Laboratory Testing (emoe527) J. Antonio H. Carraro · 2019. 1. 24. · By Carraro, Boukpeti,...

Encyclopedia of Marine and Offshore Engineering – Wiley

Last Revision: 15/03/2017 Laboratory Testing By Carraro, Boukpeti, Guadalupe-Torres and Joer (2015)

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Laboratory Testing

(emoe527)

J. Antonio H. Carraro Department of Civil and Environmental Engineering Imperial College London Skempton Building London United Kingdom E-mail: [email protected] (formerly of Centre for Offshore Foundation Systems at UWA) Nathalie Boukpeti Centre for Offshore Foundation Systems The University of Western Australia Crawley Western Australia Australia Yaurel Guadalupe-Torres Centre for Offshore Foundation Systems The University of Western Australia Crawley Western Australia Australia Hackmet Joer Fugro AG Pty Ltd. Osborne Park Western Australia Australia

Keywords: offshore sediments, geotechnical laboratory test, element test, mechanical

response, fabric analysis, basic characterization



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Abstract

Offshore infrastructure is extremely costly to design and build. Advanced and

scientifically sound testing programmes can yield substantial savings to overall

infrastructure costs due to the improved reliability that results from better and more

accurate characterization of offshore sediments. Well-designed and carefully executed

laboratory tests are paramount to the successful design and construction of offshore

infrastructure. A variety of laboratory tests are available to the 21st century engineer to

characterize the physical properties and mechanical response of offshore sediments and

derive design parameters for offshore geomechanics analyses. To be competitive,

engineers must be equally familiar with cutting-edge testing tools as well as conventional

methods used in practice for decades. The most common and relevant laboratory tests

available to assess fundamental aspects related to the constituency and mechanical

behavior of offshore sediments are discussed. These include basic characterization, soil

fabric analyses and mechanical tests. Particular emphasis is placed on the underlying

background and rationale associated with each test discussed.



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1 INTRODUCTION

Geotechnical practices vary across the world as does the rationale behind them. In pre

1970s Siberia, where engineers dealt with permafrost, soil mechanics was based on creep.

As a result, centrifuge work there focused on the understanding and optimization of this

feature of soil behavior (Schofield 2005). Far west from the Ural Mountains, Schofield’s

soil mechanics was based on plasticity theory with time effects due to primary

consolidation, which “made centrifuge models look better” to him than to Siberian

engineers (Schofield 2005). This example illustrates that the same engineering approach

(centrifuge testing, in this case) can be employed to analyze different types of problems

in various parts of the world – possibly yielding outcomes with various levels of success.

Ability to analyse fundamental mechanisms and innovate are key aspects of sound

engineering approaches.

Laboratory testing of soils can also be designed and have its results interpreted in

various ways. Laboratory tests are typically designed to provide input for a given

engineering analysis. A variety of laboratory tests and devices exist at present. Therefore,

designers must be familiar with cutting-edge testing tools available for modern

geotechnical analyses as well as conventional methods used in practice for decades.

Ultimately, the engineer’s decision relies not only on the science underpinning the

analysis, but also on the resources and capabilities available. Rather than prescribing a

particular list of testing requirements, this chapter focuses on the fundamentals and

mechanics associated with testing protocols currently available so that engineers can

make informed decisions about which choices may best suit their analyses. Offshore

infrastructure is costly to design and build. Cutting-edge testing programmes at or above

the state-of-the-practice can therefore yield substantial savings to the overall cost of such

infrastructure. This can be the difference between a seemingly successful approach

(possibly relying on inadequate laboratory testing) with grave financial consequences to

the final project costs, and a successful offshore development.

1.1 Purpose and scope of laboratory testing



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The mechanical behavior of soils is a complex field of study – soil response to

loading is influenced by many factors. A complete assessment of the combined effect of

all possible factors affecting soil response is impractical. Therefore simplifying

assumptions are often made. Fortunately, soil mechanics has rigorous conceptual

frameworks (e.g. Schofield and Wroth 1968) that are invaluable to the inexperienced

engineer and critical to the practitioner who wants to analyze a soil mechanics problem

following a sound, scientific background. Rigorous analyses of soil behavior have shown

that the mechanical response of soils is fundamentally affected by state variables related

to the current soil state as well as intrinsic parameters associated with the soil inherent

constituency (Salgado 2008). Classical examples of soil state variables include stress and

density (or fabric, rigorously speaking). Intrinsic soil parameters relate to the soil’s

inherent nature and composition (e.g. specific gravity and carbonate content, which relate

to soil mineralogy).

A well-defined boundary between state variables and intrinsic parameters may not

always be easily established though. For example, carbonate soils have particles that may

break under typical stresses imposed by offshore structures. As a result, particle shapes

and sizes (thus fabric) for these soils can change significantly during loading (Coop

1990) ultimately changing the soil itself – e.g. a carbonate clean sand may turn into a

nonplastic silty sand after loading (Carraro and Bortolotto 2015). In spite of this

limitation, the state variable versus intrinsic parameter framework can always be used to

provide insights to the engineer so that mechanistically sound design decisions can be

made and proper laboratory testing programmes designed. Next, we discuss laboratory

tests that can help the modern engineer evaluate typical soil properties and parameters

useful for the accurate assessment of soil conditions relevant to engineering design.

1.2 Types of laboratory tests

Depending on the information being sought, laboratory tests can be divided into two

groups: experiments required for qualitative or quantitative analyses. Tests required for

qualitative analyses are used to help create a picture of the soil and its physical properties.

For example, specific gravity, Atterberg limits, X-ray diffraction, particle size analysis



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and scanning electron microscopy tests can all help formulate a valid hypothesis about

the type of soil being tested (e.g. nonplastic silty sand, high-plasticity clay, etc.). None of

this information may be used directly, for example, to design the size of a foundation to

sustain structural loads imparted by an offshore platform. However, such test results can

assist the engineer to decide the type and number of mechanical tests to be conducted as

part of a site investigation program. These tests can also provide insights as to how

permeable the soil is or how stable the deposit should be expected to be upon variations

in water content, salt concentration or external loading.

