Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2012
Publication Date: 5/31/2012
Publication URL: http://handle.nal.usda.gov/10113/54081
Citation: Lu, Z., Wilson, G.V. 2012. Acoustic measurements of soil pipeflow and internal erosion. Soil Science Society of America Journal. 76:853-866. DOI: 10.2136/sssaj2011.0308. Interpretive Summary: Erosion of the inner walls of large soil pores, called soil pipes, can lead to embankment failures, landslides, and gully erosion. Methods are needed that can detect and monitor flow through soil pipes and the resulting internal erosion without disturbing the soil. The objective of this study was to test the ability of monitoring flow though soil pipes and detecting internal erosion using acoustic (sound) sensors. This paper presents a laboratory study using both active and passive acoustic techniques to monitor and assess flow through soil pipes and the internal erosion of the pipes. A 140 cm long by 100 cm wide soil bed that was 25 cm deep contained a single 6 mm diameter soil pipe at the 15 cm depth. The soil pipe extended from a water reservoir at the upper end to the soil bed face at the lower end. Flow through the soil pipes occurred when the water level was raised and maintained at a constant water level 2 cm above the soil pipe. The flow rate and sediment concentration flowing from the soil pipe were measured every 15 seconds while also measuring the water pressures at several locations within the soil bed every 30 seconds. At the same time, the active sound waves, created by a sound emitter and recorded by a sound sensor 20 cm apart, moving through the soil bed across the soil pipe were continuously measured at four locations along the soil bed. In addition, passive sound waves, listening for sounds created by water moving through the soil, were measured at one location within the soil bed. For active acoustic measurements, a method was developed that measures a specific sound wave frequency (P wave) to obtain the sound wave velocity under rapidly changing soil water conditions. The study showed that the variation of the P-wave velocity reflected the changing conditions such as pipeflow and internal erosion processes at the onset of flow through the soil pipe, the buildup of water pressure within the pipe, the saturation of soil around the pipe, the variation of soil water pressures within and in the soil adjacent to the pipe, as well as drainage of the soil after the water flow through the soil pipe was stopped. These observations were analyzed using the concept of the effective stress and its relationship with the P-wave velocity. For passive measurements, background noise was recorded by a sensor buried inside the soil, close to the soil pipe. Three sound signal processing methods were tested which revealed that soil pipeflow can be identified and monitored by acoustic methods.
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 6mm diameter soil pipe, 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 in 15 s intervals while measuring soil water pressures at several locations within the bed every 30 s and continuously monitoring the active soil wave propagation at four locations along the soil pipe and the passive sound wave at one location. For active measurements, the phase slope method was employed to measure the P-wave velocity under noisy and ever-changing 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 sound wave. 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, ambient noise was recorded by a sensor buried inside the soil and close to the soil pipe. Three signal processing algorithms were applied for the noise analysis, which revealed the common temporal characteristic of the water flow noises. The passive study suggested that soil pipeflow can be identified and assessed from the noise levels in terms of TD-RMS and FD-RMS and from the contrasts of the power spectrum image.