2nd ed. p. cm. -- (AWWA manual ; M) Rev. ed of: Fiberglass pipe design manual. c Includes bibliographical references and index. ISBN AWWA-Mpdf - Download as PDF File .pdf), Text File .txt) or read online. Manual Pdf PDF Ebooks AWWA Manual M45 second edition. Download. AWWA Manual M45 second edition - Wed, 20 Mar
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iii. Contents. AWWA Manual M List of Figures, v. List of Tables, xii. Preface, xi. Acknowledgments, xiii. Chapter 1. History and Use. Printed Edition + PDF; Immediate download; $; Add to Cart This manual is the third edition of AWWA Manual M45, Fiberglass Pipe Design. It provides. 4 days ago GMT (PDF) AWWA Manual M45 second edition | otto otto Awwa Manual m Fiberglass Pipe Design - [PDF Document].
The suitability of each method depends on the type of flow gravity or pumped and the level of accuracy required. Although not as technically correct as other methods for all velocities, the Hazen-Williams equation has gained wide acceptance in the water and wastewater industries.
The Hazen-Williams equation is presented in nomograph form in Figure , which is typical for small-diameter fiberglass pipe. When fluids other than water are encountered, a more universal solution such as the Darcy-Weisbach equation should be used. The Hazen-Williams equation is valid for turbulent flow and will usually provide a conser- vative solution for determining the head loss in fiberglass pipe.
Graphs and examples use nominal pipe size for simplicity. The actual inside diameter ID should be used in hydraulic calculations. A design value of is frequently used with fiberglass pipe. It is inversely proportional to the diameter of the pipe. The primary advantage of this equation is that it is valid for all fluids in both laminar and turbulent flow. The disadvantage is that the Darcy-Weisbach friction fac- tor is a variable. Once preliminary sizing of the pipe diameter has been completed, the next step is to determine whether the flow pattern within the pipe is laminar or tur- bulent.
This characterization of the flow is necessary in the selection of the appropriate friction factor to be used with the Darcy-Weisbach equation. The well-known Reynolds number equation is used to characterize the fluid flow: Friction factor for laminar flow is denoted as fl, and ft denotes friction factor for turbulent flow. When the flow regime is turbulent i. Fiberglass pipe has a surface roughness parameter e equal to 1.
The fric- tion factor for turbulent flow can also be calculated from the Colebrook equation: This approach has sufficient accuracy for many applications and is used most often with the Hazen-Williams or Manning equations. The approach does not consider turbulence and subsequent losses created by different fluid velocities. When tabular data are not available or when additional accuracy is necessary, head loss in fittings or valves can be determined using loss coefficients K factors for each type of fitting.
Table provides the typical K factors. Equation illustrates the loss coefficient approach. The total head loss in a system includes, but is not limited to, losses from fittings, the head loss from the straight run pipe, and head losses due to changes in elevation. This section outlines the basic procedure for determining the head loss due to friction and relative economic merits when considering different pipe materials. Calculate the head loss Eq Convert head loss to pump horsepower demand: Calculate the annual energy usage To demonstrate the calculations in a clear format, the expressions below assume the pumps run 24 hours per day at full capacity.
This is not a realistic assumption. In design situations, engineers must assess the actual expected operating conditions, e. Calculate average annual energy cost AEC: These techniques consider the installed cost of pipe in the calculation and future cash flows are discounted to present value. The pressure surge results from the rapidly moving wave that increases and decreases the pressure in the system depending on the source and direction of wave travel.
Under certain conditions, pressure surges can reach magnitudes sufficient to rupture or collapse a piping system, regardless of the material of construction.
Rapid valve closure can result in the buildup of pressure waves due to the conver- sion of kinetic energy of the moving fluid to potential energy that must be accommo- dated.
These pressure waves will travel throughout the piping system and can cause damage far away from the wave source. The relatively high com- pliance low modulus of elasticity of fiberglass pipe contributes to a self-damping effect as the pressure wave travels through the piping system. In addition to rapid valve closure or opening, sudden air release and pump start-up or shut-down can create pressure surge.
Pressure surges do not show up readily on conventional Bourdon tube gauges because of the slow response of the instrument. The net result of pressure surge can be excessive pressures, pipe vibration, or move- ment that can cause failure in pipe and fittings. In other cases, mechanical valve operators, accumulators, rupture discs, surge relief valves, feedback loops around pumps, etc. Good design practice usually prevents pressure surge in most systems.
Installation of valves that cannot open or close rapidly is one simple precaution.
