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Pumping effect of wave-induced pore pressure on the development of fluid mud layer
Ocean Engineering 189 (2019) 106391
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Pumping effect of wave-induced pore pressure on the development of fluid mud layer Zhongnian Yang a, Yongmao Zhu b, Tao Liu b, c, *, Zhongqiang Sun b, Xianzhang Ling d, Jinmu Yang b a
School of Civil Engineering, Qingdao University of Technology, Qingdao, Shandong, 266033, China Key Laboratory of Shandong Province for Marine Environment and Geological Engineering, Ocean University of China, Qingdao, Shandong, 266100, China Laboratory for Marine Geology, Qingdao National Laboratory for Maine Science and Technology, Qingdao, Shandong, 266061, China d Harbin Institute of Technology, Harbin, Heilongjiang, 150001, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Pumping effect Wave-induced Silty seabed Fluid mud layer
Fluid mud widely exists in the movement of fine sediment, which is an important potential cause of marine geological disasters. It is important to understand the generation and migration mechanism of fluid mud. The purpose of this study is to investigate the vertical migration response of fine-grained sediments by wave-fluid test of pore pressure response of silty seabed under wave action. The results show that the adhesion between the silt particles and the additional pressure of the film water have a significant effect on the liquefaction characteristics of the silt. Even if the liquefaction degree is far more than 100%, the soil does not completely liquefy. The experiment result show that, there is concentrated movement of the split channel, and mesoscopic migration of fine particles between the gaps of the particles, pumping effect provides an effective source of supply for the development of the floating mud layer, it also exacerbates modern sedimentation when strengthening the ver tical exchange of shallow soil in the seabed. Besides, the blending of the middle and lower layers of the previous soil with the recent sediments has increased the difficulty of identifying the time series of the modern sedi mentary layer to some extent.
1. Introduction Fluid mud refers to a layer of high concentration sediment-laden water near the bottom of the sea. It is mainly composed of clay and silt with obvious viscous properties, and the main particle composition is generally not more than 63 μm (Li et al., 2013). Fluid mud has a clear interface with the upper water body and has great mobility. It is a unique sediment movement pattern in the muddy coastal estuaries (Mcanally et al., 2007; Qian and Wan (1983). It has been found in Thames estuary of England, Gironde estuary of France (Granboulan et al., 1989), Amazon estuary of Brazil (Allison et al., 1995; Kineke and Sternberg (1995), Yellow River estuary and Yangtze River estuary of China (Xu et al., 1994). The liquefaction of sand and the resuspension of sediment under the action of waves were always the focus of coastal research. Most scholars focus on liquefaction mechanisms and resuspension fluxes (Wainright (1990); Gao et al., 2011; Moriarty et al., 2017; Tao et al., 2018).
However, few researchers pay attention to the vertical pumping effect of wave-induced pore pressure from a microscopic point of view, especially the development of fluid mud in silty soil. When there is a large amount of fluid mud, it can cause ecological, environmental and navigation safety problems such as burying benthic organisms Corselli and Basso (1996), sediment eutrophication (Montserrat et al., 2011) and channel sudden siltation, disturbing sounding results, affecting the judgment of navigable water depth (Niu et al., 2003), etc. Besides, its interaction with suspended load and bedload has an important impact on the topographic and geomorphological evolution of estuary areas (Yunping and Tian, 2014; Liu et al., 2018), such as channel siltation, serious time and even navigation safety. It will block the channel and cause the ship to run aground (K and Z, 2019; Zhen, 2019). The problem of fluid mud originates from the results of field obser vation (Liu et al., 2018, 2019), whose means includes frozen sampling, bathymetry, gamma-ray method, ultrasonic method, tuning fork density method and coupling method (Ju et al., 2014), which can be
* Corresponding author. Key Laboratory of Shandong Province for Marine Environment and Geological Engineering, Ocean University of China, Qingdao, Shandong, 266100, China. Tel.: 0532 66783723 E-mail address: [email protected] (T. Liu). https://doi.org/10.1016/j.oceaneng.2019.106391 Received 9 April 2019; Received in revised form 14 August 2019; Accepted 27 August 2019 Available online 25 September 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.
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Ocean Engineering 189 (2019) 106391
summarized as a direct measurement, isotope measurement and acoustic measurement by measuring principle. Laboratory tests are mainly divided into two directions, one is the experimental study of the rheological properties of the sludge itself (Maa and Mehta (1988); Hasar and Kinaci (2004); Abujdayil et al., 2010),the other is the experimental study of the movement characteristics of the sludge under dynamic ac tion, including wave action (Zhao, 1982; Wang et al., 2008; Lambrechts et al., 2010; Yang et al., 2011) and current action (Hong and Ying (1988); Kreeke et al., 1997; Almroth et al., 2009; Wu et al., 2017). On the basis of field observation and laboratory test, the formation condi tions (Xu and Dong, 2011; Xu and Yuan, 2001) of sludge and the nu merical simulation (Maa and Mehta, 1990) of sludge response under dynamic action are studied from the angle of theoretical analysis. In view of the development mechanism of floating sludge, the pre vailing view is that the conditions for the formation of floating sludge include abundant fine sediment supply, relatively weak hydrodynamic conditions and appropriate salinity. Salinity and weak dynamic force are the external conditions for flocculation and sedimentation of fine sedi ment (F.L. et al., 2019; Daoji, 2007), and abundant fine sediment pro vides the material source for the formation and development of fluid mud. Previous studies have been carried out on the flocculation envi ronment of fine sediment. Li et al., 2008 considered that the Yangtze River estuary has a good environment for flocculation of fine sediment through field observation. Li and Zhang (1998) found that salinity and sediment concentration contribute significantly to the flocculation of fine sediment. The periodic change of flow velocity is the most sensitive factor to control the flocculation size of fine sediment (Li et al., 2008; Li and Zhang, 1998). It is generally believed that there are two forms of material supply of fluid mud, one is direct input from rivers. Winterwerp (1999) and Li and Ren (1996) point out that the potential of density inflow is a special manifestation of the rapid deposition of sediment at the bottom, which provides a rich material source for the formation of fluid mud. The other is the resuspension supply of storm events, whose dynamic mechanism is that storms transmit huge energy to the nearshore water body, thereby significantly increasing wave-current linkage. The shear stress of the combined bed leads to the re-suspension of fine sediment(Yeh and Mason, 2014; Miao et al., 2016a,b; Sheremet et al., 2005). Both of these two opinions are lack of dynamic process research from a micro perspective, and insufficient to fully explain the source supply mecha nism of the development of fluid mud, which are based on field
observation, revealing from a macro perspective, In this paper, silt from the Yellow River Delta is taken as the research object, and the cumula tive process of pore pressure and liquefaction response of soil under wave action are analyzed based on the wave flume test in this paper. The vertical migration law of soil particles under wave action is studied by tracing the particle size composition. Combining with theoretical calculation, this paper reveals the source and supply ways for the development of fluid mud from a mesoscopic point of view in order to further complement and enrich the development mechanism, transport law of fluid mud. 2. Methodology 2.1. Experiment design and materials The wave flume experiment was completed in the Geotechnical Laboratory of the Ocean University of China. The experimental device is shown in Fig. 1. The size of the flume is 3 m � 0.6 m � 1.2 m, and the size of the lower soil flume is 1 m � 0.6 m � 0.5 m. The air compressor is used to provide power for the device. A double-acting cylinder is installed at one end of the tank to push the water body to and fro with the wave-making plate to produce waves. The output waveform is changed by adjusting the stroke of the cylinder and the damping of the cylinder. A wedge sponge block is installed at the other end to eliminate the reflection effect of the waves and ensure the stable output of the waveform. As shown in Fig. 1, the soil tank is filled with saturated silt with a thickness of about 40 cm, and a kaolin layer is set at a distance of 5 cm from the surface of the soil body, with a thickness of less than 0.5 cm to study the transportation process and influence of pumping effect on fine granular sediment. The silt was quiesced for 4 h before and after the experiment, and then its surface samples were taken. The particle size components were determined by laser particle size analyzer. Six pore water pressure sensors (type YY-2B) were embedded in this experiment, which were located at 5 cm, 10 cm, 15 cm, 20 cm, 25 cm, and 30 cm away from the surface of the soil. The pore pressure sensor is immersed in clear water for 24 h before burial and continuously shakes sufficiently to ensure that the internal air is completely discharged. The water is artificial seawater with 35‰ salinity. The silt was taken from the tidal flat of the Yellow River Estuary with median particle size (D50) ranging from 32 to 41 μm, and kaolin was brought from industrial
Fig. 1. Diagram of the experimental setup and the photograph of the wave generator. 2
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factories with a mesh size of 4000 and D50 ranging from 2.1 to 2.8 μm.
