A Review on Hydraulic Conductivity and Compressibility of Peat
This study reviews the results of several experimental and field investigations on the behavior of peat in hydraulic conductivity and compressibility. A study on the mechanical properties of peat is important in order to gather sufficient information on the response of the soil to preloading in terms of the soil permeability and deformation. Preloading technique is normally employed as a method to improve peat ground so that the improved ground can be used as a soil foundation to support road embankment. Findings on the initial hydraulic conductivity of peat revealed that the initial coefficient of vertical permeability (kv0) of the soil ranged from 10-5 to 10-8 m sec-1 with the value was found to be lower in amorphous peat as compared to that of fibrous peat. Such findings indicated that the initial hydraulic conductivity of peat is influenced by the soil degree of decomposition. The higher is the soil degree of decomposition, the lower is its initial rate of hydraulic conductivity. Results from oedometer tests on Portage peat showed that while the soil coefficient of secondary compression (cα1) ranged from 0.17 to 0.18, its coefficient of tertiary compression (cα2) varied from 0.6 to 0.18. At high consolidation pressure, the soil cα1 approached its cα2 indicating the merging of secondary and tertiary compression components. It can be concluded that the behavior of peat is different from that of inorganic soil in that it exhibited high to moderate initial hydraulic conductivity, rapid primary consolidation and large secondary compression.
Encountered extensively in wetlands, peat is subsurface materials considered
to be among the poorest of foundation materials in terms of its engineering
properties (Dhowian and Edil, 1980). Warburton
et al. (2004) defined peat as a biogenic deposit which when saturated
consists of about 90-95% water and about 5-10% solid material. According to
Warburton et al. (2004) further, the organic
content of the solid fraction is very high, often up to 95% and is made up of
the partly decayed remains of vegetation which has accumulated in waterlogged
areas over timescales of 102-103 years. This renders peat
as an extremely soft organic soil with very low bearing capacity and high compressibility.
As a result, road embankments constructed on peat ground are often subjected
to excessive total and differential settlements. Such excessive soil settlements
often lead to serious cracks and damages of the road embankments. Gofar
and Sutejo (2007) reported that excessive settlement of peat is attributed
to unusual compression behavior of the soil with relatively short primary consolidation
and significant secondary compression. The fact is supported by the finding
of Berry and Vickers (1975) which stated that the deformation
process of peat involves two separated but interlinked effects associated with
primary pore pressure dissipation and secondary viscous creep. Such unusual
compression behavior of the soil is attributed to several factors including
the initial water content, void ratio, initial permeability, fiber content and
arrangement and the condition in which the peat is deposited (Gofar
and Sutejo, 2007).
Because of development in some parts of the world where peat deposits are extensive,
preloading techniques through surcharging have been employed with some success
as a means of in situ improvement of engineering properties (Lea
and Barwner, 1963; Weber, 1969; Samson
and La Rochelle, 1972). For roads of a high standard, where post-construction
settlements are most undesirable, the technique of precompression of the peat
has proved to be a very effective method to reduce the settlements to acceptable
values; furthermore, the precompression of the peat has the advantage of resulting
in an appreciable increase of its shear strength which makes the preloading
technique extremely interesting for different engineering applications (Samson
and La Rochelle, 1972). Improvement of peat ground by preloading technique
often leads to a satisfactory soil foundation to support road embankments. However,
success in the implementation of such technique requires in depth understanding
on the response of hydraulic conductivity and compression of peat to preloading.
Quantification of the hydraulic conductivity and compression responses of peat
to consolidation pressure require relevant soil parameters that must be acquired
through experimental and field investigations in order to analyze, interpret
and model the behavior of the soil. Recently, several laboratory studies were
successful at developing consistent laboratory experimentation on the behavior
of peat in hydraulic conductivity and compressibility (Robinson,
2003; Wong, 2005; Gofar and Sutejo,
2007). Emphasizing on such development, this study introduces the formation
and structural arrangement of peat and reviews the previous investigations and
findings on the hydraulic conductivity and compressibility of peat. Such findings
are vital to layout the fundamental principles, to develop background and to
provide justification on the capability of peat to allow water to pass through
it and to deform as a result of the application of consolidation pressure.
