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An open-dug caisson shaft is a form of top-down construction in which a concrete shaft is sunk into the ground using the weight of the shaft and additional kentledge, if required. Excavation at the base of the caisson shaft wall allows the structure to descend through the ground. A thorough understanding of the interaction between the caisson shaft and soil is essential to maintain controlled sinking of the caisson. In this paper, the failure mechanisms developed beneath caisson blades in sand are investigated. A series of laboratory tests were carried out at the University of Oxford to explore how varying blade angles affect the performance of the bearing capacity beneath the caisson. Cutting angles of 30°, 45°, 60°, 75° and 90° (flat) were penetrated into sand under plane strain conditions; forces were monitored using a Cambridge-type load cell while soil displacements were recorded using Particle Image Velocimetry (PIV) techniques. The aim of this study is to understand how the soil failure mechanism develops and to determine the optimum cutting angle. The results of the laboratory tests can be scaled to predict the likely behaviour in the field. Results show that the bearing capacity is significantly dependent on the cutting angle; in a dense sand a steep cutting angle may be used to aid sinking of the caisson.
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ABSTRACT: An open-dug caisson shaft is a form of top-down construction in which a concrete shaft is sunk into the ground
using the weight of the shaft and additional kentledge, if required. Excavation at the base of the caisson shaft wall allows the
structure to descend through the ground. A thorough understanding of the interaction between the caisson shaft and soil is essential
to maintain controlled sinking of the caisson. In this paper, the failure mechanisms developed beneath caisson blades in sand are
investigated. A series of laboratory tests were carried out at the University of Oxford to explore how varying blade angles affect
the performance of the bearing capacity beneath the caisson. Cutting angles of 30°, 45°, 60°, 75° and 90° (flat) were penetrated
into sand under plane strain conditions; forces were monitored using a Cambridge-type load cell while soil displacements were
recorded using Particle Image Velocimetry (PIV) techniques. The aim of this study is to understand how the soil failure mechanism
develops and to determine the optimum cutting angle. The results of the laboratory tests can be scaled to predict the likely
behaviour in the field. Results show that the bearing capacity is significantly dependent on the cutting angle; in a dense sand a
steep cutting angle may be used to aid sinking of the caisson.
KEY WORDS: Caisson; tapered blade; bearing capacity; laboratory testing.
1 INTRODUCTION
An open-dug caisson shaft is a form of top-down
construction. They have many functions in industry such as
launch and reception pits for tunnel boring machines and
underground storage tanks for foul and storm water attenuation.
Caissons can be sunk through various soil types including sand,
clay and rock. It can be a very safe and efficient form of
construction as the permanent structure is used to retain the soil
and water during excavation as shown in Figure 1.
Figure 1 30 m diameter reinforced concrete caisson at
Anchorsholme Park, Blackpool, constructed by Ward and
Burke Construction
In order for a caisson to sink into the ground, both the skin
friction that develops between the soil and the concrete shaft in
addition to the bearing capacity of the soil beneath the shaft
walls must be overcome. A common technique to aid the
sinking process is the use of a tapered blade, or cutting edge,
beneath the wall of the shaft. The purpose of the cutting edge is
to cut into the ground, anchor the caisson horizontally and
maintain verticality. It mobilises the failure mechanism of the
soil towards the centre of the shaft, so that the soil in this area
can be easily excavated to allow the caisson to sink. In order to
achieve controlled sinking of the shaft, a thorough
understanding of the bearing failure that develops in different
soils is therefore essential. It ensures operatives in the field
know where to excavate to induce bearing failure beneath the
shaft walls. Moreover, a reduction in the bearing capacity
beneath the caisson blades means less kentledge will be
required to get the shaft to formation level.
The bearing capacity of sloped caisson footing depends on
the angle of the tapered blade, roughness of the interface, angle
of friction of the soil, width of the footing and the unit weight
of the soil. In this paper it is assumed that the caisson diameter
is large, 2D plane strains will develop with minimal conical
action. While some work has been undertaken on sloped
conical footings [1-2], limited information is available on 2D
plane strain conditions for tapered footings.