2.3. Experiment procedure
2.2. Quantification of Kaolin
In order to simulate the physical characteristics of silt under natural consolidation and saturate the samples, air-dried silt samples were selected to remove impurities such as grass and branches after crushing. Sifted soil samples are put into the mixer, and standard seawater is added to mix at the same time to ensure uniform mixing of the samples. The pore water pressure sensor is fixed in the middle of the soil trough in advance according to the specified depth to avoid disturbance to the soil after embedding. The mixture is slowly injected into the soil tank along the inner wall until the thickness is about 35 cm. After the soil is stabi lized, a layer of kaolin with a thickness less than 0.5 cm is evenly laid on the surface, then continue adding soil to 40 cm (Hibino et al., 2014). The experimental group without kaolin layer directly injects the mixture to 40 cm. After the soil is basically stable, the standard seawater is slowly injected into the tank along the inner wall until about 35 cm away from the surface of the soil. After 12 h, the soil was naturally consolidated under hydrostatic pressure, and then the wave loading experiment was started. Before and after wave action, four surface samples were evenly sampled in the soil trough by the sampling tube for particle size mea surement. In order to avoid component interference between different experimental groups, samples in artificial seawater and soil trough should be replaced completely after each group of experiments. Wave loads were applied by using the wave generator installed at one end of the flume, and different wave actions (wave height and wave duration) were experimentally studied. The specific experimental group settings are shown in Table 1. The experimental group A and C were the basic control group, which focused on the experimental pore pressure response and surface particle size change caused by wave action. The pumping effect of wave-induced excess pore pressure on fine sediment was studied. YY-2B pore water pressure sensors were used to record the pore pressure changes at different depths of soil before, during and after wave action. Based on this, the accumulation of pore pressure caused by wave and the migration of fine particles under pumping action were calculated and analyzed.
The results of particle size measurement can not only obtain the parameters such as D50 and average particle size (Dav), but also obtain the distribution of particle size content and its cumulative distribution. By studying the grain size characteristics of sediments, the transport mode of sediments can be determined and the environmental factors affecting the grain size change of sediments can be inverted, especially the material sources and hydrodynamic environment (Mclaren and Bowles (1985); Wang et al., 2009). The experimental silt and kaolin are obviously different in origin and particle size composition. In this experiment, the silt particle size is relatively coarser (Fig. 2a, D50 is about 32–49 μm), which is taken from the tidal flat of the Yellow River Estuary (Liu et al., 2019). It is formed by natural accumulation after long-distance transportation and long-term screening of water flow. It has good sorting and grinding roundness whose material source is relatively consistent, approximating to the normal curve. The experimental kaolin has a finer particle size (as shown in Fig. 2b, D50 is about 2.1–2.8 μm). It is manufactured in in dustrial batches and screened artificially, which has poor sorting and grinding roundness, and its shape is more sharp and fine. Fig. 3a is the particle size distribution curve of mixed kaolin and silt. Its main component is 80% kaolin mixed with 20% silt. It can be seen that there are two independent peaks in the particle size distribution curve, and the maximum of the two peaks corresponds to the particle size range of kaolin (D50 ¼ 2.1 μm) and silt (D50 ¼ 34 μm), respectively. Therefore, the number of peaks in the particle size distribution curve can be used to qualitatively determine whether there are many components in the sample and whether there are corresponding components can be qualitatively judged by the size range corresponding to the maximum value. Sample size distribution curve contains multiple peaks, which can be seen as superimposed by single peaks of different particle sizes (as shown in Fig. 3b). The particle size distribution of one component can be obtained by decomposing the particle size distribution curve. The component proportion of different particle sizes (corresponding to different material sources) can be quantitatively evaluated by cumula tive distribution.
3. Results 3.1. Experimental phenomena With the beginning of wave action, the consolidated soil gradually
80
6
60
4
40
2
20
0
0 0.01
Differential (%)
10 8
Integral (%)
8
100
(a)
0.1
1
Particle size
10
100
1000
100
(b)
80
6
60
4
40
2
20
0
0 0.01
0.1
1
Particle size
10
100
Fig. 2. Particle size distribution curves of silt (a) and kaolin (b) in the experiment. 3
1000
Integral (%)
Differential (%)
10
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Ocean Engineering 189 (2019) 106391
Fig. 3. Particle size distribution curve of mixed kaolin and silt in proportion (a) and its decomposition calculation sketch (b). Table 1 Experiment setup. Group A B C D E F G
Kaolin layer
� ✓ ✓ ✓ ✓ ✓ ✓
Pre-settling time (h) 12 12 12 12 12 12 12
Sample number 1–4 9–12 17–20 25–28 33–36 41–44 49–52
Wave action Time (min)
Wave height (cm)
20 10 20 30 20 20 90
7 7 7 7 5 3 7
Post-settling time (h)
Sample number
12 12 12 12 12 12 0
5–8 13–16 21–24 29–32 37–40 45–48 53–56
Note:"�" indicates that no kaolin layer in the experiment and "√" indicates that there is a kaolin layer in the experiment.
shows a slight fluctuation. The fluctuation period of soil is consistent with the period of wave action. The surface layer of soil has the largest fluctuation, and the amplitude is less than 2 cm. The initial time of soil fluctuation is delayed with the increase of depth, and the ultimate in fluence of wave on soil depth increases with the increase of wave height. The maximum influence depth that can be distinguished by naked eyes can reach 15 cm below the surface layer of soil. The overlying water body gradually changes from clear to turbid, and the silt on the surface of the soil is lifted up, with few bubbles escaping occasionally. When the wave action stops, the fluctuation of the upper soil disappears quickly and consolidation begins, the suspended particles settle slowly, and the water gradually recovers clarity after a long time of setting. Occasion ally, obvious small channels can be seen at the boundary of the flume (as shown in Fig. 4a). The splitting channels of 3–17 cm in length can be seen. The sediment in the lower part of the soil moves up along these small channels rapidly and forms many small hilly deposits on the sur face. After a period of time, a large number of small hilly deposits are formed on the surface of the soil (Fig. 4b). The diameter and height of the hilly deposits generally do not exceed 3 cm and 1 cm. At 5 cm below the surface of the soil, kaolin migrates rapidly to the surface along the channels and forms a series of white accumulation. The phenomena of group C and D are most obvious (Fig. 4b). At first, the white kaolin upwells in group D, and then the color gradually deepens to the normal
silt upwelling, gradually covering the white kaolin. After a long period of stationary time, the hilly accumulation center gradually presents obvious depression (Fig. 4b), and the whole process lasts for about 1 h. 3.2. Accumulative pore pressure Due to the loosening of the fixture in the process of sensor embed ding, the actual position and the desired position deviate, but the overall order will not change (Fig. 5a), which does not affect the analysis of the overall pore pressure variation law, so this paper still labels and de scribes the set position according to the experiment. Besides, all sensors were calibrated before and after the experiment, and it was found that the stability of pore pressure sensors located 5 cm below the surface of soil was difficult to meet the experimental requirements (Fig. 5b①②). There are obvious anomalies in the data, so the relevant results and analysis were not involved in this paper. Fig. 6 shows the pore pressure response before, during and after wave action (H ¼ 7 cm, t ¼ 30 min) at 10–30 cm below the surface of the soil. The pore pressure fluctuations of the sensors are basically the same. Before the wave action (0–20 s), the pore water pressure of each layer is stable after a long period of static consolidation, and its value is consistent with the overburden hydrostatic pressure. During wave ac tion (20–2100 s), pore water pressure began to fluctuate with wave load, 4
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Fig. 4. Channels (a) observed at the boundary of the flume and a large number of hilly deposits formed on the surface of the soil (b).
b
a Pore pressure sensor
Y=0.554X+21.163 R2=0.999
Y=0.453X+11.970 R2=0.999
Fig. 5. Actual position of pore water pressure sensor (a) and the calibration curve of the sensor at 5 cm (b).
