Proceedings of the Institution of
Civil Engineers
Water Management 162
June 2009 Issue WM3
Pages 221–232
doi: 10.1680/wama.2009.162.3.221
Paper 700038
Received 15/08/2007
Accepted 30/07/2008
Keywords:
embankments/floods & floodworks/
site investigation
Mark Dyer
Professor, Civil Engineering
Department, Trinity College Dublin,
Dublin, Ireland
Stefano Utili
Research Fellow, Civil Engineering
Department, University of
Strathclyde, Glasgow, UK
Marcin Zielinski
Research Assistant, Civil
Engineering Department,
University of Strathclyde,
Glasgow, UK
Field survey of desiccation fissuring of flood embankments
M. Dyer
DPhil, CEng, MICE,
S. Utili
Dipl(Eng), MSc, PhD, CEng, MICE
Early studies on the 1953 North Sea floods showed that
desiccation fissuring of clay fill can play a major role in the
failure of flood embankments under overflow conditions.
However, the onset of desiccation fissuring in embankments
and its contribution towards breach initiation has not been
fully researched. Field and laboratory studies were thus
carried out into the desiccation fissuring of clay flood
embankments in the UK. The work found that a critical
condition is reached when desiccation creates an
interconnected network of sub-vertical and sub-horizontal
fissures, which significantly increases the mass permeability
of the fill material and hence allows rapid seepage of flood
water through the surface layer of the embankment. It is
suggested high rates of seepage cause localised uplifting of
clay blocks, leading to progressive slope failure and
successive breaching. Small-scale desiccation tests carried
out on discs of soil in pressure plates showed very good
agreement between the onset of cracking in the pressure
plates and the moisture content recorded in the field. This
suggests that it is possible to assess the susceptibility of a fill
material to desiccation fissuring from the soil water
characteristic curve.
1. INTRODUCTION
Between 1998 and 2002, Europe experienced more than 100
major floods, which resulted in some 700 fatalities, the
displacement of half a million people and at least €25 billion in
insured economic losses. Flood defence embankments form a
significant proportion of all flood defence assets in each member
country. For example, there are some 34 000 km of estuarine and
river flood defence embankments in England and Wales, with an
annual budget of approximately £450 million spent on
maintenance and new constructions. In the Netherlands, one third
of the country (where 8 million people live) is situated below sea
level; without flood defence embankments (16 500 km of levees)
66% of the country would be flooded. Moreover, the total cost
involved in flood defences is approximately €600 million per year.
All around Europe, coastal and estuarine flood embankments are
subject to progressively heavier hydraulic loading conditions due
to climate change. Furthermore, a significantly larger amount of
flood embankments are expected to be built in forthcoming years
because of the larger areas in danger of flooding due to global
rising ocean levels and expected stronger storms.
Most European embankments are made of locally available
soils, namely clay, peat, silt and sand. One of the deterioration
Water Management 162 Issue WM3
and M. Zielinski
Dipl(Eng)
processes known to have an adverse effect on the stability of
flood defences is desiccation fissuring. Early work by Cooling
and Marsland1–3 on the 1953 North Sea flood identified
desiccation fissuring as a major contributor to embankment
collapse. This conclusion was corroborated in more recent work
by Dyer and colleagues.4–6 Based on this evidence, field surveys
were undertaken to gather more detailed information about the
extent and nature of desiccation fissuring and its role in breach
initiation during overflow conditions.
A typical flood embankment is shown in Figure 1.6 The main
features include
(a) an embankment body, which provides the mass obstruction
against flood water
(b) the toe of the embankment on both the outward or inward
embankment faces
(c) the outward face of the embankment, directly exposed to
water
(d ) the inward face on the landward side, not normally directly
exposed to flood water
(e) the crest at the top of the embankment, which is typically
flat and (ideally) several metres wide
( f ) a drainage ditch also known as a ‘soke’ or ‘delph’ ditch
excavated close to the inward toe of the embankment
(g) surface protection, sometimes termed revetment, in the form
of vegetation (e.g. grass), man-made material (e.g. concrete)
or a combination of different materials.