Tomlinson [3] recommends various tapered angles
depending on the soil type; flatter cutting angles are
recommended for sand compared to clay. Nonveiller [4]
describes various potential rupture surfaces. As the caisson
penetrates into the soil, the resisting forces increase until a new
state of equilibrium is achieved.
A method for quantifying the bearing capacity factor, Nγ,
using limit-equilibrium theory was proposed to quantify the
bearing capacity of tapered footings beneath caissons [5-6].
According to this approach, the value of Nγ is less sensitive to
the angle of friction compared to the blade angle and
embedment depth of the wall. However, some important
parameters are neglected in this method, such as the interface
friction between the soil and sloped blade of the caisson.
Bearing capacity beneath tapered blades of open dug caissons in sand
Ronan Royston1, 2, Bryn M. Phillips1, 2, Brian B. Sheil1, Byron W. Byrne1
1 Department of Engineering Science, University of Oxford, UK
2 Ward and Burke Construction, Kilcolgan, Co. Galway
Email: ronan.royston@eng.ox.ac.uk, bryn.phillips@hertford.ox.ac.uk, brian.sheil@eng.ox.ac.uk, byron.byrne@eng.ox.ac.uk
The aim of this study is to explore the performance of various
tapered angles in sand through a series of laboratory tests.
Particle image velocimetry (PIV) analysis is employed to track
the failure planes developed in the soil during testing. In
addition, a number of tests were carried out to examine the
influence of overburden pressure during sinking.
2 EXPERIMENTAL EQUIPMENT
A three degree of freedom loading rig, developed at the
University of Oxford [7], was used for the laboratory testing in
this study (see Figure 2 and 3). A Cambridge-type load cell,
attached to the end of the loading arm, records vertical,
horizontal and moment reaction.
Figure 2 Three degree of freedom loading rig
The loading rig was located on top of a 600 mm x 300 mm x
95 mm testing tank which has a perspex front, which allows the
sand movements to be recorded during testing, see Figure 4. A
Nikon DS3200 camera located 700 mm from the front of the
tank was used to capture images at a frequency of 1 Hz during
testing. A downward penetration rate of 0.5 mm/sec was
adopted for all tests; rate effects in sand are not expected to be
significant. PIV analysis was carried out by processing the
images using MATLAB module GeoPIV [8].
The experiments were conducted using a dry, yellow
Leighton Buzzard DA30 silica sand, the properties of which are
summarised in Table 1.
Table 1 Properties of Leighton Buzzard DA30 sand
Property
Value
D10, D30, D50, D60, D90, (mm)
0.36, 0.45, 0.51, 0.54, 0.65
Specific Gravity, Gs
2.73
Minimum dry density, min (kN/m3)
14.5
Maximum dry density, max (kN/m3)
17.1
Critical state friction angle, cs (o)
32
Loose sand samples were prepared using a sand raining
procedure in conjunction with a low drop height. Dense
samples were prepared by vibrating the testing tank after sand
raining. To ensure a repeatable bulk density was achieved for
each sample, the tank was filled using the same drop height for
all tests.
Figure 3 Test setup and soil failure wedge
A schematic of a typical cross section of a cutting edge and
possible soil failure plane is provided in Figure 3, where B is
the width at embedment, A is the length of the face, β is the
tapered cutting angle, QV and QH are the vertical and horizontal
reactions, R is the resultant reaction of the forces, and X and Y
are the width and depth of the failure plane. It is worth noting
that value of β generally used in industry is 45o, derived from
on-site experience.
Aluminum pieces were created as the test pieces of varying
cutting angles, β. In order to reduce the friction between the test
pieces and the sides of the tank, 1 mm polytetrafluoroethylene
(PTFE) sheets were placed either side of the piece with
compressible foam placed in between the PTFE and the test
piece. The test piece spanned the width of the test tank to ensure
plane strain conditions.
Figure 4 Testing tank
Figure 5 Cone Penetration
3 VALIDATION OF TESTING PROCEDURES
Cone tests were performed, with an 8 mm diameter cone and
a 60° cone angle, to examine the uniformity of the sample. The
cone was penetrated 150 mm into soil samples prepared with
three different relative densities, ID, as per Equation (1), where
γ is the density of the prepared sample. The cone was attached
to the rig as shown in Figure 5 and penetrated through the sand
at a rate of 1 mm/s. From the results presented in Figure 6, the
cone penetration resistance appears consistent with depth thus
indicating a uniform sample. Moreover, it is obvious that at
higher relative densities, there is a commensurate increase in
the cone resistance.