and the pore water pressure began to accumulate in all layers (as shown in Table 2, the maximum increment was 0.80–1.27 kPa, the average increment was 0.74–0.94 kPa). The accumulative value and velocity of pore water pressure in the upper layer were greater than those in the lower layer, and the pore water pressure reached the maximum value 5.09 kPa at about 150 s at 10 cm, then decreased slightly to stable pore pressure value 4.76 kPa, the pore water pressure tends to stabilize gradually with the passage of time. After the wave action stops, the pore
water pressure fluctuation disappears rapidly, and the pore water pressure in each layer dissipates continuously. After the wave load ends, the pore water pressure returns to the level before the wave action and remains stable about 100 min. 3.3. Particle size and kaolin content Table 3 shows the variation of surface particle size and kaolin 5
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Ocean Engineering 189 (2019) 106391
0
5
10
15 20
300
600
900
1200
1500
1800
2000 4000
6000
8000
10000 12000
2 3
Before
Wave
After
4 5 6 7 8
30cm 25cm 20cm
15cm 10cm
Fig. 6. Pore pressure response at different depths before, during and after wave action (H ¼ 7 cm, t ¼ 30 min). Table 2 Analysis of pore pressure variation during wave action. Position
10 cm 15 cm 20 cm 25 cm 30 cm
Before
Wave action
After
Initial pore water pressure u0 (kPa)
Maximum pore water pressure umax(kPa)
Maximum cumulative pore pressure Δumax(kPa)
Stable pore pressure u (kPa)
Stable cumulative pore pressure Δu (kPa)
Ultimate pore pressure u1 (kPa)
3.82 4.74 5.12 5.88 6.12
5.09 5.63 5.95 6.68 6.93
1.27 0.89 0.83 0.80 0.81
4.76 5.55 5.92 6.62 6.90
0.94 0.81 0.80 0.74 0.78
3.82 4.75 5.16 5.66 6.15
Note, Δumax ¼ umax - u0 , Δu ¼ u -.u0
action. Due to poor permeability of silt, wave action leads to continuous accumulation of excess pore pressure, and the effective stress decreases accordingly. Here, excess pore pressure is expressed as Δu. Liquefaction occurs when Δu accumulates to the maximum value, which is equal to the initial average normal effective stress. According to Sumer et al. (2010) and Jia et al. (2014) research, in order to quantify soil lique faction more intuitively, the degree parameter L is introduced to quan tify liquefaction, as shown in formula(1). The larger the L is, the closer the soil is to liquefaction and the lower its strength is, the easier it is to deform or form splitting.
Table 3 Changes of particle size and kaolin content before and after wave action. Group
The average size of the surface particles before wave action Dav0(μm)
The average size of the surface particles after wave action Dav1(μm)
Kaolin content after wave action (%)
A B C D E F G
44.9 33.6 39.0 44.0 44.4 41.2 49.9
36.5 27.6 16.1 18.6 19.8 34.0 39.2
0.0 3.9 22.2 11.2 4.5 0.0 1.7
L ¼ 3Δu=γ ’ dð1 þ 2K0 Þ � 100
(1)
where K0 is the ratio of transverse effective stress to longitudinal effective stress, i.e. K0 ¼ σ’h =σ’v . For different soils, the values are different. According to previous studies (Sumer et al., 2010), this experiment takes K0 ¼ 0.5; D is the thickness of overlying soil, unit m; and gamma is the floating bulk density of experimental silt, which is the saturated bulk density of silt minus the bulk density of water. Here, take gamma ¼ 7.9 kN/m3. Table 4 shows the liquefaction of soil at different layers during wave action. It can be seen that the liquefaction degree of the soil decreases significantly with the increase of depth. The liquefaction degree of shallow soil at 15 cm is 102%, the stable liquefaction degree at 10 cm is 178.5%, and the maximum liquefaction degree is as high as 241.4%. Combining with the cumulative response curve of pore pressure under wave action (Fig. 6), it is found that although the cumulative pore pressure at 10 cm decreases, the cumulative excess pore pressure does not decrease to 0 instantaneously, but decreases steadily and finally stabilizes to 0.94 kPa, indicating that the liquefaction of this layer does occur but does not liquefy completely.
content before and after wave action. It can be concluded that the sur face particle size of soil tends to be finer after wave action. The presence of kaolin is generally detected in the surface layer after wave action except for group A and F. The content of kaolin in surface samples in creases first and then decreases with the increase of wave action time (group B, C, and D), and decreases with the decrease of wave height (group C, E, an F). A small amount of kaolin (group G) can be detected in surface samples even without settling. 4. Discussion 4.1. Identification of soil liquefaction process According to the Terzaghi’s principle of effective stress (Terzaghi, 1965), the total stress (σ) is equal to the sum of effective stress (σ 0 ) and pore water pressure (μ), i.e. σ ¼ σ’ þ μ. For saturated silt subjected to wave cyclic loading, the total stress remains unchanged during wave 6
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state inside the soil will be separated from the soil skeleton. Under the action of seepage force, the particles will be quickly moved to the sur face along these channels, and the fine particles will be more easily moved under the same seepage force. Generally speaking, the longer the wave action time (group B, C, and D) is, the more sufficient the strength of the soil decreases, the easier to form a splitting channel, and the higher the kaolin content can be detected on the surface of the soil. In the experiment, the upwelling particles seen in group D were obviously white at first, then the color gradually deepened and turned to silt color, and the continuous upward appearance did not end. The results show that kaolin with fine particle size is the main type in the initial stage of particle upwelling. With the continuous upwelling, the kaolin near the channel decreases gradually because the kaolin layer is only a very thin layer, and the upwelling particles become mainly silt, and a large number of silt upwelling con ceals the kaolin, which makes the content of kaolin decrease relatively.
Table 4 Liquefaction of soil at different layers during wave action. Maximum cumulative pore pressure ΔUmax(kPa)
Stable cumulative pore pressure ΔU (kPa)
γ’ d (kPa)
Maximum liquefaction Lmax(%)
Stabilized liquefaction L (%)
10 cm 15 cm 20 cm 25 cm 30 cm
1.27 0.89 0.83 0.80 0.81
0.94 0.81 0.80 0.74 0.78
0.79 1.185 1.58 1.975 2.37
241.4 112.6 78.8 60.8 51.3
178.5 102.5 75.9 56.2 49.4
For sandy soil, when the liquefaction degree L exceeds 100%, it can be considered that the soil is completely liquefied. However, for silt with finer grains, because of the cohesive force between grains and the additional pressure of membrane water, the liquefaction characteristics of the soil are affected, so even if the pore water pressure exceeds the effective stress (Δu > σ ’0 ), the soil may not be completely liquefied, that is, the L value calculated according to formula (1) can be greater than 100%. However, due to excessive accumulation of pore pressure, the strength of soil decreases greatly and the spacing of particles increases, which provides a good condition for particle migration and formation of splitting channels.
4.3. General pumping migration between particles In addition to the visible channel transport, the general transport of kaolin in soil particles was also studied in this experiment (group G). Through a long-term action of waves, in the time between visible channels formed and waves stopped, the surface samples of soil were taken for particle size testing. The results also showed that 1.7% of kaolin existed (Table 3). For the upper soil (0–15 cm) with full wave action, without consid ering the interaction between particles, the vertical forces of particles under wave action mainly include effective gravity of particles, uplift force, seepage force, cohesion force and additional pressure of mem brane water (Fig. 8).
4.2. Centralized pumping migration based on splitting channels
Differential (%)
During the experiment, it was observed that a series of splitting channels were formed in the soil after wave action. Driven by waveinduced excess pore pressure, the particles in the lower layer moved rapidly along these channels to the surface and accumulated near the exit of the channel. The upstream particles in group C were collected and analyzed. The results showed that the upstream particles were finer in size, with an average particle size of only 9.5 μm. They were mainly composed of kaolin and fine silt, of which kaolin accounted for about 56.4% (Fig. 7). It can be seen from Fig. 6 that the pore pressure inside the soil can be divided into the instantaneous pore pressure (curve fluctuation) caused by the rapid change of water level and the cumulative pore pressure (curve variation trend) caused by the wave action. The cumulative pore pressure is much larger than the instantaneous pore pressure. Due to the relatively poor permeability of silt (measured by experiment, the permeability of the sample used is about 1 � 10 7m/s), the accumulated excess pore pressure of silt cannot dissipate rapidly in a short time after the wave action stops. At the same time, due to the obvious decrease of soil strength, the accumulated excess pore pressure will rapidly dissipate along the weak points (or weak paths) of the soil by breaking through the soil layer, thus forming a series of splitting channels. With the dissipation of excess pore pressure, the liquefied soil particles in the fluid
(1) Effective gravity Fg , i.e. sediment gravity minus buoyancy Fg ¼ ðρs
π ρw Þg d350
(2)
6
(2) Uplift force Fy , according to Cao and Liu (2000) it can be expressed as #2 " 1 1 π 2 π2 H2 Fy ¼ ρw d50 2 (3) cos2 ðkx ωtÞ 0:322 2 4 T sinh2 ðkhÞ 4:684ðL⁄ hÞ
(3) Seepage force Fs , according to Mei and Foda (2010) and Ling, 2006 it can be expressed as Fs ¼
n 1
m πd350 ρw gH 1 � sin kx n m þ 1 6 2coshðkhÞ δ
ωt þ
π�
10
100
8
80
6
60
4
40
2
20
0
0 0.01
0.1
1
Particle size
10
100
Fig. 7. Size distribution curve of upwelling soil particles in channels. 7
(4)
4
1000
Integral (%)
Position
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Ocean Engineering 189 (2019) 106391
Fig. 8. Forces change of particles under wave action in the vertical direction.
accumulation, and the impulse of particles with relatively large size (D50 ¼ 300 μm) is negative (Fig. 9). In summary, the pumping effect of wae-induced excess pore pressure is mainly manifested in two aspects (Fig. 9). One is the visible central ized migration of splitting channels, and the other is the general migration of fine particles between granular cracks on the mesoscale.
