A Field Method for Determining the Firmness of Colonized Sediment Substrates

Abstract

FIG. 1.-Test configuration of the modified Brinell firmness test. The parameters used to calculate the force exerted by the substrate are the diameter of the indentor (D), and the diameter of the impression (d).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.

Spatial, temporal and palaeoecological significance of exhumed firmgrounds and other associated substrate types in Netarts Bay, Oregon, USA

“…Sediment samples of 350–500 g were obtained from all substrate types to compare and contrast the different substrate types and for grain‐size analysis, if applicable. Substrate firmness measurements, using a modified Brinell firmness test as outlined in Gingras & Pemberton (), were conducted in the field on the various substrate types encountered in the study area. The diameter and mass of the ball used for the test were 2.5 cm and 68 g, respectively.…”