FORMATION AND STRUCTURAL ARRANGEMENT OF PEAT
Peats are formed by the disintegration of plant and organic matter and are characterized
by very high void ratios and very high water contents (Kulathilaka,
1999). Dhowian and Edil (1980) defined peat as a
mixture of fragmented organic material formed in wetlands under appropriate
climatic and topographic conditions and it is derived from vegetation that has
been chemically changed and fossilized. The formation of peats occurs as a result
of the decaying process of plant under acidic conditions in the absence of microbial
process. At its initial state, peats are porous with high water holding capacity
and low specific gravity. Eventually with the increasing period of time, peats
are subjected to biodegradation.
The essence to the basic understanding of the mechanical properties of peat
is the structural arrangement of the soil. The size, shape, fabric and packing
of the soil particles influence the soil permeability, compressibility and shear
strength. According to Mitchell (1993), the values of
properties such as strength, permeability and compressibility are determined
directly by the size and shape of soil particles, their arrangements and the
forces between them and as such to understand the properties require knowledge
of these factors.
Kogure et al. (1993) introduced the concept of
multi-phase system of peat and developed a physical peat soil model as shown
in Fig. 1. Observation of the physical peat model indicates
that the soil can be divided into two major components, namely organic bodies
and organic spaces. The organic bodies consist of organic particles with its
inner voids filled with water, whereas, the organic spaces of the soil model
comprises of soil particles with its outer voids fill with water. The soil model
gives a clear indication that at its initial state, peat can hold a considerable
amount of water due to the hollow, spongy and coarse nature of the organic particles.
The finding is supported by the fact that the photomicrograph of organic coarse
particle of peat soil as shown in Fig. 2 gives a clear picture
that the pore spaces within the particle are capable of holding water when fully
saturated. Similar evidence of the presence of organic particle in peat can
be observed from the scanning electron micrograph of peat in Fig.
3. The natural water content of the peat ranges from 610 to 830 % with its
void ratio ranging from 11.1 to 14.2 (Terzaghi et al.,
1996). However, the initial water content and void ratio of peat are dependent
on its type (Bell, 2000).
||Scanning electron micrograph of Banting peat (Alwi,
According to Bell
(2000), amorphous granular peat can have initial moisture content and void
ratio as low as 500% and 9, respectively, whereas for fibrous peat, its initial
moisture content and void ratio can be as high as 3000% and 25, respectively. Bell (2000) also claimed that amorphous peat tend to
have higher bulk density if compared to that of fibrous peat with the formers
bulk density can range up to 1.2 Mg m-3 whilst in woody fibrous peat,
the bulk density maybe up to half to that of amorphous peat. While peats are
normally acidic in character with its pH values varying between 5.5 and 6.5,
the range of its specific gravity is between 1.1 and 1.8 (Bell,
The fiber content within peat has a major influence on the soil behavior. The
term fiber includes all other fibrous structures, such as shrub rootlets, plant
root hairs, rhizoids (root like filaments), etc. (Landva
and Pheeney, 1980). According to Dhowian and Edil (1980),
if peat has 20% fiber content or more, then it can be classified as fibrous
peat. Otherwise, the peat is classified as amorphous peat. Karlsson
and Hansbo (1981) further differentiated fibrous peat from amorphous peat
with the description that fibrous peat has a low degree of decomposition, fibrous
structures and easily recognized plant structure if compared to amorphous peat.
A significant characteristic of amorphous peat is that it has a high degree
of decomposition and thus it has lower water holding capacity if compared to
that of fibrous peat. A more detail peat classification can be made using von
Post classification system, which classifies peat according to its degree of
humification. From an engineering point of view, the stronger the fibers and
the greater their number, the more reinforced is the peat (Landva
and Pheeney, 1980).
Amorphous peat: Based on the observation of micrographs of amorphous
peat, Landva and Pheeney (1980) reported that the oven-dried amorphous-granular
material in the soil (Fig. 4a) was practically solid.
Micrographs of amorphous peat (a) Amorphous-granular material
in its natural state and (b) Amorphous-granular material compressed under
a pressure of 7000 kPa (Landva and Pheeney, 1980
Further examination of the soil fabric under a consolidation pressure (Fig.
