Deep Foundations on Bored and Auger Piles, Van Impe (ed.)© 2003 Millpress, Rotterdam, ISBN 90-77017-76-3

Bearing capacity of bored pile packed with iron and steel slag using
precast concrete pile
S.Yabuuchi
A.Nishiwaki
K.Kobayashi
K.Hirao
N.Yoshikawa
Geotop Corporation, Osaka, Japan

Keywords: bored pile, utilization of steel slag, converter slag, blast furnace slag, fill material, precast concrete
nodular pile, bearing capacity, vertical load test
ABSTRACT: The bearing capacity of a pile with packing material around it was studied by on-site tests withspecial emphasis on the expandability and consolidating ability of iron and steel slag. Excellent bearing capacity was obtained by using iron and steel slag as the packing material, which was superior to the bearing
capacity obtained by the conventional cement milk piling method. Particularly, a significant increase in bearing capacity was obtained with the passage of time, which cannot be obtained by other packing materials. The
bearing capacity increased regardless of whether the surrounding ground consisted of sandy soil or clayeysoil. This effect was more pronounced in precast concrete nodular piles than in precast concrete straight piles.
Large skin friction developed at an early stage in wet ground up to the groundwater level.
1

INTRODUCTION

By-products are generated such as blast furnace slag
during the ironmaking stage and steelmaking slag
during the steelmaking stage of iron and steel production. Some of these slags, particularly steelmaking slag before the aging process, have physicalproperties such as strength, specific gravity, hardness and expandability that are different from thoseof natural materials.
Until now, properties such as consolidation andexpandability were considered to be undesirable
when steelmaking slag was used. However, it wasobserved that the bearing capacity of the foundation
pile was enhanced by packing slag around a precast
concrete pile. Keeping this point in mind, we
worked toward the development and realization of
the bored piling method.
The high bearing capacity obtained from model
tests of foundation piles carried out until now usingiron and steel slag1),2) was confirmed by carrying out
on-site tests using large, actual piles in various kindsof ground.
The aim in this study was to determine the optimum combination of the type of ground, the type of
packing slag and the shape of precast concrete pilethat would demonstrate the best bearing capacity forthe pile constructed by the bored and slag-packed
piling method using iron and steel slag.
The aim was also to examine the effects of loading history on the bearing capacity.
This report describes the bearing capacity of aprecast concrete pile packed with iron and steel slag
(hereafter called the “bored and slag-packed piling
method”) obtained by on-site tests.
2

OVERVIEW OF TESTS

2.1 Kinds and basic characteristics of iron andsteel slag
The kinds of iron and steel slag used in the on-site
tests were steelmaking slag (converter slag) before
the aging process and blast furnace slag (granulated
blast furnace slag). The slag packed around the pileduring the construction work was steelmaking slag
only and a mixture of two kinds of steelmaking slag.
The grain size distribution curve of each kind of slag
is shown in Fig. 1, the various kinds of packing slag
are shown in Table 1 and the chemical compositionof slag in Table 2.
The results of the expansion test of the steelmaking slag are shown in Fig. 2, while the results of triaxial compression tests (CD tests) are shown in Table 3.

275

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comparison with other methods, the construction
was also carried out by the conventional bored piling
method in which soil cement formed by mixing andstirring cement milk and excavated soil (hereafter
called “bored and cement milk-packed piling
method”) is used as the packing material.

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Table 1. Various kinds of packing slag
Steelmaking Slag: 100% Steelmaking SlagMixed

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Table 2. Chemical composition of Slag (mass %)SiO2 CaO Al3O3 MgO T-Fe

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2.3.1 Bored and slag-packed piling method (newconstruction method)
In this method, a precast concrete pile is inserted in ahole excavated using an auger, and iron and steelslag is packed around the pile. This piling method
aims to enhance the development of skin friction of
the pile through the static compaction effect forground surrounding the pile due to the consolidationand expansion of the iron and steel slag. The excavation diameter was taken as 700 mm for a bored precast concrete nodular pile of diameter 300 to 400
mm. Fig. 3 shows the construction procedure.