 
 
Figure 6 Cone penetration results
A series of preliminary tests were carried out to validate the
present sand properties, sample preparation and experimental
techniques. Careful consideration of the stress level is required
in order to extrapolate model-scale laboratory testing to
expected behavior in the field. At higher stress levels the
dilatancy of soil is suppressed; relationships proposed by
Bolton [9] are used to relate the critical state angle of friction
to the peak angle of friction based on relative density and the
stress state of the sand in the laboratory testing. Equations (2)
and (3) are based on plane strain conditions for the relationship
between critical and peak angles of friction based on the
isotropic stress and density of the sample:
  
(2)

(3)
where max and cs are the peak and critical state friction
angles, respectively, p’ is the mean effective stress in the soil at
failure and ID is the relative density of the soil.
In order to validate the test results and sample preparation, a
90° piece (flat piece) was used to compare present
measurements to published literature. The relative density of
each test sample was calculated using the data presented in
Table 1 which, in turn, was used to determine the value of max.
Bearing failure was assumed to occur at 0.1B, neglecting soil
cohesion and influence of overburden, the bearing capacity, qrd,
can therefore be defined as follows:


(4)
where B is the width of the flat footing, Nγ is a bearing capacity
factor and  is the effective unit weight of the soil.
Test results for the flat pieces are shown in Figure 7.
Theoretical bearing loads are based on the approaches for
calculating Nγ by Hansen [10] and Meyerhof [11]. Test results
are consistent with theory and provide additional confidence in
the experimental set up and the Bolton method [9].
Figure 8 shows an example of the incremental displacements
in the soil obtained using PIV; the predicted failure plane
according to Rankine theory, with a friction angle of 32°, has
also been superimposed on the image. The sample in Figure 8
is for a dense sample which has a peak angle of friction of 45o
when applying Equations (2) and (3). This is not consistent with
the 32o overlaid as the dilation of the sand is suppressed at
higher densities.
The development of a triangular active wedge beneath the
footing, in addition to the passive wedges, is obvious from this
output.
Figure 7 Theoretical and measured bearing capacities
Figure 8 PIV of flat piece test
4 TESTING OF TAPERED ANGLE PIECES
A series of tests were carried out using cutting angles, β, of
30°, 45°, 60° and 75°. The test set-up for the cutting edges is
shown in Figure 3. Each test was carried out on a medium-
dense sample. The medium-dense tests have a relative density
ranging between 0.44 and 0.54.
Influence of cutting angle, β
Figure 9 plots the influence of β on the variation of R with
penetration. The resultant forces are higher for the shallower
angles; this is attributable to the much greater bearing width of
the flatter angles for the same penetration.
The influence of β on the relationship between QH and QV is
examined in Figure 10 and Figure 11. The relationships
presented in Figure 10 are remarkably linear where the steeper
cutting angles reduce the vertical reaction. The ratio of
horizontal to vertical force is shown in Figure 11 based on a
best fit line to the results in Figure 10. In general, there appears
to be a linear variation in QH/QV with β. It is worth noting that
QH is as high as 0.9QV for a 30° cutting angle which could result
in a large hoop tension force in the wall of the caisson.
ID=0.16
ID=0.48
ID=0.67
0
20
40
60
80
100
050 100 150
Reaction (N)
Penetration (mm)
Medium
Dense
0
50
100
150
200
250
300
350
400
0.00 0.20 0.40 0.60 0.80 1.00
qrd (kPa)
Relative Density, Id
Bearing Capacity -
Hansen [10]
Bearing Capacity -
Meyerhof [11]
Bearing Capacity
Test Results
Symmetry
Figure 9 Resultant force of angle pieces
Figure 10 Horizontal against vertical forces
Figure 11 Ratio of Horizontal to vertical forces
Figure 12 shows the vertical bearing capacity of the footing,
qrd,v, against the embedment width. All angled footings
illustrate a similar increase in bearing capacity with embedded
width. The theoretical vertical bearing capacity of a flat footing
is also plotted for various values of B. The bearing capacity of
the flat footing is approximately 1.4 times the bearing capacity
of the angle piece at values of B less than 50 mm. As the
bearing pressure increases, there is a change in the rate in the
increase of the bearing capacity. This could be attributable to
the stress-state of the soil as penetration progresses; additional
numerical work is being conducted to explore this aspect.