4b) indicated that there was little visual difference between the natural
fabric and the fabric compressed under 7000 kPa (Landva
and Pheeney, 1980). Clearly, there is not much void spaces observed from
the micrographs in Fig. 4a and b and that gives a clear indication
that amorphous peat has a lower hydraulic conductivity if compared to that of
Fibrous peat: Visual inspection on fibrous peat showed that the soil
is dark brown in color, very soft, spongy and contains a large amount of fiber,
organic and water content. Fox and Edil (1996) made
detail observations of the fibrous peat microstructure using a scanning electron
||Micrographs of Middleton fibrous peat (a) horizontal plane
(b) and vertical plane (Fox and Edil, 1996)
||Schematic diagram illustrating the composition of peat
A sample of fibrous peat was loaded to 400 kPa in one-dimensional
compression and the corresponding photomicrographs with respect to vertical
and horizontal planes were examined (Fig. 5). Revelation of a fabric of interwoven fibers from the photomicrograph in the
horizontal plane proves that individual fibers tend to orient themselves horizontally
as a consolidation pressure was applied to fibrous peat. This shows that under
a consolidation pressure, the void spaces in the horizontal direction became
larger than those in the vertical direction as a result of fiber orientation
within the soil and this led to a pronounced structural anisotropy of the soil.
This suggests that the horizontal hydraulic conductivity of the soil was greater
than its vertical hydraulic conductivity when the soil was subjected to a consolidation
pressure. Based on the findings of Kogure et al. (1993)
and Dhowian and Edil (1980), a schematic diagram of
peat indicating the soil composition is shown in Fig. 6.
HYDRAULIC CONDUCTIVITY OF PEAT
The capacity of a soil to allow water to pass through it is termed its permeability
(hydraulic conductivity) (Whitlow, 2001). Earlier studies
(Adams, 1965; Weber, 1969;
Lefebvre et al., 1984) on hydraulic conductivity of peat indicated
that in general, peat is averagely porous and has a medium degree of permeability
with a good drainage characteristic in its natural state.
||Values of natural water content, w0, initial coefficient
of vertical permeability, kvo and cα/Cc
for peat deposits (Mesri et al., 1997)
A summary of the values
of initial coefficient of vertical permeability of peat found by various researchers
is shown on Table 1. The results indicate that their findings differ from one another due to the
fact that soil fabric of peat varies from location to location. The physical
and structural arrangement of constituent particles e.g., fibers and granules
in peat greatly affect the size and continuity of pores resulting in a wide
range of hydraulic conductivities (Edil, 2003). The results
from Table 1 support the fact that amorphous peat has lower
coefficient of permeability if compared to that of fibrous peat. This indicates
that the permeability of peat is very much dependent on its degree of decomposition.
In its initial state, fibrous peat undergoes decomposition with time to become
amorphous peat and therefore, the initial permeability of fibrous peat reduces
with increasing time depending on its degree of decomposition.
Results of falling-head tests conducted using oedometers on Canadian peat soils
sampled from two different sites (NBR-2 and NBR-3 sites) show that Darcys
law is valid for the soils (Lefebvre et al., 1984).
Plots of logarithm of the soils coefficient of permeability versus its void
ratio as shown in Fig. 7 and 8 indicate
that the soils coefficient of permeability reduced rapidly with decreasing void
ratio (Lefebvre et al., 1984). Dhowian
and Edil (1980) further stated that at a given void ratio of Portage peat,
a typical fibrous peat, the soil coefficient of horizontal permeability (kh)
was about 300 times greater than its coefficient of vertical permeability (kv)
which proved that the fibrous peat was anisotropic (Fig. 9).
This also implies that at a consolidation pressure, the soil coefficient of
horizontal consolidation (ch) was greater than its coefficient of
vertical consolidation (cv).
Coefficient of permeability versus void ratio for vertical
and horizontal specimens of Portage peat (Dhowian and
The change in permeability of peat as a result of compression is drastic (Dhowian
and Edil, 1980). As shown in Fig. 7-9,
the peat initially had a relatively high permeability comparable to that of
fine sand or silty sand. However, as compression proceeded which resulted in
rapid decrease in the soil void ratio, its coefficient of permeability was greatly
reduced to a value comparable to that of soft intact clay.
