For the design of pipes, the flexibility and bending characteristics are an important factor, in terms of consideration for the correct installation of the pipe system.  For buried pipe applications, the “field-bending” can help to reduce the quantity of needed bends and elbows. The allowed minimum bending radius depends on the pipe wall structure, jointing method, environmental conditions (temperature, sun radiation) and duration of the bending process. For marine application, the bending process is a must for the sinking process and one of the decisive advantage of thermoplastics in comparison to rigid pipe materials.For the design of pipes, the flexibility and bending characteristics are an important factor, in terms of consideration for the correct installation of the pipe system.  For buried pipe applications, the “field-bending” can help to reduce the quantity of needed bends and elbows. The allowed minimum bending radius depends on the pipe wall structure, jointing method, environmental conditions (temperature, sun radiation) and duration of the bending process. For marine application, the bending process is a must for the sinking process and one of the decisive advantage of thermoplastics in comparison to rigid pipe materials.

Determination of bending radius
For determination of the bending radius the strain of the outer fiber and the buckling resistance of the pipe have to be considered separately. For the design all elements of the pipe string, incl. jointing, have to be verified for the bending process. The permitted strain of outer fiber mainly depends on the load period, but typical for Polyethylene and Polypropylene is to define a strain-limit for bending of 2,5% (adequate safety included). 

OD =     Outer diameter [mm], for profiled pipes with inner wall only: OD= ID + 2 * e1
ℇ    =      Strain of outer fiber [%]
Rℇ =       bending radius, strain related [mm

The bending radius to avoid buckling depends on pipe stiffness and wall structure and is for solid wall pipes and closed profile pipes (CPR) calculated as follows:

ID  =       Inner diameter [mm]
e_equ =    equivalent wall thickness, for solid wall pipes: wall thickness
S_FF  =     Spiral factor [-], for solid wall = 1, for profiled wall ca. 0,8
R_B  =     Minimum bending radius, buckling related [mm]

The spiral factor SFF considers the helical profile around the pipe and the distance between the profiles. It should be noted that all equations assume a circular round pipe without ovality, what is especially important for more heavy solid wall pipes with higher weight/stiffness-ratio. In case an additional safety factor can be considered.   The minimum bending radius R_min must be bigger than R_B and R_ℇ  ! The final determination depends further on material characteristic (flexural modulus, creep-factor etc.), the environmental conditions (temperature, sun radiation, wind etc.) and how the bending load is applied. But the below-mentioned values of a bending radius are typical.

The relation between eSDR respectively SDR and SN values can be described as follows:

eSDR =     equivalent SDR class for profiled pipes
E =           short term flexural modulus [N/ mm²]
SN =        stiffness value/class acc. to ISO 9969 [kN/m²] Author: Dipl.-Ing. Stephan Füllgrabe,Krah Pipes GmbH & Co. KG

As part of the century project “Stuttgart 21” extensive infrastructure measures must be carried out in the region of the new underground station of the state capital of Baden-Wurttemberg. This includes the relocation of several large waste water collectors that are “in the way” of the new station. The   existing sewers will be laid (culverted) under the future underground station at several points – pipes and components made of PE100 from Frank GmbH will be used for this project. In several locations the existing sewers are placed (culverted) under the future underground station - for this purpose, PE100 pipes and components by FRANK are used.As part of the century project “Stuttgart 21” extensive infrastructure measures must be carried out in the region of the new underground station of the state capital of Baden-Wurttemberg. This includes the relocation of several large waste water collectors that are “in the way” of the new station. The   existing sewers will be laid (culverted) under the future underground station at several points – pipes and components made of PE100 from Frank GmbH will be used for this project. In several locations the existing sewers are placed (culverted) under the future underground station - for this purpose, PE100 pipes and components by FRANK are used.

 Overview of the Building Project
The terminus station, also known as “Bonatz-Bau”, was opened in 1922. Over the past centuries the infrastructure around the station has developed, so that for the new construction of the underground station many existing lines  need to be severed and re-laid. Picture 1 shows the location of the underground station compared to the terminus station and the course  of the large sewage culverts.