On the other hand, tests required for quantitative analyses provide specific

information about the mechanical response of the soil (e.g. stiffness, strength, hydraulic

conductivity, etc.) so that engineering computations can be performed based on such data.

Examples of such tests include experimental protocols in which representative soil

specimens with specific shape and geometry are required such as in triaxial, simple shear,

consolidation or permeability tests, just to name a few.

As mentioned earlier, soil behavior depends on both intrinsic parameters and

current state variables of the soil being analyzed. Ideally, laboratory tests should be

designed and carried out to uncover all relevant aspects related to the inherent

characteristics of the soil (e.g. specific gravity, critical state stress ratio (M), particle

shapes and size distribution, etc.) while systematically assessing the effect of all pertinent

state variables needed for a particular analysis (e.g. specification of representative stress

range to be used in the tests, selection of appropriate specimen reconstitution method

yielding representative fabric, etc.). In practice, laboratory testing programmes are

typically specified in terms of batches of tests that may be more representative of

historical tradition and/or local practice than the actual adequacy of the approaches per se.

Nevertheless, typical groups of laboratory tests include: basic characterization tests, soil

fabric analyses, and mechanical tests. Each group is described in more detail next.

2 BASIC CHARACTERIZATION TESTS

Tests summarized in this section are either indicators of basic intrinsic properties of the

soil (useful for soil classification) or conducted to quantify a particular aspect of the



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current soil state. Detailed discussions on most of these testing procedures are provided in

comprehensive references on soil laboratory testing such as Head (1986) and Germaine

and Germaine (2009).

2.1 Basic tests to assess intrinsic properties

Specific gravity, carbonate content, X-ray diffraction, liquid limit and plastic limit tests

are all examples of simple laboratory protocols that relate to soil mineralogy and

composition. A suite of such tests along with results from a particle size distribution

analysis (see section 3) can provide a qualitative description about the type of soil the

engineer has to deal with as well as potential problems associated with its expected

behavior in situ. Insights about further analyses to be conducted may also derive from

basic characterization tests (e.g. high carbonate content may require proper assessment of

particle breakage; high liquid limit may trigger further analysis of salinity effects, etc.).

Table 1 provides examples of standards available for basic characterization tests relevant

to offshore geotechnics.

2.2 Simple tests to evaluate state variables

Water content, density and salinity determinations, for example, are among the most

elementary types of soil state assessments that can be carried out in the laboratory. While

these are standard testing procedures (Table 1), the accuracy associated with the

measurements is critical for the proper assessment of soil state.

3 SOIL FABRIC AND UNIFORMITY ANALYSES

Particle size analysis and minimum and maximum density determinations are among the

most elementary soil fabric indicators that can be readily assessed in a geotechnical

laboratory. Typical standards for these tests are listed in Table 1. X-ray radiography,

scanning electron microscopy (SEM), environmental scanning electron microscopy

(ESEM), and micro-computed tomography (micro-CT) (Figure 1) are additional



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examples of laboratory protocols that can be used to shed light into the uniformity, fabric

and structure of soil samples. X-ray radiography is commonly used in offshore

geotechnics for uniformity assessment and pre-selection of tube samples for further

mechanical testing. SEM and ESEM devices provide qualitative, visual characterization

of soil fabric features both at the micro- and macro-scales (Mitchell and Soga 2005).

Quantitative fabric assessments from SEM results can also be carried out for soil samples

with larger particle sizes, but this requires additional specimen preparation procedures

(Yamamuro and Wood 2004). ESEM analyses allow fabric assessments of wet samples

(Figure 2), thus minimizing (or eliminating) common issues associated with sample

preparation and disturbance imparted by conventional SEM procedures. Computed-

tomography of samples of offshore sediments using micro-CT technology (Lim et al.

2017) not only allows a comprehensive visual description of soil fabric (both two- and

three-dimensional assessments are possible) but also additional insights about soil fabric

(e.g., hollow particles of carbonate sand visible in micro-CT only, as shown in Fig. 1).

Micro-CT data can also produce quantitative descriptions of rigorous aspects of the soil

fabric tensor (Fonseca et al. 2013), perhaps the latest frontier in fundamental

geomechanics.

4 MECHANICAL BEHAVIOR

The mechanical behavior of soils can be modelled and characterized in many ways. From

a fundamental standpoint, the stress state at a point within a given material (soil, in this

case) can be fully described if the stress components acting on three mutually

perpendicular faces of an element are known (Figure 3a). Knowledge of both stress and

deformation (Figure 3b) states of the soil allows characterization of its relevant

mechanical properties, which are typically required for modelling and analysis. In

engineering practice, laboratory characterization of the mechanical response of soils has

historically evolved around practical and technological constraints and is often based on

local experience. Therefore, the availability and use of specific laboratory testing

procedures in many parts of the world may simply reflect the combined effect of these



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two factors rather than a more rigorous and universal agreement on fundamental aspects

of the mechanical behavior of soils that may be critical for a given analysis.

The next sub-sections describe the main types of tests available in modern geotechnical

laboratories that are used to characterize various aspects of the mechanical behavior of

offshore sediments. The discussion focuses on the boundary conditions imposed by each

testing device as well as on key design parameters that can be derived from test results.

This alternative discussion format is intended to help engineers (who may be responsible

for the design of an experimental program – or analysis of its results) better appreciate

both advantages and limitations associated with each test (as opposed to simply following

a typical list of testing requirements, which can vary significantly depending on local

practice). Key features of each test are schematically shown in Figure 4. Corresponding

testing standards and typical parameters derived from each test are summarized in Table

2. This summary is not exhaustive and additional parameters not included in Table 2 may

also be derived in some cases.

