After backfilling ducts shall be tested for integrity (air tightness and kink-free shape). Air tightness test is done by pressurizing 2 km duct stretches at a time. One end of duct will be closed and compressed air at 5-6kg/cm2 is sent from the other end. At about 5kg/cm2 pressure the inlet of compressed air will be closed. Fall in pressure should not be more than 50% in 1(one) hour.
To check that duct has not collapsed or kinked a wooden cylindrical piece (shuttle) is blown into the duct with far end fitted with Flexible wire grip/stocking. The wooden shuttle should pass through duct at far end without any obstruction and within approx. 10 minutes or less.