In addition, pumps should never be started in empty discharge lines unless slow-opening, mechan- ically actuated valves can increase the flow rate gradually. Many fluid mechanics and hydraulic handbooks provide procedures such as the previous Talbot equation for calculating pressure surges as a result of a single valve closure in simple piping systems.
Sophisticated fluid transient computer programs are also available to analyze pressure surge in complex multibranch piping systems under a variety of conditions. Use of the Hazen-Williams equation. Compute the frictional pressure loss in a Compute the frictional pressure loss in a 1,ft long, in.
Compute the head loss per unit length of pipe using Eq Convert head loss to pressure drop using Eq Determine the pipe diameter, working pressure, and pres- sure class on a pipeline. Assume the kinematic change of 7. The flow rate is 8, gpm.
Determine minimum diameter Eq Step 2. Calculate average fluid velocity Eq Calculate the Reynolds number Eq Step 4. Calculate the friction factor Eq Calculate system friction loss using Eq and Eq Consequently, the total K factor is 4 0.
Combine friction and elevation head: Convert head loss to working pressure Eq However, a higher pressure class tentatively be selected to account for may tentatively be selected to account possible water hammer in the line.
For for possible water hammer in the line. See example to kPa class is selected. See example verify that this is adequate for pres- to verify that this is adequate for pres- sure surge. Example Comparative power cost calculation.
Assume a 10,ft long, 6-in. The engi- water on a year-round basis. Calculate the average AEC pipeline. Step 1. Calculate the head loss for each material Eq Convert head loss to horsepower demand Eq In design sit- uations, engineers must assess actual operating levels.
Calculate the AEC Eq and calculate the total energy cost over 20 years: Surge pressure calculation. Assume a full instantaneous change in Assume a full instantaneous change in velocity equal to the flow velocity in the velocity equal to the flow velocity in the pipe. The fiberglass pipe has a tensile pipe. The pipe wall thickness class of kPa.
The bulk modulus of is , psi. Calculate the wave velocity Eq Calculate the surge pressure Eq Check compliance with the maximum system pressure requirement: The This exceeds the pressure class. The engineer has three options.
The first engineer has three options. The first would be to increase the pressure class would be to increase the pressure class to accommodate the surge, maintain- to accommodate the surge, maintain- ing the same pipe diameter. The sec- ing the same pipe diameter. The larger pipe pressure requirement. The larger pipe diameter will lower operating pressure diameter will lower operating pressure due to lower friction loss and will lower due to lower friction loss and will lower fluid velocity.
The third option is to fluid velocity. The third option is to provide measures, such as a surge provide measures, such as a surge tank, to reduce the magnitude of the tank, to reduce the magnitude of the surge. For this example, the second option For this example, the second option will be used and a diameter of 20 in. Calculate the fluid velocity for the new pipe diameter Eq Step 5. Calculate the new working pressure. Reynolds number Eq Friction factor Eq Friction losses using Eq and Eq The total K factor is then 4 0.
Convert to working pressure Eq and using Htotal for Hf: Calculate the pressure surge using Eq Before final selection, the engineer would typically evaluate the economics of using the larger diameter with a higher pressure class ver- sus using the original diameter with a still higher pressure class.
New York: Fiberglass Pipe Pressure Pipe. American Institute. Water Works Association. Kent, G. Preliminary Pipeline Sizing. Benedict, R. Fundamentals of Pipe Chemical Engineering. Sharp, W. Predict- Brater, E. Handbook of ing Internal Roughness in Water Mains. AWWA, 80 If the results of any calculation indicate that a requirement is not satisfied, it will be necessary to upgrade installation param- eters or select a pipe with different properties, or both, and redo pertinent calcula- tions.
Special information and calculations not covered in this chapter may be required in unusual cases see Sec. Both rigorous and empirical methods are used to design fiberglass pipe. In addition to short-term tests, many performance limits are determined at 50 years through sta- tistical extrapolation of data obtained from long-term tests under simulated service conditions.
Design stress or strain values are obtained by reducing performance limits using appropriate design factors.
Design factors are established to ensure adequate performance over the intended service life of the pipe by providing for variations in material properties and loads not anticipated by design calculations. Design factors are based on judgment, past experience, and sound engineering principles. The design method discussed in this chapter applies in concept to pipe with uniform walls and to pipe with ribbed-wall cross sections. However, for design of pipe with ribbed walls, some of the equations must be modified to allow for the special properties of this pipe.
Also, additional calculations not addressed in this chapter may be required to ensure an adequate design for a ribbed-wall cross section. The equations are presented with inch-pound units in the left-hand column and metric units in the right-hand column.
