Sediment identification using free fall penetrometer acceleration-time histories

Abstract

Knowledge of physical properties of near-surface sediments is an important requirement for many studies of the seafloor. Dynamic or Free Fall Penetrometers (FFP), instrumented with accelerometers, are widely used to assess the mechanical properties of the sediment by deriving penetration resistance from the deceleration response of the probe as it impacts and embeds the seabed. Other field investigations, a priori knowledge or a very basic description of the type of sediment (such as a description of the sediment as soft, medium or hard) derived from studying the deceleration response (accelerometer-time histories) are used for sediment identification prior to the application of an appropriate strength determination model. In many cases this information is site-specific and in others the penetration resistance is overestimated due to the dilatory effects observed in sediment with an undetected grain fraction. In this study variables affecting a dynamic penetrometer-sediment interaction system are identified. Using data from field investigations and literature we found a relationship among five variables: peak acceleration, embedment depth, total embedment time, velocity of impact and grain size. This is used to formulate a sediment identification model. The model accounts for variables that may vary widely within one deployment and it can be applied to other FFPs with different physical characteristics (such as a different mass or size). This may lead to the increased use of FFP as a deployment tool for rapid in situ characterization of the seafloor.

In situ dynamic piezocone penetrometer tests in natural clayey soils — a reappraisal of strain-rate corrections

“…Theoretical studies have shown the difficulties in correlating v 0 with the state of consolidation and z depth (e.g., Boguslavskii et al 1996). Similar to the results from Mulukutla et al (2011), our data show a direct correlation between v 0 and z depth (Fig. 12).…”

“…However, both models demonstrate that an increase in z depth correlates directly to a reducing state of consolidation or firmness factor (Fig. 12, Mulukutla et al 2011).…”

“…12). Mulukutla et al (2011) proposed a more elaborate sediment identification model for coarse silt to medium sand taking into account the normalized z depth and firmness factor consisting of the peak acceleration, acceleration due to gravity, total embedment duration, and v 0 . However, both models demonstrate that an increase in z depth correlates directly to a reducing state of consolidation or firmness factor (Fig.…”

Rapid sediment mapping and in situ geotechnical characterization in challenging aquatic areas

“…In Yakutat Bay, 50 deployments achieved an impact velocity within the range used by Mulukutla et al () for classification using FF. For the deployments with FF = 10.5–70 m −1 , the maximum deceleration ranged between 31.5 and 70 g .…”

“…They found that the presence of coarse‐grained sediments led to decelerations of more than 60 g , while decelerations of less than 20 g were measured in areas of soft clay. Mulukutla et al () proposed the Firmness Factor (FF) to characterize sediments based on penetrometer deceleration records: where a max is maximum acceleration, v i is impact velocity, and t p is the total penetration time. The minimum values of FF for fine sand and maximum for coarse silt ranged between 10.5 and 70 m −1 for an impact velocity between 4.86 and 9.16 m/s.…”