In contrast to historic flood defences, modern flood embankments
are constructed in layers using standard compaction
specifications similar to those used in highway construction.7,8
Specifications also exist for protection against surface
erosion.9–11 The fill materials include superficial soils such as
fluvial clays and silt as well as waste materials from quarry and
mining (colliery spoil, demolition material and blast-furnace
slag). A list of earthwork materials used in the construction of
flood embankments along several major UK rivers and estuaries
is given by Dyer.4
For the sake of generality, the most common failure
mechanisms affecting flood defence embankments are
(a) erosion of the outward face and crest due to wave action
(b) erosion of the inward face and crest due to overtopping
(c) shallow slippage of the outward face due to erosion of the
toe
Field survey of desiccation fissuring of flood embankments
Dyer et al.
221
River or
coast
Landward
Crest
Outward
face
Inward face
Surface
protection
Embankment body
(fill material)
Drainage ditch
(optional)
Embankment foundation
(original material)
Water
Embankment toe
Figure 1. Typical flood embankment (after Morris et al.6)
(d ) shallow slippage or erosion of the inward face linked to
piping through animal burrows or excessive seepage
through fissured clay fill leading to breach formation
(e) shallow slippage of the outward face after rapid lowering of
flood levels due to poorly constructed revetment
( f ) deep-seated slippage of the inward face due to excessive
hydraulic uplift pressures in the underlying strata
(g) large-scale translational movement due to low-strength
organic soils acting as founding strata
(h) deep-seated slope instability caused by embankment
construction on soft clays.
Some of these mechanisms are illustrated in Figure 27 and
Table 1 lists field observations indicating the possible onset of
failure, the geotechnical and hydraulic processes and the ground
conditions to be considered in a site investigation for the
maintenance of existing embankments in correspondence of
each failure mechanism. The failure mechanism due to fissuring
is indicated by the text in bold typeface.
1.1. Literature on desiccation fissuring
Embankment breach occurs rapidly and is difficult to predict.
Not surprisingly, well-documented cases of embankment
breaches are rare. One of the few notable studies of flood
embankment failures is that of Marsland and Cooling3 and the
subsequent report by Marsland2 about the role of fine fissuring
on embankment collapse. In the case of the North Sea floods,
Cooling and Marsland reported that the debris of a breach often
comprised blocks of clay transported some considerable distance
from the embankment, and in many cases the cause of failure
was attributed to desiccation fissuring of the embankment that
led to significant seepage of overflow flood water into the
embankment. Cooling and Marsland1,2 attributed the
detrimental effects of desiccation fissuring to increased seepage
during overflow conditions, with a corresponding increase in
pore pressure that led to slope instability of the landward face.
In comparison, a different mechanism leading to embankment
breaching will be proposed in Section 3.
In 1996, Meadowcroft et al.12 carried out a field survey of a
flood embankment at Tollesbury Creek. Three clay
embankments with an extensive crack pattern along the
Blackwater estuary in Essex, UK, were failed in cofferdam tests.
The experiment was notable for the observed high rate of
seepage through the crest into the highly fissured clay in the
landward zone of the embankment. In one experiment, water
Slide circle inside slope
Settlement
Slide circle outside slope
Piping
Wave overtopping
Erosion outside slope
Micro-instability
Softening
Erosion foreland
Figure 2. Potential failure mechanisms leading to embankment instability (after Pylarczyk7)
222
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Field survey of desiccation fissuring of flood embankments
Dyer et al.
Element
Geohazard
Field observations
Risk
Geotechnical process
Ground conditions to be
considered/investigated
Founding
strata
Settlement
Low crest levels
Low crest levels
leading to
overtopping
Consolidation (dissipation of
excess pore pressures) of
underlying strata or
embankment fill material
.
Deep
rotational
failure
.
Tension cracks on
Catastrophic
embankment crest
failure of
. Settlement of part of embankment
crest
. Lateral displacement
of embankment toe
. Heave of ground in
front of toe
Shear failure during
construction or embankment
raising
.
Translational
sliding
Distortion of
Catastrophic
embankment crest
failure of
leading to bulging along embankment
inward face
Lateral hydraulic force
exceeding shear strength of
founding strata along base of
embankment or desiccation of
organic fill leading to a
reduction in deadweight
.
