Experimental Study On The Bearing Law Of Pile Foundation In Silty Seabed Due To Liquefaction

In recent years, under the construction of various offshore buildings, the pilefoundation has been applied more and more widely. Pile foundation has betteradaptability to geologic conditions and loading condition. Simple structure, smallsettlement and good stability are its advantages. Pile foundation is widely used for theconstruction of port and dock, cross-sea bridges and oil platform. But under extremeand bad weather such as storm tide, the buildings maybe incline or even collapse. Itcauses great financial losses and restricts development of economy in the coastal areas.Based on the requirement of real project，we conducted the research in this thesis.Based on the fact that silty seabed is easy to liquefy under wave action,in-lab flume experiments were carried out with silty soil taken from a harborbasin in the Yellow River delta. Under two conditions of disturbance and nodisturbance, we described the sporty character of silty soil seabed under waveaction and the change of the soil engineering properties. A primary research wasmade on the calculation method of dynamic pressure of liquefied soil and wecompared the result of theoretical calculation with the one of actual measurement.A study was made of the phase lag of wave-induced dynamic soil pressure indifferent soil layers.In the condition of seabed not disturbed, fine particles with small density onthe surface of the seabed entered into the water above, and sand vein was shapedon the surface of the seabed. With the increase of the wave height, the waterabove was more and more turbid, and the sand vein on the surface of the bed wasdeeper and deeper. After the disturbance of the bed, the soil body happened toliquefy under wave action and oscillated with it. As the time of wave action wenton, the sliding surface of the liquefied soil constantly expanded. Finally it was ina relatively stable state after it reached a certain position. With the increase of the wave height, the sliding surface expanded obviously in the horizontal direction,but not obviously in the vertical direction. With constantly oscillating sliding,intensity and density of the liquefied soil increase and the water content reduced.Meanwhile, the escape channel of the pore water became unobstructed, and thepore water with clay particles moved up by seepage, and then entered into thewater above. As a result, the liquefied soil coarsened gradually. Finally, a highconcentration sediment layer about1.5~2cm in thickness was founded near thebottom.Based on the theory of small amplitude wave and two layered wave systemmodel, the dynamic pressure of liquefied soil was calculated. Compared the data oftheoretical calculation and the one actual measurement, in general the latter is greaterthan the former. On the surface the relative error between theoretical calculation andactual measurement is small and the one at the layer-20cm is relative great. But atthe position-10cm the relative error between theoretical calculation and actualmeasurement is so great.The phase lag of liquefied soil pressure between different layers was researchedunder two conditions, disturbance and no disturbance of the bed. Under the conditionof no disturbance there was obvious phase lag between upper and lower layers. Withthe increase of the wave height, the phase lag decreased gradually. The reason is thatthe soil is not homogeneous, which mainly shows in saturation and permeability. Aconclusion can be draw if the soil is fully saturated and has the same permeability,there will not be phase lag among different layers.After the disturbance of the bed, the soil liquefied. At this time there were threeobviously different layers in the flume, which were suspended sand layer, liquefiedsoil layer and stable soil layer. The result of the experiment showed that there were nophase lags between suspended sand layer and liquefied soil layer and obvious phaselags between liquefied soil layer and stable soil layer. But in the liquefied soil layerthere was no phase lag. This maybe an evidence for that there is no phase lag inhomogeneous soil. The phase lag between liquefied soil layer and stable soil layer became smaller and smaller with the increase of wave height.