Nondestructive Requirements Research Articles

Negative pressure testing standard for welded scar airtightness of waterproofing sheet for tunnels: Experimental and numerical investigation

The airtightness detection of waterproofing sheets is directly related to the success or failure of tunnel waterproofing work. The air leakage causes of waterproofing sheets mainly include unqualified airtightness of lap welded seams and unqualified airtightness of welded scars after the repair welding of local damage. There are clear test standards for the airtightness quality of welded seams in the current relevant specifications covering tunnel waterproofing technology worldwide. But for the airtightness test of welded scars, only the negative pressure testing (NPT) method is recommended, lacking quantitative test standards. The relationship between pressure loss during NPT and adsorption pressure, adsorption area, adsorption thickness, and adsorption position was systematically studied by conducting model experiments and numerical simulations. Then, based on the stability and sensitivity requirements of the test data and the test process's timeliness and non-destructive requirements, the quantitative NPT standard for welded scars of waterproofing sheets was discussed. The results show that the adsorption pressure of −40 kPa can be taken as the initial pumping pressure value, and the percentage of pressure drop rate after holding the pressure for 15 min is within 4 % as the criterion for the airtightness qualified. Field tests verified that the proposed NPT standard can fully meet the requirements of airtightness quality detection of welded scars. Compared with the qualitative detection method, it is more accurate and can achieve traceability management of testing, which is more conducive to project management.