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Buried Flexible Pipelines AS2566
- 7. Structural Response to Loading
- 14. Design Loads due to Trench & Embankment Fill
- 40. Trench Shields
- 43. Thrust Blocks
Editor's Notes
- Presented by Geoffrey D Stone C.Eng FIMechE; CP Eng FIEAust RPEQ. Principal Blenray Pty Ltd ( Design Detail & Development) [email_address] 0402 35 2313
- The designer needs to establish the data shown in this slide in order to provide the field with a competent design. In addition if field conditions vary from those on which the design is based then reference should be made to the designer. This may occur when rock, water or other unexpected conditions arise.
- Soils data is generally provided by a soils testing company. The difficulty arises as the information provided is not in the form that can be readily used by the designer. A designer needs the modulus of the soil. Data provided may be in the form of penetration count or other criteria as covered by AS 1289.
- Extract from AS 2566.1 Table 3.2. Note that this table provides a conservative solution for native soils and embedment. In un-compacted weak soils engineering judgment should be used. It is difficult to imagine that an external load can be transferred to a pipeline in very weak soils. In fact such weak soils would not withstand the load from a truck wheel. Other solutions need to be investigated by the designer.
- The following slide shows the recommended minimum trench dimensions. However these may need to be adjusted to meet the standard width of the excavator bucket. Deep trenches where trench shields are not employed will be benched or contoured. This provides significantly different conditions in design for the soil/pipe structure.
- This detail shows a standard trench design. It is an extract from AS 2566.1 Figure 3.1 Embedment Geometry
- These diagrams show how the structural loading is carried by a soil/ flexible composite structure. The side support is provided by the embedment and the native soil resisting deflection.
- The long term properties should be used when the native soil or the embedment are not good. Poor soils allow loads to be transmitted to a flexible pipe without side support. As a result the pipe may deflect or be subjected to buckling. Refer AS 2566.1 Buried Flexible Pipelines-Design and The Supplement. 5.1.2 Long-term design basis A long-term design basis is assumed to be the critical design consideration. Unless otherwise specified, this design basis is assumed to be 50 years. If this is the case the service life can be expected to exceed 50 years. The 2-year value for the ring-bending stiffness of the pipe may be used in the equations of this Section to represent the design basis of 50-year deflection, ring-bending strain and buckling response
- Stiffness is a property of pipe used in the design of buried flexible pipelines. For a homogenous material such as ABS, PVC, PB & PE this can be calculated based upon AS2566.1 Buried Flexible Pipelines-Design using the modulus and dimensions of the pipe. The ring-bending stiffness for pipes made with homogeneous materials can be determined by calculation. Plastics materials exhibit creep when subjected to sustained long-term loading, hence the long-term modulus of elasticity for products made with these materials are lower. Calculations of long-term ring-bending stiffness for plastics pipes are therefore made using the apparent long-term bending modulus of the material determined from the regression characteristics of the material established from long-term type testing. It should be noted that the material’s modulus, when responding to instantaneous loading, is still retained at or near to its initial value. NOTE: ‘Long-term’ refers to the duration of the applied loading. It is independent of the age of the material. The 3-minute and 50-year values for the ring-bending modulus of elasticity are published for most plastics products and may be used as the basis for design. Where available, these have been listed. Where this data is not available, regression information should be obtained to determine the long-term stiffness value. RING-BENDING STIFFNESS Ring-bending stiffness is an indication of the ability of a pipe to resist deflection. It is determined by deflecting the pipe using line loading and no side support. Ring-bending stiffness has an influence on the deflection, strain and buckling performance of the pipe when buried in the ground, although this is normally of less significance than the stiffness of the embedment. However, where weak or soft soils exist or where there is a high water table, pipe ring-bending stiffness becomes significant. The method given in AS/NZS 2566.1 is only applicable to pipe products that have an initial ring-bending stiffness equal to or greater than 1250 N/m/m and long-term ring bending stiffness equal to greater than 625 N/m/m. AS/NZS 2566.1 does not exclude the use of pipes with lesser initial and long-term ring-bending stiffness. However, special requirements for design and installation need to be considered (see Paragraph C1.1).