Figure 12 Effect of cutting angle on Nγ
Figure 13 Average Face Pressure against footing width
In Figure 13, the variation of the pressure at the cutting face,
R/A, is plotted against B. This framework appears to provide
improved agreement between tests and appears to be relatively
invariant to β. The slight differences in resultant pressures
could be based on the slight differences in the relative densities
and test discrepancies.
The failure mode for the different footing varies depending
on the tapered angle, β. Figure 14 (a-d) shows the incremental
total displacements of the soil; an embedment width of B=40
mm was chosen for these comparisons. At this stage of testing,
0
250
500
750
1000
020 40 60
R (N)
Penetration (mm)
β=30
β=45
β=60
β=75
0
50
100
150
200
0 200 400 600 800 1000 1200
QH(N)
QV(N)
β=30
β=45
β=60
β=75
0.0
0.2
0.4
0.6
0.8
1.0
30 45 60 75 90
QH/QV
β
0
20
40
60
80
100
120
525 45 65 85
qrd,v (kPa)
B (mm)
Flat Footing
β=30
β=45
β=60
β=75
0
20
40
60
80
100
525 45 65 85
R/A (kPa)
B (mm)
β=30
β=45
β=60
β=75
the face pressures are similar (see Figure 13). The failure
mechanism occurs towards the excavation side, as shown in
Figure 14, as the tapered angles move into the active wedge of
soil according to the Rankine theory for soil bearing failure. For
the flatter angles, as the failure stresses in the soil develop, the
soil begins to fail towards the excavation and overburden side,
similar to a traditional flat footing.
FigureFigure 15 show how the zone of influence varies with
β. The depth and width of the failure zone is much higher for
the steeper tapered angles at B=40 mm.
(a)
(b)
(c)
(d)
Figure 14 Resultant displacement at B=40mm: (a) β=75o,
(b) β=60o, (c) β=45o, (d) β=30o
(a)
(b)
Figure 15 (a) Horizontal and (b) vertical zone of influence;
B=40mm
Influence of overburden on one side
In the field, there will always be a surcharge outside the
caisson from the overburden soil as excavation and sinking of
the shaft commences. This surcharge will increase as the
caisson sinks further into the ground. A surcharge was applied
by applying a weight on the overburden side (see Figure 3).
Surcharges of 12.5kPa, 25kPa and 50kPa were considered.
The overburden has two effects on the soil and the resulting
failure mechanism. Firstly it increases the initial stress in the
soil and secondly, it will encourage failure of the soil towards
the excavation side. Figure 16 andFigure 18 shows the effects
of the overburden against the base case of a level surface.. It
can be clearly seen that the overburden increases the bearing
capacity of the soil beneath the footing.
The increase in bearing capacity is more sensitive to the
shallower tapered angles, as the increase in resultant reaction is
larger for β equal 75° compared to the β equal 45° test. The
failure mechanism of the soil is shown in Figure 17 and Figure
19. For the higher surcharge pressures, the failure mechanism
varies and is pushed towards the excavation side and the failure
plane becomes larger.
The failure mechanism occurs towards the excavation side,
as shown in Figure 14 for flatter angles (c-d), as the tapered
angles move into the active wedge of soil according to the
Rankine theory for soil failure. For the shallower tapered
angles, as the failure stresses in the soil develop, the soil begins
to fail towards the excavation and overburden side, similar to a
traditional flat footing. This can be clearly seen in Figure 19
(g), as the soil failure mechanism develops on both sides of the
75° piece.
