- 1 -

Pile Foundation Bearing Capacity Characteristics of Volcanic Ashin Hokkaido, Japan

Jun’ichi NISHIKAWAKouiti TOMISAWA ,Civil Engineering Research Institute of Hokkaido(CERI) , Sapporo,Japan

AbstractSince volcanic ash has both friability and bulk compressibility and the foundation

strength is reduced by pile construction, no definitive pile foundation design methodfor volcanic ash has yet been established in Japan. Vertical load tests of piles andcone penetration tests were therefore conducted for volcanic ash distributed inHokkaido, Japan, to verify the pile support mechanism in volcanic ash groung.

In this study, characteristics of vertical bearing capacity were examined basedon the results of field tests with an emphasis on the engineering properties ofvolcanic ash and pile construction methods. Results from the study revealed acorrelation between the necessity of reducing the skin friction of driven andcast-in-place piles in designs in volcanic ash ground and the value of conepenetration resistance.

Keywords: volcanic ash, pile, load test, bearing capacity, cone penetration test

1. Introduction

The bearing capacity formula for pile foundations in volcanic ash ground is usually designedbased on sandy ground because volcanic ash has a certain shear strength . Volcanic ash ,1)

however, is said to be friable. It has been stated in many field reports that it is difficult to ensure thedesigned skin friction and end-bearing capacity of piles due to disturbance of the ground at the timeof pile construction , and no pile foundation design or construction management methods have yet2),3)

been established for volcanic ash ground.Vertical load tests of steel pipe and cast-in-place piles were thus conducted for Shikotsu, Mashu

and Komagatake volcanic ash distributed in Hokkaido, Japan, to verify the development mechanismof pile bearing capacity in volcanic ash ground. Electric cone penetration tests (CPT) were alsocarried out to evaluate the skin friction of piles through field investigations. In this study, thecharacteristics of vertical bearing capacity of pile foundations were examined with a focus on theworkability of piles and engineering properties of volcanic ash based on the results of field tests4), 5)

to establish a reasonable design method for pile foundations in volcanic ash ground.

2. Volcanic ash

Vertical load tests for piles were conducted to confirm the bearing capacity in volcanic ashground in Hokkaido, Japan. Tests were carried out at four sites: Kotobuki viaduct with steel piles of

500 mm, L = 23 m (Site A) and Tetsuhoku Bridge with cast-in-place piles of 1,200 mm, L =φ φ

18.5 m (Site B) in Central Hokkaido, Shintomi Bridge with cast-in-place piles of 1,000 mm, L = 8mφ

- 2 -

(Site C) in Eastern Hokkaido, and Hakodate IC Bridge with cast-in-place piles of 1,200 mm ,φ

L =14m (Site D) in Southern Hokkaido (See Fig. 1: Volcanic ash distribution in Hokkaido).Volcanic ash at each site is divided into Shikotsu pumice flow sediment (Spfl., Sites A and B),

Mashu pumice flow sediment (Mafl, Site C) and Komagatake volcanic sediment (Ko, Site D) .6)

Volcanic ash from these sites was generally classified as coarse-grained volcanic ash. As shown inthe soil test results (Table 1), volcanic ash from all four sites had a relatively high shear strength inground. According to field penetration test results, coarse-grained volcanic ash in Hokkaidogenerally has a low resistance to dynamic forces because particle breakage is likely to occur, and ahigh resistance to static forces due to the unevenness of grains.

Table 1 Basic physical propenties of Volcanic ash

6)Fig. 1 Volcanic ash distribution in Hokkaido

- 3 -

3. Vertical load test

3.1 End bearing capacity of piles (qd)Vertical load tests of piles at each site were conducted using a multi cycle reaction load

method, which consisted of column devices and used surrounding piles as a reaction force, inaccordance with the criteria of the Japanese Geotechnical Society . Each test pile was equipped7)

with strain gauges placed at regular intervals in the depth direction of the piles to verify developmentof skin friction and end bearing capacity of piles at the time of the load test.

