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Pipe Gradients 

Above ground and below ground horizontal drainage pipes should be laid to an adequate gradient. 

Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities.

 A gradient of 1 in 80 is suitable for commencing calculations for pipe schemes.

 

If a gradient is too steep i.e. steeper than 1 in 40, the liquid may run faster than the solids in the sloping foulwater pipe thus leaving the solids stranded, which could then block the pipe.

 

If the gradient is not steep enough, i.e. less than 1 in 110, then the pipe could still block if the solids slow down

and become stranded. 

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The fall in a pipe may be defined as the vertical amount by which the pipe drops over a distance. The distance can be between sections of pipe or between manholes. The diagram below show pipe fall and distance. 

A gradient may be defined as fall divided by distance.

 

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  GRADIENT = FALL / DISTANE

 

or e!ample is a "# metre section of drainage pipe has a fall of $.%$ metres, calculate the gradient.

 Gradient & $.%$ ' "#

 

Gradient & $.$(")

 This can be converted into a gradient written as a ratio or (* some number.

 

Gradient & ( ' $.$(") & +$ 

Gradient & ( in +$ The above formula may be rearranged for all if the gradient is known*

 

FALL = GRADIENT ! DISTANE 

or e!ample, calculate the fall in a )$ metre section of foul water pipework if the gradient is to be ( in +$. 

A gradient of ( in +$ is converted to a number instead of a ratio.

 ( ' +$ & $.$(")

 

all & Gradient ! istance 

all & $.$(") ! )$

 

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all & $.-") metres  or -")mm. 

The previous diagram may be completed by adding a pipe gradient.

 

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In"ert Le"els 

The In"ert Le"el of a pipe is the level taken from the bottom of the inside of the pipe as shown below.

 

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The level at the #r$%n $& the pipe is the In"ert le"el plus the internal diameter of the pipe plus the pipe wallthickness. It may be necessary to use this in calculations when level measurements are taken from the crown of a

 pipe.

 

'anh$les 

A manhole or access chamber is required to (ain a##ess to a drainage system for unblocking, cleaning, rodding or 

inspection. A typical manhole is shown below. 

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/anholes may be manufactured from masonry or precast concrete. 0ometimes several precast concrete rings are

used to form a manhole which speeds up the onsite construction process. 1ormally deep manholes below 1)0

*etre in depth require step irons to assist access for a workman.

/anholes and access chambers are also manufactured in 234. An access chamber  is not usually large enough to

admit a person but is suitable for access by cleaning rods or hose and they are used for domestic applications, a

common size of plastic access chamber is #)$mm diameter. or the domestic market plastic, fibreglass or galvanised steel lids may be used but cast iron lids are required where traffic crosses. 

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A back drop manhole is used in areas where the surface level slopes as shown below.

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If the undergroung sewer pipe is to stay below ground it must follow the average gradient of the slope. Thisinvariably means that the pipe (radient +e#$*es t$$ steep, resultingin the solids being left stranded in the pipe

therefore causing a blockage.

To overcome this problem the back drop manhole was developed, as shown below. 

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An easier way to construct a back drop manhole is to use an internal vertical section of pipe as shown below.

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