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Fig. 2. Result of expansion test
Table 3. Result of triaxial test(CD test)
C kPa
ĳ°
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Mixed Slag
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43.2

2.2 Shape, dimensions and kinds of precast
concrete pile
Table 4 shows the shape, dimensions and kinds ofprecast concrete piles used as test piles.
Table 4. Precast concrete pile types
Shape

2.3.2 Bored and cement milk-packed piling method(conventional method)
In this method, a hole is excavated using an auger
and cement milk is injected in the hole. The excavated soil and cement milk are mixed and stirred toform soil cement into which the precast concrete pileis inserted. The soil cement becomes integral withthe precast concrete pile when it hardens. The excavation diameter was taken as 500 mm for a precast
concrete nodular pile of diameter 300 to 400 mm.
Fig. 4 shows the construction procedure.
6R L O &HPHQ W

Maxdiameter(Do): 440mm
Max
diameter(Do): 450mm
Dimen Minimum
sions diameter(D): 300mm
Length: 11m
Nodularinterval: 1000mm
Length: 11m,4m
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NodularPile
A
type, B type Straight Pile B type
type

2.3 Construction method
Construction during the on-site tests was according
to the new bored and slag-packed piling method. For

276

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Fig. 4. Bored and cement milk packed piling method

Yabuuchi, S., Nishiwaki, A., Kobayashi, K., Hirao, K. & Yoshikawa, N.

2.4 Vertical load test pile and evaluation method
The static axial compressive load tests of single pile
were carried out in accordance with the standards of
the Japanese Geotechnical Society 3) (hear after
called “the vertical load test”).
The loading equipment used was an anchor pile
system. An electro-hydraulic jack was introduced
between the loading beam and the test pile. A stage
loading arrangement was used with eight or more
loading stages. The number of cycles used was fouror more, in principle. Fig. 5 shows the loading scheme.
Table 5 shows the standard load step sequence.
The main measurements were applied load, settlement of pile head and pile tip and the strain in thepile.
The bearing capacity was evaluated by comparing
the resistance values from the Q-Sh curve when Sh
reached 10% of the pile diameter Db (hereafter
called the “Conventional ultimate pile resistance
Rconv”). Where Sh did not reach 10% of Db, themaximum load retained for 30 minutes was taken as
Rconv.
Note that the skin friction of the section, fi is thevalue obtained by dividing the difference in axial
load between each cross section where the strain
gauge was installed with the surface area of the pile
(product of diameter at node and ʌ and length of thesection).
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Rate ofloading

Holdingtime
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step

Table 5. Standard load step sequenceplannedDuringmaximum load:loading
number of step
Duringunloading
New loading step
Unloadingor reloading step
Unloadingto zeroloadmin: Twice the rate used loading stage::

ON-SITE TESTS OF BORED AND SLAGPACKED PILING METHOD

3.1 On-site tests in ground with sandy soil
3.1.1 Test method
Test piles were constructed by the bored and slagpacked piling method in ground consisting mainly of
sandy soil and gravelly soil.
The test piles used were of two kinds – the precast concrete nodular pile and the precast concrete
straight pile. Two kinds of slag were used as packing
material around the pile: steelmaking slag and mixedslag.
To study the change with time of the bearing capacity of each test pile, the vertical load tests wereTable 6. Test conditions
Pile type Material ofTime of a load test
1month 3months
Nodular pile Steelmaking
S1 Bored ĳ440-300,
٤
٤
Slag

Test Method

S2

Mixed Slag
Straight pile Steelmaking
ĳ450,t=65,
Slag
Ɛ=11m

S3

٤

٤

٤

٤

carried out for the same piles cured for 1 month andfor 3 months. Table 6 shows the test conditions.
3.1.2 Test ground and locations of installation oftest piles
The test ground was reclaimed ground from the surface to a depth of 5.35 m below ground level withalternate layers of sand with an N-value of 6 andlayers of gravel mixed with clayey soil with an Nvalue of 5 to 16. At levels of 5.35 m to 13.8 m below
the ground, the layers were gravel layers with an Nvalue ranging from 10 to 22. The groundwater levelwas 2.5 m below ground level.
The tip of the test pile was embedded in a gravel
layer 10.7 m below the ground level.
Fig. 6 shows the soil boring log of the test groundand the positions of installed strain gauges on thetest pile. The strain gauges were installed at the interface of each layer and on the pile head and pile tip
ends.
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Bearing capacity of bored pile packed with iron and steel slag using precast concrete pile277

3.1.3 Results of vertical loading tests and discussion

based on the results of strain measurement of thepile. Fig. 10 shows the change with time of Rconv, thetip resistance Rp, and the skin friction Rf.

