1. No category

島根大学地質学研究報告 14，15∼20ページ（1995年12月）
GeoL Rept．Shimane Univ．，14，p．15∼20（1995）
A simplified testing procedure f6r eva且uating
the undrained shear behaviour
of anisotropicaIly consolidated clay
Takeshi Kamei＊
Abstract
A simple procedure has been developed to estimate the undrained shear behaviour of anisotropically
consolidated clay without special triaxial apparatus．The proposed testing procedure requires no special cell，
and its simplicity，speed and economy are well suited to evaluate the undrained shear behaviour after Ko−
consolidation．The comparison of data from three methods shows no significant difference in undrained
shear behaviour evaluations．Finally，the experimental results obtained by the present method are found to
be reasonable in engineering practice．The type H test can be readily used by practicing engineers．A1−
thoughapPlicabletoonlyapa丘icularsoiltype，theprocedureapPearstobesufficientlyaccurateforprac−
tical purposes．
Key
words：cohesive soil，consolidated
undrained shear， earth pressure at rest， laboratory test， shear
strength，test procedure．
paratus which closely resembles that used in engineering
Introduction
practice．Anewtest（TypeH：SCIDUandType皿：
Most commercial laboratodes conduct routine isotropi−
SCIPCC）which can dramatically reduce the cost associ−
cally−consolidated triaxial tests（CIU），whereas most in−
ated with the undrained shear behaviour of Ko−consoli−
situ natural clay deposits exist under anisotropic states of
dated cohesive soils，is described in this paper．The ma−
stress，corresponding to CAU conditions．Therefore，the
jor advantage of this technique is its simplicity because
mechanical behaviour of anisotropically cohesive soils
it does not requires special triaxial apParatus．It also has
have become of increasing importance as the number of
a low initial cost and requires neither time measurement
embankments and other earth stm、ctures being designed
nor stress control during consolidation．
and constructed on soft ground have increased．However，
The purpose of this paper is to investigate whether it
unless a device has specialized features as seen in Fig．1
is possible to estimate the undrained shear behaviour of
（Nakase and Kamei，1983；Kamei，1985），Ko consolida−
Ko−consolidated cohesive soil using standard triaxial ap−
tion with no lateral strain is a difficult and time consum−
paratus，and if so，how accurate these estimations are．Re−
ing procedure to implement．In addition，the anisotropic
sults using the proposed testing Procedures are presented
consolidation phase of triaxial test specimens is generally
and compared to data from special triaxial apparatus for
time consuming and expensive，especially if Ko condi−
Ko−consolidation．
tions are maintained．It has also become standard proce−
Experiments
dure in CIU tests to use an effective confining Pressure
equal to the estimated in situ vertical stress．Conse−
Sample and testing Programme
quently，it has become routine practice to consolidate the
The undisturbed clay（Y−38）used in the present study
specimens isotropically（Kc＝1）before shear to failure．
was sampled at16∼18m depth from a site south−west
At this stage， it is advantageous to represent the