- Thrust block design software is available from pipe material manufacturers.
- The designer needs to pay special attention to local conditions where shear stresses may be excesses. These arise from differential settlement or expansion 0reactive clays). Rocker joints can be provided to meet the expected conditions. For large D/t pipe dimension ration s care needs to be exercised as the pipe may ovalise and create leaks. Such leaks in reactive clays will cause soil swelling and pipes will be exposed to greater strains. Hydrophilic materials are generally used to provide a seal at a puddle flange embedded in the concrete wall or floor. Care needs to be exercised to prevent the expanding material destroying the concrete by shear local to the puddle flange.
- Rising water tables can lead to failures as differential loads are applied to a pipe. Large D/t ratios and strain sensitive materials (GRP) are particularly susceptible to damage. There may be a need for a geotextile liner in the trench to enable ground water to drain.
- Refer to AS 2566.1 for definitions. Particular loads may be defined by authorities such as airport managers or railways.
- Refer AS2566.1
- Refer AS 2566.1 & Commentary. National Road Codes may provide different loading data. Construction loads could exceed normal operating loads.
- The consideration of unsustained loads needs to be considered in any design by the system designer. The material manufacturer needs to understand the environment and application that the material will be considered for in the design. Hence it is necessary to understand these loads and how they are applied. In addition, should there be a failure to investigate or a system to be reviewed before placing into service the unsustained loads need to be considered. Always err on the side of caution with such loads as their extent and frequency is so difficult to predict. Local conditions and regulations may require an alternate approach to design for unsustained loading. See earthquake loading below. IN any design process a risk management study is required particularly of the unsustained loads. Earth quake is covered in Australia by AS 1170 but is very specific to Australia. Any other country has different criteria for earthquake design as earthquakes manifest themselves in different manner depending upon the ground movement. The behaviour of the building is paramount in the movement of pipe supports that impose displacements and acceleration of pipe supports and hence on piping. A rigorous analysis using displacement and acceleration combined with the natural frequency of a piping system is necessary. This is rarely undertaken by designers of piping systems because of the low risk compared to the overall damage that may result in such an event.
- The pages below cover this topic
- A further modified form of Watkins, modified Iowa formula has been adopted in AS 2566.1. It has been written in a form similar to that of the publications by the Water Research Centre (Refs 3 and 4) with the pipe ring stiffness term in the denominator. Characteristics of the equation are as follows: (a) The critical deflection criterion is assumed to be the long-term deflection, i.e. 50 years unless otherwise noted. (b) Horizontal and vertical deflections are assumed to be the same. That is for a uniform embedment, the vertical to horizontal deflection ratio is unity at the lower values of the soil to pipe stiffness ratio, but in reality increases to about 1.5 for the higher values of soil to pipe stiffness ratio (Ref. 5). (c) Deflection lag factors are replaced by long term values of all parameters, i.e. pipe stiffness, soil stiffness and loading. For pipe stiffness, the two-year value may be used. (d) The bedding constant ( K ) is dependent on the depth of the haunch support and the embedment material. For the uniform embedment support of the pipe, specified in AS/NZS 2566.1, K is assumed to be a constant and equal to 0.1 . (e) The effect of the native soil in supporting the pipeline is accounted for in the effective soil modulus ( E ′) as adopted from Ref. 5 in AS 2566 Commentary.
- Trench fill or embankment fill and superimposed dead and live loads, in the long term, cause strain in the pipe wall. Deflection of the pipe results in ring-bending and ring compression strains. Ring-compression strains are generally small compared to ring bending strains and thus have not been considered in the design approach in AS 2566.1. AS 2566.1 Equation 5.3.1 (Refs 2, 3, 4 and 6) predicts the maximum tensile ring-bending strains occurring at the obvert and invert. Equation 5.3.1(2) shows that for a given deflection and pipe diameter, ring-bending strain increases linearly with pipe wall thickness. The shape factor adjusts strain values to account for the pipe ring shape. Where the pipe ring shape is an ellipse, the shape factor is three. Where the pipe ring shape varies from a true ellipse, i.e. the horizontal deflection is some degree less than the vertical deflection, a larger value is applicable. Analysis has found (Refs 2, 6 and 7) that the shape factor increases with decrease in the pipe stiffness to the soil modulus ratio. AS 2566.1 Equation 5.3.1(3) was developed by Leonhardt to model experimental results obtained by Molin (Ref. 6).
