- The near-surface region of a coastal sediment bed is complex and dynamic. At some sites, near-surface sediment deposits are susceptible to extreme events, such as tsunamis or other large overflows, which induce high shear stresses on the sediment bed. The specific properties of sediment beds subjected to such extreme loading govern the response. Current sediment instability and transport models employ index characteristics of the sediment bed to predict the response; however, additional geotechnical factors could be considered in the models. The work herein aims to connect geotechnical and coastal engineering communities by characterizing near-surface beach sediments with geotechnical methods to inform more inclusive sediment instability and transport models as they continue to develop.
A sediment bed investigation is performed in South Beach, Newport, Oregon, which is susceptible to a sizable tsunami following the impending Cascadia Subduction Zone earthquake. A laboratory program is completed to determine the index properties and near-surface response trends of the sediment sampled from South Beach and to populate a database. More specifically, sieve analyses, particle shape analyses, specific gravity testing, minimum and maximum packing density testing, triaxial testing, simple shear testing, angle of repose testing, and inclined plane testing are performed and the methodology and results are discussed.
Low confinement strength testing is performed using a simple shear device, which is thought to roughly mimic the type of shearing caused by tsunami-induced bed-shear stresses. Angle of repose and inclined plane testing are also performed to observe the response of the exposed sediment at the surface. In addition, existing correlations are evaluated for South Beach sediment that relate index properties to the response (e.g., using particle roundness to estimate the critical state friction angle) to observe the accuracy of the correlations for the fine sand. The results show similar trends found by other researchers using various testing techniques; however, the simple shear device shows some weaknesses when performing low confinement tests.
A specific interest of the current study is understanding how the friction angle changes with decreasing confinement (i.e., as the in situ location of the deposit approaches the surface of the sediment bed) for the South Beach sand investigated. The friction angle is an important property that governs the response of sediment to various types of loading, including sediment exposed to bed-shear stresses.
The results show that the investigated cross section of South Beach appears to be a generally stable and homogeneous environment; therefore, the sediment properties and response trends defined herein are expected to be applicable to the specific location for the foreseeable future, notwithstanding significant morphological changes from impending natural disasters or human-induced changes. The results also show that established correlations that utilize particle shape to estimate limiting packing densities and friction angle are generally applicable for the South Beach sediment. Although there are challenges with performing low confinement simple shear testing, the friction angle near the surface of the respective sediment bed is found to increase following the comprehensive interpretation of the test results. In addition, the laboratory results show that at depths less than about 3 m below the surface of a simulated dry, medium density sediment bed, the critical state and peak friction angles increase, and may be well estimated by the angle of repose and pocket-friction angles at the surface.
The application of the sediment properties defined herein is focused towards sediment instability and transport models for the investigated location of South Beach; however, defining near-surface sediment properties and response trends is also important for many other engineering applications. Applications involving shallow soil response, including foundations, retaining structures, landslides, and lunar construction (where the ‘near-surface’ region would be a much larger zone due to the low stress environment of the moon), would be influenced by accounting for the effect of the altered loading response trends of soils near the surface. Considering the altered response trends (i.e., sediment strength properties) would improve the accuracy of analyses for the noted applications.
The results of the current study imply that considering near-surface sediment characteristics will lead to improved sediment instability and transport model predictions, as expected. The improved predictions will help researchers determine appropriate input parameters to model their application of interest and result in better infrastructure design when the response of the near-surface region to extreme overflow events is of concern. For similar granular soil studies, laboratory procedures are suggested following the evaluation of methods performed herein while developing the database of properties for South Beach sand.
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