2.literature review

8/2/2019 2.Literature Review

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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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2.literature review

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