Fig. 2 Geologic log at each site-Relationship between the N value and the skin friction of the pile

- 4 -

As a result of vertical loading tests, the ultimate bearing capacity (Ru) measured for all testpiles satisfied the required design bearing capacity, and the goal of the field verification test wasaccomplished. This was achieved because the design end bearing capacity (qd = 3,000 kN/m ) was2

satisfied for cast-in-place pipes, and an end bearing capacity far exceeding the relationship betweenthe pile setting depth and the N value, or the relationship of the qd at the time of design/N - settingdepth in the bearing layer/pile diameter, was secured for steel pipe piles.

3.2 Skin friction of the pile (f)Figure 2 shows the skin friction (f) of test piles at four sites obtained from vertical load test

results, as contrasted with the N value in the depth direction. It can be seen that, for steel pipe pilesat Site A, almost no skin friction was observed in the section where the N value was 30 or lower,and that a relatively large skin friction (f) that satisfied the calculated design value for sandy groundf = 2N (kN/ m ) developed in the section where the N value was 30 or higher. At Sites B and D with2

cast-in-place piles, on the other hand, the results nearly corresponded with the relationship of f = 5N(kN/ m ), which was the design value for sandy ground. For cast-in-place piles at Site C with2

confined water, however, the skin friction of piles was only 60% of the design value in the layer withN values of 30 or higher.

This result was thought to be due to the difference in construction methods for steel pipe piles,which are referred to as non-displacement piles, and cast-in-place piles, which are referred to asdisplacement piles. It was presumed that, especially in the construction of steel pipe piles, vitreousmaterial in volcanic ash was broken at the time of piling and development of skin friction of pilesdeteriorated considerably in sections with a low relative density. It was also assumed that, whenconfined water exists as in the case of Site C, the development of skin friction of piles was affectedby the generation of water flows between the piles and volcanic ash soil and other factors even afterpile construction.

3.3 Boring in the steel pipe pilesFor steel pipe piles at Site A, boring in pipes was conducted for open-end vertical load test piles

and hammer-set piles with special cross-rib processing to verify the end blockage effect and groundstrength in pipes before carrying out thevertical load test.

Figure 3 shows the results of boring inpipes of open-end and cross-end piles.While end blockage was evident in thebearing layer at approximately 20 m ordeeper, the ground strength of soil in pipeswas generally low in cross-end rib piles andthe strength tended to decrease remarkablyat the finishing point of piling in the bearinglayer.

This phenomenon most likely occurredbecause the shear strength of friable volcanicash in pipes was reduced by breakagecaused by piling and, as a result, sufficientsoil resistance or end bearing capacity of thepipes was not maintained. Fig. 3 Boring in steel pipe piles at Site A

012

101112131415161718192021222324

25

26

3456789

Depth

(m )

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲　

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

0 10 20 30 40 50

Ins ide of an open-end p ile

Ins ide of a cross rib-end p ile

Surround ing ground

N Value

012

101112131415161718192021222324

25

26

3456789

012

101112131415161718192021222324

25

26

3456789

Depth

(m )

Depth

(m )

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲　

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

0 10 20 30 40

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲　

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

▲　　 ▲　　 ▲　　 ▲　　　　 ▲　　 ▲　　 ▲　　 ▲

0 10 20 30 40 50

Ins ide of an open-end p ile

Ins ide of a cross rib-end p ile

Surround ing ground

N Value

- 5 -

4. CPT test

4.1 Foundation strength (qt)CPT is a test method for measuring the point resistance (qt), skin friction (fs) and pore water

pressure (w) as sequential ground data by penetrating the ground with a cone that has a built-incenter at its top at a speed of 2 cm/s using a 200-kN penetrator.

CPT tests were conducted at pile load test positions of Sites A, B and C. The point resistance(qt) showed a relatively high correlation of qt 1,000 N (kN/ m ) to the N value at all sites. The≒ 2

CPT test was therefore considered to be somewhat effective as a simple test method that enablesthe direct measurement of point resistance (qt), skin friction (f) and pore water pressure (w) involcanic ash ground.