Working pressure, Pw. The maximum anticipated, long-term operating pressure of the fluid system resulting from typical system operation. The maximum sustained pressure for which the pipe is designed in the absence of other loading conditions. Surge pressure, Ps. The pressure increase above the working pressure, some- times called water hammer, that is anticipated in a system as a result of a change in the velocity of the fluid, such as when valves are operated or when pumps are started or stopped.
Surge allowance, Psa. That portion of the surge pressure that can be accommo- dated without changing pressure class. The surge allowance is expected to accommodate pressure surges usually encountered in typical systems. Hydrostatic design basis, HDB. Design factor, FS. A specific number greater than 1 used to reduce a specific mech- anical or physical property in order to establish a design value for use in calculations. This is reflected in typical long-term flow coefficient values of 0.
The engineer may wish to consider this in establishing design conditions. See chapter 4 on hydraulics. Excessive surge pressures should be identified in the design phase, and the causative condition should be eliminated or automatic surge-pressure relief provided, otherwise, a higher pressure class should be selected.
Some pipe products may have sig- nificantly higher values for these properties. Pipe properties necessary for design calculations include the following: A given combination of soil type and degree of compaction will largely determine the following values required for design calculations: The calculations may be made using either stress or strain, depending on the basis used to establish a particular product performance limit.
The procedure for using design calculations to determine whether pipe meets the requirements discussed in Sec. Check working pressure, Pw Sec. Check surge pressure, Ps Sec. Calculate allowable deflection from ring bending Sec. Determine soil loads, Wc, and live loads, WL Sec. Calculate the composite constrained soil modulus, Ms Sec. Check combined loading Sec. Check buckling Sec. See Sec. The HDB of fiberglass pipe varies for different products, depending on the materials and composition used in the reinforced wall and in the liner.
The HDB may be defined in terms of reinforced wall hoop stress or hoop strain on the inside surface. Temperature and service life. The required practice is to define projected pro- duct performance limits at 50 years. Performance limits at elevated temperature depend on the materials and type of pipe wall construction used.
The manufacturer should be consulted for HDB values appropriate for elevated temperature service. Design factors. This factor ensures that the stress or strain due to the short-term peak pressure conditions do not exceed the short-term hydro- static strength of the pipe. This factor ensures that stress or strain due to sustained working pressure does not exceed the long-term hoop strength of the pipe as defined by HDB.
For fiber- glass pipe design, this minimum design factor is 1. Both design factors should be checked. Either design factor may govern pipe design, depending on long-term strength regression characteristics of the particular pipe product. Prudent design practice may dictate an increase or decrease in either design factor, depending on the certainty of the known service conditions. The pressure class of the pipe should be equal to or greater than the working pressure in the system, as follows: The pressure class of the pipe should be equal to or greater than the maximum pressure in the system, due to working pressure plus surge pressure, divided by 1.
Factory hydrotesting at pressures up to 2 Pc is acceptable and is not governed by Eq and Eq Calculated surge pressure, Ps. The surge pressure calculations should be performed using recognized and accepted theories. Because of this, the engineer should generally expect lower calculated surge pressures for fiberglass pipe than for pipe materials with a higher modulus or thicker wall or both.
Surge allowance. The surge allowance is intended to provide for rapid transient pressure increases typically encountered in transmission systems.
Special consideration should be given to the design of systems subject to rapid and frequent cyclic service. The manufacturer should be consulted for specific recommendations. Satisfaction of this requirement is assured by using one of the following formulas.
For stress basis: The shape factor relates pipe deflection to bending stress or strain and is a function of pipe stiffness, pipe zone embedment material and compaction, haunching, native soil conditions, and level of deflection. For pipe zone embedment materials with a finer grain size, use the Df value of sand with moderate to high compaction. The long-term, ring-bending strain varies for different products, depending on materials and type of construction used in the pipe wall.
Prudent design of pipe to withstand bending requires consideration of two separate design factors. The first design consideration is comparison of initial deflection at failure to the maximum allowed installed deflection.
This test requirement demonstrates a design factor of at least 2. The second design factor is the ratio of long-term bending stress or strain to the bending stress or strain at the maximum allowable long-term deflection. This require- ment may be stated as follows: When installed in the ground, all flexible pipe will undergo deflection, defined here to mean a decrease in vertical diameter.
The amount of deflection is a function of the soil load, live load, native soil characteristics at pipe ele- vation, pipe embedment material and density, trench width, haunching, and pipe stiffness. Many theories have been proposed to predict deflection levels; however, in actual field conditions, pipe deflections may vary from calculated values because the actual installation achieved may vary from the installation planned. These variations include the inherent variability of native ground conditions and variations in meth- ods, materials, and equipment used to install a buried pipe.
