Acoustic measurements of soil-pipeflow and internal erosion

Authors: LU, ZHIQU - University Of Mississippi; Wilson, Glenn

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/19/2012
Publication Date: N/A
Citation: N/A

Interpretive Summary: During a thunderstorm, surface water can infiltrate or drain into ground, not uniformly, but through some preferential channels, called soil pipes. The kinematic force of the flowing water can carried soil particles away along with its path, which enlarges the diameters of the soil pipes and leads to internal erosion. It is believed that internal erosion is one of the dominate causes of embankment failures, landslides, and gully erosion. This paper presents a laboratory study using both active and passive acoustic techniques to monitor and assess soil pipeflow and internal erosion. A 140 cm long by 100 cm wide flume with a 5 degree inclination was packed with soils to a thickness of 25 cm. A 6-mm diameter rod that extended the length of the flume, 10 cm above the bottom, was removed by sliding the outlet end after packing, thereby, creating an artificial soil pipe of known initial diameter. Water was introduced into the hollow pipe from a water reservoir that was maintained a constant water level 2 cm higher than the soil pipe. The gravitational force drove water to run through the pipe and created erosions of the pipe. During the process, it was observed that the diameter of pipe was enlarged with time and water was flowing out of the pipe at increasing rate. The study was stopped when the constant water level cannot be maintained. At the opening of the pipe, the water and soil mixture from the outflow was collected for dynamic analysis to determine soil concentration and erosion rate as a function of time. Acoustic measurements were conducted with acoustic transducers buried inside of the soil and in adjacent to the soil pipe, which consisted of two parts: actively monitoring the acoustic wave propagation at four locations along the soil pipe and passively recording water flow sounds at one location. Meanwhile, a number of tensiometers were installed to record soil water pressure at different locations to compare with the results of the acoustic tests. For active acoustic measurements, the acoustic velocities were measured using a so-called phase slope method. The study showed that the variation of the acoustic velocity reflected the ongoing internal erosion processes such as the onset of soil pipeflow, the buildup of positive water pressures within the soil pipe, the saturation of soil adjacent to the pipe, the variation of water pressures within and surrounding to the soil pipe as the soil drained following removal of the constant head, and relaxation of the soil. These observations can be analyzed and understood by using the concept of the effective stress and its relationship with the acoustic velocity. For passive measurements, passive signals (including water flow sounds and ambient noises) were recorded. Three signal processing algorithms were applied for the passive signal analysis, which revealed similar temporal characteristic of the water flow sounds. It was found that the water-flow noise levels were proportional to the measured water flow rate, which suggested that soil pipeflow can be identified and assessed from sound level measurement.

Technical Abstract: Internal erosion of soil pipes can lead to embankment failures, landslides, and gully erosion. Therefore, non-intrusive methods are needed to detect and monitor soil pipeflow and the resulting internal erosion. This paper presents a laboratory study using both active and passive acoustic techniques to monitor and assess soil pipeflow and internal erosion. A 140 cm long by 100 cm wide soil bed, 25-cm deep contained a single 6 mm diam. soil pipe at 15-cm depth that extended from an upper water reservoir to the lower bed face. The soil pipe was maintained under a constant head of 2 cm and the flow rate and sediment concentration measured at 15 s intervals while measuring soil water pressures at several locations within the bed every 30 s. Acoustic measurements were conducted every 5 s, which consisted of two parts: actively monitoring the acoustic wave propagation at four locations along the soil pipe and passively recording water fl ow sounds at one location. For active measurements, the phase slope method was employed to measure the P-wave velocity under noisy and dynamic conditions. The study showed that the variation of the P-wave velocity reflected the ongoing internal erosion processes such as the onset of soil pipeflow, the buildup of positive water pressures within the soil pipe, the saturation of soil adjacent to the pipe, the variation of water pressures within and adjacent to the soil pipe as the soil drained following removal of the constant head, and relaxation of the soil. These observations can be analyzed and understood by using the concept of the effective stress and its relationship with the P-wave velocity. For passive measurements, passive signals (including water flow sounds and ambient noises) were recorded by a sensor buried inside the soil and close to the soil pipe. Three signal processing algorithms were applied for the passive signal analysis, which revealed similar temporal characteristic of the water flow sounds. The passive study suggested that soil pipeflow can be identified and assessed from sound levels in terms of time-domain root mean square (TD-RMS) and frequency-domain root mean square (FD-RMS) and from the contrasts of the power spectrum image. Abbreviations: FFT, fast Fourier transform; FD-RMS, frequency-domain root mean square; TD-RMS, time-domain root mean square.