Scaling and clogging treatment of aging tunnel drainage pipes in karst areas using eco-friendly acid agent

In karst areas, the drainage pipes of aging tunnels are prone to be clogged by precipitated carbonates, resulting in lining cracking and tunnel leaking. As a result, not only the driving safety will be deteriorated, but also the water pressure on the lining might also be elevated significantly. For the structural stability and service lifespan of old tunnels, it is of great importance to remove these precipitated carbonates in time. Traditional treatment methods are often destructive to some extent or not efficient enough. This study aims to experimentally develop an eco-friendly acid-based chemical cleaning method to remove carbonate precipitations efficiently. The proposed chemical cleaning agent is an aqueous solution with strong acidity, consisting of sulfamic acid, water, and additives. The factors affecting the cleaning efficiency include the acid solubility, temperature and flow rate of the cleaning agent, as well as additives. Elevating the solution temperature to 50 °C or a flow rate of no less than 0.2 m/s can improve cleaning efficiency. Although the salt effect cannot work, 1 wt% of polymaleic acid as a surfactant could further promote the cleaning rate. The cleaning efficiency will increase with the flow rate in a power function. The relatively low flow rate that improves the cleaning rate considerably can avoid high-pressure-induced mechanical damage to tunnel drainpipes. The waste could be easily treated to acceptable levels using commercial sewage treatment products and can also be recycled in agriculture. With the chemical cleaning, the water pressure at the arch springing of the lining will reduce with the increased radius of transverse drainpipes in a power function. The proposed acid-based cleaning method, which is highly efficient, non- or low-destructive to aging tunnels, sufficiently safe for humans, and friendly enough to the environment, will offer a promising alternative to remove the precipitated carbonates in tunnel drainpipes efficiently.