Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Centrifuge Modelling of the Influence of Size and Geometry of HybridFoundations on Bearing Capacity
J.P. PANCHAL
Research Student, Department of Civil Engineering City, University of London, London, United Kingdom

A.M. MCNAMARA
Senior Lecturer, Department of Civil Engineering City, University of London, London, United Kingdom
Email: a.mcnamara@city.ac.uk

R.J. GOODEY
Senior Lecturer, Department of Civil Engineering City, University of London, London, United Kingdom

ABSTRACT

Piles have been used widely on commercial developments in London for about the last 60 years. The life of a commercialbuilding is about 25 – 30 years and as each building is demolished and rebuilt, the piles from the previous buildings
remain in the ground. These cause obstructions to the new foundations because removal is difficult, time-consuming andexpensive. Published research has shown that sheet piled foundations are a genuine and viable alternative to cast in-situ
concrete piles. Individual sheet piles have relatively low capacity when axially loaded, therefore it is necessary to consider
their use as a pile group in conjunction with a pilecap. In this paper these are defined as hybrid foundations. An increase
in bearing capacity was observed when the geometry of the sheet pile group was varied from a circular arrangement tosquare formation. This research aimed to understand the influence of the geometric shape and the dimensions of sheet
pile groups on their bearing capacity.
Key words: Bored piles, Sheet piles, Re-use, Physical modelling, Sustainability

1. Introduction
Early bored cast in-situ concrete piles were typicallyhave been conducted to design more sustainable

about 12m long and up to 0.5m in diameter. It was usuallycapacity of pressed in sheet pile group foundations.

foundation solutions by means of improving the axial

necessary to group together a large number of small piles
to support the load from the structure. Now it is normal
practice to construct one deep large diameter pile under

2. Background
Sheet piles arranged as a hybrid pile group achieveeach column of a building.

higher capacity; which combines deep (sheet pile) andAt the end of their lifespan these structures areshallow (pilecap) foundations. A series of publisheddecommissioned however the foundations are often left incentrifuge tests (Panchal et al., 2016; Panchal et al., 2018)

place. Developers are left with the task of removing oraimed to confirm whether this foundation type provided

avoiding existing piles which incur additional costs andcomparable bearing capacity to the conventional straightdelays to the programme. As expected, this is anshafted concrete pile and whether the shape of the sheet

environmentally damaging and unsustainable means ofpile foundation influenced its performance.

managing construction waste.

A range of pile types were investigated in anIn contrast, steel piles are easily removable and canoverconsolidated clay sample. These piles included

be recycled but have low axial capacity. Recent studies

conventional rough and smooth circular solid shafted- 57 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

piles; representative of driven or cast in-situ piles. Three

Panchal et al. (2016) demonstrated that the sheet pile

sheet pile variations were also studied which comprised

group capacity was much improved over a smooth shafted

circular and square plan formations, with and withoutsolid pile but was approximately 10% lower than a roughperforations along the shaft. Fig. 1 depicts the range ofsolid shafted pile (Fig. 2).

piles that have been tested as part of this research project.

The experiments emphasised the viability andrelevance of the hybrid foundation as a construction
solution. However, there were concerns over its
buildability. Further tests were later conducted modellinga square hybrid sheet pile group which would beconsiderably easier to set-out and construct on site
(Panchal et al., 2018). Results showed that for comparable
base areas the ultimate axial bearing capacity of a square
sheet pile was equal to that of a rough concrete pile.
3. Objectives
Previously published material suggests that the shapeof the sheet pile group influences the bearing capacity ofa pile.

Fig. 1This paper aims to back analyse new centrifuge data

Model piles used in previous centrifuge tests

to determine values of the adhesion factor (α) for a
reduced aspect ratio. The purpose of this is to evaluate theimpact of dimension on sheet pile group foundations
capacity.
4. Soil Model
The tests were conducted in a 300mm deep stainlesssteel centrifuge tub, 420mm in diameter. The final sample
was required to be flush with the top edge of the tub hence
a 300mm deep extension was bolted to the top such thatan oversized sample could be trimmed to size. Speswhitekaolin clay was mixed with distilled water to a watercontent of 120%. This was approximately twice its liquid
limit and produced a workable slurry.
Water pump grease was thinly applied to the walls of
the centrifuge tub. Sheets of porous plastic and filter paper
were placed at the base of the tub over channels that hadbeen machined into the base of the tub to facilitate
Fig. 2drainage. Slurry was carefully placed in the tub to a depth

