Research outline

Development of a device to prevent drainage pipe clogging in landslide-prevention facilities

  A large number of groundwater drainage facilities are installed in landslide slopes to prevent landslides. These facilities include horizontal drainage pipe assemblies for shallow groundwater drainage, drainage wells for slightly deep groundwater drainage, and drainage tunnels for deeper groundwater drainage. Drainage pipes with holes are inserted into the landslide slopes to collect the groundwater and drain it away to the land surface. However, such pipes often become clogged with reddish-brown muddy materials that adhere to them (Photo 1). This is because, when the groundwater is rich in iron, iron bacteria living in the soil absorb iron in the groundwater to form colloidal organic matter. Such clogged drainage pipes cannot properly drain groundwater from the landslide slopes, and this may lead to landslide occurrence. From this perspective, the Snow Avalanche and Landslide Research Center (SALRC) is studying how to prevent such clogging of drainage pipes.

Photo 1 A groundwater drainage facility

(clogged drainage pipes of a horizontal drainage pipe assembly)

  Photo 2 shows a device developed by the SALRC to prevent drainage pipe clogging. Bent pipes are attached to the open ends of the drainage pipes of the horizontal drainage pipe assembly or drainage well, and connected by a single pipe with the device attached to the end. Any such device must efficiently eliminate the materials clogging the drainage pipes, while satisfying the following requirements: structurally simple, unpowered, robust, maintenance-free, and inexpensive. We developed the device taking these functions and requirements into consideration.

Photo 2 A drainage pipe clogging prevention device attached to a horizontal drainage pipe assembly

  The developed drainage pipe clogging prevention device employs shishi-odoshi-type action, repeatedly gathering roughly 2 m of groundwater, up to its opening, and then draining it off. The water flow caused by the automatic repetition of this action prevents clogging materials from adhering to the drainage pipe assembly.

  Figure 1 shows how the device works. The device automatically repeats the actions from (1) to (3). Specifically, in (1), groundwater gathers in the drainage pipe. Then, in (2), the gathered groundwater approaches the opening of the device. Finally, in (3), the device drops due to the weight of the gathered groundwater, resulting in rapid drainage of the groundwater, along with clogging materials.

Figure 1 Operational mechanism of the drainage pipe clogging prevention device

(click to enlarge)

  Photos 3 to 6 show the conditions of clogging material adhesion on Day 204 after the beginning of the field test. Large amounts of clogging materials adhere to the drainage pipes not equipped with the prevention device (Photo 3). No clogging material is found at the opening of the prevention device (Photo 4). The groundwater is rapidly discharged from the device (Photo 5). Clogging materials are observable at the opening of the drainage pipes equipped with the device (Photo 6); however, there is far less than in the case of the unequipped drainage pipes (Photo 3). These findings suggest that the device is likely to suppress the clogging of drainage pipes.

  We are planning to continue the field test, to further observe the clogging status of the drainage pipes, and confirm the durability of the device, aiming at the practical use of the drainage pipe clogging prevention device.

Photo 3 Drainage pipes not equipped with drainage pipe clogging prevention device


Photo 4 The opening of the drainage pipe clogging prevention device

Photo 5 Discharge of groundwater from the drainage pipe clogging prevention device

Photo 6 Drainage pipes with installed drainage pipe clogging prevention device

(Contac:Snow Avalanche and Landslide Research Center)

Preventing erosion and degradation of a mixed bedrock – alluvial river

Figure–1 Causes of degradation for a

riverbed with soft bedrock

(click to enlarge)

Photo–1 An example of an eroded

riverbed with soft bedrock

  Generally, riverbeds are covered with sand and gravel transported from upstream. If sediment moving in a river channel changes in amount or quality due to a natural or artificial cause, such as slope failure at a mountain in an upper reach or river improvement work (revetment installation, river channel excavation, and so forth), then the gravel-covered riverbed is subject to scouring or sediment accumulation.

  Scouring may result in exposure of the bedrock underlying the riverbed. If the bedrock is hard, erosion will not progress further. If the bedrock is unconsolidated mudstone or sandstone, then erosion progresses by means of weathering and abrasion caused by sediment transport, ultimately leading to obvious riverbed degradation (Figure–1 and –2). Because riverbeds with soft bedrock generally have a smoother surface than those covered with gravel, gravel moving over riverbeds with partly exposed soft bedrock is less likely to accumulate on the riverbed, with the result being the rapid disappearance of gravel from the riverbed (and additional exposure of the soft bedrock). By the process explained above, the rapid expansion of exposed bedrock has occurred at rivers throughout Japan (Photo–1), and has caused damage, decreased stability, or other problems to transverse river structures such as intake weirs and bridge piers. Also, the gravel riverbed is an important component of the river ecosystem, in that it provides a habitat to benthos and a spawning ground of fish. Accordingly, the transformation from a gravel riverbed to a soft rock riverbed affects the river ecosystem.

Figure–2 Schematic depicting erosion

at a riverbed with a soft rock substrate

Photo–2 Experiment using a

model of a riverbed with soft bedrock

  Our study aims to do the following: 1) clarify the process of erosion at riverbeds with soft bedrock, 2) develop a method for assessing the risk of future erosion at such riverbeds, and 3) develop technology to restrict erosion at such sites and restore gravel riverbeds.

  in the study we conducted an experiment using samples of soft rock taken from the beds of 19 rivers in Hokkaido, including the Ishikari River, the Yubari River, and the Abashiri River. The experiment showed that susceptibility to riverbed erosion depended strongly on the tensile strength of the soft rock and the amount of gravel passing over them. We constructed a mortar bed whose hardness resembled that of natural soft bedrock (Photo-2). Using this, we have been conducting flume experiments to investigate the process of erosion caused by bed load. Detailed data have been acquired from the experiments.

  The study results were compiled in an investigation manual that features a method for estimating the bedrock erosion rate and a simplified onsite bedrock investigation method. In addition, we are clarifying the relationship between the roughness of the riverbed surface and the extent of alluvia cover. Once this relationship is clarified, it will be used as basic technical knowledge of how to cover bare bedrock with gravel.

  In the future, we will develop our study so as to clarify the process of lateral erosion as well as of vertical erosion, and we will advance the development of practical technology for restricting erosion and recovering gravel riverbeds.

(Contact:River Engineering Research Team, Civil Engineering Research Institute for Cold Region)