Definition and Operating Principle of a “Culvert

”Wikipedia [2] states that the operating principle of a sewage culvert (see Picture 2) as follows: “A culvert is a pressure pipe to pass under a road, a tunnel, a river or railway tracks etc. The pipe can, for example be a gas, sewage or drinking water pipeline or a groundwater or oil pipeline. In the culvert the liquid overcomes a barrier without pumps having to be used. This is done by using the principle of the communicating pipes, according to which liquids in interconnected pipes always pend to the same level. If now liquid keeps on flowing in on one side, it reaches the same level of height on the other side and can be forwarded almost without any loss of height.” Since the existing collectors (main collector west and Nesenbach) drain in free fall, the fill level in the sewer always corresponds to the current flow rate. In case of low discharge (dry weather - QTW) the filling of the sewer falls back into the dry weather channel. Here, a relatively high flow velocity is generated with a reduced cross-section to prevent deposits. In a culvert, the system of the weather channel no longer works as the culvert would first fill up with water completely before a drain is possible on the other side at the lower gates. This would lead to an extremely slow flow speed, a high dwell time and to depositions within the culvert line. Thus, the side streams are separated in the upper gates – namely in QTW – here, the culvert is always filled as well as in Qkrit and Qmax. Qkrit and Qmax are only flooded when the respective maximum drains are reached. In case of heavy rain all three cross sections of the culvert line are filled. In the lower gates the side streams are led back together again – the further sewage transport is carried out as gravity line.

Introduction of the Subprojects – Material Selection
Main Collector West (Picture 3)
The existing cross section of the collector around the “Kurt-Georg-Kiesinger-Platz” is a circular tubbing channel DN3700 with dry weather channel (Qmax ~ 65000 l/s), that was made in closed construction. In the upper gates of the culvert the separation of the side streams is carried out in 
- Q_TW ca. 400 l/s -> culvert DN800 PE100 pipe -> always in operation,always filled-
- Q_krit ca. 3600 l/s -> culvert DN1600 PE100 pipe -> emptied
- Q_max ca. 47000 l/s -> culvert DN3500   PE100 pipe -> emptied

1= Upper gates
2= Conventional culvert
3= Lower head
4= Receiving water
5= Culvert pipeline
6= Rain weather drainage
7= Dry weather drainage
8= RWA discharge
9= Culvert pipe
10= Drainage line

Culvert Nesenbach (Picture 4)
Here, the existing cross section is a typical tunnel vault with dry weather channel – in the upper gates of the culvert a separation of the side streams happens as follows:
-Q_TW ca. 1500 l/s -> culvert DN1000 –   PE100 pipe
-Q_krit ca. 10000 l/s -> culvert DN2400 –   PE100 pipe
-Q_max ca. 100m³ /s -> culvert rectangular  duct 7,00 m x 3,60 m -cast-in-place concrete

According to Wikipedia [4] “The Nesenbach is a tributary stream of the Neckar with a length of around 13 km. The Nesenbach has “cut through” the basin in which Baden-Wurttemberg’s capital Stuttgart has developed. The small stream used to cross the city from southwest to northeast but has now been replaced along its entire length by the main collector of the same name in Stuttgart’s combined sewer system. [...] As the Nesenbach became more and more polluted over time, it was increasingly vaulted and completely dug in. Today it is the most important main collector in Stuttgart’s sewage system and serves as a sewage and rainwater canal for the entire southern part of the city. It no longer flows into the Neckar near Berg but is fed into the Mühlhausen sewage treatment plant. […]”

Material selection
The original planning provided for reinforced concrete pipes with PE liners as pipe material, the culvert “Cannstatter Straße” was then the first culvert structure at the new station with a nominal width of DN/ID2000 to be designed in this way. During the implementation planning it was determined that the handling of reinforced concrete pipes >DN/ID2000 is not possible in the wide, deep excavation trenches. Furthermore, a solution ..for the large pipe fittings up to DN/ID3500 was needed. 
The advantages of the lightweight, flexible pipe material PE100 and the availability of wound sewer pipes made of PE100 up to a nominal width of DN/ID3500 then led to the redesign of the subprojects culvert Nesenbach and main collector West. 