As presented previ- ously and as augmented by information provided in the following sections, Eq serves as a guideline for estimating the expected level of short-term and long-term deflection that can be anticipated in the field.
This equation is the best known and docu- mented of a multitude of deflection-prediction equations that have been proposed. As presented in this chapter, the Iowa formula treats the major aspects of pipe—soil inter- action with sufficient accuracy to produce reasonable estimates of load-induced field deflection levels.
Soil Engineering. These deflections are typically small for pipe stiffnesses above 9 psi to 18 psi 62 kPa to kPa depending on installation conditions. For pipe stiffnesses below these values, consid- eration of these items may be required to achieve an accurate deflection prediction. Application of this method is based on the assumption that the design values used for bedding, backfill, and compaction levels will be achieved with good practice and with appropriate equipment in the field.
Experience has shown that deflection levels of any flexible conduit can be higher or lower than predicted by calculation if the design assumptions are not achieved.
The deflection lag factor converts the immedi- ate deflection of the pipe to the deflection of the pipe after many years. The vast majority of this phenomenon occurs during the first few weeks or months of burial and may continue for some years, depending on the frequency of wetting and drying cycles, surface loads, and the amount of original compaction of the final backfill.
Secondary causes of increasing pipe deflection over time are the time-related consolidation of the pipe zone embedment and the creep of the native soil at the sides of the pipe. These causes are generally of much less signifi- cance than increasing load and may not contribute to the deflection for pipes buried in relatively stiff native soils with dense granular pipe zone surrounds.
For long-term deflection prediction, a DL value greater than 1. The bedding coefficient reflects the degree of support provided by the soil at the bottom of the pipe and over which the bottom reac- tion is distributed.
Assuming an inconsistent haunch achievement typical direct bury condition , a Kx value of 0. For uniform-shaped bottom support, a Kx value of 0. The long-term vertical soil load on the pipe may be considered as the weight of the rectangular prism of soil directly above the pipe. The soil prism would have a height equal to the depth of earth cover and a width equal to the pipe outside diameter. The following calculations may be used to compute the live load on the pipe for surface traffic see Figure These calculations consider a sin- gle-axle truck traveling perpendicular to the pipe on an unpaved surface or a road with flexible pavement.
Direction of Travel 0. Change accounts for overlapping influence areas from adjacent wheel loads. Equations as shown are for h in inches meters. This lane load is ignored in these calculations because it has only a small effect on the total live load and may be added by the engi- neer if deemed appropriate. The above calculation method assumes that the live load extends over the full diameter of the pipe.
This may be conservative for large-diameter pipe under low fills. The OD is the outside diameter of the pipe in inches millimeters. For depths of fill less than 2 ft 0. Such an analysis is beyond the scope of this manual.
The previous calculation is for single-axle trucks. Design for tandem-axle trucks may use the same procedures; however, the following substitutions for L1 should be used if both axles load the pipe at the same time. Rigid pavements dramatically reduce live load effects on concrete pipe. The Portland Cement Association developed a calculation method to consider loads transmitted through concrete pavements Vertical Pressure on Concrete Culverts Under Wheel Loads on Concrete Pavement Slabs, Portland Cement Association, Publication ST, that is still in use today and is suitable for computing live loads on fiberglass pipe under rigid pavements.
The loads shown assume that the load extends over the full diameter of the pipe. This assump- tion will not be true for large-diameter pipes with shallow covers. Loads for this condition may be lower. See calculation note 3 for guidance on appropriate adjustments. The pipe stiffness can be determined by conducting parallel-plate loading tests in accordance with ASTM D During the parallel-plate loading test, deflection due to loads on the top and bottom of the pipe is measured, and pipe stiffness is calculated from the following equation: The vertical loads on a flexible pipe cause a decrease in the vertical diameter and an increase in the horizontal diameter.
The horizontal movement develops a passive soil resistance that helps support the pipe. The passive soil resistance varies depending on the soil type and the degree of compaction of the pipe zone backfill material, native soil characteristics at pipe eleva- tion, cover depth, and trench width see Table This change is based on the work of McGrath Design values of the constrained modulus are presented in Table The table shows that Ms increases with depth of fill, which reflects the increased confining pressure.
This is a well-known soil behavior. To determine Ms for a buried pipe, separate Ms values for the native soil Msn and the pipe backfill surround Msb must be determined and then combined using Eq Special cases are discussed later in this chapter. For stress basis HDB: ASTM D The borderline condition is indicated by an en dash between the two symbols, for example, CL—CH. SC1 soils have the highest stiffness and require the least amount of compactive energy to achieve a given density.