- Where a risk of unscheduled transient pressures due to water hammer of known magnitude (calculated) exists, an equation of the following form needs to be satisfied: P w + P s ≤ 1.25 P all where P w = internal working pressure, in megapascals (daily maximum pressure to which the pipeline will be subjected under all operating modes including transient pressures during mode changes) P s = additional internal pressure to P w, in megapascals (short duration maximum pressure to which the pipeline will be subjected, e.g. field testing [less than 24 h duration] and transient pressures due to power and equipment failure)
- Both internal positive pressure and external load generate tensile strains. Even though internal positive pressure causes a degree of ‘rerounding’, the strain from a combined load is still larger than that from an external load on its own. The combined loading is considered in AS/NZS 2566.1, using an interaction equation. In Equation 5.3.3, r c is used to represent the beneficial rerounding effect (Ref. 8). The interaction Equation 5.3.3 for combined loading is derived in accordance with overseas Standards (Refs 2 and 9), where account is taken of the combined effect of stresses due to internal pressure and external loadings.
- Because external loads lead to pipe wall compression, buried pipes have a tendency not only to become oval, but also to buckle. Unless this tendency to buckle is checked, satisfactory values of the deflections and strains determined in AS/NZS 2566.1 do not of themselves indicate that the design is satisfactory. Control of buckling is handled by ensuring a sufficiently large margin of safety between the full external load, including external hydrostatic loads and negative internal pressures, and the calculated buckling capacity. The adopted buckling equations are— (a) for H equal to or less than 0.5 m or where substantial soil support cannot be guaranteed, Timoshenko’s equation 5.4(4), (see Refs 3 and 4); and (b) for H greater than 0.5 m, AS/NZS 2566.1 uses the continuum principle developed by I. D. Moore, (see Refs 10 and 11), which in the form given in Equation 5.4(5) yields similar factors of safety to those obtained using the more conventional formulae based on Lusher’s equation (Refs 2, 3 and 4) except that the soil stiffness has greater influence and the pipe stiffness less influence on the calculation. Timoshenko’s equation predicts a buckling resistance pressure for a condition of uniform external pressure without allowance for soil support, whereas Moore’s equation is valid only where external soil support is present.
- Hoop stress is the primary design criteria for thermoplastic pipe materials. It is determined by undertaking a series of burst tests at different stress levels. The data is then statistically determined in accordance with ISO /TR 9080 using a standard extrapolation method. The 97.5% lower confidence limit is then used along with a factor of safety of 1.6 to determine the design class of thermoplastic and thermosetting materials at that temperature. The hoop stress is dependent upon the resin type, temperature and time.
- Trench fill or embankment fill and superimposed dead and live loads, in the long term, cause strain in the pipe wall. The designer is referred to AS 2566.1 Supplement for the effects of strain.
- Creep is a property of thermoplastic materials that basically allows the stresses resulting from an applied load to be reduced. When a pipe element has relaxed due to creep and the load is reversed then the original properties of the material apply initially to the negative strain. The impact on the system is that loads are also reversed until the creep properties relax the pipe. The designer needs to consider loads in the positive and negative direction of pipe elements. If the load reverses frequently the pipe is exposed to repetitive loads. Such an environment promotes fatigue damage in thermoplastic materials.
- The temperature of the pipe wall is used to determine a number of factors in the design of a piping system The difficulty comes in determining what design temperature to be used. In a practical situation the pipe will contain fluid that is at one temperature and the pipe surface at another temperature. These temperatures will vary during the life of the piping system. Thermoplastic pipe has a relatively thick wall and a temperature gradient will occur across the wall of the pipe. The temperature may vary due to Ambient diurnal temperature variations Flow rate Fluid temperature range Process conditions Installation ambient temperature The designer therefore has to determine an appropriate temperature range for design. A traditional approach is given below for aboveground pipe.
