(1)
Shiaw-Yih Tzang1,
Shan-Huei Ou2
and Tai-Wen Hsu2
1 Associate Professor, Department of Harbor & River Engineering
National Taiwan Ocean University, Keelung, Taiwan 202,
Fax: (886) 2-2463-1597; E-mail: sytzang@mail.ntou.edu.tw
2Professor, Dept. of Hydraulic & Ocean Engineering,
National Cheng-Kung University, Tainan 701, China
Abstract: In this study several wave flume tests on wave-induced seabed responses of a fine sandy soil (d50=0.85μm) were conducted to investigate the characteristics of fluidized responses. Analyses on pore pressure measurements first show three typical responses, defined as unfliudized, initial fluidized and ensuing fluidized, respectively. A fluidized response is identified as the mean pore pressure build-up approaches the soil’s static stress at a given soil depth. Comparisons with Foda ＆ Tzang’s (1994) data immediately demonstrate differences of pore pressure responses in several aspects between present fine sandy beds and their silty beds such as no mean pore pressure build-ups in the unfluidized response nor evident mechanism of resonance in the initial fluidized responses. But similarly increasing sediment suspensions were again clearly observed over the fluidized sandy seabed
Keywords: fine sandy beds, pore pressure, wave-induced soil fluidization
In
the recent decades, wave-induced seabed responses in non-cohesive soils were
vigorously studied and most of the theoretical developments (e.g. Yamamoto, et
al., 1978, Madsen, 1978, Mei & Foda, 1981 etc.) were based on Biot’s
(1941, 1956) linear poro-elasticity theory for two-phase media. They were
further verified from experimental pore pressure measurements in sandy beds,
e.g. Yamamoto, et al., 1978 (d50= 0.2 & 1.2 mm), Foda & Tzang,
1994 (d50= 0.29 mm). But for critical soil responses, such as soil
fluidization as part or total soil skeleton yields resisting stresses to pore
fluid flow, so far only two flume tests (Clucky et al., 1985; Foda & Tzang,
1994) had demonstrated such wave-induced seabed fluidization exclusively in
silty deposits. In particular, Foda & Tzang reported that their pore
pressure measurements in the test silty bed consist of three typical
wave-induced soil responses including one unfluidized (or non-fluidized) and two
fluidized responses. The initial fluidization was induced by a transient
resonance mechanism and over the fluidized beds significantly increasing
sediment suspensions were always observed, which phenomena were almost not seen
over a sandy bed. On the other hand, many field studies in different coastal
environments, e.g. Hanes (1990), Conley & Inman (1992), Ifuku (1988) and
Chang & Hwang (1995), reported intermittent or drastic sediment suspensions
over near-shore sandy beaches. Above findings in both laboratory and fields
imply that in the marine environment subsoil fluidization seems related to the
near above-bed sediment suspensions while fine-grained non-cohesive seabeds are
prone to fluidized responses under wave actions. Thus, this paper is aimed at
carrying out further wave flume tests on a fine sandy bed for comparing the
resulting pore pressure responses with those of Foda & Tzang (1994) and for
clarifying correlations between seabed’s fluidized soil responses and sediment
suspensions over a field sandy beach.
Fig. 1 shows the experimental setups consisting of a wave flume (37m (L) × 1.2m (H) × 1m (W)) equipped with a piston-type wave generator on one end and a 1:4 dissipating gravel beach on the other end. The flume bottom was lifted 50 cm high through an upstream 1:10 slope so as to accommodate a soil trench for preparing the test sands. The water level was set constantly at 50 cm and several pore pressure transducers were deployed at different depths in the middle site of the soil trench. Fig. 2 displays the particle size distribution curves of the commercial test sands adopted in this study as well as the one adopted by Foda & Tzang for comparison. From the sieve analysis data, the derived d50 of the present test soil is about 0.085 mm and is classified as a fine sandy soil while the test silt of Foda & Tzang has a d50 of about 0.05 mm. Derived soil properties for present sandy soil are summarized in Table 1. In general, experimental measuring system consists of one capacity wave gauge and seven pore pressure transducers (Kyowa, BP-500 GRS) with one set near above the sandy bed and the other six at 3, 6, 10, 15, 30 and 45 mm below the bed, respectively. Pore pressure signals are amplified through an amplifier (Kyowa, DPM-8K) and are acquired together with the wave signals by a data acquisition software DASYLab.