(1) Results of measurement of pile head load andpile head settlement
Fig. 7 shows the pile head load Q versus pile headsettlement Sh curve of each test pile based on the results of the vertical load tests. This Figure shows the
Q versus Sh curves for curing periods of 1 month(bold line) and 3 months (broken line).Table 7
shows the change with time of Rconv and the Rconv ofeach test pile.

(3) Discussion on the results
The following points regarding bearing capacity canbe understood from the load-settlement relationship:(a) Comparison of two kinds of packing slag shows that
mixed slag had high bearing capacity and it developed
1.3 times the Rconv of steelmaking slag at a curing period of one month and approximately 1.4 times the
Rconv of steelmaking slag at a curing period of 3
months. The bearing capacity increased with the increase in curing period. According to 3 months, thebearing capacity for mixed slag increase by 1.24 times
that of 1 month and for steelmaking slag increase by(2) Results of measurement of strain in the pileFig. 8 shows the axial load distribution of each test
pile and Fig. 9 shows the curve of skin friction of thesection, fi versus the settlement of the section Si,

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Conventional ultimate pile resistance Rconv(kN)
Test 1month
3months
Ratio
No.
(a)
(b)
(b/a)
S1
1563
2354
1.51

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278

Yabuuchi, S., Nishiwaki, A., Kobayashi, K., Hirao, K. & Yoshikawa, N.

(b) Comparison of the shapes of precast concrete piles
showed that the bearing capacity of nodular pile wasextremely high compared to that of straight pile. The
Rconv of the nodular pile was more than two times at acuring period of 1 month and more than 3 times during reloading at a curing period of 3 months comparedto the Rconv of the straight pile. The change with timeof bearing capacity showed an increase of 1.25 times
in the bearing capacity of the nodular pile at a curing
period of 3 months, whereas in contrast, the bearing
capacity of the straight pile decreased to about 0.9
times for the 3 months curing period.

From the distribution of axial loads and the fi - Si relationship, the following observations were made:(c) Comparison of the different types of slag showed thatthe fi in the slag in wet earth upper the groundwaterlevel (2.5 m below ground level) appeared fastercompared to the slag in earth in the saturated condition at depths below the groundwater level. Furthermore, about 3 times the fi in mixed slag upper the
groundwater level appeared faster compared to thesteelmaking slag. However, with the increase in the
curing period, the difference in fi for these two slags
reduced so that the ratio became 1.3 times at a curing
period of 3 months. The fi - Si curve in this case becomes the curve at which the settlement is 20 mm andfi is the maximum range of 170 to 210 kPa. The
consolidation/ expansion of slag progresses and theslag is considered to become integral with the pile
body at this stage.

On the other hand, the onset of consolidation andexpansion of the slag below the groundwater level ata curing period of 3 months is delayed. As a result,the development of fi for both kinds of slag is small,and no difference is observed in this value for all
kinds of soil.
(d) Comparison of the shapes and kinds of precast concrete piles showed that for the same packing slag, thedevelopment of fi for the straight pile was smaller thanthat of the nodular pile for all soil sections. Moreover,the increase in fi due to reloading was only at the piletip. At other sections, the same value or a reduced
value was observed.

3.2 On-site tests in ground with clayey soil
3.2.1 Test method
Test piles were constructed mainly in ground withclayey soil by two construction methods: the boredand slag-packed piling method and the bored andcement milk-packed piling method (conventional
method) for comparison purpose.

For both construction methods, the precast concrete
nodular pile was used as the test pile and two kindsof slag, namely, steelmaking slag and mixed slag,
were used as the material packed around the pile.
To study the change with time of the bearing capacity of each test pile, the vertical load tests were carried out for test piles cured for 1 month and for 3
months. Test piles were further added to examine theeffects of loading history of piles being cured on
Rconv. Table 8 shows the test conditions.

Table 8. Test conditions
Test
No.