4.1 One-dimensional response (compression/swell)

Laterally confined one-dimensional (1D) compression/swell tests are among the earliest

tests conceived and used in soil mechanics. These are usually carried out under a zero-

lateral-strain boundary condition imposed by a physical confinement such as that

imparted by a stiff ring surrounding a cylindrical specimen. Justification for this approach

relies on the assumptions that natural soil deposition primarily takes place along the

vertical direction (as soil layers are formed in situ) and that applied surface loads extend

infinitely in the horizontal direction. As a result, soil particles cannot freely move

horizontally. Loading scenarios associated with finite geometry (i.e., circular, square and

strip footings, embankments, anchors, etc.) are not properly modelled by data derived

from 1D tests.

4.1.1 Consolidometer (incremental loading)



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A consolidometer (also referred to as oedometer – from Greek “oidēma” for swell) is a

device within which a cylindrical soil specimen is laterally confined by a stiff ring

(Figure 4). Vertical load and displacement can either be imposed or monitored under

single (either top or bottom) or double (both top and bottom) drainage conditions. The

earliest and most common type of consolidation test involves incremental vertical loading

of the specimen while its vertical deformation is monitored. Specimens are typically

inundated but no back pressure (ubp) is applied to ensure specimen saturation. Free

drainage is allowed so that the pore pressure (u) is negligible and any pore pressure

change (Δu) induced by loading is allowed to dissipate. The implicit zero-lateral-strain

boundary condition imposed by this test is commonly referred to as the “at rest” state and

relates to a radial to axial (or horizontal to vertical) effective stress ratio K0

(=σ'r/σ'a=σ'xx/σ'zz=σ'yy/σ'zz). Usually, σ'r (=σ'xx or σ'yy) is not measured so K0 must be

estimated. Analysis of test results yields stiffness and consolidation parameters (Table 2)

that can be used to model the one-dimensional confined compression/swell response of

soils, which underpins traditional time-dependent settlement analyses.

4.1.2 Consolidometers with pore pressure measurement

4.1.2.1 Rowe cell

This device is very similar to the conventional, incremental-loading consolidometer

described earlier except that it allows specimens to be back-pressure saturated before

testing and pore pressures can be monitored throughout the test. Vertical loading is

typically applied by a pneumatic system. This facilitates testing of larger specimens

(compared to conventional, incremental-loading consolidometers). One-dimensional

confined compression and consolidation analyses can be carried out based on

interpretation of both specimen’s deformation and pore pressure response. Several

drainage boundary conditions can be used. Hydraulic conductivity (k) can also be

measured directly under either constant or falling head flow conditions. A comprehensive

description of this test is provided in Head (1986) and BS 1377 (Part 6 – Consolidation

and Permeability Tests in Hydraulic Cells and with Pore Pressure Measurement).



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4.1.2.2 Constant-rate-of-strain (CRS) consolidometer

Constant-rate-of-strain (CRS) consolidation tests can be conducted by allowing pore

pressure changes to dissipate to some extent but not in full. Such tests are typically

conducted with free drainage allowed at the upper surface of the specimen and no

drainage (but pore pressure measurement) at the bottom of the specimen. This leads to a

varying profile of effective stress through the specimen as the test proceeds. The rate of

strain may be adjusted during the test in such a manner as to maintain a target ratio of

excess pore pressure to (maximum) effective stress in the specimen. This form of

consolidation test provides a faster alternative to conventional incremental loading.

Similarly to Rowe cell tests, CRS tests can be carried out on back-pressure saturated

specimens and pore pressure changes can be monitored throughout the test. The

procedure is fully described in ASTM D4186.

4.2 Shear plane/Interface response

This section refers to testing protocols used to characterize soil response along a thin

shear plane in the soil or along an interface between two materials (e.g. soil and steel, soil

and concrete, etc.). First, brief descriptions of direct shear boxes and ring shear devices

are provided, which is followed by more recent developments used for soil-interface

characterization.

4.2.1 Direct shear box

Similar to the consolidometer, shear boxes are among the earliest devices designed and

used for soil testing. Shear boxes are widely available and relatively easy to operate.

Either load- or displacement-controlled tests can be conducted with modern devices,

which are typically fully automated. While simplicity of operation is one of the main

advantages of direct shear boxes, a critical limitation of such tests relates to their inability

to define the actual soil state of the specimen being tested. While one might like to think



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of a direct shear box specimen as an element, this is not the case due to major stress and

strain concentrations that develop within the specimen. Thus, only estimates of the

average shear and normal stresses applied to the shear plane (τzy and σzz) may be inferred

from this test along with equivalent estimates for the principal stresses from a

hypothetical Mohr’s circle (Figure 4). Full saturation is usually not attempted but

specimens may be inundated. Resulting drainage conditions depend on soil type,

specimen water content, and displacement/loading rate used in the test. Nevertheless, the

device is useful for educational purposes and has played a major role in the development

of a rigorous, mechanistically sound framework for soil dilatancy and critical states

(Taylor 1948, Schofield and Wroth 1968). Both cylindrical and square specimens can be

tested.

Direct shear tests may be conducted under conditions of constant normal stress, constant

specimen height, or a hybrid condition known as constant normal stiffness (CNS)

whereby the normal stress is adjusted linearly with changes in specimen height. This

form of test is of particular relevance to evaluating pile-soil interface shaft friction, for

example for driven and grouted piles (Johnston et al. 1987, Erbrich et al. 2010).

4.2.2 Ring shear

Soil response at very large strains (typically exceeding those measured in most testing

devices) may be characterized by ring shear tests (Figure 4). In this test, a relatively short

hollow cylindrical specimen laterally confined by rigid boundaries is sheared to very

large strains so as to capture soil response in its residual state, which can be quite

different than at critical state due to further fabric evolution, particularly for carbonate

sands prone to particle breakage or clays that form residual shear surfaces. An offshore

example of such phenomenon may be illustrated by continuous loading of carbonate sand

towards its residual state. While a carbonate sand may originally exist as a clean sand

before shearing, it will evolve into a silty sand (or sandy silt) after shearing due to

particle breakage (Yap 2013).

