海床CPT和井下CPT在砂土中贯入机理差异研究

海洋石油建设中的一个关键环节是对有关海域的工程地质条件作出准确评价。海上原位静力触探(CPT)是主要的原位勘察手段,与陆地CPT不同,其贯入方式主要有Seabed和Downhole两种形式。这两种CPT由于贯入方式不同,所得到的锥端阻力存在一定的差异。采用有限元方法对这两种形式的CPT锥端阻力和贯入模式进行计算分析。研究显示,CPT入土过程中,周围土体存在两种不同位移状态,即滑动状态和排挤状态。在入土初期,土体以滑动状态为主,土体中竖向应力随贯入深度的增加而递增。在入土一定深度以后,土体的排挤状态占主导地位,竖向应力变化趋于平稳。这两种位移状态的相互转化解释了Seabed CPT和Downhole CPT实测数据之间的差异。在贯入深度较大时,Seabed CPT受排挤状态控制,而Downhole CPT仍然受滑动状态的影响。

Study on the penetration mechanism of Seabed CPT and Downhole CPT

The in situ CPT in offshore engineering has two forms: Seabed and Downhole. Due to the difference of construction technology using different methods, there is certain difference of cone tip resistance. Using the finite element method, the cone tip resistance and penetration mode of two different forms of CPT are analyzed. The results show that during the penetration process, the soil around has two different displacement modes, the rotation mode and the compaction mode. In the initial stage of penetration, rotation mode dominates soil behavior, and the soil vertical stress increases with the depth of penetration. After drill pipe penetrates to a certain depth, compaction mode becomes dominant, and the change of vertical stress with depth tends to be steady. The conversion of two kinds of mode explains the difference of measured data between Seabed CPT and Downhole CPT. When the penetration is relatively large, the Seabed CPT is controlled by the compacted state, while the Downhole CPT is still affected by the rotated state.