MethodPile type

K2

Bored

K3

Material ofa filler

Time ofa load test
1month 3months

Mixed Slag٤

Nodular pile Mixed Slagĳ440-300
Steelmaking
t=60,l=11m
Slag

٤

٤

٤

٤

Soil cement

٤

٤

K1

Bored
K4
(soil cement)

3.2.2 Test ground and locations of installation oftest piles
The test ground had thick layers of alluvial deposits.
Up to a ground level of 5.5 m below the surface, theground had alternate sandy layers with an N-value ofabout 5 and clayey layers with an N-value of about
2. Sandy layers of medium density with an N-valueof 10-17 comprised the ground from a level of 5.5 m
to 7.6 m below the surface. At depths below 7.6 m,
the ground consisted of clayey layers with an Nvalue ranging from 2 to 5. The groundwater level
was 4.0 m below the ground level.
The tip of the test pile was embedded in a clayey
layer 10.5 m below ground level.
Fig. 11 shows the soil boring log of the test groundand the positions of strain gauges installed on the test
pile. The strain gauges were installed at the interfaceof each layer and on the pile head and pile tip ends.
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Bearing capacity of bored pile packed with iron and steel slag using precast concrete pile279

3.2.3 Results of vertical loading tests and discussion

Table 9 shows the change with time of Rconv andthe Rconv of each test pile.

(1)(2) Results of measurement of strain in the pileFig. 13 shows the axial load distribution of each test
pile and Fig. 14 shows the fi versus Si curve, basedon the results of strain measurements of the pile. Fig.
15 shows the change with time of Rconv, Rp, and Rf.

Results of measurement of pile head load andpile head settlement
Fig. 12 shows the Q versus Sh curve of each test pileaccording to the results of the vertical load tests.
This Figure shows the Q versus Sh curves for curing
periods of 1 month (bold line) and 3 months (brokenline).

Table 9. Change of Rconv with time
Conventional ultimate pile resistance Rconv (kN)
Test
1month3months
Ratio
No.
(a)
(b)
(b/a)
K1
1045
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Fig. 15. Change of Rconv,Rp,Rf with time280

Yabuuchi, S., Nishiwaki, A., Kobayashi, K., Hirao, K. & Yoshikawa, N.

(3) Discussion based on the results
The following points regarding bearing capacity canbe understood from the load-settlement relationship:(a) By kind of packing slag, the bearing capacity of bothkinds of slag at a curing period of 3 months was generally the same.

pile with no loading history at the curing period of 3
months was practically the same as the Rconv of pilewith loading history at the initial loading stage. Therefore, the consolidation of packed layers due to theloading at the initial curing period is effective for the
early development of bearing capacity.

3.3 Tests on large soil chamber in sandy ground(b) Comparison of construction methods showed  that
the Rconv of the bored and slag-packed pile increased
by approximately 1.3 times at a curing period of 1
month, and increased by approximately 1.8 times at acuring period of 3 months compared to the Rconv of thebored and cement milk-packed pile. The change withtime of Rconv shows that during the reloading at a curing period of 3 months, the Rconv of the bored andslag-packed pile increased by about 1.2 times but thatof the bored and cement milk-packed pile reduced toabout 0.9 times.

From the distribution of axial loads and the fi - Si relationship, the following observations were made:(c) Comparison of the different types of slag showed thatthe fi for the slag in the wet earth up to the groundwater level (4.0 m below ground level) appeared fastercompared to the slag in earth in the saturated condition at depths below the groundwater level, and it accounted for 60% to 70% of the total resistance. Moreover, there was no difference in the development of fi
for both slags in clayey soil up to the ground waterlevel, but in sandy soil, the fi for mixed slag was about
1.9 times the fi for the steelmaking slag.
On the other hand, the onset of consolidation and expansion of both kinds of slag at depths larger than the
groundwater level was delayed. Hence, the fi at a curing period of 3 months was small, and no difference infi was observed for the different kinds of soil.

3.3.1 Test method
Various test piles were constructed by the bored andslag-packed piling method in sandy ground made oflarge soil chamber (length 6 m, width 4 m, depth 5m).
The precast concrete nodular pile was used as thetest pile and two kinds of slag, namely, steelmaking
slag and mixed slag, were used as the material for
packing the pile.
To study the change with time of the bearing capacity of each test pile, the vertical load tests werecarried out for test piles cured for 1 month and for 3
months. Test piles were further added to examine thepresence or absence of effects of loading history of
piles being cured, on Rconv.  Table 10 shows thetest conditions.

