Culvert Pipes

These come in four different classes; 20D, 40D, 60D, 80D and 100D
The different number identifications relate to the weight capacity that they can sustain; 20D implies sustenance of a weight not exceeding 20 kilo-Newton.
40D can sustain up to 40 kilo-Newton and so on, in proportion with the class number. They differ in the amount of steel per pipe, with the 60D class increasing its thickness by 12mm compared to the 20D pipe.