1. Sanitary Engineering Lecture 13

2. The three- edge bearing test
Strength of pipes:
The crushing strength of sewer pipes is determined by the three-edge bearing test. The pipe is stressed until failure occurs. Table (3) gives the minimum crushing strength for clay pipes.
Strength requirements for reinforced concrete pipes are given in table 4, for this table the crushing force correspond to 0.25mm crack. The values in the table are pre mm diameter, that’s why they are called D-Loads (KN/m.mm)
The pipe strength in supporting loads depends –on the method of pipe bedding :
- Class (D) bedding support the three bearing load only.
- Class(C) support (1.5) the three edge load (1.5 = Load factor)
- Class (B) bedding has a load factor of 1.9.
- Class (A) bedding has a load factor of 2.3 to 3.4.
- Other pipe material has similar tables to estimate their strength
The three- edge bearing test

3. 3 4

4. Example (4)
A 610 mm concrete pipe is subjected to a load of 40 KN/m (backfill and wheel load). The D-load of the pipe to produce 0.25 mm crack is 38.3N/m.mm (The three edge test strength). The pipe bedding is Class A with a load factor of 3.4.
What is the strength of this pipe
Is the strength enough to resist the applied load if the minimum factor of safety is 1.5
What is the final factor of safety in this case
Solution
a) Strength of the pipe= the three edge test strength X bedding load factor = (38.3X610)X3.4= N/m = 79.4 KN/m
b) Factored applied stress = applied stress X load factor = 40X 1.5= 60 KN/m.
c) Since the strength is 79.4 KN/m > factored applied stress (60 KN/m), so the strength is enough.
Factor of safety = pipe strength/ applied stress= the three edge test strength X bedding load factor/ applied stress.

5. Materials:
Materials used for sewers pipes are made from:
Concrete
Reinforced concrete
Vitrified clay
Asbestos cement
Cast iron (lined with cement).
Ductile iron (lined with cement).
Steel (lined with cement).
PVC, UPVC
GRB (fiber glass).

6. There are many factors effecting the selection of pipes materials such as:
Chemical characteristics of wastewater and degree of resistance to acid, base, solvents,
Resistance to scour and flow (friction coefficient).
External forces and internal pressures.
Soil conditions.
Type of backfill
Useful life
Strength and water tightness of joints and effective control of infiltration and inflow.
Availability in diameter, length, and ease of installation.
Cost of construction and maintenance.

9. Sanitary Sewers Appurtenances
Manholes
Manholes: Manholes are constructed in the following cases:
when pipes change in diameter
change of direction
change of slope
intersection of pipes
at interval, ( m)
The following table gives the allowable intervals of manholes relative to the diameter:
Note: The distance depends on the maintenance equipments available.

10. Manhole dimensions
The diameter of the manhole or its side's dimensions depends on the depth of excavation. The following table gives their relation.
The cover of the manhole should be strong enough to withstand the loads of traffic.
It is usually made of cast iron to carry a minimum concentrated load of 25 ton.
The manhole should be supplied with steps to allow for maintenance access.
The floor of the manhole should be lined with cement mortar which is called benching.

11. Brick Manhole

12. Concrete Manhole

13. Precast Concrete Manhole with drop inlet

14. Drop manhole: are used when the difference of elevation between the inlet pipe and the outlet pipe is ≥ 60cm. The drop manhole has a vertical pipe to prevent turbulence in the manhole and to allow the maintenance works to enter the manholes safely.
Drop manhole
Grease and oil traps: For the institutions, commercial units, restaurants and other places which produce oil and grease in there effluent, a grease and oil trap should be used to remove oil and grease before they enter the sewage pipes. Grease and oil affect the sewers and the treatment plant equipments that is why they should be removed. In case of the pipes, grease sticks to the walls and collects sand and other solids leading eventually to the decrease in the pipe diameter and some times to complete clogging.
Grease and oil traps