Seepage and
piping
Seepage or ponding of
water in front of
embankment
Seepage causing
internal erosion
and piping
Under-flow of flood water
leading to erosion and slope
instability
Presence of highly permeable
strata beneath embankment
leading to excessive seepage
Uplift
pressures
Heave of embankment
toe
High pore
pressures causing
instability
Build-up of uplift pressures in
confined permeable strata due
to hydraulic continuity with
flood water
Presence of highly permeable
strata beneath embankment
leading to build-up of pore
pressures due to confinement
Shallow slope
instability
.
Shallow translational
slumping or slippage
of embankment side
slopes
. Possible tension
cracks on
embankment crest,
settlement of crest,
lateral displacement
of embankment toe
or heave of ground in
front of toe
. Instability during rapid draw- . Compaction of fill material in
Damage to
down after flood or high
relation to moisture content
outward and
water load on outward face . Build-up of pore pressures
inward faces of
. Longer-term slippage of
after lengthy period of high
embankment
slopes due to pore pressure
water load, resulting in
leading to loss of
equalisation and/or
saturation of fill material or
integrity or
reduction in soil suction
leading to uplift
reduced resistance
. Erosion of toe along
. Swelling of overto seepage or
outward face due to river
consolidated clay fill leading
overtopping
migration
to shallow slips
. Reduction in soil suction
pressures in partially
saturated soils following
infiltration of rain and/or
high water load
Internal
seepage and
erosion
.
Washout of
embankment fill
material leading
to preferential
seepage paths,
piping and
eventually
breach
.
Increased risk of
seepage or
instability
Erosion of outward face and
toe due to river/coastal
migration or wave erosion
Embankment
structure
.
Erosion of
outward face
and toe
Erosion of
inward face
.
.
.
Cracking within
embankment
body
Visible seepage on
inward face of
embankment,
particularly during
‘bank full’
conditions
Animal burrows
Bare soil, loss of
material visible
Undercutting at base
of slope
Bare soil, loss of
vegetation
Consolidation and
compression characteristics
of underlying soils
. Secondary consolidation and
creep of soils and fill
. Differences in horizontal and
vertical permeability of
foundation material
Shear strength of fill and
foundation soils, in particular
undrained shear strength of
clays
. Possible longer-term gain in
strength due to
consolidation
Shear strength of soft clays
and organic soils directly
beneath the embankment
. Desiccation of peat and
organic fills leading to a
reduction in deadweight
Excessive seepage caused . Shrinkage of medium
by desiccation and fine
and highly plastic clay
fissuring
leading to fine fissuring
. Excessive seepage due to . Excessive seepage
highly permeable fill
through coarse-grained
material
fill leading to piping at
. Loss of embankment
critical hydraulic
through burrowing or
gradients
washout of fines
Reduced resistance Erosion of inward face due to
to overtopping
overflow
.
Shear strength and grading
of embankment material
. Geomorphological
assessment of long-term
river or coastal migration
Selection of suitable
topography, topsoil and
vegetation; possible use of
geotextiles
Table 1. Geotechnical factors affecting the stability of flood embankments (after Dyer4). The part relative to desiccation fissuring is shown
in bold typeface
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Field survey of desiccation fissuring of flood embankments
Dyer et al.
223
A1 = 5–7 cm
S1 = 20–24 cm
(a)
e
Kodikara18 and Kodikara et al.19 whereas Aubeny and Lytton20
and Day21 investigated the formation of fissures specifically in
clay slopes.
Concerning the UK in particular, the clay types most susceptible
to cracking are those characterised by a plasticity index (PI)
greater than 25%, which can be typically found in London clay
and Kimmeridge clay largely used to build embankments in the
south and east of England (e.g. along the east coast and the
Humber estuary).
(b)
1.2. The role of vegetation
Since flood defence embankments are typically vegetated, it is
worth discussing the effect of vegetation on their stability. In
general, grass cover has a beneficial effect on flood defence
embankments since it boosts the resistance against water flow
induced erosion. It protects the river and seaward slope of
embankments in the run-up zone above the normal water level.