(4) Bonding force and additional pressure of film water, according to Dou, 1999 and Cao and Liu (2000) it can be expressed as
π
π
(5)
N1 ¼ φ εd50 ; N2 ¼ φρw h d50 α 2 2
where ρs ¼ 2650 kg/m3 is the density of soil particles, ρw ¼ 1000 kg/ m3is the density of water, g ¼ 9.8 N/kg is the gravitational acceleration, H ¼ 0.33 m is the water depth, T ¼ 1 s is the wave period, H ¼ 0.07 m is the wave height, L ¼ 1 m is the wavelength, k ¼ 1/L is the wave number, ω is the angular frequency, and delta is the thickness of the seabed boundary layer, δ is the thickness of the seabed boundary layer. δ ¼ vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u � kd G � u , n ¼ 0.4 is porosity, β ¼ 1.0 � 107 N/m2 is bulk elastic t nG 1 2v ω
β
þ2ð1
4.4. Geomorphological effect of pumping Rivers carry a large amount of fine sediment into the sea, which can provide abundant material supply for the development of fluid mud. Density flow is a unique flow pattern in high sediment-laden rivers such as the Yellow River. It can transport sediments rapidly and massively to the sea along the bottom, but only when runoff and sediment content meet specific conditions can it occur. In addition, there are obvious seasonal characteristics of sediment transport in rivers affected by water and sediment processes, such as the Yellow River, where water mainly comes from melting glaciers in the upper reaches and rainfall in the middle reaches, while sediment mainly comes from rainstorm scouring in the middle reaches of the Loess Plateau, so the supply of water and sediment is mainly concentrated in the flood season (Wang et al., 2017). At the same time, affected by the hydrodynamic environment of the delta sea area, the hydrodynamic force in summer is weak, mainly sedimentation and deposition. While in winter, the hydrodynamic force is strong, mainly transport erosion, and sediment transport to the sea also presents obvious characteristics of summer storage and winter transport (Yang et al., 1992). For the long-term and stable development of the fluid mud layer, especially in the dry season of sediment supply, a relatively stable and effective material source is needed to ensure the existence of the fluid mud layer. The pumping effect of wave-induced pore pressure continuously transports the fine sediment at a certain depth of the lower layer to the surface, so that the dry season with less sediment supply can be effectively supplemented. It seems that this process is an indispensable factor for the development of fluid mud.
vÞ
modulus, G ¼ 5.0 � 106 N/m2 is shear modulus, v ¼ 0.33 is Poisson’s
ratio, kd ¼
ks
ρw g,
ks ¼ 1.0 � 10
8
m/s is the permeability coefficient, m is
the relative compressibility of water and soil skeleton, m ¼ 1 16
nG ð1 2vÞβ,
φ¼
is the correction coefficient, ε ¼ εk =ρw is the coefficient of cohesion, εk ¼ 2.56 � 10 2 m/s2, α ¼ 0.213 � 10 6 m is the character istic thickness related to the size of sand gap. According to the above formulas, the forces acting on the D50 ¼ 3 μm, 30 μm and 300 μm particles are calculated respectively in Fig. 8. Soil particles whose D50 ¼ 3 μm, the vertical movement is mainly affected by uplift force, cohesive force and additional pressure of membrane water. Soil particles whose D50 ¼ 30 μm, the vertical movement is mainly affected by uplift force and seepage forces. Soil particles whose D50 ¼ 300 μm, the vertical movement is mainly affected by effective gravity, uplift force and seepage force of particles. For particles with relatively small size (D50 ¼ 3 μm, 30 μm), the resultant force (positive orientation) is positive with time integration (impulse), which indicates that the momentum of particles increases with time 8
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Wave
Flow
Wave
Coarse grain layer Medium grain layer Fine grain layer
Flow
Fine particles migrate in mud diapirism channel
Fine particles migrate between particle gaps
Fig. 9. Two types of pumping effect.
the lower layer of modern sediments, which to some extent in creases the difficulty of identifying the modern sedimentary sequence.
Previous studies have shown that the continuous scouring of water in rivers can take away fine sediment and leave coarse sediment, leading to the coarsening of the riverbed (Miao et al., 2016a,b). Similarly, for the seabed under the combined action of waves and currents for a long time, the influence depth of pumping is also limited. If there is a long-term lack of effective material supply (mainly the supply of fine particles), coupled with the long-term transport and erosion of current, the fine particles will gradually decrease and the seabed will continue to coarsen. This may be a reasonable explanation for the formation of topographic phenomena such as iron sands or hard crust (Wang et al., 2004) in the Yellow River Delta.
Acknowledgments This paper is supported by the National Natural Science Foundation of China (Project No. 41602318 and No. 41672272), the National Key Research and Development Program (2017YFC0307701) and the foun dation for basic research in Ocean University of China(Grant No. 201861041). We thank the Geotechnical Laboratory of Ocean University of China for providing equipment support for this experiment. We also thank Wei Guanliand and Yang Xiuqing for their great help in the experiment process.
5. Conclusions In this paper, the silt in the Yellow River Delta is taken as the research object, and the main components of fluid mud are simulated with 4000 mesh kaolin. The responses of pore pressure accumulation, soil liquefaction and particle movement under wave action are experi mented. Based on the experimental results and calculation analysis, the following conclusions are drawn in this paper.
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(1) Surface soil particles can be re-suspended and naturally subsided by wave action. Meanwhile, the pumping effect of wave-induced pore pressure will transport the finer Kaolin from the lower layer to the surface layer, resulting in a general thinning of surface soil particle size after wave action than before wave action. (2) The silt particle size is fine, the cohesive force between particles and the additional pressure of membrane water have significant effects on the liquefaction characteristics of silt. The maximum excess pore pressure accumulated at 1.27 kPa, the liquefaction degree of surface soil is far over 100%, and the strength of soil decreases greatly, but the instantaneous complete liquefaction of soil has not yet occurred. (3) The “pumping effect” of wave-induced excess pore pressure is manifested in two aspects: one is the formation of splitting channels in the sedimentary layer under the influence of waveinduced pore pressure, and the centralized migration of fine particles along the channels; the other is the general migration of fine particles such as floating mud between the grain cracks. (4) Pumping effect brings fine-grained sediments from the lower layer to the surface layer, which provides an effective source for the development of the fluid mud layer. In addition, pumping not only strengthens the vertical exchange of shallow seabed soils, but also intensifies the integration of old and new sediments from 9