Figure 16 Influence of surcharge on resultant force; β=45,
(a)
(b)
Figure 17 (a) 12kPa surcharge (b) 50kPa surcharge;
B=40mm, β=45, see Figure 16
0.0
2.0
4.0
6.0
30 45 60 75
X/B
β
0.0
1.0
2.0
3.0
30 45 60 75
Y/B
β
(a)
(b)
0
100
200
300
400
500
020 40 60
R (N)
Penetration (mm)
50kPa
12kPa
No Surcharge
Figure 18 Influence of surcharge on resultant force; β=75,
(a)
(b)
(c)
(d)
(e)
(f)
(g)
Figure 19 (a+b) No surcharge (c+d) 25kPa surcharge (e+f)
50kPa surcharge (g) No surcharge, B=100mm and 35mm
penetration; β=75, see Figure 18
5 CONCLUSIONS
In this paper, a suite of tests carried out on small-scale
tapered footings in medium-dense sand has been presented. The
authors have arrived at the following conclusions arising from
this study:
a) The forces that develop at the base of a caisson are
largely dependent on the tapered angle of the footing.
While steeper footings have a much reduced vertical
resistance, at the same penetration, this is offset by an
increase in the horizontal reaction. The bearing pressure
on the face of the footing appears to be invariant to the
cutting angle of the test piece and varies linearly with the
embedment width of the piece.
b) Applying an overburden pressure on one side of the
footing best models conditions in the field. The
overburden has a greater influence on flatter cutting
angles since failure wedges are developed on both sides
of the cutter unlike steep angles where the failure wedge
develops on the excavation side only. The overburden
forces failure to occur on the excavation side, thereby
increasing the bearing capacity.
c) Output from the PIV analysis of the present tests can be
used to guide excavation on site in order to induce failure
of the soil beneath the caisson.
d) Further work is being carried out at University of Oxford
to extend these results and confirm the applicability to
conditions in the field. In particular, further comparisons
with full-scale measurements will be carried out to
ensure that the proposed methods are reliable and valid.
ACKNOWLEDGMENTS
The authors gratefully acknowledge the financial support
provided by Ward and Burke Construction Ltd.
REFERENCES
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conical footings on clay,” Geotechnique, vol. 53, no. 5, pp. 513520,
2003.
[3] M. J. Tomlinson, Foundation Design and Construction, 7 edition.
Harlow, England; New York: Prentice Hall, 2001.
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vol. 113, no. 5, pp. 424439, 1987.
[5] N. B. Solov’ev, “Use of limiting-equilibrium theory to determine the
bearing capacity of soil beneath the blades of caissons,” Soil Mech.
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[7] C. M. Martin, “Physical and numerical modelling of offshore foundations
under combined loads,” University of Oxford, 1994.
[8] S. A. Stanier, J. Blaber, W. A. Take, and D. White, “Improved image-
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(c)
(e) (d)
(f)
(a) (b)
(g)
0
400
800
1200
1600
2000
020 40 60 80 100
R (N)
B (mm)
50kPa
25kPa
No Surcharge
... Moreover, these studies have not mentioned about the configuration of the cutting edge used. In order to address the interaction between the soil and the cutting edge during sinking, Royston et al. (2016) have carried out 1g model tests to evaluate the effect of different cutting angles of the cutting edge on the influence zone in the sand considering the plane strain idealisation. However, the circular open caisson problem is an axisymmetric problem and hence further investigations are needed to study the interaction between the soil and the cutting edge during sinking considering the axisymmetric nature of the problem. ...
... The same is observed from the image analysis in this study (Section 4.2). Due to the inward inclined face of the cutting edge, the soil flow is observed only within the caisson for the steeper cutting angles (Chavda and Dodagoudar, 2018;Royston et al., 2016). The experimental and numerical studies performed by Royston et al. (2016) and Chavda and Dodagoudar (2018) on the effect of cutting angles on the zone of influence have clearly shown that the extent of the influence zone in the horizontal and vertical directions is 4B and 2.5B within the caisson, respectively, where B is the width of the cutting edge. ...
... Due to the inward inclined face of the cutting edge, the soil flow is observed only within the caisson for the steeper cutting angles (Chavda and Dodagoudar, 2018;Royston et al., 2016). The experimental and numerical studies performed by Royston et al. (2016) and Chavda and Dodagoudar (2018) on the effect of cutting angles on the zone of influence have clearly shown that the extent of the influence zone in the horizontal and vertical directions is 4B and 2.5B within the caisson, respectively, where B is the width of the cutting edge. However, in this study a cutting angle of β = 90°is also used, which represents the case of the ring footing. ...
