黄河三角洲沉积物的自然固结压实过程及其影响

Natural Consolidation of Deposits and Its Consequences at the Yellow River Delta

对黄河三角洲及下伏沉积物的压实量和压实过程及其引起的地面沉降进行了分析研究。分析认为 ,在厚度为 15m的新淤积的黄河三角洲沉积重压下 ,前三角洲粘土和下伏海相粘土总固结量可达约 1.3 5m ,海湾或烂泥湾及前三角洲和海相粘土总压缩量为 1.48m。这些粘土层在亚三角洲建设时期就开始受压固结 ,但是在亚三角洲废弃后 ,仍要经过几十年自然固结才会基本完成。除去前 10年的固结量 ,前三角洲粘土与下伏海相粘土叠加形成的 14m厚粘土在随后的 2 7年内可能会压缩 5 0cm ;而海湾或烂泥湾及前三角洲和海相粘土叠加成的 2 0m厚粘土在 5 0年内可能会压缩 86cm ,两者的年平均压缩速率都超过 17mm/a。固结量和速率与上负沉积物重量成正比 ,而亚三角洲沉积厚度从沉积中心向外围逐渐减少 ,所以下伏粘土固结量和速率也向周围逐渐降低。但从亚三角洲沉积厚度分布情况看 ,在大范围内 ,下伏粘土固结引起的地面沉降在几十年内都将比较明显 ,从而可能加重三角洲地区风暴潮和海岸侵蚀灾害 。

The causes for the disasters in the highly developed deltaic areas have been a main study topic for many years. Using data of depositional structure, grain size composition, dry bulk density and other engineering properties of the deposits in the Yellow River delta and under the seabed, this study made an analysis of potential land subsidence due to post-depositional sediment consolidation. The settlement was estimated in the light of some basic equations of soil mechanics and empirical relations between dry sediment bulk density and burial depth. The degree of consolidation was calculated based on the theory of Terzaghi′s one-dimensional consolidation. It is revealed that consolidation of silt layers, which are the main component of the Yellow River delta, may finish in a short time, but consolidation of clay layers under the silt layers may last for decades and the total settlement, which mainly comes from the clay layers at the top of the seabed, seems to be considerable. The clay layers may reach 14 m in thickness where the prodelta clay layers stack on the marine ones and about 20 m in thickness where clay layers consist of those in the bay and prodelta and under the seabed. The results disclose that the total settlements due to consolidation of clay layers of 14 m and 20 m in thickness under 15 m deltaic deposits may reach about 1.35 m and 1.5 m, respectively. Over about 10 years, which is the mean lifetime of the deltaic lobes of the delta, about 37% of settlement for the 20 m thick clay layers and 60% of settlement for the 14 m thick clay layers may be completed. The settlement of the 14 m thick clay layers is about 50 cm over the next 27 years and it is about 86 cm over the next 50 years for the 20 m thick clay layers. The average rates of settlement for both cases are over 17 mm/a. The settlement and its rate at a location should approximate to the products of maximums multiplied by the ratio of the thickness of deposits in the lobe at the location to 15 m, so the total sediment settlement and rate tend to decline with the distance from the depocenter of an abandoned lobe. But, the land subsidence due to sediment consolidation should be noticeable in a large area along the coast during decades after a deltaic lobe was abandoned in the light of the isopach maps of lobes of the delta. Land subsidence will result in inland invasion of storm surge and land losses and will lower the standard of projects for preventing storm surge floods and coastal erosion, so the natural sediment consolidation must be taken into account in designing projects in the areas where it is still active.