How To Prepare a Manhole Schedule in Plumbing?

             In this article we will learn how to prepare a Manhole schedule, in details, with a worked example.

Here is an example of  Manholes laid in a villa project, schedule which we will be using to create the manhole schedule and understand in detail, step by step procedure to create the manhole schedule. As a plumbing Engineer, one should know how to create the Manhole Schedule.

As you can see you have the reference layout created for manholes in the above image. While designing the manhole layout, which is done at last, once all the drainage layout drafting in autocad is completed. This manhole layout is generated in site setting layout of the project, once manhole locations are decided , all these manholes are set in place as per the design locations selected by design engineer.

There are some rules which need to be followed in order to create this manhole layout. These rules are made by the Municipal authority of that respective country. The rules that were followed while creating this above manhole layout are listed as below.

* Upto 1300mm deep manholes, the manhole cover size should be 600 mm x 600 mm.

* The depth of First manhole shall be atleast 450 mm.

* Slopes for horizontal runs shall be , 1:60 for 100 mm, 1:90 for 150 mm, and 1:120 for 200 mm pipes.

* Distance between manhole to manhole shall not exceed 15 meters in villas and buildings.

With these above points in mind, lets us create our manhole schedule.

First we should create a table as shown below :

As per the rules we need to decide the depth of the first manhole, here we will consider the minimum depth of first manhole as a standard value of 450 mm. From the table above, we will know the meaning of the terms.

COVER.L - It is the cover level of the manhole. we can consider the ground level, where the manhole cover will sit. But during actual calculations we need to consider the FFL (Finished Floor Level) values that are mentioned in the architectural site setting layout provided by the architect.

INVERT.L - It is the invert level of the manhole. We can easily get it from the formula:

                Next Invert Level = Previous Invert Level - (distance x slope)

DISTANCE - This distance in meter is the distance between the two manholes. This distance we need to calculate accurately from the architectural site setting layout provided by the architect, after we place all the manholes in our drainage site setting layout.

Manhole depth - We can easily calculate it using cover level and invert level. We will see how, in the below steps.

Pipe Dia -  Diameter of pipe selected in mm.

Manhole cover type - Which type of manole cover we need to select. If it is a car runway , near the garage of villa, then we need to select heavy duty type of cover, else medium duty.

So, lets begin the creation of our manhole schedule.

We will start with the first Manhole, MH-01 which is shown in the image above.

Here, for first manhole MH-01, cover level is +0.00 which is the level of the ground from architectural site setting layout. Since there is no second manhole yet, hence distance is nothing. The depth of the first Manhole we have selected to be 450 mm. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Invert Level = Cover Level - Depth

                                                                              = +0.00 - (0.450)

                                            Therefore, First Invert Level =  -0.450

Since the manhole is less than 1300mm deep, we will select the manhole cover size as 600 mm x 600 mm. The pipe dia we will consider as 150 mm as per authority guidelines. And the Manhole cover type we will select medium duty since no runway or anything else. Next moving on to second manhole MH-02. 

For Second manhole MH-02, cover level is +0.00 which is the level of the ground from architectural site setting layout. The distance between first and second manhole is 10.37 meters as measured on the site setting layout of drainage, after two manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the second Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.450 - ( 10.37 x (1/90))

                 Next Invert Level = -0.450 - ( 0.115)

Therefore,           Next Invert Level = -0.565

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.00 - (-0.565)

                                            Therefore, Depth =  0.565 m.

Now, moving on to Next manhole, MH-03.

For Third manhole MH-03, cover level is +0.15 which is the level of the ground from architectural site setting layout. The distance between second and third manhole is 10.02 meters as measured on the site setting layout of drainage, after three manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the third Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.565 - ( 10.02 x (1/90))

                 Next Invert Level = -0.565 - ( 0.111)

Therefore,           Next Invert Level = -0.676

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.15 - (-0.676)

                                            Therefore, Depth =  0.826 m.

Now, moving on to Next manhole, MH-04.

For Fourth manhole MH-04, cover level is +0.15 which is the level of the ground from architectural site setting layout. The distance between third and fourth manhole is 9.56 meters as measured on the site setting layout of drainage, after four manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the fourth Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.676 - ( 9.56 x (1/90))

                 Next Invert Level = -0.676 - ( 0.106)

Therefore,           Next Invert Level = -0.782

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.15 - (-0.782)

                                            Therefore, Depth =  0.932 m.

Now, moving on to Next manhole, MH-05.

For Fifth manhole MH-05, cover level is +0.15 which is the level of the ground from architectural site setting layout. The distance between fourth and fifth manhole is 11.54 meters as measured on the site setting layout of drainage, after five manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the fifth Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.782 - ( 11.54 x (1/90))

                 Next Invert Level = -0.782 - ( 0.128)

Therefore,           Next Invert Level = -0.910

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.15 - (-0.910)

                                            Therefore, Depth =  1.06 m.

Now, moving on to Next manhole, MH-06.

For Sixth manhole MH-06, cover level is +0.15 which is the level of the ground from architectural site setting layout. The distance between fifth and sixth manhole is 7.37 meters as measured on the site setting layout of drainage, after six manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the sixth Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.910 - ( 7.37 x (1/90))

                 Next Invert Level = -0.910 - ( 0.082)

Therefore,           Next Invert Level = -0.992

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.15 - (-0.992)

                                            Therefore, Depth =  1.142 m.

Now, moving on to Next manhole, MH-07.

For Seventh manhole MH-07, cover level is +0.00 which is the level of the ground from architectural site setting layout. The distance between sixth and seventh manhole is 10.0 meters as measured on the site setting layout of drainage, after six manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the seventh Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -0.992 - ( 10.0 x (1/90))

                 Next Invert Level = -0.992 - ( 0.111)

Therefore,           Next Invert Level = -1.103

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.00 - (-1.103)

                                            Therefore, Depth =  1.103 m.

Now, once all the manholes are completed, we need to calculate and tabulate for the FIC as well. FIC stands for Final Inspection Chamber. Moving on to FIC.

For FIC manhole FIC, cover level is +0.00 which is the level of the ground from architectural site setting layout. The distance between seventh and FIC manhole is 2.80 meters as measured on the site setting layout of drainage, after seven manholes have been placed, as per the designated manhole locations by design engineer. Now we have to calculate the depth of the FIC Manhole by using the formula as shown below, but before calculating the depth, we need to calculate the next invert level using the formula as shown below (and we know for selected 150 mm dia pipe, the slope to be considered is 1:90) :

                 Next Invert Level = Previous Invert Level - (distance x slope)

                 Next Invert Level = -1.103 - ( 2.8 x (1/90))

                 Next Invert Level = -1.103 - ( 0.031)

Therefore,           Next Invert Level = -1.134

Now, We have the Next invert level, we can easily calculate the depth using the formula as shown below. 

                We have the formula, Cover level - Invert level = Depth

                                            Hence, Depth = +0.00 - (-1.134)

                                            Therefore, Depth =  1.134 m.

After getting all these values, we need to get the authority manhole details, so we can compare the depth of our FIC and the authority Manhole depth, if the slope is ok then we can proceed.

In this way, we can prepare the Manhole schedule in drianage Plumbing MEP.

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