In fact, the passage of water along a bare soil surface may
entrain and transport particles already detached and,
particularly if the flow is channelled, may also detach further
soil particles. For any given discharge intensity, the erosive
effect varies according to the length of the grass. The minimum
velocity occurs when the grass is longest and the maximum
velocity occurs when the grass is shortest, for instance after
mowing or during the winter. However, grass alone is unable to
withstand the erosion hazard posed by low-frequency but highintensity short-duration flows due to storm floodings. Guidance
on the protective role played by vegetation can be found in the
literature.9,22–24
(c)
Horizontal crack
(d)
(e)
(f)
Figure 3. Schematic illustration of mechanisms leading to an
interconnected network of vertical and horizontal fissures:
(a) initiation of primary cracks for t < 17 h; (b) crack opening and
further propagation e 4 mm at 80–100 h; (c) initiation of
secondary cracks at 80–100 h; (d) differential shrinkage –
horizontal crack propagation in mode II (in plane shear) at
100–150 h; (e) initiation of cracks below shear plane at 150 h;
(f) formation of protuberance at t < 192 h (after Konrad and
Ayad16)
seeped out of the embankment at the landward toe up to 7 m
away from the overflow section, which clearly indicated the
presence of a well-interconnected network of fissures within the
embankment. Hydraulic uplift pressures beneath the
embankment were also reported to be the cause of the failure of
a flood embankment at Crayford Marshes.13–15
More recently, the mechanisms responsible for fissuring of
desiccated clay have received greater attention in the literature.
For instance, Konrad and Ayad presented the results of a
desiccation test on an intact clay deposit at the experimental
site of Saint-Alban, Quebec, Canada.16 The study recorded an
interconnected pattern of desiccation fissures forming
hexagonal polygons (as found in Thorngumbald (see Section 2))
with the appearance of lateral cracks as shown in Figure 3.
Other notable studies on the formation of fissures and cracks in
clayey soils were carried out by Blight,17 Nahlawi and
224
Water Management 162 Issue WM3
Grass roots also have a beneficial effect on soil strength. The
increase of shear strength due to rooting can be expressed,
using the Mohr–Coulomb criterion, in terms of an apparent
cohesion, whereas the friction angle remains unchanged.25 The
magnitude of the mechanical reinforcing effect of vegetation is
a function of root properties such as density, tensile strength,
length/diameter ratio, surface roughness, alignment (i.e.
straightness/angularity) and orientation to the direction of
principal strains.
However, it has also been reported that vegetation may have
some adverse effects in terms of slope stability. Smethurst
et al.26 analysed a grass-covered cut slope in London clay,
monitoring pore water pressures and water content over one
year. They concluded that vegetation had caused a large cyclic
change in effective stresses within the major drying zone (the
top 1 m depth of the profile) through a winter–summer–winter
cycle. Recent evidence from both centrifuge model tests and
numerical analyses of clay slopes suggests that cyclic stresses
thought to be representative of those induced by vegetation can
cause strain softening to occur, starting from the toe of the
slope.27–30 Over a period of several years, these cyclic stresses
can induce progressive failure.
2. FIELD STUDY
The field study reported here was carried out on a historic and a
new flood embankment at Thorngumbald near the city of Hull
on the north shore of the Humber estuary. Figure 4 shows the
historic flood embankment and the alignment of the new flood
embankment. The historic flood embankment was replaced by a
new flood embankment in 2003 in order to create new salt
Field survey of desiccation fissuring of flood embankments
Dyer et al.
pattern described by Konrad and Ayad.16 In the lower zone B,
fissures extend vertically from the horizontal fissures and
terminate in intact soil. This simple classification of desiccation
fissuring suggests that there is an upper surface zone of fill
material that has been transformed into a rubblised soil
structure with a two-dimensional network of fissures that
significantly increases the mass permeability of the fill material
and so allows water to readily flow through and along that
portion of the embankment. This pattern results in a much
greater infiltration of water into the surface of the flood
embankment, which can potentially lead to collapse of the
inward slope. Results from infiltrometer tests are presented in
Section 2.3.
Figure 4. Satellite photograph of historic flood embankment at
Thorngumbald (source: Google Earth)
marshes. The field study involved the excavation of trial pits to
expose desiccation fissuring beneath the crest and side slopes of
the embankments, along with soil sampling for later laboratory
analysis and on-site double-ring infiltrometer tests. Site visits to
Thorngumbald were undertaken in 2003, 2005 and 2006.