of Tokyo Bay．The index properties of the soil sample is
undrained shear behaviour of Ko−consolidated cohesive
shown in Table 1．Fig．2shows the grading curve of the
soils in a simple manner，using conventional triaxial ap一
specimen．All tests were consolidated undrained triaxial
＊
Department of Geoscience，Shimane University，
shear tests with pore pressure measurements． These
1060 Nishikawatsu，
Matsue690，Japan．
undrained shear tests were performed under strain−con一
15
Takeshi Kamei
16
舷ial load
Lateral pressure
Signa1（Axial disphcement）
Control
unit
Axial displacement
transducer
聯1↑
Benofヒam cylinder
鶴副
Floppy disk
一団團
Load cell
Signa1（」咽al load）
P’ t
Lubrication
Lateral
Lεteral
displac㎝1㎝t
displa㏄m㎝t
transdu㏄r
hmsdu㏄r
Pressure transducer
﹄o駕蓉。﹂o自︶
control㎜it
匙
Micro−computer
︵o﹄3沼目
田島あ
Back pressure
System Data
soRware record
Measurement
signal
Pore water pressure
副
measurement
X−Y pbtter
Measurement
unit
Printer
Lateral displacement
meas斑ement
Fig．1Schematic diagram of automated Ko−consolidated triaxial apparatus．
TablelIndexpropertiesofsoilstudied．
Ps（9／cm3）
肌（％）
ωP（％）
2，656
77．8
39．8
Sandσの
38
5
49
／
Ko−Line／’
Clay（％）
／ノ
ρノ
46
6
’
PI
Silt（％）
／
’
（1）Ko ／
nU O O O
ΩU 6 4 ∩∠
ρ温州霞台おヨ㎏①蟹52￡
︵￡エ︶σ
100
（q／P’・一定） ダ
，／
，／
、／
ノ／
／ （H）
／ Is・・（0・67po）
ノ ↓
／
， D−Test
YOKOHAMA Clay（Y−38）
（皿）
ISO．（PO）
↓
P’ニconst．
一Test
0．67pO Po
PI＝38
P’（kPa）
0
0．001 0．01 0．1 1
Fig．3Stress paths used in the present study．
GminSize（mm）
specimens，the effective confining Pressure was usually
Fig．2Grain size distribution curve of sample．
applied in one stress increment（0．67p。：SCIDU or p。：
trolled during shear to failure．Fig．3 shows the stress
SCIPCC）， while for anisotropic consolidation， drained
paths used in the present study．Three series triaxial tests
shear and mean effective principal stress（pノ）at constant
were prepared for each of the soils considered：Ko−con−
were applied to the point of completion of Ko−consolida−
solidated （Type I：CKoU） and anisotropically consoli−
tion（qo）because of convenience and simplicity of the
dated（Type H：SCIDU and Type皿：SCIPCC）．The Type
testing procedures（p。is the mean effective stress at the
Hand Type皿proposed in the present study are conven−
point of the completion of the Ko−consolidation，and q。
ience and simplicity of testing procedures for the evalu−
is the principal stress difference at the point of the com−
ation of the undrained shear behaviour of Ko−consoli．
pletion of the Ko−consolidation）．It is important to note
dated cohesive soil，although Ko conditions are not main−
that testing procedures used in this study are limited to
tained dudng consolidation．For isotropically consolidated
theevaluati・n・fundrainedshearbehavi・ur・fanisotr・pi一
A simplified testing procedure for evaluating the undrained shear
17
behaviourofanisotropicallyconsolidatedclay
1．0
0．8
0．6
0．4
Comp．Ext．
1
0＞
レb＼ぴ
H
0．2
個
品
』▲
皿
0
一〇．2
一〇．4
一〇．6
0
10
20
ε
（％）
a
Fig．4Comparisons of stress−strain behaviour obtained from three types of anisotropic consolidation process．
Table2 Summary of triaxial test results．
Initial Cond．
ωnσ◎
1
Comp．
一 一 一
dxt．
Comp．一 一 一 一一一 一一 一 一一一一
eo
2．18
82．2一 一 一 一 一 一 一 一 甲 一 一 一 』 一
After Cons．
ω・（％）
68．8
帰 需 − 囎 一
@ 79．9
ec
1．82
Tdaxial Test Results
Kc