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Jia, Y., Zhang, L., Zheng, J., 2014. Effects of wave-induced seabed liquefaction on sediment re-suspension in the Yellow River Delta. Ocean Eng. 89, 146–156. https:// doi.org/10.1016/j.oceaneng.2014.08.004. Ju, Y., Gao, M., Wang, Y.Y., 2014. Review on field observation technologies for fluid mud. J. Chongqing Jianzhu Univ. 33 (1), 98–102. K, L., Z, G., et al., 2019. Effect of seepage flow on shields number around a fixed and sagging pipeline. Ocean Engineering 172, 487–500. https://doi.org/10.1016/j. oceaneng.2018.12.033. Kineke, G.C., Sternberg, R.W., 1995. Distribution of fluid muds on the Amazon continental shelf. Mar. Geol. 125 (3–4), 193–233. https://doi.org/10.1016/00253227(95)00013-O. Kreeke, J.V.D., Day, C.M., Mulder, H.P.J., 1997. Tidal variations in suspended sediment concentration in the Ems estuary: origin and resulting sediment flux. J. Sea Res. 38 (1–2), 1–16. https://doi.org/10.1016/S1385-1101(97)00040-3. Lambrechts, J., Humphrey, C., Mckinna, L., 2010. Importance of wave-induced bed liquefaction in the fine sediment budget of Cleveland Bay Great Barrier Reef. Estuar. Coast Shelf Sci. 89 (2), 154–162. https://doi.org/10.1016/j.ecss.2010.06.009. Li, D.S., Ren, R.S., 1996. Research for special property of gravity current with the movement of wave. Ocean Eng. (4), 54–59. Li, J., Zhang, C., 1998. Sediment resuspension and implications for turbidity maximum in the Changjiang Estuary. Mar. Geol. 148, 117–124. https://doi.org/10.1016/S00253227(98)00003-6. Li, J.F., Dai, Z.J., Liu, Q.Z., 2008. In-situ observation of floccule size and fluid mud in the Changjiang Estuary. J. Sediment. Res. (3), 26–32. Li, W.H., Shi, L.Q., Liu, M., 2013. Review of estuarine and coastal fluid mud measurement technique, characteristics and transportation researches. J. Sediment. Res. (1), 74–80. Liu, T., Li, S.P., Kou, H.L., 2018. Excess pore pressure observation in marine sediment based on Fiber Bragg Grating pressure sensor. Mar. Georesour. Geotechnol. 1–8. https://doi.org/10.1080/1064119X.2018.1486926. Liu, T., Lu, Y., Zhou, L., Yang, X., Guo, L., 2019. Experiment and analysis of submarine landslide model caused by elevated pore pressure. J. Mar. Sci. Eng. 7 (5), 146. https://doi.org/10.3390/jmse7050146. Ling, X., 2006. Sediment threshold and local scour around submarine pipelines under wave action. Zhejiang University, China, pp. 23–46. Liu, Tao, Guanli, Wei, Kou, Hailei, Guo, Lei, 2019. Pore pressure observation: pressure response of probe penetration and tides. Acta Oceanol. Sin. 38 (7), 107–113. https:// doi.org/10.1007/s13131-019-1462-4. Maa, J.P.Y., Mehta, A.J., 1988. Soft mud properties: voigt model. J. Waterw. Port, Coast. Ocean Eng.-asce 114 (6), 765–770. https://doi.org/10.1061/(ASCE)0733-950X (1988)114:6(765). Maa, J.P.Y., Mehta, A.J., 1990. Soft mud response to water waves. J. Waterw. Port, Coast. Ocean Eng.-asce 116 (5), 634–650. Mcanally, W.H., Friedrichs, C.T., Hamilton, D., 2007. Management of fluid mud in estuaries, bays, and lakes. I: present state of understanding on character and behavior. J. Hydraul. Eng. 133 (1), 9–22. Mclaren, P., Bowles, D., 1985. The effects of sediment transport on grain-size distributions. J. Sediment. Res. 55 (4), 457–470. Mei, C.C., Foda, M.A., 2010. Wave-induced responses in a fluid-filled poro-elastic solid with a free surface—a boundary layer theory. Geophys. J. R. Astron. Soc. 66 (3), 597–631. https://doi.org/10.1111/j.1365-246x.1981.tb04892.x. Miao, C., Kong, D., Wu, J., 2016. Functional degradation of the water–sediment regulation scheme in the lower Yellow River: spatial and temporal analyses. Sci. Total Environ. 51–552, 16–22. https://doi.org/10.1016/j.scitotenv.2016.02.006. Miao, L.M., Yang, S.L., Zhu, Q., 2016. Variations of suspended sediment concentrations and transport in response to a storm and its dynamic mechanism. J. Oceanogr. 38 (5), 158–167. Montserrat, F., Suykerbuyk, W., Al-Busaidi, R., 2011. Effects of mud sedimentation on lugworm ecosystem engineering. J. Sea Res. 65 (1), 170–181. https://doi.org/ 10.1016/j.seares.2010.09.003.
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Pumping effect of wave-induced pore pressure on the development of fluid mud layer Zhongnian Yang a, Yongmao Zhu b, Tao Liu b, c, *, Zhongqiang Sun b, Xianzhang Ling d, Jinmu Yang b a
School of Civil Engineering, Qingdao University of Technology, Qingdao, Shandong, 266033, China Key Laboratory of Shandong Province for Marine Environment and Geological Engineering, Ocean University of China, Qingdao, Shandong, 266100, China Laboratory for Marine Geology, Qingdao National Laboratory for Maine Science and Technology, Qingdao, Shandong, 266061, China d Harbin Institute of Technology, Harbin, Heilongjiang, 150001, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Pumping effect Wave-induced Silty seabed Fluid mud layer
Fluid mud widely exists in the movement of fine sediment, which is an important potential cause of marine geological disasters. It is important to understand the generation and migration mechanism of fluid mud. The purpose of this study is to investigate the vertical migration response of fine-grained sediments by wave-fluid test of pore pressure response of silty seabed under wave action. The results show that the adhesion between the silt particles and the additional pressure of the film water have a significant effect on the liquefaction characteristics of the silt. Even if the liquefaction degree is far more than 100%, the soil does not completely liquefy. The experiment result show that, there is concentrated movement of the split channel, and mesoscopic migration of fine particles between the gaps of the particles, pumping effect provides an effective source of supply for the development of the floating mud layer, it also exacerbates modern sedimentation when strengthening the ver tical exchange of shallow soil in the seabed. Besides, the blending of the middle and lower layers of the previous soil with the recent sediments has increased the difficulty of identifying the time series of the modern sedi mentary layer to some extent.
1. Introduction Fluid mud refers to a layer of high concentration sediment-laden water near the bottom of the sea. It is mainly composed of clay and silt with obvious viscous properties, and the main particle composition is generally not more than 63 μm (Li et al., 2013). Fluid mud has a clear interface with the upper water body and has great mobility. It is a unique sediment movement pattern in the muddy coastal estuaries (Mcanally et al., 2007; Qian and Wan (1983). It has been found in Thames estuary of England, Gironde estuary of France (Granboulan et al., 1989), Amazon estuary of Brazil (Allison et al., 1995; Kineke and Sternberg (1995), Yellow River estuary and Yangtze River estuary of China (Xu et al., 1994). The liquefaction of sand and the resuspension of sediment under the action of waves were always the focus of coastal research. Most scholars focus on liquefaction mechanisms and resuspension fluxes (Wainright (1990); Gao et al., 2011; Moriarty et al., 2017; Tao et al., 2018).
However, few researchers pay attention to the vertical pumping effect of wave-induced pore pressure from a microscopic point of view, especially the development of fluid mud in silty soil. When there is a large amount of fluid mud, it can cause ecological, environmental and navigation safety problems such as burying benthic organisms Corselli and Basso (1996), sediment eutrophication (Montserrat et al., 2011) and channel sudden siltation, disturbing sounding results, affecting the judgment of navigable water depth (Niu et al., 2003), etc. Besides, its interaction with suspended load and bedload has an important impact on the topographic and geomorphological evolution of estuary areas (Yunping and Tian, 2014; Liu et al., 2018), such as channel siltation, serious time and even navigation safety. It will block the channel and cause the ship to run aground (K and Z, 2019; Zhen, 2019). The problem of fluid mud originates from the results of field obser vation (Liu et al., 2018, 2019), whose means includes frozen sampling, bathymetry, gamma-ray method, ultrasonic method, tuning fork density method and coupling method (Ju et al., 2014), which can be
* Corresponding author. Key Laboratory of Shandong Province for Marine Environment and Geological Engineering, Ocean University of China, Qingdao, Shandong, 266100, China. Tel.: 0532 66783723 E-mail address: [email protected] (T. Liu). https://doi.org/10.1016/j.oceaneng.2019.106391 Received 9 April 2019; Received in revised form 14 August 2019; Accepted 27 August 2019 Available online 25 September 2019 0029-8018/© 2019 Elsevier Ltd. All rights reserved.