2.1. Field observations
Observations were made of desiccation fissures both on the
surface and within shallow excavations during the first site visit
in 2003. Four trial trenches were excavated along the crest and
landward slope of the historic flood embankment using a
hydraulic back-actor. The final part of the excavation was
undertaken by hand to minimise soil disturbance. In addition,
soil samples were taken using U100 sampling tubes pushed into
the side and base of the trench and excavated. Visual inspection
of the trenches revealed extensive fissuring of desiccated clay to
a depth of approximately 600 mm below the crest height of the
embankment in each excavation. A photograph of one side of
the hand-excavated trench is shown in Figure 5.
The trial pits indicate that the pattern of desiccation fissures can
be divided into two zones, labelled A and B in Figure 5. The
upper zone A shows a two-dimensional array of fissures both
perpendicular and parallel to the drying surface, similar to the
Figure 5. Desiccation fissures observed in trial trench at
Thorngumbald flood embankment at an approximate depth of
600 mm below crest level. Dashed lines indicate the proposed
division of desiccation fissures into two zones: A and B
Water Management 162 Issue WM3
During the third site visit in 2006, deep fissures were observed
in the historic embankment to a depth of 1.0 m below crest
level. The extent of the fissuring was so pronounced that the
widths of the fissures were measured at 10 cm intervals, as
shown in Figure 6. However, the network of fissures was
different from the first survey. In this trial trench the fissures
were not connected into a two-dimensional network but instead
tended to be single deep fissures that would allow seepage of
water into the depth of the embankment but not lateral flow
beneath the surface of the embankment slope as will be shown
in the next section.
During the same visit, a field survey of the new flood
embankment was also conducted. The new embankment was
constructed in 2003 from locally excavated alluvial clays
extracted from a borrow pit area of the new salt marshes. The
2006 field survey of the new embankment identified desiccation
fissures along the crest of the embankment; Figure 7a shows a
polygonal pattern of fissures characteristic of desiccation
cracking. The width of the polygonal desiccation fissures varied
from 5 to 25 mm and they were generally found in areas of poor
grass cover. This particular shape of superficial fissures is
similar to the patterns observed by Konrad and Ayad16
(Figure 7b).
Hand-excavated trial pits were dug into the new embankment
to investigate the depth of surface desiccation fissures beneath
the embankment crest. During this second field survey, two trial
trenches were hand-excavated into the crest of the new flood
embankment and disturbed soil samples at four different levels
were taken for laboratory analysis (Figure 8). A further trial
trench was hand-excavated in the historic embankment
(1.0 m 1.0 m) to a depth of 1.0 m below the crest (Figure 6).
These fissures are very similar to those observed in the trial pits
along clay embankments excavated by Cooling and Marsland
(see Figure 9).
2.2. Desiccation soil moisture profile
In addition to the visual records of desiccation fissuring, the
material properties of the disturbed soil samples were measured
to determine shrinkage limit (SL), plastic limit (PL), liquid limit
(LL) and in situ moisture content (Figure 10); PL, SL and LL for
the clay fill were determined to be 14, 25 and 49% respectively.
The SL was determined by measuring volumetric change in a
bath of mercury. The moisture content profile clearly shows that
the highly desiccated clay fill within the top 500 mm has
dehydrated towards the SL of 14%. In contrast, below 500 mm
depth, the in situ moisture content varies between the SL and
Field survey of desiccation fissuring of flood embankments
Dyer et al.
225
the PL. This distinct change in
moisture content agrees with
the visual inspection of the
trial trenches that revealed a
well-defined zone of
desiccation. At greater depth,
the moisture content increases
to approximately 30%. Similar
moisture profiles were
observed by Cooling and
Marsland after the 1953 North
Sea flooding as shown in
Figure 11; the moisture
content is close to the SL in
the upper part and then it
varies between shrinkage and
plastic limits in the deeper
layer.
2.3. Double-ring
infiltrometer tests
In addition to visual
inspection of desiccation
cracking, double-ring
Figure 6. Desiccation fissures observed in a trial trench of depth 1 m in the historic embankment at
infiltrometer tests were carried
Thorngumbald (2006)
out during the first site visit in
2003 to measure the effect of
fissuring on the mass
permeability of the clay fill.