- Refer AS2566.1 & Commentary
- The pages below cover this topic
- Material characteristics ABS (Acrylonitrile Butadiene Styrene) ABS is suitable for the conveyance of potable water, slurries and chemicals. Being non toxic ABS complies with the toxicological requirements of the British Plastics Federation and the British Industrial Biological Research Association (BIBRA) code of practice for food usage. ABS Pipe Systems are very suitable for chilled water applications, due to its low temperature properties. ABS is unsuitable for use in compressed airline systems as the aromatic hydrocarbons prevalent in compressor oils will break down the polymer and result in severe failure. Good chemical resistance Good abrasion resistance Good material strength and high impact resistance Operating temperature range -40°C to +70°C Solvent welding PVC-U (Polyvinyl Chloride, Unplasticized) PVC-U has excellent chemical resistance across its operating temperature range, with a broad band of operating pressures. Due to its long term strength characteristics, high stiffness and cost effectiveness, PVC-U systems account for a large proportion of plastic piping installations. PVC-U is resistant to most solutions of acids, alkalis, salts and organic compounds miscible with water. It is not resistant to solvents, aromatics and some chlorinated hydrocarbons. PVC-U is not at all suitable for compressed air systems. Environmental resistance to aggressive caustic and acidic fluids Good abrasion resistance Operating temperature range 0°C to +60°C Bell-and-Spigot gasket-sealed joint Solvent cement joint fusion welding PVC-C (Polyvinyl Chloride, Post Chlorinated) PVC-C has excellent chemical resistance across its operating temperature range, with a broad band of operating pressures. Due to its long-term strength characteristics, high stiffness and cost effectiveness, PVC-C systems are suitable for a wide diversity of plastic piping installations. PVC-C is resistant to many acids, bases, salts, paraffinic hydrocarbons, halogens and alcohols. It is not resistant to solvents, aromatics and some chlorinated hydrocarbons. Environmental resistance to aggressive caustic and acidic fluids Good abrasion resistance Operating temperature range 0°C to +60°C Solvent welding PP (Polypropylene) Polypropylene is suitable for use with foodstuffs, compressed air, potable and ultra pure waters, as well as within the pharmaceutical and chemical industries. Polypropylene like all plastics is adversely affected by UV radiation unless manufactured with UV inhibitors and requires insulation or a protective coating if installed outside. Environmental resistance to most organic and inorganic chemicals Good material strength and fatigue resistance Operating temperature range -20°C to +110°C Fusion Welding PE (Polyethylene) When compared to other plastics, PE shows excellent diffusion resistance, and because of this property, polyethylene has been successfully used for the safe conveyance of gases for many years. Polyethylene has good resistance to acids and caustic substances. Resistant to organic and inorganic solvents at a wide range of temperatures. It is not resistant to strong oxidising acids. Resistance to acids, caustic substances, organic and inorganic solvents Good material strength and fatigue resistance Operating temperature range -40°C to +65°C Mechanical compression joint up to 125mm or Fusion Welded PVDF (Polyvinylidene Fluoride) PVDF has excellent chemical resistance which means that it is widely used in the chemical industry as a piping system for aggressive liquids. PVDF is a homopolymer without additives such as stabilisers and processing agents. It also displays excellent flame retardant properties. PVDF is listed with many worldwide agencies as suitable for use with foodstuffs, dairy products, in semi-conductor and pharmaceutical manufacturing, and for other applications in the food and drug sector. PVDF is resistant to most acids, alkalis, salts, halogens, alcohols and chlorinated hydro-carbons. Strong polarized solvents, such as ketones and esters, can cause the material to swell. Resistant to most acids, alkalis, salts, halogens, alcohols and chlorinated hydro-carbons High mechanical strength, Operating temperature range -20°C to 140°C IR Welding
