Long distance (micro-) tunneling with polymer concrete

May 23, 2008

Polymer concrete Technology dates back to the early 1960’s. It was developed in order to provide corrosion resistant conduits especially for the chemical industry. Municipalities in Europe started around the late 1970’s experiencing that cementitious concrete products can’t meet the service lifetime expectations in the corrosive sewer environment. That was the time when polymer concrete first was deployed in the municipal sewer systems.

Since then all kind of different shaped corrosion resistant polymer concrete products have been produced and supplied throughout the world, like pipes, manholes and structures.
Especially recognized is this engineered product by microtunnelling contractors due to the high compressive strength and flexibility during installation. The high jacking capacity of the pipes helps minimizing the construction risks and provides the contractor the required flexibility to overcome unexpected hindrances during installation.
The newest development is the polymer concrete tunnel segment (PC). Segment tunneling is more beneficial compared to pipe jacking especially for larger diameters (>2.5m) and longer drives. The owners and contractors of sanitary sewer tunnel projects are currently limited to RCP tunnel segments with the post installation of a corrosion resistant liner or a separate carrier pipeline (2 pass tunneling). With the PC tunnel segments in a one pass tunnel application these cost and time consuming measures can be avoided.
1. THE HISTORY OF POLYMER CONCRETE
Polymer concrete products have been used for decades in engineering constructions like machine basis for high precision milling machines, in the building industry for facade products and sanitary parts, in electrical engineering for isolation devices and especially in the chemical industry for all types of ducts due to its favourable properties.
The development of polymer concrete products, mostly pipe, dates back to the early 1960`s. The objective was to achieve a substantial increase in resistance to chemical attack from the inside and outside and strength in respect of the stresses from external and internal loads whilst retaining the economical advantages of the pipe as a prefabricated finished part.
At the beginning of the development the well known cementitious concrete pipes have been coated with a polymer resin mix. The resin mix has been applied via a high speed rotating tool which was forwarded into the stationary pipe. Later on, fine sand was add to the resin mix and a thicker and more abrasion resistant layer was generated to protect the concrete core material. Beside pipes also bricks and tiles have been clad in a similar way at that time.
Around 1967, Zueblin, a construction company from the southern part of Germany has developed the first full wall Polymer Concrete pipe called “Polybetonrohr”.
From that time on the polymer concrete products have been installed in various applications worldwide. Due to the higher raw material costs the products were primarily used in the chemical industry. Later on, the inert material has also been used in the municipal sanitary sewer market and started to replace the cementitious concrete products, pipes, manholes and structures, which were failing in the sanitary sewer environment.
With the development of trenchless technologies (microtunneling and pipe jacking) in the 1970`s, polymer concrete pipes became popular especially by owners and engineers of sewer systems. In 1996 polymer concrete was first used in the U.S. To date, more than 300,000 m of pipes have been successfully installed in the world’s sewer systems. Over the past years, the processes for the production and manufacturing of polymer concrete products like pipes, manholes and structures have been fundamentally improved and are now available as an economical alternative to products like RCP especially in sewer applications.
2. THE MATERIAL AND ITS BENEFITS
Polymer concrete consists of unsaturated thermosetting polyester resin, silica filler, quartz sand and aggregates. All materials are inert and resistant to chemical attack. The polyester resin is the bonding agent between the mineral aggregates. Unlike Portland cement in cementitious concrete products, polyester resin forms a consistently strong bond with the evenly graded aggregates.
Due to the high chemical resistance, especially the resistance against hydrogen sulfides, polymer concrete products have found favor in sewer installation and rehabilitation projects. The physical properties including the high stiffness and compressive strength values makes the polymer concrete pipes ideal for pipe jacking and microtunneling operations.
During the mixing process, the high viscosity resin penetrates into the capillary voids of the oven dried minerals. After curing, the aggregates together with the incorporated resin forms a durable bond with a high strength and certain elasticity (less point-loading sensitiveness) compared to other products.
In contrast to cementitious concrete, where cracks can only be minimized but not eliminated, polymer concrete has no cracks. Due to the bonding mechanism of the resin and the quartz based aggregates, cracks are avoided. This bonding mechanism gives polymer concrete products their superior material behavior.
Polyester resin is a thermosetting plastic which is fully cured after the chemical polymerization process. During this curing process three dimensional molecule grids are formed. Unlike thermoplastics such as PE and PVC, polyester resin reactions within polymer concrete can not be reverted.
3. LONG DISTANCE MICROTUNNELING AND PIPE JACKING
Microtunnelling and pipe jacking has become more popular especially in high populated areas all around the world. Owners and engineers treasure the savings in social costs and the environmentally friendly installation method compared to open trench, cut and cover methods. It is a state of the art technology with minimum impact on the surface. Besides some construction activity at the launch shaft and the receiving shaft the construction works takes place beneath the surface.