The rings were driven approximately 10 mm into the crest of the
new embankment to provide a seal (see Figure 12). The recorded
infiltration rate is illustrated in Figure 13a, which shows water
seepage occurred rapidly and the corresponding mass
permeability is three orders of magnitude higher than typical
rates for clayey soils. Elsewhere on fissured sections of the new
embankment, the water drained away too rapidly to allow any
meaningful readings to be taken. For comparison, the
infiltration rate measured for a double-ring infiltrometer test
carried out on an unfissured section of the historic embankment
is shown in Figure 13b. The resulting mass permeability is
comparable to that of sand. This pronounced difference in
infiltration rates is entirely due to the presence of desiccation
fissures. The presence of fissures radically increases the mass
permeability of clay fill to that of coarse-grained soil or cracked
rock.
0
Moisture content
Liquid limit
Plastic limit
0·1
0·3
0·4
0·5
Crack spacing
at surface
Protuberance
Figure 7. (a) Pattern of desiccation fissures on crest of new
embankment (2006); (b) sketch of desiccation fissures from
Konrad and Ayad16 experiments (not to scale)
Water Management 162 Issue WM3
0·6
0·7
10 15
(b)
226
Depth: m
0·2
Cracks below
shear plane
20
25 30 35 40 45 50
Moisture content: %
55
60
65
70
Figure 8. Moisture content profile in the trial trench excavated
below crest of embankment (2006)
Field survey of desiccation fissuring of flood embankments
Dyer et al.
0
Moisture content: %
40
60
20
80
100
0
A, front crest
B, rear crest
Depth: m
0·5
B
1·0
A
1·5
SL
PL
LL
Figure 11. Moisture content profiles at North Grain2
achieved do not give any indication about local soil
permeability, the results are nevertheless reliable enough to
indicate that overflowing water is likely to seep into the crest
rather than the inward slope of the embankment.
Figure 9. Trial trench excavated at North Grain2
These observations concur with anecdotal evidence from fullscale cofferdam tests carried out by Marsland and Cooling3 in
which water overtopping the crest failed to reach the landward
side because the water drained into the crest of the trial
embankment too rapidly, which led (within a matter of minutes)
to the progressive collapse of the landward face eventually
resulting in breach. Finally, it is worth noting that double-ring
infiltrometer measurements are difficult to repeat, as noted by
Marsland and Cooling in their attempts to measure the amount
of water seeping through a bank. Although the measurements
Trench No. 1 crest
Trench No. 1 slope
Trench No. 2 crest
Trench No. 2 slope
0
10
Trench No. 3 crest
Trench No. 3 slope
Trench No. 4 crest
Trench No. 4 slope
Moisture content: %
20
30
3. POSTULATION OF A NEW FAILURE MECHANISM
The field observations and infiltrometer tests highlight the
potential for desiccation fissuring to alter the fabric and texture
of fill material and increase mass permeability by several orders
of magnitude. This is caused by interconnected patterns of
fissures that allow rapid seepage of water into a desiccated zone.
Based on these observations, Figure 14 shows a proposed failure
mechanism in which excessive internal seepage during overflow
conditions leads to the uplift of desiccated clay blocks and
progressive slope failure. It is proposed that the network of
shallow desiccation fissures could allow a critical hydraulic head
to be developed beneath the outward slope, leading to the uplift
of desiccated blocks of rubblised fill material. This failure mode
depends on hydraulic continuity between the embankment crest
and the outward slope to allow a critical hydraulic head to be
developed for uplift to take place. It would only need a small
section of desiccation to be uplifted for a progressive shallow
40
0
Depth: m
0·2
0·4
0·6
0·8
1·0
1·2
SL
PL
Figure 10. Moisture content profiles in trial trenches excavated
below crest and side slopes of embankment (2003)
Water Management 162 Issue WM3
Figure 12. Surface crack pattern within the inner ring
Field survey of desiccation fissuring of flood embankments
Dyer et al.
