Taking curves with confidence
Jan 28, 2016
Finding the weakest link and providing new solutions can help reduce pipe damage and increase scope for pipe-jacking projects
Conventional pipe joints with pressuretransfer rings made of wood or chipboard often reach their limits in such cases.
Sometimes, the problem remains unnoticed until the obligatory structural calculation for pipes according to the DWA (German Association for Water, Wastewater and Waste) worksheet A 161 (Structural Analysis of Jacking Pipes and Implications for Practical Application) shows such small permissible jacking forces that the number of required intermediate jacking stations increases unexpectedly.
Even the tried and tested oriented strand board (OSB) is overstrained.
The use of new force transmission techniques based on a hydraulic joint plus an online computation and monitoring system (CoJackHydra) represents a new method allowing for high jacking forces even with difficult alignments. Thus pipe damage can be avoided and the area of application for pipe jacking extended.
At the moment, CoJackHydra is only used by German construction firms. To extend this system abroad, developer S & P Consult is looking for local partners, which can take over the necessary support services in the field of measurement technology.
Ensuring best practice
With pipes made of reinforced concrete or vitrified clay, in particular, the pipe string does not bend continuously in curved jackings and in steering movements, but the ‘stiff’ pipe remains largely straight.
This results in angular deflections in the pipe joints, which are recognisable in the form of different joint gap widths all over the circumference of the pipe joint. If the deflection is big enough, a gaping joint is formed that reduces the pressure transmission surface, resulting in an inevitable increase of contact pressure.
The bigger the force and the number and extent of the steering movements, the more the pressure-transfer ring is loaded beyond its limit of elasticity. This load results in irreversible plastic compressions that persist even after the ring is unloaded. Each ring is loaded and unloaded many times during jacking.
Furthermore, the angular deflection changes constantly. Thus, the pressuretransfer ring changes its geometry and increasingly loses its load-distributing effect; it ‘hardens’.
Because of these effects, the contractor should include a more accurate structural calculation of the pipe stress considering the non-linear stiffness behaviour of the pressuretransfer ring to ensure a sufficiently high level of safety. Economic practicability must also be taken into account. Essential input parameters for a calculation of the pipe stresses are:
- Non-linear stiffness behaviour of the pressure-transfer ring, separating the elastic and plastic deformation portion;
- Stress history of the pressure-transfer ring at any point in time;
- Changes in the stiffness and geometry of the pressure-transfer ring in the course of jacking;
- Temporal development of the longitudinal force; and
- Chronology of the line geometry.
The list indicates that the calculation of pipe stresses must not be limited to a single point in time, but requires a consideration of the above-mentioned parameters during the construction phase.
As soon as changes in the stiffness and geometry of the pressure-transfer ring in the course of jacking becomes an important quality aspect, the operator becomes much more responsible for steering errors and an exceedance of the jacking force.
What to avoid
Often this damage is not visible from the inside of the pipes, but has such a substantial effect on the durability of the whole construction that even more severe consequential damages and a considerable need for rehabilitation must be expected after a few years, before or after the warranty period expires.
Especially for operators, minimisation of unnecessary steering movements is therefore of utmost importance. Because this is not always possible, online monitoring of all stresses in every pipe during jacking is recommended.
This particularly allows:
- The determination of the pipe stress as a result of loading in the direction of the pipe axis for each pipe and for each point in time under observation during jacking;
- The safe use of increased jacking forces, if necessary;
- The safe continuation of jacking after steering errors and an exceedance of the jacking force;
- The consideration and evaluation of scenarios regarding the development of the jacking force and to high steering movements on the remaining jacking still to be installed (remaining distance);
- A complete, comprehensive and, in particular, comprehensible documentation of jacking; and
- Permanent and location-independent observation of jacking data, displayed graphically on the Internet (corresponding access rights presumed).
Design guidelines over the decades
From 1982 to 1984 S & P Consult initiated the research programme ‘Development and testing of jacking pipes in non-accessible cross-sections’. The outcome was design guidelines for microtunnelling pipes.
In 1986 S & P Consult was involved in the development of twophase pipe jacking for non-accessible crosssections. For this development Prof. Dr.-Ing. Stein received the Bauma Medal of Honour at the German Construction Equipment Day on April 6, 1986.
This medal is for unique, trend-setting innovations, which had been developed in close co-operation with the German building industry and companies from German construction, as well as construction equipment and material industry.
In 1987 the company was involved in the development and testing of a microtunnelling machine to replace non-accessible sewers via remotely controlled traversing (pipe replacing) in Hamburg. New developments for the computation of jacking took place from 1998 to 2004.
The result was the CoJack computerassisted simulation method to determine the actual stresses of the jacking pipe faces due to the jacking force in the construction state, considering the non-linear material properties of the pressure-transfer rings, in particular in case of multiple loads and the actual line course (self-development). Until 2014 S & P Consult was involved in the revision of DWA A 161.
Other developments deal with an injection device for microtunnelling machines to homogenise the subsoil, and to stabilise and secure the annular gap in jacking projects, and the optimisation of injection media and injection methods for a continuous support of the annular gap during pipe jacking in heterogeneous subsoil. S & P Consult was also involved in the development of a calculation/ design method for pipe jacking of reinforced concrete box pipes.
As part of a research project (2014/15) funded by the German Federal Environmental Foundation, S & P Consult developed a new method for a close-to-reality computation of the cornering forces which occur during pipe jacking (contact forces between pipe and soil).
The bases were laboratory and extensive in-situ tests to provide the necessary parameter for a mathematical rigid body model. The results are approximation formulas with which the previously unknown cornering forces can be determined with sufficient accuracy. Thus we can significantly increase project safety.
Monitoring deep projects
At an early stage S & P Consult was appointed as consultants to the national environmental agencies, in the approval process. In addition to an expert opinion and a profound design evaluation, the company’s services also include the revelation of optimisation potentials regarding inter alia the drainage system, the construction of shafts and sewers, and the operational concept.
Among other aspects, design evaluation includes the preliminary assessment, design and approval planning for the sewer and pumping plants, construction methodology, operating models, disposal safety, fundamentals of water management, work safety, ventilation as well as concepts for cleaning, rehabilitation and maintenance.
Later S & P Consult took part in the final design and today it is involved in the supervision of construction. The entire pipe-jacking project is monitored via the CoJack online computation and controlling system. This allows for a safe continuation of jacking after strong steering movements and by exceeding the driving force without stability.
Enter the second generation
Demand for microtunnelling/pipe jacking will grow, as the first generation of sewer collectors comes to an end. Plus, the increasing global urbanisation requires solutions for the installation of utility tunnels without interference to the existing traffic infrastructure.
Today many cities experience daily gridlock and upgrading the underground infrastructure will therefore require trenchless solutions.
Qualification of decision makers and engineering firms in the field of pipe jacking and microtunnelling is a decisive pillar to extend the use of these trenchless technologies.
Learning material and knowhow has to become available worldwide, independent of time and location. So e-learning will play a major role in our business. Because of this, S & P Consult started with the development of e-learning modules in 2006. Today it already trains and educate engineers in six countries.
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S & P Consult GmbH
Dr.-Ing. Robert Stein
+49 (0) 234 / 5167-0
+49 (0)234 / 5167-109