In this article we will discuss about the classification, dimensions and component parts of a manhole.
Classification and Dimensions of Manholes:
Manholes are classified according to depth as:
(i) Shallow manholes,
(ii) Normal manholes, and
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(iii) Deep manholes.
(i) Shallow Manholes:
Shallow manholes are those which are about 0.75 to 0.90 m in depth. These manholes are of rectangular shape with minimum internal size 0.9 m x 0.8 m. These are constructed at the beginning of branch sewers or on sewers laid at places which are not subjected to heavy traffic. These are also known as inspection chambers and are provided with light cast iron cover and frame at the top.
(ii) Normal Manholes:
Normal manholes (or medium manholes) are those which have depth more than 0.9 m and up to 2 m. These manholes may be of square or rectangular shape with minimum internal size 1 m x 1 m or 1.2 m x 0.9 m, or of circular shape with minimum internal diameter 0.9 m. The section of square or rectangular manholes is not changed with depth.
The circular manholes are of uniform section in lower portion and slanting in top portion so as to narrow down the top opening equal to internal diameter of manhole cover. These manholes are provided with heavy cast iron cover and frame at the top.
(iii) Deep Manholes:
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Deep manholes are those having depth more than 2 m. These manholes are mostly circular in shape. Depending upon the depth of manhole, the diameter of manhole changes.
For different depths the internal diameter of circular manholes may be kept as follows:
The circular manholes are of uniform section in lower portion and slanting in top portion so as to narrow down the top opening equal to internal diameter of manhole cover.
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However, for depths above 2.0 m and up to 2.5 m, manholes may be of rectangular shape with minimum internal size 1.2 m x 0.9 m. The size of rectangular manholes is reduced in the upper portion to reduce the size of manhole cover. The reduction in size is achieved by providing an offset constructed of either R.C.C. slab or brick arch. The rectangular manholes with arch type offset are also known as arch type manholes. The arch type manholes may be constructed for depths of 2.5 m and above with minimum internal size 1.4 m x 0.9 m.
Deep manholes are provided with steps on one of the vertical wails to enable the workers to go down up to the bottom. These manholes are also provided with heavy cast iron cover and frame at the top.
According to the Indian Standard IS: 4111 (Part 1)-1991 manholes are classified on the basis of depth as:
(i) Shallow manholes and
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(ii) Deep manholes.
In this classification shallow manholes are those having depth less than or equal to 2 m, and deep manholes are those having depth more than 2 m. In other words shallow manholes of this classification includes both shallow and normal (or medium) manholes of the previous classification.
Further for manholes of different depths, minimum internal sizes have also been recommended by the Indian Standard IS: 742-1990 which are given in Table 6.2.
Component Parts of a Manhole:
A typical manhole consists of the following component parts:
(i) Access shaft;
(ii) Working chamber;
(iii) Base and side walls;
(iv) Bottom or invert;
(v) Steps or ladder;
(vi) Cover and frame.
(i) Access Shaft:
The upper portion of a deep manhole is known as access shaft. It is a vertical passage which provides access to the working chamber of the manhole from the manhole cover. The minimum size of access shaft is about 0.75 m x 0.60 for rectangular manholes and about 0.70 m diameter for circular manholes.
For rectangular manholes built of brickwork the access shaft is corbelled inwards on three sides to reduce its size to that of the opening in the cover frame, and to provide easy access on the fourth side to step irons or ladder.
Alternatively, the access shaft may be covered by a reinforced cement concrete slab of suitable dimensions with an opening for manhole cover and frame. For circular manholes the access shaft is usually made slanting inwards so as to narrow down the top opening equal to internal diameter of manhole cover.
(ii) Working Chamber:
The lower portion of a manhole is known as working chamber which provides working space to carry out cleaning and inspection of sewer line. The minimum size of working chamber for deep rectangular manholes is 1.2 m x 0.9 m with larger dimension being in the direction of flow. For deep circular manholes the minimum diameter of the working chamber is 1.2 m. The height of working chamber should preferably be not less than 1.8 m.