SC5 soils, which are not recommended for use as backfill, have the lowest stiffness and require substantial effort to achieve a given density.
Soil stiffness categories are explained in chapter 6. SC1 soils have higher stiffness than SC2 soils, but data on specific soil stiffness values is not available at the current time. Even if dumped, SC1 materials should always be worked into the haunch zone, see Sec. Vertical stress level is the vertical effective soil stress at the springline elevation of the pipe.
It is normally computed as the design soil unit weight times the depth of fill. Buoyant unit weight should be used below the groundwater level.
Engineers may interpolate intermediate values of Msb for vertical stress levels not shown on the table. In-between values of Sc may be determined by straight-line interpolation from adjacent values. Ms special cases: Geotextiles—When a geotextile pipe zone wrap is used, Msn values for poor soils can be greater than those shown in this table. Solid sheeting—When permanent solid sheeting designed to last the life of the pipeline is used in the pipe zone, Ms shall be based solely on Msb.
Buried pipe is subjected to radial external loads com- posed of vertical loads and the hydrostatic pressure of groundwater and internal vac- uum, if the latter two are present. External radial pressure sufficient to buckle buried pipe is many times higher than the pressure causing buckling of the same pipe in a fluid environment, due to the restraining influence of the soil. The summation of appropriate external loads should be equal to or less than the allowable buckling pressure.
The allowable buck- ling pressure qa is determined by the following equation: Satisfaction of the buckling requirement is assured for typical pipe installations by using the following equation: The minimum requirements for axial strengths are as specified by Sec.
When restrained joints are used, the pipe should be designed to accommodate the full mag- nitude of forces generated by internal pressure. Special consideration should be made for the following conditions: For reference, the set of design conditions, pipe properties, and installation parameters assumed for this design example are presented in Table This summary is not repeated in the body of the example design calculations.
The pipe material properties and characteristics presented in Table have been assumed for illustrative purposes and should not be used as actual design values. Values for these parameters differ for various pipe constructions and materials and should be obtained from the manufacturer.
Confirm pressure class Eq Check working pressure Eq Check surge pressure Eq Calculate maximum allowable deflection Eq Calculate soils load Eq Calculate live loads Eq Calculate the composite constrained soil modulus Eq Determine Sc from Table Calculate the predicted deflection Eq Check combined loading Eq and Check buckling Eq a.
Buckling check at 1. Buckling check at 2. American Association of Density. West Conshohocken, Pa.: American State Highway and Transportation Officials. Society for Testing and Materials. American Society for Testing and Materials.
American Soil Using Standard Effort. West Consho- Water Works Association. American Society for Testing Cagle, L. Recom- and Materials. Group on Buckling. In Proc. Modulus of Soil Reaction Plate Loading. Values for Buried Flexible Pipe. Journal of American Society for Testing and Materials. Geotechnical Engineering, Pipeline Installation.
Relativity Publishing. Soil Classification System. West Consho- Luscher, U. Buckling of Soil Surrounded hocken, Pa.: American Society for Testing Tubes. Soil Mech. In Pipelines in the Constructed Pipe and Fittings. Edited by J. Castronovo American Society for Testing and Materials. Reston, Va.: American Soci- ———. American Society of Loads, Deflection, Strain. ISO Bull. Spangler, M. Soil Engineering, 4th ed.
The structural design process, discussed in chapter 5, assumes that a pipe will receive support from the surrounding soil, and the installation process must ensure that the support is provided. The guidelines in this chapter suggest pro- cedures for burial of fiberglass pipe in typically encountered soil conditions. Recom- mendations for trenching, placing, and joining pipe; placing and compacting backfill; and monitoring deflection levels are included.
Diameters range from 1 in. Engineers and installers should recognize that all possible com- binations of pipe, soil types, and natural ground conditions that may occur are not considered in this chapter. The recommendations provided may need to be modified or expanded to meet the needs of some installation conditions.
Section 6. Guidance for installation of fiberglass pipe in subaqueous conditions is not included. The following terms are specific to this manual: Backfill material placed in the bottom of the trench or on the founda- tion to provide a uniform material on which to lay the pipe; the bedding may or may not include part of the haunch zone see Figure A measure of the ease with which a soil may be compacted to a high density and high stiffness.
Crushed rock has high compactibility because a dense and stiff state may be achieved with little compactive energy. Any change in the diameter of the pipe resulting from installation and imposed loads. Deflection may be measured and reported as change in either ver- tical or horizontal diameter and is usually expressed as a percentage of the unde- flected pipe diameter. The engineer or the duly recognized or authorized representative in responsible charge of the work.