4.2 Skin friction (fs)Figure 4 shows the skin friction (fs) in the depth direction at Sites A, B and C, obtained as a

result of CPT tests. According to the figure, the skin friction (fs) obtained by cone penetrationdeveloped in proportion to the N value of volcanic ash shown in Fig. 2, and these measurementvalues roughly corresponded with the design values based on sandy ground. In comparison withthe skin friction (f) measured using the vertical load test, which is also shown in the figure, valuesapproximately equal to the lower limit of fs were found by utilizing the CPT test for cast-in-place pilesat Site B, in the same manner as in the correlation chart with the N value shown in Fig. 2. For steelpipe piles at Site A and cast-in-place piles at Site C with confined water, however, the values wereobviously smaller than the fs value found using CPT tests, before the N value became approximately30.

Fig. 4 Skin friction (fs) determined by CPT test

- 6 -

4.3 Cone repetition methodIn addition to ordinary CPT tests, penetration was repeated in this study under the assumption

that disturbance of ground occurred at a certain depth, which is shown in Fig. 4 (referred to as therepetition method, as opposed to the uniaxial method that is usually used), to verify the energydispersion to the ground at the time of foundation pile construction and the decrease in skin frictiondue to the particle breakage peculiar to volcanic ash at Sites A, B and C. The repetition stroke wasmaintained at 5 cm, taking into account the influence of porous wall peeling.

Results showed that the skinfriction (fs) determined by the CPT testat Site A eventually converged ataround 1/100 after approximately 20repetitions. The converged skin friction( fs ' ) va lue at S i te A roughlycorresponded with the skin friction in thevolcanic ash layer with an N value of 30or lower obtained using the vertical loadtest. It was therefore consideredexplainable as a phenomenon ofdecreased skin friction caused by theplacement of steel pipe piles. Figure 5shows the relationship between the ratioof fs’ converged by the repetitionmethod at Site A to fs obtained byutilizing the uniaxial method and thepoint resistance (qt). As a result, the exponential function formula of fs’/fs = 0.1187e was-0.0007qc

found. This formula shows the reduction in the decreasing rate of skin friction (fs’/fs) by therepetition method with an increase in qt. It is therefore considered to represent the friability ofvolcanic ash due to the steel pipe pile construction at N values of 30 or lower, in the same way asthe development of skin friction obtained through the vertical load test.

Figure 6 shows the relationshipbetween the skin friction at the firststage of the CPT repetition method (fs1)and the result of vertical load tests (f) atSites B and C with cast-in-place piles.As can be seen in the figure, therelationship between fs1 and the loadtest value (f) was regular although datawere insufficient, and it will benecessary to evaluate the influence ofconfined water at Site C in the future.In assuming that the decreasing rate ofskin friction at the first stage ofrepetition was at the same level as thedisturbance of volcanic ash at the timeof cast-in-place pile construction, it was

Fig. 5 Relationship between fs’/fs of cone penetrationtest and point resistance qt for steel pipe piles at Site A

Approximate curvefs’/fs=0.1187e

-0.0007qc

0.15

0.10

0.05

0.00

fs’/fs

0 20000 40000 60000

Point resistance qt (kN/m )2

Approximate curvefs’/fs=0.1187e

-0.0007qc

0.15

0.10

0.05

0.00

fs’/fs

0 20000 40000 60000

Point resistance qt (kN/m )2

Fig. 6 Relationship between the skin friction at thefirst stage of repetition and that of the load test atsites with cast-in-place piles

0

100

200

300

400

0 100 200 300 400

load test resu lts f（kN/m )

fs1（kN/m )

2

S ite B

S ite C

2

0

100

200

300

400

0 100 200 300 400

load test resu lts f（kN/m )

fs1（kN/m )

2

S ite B

S ite C

2

- 7 -

presumed that the circulation effect in sandy ground from the point of stress to surrounding areasexpressed by the cast-in-place pile bearing capacity formula of Myhof , which is the bearing8)

capacity theory of Prandtl’s system, appeared in the state where surrounding soil had not peeled offfrom the tip of the cone in the CPT test.

At Sites A and B, recovery of the converged fs’ over time was not observed even when therepetition method was used again 12 hours after the repetition test was conducted at a certain depth.

