Date Published: January 25, 2019
Publisher: Public Library of Science
Author(s): Liquan Xie, Yehui Zhu, YanHong Li, Tsung-Chow Su, Jianguo Wang.
A geotextile mattress with sloping curtain is a newly proposed countermeasure against river and estuarine scour. In previous laboratory experiments, a geotextile mattress with sloping curtain was capable of protecting the bed downstream from scour and stimulating sediment deposition on both sides. However, the seepage scour under its geotextile mattress is inadequately researched at present. In this study, the Geotextile Mattress with Sloping Plate (GMSP) is proposed based on the simplification of the geotextile mattress with sloping curtain with the construction feasibility considered. A series of experiments was conducted to investigate the pressure distribution around the GMSP and the averaged seepage hydraulic gradient beneath its mattress. The results indicate remarkable pressure difference on two sides of the GMSP. The minimum bed pressure appears about 1.3 times the plate height downstream to the GMSP. The averaged seepage hydraulic gradient beneath the mattress increases with the sloping angle increasing from 35° to 60° in general. The averaged hydraulic gradient also ascends as the relative plate height increases, but reduces as the opening ratio increases at opening ratios greater than 0.143. The safety boundary for the averaged hydraulic gradient under the geotextile mattress of the GMSP could get much smaller than the critical hydraulic gradient of piping and can easily be overwhelmed. This phenomenon can mainly be attributed to the discontinuous contact between the mattress and the seabed. A suggestion for the parametric design of the GMSP is to extend the width of the mattress to reduce the risk of seepage failure.
Scours in river channels and coastal areas are long-lasting threats to the underwater structures and dikes. The failure of these structures and dikes could lead to considerable economic losses and heavy casualties. The scour on the river banks could shape the bank slope much steeper, which would cause bank collapses [1, 2]. Underwater scours may also endanger the structures installed on the seabed like bridge piers [3, 4], pipelines [5–7] and wind turbine foundations [8–10]. For example, degradation of river channel and scour around bridge piers may cause the exposure of the pier foundations, including the footings and the piles (see Fig 1). The exposure of bridge foundations will surely endanger the stability of bridges and may result in bridge failure and thus trigger local traffic paralysis and great economic losses. The present measures against the scour can be mainly divided into two sections according to their principles: enhancing the stability of the erodible bed and modifying the flow structure. The former mainly includes toe protection , revetments [12, 13] and mattresses , while the latter includes groins [15, 16] and submerged breakwaters [17, 18]. However, many of the measures consolidating the stability of sediment on river bed and banks, like the revetments and mattresses, would lead to some environmental and ecological problems. These structures were reported to affect the abundance of fish species in local reaches  probably because they completely cover the bank slopes and river beds and cut off the interaction between the channel flow and the sediment on banks and river beds . In China, a large quantity of revetments and mattresses with a full coverage of river banks or river beds scatter along the middle and downstream reaches of the Yangtze River (see Fig 2), bringing about satisfying protection effects and great impacts on local ecological systems as well. At the same time, measures attempting to change the flow patterns cost heavily in construction  and maintenance. Therefore, some improvements have been developed based on these adverse effects.
A series of simplifications are proposed on the structure of the GMSC to make it friendlier in the engineering construction (see Fig 6). The sloping curtain is replaced with a floating plate which will also lean to the leeside when placed in steady current. Thus the improved device is termed as the Geotextile Mattress with Sloping Plate (GMSP). The sloping plate can be made of floating materials like foam polymers or inflatable structures. In practical engineering, the dimensions and the material of the sloping plate are designed based on various factors, including the size and properties of the bed or structures to be protected, the flow parameters like flow depth and flow velocity. The floating tube on the top of the curtain is removed. The sloping plate is anchored to the geotextile mattress with a series of strings on the bottom edge. The gap between the bottom edge of the sloping plate and the mattress serves as the sand-pass openings. The belts are used on the sloping plate for strengthening the integrality. The other components of the GMSC remain the same.
Potential seepage failure under the mattress of a GMSP can be fatal to the GMSP structure, and similar phenomenon has occurred in previous experiments. It is thus important to improve our understanding on the mechanism of the seepage failure. One main aspect of the seepage failure mechanism is the parametric effects of the GMSP on the seepage hydraulic gradient under the mattress, which is a key parameter to predict the onset of seepage failure. The seepage gradient under the mattress is mainly determined by the bed pressure distribution near the GMSP, so the focus of this qualitative mechanism study is the effects of the GMSP parameters on the averaged hydraulic gradient under the mattress. Thus the tests were performed on an unmovable bed. The GMSP models were fixed on the side walls of the flume on both ends and the GMSP parameters were adjusted manually to improve the convenience in the tests.
In this study, the width of the geotextile mattress of the GMSP is selected based on the suggestions of Xie and Liu . The width of mattress on the upstream side (Lu) is 3 cm and the width on the leeside (Ld) is 4 cm (see Fig 7), i.e. the mattress covers a range from x = -0.03 m to 0.04 m (x = the distance to the anchoring point of the sloping plate, see Fig 8). The averaged seepage hydraulic gradient beneath the mattress im is calculated as:
where im is the averaged seepage hydraulic gradient beneath the mattress, Δp is the difference of bed pressure readings on two sides of the GMSP (between x = -0.03 m and x = 0.04 m in this study), ρ is the density of water (ρ = 1 × 103 kg/m3) and g is the gravity acceleration (g = 9.8 m/s2). In this study, Lu = 0.03 m and Ld = 0.04 m. In the following parts of this paper, the averaged seepage hydraulic gradient under the mattress im will be used to evaluate the potential of seepage failure under the geotextile mattress.
In this study, the Geotextile Mattress with Sloping Plate (GMSP) is proposed based on the simplification of the GMSC with the construction feasibility considered. The bed pressure distribution around the GMSP is measured and the effects of three GMSP parameters on the averaged seepage hydraulic gradient underneath the geotextile mattress of the GMSP are studied. On the basis of the analysis above, the following conclusions can be proposed.