4.2.3 Interface shear tests



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These tests may be required to assess soil-interface material characteristics such as those

existing between offshore sediments and foundation elements, pipelines or sheet pile

walls. Several approaches have been used to this end including tilt tables (Fig. 4) such as

the one described by Pedersen et al. (2003) as well as modified direct shear boxes and

ring shear devices. Modified direct shear boxes can be used to assess soil-interface

material characteristics such as those existing between offshore sediments and pipelines

(White et al. 2012). In this approach, one half of a shear box, which would originally

contain half of the soil specimen (Fig. 4), can be replaced with an interface material such

as steel, concrete, polyethylene, etc. This defines a clear soil-interface boundary, for

which average stress parameters can be quantified and a soil-interface friction angle (δ)

or coefficient deduced. Most advantages and limitations described previously for

conventional direct shear boxes apply. Special procedures are required to successfully

conduct low-stress tests, which are common in offshore applications. A more

sophisticated variation of shear boxes also allows the same specimen to be sheared both

along a soil-interface plane as well as along a soil-soil failure plane to impose shear

failure in the soil (Ganesan et al. 2014). Soil-interface material characterisation can also

be conducted using modified ring shear devices, particularly for analysis concerned with

the assessment of pile-soil interface friction angles and large-displacement response

(Lemos and Vaughan 2000, Ho et al. 2011).

4.3 Two-dimensional response

Experimental results from tests included in this sub-section are commonly analyzed by

assuming that the device used imposes a condition of plane strain to the specimen,

particularly following a simple shear mode of deformation. Schematic representations

shown for these tests in Fig. 4 also reflect this assumption. In reality, this idealisation is

applicable only to specific zones within simple shear specimens. Cylindrical simple shear

specimens loaded horizontally along either the x or y direction (Figure 3) display stress

non-uniformities even for an ideal, elasto-plastic soil (Doherty and Fahey 2011). All

commercially available direct simple shear devices and other simple shear devices used



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in engineering practice suffer from the additional limitation that complementary shear

stresses cannot be properly applied to external vertical planes (Budhu 1984). Average

shear and normal stresses applied to horizontal planes may still be measured and used

with caution in analyses of soil response for specimens tested in these devices (Airey and

Wood 1987). Despite these limitations, these devices are popular in engineering practice

because they require relatively small test samples and the testing procedure is relatively

straightforward. Simple shear devices (direct or otherwise) are not the only ones that

allow principal stress rotation but may be the simplest ones that can be used for this

purpose. Principal stress rotations induced by simple shear tests are solely applicable to

simple shear boundary conditions, as opposed to those required for general, truly 3D

analyses that include but are not limited to simple shear response.

4.3.1 Direct simple shear

As an improvement for the inherent limitations of direct shear boxes, direct simple shear

devices were originally developed in the first half of the 20th century (Kjellman 1951,

Bjerrum and Landva 1966) both at the Swedish Geotechnical Institute and at the

Norwegian Geotechnical Institute (NGI). A specific protocol for direct simple shear

testing of fine-grained soils is outlined in ASTM D6528-07. Lateral confinement of

specimens tested in these devices comprises either a wire-reinforced membrane (NGI

apparatus) or a stack of rigid hollow discs for most of the remaining commercially

available apparatuses. Usually, direct simple shear specimens are neither inundated nor

back-pressure saturated and horizontal stresses (σxx or σyy) are not measured. The

physical horizontal confinement must then be used along with specimen height control

during shearing to impose a zero-volume-change condition and simulate undrained

loading. Analysis of such test results assumes that the change in vertical stress required to

ensure constant volume during direct simple shearing is equivalent to the change in pore

pressure magnitude (Figure 4) that would be observed in truly undrained tests on fully

saturated specimens (Dyvik et al. 1987).

4.3.2 Simple shear with cell pressure confinement



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Other simple shear devices that allow assessment of horizontal stresses were developed in

different parts of the world. These include the fully instrumented cuboidal and cylindrical

devices at Cambridge University (e.g. Budhu 1984, Airey and Wood 1987) and the

cylindrical device with flexible lateral boundary at the University of California-Berkeley

(Peacock and Seed 1968). An alternative version of the UC-Berkeley simple shear device

was developed at The University of Western Australia (UWA), which has become the

industry standard in Australia (Randolph and Gourvenec 2011). One of the advantages of

the UWA simple shear device is that specimens can be fully saturated by back pressure

thus allowing pore pressures to be measured throughout the test. The cell pressure (σc)

can be controlled, which allows the stress state to be fully assessed at any time during the

test, including assessment of the intermediate principal stress (σ2) magnitude and

direction (Carraro 2016). Since Δu during undrained shearing may result both from the

supressed tendency of the specimen to change volume (i.e., specimens can be fully

saturated and vertical displacements (dzz) can be limited to negligible amounts via height

control) as well as from more complex particle breakage mechanisms, such an approach

is useful for a rigorous understanding of the true undrained behavior of carbonate

offshore sediments. As a zero vertical displacement boundary condition (dzz≈0) can be

imposed during undrained shearing of saturated specimens, Δu must be compensated by

adjusting σc by an amount equal to Δu so as to maintain Δσzz≈0 and yield a similar

effective vertical stress response to that enforced in direct simple shear tests (Figure 4).

4.4 Three-dimensional response

Soil specimens tested in a geotechnical laboratory have real dimensions that can be

defined along three perpendicular directions (including all 1D and 2D tests discussed

previously). A rigorous analysis of results from a given testing protocol should thus be

dictated by a careful examination of the actual boundary conditions imposed in the test

rather than empirical modes of interpretation. Interpretation of test results that ignores the

actual boundary conditions used in a test can be misleading and result in inadequate

generalisation or extrapolation of soil response to conditions outside the capability of the



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testing protocol used. As discussed previously, 1D consolidation test results solely apply

to analyses associated with a semi-infinite horizontal soil deposit subjected to a surface

load boundary condition extending in all horizontal directions along the soil boundary. In

the next sub-sections, discussions are presented in a similar way in an attempt to address

the effect of the actual boundary conditions associated with the various tests discussed.

