Buried water-carrying services (BWCSs) such as buried water mains, sewers and storm water pipes are critical infrastructures in Hong Kong and elsewhere in the world. During their service, pipes may become defective and water may leak from the pipes. Between 1984 and 2004, 206 landslide incidents involving water-carrying services in the vicinity of the slopes of concern were reported to Geotechnical Engineering Office (GEO). Well-known disasters such as the 1994 Kwun Lung Lau landslide invoke the public awareness of leakage-induced disasters. There are strong needs to investigate the potential risks of a slope due to leaking pipes, and to suggest protection measures to mitigate the hazards.

A series of centrifuge model tests was conducted to investigate potential failures of soil slopes...[ Read more ]

Buried water-carrying services (BWCSs) such as buried water mains, sewers and storm water pipes are critical infrastructures in Hong Kong and elsewhere in the world. During their service, pipes may become defective and water may leak from the pipes. Between 1984 and 2004, 206 landslide incidents involving water-carrying services in the vicinity of the slopes of concern were reported to Geotechnical Engineering Office (GEO). Well-known disasters such as the 1994 Kwun Lung Lau landslide invoke the public awareness of leakage-induced disasters. There are strong needs to investigate the potential risks of a slope due to leaking pipes, and to suggest protection measures to mitigate the hazards.

A series of centrifuge model tests was conducted to investigate potential failures of soil slopes inclined at 35° in accordance with the current mainlaying practice in Hong Kong when subject to leakage. To mitigate the captioned risks, three protection measures; namely (1) geotextile enclosure, (2) geomembrane enclosure, and (3) geomembrane sheathing, were suggested for mainlaying. In order to simulate the large-pressure and large-flowrate leakage process, a newly developed water-supply system was developed which can sustain a leakage pressure of 185 kPa and a maximal leakage rate of 2 L/min (30 L/min in prototype when tested under the enhanced gravity of 30g). Five model tests were performed on the 400 g-ton beam centrifuge in the Geotechnical Centrifuge Facility (GCF) of HKUST. Test results indicate that both erosion and sliding failures may occur if without any protection measures. Sliding failure even occurred at 23.4 days after applying a total leakage pressure of 35 kPa. Such short time-to-trigger may occur before the on-time inspection and remediation of the leakage, leading to catastrophic consequences. With a geomembrane enclosure or sheathing, both erosion and sliding failures can be prevented. There was no noticeable water infiltration nor slope displacements with the aid of the captioned protection measures, thus the performance of the protection measures was shown to be satisfactory.

To further investigate erosion-fluidization phenomena induced by leaking pipe, another series of centrifuge test was conducted. Since the required pressure and flow rate for erosion-fluidization is much higher than the one for the slope tests, another water-supply system was developed to serve the captioned purposes. The working principle of this water-supply system is based on Mariotte’s bottle. Water level changes in the bottle will not affect the outflow pressure. Test results indicate that multi-stage failure occurred with the increasing leakage pressure. At lower pressures (< 100 kPa), fine particle erosion is the major failure mode. As the leakage pressure increases to 200 kPa - 300 kPa, the loosely-packed models experienced cylindrical failure and fluidized cavity formed. On the contrary, the densely-packed models experienced particle erosion, wedge failure and cylindrical failure. Fluidized cavity also formed with the increasing pressure. The wedge failure zones had inclination angles of about 63.7° and 57.1° for 3 m and 6 m of burial depth, respectively. The inclination angle for the shallow model is similar to the one calculated using Rankine’s active failure (? = 45° + ?′/2) whereas the deep model has an inclination slightly less than this angle. It is shown that the wedge failure is induced by the upward seepage force which exceeds the force equilibrium condition. The extend of the fluidized cavity depends mostly on the particle Reynold’s number Re_?. For the model with a large burial depth of 6 m, the particle Reynold’s number Re_? at the crown of fluidized cavity reached 74-83. An analytical model is proposed to predict the initiation pressure and flow rate for the wedge failure, the extend of fluidized cavity as well as the pressure-flow characteristics. The validity of the analytical model is verified by the centrifuge test results in this study and test data in the literature.