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... It is evidenced that the construction scales of bridge foundations, tunnels, excavations and open caissons continue to enlarge, highlighting the sustained importance of large and deep excavation works. Recently, large diameter deeply-buried (LDDB) open caissons (inner diameter D in ≥ 15 m and buried depth H ≥ 30 m) are widely used to provide a temporary access to the subsurface for piping and tunnelling, or as permanent works, are utilized for deep foundations, elevators, underground storage, ventilations, pumping stations and sewerage purposes (Dachowski and Kostrzewa, 2017;Fischer et al., 2004;Khasawneh et al., 2017;Lai et al., 2020, Lai et al., 2021Li et al., 2022;Royston, 2018;Royston et al., 2016;Schwamb, 2014;Tomlinson and Boorman, 2001). The cross section of caisson shafts can be rectangular or circular. ...
... However, both techniques are computationally expensive and time-consuming. Some analytical solutions to estimate the ultimate bearing capacity of cutting edges (Solov'ev, 2008;Yan et al., 2011;Royston et al., 2016, Royston et al., 2021 or earth pressure (Cho et al., 2015;Kim et al., 2013;Liu, 2014;Tobar and Meguid, 2010) on the caisson shafts were thus provided as theoretical basis for numerical simulation. Nevertheless, due to their complexity, the existing analytical solutions were yet constrained to use in practice. ...
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... However, the different configuration of the cutting edge, i.e., variation in the tapered angles of the cutting edge is not explicitly accounted in their study. It is noted that only one study reports the experimental evaluation of the influence zone in sand for varying tapered angles of the cutting edge considering plane strain idealization [22]. The bearing capacity of ring footing is evaluated by experimental investigations [6,10,12,20,23,24], the limit equilibrium method, the upper-and lower-bound plastic limit analyses [16], the method of characteristics [11,15], the finite difference method [3,4,13,21,35], and the finite element method [7,9,17,18,30]. ...
... In many projects, the large diameter caissons (D o > 10 m) are used (e.g., [2,19,22,25,34]). In such projects, the sequential excavation is adopted for the uniform and control sinking of the caisson. ...
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The analytical evaluation of the bearing capacity of the ring footing and cutting edge of the open caisson requires a definition of the size of failure zone in the soil, whereas the failure zone in soil depends on the type of soil and the configuration of the ring footing and cutting edge of the open caisson. In the study, the 1 g model tests are carried out to evaluate the failure zone in the sand beneath the ring footing and cutting edge of the circular open caisson. The radii ratio of the ring footing and cutting edge of the open caisson models are varied as ri/ro = 0.615, 0.737, and 0.783, and the tapered angle of cutting edge models is varied as β = 30° and 45°. The radius ratio is the ratio of internal radius to the external radius of the ring footing and cutting edge of the open caisson. The ring footing and caisson models are fabricated using Teflon tubes, and Indian standard sand is used as soil medium. The image-based deformation measurement technique is used to evaluate the failure zone in sand. The image analysis results are validated from the rigid displacement test.
... The purpose of the cutting face is to reduce the vertical bearing capacity and encourage propagation of the soil failure mechanism towards the centre of the caisson for excavation . An accurate estimate of the vertical bearing capacity is needed to ensure the caisson self-weight overcomes soil penetration resistances in a controlled manner during the sinking process (Royston et al., 2016). ...
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Large-diameter open caissons are an increasingly common means of constructing underground storage and attenuation tanks, as well as launch and reception shafts for tunnel boring machines. A ‘cutting face’ at the base of the caisson wall, resembling an inclined ring footing, is typically used to aid the sinking phase. This paper describes a suite of over 15,000 finite element limit analyses exploring the bearing capacity of a caisson cutting face, partially- or wholly-embedded in undrained soil. The primary aim of the study is to assess the influence of the cutting face inclination angle on the vertical bearing capacity. The effects of cutting face roughness, internal overburden and surcharge, and caisson radius are also investigated. In particular, the results indicate that a steepening of the inclination angle may not always reduce the bearing capacity, if the cutting face is rough. The numerical output informs the development of a closed-form approach for application in routine design. The new design method is shown to provide an excellent representation of the numerical output.