Pipe Materials and Pipe Production
The production of the wound PE100 sewage pipes DN/ID800 – DN/ID3500 with integrated E-fusion socket (up to DN/ID2400) as well as manholes and components took place at the Frank Kunststofftechnik GmbH in Wölfersheim. As pipe material PE100 Borsafe HE3490-LS was used. Here are some selected material characteristics: Density:  940 kg/m³ acc. to ISO 1183
Poisson’s ratio: 0,38
Short-time E modulus:  1203 N/mm²
Long-time E-modulus:    193 N/mm²
Short-time tensile strength: 29,90 N/mm²
Long-time tensile strength: 18,90 N/mm² Polyethylene (PE100) is a thermoplastic which has, next to a low, specific weight, an extraordinary workability, weldability and flexibility. PE is especially resistant against aggressive media (acids and lies). Furthermore, the molecular construction of the material, which is made of carbon hydrogen, enables a material recycling. 
Polyethylene can be recycled up to 100%. In DIN 8074 “Polyethylene (PE) – Pipes PE80, PE100 – Dimensions” and DIN 8075 “Polyethylene (PE) pipes – PE80, PE100 – General quality requirements, testing” the following statement regarding long-term strength has been made: “The operating time previously estimated at 50 years can be extended to at least 100 years due to many years of testing and experience for PE pipes at application temperatures of 20°.”

Production of Spiral Wound Pipes
In a molten state, the moulding compound is spirally wound onto a metal mandrel in form of a continuous overlapping strip (Picture 5).  A second, functional and/or inspection-friendly inner layer can be applied via a co-extruder. A metal mandrel, which determines the inside diameter of the pipe, serves as calibration. The pipes are slowly cooled by a blower. In this way, residual stresses caused by volume shrinkage and the production process can be reduced. Different wall thicknesses and profile geometries can be achieved by winding the moulding compound in several layers and varying the amount of material applied (Picture 6). PKS® sewer pipes are available in the nominal sizes DN 300 to DN 3500. The basic wall thickness is determined in accordance with the minimum requirements of DIN EN 13476-3 or operational requirements. Production and quality assurance are carried out within the framework of the general building approval Z-40.26-359.

Welding Connections 
In the PKS® pipe system (profiled sewer pipe system) the single pipes, manholes and accessories are welded according to the standard with an integrated heating socket (E-fusion), this connection technology is available up to DN/ID 2400. During production socket and spigot are attached  to the pipe. When the installation is carried out on site, the spigot is put into the socket and welded together (Picture 7). 
The welding parameters are transferred to the welding device using a barcode. The welding device automatically records the welding process.

Static Calculation of the Pipes
Generally, the static calculation of buried sewer pipes is carried out in accordance with ATV-DWA A 127. For the preselection of a suitable pipe, this calculation method was also used. Much more precise and illustrating the installation conditions better is a calculation using Finite Elements (FE). With this method the installation conditions and the pipes are divided into small elements (finite elements). This results in a very precise calculation. As the culvert pipes were completely encased in concrete, the groundwater level and inner pressure values were decisive for the calculation. For flexible pipe materials three checks always have to be performed - deformation, stability and stress check (Picture 8). An essential condition of the contracting authorities was that the evidence of a service life of 100 years could be given.  Details Main Collector West
After the basic decision for the pipe material PE100 and the static calculation had been made, the work planning of the culvert sections was carried out in 3D. The individual parts for the production drawings were taken from the overview design (Picture 9). First, component 1.1 was installed in the lower head to be able to connect the necessary bypass for the duration of the construction period. Since component 1.1 (Picture 10) could not be assembled in one piece, production in the factory was divided into two parts. After delivery in Stuttgart, the homogeneous joining was carried out by hot gas extrusion welding. An essential aspect in the selection of the pipe material was the weight and thus the possible handling of the pipes and components in the deep and wide excavation pits. The concrete pipes up to DN3500 included in the original planning could only have been moved with a very large mobile crane. However, there was no space available on the narrow inner-city construction site for a corresponding mobile crane. The PE100 pipes could be completely moved and installed using the existing tower crane. 