Final backfill. Backfill material placed from the top of the initial backfill to the ground surface see Figure Soil particles that pass a No. Any permeable textile material used with foundation, soil, earth, rock, or any other geotechnical engineering-related material as an integral part of a synthetic product, structure, or system. Backfill material placed on top of the bedding and under the spring- line of the pipe; the term only pertains to soil directly beneath the pipe see Figure Initial backfill.
Backfill material placed at the sides of the pipe and up to 6 in. Backfill 6 to 12 in. Aggregates such as slag that are products or by- products of a manufacturing process, or natural aggregates that are reduced to their final form by a manufacturing process such as crushing.
Maximum standard Proctor density. The maximum dry density of soil com- pacted at optimum moisture content and with standard effort in accordance with ASTM D Native in situ soil.
Natural soil in which a trench is excavated for pipe instal- lation or on which a pipe and embankment are placed. Open-graded aggregate.
An aggregate that has a particle size distribution such that when compacted, the resulting voids between the aggregate particles are rela- tively large. Optimum moisture content.
Pipe zone embedment. All backfill around the pipe, including the bedding, haunching, and initial backfill. Processed aggregates. Aggregates that are screened, washed, mixed, or blended to produce a specific particle size distribution. Relative density. Soil stiffness.
A property of soil, generally represented numerically by a modu- lus of deformation, that indicates the relative amount of deformation that will occur under a given load. Split installation. An installation where the initial backfill is composed of two different materials or one material placed at two different densities. The lower mater- ial extends from the top of the bedding to a depth of at least 0. Other tests, such as the standard penetration and cone penetrome- ter tests, are also useful in determining soil stiffness.
Depending on actual installation conditions, such as trench geometry, the in situ soil conditions may also have a signifi- cant impact on pipe design. Refer to chapter 5 for further discussion. Consideration should also be given to seasonal or long-term variations in ground- water level when evaluating groundwater conditions. For example, if the soil exploration program is conducted in August, the groundwater level may be quite low compared to levels in April or May.
Soil SC1 indicates a soil with high compactibil- ity, i. Each higher number soil stiffness category is successively less compactible, i. See chapter 5 for a discussion of how soil stiffness affects buried pipe behavior. Table provides recommendations on installation and use of embedment materi- als based on stiffness category and location in the trench.
In general, soil conforming to SC1 through SC4 should be used as recommended and SC5 materials should be excluded from the pipe zone embedment. SC1 materials provide maximum pipe support for a given percent compaction due to low content of sand and fines. With minimum effort these materials can be installed at relatively high soil stiffnesses over a wide range of moisture contents.
In addition, the high permeability of SC1 materials may aid in the control of water, making them desirable for embedment in rock cuts where water is frequently encountered. However, when groundwater flow is anticipated, con- sideration should be given to the potential for migration of fines from adjacent materials into the open-graded SC1 material see Sec. Soil stiffness category 2 SC2. When compacted, SC2 materials provide a rel- atively high level of pipe support. However, open-graded groups may allow migration and the sizes should be checked for compatibility with adjacent material see Sec.
Soil stiffness category 3 SC3. Higher levels of compactive effort are required and moisture content must be near optimum to minimize compactive effort and achieve the required density.
These materials provide reasonable levels of pipe sup- port once proper density is achieved. Soil stiffness category 4 SC4. SC4 materials require a geotechnical evalua- tion prior to use. Even if dumped, SC1 materials should always be worked into the haunch zone see Sec. If use of these materi- als is allowed, compaction and handling procedures should follow the guidelines for SC3 materials. When properly placed and compacted, SC4 materials can provide reasonable levels of pipe support; however, these materials may not be suitable under high fills, surface-applied wheel loads, or high—energy-level vibratory compactors and tampers.
Do not use where water conditions in the trench prevent proper placement and compaction. Soil stiffness category 5 SC5. SC5 materials are not suitable for use as back- fill for flexible pipe and must be excluded from the pipe zone embedment. The moisture content of embed- ment materials with substantial fines must be controlled to permit placement and compaction to required levels. For soils with low permeability i. Obtaining and maintaining the required limits on moisture con- tent are important criteria for selecting materials, because failure to achieve required density, especially in the pipe zone embedment, may result in excessive deflection.
Do able or when minimize migration. Not conditions in trench geotextile filter suitable for use as a recommended for prevent proper media. Not blanket and under- Table Uniform 62 kPa or less.
Foundation Suitable as foundation Suitable as foundation Suitable for replacing Not suitable. Install and in 6-in. Pipe zone Suitable as restricted Suitable as restricted Suitable as restricted Suitable as restricted embedment above.