227
8000
Test 2 (new embankment)
Soil type
Constant
infiltration rate:
mm/h
Sand
Sandy loam
Loam
Clayey loam
Clay
<30
20–30
10–20
5–10
1–5
7000
Infiltration rate: mm/h
6000
5000
4000
3000
2000
1000
0
1
10
100
1000
Cumulative time: s
(a)
120
110
Test 1 (old embankment)
100
Infiltration rate: mm/h
90
80
Soil type
Constant
infiltration rate:
mm/h
Sand
Sandy loam
Loam
Clayey loam
Clay
<30
20–30
10–20
5–10
1–5
70
60
50
40
30
20
10
0
0·10
1·00
Cumulative time: h
(b)
Figure 13. (a) Infiltration rate from the double-ring infiltrometer test run on a fissured zone in the new embankment and (b) an unfissured
zone in the historic embankment
slope failure to develop and migrate upwards to the crest. The
resulting collapse of the crest would release a torrent of flood
water and lead to rapid growth of the breach. This scenario
agrees with the observations reported by Cooling and Marsland
for large-scale cofferdam experiments. However, Cooling and
Marsland attributed the slope failure to an increase in pore
pressure. Based on these new field investigations, this is unlikely
to occur or to be a primary factor in slope failure.
228
Some possible remedial measures to limit or prevent fissuring
are available, for example
4. LABORATORY STUDIES
Small-scale laboratory studies were undertaken to investigate
the onset of desiccation cracking. A record of soil types and
classification properties is shown in Table 2. Cracking was
induced by allowing discs of soil to dry on both a laboratory
bench and inside a pressure plate under increasing soil suction.
For the laboratory bench tests, soil samples were remoulded at
the liquid limit and placed into cylindrical moulds of various
diameters (66–122 mm) and thicknesses on a glass plate to
compare the effect of geometry on the onset of desiccation
fissures.
(a) removal of topsoil from the area affected by fissures and
replacement with either the same material or hoggin
(b) realisation of a berm on the landward side
(c) insertion of a sheet pile cut off longitudinally along the
centre of the embankment crest
(d ) grouting of granular material into clay, which reduces its
plasticity and therefore its tendency to fissure
(e) the use of geotextiles or geogrids.
For the pressure plate tests, soil discs were prepared using five
rubber 66 mm diameter sample rings; each was numbered,
weighed and measured. The soil had been mixed at or just
above the liquid limit, ensuring the soil was of good consistency
throughout. The samples were subjected to air pressures to
create suction within the samples. The pressure was gradually
increased to allow equilibrium of the samples’ pore water
pressure to the air pressure being applied. This allowed gradual
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Field survey of desiccation fissuring of flood embankments
Dyer et al.
Potential slippage
of inward face
Flood level
Stage A
Water flow through
the cracked surface
Flood level
Stage B
Uplifted blocks
Flood level
Stage C
Flood level
Breach
Stage D
Figure 14. Proposed failure mechanism with breach formation of uplifted clay blocks
desaturation of the samples to occur. Each pressure stage (50,
100, 200, 300, 400 and 500 kPa) was applied to the samples for
two days to ensure equilibration of pore pressures. The samples
were weighed between successive pressure stages in order to
calculate water loss. Once each stage had been completed, the
five samples were dried in an oven and weighed to determine
the soil water characteristic curve (SWCC).
Water Management 162 Issue WM3
4.1. Critical cracking ratio
Cracking was investigated in the laboratory bench tests by
progressively reducing the thickness of the soil discs. The
geometry and relative proportion of the soil discs were observed
to influence the development of desiccation fissures, in
particular the development of a pronounced crack across the
diameter of the soil sample.
Field survey of desiccation fissuring of flood embankments
Dyer et al.
229
25
Liquid Plastic Plasticity Critical
limit: limit: index: % cracking
ratio
%
%
67.8
55.5
33.5
24.7
34.3
30.8
12.0
10.2
121.4
91.0
49.1
61.8
43.9
25.1
59.6
47.0
24.0
4.5
7.3
12.0
20
Critical cracking ratio
Kaolin (China clay)
Brown Birtley clay; glacial
till from Durham county
Wyoming bentonite
50% bentonite/50% kaolin
Thorngumbald clay
Laid at LL
Laid at PL + 0·5PI
Decreasing initial
moisture content
15
10
5
Table 2. Critical cracking ratios of the five clays investigated when
dried from the liquid limit
0
20
The effect of geometry and relative sample thickness can be
characterised by considering the critical cracking ratio (CCR),
first defined by Dyer,31 which is the ratio of the diameter to the
depth of the sample when one crack extends diametrically
across the sample as shown in Figure 15.32 A unique CCR was
determined for each clay type tested.