The weight of the individual pipes DN3500 with a length of 5.50 m was approx. 5 tons. As the transports were only allowed to be carried out in times of low traffic volume within the scope of the “traffic law exemption permit”, the pipes were also unloaded and brought into the excavation pit during the night (Picture 11). A further advantage of the pipes and components made of PE100 is the high degree of prefabrication, which made it possible to prepare complex components and pipe fittings in the workshop. For example, DN3500 pipe bends (Picture 12) and access shafts could be prefabricated, delivered and installed.

Stairways (Picture 13), which were necessary to comply with accident prevention regulations, could also be prepared accordingly in the inclined and ascending sections from the head and to the lower head. For the operator of the culvert structures, the “Stuttgarter Stadtentwässerung”, a uniform, bright, inspection-friendly inner layer of the pipes and components was very important. This was made possible by the coextrusion process used to manufacture the pipes. The transitions from rectangle to round (Picture 14) in the head could be realized with unwinding made of PE sheet material.Stairways (Picture 13), which were necessary to comply with accident prevention regulations, could also be prepared accordingly in the inclined and ascending sections from the head and to the lower head. For the operator of the culvert structures, the “Stuttgarter Stadtentwässerung”, a uniform, bright, inspection-friendly inner layer of the pipes and components was very important. This was made possible by the coextrusion process used to manufacture the pipes. The transitions from rectangle to round (Picture 14) in the head could be realized with unwinding made of PE sheet material. 

Details Culvert Nesenbach
For the Nesenbach culvert, the main cross-section (Qmax) must be designed as an in-situ concrete rectangular channel, as no precast elements could be used for the dimension (7.00 m x 3.60 m). For the two “small” pipes DN2400 (Qkrit) and DN1000 (Qtw) the same construction method was used as for the main collector West. The pipes and components made of PE100 were constructed as far as possible in the same way as the main collector west. 
There, the installation in the very deep excavation trench (20 m) was also carried out with a tower crane. The individual pipes DN1000 and DN2400 were welded using the integrated electrofusion socket. The tightness test was then carried out directly with a socket testing device (Picture 15). 
The method “L” from DIN EN 1610 “Testing of individual joints” was used. This ensured that only “tight” pipe connections were encased in concrete. The lowest point of the Nesenbach culvert is the pump house. As with the main collector west, the pipe cross-sections Qkrit and Qmax are emptied after filling (after a rain event) to prevent rotting and odour. 

To be able to construct the underground building in advance, “insert parts” (Picture 16) had to be delivered long before the actual pipe sections to be set in concrete. The pipe sections “Crossing” and “Climbing” were then connected to these inserts after the construction of the excavation pits.  Conclusion(August 2018) - main collector West in operation - culvert Nesenbach under construction) - Feasibility produced by PE100 as pipe   material- High degree of prefabrication- Easy handling with tower crane due to   low weight- Permanently tight due to welded joints - Adjustment work on site possible- Flexible material compensates for   settlement differences - Guaranteed long service life – design    period --> 100 years

Literature / Sources
[1] https://www.bahnprojekt-stuttgart-ulm.de/aktuell/ 
[2]https://de.wikipedia.org/wiki/D%C3%BCker
[3] https://link.springer.com 
[4]https://de.wikipedia.org/wiki/Nesenbach
[5] https://www.bahnprojekt-stuttgart-ulm.de | Foto: Achim Birnbaum,  Stuttgart Autor: Jochen Obermayer, FRANK GmbH, Mörfelden-Walldorf / Germany

Costs optimized pipe due to the distinction of working pressure and pipe stability, while designing

Imagine: Tuesday morning at the technical department of Krah in Germany. A customer from South-East Asia is calling and asking for a pipe with 10 bar working pressure, because a big tender is new in the market DN 2000 and 10 bar, total length 3 km, application “closing an open channel”. So, should we really suggest a solid wall pipe SDR17, for a pipe DN/ID2000 – or will make it sense to ask questions and think? Our aim is it to get the tender and to reduce the investment for the client, because usually it is tax-payers money in infrastructure projects.

