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CHAPTER 2
LITERATURE REVIEW
2.1 GENERAL
This chapter reviews the literature relevant to the present study. The
theories available for quantifying the vertical bearing capacity during installation or
preloading are discussed in detail.
2.2 BEARING CAPACITY OF SPUDCAN FOOTINGS
There are two principal concerns in the assessment of whether a jack-up unit
can be safely used at a particular site:
(i) Prediction of footing penetration during preloading (vertical load only),
(ii) Assessment of footing stability under design storm conditions
(combined vertical, horizontal and moment loading).
With some basic considerations these two aspects of bearing capacity analysis are
discussed below.
2.2.1 BEARING CAPACITY IN UNIFORM CLAY
The ultimate vertical bearing capacity of a Spudcan foundation in clay at a
specific depth can be expressed by (SNAME, 1997)
vu c u oq N s p
A
= + + (2.1)
Where
Po = effective overburden pressure due to backfill.
V = combined volume of embedded spudcan
A = cross-sectional area of the spudcan
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2.2.2 BEARING CAPACITY IN SAND OVERLYING SOFT CLAY
A number of analytical procedures are available in the literature to
evaluate the margin of safety against a punch-through failure for two-layered soil
systems. One simplified procedure involves the projected area method, in which
1 1 2 2P = p A = p A (2.2)
Factor of safety against punch-through = q2/p2 (2.3)
Where
p1 = bearing stress under the footing;
A1 = area over which p2 is distributed;
P = total load of footing;
q2 = ultimate bearing capacity of soft layer;
This method assumes that the footing loads applied to a strong layer are
distributed downward through the layer. An equivalent footing, with effective
dimensions that are increased at a rate of 3- vertical to 1- horizontal through the strong
layer, is placed at the top of the weaker layer. When the pressure on the equivalent
footing equals the bearing capacity of the underlying layer, then the computed factor of
safety will be unity
In the projected area method the idealized foundation is assumed to act at the
interface between upper and lower strata, with dimensions larger than those of true
(higher) foundation. Various authors have recommended angle of projection which
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diverge towards, = , tan -1 (1/2), tan-1 (1/3). Such analysis take no direct account of
the strength of the sand itself and only few take any indirect account by considering =
f() (Jacobsen et al. 1977).
Hanna and Meyerhof (1980) method is commonly used for the analysis of the
punch-through case. This method is generally preferred by the offshore industry. This
method proposed an analysis on the assumption of a truncated cone of soil in the upper
layer being pushed down into the lower layer.
( ) tan2 1 26 2 + vu UB sDq S H K H B
+= + (2.4)
Where
qu = Ultimate bearing capacity,
Ks = Punching shear co-efficient,
Sub= Undrained shear strength of the lower stratum.
' = Effective unit weight of granular stratum,
H= Thickness of upper layer in two layer system,
D= Depth of the widest cross-sectional area,
V=Volume of soil displaced by spudcan.
Craig and Chua (1990) conducted series of centrifugal tests on sand and clay
using spudcan (circular footing) model diameter of 140 mm. They have suggested
procedure for calculating the spudcan bearing capacity in uniform clay, sand, and sand
overlying clay. They observed that while the assumed mechanism of the Hanna and
Meyerhof (1980) type of analysis of potential punch-through (which ignore any
distortion of the sand/clay interface and assume removal of the displaced soil) may be
appropriate at small penetrations. An alternative calculation which is able to
accommodate the observed mechanisms associated with gross displacement is needed.
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Teh et al. (2005) conducted a series of centrifuge tests on sand overlying clay
and suggested that the load spread angle is not a constant, as commonly assumed in
conventional bearing capacity theories in layered soils, but increases with qsand/qclay and
decreases with H/B where qsand, qclay, H and B are bearing strength of sand, bearing
strength of clay, depth of penetration and diameter of spudcan, respectively.
Hossain et al. (2006) presented a new approach that is based on soil failure
mechanism, including cavity formation and eventual back-flow of soil over the
spudcan, which was observed in drum centrifuge tests and in large deformation finite
element analysis. The centrifuge model tests included half-spudcan penetration tests, in
which the half section that coincides the side Perspex wall is allowed to penetrate.
During the penetration, the soil deformation was monitored using PIV analysis. When
the soil starts to flow back into the top of the spudcan, the existing open cavity remains
stable with no further change in depth. No evidence of cavity wall collapse, as would
be indicated by inward and downward soil movements into the open cavity, was
observed in either the model tests or the numerical analysis.
Condition for back-flow, and the limiting cavity depth, H, may be expressed
simply as a relationship between H/D and the non-homogeneity ratio suH/ D, where
suH is the shear strength at depth H. This relationship appears extremely robust over a
wide range of soil strengths and foundation diameters.