Centrifuge results of circular sheet pile with holes

compared against rough and smooth solid circular shafted pilesof 550mm using a scoop and was regularly agitated using(Panchal et al., 2016)a palette knife to prevent air entrapment before being
sandwiched between another layer of porous plastic andThe literature showed that hybrid foundations offer

filter paper.

significantly improved capacity and is a worthwhile andThe sample was transferred to a hydraulic press

sustainable foundation solution. For solid circular piles

where a tightly fitting platen was lowered onto the sample.

and circular sheet pile groups of comparable diameters

The pressure was gradually increased from 25kPa to- 58 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

500kPa over the period of a week. Pipes fitted to the180mm, protruding 20mm above ground surface.

drainage taps directed water into a bucket and holes drilled
through the platen allowed water to drain from the top of(a)the sample. This aided in accelerating the consolidation ofthe sample. he sample was swelled back to 250kPa theday prior to testing producing a highly overconsolidated
sample with an overconsolidation ratio (OCR) of 19.6 at
50g at the pile base following in-flight consolidation.
5. Apparatus
The experiments were conducted on the Acutronic
661 beam centrifuge at City, University of London. The
centrifuge tub and loading apparatus were designed by(b)Gorasia (2013). It comprised a lead screw actuatorconnected to a stiff loading beam to which load cells and
LVDTs were secured.
The sheet piles used in these tests were formed froma 0.5mm thick stainless steel plate pressed into acorrugated profile. The sheets were folded and welded sothat they were square in plan and nominally 53mm and
43mm wide along the central axis of the ribs.
A pile and loading cap were machined fromaluminium that served the purpose of securing the shape(c)of the sheet piled foundation and providing a platform on
which the LVDTs and load cell could sit.
6. Test procedure
The sample was swelled back to 250kPa the day prior
to testing. A pore pressure transducer (PPT) was installed
to the centre of the model 100mm below the soil surface
and backfilled with kaolin slurry mixed to a water contentof 120%.
In preparation for the test the sample was recoveredfrom the hydraulic press and the extension lifted off before
a wire cutter and palette knife were used to trim the sample
flush with the top of the tub (Fig. 3a). A thin layer of
PlastiDip, a synthetic rubber membrane, was sprayed
across the clay surface to prevent the sample from dryingout excessively, whilst leaving the area for the sheet pilegroups uncovered. The apparatus frame was aligned above
the sample and the loading frame lowered until it indented
the clay surface. Having marked out the centres of thepiles the frame was removed and sheet piles centrallyaligned (Fig. 3b). A hydraulic press (Fig. 3c) was used topress the piles into the soil and embed them to a depth of- 59 -(d)Proceedings of the First International Conference on Press-in Engineering 2018, Kochigroup gave 52% greater capacity compared with the small(e)pile. At the ultimate state the larger square pile offered
71% greater ultimate bearing capacity.
Test piles, either with comparable base areas orperimeters, were plotted to understand the influence ofgeometry and dimension on the behaviour of the pile.
Table 1 summarises the nominal pile widths/diameters,
the perimeter/circumference and the area of the pile base.

Fig. 3

Images taken during model making include (a)trimmed sample flush with top of tub, (b) pile centres marked
out and surface sealed with PlastiDip, (c) embed piles to correct
depth, (d) resin pile caps, (e) complete model prior spin up.

Two-part Sika Biresin was poured within the sheet
pile upstand to form a pile cap that was approximately
15mm deep, shown in Fig. 3d. This was left to cure before
securing the loading caps to each pile group. The loading
frame was bolted to the centrifuge tub and the loadingmechanism lowered until it was approximately 2mm
above the caps. The model, immediately prior to spin up,
is depicted in Fig. 3e.

Fig. 4

Results from centrifuge test comparing a large square

The package was weighed and transferred to thesheet pile group against a small squarecentrifuge swing. A standpipe was connected to the basedrain to maintain a water table 30mm below the clay

Table 1.

Summary of pile geometry used in the centrifuge

surface. The sample was left to consolidate at 50g for atests

period of 24 hours to allow the excess pore pressures toSoliddissipate; equilibrium conditions were confirmed by thecircularpore pressure transducer readings. The piles were loadedpileat a rate of 1mm/minute whilst measuring loads andNominal width

settlements.