1st Question: “What is the requested inside diameter of the pipe?” 
Sometimes the pipe is described with inside dimensions and sometimes with outside dimensions, depending on the referred standard.  For example, if the tender for a PE-Pipe is according to DIN8074, the corresponding Krah pipe would be DN/ID1800, because DN2000/SDR17 will have a hydraulic inner diameter of 1765 mm.
Due to the information that the application is “closing an open channel, for transport of raw water” you could ask the 2nd Question: “What is the filling level of the pipe?” or “Is the pipe fully filled?” 
Usually the answer is: “no” or “could be, when a valve is closed”. At this moment you know, that the 10bar requirement is because of the installation conditions and not because of a real inside working pressure. 3rd Question: “What is at the ends of the pipe (pumps, lake, river, plant etc.)” 
The reply is important for better understanding what internal pressure could be theoretical there and what is necessary for operation. 4th Question: “How the pipe is installed (exposed, buried)?” and if buried: “What is the soil quality (backfilling, proctor density etc)?”
That and also information about additional loads are crucial for the needed stiffness and stability of the pipe.  
So, finally we have an idea of the needed structural load capacity of the pipe.Assumed the customer is telling us, that the pipe is connecting a municipality with a sewage treatment plant and there is no integrated pump and no significant geodetic height difference (remember the pipe is replacing an open channel). It become very clear that the original reason for choosing PN 10 classification was not the internal pressure, but the external load capacity of a PN 10 wall thickness. So, what is the external load capacity of PN 10 pipes and how it is related to stiffness classes?
The answer is simple:  PE 100, PN 10 is equivalent to SN 20 ! Any PN can be translated to SN by considering the typical mechanical characteristic of Polyethylene*

* flexural modulus of 1000 MPa

Honestly, very often we get the reply, that there would be no difference between internal and external capacity and people insist in PN 10. Very clear that people who insist in PN 10 are not aware that the load direction is decisive for the static design and ignoring that is wasting a lot of money. The difference in internal and external load design is briefly described in the table below.

Back to the first questions, what should we quote? If we quoted standard PE100 pipes DN/OD 2000, with SDR 17, length 3km – the total weight is 2408 tons. Jointed by butt-welding (Cost of one machine approx.. 100.000 EURO, speed of one joint 8 hrs – outside the trench. If we quote Krah-Pipe with same stiffness/external load capacity, with PN 1 or PN 1,5 internal pressure load capacity, same length of 3km – the total weight is 1220 tons. Jointed by integrated electro-fusion (Cost of one machine approx.. 5.000 EURO, speed of one joint 30 min. – inside the trench). Material Saving = 1188 ton = approx. 1,2 Billion $
These Material savings of 50% for profiled pipes with same SDRis valid for all SDR classes !

Saving material means:
• Saving costs for the client (and more projects can be realized with the same budget)
• Saving installation time means saving costs again, and a quicker realisation of the project

Also: The described case is a true story and finally the necessary pipe stiffness was SN 7.2, calculated and verified by third party. That means the saving was not 50%, it was almost 70%  !

So, think! Safe money and protect the environment. Did you know? 
Simplified for Polyethylene we can use the ratio between short and longterm flexural modulus of  “5.5 : 1”  and receive by inserting the typical stiffness formula into the external load formula following the simple relation. Simplified for Polyethylene we can use the ratio between short and longterm flexural modulus of  “5.5 : 1”  and receive by inserting the typical stiffness formula into the external load formula following the simple relation. p_o=0,052 ∙ SN Author: Dipl-Kfm. Alexander Krah, Krah Group

High safety requirements in terms of hygiene and security of supply are the most important aspects in the planning and construction of drinking water supply networks. Therefore, it is necessary to use all available resources as far as possible. Local resources should be available for the local residents inside their region.

A proper transportation of potable water by appropriate equipment is essential. During the configuration of  a supply system, special attention must be paid to the efficiency of the pumps, to the inner pipe diameter  and to the proper selection of pipe material. But it is just as important to consider all constructions of  potable water systems. They must correspond to the rest of the supply system in terms of functional reliability, safety requirements and durability.