... The bearing failure at the cutting edge is important to permit the controlled sinking of the open caisson. Recently, Royston et al. (2016) conducted the experiments to identify the influence zone at the cutting edges in the sand under plane strain condition. Using Particle Image Velocimetry technique, soil displacement contours are plotted for varying cutting edge angles of 30°, 45°, 60°, 75° and 90°. ...
Chapter
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Open caissons are sunk into the ground by removal of soil within the caisson shaft. During sinking of caisson, the stresses in the soil at the cutting edge increase and result in the bearing failure of soil. The extent of soil failure in the excavation side of the open caisson is termed as influence zone. In this paper, the finite element analysis is carried out to study the effect of geometric configuration (radius ratio and tapered angle of the cutting edge), strength parameters (c’ and φ'), unit weight of soil and surcharge on the extent of the influence zone. The caisson considered in the study is having a radius ratio of 0.8, steinning thickness of 1 m and cutting edge with a tapered angle of 45°. The reaction offered by the soil at the cutting edge is also evaluated. The identification of the extent of failure zone at the cut-ting edge of the caisson helps in devising proper excavation strategy in the field for the controlled sinking of the caisson.
... A thorough understanding of the different aspects of the open caisson, such as load-penetration response, soil flow mechanism beneath the cutting edge and bearing capacity of the cutting edge, each one corresponding to the varying cutting angles, will help mitigate problems like tilting, shifting, freezingthat is, no further sinking, and sudden collapse of the caisson during the sinking (Chatterjee and Dharap, 1999;Maslik et al., 1978;Panda, 2015;Saha, 2007;Salamov et al., 1965;Ter-Galustov et al., 1966). In order to study the soil flow mechanism around the cutting edge, Royston et al. (2016) In order to observe and quantify the soil displacement around the cutting edge, the image-based deformation measurement technique is employed. The use of such techniques for measuring large soil displacements has been reported and appreciated by several researchers (Iskander, 2010;Rechenmacher and Finno, 2004;Stanier et al., 2016;Take, 2015;White et al., 2003;Yuan et al., 2017). ...
Article
Open caissons are sunk into the ground by their own weight. A cutting edge of the caisson having a tapered inner face on loading – that is, raising of the steining – results in bearing failure by displacing the soil which is in contact with the cutting edge. The bearing capacity of the cutting edge and the soil flow mechanism depend on the configuration of the cutting edge, sinking depth and soil type. This paper presents the results of a series of 1g model tests, which investigate the effect of varying tapered angles of the cutting edge on the penetration resistance of the open caisson. The vertical failure load and corresponding vertical bearing capacity factor, N′γ, and the soil flow mechanism around the cutting edge are investigated. The soil flow mechanism and the influence of surcharge formed at the top level of the cutting edge due to advancement of the caisson in the ground are examined using the image-based deformation measurement technique. The results highlight that the cutting angle of the cutting edge and sinking depth play important roles in the load–penetration response and soil flow mechanism.
... The detailed site investigation, engineering judgment, instrumentation, skilled labor, informed decisions, etc. are necessary to avoid such problems. The experimental investigations have been carried out and reported to evaluate the contact pressure distribution at the base and sides of the caisson, load settlement and deflection behaviour, earth pressure mobilization in caissons embedded in soft clay, and the formation of failure zone in sand during penetration of the cutting edge of the caisson (Katti and Dewaikar 1977;Alampalli and Peddibotla 1997;Kumar and Rao 2010;Royston et al. 2016). It is observed from the above literature that many difficulties have been faced during the execution of the open caisson. ...
Conference Paper
Open caissons are deep foundations sunk into the ground by removal of soil within the caisson shaft. A cutting edge with a tapered inner face is used at the bottom of the caisson to allow the bearing failure of the soil which is in contact with the cutting edge. The soil is removed within the shaft of the caisson during sinking which results in bearing failure of the soil due to self-weight of the caisson. The formation of influence zone in the soil at the cutting edge is termed as failure zone. In this study, the finite element (FE) analysis is carried out to evaluate the extent of failure zone in the c-φ soil at the cutting edge of the open caisson. The cutting edge is penetrated into the soil and the extent of the failure zone is evaluated. The effect of variation of tapered angles of the cutting edge, the radii ratio of the open caisson, unit weight, friction angle and cohesion of the soil, and magnitude of the surcharge on the extent of the failure zone is investigated. From the FE results, the multivariate linear and nonlinear regression analyses are performed and easy to use predictive equations are developed to estimate the extent of failure zone in the soil beneath the cutting edge. These equations help in quick estimation of the extent of failure zone based on the configuration of the open caisson adopted at the construction site.
