Lines/Levels & Gradients

Lines/Levels & Gradients

^{Note: You need 100% in this lesson on the self checks.}

Establishing line, level, and gradient

• At the start of a job, the datum needs to be established

• The datum point can be the invert level of a pipe.

• The invert level of a pipe is the lowest wetted area at the bottom of

  the inside of that pipe

• For example, ceramic pipes have walls which are approximately 20 mm thick.

Invert level

• Invert level shown at bottom of inside of pipe

Establishing gradient

• A pipe installed with a gradient is not level — it has a slope

  or fall.

• The term gradient is sometimes referred to as grade.

• Gradients are usually specified in NZBC and AS/NZS3500

Line and level and gradient

• Line is the shortest distance between two points

• Level is described as a horizontal line that is parallel to the surface

• Gradient is a slope below horizontal. It is usually expressed as a ratio or as a percentage

^{Sometimes you need to know how many millimeters per meter your drain has to fall from the gradient you have been given}

• Use the following method

• 1:60 means for every 60m you travel the fall is 1m

• The calculation is simple > 1000 (mm in 1 m)  divided by 60  (grade) = 16.6mm for every meter

​Setting up a dumpy level

​Taking readings & levels

Check out the folllowing triangle

You will need two values to calculate the third.

See the next slide

Calculate the fall for a drain 8m long with a 1:40 grade

​

• ​F = L divided by G

• Therefore

• ​8 / 40 = 0.200

• THE FALL = 0.200 m or 200 mm

​In the examples we have done, the gradient could have been expressed as a percentage 1.65% at the gradient 1:60 = 1.65%.

​

Note that the conversions between gradients and percentages are rounded, that is, they are rounded to the nearest 0.05%.

​

For this reason calculations will vary slightly depending on whether a ratio or percentage is used. The following table shows conversions between percentages, ratios and mm/m.

Drain grades are calculated by engineers to produce

• Self cleansing velocities

• The correct Fixture Discharge Unit loading to grade

• Free ventilation throughout the drainage system

​Take a look at this video, although American it still has relevance to NZ and gives you an understanding of how FDUs were calculated

This is a table from G13 that shows the different gradients for different loadings

Note: You can increase the pipe size and shallow the grade, this may be helpful if you are struggling for fall between to datum points.

​

​Some more key points:

• ​When it is necessary to install a drain below a specifed grade, an Alternative Solution might include the provision of flushing tanks and more inspection points.

• ​Drains should also be installed so that under steady flow conditions, the pipe is not completely filled. This will allow for any surge flow and prevent siphonage of any trap seals.

​Discussion point:

​Consider the following scenario below:

• What are the falls and grade required?

• ​ What can you do to achieve this?

​NZBC G13/AS2 Drainage requires the use of verifiable levelling devices such as lasers and dumpy levels where drains are laid at gradients of 1:80 or less.

Because, ​when drains are laid with a very small slope, their gradients should be checked with more accurate equipment.

Now try these

Multiple Choice

Try 1:80 grade, convert to mm per meter

• 18mm

• 180mm

• 125mm

• 12.5mm

Multiple Choice

Try this one: How many mm per mtr fall should a 1:120 drain have?

• 120mm

• 12mm

• 8.3mm

Multiple Choice

How much fall is required for a drain 15 m long with a 1:60 grade?

• 500mm

• 900mm

• 250mm

• 40mm

Multiple Choice

You have a FDU loading of 149, what grade should you lay your drain at using a 100mm pipe

• 1:40

• 1:60

• 1:20

• 1:120

• 1:80

Multiple Choice

What is 1.45% expressed as a grade

• 1:14.5

• 1:69

• 1:1.4

• 1:45

Multiple Choice

You have been asked to lay a drain at 15%, what ratio grade would it be and how much fall in mm is needed per mtr

• 1:15 and 15mm

• 1:100 and 15mm

• 1:6.6 and 151.5mm

• 1:66 and 15.1mm

Multiple Select

Calculate the overall fall from the invert level at your sewer connection of 3.540m to your drain connection at 2.770m

• 770mm

• 7.7mm

• 6.31

• 7.70

Multiple Choice

Using the previous slides calculation, what grade would the drain be laid at if it was 10m long

• 1:70

• 1:10

• 1:12.9

• 1: 77

Well Done!

Thank you for completing this lesson and your final assessment