Prefabricated constructions out of HD PE are used in drinking water supply systems for over 50 years. They are characterized by
•  Low weight to reduce the transport costs and make the installtion easy
•  High strength to withstand all internal and external loads
•  High flexibility to allow a wide range of variety of styles and shapes
•  Short time for the installation on site
•  Low maintenance based on the smooth interiors resist growth of algae and bacteria and is easy to clean
•  Highest standards in hygiene
•  High level of safety
•  Guaranteed reliability
•  Economic efficiency

They have been developed for sparsely populated areas (e.g. mountains) where central drinking water supply is not economically feasible.
But a clear separation between a central and a decentralized drinking water supply is disappearing. Drinking water is a precious good. Where drinking water can be obtained from springs or wells, it is used to supply the people. A supply system consists of pressure pipes, manholes and reservoirs. The complete system has to be tight, fast to install, easy to operate and should have a possibly long lifetime and must meet all hygienic requirements. The great advantages of the prefabricated constructions that were learned to appreciate in the mountains also help in densely built-up areas. Now, they are used regardless of the population density in the drinking water systems.

The basement for all constructions is usually a spirally wound polyethylene pipe with a double walled structure. This double wall contains an internal solid wall layer, a reinforcing profile and a final external solid wall layer. These kinds of pipes provide low weight, high ring stiffness and low costs. They can be designed in a way that they withstand all internal and external loads. Together with the characteristics of the material, it is ideal for manufacturing collecting shafts and water reservoirs.The following report shows that structural double-walled pipes are a very attractive option for  the fabrication of storage tanks and collecting shafts regarding costs and safety during installation and usage.

Situation in the drinking water supply area

The following figure shows a scheme of drinking water supply area. The water from different sources is collected and transported through pipes to the collecting shafts. Here, a quality and quantity control can be made. After this, the water is forwarded to a storage tank. The storage tank is used as a buffer; if necessary, the water can be treated here additionally. At last supply to the consumer can be carried out.
The following constructions are required for a safe drinking water supply: • Source collection chambers
•  Well shafts
•  Transport Pipes
•  Drinking water reservoirs
•  Valve Chambers
•  Firefighting Tanks

Requirements for constructions
The constructions must complete the supply system of pressure water pipes. Moreover, they must fulfil the requirements of safety and reliability. They are meant to collect and store potable water. At the same time the tanks and shafts must leave room for the measuring and controlling technology such as for pumps to  pass on drinking water. However, the design should enable for easy inspection and maintenance as well. Further requirements are: • Short installation time
• Little maintenance
• High level of safety
• Guaranteed reliability
• Economic efficiency

To fulfil all these requirements prefabricated constructions are used. The basis is always a structured double wall pipe that is made out of polyethylene. Choice of material - Polyethylene
Polyethylene has proven itself successfully for potable water applications for a long time. For more than 50 years it has been used for water supply systems as well as for collecting systems. Its neutral properties towards taste are constantly given. It has also been tested successfully for use of food storage. It is tasteless and resistant to corrosion. Pipes and other semi-finished products made from polyethylene provide a smooth and pore-less textured surface. These characteristics reduce the maintenance work and simplify the cleaning procedure. Because of the mentioned advantages it makes sense to design the complete drinking water construction out of this material as well.The welding of all inlets and outlets create a complete plastic system, which guarantees tightness and safety. Large diameter HDPE pipes with structured walls
The polyethylene pipes with structured walls have been originally designed for gravity lines.  They provide low weight, high ring stiffness and low costs. The wall structure is a combination of a solid liner and a reinforcing profile. The pipes are produced and controlled according to the international standards DIN 16961, EN 13476 and ISO 9969. The pipes are helically wound on a steel-mandrel. The mandrel ensures the inner diameter of the pipe. All pipes can be delivered from ID 300 to ID 3500.  The winding technology allows an individual and customized design of the pipes for each project to fulfil all requirements regarding internal and external loads like inner pressure, soil- and traffic loads. Thus, any ring stiffness can be achieved and pipes for high loaded drain systems and manholes, even in large diameters, can be manufactured. All pipes that are used for drinking water supply have a blue inner surface. For pipes, tanks and shafts which are buried and which have to withstand heavy soil or traffic load, the ring stiffness is one of the most important design factors. The ring stiffness SR [kN/m²] is defined as the capability of the pipe to withstand the external pressure or forces. It depends on the moment of inertia of the wall, on the mean radius and the modulus of elasticity. E_c  [Nmm²] =  Modulus of elasticity
l [mm4/mm] = Moment of inertia of the wall
r_m [mm] =  Mean radius One of the major advantages of these pipes is the high ring stiffness at low weight. A comparison to solid wall pipes with same stiffness makes it obvious. A typical pipe that is used as a storage tank with a diameter of 3000 mm is a combination of a solid inner  liner with a reinforcing profile and a final external layer. This pipe has a long time ring stiffness of S 50years = 3.1 kN/m². To achieve the same ring stiffness with a solid wall pipe a wall thickness of s = 89 mm is necessary. Whereas the structural double-walled pipe has a weight of 359 kg/m, a solid wall pipe has a weight of more than 828 kg/m.