Thesis
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Open caissons are deep foundations sunk in the ground by the removal of the soil within the caisson shaft. A cutting edge with a tapered inner face is used at the bottom of the caisson to allow the bearing failure of the soil and hence the continued sinking. In the present study, the bearing capacity factors of the cutting edge for the wide range of radii ratio (ri/ro = 0.35 to 0.95), varying friction angles of the soil ( = 5 to 35) and different tapered angles of the cutting edge ( = 30 and 45) are evaluated using finite element (FE) analysis. Before evaluating the bearing capacity of the cutting edge, the preliminary investigations are carried out considering the strip and ring footing problems to frame the guidelines for the FE evaluation of the open caisson problem. A series of 1g model tests have also been performed to investigate the load penetration response of the cutting edge at different stages of the sinking and the soil flow mechanism in the soil beneath the cutting edge. Using strip footing problem, the sensitivity analysis is carried out to examine the ultimate capacity of the strip footing considering the strength parameters, width of the footing, unit weight of the soil, surcharge at the base level of the footing, and deformation parameters as the variables. Then the effect of different material models on the ultimate capacity of the strip footing is examined. A few suggestions are given in regard to the FE analysis of the ring footing and open caisson problems to assess their bearing capacities. The bearing capacity factors N'c, N'q and N' of the smooth and rough base ring footing are evaluated using the finite element method (FEM). In the analyses, the radius ratio is varied from 0 to 0.75 with an increment of 0.25 and friction angle of the soil is varied from 5 to 35. The Mohr-Coulomb yield criterion and non-associative flow rule are used in the analyses. Then the superposition of the three components of the bearing capacity equation is assessed, i.e., cohesion, surcharge and unit weight of the soil. The methodology adopted for the ring footing is used for the open caisson problem. A series of 1g model tests are performed to investigate the load-penetration response and soil flow mechanism in the soil during different stages of the sinking of the caisson. The effect of smooth and rough base conditions, varying tapered angles, different types of penetration of the cutting edge, and varying depths of sinking on the load-penetration response of the cutting edge is investigated. The soil flow mechanism corresponding to the varying tapered angles of the cutting edge, varying magnitudes of the penetration, and varying depths of sinking is examined using the image processing technique. The values of bearing capacity factor, N' of the cutting edge of the circular open caisson are also evaluated using the results of the experimental studies. The experimental studies have been performed for the embedded and rough base conditions of the cutting edge and the same are simulated in the FE analysis of the circular open caisson. The formation of influence zone in the soil beneath the cutting edge of the caisson is termed as failure zone. The extent of the failure zone in the vertical and radial directions is evaluated using the FE analysis. The effects of variation in the tapered angles and radii ratio of the cutting edge, unit weight, friction angle and cohesion of the soil, and magnitude of the surcharge on the extent of the failure zone are studied. Using the results of FE analysis, multivariate linear regression analysis is performed and easy to use predictive equations are developed to estimate the extent of the failure zone in the soil beneath the cutting edge. The predictive equations are assessed for their practical applicability using the results of the 1g model tests. The vertical bearing capacity factors, N'c, N'q and N' of the cutting edge of the open caisson are evaluated using the FEM. Two tapered angles of the cutting edge,  = 30 and 45, varying radii ratio = 0.35 to 0.95, and  = 5 to 35 are considered in the analyses. The applicability of the methodology used for the ring footing problem is examined by evaluating the bearing capacity factors for the varying values of cohesion, surcharge and unit weight of the soil. The bearing capacity factors evaluated using the FE analyses are compared with those available in the literature. The FE results are presented in the form of design charts and tables for the practical use. A complete understanding of the different aspects of the open caisson, such as load penetration response, soil flow mechanism beneath the cutting edge, and bearing capacity of the cutting edge will help in planning and controlled sinking operation of the open caisson.
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