- The design pressure in steady state is invariably used for the selection of pipe class in applications. Codes and standards require that surge be included in the design pressure. Metal piping codes allow the use of pressure excursions during the life of a piping system to accommodate unsustained loads. Therefore, the designer needs to demonstrate that in the design life of the piping system the defined time and extent of application of unsustained pressure loads can be accommodated within the code. This can also be applied to thermoplastic pipe systems although the Codes and standards have not included the provisions. Codes and Standards for thermoplastic pipe systems are not as extensive as for metallic materials. Vacuum conditions fall under two categories. Those for aboveground pipe where there is no support to prevent buckling in accordance with Timoshenko’s equations. Alternatively for buried pipe the external pressure can be tolerated if the pipe roundness remains constant. Correctly designed embedment allow this to occur provided external loads are not excessive. Design procedures will be presented later based upon AS 2566.1 Buried Flexible Pipelines Design. Different industries have varying risk profiles, levels of abuse, standardizations etc. This may dictate the class of pipe used. For instance in sewage and heavy industrial application Class 12 is invariably used as a standard. It is imprudent to select a class of pipe lower than the risk profile because failures will invariably result in higher costs than any savings. Deflection between spans is a matter of aesthetics rather than exceeding any bending stresses. In a process plant above ground piping needs to appear to be correctly installed. The minimum spacing in AS 3690 and the Product Specification Manual will provide a professional looking facility. If existing structures require spacing at greater distances a more rigorous engineering analysis is required. This is covered separately later in this course. Wear of materials is proportional to the velocity^2.5 to 4.5. In addition, other particle parameters such as shape, sharpness, density, temperature, size distribution, % fines etc. need to be considered. Testing of samples may be necessary to determine the ability of ABS to withstand abrasive conditions.
- As 2566.1 provides some standard data for materials. The designer should only use this data for preliminary design. The design should be confirmed when the pipe material is selected by the consultant/contractor or end user. In particular the properties of GRP vary depending upon the standard used for its design. The designer needs to take care as GRP may be rated for positive pressure but fail in buckling when exposed to negative pressures commonly occurring in unsteady states . This can include valve closure or pump trip. A waterhammer analysis is highly recommended.
- Do not assume that the fittings available have the same rating as the pipe class. Very few fittings can meet the Class 15 rating. Pulled bends have thinned sections on the extremity of the bend due to the manufacturing process. GRP fittings are generally made by hand in larger sizes. These may contain the required thickness to withstand pressure. However in doing so the stress intensification factors become excessive.
- The property of modulus is measured using a standard ASTM test. The property varies with temperature, time, rate of strain and the applied load. Hence the single figure published by manufacturers represents only one value. It does not allow the designer to consider all aspects of the applied load to a piping system. The published value is at 20ºC and represents a maximum figure for the material. Other data is required to enable the engineer to design a pipe system. This data must come from test undertaken on the specific resin used in pipe and fittings manufacture. Some typical data is provided based upon a resin used in pipe line extrusion. For deflection design the minimum value of modulus at the design life of the pipe system is used. For wavespeed in a surge scenario the maximum instantaneous value of modulus is used in determining the celerity. This is used to determine the pressure transients in the system. For buried pipeline design the modulus used in determining the stress, strain, deflection, combined loading and buckling criteria varies dependent upon the properties of the soil. GRP is a mixture of glass reinforcement, resin, tissues, fillers and other chemicals. In addition the construction of the GRP has varying roving angles of reinforcement or possibly chopped strands of glass. Some is continuously made on internal mandrels whereas other GRP is manufactured in batch mode on internal mandrels. Thus there is a cacophony of differing materials with no common standard. It is extremely difficult to compare products. The user should specifiy a particular set of conditions early in a project so that the dimensions of pipe and fittings is established for use in piping design. Obiouvcls if the pipe internal diameter varies the hydraulics and thus power economics is affected. Standard ISO 14692 is recommended as the most comprehensive. Just specifying a standard does not guarantee that the material used in a design will meet the design intent.
