TECHNICAL | TUNNELLING IMPACTS


Circumferential crack Circumferential crack


Above, figure 10: Examples of circumferential cracks caused by longitudinal flexure (bending) Vitrified Clay Pipe


types are given by British Standards BS7385-2:1993.


An important factor often overlooked is the possibility that vibration can increase the density of, and cause settlement, in some soils, which can put structures at risk (see BS5228-2:2009 Section B.3: Note 3).


6.3 Longitudinal flexural (bending) strain Longitudinal flexural (bending) strain (εb


) along a


pipeline is calculated from simple beam theory and is given by: εb


= y / R


where R is the longitudinal radius of curvature and y is the lever arm to the neutral axis of a pipe.


The calculation of longitudinal radius of curvature (R) is given by: R = i2


/ [2dn – (dn-1 + dn+1 )]


Masonry Sewer


where dn


is the flexural displacement predicted at the nth


data


point along the assessment line and i is the interval between the data points (see Figure 8). An interval of 1m is generally adopted for assessment purposes. However, a smaller interval may be required to capture any abrupt changes in curvature along the pipeline. The lever arm y is a function of the pipe extrados


diameter (D) and is dependent on pipe material. Figure 9 illustrates the definition of lever arm for pipe material either capable or incapable of sustaining tension under bending. Examples of circumferential cracks caused by longitudinal flexure (bending) are shown in Figure 10.


6.4 Axial Strain The calculation of axial strain (εa


given by: εa


= [(S + ΔS) / S] ) along a pipeline is


P1 S Initial positions


P2


Right, figure 11: Development of axial tensile and compressive strains along a pipeline


P1 S - ∆S Axial compression


P2


P1 S + ∆S Axial tension


P2


22 | February 2022


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49