The total weight of a complete prefabricated collecting shaft ID 3000 mm is about 1200 kg. A comparison with rigid concrete material makes the advantage in terms of weight even more obvious: a concrete construction would have a weight of about 18.000 kg (18 tons). Factory control and assurance of quality
The whole production process is monitored by a quality management system. Which includes the internal quality control and the external (third-party) quality control. The internal quality control is divided into three phases. Phase one - intake control
When purchasing the raw material, the manufacturer’s test report must routinely be requested. On receipt, an intake control regarding melting index and density is carried out. After the material has passed the intake control, a test-pipe is produced to verify its technical properties. Melting  index and density are once more tested at this stage. Furthermore, the durability of the material under short term tension is tested, a bending test is carried out and the heat resistance is examined. After this intensive procedure, the material is preliminary approved for production. Phase two – during production control
During the production the complete production process including the production parameters and all working steps and must be continuously supervised and documented. So, corrections can be made immediately. The most important dimensions are measured continuosly. Phase three – finished product control
After the production, the final product (pipe, fitting, manhole) is tested and compared with the requirements. This is the final control of all products where all parameters like surface, dimensions, material, colour and marking are checked. The results of the internal control testing shall be recorded, and, as far as possible, statistically evaluated. Records are kept for at least five years, and submitted to the inspection body on request. If the product passes all tests a quality certificate according to DIN EN 10204 can be made.The external quality control should take place twice a year. Of course The frequency of all tests must be agreed in accordance with the external quality control party – which can be “TÜV Rheinland GmbH”.All checked samples shall be considered representative for the whole production. Raw material tests should be carried out before adding the product to the production cycle. Undamaged / non affected samples can be sold afterwards. Those tests made in Production Plants are exceeding the requirements of the international standards like DIN16961, DIN EN 13476, ISO 9969, DVGW W300, etc. Collecting shafts
The collecting shaft serves as a control station where quality and quantity of the water can be checked before it flows into the reservoir or continues to the transport lines. Every collecting shaft has to be divided in a dry chamber and one or several wet chambers. The entrance is always located in the dry chamber, which serves as a base for maintenance and service work. The different water chambers are arranged in such a way, that each spring is directed into a separate intake chamber which is also divided into an intake area and a settlement area which are separated by a scum board. Thereby it is possible to treat them individually. At the same time some particles like sand can settle down.The standard production has an inside diameter of ID 1200 up to ID 3500 mm. All pipe constructions consist of the structural double wall. They have a side entrance ore a shaft lid and can be delivered with a conical entrance. Whereas constructions with DN 1200 include only one intake, constructions with DN 3500 can include up to six intakes. A special drainage facilitates the cleaning. The water flow of each spring can be measured by installing a rectangular or a triangular-notch thin plate wire.

