A Field Method for Determining the Firmness of Colonized Sediment Substrates

Abstract

ABSTRACT Substrate firmness influences the erodibility, remobilization, and topographic expression of that substrate. Sediment distribution patterns, remobilization of sediment, and the architecture of biogenic sedimentary structures are strongly affected by the firmness and cohesiveness of the sediment. Given the potentially important role sediment firmness plays in different depositional settings it is important to have a consistent means of evaluating it. This paper demonstrates that a modified metallurgical technique, the Brinell hardness test, can be used to produce accurate and consistent firmness data in modern depositional settings. In this method a glass or metal sphere (the indentor) is dropped from a fixed height into a cohesive medium; the size of the indent produced is inversely proportional to the firmness of the media. Firmness values can be reported as a pressure exerted by the substrate (kPa). This method has some advantages over standard penetrometers, such as: ease of use, portability and simplicity of equipment, testing a large area, and flexibility of calculation. Field tests show that this method is accurate if the indentation diameter is between 10% and 80% of the indentor diameter. The method is inappropriate for dry, unconsolidated sand and thixotropic mud. It is, however, extremely useful for assessing the firmness of a wide range of soft to firmground sediments that are composed of clay through coarse sand.