In the future it will be possible to establish design methods for the skin friction of piles accordingto the type of volcanic ash by accumulating basic data using the above method and verifying thecorrelation between the vertical load test and cone test values.

5. Conclusion

(1) Vertical load test showed that it is possible to secure the required end bearing capacity (qd) ofsteel pipe and cast-in-place piles through appropriate setting of the bearing layer even in friablevolcanic ash ground. During this test, especially in the case of steel pipe piles, it wasconsidered to be difficult to display the end blockage effect due to the breakage of soil in pipeswhen special processing had been applied at the tips of piles.

(2) Development of skin friction (f) of piles in volcanic ash was determined to be a result of thedifference in workability of steel pipe piles (displacement piles) and cast-in-place piles(non-displacement piles).

(3) The CPT test, which enables direct measurements of point resistance (qt), skin friction (f) andpore water pressure (w) in volcanic ash soil was found to be effective as a simple test method.

(4) Because almost no skin friction of piles developed when the N value was 30 or lower for steelpipe piles in volcanic ash ground according to the results of vertical load tests, it was presumedthat the development of skin friction was considerably reduced through breakage of vitreousmaterial of volcanic ash due to pile driving. For steel pipe piles, an exponential function formulaof (skin friction by repetition method fs’)/(skin friction by uniaxial method fs) = 0.1187 e was-0.0007qc

also obtained using the CPT test.

(5) While skin friction of piles based on sandy ground was mostly maintained for cast-in-place pilesin volcanic ash according to the results of vertical load tests, only 60% of the design value wasachieved at the site affected by confined water. The value of one stage of CPT repetition (fs1),which was thought to have a decreasing rate at the same level as disturbance of the ground atthe time of cast-in-place pile construction, showed a relatively favorable relationship with theresult of the vertical load test (f).

6. AfterwordsThis study made clear the necessity to conduct evaluation by reducing skin friction from the

design ground constants for different types of piles. Other results related to foundation pile designfor volcanic ash ground were also obtained. To establish feasible design and construction methodsfor foundation piles in volcanic ash ground in the future, it will be necessary to clarify the engineeringproperties of volcanic ash distributed widely in Hokkaido, as well as the development mechanism ofthe bearing capacity of foundation piles.

- 8 -

References

1) Japan Road Association: Specification for Highway Bridges Instruction Manual IV - substructure,pp. 348 - 433, Mar. 2002.

2) Tomisawa and Nishikawa: Bearing capacity characteristics of driving steel-pipe-pile in volcanicash ground, Collection of lectures at the 41st technical research presentation of the Hokkaidobranch of the Japanese Geotechnical Society, pp. 29 - 35, Feb. 2001.

3) Kimiaki Akai, Yuichi Tsujimoto, Shozo Sakuma and Takeshi Hanzawa: Bearing capacitymechanism of steel pipe piles in the Shikotsu volcanic ash layer, Soil and Foundation, Vol. 132,No. 3, pp. 41 - 68, July 1975.

4) Ryosuke Kitamura: Topic on Crushable Siol and Crushable Ground in Geotechnial Engineering,Japanese Geotechnical Society, Soil and Foundation, Vol. 48, No. 1, pp. 3 - 6, Oct. 2000.

5) Seiichi Miura, Yoshikazu Yagi, Hiroyuki Tanaka and Tsuyoshi Asonuma: Mechanical Behavior ofCrushable Volcanic Coarse-grained Soil in Hokkaido and its Evaluation, Japanese GeotechnicalSociety, Soil and Foundation, Vol. 48, No. 10, pp. 15 - 22, Oct. 2000.

6) Committee for Engineering Classification of Volcanic Ash : Properties and utilization of volcanicash in Hokkaido, Hokkaido branch of the Japanese Geotechnical Society, Oct. 1997.

7) Japanese Geotechnical Society: Instruction manual for vertical load test method for piles, June1993.

8) Meyhof G.G.: The Ultimate Bearing Capacity of Foundation, Geotechnique, Vol. 2, No. 4, 1951.