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summarized as a direct measurement, isotope measurement and acoustic measurement by measuring principle. Laboratory tests are mainly divided into two directions, one is the experimental study of the rheological properties of the sludge itself (Maa and Mehta (1988); Hasar and Kinaci (2004); Abujdayil et al., 2010),the other is the experimental study of the movement characteristics of the sludge under dynamic ac tion, including wave action (Zhao, 1982; Wang et al., 2008; Lambrechts et al., 2010; Yang et al., 2011) and current action (Hong and Ying (1988); Kreeke et al., 1997; Almroth et al., 2009; Wu et al., 2017). On the basis of field observation and laboratory test, the formation condi tions (Xu and Dong, 2011; Xu and Yuan, 2001) of sludge and the nu merical simulation (Maa and Mehta, 1990) of sludge response under dynamic action are studied from the angle of theoretical analysis. In view of the development mechanism of floating sludge, the pre vailing view is that the conditions for the formation of floating sludge include abundant fine sediment supply, relatively weak hydrodynamic conditions and appropriate salinity. Salinity and weak dynamic force are the external conditions for flocculation and sedimentation of fine sedi ment (F.L. et al., 2019; Daoji, 2007), and abundant fine sediment pro vides the material source for the formation and development of fluid mud. Previous studies have been carried out on the flocculation envi ronment of fine sediment. Li et al., 2008 considered that the Yangtze River estuary has a good environment for flocculation of fine sediment through field observation. Li and Zhang (1998) found that salinity and sediment concentration contribute significantly to the flocculation of fine sediment. The periodic change of flow velocity is the most sensitive factor to control the flocculation size of fine sediment (Li et al., 2008; Li and Zhang, 1998). It is generally believed that there are two forms of material supply of fluid mud, one is direct input from rivers. Winterwerp (1999) and Li and Ren (1996) point out that the potential of density inflow is a special manifestation of the rapid deposition of sediment at the bottom, which provides a rich material source for the formation of fluid mud. The other is the resuspension supply of storm events, whose dynamic mechanism is that storms transmit huge energy to the nearshore water body, thereby significantly increasing wave-current linkage. The shear stress of the combined bed leads to the re-suspension of fine sediment(Yeh and Mason, 2014; Miao et al., 2016a,b; Sheremet et al., 2005). Both of these two opinions are lack of dynamic process research from a micro perspective, and insufficient to fully explain the source supply mecha nism of the development of fluid mud, which are based on field
observation, revealing from a macro perspective, In this paper, silt from the Yellow River Delta is taken as the research object, and the cumula tive process of pore pressure and liquefaction response of soil under wave action are analyzed based on the wave flume test in this paper. The vertical migration law of soil particles under wave action is studied by tracing the particle size composition. Combining with theoretical calculation, this paper reveals the source and supply ways for the development of fluid mud from a mesoscopic point of view in order to further complement and enrich the development mechanism, transport law of fluid mud. 2. Methodology 2.1. Experiment design and materials The wave flume experiment was completed in the Geotechnical Laboratory of the Ocean University of China. The experimental device is shown in Fig. 1. The size of the flume is 3 m � 0.6 m � 1.2 m, and the size of the lower soil flume is 1 m � 0.6 m � 0.5 m. The air compressor is used to provide power for the device. A double-acting cylinder is installed at one end of the tank to push the water body to and fro with the wave-making plate to produce waves. The output waveform is changed by adjusting the stroke of the cylinder and the damping of the cylinder. A wedge sponge block is installed at the other end to eliminate the reflection effect of the waves and ensure the stable output of the waveform. As shown in Fig. 1, the soil tank is filled with saturated silt with a thickness of about 40 cm, and a kaolin layer is set at a distance of 5 cm from the surface of the soil body, with a thickness of less than 0.5 cm to study the transportation process and influence of pumping effect on fine granular sediment. The silt was quiesced for 4 h before and after the experiment, and then its surface samples were taken. The particle size components were determined by laser particle size analyzer. Six pore water pressure sensors (type YY-2B) were embedded in this experiment, which were located at 5 cm, 10 cm, 15 cm, 20 cm, 25 cm, and 30 cm away from the surface of the soil. The pore pressure sensor is immersed in clear water for 24 h before burial and continuously shakes sufficiently to ensure that the internal air is completely discharged. The water is artificial seawater with 35‰ salinity. The silt was taken from the tidal flat of the Yellow River Estuary with median particle size (D50) ranging from 32 to 41 μm, and kaolin was brought from industrial
Fig. 1. Diagram of the experimental setup and the photograph of the wave generator. 2
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factories with a mesh size of 4000 and D50 ranging from 2.1 to 2.8 μm.
2.3. Experiment procedure
2.2. Quantification of Kaolin
In order to simulate the physical characteristics of silt under natural consolidation and saturate the samples, air-dried silt samples were selected to remove impurities such as grass and branches after crushing. Sifted soil samples are put into the mixer, and standard seawater is added to mix at the same time to ensure uniform mixing of the samples. The pore water pressure sensor is fixed in the middle of the soil trough in advance according to the specified depth to avoid disturbance to the soil after embedding. The mixture is slowly injected into the soil tank along the inner wall until the thickness is about 35 cm. After the soil is stabi lized, a layer of kaolin with a thickness less than 0.5 cm is evenly laid on the surface, then continue adding soil to 40 cm (Hibino et al., 2014). The experimental group without kaolin layer directly injects the mixture to 40 cm. After the soil is basically stable, the standard seawater is slowly injected into the tank along the inner wall until about 35 cm away from the surface of the soil. After 12 h, the soil was naturally consolidated under hydrostatic pressure, and then the wave loading experiment was started. Before and after wave action, four surface samples were evenly sampled in the soil trough by the sampling tube for particle size mea surement. In order to avoid component interference between different experimental groups, samples in artificial seawater and soil trough should be replaced completely after each group of experiments. Wave loads were applied by using the wave generator installed at one end of the flume, and different wave actions (wave height and wave duration) were experimentally studied. The specific experimental group settings are shown in Table 1. The experimental group A and C were the basic control group, which focused on the experimental pore pressure response and surface particle size change caused by wave action. The pumping effect of wave-induced excess pore pressure on fine sediment was studied. YY-2B pore water pressure sensors were used to record the pore pressure changes at different depths of soil before, during and after wave action. Based on this, the accumulation of pore pressure caused by wave and the migration of fine particles under pumping action were calculated and analyzed.
The results of particle size measurement can not only obtain the parameters such as D50 and average particle size (Dav), but also obtain the distribution of particle size content and its cumulative distribution. By studying the grain size characteristics of sediments, the transport mode of sediments can be determined and the environmental factors affecting the grain size change of sediments can be inverted, especially the material sources and hydrodynamic environment (Mclaren and Bowles (1985); Wang et al., 2009). The experimental silt and kaolin are obviously different in origin and particle size composition. In this experiment, the silt particle size is relatively coarser (Fig. 2a, D50 is about 32–49 μm), which is taken from the tidal flat of the Yellow River Estuary (Liu et al., 2019). It is formed by natural accumulation after long-distance transportation and long-term screening of water flow. It has good sorting and grinding roundness whose material source is relatively consistent, approximating to the normal curve. The experimental kaolin has a finer particle size (as shown in Fig. 2b, D50 is about 2.1–2.8 μm). It is manufactured in in dustrial batches and screened artificially, which has poor sorting and grinding roundness, and its shape is more sharp and fine. Fig. 3a is the particle size distribution curve of mixed kaolin and silt. Its main component is 80% kaolin mixed with 20% silt. It can be seen that there are two independent peaks in the particle size distribution curve, and the maximum of the two peaks corresponds to the particle size range of kaolin (D50 ¼ 2.1 μm) and silt (D50 ¼ 34 μm), respectively. Therefore, the number of peaks in the particle size distribution curve can be used to qualitatively determine whether there are many components in the sample and whether there are corresponding components can be qualitatively judged by the size range corresponding to the maximum value. Sample size distribution curve contains multiple peaks, which can be seen as superimposed by single peaks of different particle sizes (as shown in Fig. 3b). The particle size distribution of one component can be obtained by decomposing the particle size distribution curve. The component proportion of different particle sizes (corresponding to different material sources) can be quantitatively evaluated by cumula tive distribution.
3. Results 3.1. Experimental phenomena With the beginning of wave action, the consolidated soil gradually
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Fig. 3. Particle size distribution curve of mixed kaolin and silt in proportion (a) and its decomposition calculation sketch (b). Table 1 Experiment setup. Group A B C D E F G
Kaolin layer
� ✓ ✓ ✓ ✓ ✓ ✓
Pre-settling time (h) 12 12 12 12 12 12 12
Sample number 1–4 9–12 17–20 25–28 33–36 41–44 49–52
Wave action Time (min)
Wave height (cm)
20 10 20 30 20 20 90
7 7 7 7 5 3 7
Post-settling time (h)
Sample number
12 12 12 12 12 12 0
5–8 13–16 21–24 29–32 37–40 45–48 53–56
Note:"�" indicates that no kaolin layer in the experiment and "√" indicates that there is a kaolin layer in the experiment.
shows a slight fluctuation. The fluctuation period of soil is consistent with the period of wave action. The surface layer of soil has the largest fluctuation, and the amplitude is less than 2 cm. The initial time of soil fluctuation is delayed with the increase of depth, and the ultimate in fluence of wave on soil depth increases with the increase of wave height. The maximum influence depth that can be distinguished by naked eyes can reach 15 cm below the surface layer of soil. The overlying water body gradually changes from clear to turbid, and the silt on the surface of the soil is lifted up, with few bubbles escaping occasionally. When the wave action stops, the fluctuation of the upper soil disappears quickly and consolidation begins, the suspended particles settle slowly, and the water gradually recovers clarity after a long time of setting. Occasion ally, obvious small channels can be seen at the boundary of the flume (as shown in Fig. 4a). The splitting channels of 3–17 cm in length can be seen. The sediment in the lower part of the soil moves up along these small channels rapidly and forms many small hilly deposits on the sur face. After a period of time, a large number of small hilly deposits are formed on the surface of the soil (Fig. 4b). The diameter and height of the hilly deposits generally do not exceed 3 cm and 1 cm. At 5 cm below the surface of the soil, kaolin migrates rapidly to the surface along the channels and forms a series of white accumulation. The phenomena of group C and D are most obvious (Fig. 4b). At first, the white kaolin upwells in group D, and then the color gradually deepens to the normal
silt upwelling, gradually covering the white kaolin. After a long period of stationary time, the hilly accumulation center gradually presents obvious depression (Fig. 4b), and the whole process lasts for about 1 h. 3.2. Accumulative pore pressure Due to the loosening of the fixture in the process of sensor embed ding, the actual position and the desired position deviate, but the overall order will not change (Fig. 5a), which does not affect the analysis of the overall pore pressure variation law, so this paper still labels and de scribes the set position according to the experiment. Besides, all sensors were calibrated before and after the experiment, and it was found that the stability of pore pressure sensors located 5 cm below the surface of soil was difficult to meet the experimental requirements (Fig. 5b①②). There are obvious anomalies in the data, so the relevant results and analysis were not involved in this paper. Fig. 6 shows the pore pressure response before, during and after wave action (H ¼ 7 cm, t ¼ 30 min) at 10–30 cm below the surface of the soil. The pore pressure fluctuations of the sensors are basically the same. Before the wave action (0–20 s), the pore water pressure of each layer is stable after a long period of static consolidation, and its value is consistent with the overburden hydrostatic pressure. During wave ac tion (20–2100 s), pore water pressure began to fluctuate with wave load, 4
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Fig. 4. Channels (a) observed at the boundary of the flume and a large number of hilly deposits formed on the surface of the soil (b).
