Han Eng Low

Recommended Practice for Full-Flow Penetrometer Testing and Analysis

The increasing use of full-flow penetrometers for estimating the undrained and remolded shear strength as well as soil sensitivity of soft sediments by both industry and researchers has resulted in a rather rapid maturation of this new in situ test method over the past decade. Experimental, analytical, and numerical analysis results for full-flow penetrometers are now sufficient to provide recommended practices regarding equipment design, testing procedures, and data analysis. Equipment design must consider both physical attributes of the penetrometers and electronic design and data acquisition. The testing procedures presented are modified from standard test methods for the piezocone with additions for evaluation of remolded strength by cycling and rate effects through variable penetration rate testing. Data reduction includes methods for normalizing the penetration resistance data for comparison between test sites and depths and methods for estimating undrained and remolded shear strength as well as soil sensitivity. All recommendations are summarized in a guidance table.

Strength Measurement for Near-Seabed Surface Soft Soil Using Manually Operated Miniature Full-Flow Penetrometer

A manually operated penetrometer (DMS) fitted with cylindrical (T-bar) and ball penetrometer tips was developed for measuring the profiles of undisturbed and remolded undrained shear strength within box-core samples. This paper summarizes the findings of a series of miniature penetrometer tests and vane shear tests that were carried out on reconstituted clay from a local site in Western Australia. The aim of the tests was to evaluate the potential of the DMS in characterizing the shear strength of seabed surficial sediments. It was found that the DMS gave essentially identical T-bar and ball penetration resistances but these were up to 17% lower than the net cone resistance. From the comparison between the T-bar and ball penetration resistance and the shear strengths measured from vane shear tests, average N factors of 11 and 14 were obtained for intact and fully remolded conditions, respectively. The test results suggest that the DMS is a reliable and efficient means of obtaining intact and remolded shear strength profiles.

Changes in Pipeline Embedment due to Sediment Mobility: Observations and Implications for Design

This paper describes temporal variations in embedment of several existing pipelines on the North-West Shelf (NWS) of Australia, and the sediment mobility processes that cause them. Distinct and explainable patterns in the extent, distribution and rate of the development of pipeline embedment have been revealed through systematic detailed examination of repeated annual integrity surveys by ROV. This represents a unique data-set that has been used to optimize the reliability of a newly designed pipeline. This paper explains why these clear findings should not be overlooked in both the buckling and stability design of initially unburied pipelines, which is in contrast to currently established industry practice. This new information supports the presumption that conventional approaches for calculating the hydrodynamic stability of unburied pipelines may be more conservative than necessary. Conversely, and arguably more importantly, it is shown that conventionally accepted methods for calculating pipe-seabed resistance forces when planning buckling schemes should be considered unsafe if embedment due to sediment mobility is possible. Consequently, this paper proposes an innovative calculation methodology that statistically captures these sediment mobility effects, and which facilitates a more justifiable geotechnical input to pipeline engineering than what is conventionally adopted. This methodology is currently being used by the authors as a state-of-the-art design practice for unburied offshore pipelines in regions of sediment mobility.© 2013 ASME

Characterization of the solid-fluid transition of fine-grained sediments

Characterization of the strength of fine-grained sediments as they evolve from an intact seabed material to a remolded debris flow is essential to adequately model submarine landslides and their impact on pipelines and other seabed infrastructure. In the current literature, two distinct approaches for modelling this material behavior have been considered. In the soil mechanics approach, fine-grained soils are characterized by the undrained shear strength, su . The critical state framework proposes a relation between su and the water content, or void ratio of the soil. In addition, rate effects and strain softening effects are described by multiplying a reference value of su by a function of the shear strain rate or the accumulated shear strain respectively. In the fluid mechanics approach, slurries of fine-grained material are characterized by a yield strength and a viscosity parameter, which describes the change in shear stress with shear strain rate. Empirical relationships have been proposed, which relate the yield strength and the viscosity to the sediment concentration. This paper demonstrates that the two modelling approaches are essentially similar, with only some formal differences. It is proposed that the strength of fine-grained sediments can be modelled in a unified way over the solid and liquid ranges. To support this unified approach, an experimental campaign has been conducted to obtain strength measurements on various clays prepared at different water content. The testing program includes fall cone tests, vane shear tests, miniature penetrometers (T-bar and ball) and viscometer tests. Rate effects and remolding effects are investigated over a wide range of water contents spanning the domains of behavior that are usually defined separately as soil and fluid. The present paper focuses on analyzing the results of fall cone, vane shear and viscometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content — suitably normalized — for a given soil. Furthermore, the effect of strain rate and degree of remolding can be accounted for by multiplying the basic strength parameter by appropriate functions, which are independent of the current water content.Copyright

The Use of Centrifuge Model Testing to Provide Geotechnical Input Parameters for Pipeline Engineering

Pipe-soil interaction testing in geotechnical centrifuges is used as a means of providing project-specific information to support the assessment of ‘friction factors’ and other geotechnical inputs to pipeline engineering. The centrifuge testing method allows moderate-sized soil samples (−0.01–1 m3) taken from the field to be used directly to determine site-specific behavior. The tests might involve simple uplift resistance testing for buried, backfilled pipelines or complicated installation and loading sequences designed to mimic the complex laying and loading histories relevant to laterally-buckling or storm loading of unburied pipelines. The paper explains briefly the principles behind centrifuge modeling and describes more fully how such testing should be used to gain benefit for a project.© 2013 ASME