COMPRESSIBILITY OF PEAT
Peats exhibit unusual compression behavior which is different from the conventional
one of clay. Peats are often regarded as problematic soils with high rate of
primary consolidation and a significant stage of secondary compression, which
is not constant with logarithm of time in some cases (Colleselli
et al., 2000). Due to its high to moderate initial permeability,
peats have relatively short duration of primary consolidation and large secondary
compression, even tertiary compression of peats can be observed. According to
Colleselli et al. (2000), the initial permeability
of peats is between 100 to 1000 times that of soft clays and silts and its coefficient
of consolidation is between 10 to 100 times greater. The consolidation behavior
of peats had been studied and presented by some researchers.
Colleselli et al. (2000) studied the compressibility
characteristics of three types of Italian peat, namely Adria-1, Adria-2 and
Correzzola peats using a 70.5 mm diameter standard oedometric consolidation
cell and a 75.5 mm Rowe cell with pore water measurement at the base of the
specimens. Figure 10 shows a log time-settlement curves of
the peats for a single load increment from 10 to 100 kPa. The peats had quite
similar amount of organic content ranging from 71 to 72% with Adria-2 peat had fiber content
more than 75% which was much greater than those of 25% of both Adria-1 and Correzzola
peats. Figure 10 shows that the primary consolidation and
secondary compression of Adria-1 and Correzzola peats were clearly defined,
with their tertiary compressions were relatively small.
However, Adria-2 peat had low primary consolidation and both of its secondary
and tertiary compressions are significantly observed in Fig.
10. With Adria-2 peat exhibited the greatest tertiary settlement than those
of the other two peats, it can be explained that the different compression behavior
is attributed to the different fiber content of the peats (Colleselli
et al., 2000). As such, it was found from the study that the higher
the fiber content of peat, the larger was the tertiary compression of the soil.
Similar finding on the compression behavior of peat can be observed from the
study of Dhowian and Edil (1980). The study evaluated
the compression behavior of Portage peat, a typical fibrous peat with intermediate
fiber content of about 31% using Anteus consolidometer with its specimen having
a diameter of 73 mm and initial height of 228 mm. Different from the conventional
apparatus, the Anteus consolidometer was modified to have two additional features:
(1) a sensitive pressure transducer to measure the excess pore water pressure
at the bottom of the specimen while the pore water is draining from the top
and (2) burettes connected to the top and bottom of the specimen to measure
the coefficient of permeability before the application of a stress increment
during consolidation by the variable head method (Dhowian
and Edil, 1980). Figure 11 shows a typical graphical
plot from the study showing the vertical strain of the soil versus logarithm
of time for the first increment of applied pressure of 50 kPa under 560 kPa
Vertical strain, normalized effluent outflow and excess pore
pressure versus logarithmic of time for a Portage peat specimen under the
first stress increment (back pressure = 560 kPa) (Dhowian
and Edil, 1980
||Consolidation data for a Portage peat specimen (back pressure
= 560 kPa), (a) σ = 50 kPa, (b) σ = 100 kPa, (c) σ = 200
kPa and (d) σ = 400 kPa (Dhowian and Edil, 1980)
According to Dhowian and Edil (1980), four components
of strain can be clearly observed from the figure as detailed as follows:
||An instantaneous strain, which takes place immediately after
the application of a pressure increment, possibly the result of the compression
of air voids and the elastic compression of the peat
||A primary strain component, which occurs at a relatively high
rate and continues for several minutes to a time, tp
||A secondary strain component, which results from a linear
increase of strain with the logarithm of time for a number of additional
log cycles of time until a time, ts, after which the time rate
of compression increases substantially giving rise to a tertiary strain
||A tertiary strain component, which continues indefinitely
until the whole compression process ceases
The soil specimen was later subjected to consolidation pressures of 100, 200
and 400 kPa and the relationship between pore water pressure, strain and normalized
outflow of the soil versus logarithm of time were graphically shown in Fig.
12. While Table 2 shows the time of end of primary consolidation
(tp) and time of end of secondary compression (ts) for
the soil specimen subjected to the consolidation pressures, Fig.
13 provides graphical illustrations of the relationship between compression
parameters versus the consolidation pressures for the peat. As shown in Fig.
13a in relation to Fig. 12, about 10% strain for the
first pressure increment was resulted from instantaneous and primary compression.
