Structural Design

 

TESTING

The Irish Standard Specification for Concrete Sewer Pipes, I.S.6: 2004 gives minimum crushing test loads for each diameter of pipe. Loads are applied in a three edge-bearing test described in the standard. Hydraulic tests are also carried out, by applying an internal hydraulic pressure of 0.5 bar to test for porosity. We also employ a 100% gauging system of our Spigots and Sockets to ensure watertight joints to 10m head of water.

BEDDING FACTORS AND MATERIAL

In the three edge-bearing test, the vertical loading and supporting reactions are line loads. The strength of the pipes determined in the crushing test is multiplied by a bedding factor (Fm), which represents the amount by which the stresses in the pipe are reduced because of the spreading properties of the bedding for load and reaction.
The value of bedding factor (Fm) for a particular method of construction is not a precise figure but is affected by the quality of workmanship. The figures given are assuring a good standard of workmanship. Pipe settlement will be kept to a minimum by the proper selection and compaction of the bedding material. The bedding should be compacted to a density not less than that of the natural soil in the sides and bottom of the trench. The bedding material should be of a similar particle size to that in the trench sides. Where the ground is clay or silt, bedding material must consist of all-in gravels to prevent the trench from becoming a drainage channel and thus carrying away fines from the trench walls and bedding and causing settlement of the pipes. Rounded single size material is not recommended. The bedding directly beneath or above the pipeline must not be over compacted; otherwise line loading of the pipes will result. Research and long experience has shown the following rule of thumb to be acceptable with a maximum of 40mm and limit on fine sands.

 

COMPACTION

The degree of compaction shall be as specified in the structural design of the pipeline. Specified degrees of compaction shall be controlled by a method specification related to the particular equipment used (compaction means) or, where required, verified by testing.
The initial backfill directly above the pipe should be compacted by hand where required. Mechanical compaction of the main backfill directly above the pipe should not be commenced until there is a total depth of cover of least 300mm above the top of the pipe. The total depth of the cover directly above the pipe before mechanical compaction is commenced depends on the type of compaction device. The choice of compaction equipment, the number of passes and the thickness of layer to be compacted shall take account of the material to be compacted and the pipe to be installed.
Compaction by saturating the backfill or side fill is permissible only in exceptional cases and then only in suitable, non-cohesive soils.

MINIMUM DEPTH OF COVER

It is advisable that pipes laid under roads should have cover over the pipe of not less than 1.2m. Pipes laid with less than 1.2m cover require special consideration. For pipes laid in fields a minimum cover of 0.6m should be provided. At shallower depths there is a risk of damage from agricultural operations.

 

DESIGN CALCULATIONS

The required crushing strength of a concrete pipe can be calculated using the following equation.

Wt > (We x Fs) / Fm

Where: Wt = required I.S.6 crushing load (kn/m), (Fn=Wt)
We = Total applied load on the pipe (kn/m)
Fs = Factor of safety (1.25 MINIMUM)
Fm = the bedding factor

Crushing load (Wt) as stated in I.S. 6: 2004 “Perforated Pipes” Table

NOTE:

The minimum crushing load (Wt) is the load which the pipe will sustain without collapse. The proof load (Wp) is the load which reinforced pipes sustain without developing a crack exceeding 0.3mm in width over a length of 300mm.. The Design Loads used in Tables 16, 17 and 18 are calculated from the Marston’s Formula to collapse. (Fn=Wt)