NEW APPROACHES TO UNDERSTANDING THE STRUCTURE AND
GEOTECHNICAL CHARACTERISTICS OF ESTUARINE
SEDIMENTS1
Daniel Bishop
Priority Research Center for Geotechnical and Materials Modelling, The University of Newcastle
ABSTRACT
This paper presents a summary of the doctoral research undertaken by Bishop (2010), to improve our
understanding of soft estuarine soils, with the ultimate aim of supporting improved engineering practice in eastcoast Australian soft soil environments.
1
INTRODUCTION
The aim of the research was “to investigate the distribution and behaviour of soft clays and the distribution of
associated sediments in the coastal estuaries of eastern Australia, with specific application to the Richmond
River Estuary underlying the proposed route of the Ballina Bypass”. In particular, it was apparent that much
more is known about estuarine environments in the fields of geology, sedimentology and coastal geomorphology
than is ever utilized in routine geotechnical engineering practice. Hence, this work sought to review the
interdisciplinary literature on coastal estuaries and to distil from this diverse information insights that lead to a
deeper understanding of the geotechnical characteristics of estuarine deposits, in an appropriate form to be of use
to geotechnical practitioners.
2
APPROACH
As a basis for the work, it was necessary to establish the existing state of knowledge in the interdisciplinary
literature associated with the evolution of estuaries and more specifically, the evolution of estuaries along the
east coast of Australia. Evolution in this sense occurs over the span of geological time during which these
estuaries are formed, and it includes the incision of the basement into pre-existing rocks, the filling with
sediment and the cyclical overprinting of eustatic sea level change, to arrive at its present form. This is illustrated
in Figure 1.
The central aim of the thesis was achieved through the following tasks;
1) Establish a process-driven framework for understanding the spatial distribution of sediments within the
estuary.
2) Establish a mechanistic framework for understanding the behaviour of soft clay – both what it is and the
origin of its behaviour, in the context of its depositional and post-depositional environments.
3) Review existing geotechnical tools/methods for the investigation of estuarine deposits, and in particular,
4) Integrate these tools into the frameworks derived above to extract new information that can improve our
interpretation of estuary sediment distribution and our understanding of their geotechnical
characteristics2.
Many of the ideas/concepts reviewed in this research had some measure of applicability to all of the tasks above.
These tasks were differentiated and grouped in the following ways.
2.1
1)
DISTRIBUTION OF SEDIMENTS
A review was undertaken of areas of multi-disciplinary research associated with estuary evolution.
2) A suitable framework was established for interpreting the sediment distribution on a regional scale in
NSW, that can be used at a local level.
3) The most suitable investigation tools available for defining the macro-scale structure of the estuarine
deposits were identified.
1
This paper was presented at the Sydney Chapter YGP meeting in September 2008
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DANIEL BISHOP
4) The application of these tools was extended, to elucidate additional soil profile information.
5) The extended investigation tools were integrated into the evolutionary framework to establish an
investigative approach for mapping the distribution of estuarine deposits.
2.2
FORMATION OF SEDIMENTS
1) Multi-disciplinary literature associated with the formation of estuarine clays, with emphasis on the
control of structure at the micro scale, was reviewed.
2) A suitable model for understanding the biogeochemical structure of estuarine clays was established.
2.3
GEOTECHNICAL BEHAVIOUR
1) The literature associated with the geotechnical behaviour of estuarine clays was reviewed.
2) The geotechnical tools available for quantifying this behaviour were reconsidered to identify what other
useful information might be extracted from their results.
3) A suitable framework/model for understanding the behaviour based on the principles of the structured clay
framework was established.
Figure 1: Evolution of wave dominated barrier estuary (Roy et al., 2001)
3
DISTRIBUTION OF SEDIMENTS
A principal outcome of this work was the realization that the relationships between the point data of a
geotechnical investigation are not arbitrary, and that the systematic trends of sediments within deposits is not
limited to the usual presumption of simple horizontal layers. The patterns of sediments encountered in a borehole
or CPT are not random, nor are the changes in sediment type that occur laterally throughout an estuary. Rather,
the vertical and horizontal variations in sediment type that occur are described by natural “functions” that are
prescribed by the processes that are responsible for the formation of the geological deposit. These, in turn, are
dictated by the particular geological environments that existed during and after sediment deposition. The
functions describing the spatial distribution of sediments are to some extent predictable. By understanding the
processes and factors that define these functions, we can propose functions for particular situations and so make
better interpolations (and in some cases extrapolations) of soil units, that may be useful in refining a site
investigation strategy as a site investigation progresses.
The interpretation of the spatial distribution of estuarine deposits is based on the following principles.