- The property of modulus is measured using a standard ASTM test. The property varies with temperature, time, rate of strain and the applied load. Hence the single figure published based on the ASTM test represents only one value. It does not allow the designer to consider all aspects of the applied load to a piping system. The published value is at 20ºC and represents a maximum figure for the material. Other data is required to enable the engineer to design a pipe system. This data must come from test undertaken on the specific resin used in pipe and fittings manufacture. Some typical data is provided based upon a resin used in pipe line extrusion. For deflection design the minimum value of modulus at the design life of the pipe system is used. For wavespeed in a surge scenario the maximum instantaneous value of modulus is used in determining the celerity. This is used to determine the pressure transients in the system. For buried pipeline design the modulus used in determining the stress, strain, deflection, combined loading and buckling criteria varies dependent upon the properties of the soil. The above matters are discussed in further detail in the Design section below.
- Refer AS 2566.2 Buried Flexible Pipelines –Installation or alternate code.
- Trench excavation conditions may impact on the design of the soil/pipe structure. The designer needs to be fully in contact with the installation to ensure that the design intent is met. Where a construction company does not employ the original designer invariably events occur that lead to compromise of the design intent.
- Removal of trench shields disturbs the region that has been previously compacted. Local re-compaction is necessary to ensure that the design intent is not compromised.
- Refer AS 2566.2
- Refer AS 2566.2 Appendix F for details of jointing techniques.
- Refer AS 2566.2 Appendices G & H for details
- Refer AS2566.2
- Refer AS2566.2 or other applicable standard or code.
- Refer AS 2566.2 Appendix M
- Refer AS2566.1
- Refer AS 2566.2
- Refer AS 2566.2
- For underground installed the test procedure is set out in AS2566.1 Buried Flexible Pipelines-Installation
- Product is inspected in the factory to conform to standards or specialised drawings. If a fitting has a failure ensure that it is made to standard. There have been occurrences of other manufacturers fittings or even different materials have been used in some installations. There have been rare instances of fittings leaving the factory without quality control checks. Installers and fabricators should be encouraged to carry out dimensional checks and trial fittings. Any non conformances should be documented and reported immediately to the factory. Obtain samples if possible. Pipe is extruded in accordance with standards limits for ovality. There are no ovality limits for pipe of SDR 26 or greater. This is because it is considered that thin wall pipe can be re rounded during fabrication. If the pipe is out of round it should not be used to effect a joint. Socket dimensions are controlled to standard. In the event that the socket dimensions are out of tolerance the fitting should not be used to effect a joint. Any surface defects should be examined to comply with standard. Any internal surface damage should be closely inspected. As any pressure is applied to a pipe the inside surface is exposed to tensile stress and deep, sharp scratches could provide a risk. This is not as critical with ABS as it is with PE or PVC-U. Fabricated fittings by their nature are not mass produced and involve a degree of hand manufacturer. Hence the possibility of error is inherently higher than for a moulded fitting. These fittings are therefore subjected to higher levels of factory inspections during their manufacture. As they are fabricated they may also be more sensitive to damage from excessive loads during installation. Any cracks at welds are an indication of excessive loads that may or may not result in failure. Fabricated fittings are generally in the large bore pipe sizes. The cost of such fittings is inherently greater than for mass produced fittings. Therefore it is in the installers interest to check the dimension and orientation of the fittings prior to undertaking the welding procedure. A failed fitting connected into a piping system can rarely be rectified. There are in house procedures for repairing leaks. No customer wants a repaired fitting in a new facility and these are to be avoided if at all possible. Factory prepared ends on GRP have led to failures. This has occurred where the ends have been bevelled. Also failures have occurred where the ends of the GRP pipe using a batch internal mandrel has been used. The failures occurred where the ends have not been cut off. The ends of a GRP manufactured using a batch mode has a roving angle change as the machine reverses the lay up. This roving angle change weakens the material reinforcement.