Drinking water reservoirs
Drinking water reservoirs are used as buffer for fresh water. Thus, variations in consumption can be well- balanced. The size of the reservoir has to be chosen in relation to the amount of fresh water supply, so that a sufficient supply of drinking water can be guaranteed at all times. The water rervoirs consist of an operating room and two or more storage chambers. This chambers are accessible from the operating room via a stainless-steel safety door which is located in the seperating wall. The operating room includes all necessary valves and measuring equipment. Through sight glasses, the internally illuminated storage tank can be visually inspected at any time. Each water reservoir has also an intake, an overflow, an extraction, and an outlet. The standard constructions are produced with an inside diameter from ID 2000 up to ID 3600 mm. They are constructed with structural double wall pipes. The front and end walls are made of reinforced PE plates. The entrance is exclusively located in the dry chamber. Exceptions to this rule are storage tanks for fire-fighting purposes. They are constructed without a dry chamber. The storage volume can be freely chosen. Usually the storage tanks have a volume of 5 m³ to 800 m³. Valve chambers
Valve chambers are required to separate pipes, ventilate the supply lines, reduce pressure or measure the water quantities. So that a safe drinking water supply is guaranteed. Design and installation
Flexible plastic pipes provide a higher durability than conventional rigid pipes. In the case of a buried rigid pipe, it is the pipe alone that constitutes the structure and is subjected to the full load, whereas in the case of a flexible pipe, it is the interaction procedure between the pipe and the surrounding soil that constitutes the structure [1]. The pipe always carries the loads together with the surrounding soil. A correct design of plastics pipes and shafts requires consideration of the actual situation at site regarding the product itself, the soil conditions, the installation procedure and the different types of loading.

The pipe design method considers the following values:
• Information about construction site, pipeline zone and the main filling. It can havedifferent modulus of deformation and Proctor density of compaction.
• Information about external loads
• Installation including the trench excavation
• Height of ground water level
• Working conditions, such as outside temperature or operating pressure

The installation of the pipes is as important as the accurate calculation beforehand. Pipes and shafts perform only as well as they are installed. A proper soil investigation and a definition of the backfill parameters are necessary for the pipe design. It is commonly known that the deflection of all flexible pipes and their capability to withstand the external loads are strongly influenced by the installation conditions. Because of the interaction between the pipe and the surrounding soil, the installation should be done properly. The embedment is the most important part of the soil- pipe structure. Regarding the compaction around the pipes, tanks or shafts, it is necessary, that the backfilling material is easily and homogenously compactable. The backfilling material should be noncohesive soil and free of big stones. The compaction of the soil should be homogeneously and evenly distributed with the result that deflection is limited.

The installation time of the constructions should be as short as possible and include:
• Supply of all materials
• the excavation
• If necessary; wrecking of existing constructions
• Preparing the bedding
• Installation of the construction
• Connecting the service pipes
• Backfilling
• Surface reinstatement

Our Experience has shown, that depending on the constructions, the installation time can vary between one and nine days. Such a short installation time can be realized due to the high degree of prefabrication in the factory. The constructions are supplied ready for operation. Only the connection of the service pipes must be done on site.
The following pictures show the installation of a collecting shaft. The transport is done by a small truck. After preparation of the excavation and the foundation, the shaft is positioned and the service pipes are connected. A minimum working space of 0.5 m shall be provided for correct backfilling. No grain size should come below 32 mm. The material is to be filled equally in layers of 20 - 40 cm thickness and must be carefully compacted. If necessary, an extra inlet or outlet can be made at any time during the installation. After this  the old shaft can be wrecked. The last step of the installation is the reinstatement of the surface after the competition of the backfill.

The following pictures show a water reservoir during transport, the installation and after the reinstatement of the surface. After the trench excavation, the bedding must be prepared. The bedding must ensure an even pressure distribution under the pipe in the surrounding area. According to EN 1610 the bedding type 1 should be suitable. The thickness of the lower bedding shall not be less then 150 mm.  After that the compaction of side fill and main backfill can start. The initial backfill directly above the tank should be compacted by hand or light compaction equipment. The side fill and the main backfill are usually compacted with a medium weight vibration stamper. All remarks about the material, layer thickness and working space are valid for the storage tanks as well. Because of their low weight, easy handling and the fact that all constructions were prefabricated, both introduced constructions have been completely installed an extremely short time and in accordance to all hygienic requirements.
ConclusionThe structural HDPE pipe is the most important component of constructions like collecting shafts, water reservoirs aund valve chambers. It has been shown, that a fast and cost-effective installation of an absolute tight, safe and reliable drinking water supply system can be realised. As the pipes are designed individually for each project, a highly loaded system even in large diameters can be produced.
Author Andreas Wittner, Hawle Kunststoff GmbH