However, the strain decreased to a constant value of 2% for the remaining
pressure increments. This shows that the strain due to instantaneous and primary
consolidation of the soil was relatively small due to its high initial permeability.
As shown in Table 2, while the time of end of primary consolidation
of the soil increased progressively from 1.35 to 2.68 min with increasing consolidation
pressures, its time of end of secondary compression declined gradually from
2472 to 468 min with increasing consolidation pressures. Interestingly, as Fig.
13b shows that the rate of secondary compression was almost constant and
lied within the range of 0.17 to 0.18, Fig. 13c indicates
a steady decrease of rate of tertiary compression from 0.6 to nearly 0.18 at
consolidation pressures of 25 and 400 kPa, respectively.
||Change in tp and ts with pressure for
Portage peat (average values for all tests) (Dhowian
and Edil, 1980)
|NA: Not available
The trend provides
a clear indication that with increasing consolidation pressures, the rate of
tertiary compression (cα2), approached the rate of secondary
compression (cα1), which eventually resulted in the tertiary
component of compression merged with secondary compression at high consolidation
pressures (Dhowian and Edil, 1980). Despite of the finding that tertiary compression influences the long term
compression behavior of peat, Hartlen and Wolski (1996)
mentioned that there was no field evidence of tertiary compression of peat and
it may therefore be considered a laboratory effect, which needs not to be included
in the test evaluation. However, whether this is just a laboratory phenomenon
or it also exists in the field remains unresolved (Dhowian
and Edil, 1980).
The absence of tertiary compression of peat at field is evident in the field
settlement observation of Samson and La Rochelle (1972)
on a Canadian peat land. The peat land was preloaded by a sand embankment in
three loading stages with the embankment heights of the first, second and third
loading stages were 1.2 to 2.5, 0.3 and 1 to 1.5 m, respectively. Observations
of the field settlement were made using 12 square settlement plates of 1x1 m
size and 7.5 cm thick. Two typical log time-settlement curves of the peat subjected
to the three loading stages measured at plates P-1 and P-3 are shown in Fig.
14 and 15. For analysis of the field settlement, the
results of the second and third loading stages are shown in Fig.
16a and b (Samson and La Rochelle,
Analysis made on the graphs showed that the S-shaped primary consolidation
curves are clearly defined with linear secondary compression. Settlement results
from the settlement plates indicate that the time for the end of primary consolidation
(tp) for the first, second and third loading stages ranged from 5
to 10, 17 to 26 and 55 to 200 days, respectively. Such increase in the end of
primary consolidation time can be attributed to the large and rapid decrease
of the soil permeability as a result of staged preloading. Assuming constant
values for the coefficient of secondary compression from the field investigation,
it is expected that, 10 years after construction, the settlement will range
from 0.7 to 2.5 in (1.8 to 6.4 cm).
Such settlement is relatively small and
therefore it can be stated that staged preloading is effective in reducing the
long term settlement of peat appreciably but it requires a significant amount
of time before it can be ascertained that the long term settlement is reduced
to within acceptable limits.
Based on the review of both experimental and field investigations on hydraulic conductivity and compressibility of peat, the following concluding remarks can be drawn:
||Peat is characterized by high to moderate initial hydraulic
conductivity due to its porous nature. However, hydraulic conductivity of
peat reduces drastically after compression due to its short duration of
||The compression behavior of peat is quite distinct from that
of inorganic soil. It is characterized by relatively short primary consolidation
and is followed by large secondary compression which may not be constant
with logarithm of time. The presence of tertiary compression in peat maybe
considered as a laboratory effect since it is not evident in the results
of field investigation on the soil
||High compressibility of peat is basically attributed to the
presence of its multi-phase system, in which the soil consists of both macropores
and micropores due to the coarse and spongy organic particles as evident
in the photomicrographs of the soil
||Since, the properties of peat are very site specific due to
the varying degree of peat decomposition, experimental and field investigations
on hydraulic conductivity and compressibility of a typical peat are helpful
to analyze and characterize the soil in response to preloading
The authors wish to specially acknowledge Sadek Deboucha and Babak Kamali for giving constructive comments on the review article before it was sent for consideration to be published in the Journal of Applied Sciences.
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