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DANIEL BISHOP
1) The generic form of the function that describes the overall spatial distribution of sediments can be
adapted from the study of Sequence Stratigraphy3 (Miall, 1996). This idea is demonstrated in Figure 2,
which shows the general spatial distribution pattern of sediments in an incised-valley barrier estuary, as
is typical of the eastern Australian coastline (Zaitlin et al., 1994).
Figure 2 Sequence Stratigraphic framework for the infilling of an incised valley during a single phase of sea
level rise (A) Spatial relationship of depositional environments (B) Subdivision of sediment fill into facies
deposited at different stages of sea level rise (C) Key stratigraphic boundaries in the sediment profile (Zaitlin et
al., 1994).
2) The particular functions that describe the spatial distribution of sediments at a local level are defined by
three key boundary conditions (variables): the regional sea level curve, estuary geomorphology
(including geology, catchment area, relief) and climate (including rainfall intensity/distribution, wind
3
The study of Sequence Stratigraphy derives from the field of geology, and it is the study of sediment accumulations, in such a way as to
identify bodies of sediment as unconformity bounded units on a variety of scales, on the basis of their common, or related, depositional
environments.
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DANIEL BISHOP
direction/intensity etc.). These are used to calibrate the generic framework based on Sequence
Stratigraphy for a particular estuary using Morphodynamic4 principles.
3) Within an individual estuary, the type of sediment found at a given location and depth within the
deposit, is defined by the geomorphology and climatic conditions that would have existed at that point,
at the time when the regional sea level was at that elevation5. The evolution of these conditions at that
point, over time, leads to characteristic transitions in the vertical succession of sediment layers, that can
be recognized in CPTU data. This is illustrated in Figure 3. The resulting incised valley sequence
stratigraphic model (IVSSM) is linked with the CPTU data so that the piezocone response though
different segments of an incised valley fill can be used as a basis for interpreting data.
Figure 3 : CPTU patterns derived from ideal relationships and the IVSSM provide clues as to the
spatial distribution of sediments above, below and laterally to them.
4) Given the previous statement, the arrangement of sediments (central basin, tidal delta, tidal flat,
floodplain etc) about any point of interest is defined by the distance of the point from the tidal inlet
along the axis of the channel and the distance of the point from the valley margins. It is important to
note that the distances referred to, may refer to the location of the geomorphic features (in bold) when
the sea level was in a lower position. These do not necessarily correlate to the current location of these
features.
5) If the idea that there are functional relationships between sediment units in a vertical succession is
extended, then it follows that there should be some evolving relationship between soils evident in the
CPT data. If the soil type at each depth is inferred from CPT data, using (say) the Robertson et al.
(1986) chart, then soils formed under similar conditions should group together and in a characteristic
region of the chart. Further, evolving differences should be apparent from the drift in position of
spatially successive data plotted on the chart. This idea is illustrated in Figure 4, where the soils with
particular geological characteristics, from some typical depositional environments, are indicated on the
4
Morphodynamics is the study of the interaction and adjustment of fluid hydrodynamic processes, estuary morphology and sequence
dynamics, involving the motion of sediment.
5 This statement is true for estuarine sediments and associated marginal fluvial and marine deposits because in the present day environment
they are found only within a very narrow elevation window defined by the tidal range (NSW = 2-3 m about M.S.L.).
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DANIEL BISHOP
Robertson et al. (1986) chart.
100
10
11
9
8
7
12
qT (MPa)
10
5
4
6
3
1
1
2
0.1
0
2
4
6
8
FR (%)
a sand with interbedded fines (lenticular beds)
b fines with interbedded sand (flaser beds)
c uniform soft clay, stiffening with depth
d uniform stiff clay,
e fining-up sequence
f geological event (eg time gap)
g geological event
arrows indicate increasing depth
Figure 4: Idealized examples of sediment interval trends when individual CPT data points are plotted
sequentially on the soil type classification diagram (Bishop et al., 2008).
An example of the application of these ideas is presented in Figure 5 by the interpretation of data from
CPTU 141, the Richmond river estuary at Ballina. Such an interpretation leads to a better understanding
of the similarities and differences in apparently similar soils, and it identifies the likelihood of particular
geotechnical characteristics in particular sediment bodies.
6) Entire sedimentary sequences, made up of one or more of the characteristic profile segments shown in
Figure 3 and related as shown in Figure 4, can be recognized from CPT data and, in doing so, a
“context” in terms of the depositional origin can be interpreted for each of the different sediment
intervals in a soil profile. When assembled into the entire profile, the broader context of the profile and
its place in the estuary system of Figure 2, can be determined. This is illustrated in Figure 6.