To investigate the effect of initial moisture content on the onset
of cracking, soil samples were prepared with different initial
moisture contents chosen to be halfway between the liquid limit
and plastic limit (i.e. PL þ 0.5PI). A comparison of the samples
prepared at the liquid limit and those halfway between liquid
and plastic limits is shown in Figure 16.32 The results indicate a
relationship between plasticity index and CCR. In addition, the
graph shows a significant increase in CCR when samples were
prepared in the drier condition. This suggests that the extent of
cracking and the propensity of fissures to form decreases with
lower initial soil moisture content.
4.2. Comparison between laboratory and field studies
The SWCC determined from the pressure plate tests allowed a
link to be established between the onset of desiccation cracking
of the soil discs in the pressure plates and the corresponding soil
suction. The soil discs in the pressure plates were constructed
with a CCR of 12. Desiccation fissuring was observed to take
place at a matric suction of 300 kPa. Based on the SWCC, the
soil suction corresponded to a moisture content of
approximately 16%. This result is in good agreement with the
30
40
50
60
Plasticity index: %
Figure 16. Relationship between critical cracking ratio and
plasticity index, comparing samples laid at the liquid limit and
midway between the plastic and liquid limit32
moisture content determined for the disturbed soil samples
taken from the desiccated zone in the Thorngumbald flood
embankment (Figure 17).
The laboratory and field studies suggest that the susceptibility
of different clayey fills to desiccation fissuring can be replicated
in a laboratory by small soil discs with geometry corresponding
to the CCR. The moisture content or soil suction at which the
material is likely to develop significant desiccation fissures in
the field can thus be predicted and hence monitored.
5. CONCLUSIONS
The Thorngumbald field study yielded new information about the
extent, depth and pattern of desiccation fissuring that can occur
in a flood embankment constructed from a medium-plasticity
cohesive fill. Desiccation fissures typically propagated
perpendicular from the drying surface (crest surface or side
slopes) and bifurcated into lateral fissuring at a depth of
approximately 30 cm, resulting in an orthogonal network of
desiccation fissures. The orthogonal network of fissures
significantly increased the mass permeability of the embankment,
which, according to infiltrometer test data, resulted in a
permeability similar to that of coarse sand or gravel.
Based on these observations, a new failure mechanism for an
embankment subject to desiccation fissuring has been proposed:
Gravimetric moisture
content: %
60
50
40
30
20
Field cracking
10
0
Lab cracking
0
100
200
300
400
500
Matric suction: ua – uw kPa
Figure 15. Example of a major crack running from edge to edge;
note the presence of minor fissures in the same direction as the
major one32
230
Water Management 162 Issue WM3
Figure 17. Gravimetric moisture content versus matric suction
for the tested Thorngumbald clay
Field survey of desiccation fissuring of flood embankments
Dyer et al.
seepage through rubblised fill results in uplift or washout of
blocks of clay fill; this leads to progressive slope failure and
hence breach initiation when the collapse extends up to the
embankment crest. This is a new and alternative interpretation
of the mechanics of breach initiation due to desiccation
fissuring.
Furthermore, based on the results of laboratory studies, the
concept of the critical cracking ratio has been proposed to relate
sample geometry (ratio of diameter to thickness) to the onset of
desiccation fissuring. The concept was tested on five different
soils with different plasticity indices. The onset of desiccation
cracking in pressure plate tests showed good agreement with the
field studies in which soil suction and moisture content
coincided on the SWCC.
Combining the field work and laboratory results, it seems that
desiccation fissuring can take place within a relatively short
period of time after construction for medium or highly plastic
clay fills. The resulting desiccation fissuring can typically
extend to a depth of 60 cm. However, the crucial factor in terms
of embankment stability is the onset of an orthogonal pattern of
fissures that allows lateral seepage of flood water. Observed to
take place within a depth of 30 cm, relatively homogeneous fill
material is transformed into a rubblised layer of desiccated
blocks of clay. Double-ring infiltrometer testing has been shown
to be a useful tool for the detection of extensive desiccation
cracking with possible increase in mass permeability to that of
coarse sand or gravel.
ACKNOWLEDGEMENTS
The study was carried out with financial support from the
Engineering and Physical Sciences Research Council flood risk
management research consortium (FRMRC) and the Department
for Environment, Food and Rural Affairs/Environment Agency
research and development programme.
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