Y1
Y2
Y3
Y4

(d) Regardless of the kind of ground around the pile, the fivalue at a curing period of 1 month for the bored andcement milk-packed pile was uniform. In contrast, thevalue of fi up to the groundwater level for the boredand slag-packed pile was 100 to 300 kPa, accounting
for a major part of the total resistance. The settlementthat occurred at the maximum value of fi was about 10
mm in all the soil layers in case of the bored and cement milk-packed pile and the consolidated soil cement behaved as an integral part of the pile body. In
contrast, in case of the bored and slag-packed pile, thesettlement was greater than 50 mm in each layer at
depths below the groundwater level. Therefore, theonset of consolidation and expansion of the slag wasdelayed.

Table 10. Test conditionsTime of a load test
Material
Pile type3
6
of a filler 1month months months
Mixed
٤
٤
٤
Slag
Mixed

٤
٤
Slag
Nodular pile
Bored ĳ440-300, Steel٤٤٤
t=60,l=4m making
Slag
Steel
٤٤makingSlag

Test
MethodNo.

3.3.2 Test ground and locations of installation oftest piles
Artificial fill ground is sandy ground of medium
density with an N-value of about 20. After construction of the test piles, the sandy ground was considered to be in the saturated condition taking the
groundwater level as the ground level (GL ±0 m).
The test pile was installed with its tip at 3.0 m below ground level in sandy ground.
Fig. 16 shows the soil boring log of the fill groundand the positions of installed strain gauges on thetest pile. Strain gauges were installed in the pile
head, center of the pile and pile tip ends.

(e) The Rconv of the bored and slag-packed pile reloaded
at a curing period of 3 months increased to 1.25 times
the Rconv at the initial loading. However, the Rconv of

Bearing capacity of bored pile packed with iron and steel slag using precast concrete pile281

3.3.3 Results of vertical loading tests and discussion(3) Discussion based on the results
Conclusions arrived at regarding the bearing capacity based on the results of Table 11 are as follows:(1)

Results of measurement of pile head load andpile head settlement
Fig. 17 shows the Q versus Sh curve of each test pileaccording to the results of the vertical load tests.
This Figure shows the Q versus Sh curves for curing
periods of 1 month (bold line) and 3 months (brokenline) and 6 month (chain line).
Table 11 shows the Rconv of each test pile and the
Rconv with the change in time.

(a) Comparison of the various kinds of packing materials
showed that the Rconv of bored and slag-packed pile insaturated sandy ground when mixed slag was used
was about 1.4 times larger than the case when steelmaking slag was used as packing material. With thechange in time, the bearing capacity increased byabout 1.5 to 2.0 times compared to the initial loading
result for both kinds of slag when the pile was reloaded at a curing period of 3 months and 6 months.

(2) Results of measurement of strain in the pileFig. 18,19 shows the axial load distribution of eachtest pile and Fig. 20,21 shows the fi versus Si curve,
based on the results of strain measurement of thepile. Fig. 22 shows the change with time of Rconv, Rp,
and Rf.

(b) The effects of the presence/absence of loading history
during the curing period on the bearing capacity werestudied. It was found that the Rconv of pile initiallyloaded at a curing period of 3 months was about 0.65
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Test 1month 3months Ratio 6months Ratio
No. (a)
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Y1 1135 1693
1.49 2182
1.29
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1.36
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1.61 1542
1.30

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Yabuuchi, S., Nishiwaki, A., Kobayashi, K., Hirao, K. & Yoshikawa, N.

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Rconv of the pile with loading history when it was reloaded. Moreover, the Rconv was 0.96 times for mixed
slag and about 1.25 times for steelmaking slag compared to the Rconv in the initial condition of the pilewith loading history. This suggests that loading thepile in the initial stage of curing is effective in developing the bearing capacity of the pile at an early stage.

