E3S Web of Conferences 415, 01009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501009

Critical states observed in triaxial compression tests on
volcanic pumice soil related to debris flow in the 2008 IwateMiyagi Nairiku Earthquake
Hiroyuki Hashimoto1*, Itsuki Sato2, and Reiko Kuwano3
1The University of Tokyo, Department of civil engineering, 7-3-1 Hongo, Bunkyo, Tokyo, Japan
2 Port and Airport Research Institute, Geotechnical Engineering Department, 3-1-1 Nagase, Yokosuka, Kanagawa, Japan
3The University of Tokyo, Institute of Industrial Science, 4-6-1 Komaba, Meguro, Tokyo, JapanAbstract. The Iwate-Miyagi Nairiku Earthquake (M7.2) that occurred on June 14, 2008, caused a slope
failure in Dozo-zawa River near the summit of Mt. Higashi-Kurikoma. The debris flow caused by the slope
failure flowed for about 10 km and took the lives of 7 people. The mechanisms of the slope failure and longdistance debris flow have not yet been well understood. In this study, a series of triaxial compression tests
was conducted using a highly crushable pumice soil collected from the collapsed area, and shear behaviorand particle breakage were discussed. Both reconstituted specimens and intact specimens showed
contractancy and reached critical states that implies high flow potential. However, intact specimens
preserved looser structure during consolidation and showed stronger contractancy during shearing than thereconstituted specimens, due to their natural soil structure. Significant particle breakage was observed in thetests with reconstituted specimens.

1 Introduction
1.1 Dozou-sawa slope failure and debris flow
The Iwate-Miyagi Nairiku Earthquake of magnitude 7.2
hit an inland volcanic mountain area in Northern Japanon June 14, 2008. As a result of the earthquake, a large
slope failure was generated at the head of Dozou-sawa
river near the top of Mt. Higashi Kurikoma, as shown inFigs. 1(a) and (b). The scale of the slope failure wasestimated to be about 200 m long, 300 m in maximum
width, and 30 m in maximum thickness with about 1.5
million m3 in volume of debris [1]. This slope failure
triggered a debris flow that travelled 10 km and 7 people
were killed in Koma-no-yu hot spring.
The collapse site is part of the Quaternary Kurikoma
volcano. The geology around the area is composed ofandesitic lava, tuff, and fused tuff, and the bedrock is
Neogene sedimentary rock, tuff, and fused tuff [2]. As
shown in Fig. 1(c), non-welded brown pumice soil andweathered volcanic clay were found on the slip cliff andground surface in the northern part of the collapse area.
1.2 Properties of Dozou-sawa pumice soil
A particle of the Dozou-sawa brown pumice soil was
porous and contained water within it. It could be easily
crushed by fingers, and when crushed, the water in theparticle came out and became sticky.

The particle size distribution and physical propertiesof the pumice soil are shown in Fig. 2 and Table 1,respectively. The particle sizes of the soil were generally
in the range of 0.1 to 10 mm. The dry density of Dozousawa pumice soil measured at the site was below 1, andvoid ratio was 1.84, indicating that the soil had an
extremely loose structure. Plasticity index wasexamined using particles sieved to less than 425
micrometers and found to be as low as 7. The reason thenatural water content (wn) became higher than the liquidlimit (wl) is considered that the original pumice soil
contained coarser particles than those used in the liquid
limit test, and the coarser particles trapped more waterin their intra-particle voids.
1.3 Research objective
In this study, a series of triaxial tests was conducted on
intact and reconstituted specimens of Dozou-sawapumice soil to investigate mechanisms of the large slope
failure and long-distance debris flow. Particle breakage
was also discussed by examining the particle size
distributions of the specimens before and after the tests.
Table 1. Physical properties of Dozou-sawa pumice soil𝜌𝜌𝑤(g/cm3) (g/cm3)(%)

2.66 0.9362𝑒𝐷𝑤

𝑤

𝐼(mm)

(%)

(%)

(%)

1.84

1.7

5360

7

𝜌 : Soil particle density, 𝜌 : Dry density, 𝑤n : Water content innatural condition, 𝑒: Void ratio, 𝐷 : Mean particle diameter,𝑤p : Plastic limit, 𝑤L : Liquid limit, 𝐼 : Plasticity index* Corresponding author: hsmt@iis.u-tokyo.ac.jp© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 01009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501009

except for one case, where the target void ratio was 2.2
using slightly moist soil.