Axial symmetry is examined first and then the discussion is extended into the generalised

multi-directional loading scenario that relates to true three dimensional (3D) soil behavior

– the most common and relevant scenario that is most representative of the vast majority

of engineering analyses and applications.

4.4.1 Axial symmetry

If a case of radial symmetry can be specified with respect to a particular axis, an

axisymmetric condition results. For example, a circular footing resting on the seabed

imparts axisymmetric stress increments to elements located exactly under the footing

centerline. Elements away from the centreline of the footing are not under axisymmetric

states due to principal stress rotation (imparted by the finite nature of the surface

boundary load). Axisymmetric loading can be conveniently emulated in the laboratory by

testing cylindrical specimens. Due to the relative simplicity associated with cylindrical

sampling and testing, axisymmetric tests were among the earliest to be used to study soil

behavior under boundary conditions that are different than those imposed by 1D and 2D

tests.

Triaxial – this is one of the earliest mechanical tests used to analyze soil behavior under

conditions that differ from infinite surface boundary loads (1D response) or (2D) planar

analyses (e.g. plane strain, plane stress and simple shear). “Triaxial” tests are actually

axisymmetric tests conducted under lateral stress confinement. Schematic representation

of a triaxial specimen is shown in Figure 4. Many types of triaxial test can be performed

in geotechnical laboratories depending on the aspect(s) of soil behavior relevant for a

particular analysis. Controlled testing conditions typically include (but are not limited to):

degree of saturation (e.g. saturated, unsaturated), boundary condition during



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consolidation (e.g. isotropic, anisotropic, zero lateral strain), drainage during shearing

(e.g. drained, partially-drained, undrained), loading type and mode (e.g. monotonic,

cyclic; compression, extension) and loading rate (e.g. static, dynamic). Except for tests

conducted under unsaturated and/or partial-drainage conditions, all other combinations

are commonly used in offshore geomechanics. Typical parameters and/or properties

derived from triaxial tests are listed in Table 2 and Fig. 4. Use of triaxial parameters in

analyses that have little resemblance to axial symmetry is not uncommon. This includes

direct comparisons and/or mixing of triaxial parameters with parameters derived from

other tests for use in stress path analyses (Lambe 1967). In the last century, this was

justified due to inherent technical limitations associated with laboratory tests available at

the time (including but not limited to triaxial tests). Strictly speaking, conventional

triaxial tests impose a well-defined three-dimensional stress state on the specimen, but

with two of the principal stresses equal, thus defining the intermediate principal stress

magnitude and direction. Due to axial symmetry, these tests are often interpreted in

practice as if they were two-dimensional tests. While this approach may be convenient, it

will not be sufficiently accurate or appropriate for analyses that require a rigorous link

between volumetric strains (or pore pressure changes) and stress increments (Schofield

and Wroth 1968), which characterizes the real three-dimensional response of soils (Muir

Wood 1984).

Resonant column – this test relies on the analysis of the boundary-value problem

associated with the boundary conditions and modes of vibration imposed during resonant

column testing. Various types of resonant column apparatuses have been developed

including free-free (i.e., specimen is free at each end) and fixed-free (i.e., specimen is

free at one end only) devices. Fixed-free (also known as fixed-based) devices have

become increasingly popular due to simplicity of the equipment required and higher

levels of torque that can be applied (Clayton 2011). Controlled torque disturbances can be

applied to one end of a cylindrical soil specimen using a specially designed oscillator,

which allows the magnitude, shape and frequency of the oscillation to be accurately

imposed. Soil response is quantified by an accelerometer and interpreted from vertical

wave propagation analysis. Double integration yields velocities and displacements and,



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along with knowledge of specimen density, post-test inference of specimen stiffness. In

modern devices, testing under a wide range of frequencies can be easily conducted

leading to prompt determination of the specimen’s resonant frequency. Modern devices

also allow easy selection of the cyclic loading mode applied to the specimen top

(torsional or flexural) via control of the magnitude and direction of the forces imposed by

each one of four coil-magnet sets assembled above the top cap (Fig. 4). The resulting

loading mode thus allows assessment of either shear or flexural stiffness parameters of

the soil, although shear stiffness parameters are the ones more commonly measured

during resonant column testing. An alternative experiment can be carried out through the

application of a controlled disturbance to the specimen top, and then suddenly

interrupting the process and allowing the disturbance to decay over time while

monitoring the disturbance decay. This allows the specimen’s damping ratio (ξ) to be

assessed in a quick and convenient way.

4.4.2 Generalized loading

While axial symmetry is convenient in the laboratory due to the simplicity it imparts to

experimental procedures, axial symmetry is rarely found in nature and is rarely directly

relevant to most engineering analysis. In some cases, plane strain assumptions may be

justified and simple shear tests might be a good alternative for simple soil

characterization. In reality, most soils are inherently anisotropic materials due to the

prevailing geological conditions associated with the genesis of natural soil deposits.

Additional loading imparted by engineered structures and natural processes (e.g.

foundation elements and embankments with finite dimensions, wave loading, wind

loading, earthquakes, sloping ground, etc.) ultimately impart non-trivial boundary

conditions to soil deposits, as opposed to those that can be emulated by relatively simple

laboratory tests such as triaxial and simple shear tests (or their combined use). For most

engineering applications, not only the magnitude of the principal stresses but also their

direction, play a fundamental role in the response of anisotropic materials such as soil.

The following sections summarise experimental alternatives that are now available in

modern laboratories to assess non-trivial loading scenarios. These devices are very



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powerful because they allow systematic assessment of the effect of principal stress

magnitude and direction changes without having to resort to the (often unreconciled)

mixing of results originating from different tests with inconsistent boundary conditions.