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10
a 0-2.0m
b 2.0-5.2m
c 5.2-10.0m
d 10.0-13.1m
e 13.1-15.0m
f 15.0-17.2m
g 17.2-19.8m
h 19.8-21.5m
i 21.5-24.0m
j 24.0-25.6m
11
9
8
7
12
6
10.00
qT(MPa)
5
4
3
100.00
10
0-2.0m
2.0-5.2m
5.2-10.0m
10.0-13.1m
13.1-15.0m
15.0-17.2m
17.2-19.8m
19.8-21.5m
21.5-24.0m
24.0-25.6m
11
9
8
7
end
12
6
10.00
5
qT(MPa)
100.00
4
3
start
1.00
1.00
1
1
2
0.10
0.10
0.0
2.0
4.0
2
6.0
0.0
8.0
2.0
4.0
6.0
FR (%)
FR (%)
a) Plot of individual CPTU data points (half data)
b) Plot of CPTU data paths with increasing depth.
100.00
10
a 0-2.0m
b 2.0-5.2m
c 5.2-10.0m
d 10.0-13.1m
e 13.1-15.0m
f 15.0-17.2m
g 17.2-19.8m
h 19.8-21.5m
i 21.5-24.0m
j 24.0-25.6m
11
9
8
7
12
end
25.6m
10.00
6
5
j
qT(MPa)
4
generally fining upwards, within units
and throughout sequence: e to j
start
0m
i
3
1.00
h
g
f
e
13.1m
d
c
a
b
1
2
0.10
0.0
consistently fine
1.0
2.0 but stiffening
3.0 with 4.0
depth: b to e
c)
5.0
6.0
7.0
8.0
9.0
FR (%)
simplified plot of CPTU data paths for CPT141
Figure 5: CPTU data points and paths for CPT141 from the Richmond River Estuary at Ballina.
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DANIEL BISHOP
Figure 6: CPTU interpretation of the IVSSM framework for northern NSW estuaries.
4
BIOGEOCHEMICAL MODEL
On the basis of our improved understanding of the origin of estuarine sediments, we are able to identify the
factors that will control their geotechnical behaviour. Most geotechnical practitioners appreciate that the water
chemistry varies spatially within estuary systems. However, one frequently overlooked factor is that estuaries are
also biologically rich environments, and that there is more to estuarine sediment than simply soil particles. The
link between sediment bodies and their formation in chemically and biologically active environments provides
the basis for a biogeochemical model.
Key factors in a biogeochemical model of estuarine systems include:
1) East coast Australian clays are primarily chemically precipitated rather than physically deposited. This
is significant in regard to their fabric, which is a fundamental part of their structure, and hence
behaviour.
2)
The organics included during deposition and incorporated during post-depositional processes may only
represent a small mass fraction (around 4% in many Australian east coast estuary sediments), but
because of their relatively low density, they contribute a significant volume to the soil, and they may
occupy most or all of the pore space that exists. Organic material is a significant component of both
marine and terrestrial clays. Residual and decaying plant material is associated with the (terrestrial)
floodplain and meander clays, while marine derived organic biomolecules are found in the central basin
(marine) clays.
3) The literature suggests that interactions between soil particles and organic molecules are complex and
diverse in character, as is illustrated in Figure 7.
4) The literature recognizes that organic molecules in sediments can “assemble” to form an intricate web
throughout the sediment, the structure of which is dependent on pH and ionic strength of the pore water,
both during and after deposition. These affect the stability of the sediments. (Riding and Awramik,
2000; McCall and Tevesz, 1982; Sposito 1989; House, 1998; Sparks, 2003; Petka et al., 1998). These
ideas are illustrated in Figure 8.
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Figure 7: Schematic representation of the different organic-clay interactions on a particle aggregate in natural
water. (House, 1998).
Figure 8: Effects of pH and Ionic strength on biopolymer fabric (figure modified from Sparks, 2003), M is
Molar concentration of NaCl in solution.
5) The distribution of ecological conditions under which biogeochemical process operate, both past and
present, are strongly related to the distribution of sedimentary facies in the estuary system.
6) Redox processes dominate the post-depositional sedimentary environment. Evidence includes the
presence of authigenic sulphides, methane and carbonate nodules. These indicate processes including
sulphate reduction and sulphide precipitation, migration of methane, migration of divalent cations in the
pore fluid and reduction of CO2.
7) The same environmental conditions responsible for the formation of quick clays, (i.e. presence of redox
and methanogenic reactions, reduction in pore water salinity due to fresh water flushing of saline clay)
are active in the depositional and post-depositional environments of NSW estuary deposits. These
further affect the stability of the sediments.