Fig. 3. Intact specimen and disturbed sample for reconstituted
specimen

2.2 Test conditions
Table 2 summarizes the conditions of triaxial tests in thisstudy. A series of drained (CD) and undrained (CU)monotonic compression tests was conducted undersaturated conditions. The confining pressure before
consolidation was 20 kPa for intact specimens and 10kPa for reconstituted specimens. To ensure a highdegree of saturation (Skempton’s B-value > 0.95), thedouble-vacuum method and a back pressure of 200 kPa
were applied. Specimens were isotropicallyconsolidated to 20, 100 or 200 kPa and left 1 hour tocomplete primary consolidation. Undrained or drained
monotonic loading was then applied at a shear rate of
1.0 mm/min. After testing, specimens were oven-dried
and then sieved to measure the amount of particle
breakage during the tests.
In addition, one experiment (R20) was conducted inwhich only a reconstituted specimen was prepared andsaturated to check for particle breakage due to specimen
preparation, saturation, and oven drying.

Fig. 1. Slope failure in Dozou-sawa river: (a) Google Earthimage, (b) General view, (c) Exposed volcanic pumice soil

Fig. 2. In-situ particle size distribution of Dozou-sawapumice soil

2 Triaxial tests

Table 2. Test conditions of triaxial tests

2.1 Specimen preparation

Case IDTo examine the effect of soil structure, intact specimens(less disturbed specimens that maintained the in-situ soilstructure) and reconstituted specimens were prepared inthis study.
Intact samples were collected at the site by pushingtubes gently in a vertical direction into the target layer.
They were extracted from the tubes, trimmed tocylindrical specimens of 50 mm in diameter and 100 mm
in height, as shown in Fig. 3.
Reconstituted specimens of 50 mm in diameter and100 mm in height were made of disturbed sample that
had been naturally dried and passed through a 13.2 mmsieve, as shown in Fig. 3. The soils were re-mixed tomatch the particle size distribution of the naturalcondition. To minimize the particle breakage during thesample preparation, each specimen was prepared in 5layers by spooning the soil into the mold and applying
gentle tamping until a target void ratio was achieved.
The specimens were prepared to be roughly equal to thein-situ void ratio (target 𝑒=1.8) using naturally dried soil,

IU20
IU100
IU200
IU200_2ID100
RU20
RU100
RU200RD100R20RU100L

Test 𝜎type (kPa)IntactCU20CU100
CU200
CU200
CD
100Reconstituted CU
20CU100CU200
CD100Satu…
rationRecon loose
CU100Specimen type𝑒𝑒

1.96
1.65
1.85
2.10
2.07
1.76
1.74
1.58
1.74

1.96
1.59
1.67
1.88
1.95
1.75
1.60
1.46
1.58

1.82
2.29

1.53

𝜎 : Effective confining pressure, 𝑒 : Initial void ratio, 𝑒 :Void ratio after consolidation. Note that IU100 contained alarge particle approximately 30 mm in diameter.

2

E3S Web of Conferences 415, 01009 (2023)
DFHM8https://doi.org/10.1051/e3sconf/202341501009

