of the pipelines completed, permanent hydraulic joints were applied. Contractors often chose the hydraulic joint system for curved alignments because they could use pipes with usual lengths. In several cases, the hydraulic joint and the real-time monitoring system assisted in absorbing large joint rotations caused by geotechnical difficulties or steering corrections without damaging the pipes.

Compared to a conventional joint made of wood material, the eccentricity of the resulting jacking force is largely reduced by the hydraulic joint, resulting in much more uniform stress distribution at the pipe ends and hence a considerably reduced pipe damage potential.

Contrary to conventional joints the response of hydraulic joints is fully reversible, i.e. independent of the deformation history of the joint. The measurement of joint displacements and joint liquid pressures permits an effective real-time monitoring of the jacking process.

As a result of the reduced eccentricity of the resulting jacking force, higher jacking forces can be applied, longer pipes can be used, smaller radii of curvature are feasible and friction forces are reduced due to the smaller lateral bedding forces.

Navigation Systems

While the hydraulic joint makes curved pipe jacking safer and more efficient, automatic navigation systems are a basic requirement for every curved drive.

These systems provide the machine operator with continuous information about the TBM position, which guarantees a smooth steering correction and a minimum of deviation. Because of these systems, the effect of every steering correction will be displayed on the steering monitor and allow an optimum of reaction time for the operator. Avoiding larger steering correction will automatically lead to lower deflection angles between the pipes and decrease the risk of pipe damage. A good combination between smooth steering by the operator and precise information about the TBM position is the key for successful jacking.

As one of the leading survey technology supply companies, VMT GmbH have been responsible for the successful navigation of more than 500 tunnelling projects throughout the world, achieving the desired alignment in association with a wide range of tunnelling machines.

The automatic navigation systems for microtunnelling curved drives can be divided into two categories:

* Laser Tachymeter Systems with Automatic Laser Target * Gyro based navigation systems in combination with electronic hose levelling system

Both navigation systems have their field of application in curved microtunnelling and pipe jacking and each can work on the base of a theoretical principle that has to be checked frequently by the surveyor. Depending on the project circumstances and requirements, either the Laser Tachymeter System or the Gyro System is the best choice.

Laser-Tachymeter systems offer suitability for long-range and curved bores, starting from approximately ID>1000 mm. The main components are a servo motorised Laser Tachymeter, which is mounted inside of the tunnel and an electronic laser target that is mounted inside of the TBM. The system is fully automatic and is remote operated by the TBM operator.

The calculation principle of this system is based on the theory of the invariance of the pipeline, which means that all pipes must follow the same course that is given through the drive of the TBM.

Even the TBM drift will be recognised and will not influence the accuracy of the system calculations. Approximately every 500 mm, an automatic measurement cycle will be undertaken by the system in order to verify the azimuth to a reference target prism. The intervals for the necessary check measurements for calibration of the system can be chosen on quite a large scale, up to 80-100 m, depending on the inside diameter of the pipes and the curvature of the DTA. All relevant data will be presented in graphical and numerical format on a navigation monitor.

The main advantage of this systems is that all alignment geometries are achievable. A continuous authentic display of the TBM position is available, control measurement interval of 80 m to 120 m can be used and advance is possible even during system measurement work.

The Laser-Tachymeter System is the optimum choice, for all curved drives and long distance drives, where the TBM allows a high advance rate and minimum down time for the navigation system is required.

Gyro compass-based systems with an electronic hose levelling system offer navigation for straight and curved drives with small diameters – starting from ID 800mm – and can be used on compressed air drives. The main components are a north seeking gyro compass, which is mounted inside of the TBM and the electronic hose levelling system. The gyro compass calculates the horizontal TBM position and the azimuth of the TBM while the electronic hose levelling system is calculated from the vertical TBM position.

The calculations of the gyro system are based on the principle of dead reckoning, which assumes that the TBM will move exactly along the direction that is defined by its axis. The difference between the direction along which the TBM is moving and the axis of the TBM is referred to as the drift of the TBM. This drift will affect the calculations of the TBM position. Therefore, frequent check measurements for the calibration of the system must be carried out every 40 m in order to determine the TBM drift and reduce its influence to a minimum by pre-setting this value for further calculations.

The electronic hose level consists of two height sensors that are mounted in the start shaft and inside the TBM. Both sensors are connected by a hose line, which is filled with special liquid. The height sensors are pressure sensors that measure the pressure of the liquid. The different pressures indicated at the two sensors are a precise measure for the height difference between the sensors.

This measuring system is unique because of the absolute reference from the TBM to the start shaft. The accuracy of the TBM elevation is completely independent from the drive length and refraction free.

The major advantages of the system include its compact construction, simple control measurements, precise elevation data through the electronic hose levelling system and it is applicability to all advanced methods. In addition, no line of sight is required when operating the system.

Only automatic navigation system can meet the demand for longer and curved microtunnelling, instead of conventional surveying. These systems enable contractors to bore tunnels with higher efficiency and quality. Different systems are available and can be used depending on project specific requirements. The combination of navigation and monitoring system allow operators and site engineers to evaluate and analyse the boring process much better then in the past.

This article is a summary of the paper by Stefan Trumpi-Althaus and Alexander Seilert entitled Jackcontol and VMT: new dimensions for pipe jacking and microtunnelling. The paper was presented at the Trenchless Australasia conference in March.