(mm)
Measured

7. Test results
Two square sheet pile groups; nominally 53.5mm andperimeter (mm)43mm wide were tested as part of this investigation.

base (mm2)Area of pile

Sheet pile groups
Circular

Large

Smallsquaresquare

60

60

53.5

43188

217

246

214

2827

2827

2862

1849

Both sheet piles groups were embedded 180mm into thesoil and a 15mm deep resin pile cap was cast above groundFig. 5 explains how the nominal widths werelevel. A water table was established 30mm below groundobtained for each of the piles tested in these experiments,level. Shear vane readings taken immediately after the test

where the average width was taken between maximum

showed that the average undrained shear strength along

and minimum widths between the ribs. The perimeter was

the piles was 32kN/m2.

measured and the area was calculated using the nominalFig. 4 illustrates the axial load against the settlement

width or diameter.

normalised by the nominal sheet pile width. At a working
load of 1% normalised settlement the larger square pile- 60 -Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

Fig. 6 was plotted to explore the influence of piledimension on the working capacity of a square sheet pilegroup. It shows that a 15% increase in the perimeter of thesheet pile more than doubles the magnitude of normalised
axial load/Su. The initial pile response of the larger square(a)pile was also shown to be approximately twice as stiff as
the small square pile group.
For comparable base areas Fig. 7 considers thebehaviour of a rough solid shafted circular pile, the large
square sheet pile and the perforated circular sheet pile.
The rough solid pile exhibited similar behaviour to thecircular sheet pile with holes. However, the large square
sheet pile was considerably stiffer at the initial loading
stages and continued to withstand higher loads during thetest.

(b)

A final comparison was drawn between the small
square sheet pile group and the circular sheet pile, 43mm
wide and 60mm in diameter respectively. Thesefoundations were regarded as similar owing to similarities
in the pile perimeter. Fig. 8 shows that up to 0.25%
normalised settlement the stiffness of the circular sheet
pile was equal to that of the small square pile. However,the circular sheet pile tends towards the ultimate capacity
whereas the square sheet pile continues to transfer load
into the soil. Subsequently, the working load of the small(c)square sheet pile was 36% higher than the circular pile ofcomparable shaft area.

(d)Fig. 5 Nominal widths of the various piles (a) solid shafted
pile, (b) circular sheet pile, (c) large square sheet pile and (d)small square sheet pileFig. 6Comparison between pile response of large and smallsquare sheet pile groups.

- 61 -

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

foundation being analysed as an open ended tubular pilein clay (Jardine et al., 2005).
𝑄𝑢𝑙𝑡 = 𝑄𝑏 + 𝑄𝑠
𝑄𝑏 = [𝐴𝑏 (𝑁𝑐 𝑆𝑢 + 𝛾𝐻)] / 2(1)

𝑄𝑠 = 𝐴𝑠 𝛼𝑆𝑢

(3)

(2)

Where Ab is the basal area in contact with the soil, As
is the shaft area in contact with the soil, Nc is the bearing
capacity factor, Su is the undrained shear strength of theclay, γ the bulk unit weight of soil, H the total length of thepile and α the adhesion factor.
The α values of the sheet pile groups from this test,
and previously published tests, were back calculated.
Comparisons were made between these values and those
calculated for the same plan area but shallower piles. The
Comparison between pile response of a rough solid

adhesion factor is influenced by the strength of the soil andcircular pile and small square sheet pile group of comparable

the surface roughness of the pile. In addition, the α factor

base area.

tends to be lower in stiff clays than in soft soils. It alsoFig. 7

stands true that the smoother the pile the lower the α value.
This research tested two square sheet pile groups ofvarying dimensions. Theory dictates that similar α values
should be obtained. The results from this centrifuge test
were used to back analyse α for the two sizes of square
sheet pile groups.
Table 2 summarises the α values obtained at 1%
normalised settlement and at the ultimate state. The
analysis from this experiment are compared against thevalues obtained from previously published data. The
results show that α for the same pile geometry, irrespectiveof pile size, is reasonably similar.
Table 2.