In a pipe jacking operation all jacking forces are generated in the starting pit and have to be transferred from pipe to pipe to the microtunneling machine ahead of the whole pipe string. The resistance force which need to be overcome are split in two areas. The machine area and the pipeline area. On the machine part we have the tunnel face pressure which stabilizes the tunnel face and reduces subsidence on the surface, the operating resistance, which is mostly controlled by the machine operator who controls the slurry system and the steering of the MTBM, the edge resistance and the machine skin friction. On the pipe side, which by far is the largest section, we only have the skin friction. The skin friction is dependent on the friction factor of the material, the surface area and the nominal forces acting on the pipe OD from the surrounding soil.
The pipe friction, which represents the major portion in the overall calculation of the required jacking forces is dependent on the type of soil, burial depth of the pipeline, ring annulus lubrication as well as the material properties of the pipe. The smooth outer surface and non water absorbing behavior as well as the high compressive strength benefits the MTBM operator. Jacking length can be maximized and jacking forces minimized.
Polymer concrete pipes have one of the highest jacking capacities compared to other pipe jacking materials like RCP, CCFRP, and Clay. There is one material which is stronger, that is steel. But in a sanitary sewer environment it requires either additional coating or the installation of an additional carrier pipe. Clay jacking pipes are used for smaller diameter below 48” and need to be handled with special care due to their point loading sensitiveness.
The high jacking capacity can be translated in a real benefit for the contractor and this is the maximum jacking distance without the use of an Intermediate Jacking Station. The following calculation is based on a conservative average friction force of 1,0 to/m2 for the calculation of all 3 different pipe materials independently, and also not taking the different roughness values of the different pipes into account, well knowing that RCP has a much higher roughness than PC. But even without taking this benefit into account the differences in maximum jacking length is significant. More than three times compared to RCP and 4 times compared to CCFRP. (RCP* is calculated on a c-wall pipe according Sherle, and CCFRP according C-Tech data)
The longest jacking drive in the US (one push) to date, with the use of polymer concrete pipes, has been successfully finished by Paul Vadnais construction at the Murietta project in Southern California. The total drive was 1,480ft. The initial plan was to split it into two sections (620ft, 850ft). After the contractor recognized that the jacking forces can be kept at a very minimum level the contractor decided to continue the drive into the second section and drove up the two sections in one push. As a safety precaution an Intermediate Jacking Station has been installed, but never been operated.
With the use of the appropriate ring annulus lubrication, proper MTBM design and operation, as well as the use of Intermediate Jacking Stations, arbitrary jacking drives can be realized (>36”, man entry).
Intermediate Jacking Stations (IJS) break the whole jacking length into smaller pipe jacking sections. Generally only the pipe friction between two IJS’s have to be overcome by the following IJS, respectively the first IJS behind the MTBM has to overcome in addition the resistance forces of the MTBM. The last push is concluded by the jacking station in the starting pit, which finishes the time consuming operation. As a rule of thumb, the first IJS is installed when the jacking force reaches 60-70% of the max. jacking capacity of the jacking station in the starting pit or the overall allowable jacking force of the jacking pipe (including safety factor). The operation of every single IJS starts when the jacking forces monitored in the starting pit get close to the critical jacking force limitation.
With the use of polymer concrete the time and cost consuming operation of the IJS’s can be reduced.
As an example, comparing two pipe materials with the same jacking resistance during a pipe jacking operation, what means that the increase of the jacking forces per increase of drive length is identical. Pipe X (60”) has a maximum jacking capacity of 540 to and Polycrete (60”) a maximum jacking capacity of 2,500 to. That means with use of Pipe X, 4 IJS’s have to be operated, beginning with the first at about 300ft and with Polycrete only one close to the end of the drive.
In order to be on the safe side, independent from the jacking force, an IJS should be installed every 300-400ft independent of the pipe material (rule of thumb). It provides the machine operator the ability to respond in case the jacking forces spike unforeseen (unexpected soil formations etc.).
4. BENEFITS OF POLYMER CONCRETE JACKING PIPES IN CURVED APPLICATIONS
The rigidness of the Polymer concrete pipe together with the high jacking capacity and the less point loading sensitiveness makes it ideal for curved pipe jacking applications.
Allowing curved drives in the design process enables the design engineer to select the most economical rout for the tunnel operation. The amount of manholes, total jacking length, volume of excavated material, construction time and costs etc. can be reduced significantly.
A large percentage of jacking drives in Europe are curved drives. In fact every jacking operation is a kind of curved drive because of the constant steering of the MTBM operator to stay in line and grade.