This microstructural model recognises the involvement of macro-, micro- and nano-scale organo-chemical
processes at all stages of sediment evolution, i.e. transportation, deposition and post-depositional processes.
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5
IMPLICATIONS FOR GEOTECHNICAL BEHAVIOUR
The many aspects of the above biogeochemical model have significant consequences for the geotechnical
behaviour of estuarine clays. These are best evaluated using the structured clay framework, originally developed
in the work of Burland (1990) and extended by Cotecchia and Chandler (2000). According to this framework,
the behaviour of a clay is a function of its structure, which is the combination of its fabric and the stability
imparted to the fabric by subsequent influences. According to Bennet and Hulbert (1986), the fabric of a soil is
mostly determined by the physical arrangement of particles, imparted at the time of sediment deposition by the
physical and chemical conditions that prevail in the depositional environment. However, the structure of a soil
can be modified through subsequent changes to the stability of a fabric, which can occur under constant load
conditions by post-depositional changes in pore chemistry. Mechanisms for changing the fabric’s stability
include the exchange of the adsorbed cations or the deposition of cements (organic/mineralogical) (Hight and
Leroueil 2003; Leroueil and Hight 2003).
In order to assess the consequences of structure on the geotechnical characteristics of a given soil, the structured
clay framework considers the compression behaviour of any given soil relative to two ideal compression
behaviours. These are the intrinsic compression line (ICL) obtained from consolidation of a destructured soil
slurry, and the sedimentation compression line (SCL) obtained from observations of normally consolidated deep
ocean clays (Skempton 1970). The intrinsic and sedimentation compression lines are defined on a plot of void
index (Iv) against log stress, which effectively normalizes them to account for differences in the plasticity of
different clays. The void index Iv is defined by the ratio (e – e*100) to (e*100 – e*1000) where e is the void ratio and
e*x is the void ratio on the ICL for a stress of x kPa.
The effects of changes to fabric and stability of any clay soil can be evaluated by plotting its oedometer test
results on a figure containing the ICL and the SCL. In order to fully appreciate any effects, their position relative
to four reference stresses is necessary. These are:
•
•
•
•
the intrinsic stress (σ've*): the stress value where the oedometer curve crosses the ICL
the in situ stress (σ'vo): the stress determined from the geological history
the yield stress (σ'vy): the stress beyond which Iv falls rapidly as the stress increases.
the past maximum geological load (σ'vc): by definition, the preconsolidation pressure
The ICL and SCL lines are shown in Figure 9. From Figure 9, it is apparent that seemingly similar oedometer
test behaviours can belong to different soils with very different geological histories and significantly different
geotechnical characteristics.
Figure 9: Structured clay framework and relationship of key parameters for a) a normally consolidated soil with
micro-structure and b) and an over consolidated soil with no structure
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DANIEL BISHOP
From a consideration of the possible depositional and post-depositional conditions that may affect the structure
of a clay soil, it is possible to define regions relative to the ICL and SCL that correlate geological history (from a
process perspective) to geotechnical behaviour. This is shown in Figure 10.
Figure 10: Range of normalised clay behaviours within the Structured Clay framework, (modified from
Chandler, 2000)
Implications of a better appreciation of the geological origin of estuarine clays include, but are not limited to, the
following:
1) Highly sensitive clays (salt-water deposited clays de-stabilised by fresh water flushing) may exist where
central basin clays have remained saturated and normally consolidated in areas of present day estuary
maturity (now beyond the reach of tidal waters).
2) Many soils that are considered to be “over consolidated” in the classical sense, are actually normally
consolidated, but have had the stability of their fabric modified in the post-depositional environment, by
changes to the bio/chemical characteristics at the particle and/or molecular scale, without ever being
overloaded/unloaded.
3) Changes to fabric and its stability, as a result of the diurnal desaturation and desiccation which occurs
during deposition in tidal flat environments, may make tidal flat clays appear overconsolidated, even
though they are strictly normally consolidated.
4) The presence of organics in “normally consolidated” clays of high void ratio may make them appear
slightly overconsolidated when tested, and may be responsible for anomalies in the correlation between
excess pore pressure dissipation and rate of consolidation, sometimes observed beneath fills.
5) The significance of organic substances in estuarine sediments extends to the stabilization of sand
sediments, leading to the formation of “coffee” rock, which can play an important role as a
morphological constraint on the morphodynamics of a particular estuary.
6
CONCLUSIONS
There is much that can be learned about estuarine soils that is of benefit to geotechnical engineers working in
estuarine soil environments. This work is a first step toward a more scientific approach to engineering these most
difficult soils.
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7
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