Quality Assessment of Marine Sediments Recovered With a Hydraulically Tethered Piston Sampler
Authors: Kelleher Patrick Joseph , Hull Tim 
Published: October 2008

Piston corers are commonly used for the recovery of unconsolidated marine sediment samples. Compared to straight push tube samplers, they offer an improved potential to recover higher quality, longer core samples, particularly where coarse-grained marine sediments are being investigated.

There are a number of different types of piston corers deployed in geotechnical site investigations today - ranging from traditional free fall, single stroke, mechanically triggered devices to much more sophisticated, seabed-deployed, or alternatively down-hole vessel-deployed, devices.

The Hydraulically Tethered Piston Corer (HTPC) described in this paper is deployed down a borehole using a seabed founded, remotely operated drill rig. The design characteristics of this tool are described, together with the operating methodology. A number of studies, which assess the quality of the samples recovered using this tool are presented. The results demonstrate the high quality of the sample achieved with this device compared to other sampling techniques. Triaxial tests, undertaken to assess sample disturbance, indicate that the samples can be classified within the ‘very good to excellent’, ‘good to fair’ categories as defined by NGI (the Norwegian Geotechnical Institute). Thin section fabric analyses indicate that very little sample disturbance has occurred at the sample margin, and no disturbance has occurred elsewhere within the samples.

Recent literature has suggested that a larger sample diameter is perhaps the most critical factor in recovering undisturbed samples for geotechnical testing. However, the results presented herein demonstrate that the quality of the samples is perhaps more greatly influenced by careful consideration of tool geometry during tool design, together with precise control of the tool during the sampling process.

Introduction

Piston coring is a widely used methodology for reliably obtaining near continuous marine sediment samples in materials ranging from very soft fine grained sediments commonly encountered in deepwater sites and alluvial deltas, to the typically coarser grained sediments recovered from shallow water zones along continental shelves. Piston corers are differentiated from open-ended, push samplers by the inclusion of a tethered piston within the barrel. Both systems are equipped with a cutting shoe at the leading edge of the barrel and may be also equipped with a core catcher.

Continuous sampling to full barrel capacity is possible for these devices provided:The bearing capacity of the soil ahead of the cutting shoe exceeds the mobilized soil/liner interface friction capacity andSufficient thrust is available to overcome forces mobilized ahead of the cutting shoe, and friction/adhesion mobilized on the external surface of the barrel.

Piston corers usually provide improved recoveries compared to conventional, open-ended, push corers. In an ideal system, the piston imparts a controlled ‘suction’ pressure to the top of the soil sample, whenever flow of soil into the barrel is impeded due to excessive interface friction between the liner and the soil. This effect is particularly beneficial where dilatant, coarse-grained soils are being sampled.