Discussion on storm-induced liquefaction of the superficial stratum in the Yellow River subaqueous delta

Geological disasters on the superficial seafloor were revealed in geological investigation on the Yellow River subaqueous delta. Combined with dynamic triaxial tests and wave flume experiments, occurring conditions and forming patterns of liquefaction as well as motion of the liquefied soil were explained in this paper. Based on the viewpoint that the geological disasters were formed due to silty soil liquefaction under storm waves, re-stratification issue of the superficial stratum was analyzed. Movement of the liquefied soil agreed with the wave, leading to differentiation of the soil particles. Research issues in respect of geological, environment and engineering of storm-induced liquefaction were also discussed.

The characteristics of consolidation settlement and its contribution to the topographical change in the northern modern Huanghe River subaqueous delta in China

The sediments of the modern Huanghe River subaqueous delta are easily to generate settlement and lead to topography change which is due to fast deposition rate, high void ratio, moisture content and compressibility. The sediment consolidation settlements and its contribution to the topography change in the northern modern Huanghe River subaqueous delta are studied based on drilling data, laboratory experiment results, and water depth measurements of different time. The results show that the final consolidation settlement of drill holes in the study area is between 1.17 and 3.21 m, and mean settlement of unit depth is between 2.30 and 5.30 cm/m based on the one-dimensional consolidation theory and Plaxis numerical model. The final consolidation settlement obtained by Plaxis numerical model is smaller than that obtained by the one-dimensional consolidation theory, and the difference is 3.4%-39.9% between the methods. The contribution of the consolidation settlement to the topographical change is at 20.2%-86.6%, and the study area can be divided into five different regions based on different contribution rates. In the erosion area, the actual erosion depth caused by hydrodynamics is lower than the changes of measured water depth, however, the actual deposition amount caused by hydrodynamics is much larger than the changes of water depth obtained by measured data in the equilibrium and deposition areas.

Dynamics of Bottom Boundary Layers in the Yellow River Subaqueous Delta Based on Long-Term In-Situ Observations

Objective In geo-marine science,the generalized bottom boundary layer（BBL）represents a layer between sediments and seawater.The BBL plays an important role in geological,geobiochemical,geophysical and geotechnical research because it is the connection region of hydrosphere,

Inversion and Prediction of Consolidation Settlement Characteristics of the Fluvial Sediments Based on Void Ratio Variation in the Northern Modern Yellow River Subaqueous Delta, China

The modern Yellow River delta is formed near the estuary of the Yellow River with the characteristics of short formation time, efficient sedimentation rate and loose structure which make sediments prone to be compacted and consolidate under the geostatic stress and overburden stress. It is one of the key areas with land subsidence disasters in China, bringing a series of safety hazards to production and living. Based on the data of massive surface cores and ten drill holes ranging from 12 to 40 m obtained from the northern modern Yellow River subaqueous delta, the inversion method suitable for the calculation of consolidation settlement characteristics of the modern Yellow River subaqueous delta is discussed, and the consolidation settlement characteristics of the delta sediments are inversed and predicted in this paper. The actual void ratio of the delta sediments at the depth from 3 to 15 m shows a significant power function relationship with the depth, while the void ratio of the sediments below 15 m changes little with depth. The pre-consolidation settlement(from deposition to sampling) of the delta sediments is between 0.91 and 1.96 m, while the consolidation settlement of unit depth is between 9.6 and 14.0 cm m^(-1). The post-consolidation settlement(from sampling to stable) of the subaqueous delta sediments is between 0.65 and 1.56 m in the later stage, and the consolidation settlement of unit depth is between 7.6 and 13.1 cm m^(-1) under the overburden stress. The delta sediments with a buried depth of 3 to 7 m contribute the most to the possible consolidation settlement in the later stage.

