Stress tests of ODP Site 131-808 sediments

Sediments undergoing accretion in trench-forearc systems are subjected to conditions of large lateral thrusting. This stress regime controls the mechanism of faulting as well as the yield and strength properties of the sediment. Understanding them is therefore crucial for the construction of quantitative models of sediment dynamics in convergent margin settings.
For this purpose triaxial and oedometer tests were performed on six whole-round core samples recovered from Site 808 from depths between 173 and 705 mbsf. Samples from five depth intervals were subjected to a triaxial test program that was primarily designed to define yield and strength behavior. Test specimens were cut parallel and normal to the core axis. Additional five oedometer tests with similarly prepared specimens were performed on samples from four depth intervals to evaluate the directional state and degree of sediment compaction.
Test results show that the degree of sediment compaction is higher than expected from overburden. This overcompaction increases with depth. A well-developed mechanical anisotropy is evident in all samples tested, regardless of their depth and lithology. Values of yield limit, stiffness, and shear strength are up to 40% higher in the horizontal direction compared to the vertical direction. In addition the test data demonstrate that the axis of the volumetric yield loci have rotated into extensional stress field. This verifies that the mechanical state of sediment in the accretionary wedge is controlled by in-situ stress conditions of extensional nature. The coefficients of lateral stress inferred suggest that the extensional stress regime becomes increasingly effective with depth.

沟弧前（trench-forearc）系统中正在经历增生作用的沉积物，会承受强烈的侧向挤压应力。该应力状态不仅控制着断层作用机制，同时也决定了沉积物的屈服与强度特性。因此，明晰这些特性对于构建板块会聚边缘（convergent margin）环境下的沉积物动力学定量模型至关重要。
为此，研究人员对从808站位（Site 808）获取的6件完整岩心样品开展了三轴试验（triaxial test）与固结仪试验（oedometer test），样品采集深度介于173至705米海底以下（meters below sea floor, mbsf）。其中，5个深度层位的样品被用于三轴试验方案，该方案的核心目标是明确沉积物的屈服与强度特性。试验试样分别沿岩心轴平行与垂直方向切割。此外，针对4个深度层位的样品，研究人员采用相同制样方式制备试样，开展了额外5组固结仪试验，以评估沉积物压实状态的方向性与压实程度。
试验结果表明，沉积物的压实程度高于上覆岩层荷载下的预期值，且该超压实程度随深度增加而加剧。所有受试样品均表现出显著的力学各向异性（mechanical anisotropy），且不受样品采集深度与岩性（lithology）的影响。与垂直方向相比，水平方向上的屈服极限、刚度与抗剪强度（shear strength）最高可高出40%。此外，试验数据显示，体积屈服轨迹（volumetric yield loci）的轴已旋转至伸展应力场（extensional stress field）中，这证实了增生楔（accretionary wedge）内沉积物的力学状态受具有伸展性质的原位应力（in-situ stress）条件控制。通过推导得到的侧应力系数表明，伸展应力状态随深度增加其作用愈发显著。