地工合成材護岸之沖蝕行為研究

The erosion behavior of revetments using geotextiles

何嘉浚(Chia-Chun Ho)

指導教授 ： 陳榮河 ； Yves-Henri Faure

國立臺灣大學/工學院/土木工程學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.00436

摘要

河川堤岸之不當沖蝕往往會影響河岸邊坡之穩定性並危及鄰近建築物的安全，而且被沖蝕崩落並堆積於河道中之土石材料也可能誘發土石流，因此須施以人工整治，以提高其穩定性，降低災害的發生。目前對於河川的治理大都採行構築高水堤防，並且採用混凝土為築堤材料，對生態環境之衝擊甚大，且對於河川長期沖蝕下之穩定性勘慮，因此尋求其他更合適的材料來替代混凝土築堤，且能同時兼具安全性及生態考量，是一個刻不容緩的問題。
採行地工合成材保護河岸除了具有生態環境營造上的優點之外，若與混凝土堤防護岸相比，其長期穩定性亦較優越，但是如果設計不良則可能造成河岸土壤大量流失進而破壞，目前對於地工合成材護岸之沖蝕行為的研究眾多，且亦已發展出相關之設計準則，然均只針對單向水流行為進行研究，而實際情況下之地工合成材護岸的水流行為複雜，本研究將其區分為單向水流作用區、雙向循環水流作用區及切向水流作用區等三區。
在本計畫中主要以試驗的方式，針對三區不同之水流條件而發展出相應之試驗設備以分別探討無凝聚性或低凝聚性土壤之沖蝕行為。經研究結果顯示，位於單向水流作用區之地下水滲流會引起土壤內部沖蝕，其中一部份的土壤顆粒會穿過地工合成材而流失，另一部份則可能堵塞於地工合成材內部纖維或堆積於地工合成材後方形成自然土壤過濾層並造成滲流速度下降，一旦滲流速度低於臨界流速(vc)，土壤內部沖蝕便會停止。
循環水流作用區可依水流循環週期而區分為短週期及長週期循環水流，二種水流情況作用下之土壤沖蝕行為各異。因此，本研究各自發展出不同之試驗設備。採用大型水槽試驗針對短週期循環水流情況進行研究後得知，位於循環水流作用區之上部區域的土壤容易受水流反覆的作用而誘發較高的超額孔隙水壓力並引致崩潰破壞，中間區域則因水流沿著河岸邊坡產生切向流動而沖刷土壤表面，並帶至下部區域堆積，研究結果亦顯示除了地工合成材之開孔徑外，覆蓋於地工合成材上方之塊石覆蓋面積亦是控制土壤沖蝕的關鍵因素之一。在長週期循環水流作用下，經由雙向循環水流試驗儀之試驗結果得知，若循環水流的週期甚長(600 sec/cycle)，則土壤內部之滲流流速緩慢，不足以使土壤顆粒產生移動，因此並無土壤沖蝕之虞；然一旦流速加快，則土壤因滲流水壓力的提高而造成有效應力下降，並可能產生土壤砂湧而誘發大量的土壤流失及沉陷，且影響護岸土壤的深度較短週期循環水流者為深。再者，分析位於地工合成材上下二水壓計之水力梯度比，及以電子顯微鏡觀察地工合成材內部纖維構造可以發現，土壤顆粒堵塞的情形並不如單向水流之作用嚴重。
切向水流作用區之土壤沖蝕行為研究則採用平行沖蝕試驗儀進行，研究結果顯示，土壤表面之切向沖蝕行為可以依水流流速加以區分，若流速低於臨界流速(vc)，則土壤無沖蝕之虞；介於臨界流速與破壞流速(vf)之間，土壤產生穩定沖蝕；一旦流速大於臨界流速，土壤便會產生劇烈沖蝕，並可能引起護岸沖蝕破壞，再者，於護岸表面有無舖設地工合成材所產生之土壤沖蝕行為亦不盡相同，無舖設地工合成材之護岸可能產生無止盡的土壤沖蝕，直到護岸因坡趾淘空而破壞，若舖設合宜之地工合成材則可以避免此一現象，並可能在地工合成材下方形成一自然過濾層，進而防止土壤繼續遭受沖蝕，一旦自然過濾層完全形成，護岸便可趨於穩定。

關鍵字

地工合成材 ； 沖蝕 ； 過濾 ； 循環水流 ； 切向水流 ； 河岸保護

並列摘要

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.

並列關鍵字

Geotextile ； Soil erosion ； Filtration ； Cyclic flow ； Tangential flow ； Bank protection

參考文獻

Williams, N. D. and Luettich, S. M., (1990), “Laboratory measurement of geotextile filtration characteristics.” Proceedings of the 4th International Conference on Geotextiles, Geomembranes and Related Products, Hague, Netherlands, Vol. 2, pp. 273-278.

Fischer, G. R., Christopher, B. R. and Holtz, R. D. (1990), “Filter criteria based on pore size distribution.” Proceeding of the 4th International Conference on Geotextiles, Geomembranes and related products, Hague, Netherlands, Vol. 1, pp. 289-294.

Bhatia, S.K. and Huang, Q., (1995), “Geotextiles filters for internally stable/unstable soils.” Geosynthetics International, Vol. 2, No. 3, pp. 537-565.

Cazzuffi, D., Mazzucato, A., Moraci, N., and Tondello, M. (1999), “A new test apparatus for the study of geotextiles behavior as filters in unsteady flow conditions: relevance and use.” Geotextiles and Geomembranes, Vol.17, No. 5-6, pp. 313-329.

Chang, D.T. and Nieh, Y.C. (1996), “Significance of gradient ratio test for determining clogging potential of geotextiles”, Recent Developments in Geotextile Filters and Prefabricated Drainage Geocomposites, ASTM STP, Vol. 1281, pp.113-131.