Test sands are first well mixed with water in a mixing tank over the flume and then displaced unto the soil trench. The same procedures for preparing the test sands continue for three to four times until some excessive soil-water volumes are kept above the trench for assuring enough sand deposits within the test trench. After being left for one day to settle down without disturbance, the excessive sand deposits are removed from the soil trench to keep the bed surface flush with the flume bottom. The flume is subsequently filled with water until the designated depth of 50 cm and left for two more days before the initial wave generations to allow the test sands to further consolidate. For each run, information on voltage and period are first transmitted from a HP7957S main system and through HP3852A control unit to wave maker to generate waves in the flume for about 200 seconds and measuring data are acquired with a frequency of 20 Hz. Then, the wave generator is terminated and the loaded soils again are left to consolidate before it is to be re-loaded with waves after hours to days for another run. Usually for each preparation of test sand deposits, several runs of wave generation and termination are carried out and test sands are then retrieved out of the trench for preparing another new test. Design conditions of the selected test runs for analysis in this paper are summarized in Table 2.
Among the rather diverse measured pore pressure measurements, Fig.3 shows three records at a depth of 0.3 m categorized two tests displaying fluidized responses are selected in this study according to whether there is any mean pore pressure build-ups at each measuring depth and the occurrence sequence of fluidized response. Fig. 3a illustrates the unfluidized response of the sandy bed since there is no occurrence of mean pore pressure build-up with rather uniform pressure oscillations as the overloading waves. Fig. 3b illustrates the initial fluidized response with a comparatively large mean pore pressure build-up representing the first fluidized response among the runs of the same test. Fig. 3c illustrates the ensuing fluidized response with almost similar magnitudes of maximum mean pore pressure build-up as those of the initial fluidized runs (Fig. 3b). For those in the following runs of the same test with similar fluidized response are all classified into this category. However, differences of the two fluidized responses shown n Fig. 3b and 3c are not distinct but mainly consist in evolutions of magnitudes of both of the pore pressure oscillations and the mean pore pressure in the fluidized phase. For comparisons with silty bed’s responses to wave loading, the three characteristic soil responses summarized by Foda & Tzang (1994) are shown in Fig. 4, defined as unfluidized, resonant-fluidized and non-resonant fluidized, respectively. The classifications are based on magnitude of pore pressure build-up at a designated depth and the occurrence of the resonance mechanism, which is exclusively associated with the first run of fluidized response.
From both pore pressure measurements shown in Fig. 3 and 4, it is noted that soil responses in present test sands are all slightly different from those of the silty bed. The unfluidized responses of the sandy bed are not accompanied with a typical mean pore pressure build-up implying no occurrence of a thin fluidized surface layer in the sandy bed under any wave loading (Tzang, 1998). Fig. 3b demonstrates that the initial fluidized response in sandy bed begins to occur after being subject to only several wave loadings (t~20 to 30 sec). The transient stage towards a post fluidized stage with a steady mean value also lasts for several wave cycles (t~ 30 to 40 sec) while the pore pressure oscillations in the post fluidized stage are amplified gradually and then decrease to become uniform. Disregarding the surface layer fluidization, the pre-fluidized and post-fluidized stages of the pore pressure response are quite similar in amplitude with those of resonant fluidized of the silty bed. However in the transient stage, pore pressure records of the sandy bed lack evident resonance mechanism in the silty bed as identified in Fig. 4b. In the ensuing fluidized response, Fig.3c illustrates the pore pressure in the post-fluidized stage oscillates around a decreasing mean value, similar trends are usually found in the measurements at deeper depths or in later fluidized runs. On the other hand results with sustaining mean pore pressure value can be found at shallower depths and in the earlier fluidized runs.