Work mate- above. Difficult to above. Difficult to rial under pipe to rial under pipe to place and compact in place and compact in provide uniform provide uniform the haunch zone. Relative compactive Low Moderate High Very high effort required to achieve minimum density Compaction Vibration or impact Vibration or impact Impact Impact methods Required moisture None None Maintain near Maintain near control optimum to optimum to minimize minimize compactive effort.
They may be used as final backfill as permitted by the engineer. Compatibility of pipe and backfill. Experience has shown that pipe deflections and strain levels increase when low-stiffness pipe is embedded in backfill materials that require large compactive efforts. This occurs because of the local distortions of the pipe shape that result as compactive energy is applied to the backfill. Because of this, it is recommended that pipe with stiffness of 9 psi 62 kPa or less be embedded only in soil types SC1 or SC2.
Maximum particle size. Maximum particle size for pipe zone embedment is limited based on pipe diameter, as listed in Table When final backfill contains cobbles, boulders, etc. Backfill containing boulders larger than 8 in. When open-graded material is placed adjacent to finer material, fines may migrate into the coarser material under the action of hydraulic gradient from groundwater flow.
Field experience shows that migration can result in significant loss of pipe support and increasing deflections that may eventually exceed design limits. The gradation and relative size of the embedment and adjacent materials must be compatible in order to minimize migration. In general, where significant groundwater flow is anticipated, avoid plac- ing coarse, open-graded materials, such as SC1, above, below, or adjacent to finer materials, unless methods are employed to impede migration.
For example, consider the use of an appropriate soil filter or a geotextile filter fabric along the boundary of the incompatible materials. The following filter gradation criteria may be used to restrict migration of fines into the voids of coarser material under a hydraulic gradient: The aforementioned criteria may need to be modified if one of the materials is gap graded.
Materials selected for use based on filter gradation criteria should be handled and placed in a manner that will minimize segregation. Cementitious backfill materials. Although not specifically addressed by this manual, use of these materials is beneficial under many circumstances. Slope trench walls or provide supports in conformance with safety standards. Open only enough trench that can be safely maintained by available equipment.
Place and compact backfill in trenches as soon as practicable, preferably no later than the end of each working day. Place excavated material away from the edge of the trench to mini- mize the risk of trench wall collapse.
Water control. It is always good practice to remove water from a trench before laying and backfilling pipe. Although circumstances occasionally require pipe installa- tion in conditions of standing or running water, such practice is outside the scope of this chapter. Prevent runoff and surface water from entering the trench at all times.
When groundwater is present in the work area, dewater to main- tain stability of in situ and imported materials. Maintain water level below pipe bed- ding. Use sump pumps, well points, deep wells, geotextiles, perforated underdrains, or stone blankets of sufficient thickness to remove and control water in the trench.
When excavating, ensure the groundwater is below the bottom of the cut at all times to pre- vent washout from behind sheeting or sloughing of exposed trench walls.
To preclude loss of soil support, employ dewatering methods that minimize removal of fines and the creation of voids within in situ materials. Running water. Control running water that emanates from surface drainage or groundwater to preclude undermining of the trench bottom or walls, the foundation, or other zones of embedment. Provide dams, cutoffs, or other barriers at regular inter- vals along the installation to preclude transport of water along the trench bottom. Backfill all trenches as soon as practical after the pipe is installed to prevent distur- bance of pipe and embedment.
Materials for water control. Use suitably graded materials for foundation layers to transport running water to sump pits or other drains. Select the gradation of the drainage materials to minimize migration of fines from surrounding materials see Sec.
Where trench walls are stable or supported, provide a width sufficient, but no greater than necessary, to ensure working room to properly and safely place and compact haunching and other embedment materials. The space between the pipe and trench wall must be 6 in. For a single pipe in a trench, minimum width at the bottom of the trench should be 1.
For multiple pipes in the same trench, clear space between pipes must be at least the average of the radii of the two adjacent pipes for depths greater than 12 ft 3. The distance from the outside pipe to the trench wall must not be less than if that pipe were installed as a single pipe in a trench. If mechanical com- paction equipment is used, the minimum space between pipe and trench wall or between adjacent pipe shall not be less than the width of the widest piece of equip- ment plus 6 in.
In addition to safety considerations, the trench width in unsupported, unstable soils will depend on the size and stiffness of the pipe, stiffness of the embedment and in situ soil, and depth of cover.
Specially designed equipment or the use of free-flowing backfill, such as uniform rounded pea gravel or flowable fill, may enable the satisfac- tory installation and embedment of pipe in trenches narrower than specified earlier. If the use of such equipment or backfill material provides an installation consistent with the requirements of this manual, minimum trench widths may be reduced if approved by the engineer.