during operation. It is like driving a car (or truck) on a straight highway where all of a sudden the driver is facing a strong wind from one side and has to steer against it in order to stay in his lane. When the wind changes direction or strength he has to counter steer to compensate the changes in resistance forces. A similar effect is recognizable during pipe jacking with the difference that instead of the wind the operator has to respond to uneven resistance forces arising from the soil around and ahead of the MTBM. Steering is undertaken by articulating the first two machine sections where the pipes follow automatically into the by the machine excavated chamber.
This example shows a 60” standard pipe with a 10ft length and a minimum curve radius of 1,230ft. The pipes follow automatically into the excavated space of the MTBM around the curve and articulate accordingly. The overall jacking forces are transferred from pipe to pipe along the red line. There is an even load distribution in the straight section (yellow area) and an uneven, eccentric load distribution (red area) in the curved section. This eccentricity results in a reduction of the maximum allowable jacking forces and is taken into account in the calculation according the ATV A 161.
The difference of curved drives compared to straight drives in the design process is the reduction of the maximum jacking load and the calculation of the minimum jacking radius (curve radius plus steering angle of the MTBM).
5. POLYMER CONCRETE TUNNEL SEGMENTS FOR SEGMENT TUNNELING OPERATIONS
The latest development is the bolted and gasketed polymer concrete tunnel segment for one pass tunneling applications.
The difference to pipe jacking is that instead of installing the pipes in the starting pit and then pushing pipe by pipe the MTBM forward, the pipe ring is split in segments which are transported through the tunnel to the Tunnel Boring Machine (TBM) ahead of the tunnel and reinstalled there to a complete segment ring in the shelter of the tailskin (rear section of the TBM). The TBM has its own thrust ring, a ring of hydraulic cylinders, with which the TBM is pushing forward, thrusting from the last segment ring installed.
The benefits of segment tunneling compared to pipe jacking are lower risk of settlements, due to the immediate grouting of the freshly installed segment ring in place as the tunnel boring machine advances, no ring annulus lubrication necessary as well as arbitrary tunnel length and curves possible (segment design must meet the minimum curve requirements). However the major benefit is that the thrust forces of the TBM only affect the first few segment rings and not the whole pipe line like in pipe jacking operations.
The preferred material was RCP, due to the availability, economical advantages and long term history all around the world. But recognizing the weakness of cementitious concrete in a sanitary sewer environment, Portland cement concrete segments need additional protection from inside as well as protection from outside when applications in caustic soil conditions are considered. As protective coatings pre- or post-installed plastic liners are encountered. The safest, but also most time and cost consuming method, is the two pass installation, where the RCP segment tunnel acts as “host tunnel” for the post-installed carrier pipe.
With polymer concrete tunnel segments these cost and time consuming measures can be avoided. It is a one pass installation where the PC segment tunnel serves both purposes as host and carrier for the corrosive sewer.
The first Polymer Concrete tunnel segments, as glued version, have been installed in 2000 in the northern part of Germany. It was the replacement of an old corroded brick sewer with and internal diameter of 1,800mm and 2,000mm.
This design was a forerunner of the bolted and gasketed tunnel segment version. The construction took place in kind of ideal ground condition (cohesive soil) with stable tunnel face and no groundwater head pressure. That was the reason why the contractor had chosen an open face machine with a part face excavation by bucket excavator.
The open face tunnel machine with no head pressure made it possible to install the ring in one step by gluing the profiled segments with the tong and groove joints together with the use of epoxy mortar before the TBM excavated the next section and at the same time filling the gap behind the segment ring with cementitious grout.
6. CONCLUSION
Polymer concrete has definitely benefits in trenchless applications, especially when it comes to installations in a sanitary sewer environment or in corrosive soil conditions. Due to the material properties, new technologies can be used for long distance pipe jacking and curved alignments as well as segment tunneling operations. The savings involved not only benefit the contractor but also the owner and due to the higher buried asset value every individual who contributes with their tax payments for a cleaner and saver environment.


Literature:

[1] Alten, T. et al, 1999, Challenges for the 21st century. Balkema Rotterda.
[2] ATVA 161, Design calculation of driven pipes, Abwassertechnischer Vereinigung e.V.
[3] ASTM D 6783-05, 2005, Standard Specification for Polymer concrete pipe.
[4] Baklaschow, W. Chlopzow, U. Rößler, 2003, Statische Berechnungen in der Technologie des Mikrotunnelbaus.
[5] Blom, C.B.M., 2001, Damage patterns and mechanism of segmented concrete tunnel linings. Delft University of Technology, Delft.
[6] CEN TC 151: Tunnelling machines- Shield machines, thrust boring machines, auger boring machines, lining erection equipment- Safety requirements
[7] Dahl, J., Nussbaum, G., 1997, Neue Erkenntnisse zur Ermittlung der Grenztragfähigkeit von Tübbings im Bereich der Koppelfugen, Tunnelbau Taschenbuch.
[8] Herzog, Max A.M., 1999, Elementare Tunnelbemessung, Werner Verlag.
[9] M. Scherle, M., 1997, Rohrvortrieb, Band 1,2. Berlin.

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