Before analyzing the fluidized response of the adopted sandy soil under wave actions, it is worthwhile of studying its behaviors in unfluidized status for confirming soil properties. Applying soil properties listed in Table 1, comparison of depth profile of the pore pressure amplitude in Fig. 5 demonstrates that measurements are in reasonable agreement with the poro-elasticity theory (Mei & Foda, 1983). Along with previous studies, the results again confirm that sandy soil’s response under simple harmonic wave actions could well be described as poro-elastic For a porous soil, the static stresses equilibrium in soil mechanics state that any pore pressure build-up could reduce soil’s effective stresses as some portion of solid skeleton lose their inter-granular contacts and to be born by pore fluid. Thus, the maximum pore pressure build-up at a depth d below bed surface shall be equal to soil’s static stresses as expressed by:
(1)
where n is the soil porosity, g
is the gravity acceleration,
are densities of solid soil
particles and pore water, respectively. By applying soil properties in Table 1,
theoretical values of DPs
at given depths d
are derived and listed in Table 3.
For evaluating soil’s fluidized responses, mean pore pressure data are first calculated by using a Linear Moving Average Scheme (LMAS) adopted by Foda & Tzang (1994). As a result, Figure 6 shows the calculated variations of the mean pore pressure build-ups with time at four depths for the three typical responses shown in Figure 3. From Figure 6a, it is noted that mean pore pressure build-ups in unfluidized soil response are essentially very small through the whole soil trench suggesting even unlikely internal sediment suspensions to play roles in inducing critical piping pore flows (Tzang, 1998). Contrarily, Figure 6b displays that in the initial fluidized response mean pore pressures build up to attain to a stationary maximum value at each depth, which increases with depth while the build-ups initiate and attain to maximum values at almost the same time at the four depths. Referring to theoretical static soil stresses in Table 3, the values of DPs are about 800, 1200, 2400, and 3600 N/m2 at depth of 10, 15, 30, and 45 cm, respectively. The corresponding calculated maximum mean values in the beginning stage of fluidization are about 540, 970, 2030 and 3252 N/m2 representing 70% to 90 % of soil fluidization at those depths, respectively. Therefore, it is clearly verified that in the initial fluidized response the whole layers of the sandy bed are mostly fluidized. The further increase at depth of 10 cm after t~120 sec is considered to be due to settling of the pore pressure transducer in the fluidized soil-water mixture under continuous wave actions. On the other hand, a mild declining mean pore pressure trend is recognized in the same run from data at depth of 45 cm implying that the sands in that deep part has gradually redeveloped their resisting strength. Such declining trend becomes steeper from data at deeper measuring locations in the ensuing fluidized responses as typically illustrated in Figure 5c.
It can be clearly pointed out in Figure 6c for the ensuing fluidized responses that the data at depth of 45 cm display a maximum build-up value of about 2270 N/m2, which is much less than the maximum value in the previous initial fluidized response. Meanwhile, the curves at 30 cm also display a almost identical declining trend with that at depth of 45 cm after reaching a maximum mean pore pressure, which is around 2000 N/m2 and is only slightly less than that in previously initial fluidized response. In this test run, the sandy soils in the other two shallower depths sustain the same maximum mean values through the wave generation durations implying continuous fluidized status at those layers. From other data of later fluidized responses, the declining trend might spread to shallower depths after more episodes of wave loading and consolidating and even become unfluidized again. The redevelopment of soil strengths is also found in the silt tests by Foda & Tzang (1994) but generally there are fewer runs of ensuing fluidized response in present sand tests suggesting more efficient stress dissipating mechanism is associated with the sandy soils.