b
a Pore pressure sensor
Y=0.554X+21.163 R2=0.999
Y=0.453X+11.970 R2=0.999
Fig. 5. Actual position of pore water pressure sensor (a) and the calibration curve of the sensor at 5 cm (b).
and the pore water pressure began to accumulate in all layers (as shown in Table 2, the maximum increment was 0.80–1.27 kPa, the average increment was 0.74–0.94 kPa). The accumulative value and velocity of pore water pressure in the upper layer were greater than those in the lower layer, and the pore water pressure reached the maximum value 5.09 kPa at about 150 s at 10 cm, then decreased slightly to stable pore pressure value 4.76 kPa, the pore water pressure tends to stabilize gradually with the passage of time. After the wave action stops, the pore
water pressure fluctuation disappears rapidly, and the pore water pressure in each layer dissipates continuously. After the wave load ends, the pore water pressure returns to the level before the wave action and remains stable about 100 min. 3.3. Particle size and kaolin content Table 3 shows the variation of surface particle size and kaolin 5
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Ocean Engineering 189 (2019) 106391
0
5
10
15 20
300
600
900
1200
1500
1800
2000 4000
6000
8000
10000 12000
2 3
Before
Wave
After
4 5 6 7 8
30cm 25cm 20cm
15cm 10cm
Fig. 6. Pore pressure response at different depths before, during and after wave action (H ¼ 7 cm, t ¼ 30 min). Table 2 Analysis of pore pressure variation during wave action. Position
10 cm 15 cm 20 cm 25 cm 30 cm
Before
Wave action
After
Initial pore water pressure u0 (kPa)
Maximum pore water pressure umax(kPa)
Maximum cumulative pore pressure Δumax(kPa)
Stable pore pressure u (kPa)
Stable cumulative pore pressure Δu (kPa)
Ultimate pore pressure u1 (kPa)
3.82 4.74 5.12 5.88 6.12
5.09 5.63 5.95 6.68 6.93
1.27 0.89 0.83 0.80 0.81
4.76 5.55 5.92 6.62 6.90
0.94 0.81 0.80 0.74 0.78
3.82 4.75 5.16 5.66 6.15
Note, Δumax ¼ umax - u0 , Δu ¼ u -.u0
action. Due to poor permeability of silt, wave action leads to continuous accumulation of excess pore pressure, and the effective stress decreases accordingly. Here, excess pore pressure is expressed as Δu. Liquefaction occurs when Δu accumulates to the maximum value, which is equal to the initial average normal effective stress. According to Sumer et al. (2010) and Jia et al. (2014) research, in order to quantify soil lique faction more intuitively, the degree parameter L is introduced to quan tify liquefaction, as shown in formula(1). The larger the L is, the closer the soil is to liquefaction and the lower its strength is, the easier it is to deform or form splitting.
Table 3 Changes of particle size and kaolin content before and after wave action. Group
The average size of the surface particles before wave action Dav0(μm)
The average size of the surface particles after wave action Dav1(μm)
Kaolin content after wave action (%)
A B C D E F G
44.9 33.6 39.0 44.0 44.4 41.2 49.9
36.5 27.6 16.1 18.6 19.8 34.0 39.2
0.0 3.9 22.2 11.2 4.5 0.0 1.7
L ¼ 3Δu=γ ’ dð1 þ 2K0 Þ � 100
(1)
where K0 is the ratio of transverse effective stress to longitudinal effective stress, i.e. K0 ¼ σ’h =σ’v . For different soils, the values are different. According to previous studies (Sumer et al., 2010), this experiment takes K0 ¼ 0.5; D is the thickness of overlying soil, unit m; and gamma is the floating bulk density of experimental silt, which is the saturated bulk density of silt minus the bulk density of water. Here, take gamma ¼ 7.9 kN/m3. Table 4 shows the liquefaction of soil at different layers during wave action. It can be seen that the liquefaction degree of the soil decreases significantly with the increase of depth. The liquefaction degree of shallow soil at 15 cm is 102%, the stable liquefaction degree at 10 cm is 178.5%, and the maximum liquefaction degree is as high as 241.4%. Combining with the cumulative response curve of pore pressure under wave action (Fig. 6), it is found that although the cumulative pore pressure at 10 cm decreases, the cumulative excess pore pressure does not decrease to 0 instantaneously, but decreases steadily and finally stabilizes to 0.94 kPa, indicating that the liquefaction of this layer does occur but does not liquefy completely.
content before and after wave action. It can be concluded that the sur face particle size of soil tends to be finer after wave action. The presence of kaolin is generally detected in the surface layer after wave action except for group A and F. The content of kaolin in surface samples in creases first and then decreases with the increase of wave action time (group B, C, and D), and decreases with the decrease of wave height (group C, E, an F). A small amount of kaolin (group G) can be detected in surface samples even without settling. 4. Discussion 4.1. Identification of soil liquefaction process According to the Terzaghi’s principle of effective stress (Terzaghi, 1965), the total stress (σ) is equal to the sum of effective stress (σ 0 ) and pore water pressure (μ), i.e. σ ¼ σ’ þ μ. For saturated silt subjected to wave cyclic loading, the total stress remains unchanged during wave 6
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state inside the soil will be separated from the soil skeleton. Under the action of seepage force, the particles will be quickly moved to the sur face along these channels, and the fine particles will be more easily moved under the same seepage force. Generally speaking, the longer the wave action time (group B, C, and D) is, the more sufficient the strength of the soil decreases, the easier to form a splitting channel, and the higher the kaolin content can be detected on the surface of the soil. In the experiment, the upwelling particles seen in group D were obviously white at first, then the color gradually deepened and turned to silt color, and the continuous upward appearance did not end. The results show that kaolin with fine particle size is the main type in the initial stage of particle upwelling. With the continuous upwelling, the kaolin near the channel decreases gradually because the kaolin layer is only a very thin layer, and the upwelling particles become mainly silt, and a large number of silt upwelling con ceals the kaolin, which makes the content of kaolin decrease relatively.
Table 4 Liquefaction of soil at different layers during wave action. Maximum cumulative pore pressure ΔUmax(kPa)
Stable cumulative pore pressure ΔU (kPa)
γ’ d (kPa)
Maximum liquefaction Lmax(%)
Stabilized liquefaction L (%)
10 cm 15 cm 20 cm 25 cm 30 cm
1.27 0.89 0.83 0.80 0.81
0.94 0.81 0.80 0.74 0.78
0.79 1.185 1.58 1.975 2.37
241.4 112.6 78.8 60.8 51.3
178.5 102.5 75.9 56.2 49.4
For sandy soil, when the liquefaction degree L exceeds 100%, it can be considered that the soil is completely liquefied. However, for silt with finer grains, because of the cohesive force between grains and the additional pressure of membrane water, the liquefaction characteristics of the soil are affected, so even if the pore water pressure exceeds the effective stress (Δu > σ ’0 ), the soil may not be completely liquefied, that is, the L value calculated according to formula (1) can be greater than 100%. However, due to excessive accumulation of pore pressure, the strength of soil decreases greatly and the spacing of particles increases, which provides a good condition for particle migration and formation of splitting channels.
4.3. General pumping migration between particles In addition to the visible channel transport, the general transport of kaolin in soil particles was also studied in this experiment (group G). Through a long-term action of waves, in the time between visible channels formed and waves stopped, the surface samples of soil were taken for particle size testing. The results also showed that 1.7% of kaolin existed (Table 3). For the upper soil (0–15 cm) with full wave action, without consid ering the interaction between particles, the vertical forces of particles under wave action mainly include effective gravity of particles, uplift force, seepage force, cohesion force and additional pressure of mem brane water (Fig. 8).