- There have been failures of the pipe itself in the history of pipeline applications. Where pipe or joint has failed it has generally been damaged during installation, subjected to overpressure or even more rarely, manufactured from sub standard resin or regrind. When flanged joints fail it is usually the result of a faulty or poorly selected gasket or insufficient tightening of the bolted joint. This is generally rectified without recourse to the supplier. Physical damage to the stub end faces may cause weeping at the joint. Minor leaks may occur at solvent or resin lay up chemical welds due to faulty procedures or more rarely a product quality problem. The latter can be avoided if the sub contractor checks the dimensions of the product before undertaking the weld. This may be a dimensional check or a trial fitting. Rubber ring joints if fitted and lubricated correctly should not leak. Damaged seals may be a cause of a minor leak. The ovality of the pipe may be a cause of a leak however the dimensions would have to be outside of the standard tolerances for this to occur. For RRJ pipe the wall is generally thin compared to the diameter and it is expected that any ovality is self correcting when inserted into the socket. However some authorities have banned the use of PVC-O for this reason. Fabricated fittings have been known to fail if inadequately supported during installation. Such failures may manifest themselves in cracks. This may be the cause of a pin hole leak. Thin wall pipework needs to be designed for local buckling loads arising at the support. To resist these loads the use of 120 º saddle strips are recommended. The corners of the saddle strips should be fitted on site with a small radius to avoid local point loads on the pipe surface. Care should be taken in the location of saddle strips. If the pipe moves due to temperature change then the saddle may slip off the supports and create an impact load. When the pipe reverses the strip may catch the support and create a large force that can damage the support. When the pipework is put into service supports may be damaged for a variety of causes. One of the more common causes is when a clamp has insufficient clearance and the expanding/contracting pipe binds on the clamp. This occurs if the pipe comes into contact with the clamp and the clamp deflects. This can destroy the support. Scraping of the pipe surface is an indication that this has occurred. Inadequate supports such as mechanical anchors in ceilings or walls are also likely to fail. Thrust blocks are required to be designed and installed for use with rubber ring jointed pipe. The thrust block should be designed to resist the thrust loads arising from testing or surge. There is a need for the site supervisor to check that the soil conditions conform with the design values used in determining the bearing area of the thrust blocks. If the trench conditions exhibit groundwater or excessive fines then there may be a need to use a geotextile fabric to prevent the migration of the embedment of fines that would compromise the design. If the designer has not considered all scenarios of waterhammer in the design damage to pipework and supports can occur. This is particularly so when the operating conditions during this period may not be in accordance with standard operating procedures. If a failure has occurred it is imperative to obtain as many operating records and details of the damage as possible. If possible obtain details of the automated valves, pumps and motors in the system. Over pressure of the system may cause a failure. Operating records should be inspected. If a particular section of piping has been damaged the material should be retrieved for further factory investigation. Photographs of the failed surfaces will be used to determine the mode of failure. If the pipe is installed in such a manner that shear forces can occur the pie wall can be damaged. Shear often occurs at locations where the pipe penetrates a concrete structure. If the structure settles a shear force will be applied to the adjacent pipe. The designer should take this into account by allowing for “rocker” spools or flexible bellows joints. A “rocker” spool should have a RRJ or flexible coupling at each end. The spool length should be a minimum of 1.5D where D is the outside diameter of the pipe. Pipe installed at right angles to the wall with sufficient flexibility will also allow for some settlement. Where the pipe is installed in an environment where vibration or imposed frequencies apply particular attention should be taken in respect of the supports. The material may be tough but is not totally immune to vibration damage. Measures that can employed are more frequent supports, elastic supports in the form of springs, rubber cushioning or isolation with rubber bellows. When rubber bellows are under line pressure they should be restrained with tie rods or external anchors. Any external anchor must be capable of withstanding the load. The load can simply be calculated from the cross sectional area of the largest diameter of the runner bellows times the pressure. A factor of safety of 2 is usually applied to this calculated load when designing the anchor. Particular care should be taken where there are rapid changes in section such as reducing tees, reducing bushes etc. The root of the change in section can cause a stress intensification and a site for crack initiation. Wear can occur at the internal surface of pipes when a solids bearing fluid is transported. Although predictions can be made for general pipe wear there may exist local conditions that cause rapid wear. These will be at changes of direction or section and areas of direct impact. Wear can also occur on the outside surface of the pipe that vibrates or moves on supports. There have been instances of temporary wooden supports in trenches not being removed and slight movement has caused wear through the pipe wall.
- AS 2566 is mandatory if enshrined in contract or law AWWA M45 can be used if AS 2566 is not specified or required by an act Some research has been done modelling soil as a series of springs but this is not necessary if AS 2566 is used There is no mandate as to what discipline owns this work but they must be competent