Clay Pipe
Engineering ManualClay Pipe
Engineering Manual
The general practice is to allow the contractor to select the method of
excavation provided that it results in an installation in compliance with the
project plans, specifications and applicable safety requirements. Any method
which is not in accordance with normally accepted practice must receive prior
approval of the engineer.
The single, most important factor affecting the structural loading on installed pipe is the trench width at the top of the pipe. Any increase in width over the design trench width shown in the specifications or on the plans will increase the backfill load. Should the trench width exceed the specified dimensions, and provision for this condition is not covered in the specifications and plans, the method of construction must be approved by the engineer.
The trench is generally excavated in the upstream direction. Any variation in this procedure should be at the direction of the engineer. It is important that the line and grade shown on the plans be followed.
Where ground conditions are such that trench walls will not remain vertical, the contractor may elect to use sloping side walls or to use solid sheeting to support the trench wall. In all cases, the critical dimension is the trench width measured at the top of the pipe.
It may not always be necessary to use shoring, sheeting or trench boxes. The primary concern is for safety and all applicable regulations should be strictly observed. Shoring and sheeting also retains trench width integrity and reduces the risk of cavein.
When used, timber sheeting should be placed above the top of the pipe. However, sheeting placed in the pipe zone shall be left in place or cut off not lower than the top of the pipe. Pulling timber sheeting creates voids at the sides of the pipe that reduce the side support provided by the soil. Thin steel sheeting may be pulled provided no voids are created and the pipe bedding is not disturbed.
Steel trench boxes are used for trench construction and safety. If possible, the trench box should ride above the top of the pipe on the bottom of a wider steptrench. Narrow backhoe buckets are available to maintain design trench width up to the top of the pipe. In this case, dragging the trench box forward does not interfere with pipe bedding and cannot pull the pipe joints apart.
If the trench box rides below the top of the pipe, care must be taken to protect the integrity of the pipe bedding, particularly when movement of the trench box leaves a void in the pipe bedding. Care must also be taken to ensure that movement of the trench box does not pull the pipe joints apart. A suggested method would be to secure the pipe with a wood crossblock, cable and winch at a downstream manhole.
Examples of sheeting, shoring and trench boxes are illustrated on pages 88 to 90.
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Special buckets to maintain trench width | |
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Typical steel trench safety shields | |
Trench load design for all pipe is based upon stable bedding and firm foundations. It is essential, therefore, that the trench bottom remain stable during backfilling and under all subsequent trench operations. Any departure from a stable foundation can nullify the efforts of both the designer and contractor because it can result in localized pipe stress concentrations which may cause structural failure. When unstable or rocky trench bottoms are encountered, it will be necessary to over excavate and restore the trench bottom to a stable uniform foundation with selected materials capable of properly supporting the pipe. Select native materials, crushed stone, gravel, slag, coral or other granular materials are commonly used for this purpose. The amount of granular material necessary to stabilize the trench bottom will vary according to the field conditions encountered.
Care should be taken in storage, handling and installation to avoid damage to the pipe and joint surface.
A visual inspection of the pipe just prior to installation should be performed by the installer.
Pipe are generally laid with the bell or coupling pointing upgrade. Adequate bell or coupling holes must be made to provide full longitudinal support of the barrel. The pipe shall be laid to the design line and grade. Line and grade are transferred from the survey points by means of devices used for this purpose. The pipe is then laid using a laser or gradeliner. It is advisable to use short lengths of pipe in and out of manholes and structures for additional flexibility.
For additional installation information and techniques, refer to ASTM C 12 Standard Practice for Installing Vitrified Clay Pipe Lines.
The following diagrams show methods of laying pipe to line and grade.
Compression joints and couplings should be assembled in strict accordance with the manufacturer's recommendations.
Particular care should be taken to keep foreign materials from interfering with proper joint assembly. The mating surfaces of the joint should be wiped clean.
All compression joints and couplings are manufactured in accordance with ASTM C 425 Compression Joints for Vitrified Clay Pipe and Fittings.
Specialized industrial applications should be referred to the pipe manufacturer to determine the suitability of the pipe and joint.
Initial backfilling takes place after the pipe has been installed according to the engineering specifications.
The initial backfill extends from the bedding material, up the sides of the pipe, to a level approximately 12 inches over the top of the pipe. The initial backfill should be carefully placed as soon as possible to maintain proper pipe alignment and to protect the sewer line. Unless otherwise specified, this material shall be of select soil, sand or gravel, free from large stones or clods. The bedding or backfill materials shall be sliced into the haunch areas of the pipe with a shovel or other hand tool to fill the voids in this area. (See diagram below and the note on shovel-slicing on page 47.)
The final backfill extends from the initial backfill to the top of the trench. Final backfill shall be placed in lifts or stages not to exceed 10 feet when using water compaction or as required by designated methods of mechanical compaction. Final backfill shall have no rock or stones having a dimension larger than 6 inches within 3 feet of the top of the pipe. Selected backfill material may be required for the top foot or more as specified by the engineer.
Compaction of the backfill material is usually required to prevent settlement of the ground surface or to support paving or structures. In areas where support of the pavement over a trench is required, compaction of part or all of the backfill material may be specified. When it is necessary to achieve a high degree of compaction, it may be advisable for the design engineer or contractor to consult a geotechnical engineer.