0．43輔 騨 騨 一 騨 層 一 響 − 一 一 一 暑 一
c。依Pの
@ 2．13
67．9
2．08
@ 77．9
84．3
33900
@一18．19
30630
1．51
42550
26960
@ 〇．43
@ 78．8
66．9
1．78一 一 一 一 一 一 一 一 一 一
0．43一 一 一 昌 一 一 一 一 甲 − 一 卓 辱 一
114．〇一 一 一 一 一 一 一 一 一 軸 一 一
2．07
67．0
@1．78
@ 〇．43
@ 78．9
一17．89
2．24
66．1
0．43
117．3
0．85
47300
84．0
一18．87
Q9560
P．81
皿
Comp．一 一 一 一 一 一 畢 一 一 一 一 一 一
1．76一 一 雪 一 一 一 一 一 幽 一 一 一 一 一
一 一 一 』 一 一 一 一 一
@ Ext．
@ 86．8
一 一 一 一 『 一 一 一 一
2．31
68．8 @ 1．83
Af
0．51一 一 一 一 − 一 昌
Cu／P
0．41一 一 一 幽 一 甲 一 ⇔ 一 一 一 一 一
一 一 一
H
@ Ext．
E50（kPの
1．04
116．3一 一 一 一 一 一 一 一 一 − 一 一 一 一
『 一
78．3一 一 一 一 督 一 一 一 一 − − 一 一 一
εf殉
@ 〇．43
0．80
@ 〇．28
0．56一 一 一 一 一 一 一 一
0．41， 一 榊 醤 一 一 岬 『 一 一 一 甲 一
Z．77
@ 〇．28
0．40一 一 一 』 一 一 一 一 一 』 一 ｝ 一 輯
0．42｝ ｝ 一 一 一 一 一 帰 一 一 一 一 一
@ 0．76
@ 〇．30
一 一 一 一 一 − 一 一
Takeshi Kamei
18
農品H
IH皿
0．4
讐HH
0．6
の ・b＼5ぐ
0．2
0
一〇．2
一〇．4
0
10
20
ε （％）
a
Fig．5Comparisons of excess pore pressure
△u−strain behaviour obtained from three types of anisotropically
consolidation process．
cally consolidated cohesive soils only．
iour obtained from three types of anisotropic consolida−
A vertical effective consolidation pressure of 294 kPa
tion process is shown in Fig．5，where the excess pore
was used in the consolidation process．A back pressure
pressure △u is no㎜alized by dividing by the vertical
of l96kPa was applied to all the test specimens
メ
effective consolidation pressureσvc．As seen in this fig−
throughout the consolidation and undrained shear． For
ure，reasonable agreement is obtained from the excess
each soil sample，triaxial compression and extension load−
pore pressure △u−strain behaviour of undrained triaxial
ings were pe㎡omed，with a constant rate of axial strain
compression and extension loadings for undisturbed Yok−
of O．07％／min．（Kimura and Saitoh，1983；Nakase and Ka−
ohama clay（Y−38）su切ected to the three types of ani−
mei，1986）
Test results and discussions
sotropic consolidation processes．
Figure 6 compares the effective stress path results
from three types of anisotropic consolidation processes，
Figure4compares the stress−strain behaviour，results
where the principal stress difference q＝σa一σr and the
obtained from three types of anisotropic consolidation
メ ノ ノ
mean effective stress p＝（σa＋2σr）／3are nomlalized by
processes，where the principal stress difference q＝σa一σr is
dividingbytheverticaleffectiveconsolidationpressure
normalized by dividing by the vertical effective consoli一
ノ
ガ
dation pressureσvcr．As seen in this figure，there is rea−
ment between the results for effective stress paths of
sonable agreement between results for stress−strain behav−
undrained triaxial compression and extension loadings for
iour of undrained triaxial compression and extension
undisturbed Yokohama clay（Y−38）for the three types of
loading for undisturbed Yokohama clay（Y−38），su切ected
anisotropic consolidation processes considered．The proce−
tothethreetypesofanisotropicconsolidationprocess．
dureissufHcientlyvalidandaccurateforpracticalpur−
Comparison of excess pore pressure △u−strain behav一
poses， and should be used to investigate the undrained
σvc．As seen in this figure，there is reasonable agree−
A simplified testing procedure for evaluating the undrained shear
19
behaviourofanisotropicallyconsolidatedclay
