Laboratory development of a vertically oriented penetrometer for shallow seabed characterization

Abstract

Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1–2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin sample, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field.

Experiments Using a Novel Penetrometer to Assess Changing Strength of Clay during Remolding and Reconsolidation

“…The VOP (Fig. 1) is a cylindrical rod that is strain-gauged above the mudline to assess the lateral soil resistance on the buried part and therefore the soil strength based on the analysis of a laterally loaded pile as described in Sahdi et al (2016).…”

“…A total of 10 VOP tests were conducted, which involved insertion of the device (at 0.5 mm=s) to a depth, z T (distance from the mudline to the embedded tip of the VOP), followed by cycles of horizontal movement with a specified velocity, v, and horizontal amplitude, u (Table 2). These 10 VOP tests include 2 additional slow tests to those reported by Sahdi et al (2016), designated in Table 2 as VOP_NC0330 and VOP_NC045. These tests were conducted at v ¼ 0.3 mm=s and an embedment of 30 and 45 mm (model scale), respectively.…”

“…The lateral resistance during fast VOP horizontal translations, in which negligible pore pressure dissipation occurs was discussed by Sahdi et al (2016). The present paper reports the full range of tests and is focused on the response in the middle position of each cyclic range and the influence of consolidation within the cycles.…”

“…With further development, it could prove useful to examine the lateral strength heterogeneity over the zone of a planned foundation footprint and to supplement point measurements of strength profile from conventional penetrometers, for example along pipeline routes. The design and method of analysis of the VOP is described in more detail in Sahdi et al (2016).…”

Development of a novel lateral resistance measurement penetrometer for pipe–soil interaction centrifuge model test

“…The origin of LRMP is the vertically oriented penetrometer (VOP) which was designed to assess the shallow strength of soft seabed over continuous horizontal profiles, in which four pairs of strain gauges were mounted on a column for measuring the bending moments to infer the soil resistance. 14 The VOP is convenient to use for inferring the force magnitude and position of application, while it is not easy to determine the force orientation if an incident impact angle exists in practice. In this article, we develop a new device and propose that the LRMP can potentially be used as a means of determining the impact forces on offshore pipeline by submarine landslides.…”