A STUDY ON WAVE-INDUCED LIQUEFACTION OF SANDY SEA BED AND SEDIMENT TRANSPORT

直接测量碎波带有效应力和孔隙水压力，研究碎波带有效应力和孔隙水压力与应力、波浪条件等因素的关系，并根据有效应力数据评价底高程的变化。通过对这一时期的实测数据进行仔细观察和分析，发现：(1)孔隙水压力和有效应力对水面高程的响应没有时滞。测量区域经常受到破碎波的冲击，预计那里的沙子会松动。这样，作用在底部的压力几乎同时传递到土内部的孔隙压力。当波陡较大的短波经过时，孔隙压力和有效应力均表现出较大的变化。(2)碎波带内的液化机理似乎与碎波带外的液化机理不同。波顶通过时的大有效应力降和向岸方向的强水流是碎波带泥沙运动的可能原因。(3)根据有效应力记录估算的水深变化与实测水深变化吻合较好。得出以下三点结论：（1）海床有效应力与砂土液化密切相关，进而与砂土运动密切相关; (ii)海底高程的变化可以由砂层中的有效应力来估计。(iii)有效的应力传感器被证明是有用的估计本地和时间输沙。(4)当海平面较低时，可以观察到流向近海的海流。有效应力的变化对应于这些电流。在低潮时，50分钟的平均有效应力急剧下降。较浅的水深可能是一个可能的原因，其确切原因仍在研究中。近岸的地下水位很可能受到长波和潮汐变化的影响。

The effective stress and pore pressure in the surf zone were directly measured to study the relation with the stress, and the wave condition and other factors, and to evaluate the bottom elevation change from the effective stress data. From the careful observation and analysis of the measured data during this period the followings are found.(1) The pore pressure and effective stress appears to respond to the water surface elevation without any time lag. The measured area is always attacked by breaking waves and the sand there is anticipated to be loose. Then, the pressure on the bottom transmits almost simultaneously to the pore presssure inside the soil. Both the pore pressure and effective stress show larger variation when the short waves with large wave steepness pass.(2) The mechanism of liquefaction in the surf zone seems to be different from that outside the surf zone. The large effective stress drop and strong flow in the onshore direction when the wave crest passes are the possible cause for the sand movement in the surf zone.(3) The water depth change estimated from the effective stress records agrees well with the measured depth change. This leads to the following three conclusions (i) The effective stress in sea bed is closely related to liquidization of sand and then to sand movement. (ii) The sea bottom elevation change can be evaluated from the effective stress in the sand. (iii) The effective stress sensor was proved to be useful in estimating local and temporal sediment transport.(4) The currents going offshore were observed when the sea level was low. The effective stress varied corresponding to these currents. The averaged effective stress over 50 minutes drops sharply at low tide. The shallower water depth may be a possible cause, and the exact reason for this is still being studied. The ground water level near the coast is likely to affected by the long waves and tidal changes.