2.3 PUNCH-THROUGH FAILURE:
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TheSpudcan footing is normally installed by preloading processes. During the
preloading process, the load applied to a spudcan has to be reached by the bearing
resistance from the soil in order to maintain static equilibrium. In soil condition
showing increasing bearing resistance with depth, this process sets the spudcan deeper
at a rate controlled by the load increments. On the other hand, in conditions consisting
of a strong soil layer overlying a weaker layer, the bearing resistance may decreases
with depth at some point during the process, leading to temporary lose of static
equilibrium. This results in rapid uncontrolled spudcan penetration, or punch-through,
before resting at a final depth where the bearing resistance is sufficient to overcome the
preload. Punch-through may also occasionally occur due to storm overload (McClelland
et al., 1981; Baglioni et al., 1982).
Most punch-through failures happen during the preloading in stratified soil
profiles with a relatively thin layer of sand or strong clay overlying a weaker layer
(Baglioni et al. 1982, McClelland et al.1981, Young et al.1984, Craig et al.1985).
Punch-trough also can occur in normally consolidated or lightly overconsolidated clays
and silts due to partial consolidation occurring during any delay in preloading, and the
development of a localized strong crust of soil just beneath the spudcan (McClelland et
al.1981, Young et al.1984).
Craig and Chua (1990) presented the results of a series of centrifuge tests on
model spudcans (14m in prototype diameter) installed in dense sand (with friction angle
of 38) overlying a stiff clay layer with uniform undrained shear strength cu of 41 to
45 kPa). The results of three tests with different sand thickness showed that for the case
with 2.6 m prototype sand thickness no distinctive punch-through failure was observed,
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whereas for the case of 7 m and 9.5 m sand thickness, punch-through failures occurred
within the first 2 m of penetration.
Tjahyono et al., (2009) conducted a series of full-spudcan and half-spudcan
centrifuge tests on thin upper strong layer overlying soft clay with normalised upper
layer thicknessH/B ranging from 0.16 to 0.71, where H and B are penetration depth and
spudcan diameter, respectively. The strength ratio of lower-to-upper soil layers used in
this study is 0.2. The measured spudcan load-penetration response shows a change of
profile from a monotonously increasing trend for the thinner upper layers (H/B 0.31)
to a post-peak softening trend for the thicker upper layers, thereby suggesting that the
likelihood for punch-through decreases with thinner crust layer. The observed soil
deformation for the case ofH/B equal to 0.4 reveals punching-shear failure in the upper
layer initiated at shallow penetration depth (D/B less than 10%), followed by the
formation of a rigid crust block beneath the spudcan, which gets carried downward by
the advancing spudcan deep into the lower layer. The effects of the crust block on the
spudcan bearing resistance in the soft clay should be taken into account in practical
analysis.
Teh et al., (2005) conducted series of centrifuge (100g) test on sand overlying
clay with spudcan diameter of 100 mm. Four tests with prototype sand layer thickness
of 5 m, 7 m, 7.7 m, and 10.5 m were performed. The result shows that the bearing stress
increases with thickness of the overlying sand layer and Punch-through failure takes
place within a narrow range of 10% to 12% of spudcan diameter beneath the sand
surface (Fig 2.1).
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0
200
400
600
800
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
D/B
q(
kPa)
case 1 (H=5m)
case 2 (H=7m)
case3 (H=7.7m)
case 5 (10.5m)
Figure. 2.1 Punch-through failure (Teh et al. 2005)
Hosssain et al. (2005) carried out centrifuge model tests to study the Punch-
through failure of Spudcan penetrating through strong clay overlying softer clay. Half-
Spudcan models were used to examine the deformation mechanisms using PIV image
analysis. Full Spudcan tests were used to obtain profile of penetration analysis. The soil
strength ratio was Sub/Sut = 0.44 where Sub, Sut are undrained shear strength of bottom
and top layers and the top layer thickness (H/Dhalf) varied from 0.3 to 1.1 for the half
Spudcan, with H/Dfull values of 0.6 to 2.2 for the full Spudcan. Punch-through failure
occurs when a peak in the penetration resistance is reached. This is triggered by the
transition to a failure mechanism with shear zone extending from the Spudcan shoulder
to the base of the strong layer. A soil plug with the shape of a truncated cone forms in
the upper layer below the Spudcan and moves down as the Spudcan penetrates further.
A transition shear zone surrounding the soil plug disappears during further penetration,
and is not evident once the Spudcan is fully embedded in the soft layer. The soil plug
has a depth of ~80% of the initial thickness of the top layer.
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Punch-through
B=10 m
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2.4 SWISS CHEESE TECHNIQUE:
Punch-through failure leads to serious accidents during the preloading stages.