4.2. Centralized pumping migration based on splitting channels
Differential (%)
During the experiment, it was observed that a series of splitting channels were formed in the soil after wave action. Driven by waveinduced excess pore pressure, the particles in the lower layer moved rapidly along these channels to the surface and accumulated near the exit of the channel. The upstream particles in group C were collected and analyzed. The results showed that the upstream particles were finer in size, with an average particle size of only 9.5 μm. They were mainly composed of kaolin and fine silt, of which kaolin accounted for about 56.4% (Fig. 7). It can be seen from Fig. 6 that the pore pressure inside the soil can be divided into the instantaneous pore pressure (curve fluctuation) caused by the rapid change of water level and the cumulative pore pressure (curve variation trend) caused by the wave action. The cumulative pore pressure is much larger than the instantaneous pore pressure. Due to the relatively poor permeability of silt (measured by experiment, the permeability of the sample used is about 1 � 10 7m/s), the accumulated excess pore pressure of silt cannot dissipate rapidly in a short time after the wave action stops. At the same time, due to the obvious decrease of soil strength, the accumulated excess pore pressure will rapidly dissipate along the weak points (or weak paths) of the soil by breaking through the soil layer, thus forming a series of splitting channels. With the dissipation of excess pore pressure, the liquefied soil particles in the fluid
(1) Effective gravity Fg , i.e. sediment gravity minus buoyancy Fg ¼ ðρs
π ρw Þg d350
(2)
6
(2) Uplift force Fy , according to Cao and Liu (2000) it can be expressed as #2 " 1 1 π 2 π2 H2 Fy ¼ ρw d50 2 (3) cos2 ðkx ωtÞ 0:322 2 4 T sinh2 ðkhÞ 4:684ðL⁄ hÞ
(3) Seepage force Fs , according to Mei and Foda (2010) and Ling, 2006 it can be expressed as Fs ¼
n 1
m πd350 ρw gH 1 � sin kx n m þ 1 6 2coshðkhÞ δ
ωt þ
π�
10
100
8
80
6
60
4
40
2
20
0
0 0.01
0.1
1
Particle size
10
100
Fig. 7. Size distribution curve of upwelling soil particles in channels. 7
(4)
4
1000
Integral (%)
Position
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Ocean Engineering 189 (2019) 106391
Fig. 8. Forces change of particles under wave action in the vertical direction.
accumulation, and the impulse of particles with relatively large size (D50 ¼ 300 μm) is negative (Fig. 9). In summary, the pumping effect of wae-induced excess pore pressure is mainly manifested in two aspects (Fig. 9). One is the visible central ized migration of splitting channels, and the other is the general migration of fine particles between granular cracks on the mesoscale.
(4) Bonding force and additional pressure of film water, according to Dou, 1999 and Cao and Liu (2000) it can be expressed as
π
π
(5)
N1 ¼ φ εd50 ; N2 ¼ φρw h d50 α 2 2
where ρs ¼ 2650 kg/m3 is the density of soil particles, ρw ¼ 1000 kg/ m3is the density of water, g ¼ 9.8 N/kg is the gravitational acceleration, H ¼ 0.33 m is the water depth, T ¼ 1 s is the wave period, H ¼ 0.07 m is the wave height, L ¼ 1 m is the wavelength, k ¼ 1/L is the wave number, ω is the angular frequency, and delta is the thickness of the seabed boundary layer, δ is the thickness of the seabed boundary layer. δ ¼ vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u � kd G � u , n ¼ 0.4 is porosity, β ¼ 1.0 � 107 N/m2 is bulk elastic t nG 1 2v ω
β
þ2ð1
4.4. Geomorphological effect of pumping Rivers carry a large amount of fine sediment into the sea, which can provide abundant material supply for the development of fluid mud. Density flow is a unique flow pattern in high sediment-laden rivers such as the Yellow River. It can transport sediments rapidly and massively to the sea along the bottom, but only when runoff and sediment content meet specific conditions can it occur. In addition, there are obvious seasonal characteristics of sediment transport in rivers affected by water and sediment processes, such as the Yellow River, where water mainly comes from melting glaciers in the upper reaches and rainfall in the middle reaches, while sediment mainly comes from rainstorm scouring in the middle reaches of the Loess Plateau, so the supply of water and sediment is mainly concentrated in the flood season (Wang et al., 2017). At the same time, affected by the hydrodynamic environment of the delta sea area, the hydrodynamic force in summer is weak, mainly sedimentation and deposition. While in winter, the hydrodynamic force is strong, mainly transport erosion, and sediment transport to the sea also presents obvious characteristics of summer storage and winter transport (Yang et al., 1992). For the long-term and stable development of the fluid mud layer, especially in the dry season of sediment supply, a relatively stable and effective material source is needed to ensure the existence of the fluid mud layer. The pumping effect of wave-induced pore pressure continuously transports the fine sediment at a certain depth of the lower layer to the surface, so that the dry season with less sediment supply can be effectively supplemented. It seems that this process is an indispensable factor for the development of fluid mud.
vÞ
modulus, G ¼ 5.0 � 106 N/m2 is shear modulus, v ¼ 0.33 is Poisson’s
ratio, kd ¼
ks
ρw g,
ks ¼ 1.0 � 10
8
m/s is the permeability coefficient, m is
the relative compressibility of water and soil skeleton, m ¼ 1 16
nG ð1 2vÞβ,
φ¼
is the correction coefficient, ε ¼ εk =ρw is the coefficient of cohesion, εk ¼ 2.56 � 10 2 m/s2, α ¼ 0.213 � 10 6 m is the character istic thickness related to the size of sand gap. According to the above formulas, the forces acting on the D50 ¼ 3 μm, 30 μm and 300 μm particles are calculated respectively in Fig. 8. Soil particles whose D50 ¼ 3 μm, the vertical movement is mainly affected by uplift force, cohesive force and additional pressure of membrane water. Soil particles whose D50 ¼ 30 μm, the vertical movement is mainly affected by uplift force and seepage forces. Soil particles whose D50 ¼ 300 μm, the vertical movement is mainly affected by effective gravity, uplift force and seepage force of particles. For particles with relatively small size (D50 ¼ 3 μm, 30 μm), the resultant force (positive orientation) is positive with time integration (impulse), which indicates that the momentum of particles increases with time 8
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Ocean Engineering 189 (2019) 106391
Wave
Flow
Wave
Coarse grain layer Medium grain layer Fine grain layer
Flow
Fine particles migrate in mud diapirism channel
Fine particles migrate between particle gaps
Fig. 9. Two types of pumping effect.
the lower layer of modern sediments, which to some extent in creases the difficulty of identifying the modern sedimentary sequence.
Previous studies have shown that the continuous scouring of water in rivers can take away fine sediment and leave coarse sediment, leading to the coarsening of the riverbed (Miao et al., 2016a,b). Similarly, for the seabed under the combined action of waves and currents for a long time, the influence depth of pumping is also limited. If there is a long-term lack of effective material supply (mainly the supply of fine particles), coupled with the long-term transport and erosion of current, the fine particles will gradually decrease and the seabed will continue to coarsen. This may be a reasonable explanation for the formation of topographic phenomena such as iron sands or hard crust (Wang et al., 2004) in the Yellow River Delta.
Acknowledgments This paper is supported by the National Natural Science Foundation of China (Project No. 41602318 and No. 41672272), the National Key Research and Development Program (2017YFC0307701) and the foun dation for basic research in Ocean University of China(Grant No. 201861041). We thank the Geotechnical Laboratory of Ocean University of China for providing equipment support for this experiment. We also thank Wei Guanliand and Yang Xiuqing for their great help in the experiment process.
5. Conclusions In this paper, the silt in the Yellow River Delta is taken as the research object, and the main components of fluid mud are simulated with 4000 mesh kaolin. The responses of pore pressure accumulation, soil liquefaction and particle movement under wave action are experi mented. Based on the experimental results and calculation analysis, the following conclusions are drawn in this paper.
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(1) Surface soil particles can be re-suspended and naturally subsided by wave action. Meanwhile, the pumping effect of wave-induced pore pressure will transport the finer Kaolin from the lower layer to the surface layer, resulting in a general thinning of surface soil particle size after wave action than before wave action. (2) The silt particle size is fine, the cohesive force between particles and the additional pressure of membrane water have significant effects on the liquefaction characteristics of silt. The maximum excess pore pressure accumulated at 1.27 kPa, the liquefaction degree of surface soil is far over 100%, and the strength of soil decreases greatly, but the instantaneous complete liquefaction of soil has not yet occurred. (3) The “pumping effect” of wave-induced excess pore pressure is manifested in two aspects: one is the formation of splitting channels in the sedimentary layer under the influence of waveinduced pore pressure, and the centralized migration of fine particles along the channels; the other is the general migration of fine particles such as floating mud between the grain cracks. (4) Pumping effect brings fine-grained sediments from the lower layer to the surface layer, which provides an effective source for the development of the fluid mud layer. In addition, pumping not only strengthens the vertical exchange of shallow seabed soils, but also intensifies the integration of old and new sediments from 9
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