Mine Subsidence

In ground subject to earth movement or in areas affected by underground mining, pipes can be subjected to longitudinal stresses. These stresses can occur anytime after installation and result in axial stress in the pipe. Whilst PVC pipes are capable of absorbing significant strains it is advantageous to use rubber ring jointed pipe in these areas. The pipe MUST be correctly installed to the witness mark position. All Vinidex rubber ring pipes are designed to absorb some ground strain and movement in the pipe joint can accommodate a certain amount of the strain associated with mine subsidence.

Each joint’s ability to take strain can be calculated from the equation:

M = 3SL + TD + 1.5 α ΔtL

where:

M = total axial movement within the socket while still maintaining the seal (mm)
S = ground strain (mm/m)
L = length of pipe (m)
T = permitted socket deflection (radians)
D = outside diameter of the spigot (mm)
α = coefficient of thermal expansion for PVC (8.1 x 10 ^-5/°C) *
Δ t = maximum temperature variation (22°C)

* Note, this value of the coefficient of thermal expansion of PVC is used by the Mine Subsidence Board. Elsewhere, 7 x 10 ^-5/°C is used.

The witness mark is positioned to allow the optimum combination of insertion/extraction without overstressing the pipe or losing seal.

Allowable Ground Strain (%) for Series 1* spigot and socket *PVC-U and PVC-M pipes (3 & 6 metre lengths) mm/m

Allowable Ground Strain (%) for Series 2 spigot and socket PVC-U and PVC-M pipes (3 & 6 metre lengths) mm/m.

Allowable Ground Strain (%) for Series 2 spigot and socket Supermain pipes (3 & 6 metre lengths) mm/m.

These tables assume that ground strain is uniformly transferred along the pipe. Pothole subsidence or large localised fissures may result in damage to the pipe or joint.