WATER PLANT OPERATOR TRAINING PROGRAM


Pipe Installation - Thrust Blocks

Thrust blocks in water mains are created in the following ways:
  • Where the pipe changes direction horizontally or vertically;
  • where the pipe changes in size;
  • at dead ends;
  • at restrictions; and,
  • when valves or hydrants are closed quickly.
Thrust blocks are used at these locations toprevent damage to the pipe caused by unsupported pipe movement. Tees, bends, plugs, hydrants, and other appurtenances and fittings require thrust blocks to restrain the pipe. If thrust blocks are not provided, the pipe would be free to move causing joint separation, leakage, and damage to other connected structures or pipes.

INFORMATION regarding the use of thrust blocks includes the following:
  1. Normally plain unreinforced CONCRETE is used to construct blocks. Reinforcement may be used as needed. Sulphate resistant additives may be added to the concrete if the soil contains sulphates. Materials such as wood or any other degradable material are notrecommended since these materials are subject to deterioration.

  2. The LOCATION for use of thrust blocks is illustrated in the figure below.

  3. Thrust blocks must bear against UNDISTURBED soil. Disturbed soil is subject to compression upon loading and therefore should not be used as a bearing surface.

  4. The SIZE of the thrust block depends on the water pressure in th epipe, size of the pipe, type of fitting, and soil bearing strength. The table below lists the thrust force in Newtons for an internal pressure of 690 kPa for various pipe sizes and fittings. The next table lists the approximate soil bearing strengths. The figure below the tables indicates methods required to calculate the thrust block area.

  5. Machine excavation of the thrust bearing surface should be discouraged because this method usually damages the bearing surface. HAND DIGGING is the preferred method.

    Example:
    For the installation shown in the figure above, the internal pipe pressure is 690 kPa, the pipe size is 15.2 cm, and the native soil is soft clay. Calculate the required thrust block area.

    Solution:
    From the first table above, the thrust in for a 15.2 cm 90° bend is 21484 N. From the next table, the bearing capacity of the soil is 23939.5 N/m2 for soft clay. Therefore the bearing area is:

    Bearing Area = Thrust Force / Soil Bearing Capacity

    Bearing Force = 21484 N / 23939.5 N/m2 = 0.90m2

    If L = 0.9 metres, then D = .9/.9 = 1.0 metres