Size

Finding the fall

You will not always work on sites that need trenches of exactly 40 m or 50 m to make it easy to calculate the grade. You will be planning trenches of many different lengths throughout your working life. You will also have to take into account the depth of the authority connection point on each site.

It is your responsibility to calculate the slope needed in every trench to meet the minimum grade requirement and ensure adequate depth of cover at the upstream end. You need to calculate the fall.

Fall is the difference in level between two points. In plumbing work the fall is always in the direction of the flow.

Graphic showing a fall of 1000 mm. It is from 600 mm below ground level to 1.6 m.

Fall is calculated using the following formula. There is a slight difference in the formula depending if the grade is expressed as a percentage or a ratio.

Graphic showing formula for fall where grade can be a ratio or a percentage. The formula is: F equals fall in millimetres. L equals length of trench (or pipe) in millimetres. G equals grade as a percentage or ratio. So, grade as a ratio is expressed as F equals L divided by G. Grade as a percentage is expressed as F equals (L times G%) divided by 100.

Note: there will sometimes be a minor variance in the fall calculation answer depending if the grade is expressed as a percentage or ratio. This is due to the inexact rounding of the figures when the ratio is converted to a percentage. 

Worked examples

 

Worked example 1 - Finding the fall of a pipe

A 6 m length of 100 mm pipe laid at a minimum gradient of 1.65% or 1:60 would have the following fall:

With grade as a ratio, F equals L divided by G. In the worked example, F equals 6000 divided by 60. The fall therefore is 100 mm. With grade as a percentage, F equals (L times G%) divided by 100. In the worked example, F equals (6000 times 1.65) divided by 100. The fall therefore is 99 mm.

The drain, at the point it enters the sewer, must be at least 99 mm deeper than the drain at the upstream end.

Worked example 2 - Finding the fall of a trench

A 23m trench with a minimum grade of 1.25% or 1:80 would have the following fall:

With grade as a ratio, F equals L times G. In the worked example, F equals 23000 divided by 80. The fall therefore is 288 mm. With grade as a percentage, F equals (L times G%) divided by 100. In the worked example, F equals (23000 times 1.25) divided by 100. The fall therefore is 288 mm.

The bottom of the trench, where the drain enters the sewer, must be at least 288 mm deeper than the bottom of the trench at the head of the drain.