1．0
／
！
／
・虜
ノ
勺
0．5
蕊 ．／
ら ／
．窃 競ノ
δ 鷲／
ノ／
O﹀
・b＼σ
1，／
ノ／
／
！
0
凪品H
讐HH
IH皿
ゑ宅
●も餐
q窒
一〇．5
0 0．5 1．O
P亨／σ l
VC
Fig．6Comparisons of effective stress path obtained from three types of anisotropic consolidation process．
shear behavlour of Ko−consolida，ted cohesive soil．The
The author is fully aware of the limitations of the pro．
type皿（SCIPCC）test，however，is time−consuming and
posedtestingProcedure，anditshouldberegardedonly
impractical for commercial testing．The weakness of this
as a first apProximation．
method lies in its neglect of the Ko conditions during
Conclusions
consolidation，however，a limitation that the type H ap−
proach has overcome is to reduce the cost significantly．
A simple procedure has been developed to estimate the
The applicability of the proposed methods to other soils
undrained shear behaviour of εmisotropically consolidated
will constitute further research．Table2shows the sum−
clay without special triaxial apparatus．The proposed tesレ
maηゾOf triaXial teSt reSUltS．
ing Procedure requires no special cell，and it is a simple，
The combination of the proposed method and the shear
rapid and economical way，to evaluate the undrained
provide an extremely rapid，easy，reliable and economic
shear behaviour after Ko−consolidation．The comparison
means of evaluating the undrained shear behaviour of Ko
of data from three methods shows no significant differ−
−consolidated cohesive soils．
ence in undrained shear behaviour evaluations．To this
Takeshi Kamei
20
end，the experimental results obtained by the present
degree of Doctor of Engineering．
methods are found to be reasonable in engineering prac−
Kim皿a，T．and Saitoh，K．，1983：The influence of strain
tice．The type H test can be readily used by practicing
rate on pore pressure in consolidated undrained triaxial
engineers．Althoughrelevanttoonlyapa質icularsoil
tests on cohesive soils，Soils and Foundations，Vol．23，
type，theprocedureapPearstobesuf行cientlyaccuratefor
No．1，80−90．
practical purposes．
Ladd，C．C．and Foott，R，1974：New design procedure
Acknowledgement
for stability of soft clays，Joumal of Geotechnical En−
gineering Division，ASCE，Vol．100，No．7，763−786．
The author would like to express his gratitude to Mr．
Nakase， A． and Kamei， T．， 1983：Undrained shear
M．Hayashi，of Kiso−Jiban Consultants Co．，Ltd．for his
strength anisotropy of nomally consolidated cohesive
help during experimental work．
soils，Soils and Foundations，Vol．23，No．1，91−101．
References
Kamei，T．，1985：A study on the mechanical behaviour
of nomally consolidated cohesive soils，thesis pre−
Nakase，A．and Kamei，T．，1986：Influence of strain rate
on undrained shear characteristics of Ko−consolidated
cohesive soils，Soils and Foundations，Vol．26，No．1，85
−95．
sented to the Tokyo Institute of Technology，at Tokyo，
Japan，in partial fulfillment of the requirements for the
（要 旨）
亀井健史，1995．異方圧密粘土の非排水せん断挙動に関する簡便法の提案、島根大学地質学
研究報告，14．
自然地盤上に構造物を建設する際の破壊と変形のメカニズムを高精度に解析するために
は，原位置での土要素の異方力状態を正確に再現したKo圧密状態からのせん断試験結果を
適用する必要がある．しかしながら，実務における三軸試験では三軸試験装置の機能・試験
者・コスト・試験時間の制約等の観点から，ほとんどの場合等方圧密条件下での試験が実施
されている．
上記の点に着目し，本研究では三種類の異方圧密過程の違いが，その後の非排水せん断特
性に及ぼす影響を検討している．その結果，通常の三軸試験装置を用いることによっても，
本研究で提案している応力経路を土要素に作用させることにより，異方圧密粘土の非排水せ
ん断挙動を評価できることを示唆している．