Many authorities consider it impractical and unnecessary to compact backfill materials to a density exceeding the density of the insitu soils.
Trench backfill specifications generally require mechanical compaction in layers, referred to as lifts, but may allow compaction by the use of water.
Mechanical Compaction
Most soil materials may be compacted by mechanical means in lifts. However, it is necessary to determine if the field moisture content is in the optimum moisture range in order to obtain the desired compaction with normal compactive effort.
It must be recognized that success in the mechanical compaction of backfills is entirely dependent upon the control exercised during this operation.
The selection and use of suitable compaction equipment must be made with care so that the pipe will not be disturbed or damaged. Heavy mechanical compacting equipment, such as highway type sheepsfoot rollers, dozers and loaders, should not be used until a cover of at least 5 feet has been obtained over the top of the pipe. Pneumatic tampers, vibratory pads and self propelled trench compactors specifically designed for this work are available. A pavement breaking type of falling weight "stomper" or drop hammer, should never be used for compacting, even with a substantial cover over the pipe. This type of impact device can damage the pipe and or force it out of alignment.
Hand held and walk behind vibrators and small trench rollers are good examples of acceptable compaction equipment.
The water method of compaction, known as flooding or jetting, when conducted in lifts, produces super saturation of the backfill material, which, for any given soil, will produce a degree of consolidation that can be predicted with reasonable accuracy. The desired range of compaction can be obtained with water in native granular or sandy materials which would include most sandy and silty soils and even those with some clay content. However, materials which are predominantly clay cannot be satisfactorily compacted by super saturation because of cohesion and low impermeability of the soil. Water jetting should not be allowed to disturb the initial backfill or the bedding which can result in pipe displacement or damage.
All structural design information for trench loads on vitrified clay pipe is based upon stable bedding. Therefore, when the bottom of the trench is not sufficiently stable, or firm, to prevent vertical or lateral displacement of the pipe after installation, the first step is to develop a nonyielding supplementary foundation for the pipe, irrespective of other bedding requirements.
Supplementary foundations may be of various types to provide an adequate and nonyielding base. Several suggested types are described below.
The simplest supplementary foundation is to excavate native soil below the grade of bedding material and replace with a layer of crushed rock or other coarse aggregate. Conditions where a crushed rock foundation will be most advantageous are where the instability of the trench bottom exists for only a very limited depth. The size and amount of rock necessary to stabilize the trench bottom will vary greatly according to the field conditions encountered.
Under conditions where an extremely unstable area is to be crossed and that area represents a very short length of line, it is possible to reinforce the pipe by full concrete encasement and adequate reinforcing steel to produce a rigid beam. Such a beam, if stabilized at its ends, can safely perform in localized unstable conditions or in generally unstable conditions by spanning between pile bents which have been located at proper intervals. Also, it may be necessary to utilize concrete encasement in shallow trenches, utility crossings and where a higher field supporting strength is required.
Piling and other special foundations have been used successfully to provide adequate support for sewers constructed in areas having extremely unstable subgrade conditions. Wood piling can be placed and then securely capped with timber planking, which then serves as a stabilized trench floor.
In all cases, an adequate thickness of approved bedding material is placed on the flooring. Pipe is then laid on the bedding where bell or coupling holes have been dug to assure even distribution of pipe support along the entire pipe barrel.
All special foundations should be designed to support the weight of the full sewer, backfill, and other anticipated loadings. Care should be taken to locate and protect all underground structures before setting piling or sheeting.
Water should be removed from the trench before final grading of the bedding. The trench should be kept dry during all phases of pipe installation. This can be done in several ways:
Over-excavate the trench bottom and fill with crushed stone or other angular material to provide a french drain under the pipe. This drain will carry the water to interceptor sumps where it can be pumped to the ground surface. (See following diagram.)
The ground water table also can be lowered with well points wherever soil conditions permit. They should be located at intervals dictated by soil properties and placed reasonably close to the trench walls. They should be sunk to a depth below the elevation of the trench bottom. (See diagram below.) Several well points can be joined together which can be handled by one pump.
In some cases the trench dewatering system may consist of a geotextile in addition to open graded crushed rock. Fine sands in a fluctuating water table environment are vulnerable to foundation problems and may require a geotextile encapsulation of the drain.
Crushed rock or other coarse aggregate is recommended and used as a bedding material to improve the load bearing capacity of pipe. Deeper layers of these materials have been employed to stabilize the base of the trench. Loss of pipe support can occur when open-graded materials are used on sites having fine to medium sands at the base of the trench and a water table which fluctuates rapidly in the pipe zone. This is believed to be caused by water moving rapidly through the fine to the coarse material and carrying the fine sands with it. To prevent movement of the fine sands into the voids of the open-graded bedding material, the material should be encapsulated in a geotextile material. Overlaps should be provided and care must be taken to prevent entry of sands into the crushed rock or aggregate base.
Controlling migration of bedding material with geotextile
In main line sewer construction, it is important to assure proper embedment, backfill and compaction of the construction materials which support and surround all Wye's or Tee's used for service connections. Some cities use Tee's instead of Wye's based on research studies which indicate that there is an insignificant difference in turbulence of flow between Wye and Tee connections to small and intermediate diameter main line sewers.
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Copyright © 1998 National
Clay Pipe
Institute
Last
modified: April
1, 1998