In Fig. 4(d), the void ratios, 𝑒, are plotted against themean effective stresses, 𝑝′, in semi-logarithmic scale. A
linear critical state line was observed for reconstitutedspecimens. The reconstituted specimen with loose initialstructure (RU100L) collapsed during consolidation andcame on the same critical state line. On the other hand,
the intact specimens kept higher void ratios than thereconstituted specimens at the corresponding 𝑝′ ,indicating that the structure of intact specimens was notcompletely lost due to consolidation pressure.
These differences in the behavior of intact andreconstituted specimens can be attributed to dissimilarity in soil structure of the two specimen types.
The loose in-situ soil structure was found to be difficult
to reproduce in reconstituted specimens.
Fig. 5 shows the particle size distributions before andafter triaxial tests. Significant particle breakage wasobserved in all the cases with reconstituted specimens.
However, no relationship was found between effective
confining pressure and the magnitude of particlebreakage Significant particle breakage was observed
even in the test with only the specimen preparation andsaturation (R20), suggesting that large particle breakage
occurred before shearing. It is not clear at what stage andto what extent the crushing occurred. The intense
particle breakage prior to shearing made it difficult toevaluate the effect of shear behavior on the soil structure.
Particle size distributions of the intact specimens werelargely varied. Since the particle size distributions
before the tests were not known, the amount of particlebreakage in the intact specimens could not be examined.

2.3 Results
Fig. 4 summarizes the results of triaxial tests.
Referring to Figs. 4(a) and (b), in all the CU tests,
deviator stresses, 𝑞 , gradually increased with axial
strains, 𝜀 , and finally stabilized. The CU tests withreconstituted specimens initially exhibited contractancy,
then passing the phase transformation point where thedilatancy behavior, the way of excessive pore waterpressure generation, changed from contractive todilative. When 𝜀 reached 10 to 15 %, they finallyreached critical states, where deformation continues
under constant stress and constant volume, that implies
high flow potential. The critical state line is illustrated
by the dotted line in Fig. 4(b). The CU tests with intact
specimens show similar behavior to reconstituted ones.
But the dilatancy behavior did not seem to be the same.
Especially, in the cases of an effective confiningpressure, 𝑒 , of 200kPa (IU200 and IU200_2), strong
contractancy was observed. The effective stress paths
reached critical state without experiencing the phasetransformation.
Referring to Figs. 4(a) to (c), both the CD test withintact specimen (ID100) and reconstituted specimen(RD100) exhibited a large contractancy. In the CD testwith reconstituted specimen (RD100), 𝑞 and volumetricstrain, 𝜀 , became constant at 𝜀 = 15 %, and reachedcritical state. In contrast, in the CD test with intact
specimen (ID100), the increase of 𝑞 was more gradual,
and the strong contraction continued even when 𝜀reached nearly 20%.

Fig. 4. Results of triaxial tests. (a) Deviator stress and axial strain relationship. (b) Deviator stress and effective mean stressrelationship. (c) Volumetric strain and axial strain relationships. (d) Void ratio and Effective mean stress relationship.

3E3S Web of Conferences 415, 01009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341501009

Fig. 5. Particle size distribution before and after triaxial tests

3 Conclusions
In this study, a series of triaxial tests were performed on
the volcanic pumice soil collected in Dozou-sawa river,where a large slope failure and a long-distance debris
flow occurred in the 2008 Iwate-Miyagi Nairiku
Earthquake. The pumice soil was highly crushable andretained extremely loose structure in the in-situ
condition. Such a loose structure could not be
reproduced in a reconstituted specimen. Almost all thecases reached critical states that indicated high flow
potential. Intact specimens retained looser structureduring consolidation compared to reconstituted
specimens, and those with larger confining pressures
showed stronger contractancy during shearing.
Significant particle breakage occurred in reconstituted
specimens, but no notable relationship was observed
between confining pressure and the amount of particle
breakage. Further studies need to be carried out toevaluate when and how much particle breakage
occurred during the tests.

References
1.

2.

Geospatial Information Authority of Japan (GSI),
Aero-photographs of slope failure area that caused
damage to Komanoyu Hot Springs (2008)
Available at:https://www.gsi.go.jp/johosystem/johosystem6003
0.html, accessed: 05/11/2022. (in Japanese)
M. Irasawa, M. Ushiyama, H. Kawabe, M. Fujita,
Y. Satofuka, D. Higaki, T. Uchida, A. Ikeda, J. ofJSECE, 61, 3, pp. 37-46 (2008)https://doi.org/10.11475/sabo.61.3_37 (inJapanese)4