Bedding construction is very crucial in the installation of clay pipes being rigid.  To determine the adequacy of the type of bedding construction used on the site, SVCP offers free technical advice/assistance on the Static Load Calculations of clay pipes.  Static Load Calculation determines the external or design load on the pipes used which are influenced by the trench width pipes are laid, depth of backfill cover and the type of bedding in which the pipes are laid.

 

Under the principle that the maximum total external load on the pipes, we must not exceed the minimum value of the pipe strength, FN multiplied by the appropriate value of the bedding factor, BF and divided by a suitable design factor of safety, FS generally taken as 1.25.

 

          i.e       Wc  £ FN x BF         

                                       1.25

 

Computation of the trench load is done using Marston’s formula

                    ¾2

Narrow Trench load, Wc       =         1-e-2Km’ H/Bd g Bd           

                                                  2 Km’

 

Where                    Km’      =         the product of active earth pressure coefficient &       

                                                  coefficient of sliding friction within a mass of fill

                  

                             H        =       depth of cover         

                             Bd        =       trench width  

                             g         =       unit weight of backfill material

 

                                                                          ¾2

and Wide Trench load, Wc’   =       [1.50 H/B­c-0.07] g Bc when rsdp

                                                is taken as 0.5

             ¾2

or,                         Wc’     =       [1.59 H/B­c-0.09] g Bc when rsdp               

is taken as 0.7

 

where                     Bc       =       outer diameter of pipe        

rsd       =       settlement ratio       

                             p        =       projection ratio

 

Total design or external load =       lesser of the narrow or wide trench load +
                 surcharge + water load       

Bedding Factor can therefore be computed once the total external load has been determined.  The value of the bedding factor for a particular method of construction is not a precise figure but is affected by the quality of workmanship.  It is determined by the effectiveness with which bedding materials and backfill around the pipe distribute the loads and support pressures acting on it.  As design cannot be separated from construction, calculation of the bedding factor assumes a reasonable standard of workmanship and supervision on site.

 

As a guide, reference is made to the recommended types of bedding construction by the Clay Pipe Development Association Limited with the corresponding Bedding Factor.

 

References:

 

1.     Transport Research Laboratory – “Simplified Tables of External Loads on Buried Pipelines” and “Guide to Design Loadings for Buried Rigid Pipes”

 

2.   Clay Pipe Development Associated Limited – “Design Tables for Determining the Bedding Construction of Vitrified Clay Pipelines”