Next, the following conclusions were arrived at fromthe fi - Si relationship and the axial load distribution,
based on the condition that the sandy ground was
saturated with water:(c) The development of fi in case of mixed slag was more
noticeable closer to the surface. In contrast, the development of fi incase of steelmaking slag was almost
uniform throughout the depth. The fi for mixed slag at
a curing period of 6 months was about 2 times the fi
for steelmaking slag on an average.
(d) During on-site tests in sandy soil and clayey soil, consolidation and expansion of the slag layers below thegroundwater level were excessively delayed. In contrast, an increase in the bearing capacity due to consolidation of slag layers in water was verified in thistest. This is because the outflow/inflow of groundwater occurred constantly in the on-site tests, whereas nooutflow of water occurred from the large soil chambers in these tests. As a result, consolidation progressed due to the concentration of Ca ions thatleached from the slag. Reaction layers were observedat the interface of the slag and the ground when the
ground was excavated after the tests.

4

RELATIONSHIP BETWEEN SKIN FRICTION
FI OF THE BORED AND SLAG-PACKED
PILE, TIP RESISTANCE QP AND N-VALUE

Regression equations were derived from the relationship between skin friction of the section, fi, andthe average N-value of the section at the conventional ultimate pile resistance Rconv, and the relationship between the pile tip resistance qp and pile tip
Np. The sandy soil and clayey soil were assumed toaccount for more than 70% of the volume at eachsection, and the skin friction at the section and average N-value of the section were taken as fs, Ns, fc
and Nc respectively. The N-value at the pile tip, Np,
was taken as the net average N-value in the range of“1 Db above and 1 Db below” the lowest node.
Fig. 23 and Fig. 24 show the f versus N relationship, while Fig. 25 shows the qp versus Np relationship.
From the above, the following conclusions werearrived at:(1) In sandy ground, the fs of pile reloaded at a curing period of 3 months increased to approximately 1.4 times the fs at the initial loading condition. As curing progresses, the N-value of thesurrounding ground was directly proportional tofs.
(2) In clayey ground, the fc of pile reloaded at a curing period of 3 months increased by 1.2 to 2.4
times the corresponding value at the initial loading test. However, in clayey layers below thegroundwater layer during the curing period, fc
developed according to the action of the pile
body and the coarse-grained slag and showed aconstant value regardless of the N-value of thesurrounding ground. After a long curing period,
the packed slag becomes integral with the pile

Bearing capacity of bored pile packed with iron and steel slag using precast concrete pile

283

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body, resulting in a proportional relationship between the N-value of the surrounding groundand fc.
(3) The pile tip resistance stress, qp, of piles withloading history increased during reloading. This
trend is similar to that of ordinary bored precast
concrete piles. That is, qp is the stress level thatappears between the pile tip and the ground with
the increase in the relative density of the grounddue to the compression during penetration of thepile tip into the ground.
5

CONCLUSIONS(1) By using Iron and steel slag as packing material
in the bored precast concrete piling method, it
was verified that the bearing capacity of the pileincreases.

(3) By combining a precast concrete nodular pilewith iron and steel slag, the bearing capacity ofthe part below the node can be increased andhigh bearing capacity can be developed.
(4) The bored and slag-packed pile is effective insandy and clayey soils since the bearing capacity
increases. The bearing capacity particularly insandy ground is very high.
(5) The skin friction of bored and slag-packed piledevelops faster in ground in the wet condition
upper the groundwater level. However, in groundbelow the groundwater level, the onset of
consolidation and expansion of the slag isdelayed because of the inflow/outflow of water,and more time is necessary for the developmentof skin friction.
REFERENCES(2) The bored and slag-packed pile develops excellent bearing capacity compared to the bored andcement milk-packed pile constructed by the conventional method. Particularly, the bearing capacity increases remarkably with the passage oftime to a level that cannot be obtained by conventional construction methods. Accordingly, theuse of slag instead of soil cement enables reduction in costs and moreover, if such a bearing capacity at the completion of building structures
can be used during design, then further costbenefits may be anticipated.

2841) Yabuuchi,S & A.Nishiwaki & M.Himeda 2001.Applicationof steelmaking slag to pile foundation. “CAMP-Iron &Steel Institute Japan” Vol.14 (2001)-942.
2) Yabuuchi,S & M.Sumi & A.Nishiwaki & M.Himeda Construct a pile foundation with iron and steel slag. “New trend
toward utilization of steel slag as construction materials”
40-44.2002. Iron & Steel Institute Japan.
3) “Method and Explanations for Static Axial Compressive
Load Test of Single Pile” 2002. The Japanese GeotechnicalSociety.

Yabuuchi, S., Nishiwaki, A., Kobayashi, K., Hirao, K. & Yoshikawa, N.