True triaxial – a true triaxial device is required in order to control the magnitude of all

principal stresses independently, avoiding restrictive relationships between them (e.g.

σ2=σ3=σr, in triaxial compression). True triaxial devices require testing of cuboidal

specimens. Normal stresses (or strains) applied to three mutually orthogonal specimen

boundaries can be controlled and/or measured separately, thus allowing independent

control (or measurement) of the magnitudes of σ1, σ2 and σ3. Shear stresses are not

applied to the specimen boundaries so the normal stresses applied to those boundaries are

principal stresses (Fig. 4). Principal stress directions cannot be incrementally rotated in a

true triaxial apparatus.

Hollow cylinder – a more versatile and powerful tool available for analysis of the effects

of both principal stress magnitude changes and principal stress rotation on soil behavior

is the hollow cylinder apparatus (Hight et al. 1983). Hollow cylinder devices allow

independent control of boundary tractions (forces and/or pressures) imposed on specimen

boundaries (Fig. 4). Thus not only hydrostatic (change of volume) and deviatoric (change

of shape) effects associated with changes in relevant stress tensor invariants (p and q,

respectively) can be imposed but also two additional parameters, b and α, can be defined

and manipulated (Fig. 4). Parameters b and α allow the intermediate principal stress

magnitude (σ2) as well as the major and minor principal stress directions to be

systematically assessed and varied. Consequently, four independent characteristics of the

soil stress tensor can be evaluated. Compared to triaxial conditions, this extra probing

allows two additional degrees of freedom to be discovered, uncovering additional

information uniquely related to soil anisotropy (or fabric).

Hollow cylinder testing is complex and requires higher-level analytical and experimental

skills than those required in other tests. Specimen preparation (or reconstitution) requires

great care and represents the biggest challenge in hollow cylinder testing. This is

particularly critical in offshore geotechnics since the preferred approach adopted in



19

practice traditionally relies on the use of “intact” samples with relatively small

dimensions to minimize site investigation costs. Procedures for the preparation of hollow

cylinder specimens of either stiff or very soft undisturbed clays are available (Talesnik

and Frydman 1990, Nishimura et al. 2007). For uncemented clean sands or sand mixtures,

recent developments now allow underwater reconstitution of uniform specimens of these

soils (Tastan and Carraro 2013) with fabric and behavior that is consistent with those of

real alluvial and marine soil deposits of similar composition (Vaid et al. 1999, Hoeg et al.

2000, Ghiona and Porcino 2006). Thus appropriate specimen preparation/reconstitution

methods exist for these groups of soils that can preclude the need for intact samples. For

analyses involving multi-directional (3D) loading with or without principal stress rotation

(e.g. offshore wave loading, wind turbine foundations and anchoring, earthquakes),

inclined consolidation (e.g. lateral spreading, offshore submarine slopes, stress state in

the seabed under shallow or mat foundations) and other complex, fabric-dependent

features of soil behavior (e.g. rigorous validation of truly 3D numerical models), hollow

cylinder testing is no longer the new frontier but rather the latest capability in

geotechnical laboratory testing.

4.5 Thermo-electrical behavior

Thermal and electrical characteristics of soils have become increasingly relevant in

offshore applications, particularly those related to pipeline infrastructure. Fundamentally,

the thermal and electrical response of soils also directly relate to soil fabric, as particle

orientation, particle arrangements and density are some of the factors that influence both

the thermal and electrical conductivity of soils (Mitchell and Soga 2005).

4.5.1 Thermal conductivity

The use of a line heat source is one of the most common methods used to assess the

thermal conductivity of offshore sediments, at present. This method relies on the use of

an instrumented thermal probe, which both instigates (by injecting power) and measures



20

temperature changes in the specimen. The method has been standardised by ASTM

D5334-14.

4.5.2 Electrical conductivity

The flow of electricity through soil depends on the combined effect of electricity flow

through soil particles and pore fluid. Thus electrical conductivity measurements also

depend on soil density and fabric and can be conducted in a geotechnical laboratory.

Different testing protocols may be used for this purpose, including applying a well-

defined voltage difference across a specimen using electrodes and measuring the

resulting current. This allows the electrical resistance of the soil to be obtained. Knowing

the specimen geometry, the soil conductivity can be determined.

4.6 Wave propagation

4.6.1 Bender elements

Bender element testing constitutes a simple and convenient way to evaluate the

compressional (vp) and shear (vs) wave velocities of soil (Shirley and Hampton 1978).

Bender elements consist of a sandwich of piezoelectric transducers set up and wired to

allow vs and vp to be determined (Dyvik and Madshus 1985). Modern setups allow them

to be installed in virtually any apparatus. An applied voltage across a (transmitter) bender

element deforms it, imparting a mechanical disturbance onto the surrounding soil. This

mechanical disturbance propagates through the specimen and is sensed by other

(receiver) element(s) located elsewhere in the specimen. Since the distance between

transmitting and receiving elements can be quantified, and the trace of the wave used is

recorded in time domain, the wave propagation velocity can be determined. From simple

wave propagation analysis, a relationship between material (total) density, wave velocity

and stiffness is derived allowing the small-strain elastic parameters of the soil to be

determined. Given the anisotropic characteristics of soils, both directions of wave



21

vibration and propagation must be defined for proper analysis of results (Atkinson 2000),

yielding at least three independent components (e.g. vs xy=vs yx, vs xz=vs yz and vs zx=vs zy).

5 DESIGN OF LABORATORY TESTING PROGRAMS

Laboratory testing programs for offshore developments often require a combination of

the testing procedures described earlier (Randolph and Gourvenec 2011). The types and

number of tests to be conducted depend on the analyses required for a given project. The

rationale behind laboratory testing programs conducted to assess stiffness and strength

characteristics of offshore sediments may require further clarification, particularly for

cyclic loading scenarios usually associated with offshore analyses. This is discussed next.

