concrete storm sewers
There are 5 classes of concrete pipe; Class I, II, III, IV, and V. What defines the type is the D-Load rating of the pipe. The D-load is the supporting strength of a pipe when it’s loaded under a triple edge bearing test.
Triple Edge Bearing Test
D-load is measured in pounds per lineal foot of inside diameter or horizontal span. The following are ‘Ultimate D-Load’ rating that make up each class;
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Class I = 1200lb/ft/ft
Class II = 1500lb/ft/ft
Class III = 2000 lb/ft/ft
Class IV = 3000 lb/ft/ft
Class V = 3750 lb/ft/ft
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Typical sizes for concrete pipe are the following:
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12, 15, 18, 24, 30, 36, 42, 48, 54, 60, 66, 72, 84″ and higher.
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Concrete pipe can be reinforced or non-reinforced. When it comes to concrete pipe joints there is a precision joint and a non-precision joint.
Precision joints are achieved with the use of a an O-Ring or Profile gasket that’s designed to perform with the bell and spigot profiles of a particular concrete pipe’s design.
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Non-performance joints are more forgiving when it comes to water/silt infiltration but still perform well enough for most intended purposes. An example of a non-precision joint would consist of mastic sealant, butyl sealant, mortar joint, or an external wrap.
When it comes to custom bends, tees, wyes, and crosses, these parts will typically need to be manufactured to suit and will typically require significant cost and a lead time, if a certain manufacturer makes them at all. Fabrication of concrete pipe fittings can be uneconomical for standard branches and laterals entering a concrete pipe.
When it comes to concrete pipe, joint connection is a bit more critical than plastic storm pipe as ‘joint gap’ is more prevalent with concrete pipe. This is because concrete pipe is so rigid and the bells and spigots must be large enough to fit the pieces together. ‘Joint gap’ is the vertical distance from the top of the spigot and bottom of the bell when 2 pipes are connected. The amount of allowable joint gap is commonly provided by the engineer of the project or recommended by the pipe manufacturer.
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Concrete pipe being a rigid material is far less dependent than flexible pipe on the effects and stresses of the pipe envelope (backfill material and cover loading). The intrinsic strength of concrete pipe makes up for certain construction shortcomings (re: compaction, trench depths, cover/fill heights) that might affect other pipe types
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The first consideration when it comes to installing concrete storm drainage pipe is unloading the material from the truck. The pipe will typically come on a flatbed semi truck, sometimes with a boom to unload the pipe. However having a backhoe or excavator on site to unload is typically required. The pipes will have to be removed one by one using caution not to chip or break the relatively brittle concrete material. This is often done with textile slings strung through the center of the pipe, slings wrapped around the center of gravity of the barrel, or with lift holes cast into the pipe during fabrication. Smaller concrete pipe may be picked by way of large forks through the center.
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An approximation of how many pieces of pipe that will fit on a common flatbed truck for 12”-24” are:
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-12” – 36/ea
-15” 33/ea
-21” 22/ea
-24” 18/ea
Concrete pipe placed on wood dunnage
The pipes should be stored on a flat surface. A typical concrete storm pipe crew will consist of 2 operators, 2 laborers, and a foreman but can vary depending on pipe size, depth, and extent of backfill/compaction required. The equipment required will be an excavator/backhoe and a loader.
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Once the trench is excavated, a pipe bedding is sometimes required under and around the pipe. However, since the primary purpose of pipe bedding is for protection and structural support, concrete pipe will sometimes not require bedding, being a very rigid and strong material. All that is sometimes needed is a strong level natural earth surface. If bedding is used it may simply be a few inches crushed stone, gravel, or sand for basic leveling purposes.
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It is common for pipe crews to ‘string out’ the pipe along the sides of the trench. This process expedites installation once the actual pipe installation begins. The need to cut pipes may arise. If so, an accurate line must be traced around the pipe to provide a guide for the cutting tool. A good way to check that your cut marks are square is by measuring from the nearest bell or spigot end to several points along the mark assuring the distances are all equal. Concrete storm pipe can be cut with a cut-off saw with a steel diamond blade.
Once a sufficient or desired amount of pipe is strung out along the trench, the excavator will begin lifting pipe with use of ropes, chains, or slings into the trench. Pipe layers or laborers will direct the operator from outside of and inside of the trench on where and when to place the pipe.
Setting pipe with cable & hook near center of gravity
Large diameter concrete pipe will commonly have lift holes for rigging/lifting, which are plugged before backfill
Lubricated spigot & gasket
The pipe being actively laid is then lifted and the bell end is slowly moved toward the spigot end of the other piece by way of the operator in the excavator. The pipe laborers will confirm when the pipe being placed has been pushed ‘home’ and has fully interlocked with the field installed O-Ring or profile gasket. The pipe layers will continue to direct the machine operator on moving the pipe up, down, left, and right to attempt to push the pipe home. If the pipes cannot be simply guided in place by hand, the spigot end of the pipe being laid can be gently pushed with the excavator until the pipes are appropriately connected. Once the stretch of pipe run for that day is placed, it is typically backfilled that same day.