Sediment consolidation settlement of Chengbei Sea area in the northern Huanghe River subaqueous delta,China

One of the most important factors controlling the morphology of the modem Huanghe （Yellow） River delta is consolidation settlement, which is impacted by fast deposition, high water content, and low density of seafloor sediment. Consolidation settlement of the Huanghe River subaqueous delta was studied based on field data, laboratory experiments on 12 drill holes, and the one-dimensional consolidation theory. Results show that vertical sediment characteristics varied greatly in the rapidly forming sedimentary bodies of the modem Huanghe River subaqueous delta. Sediments in the upper parts of drill holes were coarser than those in the deeper parts, and other physical and mechanical properties changed accordingly. On the basis of the one-dimensional consolidation theory and drilling depth, the final consolidation settlement of drill holes was between 0.6 m and 2.8 m, and the mean settlement of unit depth was at 1.5-3.5 cm/m. It takes about 15-20 years for the consolidation degree to reach 90% and the average sedimentation rate within the overlying 50 m strata was at 5 cm/a to 12 cm/a. This study helps to forecast the final consolidation settlement and settlement rate of the modem Huanghe River subaqueous delta, which provides key geotechnical information for marine engineers.

Sedimentary facies of the subaqueous Changjiang River delta since the late Pleistocene

The sedimentary facies of the subaqueous Changjiang （Yangtze） River delta since the late Pleistocene was studied based on lithology and foraminifera analysis for two boreholes, CJK07 and CJK11, along with 14C dating. Four sedimentary facies were identified, namely fluvial, tidal flat, offshore, and prodelta facies. The fluvial sedimentary facies is comprised of fluvial channel lag deposits, fluvial point bar deposits, and floodplain deposits, showing a fining-upward sequence in general with no benthic foraminifera. A layer of stiff clay overlies the fluvial deposits in core CJK07, indicating a long-term exposure environment during the Last Glacial Maximum （LGM）. During the postglacial sea-level rise around 13-7.5 cal ka BP, the tidal flat facies was deposited in core CJK11, characterized by abundant silt-clay couplets. Euryhaline species dominate the subtidal fiat foraminiferal assemblages, while almost no foraminifera was found in the intertidal fiat. The offshore environment was the major sedimentary environment when the sea level reached its highest level around 7.5 cal ka BP, with a maximum accumulation rate of 10 mm/a found in core CJK11. Prodelta sediments have been deposited in core CJK11 since -3 cal ka BP, after the formation of the Changjiang River delta. The difference in sedimentary facies between core CJK07 and CJK11 is due to their location： core CJK07 was in an interfluve while core CJK11 was in an incised valley during the LGM. Furthermore, AMS 14C dating of core CJK07 shows poor chronological order, indicating that the sediments were reworked by strong tidal currents and that sediment deposited since -7.7 cal ka BP in core CJK07 was eroded away by modem hydrodynamic forces caused by the southward shift of the Changjiang River delta depocenter.

In Situ Observation of Silt Seabed Pore Pressure Response to Waves in the Subaqueous Yellow River Delta

The in situ pore pressure response of silt under wave action is a complex process.However,this process has not been well studied because of limited field observation techniques.The dynamic response process is closely related to engineering geological hazards;thus,this process must be urgently explored.A long-term in situ observational study of the silt sediment pore water pressure response process under wave action was conducted in the subaqueous Yellow River Delta.The response characteristics of pore water pressure are affected by tidal level and wave height.Tidal level affects the overall trend of the pore water pressure response,while wave height influences the amplitude of the pore water pressure response.This study revealed a significant lag effect in the pore pressure response.The transient pore pressure in the seabed did not respond immediately to the wave-induced pressure stress on the seabed surface.This phenomenon may be attributed to the change in soil permeability.The maximum response depth was approximately 0.5 m with a 2 m wave height.A concept model of silt soil pore pressure response under different types of wave action was developed.The accumulation rate of the pore pressure is less than the dissipation rate;thus,the developed model highlights the oscillation pore pres-sure response mechanism.The highlighted response process is of considerable importance to transient liquefaction and the startup process of pore pressure response.