5.1 Test selection based on relevant boundary conditions

Most offshore structures rely on some form of physical interaction with the seabed

whether via foundations, anchoring elements, or through direct contact with the seabed.

Rigorous design of scientifically-sound laboratory testing programs thus requires

knowledge of relevant soil-structure interaction issues as well as fundamental soil

mechanics principles critical to such analyses. Laboratory testing programmes are

typically designed to assess the effect of expected changes in the state of representative

seabed elements located within a certain zone of interest before, during and after

installation of the structure (or its components). This can be achieved by applying

controlled disturbances to soil specimens using appropriate testing protocols. Results are

then used to calibrate constitutive models or as a direct input for the analyses to be

conducted.

As a minimum, controlled disturbances to the state of soil elements should account for

the following factors: (a) natural formation of the deposit, (b) structure installation, and

(c) soil-structure interaction effects due to transient external loads imparted by waves,

wind, etc. (Figure 5). Deposit formation often takes place under (or is assumed to follow)

typical zero-lateral-strain boundary conditions associated with a K0 consolidation stress

path (Fig. 5a). Construction or installation of the structure (or its components) imposes



22

further changes to the state of soil elements within the zone of interest (Fig. 5b).

Interactions between the structure and transient loads induced by ocean waves, wind,

anchors, etc., impart additional cyclic loads to soil elements of interest (Fig. 5c). The

resulting state of soil elements schematically shown in Fig. 5c can be highly non-trivial

and vary widely depending on the boundary conditions and features of the soil-structure

interaction problem to be analyzed. Such analyses can be complex, as are the mechanics

associated with the true 3D behaviour of offshore sediments subjected to 3D loading

scenarios.

5.2 Current practice

Laboratory testing programs are commonly specified based on local experience and

equipment availability. Such specifications historically result from analyses that rely on

the assumption of a pre-specifed failure mechanism. Testing protocols are then used to

simulate as closely as possible the expected disturbances imparted to various elements

along the proposed mechanism (Lambe 1967). This practice has traditionally relied on a

combination of simple shear (either direct or cell-pressure controlled) and triaxial tests to

characterize soil response along the assumed, pre-specifed failure mechanism. Two

fundamental steps associated with this practice involve defining: (1) the stress state to be

imposed on test specimens once soil-structure interaction takes place but before cyclic

loading starts (i.e., magnitudes of Δσʹx_s, Δσʹy_s and Δσʹz_s in Fig. 5b or both the

magnitude and direction of normal and shear stresses required to fully describe the stress

state of elements away from the centerline); and (2) the characteristics of the cyclic

loading stage used to simulate transient loads (i.e., time histories of additional cyclic

components Δσʹx_c, Δσʹy_c and Δσʹz_c in Fig. 5c or of both the magnitude and direction of

normal and shear cyclic stresses required to fully describe the stress state of elements

away from the centerline).

In common practice involving simple shear and triaxial apparatuses, step (1) is equivalent

to specifying post-K0 consolidation (but pre-shearing) values for τzy_s and q_s (Fig. 4),

respectively. Likewise, step (2) involves establishing relevant variations for further cyclic

changes in τzy_c and q_c, to be used during cyclic loading, along with an appropriate



23

loading scheme (e.g. sinusoidal loading at 0.1 Hz is a commonly used pattern in many

laboratories). The approach has some shortcomings including (a) inconsistent boundary

conditions are effectively used to characterize the state of different soil elements within

the same analysis (i.e., intermediate principal stress changes and principal stress rotation

are not systematically accounted for), and (b) empirical interpolation is required to

estimate changes in stiffness and strength for soil elements that differ from those used in

laboratory tests (i.e., for most points along a pre-specified failure mechanism). The main

advantages of this approach are: (1) relatively small samples are required, and (2) the

approach is well established among practitioners.

Comprehensive examples on how the approach is used in practice are well described

elsewhere for shallow (Andersen 2015) or both shallow and deep (Randolph and

Gourvenec 2011) offshore foundations.

6 SUMMARY

Many types of laboratory test are available to the 21st-century geotechnical engineer

interested in characterizing and modelling offshore sediment behavior. Understanding the

fundamental nature of (and the reason for) the information being sought as well as the

true advantages and limitations of the various testing protocols available should be the

primary concern of modern geotechnical engineers. Testing practices vary widely around

the world and may be influenced by factors that may have little (or nothing) to do with

the science and mechanics associated with the engineering analysis at hand. This chapter

has summarized some of the most common and relevant laboratory tests available to

discover fundamental aspects related to the constituency and state-dependent mechanical

behavior of offshore sediments. Particular emphasis was placed on the underlying

background and rationale associated with each test so that engineers can make informed

decisions about what type(s) of test(s) may be most relevant to the design or analysis of a

laboratory testing program in offshore geotechnics.

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27

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28

List of Tables

Table 1. Basic characterization tests and other common geotechnical tests

Test Reference ASTM

International Other

Carbonate content ASTM D4373 WA 915.1-2012* Liquid limit ASTM D4318 BS 1377

AS 1289.3.9.1 (fall cone) Maximum density ASTM D4253 - Minimum density ASTM D4254 - Particle size analysis ASTM D6913

ASTM D422 ASTM D1140

-

Plastic limit ASTM D4318 BS 1377 AS 1289.3.2.1

Salinity ASTM D4542 Germaine and Germaine (2009) – Chapter 6 Specific gravity ASTM D854 - Specimen density ASTM D7263 - Thermal conductivity ASTM D5334 - Water content ASTM D2216 - * Test Method WA 915.1-2012 (Calcium carbonate content) issued by Main Roads, Western Australia



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Table 2. Examples of typical parameters/properties derived from laboratory tests

Test Type Standard(s) Parameter/property Consolidation/flow Stiffness/compressibility Strength Cyclic loading

Consolidometer ASTM D2435 cv,1D Ezz,1D(=1/mv,1D), Cc, Cs - - Rowe cell, CRS consolidation