Several such cases are reported in the literature. Usually punch-through failure occurs
when the thickness of hard layer is about D/2 to D/4, where D is the diameter of the
spudcans. In such situations, the hard layer is often weakened before installation of the
spudcan. One such practice in the industries, known as Swiss cheese drilling is often
used to weaken or degrade the thin sand or hard clay layer and allow controlled
penetration in multi layer soil conditions. Swiss cheese drilling typically consists of
drilling 30 to 40 holes, each having 600 mm to 900 mm diameter through the hard layer
in each planned spudcan footprint (Kosterlnik and Guerra 2007). This technique has
been recently used to reinstall jack-ups without incident of failure at several locations
in Southeast Asia, where severe punch-through failures occurred during the first
attempt of preloading through layered clays (Maung and Ahmad,2000; Brennan et al.,
2006; Kostelnik et al., 2007).
Maung et al., (2000) reported a case history at Anding, of the Malaysian
peninsulas west coast, where the bow leg of the Harvey H. ward jack-up punched
through a stiff clay layer at 11m depth. Following this incident, additional soil borings
were undertaken which indicated the presence of a stiff layer between 11 to 13.5 m
embedment (Fig.2.2). In order to eliminate the punch-through of this layer, a Swiss
cheese operation was carried out by drilling 0.66m diameter holes to a depth of 16 m.
The drilling depth was 2.5 m below the first stiff layer reaching the top of another
underlying stiff layer. A total of 32 and 43 holes were drilled within each spudcan
footprint. In total this is equivalent to 18 to 25% of the spudcan footprint area. Five and
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a half days were required to complete this Swiss cheese drilling and the preloading
operations.
Brennan et al. (2006) described reinstallation of a KFELS B class jack-up in the
Natuna Sea. The method selected was to drill 0.065D diameter holes (0.914 m) on
equivalent triangular grid with a spacing of 0.109D to 0.131D. Most of the holes were
drilled directly underneath the spudcan to a depth of 25 to 30 m. This depth corresponds
to 12 to 17 m below the stiff layer, with the different heights reflecting the different
layering at each spudcan location. The total area of holes within the perimeter of the
spudcan was approximately 21 to 31% at shallow penetration depths and 15 to 18% at
deep penetrations.
Kostellnik et al., (2007) presented two case histories in Malaysia on safe jack-
up rig installation: the first case was at Raya where a total of 43 to 73 holes, each with
diameter of 0.66 m, were drilled on equilateral grid with a spacing of 0.124D to 0.15D.
The second case was at Tapis were a large number of holes were drilled (reported to be
105). This was due to higher shear strengths of the stiff layer and 0.065D diameter
holes were opened using a holes opener. The holes were drilled outside the perimeter
of spudcans, and the area removed from inside of spudcans periphery was about 11 to
18%.
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Figure. 2.2. Conventional pattern of Swiss cheese drilling (after Maung et al. 2000)
Hossain et al., (2008) conducted a series of model tests of a 40 mm diameter (D)
spucan footing vertically installed in stiff-over soft clay deposit. In this preliminary
study, the effectiveness of Swiss cheese drilling was investigated by drilling holes of
different spacing, depth and distribution both underneath and outside the immediate
perimeter of the penetration spudcan as shown in Fig. 2.3 (a). The method of producing
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74.50 M
PLAN VIEW
MLS
SEA BED
6.6M
INITIAL SPUDCAN
PENETRATION
DURING FAILED RIG
PRELOADING
ATTEMPT
2.5 M HARD THIN
CLAY LAYER
18 M REQUIRED
SPUDCAN
PENETRATION FOR
ALL THE THREE
LEGS
26 PILOT HOLES(43 NUMBERS)LAYER
11.0 M
13.5 M
SIDE WAY VIEW
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the holes, through drilling and coring, were also investigated. Results (Fig. 2.3b)
showed that punch-through was mitigated when the layered deposit was punctured in a
zone of 0.25D immediately outside the spudcan periphery by coring holes of 0.05D
diameter on an equilateral triangular grid of 0.1D and to a depth of twice the thickness
of the stiff layer.
Figure. 2.3(a). Spudcan penetration on sample without drilling (Hossain et al.
2008).
Figure: 2.3 (b). Effect of extension of drilling adjacent to the perimeter of the
spudcan perimeter (Hossain et al., 2008)
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2.5 SUMMARY
The above review reveals that the Swiss-cheese technique may be safely
adapted to mitigate the punch-through failure of spudcan. The area reduction adapted
due to drilling in the field is in the range of 11 % to 25% of footprint area of spudcan.
While many case histories are reported in the literature, optimum drill hole pattern, that
leads to least resistance to penetration is not studied in detail. The present investigation
attempts to find out the optimum bore-hole pattern, that gives least resistance to
penetration.
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