Support of trench walls. When supports such as trench sheeting, trench jacks, or trench shields or boxes are used, ensure that support of the pipe embedment is maintained throughout the installation process. Ensure that sheeting is sufficiently tight to prevent washing out of the trench wall from behind the sheeting. Provide tight support of trench walls below viaducts, existing utilities, or other obstructions that restrict driving of sheeting.
Supports left in place. Sheeting driven into or below the top of the pipe zone should be left in place to preclude loss of support of foundation and embedment materi- als. When top of sheeting is to be cut off, make the cut 1. Leave walers and braces in place as required to support cutoff sheeting and the trench wall in the vicinity of the pipe zone. Timber sheeting to be left in place is considered a permanent structural member and should be treated against biological degradation e.
Note that certain preservative and protective compounds may pose environmental hazards. Determination of acceptable compounds is outside the scope of this manual. Movable trench wall supports. Do not disturb the installed pipe or the embed- ment when using movable trench boxes and shields.
Movable supports should not be used below the top of the pipe embedment zone, unless approved methods are used for maintaining the integrity of embedment material. Before moving supports, place and compact embedment to sufficient depths to ensure protection of the pipe. As supports are moved, finish placing and compacting embedment. Removal of trench wall support. If the removal of sheeting or other trench wall supports that extend below the top of the pipe is permitted, ensure that neither pipe, foundation, nor embedment materials are disturbed by support removal.
Fill voids left after removal of supports and compact all material to required densities. Pulling the trench wall support in stages as backfilling progresses is advised. Excavate trench a minimum of 4 in. When ledge, rock, hardpan, or other unyielding material or cobbles, rubble, debris, boulders, or stones larger than 1.
The native material may be used for bedding and initial backfill if it meets all of the criteria of the specified pipe zone embedment materials. Trench preparation is dis- cussed in Sec. The risk of unstable conditions increases dramatically with slope angle. Installing pipes aboveground may be a preferred method for steep slopes, because aboveground structures such as pipe supports are more easily defined and, therefore, the quality of installation is easier to monitor and settlement easier to detect.
This may include treatment in the backfill or on the ground surface. Proper Bedding Support b. Improper Bedding Support Source: Figure Examples of bedding support in backfill adjacent to building foundations, sanitary landfills, or in other highly unstable soils, requires special engineering and is outside the scope of this manual.
Provide a firm, stable, and uniform support for the pipe barrel and any protruding features of its joint see Figure Provide a mini- mum of 4 in. Bedding material. In general, the bedding material will need to be an imported material to provide the proper gradation and pipe support. It is preferable that the same material be used for the initial backfill.
To determine if the native material is acceptable as a bedding material, it should meet all of the requirements of the initial backfill. This determination must be made constantly during the pipe installation pro- cess because native soil conditions vary widely and change suddenly along the length of a pipeline. It is becoming common practice to leave the bedding uncompacted for a width of one third of the pipe diameter centered directly under the pipe.
This reduces concentrated loads on the invert see Figure Rock and unyielding materials. When rock or unyielding material is present in the trench bottom, install a cushion of bedding, 6 in. If there is a sudden transition from rock to a softer mat- erial under the pipe, steps must be taken to accommodate possible differential settle- ment.
Figure b illustrates one method; however, other methods are also possible. Unstable trench bottom. Use a suit- ably graded material where conditions may cause migration of fines and loss of pipe support. Place and compact foundation material in accordance with Table For severe conditions, a special foundation, such as piles or sheeting capped with a con- crete mat, may be required.
The use of appropriate geotextiles can control quick and unstable trench bottom conditions. Localized loadings. Minimize localized loadings and differential settlement wherever the pipe crosses other utilities or subsurface structures see Figures and or whenever there are special foundations, such as concrete-capped piles or sheet- ing.
Provide a in. Change in Foundation Soil Stiffness Source: If the trench bottom is excavated below intended grade, fill the overexcavation with compatible foundation or bedding material and compact to a den- sity not less than the minimum densities listed in Table If trench sidewalls slough off during any excavation or installation of pipe zone embedment, remove all sloughed and loose material from the trench.
Place pipe and fittings in the trench with the invert conforming to the required elevations, slopes, and alignment. Provide bell holes in pipe bedding, no larger than necessary, in order to ensure uniform pipe support. Visit FileOpen to see the full list. What you can do with a Secure PDF: December M Fiberglass Pipe Design, Third Edition. January M Fiberglass Pipe Design. January Fiberglass Pipe Design Manual.