BS 1377/6 ASTM D4186

cv,1D kzz,1D Ezz,1D(=1/mv,1D), Cc, Cs - -

Direct shear box ASTM D3080 - - φp, φcs -

Ring shear ASTM D6467 ASTM D7608 - - φp, φcs, φr -

Interface shear box, Tilt table - - δp, δr -

Direct simple shear

ASTM D6528 cv,1D mv,1D(=1/Ezz,1D), Cc, Cs Gzy

sup,DSS, sucs,DSS [τzy/σʹzz]p, [τzy/σʹzz]cs

N

Simple shear with cell pressure confinement

N/A cv,TX (or cv,1D for K=K0)

Γ, κ, λ Gzy

sup,SS, sucs,SS [τzy/σʹzz]p, [τzy/σʹzz]cs

b

N ξ

Triaxial ASTM D4767 ASTM D5311 ASTM D7181

cv,TX (or cv,1D for K=K0)

kzz,TX (or kzz,1D for K=K0)

Γ, κ, λ Ezz,TX (or Ezz,1D for K=K0)

ηp, M sup,TX, sucs,TX

N ξ

Bender elements (in triaxial) N/A -

Gzy max(=Gzx max) Gxz max(=Gyz max) Gxy max(=Gyx max)

- -

Resonant column ASTM D4015 - Gzy (including Gzy max)

- ξ

True triaxial N/A Various, f=(b) Exx, Eyy, Ezz

Various, f=(b) -

Hollow cylinder N/A Various, f=(α, b)

Ezz,1D, Ezz,TX, Ezz,3D

Exx, Eyy Gzy (including Gzy max)

Various, f=(α, b)

N ξ

Glossary of symbols: b: Bishop’s intermediate principal stress coefficient [=(σ2 - σ3)/(σ1 - σ3)] Cc: compression index Cs: swelling (or reloading) index cv: coefficient of consolidation in the vertical direction E: modulus of elasticity G: shear modulus K: effective stress ratio [=σʹh/σʹv or σʹr/σʹa] k: hydraulic conductivity mv: coefficient of volume compressibility N: number of loading cycles in stress-controlled tests for a certain degree of damage to occur su: undrained shear strength α: angle between vertical and major principal stress direction Γ: specific volume of critical state line at reference mean effective stress (= 1kPa) δ: soil-interface friction angle ηp: peak stress ratio q/pʹ κ: gradient of swelling line λ: gradient of compression line Μ: critical state frictional constant (i.e., stress ratio q/pʹ at critical state) ξ: damping ratio φ: soil friction angle

Notes: (1) Directional sub-indices (e.g. Ezz, Gzy, etc.) as per Fig. 3’s notation except for cv(=czz) and mv(=mzz) where original terminology for “vertical” direction was preserved; (2) Sub-indices describing test type (e.g. sup,SS and sup,TX for simple shear and triaxial tests, respectively) added to emphasize these parameters vary with boundary conditions and are not directly comparable.



30

List of Figures

Figure 2. ESEM microphotographs of transitional soil mixtures of Ottawa sand with either 15% nonplastic silt

(left) or 10% kaolin clay (right) (Carraro et al. 2009)

Figure 1. Carbonate sand particles from an offshore deposit from Western Australia: optical microscopy (left); computed tomography (right)



31

Figure 3. Element with unit volume: (a) stress and (b) strain components on three mutually perpendicular

planes with normal along x, y and z directions

σzz

σyy

σxx

z

y x

τzy τzx

τyz

τyx

τxz

τxy

εzz

εyy

εxx

γzy

γzx γyz

γyx

γxz

γxy

1

1

1

1

1

1



32

Test Type

Idealized response

Saturation /pore pressure measured

Boundary condition Notes: (C: consolidation; S: shearing)

Consolidometer 1D No/No

σxx and σyy unknown (only total stresses are measured)

Rowe cell and CRS consolidation

1D Yes/Yes

Same as (1) but with u and ubp measurements; various drainage conditions and specimen sizes possible

Direct shear box

Planar No/No

C: same as (1); S: only σzz and τzy on the failure plane are known

Ring shear

Planar No/No

σxx and σyy are unknown

Tilt table Interface No/No

Device can be placed in a water bath to inundate soil film

Interface shear box

Interface No/No

C: same as (1); S: only σzz and τzy on the soil-interface failure plane are known

Direct simple shear

2D “Simple shear”

No/No

σxx and σyy unknown; εxx=εyy=0 by physical restraint; C: similar to (1); S (constant volume): εzz≈0 by height control and Δσzz(DSS) ≈ - Δu(undrained SS)

Simple shear with cell pressure confinement

2D “Simple shear”

Yes/Yes

C: σxx/σzz selected to yield εxx≈εyy≈0≈Δu; S (undrained): εzz≈0 by height control and σxx adjusted to keep Δσzz≈0

T V

σzz

τzy

σzz dzy τzy

u=ubp+Δu

σxx=σyy dzz

σzz

γzy

σ’zz

u

γzy



33

Triaxial

3D Axisymmetric

Yes/Yes

εp=εxx+εyy+εzz

=ΔV/V p=(σa+2σr)/3 q=σa−σr

Resonant column

3D Axisymmetric

Yes/Yes

True triaxial 2D (plane strain) or 3D

Yes/Yes

Hollow cylinder

Various (3D behavior)

Yes/Yes

p=(σ1+σ2+σ3)/3

where q1=σ2−σ3 q2=σ3−σ1 q3=σ1−σ2

Figure 4. Schematic representation of test specimens and boundary conditions for mechanical tests

σzz

σyy

σxx u=ubp+Δu

€

b =σ2 −σ3σ1 −σ3

po

T V

pi

u=ubp+Δu

σzz

σyy

σxx

τzy

τyz

σ1

σ3

σ2

α

€

q =q12 + q2

2 + q32

2



34

Figure 5. Relevant factors to consider in the design of a laboratory testing program focusing on the assessment of the mechanical properties of offshore sediments

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