Summary of back calculated α values at 1%
normalised settlement
α (1%α (ultimate

circular sheet pile group and small square sheet pile group of

normalised

normalised

comparable perimeter.

settlement)

settlement)

Large square sheet pile

0.310

0.584

8. Analysis
Terzaghi (1943) presented a method of assessing theSmall square sheet pile

0.236

0.516

bearing capacity (Qult) of a pile as a summation of the base(Panchal et al., 2018)

0.305

0.815

Fig. 8Comparison of pile response of a large diameter

capacity (Qb) and shaft friction (Qs), as defined in Eq. (1)(3). Where Qb is reduced by a factor of two owing to the- 62 -Large square sheet pileProceedings of the First International Conference on Press-in Engineering 2018, Kochi

9. Discussion
Previous research (Panchal et al., 2018) concluded

and prevent the soil from drying out, had in fact seepedthat at the ultimate state the square sheet pile α was 0.815

inevitably reduce the magnitude of the α value.

down and coated the top half of the sheet piles. This would

whilst the circular sheet pile was 0.567. It was suggested
that this variation was owing to the shape of the sheet pilegroup, see Fig. 9. For instance, the ribs in the square sheet
pile were closer and may have become plugged duringloading, whereas the circular sheet, having been stretched
to form a circle had more open ribs, which enabled the soil
to shear against the entire sheet pile surface. This spacingof the re-entrant corners influenced the shearing interface(a)of the square and circular sheet piles resulting in a higher
α value for the square sheet pile group.
In practice however, each sheet pile would beinstalled individually to make the sheet pile group.
Therefore, the plugging effect is not likely to reduce themagnitude of the ultimate bearing capacity of a pile in thefield.
The difference in α between the large and small
square sheet pile group may be influenced by the numberof external ribs on the two squared sheet piles. Fig 10shows the outlines of both the large and small square sheet
pile groups. The smaller of the two piles (Fig 10a) has(b)eight external ribs and nine soil/pile shearing interfaces.
On the other hand, Fig 10b illustrates twelve external ribs
and approximately nine pile/soil shearing interfaces. The
large square is 25% larger in plan than the small pile,
however the additional ribs increase the area of soil/soil
shearing area by 50%. This results in an increase in the αvalue for larger sheet pile groups.
Upon disassembling the model, it was found that thesilicone grease used to seal the gap at the top of the pile

- 63 -Fig. 9Assumed shear zones around (a) square sheet pile and(b) a circular sheet pile (after Panchal et al., 2018)

Proceedings of the First International Conference on Press-in Engineering 2018, Kochi

solid shafted smooth and rough circular bored piles. The(a)size of the square sheet pile group has been varied and theinfluence of perforations has been briefly explored.
The results highlighted that square sheet piles offer
comparatively higher capacity than the circular sheet pilegroup, owing to reduced re-entrant corner, this resulted ina higher α value.
Smaller square sheet piles, although less than half thearea of rough solid shafted bored piles exhibited similar
ultimate capacity. The capacity of a square sheet pile wasshown to be influenced by the number of soil/soil shear
interfaces. Ensuring soil plugging between the sheet pileribs forces a soil/soil failure and can significantly increase
the capacity of a square sheet pile group.

(b)

References
Gorasia, R.J. 2013. Behaviour of ribbed piles in clay. PhD
thesis, Research Centre in Multi-scale GeotechnicalEngineering, City University London, UK.
Jardine, R., Chow, F., Overy, R. & Standing, J. 2005. 4
Design Methods for Piles in Clay. ICP design
methods for driven piles in sands and clays. January
2005, Thomas Telford, London, pp. 28-37.
Panchal, J.P., McNamara, A.M. & Goodey, R.J. 2016.
Bearing capacity of sheet piled foundations.
Proceedings of the 2nd Asian Conference on PhysicalModelling in Geotechnics. Asiafuge 2016 (ed. Ma,X.), Tongji University, Shanghai, China, pp. 1-6.
Panchal, J.P., McNamara, A.M. & Goodey, R.J. 2018.
Fig. 10

Influence of geometry on the bearing capacity ofOutline of (a) small and (b) large square sheet pilesheet piled foundations. Proceedings of the 9thgroup used in centrifuge tests

International Conference on Physical Modelling in10. Concluding remarks
To date, a number of centrifuge tests have been

Geotechnics, London, (eds. McNamara et al.) UK, pp.

conducted investigating the behaviour of corrugated

Terzaghi, K. 1943. Theoretical Soil Mechanics, John1395-1400.

circular and square sheet pile groups against conventional- 64 -Wiley and Sons, New York.