| Summary: | 博士 === 臺灣大學 === 土木工程學研究所 === 96 === Unfavorable erosion on revetments may affect the slope stability of riverbanks and jeopardize the safety of adjacent buildings, and debris can be triggered by the soils and rocks eroded from the riverbanks and accumulate on the riverbed. Improvement works are needed to increase the stability of revetments as well as to reduce the possibility of failure. Current practices usually involve building tall concrete revetments, causing negative environmental impacts and instability of riverbanks under long-term erosion. Therefore, it is crucial to find materials suitable for building revetments which are safe and environmentally friendly as well.
Geotextiles used as a riverbank protection material is not only more environmentally-friendly but also more stable in long-term compared to concrete. However, improper design of geotextile revetments can cause considerable loss of riverbank soil, which might result in failure. Today numerous studies on erosion behavior of geotextile revetments have been completed, but most of them focused on only one directional flow behavior. The actual flow behavior in geotextile revetments is rather complicated and can be categorized into uni-directional flow zone, cyclic flow zone, and tangential flow zone.
In this study, the erosion behavior of non-cohesive or low-cohesive soil under the aforementioned three flow conditions was studied by tests using the equipments developed separately. The test result reveals that ground water seepage in the uni-directional flow zone may cause internal erosion of soil, and part of soil particles may be lost through the openings of the geotextile. The rest may be clogged inside the fibers of the geotextile or accumulated behind the geotextile, forming a natural filter layer, thereby causes the decrease of seepage velocity. Once the seepage velocity is lower than the critical velocity, the internal erosion of soil will cease.
Bi-directional cyclic flow zone can be categorized into the short term and the long-term cyclic flow conditions, depending on the flow period, which may induce different soil erosion behaviors. Thus, two test instruments were developed respectively. The result of the large-scale tank test for the short term cyclic flow condition reveals that the soil in the upper layer of the bi-directional cyclic flow zone subjected to cyclic wave loadings may trigger higher excess pore water pressure and result in collapse, while the soil in the middle layer may be eroded by the tangential flow along the riverbank and accumulated downstream. In additional to the opening size of the geotextile, the coverage area of rocks on the geotextile is also a key factor affeeting soil erosion.
The test results using cyclic flow instrument show that under the long-term cyclic flow action, with long cyclic flow period (600 sec/cycle), the seepage velocity in the soil layer is too slow to move soil particles, therefore no erosion is observed. However, as the seepage velocity increases, the effective stress in the soil will decrease due to the rising seepage pressure, thereby causing boiling and triggering considerable loss of soil and settlement. Besides, the influence depth in this flow condition is greater than that of short-term cyclic flow condition. Furthermore, through the hydraulic gradient ratios between the two piezometers installed above and underneath the geotextile, as well as from electron microscopy images of fibers inside the geotextile, it can be found that clogging of soil particles is not so serious as that in uni-directional flows.
Erosion behavior in the tangential flow zone was studied with a parallel erosion test instrument. The result reveals that tangential erosion behavior on the soil surface can be categorized by flow velocity. If the flow velocity is lower than the critical velocity (vc), no erosion will occur. If the flow velocity is between the critical velocity and failure velocity (vf), steady erosion will occur on the slope face. If the flow velocity is higher than the failure velocity, intense erosion will occur on the slope face and erosion failure of the revetment may thus be triggered. Moreover, the existence of geotextile on the surface of revetment has less influence on soil erosion. Revetments without geotextile on the slope surface are subjected to continuous erosion and may finally collapse due to cave-in at the toe of slope. A suitable geotextile covered on the surface of the revetment can not only avoid erosion but also form a natural filter layer underneath the geotextile, which prevents the soil from being continuously eroded. Once a natural filter layer is completely formed, the revetment will become stable.
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