Pore pressure build-ups of fluidized responses at the four depths from another set data of Test C are also summarized in Table 4. Similarly, the attained soil fluidization could reach to about 90 % of the theoretical values while the declining trend of the mean pore pressure build-up is more significant at depth of 30cm and spreads to shallower depths in the ensuing fluidized response. As a result, the typical fluidized responses of the sandy soil are in some aspects different from those of the silty soil, especially on the resonance mechanism. Thus, the initiation of soil fluidization in present sandy bed is essentially critical and the solution finding requires more laboring tasks to be carried out. From comparisons with the study by Foda & Tzang, the facts of quite similar wave conditions and physical dimensions of the soil trench in both experimental studies, however, do suggest soil properties associated with represented particle size distributions should play an important role in the resulting bed soil’s fluidized responses under wave actions. The study is undergoing and will be reported in other paper.
In the experimental tests on sandy soil under wave actions, it is constantly visually observed that sediments vividly suspend over the sandy bed in fluidized response. Not evident sediment suspensions could be identified over an unfluidized sandy bed. The suspension events used to start to occur at sparse sites and spread over the entire bed surface but limited in the water column near the bottom. Typically, the sandy bed forms small-scale ripples after only few runs of wave loading in unfluidized test runs but keeps flat after fluidized runs. Similar to the observations in silt tests over fluidized beds, the sediment suspensions also result in the loss of the soil volume in the trench to decrease in thickness by an order of within few centimeters. In spite of some different mechanism for soil fluidization in a sandy bed, the similar observations of sediment suspensions over a fluidized bed strongly support that sediment suspensions in the filed is not solely due to bed forms as proposed from traditional points of views but contributions from the bed stability are expected to be an optional key factor.
From among a series of flume tests on wave-induced pore pressure responses in a sandy bed, two tests consisting of fluidized responses are selected for study and compared with previous fluidized responses of a silty bed as reported by Foda & Tzang. Another test with unfluidized runs are first analyzed to demonstrate the poro-elastic behaviors of the adopted sand without any evident pore pressure build-ups under simple wave loading. From the pore pressure measurements, the fluidized responses consisting of significant pore pressure build-ups during wave loading through most of the soil layers inside the test soil trench can be categorized as initial fluidized and ensuing fluidized responses. The former stands for those first runs with occurrences of soil fluidization in each test and the later for those following runs with fluidized soil responses but usually with decreasing maximum pore pressure build-ups in the deeper soil layers as the wave loading events increase. The comparisons with silty bed’s fluidized responses illustrate that the initial fluidized response of the fine sandy bed consists of neither fluidized surface soil layer nor evident resonance mechanism but pore pressure build up almost simultaneously at whole layers of soils. The ensuing fluidized responses display more rapid mean pore- pressure declining trend from the deeper layers and used to last for few runs until unfluidized responses occur. This implies that more efficient dissipating mechanism is associated with the sandy bed. In spite of the differences in pore pressure responses for both silty and sandy beds, significant sediment suspensions over fluidized soil beds and the resulting flat bed forms and losses of soil thickness are quite similar and again confirm the linkage of near above sediment suspension and underneath bed stability.
Acknowledgements
The financial support from the National Science Council under Grant No. NSC 88 – 2611 – E – 019 – 026 is much appreciated. The authors are also very grateful to Mr. Mei-Kuan Su and Mr. Chih-Ming Wang at National Cheng-Kung University for their help on experimental work and data processing.
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Fig.1 Experimental setups of the wave flume and soil french
Fig. 2 Particle sizes distributions of fine sands and the silt tested by Foda & Tzang (1994)
(a)
(b)
(c)
Fig. 3 Three typical pore pressure measurements of the fine sandy beds representing (a) unfluidized, (b) initial fluidized and (c) ensuring fluidized soil responses
(a)
(b)
(c)
Fig. 4 Three typical pore pressure measurements of the silty bed representing (a) unfluidized ,(b) resonant fluidized and (c) non-resonant fluidized soil responses (from Foda & Tzang, 1994)
Fig. 5 Comparison of depth profiles of pore pressure measurements in the fine sandy beds and poro-elasticity theory (Mei & Foda, 1981)
(a)
(b)
(c)
Fig.
6 Mean pore pressure variations at four depths of the fine
sandy beds for
(a) unfluidized (b) initial fluidized and (c) ensuring
fluidized soil responses