The size of working chamber of a manhole is usually larger than that of its access shaft and hence the working chamber is constructed by enlarging the access shaft at its bottom by providing an offset constructed of R.C.C slab or brick arch or by corbelling.
(iii) Base and Side Walls:
A bed, generally of plain cement concrete, is provided at the base to support the side walls of the manhole and to prevent the entry of groundwater. The minimum thickness of concrete bed is 15 cm for manholes of depth up to 0.8 m. 23 cm for manholes of depth above 0.8 m and up to 2.1 m and 30 cm for manholes of depth more than 2.1 m. The concrete bed may be provided with adequate reinforcement if necessary to withstand excessive uplift pressure.
The side walls of manholes are made of brick or stone masonry or reinforced cement concrete. The brick walls are very common. The minimum thickness of brick walls is 20 cm (or one brick) for manholes of depths up to 1.5 m and 30 cm (or one and a half brick) for manholes of depths more than 1.5 m.
The following thumb rule may be used for determining the thickness of brick walls-
t = 10 + 4d …(6.1)
Where
t = thickness of wall in cm. and
d = depth of manhole in m.
The inside and outside of brick work is plastered with cement mortar 1:3 (1 cement and 3 coarse sand) and inside finished smooth with a coat of neat cement.
The thickness of reinforced cement concrete (R.C.C.) walls will be much less as compared to that of brick walls and can be designed by the usual methods of structural analysis. However, R.C.C. walls are costly and hence these are adopted only under special circumstances.
(iv) Bottom or Invert:
At the bottom of the manhole a semicircular or U-shaped channel of cement concrete of diameter equal to that of sewer is constructed. Above the horizontal diameter the sides of this channel are extended vertically, nearly up to the crown of the sewer and then their top edge is suitably rounded off and made to slope towards the channel to form benching.
The slope provided for benching varies from 1 in 10 to 1 in 6. The benching enables the floor of the chamber to be drained of backed up sewage. The bottom of the channel lies in line with the invert of the sewer line.
When two or more sewers enter a manhole at the same level at the bottom of the manhole, in addition to main channel branch channels are similarly constructed with respect to the benching. At the junction with the main channel the branch channels are provided with easy curves.
Where the sewers entering and leaving a manhole are of different diameters, the entering and leaving sewers are placed with their crowns at the same level and necessary slope is given in the invert of the manhole chamber. This is done to prevent backflow in the smaller sewer when the larger sewer is flowing full. In exceptional cases and where unavoidable, the crown of entering sewer may be fixed at lower level but in such cases to the peak flow- level of the two sewers is kept the same.
(v) Steps or Ladder:
In order to facilitate entry and exit of workers steps or rungs are provided in all manholes of depth more than 0.8 m. The steps are made of cast iron and these are placed staggered at a horizontal centre to centre distance of 38 cm and a vertical centre to centre distance of 30 cm.
The top step is placed 45 cm below the manhole cover and the lowest step not more than 30 cm above the benching. The width of the step is usually 15 cm. However, if steps are made of double width staggering is not required. The steps are firmly embedded in the wall so that they do not overturn.
In very deep manholes it is desirable to provide a ladder instead of steps. The ladder gives a high sense of security to the workers.
(vi) Cover and Frame:
The opening at the top of a manhole is provided with a cover set in a frame. Mostly the openings are of circular shape and hence the manhole covers of circular shape are most commonly used. The size of manhole covers is such that there is a clear opening of at least 56 cm in diameter for manholes of depth more than 0.9 m.
Both cover and frame are of cast iron. The frame supporting the cover is generally 20 to 25 cm high and its base is 10 to 12 cm wide. The weight of cover and frame varies from 90 to 270 kg. The light type is adopted where light traffic load is to be borne and heavy type is adopted where heavy traffic load is to be borne.
The frame is firmly embedded in cement concrete on the top of masonry and the cover rests in the groove provided inside the frame. The top of manhole cover should be properly adjusted in relation to the road surface. It should be in the plane of the pavement so that it does not interfere with the traffic. The top surface of manhole cover is provided with small projections or bosses to make it rough so that.it does not become slippery.